NOTIZIARIO Neutroni e Luce di Sincrotrone - Issue 5 n.1, 2000

NOTIZIARIO Neutroni e Luce di Sincrotrone Rivista del Consiglio Nazionale delle Ricerche

SOMMARIO

Cover photo: The lower thick polystyrene layer is tiled with the two-dimensional neutron scattering pattern exhibited by the thin fuilm of poly(methyl-methacrylate) lying on top of it. The Bragg peaks, the result of interference between the front and back of the top film, are clearly visible. The van der Waals forces across the film tends to destabilise the system, resulting in spinoidal dewetting which manifests itself as a pattering of the film surface (data taken with the SURF Reflectometer at ISIS, courtesy of ISIS facility).

EDITORIALE

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C. Andreani

RASSEGNA SCIENTIFICA Insight into Biosciences from Synchrotron Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 A. Congiu Castellano

Neutron Diffraction for Residual Stress Measurements: Applications to Materials and Components for Automotive Technology . . . . . . . . . . . . . . . . . . . 10 F. Fiori Il

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Critical Behaviour of a Fluid Mixture Confined in a Porous Glass Investigated Through SANS . . . . . . . . . . 16

cura del C.N.R. in collaborazione con il Dipartimento di Fisica dell’Università degli Studi di Roma “Tor Vergata”.

F. Formisano and J. Teixeira

Vol. 5 n. 1 Giugno 2000 Autorizzazione del Tribunale di Roma n. 124/96 del 22-03-96

Nano-Scale Spectroscopy and its Applications to Semiconductors . . . . . . . . . . . . . . . . . . . . . 23 S. Heun and G. Salviati

DIRETTORE RESPONSABILE:

C. Andreani

Chain Deformation in Unfilled and Filled Polymer networks: a SAS Approach . . . . . . . . . . . . . . . . . . . . . . . . . 34

COMITATO DI DIREZIONE:

M. Apice, P. Bosi

W. Pyckhout-Hintzen et al.

COMITATO DI REDAZIONE:

L. Avaldi, F. Boscherini, F. Carsughi, U. Wanderlingh

Stress-Texture Studies in Thin Films and Coatings by Synchrotron Radiation XRD and Neutron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

SEGRETERIA DI REDAZIONE:

D. Catena

P. Scardi

HANNO COLLABORATO A QUESTO NUMERO:

G. Chiarotti, A. Paoletti, E. Rimini, F. Sette, A. Stella GRAFICA E STAMPA:

om grafica via Fabrizio Luscino 73 00174 Roma Finito di stampare nel mese di Giugno 2000

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FACILITIES

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PER NUMERI ARRETRATI:

Paola Bosi, Tel: +39 6 49932468 Fax: +39 6 49932456 E-mail: p.bosi@dcas.cnr.it.

PER INFORMAZIONI EDITORIALI:

Desy Catena, Università degli Studi di Roma “Tor Vergata”, Dip. di Fisica via della Ricerca Scientifica, 1 00133 Roma Tel: +39 6 72594364 Fax: +39 6 2023507 E-mail: catenadesy@roma2.infn.it

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EDITORIALE

con molta tristezza che mi accingo a scrivere questo primo editoriale, nella veste di nuovo direttore del Notiziario, dal momento che in questi primi mesi del 2000 tre gravi lutti hanno colpito la nostra comunità scientifica. La scomparsa di Umberto Grassano, di Vittorio Mazzacurati e di Francesco Paolo Ricci ha lasciato un grande vuoto in tutti noi e in questo numero alcuni colleghi che più da vicino li hanno conosciuti gli dedicano un breve ricordo.

Come direttore del Notiziario voglio dedicare alcune parole, anche a nome del comitato di redazione, all’amico, prima ancora che ricercatore e maestro, Francesco Paolo Ricci, scomparso il 27 Febbraio, che questa rivista ha fondato e diretto fin dalla sua nascita. Paolo ha vissuto gli ultimi anni della sua esitenza, affetto da una incurabile e dolorosa malattia, con lo stesso spirito sereno, ironico, riservato e distaccato di sempre, quasi a volersi scusare con chi gli era più vicino per il distrurbo che gli sembrava di arrecare. Chi ha lavorato insieme a lui ricorda una tensione intensa, eccessiva forse per coloro che gli erano vicino e faticavano a tenere il passo, che scaturiva dalla riflessione continua e da quel bisogno incessante di conoscenza che era al centro delle sua vita. Questa rivista rappresenta un esempio concreto della miriade di atti, interventi, proposte, azioni che hanno caratterizzato il suo lavoro. Paolo era persona dalle forti convinzioni e dal carattere testardo; questo suo modo di essere ha spesso generato divergenze, a volte scontri con colleghi, amici e collaboratori, ma tutto questo non ha mai intaccato l’affetto profondo e la stima che sapeva generare intorno a sé con il suo sorriso aperto, quasi uno specchio di un intimo candore che troppo spesso mascherava con atteggiamenti gigioneschi. Ci mancherà la sua scuola, il suo esempio, ci mancherà immensamente la sua presenza.

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t is with great sadness that I am writing my first editorial, as new director of Notiziario.

I would first of all like us all to remember the three scientists of our community who have recently passed away: Umberto Grassano, Vittorio Mazzacurati and Francesco Paolo Ricci. In this issue, three colleagues who knew them well describe their life and work.

I would like to dedicate a few words, also on behalf of all the editorial committee, to the friend Francesco Paolo Ricci who passed away on February 27th and was founder and director of this Journal from the very beginning. Paolo lived the last years of his life, while he suffered from a painful and incurable disease, with the the same serene, ironical, detached and reserved character so typical of him; it seemed he wanted to excuse himself with those close to him for any disturbance he might cause. Those who worked closely with him know of his intense drive, which sometimes appeared excessive to those close to him and who could not keep pace; this drive originated in his continuous need for understanding which was at the center of his life. This Journal is a concrete example of the many proposals and initiatives which characterized his work.

Paolo was a man with strong opinions who sometimes appeared stubborn. This often gave rise to differences of opinion with colleagues, friends and collaborators; however, this never stopped the strong affection and esteem which surrounded him due to his qualities and his open smile, a mirror of his candour which he too often masked with apparent lightheartedness. We will miss his example and his teaching and we will immensely miss his presence.

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Carla Andreani

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Articolo ricevuto in redazione nel mese di Luglio 1999

INSIGHT INTO BIOSCIENCES FROM SYNCHROTRON RADIATION A. Congiu Castellano Dip. di Fisica and I.N.F.M., Università La Sapienza Piazzale A. Moro 00185 Roma

Recent advances in instrumentation and techniques, combined with high brightness X-ray sources, such as those provided by third generation synchrotrons afford new opportunities for studying biological systems.

This paper describes some results obtained using Time-resolved diffraction, X-ray absorption spectroscopy, Small angle scattering, and Microscopy.

The First International Conference on Biophysics and Synchrotron Radiation [1] was held in July 1986 at Frascati.

Since that year, the usage of synchrotron radiation by biologists has tripled, and the new strategies have been implemented to cope with the dramatic increase in demand. Although it is crystallographers who are mainly responsible for this increased use, biological research at the synchrotron involves a wide variety of experimental approaches.

This is a brief review of some important techniques such as time-resolved crystallography, X-ray spectroscopy, non crystalline diffraction, and imaging that play a key role in life science research.

Three third generation X-ray synchrotron radiation facilities, characterized by high brilliance, have become available within the last few years: the European Synchrotron Radiation (ESRF) facility in Grenoble, France (a 6 GeV ring with a circumference of 850 m), the Advanced Photon Source (APS) in Chicago ( at 7 GeV and ring with a circumference of 1.1 Km) and the Super Photon ring (SPring-8) near Himeji in Japan (at 8 GeV and 1.4 Km circumference). Approximately 30% of the beamlines are devoted to structural biology research, including crystalline and non-crystalline diffraction, spectroscopy and imaging techniques.

Time-resolved crystallography A number of technical advances have helped make SR widely available as a standard technique for protein crystallography. Currently the use of SR has moved away from being a technique used only by a small number of specialists to one that is now routinely applied by the vast majority of protein crystallography groups world-wide.

There have been great advances in techniques for recording multi-wavelength anomalous dispersion

(MAD) data on metalloproteins [2] and more importantly on crystals of proteins or nucleic acids in which the sulfur atom of the methionine has been substituted by Se or Te. The use of area detectors, particularly imaging plates, and Charge Coupled Devices (CCDs), as well as the development of cryogenic freezing techniques, have been critical to recent advances in structural biology.

The SR source has allowed time-resolved crystallographic experiments with nanosecond time resolution to be conducted on myoglobin and photoactive yellow protein.

In both experiments, the molecules are first stimulated and a structural change initiated by a brief laser flash - the so-called pump; after a suitably adjustable time delay in the range from ns to ms, an X-ray pulse falls on the crystal and generates a diffraction pattern - the probe.

Myoglobin, defined as the hydrogen atom of biology, is a heme protein found in muscle tissue. It acts as an oxygen storage protein by reversibly binding oxygen molecules that have been transported to muscle by hemoglobin.

Oxygen (or diatomic molecules such as carbon monoxide) binds to an iron ion located near the centre of the protein.

The iron is part of an organic prosthetic group, the heme, the geometry of which changes with the ligand binding; this change is communicated to the protein through proximal histidine and non-bonded contacts. These structural variations can be triggered by a light flash that breaks the covalent bond between the carbon monoxide and the iron atom: the CO migrates away from the heme; the heme and the protein relax towards the stable deoxyMb structure; if eventually the CO rebinds with the heme the MbCO structure occurs. At room temperature in the crystal, the entire fully-reversible process (a protein quake) is complete in a few hundred microseconds. In October 1994 two groups [3,4] published atomicresolution structures of the photolysed state; the data were collected at 40K at beamline X12-C and X26-C of the National Synchrotron Light Source Brookhaven National laboratory.

Later, at ESRF nanosecond time-resolved crystallographic data were collected [5], with 1.8 Å resolution, during the process of heme and protein relaxation, after carbon monoxide photodissociation and during rebinding.

Photolysis was initiated by 7.5 ns laser pulses.

Similarly the structure of the light-activated, long-lived

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intermediate in the photocycle of PYP (photoreceptor photoactive yellow protein) was determined by timeresolved, multiwavelength Laue x-ray diffraction at a spatial resolution of 1.9 Å and a time resolution of 10 ms [6].

The importance of this research is due to fact that so far three-dimensional structures of photoactive proteins have described proteins in their dark-state conformation.

However , understanding the molecular mechanism for light-induced signal transduction requires clarification of the conformational changes of proteins during the photocycles. Time-resolved crystallography experiments were performed at X26-C beamline of National synchrotron Light Source Brookhaven National laboratory.

X-ray Absorption Spectroscopy EXAFS and XANES spectroscopy has become a structural tool, experimentally accessible to the general scientific community since the advent of XAS beamlines at Synchrotron radiation sources and the contemporary understanding of the basic physics underlying this technique. XAS of dilute systems (such as biological molecules) is only feasible with SRS and these developments have thus led to the application of XAS in structural molecular biology. Accurate bond lengths and active site geometries, as determined by XAS, are particularly important in deriving an understanding of the structure-function relationship in metalloproteins.

The spectrum can be divided into two regions: the edge region (XANES) which contains transitions to bound state orbitals including valence levels, and at higher energies where the core electron is ejected as a photoelectron, the extended X-ray absorption fine structure (EXAFS) region. When neighboring atoms are present these can backscatter some of the emitted photoelectrons. Interference between outgoing and backscattered waves causes the modulation of the X-ray absorption. The EXAFS provides very accurate values for bond lengths, typically to better than 0.02 Å, which is superior by approximately one order of magnitude to accuracies available by protein crystallography. XAS contributes to structural biology in two major ways. It can provide information on species for which crystallography is not available and can also provide corrections and complementary information on systems for which a crystal structure is available. The question of whether there is a difference between an active site in a protein solution and in the crystalline state can be addressed by performing comparative measurements.

XAS edge studies provide the means for determining electronic structure, including such properties as oxidation state, spin state and covalency for specific atomic sites in a macromolecule.

Temperature dependent variations of the XANES of

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Fig. 1. XANES spectra of MbOH at 80K (low spin) and 300K (high spin) and their derivative spectra are compared (Oyanagy et al. 1987)

alkaline metmyoglobin from sperm whale, clearly related to the iron spin state, have been reported by Oyanagy et al. [7] (fig 1). Magnetic susceptivity data showed that Fe(III) in this derivative is in a high spin state (S=5/2) at room temperature and in a low spin state at 80 K (S=1/2).

Recently a study of the spin-structure relationship in metmyoglobin was published [8] describing the analysis of K-XANES spectra in the framework of the multiple scattering approach using spin resolved self consistent potentials. XANES spectra of ferric myoglobin have been acquired as a function of pH (between 5.3 and 11.3). At pH=11.3 temperature-dependent spectra (between 20 and 293 K) have been collected.

The XANES spectra of metmyoglobin exhibit various features evolving with temperature and sensitive to both Fe spin state and Fe-heme structure. The method allows spin effects and local structural effects to be computed separately. It is possible to detect at least three structural states of the Fe-heme complex related to the multistate equilibrium: high-spin Mb+OH2 – high spinMb+ OH– – low spin Mb+ OH–.

In fig. 2 from up to bottom: A) the temperature dependence of the XANES derivative spectrum of alkaline Mb+ OH- ( pH 11.3, T= 293, 220, 150 and 100 K); B) pH dependence of XANES derivative spectra of Mb+; C) XANES spectra of Mb+OH2 (pH 5.3,T= 293 K top curve) and Mb+ OH- (pH 11.3, T= 293 K, middle curve; pH 11.3, T=100 K bottom curve) Bond specific geometric information due to the approximately cos2 α dependence of the EXAFS resulting from the plane-polarized nature of SR, can be obtained from analysis data. Geometrical information is usually not available from non-polarized single-scattering EXAFS. However, in some special cases such information

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can be obtained via the multiple scattering approach.

Single scattering EXAFS refers to the case when the outgoing photoelectron is scattered only once before returning to the absorber. Multiple scattering (MS) occurs when multiple atoms backscatter the photoelectron; in this case the EXAFS is sensitive to the relative arrangement of atoms. Information on the number, the type and the geometrical arrangement around the absorber of nearby and more distant scattering centers [9,10,11,12] can be obtained by fits to experimental data that also include MS contributions.

Using crystallographic data of similar compounds and

structural indications from other techniques, the atomic group coordinated to the metal in the active site of metalloproteins may be identified.

Recently [13] a detailed study of EXAFS spectrum of tetanus neurotoxin (TeNT) at Zn K-edge was performed which allows the complete identification of the amino acid residues coordinated to the zinc active site.

In this case the XAS study is the only structural probe because there are no crystallographic data available for TeNT. Understanding the structure-function relationship and therefore the biological mechanism responsible for a devastating disease such as tetanus can help to devise

Fig. 3. The results of the best fits on thermolysin, astacin and TeNT reported : (Panel a) n-body independent contributions considered in fit are reported; (Panel b) the total simulated signals, dotted line, superimposed on the experimental points. At the bottom of panels b, residual functions are plotted ( Meneghini et al. 1998)

are the are the

new therapeutic strategies. Comparing the absorption spectrum of tetanus neurotoxin to that of two other structurally similar zinc-endopeptidases - thermolysin and astacin - the authors inferred that in tetanus neurotoxin, zinc is coordinated to two histidines and a tyrosine. The EXAFS data were analysed using the theoretical approach developed by Benfatto et al. (1986) and Filipponi et al. (1995a) and the GNXAS package. The usual MS expansion is replaced by an equivalent expansion in terms of irreducible n-body signals, Îł(n)..

Fig. 3 shows the results of best fits on thermolysin, astacin and TeNT: (Panel a) n-body independent contributions considered in the fit are reported; (Panel b) the total simulated signals (dotted line) are superimposed on the experimental points. At the bottom of panels b, the residual functions are plotted. The hypothesized structure of Zn site in TeNT is compared with those in thermolysin and astacin in fig. 4.

Fig. 2. Up to bottom: A) the temperature dependence of the XANES derivative spectrum of alkaline Mb+ OH- ( pH 11.3, T=293,220,150 and 100 K); B) pH dependence of XANES derivative spectra of Mb+ ; C) XANES spectra of Mb+OH2 (pH 5.3, T= 293K top curve) and Mb+ OH- (pH 11.3, T= 293 K, middle curve; pH 11.3, T=100 K bottom curve) (Della Longa et al.1998)

Non-crystalline diffraction Many biological macromolecules, such as enzymes and complexes, viruses and photosynthetic reaction centers, may be investigated by crystallographic analysis. Over the last decade a large number of new structures has been

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Fig. 4. The structure of Zn site in TeNT (deduced from EXAFS analysis) is compared with those in thermolysin and astacin (Meneghini et al. 1998)

solved and the increasing availability of SR has been a key element in this spectacular success. Diffraction methods have several limitations that are important for biological systems.

Despite the utility of determining static crystal structures, many biological systems are non-crystalline in vivo or are simply not amenable to crystallographic study either because they cannot be crystallized, or their structures or dynamic behaviours change upon crystallization. An even more important consideration is that time resolved information concerning changes in structure, such as biological macromolecules perform their functions, is experimentally inaccessible by the techniques of macromolecular crystallography.

All biological structures have some degree of spatial or dynamic order which can potentially be probed by non crystalline diffraction and small angle X-ray scattering techniques.

In recent years many experiments have been performed on muscle fibre and connective tissues by means of diffraction at Synchrotron radiation facilities.

X-ray diffraction is currently the only technique which provides direct structural information at molecular level

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on the mechanism of the conversion of chemical energy into the production of strength and movement in muscle tissues. The high resolution of diffraction patterns allowed by the use of Synchrotron radiation sources suggests that a revision of some hitherto accepted models of contraction is necessary.

Time resolved X-ray diffraction has also been used to study the physical properties of synthetic model membrane systems in order to understand such processes as membrane fusion using various kinetic techniques such as T-jumps induced with lasers or rapid pressure changes.

Another experimental approach uses oriented lipid bilayer systems to obtain structural information from integral membrane proteins.

In the experiment performed at ELETTRA (Trieste) an infrared laser that emitted 1-2 kJ pulses for duration of about 1 ms was used. Using suitable crystal optics the laser pulse can be deposited onto a capillary sample holder for SAXS such that temperature jumps of 10-20 0C are made in 1ms time, which corresponds to heating rate of 104 K/sec. For the investigation of liquid crystalline phase transitions which frequently span a transition range of 0.1 0C or less, this means that transition can be triggered within tens of microseconds.

Solution scattering can be used to measure a small number of important, molecular parameters: radius of gyration, molecular weight, molecular volume and distance distribution function P(r).

An emerging application is that of stopped flow and other kinetic techniques for studying protein folding.

The third generation sources combine high flux (1013 ph s-1) with small source sizes (100 micron) and very

Fig. 6. An image of EGF labeled with a fluorescein derivative, BODIPY in A431 cells. Bar=5 Âľm. (With permission of authors) ( Tobin et al. 1998)

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low angular divergences (50 microradians). The small intense beams from undulator sources could also make more effective use of continuous flow devices for time resolved solution scattering and deliver more flux to very small specimens. For further progress to be made novel technologies such as advanced charged coupled device (CCD) based detectors will be required.

Another obstacle is that most small angle scattering and non-crystalline diffraction experiments cannot make use of cryogenic techniques since freezing is incompatible with the physiological phenomenon.

The compactness of a protein structure is an important parameter that characterizes its degree of folding.

Although the information about the secondary and tertiary structure is important, changes in the size and shape of a protein are crucial to an understanding of the mechanism of protein folding.

As protein unfolding often involves the exposure of hydrophobic groups, protein aggregation can be a factor limiting the utility of scattering techniques for probing compactness in unfolded structures; hence the utilization of Synchrotron Radiation sources can be a great advantage as it facilitates working at the lowest protein concentration.

The SAXS intensity distribution of a protein in solution can be approximated by the Guinier relation: I(q)= I0 exp (-Rg 2q 2/3) where I0 denotes the intensity at zero scattering angle, q=2πS (named momentum transfer) and Rg is the radius of gyration. The Kratky plot, I(q)q2 versus q, is a useful tool of the scattering profile for characterizing the structure of an unfolded or folded chain: a peak in the Kratky plot (panel b and c) indicates a compact globular structure, and the absence of a peak is an indication of a loss of globularity. In fig. 5 Guinier plots (panel a) and Kratky plots of ovalbumin, studied at LURE beamline, in

1540 cm-1 (amide II)

2925 cm-1 (lipids)

Position [µm] Fig. 8. A sample of a living, dividing cell, whose optical image is shown at the top, was mapped and the distributions of protein and lipid are shown. (With permission of authors) (Bantignies et al. 1998).

Fig. 7. Fluorescence lifetime of the region outlined in fig 6 is reported. A good fit was obtained for a two exponential decay profile. (With permission of authors) (Tobin et al. 1998)

the native and in some thermal and chemical denatured states, are reported [14]. The refolding and unfolding of the apomyoglobin were studied at the Stanford synchrotron radiation laboratory [15] by time resolved small angle x-ray scattering. Refolding was triggered by rapid dilution of 10 mg/ml protein in 5.6 M urea to 1.4 mg/ml in 0.8 M urea.

Time resolved stopped flow X-ray scattering measurements using the integral intensity of scattering allowed the kinetic refolding of β-lactoglobulin [16] to be

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studied. The experiment performed on SAXS beamline of the Photon Factory (Tsukuba, Japan) showed that both compaction and secondary structure formation in protein folding are rapid processes, taking place on a millisecond time-scale.

Imaging One of the major advances at the high brightness third generation synchrotron is the dramatic improvement of imaging capability. Considerable efforts have been made to develop imaging X-ray, UV and infrared spectroscopic analysis on a spatial scale ranging from a few microns to 10 nm. These developments make use of infrared light, ultraviolet light and X-rays from 100 eV to 10 keV. X-ray microscopy Below 10 keV, X-ray photons are primarily absorbed or phase shifted as they pass through matter. This means that X-ray microscopes offer unique opportunities for quantitative imaging without inelastic or multiple elastic scattering effects. Furthemore the penetration of X-rays in hydrated biological materials can be large: at energies between the carbon and oxygen absorption edges at 284 and 534 eV, respectively (the so-called water window), water layers of up to 10 microns thick are easily penetrated, while thin cellular structures provide sufficient contrast. In spite of this high contrast, the specimen must be illuminated with sufficient X-rays per pixel and therefore the dose (energy deposited per unit mass) can produce radiation damage. The introduction of cryogenic methods for studying frozen hydrated specimens mitigates (primarily by the immobilization of radiolytical products in the ice matrix) many of the problems of radiation damage. A number of approaches have been used to construct successful Synchrotron X-ray microscopes. In particular two use zone plates as focusing optics to obtain magnification images in transmission X-ray microscopes (TXMs) or to produce finely focused spots through which the specimens are rastered in Scanning Transmission X-ray Microscopes (STXMs). The spatial resolution of these microscopes can be 30-50 nm with soft X-rays (< 1 keV).

Whereas TXMs tend to use condenser zone plates as monochromators with an energy resolving power E/â&#x2C6;&#x2020;E of ~300, STXMs tend to use reflective grating monochromators with a resolving power of 3,000 or more. The higher energy resolution makes STXMs especially well suited to map the distribution of the chemical bonding states of the major constituents of a sample.

One advantage of X-ray microscopy is its ability to study single cells in their entirety, rather than be limited to a thickness of 400 nm, as is the case in electron tomography of frozen hydrated specimens.

Magowan et al. [17] used X-ray microscopy to investiga-

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Fig. 5. Guinier plots (panel a) and Kratky plots (panel b and c) of ovalbumin, in the native and some thermal and chemical denatured states are reported (Congiu Castellano et al. 1996)

te the structural development of Plasmodium falciparum malaria parasites in normal and genetically abnormal erythrocytes treated with cysteine protease inhibitors. X-ray microscopy revealed the relationship between the host erythrocyte membrane and the intraerythrocytic malaria parasite by demonstrating for the first time that constituents of the erythrocyte membrane play a role in normal parasite structural development.

UV microscopy At the Daresbury Laboratory Synchrotron Radiation Source (SRS) a confocal microscope provides a powerful new tool for studying biological systems (18). Most commercial instruments operate with fixed wavelength lasers, because confocal microscopy requires a small, high intensity light source. The synchrotron radiation

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source provides easily tunable continuous UV-visible radiation which, with the storage ring operating in single bunch mode, has a pulsed time structure ideally suited for fluorescence lifetime spectroscopy.

The collection of fluorescence lifetime data from microscopically small samples and regions within samples is allowed by the combination of single photon counting and confocal microscopy. A resolution of 99 nm (lateral) and 483 nm (axial) at wavelength 290 nm has been obtained.

Time resolved microvolume fluorescence measurements have been employed in the study of steroid hormone interaction in Leydig cells as the lifetime of a fluorophore is sensitive to its environment and can therefore be used to investigate the binding of signalling molecules in microscopic regions of individual cells. This method is also being used in the study of growth factor binding and internationalization in mammalian cells.

Fig. 6 is an image of EGF labeled with a fluorescein derivative, BODIPY, in A431 cells.

Fig. 7 shows a fluorescence lifetime of the region outlined in fig. 6. A good fit was obtained for a two exponential decay profile.

functional groups in living cells is technically very challenging, it may offer substantial fresh insight into the cell's chemical kinetics. A sample of a living, dividing cell, whose optical image is shown at the top of fig. 8 was mapped and the distributions of protein and lipid have been shown [19]. Infrared spectroscopy is an analytical technique that is sensitive to the mineral as well as to the biological components in bone. The technique can be used to determine the chemical composition of subchondral bone as a function of subchondral bone thickness and severity of osteoarthritis.

Infrared microscopy The middle infrared spectral range, which covers wavelengths from 3-15 microns, is known as the "fingerprint region". It is here that the intramolecular vibrational modes exist and these play an important role in analytical work. Infrared Synchrotron radiation is an ideal source for microspectroscopy for two reasons: -high spatial resolution -the ability to work with samples of higher optical density due to the superior signal levels available through the small apertures. Biological materials, such as cells, hair and bone, all contain molecules with a rich selection of intramolecular vibrational modes due to proteins, lipids and nucleic acids.

The dominant features in the spectra are: the broad bands from 2500-3500 cm-1 due to the NH and OH groups found in water, proteins and polysaccharides (3063-3290 cm-1) and both the symmetric and asymmetric stretching modes of methylene (CH2) and methyl (CH3) groups (2850-2960 cm-1) in lipids and proteins. The other main modes arise from CO-NH vibrations which are called amide1 (1650 cm-1) and amide 2 (1540 cm-1). Sharper modes occur below these bands due to d-CH vibrations (1450 and 1390 cm-1) and there is an amide 3 band at 1238 cm-1.

These modes could be used to make chemical images of proteins, lipids and nucleic acids in living biological cells without the use of stains and fixatives. The functional groups could be identified and their concentration profiles mapped with a spatial resolution of a few microns. While the identification and mapping of such

References

Conclusions Great potential is offered to the biosciences by the use of Synchrotron radiation.

Structural determination of biological systems at high resolution, time-resolved spectroscopies and spectromicroscopies applied to proteins and living cells have allowed the acquisition of very important results concerning fundamental problems in the life sciences, such as the structure-function relationship, the folding process of proteins and the cell's chemical kinetics.

1. Biophysics and Synchrotron Radiation (1987) Ed. A. Bianconi and A. Congiu Castellano Springer Verlag Berlin Heidelberg New York London Paris Tokyo 2. Zanotti G. Notiziario neutroni e Luce di sincrotrone Vol.3 n.2 (1998) 3. Schlichting I., Berendzen J., Phillips G.N. Jr and Sweet R.M. Nature, 371, 808-812 (1994) 4. Teng T.Y., Srajer V. and Moffat K. Structural Biology 1, 701-705 (1994) 5. Srajer V., Teng T.Y., Ursby T., Pradervand C., Ren Z., Adachi S., Schildkamp W., Bourgeois D., Wulff M., Moffat K. Science 274,1726-1729 (1996) 6. Genick U.K., Borgstahl G.E.O., Kingman N., Ren Z., Pradervard C., Burke P.M., Srajer V., Teng T.Y.,Schildkamp W., McRee D.E., Moffat K., Getzoff E.D. Science 275,1471-1475 (1997) 7. Oyanagy H., Iizuka T., Matsushita T., Saigo S., Makino R., Ishimura Y., and Ishiguro T. J. Phys. Soc. Jpn 56, 3381-3388 (1987) 8. Della Longa S., Pin S., Cortes R., Soldatov A.V., and Alpert B. Biophysical J. 75,3154-3162 (1998) 9. Strange R.W., Blackburn N.J., Knowles P.F., and Hasnain S.S. J.Am. Chem. Soc. 109, 7157-7162 (1987) 10. Benfatto M., Natoli C.R., Bianconi A., Garcia J., Marcelli A., Fanfoni M., and Davoli I. Phys. Rev. B 34, 6426-6433 (1986) 11. Filipponi A., Di Cicco A., and Natoli C. R. Phys. Rev. B 52, 15122-15134 (1995) 12. Filipponi A., Di Cicco A., M.J. Scott, Holm R.H., Hedman B., and Hodgson K.O. J. Am. Chem. Soc. 119, 2470-2478 (1997) 13. Meneghini C. and Morante S. Biophysical J. 75, 1953-1963 (1998) 14. Congiu Castellano A., Barteri M., Bianconi M., Bruni F., Della Longa S., and Paolinelli C, Z. Naturforsch. 51c,379-385 (1996) 15. Eliezer D., Jennings P. A. and Wright P. E. Science 270,487-488 (1995 ) 16. Arai M., Ikura T., Seminosotnov G.V., Kihara H., Amemya Y., and Kuwajima K. J.Mol. Biol 275:149-162 (1998) 17. Magowan C., Brown .T., Liang J., Heck J., Coppel R. L., Mohandas N., and Meyer-Ilse W. Proc.Natl.Acad.Sci. 94, 6222-6227 (1997) 18. Tobin M.J., Martin-Fernandez M., and Jones G.R. Synchrotron radiation News 11, 24-30 (1998) 19. Bantignies J.L., Carr L., Dumas P., Miller L., and Williams G.P. Synchrotron radiation News 11, 31-37 (1998)

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Articolo ricevuto in redazione nel mese di Aprile 2000

NEUTRON DIFFRACTION FOR RESIDUAL STRESS MEASUREMENTS: APPLICATIONS TO MATERIALS AND COMPONENTS FOR AUTOMOTIVE TECHNOLOGY F. Fiori Istituto Nazionale per la Fisica della Materia, Unità di Ricerca di Ancona and Università di Ancona, Istituto di Scienze Fisiche Via Ranieri 65, I-60131 Ancona (Italy)

The application of neutron diffraction technique for the determination of residual stresses in some materials and components for automotive technology is presented. After introducing the basic principles of the method, experimental results on the following samples are reported: AA6082 cylinders submitted to different quenching and ageing treatments, and a crown gear submitted to an innovative multi-frequency induction surface treatment. In the latter case comparisons with complementary measurements by X-ray diffraction are also presented.

1. Introduction Several manufacturing industrial processes and thermal or mechanical treatments leave residual stresses (RS) within materials and components. RS can be beneficial or detrimental, depending if they counteract or not external loads: tensile RS, when added to external loads (e.g. in welded samples), can accelerate the fatigue process and induce earlier failure of the component. On the other hand, surface treatments (e.g. shot-peening) are frequently used to improve hardness properties and to induce compressive RS, thus enhancing the surface toughness and wear resistance under operating conditions.

Therefore, the importance of a detailed knowledge of the spatial and directional distribution of RS in the component is evident, as it can permit to get a feeling of the lifetime of the component with respect to the environment it has to be used in. Furthermore, in many cases a precise measurement of RS can be used to validate mathematical and numerical models of technological processes, leading to more efficient and saving-cost manufacturing procedures.

To this end, several experimental methods exist, both destructive (e.g. hole drilling [1]) and non-destructive, such as acoustic-elastic methods by ultrasounds [2,3,4], micromagnetic methods (e.g. Barkhausen noise) [2,4], and diffraction techniques by X-rays or neutrons [4,5,6,7].

In particular, diffraction techniques are nowadays probably the most important and reliable ones. X-ray diffractometers dedicated to RS measurements are available in

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many research and industrial laboratories, and highenergy X-rays (synchrotron radiation) are gaining more and more importance for this purpose. Furthermore, spectrometers dedicated to RS determination are present in all of the European Large Scale Facilities at neutron sources, like ILL and LLB (F), ISIS (UK), HMI-BENSC (D) and Risø National Laboratory (DK).

Fig. 1. Schematics of a neutron diffraction experiment for residual stress measurements.

The main difference between “traditional” X-rays and neutrons (and synchrotron radiation as well) is that only stress states in surface layers (≈10 µm) can be investigated

by X-rays, as they are strongly absorbed in metals, while neutrons can penetrate down to few centimetres inside the bulk material. Therefore, in principle, the complete determination of the stress field can be obtained by a combined use of the two techniques.

The principles of neutron diffraction for RS determination, together with some relevant experimental aspects, are briefly described below. Then two typical applications to materials and components for automotive technology are presented, namely an investigation of RS induced by quenching in AA6082 alloy and by a particular surface thermal treatment (multi-frequency

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where d0,hkl is the unstrained interplanar distance. Finally the stress components can be calculated according to the Hooke’s law:

σij = Cijmnεmn

Fig. 2. The DIANE diffractometer at LLB, Saclay, France.

induction tempering) in a steel crown gear. In the second case comparisons with complementary measurements by X-ray diffraction are also presented.

2. Neutron diffraction for residual stress measurements 2.1. Theoretical principles The interplanar distance of a particular set of lattice planes (dhkl) is determined by means of the Bragg’s law:

λ = 2dhkl sinθ hkl

(1)

where 2θhkl is the diffraction angle with respect to the incident beam direction and λ is the neutron wavelength (fig.1). The lattice strain, whose direction is parallel to the exchanged wave vector Q, is calculated as

ε hkl =

εij = Sijmnσmn

(3)

The C and S 4th-rank tensors are called stiffness and compliance, respectively. In principle they have 34=81 components, but this number is reduced by their symmetry, and also by symmetry properties of the crystals. For isotropic crystals, such as Aluminium, there are only 2 independent components, which can be expressed in terms of the Young’s (E) and Poisson’s (ν) moduli. Ehkl and νhkl, referred to the investigated lattice planes, can be calculated according to existing models (Voigt, Reuss, Kröner) [4,7]. For isotropic crystals the elastic constants do not differ sensibly from a plane to another and in such cases their macroscopic values can be used as a good approximation.

Strain and stress are 2nd rank symmetric tensors, thus having 6 independent components. Therefore, in principle, measurements should be carried out in at least 6 different spatial directions of the Q vector. Anyway, in most cases, the principal strain/stress directions can be assumed to coincide with the sample principal axes.

Measurements in these three directions are thus sufficient for the complete determination of ε and σ and.

Including eq.2 in the first of eqs.3, principal stresses can be directly written as functions of the interplanar distances measured along the principal directions (i, j, k):

dhkl − d0 , hkl d0 , hkl

or

σi =

(2)

(

)

  E  (1 − ν ) di + ν d j + dk − 1  1 − 2ν  (1 + ν ) d0  

(4)

Incident beam

Slits Radial collimator Sample

Detector

Sample table

Radial collimator

Gauge volume

Detector

Beam stop Fig. 3. The ENGIN diffractometer at ISIS, Rutherford Appleton Laboratory, UK.

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2.2. Experimental aspects As already remarked above, diffractometers dedicated to RS measurements are available at the main European neutron sources. The DIANE spectrometer is shown in fig.2. It was built at the Laboratoire Léon Brillouin (LLB), Saclay, France in the framework of a project carried out in collaboration between LLB and INFM [8]. The ENGIN diffractometer at the ISIS pulsed spallation source, built in the framework of a Brite/Euram project which the Research Unit of Ancona of INFM took part in [9], is shown in fig.3.

In a neutron diffraction experiment the incident monochromatic neutron beam (wavelength λ) is diffracted by the polycrystalline sample, and the scattered intensity is recorded in a Position-Sensitive Detector (PSD), at an angle 2θ with the incoming beam direction (fig.1). Two collimators placed before and after the sample define the cross-sections of the incident and the diffracted beams, so that the best geometrical definition of the gauge volume inside the sample is obtained with 2θ≈90°. Its size is usually in the range from about 1 mm3 up to few cm3, and the measured value of RS must be intended as an average over the sampling volume itself.

The interplanar distance dhkl of the investigated (hkl) lattice planes is obtained from the Bragg's law (eq.1). In monochromatic neutron beams coming from steady nuclear reactors, dhkl is determined by precisely measuring the diffraction angle 2θ, corresponding to the intensity maximum of the Bragg peak. In the case of spallation sources, such as ISIS, where neutrons in a wide wavelength range are produced in the collision of high energy protons with a heavy target, 2θ is fixed at 90°. In this case dhkl is determined by measuring the Time-OfFlight (TOF) of neutrons between a fixed “start” point and the detector. In fact, according to the De Broglie relation,TOF is given by TOF =

mL λ h

(5)

where m is the neutron mass, L the flight path length and h the Planck’s constant. By the TOF technique the whole diffraction pattern is recorded, and the lattice parameter (a) is obtained by Rietveld refinement algorithms [10].

The determination of the unstrained interplanar distance d0 (or lattice parameter a0) is a crucial point of the experimental technique, as the definition of a true stress-free sample is not always trivial. Strains to be measured are usually of the order of 10-4 so that, from eqs. 2 and 4, it is obvious that a not precise determination of d0 can lead to big errors and even to unreliable RS values. In most cases, the following methods are commonly considered to be sufficiently “safe” from this point of view:

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1.

2.

measurements in powders of the same material of the investigated sample (often ground from the sample itself): the powder, even if submitted to plastic deformations, can be considered as stressfree, especially if submitted to stress-relieving heat treatments; measurements in small coupons, completely immersed in the neutron beam, so that the equilibrium condition

σ =

1 V

∫ σ ⋅dV = 0 V

(6)

can be applied. From eqs.6 and 4 it is straightforwardly shown that, in the case of cubic coupons, d0 can be calculated as the mean value among the interplanar distances measured in the three principal directions; 3. measurements in regions of the sample which can be assumed not to be influenced by localised stressinducing treatments (for instance, regions far from welds); 4. when allowed by the sample geometry, application of mathematical conditions to the sample itself (equilibrium, plane stress).

In any case, the validity of all of these methods should be checked at least a posteriori, as some problems can raise and make them unuseful. Among them, for methods 1 and 2 we mention the presence of II order microstresesses, at the distance scale of a few grains, which otherwise should be assumed to be vanishing, and the presence of RS pre-existing to the investigated treatments for method 3. Furthermore, when using powders or coupons, systematic experimental errors can be very difficult to be foreseen and eliminated. Finally, microstructural changes (e.g. precipitation) induced by thermal treatments, leading to different unstrained lattice parameter from one point to another in the sample, should be taken into account. Typical cases of this are welded components. In principle, in such cases, a different d0 should be used for each of the investigated gauge points [11,12].

3. Measurements 3.1. Quenched AA6082 Al alloy Fast quenching of metals from high temperatures causes high thermal gradients moving from surface zones to the bulk, with subsequent inhomogeneous cooling. Different cooling rates generate plastically deformed zones and the surface layers, more deformed than the inner ones, will result in a compressive stress state.

AA6082, a well known Al-Mg-Si alloy, widely used in

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automotive technology, exhibits mechanical properties that are highly sensitive to thermal treatments. In fact, starting from the solubilised state, too slow cooling rates can lead to the reduction of several important properties (strength, formability, toughness, etc.), while too rapid cooling can generate undesired macroscopic deformations. From the microscopic point of view, the alloy elements are usually solubilised by high temperature (520°C) annealing. In the subsequent cooling procedure, rapid quenching is needed to create the Guinier-Preston zones, whose size increases as a consequence of the subsequent ageing, leading to an improvement of the mechanical behaviour. However a too rapid quench can enhance RS. On the other hand, too slow quenching generates coarse incoherent precipitates leading to a reduction of the mechanical performances.

In order to optimise the quenching rate and the subsequent thermal treatment, a research project aiming to determine the correlation between RS field and microstructure changes induced by different quenching rates were undertaken. In this framework, combined studies of RS by neutron diffraction [13] and microstructure evolution by Small Angle Neutron Scattering (SANS) [14], together with the use of mechanical tests and electron microscopy observations, were carried out.

Four cylindrical samples (10 cm high, radius 5 cm) were investigated in neutron diffraction experiments. They were submitted to two different quenching rates, and to two subsequent ageing treatments, as reported in tab.I. Sample n.

Quenching In water @ 20°C (30°C/s)

T6 (16h @ 165°C)

3

In water @ 20°C (30°C/s)

Natural (72h @ 40°C)

8

In boiling water (1°C/s)

T6 (16h @ 165°C)

9

In boiling water (1°C/s)

Natural (72h @ 40°C)

the Q vector parallel to the three principal strain/stress directions, assumed to be coincident with the radial, hoop and axial ones.

In all samples different points lying along a cylinder radius were considered, at two different depths: 3 mm under one of the bases and 25 mm (middle of the specimen). The unstrained lattice parameter (interplanar distance in the ILL measurements) was calculated by imposing the equilibrium condition

∫

R

σ ⊥ dA =

A

∫σ

axial ( r ) ⋅2πr ⋅ dr

=0

0

(7)

where A is the surface area of a whole section of the sample and σ ⊥ is the RS component normal to it. For each sample, the condition (7) was applied to both the radial scans performed, giving coincident values within experimental errors. The residual stresses measured

Ageing

2

Fig. 4. Residual stresses in AA6082 quenched cylinders; Z=25 mm (middle of the specimen).

Tab. I. Investigated samples and thermal treatments.

The AA6082 macroscopic elastic constants have been taken for stress evaluation: E=69 GPa, ν=0.383.

Samples n. 2, 3 and 8 (tab.I) were investigated at the ENGIN diffractometer of the ISIS spallation source at the Rutherford Appleton Laboratory (UK). Two different detector banks, at ±90° with respect to the incident beam direction, allowed the simultaneous measurement in two mutually perpendicular Q directions. The sampling volume was 2x2x10 mm3.

Sample 9 was investigated at the D1A diffractometer of the ILL, Grenoble (F). In this case the Al (200) peak was considered (d200≈2.024 Å). The used wavelength was λ=2.99 Å, thus giving a scattering angle 2θ≈95°. The sampling volume was 4x4x1 mm3.

In both experiments measurements were carried out with

Fig. 5. Residual stresses in AA6082 quenched cylinders; Z=3 mm under the specimen surface

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along the radial direction at 25 mm from the cylinder bases (middle of the sample) are shown in fig.4. The effect of the quenching rate is evident comparing sample #2 to #8 and #3 to #9: independently on the subsequent ageing treatments, the cylinders appear to be free of RS when submitted to slow quenching.

Samples submitted to fast quenching show compressive RS in the outer zones, and tensile ones in the bulk, as expected according to above consideration. The effect of ageing on fast-quenched samples is a reduction of the RS values by a factor ≈2 in the sample submitted to T6 treatment (#2), with respect to the one submitted to natural ageing (#3).

Concerning measurements along the radial direction at 3 mm under one of the cylinder bases (fig.5), again samples

Fig. 6. Residual stresses in steel crown gear submitted to MFIT surface treatment; neutron diffraction measurements.

submitted to slow quenching are essentially stress-free.

As expected, in the fast-quenched samples compressive RS are found, higher by a factor ≈2 in sample #3 (natural ageing) with respect to #2 (T6 ageing).

Also as expected, in all cases the radial component approaches zero near the side cylinder surface, thus fulfilling boundary conditions (the RS component perpendicular to the sample surface should be zero on the surface itself).

3.2. Steel crown gear Thermal austenitising and tempering treatments are being developed in automotive industry to prevent crack initiation and propagation, especially in components where stress intensity factors influence the stress field and ultimately the fatigue life of the component. This is the case of crown gears, where the tooth root typically undergoes impulsive and very high loads which frequently cause cracking if tensile RS are present at the surface. The sign reversal of these stresses

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is the aim of austenitising and tempering treatments.

In the case of automotive crown gears, failure can occur mainly due to fatigue under bending (tooth breaking), tooth side surface degradation (pitting) and collapse of sub-surface layers (spalling). According to finite-element calculations, the applied loads during operation are maximum at the surface or just below it. The most critical region is the tooth root, where fatigue cracks are initiated and act as stress concentrators. The generation of a compressive RS is then fundamental to prevent the initiation of fatigue cracks. To this end the MultiFrequency Induction Tempering (MFIT) technique was developed [15], whose effectiveness was checked by neutron and X-ray diffraction [16].

The investigated sample is a UNI55Cr3 steel crown gear.

The material composition is the following (wt%, Fe bal.): C=0.52-0.59, Si=0.1-0.4, Mn=0.7-1, Cr=0.6-0.9, P=S=0.035.

The gear has a tooth height of 9.25 mm and an axial thickness of 25 mm; the tooth helix inclination is 30°.

From the microstrucural point of view, the final result of MFIT is the generation of a martensitic structure (hardness > 750 HV) up to a depth of about 1/2 of the tooth height, and 0.4 mm under the root, and to a sorbitic (fine globular pearlitic phase) inner structure (hardness 370÷420 HV) up to 0.8 mm under the tooth root.

According to numerical simulations [15], MFIT can generate comparable or lower compressive RS values with respect to other conventional techniques (e.g. thermochemical treatments, shot-peening), but for a depth about twice as big.

X-ray diffraction measurements were carried out with Co-Kα radiation, applying the "sin2ψ" technique [4,7].

The investigated Bragg peak was α-Fe (310).

Neutron diffraction measurements were carried out at the E3 diffractometer of the HMI-BENSC, Berlin (D). The used neutron wavelength was 1.370 Å, and the

Fig. 7. Residual stresses in steel crown gear submitted to MFIT surface treatment; X-ray diffraction measurements.

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Fig. 8. Residual stresses in steel crown gear submitted to shot-peening; X-ray diffraction measurements

investigated peak was α-Fe (211). In this case measurements on UNI55Cr3 powders were performed for the evaluation of the unstrained interplanar distance.

The powders were ground from the martensitic layer and the parent material of the investigated specimen.

Both in neutron and X-ray experiments gauge points at different depths under the tooth root were considered.

The results for the hoop RS component measured by neutron and X-ray diffraction are shown in fig.6 and 7, respectively. The agreement between the two techniques is very good, taking into account that neutrons results are averaged on a wide gauge volume (2x2x2 mm3).

According to X-ray measurements, the maximum RS level introduced by MFIT is about -700 MPa. Far from the surface the RS are tensile (+200 MPa), as expected according to equilibrium conditions. The compressive zone extends up to 500 µm from the surface. This is due to the martensitic layer at the tooth root surface which results in compression after quenching, as its expansion is not allowed by the tough sorbitic substrate.

In neutron measurements the gauge point nearest to the surface actually includes the whole range investigated by X-rays, thus explaining the lower level detected (-400 MPa). The +200 MPa level far from the surface is confirmed. X-ray measurements have been also carried out on a sample submitted to a conventional shotpeening treatment. In this case a higher peak RS is reached (-1000 MPa), but the resulting compressive zone is about twice as narrow (fig.8).

4. Conclusions The neutron diffraction technique for residual stress determination has been presented, and its main theoretical and experimental features have been briefly discussed.

Some results concerning its applications to materials and components for automotive technology have been shown. The influence of quenching rate and subsequent ageing treatments in AA6082 alloy has been investigated, showing that the slow quenching rate leads to vanishing stresses. Fast quenching gives rise to residual stresses, compressive at the surface and tensile in the bulk, as expected. A reduction of these stresses by a factor ≈2 was found in samples aged according to the T6 process.

The effectiveness of Multi-Frequency Induction Tempering technique to induce compressive residual stresses at the surface of a steel crown gear was demonstrated by neutron and X-ray diffraction. The stress level found is compatible with expectations, and its comparison with the one induced by a conventional technique (shot-peening) shows a twice as narrow compressive zone, though the peak stress is slightly lower.

References 1. A.M. Jones, AERE Rep. R13005, Materials Department Division, Harwell Lab., Oxfordshire, 1989.

2. M.R. James, O. Buck, CRC Crit. Rev. in Solid State and Materials Science, 9 (1981) 61.

3. E. Schneider, K. Goebbels, Non-Destructive Detection and Analysis of Residual Stress States using Ultrasonic Techniques, in Residual Stress, ed. V.Hauk, E.Macherauch, DGM Verlag, 1983.

4. V. Hauk, Structural and Residual Stress Analysis by Nondestructive Methods, Elsevier Publ., 1997.

5. A.J. Allen, MT. Hutchings, C.G. Windsor, C. Andreani, Adv. in Phys.

34 (1985) 445.

6. Proc. of the NATO Adv. Res. Workshop on Measurement of Residual Stress Using Neutron Diffraction (Oxford, March 1991), Kluwer Acad.

Publ., 1992.

7. I.C. Noyan, J.B. Cohen, Residual Stress - Measurement by Diffraction and Intrerpretation, Springer-Verlag, New York, 1987.

8. M. Ceretti, R. Coppola, A. Lodini, M. Perrin, F. Rustichelli, Physica B, 213-214 (1995) 803.

9. I. Harris, P.J. Withers, M.W. Johnson, L. Edwards, H.G. Priesmeyer, F. Rustichelli, J.S. Wright, Proc. of the 4th Eur. Conf. on Adv. Materials and Processes – Symposium F (Materials and Processing Control), Padova, Italy, 25-28/9/1995, Associazione Italiana di Metallurgia, p.107.

10. R.A. Young (ed.), The Rietveld Method, International Union of Crystallography, Oxford University Press (1993).

11. G. Albertini, G. Bruno, B.D. Dunn, F. Fiori, W. Reimers, J.S. Wright, Mat. Sci. Eng. A 224 (1997) 157.

12. A.D. Krawitz, R.A. Winholtz, Mat. Sci. Eng. A 185 (1994) 123.

13. G. Albertini, G. Caglioti, F. Fiori, T. Pirling, V. Stanic, J. Wright, Materials Science Forum, in print (2000).

14. G. Albertini, G. Caglioti, F. Fiori, R. Pastorelli, Physica B, 276-278 (2000) 921.

15. F. Romani, Graduation Thesis, Faculty of Engineering, University of Ancona, Italy, 1998.

16. G. Albertini, G. Bruno, F. Fiori, E. Girardin, A. Giuliani, E. Quadrini, F. Romani, Physica B, 276-278 (2000) 925.

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CRITICAL BEHAVIOUR OF A FLUID MIXTURE CONFINED IN A POROUS GLASS INVESTIGATED THROUGH SANS F. Formisano I.N.F.M., Largo Enrico Fermi, 2 Arcetri, I-50125 Firenze, Italy.

J. Teixeira Laboratoire Léon Brillouin, C.E.A./C.N.R.S., F-91191- Gif-sur-Yvette CEDEX, France.

Abstract Collective phenomena occurring in fluids such as phase transitions are completely modified when they are confined in a porous matrix, and many aspects related to the critical behaviour are actually unsolved. We show here how small angle neutron scattering (SANS) experiments can represent a very effective experimental probe to investigate this class of problems, by describing some of the results obtained through a SANS experiment on a binary fluid mixture confined in porous Vycor glass.

the liquid-gas transition in a fluid, the main differences being that the order parameter is here the density instead of the concentration, and that are density fluctuations which take place.

In the last years many theoretical and experimental studies have been devoted to a better understanding of the confined fluid behaviour, either for fundamental and for technological reasons (catalysis, properties of fluids confined in rocks or clays, simulation of complex systems). Despite of these efforts, many questions are still open specially in the case of critical phenomena, such as the existence of a macroscopic phase separation, or the universality class of this (eventual) transition [3]. The confinement of the fluid in a porous system leads in fact to a drastic alteration of the bulk static and dynamic properties; a first obvious effect lies in the highly diminished role of gravity, which ultimately drives the transition in a pure fluid. Also, the properties of the particular porous system, fluid, and of their coupling, cause additional strong complications on both theoretical and experimental side.

A simple classification of porous material is based on their porosity. Well known examples of high-porosity materials are aerogels and xerogels (porosity up to 98%), which are not rigid systems displaying volume fractal character over distances typically ranging from 10 to few thousands of Å (i.e., the pores size may vary continuously over few orders of magnitude). In highly porosity silica aerogels, confinements is expected to be not very important; indeed a liquid gas-transition has been observed in confined N2 and 4 He: a critical point at a temperature below that of the bulk case has been measured, as well as a narrower coexistence curve [4].

Differently, rigid low-porosity systems such as Vycor silica glasses present the relevant feature of having a pore diameter almost constant, φv≈75 Å, and therefore a reference length in the system. This has been one of the reasons which induced us to study the critical behaviour of a binary fluid in this porous matrix through a SANS experiment [5]. Before going in some of the experimental details, a resume of the theoretical and experimental state of art will be given, followed by a short description of the SANS technique.

Introduction Phase transitions occurring in fluids close to the critical point (CP) such as the liquid-gas transition in a simple fluid, or the liquid-liquid one in a binary fluid mixture, are transitions of second order, thus described in terms of the 3-D Ising model, similarly to the case of magnetic transitions. At high temperatures, a binary mixture AB at the critical concentration xc is in the homogeneous phase (we consider in the following a normal mixture: at temperatures higher than the critical temperature Tc, the mixture is homogeneous). Then, approaching Tc, fluctuations of concentration begin to appear, and the formation of domains richer in one of the two components occurs [1], the extension of which is measured by the correlation length ξ. This quantity is strongly dependent on how close the temperature is to the CP where it diverges according to scaling laws: ξ=ξ(T)=ξ0 t-ν, where ξ0 is a constant depending on the system (typically few Å), t=(T-Tc)/Tc is the reduced temperature, and ν≈0.64 is a critical index [2]; finally, at T<Tc, the system separates into two phases.

The static structure factor S(q) that is measured through a radiation scattering experiment (q being the wavevector exchanged from the radiation) on a system close to a phase transition follows the Ornstein-Zernike law (OZ), i.e. a Lorentzian given by

I (q ) =

I OZ 1 + q 2ξ 2

(1)

with IOZ scaling like ξ2 apart from small corrections not relevant in this context. A similar scenario characterises

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Binary fluid mixtures confined in Vycor Vycor glass [6] is a low porous (28%) silica glass obtained by quenching a borosilicate glass forming a melt below its Tc. The mixture is thus forced to undergo spinodal decomposition which induces the formation of two phases, one richer in SiO2 and the second one richer in B2O3; the latter is then leached out and the final result is a highly interconnected random 3D porous network, with a pore size distribution sharply peaked at a mean diameter value of φv≈75 Å [7]; a 2D reconstruction is reported in Fig. 1. Among the various effects which influence the critical behaviour of the confined fluid, the following processes play a major role:

Fig.1. Digital reconstruction of a transmission electron micrograph of a Vycor thin section which shows the random nature of the pore network (in black) immersed in the silica matrix (in white) (from [7]).

• finite-size effects due to the confinement, i.e. the spatial constraint of the mixture to the tortuous interconnected pore network; • the quenched random disorder introduced by the host matrix, which represents a random source of interaction between fluid and glass; • the different interaction between the two components of the mixture and the silica walls, which may induce a gradient of the fluid composition inside the pores; • metastability, hysteresis, strong history-dependent effects induce a strong increase of the times the system needs to reach equilibrium [8], which may add to the critical slowing down occurring in proximity of the CP. All these effects are, with a different weight, sampledependent, thus giving rise to the real difficulties which have to be faced, both theoretically and experimentally.

On the theoretical side, two main models have been proposed in order to describe such phenomena. The Random Field Ising Model (RFIM) [9] is based on the importance of the preferential attraction between the silica walls and one of the fluid components, whereas confinement is neglected. Preferential adsorption may be seen as a random field acting on the fluid, because of the

randomness of the pore network. Therefore, the presence of quenched, i.e., spatially fixed, impurities can be described by using the RFIM formalism, in analogy with the case of magnetic materials affected by structural disorder (impurities, dirty). A RFIM transition is predicted to occur, characterised by a static structure factor S(q) simply given from the OZ term of Eq.(1) superposed to a Lorentzian squared term (LSQ):

S( q ) =

I LSQ IOZ + 1 + q 2ξOZ 2 1 + q 2ξ LSQ 2

(

)

2

(2)

with ξOZ≡ξLSQ. Phase separation would occur at a temperature much lower than the pure case, similarly to what is found for a single fluid in gel [4].

On the contrary, the single pore model was successively introduced [10] as finite size effects are predicted to dictate the confined fluid behaviour, specially in the case of narrow pores such as those occurring in Vycor.

Randomness is not here taken into account, and the glass is considered as an assembly of cylindrical independent pores. According to this model, when ξ becomes comparable to φv, phase separation may occur only at a microscopic level, because of the extremely long times (even not experimentally accessible) that fluctuations need for taking place throughout the complex pore network. Various configurations inside the pore may exist which depend on the temperature and on the length scales involved: bubbles richer in one liquid occupying the tube, layer richer in one liquid and core richer in the other, and so on.

This dualism has been not clarified by means of two SANS experiments on water+lutidine imbibed inside Vycor [11, 12], both performed close to bulk critical conditions. The measured S(q), even though very similar [13], were interpreted according to the two models by using the same Eq.(2) to describe the experimental results but (i) in [12] ξOZ≠ξLSQ, and (ii) a very different model was used to describe an unexpected peak in the data (this point will be discussed in more detail later).. A common finding was that no true critical behaviour was observed, and only fluctuations up to few tens of Å were detected Some SANS principles Basically two reasons make the SANS technique a well suited experimental approach to this kind of investigation: • SANS probes lengths in the 10-1000 Å range, therefore distances over which critical fluctuations occur; • the large strength variation of the neutron-nucleus interaction as a function of the atomic number allows to vary the scattered intensity without changing the physical properties of the sample (contrast matching

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technique); this effect is particularly important in the case of hydrogen and deuterium nuclei, making deuteration a frequent tool in neutron scattering.

This section is dedicated to a short resume of the SANS principles, in order to give the basic ideas of this technique; the relations between the intensity I(q) measured at small values of q, and the quantity of interest, the static structure factor S(q), can be found in many text books [14].

By SANS, it is generally meant the neutron diffraction at scattering angles θ<5°. In fact, in a neutron reactor, neutrons with the longest available wavelengths λ have to be used for reaching low values of the exchanged momentum q=4π/λ sin(θ/2)~2πθ/λ, thus giving access to length scales of the order of r~q-1. To keep the flux at a good level, the beam is monochromatised through a mechanical velocity selector at values of λ in the 4-20Å range (cold neutrons) and then collimated. Finally, the intensity scattered from the sample is collected by a twodimensional circular detector placed at distances from 1 to 10 m; the position sensitive detector consists of around 4000 (or 16000) cells, 1×1cm2 (0.5×0.5cm2) each. The overall resolution is usually ∆q/q≈10%, mainly due to the beam monochromatisation; despite of this large value, it is worth reminding that, apart from special cases, the low-q signal displays a smooth behaviour, therefore resolution is not crucial.

Once obtained the intensity at constant θ, it is immediately converted in I(q) at constant q, apart from eventual corrections due to multiple and inelastic scattering. From I(q), the determination of the differential cross-section dσ/dΩ is then straightforward, as these two quantities are proportional; the coefficient of proportionality depends on the experimental geometry (both sample and instrument), detection efficiency, incoming flux, sample transmission, and can be evaluated separately. In such way it is possible to determine I(q) approximately in the 10-1-10-3 Å-1 q-range.

For deriving the relation between S(q) and I(q), it is necessary to remind that dσ/dΩ is the results of all the possible interferences of the neutron waves scattered by the nuclei, weighted by the amplitude of the nucleineutron interaction, that is N dσ = ∑ bi b j exp(iq ⋅ rij ) dΩ i

18

S( q ) =

∑ exp(iq ⋅ rij ) N

i, j

(4)

that is a quantity reflecting the spatial distribution of the centres of mass, as can be understood because the Fourier transforms of the atom individual positions δ(r-ri) are the exponential term exp(-iq⋅ri) appearing in Eq.(4). It is also worth noticing that Eq.(3) can be seen as the Fourier transform of the spatial fluctuations of the scattering length density b. Through these hypotheses, the simple formula which relates the scattered intensity IAB(q) to S(q) via the scattering lengths densities ρA and ρB of the twocomponents system A+B is derived: 2 I A B (q) = (ρ A − ρ B ) S (q) = K AB S (q ) 2

(5)

The structure, i.e. S(q), is therefore uncoupled from the neutron interaction properties of the sample, represented from the contrast KAB=ρA-ρB. As, by isotopic substitution, it is possible to largely modify the contrast of the system, the scattered intensity can be thus modulated, without affecting the structure.

(3)

where the summation extends over all the N nuclei of the sample, rij is the distance between ith and jth atoms, <…> indicates an averaging over all the possible configurations, and bi is the coherent scattering length of nucleus i, which is of the order of 10-12 cm, reflecting the short range of the nuclear forces; the reason why isotopic

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substitution is very common in neutron diffraction is that bi may vary strongly with the atomic number, the more striking case being H and D nuclei where bH=-0.37 10-12 cm and bD=0.67 10-12 cm.

The exact relation (3) needs to be worked in order to connect it to S(q) at small q; without entering into the details of the complicated formalism which is needed to fulfil this task, it is more useful to describe its relevant physical aspects: • in Eq.(3), mainly the components with q⋅r~1 give a significant contribution to the sum; • the SANS technique probes distances much larger than the typical atomic size, therefore it is useless to look at the individual atom positions, but it makes sense to average over spatial regions containing large number of atoms.

As a consequence, the notion of scattering length density ρ is naturally introduced, ρ=n⋅b, where n is number density; • the static structure factor is defined as

•

Experimental results In order to elucidate some of the open questions previously mentioned, we have performed a new SANS experiment on a binary mixture fluid confined in Vycor glass [5], with the intention of determining S(q) in a thermodynamic region close to the bulk critical one.

Among the effects which may lead to complications in

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the data interpretation, particularly severe is the role played by the different wetting properties of the mixture; this effect could induce a strong (and uncontrolled) shift of the fluid composition inside the pores and, possibly, also a true phase separation, not of thermodynamic origin, but driven by wetting forces. We chose to perform the experiment on the mixture n-C6H14+n-C8F18 (hexane+perfluorooctane, in the following named hex/PFO), because: • the two fluids have a similar affinity to the silica, and preferential adsorption is expected to be minimised; • fluorine compounds has high values of b, thus a good counting rate is allowed; • the bulk critical temperature is experimentally easily accessible; • by partially deuterating with C6D14, it is possible to vary the scattering length density of the fluid, and match the contrast with Vycor (ρv≈3.5 1010 cm-2).

We have measured (PAXE small angle diffractometer, Laboratoire Léon Brillouin, CEA/CNRS, Saclay) S(q) of

Fig.2. Logarithmic plot of the intensity (in absolute units) scattered from the dry Vycor (stars) and from the samples at the lowest (open symbols) and highest (full symbols) temperature. From the bottom: #HD (circles), #H (diamond), and #D (squares); the line represent the result of the fitting procedure according to Eq.(8). In the inset at the bottom a pore section with a layer richer in fluid B and a core part richer in fluid A is depicted; V stands for the Vycor. Such structure would give rise to a double peak similar to the one observed in the experimental data.

the hex/PFO mixture imbibed inside Vycor at different values of C6D14 concentration, but all of them with the same bulk critical concentration of PFO. We could then study three bulk critical mixtures with different contrast with the Vycor. Deuteration induces a slight decrease of the bulk critical temperatures, which is Tc=38°C for the sample completely hydrogenated (#H), Tc=29°C for the sample completely deuterated (#D); the third sample (#HD, Tc=33°C) had an intermediate degree of deuteration, in order to match the contrast with the glass.

The samples were imbibed inside the porous glass at T=60°C, therefore deeply inside the single phase region; the quartz cells containing the slabs of Vycor were then sealed, in order to work at fixed concentration. Another difficulty in such investigations results from the very large relaxation times which may occur in the evolution towards equilibrium. We decided to store #HD very long times at low temperature before the experiment (two months at T=0°C). The presence of long times characterising the relaxation of concentration fluctuations was confirmed by the first set of measurements at T=55 and 50°C: for #H and #D an exponential decrease of the signal of the order of several tens of hours was observed, while #HD reached quickly equilibrium. Therefore, as in neutron scattering experiments the beam time is rare, we could not wait until the system reached equilibrium at these temperatures. At T=44.5°C, equilibrium was reached within few hours, and we can consider these like true experimental stable measurements, in the sense that repeated runs showed a constant signal; it is worth specifying that for such systems the concept of equilibrium is different from the classical one, as there is no a definite transition from metastability to stability, but a slow continuous time evolution of metastable states which can be assumed to be quasiequilibrium states [15].

Data have been collected at five temperatures from T=44.5°C to T=15.5°C, therefore up to temperatures where the pure mixture would be phase separated. The intensity I(q) corresponding to the highest and lowest temperature are shown in absolute units in Fig. 2, together with the signal coming from the dry Vycor. We may note the following: • Dry Vycor. This signal is dominated by the huge broad peak (Imax≈200 cm-1) centred at qv=0.023 Å-1, and interpreted in terms of Eq.(5): Iv=K2v Sv(q)=ρ2v Sv(q), as the pores are empty. The presence of such peak reflects the existence of a characteristic length, given by the almost constant diameter of the quasiperiodic pore network immersed in the silica glass.

• #H and #D. The amplitude of the dry Vycor peak is now decreased because of the lower contrast due to the presence of the fluid (see Eq.(5)). A strong increase of the signal at low q is detected, when temperature is decreased, in a similar way to what occurs in presence of

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critical fluctuations. A small bump in the intensity at qsv=0.05 Å-1 is noted, and later commented.

• #HD. Because of the contrast matching, in this sample a large reduction of the signal level (of a factor ~200) has been found. No particular features at low q are present, leading us to conclude that, unlike the other two samples, a thermodynamic phase separation took place during the time it was kept at low temperature. The increase of temperature from 0°C to 60°C (thus again in the single phase of the pure mixture) at the moment of the experiment, did not allow the remixing of the confined mixture; this irreversible phenomenon can be understood reminding that hysteresis effects appear also in the bulk case, and the mixing of a mixture can be a very long process (differently from the separation) without hardly stirring the sample. The second peak, here largely enhanced with respect to the other two samples, was already observed in [11, 12], and can be interpreted as coming from a wetting layer, richer in one of the two fluids (fluid A for instance, as shown in the inset of Fig.2) which coats the pores walls, leaving a core part richer in the other fluid [12]. As a consequence, the appearance of a new characteristic length shorter than φv, and therefore of a new peak at qsv>qv in S(q), are induced. The variation of temperature causes simply a remixing of the two phases, i.e. a modification of their contrasts, and therefore of the peaks amplitudes.

On the basis of the latter considerations, we have analysed the results of #H and #D trying to write down a model which takes in account the presence of this double-layer structure. Such configuration can be seen as the one coming from two interpenetrating quasiperiodic networks, analogously to the bicontinous phase. It is clear that this is an ideal representation, as it is realistic to assume the presence of a single phase (A or B) in some of the necks, corners, and narrower pores which are present in this glass. But it is important to remark that it is this double-layer configuration that is responsible for the two peaks, while a pore occupied by a single fluid would give a contribution proportional to the dry Vycor peak. This can be better understood when considering that the double-layers configuration can be represented by [16]

[ Kbv Fv (q) − Kba Fsv (q)]2

20

 q  −d I (q ) = Cf   qmax  qmax 

(7)

where C is a constant, f(q/qmax) is an universal function of the ratio q/qmax, with qmax corresponding to the maximum of the structure factor (qv in case of Sv), and d=3 is the dimension of the system. This relation is a dynamical law in the sense that describes the later stages of spinodal decomposition of quenched unmixing mixtures, where the slow formation of pores of increasing size, and therefore of time-dependent decreasing values of qmax(t), occurs; this time dependence is not explicitly indicated in Eq.(7) as it refers to the static situation.

As mentioned above, it is possible to represent the new double-layer structure as a new quasiperiodic structure homotetic of the dry Vycor one, therefore as a previous stage of the spinodal decomposition which forms the glass: similarly to what we said about Sv, Ssv≡Fsv2 represents a new dry Vycor with narrower pores (as it can be

(6)

where Kbv (Kba) is the contrast between fluid B and Vycor (fluid B and A), and Fv (Fsv) is the corresponding form factor. It is immediate to conclude that F2v is exactly the dry Vycor structure factor Sv, as obtained in the case B=A (eventually the vacuum), and Eq.(6) reduces to (KbvFv)2, that is the dry Vycor structure factor modulated with a different contrast due to the presence of B. More care has to be paid as far as the second term is concerned. The

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starting point is that Vycor is obtained by spinodal decomposition, for which the more general dynamical scaling law is given from [17] :

•

Fig.3. Experimental intensity scattered from dry Vycor Iv(q) (stars) compared with the calculated value Isv(q) (circles) as obtained fromIv(q) by applying the scaling law (7).

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seen equating B=V in Eq.(6) and in the inset of Fig.2), with Kba being the corresponding contrast. On the basis of these ideas, Ssv was calculated by (i) scaling the experimental values of Sv, in order to obtain f(q/qmax), and (ii) applying Eq.(7) with qmax≡qsv; the result is shown in Fig.3.

Once defined a model to describe the structure, the data of #H and #D were fitted by adding to Eq.(6) the OZ term (1) in order to take into account the low-q behaviour, that is:

I (q) =

[

IOZ + Kbv Sv (q ) − Kba Ssv (q ) 1 + q 2ξ 2

]

2

(8)

where Ioz, ξ, and the two contrasts are parameters of the fit; the good agreement with the experimental data (see Fig.2) suggests the validity of the model we adopted.

Before looking at some of the results of the fit, it is instructive to see the relative importance of the single terms of Eq.(8) as obtained from the fit, which are shown in Fig.4 for #D at the lowest temperature; we observe that: • as expected, the OZ term dominates at low q; the absolute value of intensity is large, therefore critical fluctuations extend over long distances.

Fig. 5. F Fit parameters relative to #H (full symbols) and #D (open symbols) as function of the temperature. In (a) the contrasts Kbv (square) and Kba (circles) are shown, while the dashed line indicates the zero level. ξ data are reported in (b), and compared with the mean pore diameter φv (dashed line); two arrows mark the two bulk critical temperatures.

Fig. 4. Intensity scattered from #D at the lowest temperature (full squares), compared with the result of the fit to the data through Eq.(8). The symbols indicating the different components of the fit are shown in the legend.

• In these two samples, the contrast of the mixture with the Vycor was rather large, and consequently also the term K2bvSv(q).

• The term K2baSsv(q) is very low (almost zero in this scale), meaning that the contrast between B and A is small, that is the composition of the two phases is very similar.

• The cross term which describes the coupling between the Vycor and the two phases is negative, and its contribution extend over a wide q-range.

These considerations apply for both samples at all the investigated temperatures, as shown in Fig.5, where the values of ξ and of the two contrasts are reported.

Globally, we may note that the temperature influences slightly the fluids behaviour, at least in this range of temperatures, while in the pure case a phase transition is observed. As far as the contrasts are concerned, the main difference lies in their opposite signs, a fact which made us to conclude [5] that is the hexane rich phase which wets the silica walls (phase A in our representation), confirming previous observations on a similar perfluoroalkane+alkane mixture imbibed in silica glasses. Quantitatively, the evaluation of the ratio between Kba=ρb-ρa and the ρ value of the imbibed critical concentration, gives the concentration composition shift

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in the pore induced by the preferential adsorption. We thus found a variation of ~10% for #H and of ~2% for #D, meaning that the confined fluid were still close to criticality.

The large values of the correlation length confirm that the two samples, mainly #D, were close to critical conditions, but the qualitative behaviour is very different than the bulk mixture where, close to CP, ξ varies rapidly. ξ increases slowly when lowering the temperature, even well below the bulk critical point, therefore the system is not yet separated, unlike #HD where a thermodynamic phase separation had taken place. The critical point, if any, is placed at lower temperatures than the pure case, as predicted. An important difference with the previous results lies in the detection of ξ>φv, meaning that fluctuations may extend along several pores, and that φv does not represent a length cut-off. It seems significant and realistic, even though based only on few points, that when ξ is of the order of φv, the temperature dependence ξ=ξ(T) is rounded off, as if the effect of finite pore dimension consisted in a smearing of the correlation length distribution.

Conclusions and perspectives We believe that the class of mixtures we have investigated are very suited for these studies as the wetting effect results of minor importance, and critical fluctuations are enhanced. It is reasonable in fact to conclude that the adsorbed layer consists of a thin film blocked on the pore while, in the free core part, critical fluctuations of rather extended lengths can take place as the concentration is still close to the critical one. This suggests that the RFIM is the proper model at least for the system here studied. We did not observe any presence of a LSQ term, but a huge precision would be needed to distinguish such contribution from the remaining components; also, the presence of a LSQ term, manifestation of a RFIM transition, could occur at lower temperatures. It is clear that such subject deserves further investigation for a deeper understanding of these phenomena. It would be interesting to study the liquidgas transition in a simple fluid, but more complicate would be the choice of the system, because the density fluctuations scatter much less than concentration fluctuations. In order to discriminate the OZ contribution from the other ones, a very good contrast should be achieved, but this is not feasible with the majority of fluids. We are going to carry out new SANS experiments on perfluoroalkane+alkane mixtures with Tc higher than the one of the investigated mixture; in this way it will be possible to study the critical phenomena on a more extended temperature range, that represents a condition necessary for a better comprehension of this subject. As a matter of fact, SANS represents the good technique for

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•

this purpose, but the real experimental difficulty which is emerged lies in the long relaxation times needed to perform equilibrium measurements. To partially bypass this problem, which is in conflict with the low availability of neutron beam time, we believe that a contemporary static and dynamic study by light scattering is fundamental.

References 1. It is worth to remind that the transition into the two-phase region does not imply that fluid A is completely separated from the fluid B; 2. For an introduction to this subject see Stanley H E 1971, Introduction to phase transitions and critical phenomena (Clarendon Press, Oxford); 3. A resume of the experimental and theoretical state of art as well as a complete list of the relevant literature can be found in Pitard E, Rosinberg M L and Tarjus G 1996, Mol. Sim. 17, 399; 4. Wong A P Y and Chan M H W 1990, Phys. Rev. Lett. 65, 2567; 5. Formisano F and Teixeira J 1999, to be published in Europ. Phys. J. E; Formisano F and Teixeira J 1999, to be published in J. Phys.: Condens.

Matt.; 6. Vycor Glass No. 7930, Corning Glass Works, Corning, NY 14830; 7. Levitz P, Ehret G, Sinha S K and Drake J M 1991, J. Chem. Phys. 95, 6151; 8. Fisher D S 1986, Phys. Rev. Lett. 56, 416; 9. Brochard F and de Gennes P G 1983, J. Phys. Lett. (Paris) 44, 785; de Gennes P G 1984, J. Phys. Chem. 88, 6469; Andelmann D and Joanny J -F 1985, Scaling Phenomena in Disordered Systems, ed Pynn R and Skjeltorp A (New York: Plenum), p. 163; 10. Liu A J, Durian D J, Herbolzheimer E and Safran S A 1990, Phys. Rev. Lett. 65, 1897; Liu A J and Grest G 1991, Phys. Rev. A 44, R7894; Page J H, Liu J, Abeles B, Deckman H W and Wez D A 1993, Phys. Rev. Lett.

71, 1216; 11. Dierker S B and Wiltzius P 1991, Phys. Rev. Lett. 66, 1185; 12. Lin M Y, Sinha S K, Drake J M, WU X -l, Thiyagarajan P and Stanley H B 1994, Phys. Rev. Lett. 72, 2207; 13. A detailed analysis of these two experiments in terms of the two models can be found in Monette L, Liu A J, and Grest G 1992, Phys.

Rev. A 46, 7664; 14. See for instance Guinier A and Fournet G 1955, Small Angle Scattering of X-Rays (Wiley, New York), and Glatter O and Kratky O 1982, Small Angle X-Ray Scattering (Academic, London); 15. Fisher D S, Grinstein G M, and Khurana A 1988, Physics Today 41, 56 16. See for instance Ottewill R H 1991, J. Appl. Cryst. 24, 436; 17. Furukawa H 1984, Physica 123A, 497.

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Articolo ricevuto in redazione nel mese di Marzo 2000

NANO-SCALE SPECTROSCOPY AND ITS APPLICATIONS TO SEMICONDUCTORS S. Heun Sincrotrone Trieste, S.S.14, km 163.5, Basovizza, 34012 Trieste, Italy.

G. Salviati Istituto MASPEC, CNR, Parco Area delle Scienze 37A, 43010 Loc. Fontanini, Parma, Italy.

Nano-structured semiconductor materials with a lateral dimension less than the de Broglie wavelength of electrons are expected to exhibit quite different electronic properties from those of common devices. Fabrication technologies for nanostructured devices have been developed recently, and the electrical and optical properties of such nanostructures are a subject of advanced research. However, classical spectroscopic techniques can not be applied to these structures because their lateral resolution is not sufficient to resolve them. In order to understand and to control their physical properties, it is indispensable to evaluate the nanostructures by using nanospectroscopic techniques. In this review article, the different approaches to nanospectroscopy will be discussed, that is, photon and electron probe nanospectroscopy and proximal probe techniques. Particular emphasis will be put on the synchrotron radiation photoelectron nanospectroscopy.

The fabrication techniques at least for prototype nanoscale devices have already been developed. While the traditional UV lithography, which is used for today's devices, probably will not go below 200 nm [3], x-ray lithography allows feature sizes from 500 nm to 30 nm [12]. Electron-beam lithography can even do better with minimal structure sizes of a few tens of nanometers [13].

There is also a strong effort to use proximal probe techniques for nanomanipulation. With the scanning tunneling microscope (STM) it is now possible to move single atoms in a controlled way on a surface [14]. STMs and atomic force microscopes (AFMs) have been used to build working nanodevices [15,16]. These scanning methods are still too slow for real production, but there are efforts to put several hundred tips or even microscopes on one chip to speed things up [17,18].

All practical semiconductor elemental analysis employs spectroscopy. A probe in a well defined quantummechanical state (usually a monochromatic photon or electron beam) is interacting with the sample. Any changes in the state of the probe beam are then measured, or other particles excited by the probe are detected. In brief, the main advantage of an electron probe is the relative ease of beam handling, especially also in focusing the beam on a small spot on the surface [19]. On the other hand, the use of a photon probe strongly reduces beam damage on the sample [20]. The surface sensitivity of the method is mainly determined by the choice of the detected particles. Electrons with typical energies of 10 to 2000 eV have a very small escape depth (less than 5 nm) [21]. Methods detecting low energy electrons are therefore very surface sensitive. The attenuation length of photons in semiconductors is much larger, these methods therefore are more bulk-sensitive [22].

Among the classical spectroscopies, x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are by far the most common ones in surface science [21,23]. In AES, Auger electrons are excited by a photon or electron beam. Since Auger electrons have discrete kinetic energies that are characteristic of the emitting atom, AES is particularly useful for an elemental analysis of the sample surface. In XPS, the sample is excited by a monochromatic x-ray beam, and the photoemitted electrons are energy selected. The energy distribution curve of the photoelectrons is, again, characteristic of the

1. Introduction The semiconductor industry has grown rapidly in recent decades. The main reasons for such phenomenal market growth are the continued technological breakthroughs in integrated circuits (ICs). The metal-oxide-semiconductor field effect transistor (MOSFET) is by far the most common type of transistor in IC technology [1]. In the 1960s, Gordon Moore observed that the feature size in MOSFETs was decreasing by a factor 2 roughly every 18 month [2]. This empirical trend has continued until today, where structure sizes below 0.35 µm are used [3].

Device miniaturization results in reduced unit cost and in improved performance. This is illustrated with the performance of a typical personal computer over the years. Another benefit of miniaturization is the reduction of power consumption.

However, researchers have projected that below 100 nm in size, the laws of physics will prevent further reduction in the minimum size of today's MOSFETs, and new device concepts will have to be found which take advantage of the quantum mechanical effects that dominate on such a small scale [3,4]. A number of nanometer-scale devices have already been realized: Resonant-tunneling devices [5], single-electron transistors [6], and quantum dot arrays [7]. These devices have minimal structure sizes of typically 50 nm [8-11]. All these designs have in common that the active region of the device is in the surface region of the wafer (topmost µm).

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emitting atoms and, more importantly, of their chemical environment. Therefore, XPS goes beyond elemental analysis to provide chemical information. It should be stressed here that also with AES a chemical analysis of the sample is possible in selected cases. However, the analysis of such results is more complicated than in the case of XPS. The reader will find a detailed discussion of this topic in Ref. [23]. A typical probe spot size in XPS and AES is of the order of 300 µm [24-26]. Another method very commonly used in semiconductor science is photoluminescence (PL) spectroscopy [27]. Light is directed on a sample, where it is absorbed and can excite charge carriers. When these carriers relax to their equilibrium state, the excess energy can be emitted in the form of light. The energy of the light is related to the energetic levels within the semiconductor. PL is therefore useful to determine the band gap of a semiconductor, impurity levels, and recombination mechanisms.

However, all these classical spectroscopic techniques are of limited use in the study of semiconductor nanostructures, because their lateral resolution is not sufficient. Therefore the use of spatially resolved spectroscopic probes is required. In this review article, the different approaches to nanospectroscopy will be discussed. We will restrict ourself to photons and electrons as sample probes, their main advantages being a limited modification of the sample surface and their compatibility with ultra high vacuum [28]. Other kinds of

probes are less commonly used and are discussed in detail in Ref. [28]. After two sections dealing with photon and electron probe nanospectroscopy, we will dedicate a third section to proximal probe techniques.

2. Photon Probe Nanospectroscopy 2.1. General Considerations Since the spatial resolution of normal optical microscopes is diffraction-limited to approximately half the light wavelength [29,30], nanometer resolution requires the use of ultraviolet light or x-rays. Laboratory sources do not provide enough intensity for high resolution studies [28]. X-ray lasers are being considered as light sources for nanospectroscopy [31], but so far no working instrument has been demonstrated. Therefore the only light source available at this moment for photon probe nanospectroscopy are synchrotrons. Detection of light is possible [32], but due to the large x-ray attenuation length of a solid this results in a rather bulk-sensitive measurement (100 nm sampling depth and more) [22]. Work in transmission requires thinned samples (thickness ≈ 100 nm) which is not straightforward [19]. Therefore the detection of photoexcited electrons with an escape depth of only a few nanometers is preferible in most semiconductor applications. Here, the detection of photoelectrons is advantageous with repect to the detection of Auger electrons, because the photoelectron characteristic peaks are much narrower than Auger electron peaks [28]. Further-

monochromator

HS analyser undulator

focusing optics (ZP&0SA)

scanning stage Fig. 1. A sketch of the ESCA microscopy beamline at Elettra. X-rays from the undulator are monochromatized and focused by a zone plate (ZP) through an order selecting aperture (OSA) on the sample, which is mounted on a scanning stage. A hemispherical (HS) analyzer is collecting the photoelectrons.

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more, the signal-to-background ratio for Auger electrons is smaller than for photoelectrons [28].

There are two basic ideas about how to realize photon probe nanospectroscopy: scanning mode and imaging mode. In the scanning type design, the spatial resolution is obtained by focusing the illuminating light on a small spot on the sample. The signals of interest such as the photoelectrons, the fluorescence, or the transmitted x-rays are integrally collected. Images are obtained by scanning the sample relative to the beam. In imaging mode, the sample is homogeneously illuminated with photons, and an electron optics forms a magnified image of the sample surface on a screen. The spectroscopic information is obtained by either scanning the photon energy or by providing the microscope with an energy filter.

2.2. Photoelectron Microscopes 2.2.1. Scanning Type Photoelectron Microscopes In a scanning microscope, the light is focused by a Fresnel zone plate or a Schwarzschild objective on a small spot (diameter ≈ 100 nm) on the sample [30]. Both need a high brilliance light source. The excited photoelectrons are collected by a commercial hemispherical analyzer with a typical energy resolution of 200 meV [30]. This is sufficient for the detection of chemical shifts in core level peaks [33] and for valence band spectroscopy [34]. Samples do not need to be flat, so that, for example, measurements on cross-sectioned samples can be performed [35]. Drawbacks of this design are the bad time resolution caused by the need to scan the sample relative to the light spot, and the high photon flux in a small spot, which might locally charge or damage the sample [36].

e–

Microscope

Refocusing Mirrors

lightwaves will have a phase difference of 2πn (n integer), i.e. the condition of constructive interference is met.

Therefore zone plates act as lenses with several foci.

To block the higher order foci, an order selecting aperture (OSA) is used. The (unfocused) zero-order light is blocked by a central stop in the zone plate. The efficiency of modern zone plates can reach 55 % for hard x-rays (7 keV). For soft x-rays (hv < 1 keV), an efficiency of 10% is routinely achieved. The minimum spot size that can be obtained is 1.22 times the width of the outermost ring [38]. Nanolithography is therefore required to produce zone plates with nanometer spot size. A typical focal length of a zone plate lens for light with 500 eV is some millimeters. Since the focal length is proportional to the photon energy, work at much lower photon energies would result in unpractically short working distances.

Therefore, zone plates are not used in photoemission at photon energies below ≈ 300 eV. A Schwarzschild objective is consisting of a concave and a convex mirror. Fig. 2 shows as an example the spectromicroscopy beamline at Elettra [39]. The radiation impinges nearly orthogonal on the Schwarzschild mirror surfaces. Since the reflectivity of metals for normal incidence in the VUV and for x-rays is very small (< 1 % [22]), multilayer coatings have to be employed. They enhance the reflectivity by constructive interference of the wavefronts reflected at the single layer boundaries.

This condition is met if the wavelength equals 2 times the layer thickness. Modern multilayer coatings can reach in a small energy range reflectance values of the order of 50 % [24]. This implies that for each photon energy a dedicated Schwarzschild objective has to be build. Scans

Source Monochromator

Fig. 2. Schematic optical layout of the Spectromicroscopy beamline at Elettra. X-rays from the source are directed to a pinhole (P), from where they are focused by a Schwarzschild objective on the sample which is mounted on a scanning stage (SS). Photoelectrons (e-) are collected by a hemispherical energy analyzer.

A typical setup for a scanning photoelectron microscope (SPEM) employing a Fresnel zone plate is shown in Fig. 1. It shows the ESCA microscope at the Elettra synchrotron radiation source in Trieste, Italy [37]. Fresnel zone plates are circular diffraction gratings made of an alternating sequence of absorbing and transparent rings.

In certain distances from the zone plate, all transmitted

Focusing Mirrors

Switching Mirror

of the photon energy are practically impossible [24]. Since each layer of the coating has a thickness comparable to the photon wavelength, surface and interface roughness prevents the use of Schwarzschild objectives for shorter wavelength, i. e. higher energies (E > 300 eV) [40].

This particular property of Fresnel zone plates and Schwarzschild objectives explains also why there are still

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two different systems in use, why it is not possible to find the best technical solution. The use of Fresnel zone plates for photoelectron spectroscopy at photon energies below about 300 eV is unpractical due to their short focal length, and exactly in this low energy range, the Schwarzschild objectives show their best performance.

2.2.2. Imaging Type Photoelectron Microscopes The virtually only imaging photoelectron microscope is the photoemission electron microscope (PEEM). In this setup, the light is illuminating a spot of several micrometer diameter on the sample. This spot size is still small relative to what is used in classical (integral) setups, but it is large compared to what is required in the scanning type design. A PEEM employs electrostatic or magnetic lenses to form a magnified image of the sample on a screen. It allows continuous imaging with video rate [41]. A basic PEEM is shown in Fig. 3 [42]. Such modern systems reach a lateral resolution of better than 50 nm [42-45]. The early PEEM work has been performed with deuterium or mercury lamps [41]. In these experiments, lateral variations of the work function of the sample were

Fig. 3. A schematic drawing of the IS-PEEM. The sample is illuminated by x-rays of energy hv. The PEEM consists of three electrostatic lenses. The lateral photoelectron distribution is detected by a multichannelplate, a screen (YAG crystal), and a CCD camera.

used as contrast mechanism. However, to do real elemental sensitive work, higher photon energies are necessary to excite atomic core levels. But even with sufficiently high photon energies, a standard PEEM can not be used for micro-XPS because it is not equipped with a photoelectron energy analyzer. However, it has been

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Fig. 4. Schematics of the spectroscopic photoemission and low energy electron microscope (SPELEEM). The sample can be illuminated by x-rays or with an electron gun. The sector field is used to seperate the incoming and outgoing electrons. The analyzer selects electrons with a certain kinetic energy. The projector is used to display the magnified image of the sample on the screen.

shown that the total photoelectron yield is almost proportional to the photoabsorption coefficient [46]. So, by scanning the photon energy one can perform optical absorption edge spectroscopy with the lateral resolution of the PEEM. This technique is called Âľ-XANES (x-ray absorption near edge spectroscopy). It is very useful for the study of organic and of magnetic materials. A requirement for this kind of experiments is a tunable xray source. Therefore spectroscopic work with PEEM requires the use of a synchrotron. In summary, the PEEM is a simple instrument with by far the best time resolution of all nanospectroscopies. No complicated x-ray optics is needed, no sample scanning necessary. One drawback is the need for a flat sample.

The lateral resolution of this instrument is not limited by diffraction, but rather by lens aberrations. Therefore, the spatial resolution of a PEEM can be increased by reducing the aberrations. One way to do this is to add an energy filter to the PEEM [43,47,48]. This reduces chromatic aberrations and allows to collect photoemission spectra, which in the case of semiconductors provide more information than XANES spectra. A lateral resolution of 22 nm and an energy resolution of better than 0.5 eV have been achieved with such an instrument at Elettra. A schematic drawing of it is shown in Fig. 4 [49]. It is called spectroscopic photoelectron and low energy electron microscope (SPELEEM) because it is also equipped with an electron gun to perform low energy electron microscopy with a lateral resolution of 8 nm [50]. The separation between incoming and outgoing electrons is achieved by a magnetic prism (sector field). The electrons emitted or reflected from the surface are transferred into the image plane of the microscope, where a magnified image of the sample can be observed with a video camera or a slow scan CCD camera. When used as an electron microscope, the SPELEEM can be used to obtain real

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Fig. 5. Layout of the field effect transistor used for the measurements, and PEEM images from it taken with Ga 3d, Ti 3p, and Al 2p photoelectrons.

space images of the sample (LEEM) or to measure the intensity distribution in reciprocal space (low energy electron diffraction (LEED)). Furthermore, the use of the energy analyzer allows measurement of the energy distribution of the electrons (electron energy loss spectroscopy (EELS)). Both LEED and EELS can be measured from a micrometer spot on the sample. In complete analogy to this, three modes of operation are available when working with photons: PEEM as well as photoelectron diffraction (PED) and photoelectron spectroscopy (PES). Details on the use of this instrument can be found in Refs. [43,49].

Fig. 6. SPELEEM images of an AFM locally oxidized sample showing a strong contrast inversion. Field of view 12 µm. The intensity (gray scale) is proportional to the photoemitted electron intensity at given kinetic energy. (a) Kinetic energy = 26.8 eV, binding energy 104.7 eV corresponding to the high binding energy side of the Si 2p oxide component. (b) Kinetic energy = 29.2 eV, binding energy 102.3 eV corresponding to the low binding energy side of the Si 2p oxide component.

2.3. Examples In the following we will give two examples which will illustrate what photoelectron nanospectroscopy can contribute to semiconductor industry. The experiments were performed with the SPELEEM at Elettra. Fig. 5 shows spectromicroscopic images of a field effect transistor (FET) at three different photoelectron energies, corresponding to the Ga 3d, Ti 3p, and Al 2p core levels [49]. The black circle in the FET-sketch indicates the field of view (FoV) for the images. It has a size of 19 µm. The photon energy used for the measurements was 131.3 eV. In the FET-structure, the GaAs substrate is visible between drain and source and the gate. These regions are clearly visible as bright lines in the images taken at the Ga 3d and As 3d (not shown) core levels. Also the Al gate is identified as a bright line at the energy of the Al 2p core level. Drain and source are highlighted at the electron energy of the Ti 3p core level. Fig. 5 also illustrates that defect analysis on devices can be performed by SPELEEM. The defects seen in the images can be analyzed with nanospectroscopy.

As we have already pointed out in the introduction, proximal probes have attracted attention as potential new tools for nanofabrication because of their demonstrated ability to image and manipulate matter on the atomic level [14]. One of the most promising techniques that uses AFM and STM to produce a pattern on a semiconductor surface and to fabricate working nanodevices is local anodic oxidation (LAO) [16]. In a humid environment, the biased tip of a scanning microscope can write nanometer wide oxide lines on a semiconductor- or metal-surface. In spite of the ease and of the effectiveness of the LAO process, no chemical information is yet available on the nature of the patterned oxide. In the following we will give an example that

Fig. 7. Si 2p core level spectra from native oxide and AFM written oxide.

They are composed of a bulk component and an oxide component, shifted towards higher binding energies. For details, see text.

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addressed this problem by spatially resolved photoemission spectroscopy.

Fig. 6 shows two images of AFM oxide lines on Si(001) at two different photoelectron kinetic energies [51]. The four rectangular patches appearing in both pictures correspond to the LAO structures. The field of view is 12Âľm. The image on the left was taken with an electron kinetic energy of 26.8 eV, corresponding to a binding energy of 104.7 eV on the high binding energy side of the oxide component of the Si 2p core level emission peak.

The image on the right was taken with an electron kinetic energy of 29.2 eV, corresponding to a binding energy of 102.3 eV, on the low binding energy side of the same peak. A remarkable contrast inversion is visible.

Fig. 7 shows spectra obtained by plotting the spatially resolved photoelectron intensity versus the binding energy for the LAO patches and for the region covered by native oxide. The Si 2p core level emission peak is composed of two main components. The weak component at 99.3 eV binding energy is related to the emission of electrons from the Si 2p core level in bulk silicon. It is attributed to emission from silicon below the oxidized surface. The strong component at higher binding energy is related to the emission of electrons from the Si 2p core level in the silicon oxide on the surface. The component taken from the native oxide shows an energy shift of 3.9 eV relative to the bulk peak, in perfect agreement with literature.

This shift reflects the degree of oxidation of Si atoms, is therefore a chemical shift. The component from the AFM oxide appears at a binding energy higher than the native oxide component. This can be attributed to a charging of the thick AFM oxide due to the photoemission, in quantitative agreement with an integral photoemission study on SiO2/Si structures [52]. The experiment also evidenced a high degree of homogeneity and the stoichiometry of the AFM oxide. Furthermore, the data indicate that the density of the AFM oxide is lower than that of the native oxide. These topics must be addressed to develop a reliable and reproducible lithographic process. The data show the need for laterally resolved spectroscopic analysis of such nanoscopic structures.

3. Electron Probe Nanospectroscopy 3.1. Transmission Electron Microscope (TEM) In a conventional Transmission Electron Microscope (CTEM or TEM), a thin specimen (typically in the range of 5-500 nm for 100 keV electrons) is irradiated by an electron beam with energies ranging from 60 keV to 3 MeV. The choice of the accelerating voltage and specimen thickness depends on the sample density and elemental composition and on the resolution desired. The point-topoint resolution of a TEM rougly depends on the energy of the impinging electrons and on the spherical aberration coefficient of the magnetic coils. Appropriate

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preparation techniques (chemical polishing followed by ion milling for semiconductors and metal foils, ultramicrotomy for biological specimens) are necessary to thin down the samples to the desired thickness.

Electrons are emitted by thermoionic emission by tungsten airpin cathodes, LaB6 rods, or by a field emission gun (FEG) by pointed tungsten filament. A two-stage condenser lens system allows to change the illumination aperture and the illuminated specimen area.

The electron intensity distribution below the sample is imaged with a three-stage lens system onto a fluorescence screen; the image can be recorded by a photographic plate or by CCD cameras and YAG scintillators (see Fig. 8) [53].

Bright field contrast is due either to scattering contrast (absorption of electrons scattered through angles larger than the objective aperture) or by interference between the scattered wave and the incident wave at the image point (phase contrast). Atomic resolution is achievable in a TEM because elastic scattering is strongly localized to the region occupied by the screened Coulomb potential of nuclei: a very recent prototype of an high resolution TEM (HREM) reached 0.1 nm point-to-point resolution.

Commercial HREMs working at intermediate accelerating voltages normally provide resolutions in the range of 1.5-1.7 nm.

Fig. 8. Sketch of a transmission electron microscope (TEM). The Si(Li) detector is used for energy dispersive x-ray microanalysis (EDS).

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A special capability in the most recent TEMs equipped with a field emission gun is the formation of electron probes with diameters < 1 nm. The instruments operate in the Scanning Transmission (STEM) mode and it is possible to image thick or crystalline specimens, to record secondary and backscattered electrons, cathodoluminescence and electron beam induced currents etc. The main advantage of the STEM mode is that it is possible to perform microdiffraction and elemental analysis (Xray microanalysis, electron energy loss spectroscopy) from very small specimen areas. The strenght of the TEM is that it is possible to provide atomic resolution images (containing information down to 0.1-0.2 nm) and to operate in various microanalytical modes (by using electron probes in the STEM in the range 1-5 nm). This results in an high resolution analytical instrument which is nowaday essential for high level advanced research in material science. In the following, some of the most interesting capabilities of an analytical TEM will be briefly discussed [54].

3.1.1. Energy Dispersive X-ray microanalysis (EDS) The EDS is commonly used to determine the elemental composition of the specimens investigated both qualitatively and quantitatively. X-ray spectrometers can be coupled to a (S)TEM to acquire X-ray quanta emitted by the specimen under electron irradiation. In the imaging mode elemental mapping is also possible. An energy dispersive detector (Si-Li or Ge) with an energy resolution ∆Ex≈120-130 eV allows the simultaneous recording of all the characteristic lines with X-ray quantum energy Ex=hv greater than 1 keV. A disadvantage of the EDS is that neighbouring characteristic lines are not well separated and the analytical sensitivity is not very high (about 5x10-5). However, since the X-ray quanta are generated by thin foils, only small corrections are needed for a reliable quantitative elemental determination.

3.1.2. Electron Energy Loss Spectroscopy (EELS) Electrons that have been inelastically scattered in ionization processes contain the fine structure of the innershell ionization steps. In the energy-loss spectrum, steep steps are seen at energy losses, ∆E, above the ionization energy of a K, L, or M shell from which atomic electrons are excited to an unoccupied energy state above the Fermi level. Electron spectrometers incorporated in the TEM column or placed below the fluorecent screen can be employed to record electron energy-loss spectra. In addition to the elemental composition, EELS spectra also contain information about the electronic structure of the specimens (Z>4). A study of the extended fine structure allows to measure the nearest-neighbour distances.

The EELS technique is therefore an optimum method to analyze elements of low atomic number in thin film

with thicknesses smaller than or comparable to the mean free path for inelastic scattering. Further, it is more efficient than the EDS because the spectrometer collects a large fraction of inelastically scattered electrons that are concentrated within small scattering angles (the X-ray quanta are isotropically emitted and only a small solid angle, ~10-2 rad, is collected by an EDS detector).

Also for EELS the elemental mapping in the electron spectroscopic imaging mode is possible when an electron energy filter is used.

3.1.3. Cathodoluminescence Spectroscopy and Imaging (SRCL) Cathodoluminescence is the physical process during which a system (in this case a semiconductor) which is in an excited state because of the irradiation from energetic electrons (possibly in an STEM or in a SEM), emits photons during the relaxation to a lower energy state. The light emitted by the sample (that can be cooled from room temperature to liquid helium temperature) is collected by a mirror (normally parabolic in shape) and then sent to a monochromator equipped with different gratings and detectors; finally the signal is sent to a computer.

If the temperature is sufficiently low (for instance 20 K for GaAs) excitons (electron-hole-pairs that form a bound state) both free or bound to impurities can be easily studied. In addition to band-to-band transitions, electrons from the conduction band can recombine with neutral acceptors, neutral donors can recombine with holes in the valence band, electrons bound to donors may directly recombine with holes bound to acceptors (at higher impurity concentration) giving rise to donoracceptor pair (DAP) recombinations. In an STEM the electron beam is well defined in energy and can be focused to a very small spot (1-10 nm) with the possibility to scan the sample surface. This results in a map on a submicrometer scale of the intensity variations of the optical transitions on the growth plane and in the possibility of monochromatic imaging of selected emissions with high spatial resolution. When the electron beam is blanked, time resolved cathodoluminescence spectroscopy can also be carried out. In contrast to PL, a disadvantage of this technique is that it is not straighforward to selectively excite the specimens by electrons of such high energy.

3.1.4. Selected Area Electron Diffraction (SAD) In a TEM, on the exit side of a specimen, several diffracted beams are present in addition to the transmitted beam. These are focussed by the objective lens to form a spot pattern in its back focal plane. Electron diffraction methods are used to identify different phases by measuring the lattice plane spacing and to determine crystal orientations in polycrystalline films or single crystal foils. Normally quantitative information can be

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obtained on the identity of phases and their orientation relationship to the matrix, on the habit planes of precipitates, slip planes in materials, on the exact crystallographic descriptions of crystal defects induced by deformation, irradiation etc. and on order/disorder, spinodal decomposition, magnetic domains etc. Extra spots and strikes due to superstructures, antiphase structures, plate-like precipitates etc. can also be studied if a selected area is imaged by inserting a diaphragm in the first intermediate image.

3.1.5. Convergent Beam Electron Diffraction (CBED) Convergent beam diffraction gives two dimensional maps of the diffraction intensity as a function of the inclination between incident electrons and a particular crystal direction. They are normally composed of a series of discs each one corresponding to a different Bragg reflection. The intensity variation inside the discs carries information about specimen orientation, thickness, local strain etc. One of the most interesting fields, CBED can be employed in, concernes the chemical composition and local strain determination in semiconducting heterostructures and devices made by lattice mismatched layers. This is normally done by studying the shifts in the High Order Laue Zone (HOLZ) line positions. The spatial resolution depends on the specimen thickness and probe size. With the modern FEG-TEMs, resolutions of about 1-2 nm can be achieved in the CBED mode.

Fig. 9. Schematics of a scanning electron microscope (SEM). For details, see text.

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3.2. Scanning Electron Microscope (SEM) In a SEM (Fig. 9), electrons emitted by a thermoionic or field emission cathode are accelerated from 1 to 40 keV between cathode and anode [55]. The smallest beam cross-section at the gun (crossover) is demagnified by a three-stage electron lens system so that the electron beam diameter at the specimen surface can be of the order of 110 nm with probe currents of 10-10 to 10-12 A. A deflection coil system scans the electron probe in a raster across the specimen in synchronism with the electron beam of a separate TV monitor. The intensity on the monitor is modulated by one of the signals coming from the electron probe-specimen interaction to form the image.

Some of the advantages of the SEM are the large depth of focus (some mm), the excellent image contrast, the very easy preparation of solid specimens (for semiconducting or conducting samples no preparation is necessary) and the wide variety of products due to the electron beamspecimen interaction (Fig. 10) that can be employed for elemental and spectroscopic analyses of the specimens investigated. The most commonly employed signals are Secondary Electrons (SE), Backscattered Electrons (BSE), Auger Electrons (AE), X-ray quanta (XR), Cathodoluminescence (CL), and Electron Beam Induced Current (EBIC). When SRCL is performed in an SEM, depth resolved and excitation dependent spectra from the specimen can be easily achieved due to the possibility of varying the energy of the electron beam. Linearly polarized cathodoluminescence can also be used to examine for instance the presence of strain and to probe the character of the hole states in the optical transitions by using a rotable vacuum linear polarizer inside the SEM chamber.

The EBIC technique consists of measuring the current flowing through the contacts of a semiconductor junction after it has been excited by an electron beam. Typically a p-n junction or a Schottky barrier arrangement are used. A sensitive current meter, coupled with a current amplifier is connected across the contacts and is used to measure the current produced in the junction.

As a final remark it must be stressed that, unlike for the TEM where thin foils are investigated, in the SEM mode the lateral resolution achieved on the specimen surface does not necessarily correspond to the analytical resolution. This is due to the different interaction volumes inside the specimen where the various signals come from (Fig. 10). However, when the above mentioned techniques are attached to an SEM, a higher signal-to-noise ratio is achieved with respect to the TEM mode, due to the larger thickness of the SEM specimens. However, there is a new technique, not commercially available yet, that can offer unique advantages inside an SEM. This new technique is the ensamble of the SRCL and the Scanning Near Field Optical Microscopy (SNOM), namely CL-SNOM. Using this technique it is possible to perform CL-measu-

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bution of the 362 nm line was found (Fig. 12 b) [59].

Depth resolved CL investigations on the samples (Fig. 13) evidenced an increase of the 362 nm line with respect to the NBE only in sample #105 [59]. Further, CL spectra obtained in different areas of the two specimens showed the onset of an anticorrelation between the 362 nm and the NBE bands. This is in agreement with the inhomogeneous distribution of the 362 nm line as shown in Fig. 12 b [59].

Due to the different growth conditions, the role of structural defects inhomogeneously distributed across the layers is taken into account. Cross sectional TEM micrographs (not shown here) revealed the presence of stacking faults (SF) only in sample #105. Due to the increase of the 362 nm CL peak intensity by increasing the accelerating voltage, the SF density depth distribution has been studied by cross sectional TEM investigations. It has been found that

Fig. 10. Origin and information depth of secondary electrons (SE), backscattered electrons (BSE), Auger electrons (AE), and x-ray quanta (X) in the diffusion cloud of electron range R for normal incidence of the primary electrons (PE)

rements in a SEM with the lateral resolution of a SNOM, which can be as good as 50 nm [56-58].

3.3. Example To conclude this part, an example concerning a study by SEM-SRCL of excitonic transitions in GaN epilayers will be presented. We have performed CL studies of nominally hexagonal GaN grown on different substrates.

We find an additional CL emission at about 362 nm only when a very high density of planar defects is present in the epilayers. We determine the nature of the structural defects and we ascribe the additional CL line to excitons bound to stacking faults (SFE) [59].

In Fig. 11 a comparison between 20 K CL spectra of the emissions of two GaN samples (#105 and #84) grown by gas source molecular beam epitaxy (GSMBE) under different conditions on sapphire is reported [59]. The two spectra present a strong emission line at about 357 nm and the usual DAP band around 386 nm. The temperature dependence of this line in the two samples and the comparison of our data with those reported in the literature for hexagonal GaN, suggest they are near band edge (NBE) radiative transitions (e.g. free and bound excitons).

In the following we will focus on the CL additional emission found in sample #105 at 362 nm. Plan view 77 K monochromatic CL imaging at 357 nm and 362 nm revealed a different intensity distribution across the specimen surface. In particular, a close correspondence between the sample surface as imaged by atomic force microscopy (Fig. 12 a) and the monochromatic CL distri-

Fig. 11. Low temperature CL spectra of samples grown by gas source molecular beam epitaxy on Al2O3.

in sample #105, it decreased from 6x1013 cm-3 in a 150 nm tall region near the interface to 1.5x1013 cm-3 in the 700 nm thick top portion of the sample.

On the basis of the previous results, as a preliminary conclusion, we may state that the line at 362 nm is correlated with (and thus caused by) the presence of stacking faults irrespectively of the growth mode. The role of inversion domain boundaries on the onset of extra lines in addition to the free exciton one has been considered and then ruled out on the basis of literature data. Further, also misfit dislocations in the (0001) plane, that are highly charged due to the strong ionic character of the bond are not expected to induce excitonic transitions usually responsible for the dislocation luminescence.

The high concentration of SFs revealed by detailed TEM investigations of samples #105 the anticorrelation of the additional line with respect to the NBE emission and its increase in intensity near the buffer/layer interface, sug-

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Fig. 12. Comparison between AFM imaging (a) of the #105 sample surface and monochromatic CL micrographs obtained at (b) 362 nm and (c) 357 nm.

gest the emissions at 362 nm are strictly related to excitons bound to SFs within the epilayer.

4. Proximal Probe Techniques After the nobel-prize winning invention of STM in the 80s, the field has seen a rapid development. New surface probes were proposed and implemented. It is common to all these scanning probe microscopes that a surface probe is held in close proximity or in contact to the sample surface, which is then scanned relative to the probe.

Having established itself as a leading-edge microscopy, there is now a tremendous effort to utilize the high lateral resolution obtained by scanning probe microscopes for spectroscopy with high spatial resolution. Electrons and photons are used for excitation and detection. Scanning tunneling spectroscopy is a local measure of the density of states (occupied and unoccupied) close to the Fermi level [60]. However, the tunneling electrons can also be used as a source of cathodoluminescence, which is then collected by a lens or optical fiber and analyzed by a monochromator [61,62]. With scanning near-field optical microscopy (SNOM), even the diffraction limit of farfield optics can be avoided, resulting in a lateral resolution of these microscopes of 50 nm and better for light in the visible range [63,64]. Using this principle, it is possible to perform photoluminescence [65] or cathodoluminescence [56-58] mesurements or to detect an optical beam induced current (OBIC) [66], all with better than 100 nm lateral resolution. These methods have the potential to clarify the relationship between atomic-scale structures and optical properties.

5. Conclusions Each of the instruments that we have discussed has its special features and technical limitations that make it appropriate for certain experiments but not for others.

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Most of the reviewed instruments are of the scanning type, which is more flexible because it allows to use different detector systems, but is inherently slow.

Depending on the signal level, it typically takes seconds to minutes to acquire an image. Imaging techniques are faster by construction; with PEEM and TEM, video rate can be achieved, making it a very valuable tool for the study of the temporal evolution of a system. Work in transmission requires extensive sample preparation, but in the case of electron microscopy the reward is a superior lateral resolution. Detecting low energy electrons increases the surface sensitivity of a method (typical sampling depth some nanometers), while the detection of photons increases the sampling depth to some 100 nm. Elemental analysis can be performed by several methods, but real chemical analysis requires the use of photoelectron spectroscopy. Electron microscopy offers the better lateral resolution (1 nm and better) but suffers from sample damage due to the high energy electron beam. The use of a photon probe reduces beam damage on the sample, but the lateral resolution is somewhat worse (10 nm at best).

However, there are efforts to push the lateral resolution of photon probe nanospectroscopies. The art of making Fresnel zone plates has seen tremendous progress in the last years, and a best resolution of 10 nm seems possible.

Furthermore, a new generation of PEEMs is under construction in different laboratories: the SMART project at BESSY [67], the PEEM-III project at the ALS

Fig. 13. Low temperature CL spectra of the sample #105 at different beam energies.

[68], and the XPLEEM project from Delong Instruments at Elettra [69]. These instruments will be similar to the SPELEEM (Fig. 4), but they will use an electron mirror in the electron-optical path for aberration correction [70]. Their lateral resolution is calculated to be a few nanometers [71].

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Acknowledgements Helpful discussions with L. Sorba, M. Sancrotti, M. Bertolo, E. Di Fabrizio, M. Lazzarino, P. Pingue, F. Beltram, R. Vasina, and J. Westermann are greatfully acknowledged

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Articolo ricevuto in redazione nel mese di Maggio 2000

CHAIN DEFORMATION IN UNFILLED AND FILLED POLYMER NETWORKS: A SAS APPROACH W.Pyckhout-Hintzen, S.Westermann, A.Botti, D.Richter Forschungszentrum Jülich, IFF, D-52425 Jülich, Germany

E. Straube Universität Halle, FB Physik, D-06099 Halle, Germany

Small Angle Neutron Scattering investigations into the microscopic deformation of chains in uniaxially strained polymer networks are presented. The experimental data are succesfully interpreted in terms of a mean-field tube model of Heinrich and Straube which describes the confinement of chains due to the chemical crosslinking and chain entanglements by an effective tube which is anisotropic and depends on strain non-affinely as d(λ)=d0√λ. Different length scales in the rubber are studied and a transition from affine to phantom-like behaviour of chain deformation is found for decreasing lengths of the investigated part of the chain.

A model system for the reinforcement phenomenon in filled elastomers, induced by microphase separation is studied by both SANS and SAXS. The strain amplification concept could be proved unambigously for the first time.

obtain information like deformation on several length scales covering the level of one elastic chain, the tube as well as global information at tens or hundreds of elastic chain lengths.

For micro-heterogeneous systems the contrast matching technique must be applied. The contrast matching technique bases on the fact that scattering lengths vary from one monomer to the other and enables one to match certain components. To analyze the structure of chains in the rubbery phase in a microphase- separated crosslinked blend or two-phase material in general, the solid phase can be made invisible and especially information about the matrix that cannot be obtained separately from other methods, is extracted and allows the testing of theoretical models [9,10]. This contribution will deal therefore with the microscopic deformation at the chain level to test fundamental models of rubberelasticity and apply the concept in the field of filled elastomers on a model-filler.

Introduction The chain dynamics in entangled polymer melts has been tackled succesfully using the well known reptation model [1,2]. Here, entanglement constraints are modeled by a tube and the fluctuation of segments is restricted to its interior. Applied to rubbers, the neglect of chain interactions which is the basic deficit of the phantom theory of rubberelasticity can be relieved hereby [3,4].

When real networks are strained, the polymer chains are deformed and entanglements also become active. In the tube model, the latter chain interactions are simulated by a harmonic potential and their deformation behaviour is derived [5-7]. The tube diameters dµ are defined as the root-mean-square segmental fluctuations in the directions of the principal axes and follow a non-affine deformation law as

d µ = d0 ⋅ λνµ

(1)

with ν = 1/2. λµ is the strain ratio along the principal axes µ = x,y,z.

The size and shape of the deformed polymer chains and their fluctuation width can be conveniently measured by SANS, thereby taking profit of the difference in the scattering length between hydrogen and deuterium [8].

The chains can be made visible in a dense system just by implanting deuterated chain sections along the path or replace part of the normal chains by their deuterated analog. It is clear that it is a powerful tool to selectively

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Experimental Narrow molecular weight distributed Polyisoprenes with high 1,4 microstructures are obtained by anionic polymerization using sec-BuLi as the initiator in apolar solvents. Isotopic triblock structures of the HDH-type were obtained from sequential addition of carefully weighed amounts of the respective monomers and under the necessary precautions to limit chain degradation due to impurities and to preserve symmetry in the wings. The labelled center fraction was varied between 3 and 35% by volume. A block-copolymer PI-PS-PI with χ ⋅ ≈ 80 was prepared by coupling a monodisperse diblock PI-PS with dimethylchlorosilane. The styrene content of 18.0 vol.% was confirmed by fluorescence spectroscopy. Molecular weights of homopolymers and total block copolymers were determined by GPC to be 200000 g/mol and 135000 g/mol respectively. The overall-polydispersities are less than 1.05. They were further confirmed by lowangle laser-light-scattering experiments and membrane osmometry.

Randomly crosslinked networks were prepared using dicumylperoxide (DCP) as the crosslinker. Samples and DCP were mixed in THF and evaporated over about 1 week under high vacuum to assure solvent-free polymer.

The crosslinking was performed under Argon atmosphere at a temperature T = 160° for 2-3 hours to

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ensure complete decomposition of the DCP. Elastic chain lenghts, MC, were estimated from the swelling degree in cyclohexane. In the case of the microphase-separated polymer, the block copolymer was 100000 g/mol and the homopolymers about 90000 g/mol. Blends from a pure homopolymer mixture and PIPSPI were made to give 90% and 50% of the triblock-copolymer, yielding PS-filler degrees of Φ1 = 0.16 and Φ2 = 0.09, respectively. MC, the mass of the elastic chains, was estimated from swelling in cyclohexane and yields for the unfilled sample (8.880±350) g/mol, identical to (8.530±340) g/mol for the Φ1 = 0.16 and (8.490±340) g/mol for the Φ2 = 0.09 sample.

The average gel fraction was wgel=(0.962±0.004). The swelling method yields directly real elastic masses since the PS-domains are equally dissolved in the good solvent. No corrections for insoluble filler materials are necessary. Networks were strained in a calibrated straining device and extension ratios were determined from reference marks on the sample.

SANS experiments were recorded in 2D-detection at PAXY (LLB, Saclay), NG-7 (NIST, Washington), D22 (ILL, Grenoble) and KWS1 (FZ, Jülich) using wavelengths λ of respectively 8, 7, 10 and 7 Å and ∆λ/λ ~ 0.1-0.15. SAXS data were obtained at the synchroton beamline JUSIFA (DESY, Hamburg) using a wavelenght of 1.54Å.

Scattering intensities were corrected for empty beam, cell and detector efficiency and absolutely calibrated against water, silica or lupolen depending on the instrument used. Cross-checks yielded a good agreement in the absolute level of intensities. Incoherent backgrounds measured from fully protonated samples were subtracted, weighed with their volume fraction present.

Data reduction was carried out pixel-wise prior to radialaverage isotropic data. Cross sections δΣ/δΩ were ➙ transformed to the structure factors S(q→) of the system [11] according to

r 2 dΣ dΩ = ( a D − a H ) ⋅ Nchains ⋅ S(q→ )

(2)

a is the total scattering length for deuterated and protonated monomers, summed over the atoms of one unit. Nchains is the number density of labeled chains of polymerization degree Z. (see later) Anisotropic data along principal axes were obtained from Zimm-like extrapolations [12] following

(

S q , q⊥

)

−1

( ) ⋅ (1 + q

= S q ,0

−1

2 ⊥

)

⋅ Rg2, app 3

mode were performed using an ARES-Rheometrics instrument in the frequency range 0.1 to 100 rad/s in the parallel plate-plate geometry in an in situ investigation of the crosslinking reaction under a nitrogen blanket. The strain amplitude γ was 1%. The distance between the plates was adjusted to allow for thermal expansion of plates and sample. The temperature program was identical to the temperature profile of a parallel SANS study. Details of this experiment will be published separately [13].

SANS Structure factor The normalized structure factor for a single, labeled chain, crosslinked into a network in the Warner-Edwards approach [4] can be simply recasted in terms of the r x 1 Slab (q→, λ ) = 2 ∫0 dx ∫0 dx'

(

−Qµ2 1 − λ2µ

)2

∏ exp −(Qµ λ µ ) ( x − x' ) 2

µ

dφ2 6 Rg2

( 1 − exp ( −

( x − x' ) dφ2

(4)

)) ]

2 6 Rg2 Here, Q is defined as Q = q ⋅ Rg and λµ is λ and 2 1 λ respectively following incompressibility. dφ is defined as the mean-square tube dimension in the direction of observation f relative to the parallel axis of straining as

dφ2 = d02 ⋅

λ2 ⋅ cos 2 (φ ) + 1 / λ ⋅ sin 2 (φ )

(5)

Eq. 1 is retrieved along parallel (φ = 0°) rand → perpendicular (φ = 90°). As a consequence, Slab (q, λ ) does not separate into the two principal axes. x, x' are reduced chain length coordinates over which is integrated. The structure factor can be understood as the product of an affinely deformed random path with the contributions from the restricted fluctuations around it.

The parameters of the model are the tube diameter d0 and the microscopic deformation λ.

(3)

e.g. for the parallel direction. The method proved to do better than sector-binned data and is physically correct. If necessary 2D-data were fitted immediately.

Dynamic mechanical experiments in oscillatory shear

Fig. 1. Left: 2D-representation of strained homopolymers. The strain directions is vertical. Right: Evaluation of the isotropy angle for the estimate of the deformation.

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Homopolymers The model perfectly describes two-dimensional SANS patterns on networks made from long primary chains [12,14,15]. The typical appearance of lozenges as in Fig. 1 is fully explained by the introduction of the f-dependent fluctuations in the off-axis directions and provides us with the only possible model to date which fits at the same time also both principal axes correctly. It has been clearly shown in the appropriate references how a change in the deformation exponent of the tube induces subtle but consistent different shapes. A special angle for which the effective deformation is isotropic for both chain and tube part can be identified from inspection of Eq. 5 and is experimentally easily accessible. From this, a direct and almost model-independent estimate for the microscopic chain deformation is rendered possible. Typical scattering pattern with theoretical fits to the structure factor are shown below for good and virtually defectless networks. Here, the deformation that the chain experiences, λ is affine and for the segment fluctuation parameter, which follows the direction cosines as introduced before, a size d0 = 42Å is obtained. We remark that an alternative model in which the tube is not allowed to deform but restrictions are varied over the length of the chain can indeed yield a comparable quality of fitting [16]. However, the outcome is consistently wrong and even contradictory to the own-made assumption of nondeformability for this tube diameter which effectively becomes a function of λ in order to fit the shape [17].

Triblock HDH copolymers In contrast to the common homopolymer systems, triblock-copolymers of the HDH-type enable one to change the length of the labeled path without affecting

Fig. 2. Isotropic (open symbols) and deformed correlation hole (closed

symbols) for the triblock HDH network with φD = 0.35 in parallel and perpendicular direction.

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the network properties. Then, the parameter d0/Rg as it occurs in the structure factor becomes a static tool for the study of segmental dynamics in networks. The correlation hole under deformation is well described by an RPA approach which is modified for the presence of the tube-like constraints. It is assumed that the interchain correlations can be decoupled from short-range fluctuations as e.g. on the length scale of the tube. A factorization of both effects was suggested. The structure factor for the mean configuration of a block copolymer is then proposed as Stot,RPA =

SDDSHH – S2DH SDD + SHH + 2SDH

= SDD –

(SDD + SDH)2 SDD + SHH + 2SDH

(6)

for both isotropic and deformed system. The deformation is included as before by rescaling Q ⋅ Rg with λ. The bare partial structure factors SDD, SHH and SHD comprise the architecture of the copolymer. We have introduced here C(λ) as

C(λ) = SDD,RPA,λ/SDD,λ

(7)

which can be interpreted as the inter-chain structure factor of the system. With the assumptions above the structure factor for a system of constrained chains can then easily be obtained from ➙

SDD,λ = C(λ) • Slab,λ

(8)

Peak positions as well as relative heights are in perfect agreement with experiment [12]. Due to the peak shape, effects like thermal chain degradation and ageing upon crosslinking can be quantified directly from the non-zero intensities at Q = 0. A kinetic degradation scheme basing on random scissioning of chains was developed ,solved numerically and included in the data description, yielding an multi-component RPA treatment. As main result, DCP seemed to leave only between 40 and 80% of the triblock chains intact. Fig.2 presents a deformed correlation peak and fit for a HDH-polyisoprene network with a labelled section φD = 0.35, d0/Rg = 0.6 and strain 1.8. The tube diameter is consistent with d0 = (42±1) Å.

Experiments were performed on copolymers with φD = 0.35, 0.12 and 0.02, thereby varying d0/Rg from 0.6 to 2.2. The affine deformation of the labeled path should be maintained as long as it is well embedded and constrained in the network. However, if the length scale of interest is reduced, this inevitably necessitates the introduction of reduced chain deformations for the case that the elastic chain lengths now exceed the length of the labeled segment fraction. This local non-affinity of the primitive labeled path is then derived from

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the number of chemical knots per chain, nC = MW/MC - 1 to give β = tanh(0.065 nC). More elaborate theories involve a detailed knowledge of the crosslinking and scissioning chemistry of rubbers which is lacking. Fig.4 present the dependence. In 2D, less anisotropic patterns are observed. It is of particular interest to note that the affinity of deformation is reached at crosslink numbers per primary chain of ≈15-20 as are used in typical high performance rubbers.

Fig. 3. Strain relaxation factor α for three different situations fc = 0.3, fc = 0.5 and the affine case of fc =1.

rij2, λ =

(

1 − ( j − i / N ) ⋅ 1 − λ2

λ2

)λ

2

rij2,1 = αλ2 rij2,1

(9)

(0 < α < 1). N is the number of monomers per elastic chain, MC, and (j-i) becomes the length of the deuterated part. This correction factor to λ depends on j − i / N = fc = Mlab / MC . The the length scale with agreement of the experimentally fitted values for fc with estimates from the crosslink density from swelling i.e.

0.45 and 0.55 respectively for the samples with φD =0.12 and 0.02 is excellent.

In the case that constraints are not felt anymore, i.e. the labeled segment is shorter than the tube dimensions, a description of the scattering behaviour can only be achieved by the phantom model i.e. vanishing constraint terms, and chain ends which are displaced as above. It is the first proof of the existence of the tubeconstraining potential and the phantom limit at the same time in a well entangled network with minor degrees of defect structures.

Defect Networks The existence of non-affine deformations in rubbers is not new and is believed to be correlated to the presence of defect structures [18]. An initial characterization of defects or the effect on the microscopic deformation can be achieved by the introduction of a non-affinity index β as λi = λb, introduced first to describe the fast decrease of the C2-constraint term upon swelling. It comprises both limiting cases of affine and no deformation at all. An empirical relationship was set up by us by fitting through a set of data, obtained from a variation in primary molecular weights and crosslinking density, in terms of

In situ experiments For the study of latter effects, the HDH copolymer with φD=0.35 was selected in view of its physical and scattering advantages over the shorter-labeled triblocks.

In order to get more insight in the simultaneous crosslinking and scissioning kinetics upon crosslinking with DCP radically, in situ SANS and parallel dynamic mechanical experiments were undertaken to study the low frequency, i.e large scale structure, and high frequency behaviour, i.e. local length scale structure at T = 140°C. The kinetic degradation scheme, derived for the networks before, is in agreement with the new findings and proves following: no degradation is found for the pure triblock and two steps in the degradation in the presence of DCP can be recognized from SANS. Rheology is insensitive to these changes. Both steps are correlated to DCP by 1: its decomposition temperature and 2: the gelpoint of the system. The rise in intensity at Q = 0 is due only to this mechanism of scission and no crosslink influence can be derived from the early stages.

Since the scission process is an activated process, the rate of scission will be larger at T = 160°C at which the samples in the previous section were crosslinked. The fraction of intact chains vs. temperature is shown in Fig.5 for a reference melt and an in situ forming network.

Thermal and crosslinking degradation can be clearly distinguished.

Fig. 4. Comparison of the experimentally obtained non-affinity exponents β with an empirical approach.

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Filled Elastomers The basic theories of rubberelasticity should still apply for real, filled compounds. In filled elastomers, however, several new aspects enter and the total response is the sum of several mechanisms [19-21]. To unravel the microscopic origin of the reinforcement process it is a prerequisite to isolate the effects through an adequate modelling. Strain amplification due to the hydrodynamic effect [22] is one of these and is described as

λ − 1 = f ⋅ (λ − 1)

(10)

The overstrain factor, f, can be understood to parallel the increase of the modulus E of the filled system compared to the modulus E0 of the pure matrix, E = f ⋅ E0

(11)

In a Padé approximation of the expansion of f up to second order in the volume fraction Φ for a system of polydisperse undeformable spheres derived theoretically [23,24],

f ≈1+

2.5 ⋅ Φ 1 − 2⋅Φ

(12)

a strong dependence on the volume fraction is expected.

To simulate model filler properties, a triblock copolymer of the type PI-PS-PI ΦPS = 0.18 was selected. This block copolymer undergoes a thermodynamically driven microphase separation, which favours spherical PS domains. The degree of the in situ filling is adjusted by blending the PI-PS-PI starlike micelles with a PI

Fig. 5. Fraction of intact triblock chains versus reaction time. Upper curve corresponds to the melt sample without DCP addition, lower curve to the network with DCP. The decomposition and gelpoint temperatures are situated around rt = 30 and 40.

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homopolymer matrix as the soft, rubbery phase. Effective PS volume fractions of Φ1 = 0.16 and Φ2 = 0.09 were so prepared. Unlike the usual inverse block copolymer PSPI-PS, our system has to be crosslinked to obtain a permanent network structure since the elastic, soft chains are only connected to one single domain. The advantage, however, is a direct analogy with the pure matrix network. These systems are believed to closely resemble the structure of a carbon-black or silica reinforced network whereas the typical fractal character of both latter industrial fillers can be avoided.

Two contrast conditions, termed composition matching and achieved by means of 2 homopolymers to match the scattering length of the PS-phase , and phase matching to eliminate the scattering due to intra-block correlations in the block copolymer must be considered. Ideally a blend of equally sized labeled and unlabeled homopolymers is diluted with a phase-matched copolymer. The scattering intensity for this phasematched copolymer with a blend of homopolymers, compositionally matched to this is [9,10]

r dΣ r (q, Λ ) = ∆2H N H + ∆2D N D ⋅ Z D2 ⋅ SD (q, Λ ) dΩ

(

)

(13)

Here, Ni (i = H, D) is the number density of a homopolymer component, ZD the polymerization degree of the D-homopolymers, ∆i = (bi - bPS) (i = H, D) is the scattering length contrast between the homopolymers and both blocks of the copolymer. bi is the scattering length density. Λ now represents the microscopic deformation tensor acting on the chain level with the components Λµ (µ - x,y,z).

The phase matching condition applies for a triblock copolymer with polyisoprene arms statistically built up from 16 vol-% deuterated and 84 vol-% protonated monomers. The composition-matching is achieved by mixing 16 vol-% deuterated with 84 vol-% protonated homopolymers. The superstructure of the PS domains allows the easy identification of additional scattering contributions at non-ideal contrast conditions. The microscopic matrix chain deformation in the reinforced network is evaluated using the tube model of rubber elasticity [5] which successfully describes the chain deformations of unfilled networks in SANS [12,15]. SAXS enables us to determine the geometrical properties of our systems, the scattering contrast now due to differences in the electron densities of polystyrene and polyisoprene. The structure of PS domains in comparable block-copolymer systems can be modeled by a liquid-like PercusYevick structure factor [25] or a BCC lattice-like structure factor [26-28]. However, it is beyond the scope of this paper to discriminate between different structural models.

In the case of a liquid-like structure of spheres, the

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scattering intensity is written as the product of the average formfactor P (q, Rm ) of polydisperse spheres with smeared boundary, σ, and the hard sphere structure factor in the Percus-Yevick approximation SPY(Q). The diffuse boundary, σ, represents the width of a boundary layer, whose electron density profile is the convolution of the density profile for a sharp interface with a Gaussian smearing function.

For the isotropic network, the average radius of the

Fig. 6. Upper: 2D and axis description of blend with ΦPS = 0.16. Solid curves are best fit to the theory without hydrodynamic correction f.

Lower: 2D and axis description taking into account the strain amplification factor f=1.9.

spheres of Rm = (83.0±1.2)Å with gaussian polydispersity of σR = (12.0±0.6)Å. The hard sphere radius was RHS=(120.0±0.6)Å corresponding to a hard sphere volume fraction ηHS = (0.49±0.01). For σ we have used a Porod analysis to obtain 4Å.

An affine correlation of the filler displacement with macroscopic strain as well as a deformation of the PSdomains are observed. The PS volume fraction can again be calculated to φPS = η(Rm/RHS)3 = (0.16±0.20) in excellent agreement with the chemical characterization.

Further insight into details of order or disorder of the microphase domains are unimportant for the purpose of this investigation of matrix properties.

The SANS data on the block copolymer showed that the matching condition was chemically not perfect. The excess intensity, however, is isolated in a peak at QRg about 2. A biased subtraction of this intensity from the blends to obtain Debye curve scattering yields corrected scattering intensities [29]. For the representative case of Φ1 = 0.16 and a macroscopic deformation of λ = 1.65 the theoretical curve calculated from the tube model (eq.5) based on the parameters of the unfilled reference sample

without an overstrain factor considerably disagrees as is shown in Fig. 6. Introducing the microscopic overstrain factor f, given in eq.10, into eq.5, using the transformation of λ to λ, the average microscopic overstrain factor f becomes the only relevant parameter to be optimized further. Since the network parameters are similar, besides the strain amplification all chain parameters can be estimated from the unfilled case. An extra confinement effect is not present. In the representative example a value of f = 1.9 was determined, which gives an excellent fit for the whole q-range studied.

The model as such therefore represents the first direct microscopic proof of a matrix chain overstrain in filled elastomers. To determine whether f also depends on strain - unlike eq.12 - all anisotropic spectra were evaluated in the same way. A fitting of the tube diameter again proved to be unnecessary.

The reinforcement factor in Fig. 7 seems to follow perfectly the known relationship f (Φeff) as in Eq.12 with Φeff =Φ (1+δ) as a correction for the strongly bound phase on the surface of the microphase separation [23].

Summary We have studied the microscopic deformation and tube confinement in peroxide-crosslinked networks by means of the Small Angle Neutron Scattering technique. We have shown that the level of deformation depends strongly on the length scale of investigation. For wellcrosslinked networks with typically 15-20 knots per primary chain, affinity is found down to the elastic chain.

Latter length scale was thoroughly investigated by means of selectively labeled HDH block copolymers with varying deuterated fraction. It is observed that for labeled parts that are shorter than the typical tube

Fig. 7. Dependence of strain amplification f (right) and radius of gyration (left) on the volume fraction of PS-filler. The agreement of f is perfect with the suggested Pade-approach and an effective filler volume fraction.

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diameter the tube constraints vanish and the phantom model re-appears. Also the effect of reduced deformation in the case that the labeled part is shorter than the shortest elastic chain is included in the investigation.

Basing on the information from the unfilled rubbers, we have studied the phenomenon of rubber reinforcement on a model system by simulating the filler by microphase-separated spherical PS-domains. All length scales of interest as well as sizes of the domains themselves are available. This tailor-made block copolymer provides a one-to-one correspondence to real filled elastomers. By judiciously matching the scattering properties of the filler component by that of the matrix, the effect of the reinforcement on the matrix chains was studied by SANS, whereas the response of the filler could be studied by SAXS. The SAXS experiment yielded a filler radius of about 84Ă&#x2026;, which compares well to typical sizes of carbon black or silica fillers, and an affine correlation of the filler displacement with macroscopic sample strain was found.

The variation of the domain radius with the strain indicated that the PS spheres are softer than expected, which may be attributed to the presence of some polyisoprene chains within the PS domains reducing the density of the micelle-like core.

The SANS experiments were evaluated using an extension of the tube model for randomly cross-linked networks and the molecular parameters derived were in good agreement with the expected ones for the unfilled case. The reinforcement factor f depended on the filler concentration according to the overstrain picture and varied slightly with strain. The data were fitted to a very high degree of accuracy with the assumption of an homogenous over-strain in the matrix but first signs of a break-down of this crude treatment have shown up.

This work reports for the first time a direct experimental determination of the overstrain factor which has to be considered in reviewing semi-microscopic theories of the rubber reinforcement with real filler materials. This work encourages the analysis of silica-filled systems which is currently still under progress.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

Acknowledgements The authors thank Dr. G. Heinrich, Continental AG, for valuable discussions during the progress of the work and Mrs. M. Hintzen for the kind preparation of the network samples.

References 1.

2.

3.

S.F. Edwards and T.A. Vilgis, Rep. Prog. Phys. 1988, 51, 243-297.

M. Doi and S.F. Edwards, The Theory of Polymer Dynamics, Clarendon Oxford, 1986.

R.T. Deam and S.F. Edwards, Philos. Trans. R. Soc. London, Ser. A 1976, 280, 317.

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M. Warner and S.F. Edwards, J. Phys. A. 1978, 11, 1649.

G. Heinrich, E. Straube, and G. Helmis, Advances in Polymer Sciences 1988, 85, 33-87.

G. Heinrich and E. Straube, Acta Polymerica 1983, 34, 589-594.

G. Heinrich and E. Straube, Acta Polymerica 1984, 35, 115-119.

A. Kloczkowski, J.E. Mark and B. Erman, Comp. Polym. Sci. 1992, 2, 8.

X. Quan, I. Gancarz, and J.T. Koberstein, J. of polymer Science Part B: Polymer Physics 1987, 25, 641.

X. Quan and J.T. Koberstein, J. of Polymer Science Part B: Polymer Physics 1987, 25, 1381 J.S. Higgins and H.C. Benoit, Polymers and Neutron Scattering, Clarenden Oxford1994.

S. Westermann, V. Urban, W. Pyckhout-Hintzen, D. Richter, and E. Straube, Macromolecules 1996, 29, 6165-6174.

A. Botti, W. Pyckhout-Hintzen, and D. Richter and E. Straube, in preparation 1999.

E. Straube, V. Urban, W. Pyckhout-Hintzen, and D. Richter, Macromolecules 1994, 27, 7681.

E. Straube, V. Urban, W. Pyckhout-Hintzen, D. Richter and C.J. Glinka, Phys. Rev. Lett. 1995, 74, 4464.

D. Read, Phys. Rev. Lett. 1997, 79, 87.

D. Read, Phys. Rev. Lett. 1998, 80, 5449.

J. Bastide, J. Herz, and F. Boue, J. Phys. (Paris) 1985, 46, 1967.

J.-B. Donnet, Carbon Black, M. Dekker New York, Basel, Hong Kong 1993.

D.C. Edwards, J. of Material Science 1990, 25, 4175-4185.

J. Donnet, A. Vidal, Prog. Coll. Polym. Sci. 1987, 75, 201-212.

E. Guth and O. Gold, Phys. Rev. 1938, 53, 322.

R.M. Christensen, Mechanics of Composite Materials, Wiley New York, Chichester, Brisbane, Toronto 1979.

H.-S. Chen and A. Acrivos, Int. J. Solids Structures 1978, 14, 349.

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L. Leibler, Macromolecules 1980, 13, 1602.

M. Schwab and B. Stuehn, Phys. Rev. Lett. 1996, 76, 924.

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S. Westermann, M. Kreitschmann, W. Pyckhout-Hintzen, D. Richter and E. Straube, Macromolecules 1999, 32, 5793.

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Articolo ricevuto in redazione nel mese di Aprile 2000

STRESS-TEXTURE STUDIES IN THIN FILMS AND COATINGS BY SYNCHROTRON RADIATION XRD AND NEUTRON DIFFRACTION Paolo Scardi Dipartimento di Ingegneria dei Materiali, Università di Trento 38050 Mesiano (TN), Italy

Solo dopo aver conosciuto la superficie delle cose ci si può spingere a cercare quel che c’è sotto. Ma la superficie delle cose è inesauribile.

(I. Calvino, Palomar, Einaudi, 1983) Only after knowing the surface of things can one explore what is underneath. But the surface of things is endless. (free translation) Introduction In the last decade non-conventional radiation sources, like Synchrotron Radiation (SD) storage rings and reactors or spallation neutron sources, have been increasingly used in structural and materials science studies.

Many of these researches were based or Neutron Diffraction (ND) and X-ray Diffraction (XRD) techniques, whose potentiality is greatly enhanced by SR. Despite the large number of scientific activities that are being conducted, and new Large Scale Facilities (LSF) that are planned for the near future (e.g., Diamond in UK), relatively few applications directly concerned technological issues [1].

A deeper discussion on this point would be far beyond the scope of the present paper, but two factors at least should be considered: the access to LSF is not straightforward, and costs and time length or researches can be incompatible with industrial R&D and diagnostic requirements. In addition, most companies are probably not aware of the great potentiality of SR XRD and ND; on the other hand, it is also possible that many scientists engaged in SR XRD and ND studies are not completely aware of the industrial needs and of the many possible applications of technological interest. Therefore, the role of popularisation and information within the scientific as well as industrial community cannot be overemphasised.

It is the main purpose of this work to review some recent applications of SR XRD and ND to typical examples of Materials Science & Engineering studies concerning thin films and coatings for metallurgical and thermal engine applications of direct technological interest.

SR XRD has several advantages over to corresponding laboratory XRD: the most obvious is the high brilliance

that can reduce greatly measurement times, and allow measurements that would be unrealistically long on a lab-scale equipment. More specifically, thin films and coating studies can benefit of the following unique experimental conditions: • Highly parallel (line or point) beam: ideal for texture and strain studies.

• Variable wavelength: necessary to measure gradients of properties in coatings and surface layers.

• Monochromatic, narrow instrumental profile: best conditions for powder (θ-2θ) measurements, and Line Profile Analysis (LPA) studies in particular.

Beam penetration is probably the main limit of SR XRD; even if a transmission geometry can permit a throughthickness scansion of properties (e.g., residual strain [2,3]), thickness up to a few mm or cm can be reached for low-absorption materials only (e.g., Al), and very high energy. X-rays are intrinsically not suitable to produce diffraction phenomena of practical interest for stresstexture measurements with very high energies (above ≈20 keV). Therefore, apart from particular cases, typical measurement depths range from fractions of microns to 50÷100 µm.

Much higher penetrations can be achieved by ND; metal and ceramic components of the order of several cm or more can be easily crossed by a neutron beam [2,3]. The limit, in this case, is on the opposite side, i.e., the smallest depth difference that can be distinguished, and the average over a relatively large sampling volume.

In ordinary conditions (ENGIN, ISIS), strain depthprofiling cannot be measured with steps smaller than ~0.25 mm. Therefore problems can arise when studying coatings of “intermediate” thickness, of the order of 100÷200 µm, which can be viewed as too thick for SR XRD and too thin for ND. SR XRD and ND can be especially useful in problems of residual stresses, texture and phase compositions (including also the determination of nature and amount of lattice defects) of thin films and coatings; technological properties (mechanical, thermal, electrical etc., but also stability and service life) are closely related to these factors. In the following I will describe variable-

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wavelength techniques to study through-thickness variation of the residual stress field in thin polycrystalline diamond coatings for cutting tool applications, and analogous topics in thick ceramics for Thermal Barrier Coatings (TBCs) on engine components.

Ample bibliography is reported for details on these studies.

Polycrystalline diamond coatings Polycrystalline diamond coatings (PDCs) produced by Chemical Vapour Deposition (CVD) techniques find a number of applications as hard layers to improve wear resistance of cutting tools and metal components [4].

Practical uses, however, can be limited by adhesion and coating stability in service conditions, and both features are strongly connected with the stress field produced during deposition or in service. Residual stress can be of thermal or intrinsic nature: the former is due to the high deposition temperature and differences in thermal expansion coefficient (TEC) of substrates and PDC, whereas the latter is typically due to grain growth or phase transformations [5,6].

PDCs have a peculiar microstructure, with single-crystal grains whose shape and orientation is closely related to the CVD process conditions [7]. Surface pretreatments affect nucleation and final microstructure, thereby adhesion and wear resistance change considerably [8,9].

Typical pictures at the early stages of diamond growth and at the surface of a thick (5µm) PDC produced by HFCVD (Hot Filament CVD, [8]) are shown in Figure 1.

In the following case of study, substrates were made of Ti-6Al-4V alloy; preliminary XRD lab measurements revealed the presence of a TiC layer between diamond and α-Ti substrate matrix, whereas the residual stress field in the PDC proved to be planar and rotationally symmetric (i.e., the only non-zero stress component is in the surface plane, with σ11 = σ22) [10,11]. SR XRD data

Fig. 1. Early stages of polycrystalline diamond on a WC-Co component (Courtesy of R. Polini) (a); surface of a 5 micron PDC on WC-Co (b).

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were collected at British facility of Daresbury, station 2.3, using a 4-circle goniometer: measurement geometry is shown in Figure 2.

Texture and residual strain in diamond coating and phases underneath were studied by selecting the most appropriate wavelengths. Diamond (111) and (220) pole figures are shown in Figure 3, together with an analogous picture for (024) α-Ti. PDC exhibits a broad (hh0) fibre texture (i.e., grains with [h00] growth direction and lack of any order in the growth plane), whereas no texture is observed in TiC and α-Ti matrix.

Based on the above information, residual strain was measured on the three phases, stacked in the sequence αTi/TiC/PDC, using different wavelengths (two different values for α-Ti matrix). Figure 4 shows the results in the form of sin2ψ plot (interplanar distance or strain as a function of the sin2 of the ψ-tilting angle) [12]: trends are linear, as expected from the planar stress hypothesis, but slopes are markedly different in the three phases.

Average residual stresses, calculated by using appropriate X-ray Elastic constants (Table 1), are displayed in Figure 3. diamond

TiC

α-Ti(C)

wavelength (Å)

2.15

1.66

1.68 1.4

(hkl)

220

024

9.2 x10-4

2.73 x10-3 1.13 x10-2 (°) 1.94 x10-3 (°)

-5.5 x10-5

-4.50 x10-4

-2.7 x10-3 (°) -3.49 x10-4

72.2

4.8

8.6

Phase:

S1

(GPa)-1

_ S2 (GPa)-1

WC 2.2

0.9 0.65

(°) ξ (at ψ=45°) (µm)

2.8

0.54 1.25 2.96

(°) calculated as (1+ν)/E and –ν/E, respectively (E=106 GPa; ν=0.31); (°°) (E=640 GPa; ν=0.26).

Table 1. Wavelengths, information depth (ξ), XECs and Miller indices of studied reflections.

The strong compressive stress in the PDC turns to weakly tensile in the substrate. This study was particularly useful to understand the role played by the TiC reaction layer in adapting the strongly compressed PDC to the metal substrate. All the present phases could be accessed within the same measurement, profiting from the possibility of tuning appropriately wavelengths.

The thermal component of PDC residual stress can be estimated as σT ≈ ∆α ⋅ ∆T ⋅ E/(1-ν) (∆α is the difference between TEC of coating and substrate, E and ν are Young modulus and Poisson ration of PDC, respectively), which gives σT = -6 GPa (for a PDC deposited on a Ti-alloy at Tdep=650°C), very close to the measured value. This result strongly suggests that thermal stresses are dominant in these systems.

The same was also observed for PDCs on WC-Co cutting tools [9,13]. In this case, SR XRD measurements on samples produced at different Tdep gave interesting

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Fig. 2. Schematic of the instrumental set-up, with definition of 2θ, ψ-tilting and azimuth (φ) angles.

information on coating adhesion and stability. Given the thermal nature, compressive stress increases with the deposition temperature; this is frequently observed in PDCs because diamond TEC is usually much lower than that of metals and many ceramics.

Even if compressive stresses in coatings and surface layers can be beneficial, for they can improve the protective action against wear, very high values can lead to coating detachment. It is therefore necessary to find an optimal balance between the two needs: high surface compression and good adhesion. Figure 5 shows planar residual stresses measured by SR XRD in PDCs on WCCo components as a function of Tdep; experimental values are close to the calculated thermal stress up to a critical value beyond which measured residual stress reduces, due to a progressive coating failure. XRD patterns are

collected on a relatively large surface area (≈1 cm2), so localised failures can reduce without eliminating the measured residual strain; Figure 4 can then be used to establish the optimal (maximum) Tdep. Coatings deposited above Tdep>750°C are likely to be damaged: in fact, above this temperature PDCs frequently spalled soon after deposition, or did not resist the SR XRD measurements. Some of them were partially lifted from the substrate, without apparent damages, and only residual strain measurements could disclose conclusive evidence on their adhesion.

Therefore, an important process parameter could be tuned by means of SR XRD, and coating quality (and adhesion in particular) could be tested in a nondestructive way. Interestingly, analogous considerations could also be done on the basis of a LPA study (details in ref. 13,14).

Given the marked thermal nature of residual stress, numerical FEM models (based on the TEC differences and other properties of coating and substrate) can be used to predict the stress field through the thickness of coated components. Results of these modelling usually show a sharp stress change from compressive to tensile at the PDC-substrate interface [15]. However, direct experimental evidence of this important feature is missing, even if, as we have already underlined, PDCsubstrate interface stresses are determinant in controlling adhesion and durability of coated components. A deeper insight on this important point was made possible by SR XRD [16].

Sample was a WC-6%Co (K10 alloy) component coated

by PDC. The 5µm diamond coating was deposited at 750°C (further details in references 13,14) and did not

Fig. 3. Pole figures ((220) diamond, (111) diamond, (220) TiC) and average residual stress in a Ti6Al4V component with PDC and TiC interface [11].

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constants data with reasonable reliability [17]. In addition we can assume a rotationally symmetric planar stress, so that a gradient can be described simply as: S S σ 11 = σ 22 = σ S ( z ) = a + b z + c z 2 + .....

(2)

where z is the position below the surface. The average sample stress in Eq. 1 is averaged over the X-ray absorption low, and can be written as: t

σS =

−z ξ S ∫ σ ( z ) ⋅ e dz 0

t

∫e

−z ξ

dz (3)

0

Fig. 4. sin2ψ plot for diamond, TiC and α-Ti phases in the sample of Figure 3 [11].

show significant texture. SR XRD revealed a compressive stress of about –1.7 GPa, comparable to the thermal stress (see Figure 5); the same value was obtained by using two different wavelengths (Figure 6a), both with positive and negative ψ-tilting, as well as at different azimuth (φ), confirming the planar, rotationally symmetric nature of the residual stress field [10].

To investigate the residual stress in the interface region of the substrate, we selected three different wavelengths, in order to collect residual strain data at different average depths in the substrate (Table I). The results are markedly different, and clearly indicate the presence of a stress gradient that was studied as follows. The basic equation in X-ray residual stress analysis (XRSA) is [17]: L 〈ε 33 〉 = Fij σ ijS

(1)

where the strain is measured in the laboratory system (L) along the scattering vector direction (ε33 component), and is averaged ( ) over all the crystalline domains whose hkl planes are in Bragg condition.

On the other hand, the quantity of interest is the macroscopic residual strain, or the corresponding residual stress, in the sample system (S). Fij are stress factors, calculated from elastic constants accounting for texture effects [18,19]. Unlike elastic constants, stress factors are not tensor properties; their calculation can be quite complex and depends on the mechanism of mechanical grain coupling in the material, which in principle is not known [17-19].

In our case texture is absent, and the material can be considered as quasi-isotropic (texture-free, macroscopically isotropic) and homogeneous. Therefore Fij can be replaced by XECs that can be calculated from elastic

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where t is thin film thickness, and ξ = (sinθ cosψ)/2µ is the information depth, which depends on θ, ψ (Figure 2) and µ, the linear absorption coefficient. Introducing XECs (ξ=(sinθ cosψ)/2µ) we can write the sin2ψ equation in the presence of a stress gradient [16,18]:    t ξ (1 + t 2ξ )  tξ   L = 2 S 1hkl + 12 S hkl sin 2 ψ ⋅ a + bξ 1 − t ξ  + 2cξ 2 1 − ε 33  + ... 2  e − 1 et ξ − 1    

[

]

(4)

The biaxial stress hypothesis also allows the use of the S2 condition sin ψ ε = 0 = −2 ⋅ S1 ½ to calculate the strain from interplanar distance [17,18]. Eq. (4) can be used to simultaneously fit strain data obtained from different wavelength measurements, in order to refine the coefficients (a,b,c..) of the stress gradient (Eq. 2). The result of this procedure in shown in Figure 6b, whereas the stress distribution in the surface and interface region of the coated component is reported in the drawing of Figure 6c. No sharp change from compression to tension is observed, and the neutral axis lays below the interface, inside the substrate. This clearly demonstrate that a modelling simply based on TEC differences is an 2

hkl

hkl

Fig. 5. Planar residual stress measured by SR XRD in PDCs deposited on WC-Co at different deposition temperatures.

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oversimplification that doesn’t reproduce the actual features of the coating-substrate interface in real components. These results find a direct correlation with the wear behaviour, that was tested against several metallic and ceramic counterface materials. Details on these studies can be found in reference 9.

Ceramic Thermal Barrier Coatings (TBCs) Thick layers of zirconia-based materials find important applications as TBCs on gas turbine hot components like combustor cans, transition ducts and 1st stage vanes and airfoils, but also in Diesel engines parts, like piston heads and valves [20-22]. Thermal insulation of TBCs can

reduce the temperature of the underlying metal component by 100°C or more, with a remarkable increase in durability and a reduction in fuel consumption; estimated fuel economy for a 250-aircraft fleet using TBCs on turbine blades is of the order of 10 million gallons per year [21]; conversely, an increase of 110°C in the inlet gas temperature may allow a 20% increase in thrust of jumbo-jet gas turbines [23]. Therefore, the great technological and economical impact has fostered many researches in this field, for the production of better TBCs and cost reduction. Plasma Spray and EB-PVD are the most used (and suitable) techniques to produce ceramic

Fig. 7. Microstructure of a typical Air Plasma Spray coating of Y-PSZ (fracture surface).

Fig. 6. sin2ψ plot for a PDC on WC-Co, as obtained by SR XRD by using two different wavelengths (a); experimental data and modelling by means of Eq. (4) for the WC phase interface (b); trend of residual stress in coating and WC interface layer (c) [16].

coatings in the thickness range from 0.1 to ca 2 mm (or more). One of the main issues in the design of long durability coatings is improving adhesion; coating debonding (spalling), in fact, is a primary failure mechanism. In turn, adhesion is directly connected with the thermal stress developed during deposition and, especially, in service conditions [24].

Modelling the behaviour of typical ceramic TBCs is a formidably difficult task [25]. The main reason lays in the complex microstructure (Figure 7), and plasticity and creep phenomena at the metal interface that can be difficult to model; in addition, corrosion (and oxidation in particular), phase transformations, action of contaminants in the fuel, stability of metal and oxide phases, ceramic sintering, are all important factors that make modelling and life prediction a complex issue. A direct measurement of residual stresses is therefore required.

SR XRD and ND are the only viable non destructive techniques to investigate the through-thickness stress distribution in coated components. The first example concerns a 300 µm TBC of Y-PSZ (Yttria Partially Stabilised

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Fig. 8. Least squares fitting of residual strain data collected by SR XRD on a Y-PSZ TBC deposited on an Al-alloy component. Data were collected by using two different wavelengths (a); residual strain gradient resulting from the analysis of (a) (b) [16,27].

Zirconia) deposited on Al-alloy bars (9mm x 6mm x 130mm) by Plasma Spray under controlled conditions of temperature and atmosphere. After low temperature ageing, phase transformations taking place in the ceramic led to a single-phase tetragonal material, seemingly in a compressive in-plane stress state [26]. Even if thickness exceeds the depth accessible by X-rays, a measurement of gradient in the outer ≈50 _m layer can be carried out by SR XRD, and the result can be integrated by information on the average stress in the coated component obtained by measuring coating length and curvature change after debonding by chemical attack [26].

The approach proposed by Eq.(5) was used again, considering that the thickness t tends to infinity (values above ca 100 µm can be considered as infinity for the absorption of X-ray wavelengths of the present study), and using two sets of data collected at different wavelengths. The results of the least square fitting are shown in Figure 8a, and the corresponding residual strain trend in Figure 8b. This result, which confirmed the

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compression in the coating and revealed the presence of a steep gradient, can be used together with values of length and curvature changes after debonding as an input of a suitable mechanical model, based on equilibrium conditions of forces and bending moments [27]. In this way it was possible to calculate the stress trend inside the component, which is reported in Fig. 9.

This result was of great interest from the methodological point of view, as it gave a through-thickness residual stress trend in a coated component, and demonstrated the importance of phase transformations on residual stress evolution in TBCs. However, a direct analysis of the stress at the interface was out of reach. To this purpose we can resort to ND. The following example concerns a thick (1.6 mm) Y-PSZ TBC deposited by Air Plasma Spray (APS) on an Al-alloy piston head semicomponent (diameter 87 mm, height 10 mm) [28-30]. A 0.2mm bonding layer of Ni-CrAlY was deposited on the metal before APS. The component underwent a severe thermal cycling, reproducing service conditions, in order to study the failure mechanism. Thermally cycled components started to develop a crack at the edge of the disk (along the rim), near the ceramic side of the interface region; the central area of the studied disk, instead, was still perfectly adherent and ND measurements were then conducted in that area.

ND was done at the British facility of ISIS, using ENGIN on the PEARL beamline. The instrument was specifically designed to make strain measurements, and in particular through-thickness scansions. The set-up used in our study is reported in Figure 10: Time of Flight (TOF) patterns were collected at different z position, in order to place the sampling volume at increasing depth inside the component. Figure 11a shows some selected patterns, where we can see the various phases appearing at different depth. The information on the strain is obtained from peak position shifts that, under the adopted geometry, give an ε33 component.

Data processing is not straightforward, and requires some additional work. Besides the problem of determining the interplanar distance for strain-stress free samples [30], several corrections for aberrations are required; in addition, the trend of Figure 11b, obtained from peak position shifts, is an average strain over the sampling volume (strictly, that part of sampling volume that is actually intersecting the material for any given z position).

A suitable model is then necessary to obtain the residual strain trend as a function of depth [30]; however, most valuable conclusions can be drawn on the basis of Figure 11b without further processing. In fact, if we assume that the main stress component is a rotationally symmetric plane stress (only non-zero component σ11 = σ22), then measured strain along the 33 direction can be interpreted

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as an in-plane compression or tension for positive and negative strain values, respectively (simply a Poisson effect). From Figure 11b we can then conclude that: • TBC surface is in compression, in agreement with preliminary XRD observations; a compressive surface stress is attributed, among other mechanisms, to ceramic sintering and filling of porosity with corrosion products (also observed by XRD).

• in-plane compression decreases toward the inside, and turns to tension around the mid of the coating thickness; • compression increases again near the Y-PSZ/NiCrAlY interface. A steep strain gradient is present within the ceramic, near the interface; • the Ni-CrAlY bonding layer is in compression (experimental resolution didn’t allow the collection of more than one point for that phase); • Al-alloy substrate is in tension at the interface; total stresses and bending moments tend to equilibrate (even if, strictly, stress and moments should equilibrate over the entire component, and not necessarily locally).

These observations are compatible with a model of

Fig. 9. Residual stress trend across the coated component of Figure 8; the result was obtained by using SR XRD data and coating length and curvature change after debonding [27] (a). Detail of the stress trend in the TBC, with stress gradient obtained by SR XRD, with indication of the information depths for the two wavelengths used (b).

Fig. 10. Instrumental geometry of TOF data collection on ENGIN at ISIS (Didcot, UK). Schematic cross section of the studied coated component.

thermal cycling where substrate creep takes place in the interface region. In fact, we can assume that thermal expansion during the heating stage of each thermal cycle puts the Al-alloy in a compression, because TEC is almost double than that of TBC and bonding layer. During the high temperature part of the cycle, compression tend to be released by substrate creep in the interface region. On cooling the situation reverses, and when temperature is sufficiently low to arrest creep, the coating tend to be in compression while the substrate is in tension. The resulting stress tends to develop shear components along the edge of the coated disk, and cracks are most likely to nucleate and propagate in the interface area, inside the ceramic (which is brittle), in the region where a steep strain gradient was observed.

This interpretation is well supported by other experimental observations, and suggested an interesting failure model in thermally cycled TBCs [30].

Phase stability, besides adhesion, is an important problem in TBCs technology. Zirconia has several polymorphs: at room pressure we can find monoclinic (m), tetragonal (t) or cubic (c) zirconia (in order of stability from low to high temperature). High temperature phases can be retained at RT for kinetic reasons, but also by adding suitable stabilising oxides (like Yttria and /or Scandia in our TBCs). The aim is obtaining a single-phase material, stable in a wide temperature range, in order to prevent the catastrophic effects of volume expansion associated with polymorphic phase transformations (especially the 4% volume change of t-m transformation). Considerable efforts are thus addressed to the development of new zirconia materials, and stabilising oxides in particular. Measuring phase content is an important issue: XRD is typically used, also as a routine-basis technique, even if overlapping among peaks of zirconia polymorphs can hinder the analysis and make results unclear.

In a recent study on a new stabilising oxide mixture

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based on the Scandia-Yttria system [31], SR XRD was used to disclose the complex phase equilibrium determined by a high temperature ageing of zirconia powders and TBCs. The purpose of the study was to verify the stability of phase composition after an accelerated high temperature ageing. Verifying the

Fig. 11. Selected TOF patterns collected at different z values (a). Residual strain (ε33 component) in the coated component. Data are averaged over the sampling volume (curves reported just to drive the eye) (b). [29,30].

formation of m-phase does not need, usually, very sophisticated instruments: two m-phase peaks at low angle are easily distinguished from the t or c reflections.

Understanding the composition of t and c phases is more complex. SR XRD patterns collected on TBCs heat treated at 1400°C [31] are shown in Figure 12, together with the result of the modelling by means of a program based on the Rietveld method [32], which permits structure refinement as well as quantitative phase analysis [33].

The intense SR beam allowed the collection of high signal-to-noise patterns: narrow, monochromatic profiles were the best conditions for clearly identifying the present phases. Analogous data collected on lab XRD

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instruments led to no conclusion. The problem was in the identification of three different t-phases, recognised on the basis of the c/a ratio and considerations based on the Zirconia-Scandia and Zirconia-Yttria phase diagrams [31,34]. In particular, after thermal cycling, the TBC made of the new Scandia-Yttria stabilised Zirconia (SYSZ) showed an almost negligible m-phase content (Figure 12a), and was made mostly of a t’ (so-called nontransformable) tetragonal phase composing as-sprayed coatings; very little t1 phase was found (zircon was present as a contaminant due to the preparation in quartz crucibles of the new SYSZ powder used for Plasma Spray). The state-of-art YSZ coating, in analogous heat-treatment conditions developed a much bigger amount of m-phase (Figure 12b); in addition, the t’ phase completely demixed in two t-phases, t1 and t2, leading to a complete destabilisation of the material, and progressive m-phase formation. The conclusive evidence for the presence of the two t-phases, never observed clearly before, was given by long wavelength measurements, like that shown in Figure 12c, that allowed a better separation between peaks of the present polymorphs.

Highly textured thin films SR XRD and ND applications discussed so far concerned weakly textured or random coatings. However, a very large number of thin film systems of technological interest are strongly textured (typical pole figures of differently textured thin films are reported in Figure 13): epitaxial or etheroepitaxial thin films, frequently used for electronic applications, tend to develop a single-crystal like texture, with order both along growth direction and in the growth plane; concerning strain-stress relationships they can be described as single crystals, even if grain boundaries certainly play an important role [6].

The case of fibre textured (FT) thin films in more complex. Most PVD thin films for metallurgical applications (e.g., hard nitride coatings, TiN, CrN etc.) belong to this category. Fibre texture can be more or less sharp, and fibre components can be one or more; in addition this type of PVD thin films are known to develop intense residual stresses (frequently the order of 5-10 GPa or more, mostly of intrinsic nature [35,36]), which area key-factor in determining adhesion and stability.

In this case stress-texture interaction is very strong and cannot be overlooked. In addition to the effect of texture on the stress-strain mechanical model (see the discussion above on stress factors), texture measurements by conventional pole figures can be difficult to carry out; peak positions (fixed during pole figure measurements) change dramatically with ψ-tilting, due to the strong residual stresses.

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Texture and stress must be considered simultaneously.

Recently we proposed a data collection strategy to study FT thin films, based on the measurement of several θ-2θ patterns at increasing ψ-tilting values [36,18]. Due to the axial symmetry, these θ−2θ/ψ maps can be collected for any arbitrary value of azimuth angle (φ). Figure 14 shows

Fig. 12. SR XRD data collected at Daresbury, UK (station 2.3) in θ−2θ configuration (λ=1.5406Å) on Yttria-Scandia-Zirconia (a) and YttriaZirconia (b) TBCs heat treated for 100h at 1400°C plus 24h at 1480°C. Detail of the pattern of (b), collected with λ=2.2 Å (c) [31]

a map for the (200) reflection of a TiN thin film on austenitic steel; besides nitride, signal from the substrate is also visible, consisting of the (111) reflection of austenite and a weaker reflection from martensite (110) [10,18,36]. Schematically, intensity distribution as a function of ψ gives an indication on texture, whereas peak shift (in 2θ) is related to strain; further information on lattice defects and domain size can be obtained from profile width and shape. From these data it is possible, by suitable least squares fitting and assuming an appropriate model of mechanical grain-coupling, to have a valuable description of growth mechanisms and growth stresses, that frequently involve gradients [37,18].

SR XRD can be very important, especially in connection

with the last point, i.e., the presence of gradients: present day investigations [37,18,19], in fact, usually consider thin films as homogeneous. This may be true of composition (although compositional gradients are also observed, for instance in TiN coatings, but they can be taken into account, at least in principle), but is not easily verified a priori for texture.

FT thin film microstructure, including grain shape and orientation, usually changes in the initial growth stages: interfacial layers of different orientation are frequently observed [6], and texture may change dramatically with thickness [18]. It is therefore important to collect data by using different wavelengths, as shown in Figure 15 for a CoNiCrAlY superalloy thin film, in order to access different depths in the thin film. This type of investigations could hardly be done on a lab XRD system, and SR XRD is again a unique tool in the hands of materials scientists and engineers. Promising developments are expected in the future, to develop 3D texture descriptions.

LPA studies A final word concerns the use of SR XRD in LPA studies.

X-ray Powder Diffraction (XRPD) is probably one of the first and most successful applications of SR to structural studies. Station 2.3, used for many of the measurements described in this work, was specifically designed for powder diffraction [38], and most LSF around the world have at least one XRPD station or more. It is not the purpose of this work to review SR applications to XRPD, but it should be underlined that besides structural studies, materials science can greatly benefit of SR XRPD. So far we have described an application of XRPD to a delicate problem of quantitative phase analysis, which is certainly a valuable application of SR. LPA can be greatly improved by using SR XRPD, because of the monochromatic beam conditions that can be obtained together with narrow instrumental profiles and high brilliancy. These features can be extremely important when studying problems of anisotropic line broadening (dependence of profile width and shape on hkl [39,40]) like in the case shown in Figure 16 [40]. A new approach to model the whole XRPD pattern on the basis of line and plane lattice defects and domain size has been recently proposed, and could be tested in a complex case involving 33 profiles of a spinel phase, including also 9 peaks of a position standard (Si) [40]. The unique features of SR XRPD where fully exploited to reach the required data quality to effectively test the procedure. In the detail of Figure 17a, we can appreciate the effect of line broadening anisotropy, that in the spinel case was mostly due to the anisotropic effects of dislocations.

Such an effect can be described by the average contrast factor [40.41], whose trend as a function of the

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orientational parameter (H = (h2k2+h2l2+k2l2)/(h2+k2+l2)2 ) is also shown in Figure 17b. More details on this interesting and promising application can be found in references 39,40.

0.4 Å

Acknowledgements SR XRD and ND applications reviewed in this paper were the result of the work of several people, among which I wish to thank S. Setti, M. Leoni, S. Veneri, M. D’Incau (Univ. of Trento), R. Polini (Univ. Roma II), G. Cappuccio (CNR-INFN, Frascati (Rome)), L. Bertini (Univ. Pisa), J. Wright (ISIS, Didcot UK), C. Tang, R. Cernik and A. Neild (Daresbury SRS, UK).

0.8 Å

Polycrystalline thin films

Random

Fibre-textured

‘Single-crystal’

Fig. 13. (h00) pole figures of polycrystalline thin films with different texture. Fibre-axis is [hhh] in the FT thin film. Growth normal is [hhh] for the single-crystal like thin film.

Fig. 15. θ−2θ/ψ maps collected on a Co-Ni-base superalloy thin film by SR XRD at different wavelengths. Pictures refer to the high ψ-tilting range (44-80°) for the (111) (left, lower 2θ) and (200) (right, higher 2θ) fcc reflections.

Ψ References

70

M

50 30 10 -10 -30 -50 -70 40.5

42.5

TiN

44.5 2θ

46.5

A

Fig. 14. θ−2θ/ψ map for the (200) reflection of a fibre-textured TiN thin film on AISI 304 steel. Features attributed to the coating and substrate phases are indicated (A: austenite, M: martensite).

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1. See e.g.: Synchrotron Radiation Department, Scientific Report 1995-96 (Daresbury Laboratory, UK), 1996; ibidem, Scientific Report 1996-97, 1997-98, 1998-99; The Rutherford Appleton Laboratory, ISIS facility annual report, 1996-97 (Didcot, UK); ibidem, ISIS facility annual report, 1997-98, 1998-99.

2. P.J. Webster, G.B.M. Vaughan, G. Mills and W.P. Wang, Mat. Sci. Forum, 278-281 (1998) 323; Å. Kvick (ed), Local characterisation of materials, Internal ESRF Publ. N°ESRF97KV10T, (ESRF, Grenoble), 1997.

3. P.J. Webster and X. Wang, Surf. Eng., 10 (1994) 287; P.J. Webster, G. Mills, X.D. Wang, W.P. Wang and T.M. Holden, J. Neutron Res., 3 (1996) 223.

4. J. Wilks, E. Wilks, Properties and applications of diamond, (Butterworth, Oxford), 1991. J.R. Davis (ed.), Tool Materials, (ASM International, Materials Park, USA), 1995.

5. I.C. Noyan, C.C. Goldsmith, Adv. X-ray Anal., 34 (1991) 587.

6. P. Scardi, in Science and Technology of Thin Films, edited by F.C. Matacotta and G. Ottaviani, (World Scientific Publisher Co., Singapore), 1995. p. 241.

7. Ch. Wild, N. Herres, P. Koidl, J. Appl. Phys, 68 (1990) 973.

8. R. Polini, G. Marcheselli, and E. Traversa, J. Am. Ceram. Soc., 77 (1994) 2043; R. Polini, G. Marcheselli, G. Mattei and E. Traversa, J. Am. Ceram. Soc., 78 (1995) 2431.

9. G. Straffelini, P. Scardi, A. Molinari, J. Mater. Res. (2000). Submitted; C. Carlando, Comportamento tribologico di rivestimenti di diamante policristallino su WC-Co, Thesis, Università di Trento, 1997. (In Italian).

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10. M. Leoni, Residual Stress Gradients in Polycrystalline Coatings, PhD Thesis. Università di Roma ‘Tor Vergata’, 1999.

11. P. Scardi, M. Leoni, G. Cappuccio, V. Sessa, M.L. Terranova, Diamond and Related Mat., 6 (1997) 807.

12. I.C. Noyan and J.B. Cohen, Residual Stresses, (Springer-Verlag, New York), 1987.

13. S. Veneri, Influenza dei parametri di deposizione sulle caratteristiche microstrutturali di rivestimenti di diamante policristallino, Università di Trento, 1998. (In Italian).

14. P. Scardi, S. Veneri, M. Leoni, R. Polini and E. Traversa, Thin Solid Fims, 290-291 (1996) 136.

15. J.K. Wright, R.L. Williamson, K.J. Maggs, Mat. Sci. Eng., A187 (1994) 87.

16. P. Scardi, M. Leoni, S. Veneri, Advances in X-Ray Analysis, 40 (1997) (on CD-ROM).

17. V. Hauk, Structural and Residual Stress Analysis by Nondestructive Methods, (Elsevier Amsterdam), 1997.

18. P. Scardi, Y.H. Dong, Mat. Sci. Forum (2000). In press. P. Scardi, Y.H. Dong, J. Mater. Res. (2000). Submitted.

19. J,-D Kamminga, Origin and measurement of stresses in thin layers, Delft University of Technology, 1999.

20. R.A. Miller, Surf. Coat. Technol., 30 (1987) 1; K. Ebert, C. Verpoort, Ch. Karsten, Mat. Sci. Forum, 163-165 (1994) 587; T.E. Strangman, Thin Solid Films, 127 (1985) 93; S.M. Meier, D.M. Nissley and K.D. Sheffler, NASA Contractor Report 18911, (NASA Lewis Research Center, Cleveland, OH), 1991.

21. S.M. Meier, D.K. Gupta and K.D. Sheffler, J. Of Minerals (JOM), 43 (1991) 50.

22. I. Kvernes, J.K. Solberg and K.P. Lillerud, in Proc. 2nd Conf.

Advanced Materials for Alternative Fuel Capable Directly Fired Heat Engines, ed. J. W. Fairbanks and J. Stringer, (RD-2369-SR, EPRI, Palo Alto, CA), 1982. p. 6-8; Ibidem, p. 6-117.

23. W.P. Danesi and M. Semchyshen, in The Superalloys, ed. C.T. Sims and W.C. Hagel, (J. Wiley and Sons, New York), 1972, p. 565.

24. Kuroda and T.W. Clyne, Thin Solid Films, 200 (1995) 49; S.C. Gill and T.W. Clyne, Metall. Trans., B21 (1990) 377.

25. BRITE/EURAM Project BE-4212-90, Modelling and Characterisation of the manufacturing process of ceramic thermal barrier coatings, Final Report, Commission of the European Communities, 1995; BRITE/EURAM Project BE-4272-90, Finite Elements Modelling of Ceramic TBCs to Extend the Operating Range of Heat Engine Components, Final Report, Commission of the European Communities, 1995.

26. P. Scardi, E. Galvanetto, A. Tomasi, L. Bertamini, Surf. & Coat. Techn., 68/69 (1994) 106; P. Scardi, M. Leoni, L. Bertamini, Surf. & Coat.

Techn., 76-77 (1995) 106.

27. P. Scardi, M. Leoni, L. Bertamini, L. Bertini, Surf. and Coat. Technol., 94-95 (1997) 82 28. P. Scardi, M. Leoni, L. Bertamini, M. Marchese, Surf. and Coat.

Technol., 86/87 (1996) 109

Fig. 16. Result of Rietveld refinement of a Li,Mn spinel sample with line broadening anisotropy (SR XRD data, λ=1.2 Å) [40].

29. P. Scardi, A. Gualtieri, M. Bellotto, ”Industrial Applications of Powder Diffraction”, CPD Newsletter, 19 (1997) 1 (http://www.iucr.org/iucr-top/comm/cpd/).

30. P. Scardi, M. Leoni, F. Cernuschi and L. Bertini, J. Am. Ceram. Soc., (2000). Submitted.

31. M. Leoni, R.L. Jones and P. Scardi, Surf. Coat. Technol., 108-109 (1998) 107.

32. R.A. Young (ed.), The Rietveld Method, (Oxford Univ. Press, Oxford), 1993.

33. M. Leoni, P. Scardi, Mat. Sci. Forum, 278-281 (1998) 177; Y.H. Dong & P. Scardi, J. Appl. Cryst., 33 (2000). In press.

34. R.L. Jones, Experiences in Seeking Stabilizers for Zirconia Having Hot Corrosion-Resistance and High Temperature Tetragonal (t’) stability, NRL/MR/6170—96—7841 (Naval Research Laboratory, Washington, DC), 1996.

35. L. Chollet, A.J. Perry, Thin Solid Films, 123 (1985) 223; R.Y. Fillit, A.J. Perry, Surf, Coat. Technol., 36 (1988) 647.

36. P. Scardi, M. Leoni, Y.H. Dong, Adv. X-ray Anal., 42 (1999) (On CDROM).

37. M. Leoni, U. Welzel, P. Scardi, (2000). In preparation.

38. W. Parrish, M. Hart, Adv. X-ray Anal., 32 (1989) 481; R.J. Cernik, P.K. Murray, P. Pattison and A.N. Fitch, J. Appl. Cryst., 23 (1990) 292.

39. P. Scardi, in X-ray Powder Diffraction Analysis of Real Structure of Matter, eds. H,-J Bunge, J. Fiala, R. L. Snyder (IUCr series, Oxford Univ. Press), 1999. p. 570.

40. P. Scardi and M. Leoni, J. Appl. Cryst., 32 (1999) 671.

41. M. Wilkens, Phys. Stat. Sol., (a) 2 (1970) 359.

Fig. 17. Average contrast factor of dislocations as a function of the orientational parameter, H (a); detail of the pattern of Figure 16, with Miller indices of present reflections of spinel phase and Si internal standard [40].

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ESS R&D Activities on Neutron Instrumentation Introduction Accelerator based neutron scattering sources are being recognised worldwide as the most feasible route for the next generation high-flux neutron sources.1 The premier spallation sources exhibit neutron fluxes and brilliances which are of the order from ten to several hundred times greater than existing steady state neutron sources. As a result, several pulsed spallation sources are currently under development around the world (i.e., the second target station at ISIS, AUSTRON, Japanese Hadron Project and the European Spallation Source (ESS)). In the US, construction of the new 2 MW Spallation Neutron Source (SNS) commenced last year at Oak Ridge National Laboratory (ORNL) and is scheduled for completion in 2005.

With an average proton beam power of 5 MW and a 50 Hz repetition rate, ESS was designed as the most powerful neutron source. With the proposed ESS design and appropriate instrumentation experiments were expected to gain up to three orders of magnitude in data collection rates. A suite of 44 instruments and two target stations has been discussed.3 With this in mind along with the vision outlined in the ESS feasibility study an ESS R&D Council was established in 1997 and an ESS R&D project phase was initiated. At the R&D phase, a unique possibility is offered to connect the design of specific neutron instruments directly with the design of the target station, moderators and delivered proton beam. Thus, even at the earliest stages of the future neutron source scientific interest and demand in neutron instrument usage can be combined with the technical layout and construction of the future facility.

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Potential instrument set-ups can be optimised by simulation techniques and the best possible moderator and neutron beam pulse structure can be defined which will lead to an iterative optimisation process between the accelerator design and the proposed spectrometers.

At several European laboratories a broad range of ESS R&D efforts are organised currently to achieve the expected aim of a new and powerful neutron source. The efforts with regards to instrumentation are concentrated on a detailed concept for the critical re-evaluation and optimisation of all aspects of a particular instruments. Main topics, at present, are instrument simulation packages, detector development, new instrument concepts and prototyping.

Presently, these efforts involve nine European laboratories and institutions, all of which are members of

Fig. 1. Three simulated spectra from an ensemble of monodispers hard spheres for SANS instruments housed at a continuous source (ILL) and on the planned second target station of ESS operating either in the LPSS or SPSS mode shown.

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the ESS R&D Council. Several of these activities are actively supported by projects within the EU-RTD. Results from these efforts were presented recently at the 6th ESS General Meeting [Sept. 20.-22. 1999 in Portonovo ANC (Italy)]4.

Instrument Simulation The development of sophisticated and appropriate simulation programs is an important effort. New simulation packages for the refinement of neutron instruments have been developed at Risø National Laboratory (Denmark) and the Hahn-Meitner-Institute (HMI, Berlin, Germany). Additional work is being carried-out at the Ciemat research centre in Spain. In part, these activities are supported by the EU project SCANS. Based on Monte Carlo simulation procedures the Risø program McStas currently supports simulation of both triple-axis and time-of-flight instruments, and has preliminary support for polarised neutrons (neutron.risoe.dk/ mcstas/).

The virtual instrumentation tool VITESS, developed at HMI (dsora.hmi.de: 8888/projects/ess/ vitess/), simulates the performance of instruments at continuous and pulsed neutron sources. Recently the performance of a high resolution Time-Of-Flight (TOF) powder diffractometer on a long pulsed spallation source has been simulated in connection with experiments at Budapest Neutron Center (BNC, Hungary) (see below). Calculations on a crystal analyser spectrometer, a reflectometer and several SANS instruments are available, some results are shown in Fig. 1. An important next step in the use of these simulation efforts will be the incorporation of the moderator design which will be used at the ESS target station.

Detector Developments The pulsed high-flux neutron beams at the ESS require novel imaging de-

VARIE

tector systems with improved resolution and faster response times, necessary for exploiting the improved beam intensity, flux density and time structure. At HMI a new neutron detection system is under construction based on developments of highly efficient large-area multilayer micro-strip gas chamber (MSGC) detectors optimised for lowpressure, two-stage amplification (www.fz-juelich.de/ess/CUR/Detectors. html). Such detectors will achieve a count rate capacity which is intrinsically >106 mm-2s-1, however, with economical readout modes limited to >106 cps local rate and 107 cps per detector segment.

At Interfaculty Reactor Institute (IRI, Delft, Netherlands) new gas electron multiplier systems for similar count rates and resolution are being investigated and improved (wwwiso.iri. tudelft.nl). Further studies are under way on solid state detectors and large crystal monochromators at the University of Perugia in Italy, and on deposition techniques of Gd for neutron converters and scintillators at Ciemat. Fundamental support on most of these R&D efforts is obtained by the EU project TECHNI, which is dedicated to the development of new neutron detectors.

New Instrument Concepts and Prototyping A major area of R&D activities is the development of new instrument concepts and prototyping. The research centres involved cover a wide range of applications and instrumentation. In collaboration between HMI and the Central Research Institute for Physics (KFKI, Budapest, Hungary) a chopper test facility has been built at BNC to explore new instrument configurations and to benchmark simulation calculations.

Recent calculations for a high resolution TOF powder diffractometer on a long pulse spallation source were verified (see above). An example of high resolution powder data mea-

sured is shown in Fig. 2. The chopper system developed can be used as a dedicated TOF monochromator in future spallation source based instruments.

A project at HMI is dedicated to employing the Laue technique at an advanced spallation source. This technique will have a major impact on neutron crystallography expanding the scope of accessible experiments to atomic resolutions of biological macromolecules (www.kfa-juelich. de/ess/CUR/Single_ Cryst.html).

Methodological improvements in single crystal TOF diffraction at pulsed spallation neutron sources are required. This aspect will be tackled by developing an analytical correction for the effect of thermal diffuse scattering around Bragg reflection measurements.

In collaboration with FZ JĂźlich and HMI the feasibility for neutron spinecho spectroscopy at a pulsed spallation source is being examined ( w w w. k f a - j u e l i c h . d e / e s s / CUR/NSE. html). A pulsed TOF option consisting of a set of choppers is being developed in hopes that it will replace the standard velocity selector presently being used to produce a broadband pulsed neutron

Fig. 2. High resolution (?d/d ca. 2*10^-3) powder data measured with the TOF powder diffractometer at the Budapest Neutron Centre.

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â&#x20AC;˘

beam. Recently, implemented and successfully tested at the IN15 spectrometer at ILL (Grenoble, France), it serves as a test facility to perform spin-echo spectroscopy under conditions of a pulsed neutron source. The results will be used to design a new spin-echo spectrometer at a pulsed spallation source for which funding has been applied for.

The idea to combine TOF and Larmor precession for measuring the initial as well as the final neutron wavelength in a 2D experiment is being studied at IRI. This concept may be used for the development of a new flexible range quasi-/inelastic spectrometer or a resolution enhanced TOF powder diffractometer.

For the latter, repeat spacings beyond the limit of the initial neutron pulse width maybe resolved using a Larmor frequency of up to 104 precessions and enhanced collimation.

Labelling the wavelength as well as the scattered angle by Larmor precession opens the possibility to develop a spin-echo small angle neutron scattering instrument (www. iri.tudelft.nl/~sfwww/ sesans/), which would enable measurements with high intensity in the correlation range of 5-1000 nm.

A novel instrument for spectroscopic studies in condensed matter with eV neutrons is being developed by the University of Rome and INFM (Italy) in collaboration with the University of Liverpool (England) and CLRC-RAL (England). The VESUVIO project aims to provide prototype instrumentation at the ISIS pulsed neutron source in order to establish a routine experimental and theoretical program in neutron scattering spectroscopy at eV energies (www.roma2. infn.it/infm/vesuvio/). A workshop to outline the new developments under the VESUVIO project was held recently [Nov. 26.-27. 1999 in Abingdon (England)].

R&D efforts for prototyping inelastic spectrometers at pulsed spallation sources are done within the TOSCA

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project by CNR (Italy) and ISIS. Their aim is to improve crystal analyzer inverse geometry inelastic spectrometers for vibrational spectroscopy (laser.ieq.fi.cnr.it/projects/ tosca/main.htm). Test experiments carried-out in order to optimize the geometry and the performances of the TOSCA spectrometer will open the way to new instrument geometries relevant to ESS. Additional R&D Efforts Additional R&D efforts with respect to ESS instrumentation are established at the Atominstitut Vienna (Austria), with research related to new optical components for neutron beam focussing and the development of moderators done at FZ Jülich, Paul Scherrer Institute (PSI, Switzerland) and Risø National Laboratory. 3He neutron spin filter cells active over a broad range of neutron wavelengths, to be used particularly

in white beams of pulsed spallation sources, are another R&D effort at HMI (www.kfa-juelich.de/ess/ CUR/Spin_ Filter.html). Prototype filter cells are already being used and a special coil/shielding device capable of keeping the filter cells in a homogeneous magnetic guide field when under transport or during the course of the experiment, have been tested. These efforts are supported also by the EU project ENPI. Further R&D efforts are continuously occurring through research in all laboratories and institutions engaged in neutron research throughout Europe. Workshops and meetings are organised on specific R&D instrumentation topics and progress reports are presented. Thus, supported by the TMR neutron round table a workshop on "Protein Crystallography with Neutrons" was held [Feb. 25.-26. 2000 in Berlin (Germany)] and a workshop on neutron

ATTIVITA’ DEL COMITATO CNR DI COORDINAMENTO LUCE DI SINCROTRONE Il Comitato, nel suo primo anno di attività, ha fatto un attento esame delle azioni fin qui intraprese dal CNR nel settore, in particolare le realizzazioni strumentali presso i Sincrotroni ESRF di Grenoble ed ELETTRA di Trieste. Con una panoramica sintetica delle possibilità di utilizzo della strumentazione sinora realizzata e di quella futura che si propone di realizzare, ha voluto dare indicazioni per sviluppare e gestire questa attività strategica interdisciplinare per il CNR. Su questa base ha preparato un piano triennale 2001-2003 sulla attività con luce di Sincrotrone, da inserire nel piano triennale dell’Ente.

Alla fine del 1999 è scaduto il contratto stipulato da CNR, INFM e INFN, per la parte italiana, con l’European Synchrotron Radiation Faci-

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lity (ESRF) per la gestione della linea italiana GILDA a Grenoble.

Il Comitato ha esaminato la proposta di proroga, con piccoli emendamenti, del contratto dal 1/1/2000 al 31/12/2004 presentata da ESRF, esprimendo parere positivo ed avviando così l’iter formale che ha portato alla firma del medesimo. Come avvenuto per il primo contratto, i tre Enti italiani hanno concordato tra loro una “Dichiarazione congiunta”, in cui si sono impegnati a garantire il funzionamento di GILDA, sostenendone gli oneri finanziari.

Il 31 dicembre 1999 sono scaduti gli accordi biennali stipulati tra il CNR e la Società Sincrotrone Trieste ( ST ) relativi alle prime due linee sperimentali realizzate in compartecipazione tra i due Organismi: • Fotoemissione ad Alta Risoluzione

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spin-echo spectroscopy and on moderator concepts is being announced.

Information on these meetings and additional news on the on-going R&D efforts regarding instrumentation and the ESS project as a whole are available through the web at www.kfa-juelich.de/ess/ess.html.

References 1. D. Richter, T. Springer; A twenty years forward look at neutron scattering facilities in the OECD countries and Russia, Technical Report, ESF/OECD, Jülich, 1998 2. ESS – A Next Generation Neutron Source for Europe, Volume I-III, 1997 3. J. Carlile; A Reference Instrument Suite for the European Spallation Source and the Reports from the Instrument Working Groups, Jülich, 1998 4. F. Carsughi; Notiziario, 4(2), 38-39, 1999

T. Gutberlet, F. Mezei, M. Steiner Hahn-Meitner-Institut Berlin, Germany

Energetica (VUV), progettata e costruita in compartecipazione tra l’Istituto di Struttura della Materia dell’Area di Ricerca di Tor Vergata (CNR) e la ST. La linea è stata finanziata al 50% dal CNR e dalla ST e la gestione è interamente a carico del CNR. • Cristallografia Diffrattometrica da Cristallo Singolo (XRD1), progettata e costruita in compartecipazione tra il Dipartimento di Chimica dell’Università “La Sapienza” di Roma, l’Istituto di Strutturistica Chimica di Montelibretti (CNR) e la ST. La linea è stata aperta all’utenza nel febbraio ’96. La costruzione è stata finanziata al 50% dal CNR e dalla ST e la gestione è ripartita in ugual misura tra i due Enti.

Sono stati pertanto predisposti, d’intesa con la ST, due schemi di accordo, che il Comitato ha esaminato attentamente, già approvati dal Consiglio Direttivo e prossimi alla firma.

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Attività della Commissione per la Spettroscopia Neutronica del CNR La Commissione ha proseguito la sua attività con regolarità, dedicandosi prevalentemente al coordinamento con gli Organi di gestione del CNR. Come noto, è in corso una riorganizzazione di grandi proporzioni del Consiglio Nazionale delle Ricerche, basata su due punti sostanziali: decentramento delle attività scientifiche sugli Organi di ricerca e snellimento degli Organi di controllo e gestione, con la creazione di due Consigli formati da esperti in vari settori, che affiancano la Presidenza dell’Ente. La ristrutturazione è tuttora in corso e solo recentemente si è avuta l’approvazione del nuovo regolamento. La Commissione ha cercato, quindi, di far sì che le attività di spettroscopia neutronica in corso potessero trovare spazio nella nuova struttura del CNR. Il risultato di maggior rilievo da menzionare è il completamento della seconda fase del progetto TOSCA, che vede l’installazione della seconda fase dello spettrometro ormai prossima. Più importante ancora, se possibile, è la disponibilità di un sito per la futura stazione Italiana.

Detta stazione, fortemente voluta dalla nostra comunità, permetterà la gestione di uno strumento adatto all’addestramento del personale ed alla preparazione delle esperienze, oltre che di ricerche scientifiche. È stata già fatta richiesta di un adeguato finanziamento per detta stazione. Tale richiesta è stata fatta direttamente da Marco Zoppi dell’Istituto di Elettronica Quantistica

di Firenze, cioè direttamente da un Organo di ricerca fortemente impegnato nel campo della Spettroscopia Neutronica. In ogni caso un piccolo diffrattometro sarà installato sul sito quanto prima, facendo uso di componenti già disponibili e dismessi da altri progetti. Appena disponibili i finanziamenti si procederà al disegno e sviluppo di uno strumento multi-uso più sofisticato. Una discussione sull’impiego della stazione Italiana sarà stimolata durante il prossimo congresso della SISN. È utile ricordare infine che la Commissione ha provveduto a presentare al CNR un piano triennale (2001-2003) per la Spettroscopia Neutronica. È questo il primo piano di questo tipo, in quanto il CNR si dota per la prima volta di un suo piano triennale. Il prossimo triennio è molto importante poiché nella primavera del 2002 scade l’attuale accordo con ISIS. Un suo rinnovo necessiterà di un considerevole impegno da parte della Comunità per far sì che il CNR continui in questa collaborazione. Nel piano triennale si è anche presa in considerazione la possibilità di partecipare in modo adeguato allo sviluppo del progetto della nuova sorgente Europea ESS. Tale progetto, sebbene sia ancora in una fase di valutazione tecnica e scientifica, sarà sicuramente di grande importanza nel prossimo decennio e la Comunità Neutronica Italiana dovrà confrontarsi con esso con grande attenzione.

Prof. Sacchetti

Situazione delle ricerche sostenute dal CNR e proposta per un piano triennale. Attività di spettroscopia Neutroni Piano triennale proposto dalla Commissione Neutroni del CNR Premessa Le ricerche con le tecniche di spettroscopia neutronica effettuate dai ricercatori italiani presso la sorgente pulsata di neutroni ISIS operante al Rutherford-Appleton Laboratory, (Oxford, U.K.) sono state sostenute dal CNR nel decennio 1985-1995, grazie ad un accordo stipulato tra l’Ente e il SERC (Science and Engineering Research Council). Vista la notevole ricaduta scientifica, tale accordo è stato successivamente rinnovato, nel marzo 1996, (con il CLRC - Council for the Central Laboratory of the Research Councils, Ente che ha preso in carico le attivita’ del SERC) fino a marzo dell’anno 2002.Attraverso questo accordo internazionale l’Ente:

• garantisce l’accesso alla strumentazione di ISIS a tutta la comunità italiana, con una percentuale di utilizzo pari al 5% del tempo totale disponibile. La Tabella 1 riporta la percentuale di utilizzo di tempo assegnata per l’attività di ricerca dei gruppi italiani nel corso degli anni.

• finanzia direttamente lo sviluppo di strumentazione, progettata e costruita presso i propri organi di ricerca, anche in collaborazioni con gruppi universitari. In particolare si ricordano il Progetto PRISMA realizzato presso l’ISM (Istituto di Struttura della Materia, Frascati) nel periodo dal 1984 al 1991 ed il Progetto TOSCA, realizzato presso l’IEQ (Istituto di Elettronica

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Quantistica, Firenze), avviato nel 1996 e la cui conclusione è prevista contestualmente alla conclusione dell'accordo in atto.

Questi accordi internazionali, insieme ad analoghi accordi stipulati più recentemente dall’INFM, hanno avuto un ruolo decisivo nello sviluppo della comunità scientifica italiana che impiega le tecniche di diffusione dei neutroni.

Tale comunità è cresciuta da un numero molto ridotto di ricercatori nella prima metà degli anni ’80, fino a circa 200 ricercatori oggi operanti presso le Università e gli Enti di Ricerca (CNR ed INFM). Detta comunità, sebbene sia ancora relativamente ridotta rispetto alle comunità analoghe che sono presenti in tutti i paesi avanzati, assume oggi una dimensione più appropriata in confronto con le principali nazioni dell’Unione Europea. In particolare, i vari gruppi di ricerca conducono attività sperimentali, sia presso i principali reattori che presso sorgenti pulsate di neutroni, su un ampio spettro di tematiche scientifiche e sono anche attivamente impegnati nello sviluppo e realizzazione di nuova strumentazione per l’impiego dei neutroni.

Il CNR inoltre ha aderito nel 1998, assieme all’INFM, al ESS R&D Council, un consorzio di Enti e Istituzioni Europee, che ha l’obiettivo di predisporre il progetto per la costruzione di una nuova sorgente pulsata di neutroni, European Spallation Source (ESS), da sottoporre ai Governi dell'Unione Europea.

Detto progetto è molto importante nel panorama mondiale di queste attività e rappresenta il mezzo che può consentire all’Europa di mantenere la sua posizione di rilievo nei confronti di Stati Uniti e Giappone. È opportuno osservare che, mentre l’INFM è un Istituto che sostiene ricerche tematiche di Fisica della Materia, il CNR ha l’importante ruolo di sostenere anche ricerche multidisciplinari. Le attività di ricerca e di sviluppo di strumentazione ad ISIS ed in ambito ESS, per quello che concerne il CNR, sono coordinate da una apposita Commissione per la Spettroscopia Neutronica.

Nel decennio 1985-1995 l’impegno diretto del CNR, per tramite dei suoi organi, nelle attività di ricerca e sviluppo nel campo della strumentazione per spettroscopia neutronica è quantificabile, in media, in 4 ricercatori dedicati per tutto il periodo, con un investimento per la strumentazione pari a 3000 ML (Progetto PRISMA e sviluppo di cristalli monocromatori). In aggiunta, nello stesso periodo, l’accesso alla sorgente ISIS ha comportato un investimento oneroso pari a 20.000 ML. Nel periodo 1996-2001 l’impegno complessivo del CNR è quantificabile in media in 4.5 ricercatori, con un investimento per strumentazione pari a 3200 ML (Progetto TOSCA e sviluppo di cristalli monocromatori) e un investimento oneroso per l’accesso alla sorgente ISIS pari a 12300 ML.

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Attività di ricerca e sviluppo di strumentazione per l’anno 2000 Per l’anno corrente è già previsto: a) l’impegno per l’accordo in atto con il CLRC Inglese per l’accesso ad ISIS. b) il completamento del progetto TOSCA, per il quale è opportuno prevedere un assegno di ricerca per un giovane ricercatore da affiancare, in una fase successiva, al ricercatore attualmente distaccato ad ISIS (assunto ex. Art. 36) già in servizio. c) l’avvio effettivo delle attività previste dall’accordo ESS il quale prevede una quota onerosa relativamente modesta e l’impegno di due unità di personale. Per questi ricercatori vanno previste le spese di missione necessarie per operare nell’ambito di questa collaborazione.

Previsione di spesa per l’esercizio 2000 Denominazione Tipologia dell’attività

ISIS

Acc. Internaz.Attivo

TOSCA

Costruzione shutter

Personale Investimento (anni/uomo) (ML) 2050*

intermedio TOSCA/ stazione italiana ESS ESS

1 Ass. di ricerca

Acc. Internaz.Attivo

400# 60@

Missioni di 2 un. di personale CNR

Totale

30 2540

* Quota onerosa prevista dall’accordo (Bilancio Uff. Relazioni Internazionali) # Richiesta imputabile a IEQ-CNR (Firenze) @ Quota prevista dall’accordo in via di stipula (Bilancio Uff. Relazioni Internazionali)

Attività di ricerca e sviluppo per il periodo di validità del piano triennale 2000-2001 Nel corso del triennio 2001-2003 giungerà a scadenza l’accordo CNR-CLRC per l’accesso italiano ad ISIS Questa possibilità deve essere mantenuta per il futuro almeno al livello attuale. Occorre quindi prevedere nell’ambito del piano triennale il rinnovo dell’accordo con ISIS, essenzialmente alle stesse condizioni di quello attualmente in atto. Dato che nel decennio 1988-1998 la percentuale di utilizzo del tempo macchina ad ISIS si è mantenuta al di sopra del livello del 5% (il valore medio nel decennio risulta pari al 6%, vedi Tabella 1) appare ragionevole prevedere un rinnovo dall’accordo con il CLRC almeno per le stesse percentuali di utilizzo dell’accordo precedente, pari cioè al 5%. In analogia con l’accordo attualmente in essere si può prevedere una quota onerosa pari al 4% (stimabile in 2250 ML/anno), mentre l’ulteriore quota dell’1% potrebbe essere coperta

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dalla realizzazione di un nuovo strumento italiano da installare ad ISIS. Questo nuovo progetto dovrà essere iniziato contemporaneamente all’avvio del nuovo accordo, cioè all’inizio del 2002. Il suo costo (prevedibilmente dell’ordine di 3500 ML) dovrebbe essere ripartito su un arco di 3-4 anni, investendo quindi competenze relative al piano triennale 2004-2007.

Progetto TOSCA Con l’installazione del secondo banco di analizzatori previsto per la primavera del 2000 si concluderà la fase di costruzione dello spettrometro e, a partire dall’esercizio 2001, è necessario prevedere quindi solo le spese di manutenzione ordinaria e di personale (un ricercatore a tempo determinato - attualmente ex. Art. 36 - ed un Assegno di ricerca che dovrebbe essere distaccato ad ISIS e potrebbe investire la stessa persona che coprirà l'assegno di ricerca in Italia nel corso dell'anno 2000).

Il progetto TOSCA, secondo gli accordi siglati a suo tempo con il CLRC, prevede la realizzazione, a valle della stazione sperimentale principale di TOSCA, una stazione di test e sviluppo, sulla quale i ricercatori italiani disporranno del 50% del tempo. La realizzazione di una tale facility è di estrema importanza per la comunità nazionale. Va infatti ricordato che l’Italia non dispone al momento di alcuna sorgente neutronica nazionale, anche di bassa intensità. La disponibilità di una stazione con accesso facilitato è di grande importanza per lo sviluppo di esperimenti preliminari, per test di nuova strumentazione, e per attività di formazione di giovani ricercatori.

La stazione sperimentale a valle dello di TOSCA potrebbe essere efficacemente impiegata anche nelle attività di Ricerca e Sviluppo previste dall’accordo ESS. Detta attività non potrebbe essere condotta sulle linee “pubbliche” di ISIS, le quali sono completamente dedicate alla ricerca scientifica. Per la realizzazione di tale stazione è necessario un investimento di 600 ML, distribuito sui due esercizi 2001 e 2002, oltre ad un’unità di personale ricercatore. Vengono riportate di seguito le tabelle che descrivono la previsione di spesa sulla base delle proposte presentate in precedenza. Dette previsioni non considerano il personale ricercatore in servizio con contratto a tempo indeterminato o determinato. Sulla base di quanto descritto in precedenza, oltre al personale con contratto a tempo indeterminato, in servizio presso organi CNR, si prevede la stipula o il mantenimento di due contratti per ricercatore a tempo determinato presso ISIS. Considerazioni sul personale Oltre agli assegni di ricerca, i cui oneri sono stati specificati nelle tabelle precedenti, si considera necessario che vengano mantenute ad ISIS due unità di personale ricercatore, con contratto a tempo determinato, con il compito di gestire la strumentazione (TOSCA + il

nuovo strumento) e di fornire supporto agli utenti italiani. Non si ritiene di poter quantificare la spesa in oggetto, anche se questa è dell’ordine di 160 ML per anno per le due unità.

Previsione di spesa per l’esercizio 2001 Denominazione

Tipologia dell’attività

Personale Investimento (anni/uomo) (ML) Acc. Internaz. Attivo 2050* 1 Ass. di Ricerca (Estero) 40 Manutenz. TOSCA 300 Staz. Italiana ad ISIS 1 Ass. di Ricerca (Estero) 400+40 Acc. Internaz.Attivo 130@ Missioni 2 Unità personale CNR 60 3020

ISIS TOSCA

ESS

Totale

Previsione di spesa per l’anno 2002 Denominazione

Tipologia dell’attività

ISIS

Personale Investimento (anni/uomo) (ML)

Acc. Internaz.

Rinnovo 03/2000

TOSCA Staz. Italiana ad ISIS

2250* 1 Ass. di Ricerca (Estero) 1 Ass. di Ricerca (Estero)

40 200+40

Nuovo Strumento ad ISIS ESS Acc. Internaz.Attivo

800# 150@ Missioni 2 Unità personale CNR

Totale

60 3540

* Quota onerosa prevista dall’accordo (Bilancio Uff. Relazioni Internazionali). @ Quota onerosa prevista dall’accordo (Bilancio Uff. Relazioni Internazionali) # Lo strumento è valutato complessivamente 3500 ML da distribuire negli anni di durata dell’accordo con ISIS (2002-2007)

Previsione di spesa per l’anno 2003 Denominazione

Tipologia dell’attività

ISIS TOSCA

Acc. Internaz.

Personale Investimento (anni/uomo) (ML) 2250* 1 Ass. di Ricerca (Estero) 40

Nuovo Strumento Nuovo Strumento ad ISIS ESS Acc. Internaz.Attivo

1200# 50@ Missioni 2 Unità personale CNR

Totale

60 3600

* Quota onerosa prevista dall’accordo (Bilancio Uff. Relazioni Internazionali) @ Quota onerosa prevista dall’accordo (Bilancio Uff. Relazioni Internazionali) # Lo strumento è valutato complessivamente 3500 ML da distribuire negli anni di durata dell’accordo con ISIS (2002-2007)

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Utilizzo del tempo macchina presso ISIS da parte di gruppi di ricerca italiani

ITALIA

ITALIA

Isis

Giorni

Percentuale

Giorni

Percentuale

Ciclo

richiesti

%

assegnati

%

88/1 89/1 89/2 90/1 90/2 91/1 91/2 92/1 92/2 93/1 93/2 94/1 94/2 95/1 95/2 96/1 96/2 97/1 97/2 98/1 98/2 99/1 99/2

222.3 203.0 198.3 198.8 170.3 176.0 169.3 135.3 167.4 172.0 158.6 112.3 123.6 190.4 141.8 119.7 143.7 102.0 164.0 123.0 189.0 127.0 205.0

11.8 10.3 11.4 10.7 8.8 7.9 8.1 4.7 7.3 6.1 6.7 4.6 5.0 8.2 5.7 5.3 5.9 4.6 7.5 5.7 8.8 6.6 10.6

100.1 55.1 64.8 33.0 53.5 61.8 57.4 49.8 61.3 45.9 51.9 39.4 51.1 72.4 56.5 53.3 57.7 56.3 71.0 52.0 92.0 68.0 105.0

10.5 6.0 9.8 5.5 7.6 7.9 5.8 4.6 7.5 4.9 5.1 4.2 4.9 6.5 4.7 4.6 4.7 4.5 6.0 4.1 6.6 5.6 7.3

TOTALE

3712.8

1409.3

14

Italia % tempo

12 10 8 6 4 2

Richiesti Assegnati

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â&#x20AC;˘

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99/2

98/2

97/2

96/2

95/2

94/2

93/2

92/2

91/2

90/2

89/2

88/1

0

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ENSA – The European Neutron Scattering Association Prof. Bob Cywinski Chairman of the European Neutron Scattering Association University of St Andrews, Scotland There is little doubt that neutron scattering plays a major infrastructural role in underpinning much of condensed matter science and technology within the disciplines of physics, materials science, chemistry, the life sciences, the earth sciences and engineering. Consequently there is also little doubt that Europe can legitimately claim a significant strategic advantage in these fields of research, not only because Europe boasts the world's premier neutron scattering sources but also because Europe hosts the largest, most experienced and broadest-based community of neutron beam users. Indeed over 4,500 neutron scatterers, almost two thirds of the world's total number, reside in Europe and exploit European neutron facilities.

It is therefore tempting to conclude that European neutron scattering science is currently enjoying a "golden age". From a short-term perspective such a view is well justified: the European neutron scattering community can be proud of its achievements, and confident in its world lead.

However a medium- to long-term perspective reveals that this lead is not unassailable. On the one hand Europe in particular faces the impending reality of the much discussed "neutron drought". The drought, originally forecast by the late Tormod Riste in a 1994 Analytical Report commissioned by the OECD Megascience Forum, is a consequence of the continuing expansion of a multidisciplinary neutron scattering community and the imminent closure of many ageing research reactors. On the other hand a very serious challenge to European

supremacy in the field of neutron science has been mounted by the USA and Japan, both of whom are well advanced with their plans to alleviate their local "neutron droughts" through major financial, scientific and technological investments in third generation advanced neutron sources.

With these concerns in mind, delegates from the neutron scattering communities and societies of several European nations met in Grenoble in September 1994 to propose the foundation of a European Neutron Scattering Association, ENSA. From the start it was clear that ENSA had a vital role to play in providing a platform for discussion and a focus for action in neutron scattering science and technology in Europe and, at the inaugural ENSA meeting in December 1994 in Madrid, the delegates identified several specific aims which are now enshrined in the ENSA Articles of Association. Specifically ENSA seeks to • Identify the needs of the neutron scattering community in Europe.

• Optimise the use of present European neutron sources • Support long-term planning of future European neutron sources • Assist with the co-ordination of the development and construction of instruments for neutron scattering • Stimulate and promote neutron scattering activities and training in Europe, and in particular to support the opportunities for young scientists • Promote channels of communications with industry • Disseminate to the wider community information which demonstrates the powerful capabilities of

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neutron scattering techniques and other neutron methods • Assist, if appropriate, national affiliated bodies in the pursuit of their own goals Each of these aims is actively pursued with dedication and vigour by ENSA, which has now grown into a thriving affiliation of seventeen national neutron scattering societies and organisations that directly represent neutron beam users. Delegates from Austria, Belgium, the Czech Republic and Slovakia, Denmark, France, Germany, Hungary, Italy, the Netherlands, Norway, Poland, Portugal, Russia, Spain, Sweden, Switzerland and the United Kingdom, meet together twice yearly. Recently Romania and Greece have also sought membership of ENSA. Representatives of the major European neutron facilities and projects, the Neutron Round Table and the European Science Foundation all attend ENSA meetings with the status of observers and, correspondingly, the Chairman of ENSA has a seat at the Neutron Round Table and on the European Spallation Source (ESS) Research and Development Council.

Over the last five years ENSA has succeeded in establishing an entirely unique forum in which neutron beam users and providers can meet together to co-ordinate research and development programmes and optimise and promote neutron beam utilisation at facilities across Europe.

Indeed, ENSA initiatives in the development of neutron instrumentation and the creation of a neutron software database, are ongoing activities, carried out in collaboration with the neutron sources and the Round Table, and are well documented on the ENSA web pages (http://www1.psi.ch/www_ensa_h n/welcome_ensa.html).

Throughout its existence ENSA has also worked in close collaboration with the European Science Foundation. As part of this collaboration

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ENSA organised the "ESF Workshop on Scientific Prospects of Neutron Scattering with Today's and Future Neutron Sources", held at Autrans, near Grenoble, in January 1996 and more recently conducted a comprehensive "Survey of the Neutron Scattering Community and Facilities in Europe". Both activities have resulted in extremely informative and widely quoted ESF publications which not only place European neutron scattering in perspective but also provide a framework upon which future strategic decisions can be based. Both reports can be downloaded in full directly from either the ESF or ENSA websites.

The ENSA survey of the neutron community has, in particular, provided a remarkable and self-consistent insight into the nature and extent of neutron science within Europe. The survey dispels once and for all the widely held myth that neutron scattering is a specialist technique employed principally by physicists. Instead it emerges as a widely applicable tool exploited by a

broad and vibrant condensed matter community (figure 1a). Moreover, whilst the survey highlights the vital role of the pre-eminent high flux neutron sources, ILL and ISIS (figure 1b), it also provides a clear indication that the future health of neutron scattering science within Europe is intimately linked to the development and construction of a major third generation high flux facility, such as the ESS, and that such a project must be considered as a matter of great urgency.

Perhaps the best-known ENSA activity has been the inauguration and organisation of the innovative European Conferences on Neutron Scattering. The first ECNS conference, held in Interlaken in 1996 in co-operation with the Paul Scherrer Institute (Villigen), proved to be the largest neutron scattering conference ever held with almost 700 delegates from 40 countries presenting over 650 published papers. The second conference in the series (ECNS'99) held in Budapest in co-operation with the Budapest Neutron

A Materials Science 19.4%

Centre, was equally successful.

There is every reason to believe that ECNS'03 in Montpelier will continue the tradition.

ENSA has an extremely strong commitment to nurturing and promoting the younger members of the European neutron scattering community.

Consequently an emerging hallmark of the ECNS conference series is the high profile afforded to young scientists. Both ECNS’96 and ECNS'99 were preceded by a Training Course, in each case attended by well over a hundred young scientists, many of whom received generous bursaries.

At each meeting ten ENSA Young Scientist Awards were presented for outstanding scientific contributions.

Also, as part of a new initiative to secure the active involvement of young scientists in the future development of neutron scattering science, techniques and facilities within Europe, ENSA convened, prior to ECNS'99, four Young Scientist Panels. The Panels, with a combined membership of 31 young experts from 14 European countries elected

Life Sciences Engineering Science 2.9% 3.6% Earth Science 0.9%

Chemistry 26.9%

Physics 46.3%

Figure 1 (a) European exploitation of neutron scattering techniques across the major scientific and technological disciplines (b) European neutron beam usage by source type. (From the ENSA Survey of the Neutron Scattering Community and Facilities in Europe – an ESF/ENSA publication ISBN 2-912049-00-8, 1998)

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from over a hundred nominations, have considered issues as wideranging as neutron sources, instrumentation, sample environment and data analysis software, all viewed from a largely new perspective. It is hoped and intended that the Young Scientists Panels will continue to operate in conjunction with ENSA well beyond ECNS'99.

It is also important that the community should celebrate and publicise the tremendous achievements of the more experienced neutron scientists. In this context ENSA has established the Walter Hälg Prize for European Neutron Scattering, with the help from a generous donation

by Professor Hälg, the founder of neutron scattering in Switzerland.

The prize of 10,000 CHF is awarded biannually to a European scientist for "outstanding, coherent work in neutron scattering with long-term impact on scientific and/or technical neutron scattering applications".

A particular highlight of ECNS'99 in Budapest was the ceremony and plenary associated with the presen-

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tation of the first ENSA Hälg Prize.

Ferenc Mezei (HMI Berlin) was the very worthy recipient of this prestigious award and it was quite fitting that he should receive it in the very city where he carried out his pioneering work on the neutron spin echo technique. The second ENSA Hälg Prize is to be awarded next year at ICNS'2001 in Munich, and a call for nominations will be announced shortly.

Although the twelfth committee meeting is about to be held in May 2000 in Munich, it is clear that the work of ENSA has only just begun.

From the perspective of ENSA the future of European neutron scattering science and research infrastructure is both exciting and challenging.

The excitement stems from the wonderful opportunities that are provided by the continuing optimisation of existing neutron sources alongside the developing scientific and technical case for the European Spallation Source which promises a strategic facility that will keep Europe ahead of the field for at least the next half

Low flux sources 21.3%

century. The challenge is to secure appropriate funding mechanisms to maintain our major facilities at the cutting edge of neutron science and to allow the ESS project to move rapidly ahead to realisation.

My own personal challenge as ENSA Chairman is to prove a worthy successor to Dieter Richter and Albert Furrer who, as first and second Chairmen respectively, have dynamically and skilfully steered the Association through its formative years to establish ENSA as a major scientific and political force in European neutron scattering. This challenge, however, is made considerably easier by the strong support and expert advice offered by the previous chairmen and the current members of the ENSA executive, Lars Borjesson (Swedish delegate and ENSA Secretary) and Fabrizio Barrocchi, the Italian delegate and ENSA Vice-chairman.

■

ISIS 17.4%

Institut Laue Langevin 21.2%

Medium flux sources 40.2%

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S.I.S.N. Nel panorama attuale della neutronica uno degli sviluppi più attesi e importanti è il Millennium Programme dell'Institut Laue-Langevin di Grenoble. La sua formulazione e definizione finale hanno preso un considerevole periodo di tempo, col coinvolgimento di tutte le comunità di utilizzatori. E' un programma ambizioso di costruzione di nuovi strumenti, di upgrading di altri, e di miglioramenti importanti nelle infrastrutture. Coinvolgendo poi lo sviluppo per i prossimi cinque anni della facility migliore nel mondo, questo programma ha importanti valenze di innovazione e progresso scientifico.Un altro punto di interesse è l'inquadramento di questo programma nel dibattito sulle future sorgenti. Chiaramente l'attuazione dei miglioramenti proposti per ILL darà un'idea più precisa della scienza che si potrà fare con le sorgenti future in discussione, e dunque anche del tipo di scelte che la comunità internazionale sarà chiamata a fare.

Per tutti questi motivi è bene che la nostra comunità sia presente in questi dibattiti, questi sviluppi, e nel lavoro che sta già cominciando per realizzare i progetti proposti.

L'ultima riunione del Consiglio Scientifico dell'ILL è stata fortemente caratterizzata dalla discussione sul Millennium Programme. Le decisioni prese sono importanti e avranno una certa rilevanza anche per la nostra comunità, in quanto configurano gli sviluppi della migliore sorgente neutronica per i prossimi trequattro anni. In particolare: 1. Partono quest'anno i due progetti Super D2B e nuovo D7; 2. Per il prossimo anno verranno messi in cantiere: a. Studio di Fattibilità della sorgente ultrafredda basata su nanoparticelle di carbonio nell'elio liquido raffreddato. b. Upgrade di IN3 per farne un Resonance Spin Echo spectrometer.

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c. Laboratorio deuterazione molecole biologiche (in coll. con EMBL). d. Upgrade di IN14.

Il resto dei progetti proposti per il Millennium Programme verranno ridiscussi nel CS di autunno. Riassumo qui brevemente i Progetti: Super D2B: upgrade di D2B mediante installazione di un nuovo stack di 128 rivelatori a filo e collimatori di mylar, per avere un angolo solido di collezione molto più grande, ma allo stesso tempo altissima risoluzione nel piano orizzontale. Si prevede un aumento del counting rate di un fattore 6. D7: Viene previsto un aumento di un fattore 30-40 del counting rate installando analizzatori supermirror in uno dei quattro stacks di rivelatori Laboratorio Deuterazione: si prevede di creare una vera e propria facility per permettere a users esterni e locali di deuterare campioni biologici. Verrà fatto in collaborazione con EMBL; non si sa ancora dove.

Sorgente Ultrafredda: è stato solo approvato lo studio di fattibilità, che prevede lo studio delle caratteristiche fisiche di sospensioni di nanoparticelle di carbonio in elio liquido in funzione della dimensione, polidispersità, concentrazione. In caso di risultati positivi, anche se non formalmente deciso, è molto probabile che si proceda col progetto principale. IN3: si prevede di sviluppare la tecnica di neutron spin eco risonante su IN13, per aumentare la risoluzione in energia a meglio di 10 meV, ossia un ordine di grandezza superiore a quella di un tre assi standard. L'applicazione principale prevista è allo studio dei rilassamenti delle eccitazioni magnetiche o reticolari. IN14: Verrano installate ottiche supermirror su IN14 (triplo assi con sorgente fredda); si prevede un guadagno di 3-6 nel flusso. Questi i progetti che sicuramente partiranno questo o il prossimo anno.

Altro argomento discusso è stato il ruolo dei CRG (Collaborating Research Groups), e la politica di ILL per il loro futuro. Si è deciso di avere

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una discussione approfondita alla prossima riunione del CS. Anche per questo aspetto siamo fortemente interessati, in quanto la nostra comunità è impegnata in due CRG, uno già operativo (IN13), e l'altro (BRISP) negli stadi finali della definizione del progetto operativo (il contratto è già stato firmato dai due Presidenti).

Per quanto riguarda IN13, sono state date garanzie di interesse per il suo upgrading. Il problema però rientra in quello più generale dell'upgrading di tutte le guide originali (e cioè vecchie di più di venti anni), il cui costo previsto è di 64 Megafranchi, che non ci sono. Il CS all'unanimità ha chiesto allo Steering Committee dell'ILL di trovare questo finanziamento aggiuntivo, per riportare il potenziale scientifico dell'ILL al meglio delle possibilità permesse dalle attuali tecnologie.

Chiaramente l'upgrading di IN13, che prevede appunto a sostituzione della guida con supermirrors, è direttamente coinvolto in questo progetto generale. In ogni caso il Management si è espresso molto favorevolmente sulla prosecuzione e sul-

Al XI Convegno Nazionale della Società Italiana di Spettroscopia Neutronica (SISN) farà seguito, il giorno 20 Ottobre, un Convegno Internazionale di una giornata in memoria di Francesco Paolo Ricci -recentemente scomparsodal titolo "Francesco Paolo Ricci memorial: his legacy and future perspectives of neutron spectroscopy", per ricordare il suo contributo allo sviluppo della spettroscopia neutronica in Italia e il suo impegno per l'accesso degli studiosi Italiani alle facilities internazionali e per la formazione dei giovani ricercatori.

Ulteriori informazioni possono essere richieste alla Sig.ra Grazia Ianni (ianni@caspur.it) e alla Prof.ssa Maria Antonietta Ricci (riccim@fis.uniroma3.it).

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l'upgrading di IN13; un pò più freddino è stato il CS. In ogni caso il progetto dovrebbe proseguire, e questo vuol dire che la nostra comunità sarà chiamata ad una condivisione delle spese dell'upgrading: ILL curerà le guide (se otterrà i soldi), e i membri del CRG avranno la responsabilità di finanziare e costruire i previsti nuovi monocromatori e analizzatori.

Dato che ci sarà la discussione sui CRG nel prossimo Consiglio Scientifico dell'ILL, sarebbe opportuno che la nostra comunità esprimesse fortemente (se così valuta) l'interesse per IN13 e il suo upgrading. Da questo punto di vista sarebbe particolarmente importante il contributo di colleghi biologi, chimici etc, dato che IN13 è

risultato essere particolarmente interessante per la sua valenza interdisciplinare, e questa è una delle priorità che la Direzione dell'ILL ha indicato per gli sviluppi futuri.

Concludo ricordando che sia i CRG che i progetti del Millennium Programme costituiscono un'importante occasione di cimentarsi nella progettazione, costruzione o miglioramento di spettrometri e strutture connesse. Considerando che la nostra comunità è ancora sottodimensionata fortemente nel campo della strumentistica, sarebbe oltremodo auspicabile che venissero dedicate risorse sia finanziarie che umane per partecipare a queste attività. Particolarmente importante ritengo sia la formazione

LO SPETTROMETRO IN13 ALL’ISTITUTO LAUE-LANGEVIN The backscattering spectrometer IN13 at the ILL became a CRG (Collaborative Research Group) instrument in July 1998 under a contract between the Université J. Fourier and the ILL. It is now operated in the frame of a French-Italian collaboration between the Université J. Fourier, the Institut de Biologie Structurale (CEA-CNRS, Grenoble), the Laboratoire Léon Brillouin (CEA-CNRS, Saclay) and the Istituto Nazionale per la Fisica della Materia (INFM, Italy).

A major upgrade of the electronics and instrument control programs has been performed in August 1999 retaining the previous instrument characteristics: energy resolution of about 10 meV, Qrange 0.3 to 5.5 Å-1, energy transfer up to ca. 250 meV. These characteristics are extremely well suited for the study of large molecular assemblies held together by weak interactions. Main purpose of the CRG is to investigate the low energy dynamics of biological macromolecules and the relationship between function and microscopic dynamics. Some examples of experiments recently performed on the instruments will be described.

Lo spettrometro in backscattering IN13 all’ILL è stato rimesso in funzione nel Luglio del 1998 grazie ad un accordo tra enti di ricerca Italiani e Francesi: l’Istituto Nazionale per la Fisica della Materia, l’Istituto di Biologia Strutturale (CEA-CNRS, Grenoble), il Laboratorio Léon Brillouin (CEA-CNRS, Saclay) e l’Università J. Fourier di Grenoble. E’ stato così realizzato un CRG (Collaborative Research Group) per la gestione dello spettrometro con l’obiettivo di impiegarlo principalmente per lo studio della dinamica a bassa energia di macromolecole biologiche e della relazione tra funzione biologica e dinamica microscopica. Un ulteriore importante obiettivo di questo progetto, soprattutto per la comunità italiana, è stato quello di favorire l’interazione di ricercatori con esperienza nel campo dei neutroni, operanti per lo più nel campo della Fisica dei solidi e dei liquidi, con gruppi attivi nel campo della Biofisica e quindi di espandere e consolidare la comunità degli utilizzatori delle tec-

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di giovani ricercatori in questo campo, e dunque l'accensione di borse di dottorato, e di contratti di ricerca post doc sarebbe particolarmente opportuna. Naturalmente dovrebbe esserci una precisa finalizzazione, e dunque scelte scientifiche e strategiche sui progetti su cui intervenire.

Non dimentichamo inoltre che il progetto Millennium è solo nella sua prima fase, e altre proposte e progetti verranno valutati nel prossimo hanno. Dunque un momento di grandi opportunità, ma anche di valutazioni e di scelte, cui sarà chiamata la nostra comunità.

Marco Fontana Presidente della SISN

niche neutroniche soprattutto nei settori della “soft matter” e dei sistemi molecolari complessi.

Tra gli strumenti ad alta risoluzione dell’ILL, IN13 è il solo in geometria di backscattering che utilizza un fascio di neutroni termici; in tal modo può accoppiare una elevata risoluzione in energia (∆E ~ 10 µeV) con la possibilità di accedere ad elevati momenti trasferiti, Q (l’intervallo di Q accessibili va da 0.3 a 5.5 Å-1) Uno schema dello strumento è mostrato in Fig. 1. Il fascio di neutroni termici (E=16.45 meV) viene diffratto alla Bragg da un monocromatore di CaF2 posto in una criofornace la cui temperatura può essere variata tra 80 K e 350 K; in tal modo il passo reticolare del monocromatore viene modificato e conseguentemente si può variare l’energia dei neutroni in un intervallo compreso tra –120 µeV e 230 µeV attorno all’energia incidente. I neutroni vengono quindi deflessi sul campione e, dopo lo scattering da quest’ultimo, vengono analizzati in energia da un sistema di cristalli analizzatori, anch’essi di CaF2, che sono mantenuti a temperatura ambiente ed operano in geometria di backscattering. La scansione in energia si ottiene quindi variando

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energia dei neutroni incidenti sul campione. La maggiore limitazione dello strumento è tuttora il suo basso flusso, infatti IN13 è collocato lungo una guida di neutroni termici e dista oltre 70 m dal reattore. Per ottenere dati con una buona statistica sono necessari tempi di acquisizione piuttosto lunghi: circa 2 ore per una misura elastica (hω =0) e da 3 a 4 giorni per ottenere lo spettro quasielastico completo. Un obiettivo importante nell’ambito di un possibile rinnovo del progetto, che nella forma attuale scade nel Giugno 2001, dovrebbe quindi essere un sostanziale aumento del flusso di neutroni sul campio-

Fig. 1. Schematic view of the IN13 backscattering spectrometer at the ILL

la temperatura del monocromatore rispetto a quella degli analizzatori.

La possibilità di accedere a valori elevati del momento trasferito, mantenendo al contempo una elevata risoluzione in energia, è una caratteristica peculiare di IN13 che risulta particolarmente utile per mettere in evidenza le anarmonicità nella dinamica vibrazionale del campione ed anche per studiare i moti diffusivi in un ampio intervallo di momenti trasferiti potendo così analizzare in dettaglio le caratteristiche geometriche di questi processi. Questo tipo di informazioni è particolarmente interessante per lo studio della dinamica dei sistemi disordinati: transizioni vetrose negli amorfi, dinamica atomica nei fluidi semplici e associati,

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transizioni conformazionali nei polimeri, dinamica delle biomolecole, interazioni molecola-solvente nei fluidi complessi.

Nel corso del primo anno del progetto si è provveduto ad un sostanziale ammodernamento degli apparati di controllo ed acquisizione dati dello spettrometro. L’elettronica di tipo CAMAC è stata sostituita da un sistema VME controllato da un PC, ed il nuovo software che è stato sviluppato permette un controllo molto più semplice ed accurato di tutti i parametri sperimentali durante la misura. Inoltre, grazie alla aumentata velocità dei controlli, è stato possibile ottenere una migliore accuratezza nella definizione della temperatura del monocromatore e quindi della

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Fig. 2. Elastically scattered intensity by ice Ih at 20, 100, 180 and 260 K (from top to bottom.

Lines are fits by a model in which protons are able to jump between two sites of different energies [L. Bove, F. Sacchetti and A. Paciaroni, INFM-Perugia, Italy and INFM-OGG-Grenoble, France].

Fig. 3. Incoherent scattering from entire cells, and comparison with myoglobin as hydrated powder (Doster et al. Nature 1999), and in solution. [G. Zaccai, M. Tehei, B. Franzetti, C. Pfister, IBS-Grenoble and B-Z. Ginzburg, Jerusalem, Israel].

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ne; a questo scopo verranno condotti sullo strumento attuale dei test nel corso del 2000, una stima realistica sembrerebbe indicare come tecnicamente realizzabile un aumento del flusso di un fattore 8 – 10.

Grazie al CRG-IN13 diversi ricercatori INFM dell’area biofisica (Sezione B) e dei sistemi disordinati (Sezione C) hanno proposto ed avviato dei progetti di ricerca che prevedono l’utilizzo dello scattering dei neutroni ad alta risoluzione; nel seguito illustreremo alcuni esempi di studi condotti da ricercatori Italiani e Francesi.

Il primo (L. Bove et al. INFM-Perugia) riguarda lo studio della dinamica dell’idrogeno nel ghiaccio Ih a differenti temperature. Questo rappresenta un sistema modello di partenza relativamente semplice per analizzare la dinamica dei protoni in sistemi più complessi di reti di legame idrogeno, come ad esempio le proteine idratate. Lo scattering dal campione è risultato totalmente elastico anche alle temperature più elevate impiegate (260 K). La dipendenza da Q dell’intensità di scattering (nell’intervallo di energia da –20 a +20 µeV) riportata in Fig.2, appare non armonica anche alle temperature più basse misurate (20 K). I dati sono stati analizzati con un modello in cui i protoni possono saltare tra due siti con energie differenti verosimilmente lungo le direzioni dei legami ossigeno-ossigeno nei “loop” esagonali

Fig. 4. Energy resolved incoherent scattering from ribosomes (70S) purified from E. coli and T. thermophilus. [C. Pfister, G. Zaccai, IBSgrenoble, I. Serdyuk and I. Scherbakova, Puschino, Russia].

Fig. 5. Elastic scattered intensity vs Q2 at different temperatures for an amylose sample hydrated at 0.47 (g. D2O/g. dry saccharide).

The solid lines are fits to an asymmetric doublewell potential. [M. Di Bari, G. Albanese, F. Cavatorta, A. Deriu, INFM-Parma, Italy].

presenti nella struttura disordinata del ghiaccio.

Alcuni esperimenti condotti da ricercatori dell’IBS di Grenoble si sono posti l’obiettivo di studiare l’effetto dell’influenza dell’ambiente intracellulare sulla dinamica delle proteine.

Le misure sono state condotte su cellule di E. coli, di H. marismortui e globuli rossi del sangue a temperature comprese tra 280 e 320 K per evitare una denaturazione da eccessivo raffreddamento. Dopo aver sottratto il contributo allo scattering dovuto ai moti diffusivi del solvente, dalla dipendenza da Q dell’intensità elastica è stato dedotto, in approssimazione Gaussiana, uno spostamento quadratico medio in funzione della temperatura che è stato interpretato in termini di “rigidità” dell’ambiente intracellulare. I dati riportati in Fig. 3 mostrano il confronto con gli spostamenti quadratici medi misurati in polveri idratate di mioglobina (dati da Doster et al. Nature 337, 754, 1989) e in soluzioni di mioglobina (media tra due misure con concentrazioni di 100 e 200 mg/ml in D2O).

Appare evidente che la dinamica macromolecolare nelle cellule dipende dal tipo di cellula: infatti una “rigidità” via via maggiore si osserva nei globuli rossi, nell’E. coli e nell’H.marismortui. Questo tipo di misure apre delle interessanti prospettive per una migliore comprensione del-

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l’effetto degli ambienti intracellulari in vivo e gli autori intendono estendere queste misure ad altri tipi di celle di organismi termofili ed ipertermofili.

Misure interessanti sono anche state condotte su ribosomi, aggregati macromolecolari di proteine ed acidi nucleici. I campioni utilizzati per gli esperimenti sono stati ottenuti da E. coli (un organismo che ha raggiunge la funzionalità ottimale a 35 C) e Thermus thermophilus (un batterio termofilo che vive a temperature di 70-80 C). I risultati degli spostamenti quadratici medi in approssimazione Gaussiana (Fig. 4) indicano che mentre i valori assoluti di 〈u2〉 sono, a 280 K, leggermente superiori per il Thermus thermophilus rispetto al E. coli (ciò potrebbe essere dovuto ad una leggera differenza di idratazione tra i due campioni), la variazione di 〈u2〉 con la temperatura è leggermente ma significativamente inferiore per il batterio termofilo. Ciò indica che un organismo termofilo deve raggiungere una temperatura più elevata perchè le biomolecole raggiungano il grado di “flessibilità” strutturale necessario alla ottimizzazione della loro funzione biologica.

L’ultimo esempio qui riportato riguarda la dinamica di alcuni polisaccaridi: amilosio ed amilopectina che sono i due principali componenti dell’amido. Nella Fig. 5 è mostrata

Fig. 6. Temperature dependence of the proton mean square displacement 〈u2〉 (normalised at 20 K) for amylose (top) and amylopectine (bottom). The hydration is expressed as g.

D2O/g. dry saccharide. The continuous line is a fit of the low temperature data to an Einstein model of independent harmonic oscillators [M. Di Bari, G. Albanese, F. Cavatorta, A. Deriu, INFM-Parma, Italy].

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l’intensità elastica ottenuta da misure su amilosio per differenti idratazioni. I dati, che per T>200 K mostrano un andamento chiaramente anarmonico, sono stati analizzati utilizzando un modello di transizioni conformazionali delle catene polisaccaridiche descritte da un potenziale a doppia buca per la dinamica dei protoni. Gli spostamenti quadratici medi così ottenuti (Fig. 6) mostrano un comportamento armonico fino a 320 K nel campione secco. Viceversa nei campioni idratati si osserva una transizione cinetica, simile alle tran-

sizioni vetrose nei sistemi amorfi, a circa 230 K. Le temperature di transizione dipendono marcatamente dall’umidità e, a parità di idratazione, sono molto più alte di quelle osservate nelle proteine globulari (tipicamente 150-180 K). Questi studi possono avere anche un interesse applicativo nel settore della “food science”: infatti in questo campo di ricerca oggi si applicano sempre più spesso concetti e modelli derivati dalla fisica e chimica dei polimeri (processi cinetici metastabili, stati “vetrosi” dinamicamente confinati)

in luogo di modelli termodinamici all’equilibrio. Da questo punto di vista la temperatura di transizione vetrosa, TG, può costituisce un parametro di interesse significativo in relazione alle proprietà fisiche ed alla stabilità e dei prodotti alimentari.

A. Deriu Università di Parma e INFM, Parma A. Paciaroni INFM Operative Group Grenoble C. Pfister, IBS e Università J. Fourier Grenoble

Early Neutron Diffraction in Italy: F.P. Ricci

Francesco Paolo Ricci, the Founder and Editor of this Journal died in Rome on 27 February. He was professor of Physics at "Università di Roma Tre" which he joined a few years ago at its start to help the new Physics Department to attain the high standard of the old Physics Department of "Università Roma - La Sapienza" where he had been Professor for about 30 years, serving as Chairman during "the difficult seventies".

With Paolo Ricci disappears an exceptionally good man beloved by his family and friends and a distinguished scientist who gave a remarkable contribution to neutron research.

He is survived by the wife Silvana Piermattei a well known Medical Physicist and his former University mate, three sons and one daughter.

The last son of a large family and grandson of a painter who was very famous at the turn of the century, Paolo Ricci always had a particular taste for beauty,

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under every form it was appearing and in every field. His roots belonged to Abruzzo, a region east of Rome of great natural beauty which includes the highest mountains of peninsular Italy, old villages, green pastures and wide white beaches along the Adriatic sea.

After fullfilling the requirements for graduating in Physics he joined for his thesis and then postgraduate work the Low Temperature Laboratory at Roma La Sapienza performing ion diffusion and mobility experiments in liquid Helium until 1957 when E. Amaldi asked him to start in Italy research in neutron diffraction together with G. Caglioti and A. Paoletti.

At that time neutron diffraction was still at its childhood and work was being performed by about half a dozen groups around the world where research reactors were available: Argonne, Brookhaven, Chalk River, Harwell, Oak Ridge and Saclay. In Rome a small reactor was under construction at Casaccia mostly for training, with few experimental beam-holes for neutron work.

The first task of the group was obviously to design a spectrometer. That led immediately into the problem of optimizing the physical parameters of the ap-

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paratus to be built. Such a problem had been faced already by all other groups performing neutron work, but the criteria and results were mostly dispersed in local reports if not in hand written notes and were barely mentioned in the "experimental" sections of the existing literature reporting the new exciting results obtained in crystal and magnetic structures or were just orally transmitted from scientist to scientist.

The Rome group took the occasion to work out, starting from simple models, closed formulas for powder and single crystal diffraction taking into proper account the characteristics of monochromators, samples, collimators, detectors.

That was the first work of the group, the only one which could have been possibly performed on paper, without a reactor available. After that the three friends went across the Ocean to learn the neutron diffraction technique: G. Caglioti to Chalk River with B.N. Brookhouse, A. Paoletti to Brookhaven with R. Nathans and F.P. Ricci to MIT with C.G. Shull.

Paolo was considered the luckiest as he was the only one living in a beautiful town and working with the father of neutron diffraction. But it turned out that the MIT reactor was behind sche-

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dule at such an extent that after one year Paolo came back to Rome without having "seen" a single neutron. Hovever he had plenty of opportunities for preparing and discussing forthcoming neutron work with Cliff Shull. That meant that after all he gained the best training since he could join in setting up the refined experiments planned "Cliff's style", without even bothering collecting data which of course is a most important business but may also turn in a tedious one if it leads only to expected results.

To that time dates the deep friendship and esteem Cliff Shull always had for Paolo Ricci that means that Cliff evaluated at a good rate the cooperation of Paolo who for a while was the only person working with him. Of course in a way neutron work was more simple than now, but it was absolutely new and someone doing research was always on his own, in "no man's land".

In meantime Caglioti was learning lattice dynamics by inelastic neutron scattering from its pioneer B.N. Brookhouse and Paoletti was looking at departures from spherical symmetry of 3d electron distribution with polarized neutrons, under the guide of Bob Nathans.

At the end of '59 all members of the group were back in Italy and at that time they separated. Caglioti went to the CP5Mw reactor in Ispra center which just at that time was becoming an European Establishment and took there the new built spectrometer. Paoletti and Ricci preferred to stay in Rome at the Casaccia TRIGA reactor of which in the meantime had been decided the upgrading to 1Mw, which meant a full year shutdown.

However, before it, there was the possibility to test and perform the first measurements with a polarized neutron spectrometer, the second coming to operation after the one in Brookhaven. It had been quickly built from a standard Xray diffractometer taking advantage for the spin flipping equipment of the laboratories in Frascati where it had been recently built the R.F. system for a new 1 Gev Electrosynchrotron.

For about 10 years I had then the chance and the honour of working with Paolo

Ricci. It was a great experience as he had a deep vision of the problems and without underestimating difficulties he found always a good reason for a joyful approach to work.

In those years neutron work in Italy went along predictable paths with major emphasis on lattice dynamics and mechanical properties in Ispra and magnetic properties in Rome where also a three axis spectrometer for the study of magnetic excitation had been set up.

The results were not exceptional but of good quality and on line with the problems being investigated through the world at that time. But it was becoming increasingly evident that the neutron study of condensed matter was looking for new fields.

In September 1968 a Symposium on "Current Problems in Neutron Scattering" was held in Rome at Casaccia with the participation of practically all the scientists working with neutron scattering in Europe and U.S.A. It appeared clear that Liquids and Phase Transitions were catching more attention as a new generation of high flux reactors was coming into operation, at Brookhaven first and later in Grenoble.

A new class of experiments was made possible, paving the way to the present investigations in Complex Molecules, Multilayers and Biological Systems. The groups operating at small reactors were rather discouraged and the groups of Casaccia and Ispra made no exception.

At that time travelling was more difficult and work at the neutron sources was not possible if one did not take care of the equipment which belonged to the operating research group rather than to the neutron source. That practically excluded from regular work the groups of the countries which did not own the neutron source.

Italy did not enter the ILL consortium either at its foundation or later at the time when UK joined in. After losing this opportunity Caglioti and Paoletti decided to quit neutron work believing that it would have been impossible to perform good work without a good source. But Paolo Ricci decided to go on,

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gathering the few people left and starting at the 1 Mw reactor of Casaccia a line of ingenious experiments which eventually led to a renewed interest in Italy for the subject and to a new generation of Italian neutron investigators who later were able to take advantage of the research opportunities provided by the European cooperation in the construction and operations of intense sources.

Paolo acted with humility and vision at the same time, suggesting problems, teaching young students, encouraging those who were dubious. He had a wide and solid culture and not only in Physics. Until now, at 70 he was really the yeast of a wide scientific community which grew under his continous care and gradually was accepted as a significant component of the international neutron community, as he was able to extract the best from his coworkers.

Still he did not like the limelights: he enjoyed the back stage better as he was an exceptional Director rather than an Actor, discovering, discussing, testing, rehearsing that permanent, brilliant often unexpected show which is Research in Physics.

Because of his ancestry he was always looking for beauty and not only in Art toward which he felt a strong attraction but in Science as well. In a way he looked for an aesthetic dimension of Science. Maybe he thought that beauty was a common component of human activity which indicated the presence of at least some hint of the Truth mankind is always pursuing and never reaching.

His comment after a good seminar throwing a new light on some difficult problem invariably was "E' proprio bello!" and saying that, he looked absolutely happy and indeed he was.

Paolo Ricci was one of the very few persons to work with was fullfillment, joy, even pleasure. We are going to miss him greatly, but we consider as a great gift of life to have been for a long time his colleagues, his coworkers, his friends.

Antonio Paoletti Università degli Studi di Tor Vergata Facoltà di Ingegneria

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In ricordo di Francesco Paolo Ricci Vogliamo qui brevemente ricordare ai colleghi ed ai giovani ricercatori della Fisica della Materia Francesco Paolo Ricci (che si è spento dopo una lunga malattia lo scorso febbraio a Roma) con tutto l’affetto, la profonda amicizia e stima che sono frutto di tanti anni di lavoro in comune. Francesco Paolo è stato per lungo tempo elemento di stimolo e di discussione critica e vivace all’interno della nostra comunità scientifica in anni particolarmente critici (tra gli anni ottanta e l’inizio degli anni novanta) nei quali si assisteva ad una notevole crescita culturale e scientifica ma alla quale non corrispondeva né una adeguata struttura organizzativa né i mezzi finanziari erano sufficienti a garantirne il naturale sviluppo. Come Segretario

scientifico e, successivamente, come Direttore del gruppo Nazionale di Struttura della Materia ha avuto un ruolo incisivo e determinante nell’evoluzione strategica della organizzazione della ricerca in struttura della materia (poi divenuta Fisica della Materia con l’apporto di Elettronica Quantistica, Plasmi, Cibernetica e Biofisica) verso gli assetti che si sono più recentemente consolidati, avendo ben chiari alcuni riferimenti, ed in particolare: - La struttura del GNSM in settori Nazionali Tematici (che furono il primo modello delle attuali sezioni INFM) con notevole autonomia gestionale e con la partecipazione anche della componente industriale interessata alla ricerca.

- Una forte interazione tra Unità di Ri-

In memoria di Umberto Maria Grassano Umberto Maria Grassano ci ha lasciati nella notte tra il 3 e il 4 Maggio 2000.

Aveva 61 anni e da qualche tempo era affetto da un male incurabile. Lascia un grande vuoto specie in coloro, e sono tanti, che ne hanno apprezzato le elevate qualità morali, la dedizione alla ricerca e all'Università, l'impegno organizzativo a livello nazionale, la disponibilità a farsi carico dei problemi degli altri, l'assoluto disinteresse personale. Si era laureato in Fisica all'Università di Pavia, alunno del Collegio Ghislieri, nel 1961. Nel 1963 si era trasferito all'Università di Messina e poi, nel 1965, all'Università di Roma "La Sapienza". Nel 1980 era tornato a Messina come professore straordinario di Fisica Molecolare e dal 1981 era diventato professore straordinario e poi ordinario di Fisica dello Stato Solido all'Università di Roma "Tor Vergata".

Aveva trascorso vari periodi di studio all'estero: all’Imperial College di Londra nel 1962, alla Cornell University nel 1975, all’Università di Nimega nel 1981 e

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82. Condirettore del corso “Excited state spectroscopy in solids” della Scuola “E. Fermi” di Varenna nel 1985, Direttore del GNSM dal 1996 al 1998, membro del Consiglio direttivo e della Giunta esecutiva dell‘INFM dal 1987 al 1997, Direttore del Dipartimento di Fisica dell’Università di Roma “Tor Vergata” dal 1991 al 1993, Presidente del Consiglio di Corso di Laurea in Fisica nella stessa Università dal 1983 al 1986 e dal 1999 alla sua morte.

L’attività scientifica di Umberto Grassano ha riguardato principalmente le proprietà ottiche dei cristalli ionici e si è sviluppata in modo coerente nell’arco di un quarantennio: dai centri di colore, ai laser per infrarosso, all’ottica non lineare, ai nuovi materiali (per laser). Quest’ultima attività lo aveva portato ad organizzare, con i colleghi chimici, il corso di diploma in Scienza dei materiali che in futuro si trasformerà in una laurea di primo livello.

Numerosi sono stati i suoi contributi

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Vol. 5 n. 1 Giugno 2000

cerca Universitarie ed Istituti e Centri di Ricerca CNR, con l’intento di arrivare ad una struttura complessiva con una evidente composizione organica ed il più possibile efficiente. Si deve alla sua azione la realizzazione dei progetti coordinati con la valutazione di referee internazionali.

- Il potenziamento di progetti ed iniziative internazionali che facessero riferimento alla grandi facilities europee (riguardanti in particolare l’utilizzo di fasci di neutroni e la radiazione di sincrotrone).

Non dimenticheremo mai il vigore e la passione con cui ha vissuto le vicende scientifiche ed organizzative più importanti degli ultimi 30 anni, l’apertura ai giovani, la lealtà e la sincerità con cui si è battuto per affermare le proprie idee e trasmetterle agli altri.

Angiolino Stella Emanuele Rimini

scientifici di grande rilievo. Senza pretesa di completezza, desidero elencare i seguenti: Nel 1966 ha introdotto (in collaborazione con lo scrivente) il metodo che oggi viene chiamato “pump and probe o PAP”, applicandolo, con sorgenti ottiche tradizionali, allo studio degli stati eccitati del centro F(PRL 16,124 (1966)).

Nel 1966 ha pubblicato (con lo scrivente e con Renzo Rosei) la prima osservazione dell’effetto Stark del centro F (PRL 17, 1043 (1966)). Ha esteso in seguito questa tecnica a molti altri centri facendone uno strumento per lo studio degli stati eccitati non raggiungibili da transizioni a un fotone (lavori in collaborazione con G. Margaritondo, R. Rosei, M. Bonciani, A. Scacco, A. Tanga).

Nel 1974 ha pubblicato la prima osservazione dell‘emissione stimolata da centri F eccitati (Optics Comm. 11, 8(1974) in collaborazione con F. De Martini e F. Simoni). L’articolo contiene la proposta di costruire un laser a centri di colore. Ha in seguito collaborato alla realizzazione dei primi laser italiani a centri di colore (Revue Phys Appl. 18, 301 (1983), in collaborazione con G. Baldacchini, P. Violino e altri).

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A seguito della sua lunga attività nel campo dell’ottica non lineare, nel 1986 ha esteso il range dell’assorbimento a due fotoni al campo spettrale della radiazione di sincrotrone, osservando gli eccitoni 2p e 3p nel KCl a 8.5 eV (Europhysics Lett. 2, 571 (1986), in collaborazione con F. Bassani, M. Casalboni, M. Piacentini ed altri) che non sono accessibili con le normali sorgenti ottiche.

Ha sviluppato negli anni una pregevole attività di ricerca sul dicroismo circolare magnetico dei centri di colore (Solid State Comm. 21, 225 (1977); Phys. Rev. B16, 5570 (1977); Phys. Rev. B20, 4357 (1979), in collaborazione con G. Baldacchini e A. Tanga).

Non ha avuto la soddisfazione di veder pubblicato il libro di Fisica dello stato solido (Bollati-Boringhieri in corso di stampa) che ha scritto con Franco Bassani. Resterà una delle testimonianze che ci ha lasciato, a compimento della sua lunga attività di studio e di ricerca.

Vorrei ricordare ancora un aspetto della sua attività di ricercatore: l'estremo ‘understatement’ con cui parlava dei suoi risultati scientifici e il suo rifiuto di ogni abbellimento, enfatizzazione, richiesta

di priorità. Se dovessimo accettare l'ingiusta etichetta che ci è stata appiccicata anni or sono, quella di "baroni della scienza", dovremmo dire che Umberto era l'anti-barone per vocazione e per convincimento. Aveva sviluppato molte collaborazioni con sedi e gruppi diversi senza mai attribuirsi né richiedere il ruolo del ‘principal investigator ’ né preoccuparsi se altri se lo attribuissero.

Umberto era un uomo buono, dotato di intime convinzioni religiose, che mai ostentava e che lo hanno molto aiutato negli ultimi mesi di sofferenza. Era sempre disponibile a farsi carico dei problemi di tutti, sia a livello individuale che delle istituzioni. Nell'ultimo anno, già provato da un male inesorabile, in condizioni difficili per la nostra Università a causa del passaggio al nuovo ordinamento didattico, aveva accettato, con giovanile determinazione, l'incarico di presidente del Consiglio di corso di laurea in Fisica. Esempio per tutti noi di spirito di servizio e anche velato rimprovero per coloro che preferiscono rinchiudersi nei loro studi o laboratori, senza rendersi conto che l'interesse generale è alla base degli interessi particolari.

Ricordando Vittorio Mazzacurati Il Foundation Phase Report (FPR) della European Synchrotron Radiation Facility (ESRF), pubblicato nel 1987, prevedeva la costruzione di una linea di luce dedicata alla spettroscopia Mössbauer e allo scattering anelastico dei raggi X (IXS).

La parte dedicata allo IXS evidenziava l’interesse di poter studiare eccitazioni di tipo fononico ed elettronico con i raggi X come un complemento ed una alternativa a metodi esistenti quali le spettroscopie neutroniche e di fotoemissione. Questo progetto trovava la principale motivazione nelle applicazioni di tipo fononico, i.e. nella parte a piú alta risoluzione. Infatti, in questa direzione, c’erano giá stati dei tentativi fatti da un Gruppo operante al Sincrotrone di Amburgo, HASY-Lab, il quale aveva realizzato uno spettrometro per IXS. Questo

strumento aveva come scopo il raggiungimento di una risoluzione totale in energia di 7 meV, e aveva dimostrato sperimentalmente la possibilitá di poter misurare eccitazioni fononiche con una risoluzione di circa 20 meV – il progetto auspicato nel FPR era quello di ottenere i 7 meV di risoluzione all’ESRF, grazie all’attesa maggiore collimazione della nuova sorgente, e grazie ad un programma da lanciare per la costruzione di analizzatori dell’energia dei fotoni con altissima risoluzione (ottenere 5 meV a 14 KeV, i.e. ∆E/E~10-7).

Accettai nel 1990 di venire all’ESRF, rientrando da una esperienza di otto anni in America, per occuparmi dello sviluppo di questa linea di ricerca – lo sviluppo dell’IXS all’ESRF. Grazie al mio rientro, mi potei riavvicinare ad alcune

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Ha trascorso l'ultimo mese della sua vita sereno e distaccato, quasi scusandosi per il disturbo che arrecava agli amici. A tratti era ironico, con l'ironia sfumata di chi ormai sta sopra alle vicende del mondo. Anche se l'ironia ha fatto parte del suo modo di vivere da sempre. A tratti, tuttavia, partecipava ai nostri piccoli problemi, discuteva degli sviluppi futuri, dava consigli, si preoccupava delle ricerche in corso. Insomma ha vissuto compiutamente fino alla fine.

Ricordo che l'ultima volta che l'ho visto, pochi giorni prima della morte, un suo collaboratore gli portò il testo dattiloscritto di un suo lavoro. Egli lo prese e assicurò che lo avrebbe letto e commentato! E ancora: la lettera di dimissioni da Presidente di CCL fu firmata il giorno prima della morte "perché le mie attuali condizioni di salute mi impediscono di esplicare l'incarico in modo adeguato".

Questo era Umberto! Addio, continuerai a vivere nel ricordo di tutti noi! Potremo dire di te con il poeta: non omnis moriar multaque pars mei vitabit Libitinam.

Gianfranco Chiarotti Maggio 2000

persone con le quali avevo studiato durante il periodo univerisitario, e in particolare con Giancarlo Ruocco e Vittorio Mazzacurati. In questo ritrovarsi, la conversazione si focalizzó subito sul mio nuovo progetto, e in particolare sulle sue potenziali applicazioni nella fisica dei sistemi disordinati e sulla sua complementarietá ai neutroni e alle tecniche di simulazione numerica.

L’importanza di questo scambio di idee, iniziato nel 1990 e protrattosi fino a metá 1992, é stato di fondamentale importanza: risultó infatti chiaro che 7 meV di risoluzione erano assolutamente insufficienti per sperare di poter accedere con impatto e rilevanza a problemi aperti e di notevole interesse – 3 meV erano assolutamente necessari e possibilmente si doveva scendere a 1-2 meV. Convincersi di questo punto e’stato di cruciale importanza per tutta una serie di motivi logistici: Tornare in America?

Era fisicamente possibile spingere le tec-

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niche di diffrazione da cristalli perfetti al punto di poter ottenere ∆E/E~10-8?

“I Tedeschi” non erano riusciti a costruire analizzatori con risoluzione in energia meglio di 20 meV – potevamo noi arrivare a 1 meV? Utilizzando tecniche di diffrazione con angoli di Bragg vicini a 90o, per ottenere fino a 1 meV di risoluzione, si deve poter cambiare l’energia dei raggi X per accedere a differenti riflessioni del cristallo – Questo implica che la coesistenza dell’esperimento di IXS e di Mössbauer sulla stessa linea diventa impossibile. Quale sarebbe stato l’atteggiamento dei direttori ESRF dinanzi alla domanda di dedicare una intera linea allo scattering anelastico?

Grazie a infinite discussioni, calcoli (calcoletti e calcoloni), innumeri bottiglie di vino (del migliore) ed anche un intossicamento da nicotina (passivo e attivo) non indifferente, si venne a costituire fra noi tre un enorme entusiasmo basato sul convincimento che, non solo si poteva arrivare a spingere la risoluzione fino a 1 meV, ma anche che rimanevano abbastanza fotoni per poter fare degli esperimenti.

A capo delle discussioni sulla realizzabilitá tecnica di un tale strumento, la convinzione dell’importanza di dover scendere ad almeno 3 meV veniva dal fatto che usando l’IXS si poteva accedere alla regione di momento (Q) ed energia (E) scambiate caratteristiche delle zona in cui, in sistemi disordinati come vetri, liquidi e fluidi densi, ci si aspetta la transizione della dinamica collettiva da una situazione descrivibile in termini idrodinamici a quella caratteristica di particelle quasi-libere fra collisioni successive.

Questa transizione comporta cambiamenti qualitativi nel fattore di struttura dinamico, S(Q,E), che é la quantitá direttamente misurabile in una esperienza di scattering anelastico coerente quale quello dei raggi X. Il grande entusiasmo nel progetto proveniva dal fatto che, per motivi cinematici, questa zona della S(Q,E) é praticamente inaccessibile alle spetroscopie neutroniche ed era nota principalmente grazie alla simulazione numerica – conseguentemente ci stavamo convincendo che forse saremmo arrivati a mettere a

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punto un nuovo metodo sperimentale per accedere a una regione molto poco conosciuta ma di fondamentale interesse per capire la dinamica microscopica dei sistemi disordinati. La convinzione proveniva dal fatto che, facendo una stima sull’acqua, dove si tenne conto con attenzione ed in modo conservativo di tutti gli effetti strumentali e fisici, ottenemmo – in modo ineluttabile – un countrate integrato in energia di 1 conteggio/s con lo spettrometro operante a una risoluzione di 1 meV. Un tale segnale, confrontato con quello caratteristico di esperienze neutroniche, non é enorme ma é quanto basta per cominciare a lavorare!

Ed infatti segnó l’inizio dell’avventura: Ottenemmo il “divorzio” dal Mössbauer.

In parallelo a uno spettrometro simile a quello realizzato ad HASY-Lab, ottenemmo i fondi per costruire, come progetto ad alto rischio, uno spettrometro per spingere la risolzione fino a 1.5 meV – questo progetto venne catalogato come “in-House Research” e non come uno strumento per “Routine User ’s Operation”. La sua principale caratteristica é la lunghezza di 7 m del braccio rotante che contiene l’analizzatore sferico a cristallo di silicio. Nonostante le dimensioni, questo braccio deve avere riproducibilitá e precisione meccaniche nel range di 10-6 m e 10-6 rad.

Lanciammo un programma completamente nuovo per la costruzione di analizzatori a cristallo con grande accettanza angolare (quattro volte superiore alla tipica risoluzione in momento) e altissima risoluzione. Questo perché ci rendemmo conto che le soluzioni adottate precedentemente dai “Tedeschi” soffrivano di alcuni problemi di fondo che compromettevano in modo insolubile la risoluzione e l’accettanza angolare.

Il periodo 1992-95 fú tutto un fervore di disegni, preparativi, test e pesanti costruzioni con poli a Grenoble e L’Aquila. In questo periodo l’impegno di Vittorio su questo progetto fu quasi totale, ed, in particolare, con Giancarlo si prese la responsabilitá della costruzione del braccio. Nel Giugno 1994, un TIR targato L’Aquila approdava a Grenoble con l’intero sistema meccanico. Il seguito dell’avventura costituisce ormai dominio della letteratura scientifica ed é

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esemplificato da: i) Una lista di publicazioni tra il 1995 e il 2000 comprendente circa 25 lettere a Physical Review Letters, Nature and Science, ii) Lo spettrometro dell’Aquila é dall’inizio del suo funzionamento lo strumento piú richiesto dagli Users della linea ESRF di IXS – ID16 …. anche dai “Tedeschi”, iii) Una nuova linea é stata costruita all’ESRF con un braccio di 12 m, con il quale si vorrebbe arrivare a 0.8 meV di risoluzione in un prossimo futuro, ed infine iv) Spettrometri praticamente identici a quello di ID16 sono in costruzione all’Advanced Photon Source (Argonne) ed a SPRING-8 in Giappone. Tutto questo é stato possibile anche grazie all’impegno, all’interesse sia nella parte scientifica che strumentale, ed all’entusiasmo di Vittorio. Tutto questa confidenza é stata ampiamente ripagata: basti dire che oggi l’IXS viene considerato come uno dei grandi successi delle sorgenti di luce di sincrotrone di terza generazione, e come una nuova tecnica spettroscopica con vaste aree di applicazione.

Caro Vittorio, per fortuna abbiamo avuto il tempo di fare alcuni esperimenti insieme che ripagano il tuo lavoro, ma che, inoltre, segnano la storia scientifica dei sistemi disordinati! In particolare: Abbiamo svelato il mistero del suono veloce nell’acqua e abbiamo fatto vedere che esso corrisponde al limite elastico dove la dinamica collettiva microscopica del liquido e del cristallo diventano equivalenti. Abbiamo fatto vedere che nei vetri esistono modi collettivi propaganti ad alto Q. Questo, oltre a permettere di descrivere in modo completo la dinamica dei sistemi vetrosi, ha messo nel giusto contesto gli aspetti microscopici che influenzano le anomalie termodinamiche dei vetri rispetto ai cristalli corrispondenti – In particolare abbiamo messo la parola fine a tante teorie e idee, spesso ideate grazie alla mancanza del dato sperimentale, sulle proprietá dei sistemi disordinati. Questo lavoro continua e continuerá, purtroppo senza il tuo contributo, ma sicuramente nel tuo ricordo.

Francesco Sette ESRF – BP220 F-38043 Grenoble Cedex – France Tel: +33.4.7688.2224, e-mail: sette@esrf.fr

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TMR TMR

Training and Mobility of Researchers

Support for activities in the field of neutron scattering is available from the neutron round-table.

The neutron round-table is funded by the EC (DGXII) with approximately 100.000 Euro per year. The mission of the round-table is:

1.

To actively encourage

3.

5.

To support training of young scientists

The round-table consist of

co-ordination and collaboration

and other scientists, new to the

representatives from all major

between user facilities - such

field of neutron scattering about

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that the European users will

the potential of the method.

from EC supported networks

benefit through a better quality

developing novel

access to the European neutron

4.

scattering facilities.

national access to summer

user representatives appointed

schools, workshops, training

by ENSA (European Neutron

To spread the

courses, co-ordination activities

Scattering Association). The

knowledge about the

etc. Detailed information on

name of all contact persons can

potential of neutron scattering,

how and when to apply for

be found on the web page

and support studies on future

support can be found on the

mentioned above. The present

prospects with neutron

round-table web page:

chairman/co-ordinator of the

scattering.

http://www.risoe.dk/fys/TMR.htm

round-table is Kurt Nørgaard

and an increased quantity of

2.

The round-table

instrumentation and techniques

supports non-

for neutron scattering plus 5

Clausen, and can be contacted as kurt.clausen@risoe.dk

Vol. 5 n. 1 Giugno 2000

â&#x20AC;˘

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SCUOLE E CONVEGNI

ELETTRA Users Meeting Circa 170 ricercatori hanno partecipato al settimo Users’ Meeting di Elettra che si è tenuto nel Main Building dell’ICTP (Trieste) nei giorni 29 e 30 Novemnbre scorsi. L’idea di estendere lo Users’ meeting con un workshop satellite da tenersi nei giorni immediatamente precedenti o seguenti si è rivelata molto buona e circa 70 persone hanno partecipato al meeting satellite, quest’anno dedicato a "Reactions at Surfaces". Nei due interventi di apertura M. Altarelli e R. Walker hanno illustrato lo stato ed i progetti a medio e lungo termine sia della facility che della macchina. Nove beam-line sono completate, una è in stadio avanzato di commissioning ed altre nove sono in costruzione. Le prospettive a medio termine includono la costruzione di altre tre beam-line, il completamento

della fase di studio e l’implementazione di insertion device nelle sezioni dritte piu’ corte, la sostituzione dell’attuale Linac con un Linac più breve ed un anello di booster. Quest’ultimo dovrebbe garantire iniezioni più rapide e la possibilità di iniezioni con una procedura "top-up", in cui il fascio non viene mai completamente azzerato.

Le relazioni seguenti hanno illustrato i risultati delle varie ricerche svolte ad Elettra, che si estendono dagli studi su singoli atomi e molecole in fase gassosa alla ricostruzione della struttura di proteine ed alla microfabbricazione. W. Gudat , guest speaker per il 1999, ha poi illustrato lo stato di avanzamento della nuova sorgente di radiazione di sincrotrone tedesca Bessy II. Durante il meeting è stato assegnato il premio Fonda-Fasella a A. Riboldi-

ESRF Users Meeting 2000 Si è svolto nei giorni 10-12 febbraio 2000 il decimo Users’ Metting di ESRF. Come ormai consueto la riunione si è articolata in una giornata di relazioni plenarie e in tre workshop tematici.

Quest’anno i titoli dei workshop sono stati "Fast structural changes", "Surface Science 2000" e "Challenging problems in structural biology".

Durante la giornata comune vi sono state quattro relazioni scientifiche su invito che hanno avuto come tema recenti risultati ottenuti ad ESRF. E. Dooryhee (ESRF) ha spiegato come la diffrazione da polveri ha contribuito alla caratterizzazione dei cosmetici utilizzati dagli antichi egizi; questa relazione è un interessante esempio delle nuove e crescenti applicazioni della luce di sincrotrone alla archeometria. La seconda relazione su invito, di A. Liljas (Lund), ha avuto come tema la cristallografia macromolecolare, un settore in cui l’uso della luce di sincrotrone è tuttora in fortissima espansione; l’oratore ha parlato dei suoi recenti studi della funzione ribosomica. Il campo della alta pressione è stato oggetto della relazione di R. Luebbers (Paderborn) il

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quale ha applicato la diffusione anelastica nucleare per studiare il ferro ad alta pressione ed ha illustrato le ripercussioni geologiche dei risultati; questa relazione è un ulteriore esempio di come nuove tecniche con luce di sincrotrone vengono applicate a problemi di interesse per le Scienze della Terra.

Nell’ultima relazione, C. Meneghini (INFM – Grenoble) ha riportato i risultati di misure XAFS su manganiti perovskitiche, spiegando come essi mettono in luce la relazione fra struttura locale, magnetismo e magnetoresistenza colossale; questo lavoro illustra come la spettroscopia con luce di sincrotrone continui ad avere un ruolo di primo piano nella indagine dei sistemi elettronici correlati.

Oltre alle relazioni su invito vi sono state altre due comunicazioni a carattere scientifico. La prima è stata tenuta dal vincitore dello "Young Scientist Award" del 2000, R. Neutze (Uppsala); egli ha presentato una stimolante relazione riguardante lo sviluppo di tecniche con risoluzione temporale dal femto- al pico- secondo e la loro applicazione a problemi di interesse chimico e

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Vol. 5 n. 1 Giugno 2000

Tunniclife per il suo lavoro sulla struttura cristallina della proteina portatrice dell’infezione della "Legionella pneumophila". Questo premio stabilito quest’anno per la prima volta, in memoria di L. Fonda e P.M: Fasella, che hanno dato un fondametale contributo alla costruzione e sviluppo di Elettra, verrà attribuito ogni anno ad un giovane ricercatore per i risultati ottenuti ad Elettra.

L’assemblea degli utenti di Elettra ha eletto tre nuovi membri per il Comitato degli Utenti. M. Sancrotti, N. Zema e H. Hamenitsch sostituiranno G. Stefani, P. Carra e T. Prosperi. Mentre ringraziamo i membri precedenti del Comitato per il lavoro svolto, facciamo i nostri migliori auguri ai nuovi membri per il loro non facile ruolo di inerfaccia tra un’"effervescente" comunità di utenti e la facilty.

L. Avaldi

biologico. La seconda è stata tenuta da W. Thomlinson (ESRF) ed ha descritto le prime angiografie eseguite a Grenoble su pazienti umani; questi esperimenti sono stati effettuati nell’ambito di una studio condotto in collaborazione con l’ospedale di Grenoble, ed hanno suscitato grande interesse per le potenziali ricadute di notevole interesse medico e sociale.

Le tecniche con luce di sincrotrone hanno sempre maggiore interesse nel campo industriale. Per illustrare alcune di queste applicazioni tre ditte sono state invitate a spiegare in quale modo utilizzano la luce di sincrotrone: Unilever (detergenti), Aventis (biotecnologie) e L’Oreal (cosmetici).

Questa sessione è stata organizzata dall’apposito ufficio per le relazioni industriali di ESRF. Infine, lo Users Meeting è stato l’occasione per la presentazione del CDROM divulgativo "Synchrotron Light" prodotto da ESRF. Si tratta di uno stimolante e divertente CD interattivo il cui scopo è di illustrare le proprietà e le applicazioni della luce di sincrotrone; il CD sarà distribuito tra breve e può essere utilizzato a vari livelli di approfondimento.

F. Boscherini

SCUOLE E CONVEGNI

OTTAVO CONVEGNO SILS 29 Giugno-1 Luglio 2000 Università di Palermo – Palermo

Caro collega, ti ricordo la scadenza del 15/4/2000 per l’invio degli abstract all’indirizzo e-mail: sils2000@ictpn.pa.cnr.it.

Le sedi del convegno saranno due: la seduta inaugurale del 29/6/2000 si terrà presso la Sala delle Capriate, a Palazzo Steri, sede dell’Università di Palermo, piazza Marina 61.

Le sedute del 30/6 e 1/7/2000 avranno luogo presso la Sala Consiliare della Provincia di Palermo, Palazzo Comitini, via Maqueda 100.

Accludo un elenco di alberghi che offrono un prezzo scontato per i partecipanti al convegno e che sono abbastanza vicini alle due sedi (il più lontano è il Politeama, a circa 20 minuti a piedi). Ulteriori informazioni su alberghi, mappa della città, ecc. si possono trovare al sito: www.comune.palermo.it NB: ti consiglio di prenotare l’albergo con ampio anticipo, perché nel periodo del convegno l’afflusso turistico sarà notevole.

Antonino Martorana

Hotel

singola

doppia

Tel /Fax

Politeama Palace Hotel ****

180.000

250.000

322777 / 6111589

Grand Hotel des Palmes ****

180.000

250.000

540350 / 540330

Crystal Palace Hotel ***

140.000

198.000

6112580 / 112589

Hotel Tonic ***

100.000

130.000

581754 / 581754

Hotel Europa ***

115.000

170.000

6256323 / 256323

Albergo Mediterraneo ***

110.000

160.000

5811337 / 586974

Massimo Plaza Hotel ***

180.000

240.000

325657 / 325711

Grande Albergo Sole ***

150.000

200.000

6041510 / 110182

(prefisso 091)

I prezzi includono la prima colazione. Alla prenotazione, le offerte sono valide entro il 15/5/2000, fare riferimento al congresso SILS e/o convenzione CNR.

Vol. 5 n. 1 Giugno 2000

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SCUOLE E CONVEGNI

V Scuola di Spettroscopia Neutronica “Francesco Paolo Ricci” Diffusione Anelastica dei Neutroni Hotel Capo d’Orso - Località Cala Capra - Palau (SS) 23 settembre - 3 ottobre 2000 Generalità sullo scattering dei Neutroni Sorgenti e strumentazione Diffusione anelastica coerente: eccitazioni collettive Diffusione anelastica incoerente: spettroscopia vibrazionale Diffusione quasielastica: moti diffusivi Distribuzione di impulso nei sistemi classici e quantistici Dinamica microscopica dei liquidi Dinamica dei sistemi macromolecolari Spettroscopia di tunneling Applicazioni a: biologia, chimica, materiali

A. Albinati, Università di Milano C. Andreani, Università di Roma “Tor Vergata” U. Bafile, Istituto di Elettronica Quantistica, CNR, Firenze U. Balucani, Istituto di Elettronica Quantistica, CNR, Firenze M. Bée, Università “J. Fourier”, Grenoble R. Caciuffo, Università di Ancona C.J. Carlile, ILL, Grenoble D. Colognesi, CNR-ISIS, Chilton, U.K. M.T. Di Bari, UdR-INFM, Parma B. Dorner, ILL, Grenoble J. Eckert, Los Alamos, USA A. Paciaroni, OGG-INFM, Grenoble C. Petrillo, Politecnico di Milano F. Sacchetti, Università di Perugia U. Wanderlingh, Università di Messina

Il costo di partecipazione di Lit. 1.300.000 dà diritto alla frequenza delle lezioni, delle esercitazioni pratiche ed alla pensione completa presso l’Hotel Capo d’Orso (www.delphina.it/orso.htm) per tutta la durata della Scuola.

SCADENZA ISCRIZIONI: 30 GIUGNO 2000 Direttori A. Deriu, Dip. di Fisica, Università di Parma – M. Zoppi, Consiglio Nazionale delle Ricerche, IEQ, Firenze

Segreteria Organizzativa G. Ianni, Gruppo Nazionale Struttura della Materia del CNR, Roma

Spettroscopia Neutronica “F.P. Ricci” Diffusione Anelastica dei Neutroni

Informazioni generali e Modulo per l’iscrizione a. A partire da questa edizione la Scuola viene intitolata alla memoria del Prof. Francesco Paolo Ricci che ne era stato il promotore ed aveva sostenuto questa iniziativa fin dalla sua prima edizione nel 1981. b. La domanda di iscrizione deve essere fatta compilando il modulo di partecipazione reperibile presso il sito Web della Scuola. c. Oltre alle lezioni ufficiali la Scuola prevede seminari ed esercitazioni pratiche che completeranno il programma didattico. d. Il numero degli studenti è limitato a 30. Persone con provata esperienza nel campo potranno essere ammesse come osservatori. Dietro loro richiesta la segreteria potrà occuparsi della loro sistemazione alberghiera. e. L’accettazione delle iscrizioni e l’eventuale contributo verranno comunicati via e-mail entro il 31.07.2000. f. È disponibile un certo numero di borse per coprire il costo della partecipazione.

Per informazioni rivolgersi alla Segreteria della Scuola: Grazia Ianni - GNSM, Viale dell’Università 11, 00185 Roma Tel.: 06 4452258 - Fax: 06 4941159 - e-mail: ianni@axcasp.caspur.it Sito Web: http://SISN.unime.it/scuola_neutroni.html

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Hotel Capo d’Orso - Località Cala Capra - Palau (SS) 23 settembre - 3 ottobre 2000 MODULO D’ISCRIZIONE Nome …………………… Cognome ………………………

Posizione attualmente ricoperta (laureando, dottorando, borsista,…) …………………………………………………

Affiliazione ………………………………………………… ………………………………………………………………

Indirizzo

……………………………………………………

………………………………………………………………

Telefono …………………… Fax ……………………………

E-Mail …………………………………………………………

Campo di attività …………………………………………… …………………………………………………………………

La scheda va inviata entro il 30 giugno 2000 a: Grazia Ianni, GNSM, Viale dell’Università 11, 00185 Roma e-mail: ianni@axcasp.caspur.it • fax 06 4941159

Vol. 5 n. 1 Giugno 2000

CALENDARIO

10-12 luglio 2000

OXFORD, U.K.

2-6 ottobre 2000

The Sixth International Conference on Residual Stresses.

P. Farrelly, IoM Conferences & Events.

Tel: 44 171 4517391; Fax: 44 171 8392289 E-mail: Pauline_Farrelly@materials.org.uk

NOLPC 2000 - 8th International Conference on Nonlinear Optics of Liquid and Photo Refractive Crystals http://www.isp.kiev.ua

31 ottobre - 2 novembre 2000 26-29 luglio 2000

HALLE/SAALE, GERMANY

Many Particle Spectroscopy of Atoms, Molecules and Surfaces e-mail: jber@mpi-halle.de

NCM8, 8th International Conference on the Structure of Non-Crystalline Materials e-mail: ncm8@glass.demon.co.uk http://www.sgt.org

6-9 novembre 2000

BERLIN, GERMANY

27 novembre-1dicembre 2000

EDIMBURG, UK

MURCIA, SPAIN

European Conference on Iteration Theory Faculdad de Matematica, Campus de Espinardo Tel: 34 968 364176; Fax: 34 968 364182

4-16 settembre 2000

ROMA, ITALY

II Scuola Sperimentale di Diffrazione di raggi X a Dispersione di Energia (EDXD) ed Angolare (ADXD) Dipartimento di Chimica, Università “La Sapienza”

23 settembre - 3 ottobre 2000

BOSTON, MA, USA

MRS Fall Meeting http://dns.mrs.org

5th International Conference on Quasi-Elastic Neutron Scattering

4-9 settembre 2000

TSUKUBA, JAPAN

ICANS-XV: 15th Meeting of the International Collaboration on Advanced Neutron Sources

7th International Conference on Synchrotron Radiation Instrumentation http://sri2000.tu-berlin.de

31 agosto 1 settembre 2000

DENTON, USA

CAARI 2000: XVIth International Conference on the Application of Acceleratoes in Research and Industry http://www.phys.unt.edu/accelcon/

ABERYSTWYTH, WALES, U.K.

21-25 agosto 2000

IBARAKI, JAPAN

ASR 2000: 1st International Symposium on Advanced Science Research

1-4 novembre 2000 6-11 agosto 2000

CRIMEA, UKRAINE

9-13 settembre 2001

MUNCHEN, GERMANY

International Conference on Neutron Scattering 2001 (ICNS 2001) Physik Dept. E13, Technische Univ. München , D-85747 Garching, Germany Tel: +49 89 28912452; Fax: +49 89 289 12473 e-mail: info@icns2001.de http://www.icns2001.de

maggio 2002

NIST, USA

American Conference on neutron Scattering

PALAU (SS), ITALY

V Scuola di Spettroscopia Neutronica “F.P. Ricci” Diffusione Anelastica di neutroni

Vol. 5 n. 1 Giugno 2000

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SCADENZE

Scadenze per richieste di tempo macchina presso alcuni laboratori di Neutroni

Scadenze per richieste di tempo macchina presso alcuni laboratori di Luce di Sincrotrone

ISIS

ALS

La scadenza per il prossimo call for proposals è il 16 aprile 2000 e il 16 ottobre 2000

Le prossime scadenze sono il 15 marzo 2000 (cristallografia macromolecolare) e il 1 giugno 2000 (fisica)

ILL BESSY

La scadenza per il prossimo call for proposals è il 15 febbraio 2000 e il 15 agosto 2000

Le prossime scadenze sono il 15 febbraio 2000 e il 4 agosto 2000

LLB-ORPHEE-SACLAY La scadenza per il prossimo call for proposals è il 1 ottobre 2000 per informazioni: Secrétariat Scientifique du Laboratoire Léon Brillouin, TMR programme, Attn. Mme C. Abraham, Laboratoire Léon Brillouin, CEA/SACLAY, F-91191 Gif-sur-Yvette, France.

Tel: 33(0)169086038; Fax: 33(0)169088261 e-mail: abraham@bali.saclay.cea.fr http://www-llb.cea.fr

DARESBURY La prossima scadenza è il 30 aprile 2000 e il 31 ottobre 2000

ELETTRA Le prossime scadenze sono il 28 febbraio 2000 e il 31 agosto 2000

ESRF BENSC La scadenza è il 15 marzo 2000 e il 15 settembre 2000

Le prossime scadenze sono il 1 marzo 2000 e il 1 settembre 2000

GILDA

RISØ E NFL La scadenza per il prossimo call for proposals è il 1 aprile 2000

(quota italiana) Le prossime scadenze sono il 1 maggio 2000 e il 1 novembre 2000

HASYLAB (nuovi progetti) Le prossime scadenze sono il 1 marzo 2000, il 1 settembre 2000 e il 1 dicembre 2000

LURE La prossima scadenza è il 30 ottobre 2000

MAX-LAB La scadenza è approssimativamente febbraio 2000

NSLS Le prossime scadenze sono il 31 gennaio 2000, il 31 maggio 2000 e il 30 settembre 2000

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FACILITIES

LUCE DI SINCROTRONE SYNCHROTRON SOURCES WWW SERVERS IN THE WORLD (http://www.esrf.fr/navigate/synchrotrons.html)

DAFNE INFN Laboratori Nazionali di Frascati, P.O. Box 13, I-00044 Frascati (Rome), Italy tel: +39 6 9403 1 fax: +39 6 9403304 http://www.lnf.infn.it/ Tipo:P Status: C

ALS Advanced Light Source MS46-161, 1 Cyclotron Rd Berkeley, CA 94720, USA tel:+1 510 486 4257 fax:+1 510 486 4873 http://www-als.lbl.gov/ Tipo: D Status: O AmPS Amsterdam Pulse Stretcher NIKEF-K, P.O. Box 41882, 1009 DB Amsterdam, NL tel: +31 20 5925000 fax: +31 20 5922165 Tipo: P Status: C

DELTA Universität Dortmund,Emil Figge Str 74b, 44221 Dortmund, Germany tel: +49 231 7555383 fax: +49 231 7555398 http://prian.physik.uni-dortmund.de/ Tipo: P Status: C

APS Advanced Photon Source Bldg 360, Argonne Nat. Lab. 9700 S. Cass Avenue, Argonne, Il 60439, USA tel:+1 708 252 5089 fax: +1 708 252 3222 http://epics.aps.anl.gov/welcome.html Tipo: D Status: C

ELETTRA Sincrotrone Trieste, Padriciano 99, 34012 Trieste, Italy tel: +39 40 37581 fax: +39 40 226338 http://www.elettra.trieste.it Tipo: D Status: O

ASTRID ISA, Univ. of Aarhus, Ny Munkegade, DK-8000 Aarhus, Denmark tel: +45 61 28899 fax: +45 61 20740 Tipo: PD Status: O

ELSA Electron Stretcher and Accelerator Nußalle 12, D-5300 Bonn-1, Germany tel:+49 288 732796 fax: +49 288 737869 http://elsar1.physik.uni-bonn.de/elsahome.html Tipo: PD Status: O

BESSY Berliner Elektronen-speicherring Gessell.für Synchrotron-strahlung mbH Lentzealle 100, D-1000 Berlin 33, Germany tel: +49 30 820040 fax: +49 30 82004103 http://www.bessy.de Tipo: D Status: O

ESRF European Synchrotron Radiation Lab. BP 220, F-38043 Grenoble, France tel: +33 476 882000 fax: +33 476 882020 http://www.esrf.fr/ Tipo: D Status: O

BSRL Beijing Synchrotron Radiation Lab. Inst. of High Energy Physics, 19 Yucuan Rd.PO Box 918, Beijing 100039, PR China tel: +86 1 8213344 fax: +86 1 8213374 http://solar.rtd.utk.edu/~china/ins/IHEP/bsrf/bsrf.html Tipo: PD Status: O CAMD Center Advanced Microstructures & Devices Lousiana State Univ., 3990 W Lakeshore, Baton Rouge, LA 70803, USA tel:+1 504 3888887 fax: +1 504 3888887 http://www.camd/lsu.edu/ Tipo: D Status: O CHESS Cornell High Energy Synchr. Radiation Source Wilson Lab., Cornell University Ithaca, NY 14853, USA tel: +1 607 255 7163 fax: +1 607 255 9001 http://www.tn.cornell.edu/ Tipo: PD Status: O

EUTERPE Cyclotron Lab.,Eindhoven Univ. of Technol, P.O.Box 513, 5600 MB Eindhoven, The Netherlands tel: +31 40 474048 fax: +31 40 438060 Tipo: PD Status: C HASYLAB Notkestrasse 85, D-2000, Hamburg 52, Germany tel: +49 40 89982304 fax: +49 40 89982787 http://www.desy.de/pub/hasylab/hasylab.html Tipo: D Status: O INDUS Center for Advanced Technology, Rajendra Nagar, Indore 452012, India tel: +91 731 64626 Tipo: D Status: C

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FACILITIES

KEK Photon Factory Nat. Lab. for High Energy Physics, 1-1, Oho, Tsukuba-shi Ibaraki-ken, 305 Japan tel: +81 298 641171 fax: +81 298 642801 http://www.kek.jp/ Tipo: D Status: O Kurchatov Kurchatov Inst. of Atomic Energy, SR Center, Kurchatov Square, Moscow 123182, Russia tel: +7 95 1964546 Tipo: D Status:O/C

SOR-RING Inst. Solid State Physics S.R. Lab, Univ. of Tokyo, 3-2-1 Midori-cho Tanashi-shi, Tokyo 188, Japan tel: +81 424614131 ext 346 fax: +81 424615401 Tipo: D Status: O SRC Synchrotron Rad. Center Univ.of Wisconsin at Madison, 3731 Schneider DriveStoughton, WI 53589-3097 USA tel: +1 608 8737722 fax: +1 608 8737192 http://www.src.wisc.edu Tipo: D Status: O SRRC SR Research Center 1, R&D Road VI, Hsinchu Science, Industrial Parc, Hsinchu 30077 Taiwan, Republic of China tel: +886 35 780281 fax: +886 35 781881 http://www.srrc.gov.tw/ Tipo: D Status: O

LNLS Laboratorio Nacional Luz Sincrotron CP 6192, 13081 Campinas, SP Brazil tel: +55 192 542624 fax: +55 192 360202 Tipo: D Status: C LURE Bât 209-D, 91405 Orsay ,France tel: +33 1 64468014; fax: +33 1 64464148 E-mail: lemonze@lure.u-psud.fr http://www.lure.u-psud.fr Tipo: D Status: O

SSRL Stanford SR Laboratory MS 69, PO Box 4349 Stanford, CA 94309-0210, USA tel: +1 415 926 4000 fax: +1 415 926 4100 http://www-ssrl.slac.stanford.edu/welcome.html Tipo: D Status: O

MAX-Lab Box 118, University of Lund, S-22100 Lund, Sweden tel: +46 46 109697 fax: +46 46 104710 http://www.maxlab.lu.se/ Tipo: D Status: O NSLS National Synchrotron Light Source Bldg. 725, Brookhaven Nat. Lab., Upton, NY 11973, USA tel: +1 516 282 2297 fax: +1 516 282 4745 http://www.nsls.bnl.gov/ Tipo: D Status: O NSRL National Synchrotron Radiation Lab. USTC, Hefei, Anhui 230029, PR China tel:+86 551 3601989 fax:+86 551 5561078 Tipo: D Status: O Pohang Pohang Inst. for Science & Technol., P.O. Box 125 Pohang, Korea 790600 tel: +82 562 792696 f +82 562 794499 Tipo: D Status: C

SRS Daresbury SR Source SERC, Daresbury Lab, Warrington WA4 4AD, U.K. tel: +44 925 603000 fax: +44 925 603174 E-mail: srs-ulo@dl.ac.uk http://www.dl.ac.uk/home.html Tipo: D Status: O SURF B119, NIST, Gaithersburg, MD 20859, USA tel: +1 301 9753726 fax: +1 301 8697628 http://physics.nist.gov/MajResFac/surf/surf.html Tipo: D Status: O TERAS ElectroTechnical Lab. 1-1-4 Umezono, Tsukuba Ibaraki 305, Japan tel: 81 298 54 5541 fax: 81 298 55 6608 Tipo: D Status: O UVSOR Inst. for Molecular ScienceMyodaiji, Okazaki 444, Japan tel: +81 564 526101 fax: +81 564 547079 Tipo: D Status: O

Siberian SR Center Lavrentyev Ave 11, 630090 Novosibirsk, Russia tel: +7 383 2 356031 fax: +7 383 2 352163 Tipo: D Status: O SPring-8 2-28-8 Hon-komagome, Bunkyo-ku ,Tokyo 113, Japan tel: +81 03 9411140 fax: +81 03 9413169 Tipo: D Status: C

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D = macchina dedicata; PD = parzialmente dedicata; P = in parassitaggio.

O= macchina funzionante; C=macchina in costruzione.

D = dedicated machine; PD = partially dedicated; P = parassitic.

O= operating machine; C= machine under construction.

Vol. 5 n. 1 Giugno 2000

FACILITIES

NEUTRONI NEUTRON SCATTERING WWW SERVERS IN THE WORLD (http://www.isis.rl.ac.uk)

BENSC Berlin Neutron Scattering Center, Hahn-Meitner-Institut, Glienicker Str. 100, D- 14109 Berlin-Wannsee, Germany Rainer Michaelsen; tel: +49 30 8062 3043 fax: +49 30 8062 2523 E - Mail: michaelsen@hmi.de http://www.hmi.de BNL Brookhaven National Laboratory, Biology Department, Upton, NY 11973, USA Dieter Schneider; General Information: Rae Greenberg; tel: +1 516 282 5564 fax: +1 516 282 5888 http://neutron.chm.bnl.gov/HFBR/ GKSS Forschungszentrum Geesthacht, P.O.1160, W-2054 Geesthacht, Germany Reinhard Kampmann; tel: +49 4152 87 1316 fax: +49 4152 87 1338 E-mail: PWKAMPM@DGHGKSS4 Heinrich B. Stuhrmann; tel: +49 4152 87 1290 fax: +49 4152 87 2534 E-mail: WSSTUHR@DGHGKSS4 IFE Institut for Energiteknikk, P.O. Box40, N-2007 Kjeller, Norway Jon Samseth; tel: +47 6 806080 fax: +47 6 810920 telex: 74 573 energ n E-mail: Internet JON@BARNEY.IFE.NO ILL Institute Laue Langevin, BP 156, F-38042, Grenoble Cedex 9,France Herma Büttner; tel: +33 76207179 E-mail: sco@ill.fr fax: +33 76 48 39 06 http://www.ill.fr IPNS Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4814, USA P.Thiyagarajan,Bldg.200,RM. D125; tel :+1 708 9723593 E-mail: THIYAGA@ANLPNS Ernest Epperson, Bldg. 212; tel: +1 708 972 5701

fax: +1 708 972 4163 or + 1 708 972 4470 (Chemistry Div.) http://pnsjph.pns.anl.gov/ipns.html ISIS The ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxfordshire OX11 0QX, UK Richard Heenan; tel +44 235 446744 E-mail: RKH@UK.AC.RUTHERFORD.DEC-E Steve King; tel: +44 235 446437 fax: +44 235 445720; Telex: 83 159 ruthlb g E-mail: SMK@UK.AC.RUTHERFORD.DEC-E http://www.isis.rl.ac.uk JAERI Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-11, Japan.

Jun-ichi Suzuki (JAERI); Yuji Ito (ISSP, Univ. of Tokyo); fax: +81 292 82 59227 telex: JAERIJ24596 http:// neutron-www.kekjpl JINR Joint Institute for Nuclear Research, Laboratory for Neutron Physics, Head P.O.Box 79 Moscow, 141 980 Dubna, USSR A.M. Balagurov; E-mail: BALA@LNP04.JINR.DUBNA.SU Yurii M. Ostaneivich; E-mail: SANS@LNP07.JINR.DUBNA.SU fax: +7 095 200 22 83 telex: 911 621 DUBNA SU http://www.jinr.dubna.su KFA Forschungszentrum Jülich, Institut für Festkörperforschung, Postfach 1913, W-517 Jülich, Germany Dietmar Schwahn; tel: +49 2461 61 6661; E-mail: SCHWAHN@DJUKFA54.BITNET Gerd Maier; tel: +49 2461 61 3567; E-mail: MEIER@DJUKFA54.BITNET fax: +49 2461 61 2610 telex: 833556-0 kf d

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FACILITIES

LLB Laboratoire Léon Brillouin, Centre d’Etudes Nucleaires de Saclay, 91191 Gif-sur-Yvette Cédex France J.P Cotton (LLB); tel: +33 1 69086460 fax: +33 1 69088261 telex: energ 690641 F LBS+ E-mail: COTTON@BALI.CEA.FR http://bali.saclay.cea.fr/bali.html NIST National Institute of Standards and TechnologyGaithersburg, Maryland 20899 USA C.J. Glinka; tel: + 301 975 6242 fax: +1 301 921 9847 E-mail: Bitnet: GLINKA@NBSENTH Internet: GLIMKA@ENH.NIST.GOV http://rrdjazz.nist.gov ORNL Oak Ridge National Laboratory Neutron Scattering Facilities, P.O. Box 2008, Oak Ridge TN 37831-6393 USA George D. Wignall, Small Angle Scattering Group Leader; tel: +1 423 574 5237 fax: +1 423 574 6268 E-mail: wignallgd@ornl.gov http://neutrons.ornl.gov PSI Paul Scherrer Institut Wurenlingen und Villingen CH-5232 Villingen PSI tel: +41 56 992111 fax: +41 56 982327 RISØ EC-Large Facility Programme, Physics Department, Risø National Lab.P.O. Box 49, DK-4000 Roskilde, Denmark K. Mortenses; tel: +45 4237 1212 fax: +45 42370115 E-mail: CLAUSEN@RISOE.DK or SANS@RISOE.DK NFL-Studsvik in Sweden E-mail: mcgreevy@studsvik.uu.se

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Vol. 5 n. 1 Giugno 2000