Effect of SiO 2 Addition on Microwave Dielectric Properties of Ca 0.61 La 0.39 Al 0.39 Ti 0.61 O 3

International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 www.ijm‐me.org doi: 10.14355/ijmme.2016.05.005

Effect of SiO2 Addition on Microwave Dielectric Properties of Ca0.61La0.39Al0.39Ti0.61O3 Ceramics1 Hao Li1, Bin Tang *2, Lin Liu3, Shuren Zhang4 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of china, Chengdu 610054, China *1

haouestc@gmail.com; *2tangbin@uestc.edu.cn;34923778334@qq.com;4zsr@uestc.edu.cn

Abstract The effects of SiO2 addition on microwave dielectric properties of Ca0.61La0.39Al0.39Ti0.61O3 ceramics prepared by a conventional solid‐state ceramic route were investigated. The x‐ray diffraction (XRD) results showed that all the samples with less than 2.0 wt% SiO2 addition exhibited only a single phase Ca0.61La0.39Al0.39Ti0.61O3 with tetragonal structure formed. Scanning electron microscopy (SEM) images of SiO2 doping ceramics sintered at 1340 ℃ for 12h showed a compact microstructure. The silica entered the crystal lattice or was diffused to the interstitial sites in ceramic matrix causing the increase of the lattice parameters.

SiO2 addition formed the small ambiguous grains, which effectively lowered the sintering temperature. The SiO2‐doped Ca0.61La0.39Al0.39Ti0.61O3 ceramics have a low sintering temperature of 1340°C and a near zero τf value. Good microwave dielectric properties with a εr = 41.5, Q × f = 39,491 GHz and τf = ‐0.1 ppm/°C are obtained for 0.02wt% SiO2 doped ceramics sintered at 1340 ℃ for 12h.

Keywords Ca0.61La0.39Al0.39Ti0.61O3; Microwave ceramics; Dielectric properties; Microstructure

Introduction With the development of modern communication systems, microwave dielectric ceramics have been a hotspot in the past two decades. They are frequently used for the production of passive microwave components like substrates for integrated circuits, mounting, dielectric filters, dielectric antennas and dielectric resonators.1, 2 In particular, the microwave dielectric materials used in the base stations of telecommunications are required to have high quality factor (Q × f) value for high power, high dielectric constant (εr) for miniaturization and low temperature coefficient of resonant frequency (τf) for frequency stability.1, 3, 4 Commonly, it is tough to acquire microwave dielectric materials with high εr and high Q × f simultaneously because of the generally negative correlation between εr and Q × f. The cation deficient perovskites such as Ba5Nb4O15 1 and Ba5‐xSrxNb4O15 2, exhibit dielectric constant between 40 and 50 and high Q × f, but their high τf diminish their practical applicability. In our previous studies,7, 8 we have developed ceramics systems with high εr and Q × f with potential applicability, for example, Zr(Zn1/3Nb2/3)0.6Ti1.4O6: εr = 41.7, Q × f = 42,100 GHz and τf = ‐15.5 ppm/°C and BaTi4O9‐BaZn2Ti4O11: εr = 36.4, Q × f = 62,600 GHz and τf = +0.2 ppm/°C. However, they did not perform very well due to the excess negative τf value or relatively small εr. Current progress in the field of wireless telecommunications has resulted in demands for temperature stable materials (τf ≈ 0 ppm/°C) with at least εr ~ 40 and Q × f ~ 40,000 because the requirements of narrow bandwidth and extremely low insertion loss (< 0.3dB). Fortunately, notable contributions to the literature on microwave dielectrics found that the MAlO3‐NTiO3 (M = La, Nd; N = Ca, Sr) ceramics showed high permittivity and high quality factor with low τf .9‐12 Many researches have indicated that by mixing two or more ceramics which show positive and negative τf, respectively, could yield a new solid solutions or composite dielectric material with near‐zero τf .3 It has already

1

This paper was selected from the 2nd International Conference on Advance Materials Research and Application (AMRA 2015).

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been demonstrated that most rare‐earth aluminates have suitable permittivity, high quality factors and a large negative τf value.4 LaAlO3 is a rhombohedral perovskite with εr ~ 22, Q × f ~ 46,000 GHz and τf ~ ‐44 ppm/°C.5 CaTiO3 (εr ~ 170, Q × f ~ 3500 GHz, τf = 800 ppm/°C)5 has often been used in controlling microwave dielectric properties and the sintering behavior.6 Relative permittivity higher than 40 can be obtained with high quality factors and a temperature stable τf in LaAlO3‐CaTiO3 systems. Moon et al.7 studied the sintering behavior of LaAlO3‐CaTiO3 ceramics and the microwave dielectric properties of εr ~ 37, Q × f ~ 47,000GHz and τf = 5 ppm/°C were obtained at 0.65LaAlO3‐0.35CaTiO3. Suvorov et al.8 reported dielectric properties of εr ~ 44, Q × f ~ 30,000 GHz and τf ~ ‐3 ppm/°C for undoped 0.7CaTiO3‐0.3LaAlO3 ceramics and the sintering temperature was as high as 1450°C for 12h. Recently, Hou et al.9 proposed that the sintering temperature should be 1550°C when x>0.5 in (1‐ x)CaTiO3‐xLaAlO3 ceramic systems. Therefore, in order to improve the sinterability, microstructure and microwave dielectric properties, LaAlO3‐CaTiO3 ceramics have been modified by Al2O3,10 CuO and MnCO3 11 additives.

However, only undesirable microwave dielectric properties of Q × f ~ 38,000GHz with τf ~ 12 ppm/°C 10 and Q × f ~ 32,800 GHz with τf ~ 10 ppm/°C11 were reported in the doped LaAlO3‐CaTiO3 ceramics.

Adding a sintering aid with a low melting temperature generally can help reduce the sintering temperature of the dielectric ceramic materials.21, 22 For several researches, SiO2 sintering aids were chosen to improve the dielectric properties and reduce the sintering temperature of microwave ceramics.23, 24 SiO2 concentration was found critical for obtaining uniform directional domain microstructures in BaTiO3 ceramic.12 Although silica addition is very interesting as silica atoms are very small in size and they can easily diffuse at grain boundaries or react with the main phase, however, the dielectric properties and microstructure of LaAlO3‐CaTiO3 ceramics with the addition of SiO2 have not been well understood. In this study, effect of SiO2 addition on sintering performance, microstructure and microwave dielectric properties of LaAlO3‐CaTiO3 ceramics was investigated. In addition, X‐ray diffraction (XRD) patterning and scanning electron microscopy (SEM) analysis were also used to study the crystal structures and microstructures of the ceramics.

Experimental procedures The samples used in this study were prepared by conventional solid‐state reaction technique. The starting materials were reagent grade powders (purity>99%) CaCO3, La2O3, Al2O3 and TiO2. The starting materials were weighed according to ratio of 0.61CaTiO3‐0.39LaAlO3. The mixtures of the powders were milled in plastic jars using deionized water and zirconia balls for 4h. The powders were calcined at 1100ºC for 2h in air. Then the calcined powders were re‐milled for 4 h with 0.02‐2 wt% SiO2 of >99.5% purity. The fine powder with 3wt% of a 10% solution of PVA as a binder was pressed into pellets with dimensions of 30mm in diameter and 10mm in thickness under the pressure of 30 MPa. These pellets were sintered at a high sintering temperatures ranging from 1260 to 1380 oC for 12h in air.

The bulk densities of the sintered samples were measured by the Archimedes method. The phase composition was identified by X‐ray diffraction (XRD) using CuKα radiation (Philips x’pert Pro MPD, Netherlands). Microstructure observation was conducted on the surface of samples by using scanning electron microscopy (SEM, FEI Inspect F, United Kingdom). The dielectric characteristics at microwave frequencies were measured by the Hakki–Coleman dielectric resonator method in the TE011 mode using a network analyzer (Agilent Technologies E5071C, the United States) and a temperature chamber (DELTA 9023, Delta Design, USA). The temperature coefficient of resonant frequency was determined from the difference between the resonant frequency obtained at 25ºC and 85ºC using the equation:  f

 ( f t2  f t1 ) ( f t1  (t 2  t1 )) , where f t1 and f t2 are the resonant frequencies at t1=25°C and t2=85°C,

respectively.

Results and discussion Figure 1 shows the XRD patterns of Ca0.61La0.39Al0.39Ti0.61O3 ceramics doped with different SiO2 addition sintered at1340oC for 12 h. In all cases, only Ca0.61La0.39Al0.39Ti0.61O3 phase (ICDD‐PDF #52‐1773) with a slight difference in peak intensity was found. It is observed that presented ceramics belongs to the tetragonal‐type crystal structure,

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(400) (330) (420)

(312)

(220)

(222)

Intensity

(110)

(211) (202)

(200)

the present findings seem to be consistent with other research which found Ca0.7Ti0.7La0.3Al0.3O3 ceramics has the same type of crystal structure when excess Al2O3 up to 0.5 wt% was added.10 No obvious peak of secondary phase is found for the samples with additive SiO2 in weight percent of 0.02 to 2.00. It suggested that the silica formed a solid solution with Ca0.61La0.39Al0.39Ti0.61O3. Identical XRD patterns were obtained for the ceramics, irrespective of the amount of SiO2 addition. Therefore, SiO2 can be used as a useful additive to lower the sintering temperature of Ca0.61La0.39Al0.39Ti0.61O3 ceramics without changing the phase component.

2.00 wt% 1.00 wt% 0.20 wt% 0.02 wt% 20

40

60 2(degrees)

80

FIG.1 THE XRD PATTERNS OF CA0.61LA0.39AL0.39TI0.61O3 CERAMICS WITH VARYING AMOUNTS OF SIO2 ADDITIONS SINTERED AT 1340℃ FOR 12H

a-axis ()

5.412 5.410 5.408 5.406

3

Cell volume ( )

c-axis ()

7.668 7.656 7.644 7.632 224.4 224.0 223.6 223.2 0

5

10 15 SiO2 (wt%)

20

FIG.2 THE SIO2 DOPING DEPENDENCE OF THE REFINED LATTICE PARAMETERS OF CA0.61LA0.39AL0.39TI0.61O3 CERAMICS SINTERED AT 1340℃ FOR 12H

After more careful observation, we found there was a movement of the peaks for Ca0.61La0.39Al0.39Ti0.61O3 phase, so the lattice parameters and unit cell volume for the solid solution Ca0.61La0.39Al0.39Ti0.61O3 phase were calculated and the results are shown in Fig. 2. When increasing the doping amount of SiO2 from 0.02wt% to 2.00wt%, both the a‐ axis and c‐axis are raised, and thus the unit cell volume is increased. It’s well known that if the small silica atoms would incorporate into the lattice sites, a decrease of the lattice parameters will be observed. However, the Si4+

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ionic would be hard to substitute for other elements of Ca0.61La0.39Al0.39Ti0.61O3 ceramics, since the ionic radius of Si4+ (0.26 Å) is much smaller than the ionic radius of Ca2+ (0.99 Å) , Al3+ (0.50 Å) , La3+ (1.06 Å) or Ti4+ (0.68 Å).13 It seems possible that the expansion of the lattice are due to the Si4+ ionic was inserted into the lattice of crystal14 or was diffused to the interstitial sites.15 Therefore, the incorporation of SiO2 into the crystal lattice of Ca0.61La0.39Al0.39Ti0.61O3 ceramics resulted in increases of lattice parameters of the crystal.

(a)

(b)

(c)

(d)

(e)

FIG.3 SEM PHOTOGRAPHS OF CA0.61LA0.39AL0.39TI0.61O3 CERAMICS WITH (A)NO SIO2 SINTERED AT 1380℃, (B) 0.02WT%, (C) 0.20WT%, (D) 1.00WT% AND (E) 2.00WT% SINTERED AT 1340℃ FOR 12H

The SEM photographs of Ca0.61La0.39Al0.39Ti0.61O3 ceramics sintered at 1380°C for 12h and SiO2‐doped samples with sintering temperatures at 1340°C for 12h are shown in Fig. 3. It can be seen that, for the SiO2‐free samples [Fig. 3(a)], the ceramic has a relatively dense and uniform microstructure. When SiO2 was added, the dense microstructure was obtained at the sintering temperature with a reduction of 40°C. The pores are lowered by adding 0.02wt% SiO2, which would improve the bulk density and dielectric properties of the samples. Comparing the microstructure of samples without and with SiO2 additive, the ambiguous grains in relatively small size could be observed at the grain boundary by the grain morphology, as shown by the white arrows. It is noted that with the increasing amount of SiO2, the amount of ambiguous grains increased. This indicated that during the sintering at the high temperature SiO2 could cause the formation of ambiguous grains and effectively enhance the sintering ability of Ca0.61La0.39Al0.39Ti0.61O3 ceramics. At the level of 0.02 wt % SiO2 additive [Fig. 3(b)], a little of ambiguous grains was found in the surface, but as SiO2 content was increased further to 2.00 wt%, the amount of ambiguous grains was increased obviously. For the grain size and porosity, the grain size and porosity shown in Fig. 3(b) was less than

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that in Fig. 3(a), because although the ambiguous grains could promote the sintering process, the reverse effect caused by the impurities in the ceramic matrix played the major role in the sintering process. For Ca0.61La0.39Al0.39Ti0.61O3 ceramics with additions of SiO2, Fig. 3(b)–(e) showed that grain sizes become apparently smaller, which should be attributed to too much small ambiguous grains being formed during sintering as a result of the excessive addition of SiO2. This also accords with other research,16 which showed that SiO2 can inhibit the grain growth of microwave ceramics. Moreover, the porosity increased as increasing the amount of SiO2 additive.

With 2.00 wt% SiO2 additions [Fig. 3(e)], porous specimens were observed for Ca0.61La0.39Al0.39Ti0.61O3 ceramics sintered at 1340oC and this was just due to the ambiguous grains. The sample with 0.02wt% SiO2 addition sintered at 1340°C for 3 h shows dense and uniform microstructure as well as low porosity.

(a) 4.9 

Non-doped 0.02wt% 0.20wt% 1.00wt% 2.00wt%

4.8 4.7 42 (b)

r

41 40 39

Qf

60000 (c) 40000 20000 0 (d) 0

f

-8 -16 -24 1260

1300

1340

1380

Sintering Temperature(℃) FIG.4 THE BULK DENSITY AND MICROWAVE DIELECTRIC PROPERTIES OF CA0.61LA0.39AL0.39TI0.61O3 CERAMICS WITH VARYING AMOUNTS OF SIO2 ADDITIONS SINTERED AT DIFFERENT TEMPERATURES FOR 12H

The bulk density of Ca0.61La0.39Al0.39Ti0.61O3 ceramics doped with various amounts of SiO2 as a function of sintering temperature is shown in Fig. 4a. For all these samples except the sample with no SiO2, when the sintering temperature is above 1340°C, they show a decrease in bulk density due to over‐sintering. Meanwhile, the bulk densities decreased with increasing of SiO2 content. As the SiO2 content increased from 0.02wt% to 2.00wt%, the apparent density of specimens sintered at 1340°C decreased from 4.93 g/cm3 to 4.87 g/cm3. It can be evidenced by comparing the SEM images that the decreased densification could be due to the formation of small ambiguous grains and pores in the ceramics.

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The dielectric constant of Ca0.61La0.39Al0.39Ti0.61O3 ceramics doped with various amounts of SiO2 as a function of sintering temperature is illustrated in Fig. 4b. The change of dielectric constant occurred in similar manner as densities, since higher bulk densities means to lower porosity and small grains. Increasing sintering temperature will improve the dielectric constant by increasing density and decreasing porosity. The occurrence of over‐ sintering, however, will result in the decrease of density and dielectric constant when sintered at excessively high temperatures. As we know, the εr value was strongly determined by pores (εr ≈ 1) in samples before the ceramics is well sintered. As SiO2 addition increases, the dielectric constant was declined by the increasing amount of trapped porosity.17 Therefore, it can be also understand that the εr value of specimens sintered at 1340°C decreased from 41.5 to 40.3 with the changing of SiO2 content from 0.02wt% to 2.00wt%.

Fig.4c shows the Q × f value of Ca0.61La0.39Al0.39Ti0.61O3 ceramics with varying amounts of SiO2 additions sintered at different temperatures for 12h. It can be seen that the variation of Q × f value showed a similar tendency with the bulk density. On one hand, when increasing the sintering temperature, the Q × f value of the SiO2 doped ceramics were increased firstly and reached the maximum at 1340°C, then declined with sintering temperature further increased, but for no SiO2 sample, the Q × f value strongly raised till 1380°C. These changes were believed to correlate with the compactness of the samples, and a higher Q × f value was usually corresponding to a denser microstructure.17 On the other hand, as the SiO2 content was increasing from 0.02wt% to 2.00wt%, a sustained downward trend in the Q × f value decreased from 39,491 GHz to 19,027 GHz was observed, as illustrated in Figure 4c. The decrease of the Q × f value with increasing SiO2 can be attributed to the relative lower Q × f value of pores and small ambiguous grains comparing with Ca0.61La0.39Al0.39Ti0.61O3 which has a Q × f value about 47,000 GHz.7 Fig. 4d shows the temperature coefficient of the resonant frequency of Ca0.61La0.39Al0.39Ti0.61O3 ceramics sintered at different temperatures as a function of the SiO2 content. As increasing SiO2 contents, the τf value shifts negative direction. In general, the τf is related to the composition18, secondary phases19, porosity20 and crystal lattice21. When SiO2 content was increased from 0.02wt% to 2.00wt%, the τƒ value of specimens sintered at 1340°C was observed to decrease from ‐0.1 ppm/°C to ‐7.6 ppm/°C. When SiO2 content was 0.02wt%, a near zero τƒ value in this system was obtained. Combining Figs. 2 and 3, it is reasonably believed that the porosity and distortion of crystal lattice could be responsible for the τƒ variation, because the SiO2 addition did not change the composition of this material system.

Conclusions The sintering temperature of Ca0.61La0.39Al0.39Ti0.61O3 ceramics were reduced by SiO2 doping from 1380°C to 1340°C without obvious degradation of the microwave dielectric properties. Only a single phase Ca0.61La0.39Al0.39Ti0.61O3 with tetragonal structure forms in the ceramics with less than 2.0 wt% SiO2 sintered at 1340°C, which indicated that the silica formed a solid solution with Ca0.61La0.39Al0.39Ti0.61O3. Good microwave dielectric properties with a εr = 41.5, Q × f = 39,491 GHz and τf = ‐0.1 ppm/°C are obtained for 0.02wt% SiO2 doped ceramics sintered at 1340°C for 12h , which shows dense and uniform microstructure as well as low porosity.

ACKNOWLEDGMENT

This work is supported by National Natural Science Funds of China (Grant No. 51402039) REFERENCES

[1]

H. Sreemoolanadhan, J. Isaac, S. Solomon, et al. ʺDielectric properties of Ba5Nb4O15 ceramic.ʺ Physica status solidi (a) 143 (1994): 45‐48

[2]

R. Ratheesh, H. Sreemoolanadhan and M. T. Sebastian. ʺVibrational Analysis of Ba5‐xSrxNb4O15 Microwave Dielectric Ceramic Resonators.ʺJournal of Solid State Chemistry 131(1997): 2‐8

40

International Journal of Material and Mechanical Engineering (IJMME), Volume 5 2016 www.ijm‐me.org

[3]

Y.‐C. Chen, S.‐M. Tsao, C.‐S. Lin, et al. ʺMicrowave dielectric properties of 0.95MgTiO3‐0.05CaTiO3 for application in dielectric resonator antenna.ʺ Journal of Alloys and Compounds 471 (2009): 347‐351

[4]

S.‐Y. Cho, I.‐T. Kim and K. S. Hong. ʺMicrowave dielectric properties and applications of rare‐earth aluminates.ʺ Journal of Materials Research 14 (1999): 114‐119

[5]

R. C. Kell, A. C. Greenham and G. C. E. Olds. ʺHigh‐Permittivity Temperature‐Stable Ceramic Dielectrics with Low Microwave Loss.ʺ Journal of the American Ceramic Society 56 (1973): 352‐354

[6]

M. Tuhkala, J. Juuti and H. Jantunen.ʺAn indirectly coupled open‐ended resonator applied to characterize dielectric properties of MgTiO3‐CaTiO3 powders.ʺ Journal of Applied Physics 115(2014): 184101‐184105

[7]

J. H. Moon, H. M. Jang, H. S. Park, et al. ʺSintering behavior and microwave dielectric properties of (Ca,La)(Ti,Al)O3 ceramics.ʺ Japanese Journal of Applied Physics 38(1999): 6821‐6826

[8]

D. Suvorov, M. Valant, B. Jancar, et al. ʺCaTiO3‐based ceramics: Microstructural development and dielectric properties.ʺ ACTA CHIMICA SLOVENICA 48(2001):87‐100

[9]

G. Hou, Z. Wang and F. Zhang. ʺSintering behavior and microwave dielectric properties of (1‐x) CaTiO3‐xLaAlO3 ceramics.ʺ Journal of Rare Earths 29 (2011): 160‐163

[10] G. A. Ravi, F. Azough and R. Freer. ʺEffect of Al2O3 on the structure and microwave dielectric properties of Ca0.7Ti0.7La0.3Al0.3O3.ʺJournal of the European Ceramic Society 27 (2007): 2855‐2859 [11] J. Zheng and et al.ʺInfluence of CuO or MnCO3 additive on the dielectric properties of Ca0.65Ti0.65La0.35Al0.35O3 Ceramics.ʺ Journal of Physics: Conference Series 152 (2009): 012062 [12] G. S. Liu and R. D. Roseman. ʺEffect of BaO and SiO2 addition on PTCR BaTiO3 ceramics.ʺ Journal of Materials Science 34 (1999): 4439‐4445 [13] R. D. Shannon and C. T. Prewitt.ʺEffective ionic radii in oxides and fluorides.ʺ Acta Crystallographica Section B 25 (1969): 925‐946 [14] X. Lu, Y. Zheng and B. Zhou. ʺEffect of B2O3 addition on the microwave dielectric properties of Li2ZnTi3O8 ceramic.ʺ Journal of Electroceramic 31 (2013): 360‐363 [15] L.‐C. Chang and B.‐S. Chiou. ʺEffect of B2O3 Nano‐Coating on the Sintering Behaviors and Electrical Microwave Properties of Ba(Nd2‐xSmx)Ti4O12 Ceramics.ʺ Journal of Electroceramics 13(2004):829‐837 [16] S.‐F. Wang, S.‐J. Wang, Y.‐R. Wang, et al. ʺEffect of SiO2 addition on the microstructure and microwave dielectric properties of ultra‐low fire TiTe3O8 ceramics.ʺ Ceramics International 35 (2009): 1813‐1817 [17] S. Yu, S. Zhang, B. Tang, et al. ʺMicrowave dielectric properties of BaO‐2(1‐x)ZnO‐xNd2O3‐4TiO2 (x=0‐1.0) ceramics.ʺ Ceramics International 38 (2012): 613‐618 [18] Y. Li, H. Li, J. Li, et al.ʺEffect of TiO2 Ratio on the Phase and Microwave Dielectric Properties of Li2ZnTi3+xO8+2x Ceramics.ʺ Journal of Electronic Materials 43 (2014): 1107‐1111 [19] C.‐L. Huang, R.‐J. Lin and J.‐F. Tzeng. ʺDielectric properties of copper oxide doped 0.95Ba(Zn1/3Ta2/3)O3‐0.05BaZrO3 ceramics at microwave frequency.ʺ Materials Chemistry and Physics 97(2006): 256‐260 [20] Y. Lv, R. Zuo and Z. Yue.ʺStructure and microwave dielectric properties of Ba3(VO4)2‐Zn2‐xSiO4‐x ceramic composites.ʺ Materials Research Bulletin 48(2013):2011‐2017 [21] H.‐f. Zhou, H. Wang, D. Zhou, et al. ʺEffect of ZnO and B2O3 on the sintering temperature and microwave dielectric properties of LiNb0.6Ti0.5O3 ceramics.ʺ Materials Chemistry and Physics 109 (2008): 510‐514

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