The effect of homogeneous dispersion of Carbon Nano Tubes on Mechanical properties of Aluminium-Carb

Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

The effect of homogeneous dispersion of Carbon Nano Tubes on Mechanical properties of Aluminium-Carbon Nano Tube nanocomposite-A review 1

1,2

Lubna Rais, 2Rajnish Sharma

Chitkara University, Himachal Pradesh, India.

Abstract:- The present work reviews the work done in the field of synthesis of Aluminium-Carbon Nano Tube nanocomposite using few dominant techniques. The homogeneous dispersion of Carbon Nano Tubes (CNTs) on Aluminium (Al) matrix is a major issue in improving the mechanical properties of Al-CNT nanocomposite. The fact that the importance of CNT dispersion depends on mixing technique used is highlighted. The effect of uniform dispersion on different mechanical properties of Al-CNT nanocomposite is also summarized. The effect of volume percentage of CNTs used in a composite is also discussed.

Keywords: CNTs, mixing techniques, Aluminium.

INTRODUCTION In order to develop metal matrix nanocomposite (MMC) significant efforts have been taken till now [1-12]. Many disadvantages that existed in conventional MMCs such as poor conductivity, lower toughness and tensile strength etc. diminished with the arrival of reinforcement of MMC with CNTs. The researchers have very efficiently used the unusual electrical, thermal and mechanical properties of CNTs in transistors [13], inter connects [14], artificial muscle fibers [15], super capacitors [16] etc. The properties of the MMCs can be considerably increased by using nanoparticles like CNTs. The improvement in properties like thermal conductivity, strength and lower thermal expansion coefficient in MMC due to reinforcement of CNTs has attracted automobile industry, spacecraft industry, and even VLSI industry [17].Several methods have been developed by researchers for homogeneous mixing of the CNTs and Aluminium matrix ranging from conventional ball milling mixing technique to novel technique like molecular level mixing. These studies have reported enormous improvement in the thermal, mechanical and electrical properties of the resulting composite over the metal matrix [18]. Having a seamless cylindrical structure, CNTs prove to be the most desirable reinforcement material for Al matrix. But the improvement in mechanical properties of the composite depends on the homogeneous dispersion of CNTs over metal matrix [19].

The present study investigates about the improvement in the properties of the Aluminium metal composite when CNTs are used as the reinforcement material. It illustrates the various fabrication techniques as well as the effect of homogeneous dispersion of CNTs on various properties of the Al-CNT nanocomposite.

Al-CNT powder preparation Different proportions of Al and CNTs were used to create specimen varying from 0.5 wt % CNT to 5wt% CNT [21-

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28]. The different CNT dispersion and powder mixing techniques used to create specimen are discussed below.

Method for uniform CNT dispersion Nano Scale Dispersion In this process, energy at ultrasonic frequency is applied to agitate particles in process control agents like distilled water, ethanol, ethyl alcohol etc. The ultrasound waves propagate through the medium in the form of shock waves agitating the tubular particles of CNTs which get released from their entanglement. These sonicators run on low power (50W) as well as high power (100-1500W) [20]. In their study Seung I. Cha et al, sonicated CNTs for 24h in acid bath to functionalize them [27]. Prior to mechanical milling, R. P´erez-Bustamante et al, also sonicated powder mix for 5min for uniform dispersion of CNTs [26].

Techniques used for mixing of Aluminium and CNT powders: Spray Drying: An alloy of Al and Si used as matrix reinforced with CNTs (> 95% purity & 40-70nm dia.). The process involved dispersing the 5 wt % CNTs within the agglomerates of Al-Si alloy powder by spraying CNT powder at supersonic velocity by a carrier gas from a nozzle on to the Al alloy. This mix was then mixed with 99.7% pure Al powder in a turbula mixer for 1h to form spray powder [21].

Roll milling: This process was followed for the mixing of Al powder(99.85% purity, 14.8µm particle size) and CNTs(99.5% purity, 20nm diameter)by pouring these powders in a Roll mill having cylindrical rollers opposite to each other or flat plates which crush the material to a particle size of 2nm[22].

Dry ball milling: MWCNTs (50nm dia.) and Al (10 µm diameters,99.5% purity) were poured in a cylindrical container having spheres of steel and this ball mill was run at 423 rpm with 20:1 wt. ratio to process spherical Al particles in flakes. This was done to convert 3D structure of Al particles into 2D to match up to the 1D nature of CNTs. Further, the CNTs and Al flakes were mixed using powder ball milling for 2h at 5000C. Prior to milling ultrasonication is also done to the mix[28].

Wet ball milling: In wet ball milling the liquid process control agent (PCA) ,which avoids sticking of powder to container walls and balls, is used along with metal balls. In experiment carried out by A.M.K. Esaki et al, ball milling of Al(99.7% purity ) and MWCNTs(140nm diameter) was done for 6h at 200rpm in presence of methanol(PCA)[24].

Nouari Saheb also used this method to mix Al6061 and Al2124 with CNTs(24nm) in presence of steel balls(10mm) for 1h at 200rpm[23]. In another study NITTTR, Chandigarh

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A.M.K. Esawi et al, used this method for 3min at 400rpm in presence of methanol[25].

High energy shaking: In their investigation, R.P´erez Bustamante et al, mixed Al(99.9% purity) and MWCNT by using mechanical milling in high energy shaker mill at different milling times[26].The container having materials to be mixed is shaken back and forth at the rate of 1080 cycles per minute.

Ultrasonication: After sonication of only CNTs the mixture of Alumina and functionalized CNTs was sonicated for 24h in molecular mixing method carried out

by Seung I. Cha et al . This was followed by vaporization and oxidation at 3500C for 6h[27].

Techniques for processing of synthesized composite powder Cold Spray method: The spray powder prepared using spray drying method was again injected into a high pressure powder feeder. The spray powder was then injected into a converging-diverging nozzle which sprayed the powder on to the Al substrate. The substrate was fixed on a microcontrolled X-Y transverse table. In order to obtain desired thickness eight layers were sprayed[21].The process of cold spray is depicted in fig 1.

STRUCTURAL CHARACTERIZATION OF AL-CNT NANOCOMPOSITE Densification:According to investigation done by Nouari Saheb, the sintering of specimen with 1 wt % CNT and 2wt % CNT resulted in increase in densification at different temperatures. It was found that in Al6061+ 1wt%CNT composite almost 100% densification was achieved at 5000C. This happened due to decrease in number of pores due to increase in diffusion rate at higher temperature[23].In another study by R. P´erez-Bustamante et al, the densification of Al-CNT composite was found to be 99-95% for almost all specimen tested at different milling time[26].Complete densification was also observed in molecular mixing method for synthesis of Al-CNT composite[27].

A summary of effect of mixing on morphology of Al and CNT and dispersion of CNTs on Al matrix on the basis of SEM, TEM and XRD analysis is given in Table I shown below[21-28].

TABLE I Mixing Technique

Extrusion: In order to test the various mechanical properties of the specimen prepared using above discussed methods it is required to create desired cross sectional profiles out of material mass. It is passed though a die of desired cross section under compressing and shear stress.

This forms parts with excellent surface finish .

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Morphology CNT dispersion Aluminium powder

CNTs Slightly shortened due to impact and shearing between Al-Si particles and Al matrix

Spray Drying

Deformed

Roll milling

Maintained spherical morphology and particle size

No deformation reported

Uniform with agglomerations at certain regions due to removal of NR

Wet milling

Decreased particle size owing to increased milling time(3 to 6h)

No damage

Well dispersed

High energy shaking

In form

Slightly worn out at outer dia

uniform

Molecular level mixing

-

No damage

homogeneous

Figure 1 Steps involved in composite coating.

Sintering: After heating the powder prepared using roll milling it was heat treated in a quartz tube furnace(5000C) for about 2h. Finally, sintering is done at about 6000C in which the powdered material is solidified due to high heat and pressure application without getting liquefied [22].

The powder mixed using wet ball milling by Nouari Saheb in his experiment was also sintered under 35MPa pressure and different temperatures(4000C, 4500C, 5000C) for 20 minutes[23].The specimen prepared by R. P´erezBustamante et al, were sintered at 823K for 3h[26]. Spark plasma sintering was used by Sueng I. Cha et al in their study[27].Lin Jiang et al also used sintering at 5500C for 2h[28].

Compaction: The powdered composite is compacted in a die cavity of fixed dimension by applying high pressure from top and bottom so that the particles squeeze together closely and surface welding takes place. Most of the researchers have used this method at different pressures ranging from 475MPa[24,25], 500MPa[28]and 1Pa vacuum pressure at 1000C[27].

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ball

Uniformly distributed

PROPERTIES Microhardness In their investigation Srinivasa R. Bakshi et al, found that the hardness of the Al-CNT nanocomposite has slightly increased as compared to pure Aluminium.[21] The results of Vicker’s test shown in fig.2 obtained by A.M.K. Esawi et al, showed that the hardness of all the milled samples(pure Al and Al + 2 wt % CNT) has increased approximately three times as compared to the unmilled Al. But this result did not any enhancement in hardness due to addition of CNTs. This happened because of increasing milling time to 6 hours, which resulted in reduction of grain size of pure Al to about 48 nm. But due to CNT 8

Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

reinforcement, the grain size of Al-CNT composite particle was found to be greater (̴ 56nm) even milling. This establishes the fact that although addition of CNTs to Al matrix did not increased the hardness as expected but slightly larger grain size after milling confirms retained microstructure[24].

The results of hardness test obtained by Nouari Saheb, brought about another fact that on increasing the volume content of CNTs in the composite or increasing the sintering temperature did not help in improving the hardness. In this study, hardness was found to be increased only in Al6061 +1 wt%CNT specimen when sintered at 4500C. This may be attributed to the increase in clustering or agglomeration of CNTs with the increase in wt.%[23]. This was also reported by Seung I. Cha et al, where harness increased with 1wt % CNTs but decreased with 2wt % CNT due to clustering of CNTs[27].The milling time in investigation done by R. P´erez-Bustamante was kept limited( 2 hours). The result of hardness test showed increase in hardness with the increase in wt % of CNTs[26].

Figure 2. Vicker’s micro-indentation and nano-indentation hardness of un-milled and milled pure Al and Al–2 wt% CNT extruded samples [24].

Figure 3 Tensile performance of CNT/Al composites achieved by flake PM: (a) Tensile curves of the CNT/Al composites and the relevant Al matrix; (b) Young’s modulus and engineering tensile strength as a function of the volume fraction of CNTs[28].

in uniform distribution of CNTs over Al matrix[24].Hansang Kwon et al, found 300% increase in tensile strength compared to pure Al. Prior to addition of CNTs, extrusion of pure Al did not showed any increase in Tensile strength, therefore, this much improvement can be attributed to addition of CNTs. Presence of traces of Al4C3 also strengthened the material by efficient stress transfer effect owing chemical bonding between CNTs and Al matrix[22].

Young’s Modulus: In their study Lin Jiang et al, found that Young’s Modulus increased with the increase in vol. % of CNTs owing to extremely uniform distribution of CNTs over Al matrix as shown in fig.3[28] In another study by A.M.K. Esawi et al, the increase in Young’s Modulus in the specimen having lower wt % CNT may be attributed to the uniform distribution of CNTs. Its value for specimen with higher wt % CNT was found to be lesser as expected due to clustering of CNTs. Also use of CNTs with smaller diameter made it difficult to disperse [25].

Elastic modulus: Srinivasa R. Bakshi et al, in their investigation established that the elastic modulus of the composite depends on densification. Elastic modulus varied in different regions , with porous regions possessing lower elastic modulus while regions rich in CNTs it ranged between 229 GPa to 191 GPa[21].

Tensile strength: Investigation on Al –CNT nanocomposite carried out by A.M.K. Esawi et al, revealed that the tensile strength of Al-CNT composite increased as compared to the pure Al. Results for specimens with 0.5 wt% CNT to 2 wt % CNT showed increased tensile strength. But further increasing the CNT volume did not result in increase in tensile strength due to the fact that CNTs have tendency to agglomerate[25]. In flake powder metallurgy method of Al-CNT composite fabrication, Lin Jiang et al, showed that with the increase in vol. % of CNTs the tensile strength increased twice the pure Al as shown in fig.3. This attributed to the uniform distribution of CNTs on the Al flakes[28]. In another investigation carried out by A.M.K. Esawi et al, to increase the tensile strength of the Al-CNT nano composite, the milling time was reduced from 6 h to 3h at 4000C. It had to be annealed after extrusion which further helped

CONCLUSION The homogeneous dispersion of CNTs was achieved in most of the investigations when lower volume percent of CNTs was used .The increase in volume % of CNTs (more than 2 wt%) didn’t help in improvement of the mechanical properties. The change in 3D nature of Al particles successfully achieved the aim of uniform dispersion and enhancement in tensile strength was achieved even when more than 2 wt % CNT was used. This established that the more surface contact between CNT and Al particle the more is the mechanical strength and densification. The bonding between CNTs and Al at molecular level is also aspect that resulted in homogeneous dispersion and hence improvement in properties.

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