Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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The effect of homogeneous dispersion ofCarbon Nano Tubes on Mechanical properties

of Aluminium-Carbon Nano Tubenanocomposite-A review

1Lubna Rais, 2Rajnish Sharma1,2Chitkara University, Himachal Pradesh, India.

Abstract:- The present work reviews the work done in thefield of synthesis of Aluminium-Carbon Nano Tubenanocomposite using few dominant techniques. Thehomogeneous dispersion of Carbon Nano Tubes (CNTs) onAluminium (Al) matrix is a major issue in improving themechanical properties of Al-CNT nanocomposite. The factthat the importance of CNT dispersion depends on mixingtechnique used is highlighted. The effect of uniformdispersion on different mechanical properties of Al-CNTnanocomposite is also summarized. The effect of volumepercentage of CNTs used in a composite is also discussed.

Keywords: CNTs, mixing techniques, Aluminium.

INTRODUCTIONIn order to develop metal matrix nanocomposite (MMC)significant efforts have been taken till now [1-12]. Manydisadvantages that existed in conventional MMCs such aspoor conductivity, lower toughness and tensile strength etc.diminished with the arrival of reinforcement of MMC withCNTs. The researchers have very efficiently used theunusual electrical, thermal and mechanical properties ofCNTs in transistors [13], inter connects [14], artificialmuscle fibers [15], super capacitors [16] etc. Theproperties of the MMCs can be considerably increased byusing nanoparticles like CNTs. The improvement inproperties like thermal conductivity, strength and lowerthermal expansion coefficient in MMC due toreinforcement of CNTs has attracted automobile industry,spacecraft industry, and even VLSI industry [17].Severalmethods have been developed by researchers forhomogeneous mixing of the CNTs and Aluminium matrixranging from conventional ball milling mixing technique tonovel technique like molecular level mixing. These studieshave reported enormous improvement in the thermal,mechanical and electrical properties of the resultingcomposite over the metal matrix [18]. Having a seamlesscylindrical structure, CNTs prove to be the most desirablereinforcement material for Al matrix. But the improvementin mechanical properties of the composite depends on thehomogeneous dispersion of CNTs over metal matrix [19].The present study investigates about the improvement inthe properties of the Aluminium metal composite whenCNTs are used as the reinforcement material. It illustratesthe various fabrication techniques as well as the effect ofhomogeneous dispersion of CNTs on various properties ofthe Al-CNT nanocomposite.

Al-CNT powder preparationDifferent proportions of Al and CNTs were used to createspecimen varying from 0.5 wt % CNT to 5wt% CNT [21-

28]. The different CNT dispersion and powder mixingtechniques used to create specimen are discussed below.Method for uniform CNT dispersion

Nano Scale DispersionIn this process, energy at ultrasonic frequency is applied to

agitate particles in process control agents like distilledwater, ethanol, ethyl alcohol etc. The ultrasound wavespropagate through the medium in the form of shock wavesagitating the tubular particles of CNTs which get releasedfrom their entanglement. These sonicators run on lowpower (50W) as well as high power (100-1500W) [20]. Intheir study Seung I. Cha et al, sonicated CNTs for 24h inacid bath to functionalize them [27]. Prior to mechanicalmilling, R. P´erez-Bustamante et al, also sonicated powdermix for 5min for uniform dispersion of CNTs [26].Techniques used for mixing of Aluminium and CNTpowders:Spray Drying: An alloy of Al and Si used as matrixreinforced with CNTs (> 95% purity & 40-70nm dia.). Theprocess involved dispersing the 5 wt % CNTs within theagglomerates of Al-Si alloy powder by spraying CNTpowder at supersonic velocity by a carrier gas from anozzle on to the Al alloy. This mix was then mixed with99.7% pure Al powder in a turbula mixer for 1h to formspray powder [21].Roll milling: This process was followed for the mixing ofAl powder(99.85% purity, 14.8µm particle size) andCNTs(99.5% purity, 20nm diameter)by pouring thesepowders in a Roll mill having cylindrical rollers oppositeto each other or flat plates which crush the material to aparticle size of 2nm[22].Dry ball milling: MWCNTs (50nm dia.) and Al (10 µmdiameters,99.5% purity) were poured in a cylindricalcontainer having spheres of steel and this ball mill was runat 423 rpm with 20:1 wt. ratio to process spherical Alparticles in flakes. This was done to convert 3D structureof Al particles into 2D to match up to the 1D nature ofCNTs. Further, the CNTs and Al flakes were mixed usingpowder ball milling for 2h at 5000C. Prior to millingultrasonication is also done to the mix[28].Wet ball milling: In wet ball milling the liquid processcontrol agent (PCA) ,which avoids sticking of powder tocontainer walls and balls, is used along with metal balls. Inexperiment carried out by A.M.K. Esaki et al, ball millingof Al(99.7% purity ) and MWCNTs(140nm diameter) wasdone for 6h at 200rpm in presence of methanol(PCA)[24].Nouari Saheb also used this method to mix Al6061 andAl2124 with CNTs(24nm) in presence of steelballs(10mm) for 1h at 200rpm[23]. In another study

Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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A.M.K. Esawi et al, used this method for 3min at 400rpmin presence of methanol[25].High energy shaking: In their investigation, R.P´erezBustamante et al, mixed Al(99.9% purity) and MWCNTby using mechanical milling in high energy shaker mill atdifferent milling times[26].The container having materialsto be mixed is shaken back and forth at the rate of 1080cycles per minute.Ultrasonication: After sonication of only CNTs themixture of Alumina and functionalized CNTs wassonicated for 24h in molecular mixing method carried outby Seung I. Cha et al . This was followed by vaporizationand oxidation at 3500C for 6h[27].

Techniques for processing of synthesized compositepowder

Cold Spray method: The spray powder prepared usingspray drying method was again injected into a highpressure powder feeder. The spray powder was theninjected into a converging-diverging nozzle which sprayedthe powder on to the Al substrate. The substrate was fixedon a microcontrolled X-Y transverse table. In order toobtain desired thickness eight layers were sprayed[21].Theprocess of cold spray is depicted in fig 1.

Figure 1 Steps involved in composite coating.

Sintering: After heating the powder prepared using rollmilling it was heat treated in a quartz tube furnace(5000C)for about 2h. Finally, sintering is done at about 6000C inwhich the powdered material is solidified due to high heatand pressure application without getting liquefied [22].The powder mixed using wet ball milling by Nouari Sahebin his experiment was also sintered under 35MPa pressureand different temperatures(4000C, 4500C, 5000C) for 20minutes[23].The specimen prepared by R. P´erez-Bustamante et al, were sintered at 823K for 3h[26]. Sparkplasma sintering was used by Sueng I. Cha et al in theirstudy[27].Lin Jiang et al also used sintering at 5500C for2h[28].Compaction: The powdered composite is compacted in adie cavity of fixed dimension by applying high pressurefrom top and bottom so that the particles squeeze togetherclosely and surface welding takes place. Most of theresearchers have used this method at different pressuresranging from 475MPa[24,25], 500MPa[28]and 1Pavacuum pressure at 1000C[27].

Extrusion: In order to test the various mechanicalproperties of the specimen prepared using above discussedmethods it is required to create desired cross sectionalprofiles out of material mass. It is passed though a die ofdesired cross section under compressing and shear stress.This forms parts with excellent surface finish .

STRUCTURAL CHARACTERIZATION OF AL-CNTNANOCOMPOSITE

Densification:According to investigation done by NouariSaheb, the sintering of specimen with 1 wt % CNT and2wt % CNT resulted in increase in densification atdifferent temperatures. It was found that in Al6061+1wt%CNT composite almost 100% densification wasachieved at 5000C. This happened due to decrease innumber of pores due to increase in diffusion rate at highertemperature[23].In another study by R. P´erez-Bustamanteet al, the densification of Al-CNT composite was found tobe 99-95% for almost all specimen tested at differentmilling time[26].Complete densification was also observedin molecular mixing method for synthesis of Al-CNTcomposite[27].A summary of effect of mixing on morphology of Al andCNT and dispersion of CNTs on Al matrix on the basis ofSEM, TEM and XRD analysis is given in Table I shownbelow[21-28].

TABLE I

PROPERTIESMicrohardnessIn their investigation Srinivasa R. Bakshi et al, found thatthe hardness of the Al-CNT nanocomposite has slightlyincreased as compared to pure Aluminium.[21] The resultsof Vicker’s test shown in fig.2 obtained by A.M.K. Esawiet al, showed that the hardness of all the milledsamples(pure Al and Al + 2 wt % CNT) has increasedapproximately three times as compared to the unmilled Al.But this result did not any enhancement in hardness due toaddition of CNTs. This happened because of increasingmilling time to 6 hours, which resulted in reduction ofgrain size of pure Al to about 48 nm. But due to CNT

MixingTechnique

MorphologyCNT dispersion

Aluminiumpowder

CNTs

Spray Drying DeformedSlightlyshortened dueto impact andshearingbetween Al-Siparticles and Almatrix

Uniformlydistributed

Roll milling Maintainedsphericalmorphologyand particlesize

No deformationreported

Uniform withagglomerations atcertain regions dueto removal of NR

Wet ballmilling

Decreasedparticle sizeowing toincreasedmilling time(3to 6h)

No damage Well dispersed

High energyshaking

In form Slightly wornout at outer dia

uniform

Molecularlevel mixing

- No damage homogeneous

Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

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reinforcement, the grain size of Al-CNT composite particlewas found to be greater (̴ 56nm) even milling. Thisestablishes the fact that although addition of CNTs to Almatrix did not increased the hardness as expected butslightly larger grain size after milling confirms retainedmicrostructure[24].The results of hardness test obtained by Nouari Saheb,brought about another fact that on increasing the volumecontent of CNTs in the composite or increasing thesintering temperature did not help in improving thehardness. In this study, hardness was found to be increasedonly in Al6061 +1 wt%CNT

specimen when sintered at 4500C. This may be attributedto the increase in clustering or agglomeration of CNTswith the increase in wt.%[23]. This was also reported bySeung I. Cha et al, where harness increased with 1wt %CNTs but decreased with 2wt % CNT due to clustering ofCNTs[27].The milling time in investigation done by R.P´erez-Bustamante was kept limited( 2 hours). The resultof hardness test showed increase in hardness with theincrease in wt % of CNTs[26].

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

Elastic modulus: Srinivasa R. Bakshi et al, in theirinvestigation established that the elastic modulus of thecomposite depends on densification. Elastic modulusvaried in different regions , with porous regions possessinglower elastic modulus while regions rich in CNTs it rangedbetween 229 GPa to 191 GPa[21].

Tensile strength: Investigation on Al –CNTnanocomposite carried out by A.M.K. Esawi et al, revealedthat the tensile strength of Al-CNT composite increased ascompared to the pure Al. Results for specimens with 0.5wt% CNT to 2 wt % CNT showed increased tensilestrength. But further increasing the CNT volume did notresult in increase in tensile strength due to the fact thatCNTs have tendency to agglomerate[25]. In flake powdermetallurgy method of Al-CNT composite fabrication, LinJiang et al, showed that with the increase in vol. % ofCNTs the tensile strength increased twice the pure Al asshown in fig.3. This attributed to the uniform distributionof CNTs on the Al flakes[28]. In another investigationcarried out by A.M.K. Esawi et al, to increase the tensilestrength of the Al-CNT nano composite, the milling timewas reduced from 6 h to 3h at 4000C. It had to be annealedafter extrusion which further helped

Figure 3 Tensile performance of CNT/Al composites achieved by flakePM: (a) Tensile curves of the CNT/Al composites and the relevant Al

matrix; (b) Young’s modulus and engineering tensile strength as afunction of the volume fraction of CNTs[28].

in uniform distribution of CNTs over Almatrix[24].Hansang Kwon et al, found 300% increase intensile strength compared to pure Al. Prior to addition ofCNTs, extrusion of pure Al did not showed any increase inTensile strength, therefore, this much improvement can beattributed to addition of CNTs. Presence of traces ofAl4C3 also strengthened the material by efficient stresstransfer effect owing chemical bonding between CNTs andAl matrix[22].Young’s Modulus: In their study Lin Jiang et al, found thatYoung’s Modulus increased with the increase in vol. % ofCNTs owing to extremely uniform distribution of CNTsover Al matrix as shown in fig.3[28] In another study byA.M.K. Esawi et al, the increase in Young’s Modulus inthe specimen having lower wt % CNT may be attributed tothe uniform distribution of CNTs. Its value for specimenwith higher wt % CNT was found to be lesser as expecteddue to clustering of CNTs. Also use of CNTs with smallerdiameter made it difficult to disperse [25].

CONCLUSIONThe homogeneous dispersion of CNTs was achieved inmost of the investigations when lower volume percent ofCNTs was used .The increase in volume % of CNTs (morethan 2 wt%) didn’t help in improvement of the mechanicalproperties. The change in 3D nature of Al particlessuccessfully achieved the aim of uniform dispersion andenhancement in tensile strength was achieved even whenmore than 2 wt % CNT was used. This established that themore surface contact between CNT and Al particle themore is the mechanical strength and densification. Thebonding between CNTs and Al at molecular level is alsoaspect that resulted in homogeneous dispersion and henceimprovement in properties.

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