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Influence of functionalization onmechanical and electrical propertiesof carbon nanotube-based silvercompositesHemant Palab, Vimal Sharmaa & Manjula Sharmaa

a Department of Physics, National Institute of Technology,Hamirpur (H.P.) 177005, Indiab Department of Physics, Government College Chamba, Chamba(H.P.) 176310, IndiaPublished online: 08 Apr 2014.

To cite this article: Hemant Pal, Vimal Sharma & Manjula Sharma (2014) Influence offunctionalization on mechanical and electrical properties of carbon nanotube-based silvercomposites, Philosophical Magazine, 94:13, 1478-1492, DOI: 10.1080/14786435.2014.892221

To link to this article: http://dx.doi.org/10.1080/14786435.2014.892221

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Influence of functionalization on mechanical and electrical propertiesof carbon nanotube-based silver composites

Hemant Pala,b, Vimal Sharmaa* and Manjula Sharmaa

aDepartment of Physics, National Institute of Technology, Hamirpur (H.P.) 177005, India;bDepartment of Physics, Government College Chamba, Chamba (H.P.) 176310, India

(Received 26 July 2013; accepted 31 January 2014)

In this study, we have extended the molecular-level mixing method tofabricate multiwall carbon nanotube (CNT)-reinforced silver nanocomposites.The multiwall nanotubes used in the synthesis process were dispersed by twoways viz. covalent and non-covalent functionalization techniques. To elucidatethe comparative effects of functionalization, structural, mechanical and electri-cal properties of nanocomposites were evaluated before and after sintering.The structural characterization revealed that the nanotubes were embedded,anchored and homogenously dispersed within the silver matrix. Hardness andYoung’s modulus of nanotube-reinforced nanocomposite were increased by afactor of 1–1.6 times than that of pure silver, even before and after the sinter-ing. Covalently functionalized nanotube-based composites have shown moreenhanced mechanical properties. The CNT reinforcement also improved theelectrical conductivity of low-conducting nanosilver matrix before sintering.Non-covalently functionalized nanotube-based nanosilver composites showedmore increased electrical conductivity before sintering. But a negative rein-forcement effect was observed in high-conducting bulk silver matrix after thesintering. Thus, covalent functionalization might be appropriate for mechanicalimprovement in low-strength materials. However, non-covalent functionaliza-tion is suitable for electrical enhancement in low-conducting nanomaterials.

Keywords: metal matrix nanocomposites; carbon nanotubes; mechanical prop-erties; electrical conductivity; functionalization

1. Introduction

Silver (Ag) is an attractive metal due to its extremely high thermal and electricalconductivity. Silver and silver-graphite composites are used in electronics as intercon-nectors in integrated circuits, plug coaters, electric brushes and circuit breakers [1,2].Carbon nanotubes (CNTs) are considered as versatile filler for fabrications of electricalcontact materials instead of graphite by virtue of their unique properties. CNTs are char-acterized by their excellent properties: real density similar to that of a polymer, betterelectrical conductivity of individual CNT (107–108 S/m) than that of copper, excellentelasticity, high aspect ratio (50–1000) and mechanical strength better than that of steel[3,4]. CNT-based metal matrix composites (MMCs) have been reviewed critically byBakshi et al. to summarize the state of art of this field [5]. They have shown that the

*Corresponding author. Email: vml16sharma@yahoo.co.in

© 2014 Taylor & Francis

Philosophical Magazine, 2014Vol. 94, No. 13, 1478–1492, http://dx.doi.org/10.1080/14786435.2014.892221

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mechanical, thermal and electrical properties of CNTs could be exploited byincorporating them into some form of metal matrix. Most of the studies are focused onCNT/Cu nanocomposite. However, a limited research has been reported in preparationand characterization of CNT-based silver nanocomposites. Early study of Yi Feng et al.on MWCNT/Ag composite by powder metallurgy technique showed increased hardness,density and electrical resistivity of synthesized nanocomposites up to 8 vol.% CNTs [6].Daush et al. recently studied MWCNT/Ag nanocomposites using chemical reductionmethod. The hardness and elongation properties have reported to increase up to 7.5volume content of MWCNTs [7].

The main goal of fabricating CNT/metal nanocomposite is to take advantage of theexceptional properties of CNTs in MMCs. But, there have been very little investigationon MMCs based on CNT reinforcement due to difficulties in obtaining homogeneousdispersion of CNTs in metallic matrix and their undesirable reactions with metal matrix.However, the recently developed novel fabrication process known as molecular-levelmixing, which can be used for achieving a homogeneous dispersion of CNTs in metalmatrix materials, is quite advantageous [8]. This is due to covalent bonding at theCNT/metal interface, as a result of the reaction between surface-modified CNTs andmetal ions at molecular level. CNTs are spontaneously bundled because of the strongVander Waals interaction. Functionalization has been widely used to facilitate the dis-persion of nanotubes in solvents as a prelude to the preparation of composite materialsand can also help to improve bonding between nanotubes and matrix. The CNTs can bedispersed either by covalent or non-covalent functionalization techniques. In covalentfunctionalization acid treatment results in the formation of carboxylic acid and othergroups on the tube surface. These acid groups can further be modified in a variety ofways. It may improve both the dispersion and interfacial bonding of CNTs with metalmatrix, but it significantly degrades the physical properties of CNTs. On the other hand,non-covalent functionalization of CNTs is achieved by Vander Waals interactions. Thismakes it possible to exclude any significant degradation of the inherent properties ofCNTs [9,10]. However, comparative investigation on the effect of functionalization ofCNTs on mechanical and electrical properties of MMCs has not yet been performed.

In the present study, MWCNT/Ag nanocomposites were fabricated by modifiedmolecular-level mixing method followed by sintering process. The influence ofcovalently functionalized multiwall carbon nanotubes (C-MWCNTs) and non-covalentlyfunctionalized multiwall nanotubes (N-MWCNTs) reinforcement on hardness, Young’smodulus and electrical conductivity of MWCNT/Ag nanocomposite have beenevaluated before and after the sintering process.

2. Experimental procedure

2.1. Fabrication of CNT/Ag nanocomposite powder

MWCNTs of purity 90–98%, average length 15–30 μm and diameter 4–12 nm were pur-chased from Nanoshell, USA. A part of CNTs was covalently functionalized and rest ofthem were treated with surfactant, sodium dodecyl sulphate (SDS) known as non-covalentfunctionalization. In covalent functionalization MWCNTs were treated with H2SO4:HNO3

(3:1) and subsequently washed with deionized water and dried in an oven at 120 °C [8].In non-covalent strategy the CNTs were sonicated in ethanol in the presence of SDS for 2h. C-MWCNTs and N-MWCNTs obtained by the respective functionalization processes

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were used in the synthesis process. The metal salt was reduced chemically instead ofthermal reduction as a modification in molecular-level mixing method. The synthesis pro-cess is called as modified molecular-level mixing method [11]. Different samples ofMWCNT/Ag nanocomposites were prepared by the process shown in schematic Figure 1.

The C-MWCNTs and N-MWCNTs of said percentages were dispersed in 200 mlethanol by probe sonication. AgNO3 (Sigma Aldrich, purity 99%) was taken as themetal salt for synthesis of nanocomposites. Three grams of silver nitrate was poured inthe dispersed CNT solution with magnetic stirring for 12 h and 2 ml of hydrazinehydrate (Merck, purity 99–100%) was added to this solution as a reducing agent. Allthe chemicals employed in the synthesis process were of analytical reagent grade andused without further purification. The resultant solution was centrifuged and the precipi-tates were washed many times with deionized water to remove the surfactant com-pletely. Finally, MWCNT/Ag nanopowder was obtained by drying the precipitates at50 °C on a hot plate.

2.2. Consolidation of CNT/Ag nanocomposite powder

The nanopowder was compacted by uniaxial molding press at a pressure of 320MPa.The pellets of size Ø13 mm × 2mm of C-MWCNT/Ag and N-MWCNT/Ag nanocom-posites containing 0, 1.5, 3, 4.5 and 6 vol.% CNTs were prepared. All the fabricatednanocomposite samples were sintered in horizontal tube furnace with attached program-mable temperature controller. The nanocomposite samples were placed in alumina tubeby keeping them in silica crucible. The sintering temperature of 800 °C was attained atthe sintering rate of 5 °C/min in an inert atmosphere. The sintering temperature waskept constant for 12 h. Theoretical and measured densities of samples evaluated beforeand after the sintering are shown in Table 1.

Figure 1. (colour online) Schematic view of the synthesis process.

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The morphology and micro-structure of the fine nanopowder have been analysed byX-ray diffraction (XRD), electron diffraction spectroscopy (EDS), scanning electronmicroscope (SEM) and high resolution transmission electron microscope (HRTEM)(JEOL, JEM-2100F) at an accelerating voltage of 125 kV. The hardness, Young’s modu-lus and electrical conductivity of MWCNT/Ag nanocomposite compacts were measuredby a nanoindenter (bruker) and four probe apparatus using Kiethly 2400 source meterand nanovoltmeter, respectively.

3. Micro-structural characterization

In the present synthesis technique, the nucleation of silver nanoparticles occurs in stableCNT suspension. It leads to homogenously dispersed and embedded CNTs in silvermatrix at the molecular level. The confirmation of synthesis of silver nanoparticles wasdone by XRD. Figure 2 (a) displays the XRD spectrum of the CNT/Ag nanocomposite.

It indicates the crystalline nature of the nanocomposites. The peaks at 2θ values38.9°, 45.1°, 65.1°, 78.2° and 82.4° correspond to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (22 2) reflections of silver as per JCPDS (file No. 04-0783) data. A weak peak is observedat 26.5° only in C-MWCNT (6 vol.%)/Ag nanocomposites (shown in inset), indicatingthe presence of CNT in the nanocomposite. However, the absence of this CNT peak inother samples indicate the uniform dispersion of CNTs in MMCs or can also be due tolow CNT content, low scattering angle and limited resolution of XRD instrument. Theclear pattern of XRD lines shows that crystalline phase of Ag grew between MWCNTsand eventually soldiered them. The quantitative presence of carbon and silver in thenanocomposite powder is indicated by EDS characteristics profile of CNT/Ag nanocom-posites (Figure 2(b)). The dark contrast encircled area was analysed by EDS. It shows97.14% Ag and 2.86% carbon. It confirms the presence of silver particles on the surfaceof CNTs. The peaks observed at 3.0, 3.2 and 3.4 keV correspond to the binding ener-gies of Ag, while the peaks situated at the binding energies of 0.25 keV correspond toC. No peak of other impurity has been detected.

The SEM images of nanocomposite powder are shown in Figure 3. CNTs arehomogeneously implanted inside or on the grain boundaries of silver nanopowder attheir low volume percentage (Figure 3(a) and (b)). The agglomeration becomes

Table 1. Theoretical, measured and relative density of specimens.

Composition

Theoreticaldensity (gm/

cm3)

Before sintering After sintering

Measureddensity(gm/cm3)

Relativedensity(%)

Measureddensity(gm/

cm3)

Relativedensity(%)

Ag 10.40 6.96 66.35 10.20 97.23N-MWCNT(1.5 vol. %)/Ag 10.35 8.74 84.45 9.86 95.27N-MWCNT (3 vol. %)/Ag 10.22 7.13 69.77 9.57 93.63N-MWCNT (4.5 vol. %)/Ag 10.02 5.06 50.50 9.34 93.21N-MWCNT (6 vol. %)/Ag 9.96 4.09 41.06 9.14 91.76C-MWCNT(1.5 vol. %)/Ag 10.35 7.97 77.00 9.97 96.32C-MWCNT (3 vol. %)/Ag 10.22 8.93 87.37 9.73 95.20C-MWCNT (4.5 vol. %)/Ag 10.02 6.91 68.96 9.19 91.71C-MWCNT (6 vol. %)/Ag 9.96 5.68 57.02 9.05 90.86

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dominant at high volume percentage of CNTs in MMCs (Figure 3(c) and (d)). Theseimages reveal that the tendency of CNT agglomeration increases with increase of theirconcentration in the matrix.

Figure 2. (colour online) (a) XRD spectrum of C-MWCNT /Ag nanocomposites and (b) EDSspectrum of C-MWCNT (3 vol.%) /Ag nanocomposite.

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The C-MWCNTs (3 vol.%) /Ag nanocomposites were analysed by high-resolutionSEM as shown in Figure 4. The HRSEM images show that the C-MWCNTs areembedded in the matrix. It appears from Figure 4(b) and (c) that the CNTs are short inlength due to covalent functionalization and sonication. Figure 4(d) and (e) elucidatethe comparative effect of the mode of functionalization i.e. the C-MWCNT is fullycovered with bonded nanopowder; whereas no directly bonded particle appears in thecase of N-MWCNT.

The micro-structure of consolidated nanocomposite samples was also analysedbefore and after sintering by the SEM to visualize the effects of sintering. The SEMmicrograph shown in Figure 5(a) indicates the large voids and homogenously anchoredCNTs in consolidated un-sintered MWCNT/Ag nanocomposites. The silver nanoparti-cles bonded with CNTs could not merge completely into the silver matrix continuumon consolidation up to the pressure of 320 MPa. The silver nanoparticle clustersattached with CNTs and rest of the silver matrix continuum have created two differentcontrasts as shown in HRSEM Figure 5(c). On sintering they have merged into a singlecontinuum (Figure 5(b)). The porosity has reduced to a large extent by sintering due tograin growth. Presence of physically intact MWCNTs shows that they are not damageddue to prolonged sintering.

The fabricated MWCNT/Ag nanocomposite powder was further analysed byHRTEM. The HRTEM micrographs revealed that the CNTs are properly dispersed andimplanted in the matrix at molecular level [Figure 6(A) (a)]. The bonded silver nanopar-ticles over the C-MWCNT surface are visible in Figure 6(A) (b). The HRTEM image

Figure 3. The SEM micrographs of MWCNT/Ag nanocomposite powder. (a) N-MWCNT (1.5vol.%)/Ag. (b) C-MWCNT (1.5 vol.%)/Ag. (c) C-MWCNT (4.5 vol.%)/Ag. and (d) N-MWCNT(6 vol.%)/Ag.

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of silver-CNT contact surface is shown in Figure 6(A) (c). Micrographs of dispersedN-MWCNTs/Ag nanocomposites in ethanol before consolidation are shown inFigure 6(A) (d, e). These micrographs show that N-MWCNTs are well embedded in thesilver matrix but silver nanoparticles are neither directly bonded to the CNT surface norencapsulated inside it.

Apart from the homogenous and embedded distribution of CNTs in the matrix, theinterfacial metal-CNT contact surface plays an important role. Figure 6(B) elucidates

Figure 4. High resolution SEM micrographs of (a–d) C-MWCNT (3 vol.%)/Ag nanocompositeand (e) N-MWCNT (3 vol.%) /Ag nanocomposite.

Figure 5. SEM micrographs of consolidated CNT/Ag specimens. (a) before sintering and (b) aftersintering.

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the HRTEM images of Ag-CNT contact surface. The presence of different materials canbe noticed on functionalized CNTs. The graphitic walls of a MWCNTs with inter planermagnification are shown. It is confirmed that there are no inclusions between the CNTside wall and the Ag matrix, as shown in Figure 6(B) (a–c). However, atomically disor-dered regions on the surface of multi-walled CNTs are observed Figure 6(B) (d). Theencircled interfacial regions are shown at higher magnification in the adjoining micro-graphs. No carbide or other impurity layer is visible at the interface.

Figure 6. (colour online) HRTEM micrographs (A) C-MWCNT (1.5 vol.%)/Ag (a–c),N-MWCNT (1.5 vol.%)/Ag nanocomposite (d, e) and (B) HRTEM analysis of CNT-metalInterface surface.

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FTIR is a qualitative technique for observing the interactions and perturbations thatoccurs at the CNT surface during adsorption of functional moieties. To indentify thesefunctional groups, the FTIR studies have been performed on N-MWCNTs, C-MWCNTsand C-MWCNT/Ag composite in the range of 400–4000 cm−1 [Figure 7(a)]. The indi-vidual spectrum is vertically shifted with the purpose of improving the visualizationeffect. The characteristic stretching vibration modes of N-MWCNT, C=C (1647 cm−1)and O–H (3476 cm−1), are apparent in the spectrum. However, the low intensity ofthese vibration peaks shows lesser amount of defects in N-MWCNTs. Also, the appear-ance of weak absorption peaks at 1386, 1647 and 3476 cm−1 assigned to C–O, C–Cand O–H bond stretching, indicate the presence of some functional groups. These mightarise during the synthesis and purification processes.

Compared to the IR spectrum of pristine CNTs, the acid-treated CNTs clearly showthe presence of strong absorption peaks at 1600–1700 cm−1 corresponding to the car-bonyl and carboxyl groups. A prominent absorption peak at 1631 cm−1 indicates thestretching vibration of C=C–C bond due to bonded oxygenated functional groups. Thebroad peak at 1031–1130 cm−1 could be assigned to C–O and C–C bonds stretching.Moreover, a broad shoulder absorption band in the region 2900–3500 cm−1 can beassigned to the O–H stretching vibrations of carboxyl groups (O=C−OH and C−OH).Also the stretching of C=O of the carboxylic acid (–COOH) group is observed at 1725cm−1. These results indicate the successful insertion of the hydroxyl and carboxylicfunctional groups on the CNT surface by their treatment with H2SO4/HNO3 [12,13].

However, in IR spectrum of C-MWCNT/Ag nanocomposite, a shift in peakpositions has been observed. It is a clear indication of the interaction of theMWCNTs with silver. The suppression of prominent absorption peaks of C-MWCNTsat 1031, 1631 and 3421 in silver composites are correlated with stretching of bondsdue to interaction between CNTs and metal particles. The appearance of a peakapproximately at 1400 cm−1 corresponds to the C–O stretching, which indicates thatthe interaction and perturbation occurs at the CNT surface due to adherence of metalparticles [14].

Raman spectroscopic measurements were carried out to assess the structuralintegrity of CNTs. Figure 7(b) shows the Raman spectrum of N-MWCNTs andC-MWCNT along with C-MWCNT/Ag nanocomposite recorded at room temperatureand ambient pressure. The two main typical graphitic peaks are present in the spectrumof all the samples, indicating graphite, G band (1575 cm−1) and the disorder, D band(1345.54 cm−1) of CNTs. The corresponding second-order harmonics, 2D band is alsopresent around 2680 cm−1 in the spectrum of N-MWCNTs. The presence of these char-acteristic peaks even after chemical treatment proves that the acid treatment does notdamage the structure of CNTs. The slight increase in D to G peak intensity ratio (0.97for C-MWCNTs compared to 0.75 for N-MWCNTs), indicates that covalentfunctionalization has increased the defect density by introducing carboxylic, carbonyland hydroxyl groups [15,16]. The presence of broad G band shoulder in the spectrumof CNT/Ag nanocomposite, indicates the bond-angle distortion at the C-atom in sixfoldaromatic rings, due to covalent functionalization and interaction between silver and theCNT surface.

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4. Results and discussion

4.1. Evaluation of mechanical properties

The mechanical properties of fabricated MMCs were analysed against MWCNTs con-centration. The nanocomposite compacts were in nanorange before sintering and getsconverted into bulk after prolonged sintering. Therefore, mechanical properties viz.hardness and Young’s modulus of C-MWCNT/Ag and N-MWCNT/Ag nanocomposites

Figure 7. (colour online) FTIR (a) and Raman spectrum (b) of N-MWCNTs, C-MWCNTs andC-MWCNT/Ag nanocomposite.

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were evaluated before and after the sintering process [17,18]. Figure 8 shows thehardness and Young’s modulus of C-MWCNT/Ag and N-MWCNT/Ag nanocompositesbefore sintering.

Both mechanical parameters increase with the CNT content in the silver matrix.Comparatively, more increase in hardness and Young’s modulus of C-MWCNT/Ag thanN-MWCNT/Ag nanocomposites has been observed. The hardness and Young’s modulushave been increased by a factor of 1.1–1.6, respectively. Mechanical properties ofMWCNT/Ag nanocomposites have improved more rapidly up to 3 vol.% of MWCNTsand, thereafter, the rate of enhancement of mechanical properties decreases or becomesconstant. It is probably due to the increase in MWCNT agglomeration tendency anddecrease in density at higher CNT volume content in silver matrix as revealed byFigure 3 and Table 1. The hardness and Young’s modulus of same MMCs aftersintering are shown in Figure 9.

The results reveal that the mechanical properties are improved twice on sintering.The increased strength is attributed to good distribution of MWCNTs as well as interfa-cial bonding between MWCNTs and silver realized by the fabrication method. Sinteringreduces the porosity and increases the density of MWCNT/Ag nanopowder by coales-cence of metal nanoparticles. The overall improvement of hardness and Young’s modu-lus of the sintered nanocomposites are influenced by a combined effect of reduction inporosity, residual stress removal and MWCNT distribution [19]. Also homogenouslydispersed MWCNTs inside the grain boundaries could inhibit dislocations in nucleationand motion, resulting in an increase in the mechanical strength and stiffness [20]. Asexpected, the nanocomposites including C-MWCNTs had the high modulus and hard-ness values due to fine dispersion and adhesion of C-MWNTs in MMCs.

Figure 8. (colour online) Hardness and Young’s modulus of MWCNT/Ag nanocomposite beforesintering.

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4.2. Evaluation of electrical conductivity

The electrical conductivity of a conductor is always proportional to the mean free pathi.e.

r ¼ q2nl

vm

where q is the charge on an electron, v is the velocity of the electron, n is the electrondensity and m is the mass of the electron. A defect-free CNT shows ballistic conductiondue to prolonged mean free path in comparison with its length. Low resistivity in CNT/metal nanocomposites is possible because the ballistic conducting CNTs have electronmean path in several orders of magnitude longer than the conventional conductors.Using a simple effective-medium model, Hjortstam et al. [21] have proposed thatMMCs based on aligned, ballistic CNTs embedded in a metal matrix might work as anultra-low-resistive material. However, the improvement in electrical conductivity of met-als by CNT reinforcement has not yet been achieved experimentally, although certainexperimental evidences have been reported by researchers in this direction [22–26]. Inthe present part of work, we have studied the electrical conductivity of C-MWCNTsand N-MWCNTs-reinforced silver nanocomposites against MWCNTs volume percent-age and sintering, thereof to add few facts in the same direction. The electrical conduc-tivity of the MWCNT/Ag nanocomposites measured before and after sintering is plottedin Figure 10.

The electrical conductivity of pure nanosilver sample before sintering was1.5 × 107 S/m, which is smaller than the electrical conductivity of bulk silver. It is dueto the presence of large pores and less density of nanocomposites before sintering(Table 1). It is interesting to find an increase in electrical conductivity of MWCNT/Ag

Figure 9. (colour online) Hardness and Young’s modulus of MWCNT/Ag nanocomposite againstMWCNT volume percentage after sintering.

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nanocomposites in comparison with their counterpart pure nanosilver fabricatedunder similar conditions (Figure 10(a)). More increase in electrical conductivity ofN-MWCNT/Ag than C-MWCNT/Ag has been correlated with the effect of non-covalentfunctionalization of MWCNTs. At 1.5 vol.% MWCNTs, electrical conductivity ofMWCNT/Ag nanocomposite shows a sharp increase due to uniform dispersion ofMWCNTs in MMCs as indicated by SEM and TEM micrographs. Increase in the con-ductivity of MWCNT/Ag nanocomposites is attributed to the flow of high percentage ofelectrons through anchored MWCNTs across the pores or grain boundaries through bal-listic MWCNT flyover. The embedded MWCNTs in silver matrix facilitates electrontransfer along with the ballistic transport by prolonging the electron’s mean free path[27]. However, scattering of electrons by MWCNT agglomerates at their high-volumepercentage decreases the electrical conductivity. After sintering, the electrical conductiv-ity of MWCNT/Ag nanocomposites decreases with the increase of MWCNT content(Figure 10(b)). The sintering results in reduction of porosity due to coalescence of silvernanoparticles and grain growth. Prolonged sintering process converts the low-conduct-ing nanoparticles into high-conducting bulk metal. Although, the density of silvermatrix increases with sintering but anchored CNTs across the pores or grain boundarieswhich were acting initially as ballistic flyover across the pores will now act as impurityor scattering centre for flow of electrons through highly conducting crystalline silvermatrix continuum. Since it is very difficult to achieve theoretical density and removeporosity completely even after sintering, the combined effect of multiple factorsi.e. residual porosity, MWCNT agglomeration and interfacial contact resistance areresponsible for decrease in electrical conductivity.

The mechanism of increase in mechanical strength and electrical conductivity isreciprocal. The mechanical improvement requires bonding of free electrons; whereas theelectrical conduction needs availability of free electrons. It has now been an establishedfact that chemical treatment of CNTs with strong acids creates defects on its surfacedue to cleavage of C–C bonding structure. The final products are nanotube of smallerlength, whose ends and sidewalls are decorated with a high density of various oxygen

Figure 10. (colour online) Electrical conductivity of MWCNT/Ag nanocomposite (a) beforesintering and (b) after sintering.

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containing groups [28]. This fact is also confirmed by FTIR and micro-structural analy-sis. The strengthening mechanism of CNT/metal nanocomposites fabricated by molecu-lar-level mixing method has been correlated to the special bonding at the CNT/metalinterface due to the reaction between surface modifies CNTs and metal ions [29]. Thiscovalent bonding between MWCNTs and metal ions might increase the mechanicalstrength but decrease the electrical conductivity due to the decrease in availability offree electrons for conduction. However, non-covalent approach involves adsorption ofthe metal ions on nanotube surface, either via weak π–π stacking interaction or throughcoulomb attraction without disturbing the free electron system of MWCNTs. Therefore,in non-covalent functionalization the mechanical strength of bonding is weak but itboosts the electrical conduction. Thus, non-covalent functionalization can improve theelectrical conductivity of low conducting nanosilver matrix; whereas the mechanicalstrength of MMCs is more effectively improved by covalent functionalization.

5. Conclusions

As a summary, an innovative molecular-level mixing method has been extended suc-cessfully to fabricate CNT/silver nanocomposite followed by sintering. The hardnessand Young’s modulus of C-MWCNT /Ag nanocomposites were higher thanN-MWCNT/Ag nanocomposites. Mechanical properties were improved significantly byC-MWCNTs reinforcement over unreinforced silver matrix before and after the sinteringprocess. The N-MWCNTs could improve the electrical conductivity of low conductingnanosilver matrix, however, electrical conductivity of highly conducting bulk silvermetal decreases by MWCNT reinforcement. The observed difference in mechanical andelectrical properties for both types of MWCNTs at the same MWCNT content is attrib-uted to the influence of their mode of functionalization.

Funding

We are grateful for the financial support from the Department of Science and Technology [Pro-ject-SR/FTP/PS-054/2011(G)], India.

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