Effect of CNT decoration with silver nanoparticles on electrical conductivity of CNT-polymer composites

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Effect of CNT decoration with silver nanoparticles on electrical conductivity of CNT-polymer composites Peng Cheng Maa, Ben Zhong Tangb, Jang-Kyo Kima,* a

Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

b

A R T I C L E I N F O

A B S T R A C T

Article history:

A simple approach to decorate carbon nanotube (CNT) with silver nanoparticles (Ag-NPs)

Received 22 January 2008

was developed to enhance the electrical conductivity of CNT. CNTs were functionalized

Accepted 24 June 2008

using ball milling in the presence of ammonium bicarbonate, followed by reduction of

Available online 2 July 2008

silver ions in N, N-dimethylformamide, producing silver decorated CNTs (Ag@CNTs). The Ag@CNTs were employed as conducting filler in epoxy resin to fabricate electrically conducting polymer composites. The electrical, thermal and mechanical properties of the composites were measured and compared with those containing pristine and functionalized CNTs. It was found that when pH was about six, highly dispersed Ag-NPs can be decorated on functionalized CNTs. The electrical conductivity of composites containing 0.10 wt% of Ag@CNTs was more than four orders of magnitude higher than those containing same content of pristine and functionalized CNTs, confirming the advantage of the Ag@CNTs as effective conducting filler. The ameliorating effect of improved electrical conductivity was not at the expense of thermal or mechanical properties.  2008 Elsevier Ltd. All rights reserved.

1.

Introduction

Since the discovery of carbon nanotubes (CNTs) [1], fundamental research on CNTs and their applications have made rapid progresses [2–5]. Unlike other carbon materials, such as graphite and diamond, CNTs are one dimensional carbon material. The graphene layer is rolled up cylindrically with diameter on a nanometer scale. The structure and morphology of CNTs enable them to serve as a specific template for preparing metal nanoparticle-CNT nanohybrids [6,7]. The combination of these two materials (i.e. CNTs and nanoparticles) is particularly useful to integrate the properties of the two components in hybrid materials for use in catalysis, energy storage and nanotechnology [7]. Many studies have been devoted to fabricating metal nanoparticle decorated CNTs as well as measuring their unique electrical, magnetic and optical properties [6–13]. Among them, Ag nanoparticles (Ag-NPs)

attached onto CNTs (Ag@CNTs) gained significant attention due to their potential applications as catalyst [14], optical limiter [15] and advanced material [16]. To prepare well-defined Ag@CNTs, several different methods, including thermal decomposition reaction [9], vapor deposition [14], surface chemical reduction [17] and gamma-irradiation [18], have been proposed. However, there were two major drawbacks identified in applying these methods: one was the weak interaction between the Ag-NPs and CNT surface [19,20]. In most cases, Ag-NPs were formed inside the tubes due to the capillary effect [21,22], leading to poor synergic effect of the Ag@CNT nanohybrids. The other was agglomeration of AgNPs, causing difficulties in uniform decoration of CNT surface. To realize the effective production of Ag@CNTs, an easy and low-cost route is desired. Epoxy resin-based CNT composites have been extensively investigated in view of their potential applications in the elec-

* Corresponding author: Fax: +852 2358 1543. E-mail address: [email protected] (J.-K. Kim). 0008-6223/$ - see front matter  2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.06.048

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tronics, aerospace and automotive industries [23]. It was shown that by adding less than 1.0 wt% of CNTs into epoxy [24], the electrical conductivity of the composite was improved from around 1013 to 104 S/cm. The measured electrical conductivities of CNT/epoxy composites typically ranged from 10 5 to 103 S/cm above the percolation threshold [24–28]. While further increase in nanotube content above the percolation threshold can enhance marginally the electrical conductivity of composites, the solution viscosity becomes too high to produce void-free composites when the CNT content is higher than 1.0 wt%. This limits the use of CNT-polymer composites for applications where high CNT contents are necessary. Therefore, processing techniques for improving the electrical conductivity of composites below or near the percolation threshold became crucial to get highly conducting composites. Simulation results indicate that the contact resistance of CNTs in composites plays a dominant role in contrast to the dominant role of the intrinsic resistance of CNTs [29,30]. It is expected that the electrical conductivity of composites can be significantly improved if the contact resistance of CNT junctions in polymer matrix can be reduced. This paper is part of a larger project on the development of conducting polymer composites containing CNTs. In this paper, a simple approach is proposed to decorate CNTs with AgNPs based on the wet chemistry reaction, aiming at enhancing the electrical conductivity of CNTs and thus reducing the contact resistance of CNT junctions in a polymer matrix. Several characterization tools were employed to characterize the structure and morphology of Ag@CNTs. The Ag@CNTs were incorporated into an epoxy resin as conducting fillers to prepare electrically conducting composites. The CNT dispersion state in epoxy, as well as the electrical, thermal and mechanical properties of the composites containing Ag@CNTs, were investigated and compared with those containing pristine and functionalized CNTs. The implication of the findings was discussed regarding to the potential application of Ag@CNTs as conducting fillers in electronic packaging industries.

2.

Experimental

2.1.

Materials and Ag decoration onto CNT

The CNTs used in this study were basically the same as those employed previously [24,25,31,32]. They were multi-walled CNTs (MWCNTs) and synthesized by a chemical vapor deposition method (supplied by Iljin Nanotech, Korea). The diameter and length ranged between 10–20 nm and 10–50 lm, respectively, according to the supplier’s specification. The CNTs were functionalized using ball milling in the presence of ammonium bicarbonate (NH4HCO3) [33]. A cylindrical ball milling container was made of stainless steel (70 mm in inner diameter · 110 mm in height), which contained a total of 450 ZrO2 milling beads, comprising 150 pieces each with diameter of 2.0, 3.0, and 5.5 mm. The container with 1.58 g (0.02 mol) of NH4HCO3 and 0.25 g of CNTs was rolled at a speed of 250 rpm for 2 h. After ball milling, the residual gases in the container were removed in a vacuum oven at 100 C for 24 h. Ag-NP decoration onto the CNT surface was realized based on the reducing reaction of silver ions (Ag+) using N, N-

dimethylformamide (DMF). Typically, 15 mg of functionalized CNTs and 1 mg of sodium dodecyl sulfate were added into 80 ml of DMF, then the mixture was subjected to ultrasonication (Digital Sonifier supplied by Branson Ultrasonics, USA) for 1 h. 1/4 of the sonicated mixture (20 ml) was transferred into a 100 ml three-neck flask, and its pH was adjusted using nitric acid (HNO3, 0.01 mol/L). To achieve Ag-NP decoration, 10 ml of silver nitrate solution (0.02 mol/L) was added into the mixture within 1 h at 60–65 C while the mixture was continuously stirred. The solution was kept without stirring at room temperature for 24 h for Ag deposition. The final product was obtained by filtration and washing with ethanol, water and acetone sequentially.

2.2.

Fabrication of conducting composites

The composites were made from epoxy, a diglycidyl ether of bisphenol A (DGEBA, Epon 828, Shell Chemical), and a curing agent, m-phenylenediamine (mPDA, Sigma-Aldrich). CNTs were dispersed in ethanol before adding the monomer epoxy, and the mixture was ultrasonicated for 1 h each at 60 C and 80 C. The mixture was then degassed at 80 C for 5 h to eliminate the entrapped air and the remaining ethanol. Then the mPDA hardener was added into the mixture at a resin-to-hardener weight ratio of 1:0.145. The composite was molded into a flat plate and cured at 80 C for 2 h, followed by post cure at 150 C for 2 h. Fig. 1 shows the processing steps used to fabricate the CNT/epoxy composite [25]. Composites containing different weight fractions (0.05, 0.10, 0.25, and 0.50 wt%) of pristine CNTs, functionalized CNTs and Ag@CNTs were prepared.

2.3. Characterization composites

of

CNTs

and

CNT-polymer

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to characterize the morphology and structure of CNTs before and after Ag decoration. TEM images were taken on a JEOL 2010F (JEOL Ltd., Japan) with an energy dispersion X-ray (EDX) analyzer. XRD studies were performed in a powder XRD system (PW1830, Philips) with CuKa radiation (k = 0.154 nm). Time-of-flight secondary ion mass spectroscopy (ToF-SIMS, Physical Electronics 7200 ToFSIMS spectrometer) was used to identify the functional groups on the CNT surface before silver decoration. The primary Cs+ ion source was operated at 8 keV and the scanning area was 200 lm square. Dispersion state of CNTs in epoxy matrix was visualized by examining the fracture morphologies of the composites on a scanning electron microscope (SEM, JEOL-6700F). The presence of Ag-NPs on CNT surface was further confirmed by measuring the electrical conductivity of CNTs using a four-probe resistivity/Hall measurement system (HL5500PC, Bio-Rad). Sheet samples of 40–60 lm in thickness were prepared by pumping 5 mg of CNT between two iron plates at a pressure of 150 KN/cm2. The bulk electrical conductivity of composites was measured at room temperature using a programmable curve tracer (Sony Tektronix 370A). Specimens were polished on both sides into a thickness of 1 mm and a very small amount of silver paste (of thickness about

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Epoxy (DGEBA) Degassing at 80 oC for 2 h CNTs

1. Ultrasonication at 60 oC for 1h

Disperse in ethanol

Sonicated mixture 2. Ultrasonication at 80 oC for 1h

Vacuum oven at 80 oC for 5h

1. Curing at 80 oC for 2h 2. Curing at 150 oC for 2h

Composites

Curing agent (mPDA) (14.5 wt% of epoxy) Fig. 1 – Schematics of CNT/Epoxy composite preparation [25].

0.05 mm) was applied on the sample surface to reduce the contact resistance between the samples and electrodes. To minimize any potential problems associated with silver paste, the samples were heated at 40 C to remove solvent quickly. Then, the edges of the samples were ground again to remove silver paste attached on them. FlashLineTM 3000 thermal conductivity measurement system (Anter Corporation, the testing method is based on American Society for Testing and Materials (ASTM) Standard E1461), was used to measure the thermal conductivity of the composites. The disc-like specimens with a diameter of 12.7 mm and a thickness in the range of 0.3–0.4 mm were used for the measurement. Threepoint flexure test was performed to measure the mechanical properties of neat epoxy and CNT/epoxy composites according to the specification, ASTM standard D790-96. Five specimens were tested for each set of conditions on a universal testing machine (MTS Sintech 10/D).

3.

Results and discussion

3.1.

Interactions between Ag and CNTs

The degree of interactions between Ag and CNTs was evaluated on CNTs with and without ball milling functionalization. It was interesting to note that when pristine CNTs were used as template for Ag decoration, most Ag-NPs were deposited onto the carbon film of the copper grid used for TEM characterization (Fig. 2a). The high resolution TEM (HRTEM) image confirmed that the CNT surface was relatively clean without any evidence of Ag-NPs on it. Several Ag-NPs were found inside the CNTs, which were introduced due to the capillary effect (Fig. 2b) [21,22]. In sharp contrast, when the CNTs were functionalized using ball milling in the presence of NH4HCO3, very different results were obtained: Fig. 2c and 2d present a number of tiny Ag-NPs attached to the CNT surface. The Ag-NPs were uniformly distributed without agglomeration, and their sizes were also uniform ranging from sub-nanometer to about 2–4 nm. The strong signal at about 3 keV of the EDX spectrum (Fig. 2e) obtained from the TEM was a testament to the decoration of Ag-NPs on CNT surface. The mechanism behind the successful Ag decoration onto the CNT surface after functionalization is schematically illustrated in Fig. 3: during the functionalization of CNTs using ball milling, ammonia gas was introduced into the system due to the decomposition of

NH4HCO3. The ball milling also introduced defects and broke the –C–C– bonds on the surface layer of CNTs [34,35], which was further facilitated by the presence of NH4HCO3 [33]. These CNT surface modifications, in turn, allowed the amine groups to form covalent bonds with the broken –C–C– bonds. The coordination interaction between the Ag cations and amine groups resulted in the attachment of Ag particles onto the functionalized CNTs, as illustrated in Fig. 2c and 2d. The ToF-SIMS analysis was carried out to verify the chemical states of nitrogen on the surface of functionalized CNTs. Based on the ToF-SIMS results shown in Fig. 4 for the CNTs milled with NH4HCO3 (where N = 0.37 at% as determined by X-ray photoelectron spectroscopy), three different types of nitrogen compounds were identified. They include (i) the ammonium gas adsorbed onto the CNT surface, as verified by m/z = 18; (ii) the amines that can be verified by the peaks of –CH4N (–CH2NH2), –C2H6N (–CH2CH2NH2) and –C3H8N (–CH2CH2CH2NH2) at m/z = 30, 44 and 56, respectively; (iii) the amide which is present in the form of –C3H6NO (–CH2CH2–CO–NH2) at m/z = 72. The comparison between the relative intensities of these nitrogen compounds suggested that only a small amount (15%) of NH3 gas was adsorbed onto the CNT surface while the majority of these compounds were covalently bonded to the CNTs, effectively supporting the above assumption that nitrogen functional groups were attached onto the functionalized CNTs (Fig. 3). Fig. 5 shows the XRD patterns for the pristine CTs and the CNTs after Ag-NP decoration. The pristine CNTs revealed reflections corresponding to the C(100), C(102), and C(110) planes of crystalline graphite-like materials [20], whereas the Ag@CNTs showed those corresponding to four main crystallographic planes, namely, Ag(111), Ag(200), Ag(220), and Ag(311). A previous study based on theoretical calculations [19] demonstrated that the binding energy between the Ag atoms and pure CNTs was very weak. In contrast, the AgNPs were strongly attached to the CNT surface due to the amino functionalization of the CNTs in our experiment, which enhanced the reactivity with Ag. A similar conclusion was reported previously for N-doped CNTs [20] in a study involving the CNT fabrication using the thermolysis of ferrocene in benzylamine at high temperature. When comparing these two techniques, our method is considered to be easier to apply and less costly. To evaluate the influence of pH on the distribution of AgNPs on CNTs, experiments were performed using different

1500

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Fig. 2 – TEM images of the Ag@CNTs without (a) and with (c) functionalization and the corresponding HRTEM (b, d) and (e) EDX Spectrum (spot A in d).

Fig. 3 – Schematics of CNT functionalization and interaction with Ag cations.

Fig. 4 – ToF-SIMS results of CNTs after functionalization.

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Intensity (a.u)

1200 Ag@CNTs Pristine CNTs

900 C(100)

600 Ag(200)

300

Ag(220)

C(102)

Ag(311)

C(110)

0 20

30

40

50 2θ (degree)

60

70

1501

that at pH = 2 the neutral nature of the CNT surface was protonated, where the CNT surface completely lost its role as nucleation sites for Ag attachment. When the pH was 4, many Ag particles of hexagonal shape were formed without apparent bonding to the CNT surface, and these particles were heavily agglomerated to form larger clusters (Fig. 6b). When the pH was 6, Ag-NPs of spherical shape were tightly attached onto the CNTs (Fig. 6c). These observations indicate that a suitable pH value of mixture is crucial to the formation of dense Ag-NPs on CNTs.

80

Fig. 5 – XRD patterns of the CNTs before and after Ag decoration.

pH values by varying the HNO3 concentration. Typical TEM images shown in Fig. 6 indicate that pH of the mixture solution plays a significant role in determining the size and distribution of Ag particles. The Ag particles tended to become larger and agglomerated with decreasing the pH value. When the pH value was 2 as an extreme case, characteristic Ag nanowires with a diameter of about 300–400 nm were produced instead of Ag particles (Fig. 6a). This finding indicated

3.2.

CNT dispersion in epoxy matrix composites

The fracture morphologies of 0.25% CNT/epoxy composites shown in Fig. 7, shed some insight into the nanoscopic dispersion states of CNTs. Large CNT agglomerates are seen for the composites containing pristine CNTs (Fig. 7a), whereas these agglomerates are almost absent for the composites containing functionalized CNTs and Ag@CNTs (Fig. 7b and c). There is significant analogy between the fracture surface of the latter two composites and that with the CNTs functionalized using an organic silane [25], which was characterized by improved dispersion of CNTs in the matrix. This observation was attributed to the functionalization of the CNTs using ball

Fig. 6 – Ag distribution and interaction with CNTs at different pH. (a: pH = 2; b: pH = 4 and c: pH = 6).

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Fig. 7 – Fracture surface morphologies of the 0.25% CNT/epoxy composites (a: Pristine CNTs; b: Functionalized CNTs; c: Ag@CNTs).

3.3.

Electrical conductivity of composites

1.00E+00 Electrical conductivity (S/cm)

milling. This process disentangled the CNT agglomerates [33], resulting in easier dispersion of CNTs in the epoxy resin upon ultrasonification. In addition, the functional groups like amine and amide on the surface of CNTs introduced during milling (Figs. 3 and 4) not only facilitated the Ag decoration onto CNTs, but also encouraged the dispersion of CNTs in the matrix.

1.00E-02 1.00E-04 1.00E-06 Ag@CNTs Functionalized CNTs Pristine CNTs

1.00E-08 1.00E-10 1.00E-12 1.00E-14

It was expected that the Ag-NP decoration would have a beneficial effect on the electrical conductivity of CNT because the inherent electrical conductivity of Ag (rAg = 6.30 · 105 S/ cm) is much higher than that of the MWCNTs used in this study. Table 1 summarizes the electrical conductivities of different CNTs before incorporating into composites. The electrical conductivity of functionalized CNTs was increased from 5.15 to 9.33 S/cm, this enhancement was attributed to the charge transfer between CNTs and nitrogen compounds that were covalently-bonded onto the CNTs [33,36]. The conductivity of CNTs increased significantly to 30.53 S/cm after silver decoration, showing the effectiveness of silver in improving the conductivity of CNTs. This observation has a practical implication in that the Ag@CNTs can be potentially useful as the conducting filler in polymer composite with unique properties that other filler materials seldom possess, such as increased electrical conductivity, high aspect ratio and reduced contact resistance in CNT junctions. The electrical conductivity of the composites is plotted as a function of CNT content in Fig. 8. All the composites containing CNTs with different functionalities presented a transition from the insulator to conductor: the approximate percolation thresholds were found to be around 0.10 wt% CNT content. The composites with different CNT surface conditions displayed significantly different electrical conducting behaviours. The incorporation of 0.50 wt% pristine CNTs in-

0

0.1

0.2 0.3 CNT content (wt%)

0.4

0.5

Fig. 8 – Electrical conductivity of composites varied with CNT content.

creased the conductivity of composite by about nine orders of magnitude. This observation is consistent with previous findings on CNT composites based on similar matrix materials [24–28]. The composites containing Ag@CNTs exhibited a more pronounced enhancement in electrical conductivity than their counterparts containing pristine CNTs. For the same CNT content of 0.50 wt%, a remarkable - about twelve orders of magnitude–improvement in electrical conductivity was observed (from 2.2 · 1013 to 0.81 S/cm), showing the advantage of Ag@CNTs as an effective conducting filler to improve the conductivity of the composites. However, the composites containing functionalized CNTs exhibited consistently lower conductivities than those containing pristine CNTs and Ag@CNTs for the whole CNT contents studied although the functionalized CNTs had a slightly higher electrical conductivity than the pristine CNTs. It appears that the CNT length played a dominant role in determining the electrical conductivity of the composites with functionalized CNTs. It was shown that CNT lengths were reduced after the ball milling process [33], and the CNT length was one of

Table 1 – Electrical conductivities of different CNTs Sample Electrical conductivity (S/cm)

Pristine CNTs

Functionalized CNTs

Ag@CNTs

5.15 ± 0.57

9.33 ± 0.56

30.53 ± 1.28

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the most important factors, along with the degree of CNT dispersion, that determine the electrical conductivity of CNTpolymer composites [24]. However, the Ag-NPs that were tightly attached onto the defect sites of CNT surface (Fig. 2d) compensated the above negative effect by enhancing the conductivity of CNTs and reducing the contact resistance of CNT junctions in matrix.

3.4. Thermal conductivity of the composites containing Ag@CNTs Apart from the electrical conductivity, thermal conductivity is also an important requirement for many applications in electronic components and assemblies. As silver is highly thermal conducting, the Ag decoration of CNTs may also give rise to the thermal conductivity of CNT composite. The measured thermal conductivities of composites with varying CNT content are shown in Fig. 9, which are compared with those obtained from theoretical models, including the rule of mixtures (Eq. (1)) and Nan’s model (Eq. (2)) [37], in an effort to understand the mechanisms involved. The respective equations used for calculation are: jc ¼ jm ð1  Vf Þ þ jf Vf

ð1Þ

jc ¼ ð3jm þ jf Vf Þ=ð3  2Vf Þ

ð2Þ

where jc, jm and jf are the thermal conductivities of composite, epoxy matrix and CNTs, respectively. The measured value of jm = 0.48 and jCNT = 740 W/(m K) [38] were used for the calculation. Vf is the volume fraction of CNTs in composites: to convert the weight friction of CNT to volume fraction, the densities, 1.2 and 1.8 g/cm3, were used for epoxy matrix and CNT, respectively [24]. The experimental results indicate that the thermal conductivities of composites gradually increased with increasing CNT content, and more than 30% increase was achieved with a small amount of 0.10 wt% CNTs, confirming the effectiveness of CNTs as a multi-functional filler in enhancing the thermal conductivity of composites [39–41]. The increase in thermal conductivity became significantly slowed at CNT contents above 0.10 wt%, resulting probably from secondary agglomeration of CNTs [24] or difficulties associated with uni-

Thermal conductivity (W/(m-K)

1 (94%) (84%)

0.9 (72%) (70%)

0.8

3.5.

Mechanical properties of composites

(71%)

(60%) (43%)

Ag@CNTs Functionalized CNTs Pristine CNTs Rule of mixtures Nan's model: Ref. [37]

(36%) (33%)

0.6

(24%) (14%)

0.4

form dispersion of CNTs [42,43] in the matrix. The thermal conductivities of composites were consistently higher in the ascending order of functionalized CNTs, pristine CNTs and Ag@CNTs for a given CNT content. The lower conductivity of the composites with functionalized CNTs may be associated with the reduction in CNT aspect ratio [33] and improved interfacial interactions between the functionalized CNTs (containing amine and amide groups on the surface) and epoxy matrix. For a given volume fraction, the number of CNT-polymer-CNT contacts increased with decreasing aspect ratio, and a lower aspect ratio of CNTs deteriorated the thermal transport across the CNT-polymer interface [40]. Additionally, the improved interfacial interactions between CNTs and epoxy inhibited the phonon transportation along CNTs and increased the interfacial thermal resistance by affecting the damping behavior of the phonons’ vibration [44]. In contrast, the higher conductivity of Ag@CNTs composites than the other composites attributed to improved dispersion of CNTs in the matrix, as well as the silver decoration which enhanced the connecting conductance of the CNTs through conducting networks in the matrix. Because silver itself is a good thermal conductor (jAg = 429 W/(m K)), the Ag-CNT interface facilitates phonon conduction by reducing boundary scattering losses [43] and interfacial resistance to heat flow [45,46] between Ag@CNTs and the polymer matrix. Therefore, the decoration of CNTs with Ag-NPs offers an excellent means to recover the potential reduction in thermal conductivity arising from functionalization of CNTs. The results of two models exhibited different conductivity trends as a function of CNT content: the rule of mixtures gave an upper bound estimate because this model assumed a direct influence of filler phase with a higher conductivity. In contrast, the Nan’s model that was developed based on the random distribution of CNTs and the averaging of thermal conductivities of matrix and filler, correlated better to the experimental data than the rule of mixtures model, especially at low CNT contents in the range of 0–0.10 wt%. Nevertheless, the Nan’s model also overestimated compared to experiments at higher CNT contents. Again, the increasing difficulties involved in dispersing CNT agglomerates into individual CNTs at high CNT contents are mainly responsible for this observation. Note that both two models are based on the assumption of perfect dispersion of individual CNTs and direct contact of all CNTs with matrix material, which is the case seldom in reality.

(63%)

0.7

0.5

1503

(0.0%)

0

0.1

0.2 0.3 CNT content (wt%)

0.4

0.5

Fig. 9 – Comparison of thermal conductivities of composites between experiments and theoretical models (Data in bracket indicate the increase of thermal conductivity compared to neat epoxy).

Fig. 10 shows the elastic modulus and strength measured from the flexural test of the composites, which are plotted as a function of CNT content. The modulus increased rapidly with increasing CNT content, before being saturated at about 0.10 wt% CNT content. The composites containing functionalized CNTs exhibited slightly higher modulus than their counterparts containing pristine CNTs and Ag@CNTs. This observation was expected judging from the fact that the amino functionalization improved the dispersion of CNTs in the matrix as well as the interfacial interactions between CNTs and epoxy matrix, as confirmed by SEM image (Fig 7b). The flexural strength of the composites showed a peak similarly

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3.4

b 130

3.3

Flexural strength (MPa)

Flexural modulus (GPa)

a

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3.2 3.1 3

Ag@CNTs Functionalized CNTs Pristine CNTs

2.9 2.8 0

0.1

0.2 0.3 CNT content (wt%)

0.4

0.5

120

110 Ag-CNTs Functionalized CNTs Pristine CNTs

100 0

0.1

0.2

0.3

0.4

0.5

CNT content (wt%)

Fig. 10 – Variations of (a) flexural modulus and (b) flexural strength of composites with CNT content .

at about 0.05–0.10 wt% CNT, followed by gradual drops with further increase in CNT content, similar to our recent study on composites with silane-treated CNTs [25]. It is suspected that the curing reactions between the DGEBA epoxy and amine hardener was adversely affected by the high CNT content, and this effect became more pronounced when functionalized CNTs containing amine and amide groups were employed in composites, as verified by the lowest strength when CNT content reaches at 0.50 wt%. Nevertheless, the strengths of the composites containing Ag@CNTs were consistently higher than their counterparts containing pristine CNTs, suggesting the reinforcement effect due to CNTs in composites was not sacrificed after functionalization and Ag decoration.

4.

the important properties of the Ag@CNTs and the composites under the end-use conditions is underway with these useful applications in mind.

Acknowledgements This project was supported by the Research Grant Council of Hong Kong SAR (Project No. 614505) and University Granted Council for NANO Concentration. Technical assistance from the Materials Characterization and Preparation Facilities (MCPF) of HKUST is appreciated.

R E F E R E N C E S

Conclusions

A simple method was established for Ag particle decoration onto CNTs after functionalization using ball milling in the presence of NH4HCO3. The pH value of the reaction played an important role for distribution of Ag-NPs on CNTs: well dispersed Ag-NPs were achieved at a pH value about 6. Epoxy-based composites containing CNTs with and without Ag particles were prepared. The electrical conductivity of the composites with Ag@CNTs was significantly higher than those containing pristine and functionalized CNTs for CNT content higher than 0.10 wt%, confirming the advantage of Ag@CNTs as an effective conducting filler. The thermal and mechanical properties of the composites with different CNTs were comparable, indicating that the role of CNTs as the reinforcement in composites was not sacrificed after Ag decoration. The findings of this paper also indicate potential applications of the Ag@CNTs for electronics packaging and assemblies, such as isotropic conductive adhesives (ICAs). ICAs have been developed to replace the lead-containing solder joints for over a decade, offering many advantages [47,48]. However, ICAs also possess important drawbacks, such as poor electrical and mechanical properties especially after hydrothermal ageing and thermal cyclic loading, requiring further efforts to address these limitations. Materials with both high aspect ratio and high electrical conductivity, such as the Ag@CNTs developed in this study, are regarded as promising conductive fillers for ICAs. Further evaluation of

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