Improvement of Insulating and Thermal Properties of SiO2-Coated

Feb 23, 2016 - Copper nanowire/epoxy terminated poly(dimethylsiloxane) (ETDS) composites were prepared and used as thermal interface materials (TIMs)...
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Improvement of Insulating and Thermal Properties of SiO2‑Coated Copper Nanowire Composites Kiho Kim, Kisang Ahn, Hyun Ju, and Jooheon Kim* School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 156-756, Korea ABSTRACT: Copper nanowire/epoxy terminated poly(dimethylsiloxane) (ETDS) composites were prepared and used as thermal interface materials (TIMs). The surfaces of copper nanowire were coated with SiO2 to enhance their thermal and mechanical properties in the ETDS matrix. The thermal conductivity of CuNW/ ETDS composites continuously increased with an increasing loading of CuNW filler. Moreover, at fixed particle loadings, an increased thermal conductivity was observed in surface-modified composites (CuNW@SiO2/ETDS). The effects of SiO2-coated copper nanowire were investigated using thermal conductivity prediction models. The storage modulus graphs illustrated the enhancement achieved by surface treatment of CuNW. In addition, the SiO2-coated copper nanowire composites showed decreased electrical conductivity as compared to the raw copper nanowire composites. Thus, these SiO2-coated copper nanowire/ETDS composites may be used as thermal interface materials in the electronics industry.

1. INTRODUCTION In electronic devices, thermal management is one of the most important issues to grapple. Because electronic devices consume high power, the use of heat sinks has become indispensable in protecting modern electronic devices from thermal failure.1 The reliability of an electronic device is exponentially dependent on the operating temperature of the junction. A small difference in the operating temperature (10− 15 °C) can cause a 2-fold reduction in the lifespan of a device.2 Therefore, the unwanted heat must be removed to maintain device temperature and lifespan. In order to remove generated heat from a device, a heat sink which was made up of high thermal conductive metal with large surface area was equipped on the opposite side to release the heat to the atmosphere. However, the rough surface between the heat sink and the device caused a microvoid which acts as thermal conductive resistance and prevents effective transfer.3,4 Therefore, it is essential to effectively remove heat, and air voids must be filled with high thermal conductive material. In order to achieve this objective, scientists have employed the strategy of sandwiching an elastomeric material at the interface of the heated device and thermal sink which is called thermal interface material (TIM). Thermal interface materials are thermally conductive materials that are used to reduce thermal resistance at the interface of jointed solid surfaces, such as those between microprocessors and heat sinks. They are used to increase thermal transfer efficiency. Compared to air, thermal interface materials are better conductors of heat in electronic devices, since they reduce the air voids causing resistance to heat transfer. For improving the thermal conductivity of TIMs, the most common approach is to introduce highly conductive fillers into the TIMs. In order to obtain TIMs with enhanced thermal conductivity, scientists have used various fillers, such as metals,5 ceramics,6 and carbon materials.7 These fillers have been © XXXX American Chemical Society

applied to thermosetting or thermoplastic polymers to form composite TIMs. For fabricating materials with high thermal conductivity, scientists have also used a very high filler loading. However, high filler loading often deteriorates the mechanical, electrical, and other related properties of a filled polymer. Therefore, we need to develop a novel strategy to obtain TIMs with high thermal conductivity; the content of fillers should be relatively lower in these TIMs. In addition to the inherent thermal conductivity, the morphology of the filler also plays an important role in the performance of TIMs.8 Compared with microsized fillers, the thermal and mechanical properties of nanosized fillers are much superior.9 Among the varieties of nanostructures, it is preferable to use nanowires in TIMs because of their inherent continuity. Moreover, since nanowires have a large aspect ratio, the percolation threshold of TIMs is lowered with their use.10 Recently, silver nanowire (AgNW) has been extensively used as a filler in TIMs, because they have high thermal conductivity. Moreover, the fabrication process of AgNW is more advanced. Composites made from AgNW exhibit superior thermal conductivity. However, the industrial application of AgNW is limited because it is very costly. On the other hand, copper is an abundant metal that is 100 times less expensive than Ag. Moreover, copper has a high thermal conductivity of 390 W/ mK and good mechanical strength. Therefore, copper nanowire (CuNW) can be conveniently used as a filler in TIMs. However, in spite of their desirable properties, CuNW has only been employed in heat management systems of TIMs, because they have excellent electrical conductivity. However, the Received: November 2, 2015 Revised: February 18, 2016 Accepted: February 23, 2016

A

DOI: 10.1021/acs.iecr.5b04141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the surface treatment procedure of CuNW.

which was manufactured by TCI Korea, was used as a curing agent without performing further purification. 2.2. Preparation of Copper Nanowires. The copper nanowires were synthesized by reducing Cu(NO3)2 with hydrazine in an aqueous solution containing NaOH and ethylenediamine (EDA).16,17 Aqueous solutions of NaOH (2000 mL, 15 M) and Cu(NO3)2 (100 mL, 0.2 M) were added into a reaction flask. Then, EDA (30 mL) and hydrazine (2.5 mL, 35 wt %) were added sequentially, and all the reagents were mixed thoroughly in the flask. The resultant solution was then placed in a reactor, which was heated to 80 °C for 60 min. After the reaction, the CuNWs were washed with a 3 wt % aqueous solution of hydrazine. Thereafter, they were stored in the same hydrazine solution at room temperature in order to minimize their oxidation. 2.3. Surface Treatment of Copper Nanowires with SiO2. The surface-modified CuNWs were prepared by dipping the copper nanowires in solutions containing tetraethyl orthosilicate (TEOS), ethanol, and deionized (DI) water. Then, ethanol was used to eliminate surface impurities of these nanowires. After completing the cleansing process, 5 g of CuNW were dispersed in 100 mL of ethanol. Then, TEOS (1 mL) and DI water (1 mL) were added sequentially. The resultant solution was placed in a magnetic stirrer at 25 °C for 12 h. After stirring, the coated particles were vacuum-filtered and washed with ethanol to eliminate any residual gel that did not cover the particles. Thus, we prevented the agglomeration of CuNW. Finally, the crude products were heat treated at 200 °C for 1 h in N2 atmosphere. This heat treatment produced a homogeneous, uniform, and transparent SiO2 coating on CuNW reinforcements. The procedure used for fabricating SiO2 coated CuNW has been illustrated in Figure 1. In order to control the thickness of the SiO2 layer, the coating procedure was repeated two and three times, and those products were denoted as CuNW@SiO2(2) and CuNW@SiO2(3), respectively. 2.4. Synthesis of ETDS. In our previous study,18 we optimized the weight ratio of epoxy resin to the curing agent in order to provide efficient flexibility to the matrix. In this study, the equivalent weight ratio of ETDS to DDM was 5:1. We placed 2 g of DDM in a four-neck round flask, which was equipped with a reflux condenser. Then, we preheated the flask to 90 °C temperature. Then, we added 10 g of ETDS resin into the flask. We heated the resultant mixture in an oil bath at 90 °C for 1 h under N2 atmosphere. Bubbles were removed from the mixture by placing it in a vacuum oven for 30 min at room temperature. The mixture was then placed in an oil bath at 50 °C for 10 min under N2 atmosphere. Finally, to remove the residual air bubbles, we performed degassing in a vacuum oven for 1 h at room temperature. 2.5. Preparation of CuNW/ETDS Composite. Asprepared CuNWs were used in the preparation of the composite. The ETDS-based composite was prepared by

electrical conductivity of copper is generally greater than that suitable for TIMs. Therefore, in order to effectively use CuNW as a filler, we need to devise a novel strategy that can reduce the electrical conductivity of CuNW composites. CuNW and AgNW are materials having high electrical conductivity and so they have been used as fillers in TIMs. However, these fillers can change the intrinsic insulating properties of polymer materials. Therefore, they cannot be used to prepare composites for dielectric applications as they cannot withstand high thermal conductivity. Alumina (Al2O3), aluminum nitride (AlN), and boron nitride (BN) are fillers with excellent insulating properties, exhibiting high thermal conductivity. These materials have been used as fillers in TIM applications.11−13 However, such composites need a very high filler loading (up to 70 vol %) to achieve the necessary thermal conductivity. As a result, the mechanical properties of these composites are impaired. Among the various methods used to improve the properties of TIMs, one of the most effective ways is by surface coating of electrically conductive fillers. Cui et al. have reported that silica-coated multiwall carbon nanotubes (MWCNT@SiO2) can effectively enhance the electrical and thermal properties of epoxy composites.14 In another study, Choi et al. have reported that silica-coated graphite flakes can enhance the electrical insulating properties and high thermal conductivity of polyvinylpyrrolidone through a sol−gel reaction.15 In this study, we synthesized CuNW. Thereafter, we coated the surface of CuNW with SiO2 (CuNW@SiO2) to enhance the thermal and electrical properties of the polymer’s matrix. We also comprehensively investigated the morphological and microstructural characteristics of the CuNW and CuNW@ SiO2. ETDS composites were fabricated using both raw CuNW and surface-treated CuNW. Then, we examined the thermal and electrical conductivities of these fabricated composites. The effects of surface treatments were investigated by analyzing thermal conductivity prediction models. By performing dynamic mechanical analysis (DMA), we investigated whether the surface-treated CuNW enhanced the mechanical properties of composites.

2. EXPERIMENTAL METHODS 2.1. Materials. Ethylenediamine (C2H8N2), sodium hydroxide (NaOH), and deionized water (DI water) were purchased from Sigma-Aldrich (Seoul, Korea). Hydrazine monohydrate (N2H4·H2O, 80%) and copper nitrate trihydrate [Cu(NO3)2· 3H2O] were purchased from Daejung Chemicals & Metals Co., Ltd. (Seoul, Korea). Ethanol (C2H5OH) and tetraethyl orthosilicate (TEOS) were purchased from Samchun Chemicals (Seoul, Korea). Epoxy-terminated dimethylsiloxane (ETDS) was purchased from Shin-Etsu Silicon (KF-105, equivalent weight (E.E.W.) = 166.6 g/equiv, density =1.20 g/ cm3), and it was used after drying it completely under vacuum at 50 °C for 24 h. 4,4′-Diaminodiphenylmethane (DDM), B

DOI: 10.1021/acs.iecr.5b04141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION The preparation of CuNW and CuNW@SiO2 was confirmed by XRD analysis. Figure 2 shows the XRD patterns of the

mixing various amounts of CuNW with the ETDS matrix. After mixing, the composites were cast onto a Teflon pan. Thereafter, the mixture was degassed in a vacuum oven at 60 °C until air bubbles stopped appearing on the surface of the mixture. After degassing, the CuNW/ETDS mixtures were cured at 180 °C for 2 h. 2.6. Characterization. X-ray diffraction (XRD, New D8Advance/Bruker-AXS) was performed at 40 mA and 40 kV; a scan rate of 1°/s was maintained in a 2θ range of 15 to 80°. Furthermore, Cu Kα radiation (λ = 0.154056 nm) was used to characterize the crystal structures of synthesized materials. The raw and surface-modified CuNW were characterized by performing X-ray photoelectron spectroscopy (XPS, VGMicrotech, ESCA2000) using a Mg Kα X-ray source (1253.6 eV) and a hemispherical analyzer. During curve fitting, the Gaussian peak widths were kept constant in each spectrum. Thus, we investigated the variation in the surface structures of CuNW. Field emission scanning electron microscopy (FESEM, SIGMA, Carl Zeiss) was used to examine the morphology of CuNW and fabricated composites. An instrument of field emission transmission electron microscopy (FETEM) (JEM-2100F, JEOL) was operated at 200 kV. Thus, we captured TEM and high-resolution (HR)-TEM images in the scanning TEM mode of the instrument. The thermal conductivities of the composites were calculated at room temperature using the following equation: k = α ·ρ ·Cp

Figure 2. XRD patterns of synthesized CuNW and CuNW@SiO2.

(1)

where k, α, ρ, and Cp denote the thermal conductivity (W/(m· K)), thermal diffusivity (m2/s), density (kg/m3), and specific heat capacity (J/(kg·K)) of the composite, respectively. The thermal diffusivity of all the films was measured by performing laser flash analysis (LFA) using a Netzsch 447 nanoflash. After measuring thermal diffusivity, the specific heat capacity (Cp) of the samples was examined at room temperature (25 °C) using differential scanning calorimetry (DSC; Netzsch DSC 200F3). The density of the samples was determined by the water displacement method. We measured the storage modulus of epoxy resin and its composites by performing dynamic mechanical analysis (DMA, SS6100, Seico Instruments, Chiba, Japan) at a constant frequency of 1 Hz. Before testing the samples, we dried them in a vacuum oven at 80 °C for 1 h to remove moisture. The dimension of the test sample was 25 mm (height) × 7 mm (width). The specimens were cooled under liquid N2, and then they were heated from −120 to 80 °C at a rate of 5 °C/min. The electrical conductivity of the CuNW composites was measured by performing impedance spectroscopy (IM-6ex, Zahner) at a voltage of 20 mV. For this measurement, we used cylindrical samples, which were 10 mm in diameter. We placed them between Pt-coated plates. Five measurements were carried out under each set of conditions. The electrical conductivity was calculated using the following equation: S=

h (R · A )

prepared CuNW and CuNW@SiO2. The XRD pattern of prepared CuNW shows three typical diffraction peaks at 2θ = 43.5, 50.8, and 74.4°, corresponding to the diffractions from (111), (200), and (220) planes of face-centered cubic copper (JCPDS # 03-1018), respectively.19 No other phases, such as Cu2O and CuO, were detected. As compared to the XRD pattern of raw CuNW, the XRD pattern of CuNW@SiO2 exhibited a weak and wide peak at 2θ = 22°. This peak can be assigned to an amorphous SiO2 layer.20 The XPS analyses were performed for comprehensively evaluating the surface treatment of CuNW. The XPS wide-scan spectra of raw CuNW and CuNW@SiO2 are shown in Figure 3. After surface treatment of CuNW with SiO2, new Si 2p and Si 2s peaks appeared at 105 and 156 eV, respectively. The appearance of these peaks can be attributed to the presence of Si atoms in CuNW@SiO2. In addition, the O 1s peak appears at 532 eV, which is attributed to O atoms in SiO2. These results indicate that SiO2 particles are effectively introduced into the surface of CuNW. The morphologies of as-prepared CuNW and CuNW@SiO2 were characterized by FE-SEM analysis. FE-SEM images of CuNW and CuNW@SiO2 are shown in Figure 4. As shown in Figure 4, the smooth surface of CuNW (Figure 4a) transformed into an uneven and bumpy surface of CuNW@ SiO2 (Figure 4b), indicating that the surfaces of CuNW were completely encapsulated with SiO2. The core−shell structure of CuNW@SiO2 was easily distinguished by TEM images. Figure 5 shows the TEM image of as-prepared CuNW and CuNW@ SiO2 according to the number of coatings. The morphology of the surface treated CuNW was directly established by transmission electron microscopy. As shown in Figure 5a, the raw CuNW has a relatively uniform, smooth surface. Figure 5c shows TEM image of CuNW@SiO2(1), which was observed on the external surface, different from that of the raw CuNW. The

(2)

where S, h, R, and A represent the electrical conductivity (S/ cm), thickness (cm), electrical resistance (Ω), and surface area (cm2) of the composite, respectively. C

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ETDS). To determine the influence of surface treatment on the thermal conductivity of CuNW, the thermal conductivities of fabricated CuNW composites were examined as a function of CuNW concentration; the results are shown in Figure 6. As

Figure 3. XPS spectra of as-synthesized CuNW and CuNW@SiO2.

Figure 6. Thermal conductivity of raw and surface-treated CuNW composites as a function of filler content and surface treatment.

shown in Figure 6, the weight fraction of the filler increased, and the thermal conductivity also increased significantly. Using raw CuNW loadings of 3, 6, 9, 12, and 15 wt %, the thermal conductivity was increased by factors of 1.25, 1.46, 2.09, 3.34, and 4.33, respectively, as compared to pure ETDS (0.19 W/(m· K)). Moreover, using fixed particle loadings, we increased the thermal conductivity of surface-modified composites (CuNW@ SiO2/ETDS). These results are attributed to the internal structure of composites. The surface treatment of CuNW enhanced the interfacial interaction between CuNW and the ETDS matrix. When the interfacial interaction between the filler and polymer matrix is improved, the interfacial thermal conductivity also gets enhanced effectively.21 This enhanced interfacial interaction can be attributed to the interfacial bonding between the filler and the matrix. The silica-coated CuNWs are dispersed in the polymer’s network due to the formation of a hydrogen bond between the basic group of the hydrogen acceptor in the polymer and the silanol group of SiO2.22,23 The silanol groups in the CuNW@SiO2/CuNW form hydrogen bonds with the residual polar groups of the polymer matrix.24 This enhanced interaction reduces both the interfacial

Figure 4. FE-SEM images of (a) raw CuNW and (b) CuNW@SiO2.

coated SiO2 layer is clearly revealed in a high-magnification TEM image. These results indicate that the SiO2 layer is effectively introduced onto the surface of the CuNW. Thus, the surface treatment process was successfully completed in this experiment. Moreover, the thickness of the SiO2 layer almost linearly increases in accordance with repeating the coating. Owing to the SiO2 layer being physically introduced onto the surface, the surface condition seems not to be influenced on continuous introduction of SiO2. The ETDS composites were fabricated using different weight ratios of CuNW@SiO2 (denoted as CuNW@SiO2/ETDS) in a simple mixing method. In order to compare the effect of surface modification on raw CuNW, we also fabricated ETDS composites with raw CuNW filler (denoted as CuNW/

Figure 5. FE-TEM images of (a) raw CuNW, (b) CuNW@SiO2(1), (c) CuNW@SiO2(2), and (d) CuNW@SiO2(3). D

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fraction of the filler. The detailed explanations of each model have been provided in a previous study.18−31 Figure 7 displays

thermal resistance and phonon scattering. Moreover, Figure 6 also contains the thermal transfer property according to filler concentration and the number of SiO2 coatings. As previous mentioned, the first SiO2 coating clearly increased thermal conductivity while the enhancement range was continuously decreased according to repeat of the SiO2 coating. This behavior was caused by mainly two reasons. First is the low thermal conductivity of the SiO 2 layer. The thermal conductivity of SiO2 is 1.4 W/(m·K) which is a very small value compared CuNW.25 Therefore, the increase of the portion of SiO2 reduces the portion of high thermal conductive copper, resulting in the thermal conductivity of filler being decreased. The other reason is phonon scattering at the surface. It can be confirmed by SEM and TEM morphological images that the SiO2 layer was not a uniformly composed and smooth layer but aggregated small particles and rough surface. Compared with the uniform layer, the aggregated small particles increase the number of material interfaces in a linear line. This number of interfaces according to the heat flow path and rough surface was the major cause of phonon interface scattering. As a result, the strong interfacial affinity between SiO2 and the ETDS matrix increased the thermal conductivity whereas the thick SiO2 layer also increased phonon scattering and decreased thermal conductivity. We summarized the thermal conductivity values before and after surface treatment with various one-dimensional materials in Table 1.26,27 As shown, our work shows outstanding results compared to those of other reports which used a similar nanowire to thermal conductive filler.

Figure 7. Comparison of the measured thermal conductivities of CuNW composites with the conductivities calculated using theoretical models.

the predicted values of thermal conductivity in both models as well as the experimental data. As shown in Figure 7, it is clear that the experimental data does not agree very well with the Maxwell−Eucken model. Because Maxwell’s model neglects the contact of particles within the polymeric matrix, it predicts a lower thermal conductivity. Agari’s model yielded better predictions because two adjustable constants C1 and C2 were obtained from the experimental results and applied in this model. To verify the differences in the interfacial states of CuNW filler and ETDS matrix, we observed the cross sections of the composites by FE-SEM, and Figure 8a shows a micrograph of the cross-sectional area of the CuNW 15 wt %/ETDS composite, whereas the cross-sectional area of CuNW@SiO2 15 wt %/ETDS composite is displayed in Figure 8b. As compared to the CuNW@SiO2 15 wt %/ETDS (Figure 8b), the composites fabricated from raw CuNW contained air voids between the CuNW and the polymeric matrix Figure 8a). The presence of air voids is attributed to the absence of interfacial interactions between raw CuNW and the polymeric matrix. Figure 8b reveals an enhanced compatibility between the filler and the matrix in the case of composites containing SiO2. The air voids present in the untreated composite were mostly removed; these air voids were attributed to the active interfacial adhesion between CuNW/SiO2 and ETDS. To determine the effect of surface modification by filler particles, we characterized the dynamic mechanical properties of CuNW/ETDS composites using DMA. As shown in Figure 9, the storage modulus was measured to confirm the strong interactions between CuNW@SiO2 and ETDS matrix. These properties are highly dependent on the existence of fillers, their dispersion within the matrix, volume fraction, geometrical characteristics, and load transfer from the filler to the matrix.32

Table 1. Thermal Conductivity Comparison with Other Research According to Surface Treatment thermal conductivity [W m−1 K−1] Chen et al. Wang et al. this work

AgNW/epoxy AgNW@SiO2/epoxy CuNW/epoxy AgNW/epoxy CuNW/ETDS CuNW@SiO2/ETDS

0.35 (1 vol %) 0.42 (1 vol %) 0.40 (1 1.23 (1 0.42 (1 vol %) 0.50 (1 vol %)

0.47 (2 vol %) 0.67 (2 vol %) vol %) vol %) 0.83 (2 vol %) 1.1 (2 vol %)

In order to predict the thermal conductivity of the composite materials, many theoretical and empirical models have been reported. The Maxwell−Eucken model is often used to describe the thermal conductivity of composites consisting of particles dispersed into a polymeric matrix.28,29 This model is generally used to predict the thermal conductivity of composites containing a low volume fraction of randomly distributed filler.30 For these reasons, the Maxwell−Eucken model was adopted to describe the thermal conductivity of CuNW@SiO2/ ETDS composites. To further investigate the thermal conductivity of CuNW@SiO2/ETDS composites, we applied the Agari model. Maxwell−Eucken model: kc k + 2k m − 2Φf (k f − k m) = f km k f + 2k m + Φf (k f − k m)

(3)

Agari model: log kc = Φf C2 log k f + (1 − Φf ) log(C1k m)

(4)

where kc, km, and kf are the thermal conductivities of composites, polymer, and filler, respectively. Φf is the volume E

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Figure 9. Storage modulus of ETDS and CuNW/ETDS composites with 15 wt % filler concentration.

actions between CuNW@SiO2 and ETDS matrix, restricting the segmental mobility of ETDS molecules during glass transition. The effect of surface treatment of CuNW on the electrical properties of composites was examined by measuring their electrical conductivity. At room temperature, we measured the electrical conductivity of both raw ETDS and ETDS composites containing CuNW@SiO2 with various SiO2 coating numbers. Under identical conditions, we also measured the electrical conductivity of ETDS composites containing the same weight of CuNW loading. The results are presented in Figure 10. When 15 wt % CuNW was added to EDTS matrix, the electrical conductivity of the ETDS matrix increased from 1.10 × 10−14 S cm−1 to 1.13 × 10−6 S cm−1. Thus, the electrical conductivity of the ETDS matrix increased by about 8 orders of magnitude owing to CuNW loading into the ETDS matrix. This increase in electrical conductivity with increasing CuNW content can be attributed to the presence of more CuNW inside the ETDS composite, leading to a higher probability of CuNW forming a conductive network within the matrix. Thus, the electrical conductivity of CuNW/ETDS composites is significantly higher. Moreover, compares to the electrical conductivity of CuNW/ETDS composites, the CuNW@ SiO2/ETDS composites exhibited lower electrical conductivity values. The electrical conductivity of the CuNW@SiO2/ETDS composite was 9.10 × 10−10, which is lower by about 3 orders of magnitude than that of CuNW/ETDS composite. In this range of electrical conductivity, CuNW@SiO2/ETDS composites can be conveniently used in electrical insulators and electronic packaging materials.35 This indicates that the SiO2 layer on the CuNW@SiO2 surface effectively hinders the formation of electrical networks in CuNW@SiO2/ETDS composites, because the SiO2 layer disrupts the transport of conjugating electrons, thereby increasing the tunneling energy barrier. However, there was not observed significant change according to the increase of SiO2 thickness. Due to the electrical conductivity of raw ETDS being sufficiently lower

Figure 8. Cross-sectional images of (a) CuNW 15 wt %/ETDS composite and (b) CuNW@SiO2 15 wt %/ETDS composite.

As shown in Figure 9, the storage modulus of CuNW/ETDS composites was higher than that of raw ETDS. This is attributed to the mechanical reinforcement elicited by the CuNW. The increased stiffness provided by the filler ensures that a greater degree of stress is transferred from the matrix to the filler.33 Furthermore, the storage modulus of the ETDS composite clearly demonstrates a correlation with the surface modification of CuNW. Within the experimental temperature range, the storage modulus of the CuNW@SiO2/ETDS composite was consistently higher than that of the CuNW/ ETDS composite. The observed increase in the storage modulus of the CuNW@SiO2/ETDS composites is attributed to the more effective load transfer between CuNW@SiO2 and ETDS matrix.14 This is a direct result of the interaction between CuNW@SiO2 and ETDS matrix, imparting an enhanced interfacial adhesion that restricts the motion of ETDS segmental chains.34 This indicates that the SiO2 layer on the surface of CuNW@SiO2 enhances the interfacial interF

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REFERENCES

(1) Yu, A.; Ramesh, P.; Itkis, M. E.; Bekyarova, E.; Haddon, R. C. Graphite nanoplatelet-epoxy composite thermal interface materials. J. Phys. Chem. C 2007, 111, 7565. (2) Sim, L. C.; Ramanan, S. R.; Ismail, H.; Seetharamu, K. N.; Goh, T. J. Thermal characterization of Al2O3 and ZnO reinforced silicone rubber as thermal pads for heat dissipation purposes. Thermochim. Acta 2005, 430, 155. (3) Uetani, K.; Ata, S.; Tomonoh, S.; Yamada, T.; Yumura, M.; Hata, K. Elastomeric thermal interface materials with high through-plane thermal conductivity from carbon fiber fillers vertically aligned by electrostatic flocking. Adv. Mater. 2014, 26, 5857. (4) Shahil, K. M. F.; Balandin, A. A. Thermal Properties of Graphene and Multilayer Graphene: Applications in Thermal Interface Materials. Solid State Commun. 2012, 152, 1331. (5) Moghadassi, A. R.; Hosseini, S. M.; Henneke, D. E. Effect of CuO nanoparticles in enhancing the thermal conductivities of monoethylene glycol and paraffin fluids. Ind. Eng. Chem. Res. 2010, 49, 1900. (6) Batmunkh, M.; Tanshen, R.; Nine, J.; Myekhlai, M.; Choi, H.; Chung, H.; Jeong, H. Thermal Conductivity of TiO2 Nanoparticles based Aqueous Nanofluids with an Addition of a Modified Silver Particle. Ind. Eng. Chem. Res. 2014, 53, 8445. (7) Zhou, J.; Ventura, I. A.; Lubineau, G. Probing the Role of Poly(3,4-ethylenedioxythiophene)/Poly(styrenesulfonate)-Coated Multiwalled Carbon Nanotubes in the Thermal and Mechanical Properties of Polycarbonate Nanocomposites. Ind. Eng. Chem. Res. 2014, 53, 3539. (8) Wu, H. P.; Liu, J. F.; Wu, X. J.; Ge, M. Y.; Wang, Y. W.; Zhang, G. Q.; Jiang, J. Z. High Conductivity of Isotropic Conductive Adhesives Filled with Silver Nanowires. Int. J. Adhes. Adhes. 2006, 26, 617. (9) Pashayi, K.; Fard, H. R.; Lai, F.; Iruvanti, S.; Plawsky, J.; BorcaTasciuc, T. High Thermal Conductivity Epoxy-Silver Composites Based on Self-Constructed Nanostructured Metallic Networks. J. Appl. Phys. 2012, 111, 104310. (10) Tao, Y.; Yang, Z. G.; Lu, X. L.; Tao, G. L.; Xia, Y. P.; Wu, H. P. Influence of Filler Morphology on Percolation Threshold of Isotropical Conductive Adhesives (ICA). Sci. China: Technol. Sci. 2012, 55, 28. (11) Im, H.; Kim, J. The effect of Al2O3 doped multi-walled carbon nanotubes on the thermal conductivity of Al2O3/epoxy terminated poly(dimethylsi-loxane) composites. Carbon 2011, 49, 3503. (12) Xu, Y.; Chung, D. D. L. Increasing the thermal conductivity of boron nitride and aluminum nitride particle epoxy-matrix composites by particle surface treatments. Compos. Interfaces 2000, 7, 243. (13) Zhi, C.; Bando, Y.; Terao, T.; Tang, C.; Kuwahara, H.; Golberg, D. Towards thermoconductive, electrically insulating polymeric composites with boron nitride nanotubes as fillers. Adv. Funct. Mater. 2009, 19, 1857. (14) Cui, W.; Du, F.; Zhao, J.; Zhang, W.; Yang, Y.; Xie, X.; Mai, Y. W. Improving thermal conductivity while retaining high electrical resistivity of epoxy composites by incorporating silica-coated multiwalled carbon nanotubes. Carbon 2011, 49, 495. (15) Choi, S.; Kim, K.; Nam, J.; Shim, S. E. Synthesis of silica-coated graphite by enolization of polyvinylpyrrolidone and its thermal and electrical conductivity in polymer composites. Carbon 2013, 60, 254. (16) Rathmell, A. R.; Bergin, S. M.; Hua, Y.-L.; Wiley, B. J.; Li, Z.-Y. The growth mechanism of copper nanowires and their properties in flexible, transparent conducting films. Adv. Mater. 2010, 22, 3558. (17) Yu, J. C.; Zhao, F. G.; Shao, W.; Ge, C. W.; Li, W. S. Shapecontrollable and versatile synthesis of copper nanocrystals with amino acids as capping agents. Nanoscale 2015, 7, 8811. (18) Im, H.; Kim, J. Enhancement of the thermal conductivity of aluminum oxide−epoxy terminated poly(dimethyl siloxane) with a metal oxide containing polysiloxane. J. Mater. Sci. 2011, 46, 6571. (19) Jin, M.; He, G.; Zhang, H.; Zeng, J.; Xie, Z.; Xia, Y. ShapeControlled Synthesis of Copper Nanocrystals in an Aqueous Solution with Glucose as a Reducing Agent and Hexadecylamine as a Capping Agent. Angew. Chem., Int. Ed. 2011, 50, 10560.

Figure 10. Electrical conductivity of the composites with raw CuNW and CuNW@SiO2 as a function of the filler content and surface treatment.

than that of SiO2, the thickness of SiO2 did not effectively interrupt the path but only influenced the filler junction.

4. CONCLUSION In the present study, copper nanowire/ETDS composites were prepared using both raw copper nanowire as well as surfacetreated copper nanowire. The surfaces of the copper nanowire were coated with SiO2. The surface treatment of CuNW was found to enhance the interfacial interaction with the ETDS matrix. As a result, the SiO2-coated CuNW composites possessed superior thermal conduction properties as compared to the composites prepared from raw CuNW. In addition, we determined the effect of SiO2 coating on the electrical insulating properties of the composite by measuring its electrical conductivity. Owing to the electrical insulating properties of SiO2, the CuNW@SiO2/ETDS composites showed lower electrical conductivity as compared to the CuNW/ETDS composites. The storage modulus graphs illustrated the enhancement achieved by surface treatment of CuNW. Therefore, it is concluded that the introduction of SiO2 onto CuNW is an effective method for producing TIMs with reasonably high thermal conductivity and lower electrical conductivity.

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AUTHOR INFORMATION

Corresponding Author

*(J.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Midcareer Researcher Program through an NRF grant funded by the MEST (2014R1A2A1A11049625). G

DOI: 10.1021/acs.iecr.5b04141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research (20) Li, C.; Mei, J.; Li, S.; Lu, N.; Wang, L.; Chen, B.; Dong, W. Onepot synthesis of Ag@SiO2@Ag sandwich nanostructures. Nanotechnology 2010, 21, 245602. (21) Yang, S. Y.; Ma, C. C. M.; Teng, C. C.; Huang, Y. W.; Liao, S. H.; Huang, Y. L.; Tien, H. W.; Lee, T. M.; Chiou, K. C. Effect of functionalized carbon nanotubes on the thermal conductivity of epoxy composites. Carbon 2010, 48, 592. (22) Saegusa, T.; Chujo, Y. An Organic/Inorganic Hybrid Polymer. J. Macromol. Sci., Chem. 1990, 27, 1603. (23) Chujo, Y.; Ihara, E.; Kure, S.; Saegusa, T. Synthesis of triethoxysilyl-terminated polyoxazolines and their cohydrolysis polymerization with tetraethoxysilane. Macromolecules 1993, 26, 5681. (24) Naskar, S.; Wolter, S. D.; Bower, C. A.; Stoner, B. R.; Glass, J. T. Verification of the O−Si−N complex in plasma-enhanced chemical vapor deposition silicon oxynitride films. Appl. Phys. Lett. 2005, 87, 261907. (25) The General Properties of Si, Ge, SiGe, SiO2 and Si3N4. Virginia Semiconductor: Fredericksburg, VA, 2002. http://www.virginiasemi. com/pdf/generalpropertiesSi62002.pdf (last accessed March 2012). (26) Chen, C.; Tang, Y.; Ye, Y. S.; Xue, Z.; Xue, Y.; Xie, X.; Mai, Y. W. High-performance epoxy/silica coated silver nanowire composites as underfill material for electronic packaging. Compos. Sci. Technol. 2014, 105, 80. (27) Wang, S.; Cheng, Y.; Wang, R.; Sun, J.; Gao, L. Highly Thermal Conductive Copper Nanowire Composites with Ultralow Loading: Toward Applications as Thermal Interface Materials. ACS Appl. Mater. Interfaces 2014, 6, 6481. (28) Maxwell, J. C. A treatise on electricity and magnetism, 3rd ed.; Dover: New York, 1954. (29) Progelhof, R. C.; Throne, J. L.; Ruetsch, R. R. Methods for predicting the thermal conductivity of composite systems: a review. Polym. Eng. Sci. 1976, 16, 615. (30) Ha, S. M.; Lee, H. L.; Lee, S. G.; Kim, B. G.; Kim, Y. S.; Won, J. C.; Choi, W. J.; Lee, D. C.; Kim, J.; Yoo, Y. Thermal conductivity of graphite filled liquid crystal polymer composites and theoretical predictions. Compos. Sci. Technol. 2013, 88, 113. (31) Ahn, K.; Kim, K.; Kim, J. Fabrication of surface-treated BN/ ETDS composites for enhanced thermal and mechanical properties. Ceram. Int. 2015, 41, 9488. (32) Zhao, Y.; Barrera, E. V. Asymmmetric Diamino Functionalization of Nanotubes Assisted by BOC Protection and Their Epoxy Nanocomposites. Adv. Funct. Mater. 2010, 20, 3039. (33) Mohanty, S.; Verma, S. K.; Nayak, S. K. Dynamic mechanical and thermal properties of MAPE treated jute/HDPE composites. Compos. Sci. Technol. 2006, 66, 538. (34) Mai, Y. W.; Yu, Z. Z. Polymer nanocomposites; Woodhead Publishing: 2006. (35) Dai, W.; Yu, J.; Liu, Z.; Wang, Y.; Song, Y.; Lyu, J.; Bai, H.; Nishimura, K.; Jiang, N. Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers. Composites, Part A 2015, 76, 73.

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DOI: 10.1021/acs.iecr.5b04141 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX