Vertically Aligned and Interconnected SiC Nanowire Networks

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Vertically Aligned and Interconnected SiC Nanowire Networks Leading to Significantly Enhanced Thermal Conductivity of Polymer Composites Yimin Yao, Xiaodong Zhu, Xiaoliang Zeng, Rong Sun, Jian-Bin Xu, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00328 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Materials & Interfaces

Vertically Aligned and Interconnected SiC Nanowire Networks Leading to Significantly Enhanced Thermal Conductivity of Polymer Composites Yimin Yao, †,‡ Xiaodong Zhu, †,║ Xiaoliang Zeng, *, † Rong Sun, *, † Jian-Bin Xu, ⊥ Ching-Ping Wong †, ⊥, # †

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen

518055, China. ‡

Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences,

Shenzhen 518055, China. ║

Department of Nano Science and Technology Institute, University of Science and

Technology of China, Suzhou 215123, China ⊥

Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong

999077, China. #

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta,

Georgia 30332, United States. *

Address corresponding to [email protected] and [email protected]

ACS Paragon Plus Environment

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ABSTRACT Efficient heat removal via thermal management materials has become one of the most critical challenges in the development of modern microelectronic devices. However, previously reported polymer composites exhibit limited enhancement of thermal conductivity even when highly loaded with thermally conductive fillers due to the lack of efficient heat transfer pathways. Herein, we report vertically aligned and interconnected SiC nanowire networks as efficient filler for polymer composites achieving significantly enhanced thermal conductivity. The SiCNW networks are produced by freeze-casting nanowire aqueous suspensions followed by thermal sintering to consolidate the nanowire junctions, exhibiting a hierarchical architecture in which honeycomb-like SiCNW layers are aligned. The composite obtained by infiltrating SiCNW network with epoxy resin, at a relatively low SiCNW loading of 2.17 vol %, represents a high through-plane thermal conductivity (1.67 Wm-1K-1) compared to the pure matrix that is equivalent to a significant enhancement of 406.6% per 1 vol% loading. The orderly SiCNW network which can act as macroscopic expressway for phonon transport is believed to be the main contributor for the excellent thermal performance. This strategy provides an insight for the design of high-performance composites with potential to be used in advanced thermal management materials. KEYWORDS:

freeze

casting,

thermal

conductivity,

silicon

carbide

nanowire,

three-dimensional network, polymer composite 1. INTRODUCTION Stricter demands for high-performance electronic devices put forward higher packing and power density of modern electronic components, inevitably causing thermal failure.1-2


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Efficient thermal dissipation has become a stringent necessity in miniaturized electronic and photonic devices for longer lifetime and higher speed.1, 3-10 Thermally conductive materials are believed to be the best choice to help address the heat-removal issue.11-14 Usually, thermally conductive materials are composed of thermally conductive fillers and a polymer matrix.15 Solution mixing has been commonly used to fabricate thermally conductive polymer composites by randomly dispersing fillers in the polymer matrix.16 However, enhancement of thermal conductivity (K) for the polymer composites with disorderly dispersed fillers is limited due to the lack of efficient thermal transfer pathways. These pathways are required for enhanced K because phonon heat conduction can occur with much less scatterings among fillers when compared with the polymer matrix.17 Although many methods with high loading fillers have been introduced, the mechanical properties of composites deteriorate in efforts to surmount their low K. Therefore, it remins a challenge to achieve high efficiency of K enhancement at low filler loading. As filler is a better heat conductor than the polymer matrix, the formation of continuous pathways of fillers is the key to achieve high K in polymer composites. Thus, controllable algnment of fillers or construction of three-dimensional (3D) filler networks in polymer has been intensively studied.11,

15-16, 18-28

This thought could ensure that most of the energy

transmits through the directional filler network, reducing unnecessary filler/matrix interficial thermal resistance. As a result, high efficiency of K enhancement can be achieved in minimized loading concentration of fillers.5,

29-32

The development of film-making

technologies (e.g., vacuum-assisted filtration, bar-coating, spin-casting, and mechanical stretching) has pushed forward the high in-plane thermal performance of polymer composites,



which is a result of in-plane stacking of filler to form conductive network in the polymer.33-38 On the contrary, the lack of through-plane assembly technology has led to the low efficiency of through-plane K enhancement in the polymer composites. Thus, freeze casting or so called ice-templated assembly as an effective strategy to align fillers in through-plane direction has attracted much attention in fabricating thermally conductive composites recently. By placing aqueous slurry on a copper pillar which was pre-cooled in liquid nitrogen, ice crystals will grow from the bottom of the slurry and grow along the temperature gradient.39-40 The ice crystals will repel the suspended fillers as they grow within the slurry, effectively templating the fillers.40-41 After removing the ice crystals by freeze drying, a 3D oriented network structure is obtained. Most of the previous studies focus on ice-templated assembling two-dimensional (2D) materials within the polymer matrix to enhance the through-plane K due to the high aspect-ratio of 2D materials.4-5, 42-44 For example, we previously fabricated 3D boron nitride nanosheets network and its polymer composites.42 The resultant composites exhibit a K value of 2.85 Wm-1K-1 at a filler loading of 9.29 vol%. Vertically aligned graphene networks are also used as the filler, contributing to a high K (2.13 Wm-1K-1) at a ultralow graphene loading of 0.92 vol%.45 When compared to 2D fillers, one-dimensional (1D) fillers with high aspect ratio were also expected to construct long heat conductive pathways along the longitudinal direction of 1D fillers in the composites.46 However, such inspiring structures are less studied in previous thermally conductive composites. Silicon carbide is a promising filler for polymer composites, which combines high thermal conductivity (~120 Wm-1K-1), excellent chemical stability, low thermal expansion coefficient and strong mechanical strength.

25, 47-50

Herein, we employ freeze casting to


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directly assemble silicon carbide nanowires (SiCNWs) into 3D macroscopic architecture as filler for epoxy resin. The 3D monoliths obtained have a hierarchical binary network architecture (2D nano-networks of SiCNWs compartmental films and 3D SiCNW micro-networks of the interconnected compartmental units). Thermally conductive composites were then prepared by impregnating epoxy resin into the 3D SiCNWs network. At a low SiCNW loading of 2.17 vol %, the composite performed a dramatically improved K value (1.67 Wm-1K-1) compared to that of neat polymer (0.18 Wm-1K-1), corresponding to an enhancement efficiency (η) of 406.6% per 1 vol% filler loading. The η value is 3~8 times higher than other reported thermally conductive and electrical insulating composites with a similar level of filler loading. The excellent thermal conduction performance is mainly attributed to the oriented SiCNW network which is believed to provide expressway for phonon transport, leading to significantly enhanced thermal conductivity. In addition, the 3D SiCNW/epoxy composites also exhibit good electrical insulation with the volume resistivity over 7.1×1014 Ω·cm. The 3D interconnected structure which leads to an improved thermal conductivity exhibits strong potential for thermal management applications for a variety of technological needs, particularly thermal interface materials. 2. EXPERIMENTAL SECTION 2.1 Materials. SiCNWs with a purity of 99% were purchased from Changsha Sinet Advanced Materials Co., Ltd, China. Sodium carboxymethyl cellulose (SCMC) as adhesive was obtained from Sinopharm Chemical Reagent Co. Ltd, China. Epoxy resin was offered by Hexion Specialty Chemicals. Curing agent methyltetrahydrophthalic anhydride and catalytic agent 2-ethyl-4-methylimidazole were offered by Shanghai Lingfeng Chemical Reagent Co.,



Ltd., China. 2.2 Preparation of 3D SiCNW Networks. Before preparation of SiCNW networks, SiCNWs were first ball-milled in sodium dodecyl benzene sulfonate solution (1 wt%) for 6 h to remove the SiCNW agglomerations. Free SiCNWs were then separated via centrifugation at 400 rpm for 10 min, followed by drying in oven. In a typical preparation process, a certain mass of SiCNWs were added to the SCMC solution followed by sonication for 30 min and ball-milling for 4 h. The suspension with a certain concentration was placed in a cylindrical mold and then immediately placed on a metal block pre-cooled in liquid nitrogen. The frozen scaffolds were freeze dried for 48 h (SCIENTZ-10N) and subsequently sintered at 1000 °C for 6 h. 2.3 Preparation of the 3D SiCNW/epoxy composites. The 3D SiCNW/epoxy composites were fabricated by vacuum-assisted impregnation of epoxy. Before the impregnation, the epoxy resin along with methyltetrahydrophthalic anhydride and 2-ethyl-4-methylimidazole were uniformly mixed. The 3D SiCNW networks were then immersed into the mixture under the assistance of vacuum. After infiltration for 6 h, the samples were cured in oven at 120°C for 2 h, 160 °C for 3 h, and 180 °C for 2 h, respectively. 2.4 Measurements and Characterizations. The FEI Nova NanoSEM 450 field emission scanning electron microscopy (SEM) was applied to examine the microstructure of the samples. The morphology of SiCNWs was further characterized by FEI Tecnai G2 F20 transmission electron microscope (TEM). The porosity of the SiCNW networks was measured by Archimedes’ method. X-ray diffraction (XRD) patterns of SiCNW were characterized by a Rigaku X-ray diffractometer using Cu Kα radiation (λ=1.5418 Å).


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Elemental scanning was achieved by X-ray photoelectron spectroscopy (XPS) using RBD PHI-5000CESCA (Perkin Elmer, USA). The measurement of volume resistivity was carried out with a Keithley-6517B high-resistance meter at an applied AC voltage of 1000 V. K was measured using LFA 467 NanoFlash apparatus (NETZSCH), calculated by (1)

‫ ܭ‬ൌ ߮ ൉ ߩ ൉ ‫ܥ‬௣

where, φ is the thermal diffusivity, ρ is the density of composite obtained according to ρ=m·V−1, where m and V are the mass and volume of the sample, respectively. The variations of the surface temperature of 3D-SiCNW/epoxy composites were recorded by an infrared thermograph (FLIR T1040). 3. RESULTS AND DISCUSSION

3.1 Fabrication and characterization of 3D SiCNW network

Figure 1. Characterization of SiCNWs. SEM images of (a) a pile of SiCNWs and (b) a single slice of SiCNW. (c) Length distribution of SiCNWs. (d) XRD pattern of SiCNWs.



Figure 1a shows the typical morphology of SiCNWs which posses a typical length of 5-12 µm, and accompanied by a small amount of short SiCNW whiskers. SEM characterization (Figure 1b) further indicates a uniform SiCNW diameter of 200-250 nm, which is also confirmed by the TEM image (Figure S1, Supporting Information). By statistic evaluation of one hundred SiCNWs, we can see that 66% of the SiCNWs are in the length range from 5 to 9 µm (Figure 1c). The XRD pattern of SiCNWs shows that the characteristic peaks of cubic β-SiC appear at 35.8°, 41.3°, 60.0°, 71.9°, and 75.5°, corresponding to (111), (200), (220), (311), and (222) crystal face, respectively, as shown in Figure 1d.34, 36 XPS detection reveals that besides the characteristic peaks of SiC, the elemental scan of Si2p was centered at 100.3 eV (Figure S2, Supporting Information). As schematically illustrated in Figure 2a, the entire procedure of obtaining 3D SiCNWs/epoxy composites consists of three steps: (1) oriented freeze casting of SiCNW suspension; (2) lyophilization of vertically aligned SiCNW networks; (3) sinter of SiCNW networks and impregnation of epoxy resin. SiCNWs were used as the building blocks to fabricate directional aerogels. In freeze casting, along the temperature gradient direction, phase separation takes place between SiCNWs and the developing ice crystals, and the SiCNWs are mounted up and vertically aligned by the developing ice templates. The frozen SiCNWs suspension was then lyophilized, leaving behind a highly porous percolated network of nanowires, with SiCNW junctions being connected by SCMC. Figure 2b shows the digital image of a typical aerogel cylinder placed in a plastic tube. The SiCNW aerogels were then sintered in air atmosphere at 400 °C for 4 h to fully burn out the SCMC. Multi-dimensional SiCNWs were used here to provide enough mechanical strength which originated from the


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friction force of SiCNW/SiCNW interfaces, ensuring the SiC network wouldn’t shrink or collapse after the removal of SCMC. The sample is then thermally treated at 1000 °C to sinter the individual nanowires and consolidate the structure. The high temperature promotes the formation of liquid SiCNWs which can fill the space formed by the removal of SCMC and facilitates fiber-fiber bonding. Typical SEM images of SiCNW junction after sintering are shown in Figure S3 (Supporting Information), where some of the remaining liquid phase formed during sintering can still be seen. The sintered junctions could provide much stronger interactions than chemical or non-covalent bonds, leading to strong mechanical strength of the network.25 The 3D SiCNW/epoxy composites were finally fabricated after impregnation of epoxy into SiCNW network. A macroscopic and aligned network can be clearly seen from Figure 2c-e. The sintered lattices exhibit a structure with high porosity and a hierarchical architecture formed by aligned, thin and highly porous SiCNW layers. The frontal and lateral morphology of the thin SiCNW layer were further characterized as shown in Figure 2d and 2e. The thickness was estimated to be 4 µm. It should be noted that SiCNWs exhibit macroscopically orderly instead of microcosmically arranged.



Figure 2. Fabrication of 3D SiCNW network. (a) Schematic illustration of forming SiCNW network. (b) Digital photograph of SiCNW network placed in a plastic tube. (c) SEM image showing the layered structure of a sintered SiCNW network. Frontal (d) and lateral (e) morphology of the thin SiCNW layer in (c). As the SiCNWs act as the assembly units of the network, the final architecture can be controllable fabricated by varying the composition of the starting suspension including the solid concentration and the content of SCMC. By increasing the solid loading of the suspension from 0.76 to 2.17 vol%, the porosity decreases from 99.0% to 97.5% with the corresponding morphology shown in Figure 3a-d. Some nanowires are trapped by the growing ice crystals during freezing and laid perpendicular to the ice growing direction after the healing of the crystal tips (Figure 3b). The number of bridges increases with the increasing nanowire concentration in the suspension. Meanwhile, the SEM images of the SiCNW networks show that the increasing solid loading leads to a decrease in the pore size but an increase in the pore numbers. Higher solid loading might restrict the movement space


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of SiCNWs during the freeze casting which leads to annular and upward trajectory for ice-templated

assembly.

As

a

result,

the

corresponding

microstructure

appears

honeycomb-like morphology when compared with that of lower solid loading. Figure S4 (Supporting information) further indicates the assembly unit of honeycomb-like structure becomes more solid simultaneously.

Figure 3. Structural characterization of SiCNW networks with different solid loadings and SCMC contents. (a-d) SEM images of SiCNW networks with a solid loading of (a) 0.76, (b)



1.25, (c) 1.79 and (d) 2.17 vol%, respectively. (a, e-g) SEM images of SiCNW networks with a SCMC content of (a) 1.0, (e) 3.0, (f) 7.4 and (g) 13.6 wt%, respectively. Since the viscosity of the suspension is the main parameter to determine the kinematic velocity of SiCNWs, which in turn control the number of the bridges, the microstructure and pore structures of 3D SiCNW network can also be tuned by varying the SCMC content. The 3D interlocking structure is a comprehensive result of the competion between the ice growing rate and the nanowire moving speed.51 Under the situation where the ice growing velocity is too fast or the movement of SiCNWs is too slow, the ice crystals would grow forward before all the SiCNWs shift to the gaps, resulting in the formation of perpendicular bridges. SCMC was used here not only as adhesive but also as a tool to control the movement speed and linear arrangement of SiCNWs. Figure 3a and 3e-g show the macroscopic morphology of the freeze-dried bodies created from suspensions with the SCMC concentrations of 1.0, 3.0, 7.4, 13.6 wt%, respectively. Clearly, the architecture has a strong dependence on the concentration of SCMC, which controls the viscosity of the suspension and the degree of alignment for forming various fashions of synapses and bridges. It is interesting that the vertical SiCNW walls are replaced by helical arrays in the microstructure as the solid loading reaches 13.6 wt%.

3.2 Structural characterization of 3D SiCNW/epoxy composite After impregnation by epoxy, the pores of 3D SiCNW network are fully filled with resin, as shown in Figure 4a-c. Negligible deformation occurred during the infiltration process due to the stable interconnected structures. Importantly, the SiCNW network exhibits good


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adhesion with the epoxy resin; no obvious interfacial debonding between the epoxy and the 3D SiCNW network is observed according to the cross sections of fractured 3D SiCNW/epoxy composites. The existence of aligned white lines indicates that the 3D SiCNW interconnected network is well maintained within epoxy resin. The parallel polymer blocks in the composites were confined by the vertically aligned SiCNW walls, which existed between these blocks. The thickness of individual polymer block is ∼10 µm, similar to the distance between the adjacent SiCNW walls in the 3D SiCNW network. The orderly distribution of SiCNWs within the composites was further confirmed via EDS (Figure 4d-f). The spectral results of elements Si and O exhibit that the SiCNW and epoxy resin are well stacked one by one along the aligned direction. These features would be beneficial for enhancing the thermal conductivity of the 3D SiCNW/epoxy composites. The cross-section morphology of epoxy composite with higher SiCNW loading was also investigated (Figure S5, Supporting Information). The in-plane SiCNW bridges begin to occur as shown in the red ellipse. However, the in-plane bridges are not that obvious as the vertically aligned SiCNW walls due to the thinner thickness. Meanwhile, the in-plane bridges were constituted by less SiCNWs than vertical SiC walls which make them more easily filled with epoxy resin.



Figure 4. Characterization of 3D SiCNW/epoxy composites. (a-c) SEM images of fractured morphology in SiCNW/epoxy composites at different magnifications. (d) EDS scanning of cross sections of fractured 3D SiCNW/epoxy composites. The spectral results of elements Si (e) and oxygen (f).

3.3 Thermal conductivity of 3D SiCNW/epoxy composite The aligned and interconnected SiCNWs chains generate a “directed percolated” thermally conductive pathway array throughout the composites (Figure 5a). The thermal conductivity of the composites in both through-plane and in-plane directions is shown in Figure 5b as a function of SiCNW loading. The pure epoxy has a low K, 0.18 Wm-1K-1 at room temperature, which agrees well with previous results.52 Enhancement of K values has been found for all measured composites. Since the anisotropic microstructure of 3D SiCNW network, the 3D SiCNW/epoxy composites also show the anisotropic nature for K enhancement. There is a distinct contrast of K values between the through-plane (0.36 Wm-1K-1) and in-plane (0.20 Wm-1K-1) direction under a loading of 0.76 vol %. At the


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highest loading (2.17 vol%), K values in the through-plane and in-plane directions for the composites reach 1.67 and 1.45 Wm-1K-1, respectively. It is interesting to note that the anisotropy in K is gradually diminished in 3D SiCNW/epoxy composites with the increase of SiCNW loadings, which can be explained by the fact that the microstructure is changed from well-ordered wall-like to honeycomb-like morphology at higher SiCNW loading. Figure 5c exhibits the negative effect of the increasing SCMC content on through-plane K of the composites. This is due to that the cross-linked bridges formed on the in-pane direction weaken the through-plane phonon transport. The increased in-plane K in Figure S6 (Supporting Information) further proves our conclusion. Another factor for K of 3D SiCNW/epoxy composites is the filler alignment. Besides the 3D anisotropic SiCNW network

with

alignments

(3D

Aniso-SiC

network),

aerogels

prepared

by

an

“isotropic-freezing” (3D Iso-SiC network), that is, submersion of the SiCNW suspension into liquid nitrogen which causes solidification to converging inward, was also fabricated as filler. The structural characterization of 3D Iso-SiC network and its composite was exhibited in Figure S7 (Supporting Information). The corresponding through-plane thermal conductivity of composites filled with 3D Aniso-SiC and 3D Iso-SiC network along with randomly dispersed SiCNWs at the same filler loading was shown in Figure 5d. The K variations in different samples reveal the importance of interconnected network and alignment along the heat flux. The oriented SiCNWs support the thermal pathway, where phonons tend to transmit between SiCNWs because of their contacting with each other. The SiCNW-SiCNW interface thermal resistance in the oriented and interconnected SiCNWs composites can be considered to be much smaller than the SiCNW−epoxy interfacial thermal resistance in



randomly dispersed SiCNWs composites.42,

52

Figure 5e shows temperature-dependent

thermal conductivity of the 3D SiCNW/epoxy composites with 2.17 vol % of SiCNWs upon multiple heating and cooling cycles alternating between 25 and 125 °C. The thermal conductivity exhibits a slight change (±5%) within the 20 cycles, suggesting stable capability of heat conduction in this temperature range. Such a small variation of thermal conductivity with temperature would be beneficial for the long-term device operation.


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Figure 5. Thermal conductivity of 3D SiCNW/epoxy composites. (a) Schematic illustration of the enhanced through-plane heat transfer of the composites. (b) Through-plane and in-plane thermal conductivity of 3D SiCNW/epoxy composites with different filler loadings. Through-plane thermal conductivity of composites as function of (c) SCMC content and (d)



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filler alignment. Cyclic thermal conductivity (e) and K enhancement efficiency (f) of 3D SiCNW/epoxy composites. (g) Comparison of K enhancement efficiency of 3D SiCNW/epoxy composite and other electrically insulating composites reported in previous work.5,

22, 24, 52-60

(Abbreviations: BNNT, boron nitride nanotubes; BNNS, boron nitride

nanosheets; AgNPs, silver nanoparticles; M-h-BN, modified h-BN; MGF, modified graphene foam;

SEBS,

styrene-(ethylene-co-butylene)-styrene

tri-block

copolymer;

PP,

polypropylene.) In order to demonstrate the superiority of anisotropic alignment of SiCNWs, the thermal conductivity enhancement efficiency (η, enhancement per 1 vol% loading) was calculated. Here, η is defined as

ߟൌ

௄ି௄೘

(2)

ଵ଴଴௏೑ ௄೘

where K and Km are the thermal conductivities of the composites and epoxy resin, respectively, and Vf is the loading of SiCNW in composites. Figure 5f shows that η increases with the increasing filler loading, and reaches 406% as the filler loading of 2.17 vol%. η values of some previously reported thermally conductive and electrically insulating composites are summarized in Figure 5g, where the detailed data is shown in Table S1 (Supporting Information).5, 22, 24, 52-60 Most of the reported composites fall in the η range of 30～100%, where 3D BNNS aerogel exhibits a relatively better performance with a η value of 167%.

It’s worth noting that the vertically aligned and interconnected SiCNW network

exhibits the highest η value of 406.61% among the reported fillers. This result suggests that the aforementioned structure has the better capability to improve heat transfer performance of


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polymer composites. In addition, the sintering process could bring close connected SiCNW junctions which possess much stronger interaction than covalent bonds.25

Figure 6. Illustration of thermally conductive paths for composites filled with (a) oriented SiCNW network and (b) randomly dispersed SiCNWs. To better understand the mechanism of oriented SiCNW network, the thermal conduction model of 3D SiCNW/epoxy composites was proposed, as illustrated in Figure 6. Polymer usually has a low thermal conduction because no effective thermally conductive networks could be formed because of the strong phonon scattering of the randomly entangled molecule chains.

16

Without any filler, the phonons would require considerable time to across over the



polymer matrix.17 If some fillers are dispersed in the matrix, we can imagine that they are similar to expressway, which the phonons can use to cross the polymer matrix faster by running on them. It might naturally be thought that the best way to cross the matrix is to build a straight expressway, as shown in Figure 6a. The phonons transport unidirectional with little scattering along the aligned SiCNWs. Therefore, the thermal conductivity of sample with highly conductive fibers shows a huge improvement. On the other hand, the polymer resin with randomly-dispersed fillers can be pictured as a matrix with randomly-dispersed expressways on its surface. The phonon can still across the polymer matrix by jumping from one expressway to another, but it would require a relatively longer time, as it would be slowed down by each jumping step. Thus, the higher contact resistance was generated because of the relatively greater number of gaps and a small contact area (Figure 6b), leading inevitably to low thermal conductivity.61


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Figure 7. Thermal management application of 3D SiCNW/epoxy composite. (a) Experimental setup for thermal infrared imaging. (b) Optical images of epoxy, commercial substrate, and 3D SiCNW/epoxy composites. (c) Surface temperature variation with heating time. Corresponding infrared thermal images of (d) epoxy, (e) commercial substrate and (f) 3D SiCNW/epoxy composite. In order to demonstrate the thermal management application of 3D SiCNW/epoxy composites, the surface temperature variations of the composites with time during heating were recorded by an infrared thermal imager. To do this, the samples of epoxy, commercial substrate, and 3D SiCNW/epoxy composites (Figure 7a) were vertically placed on the heating plate. Commercial substrate which contains 70 wt% Al2O3 particles and 30 wt% glass fiber reinforced-epoxy was set up as a contrastive sample here. The substrate possesses a through-plane K of 0.89 Wm-1K-1. Surface temperature variation and the corresponding infrared thermal images with time are shown in Figure 7c-f. To investigate the heat absorption capability, all the samples were placed on a stainless steel plate (50 °C, 5 mm in thickness). Figure 7c indicates that the surface temperature of 3D SiCNW/epoxy composite continuously increases with time at a higher rate. It is worth noting that, after 15 s, the surface temperature of 3D SiCNW/epoxy composite is very close to 50 °C due to its high thermal conductivity (Figure 7c, 7f), but the temperature of other samples is always much lower than 50 °C. This result illustrates that 3D SiCNW/epoxy composites have great potential for thermal management applications.



Figure 8. Volume resistivity of 3D SiCNW/epoxy composites with different SiCNW loadings. For electrical insulation applications, a high volume resistivity of the composites is of crucial importance. Figure 8 presents the resistivity property of the 3D SiCNW/epoxy composites. The introduction of SiCNW network (2.17 vol%) into epoxy results in a volume resistivity decrease of 2 orders of magnitude, which should be attributed to the enhanced mobility of charge carriers along the SiCNWs. Even though, the ultimate value reaches 7.1×1014 Ω cm, which is still far beyond the critical resistance for electrical insulation (109 Ω cm), indicating good electrical insulation of 3D SiCNW/epoxy composites.

4. CONCLUSIONS In summary, 3D SiCNW/epoxy composites with an ultrahigh efficiency of thermal conductivity enhancement were successfully prepared by constructing a simultaneously aligned and interconnected structure of SiCNWs via freeze casting. At a small filler loading of 2.17 vol %, the through-plane thermal conductivity of SiCNW/epoxy composite reaches 1.67 Wm-1K-1, enhanced by 400.6% per 1vol% loading when compared with pure matrix, performing the highest value of efficiency among previously reported thermally conductive


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and electrically insulating composites. Ordered network structure of SiCNW is responsible for the high thermal conductivity as such low loading percentage. Such excellent performance indicates that this approach can provide a new insight in fabricating high quality composites applied in advanced electronic packaging fields, such as thermal interface materials. ASSOCIATED CONTENT

*Supplementary Information Additional data (Figure S1-S7, Table S1) including TEM image of a single slice of SiCNW, full XPS spectrum of SiCNWs, typical SEM images of SiCNW junctions at different magnifications, SEM images of the assembly unit of SiCNW network at different solid loadings, cross-sectional morphology of epoxy composite with isotropous SiCNW network, in-plane thermal conductivity of composites as function of SCMC content with filler loadings of 2.17 vol%, morphology of 3D Iso-SiC/epoxy composite, and comparison of thermal conductivity and enhancement efficiency of our 3D SiCNW/epoxy composite with other thermally conductive and electrically insulating polymer composites. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Address corresponding to: [email protected] and [email protected]

Notes The authors declare no competing financial interest.



ACKNOWLEDGMENT The authors would like to acknowledge the financial support from National Key R&D Program of China (No. 2017YFB0406000), Frontier Sciences Key Research Program of the Chinese Academy of Sciences (No. QYZDY-SSW-JSC010), and Guangdong Provincial Key Laboratory (No. 2014B030301014).

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For Table of Contents Only


Vertically Aligned and Interconnected SiC Nanowire Networks

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