A Paper-Like Inorganic Thermal Interface Material Composed of

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A Paper-Like Inorganic Thermal Interface Material Composed of Hierarchically Structured Graphene/Silicon Carbide Nanorods Wen Dai, Le Lv, Jibao Lu, Hao Hou, Qingwei Yan, Fakhr E. Alam, Yifan Li, Xiaoliang Zeng, Jinhong Yu, Qiuping Wei, Xiangfan Xu, Jianbo Wu, Nan Jiang, Shiyu Du, Rong Sun, Jianbin Xu, Chingping Wong, and Cheng-Te Lin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07337 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Paragon Plus Environment

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A Paper-Like Inorganic Thermal Interface Material Composed of Hierarchically Structured Graphene/Silicon Carbide Nanorods Wen Dai,†, ‡,

⊥

Le Lv,†, ⊥ Jibao Lu,*, # Hao Hou,† Qingwei Yan,† Fakhr E. Alam,†, ‡ Yifan Li,‖ Xiaoliang

Zeng,# Jinhong Yu,†, ‡ Qiuping Wei,§ Xiangfan Xu,¶ Jianbo Wu,€ Nan Jiang,*, †, ‡ Shiyu Du,‖ Rong Sun,# Jianbin Xu,₴ Ching-Ping Wong∇ and Cheng-Te Lin*,†, ‡ †Key

Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine

Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences, Ningbo 315201, P.R. China ‡Center

of Materials Science and Optoelectronics Engineering, University of Chinese Academy of

Sciences, Beijing 100049, P.R. China #Shenzhen ‖Ningbo

Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China.

Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences,

Ningbo 315201, P. R. China. §State

Key Laboratory of Powder Metallurgy, School of Materials Science and Engineering, Central South

University, Changsha 410083, P.R. China. ¶Center

for Phononics and Thermal Energy Science School of Physics Science and Engineering, Tongji

University, Shanghai 200092, China. €State

Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering,

Shanghai Jiao Tong University, Shanghai, 200240, P. R. China. ₴Department

of Electronics Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong

999077, China. ∇School

of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332,

United States.

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ABSTRACT: With the increasing integration of devices in electronics fabrication, there are growing demands for thermal interface materials (TIMs) with high through-plane thermal conductivity for efficiently solving thermal management issues. Graphene-based papers consisted of layer-by-layer stacked architecture have been commercially used as lateral heat spreaders, however, they are lack of in-depth studies on their TIM applications due to the low through-plane thermal conductivity (< 6 W m-1 K-1). In this study, a graphene hybrid paper (GHP) was fabricated by the intercalation of silicon source and then in-situ growth of SiC nanorods between graphene sheets based on the carbothermal reduction reaction. Due to the formation of covalent C–Si bonding at the graphene/SiC interface, the GHP possesses a superior through-plane thermal conductivity of 10.9 W m-1 K-1, and can be up to 17.6 W m-1 K-1 under packaging conditions at 75 psi. As compared to the current graphene-based papers, our GHP has the highest through-plane thermal conductivity value. In TIM performance test, the cooling efficiency of the GHP achieves significant improvement compared to that of state-of-the-art thermal pads. Our GHP with characteristic structure is of great promise as an inorganic TIM for highly efficient removal of heat from electronic devices. KEYWORDS: graphene hybrid paper, silicon carbide nanorods, hierarchical structure, through-plane thermal conductivity, thermal interface materials

Efficient heat dissipation is of crucial importance for high-power and high-frequency electronic devices to avoid working under overheating conditions, which are harmful to the device service life and reliability.1 In the real case, microgaps between the mating surfaces of 2


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heat generating electronic components and the heat sink are full of air, which is a very poor heat conductor with ultralow thermal conductivity (κ: ≈ 0.032 W m-1 K-1),2 leading to a high thermal interface resistance for the suppression of thermal percolation.3 To solve this problem, thermal interface materials (TIMs) are commonly applied to fill these microgaps and bridge between the heat source and the heat sink for removing excess heat from the devices.4 Conventional TIMs are made of polymer-based composites filled with thermally conductive materials, such as BN, AlN, Al2O3 and etc, to achieve thermal conductivity values of 1 − 5 W m-1 K-1 (50 − 70 wt% filler content is required).5-7 However, with the rapid increase of integration density and power consumption in electronics, the accompanying demand for highly efficient heat dissipation is beyond the ability of conventional TIMs to deal with. Therefore, the development of TIMs with improved through-plane thermal conductivity has become a subject of considerable scientific and technological interests. Graphene is a single layer of densely packed sp2-hybridized carbon atoms, which exhibits a ultrahigh intrinsic thermal conductivity of 3,500 – 5,300 W m-1 K-1 and large specific surface area of ≈ 2,630 m2 g-1.8-10 Due to the above characteristics, such as graphene/polymer composites with enhanced thermal conductivity,11-13 graphene coatings for efficient radiative cooling,14 and graphene papers as heat spreaders,15-17 have been extensively studied in recent years for development of the next generation of thermal management applications. Among them, graphene papers composed of layer-by-layer stacked graphene sheets have been commercially used as high-performance heat spreader in portable devices, because of its excellent in-plane thermal conductivity(κ: 1,000 – 2,000 W m-1 K-1)18, 19 and ease of mass production.20 Lian et al. fabricated a graphene paper with the size of 18 × 4.5 square inch and 3


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in-plane thermal conductivity of 1434 W m-1 K-1, by integration of electro-spray deposition and continuous roll-to-roll process.16 However, in contrast to the superior in-plane thermal conductivity, conventional graphene paper exhibits a much lower through-plane conductivity (κ: 0.1 – 3.4 W m-1 K-1),20-23 due to the high barrier for phonon transmission across a Van der Waals interface between graphene layers.20 Recently, some efforts have been done to solve this issue on improvement of the through-plane thermal conductivity of graphene papers. Jiang et al. constructed a graphene hybrid paper by growth of carbon nanoring between graphene sheets through annealing with the assistance of metal catalysts.20 Although the through-plane thermal conductivity of the paper was improved by 3 times after hybridization, the obtained value of 5.8 W m-1 K-1 is just close to that of commercial TIMs.4 Moreover, in the practical use, a proper packing pressure of 50 – 100 psi needs to be given to guarantee a good contact between TIMs and two mating surfaces.4 However, for the case of conventional graphene paper, the horizontally-aligned graphene sheets would become more ordered under normal compression, resulting in a further decrease of through-plane heat transfer capability. Drzal et al. applied a cold compaction process with 100 psi to graphene paper and found that the through-plane thermal diffusivity decreased by 60%.22 Therefore, so far it is still lack of in-depth studies on potential TIM applications based on paper-like graphene structure. As a result, in order to meet the requirement for use as TIM, it is a major challenge to create a paper-like graphene structure with not only high through-plane thermal conductivity but also appropriate mechanical properties. In this work, a graphene/SiC nanorod hybrid paper with hierarchical structure was fabricated by vacuum filtration of a mixture of SiO2 nanoparticles (NPs)-decorated graphene 4


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sheets, followed by high-frequency heat treatment to grow SiC nanorods between graphene layers. The through-plane thermal diffusivity of as-prepared graphene hybrid paper (GHP) can be improved by ≈ 2.2 times compared to pristine paper. Interestingly, under compression of 75 psi, the through-plane thermal conductivity of GHP can be further increased up to 17.6 W m-1 K-1, which significantly outperforms conventional TIMs.4 The comparative tests between GHP and state-of-the-art commercial TIMs also demonstrate the superior performance of GHP for cooling electronic devices.

RESULTS AND DISCUSSION

Figure 1. Surface morphologies of (a) pristine and (b) SiO2 NPs-decorated GO in TEM. (c) The corresponding SEM image and (d) EDS spectrum of (b). (e) Schematic of the fabrication process and the photograph of GHP.

The hierarchically structured graphene/SiC nanorods was developed by the interaction of SiO2 NPs between graphene layers, followed by the growth of SiC nanorods by rapid heat 5


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treatment at atmosphere. To act as the nucleation sites and silicon sources, pristine GO was decorated with SiO2 NPs through hydrolysis of TEOS in alkaline environment. The morphology of GO before and after SiO2 NPs decoration is present in Figure 1a–c, respectively, in which a uniform distribution of SiO2 NPs can be seen on the GO surface after hydrolysis. The composition of SiO2 NPs-decorated GO was identified using EDS analysis in TEM, by which a strong Si peak can be found in Figure 1d, and others peaks are assigned to GO and copper grid, respectively.25 Figure 1e shows the schematic diagram of the fabrication process of GHP, and also illustrates our concept of this study to create a hierarchical microstructure composed of graphene and SiC nanorods by a simple filtration/post-annealing process. As displayed in Figure 1e, a paper was first made by vacuum filtration of a mixture of the as prepared SiO2 NPs-decorated GO and graphene sheets with the mass ratio of 1 : 9, to obtain SiO2 NPs-intercalated paper-like structure. After filtration, a rapid annealing process was performed to grow SiC nanorods between graphene layers at 1400 °C in air. The formation of SiC nanorods is based on carbothermal reduction reaction according to reaction (1) – (3),26 and the comparative XRD patterns and Raman spectra of the papers before and after annealing can be found in Figure S1. SiO2(s) + 3C(s) = SiC(s) + 2CO(g)

(1)

SiO2(s) + CO(g) = SiO(g) + CO2(g)

(2)

3SiO(g) + CO(g) = SiC(s) + 2SiO2(l)

(3)

According to the previous report, a relatively strong covalent bonding at the interface of graphene/SiC nanorods can be achieved.27 As a result, a GHP with the diameter of 40 mm and the thickness of 500 μm could be obtained. 6


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Figure 2. Top-view and cross-sectional SEM images of (a) (c) GHP and (b) (d) GP, respectively. The comparison of (e) XRD patterns, (f) Raman spectra, (g) high-resolution XPS C1s spectra, and (h) TGA curves between GHP and GP.

The comparison of the microscopic morphologies of our GHP and graphene paper (GP) prepared with the same process are presented in Figure 2a–d. In Figure 2a and S2a–c, with SiO2 NPs intercalation and post-annealing, the top surface of GHP is fully covered by SiC nanorods with diameter of 20 – 100 nm and the length of 5 – 15 μm, in contrast to the smooth top surface of GP (Figure 2b). TEM observations confirmed that the crystalline 3C-SiC nanorods were synthesized with a [111] preferred growth direction, and a thin amorphous SiO2 layer (≈ 1 nm) can be seen on the outer surface of the nanorods (see Figure S3 and S4). Moreover, SiC nanorods are not only grown on the surface, but also act as the bridge between graphene layers within GHP to form a hierarchical structure, as shown in cross-section in Figure 2c and S2d–f. The cross-sectional morphology of GHP is fundamentally different

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from that of GP (Figure 2d), which is consisted of layer-by-layer stacking of graphene sheets and in agreement with the microstructure of conventional filtrated paper.18 The crystal information and chemical composition of graphene papers with and without the hybridization of SiC nanorods are identified, as presented in Figure 2e–g. In Figure 2e, apart from the signal at 26.2° originated from (002) plane of graphene sheets, other diffraction peaks at 35.8°, 60.2°, and 71.9° can be assigned to the (111), (220), and (311) crystal planes of 3C-SiC (JCPDS Card no. 29-1129), respectively. Two tiny peaks can be seen at 28.6° and 47.3°, which may derive from the fully reduced silicon. Raman spectroscopy (Figure 2f) indicates that the characteristic peaks of graphene can be found at 1337 (D-band), 1556 (G-band) and 2685 cm-1 (2D-band), respectively.28 The evaluated I2D/IG ratio of 0.57 suggests the multilayer nature of graphene sheets. Two peaks at 772 and 906 cm-1 are in correspondence with the vibrational frequencies of transverse and longitudinal optical modes of 3C-SiC at the Γ point, respectively.29 The chemical composition of GHP and GP was also analyzed by XPS, as exhibited in Figure 2g and S5. In Figure 2g, the C1s peak of GHP can be deconvoluted into four major components: C=C (44%, at 284.6 eV), C– O (13%, at 285.5 eV), C=O (6%, at 286.3 eV) and C–Si (37%, at 283.3 eV), respectively.25, 30 The deconvolution of high-resolution Si2p signal in Figure S5b reveals two peaks at 101.4 (Si-C, 95.7%) and 103.8 eV (Si-O, 4.3%), respectively, 25 indicating that the mass ratio of SiC component in nanorods is ≈ 93.7 wt%. In order to investigate the mass ratio of SiC nanorods in GHP, both GP and GHP were analyzed by TGA in air, as the curves shown in Figure 2h. Compared to the complete decomposition of GP, the TGA curve of GHP exhibits a residual weight of ≈ 6.94 wt% at 900 °C. In addition, the pure SiC nanorods were 8


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individually analyzed in the same conditions, showing a 1.3 wt% weight increment at 900 °C. Accordingly, the mass ratio of SiC nanorods in GHP can be obtained to be ≈ 6.85 wt%, which is composed of mainly crystalline SiC (≈ 6.42 wt%) and small amount of amorphous SiO2 (≈ 0.43 wt%), according to the fitting results of Si2p XPS spectrum.

Figure 3. (a) In-plane and through-plane thermal diffusivities of GHP and GP. (b) The calculated thermal conductance at the graphene/SiC nanorod interface bonded by covalent and VdW interactions. (c) The test system configuration for determining the through-plane heat transfer capacity. (d) Surface temperature evolution and the (e) corresponding IR images of GHP and GP as a function of heating time.

In order to investigate the effect of the formation of hierarchical structure within GHP on the thermal properties, thermal diffusivity (α) of GHP and GP was measured by the laser flash method. The corresponding thermal conductivity (κ) can be calculated by the equation31: κ = α × ρ × Cp, where ρ is the sample density and Cp is the specific heat capacity (0.75 J g-1 K-1, see Figure S6). As shown in Figure 3a, with the development of graphene/SiC 9


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nanorod hybrid, the in-plane thermal diffusivity of GHP is ≈ 6% lower than that of GP. The decrease is attributed to the formation of graphene/SiC junctions on the graphene surface as scattering centers, which hinder the phonon transport along the basal plane of graphene.20 In contrast, the through-plane thermal diffusivity of GHP (18.4 mm2 s-1) is ≈ 2.2 times higher than that of GP (8.2 mm2 s-1). Due to the fact that the spacing between graphene sheets was expanded by the intercalation of SiC nanorods, the density of GHP (0.8 g cm-3) is lower than that of GP (1.1 g cm-3). As a result, the through-plane thermal conductivity of GHP (10.9 W m-1 K-1) shows a 60% enhancement compared to 6.8 W m-1 K-1 of GP (see Table S1). The more efficient heat transfer capacity of GHP along the through-plane direction is contributed by both the intrinsic high thermal conductivity of 3C-SiC nanorods (bulk: 490 W m-1 K-1, nanorod: 76 – 88 W m-1 K-1),32, 33 and the creation of covalent C–Si bonding at the interface of graphene/SiC nanorods due to the in-situ growth. As Drzal et al. reported, the insertion of Au NPs into graphene layers would lead to a reduction of through-plane thermal conductivity of graphene/Au NPs paper by 23%, owing to the weak interfacial heat conduction between them based on the weak Van der Waals (VdW) interaction.22 In our case, based on the result of non-equilibrium molecular dynamics simulation (Figure 3b), the interfacial thermal conductance between graphene/SiC nanorods connected by covalent C–Si bonding (1.43 GW m-2 K-1) is one order of magnitude higher than VdW interactions (0.14 GW m-2 K-1) (details can be seen in the Supporting Information, Figure S7).The high interfacial thermal conductance at the covalently bonded interface using our proposed method is of significance to phonon transport across graphene and SiC nanorods. To compare the through-plane heat transfer capacity between GHP and GP, two papers 10


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cut into the size of 10 mm × 10 mm × 0.5 mm were placed on a ceramic heater (60 W) and kept at room temperature initially, followed by heating at the same time (Figure 3c). The time-dependence of their surface temperature evolution on the heating time was recorded using a thermocouple and calibrated infrared (IR) camera, respectively, as shown in Figure 3d and e. We found that the surface temperature of GHP increases faster than that of GP, and always shows a higher value (e.g.: ΔT = 27 °C at 300 s), confirming the superior thermal conductivity of GHP along the through-plane direction.

Figure 4.

(a) Through-plane thermal properties of compressed GHP and compressed GP

under 75 psi packing pressure. (b) Schematic illustrating the difference of heat-flow across compressed GHP and compressed GP vertically. (c) Comparison of the through-plane thermal conductivity of our GHP with the reported graphene-based papers. (d) Through-plane thermal conductivity as a function of environmental temperature.

The GHP composed of hierarchically structured graphene/SiC nanorods has been demonstrated to exhibit a high through-plane thermal conductivity (up to 10.9 W m-1 K-1), by which the GHP presents a great potential for TIM applications. However, as the TIMs used in 11


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electronic packaging, generally a packing pressure between 50 – 100 psi is required to guarantee good contact between the mating surfaces and TIMs to eliminate the microgaps at their interface.4 Therefore, the change of through-plane thermal conductivity of GHP and GP under vertical compression (75 psi) was studied. As revealed in Figure 4a, both the samples show a lower thermal diffusivity of 14.7 mm2 s-1 for compressed GHP and 4.8 mm2 s-1 for compressed GP, respectively, compared to that of pristine ones. This is because of the rearrangement of graphene sheets toward the horizontal direction during vertical compression (Figure S8), leading to higher in-plane and lower through-plane thermal diffusivities of the resulting paper with an increase of thermal anisotropy.16 Interestingly, we noticed that the reduction in thermal diffusivity of compressed GHP (19.7%) is lower than that of compressed GP (42.2%). This can be ascribed to the fact that the SiC nanorods covalently bonded between graphene sheets lower the phonon diffusion barrier across the basal planes of graphene (see Figure 4b), thus compensating the decrease of through-plane heat conduction capability caused by the rearrangement of graphene sheets. As a result, after applying the same vertical compression at 75 psi, the through-plane thermal conductivity of compressed GHP (17.6 W m-1 K-1) is more than 3 times higher than that of compressed GP (5.8 W m-1 K-1) under packaging conditions. To the best of our knowledge, the GHP possesses the highest through-plane thermal conductivity compared to the other graphene-based papers in the previous literature (see Figure 4c and Table S2),20-23, 34 and also much better than that of commercial TIMs, such as thermal gels, pads, and greases (up to 4 – 5 W m-1 K-1).4 Besides, the environmental temperature dependence of through-plane thermal conductivity of GHP was investigated, and 12


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it shows a linear drop by 30% from 25 °C to 145 °C in Figure 4d. Such behavior can be attributed the increased relaxation time of phonons as the temperature increases and is in reasonable agreement with Umklapp scattering mechanism of crystalline materials.35-37

Figure 5. (a) Schematic configuration of TIM performance evaluation system. Temperature evolution of ceramic heater as a function of (b) heating time at the heater power of 30 W and (c) various applied powers after heating for 200 s. (d) The calculated heater temperature as a function of effective thermal conductivity. (e) The comparison of heat dissipation capability based on simulated profiles.

Based on its high through-plane thermal conductivity, GHP may have the potential to act as a high-performance TIM to efficiently transfer heat across the heater/heat sink interface. Therefore, a verification system for simulating heat dissipation process in electronic components was developed, as schematically shown in Figure 5a. A state-of-the-art thermal 13


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pad (≈ 5 W m-1 K-1, 5000S35, Bergquist, USA) was employed for comparison. The GHP and 5000S35 with the size of 2 cm × 2 cm was placed, respectively, between ceramic heater and heat sink with a same bond line thickness (BLT) of 250 μm and vertical pressure of 75 psi. A water cooling system was attached to keep the heat sink at room temperature, and the evolution of the heater temperature was recorded. In Figure 5b, after the heater (30 W) turned on at 35 s, the heater temperature rise rapidly and then reached the equilibrium state. Obviously, compared to the heater temperature without TIM, the cooling performance of GHP as TIM by a decrease of 18.3 °C at 200 s is much better than that of 5000S35 thermal pad (9.8 °C). The saturated temperatures of the heater versus applied power with and without TIMs are plotted in Figure 5c, in which the slope values are 1.32 (without TIM), 0.99 (5000S35), and 0.72 °C W-1 (GHP), respectively, indicating that GHP achieves 45.5% and 27.3% improvements of cooling efficiency compared to the system without TIM and with 5000S35 thermal pad. A commercial computational fluid dynamics software (ANSYS Icepak) was then utilized for thermal analysis of our test configuration at 30 W applied power (Figure S9), and the effective thermal conductivity (κeff) of two TIMs at the same measurement conditions can be estimated (see Figure 5d). Based on the values (κeff) for GHP (≈ 4.1 W m-1 K-1) and 5000S35 (≈ 2.1 W m-1 K-1), not only superior through-plane thermal conductivity, the calculated thermal contact resistance (two sides) of our GHP (47 K mm2 W-1) is also significantly lower than that of 5000S35 thermal pad (69 K mm2 W-1). The detailed calculation process can be found in the Supporting Information (Figure S10). The lower thermal contact resistance can be attributed the characteristic hierarchical structure of graphene/SiC nanorods, which gives 14


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GHP not only a high through-plane thermal conductivity, but also lower compressive modulus compared to that of 5000S35 (Figure S11). At 75 psi, the compressive modulus of GHP (≈ 0.64 MPa) is only about one third of that of 5000S35 (≈ 1.95 MPa). As a result, GHP with a soft compressibility is easier to fill up the microgaps between the mating surfaces under packaging, leading to a maximum of contact area at the microscale and lower thermal contact resistance.38 Combining the high through-plane thermal conductivity and low thermal contact resistance, the simulated temperature profiles in Figure 5e confirm the outstanding heat transfer properties of our sample for TIM use. A real TIM application for cooling computer CPU was carried out to demonstrate superior heat dissipation performance of GHP compared to 5000S35 thermal pad. In Figure 6a, GHP and 5000S35 with the same area and BLT were directly physically fastened between the CPU (Intel® Core™ i5-7500) and the aluminum heat sink by vertically applying a packing pressure, which was carried out using spring screws. The CPU is fixed on the motherboard and the heat sink is connected to a cooling fan through the screws to transfer heat to the air flowing. A professional system diagnostics software (AIDA6439) was employed to drive the CPU running at maximum heat-generating conditions (42 W) and record the CPU core temperature (TCPU core). As the results presented in Figure 6b, it can be seen that TCPU core of the case using GHP as TIM increases more slowly than that of 5000S35 and always shows a lower value (e.g.: ΔT = 6.3 °C at 780 s). And the comparative temperature profiles of the computer motherboard in Figure 6c also indicate a lower temperature when GHP was used as TIM. As a result, our GHP is a very promising candidate to replace the state-of-the-art thermal pad for real electronic cooling applications. 15


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Figure 6. (a) The experimental setup and the schematic configuration for comparing the cooling efficiency between GHP and state-of-the-art thermal pad based on desktop computer systems. (b) CPU core temperature evolution as a function of running time. (c) Comparative IR images of the motherboard when 5000S535 and GHP were used as TIMs, respectively.

CONCLUSIONS In summary, a graphene/SiC nanorod hybrid paper was developed by a facile and easy to scale-up filtration method, followed by rapid heat treatment to in-situ grow SiC nanorods between graphene sheets based on carbothermal reduction reaction. The GHP shows a characteristic structure composed of hierarchical graphene/SiC nanorod architecture, leading to an enhanced through-plane thermal conductivity (10.9 W m-1 K-1) by 60% compared to that of GP. In addition, conventional graphene-based papers lose their through-plane thermal conductivity as a compressive force is vertically applied to them, which is not capable of developing practical TIM applications. In contrast, due to the formation of relatively strong covalent C–Si bonding, the through-plane thermal conductivity of compressed GHP can be 16


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even higher (up to 17.6 W m-1 K-1) with a compression force of 75 psi. Accordingly, we demonstrated that the heater temperature of compressed GHP by a decrease of 18.3 °C in TIM performance test is superior than 9.8 °C of commercial thermal pads (κ: 5 W m-1 K-1) with the cooling efficiency improving by 27.3%, which is also verified under actual operating conditions. By getting rid of the aging issue of conventional polymer-based TIMs, our GHP with characteristically inorganic structure has great potential to be used as high performance TIMs with good thermal and chemical stability.

METHODS Materials: Graphene sheets prepared by intercalation and exfoliation of graphite were supplied by Ningbo Morsh Technology Co., Ltd. (China), and the sample characterizations have been done in our previous work.24 The lateral size and thickness of graphene sheets are 5.4 ± 0.3 µm and 10.6 ± 0.3 nm, respectively. Graphene oxide (GO, Type: HG03-1) with the typical lateral size and thickness of 3.0 µm and 2.5 nm was supplied by Qingdao Huagaomoxi Technology Co., Ltd. (China). Tetraethoxysilane (TEOS), ammonia, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Pure SiC nanorods were obtained from Changsha Sinet Advanced Materials Co., Ltd. (China). All chemicals were of analytical reagent grade and used without further purification. Preparation of SiO2 nanoparticles-decorated GO: A hydrolysis method was employed to prepare SiO2 NPs-decorated GO. GO was dispersed in a mixture of ethanol (240 ml) and deionized water (24 ml) with ultrasonication for 120 min. Then, 4 ml ammonia was added to 17


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this dispersion for adjusting the pH value, following by the addition of 0.5 ml TEOS. After hydrolysis with bath ultrasonication for 180 min, the product was collected by vacuum filtration and washed with deionized water for several times until a neutral pH was reached. SiO2 NPs-decorated GO was finally obtained by drying the product at 60 °C. Fabrication of graphene hybrid paper: Graphene sheets (450 mg) and as prepared SiO2 NPs-decorated GO (50 mg) were uniformly dispersed in ethanol via ultrasonic treatment. The dispersion was then filtered through a Teflon filter membrane (pore size: 0.22 μm) to obtain a thin paper. The paper with the intercalation of SiO2 NPs was treated in high-frequency furnace at 1400 °C for 4 min. A graphene paper (GP) as a control sample was also made of 500 mg graphene sheets without SiO2 NPs decoration using the same experimental process. Characterizations: Raman spectra were recorded using a Reflex Raman System (Renishaw plc, Wotton-under-edge, UK) employing a laser wavelength of 532 nm. Transmission electron microscopy (TEM) images were taken by JEM-2100 (JEOL, Japan) with an acceleration voltage of 200 kV. Electron energy loss spectroscopy (EELS) was performed with a Gatan parallel spectrometer, and the corresponding analysis was conducted using scanning transmission electron microscopy (STEM) mode of TEM. The surface morphology of the samples was examined with field emission scanning electron microscopy (FE-SEM, Quanta FEG250, FEI, USA). X-ray diffraction (XRD) patterns were recorded by D8 Discover with GADDS (Bruker, Germany) with CuKa radiation (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) was carried out with AXIS Ultra DLD spectrometer (Kratos Analytical, UK). The compression tests were carried out on an electron omnipotence tester of universal testing machine (UTM, 5567A, Instron, USA). All samples were fabricated 18


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into standard size. The loading rate was controlled as 0.1 mm min-1. Thermal conductivity of the samples was measured with LFA 467 NanoFlash apparatus (Netzsch, Germany). The infrared (IR) photos were captured by using an infrared camera (Fluke, Ti400, USA).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns and Raman spectra of GHP before and after annealing; SEM images of GHP; TEM images of SiC nanorod grown on the surface of graphene; STEM image and EELS spectrum of an individual SiC nanorod; XPS spectra of GP and GHP; DSC curves of GP and GHP; Schematic and parameter setting of the NEMD implementation; Cross-sectional SEM images of GP before and after compression; Comparison of through-plane thermal conductivity of graphene-based papers; Schematic, parameter setting and results of Icepak implementation; Schematic illustrating the calculative process of the thermal contact resistance; Stress-strain curves of GHP and 5000S35. Financial interest statements: The authors declare no conflict of interest.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. 19


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*E-mail: [email protected]. ORCID Xiaoliang Zeng: 0000-0002-1389-8484 Xiangfan Xu: 0000-0001-7163-4957 Jianbo Wu: 0000-0002-3574-5585 Shiyu Du: 0000-0001-6707-3915 Rong Sun: 0000-0001-9719-3563 Jianbin Xu: 0000-0003-0509-9508 Ching-Ping Wong : 0000-0003-3556-8053 Cheng-Te Lin : 0000-0002-7090-9610 Author Contributions ⊥

W. D. and L. L. contributed equally in this work for conceiving and designing the

preliminary experiments. W. D. and C. T. L. wrote the paper and revised the manuscript at all stages. H. H., Q. Y. and X. T. performed partial materials characterization including Raman, XRD and XPS. J. L., Y. L.,S. D. and X. X. provided the NEMD and heat flux simulation&data analysis. N. J. measured the EELS sprectra of the sample and contributed reagents/materials/analysis tools. X. Z., J. Y., Q. W. and J. W. helped the mechanism explanation and assisted with the revision. R. S., J. X., N. J. and C. P. W. provided some constructive suggestions and supervised this study.

ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Key R&D Program of 20


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China (2017YFB0406000), Scientific Instrument Developing Project of the Chinese Academy of Sciences (YZ201640), the Project of the Chinese Academy of Sciences (KFZD-SW-409), Science and Technology Major Project of Ningbo (2016S1002 and 2016B10038), and International S&T Cooperation Program of Ningbo (2017D10016) for financial support. We also thank the Chinese Academy of Sciences for Hundred Talents Program, Chinese Central Government for Thousand Young Talents Program, 3315 Program of Ningbo, and the Key Technology of Nuclear Energy (CAS Interdisciplinary Innovation Team, 2014).

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TOC

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A Paper-Like Inorganic Thermal Interface Material Composed of

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