High-performance thermal interface materials

Carbon 109 (2016) 552e557

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Carbon journal homepage: www.elsevier.com/locate/carbon

High-performance thermal interface materials consisting of vertically aligned graphene film and polymer Ya-Fei Zhang, Dong Han, Yun-Hong Zhao, Shu-Lin Bai* Department of Materials Science and Engineering, HEDPS/CAPT/LTCS, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Engineering, Peking University, 100871, Beijing, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 May 2016 Received in revised form 13 August 2016 Accepted 18 August 2016 Available online 20 August 2016

Owing to the unique thermal transfer property, graphene attracts great attention as heat dissipation material. Many works have been devoted to increase thermal conductivity of graphene filled polymer composite by increasing the graphene loading. In this work, a novel composite was fabricated by rolling graphene sheets into vertically aligned graphene film (VAGF) and then penetrating liquid polydimethylsiloxane (PDMS) into it. The thermal conductivity of the VAGF/PDMS composite is up to 614.85 Wm1 K1, i.e. an enhancement per wt% of as high as 3329% compared to pure PDMS. This enhancement is due to the vertical alignment of graphene films with high in-plane thermal conductivity, which form a rapid and effective heat-transfer path. The heat dissipation performance of VAGF/PDMS composite is further confirmed by infrared thermal imaging technology. The temperature of the VAGF/ PDMS composite rises fastest under the same heating condition, compared with PDMS and copper. The results prove that the VAGF/PDMS composite most likely becomes a good candidate of high performance thermal interfacial materials. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction In order to avoid damage of electronic components from excess heat, thermal interface materials (TIMs) are commonly used to dissipate the heat from heat spots to environment. The polymer matrix TIMs filled with dispersed carbon materials, such as graphene, carbon nanotubes and carbon fibers, have relatively low values of thermal conductivity. Therefore, considerable efforts were made to design and fabricate new TIMs with high thermal conductivity. Bulk graphite is an anisotropic heat conductor with the in-plane thermal conductivity of 2000 Wm1 K1 and the crossplane one of 19 Wm1 K1 at room temperature [1,2]. However, bulk graphite is hard to be used for thermal management because it does not meet specific industry requirements, such as processability, flexibility and low cost, etc. Single-layer graphene owns extraordinarily high in-plane thermal conductivity, much higher than that of graphite [3,4]. Single and few-layer graphene have been studied as fillers in TIMs [5e8] for thermal management applications or heat spreader in electronics and optoelectronics [9e11]. Reduction of graphene oxide

* Corresponding author. E-mail address: [email protected] (S.-L. Bai). http://dx.doi.org/10.1016/j.carbon.2016.08.051 0008-6223/© 2016 Elsevier Ltd. All rights reserved.

(GO) is one of most economic and feasible method to industry-scale applications of graphene-based materials for thermal management. Hummer's method and its modifications [12e14] are widely used to massively produce GO from natural graphite. Even though owning outstanding mechanical properties [15,16], the thermal conductivity of GO is very low, usually 0.5e1 Wm1 K1at room temperature [17,18]. The thermal reduction of GO to obtain reduced graphene oxide (rGO) can overcome the shortcoming. Thermal reduced graphene film obtained from GO or graphene hybrid papers possesses high in-plane thermal conductivity of 890e1390 Wm1 K1 [19e22]. However, the process of preparing graphene oxide by Hummer's method is a little time-consuming and produces large amounts of waste water. While, the process to get graphene sheets from expanded graphite is fast and massive for production. Besides, this method to obtain graphene is easy to handle and produces less waste, but the cost maybe a little higher. Despite of having high in-plane thermal conductivity, two dimensional (2D) films are not widely used in industry for the reason that bulk materials are usually needed and high out-of-plane thermal conductivity is often required. Aligned carbon fibers [23] and network graphene foam [24] were filled into polymer to make bulk materials. However, the enhancement of these materials is not as satisfactory as expected.

Y.-F. Zhang et al. / Carbon 109 (2016) 552e557

In our previous work, we reported an efficient and simple method to fabricate a thermal-reduced vertically aligned reduced graphene oxide/epoxy composite, which has a high thermal conductivity of 2.645 W m1 K1 [25]. Theoretical prediction and finite element calculation show that the thermal conductivity of the composite will be extremely high if the thermal conductivity of rGO film can be greatly employed and its volume fraction is high. In this paper, we use synthetic graphene films made of expanded graphite, instead of reduced graphene oxide, to fabricate vertically aligned graphene films (VAGF)/PDMS composite, which has an in-plane thermal conductivity of as high as 614.85 Wm1 K1. 2. Experimental 2.1. Material preparation Synthetic graphene films were purchased from Hefei AOQ Electronic Technology Co., Ltd and have in-plane thermal conductivity of 1800 Wm1 K1. The preparation process of vertically aligned graphene film/PDMS composite is shown in Fig. 1. The purchased films (300 mm 300 mm) with thickness of 25 mm were tailored into narrow strips of 300 mm 20 mm. As shown in Fig. 1B, first strip was clamped and enwound around two needles, then second strip was interlinked with the first one, and enwound continuously. This process was repeated until a cylinder with a diameter of about 12.7 mm was formed. The needles were pulled out to give a whole VAGF cylinder (Fig. 1C). The cylinder was put into a beaker full of liquid polydimethylsiloxane (PDMS) and placed in a vacuum drying oven to remove bubbles for 5 h at room temperature. After that, the temperature was set to 80 C for 90 min to solidify (Fig. 1D). At last, the cylinder of composite was cut into desired thickness using computer numerical control blade cutting machine (SYJ-400, Shenyang Kejing automation equipment co., Ltd, China) (Fig. 1E and F). 2.2. Characterization The morphological and microstructural characterizations of obtained samples were performed using a field emission scanning electron microscope (FE-SEM, S-4800, HITACHI, Japan) operated at

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10 kV. The compression tests were carried out on an Instron 5969. The displacement was measured using an automatic extensometer AutoX750 mounted to the Instron. Instron® 2580 Series load cells are adopted for force measurement. All samples were fabricated into standard size (length/diameter ratio > 2.5). The loading rate was controlled as 1 mm min1. The measurements of thermal diffusivity were performed using a laser flash system (LFA 447 Nanoflash, NETSZCH, Germany). This method conforms to ASTM E1461. Thermal diffusivity was obtained by fitting the curves of time and temperature. Specific heat capacity was obtained by a differential scanning calorimeter (DSC, Q2000, TA Instruments, America) with a heating rate of 20 Cmin1 from 25 to 100 C. Bulk density was calculated according to sample weight and volume. The thermal conductivity is calculated by following expression:

l ¼ Dcp r

(1)

Where l is the thermal conductivity of sample, D, cp and r are the thermal diffusivity, specific heat capacity and density, respectively. The heat transport properties of PDMS, VAGF/PDMS and Cu were characterized by infrared thermal imaging camera (SC7300M, FLIR systems Inc). A platform made of aluminum was heated to a certain temperature. Then, all samples were put on it at the same time. The infrared thermal imaging camera recorded the change of temperature of the samples during the heating process. 3. Results and discussion The sample and morphologies of the top surface and longitudinal section of VAGF/PDMS composite are illustrated in Fig. 2. The VAGF loading is calculated to 92.3 wt% by measuring the weight of VAGF before and after infiltrating PDMS. Fig. 2a shows the top surface of sample, and the inset is the optical photograph of wholesize sample. It can be clearly seen that the arc-shaped graphene films are tightly stacked together with the PDMS between them. Some residues of graphene films pulled out by mechanical cutting were left on the surface (Fig. 2b). The fractured longitudinal section in Fig. 2c shows the alignment of continuous graphene films along the thickness of sample. The graphene films composed of abundant stacked graphene sheets are oriented almost in the identical direction. Graphene sheets obtained from expanded graphite have thickness of from

Fig. 1. Schematic of the fabrication procedure of the vertically aligned graphene film/PDMS composite. (A colour version of this figure can be viewed online.)

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50 nm or lesser. In fact, the decrease of the distance means the increase of graphene content in the composite, so the thermal conductivity will increase. The hot-press treatment makes graphene layers be tightly compacted together. Close contact and alignment of graphene films can minimize the phonon scattering, hence improve the thermal properties of composite. As TIMs, excellent thermal properties are the first priority, but mechanical properties also have a great influence on the service life of TIMs [28]. Fig. 3 presents the compressive stress-strain curves of studied materials. PDMS is a kind of elastomer, and owns low Young's modulus and strength, but large failure strain. VAGF is a relatively loose material with sheet stacked together. Under compression loading along VAGF plane, the sliding of sheets makes the curve to be abnormal. The first peak on stress-strain curve corresponds to the yielding point and second is the initiation of splitting of several VAGF sheets. The stress-strain curve of VAGF/ PDMS composite has a regular shape with linear elastic part and strain hardening part. Young's modulus and compression strength of studied materials are listed in Table 1. The reinforcing effect of VAGF is rather significant: Young's modulus and compression strength of 92.3 wt% VAGF/PDMS composite are increased by 193 times and 119 times compared to PDMS, respectively. The vertical alignment of graphene films contributes mainly to the increase of modulus and strength. Fig. 4 shows the failure modes under compression tests with sample length/diameter ratio z2.5. Pure PDMS usually produces buckling failure instead of fracture failure at a strain of 31%, just as shown by the inset in Fig. 3. For VAGF, graphene films were physically stacked together with poor in-plane shear resistance. It is fragile and yielded early at the strain of 0.12%. The continuous loading resulted in the splitting of VAGF sample. For VAGF/PDMS composite, the combination of rigid vertically aligned graphene films and tough PDMS endows it with good mechanical performance. When the maximum stress was reached, the damage usually started from one end of the sample, which was crimped into flower shape (Fig. 4c). This process of damage evolution can extend over 50% strain without buckling. The phenomenon proves the good deformation ability of VAGF/PDMS composite due to good interfacial adhesion between PDMS and graphene films. The TIMs are often employed under high temperature condition, so thermal stability of VAGF/PDMS composite must be good enough for practical application. Fig. 5 shows the temperature dependence

Fig. 2. SEM images of VAGF/PDMS composite: (a) top surface; (b) local magnification of top surface; (c) longitudinal section. The average thickness of graphene films is 25 mm. (A colour version of this figure can be viewed online.)

nanometers to micrometers and similar properties with that obtained by other methods. This hierarchical and aligned structure benefits the in-plane transmission of phonon and so heat. Most oxygen groups of expanded graphite have been removed during the hot compression shaping process, making the films compacted [26,27]. The average distance between graphene layers is about

Fig. 3. Typical compression stressstrain curves of studied materials (insert: PDMS curve). (A colour version of this figure can be viewed online.)

Y.-F. Zhang et al. / Carbon 109 (2016) 552e557 Table 1 Results of compressive tests. Properties

PDMS

VAGF

VAGF/PDMS composite

Young's modulus (MPa) Compression strength (MPa)

2.581 0.054

86.023 2.255

500.929 6.506

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VAGF. The thermal conductivity of the VAGF/PDMS composite is up to 614.85 Wm1 K1 at room temperature. However, it decreases slightly with temperature, which is related to the enhanced phonon scattering. Phonon scattering will be more and more important with the increase of temperature [30]. Phonon scattering mean free

Fig. 4. Compressive failure modes of pure PDMS, VAGF and VAGF/PDMS composite (left to right). (A colour version of this figure can be viewed online.)

Fig. 5. Thermal conductivity of PDMS and VAGF/PDMS composite as a function of temperature. (A colour version of this figure can be viewed online.)

of thermal conductivity of PDMS and VAGF/PDMS composite. The pure PDMS has a low and almost constant thermal conductivity of 0.20 W m1K1, which is in agreement with the previously reported value [29]. In VAGF/PDMS composite, PDMS just plays a role of adhesive to stick the graphene films together. The real heat conduction pathway is formed by vertically aligned and continuous

path will decrease with temperature rising. Therefore, thermal conductivity is inversely proportional to the temperature at the range of 200Ke400 K for both graphene [30,31] and graphite [32]. The properties measured of VAGF and VAGF/PDMS composite depend strongly on their microstructures which are related to the preparation process. Due to the loose structure of VAGF, the buckling of VAGF sample by compression may occur at different force level when several samples are tested. Besides, it is impossible to measure its thermal conductivity. Fortunately, VAGF/PDMS composite is so well prepared that both mechanical and thermal properties are stable once composite sample is cut from the same cylinder as shown in Fig. 1. This means that the dispersity of composite samples is very small. Here, both Instron 5969 and LFA 447 Nanoflash are well-known commercial equipment with high accuracy. Therefore, all results obtained are credible. The heat transfer mechanism is shown in Fig. 6. The heat flux strongly depends on the orientation of graphene [25]. In VAGF/ PDMS composite, the heat mostly flows through graphene sheets which are parallel to heat flowing direction. Because the plane of VAGF/PDMS interface is along the heat flowing direction, so, its influence is very weak to the thermal conductivity along the graphene sheet direction, but very strong in the vertical direction to heat flux. So, the interface doesn't play a key role in the thermal conductivity of our composites (we are interested only in the vertically aligned graphene sheets direction). In real application as TIMs, the end of continuous graphene films in the composites can directly contact the heat spot or heat environment, and so make the heat dissipate be very fast. Therefore, the hierarchical structure of

Fig. 6. Schematic of heat conduction mechanisms for VAGF/PDMS composite. (A colour version of this figure can be viewed online.)

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Fig. 7. Comparison of thermal conductivity and its enhancement per wt% of composites [36e38,34,39,40,25].

VAGF/PDMS composite provides fast channels for heat removing and endows the composite with superiority over common composites reinforced with dispersed filler [33]. It is noticed that most of the TIMs reported in the literature show increasing thermal conductivity by increasing the loading of fillers as much as possible. Even if graphene owns ultrahigh thermal conductivity, it is not easy to drastically improve the thermal conductivity of dispersed graphene sheets filled composites using limited loading. Fig. 7 summarizes the results of thermal conductivity of different composites reported in literature and this work. Increasing the graphene loading is the most effective way to improve the thermal conductivity. With a loading of graphite

nanoplates to 25 vol%, the thermal conductivity of graphite nanoplate/epoxy composite increased to 6.44 W m1K1, i.e. enhancement of more than 3000% [34]. Vertically aligned architecture made of functionalized multilayer graphene sheets [35] has the thermal conductivity of 75.5 W m1K1, while that of functionalized multilayer graphene sheet is 123 Wm1 K1, much lower than that of our graphene sheet. Besides, the graphene content in our composites (92.3 wt%) is higher than that of ref. [35]. As illustrated in Fig. 7, the VAGF/PDMS composite has very high thermal conductivity and thermal conductivity enhancement (TCE) per wt%, about 3329%, which proves the superiority of alignment of graphene films due to its high in-plane thermal conductivity. The VAGF/PDMS composite also outperforms high thermal conductive copper and has superior thermal management capability. Heat transfer ability from a hot platform has been compared among PDMS, VAGF/PDMS and Cu, as shown in Fig. 8. The infrared thermal imaging camera was used to capture the temperature distribution on samples. Before the tests, the surfaces of the samples were coated with a thin layer of amorphous carbon to ensure consistent emissivity of the surfaces and thermal radiation transfer between the hot platform and sample. The hot platform was continuously heated and then kept at a constant temperature. The thermal imaging camera recorded the temperature of samples with the time elapse. It can be seen from Fig. 8a that temperature of VAGF/PDMS composite rises fastest, which also proves its high thermal conductivity. Fig. 8b shows the temperature-time curves of different samples. The center of the sample surface was choose as observation spots. From Fig. 8b, the temperature of VAGF/PDMS composite began to stabilize at a time of 19s, i.e. 17.2 s earlier than the Cu. This means that the heat transfer in VAGF/PDMS composite is much fast than in Cu, which is also proved by the higher slope of former's curve. Finally, all samples stabilize at a constant temperature, which represents the steady state heat conduction. It can be seen that the stabilized temperature of PDMS slightly fluctuates at 49.9 C. However, the value of both VAGF/PDMS and Cu is higher than PDMS and appear no obvious fluctuation, which is related to intrinsic high thermal conductivity of materials. Therefore, the VAGF/PDMS composite is an ideal material for thermal management in the field of electronic packaging.

Fig. 8. (a) Images recorded by an infrared camera with samples heated on a homothermal platform (Samples from left to right are: PDMS, VAGF/PDMS and Cu); (b) TemperatureTime curves. (A colour version of this figure can be viewed online.)


4. Conclusion The vertically aligned graphene films (VAGF)/polydimethylsiloxane (PDMS) composite was prepared by simple process of graphene films rolling and liquid polymer filtration. PDMS serves as the adhesive to keep graphene films tightly stacked together, while vertically aligned graphene films function as heat transfer path with great effectiveness. Dense alignment of graphene films also provides enough mechanical strength to the composite. The composite has ultrahigh thermal conductivity of 614.85 W m1K1 at room temperature and excellent mechanical properties. Therefore, the VAGF/PDMS composite has very good potential applications in heat management field of electronic devices. Acknowledgements The work is supported by NSFC and NSFC-RGC Joint Research Scheme (Nos. 11272008, 11361161001 and CUHK450/13). References [1] P.G. Klemens, D.F. Pedraza, Thermal conductivity of graphite in the basal plane, Carbon 32 (4) (1994) 735e741. [2] A. Alofi, G.P. Srivastava, Thermal conductivity of graphene and graphite, Phys. Rev. B 87 (11) (2013). [3] S. Ghosh, W. Bao, D.L. Nika, S. Subrina, E.P. Pokatilov, C.N. Lau, et al., Dimensional crossover of thermal transport in few-layer graphene, Nat. Mater 9 (7) (2010) 555e558. [4] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al., Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (3) (2008) 902e907. [5] K.M.F. Shahil, A.A. Balandin, Thermal properties of graphene and multilayer graphene: applications in thermal interface materials, Solid State Commun. 152 (15) (2012) 1331e1340. [6] Y.-H. Zhao, Z.-K. Wu, S.-L. Bai, Study on thermal properties of graphene foam/ graphene sheets filled polymer composites, Compos. Part A 72 (2015) 200e206. [7] H. Wang, J. Gong, Y. Pei, Z. Xu, Thermal transfer in graphene-interfaced materials: contact resistance and interface engineering, ACS Appl. Mater Interfaces 5 (7) (2013) 2599e2603. [8] J. Renteria, S. Legedza, R. Salgado, M.P. Balandin, S. Ramirez, M. Saadah, et al., Magnetically-functionalized self-aligning graphene fillers for high-efficiency thermal management applications, Mater. Des. 88 (2015) 214e221. [9] H. Malekpour, K.H. Chang, J.C. Chen, C.Y. Lu, D.L. Nika, K.S. Novoselov, et al., Thermal conductivity of graphene laminate, Nano Lett. 14 (9) (2014) 5155e5161. [10] P. Goli, H. Ning, X. Li, C.Y. Lu, K.S. Novoselov, A.A. Balandin, Thermal properties of graphene-copper-graphene heterogeneous films, Nano Lett. 14 (3) (2014) 1497e1503. [11] M.C. Lemme, T.J. Echtermeyer, M. Baus, H. Kurz, A graphene field-effect device, IEEE Electron Device Lett. 28 (4) (2007) 282e284. [12] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958) 1339. mez-Navarro, R.T. Weitz, A.M. Bittner, M. Scolari, A. Mews, M. Burghard, [13] C. Go et al., Electronic transport properties of individual chemically reduced graphene oxide sheets, Nano Lett. 7 (11) (2007) 3499e3503. [14] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, et al., Colloidal suspensions of highly reduced graphene oxide in a wide variety of organic solvents, Nano Lett. 9 (4) (2009) 1593e1597. [15] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, et al., Preparation and characterization of graphene oxide paper, Nature 448 (7152) (2007) 457e460. [16] C. Chen, Q.-H. Yang, Y. Yang, W. Lv, Y. Wen, P.-X. Hou, et al., Self-assembled free-standing graphite oxide membrane, Adv. Mater. 21 (29) (2009) 3007e3011. [17] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, et al., Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater 22 (35)

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