GrapheneâCarbonâMetal Composite Film for a Flexible Heat Sink

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The Graphene-carbon-metal Composite Film for a Flexible Heat Sink Hyunjin Cho, Hokyun Rho, Jun Hee Kim, Su-Hyeong Chae, Thang Viet Pham, Tae Hoon Seo, Hak Yong Kim, Jun-Seok Ha, Hwan Chul Kim, Sang Hyun Lee, and Myung Jong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11485 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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ACS Applied Materials & Interfaces

The Graphene-carbon-metal Composite Film for a Flexible Heat Sink Hyunjin Cho1,2,‡, Hokyun Rho1,3,‡, Jun Hee Kim1,5, Su-Hyeong Chae4, Thang Viet Pham,1,6 Tae Hoon Seo1, Hak Yong Kim2,4, Jun-Seok Ha3, Hwan Chul Kim2, Sang Hyun Lee1,* and Myung Jong Kim1,6,* 1

Applied Quantum Composites Research Center, Korea Institute of Science and Technology,

Chudong-ro 92, Bongdong-eup, Wanju, Jeollabuk-do 55324, Republic of Korea 2

Department of Organic Materials and Fiber Engineering, Chonbuk National University, 567,

Baekjedaero, Deokjin-gu, Jeonju, Jeollabuk-do, 54896, Republic of Korea 3

Department of Advanced Chemicals & Engineering, Chonnam National University, 77,

Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of Korea. 4

Department of BIN Convergence Technology, Chonbuk National University, 567, Baekjedaero,

Deokjin-gu, Jeonju, Jeollabuk-do, 54896, Republic of Korea 5

Department of Bionanosystem Engineering, Chonbuk National University, 567, Baekjedaero,

Deokjin-gu, Jeonju, Jeollabuk-do, 54896, Republic of Korea 6

Nanomaterials Science and Engineering, Korea University of Science and Technology (UST),

Daejeon 34113, Korea E-mail: [email protected], [email protected] KEYWORDS: Heat sink; graphene; Electrospinning; Electroplating; Composite

1 Environment ACS Paragon Plus


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ABSTRACT The heat generated from electronic devices such as light emitting diodes (LEDs), batteries, and highly integrated transistors is one of the major causes to obstruct the improvement of their performance and reliability. Herein, we report a comprehensive method to dissipate the generated heat to the vast area by using the new type of graphene-carbon-metal composite film as a heat sink. The unique porous graphene-carbon-metal composite film that consists of an electrospun carbon nanofiber with arc-graphene (Arc-G) fillers and an electrochemically deposited copper (Cu) layer showed not only high electrical and thermal conductivity but also high mechanical stability. Accordingly, superior thermal management of LED devices to conventional Cu plates and excellent resistance stability during the repeated 10,000 times bending cycles has been achieved. The heat dissipation of LED has been enhanced by the high heat conduction in the composite film, heat convection in the air flow and thermal radiation at low temperature in the porous carbon structure. This result reveals that the graphene-carbonmetal composite film is one of the most promising materials for a heat sink of electronic devices in modern electronics.


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1. Introduction The remarkable advances in science and technology in all over the world have brought convenience and efficiency to human beings over the past few decades. In recent years, highperformance electronic devices such as computers, televisions, and mobile phones in modern electronics have transformed into further miniaturized with attractive structures. However, the more the electronic devices are miniaturized and developed, the more heat is generated.1-4 Notably, heat emitted from the electronic devices affects its functions and reduces its operation time. Also, it leads to malfunction of other parts around the electronic devices.5 Thus, the demand for a heat sink to dissipate the heat from the electronic devices to the air is rapidly increasing. Numerous researchers are focusing on developing the cost-effective materials with unique properties such as high thermal conductivity and excellent heat dissipation effect.6-8 The materials such as polymer matrix composites (PMCs),9 metal matrix composites (MMCs),10 and metal foam composites (MFCs).11 used as heat sinks in electronic devices. Polymers in PMCs have advantages, such as strong mechanical strength, high flexibility, low density, and low cost of production, but they also have a disadvantage such as low thermal conductivity.9 In this regard, the researchers focused on developing high thermal conductivity of PMCs with attractive structure. The new PMCs adopt high thermal conductive fillers, such as gold (Au),12 aluminium (Al),13 Cu,14 nickel (Ni),14 aluminium oxide (Al2O3),15 boron nitride(BN),16 aluminum nitrate (AIN),17 silicon nitride (Si3N4),18 silicon carbide (SiC),19 graphene,20 carbon nanotubes (CNTs),21-22 and carbon fiber.23 Among them, carbon materials such as graphite, graphene, CNTs, and carbon fibers have - higher thermal conductivity, compared with other materials9, and thus they are regarded as the promising materials as a heat



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sink. BN fillers in inorganic materials have excellent thermal conductivity, but the thermal conductivity of BN fillers are lower than carbon materials.24 Metal fillers also show suitable thermal conductivity, but the amount of metal fillers that can be added to PMCs is limited.9 The materials designed as a heat sink should have a minimum dimensional deformation at high temperature. Several researchers have attempted to add the highly conductive fillers to the MMCs to enhance the tensile strength and stiffness, to decrease the coefficients of thermal expansion (CTE), and to improve the wear resistance.10 The fillers such as various metals and carbon/ceramic materials including Al/SiC,10 Cu/graphite,25 tungsten(W)/graphite,26 Cu/boron (B)/diamond,27 Cu/Y2W3O12,28 Al/CNTs,29 and Cu/CNTs.30 have been utilized. Though the MMCs with those fillers show high-performance as a composite material for heat transfer, the MMCs still have a limitation for heat sinks because they do not have a porous structure to induce the air convection for natural cooling. On the other hand, MFCs have many voids within metal compared to an original metal. This material has low weight, high specific surface area, high mechanical strength, and unique porosity. Since the material initially comes from a metal, it has high electrical conductivity and thermal conductivity.11, 31 Recently, Al foam32 and Cu foam33 have been developed as a heat sink. Even though MFCs shows the high-performance as a heat sink, it is not scalable and costeffective. Thus, it can be used for particular purposes. In this context, numerous materials and methods have been developed for a heat sink, none of the methods can fulfill the requirements for the high output LED packages. In this paper, we report a novel and facile method that provides a comprehensive solution to fabricate the graphene-carbon-metal composite film that maximizes the heat dissipation effect by


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enhancing thermal conductivity as well as structural stability. Impressively, the composite film has excellent advantages to transfer the heat generated from the heat source by the heat transport mechanisms such as the heat conduction in a carbon-metal composite, natural heat convection in the air flow, and thermal radiation at low temperature in the carbon material. The package described here has a unique structure in which the composite film is applied to the electrode of the LED electronic devices and a heat sink material simultaneously. In particular, these carbon nanofibers (CNFs) had porous structures of several hundred square micrometers, which could naturally induce the convection of air due to the effective pathways. Since high-quality Arc-G was added to CNFs as a filler, Arc-G/CNFs composite film also exhibited high specific surface area and enhanced electrical conductivity compared to original CNFs. Furthermore, copper nanoparticles (Cu NPs) were electrochemically deposited on one side of the Arc-G/CNFs to form the graphene-carbon-metal composite film that has enhanced mechanical stability and electrical conductivity. Thus, the composite film reported here will be utilized for LED package because of their tailored properties such as high flexibility, excellent electrical and thermal conductivity, and outstanding mechanical stability.

2. Experimental section (1) The synthesis of Arc-G and the preparation of RGO Arc-G was synthesized by an arc-discharge method with slight modification (Arc Discharge, A-Tech System Ltd.) from the previous report.34 In brief, a 15 cm hollow graphite rod with 6 mm diameter was filled with the graphite powder (1g) without any metal catalyst. During the synthesis process, we utilized hydrogen (H2) and helium (He) as the buffer gasses, then



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maintained at 550 torr with a flow rate of H2 400 sccm and He 400 sccm, respectively. The discharge voltage and current were 30V and 150A, respectively. Also, we prepared RGO by a previous reported method.35

(2) Electrospinning Method 10wt% polyacrylonitrile (PAN) was dissolved in N, N-dimethyl formamide (DMF) solvent as an electrospinning solution. The Arc-G was added at a different concentration of 1, 3, 5, 10, and 15 wt% compared to PAN. The mixtures of the materials in the solutions were heated on the hot plate with a magnetic stirring bar for 12 hours at 60 oC. The electrospinning apparatus including a high voltage generator (ESN-HV30, NanoNC Co., Ltd.) was utilized to fabricate the composite nanofiber of Arc-G and PAN. Subsequently, the electrospinning process was carried out in the vertical direction in a protective chamber for 9 hours. The tip-to-collector distance (TCD) and voltage in all experimental conditions were fixed at 15cm and 15kV, respectively.36-39 Finally, Arc-G (0%~15%)/PAN NFs were dried for 24 hours in a vacuum oven to remove the residual solvent.

(3) Stabilization and Carbonization method All the Arc-G/PAN NFs samples were placed and stabilized in a stabilization apparatus (LIHTO01, LK labkorea Co., Ltd.) for 2 hours at 250 oC with 5 oC /min of heating rate (from room temperature up to the stabilization temperature) in an ordinary atmosphere to carry out the stabilization process. The carbonization of the samples was subsequently performed in an alumina tube furnace (Korea Furnace Development Co., Ltd.) at 1500 oC in the heating rate of 5 o

C/min (from room temperature up to the carbonization temperature) at nitrogen (N2) atmosphere


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with 2000 cc/min of the flow rate. The final samples of Arc (0%~15%)/CNFs (after stabilization and carbonization) were prepared.

(4) Electroplating process Electroplating was carried out in an electrolytic bath, consisting of DI water, 200 g/L copper sulfate (CuSO4), and 40 mL/L sulfuric acid (H2SO4). The temperature of the electrolytic bath was maintained at room temperature. Electrodeposition was performed with two electrode systems, using a DC power supply with Arc-G 15%/ CNFs as the anode and a titanium basket with Cu balls(10mm in diameter) as the cathode. The electrodeposition was accomplished at an applied current density of 0.03 A/cm2 according to the chemical formulism of reaction (Cathode: Cu2+ + 2 e– → Cu, Anode: Cu → Cu2+ + 2 e– ). The deposition time of Cu NPs controlled the thickness of the composite films.

(5) Fabricating LED (Light Emitting Diode) device on the graphene-carbon-metal composite To assemble the test samples for a heat sink application, we prepared Arc-G 15%/Cu CNFs as graphene-carbon-metal composites and pure Cu sheet (size 2cm×2cm×150 ㎛) on a slide glass substrate, respectively. LED chip (size 1mm×1mm) was mounted on the center of Arc-G 15%/Cu/CNFs and pure Cu sheet with a silver paste and gold wires. The detailed schematic diagrams about the test sample are shown in Figure S1 (a), (b), and (c). Figure S1 (a) shows a mounted LED chip on the center of Arc-G 15%/Cu/CNFs/glass. Figure S1 (b) illustrate the fabricated sample. Lastly, Figure S1 (c) indicates the mounted LED chips on the center of Arc-G 15%/Cu/CNFs/PET substrate for the flexible and repeated test. The injected current of the LED chip was 350mA DC for 24 hours. An infrared thermal imaging camera (FLIR T-335, spectral



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range 7.5–13 ㎛, FLIR Systems) was utilized for the thermal images and the temperatures of the samples. The flexible electric circuit was fabricated to investigate the performance of device’s flexibility. Five LED chips were mounted on Arc-G 15%/Cu CNFs (size 1cm×5cm) on PET sheet using a silver paste and gold wires. The repeated bending test for the flexible electric circuit board was conducted with a bending machine (Zeetech, ZBT-200, Korea). The repeated test conditions of the flexible electric circuit board were 10,000 cycles at a speed of 30 mm/s over an angle range of 0 – 90°.

(6) Characterization The morphological and structure analyses of all samples above were observed by scanning electron microscope (SEM, Nova SEM, FEI), transmission electron microscope (TEM, Tecnai G2 F20, FEI), and Raman spectroscopy (Horiba, LabRAM HR-UV-visible-NIR, 514nm Ar laser and Renishaw, InVia Reflex Raman Microscope, 488nm Ar laser, 514nm Ar laser, 785nm Diode laser). The element analysis of Arc-G was carried out by X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo Scientific). To confirm the crystallization, X-ray diffractometer (XRD) with integrated azimuthal profiles (2 Theta scans) was utilized. The electrical conductivity data of the carbonized Arc-G (0%~15%)/CNFs was acquired by sheet resistance measurement apparatus (FPP-RS 8, ASRMS-1000, DASOL ENG). The specific surface area of PAN NFs and Arc-G (0% and 15%)/CNFs was analyzed by Brunauer–Emmett–Teller (BET, 3 FLEX, Micromeritics). The measurement of thermal diffusivity and the specific heat depending on the temperature (from room temperature to 500 oC) were performed by Thermal Conductivity Measuring System (Laser Flash Apparatus, LFA 467, NETZCH) and Differential scanning


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calorimeter (DSC, DSC 200 F3 Maia, NeTZSCH). The thermal images and point temperature of the LED chips that were attached to the graphene-carbon-metal composite structures with silver paste were analyzed by an infrared thermal imaging camera (FLIR T-335, FLIR systems, spectral range 7.5–13 µm).

3. Results and Discussion The essential factors for a highly efficient heat sink material are high thermal conductivity, ease of convection inside a material as a porous structure, and high specific surface area. Thus, we designed and prepared the graphene-carbon-metal composite film for the efficient heat sink application by using a convenient electrospinning and electroplating process. The scheme of Figure 1 (a) and (b) described the synthesis of Arc-G and the preparation of a graphene-carbon composite, respectively. The detailed experimental process and discussion are described in Experimental Section.

To confirm the quality of Arc-G, we carried out various analytical instruments. Figure 2 (a) and (b) shows the scanning electron microscope (SEM) image and transmission electron microscope (TEM) images of Arc-G thin layers synthesized by an arc-discharge method. The inset image of Figure 2 (a) is a photographic image of as-grown Arc-G. As shown in Figure 2 (a), (b) and the inset image of Figure 2 (b), the Arc-G had hundreds of square micrometers size and entangled each other. The inset image of Figure 2 (b) shows the morphology of Arc-G dispersed on a TEM grid. Arc-G in Figure 2 (b) has the smooth surface morphology and a layered structure which is similar to the that of chemical vapor deposition graphene (CVD-G) in a previous



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paper.40 To additionally investigate the quality of Arc-G compared to CVD-G40-41, we acquired the Raman spectra by Raman spectroscopy with the 514nm excitation laser. The Raman spectra of Arc-G showed ID peak (1341.6 cm-1), IG peak (1568.7 cm-1), and I2D peak (2680.5 cm-1), respectively. The ID peak is derived from various disorder structures and defect sites in sp2 carbon materials, and the IG peak presents the stretching mode of the in-plane carbon-carbon bond (E2g mode).42-43 From the result in Figure S2 (a), (b), and (c), we found the ID/IG ratio of Arc-G was relatively higher than that of the CVD-G in Figure S2 (a). Moreover, we also confirmed the ID/IG ratio of Arc-G was much lower than that of reduced graphite oxide (RGO), followed by calculating the crystalline length (‫ܮ‬௔ ) and inter-defect distance (‫ܮ‬஽ ) by the equations in Table S1.44-47 In XPS analysis data (Figure S3 (b), (c), and (d)), the carbon peak was significantly higher than the oxygen peak.48 The central peak of the C-C bond at 284.0 eV, the low peak of the C-O bond at 285.5 eV, and another low peak of the O=C-O bond at 288.8 eV in Figure S3 (c) were found. As shown in Figure S3 (b) and (d), the low peak of oxygen was found, and the individual bonds such as the C-O and the H-O-C bonds were confirmed at 532.2 and 533.9 eV, respectively. The results of Raman and XPS, thus, indicated that Arc-G has high quality than GO and RGO.44-45

The Arc-G prepared by Arc-discharge method was applied as the filler of polyacrylonitrile nanofibers (PAN NFs) to improve the efficiency of the electroplating process as well as the electrical conductivity. Figure 2 (c) and (d) illustrated the pristine PAN NFs. In particular, the surface morphology of PAN NFs was immaculate and smooth. However, after adding Arc-G to the PAN solution for the electrospinning process, the surface morphology was uneven as shown in Figure 2 (e) and (f). Consequently, Figure 2 (e) and (f) indicated that the Arc-G was well embedded and distributed in PAN NFs. For the next step, we prepared the graphene-carbon


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composite film by using the electrospinning method mentioned in the experimental section. After the electrospinning of Arc-G/PAN NFs, the electrical conductivity of Arc-G/PAN NFs was still as low as a dielectric layer due to the relatively low weight ratio of Arc-G to Arc-G/PAN NFs and low graphitic crystallinity of pristine PAN NFs. We compared the level of crystallization from XRD data with integrated azimuthal profiles (2 Theta scans). From the results of XRD analysis in Figure S3 (a), we found the main peaks about (100) plane of pure PAN nanofibers were shown at 17°. Another graphite peak about (101) plane of all samples was additionally confirmed at about 22.5°. After the stabilization and carbonization of the Arc-G/PAN NFs, the sharp graphite peak about (002) plane appeared at 26.5°. Highly crystalline graphitic structure including the turbostratic carbon structure might be due to the use of the Arc-G as seed crystal like the use of other nanocarbon materials,49 and our XRD results corresponded with other reports in the literature.49-52 As a conclusion, the stabilization and carbonization of Arc-G/PAN NFs were required to crystalize structure and thus improve the electrical conductivity of ArcG/PAN NFs. Finally, the stabilization and carbonization of Arc-G/PAN NFs were conducted to improve the electrical conductivity of Arc-G/PAN NFs.

After the stabilization and carbonization processes, electrical conductivity increased by approximately 48 %, at the addition of Arc-G (15 wt%) (Figure S5). We adopted the Brunauer– Emmett–Teller (BET) analysis technique to compare the level of the specific surface area among the Arc-G, pristine CNFs, and the optimized sample (Arc-G 15%/CNFs) prepared under the same experimental conditions. As shown in Table S2, the values of the Arc-G, pristine CNFs, and Arc-G 15%/CNFs were 40.78 m2/g, 14.03 m2/g and 29.00 m2/g, respectively. The value of Arc-G 15%/CNFs was lower than that of the Arc-G because a small fraction of the Arc-G was embedded in CNFs as shown in Figure 2 (e) and (f). However, the value of Arc-G 15%/CNFs



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increased by ~200% compared to the pristine CNFs, and the tendency of the value corresponded with a previous literature which reports the correlation between the CNFs and GO used as a filler.53 The 2D polygonal structure (Figure 2 (f)) and the high weight ratio of Arc-G affected the morphology of pristine CNFs as shown in Figure 1 and Figure S4. Consequently, the difference in morphology may induce the increase of the specific surface area.

We assume that Arc-G plays two roles during stabilization and carbonization process. Arc-G was applied to conducting carbon fillers like CNTs to improve the electrical conductivity54, and the embedded Arc-G enhanced the graphitic crystallinity.49-50 After the stabilization and carbonization process, the graphene-carbon composite film became fragile and crumbled due to the stiffness of carbonized materials, and it was not easy to directly handle the graphene-carbon composite films. Thus, it was necessary to reinforce the mechanical properties and to enhance the electrical conductivity of graphene-carbon composite films. The optimized sample (Arc-G 15%/CNFs) which had the highest electrical conductivity was selected among the various conditions from Arc-G 1%/CNFs to Arc-G 15%/CNFs from the results of Figure S5 (a) and (b). Then, the Arc-G 15%/CNFs sample was electroplated on only one side for the optimized operation time as shown in Figure 3 (a), (b), and (c). Also, we conducted the tensile strength measurement by using the tensile strength machine according to the test rule of American Society for Testing and Materials (ASTM D638). The tensile strength of CNFs increased by adding Arc-G to the CNFs as shown in Figure S6 (a) and (b), and the tensile strength of Arc-G 15%/CNFs further increased by depositing Cu NPs by the electroplating process. Consequently, two methods adding the Arc-G to CNFs and electroplating Cu NPs enhanced strength as well as the electrical conductivity.


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The morphology of Cu-deposited side of the graphene-carbon-metal composite film was revealed in Figure 3 (b), (c), and (d). While one side is almost covered with Cu NPs, the other side of the graphene-carbon-metal composite film still retain the porous carbon structure as shown in Figure 3 (b) and (c). The porous structure was also found around the CNFs in the Cudeposited side in Figure 3 (d). The porosity can be the pathway of air convection. Hence, the porous structure can help induce natural heat convection because the heat convection is based on the different density of the fluid occurring from temperature gradients. Consequently, the graphene-carbon-metal composite film can efficiently dissipate the heat generated from a heat source because the heat transfer and heat dissipation are due to the heat conduction of carbon and metal matrix, the natural heat convection through abundant air pathways, and the thermal radiation at low temperature in the carbon material.55-57 To evaluate thermal properties, we measured the thermal conductivity of both Arc-G 15%/CNFs (graphene-carbon composite films) and Arc-G 15%/Cu/CNFs (graphene-carbonmetal composite film) as a function of temperature. We suppose that Arc-G 15%/CNFs (graphene-carbon composite films) could show phonon dominant heat conduction because the element of the materials is carbon which is packed into hexagonal lattices. In order to confirm that our Arc-G 15%/CNFs has phonon dominant heat conduction, we attempted the phonon dispersion analysis using different lasers (488nm, 514nm, 785nm) of Raman spectroscopy (Renishaw, InVia Reflex Raman Microscope, 488nm Ar laser, 514nm Ar laser, 785nm Diode laser). In carbon materials, most of the heat can be theoretically carried by longitudinal acoustic (LA) and transverse acoustic (TA) phonons, and the contribution of out-of-plane acoustic (ZA) phonons.47 In particular, heat conduction is usually carried out by acoustic phonons, and the lattice vibration in the strong covalent bonding (sp2) of carbon materials induces the efficient



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heat transfer.47 Since the acoustic phonons about LA and TA branches have very low intensity,58 as shown in Figure S7 (a) and (b), it was hard to identify peaks related to LA and TA branches in our results. The low signal to noise is due to poor crystallinity of CNFs that are the majority of the materials (Arc-G 15%/CNFs). As an alternative, we proved the phonon dominant thermal conduction of Arc-G 15%/CNFs from the comparison of Ke with Kp in the equation K=Kp+Ke (Kp : the phonon contribution, Ke : electron contribution).47 Using Wiedemann-Franz law,47 [Boltzmann constant (݇஻ ), Charge of an electron ( ݁ )] : ‫ܭ‬௘ /(ߪܶ) = ߨ ଶ ݇஻ଶ /(3݁ ଶ ) , we have determined that Ke of Arc-G 15%/CNFs is 0.2% of K (total). This result is the evidence of phonon dominant thermal conduction and corresponds with previous results in the literature.59-60 Arc-G 15%/Cu/CNFs (graphene-carbon-metal composite film) shows both electron and phonon heat conduction because the material is comprised of carbon and metal.61-63 Hence, as shown in Figure S8 (a), the tendency of thermal conductivity concerning temperature corresponds with that of metal and carbon structure respectively. Since our composite material was made of porous structures, we calculated the theoretical thermal conductivity of porous Cu, using the power law reported in the literature.63 The void volume fraction (p) of the graphenecarbon-metal composite was calculated as approximately 42% by using the method to compare the density of the materials. Thus, the theoretical thermal conductivity was also calculated in Figure S8 (b) as 138.9 W·m-1·K-1 (a solid state) by Equation S1. Compared to the experimental value, the theoretical thermal conductivity is higher. We suppose that Arc-G might reduce the mean free path of a phonon in the graphene-carbon-metal composite due to the complex morphology of Arc-G hampers the phonon’s effective transfer. However, the thermal conductivity value (in-plane and a solid state, 112.8 W·m-1·K-1) of the graphene-carbon-metal composite is high enough for a heat sink application.


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For a practical application as a heat sink using the graphene-carbon-metal composite, we fabricated the test samples (2 cm × 2 cm × 150 µm) using a pure Cu and the graphene-carbonmetal composite. Commercial LED chips were mounted at the center of the two samples as shown in Figure 4 (a), (b) and Figure 5 (a). An infrared thermal imaging camera was utilized to measure the thermal data of LED chip after 350mA of current was applied to two samples for 24 hours. The temperature of LED chips was measured 61.5 oC at the pure Cu sample in Figure 4 (c) and 49.7 oC at the graphene-carbon-metal composites sample in Figure 4 (d), respectively. According to our results, the temperature of LED chips on the graphene-carbon-metal composites during the operation showed 11.8 oC lower than that of the pure Cu sample. We suppose that this phenomenon is due to the principles we introduced above which are heat convection in the air flow and thermal radiation at the low temperature of carbon materials.55-57 Since it is calculated that the surface area of the Arc-G 15%/CNFs has 680,000 times larger than that of Cu, the heat exchange with air in the graphene-carbon-metal composite film would be overwhelming. At high temperatures, the rate of which heat radiation accounts for the total heat release is large. Heat dissipation at high temperatures is predominant in graphene because the thermal emissivity of graphene(~2.3%) is greater than Cu(0.01~0.07%).55, 64 As shown in Figure S9 (a) and (b), the temperature of the LED chips on the graphene-carbon-metal composites was rapidly decreased in our experiments. Hence, the LED chips could be maintained at the lower temperature for a long time due to the efficient heat transfer from a heat sink to the air. Consequently, our results suggest that the performance of a heat sink using the graphene-carbonmetal composites can improve the reliability and durability of the electrical devices such as LED chips.



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Then, we fabricated the electric circuit in Figure 5 (a) and (b) to confirm the flexibility of the graphene-carbon-metal composites. The LEDs were mounted in five parallel shapes. After applying the driving current of 350mA to the LED, tests of bending and twisting the samples were performed. Even if the radius of curvature was reduced to 0.6 cm, the LED emitted no abnormality. Moreover, LEDs emitted light when the substrate was twisted to 180 degrees. As shown in the result of repeated bending test in Figure 5 (c), the resistance increased by approximately 1% while the repeated bending test worked 10,000 times. The results above indicated that the flexible electric circuit was very stable. We assume the outstanding stability of graphene-carbon-metal composite sample stems from Arc-G15%/CNFs which inhibit the necking of our partial electroplated Cu section from the mechanical stress.65-67 Moreover, the additional experiments to compare the thermal data of Arc-G 15%/Cu/CNFs film after repeated 10,000 times bending test. We prepared the samples of Arc-G 15%/Cu/CNFs which have the size of 50mm × 10mm. Then, the heat by using a soldering iron was continuously applied to the samples for analyzing the characteristics of heat dissipation. From the result of Figure S10, we found the heat dissipation characteristics were not significant difference after 10,000 bending tests between two samples. We believe that the intrinsic contact between Cu and Arc-G 15%/CNFs was maintained because the copper and Arc-G 15%/CNFs were strongly entangled.”

4. Conclusion The high-performance graphene-carbon-metal composite film that provides comprehensive thermal management for a heat sink has been fabricated by the simple electrospinning and electroplating methods. The stabilization, carbonization, and electrochemical plating processes have reinforced the composites structure and improved the electrical and thermal properties.


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During the operation of LED chips adapting the graphene-carbon-metal composite film as a heat sink, the temperature of LED chips on the graphene-carbon-metal composites was 11.8 oC lower than that of the pure Cu sample due to the superior heat conduction, heat convection, and heat radiation. Moreover, the repeated bending test was conducted to evaluate the fatigue resistance of the flexible electric circuit during the operation. The value of resistance increased by approximately 1% while the repeated bending test worked 10,000 times in our result. More importantly, it can be simultaneously useful both as an electrode and in a heat sink application, which has the flexibility, reliability, and durability. Thus, the graphene-carbon-metal composite film can be the promising heat sink material for the flexible electronics.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications Web site The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06665. The additional figure data was indicated in Supporting Information such as the equations, XRD, XPS, SEM, the value of tensile strength, and BET data of the various samples, the data of the normalized electrical conductivity against the Arc-G contents (%), the table of electrical conductivity (S/cm) against Arc-G contents, the tendency of thermal conductivity of the sample according to the increasing temperature, the extra schematic diagrams about the test sample, the Raman spectra of the samples, the theoretically calculated graph using the equation of power law. (PDF)



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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by grants from the Korea Institute of Science and Technology (KIST) Institutional Program, Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4900135), and the Graphene Materials/Components Development Project (10044366) through the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea.


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Figure caption Figure 1. (a) The scheme of preparing of Arc-G, the photos of Arc-G&PAN solutions, and the scheme of different morphology between PAN NFs and Arc-G/PAN NFs by an electrospinning process (b) The photos and schemes of the electrospinning, stabilization, and carbonization process Figure 2. (a) SEM image of Arc-G, [A] as-grown Arc-G sample as a solid phase, (b) Highmagnified TEM image of Arc-G, [B] low-magnified TEM image of Arc-G, (c) SEM image and (d) TEM image of PAN NFs (before stabilization, carbonization), (e) SEM image and (f) TEM image of Arc-G 15%/CNFs (after stabilization, carbonization) Figure 3. (a) Scheme of the preparation process for the graphene-carbon-metal composite film by an electrospinning and electroplating process (b) Cross-section side (SEM image of the graphene-carbon-metal composite film) (c) Opposite of deposited side (SEM image of the graphene-carbon-metal composite film) (d) Deposited side (SEM image of the graphene-carbonmetal composite film) Figure 4. (a) The scheme of the heat transfer in the graphene-carbon-metal composite film by the thermal conduction, convection, and radiation effect (b) The photo of the mounted LEDs chip on the graphene-carbon-metal composite film and pure Cu (c) The temperature of LEDs chip on pure Cu (d) The temperature of LEDs chip on the graphene-carbon-metal composite film



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Figure 5. (a) The photos of the electric circuit for the bending test and (b) twisting test of the graphene-carbon-metal composite film (c) the normalization of resistance of 10,000 times of the repeated bending test

Figure legends

Figure 1. By Cho et al.


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GrapheneâCarbonâMetal Composite Film for a Flexible Heat Sink

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