Metal-Level Thermally Conductive yet SoftGraphene Thermal Interface MaterialsWen Dai,†,‡ Tengfei Ma,§ Qingwei Yan,† Jingyao Gao,†,‡ Xue Tan,†,‡ Le Lv,†,‡ Hao Hou,†

Qiuping Wei,∥ Jinhong Yu,†,‡ Jianbo Wu,⊥ Yagang Yao,# Shiyu Du,¶ Rong Sun,□ Nan Jiang,*,†,‡

Yan Wang,*,§ Jing Kong,■ Chingping Wong,○ Shigeo Maruyama,●,▽ and Cheng-Te Lin*,†,‡

†Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and ProtectiveTechnologies, Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences, Ningbo315201, China‡Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China§Department of Mechanical Engineering, University of Nevada, Reno, Reno, Nevada 89557, United States∥State Key Laboratory of Powder Metallurgy, School of Materials Science and Engineering, Central South University, Changsha410083, China⊥State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University,Shanghai, 200240, China#National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Jiangsu Key Laboratory ofArtificial Functional Materials, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing210093, China¶Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences, Ningbo 315201, China□Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China■Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts02139, United States○School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States●Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan▽Energy Nano Engineering Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba305-8564, Japan

*S Supporting Information

ABSTRACT: Along with the technology evolution for dense integration ofhigh-power, high-frequency devices in electronics, the accompanying interfacialheat transfer problem leads to urgent demands for advanced thermal interfacematerials (TIMs) with both high through-plane thermal conductivity and goodcompressibility. Most metals have satisfactory thermal conductivity but relativelyhigh compressive modulus, and soft silicones are typically thermal insulators (0.3W m−1 K−1). Currently, it is a great challenge to develop a soft material with thethermal conductivity up to metal level for TIM application. This study solves thisproblem by constructing a graphene-based microstructure composed of mainlyvertical graphene and a thin cap of horizontal graphene layers on both the top andbottom sides through a mechanical machining process to manipulate the stackedarchitecture of conventional graphene paper. The resultant graphene monolith has an ultrahigh through-plane thermal conductivity of 143 W m−1

K−1, exceeding that of many metals, and a low compressive modulus of 0.87 MPa, comparable to that of silicones. In the actual TIM performancemeasurement, the system cooling efficiency with our graphene monolith as TIM is 3 times as high as that of the state-of-the-art commercial TIM,demonstrating the superior ability to solve the interfacial heat transfer issues in electronic systems.

KEYWORDS: vertically aligned graphene, hierarchical structure, metal-level thermal conductivity, low compressive modulus,thermal interface materials

With the rapid progress of electronic device perform-ance in recent decades, the efficient removal of theheat generated from the integrated circuit (IC) chips

Received: July 1, 2019Accepted: September 24, 2019Published: September 24, 2019

Artic

lewww.acsnano.orgCite This: ACS Nano 2019, 13, 11561−11571

© 2019 American Chemical Society 11561 DOI: 10.1021/acsnano.9b05163ACS Nano 2019, 13, 11561−11571

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(such as CPU and GPU) is becoming more crucial to guaranteethe continuous, stable, and smooth operation of the system.1−3

Since the reliability of devices depends exponentially on theoperating temperature, a small temperature increase of 10−15°C may lead to a 50% reduction in the device’s lifespan.4,5 Toundertake the essential task of heat dissipation to maintain thedevice operating temperature at a desired level, thermal interfacematerials (TIMs), such as thermal greases, phase changematerials, and thermal pads, hold a dominant position inmicroelectronics and microsystems.6 Conventionally, TIMs areapplied to enhance the thermal conductance between thematingsurfaces of heat-generating components (heater) and the heatsink/spreader in electronic devices.7 To maximize the heattransfer efficiency, TIMs are required to possess not only a highthrough-plane thermal conductivity but also a low contactthermal resistance under normal packaging conditions.8 Thelatter can be achieved by ensuring good compressibility and easydeformation of the applied TIM for sufficient filling of themicrogaps between the heater and the heat sink at low contactpressure.9

Some inorganic materials have a high intrinsic thermalconductivity (κ), for example, AlN (130−260 W m−1 K−1) andCu (400 W m−1 K−1), but their rigidity (compressive modulus:around 100 GPa) leads to high contact thermal resistance forTIM applications.10−12 In contrast, organic materials such asresins and silicone rubbers can easily fill the gap at low appliedpressure (<650 kPa), but the TIMs made up of purely organicmaterials show poor performance due to their low intrinsicthermal conductivity (0.1−0.3 W m−1 K−1).13−15 Therefore,conventional TIMs combine the advantages of the above two byusing a polymer matrix reinforced with highly thermallyconductive fillers, such as AlN, Al2O3, BN, etc., to fabricate

composites with a thermal conductivity of 1−5 W m−1 K−1 at afiller content of 50−70 wt %.16−18 However, conventional TIMsface a great challenge to meet the future demand for coolinghigh-power and high-frequency devices with significantlyincreased power consumption.19 Recently, low-dimensionalmaterials with ultrahigh thermal conductivity, for instance,hexagonal BN nanosheets (up to 600 W m−1 K−1), carbonnanotubes (CNTs, higher than 3000Wm−1 K−1), and graphene(3500−5300 W m−1 K−1), have been employed extensively toprepare composites with polymers as matrix by melt or solutionblending.20−22 As a result, while the microstructure of the fillersin the polymer matrix is randomly distributed, the thermalconductivity of the composite TIMs can approach approx-imately 10 W m−1 K−1 (filler content: 20−50 wt %).23−25

Nevertheless, these low-dimensional fillers, in which phononscan efficiently propagate only along a certain direction in thematerial lattice, have highly anisotropic thermal conductivity.26

Therefore, in order to further improve the through-planethermal conductivity, the microstructural modulation towardvertical alignment of low-dimensional materials, instead ofrandom orientation, is highly desirable to break through theperformance bottleneck of current TIMs.Some techniques for the formation of a vertically aligned

architecture of CNTs or graphene have been proposed.27,28 Forthe case of CNTs, a popular approach is to grow verticallyaligned CNT arrays on the substrate by catalytic chemical vapordeposition.29 With low packing density (0.02−0.07 g cm−3), thecontact thermal resistance of CNT array-based TIMs could bevery low owing to its good compressibility.30−32 Correspond-ingly, the through-plane thermal conductivity is limited (1.21−4.87 W m−1 K−1) because of the small contact area betweenvertically aligned CNTs and the mating surfaces, leading to a

Figure 1. (a) Schematic illustration, (b) photograph, and (c) surface topographic images presenting the fabrication process of a graphenemonolith from graphene paper. Scale bar in (c) is 500 μm. The obtained graphene monolith showing (d, e) good bendability and (f) ease ofcutting.

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reduced heat flux through the basal plane of CNT array-basedTIMs.33−35 In addition, the graphene monolith with a verticallyaligned arrangement was fabricated by slicing paper consisting oflayer-by-layer stacked graphene, followed by making graphenestrips stand and then lateral compaction to form a denselypacked structure with a density of 1.3−1.6 g cm−3.28,36 Incontrast to CNT arrays, the vertically aligned graphenemonolithshows ultrahigh through-plane thermal conductivity (615 Wm−1 K−1) but is still not suitable for use as TIMs, due to its highcontact thermal resistance originating from the surface rigidity(compressive modulus: up to 0.5 GPa).37 In summary, to date, itstill remains a great challenge for emerging TIM applications tocreate a monolith composed of vertically aligned low-dimen-sional materials that exhibit both high through-plane thermalconductivity and good compressibility.Here, we developed a feasible machining-based process to

prepare a graphene monolith with an aligned microstructureusing conventional graphene paper as starting material. Byapplying a lateral mechanical force to the graphene paper, theresulting monolith is mainly constituted by vertically alignedgraphene and a thin cap of horizontal graphene layers at both thetop and bottom sides. Based on this characteristic architecture,the resultant graphene monolith exhibits both an ultrahighthrough-plane thermal conductivity of 143 W m−1 K−1

(exceeding that of many metals) and a low compressivemodulus of 0.87 MPa, which is much smaller than that ofsilicone-based composites (2−10 MPa). The combination ofmetal-level thermal conductivity and the good compressibilitycomparable to that of silicone endows our graphene monolithwith great potential for TIM applications. The comparative testsbetween our graphene monolith and state-of-the-art commercialTIMs also demonstrate the superior performance of ourgraphene monolith for cooling electronic devices. Our finding

provides insight for the construction of 2Dmaterial-based TIMs,which may satisfy the demanding heat dissipation requirementsof next-generation high-power ICs.

RESULTS AND DISCUSSIONThe graphene monolith was developed by a mechanicalmachining process, in which a shear force was first appliedaround the graphene paper, leading to the formation ofcrumpled graphene, and then a compact monolith could beobtained by laterally compressing this crumpled structure. Thetwo-step re-forming process is schematically illustrated in Figure1a. The detailed description of sample preparation can be seen inthe Methods section. In brief, graphene paper made by simplevacuum filtration was adhered to a biaxially prestretchedpolyacrylate elastomer with a stretching ratio of 400% ×400%. The graphene paper-bonded polyacrylate tape was thenshrunk naturally to deform the paper into a crumpled structureby using a self-developed automation equipment, with thedeformation process displayed in Movie S1. After beingdetached from the tape via an ethanol immersion treatment,the crumpled graphene paper was then deformed into amonolith with an optimal density of∼0.77 g cm−3 by performinga biaxial compression operation. In Figure 1b, it can be seen thatduring the deformation process the sample transforms from a 26cm diameter circle (graphene paper) to a 3.6 cm× 3.6 cm square(the monolith), whereas the thickness increases from 25 μm toapproximately 0.9 mm. Correspondingly, as shown in Figure 1c,the surface roughness of crumpled graphene (∼38.9 μm) ishigher than that of the pristine paper (∼2.8 μm), and it thendecreases to around 8.3 μm when the graphene monolith wasmade. The resulting graphene monolith is a thin pad exhibitingexcellent flexibility, bendability, and ease of cutting, as displayedin Figure 1d−f.

Figure 2. Microstructural variations during mechanical processing: Cross-sectional SEM images of (a) pristine graphene paper and (b) themonolith. (c, d) Detailedmicrostructure close to themonolith surface. (e, f) Regions enlarged from (c) exhibiting distinct graphene alignments.(g−i) Top view of the correspondingmorphological changes from (a) to (b). (j) Illustration and (k) top-view SEM image showing the verticallyaligned graphene underneath the cap. (l) Schematic illustrating the structural change of the graphene based on the proposed method.

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The concept of this study is to modulate the alignment ofgraphene microstructure based on a simple machiningoperation. In Figure 2a, the cross-sectional scanning electronmicroscopy (SEM) image of filtrated graphene paper exhibits acompact layer-by-layer stacking structure, as commonly seen inthe literature.38 Interestingly, after the proposed two-stepprocessing, we clearly observed that a vertically aligned graphenearchitecture has been formed in the resulting monolith (Figure2b−d), through an intermediate crumpled form (as displayed inFigure 1c). In addition, a thin cap consisting of horizontallyoriented graphene can be found adjacent to both the top andbottom surfaces of the monolith, as shown in Figure 2c and d.This demonstrates that our graphene monolith is composed of avertically aligned architecture (Figure 2e) with a thickness of∼850 μm (proportional thickness: 94.4%) and a thin layer of∼25 μm (proportional thickness: 2.8%) for laminated grapheneat both ends, as displayed in Figure 2f. It should be emphasizedthat the structural transition at the joint between vertical andhorizontal graphene is smooth and continuous (Figure 2d),which originates from the dense packing of the folds of crumpledgraphene paper. The top view in Figure 2g−i shows thecorresponding morphological changes of the sample surfacefrom the pristine paper to the monolith. In order to verify theinner microstructure of the monolith, the upper cap wascarefully removed using hydrogen plasma etching for 20 min(etching depth: ∼100 μm),39 as schematically illustrated inFigure 2j. After the cap removal, the surface morphology of thesample (Figure 2k) becomes significantly different from that inFigure 2i, demonstrating the formation of closely packed andvertically aligned graphene architecture based on the proposedmethod. As a result, Figure 2l illustrates the variation of theorientation of graphene layers during machining, and thehierarchical microstructure of the resulting monolith can beregarded as a mimetic structure of a honeycomb panel, whichhas a characteristic structure composed of vertical plates in the

middle and a cap of horizontal plates at both top and bottomsides.40 Herein, we named this honeycomb panel-like graphenepad as HLGP.Based on the transformation of the stacked architecture of

graphene paper from a horizontal to a vertical orientation,leading to the formation of the HLGP composed of mainlyvertically aligned graphene, we anticipated that theHLGPwouldhave a preference for heat conduction along the through-planedirection, due to the high in-plane heat transfer properties ofgraphene paper.41 Therefore, we measured the thermaldiffusivities (α) of the starting graphene paper along the in-plane direction and theHLGP along the through-plane directionby the laser flash method. The corresponding thermalconductivity (κ) can be estimated by the equation42 κ = α × ρ× Cp, where ρ is the sample density (graphene paper: 0.68 gcm−3, HLGP: 0.77 g cm−3) and Cp is the specific heat capacity(0.78 J g−1 K−1), determined by differential scanning calorimeter(DSC) analysis (see Figure S2 and Table S1). As demonstratedin Figure 3a and Table S2, the in-plane thermal conductivity ofstarting graphene paper is 273 W m−1 K−1. After the structuraltransformation, the high in-plane heat transfer property of thegraphene paper was converted into that of the HLGP along thethrough-plane direction, with the conversion rate of ∼52%,endowing the HLGP with a high through-plane thermalconductivity of 143 W m−1 K−1. This thermal conductivityachieved outperforms that of most previously reported thermallyconductive materials based on a vertically aligned orinterconnected structure, as displayed in Table S3. In order tofurther demonstrate the high through-plane thermal conductiv-ity of the HLGP, the comparative test on through-plane heattransfer capacity between the HLGP and pure molybdenum(Mo, 138 W m−1 K−1) was carried out. As shown in Figure 3b,two samples cut into the size of 10 mm× 10 mm× 0.9 mmwereplaced on a ceramic heater (60 W) and kept at roomtemperature initially, followed by heating at the same time.

Figure 3. (a) Thermal diffusivity and conductivity of graphene paper along the in-plane direction and the HLGP along the through-planedirection. (b) Test system configuration for demonstrating the through-plane heat transfer capacity. (c) Corresponding IR images and (d)surface temperature evolution of the HLGP and pure Mo versus heating time. (e) Thermal conductivity evolution of the HLGP versuscompressive strain. (f) Compressive stress−strain curve of the HLGP and PDMS. (g) Comparison of thermal conductivity versus compressivemodulus between the HLGP and common metal materials. (The compressive modulus of metals is from refs 43−49.)

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The two samples were coated by a thin graphite layer (∼5 μm)to ensure the same infrared emittance for accurately recordingtheir time-dependent surface temperature using a calibratedinfrared (IR) camera. As the results show in Figure 3c,d andMovie S2, the temperature of the HLGP increases relativelyfaster than that of pure Mo and always shows a slightly highervalue (e.g.,ΔT = 6 °C at 140 s), manifesting the higher through-plane thermal conductivity of the HLGP compared to that ofpure Mo.Based on the ultrahigh through-plane thermal conductivity of

143 W m−1 K−1, over 1 order-of-magnitude greater than that ofconventional thermal pads (on average 10 W m−1 K−1), theHLGP exhibits a great potential for TIM applications. Inpractical use, a vertical deformation of TIMs is needed under amoderate packaging pressure (<650 kPa) for guaranteeing agood contact between TIMs and two mating surfaces toeliminate the air gaps.8 Therefore, the compressibility and thethrough-plane thermal conductivity of the HLGP as a functionof vertical compressive strain were studied. As the resultspresented in Figure 3e and Table S4, the through-plane thermalconductivity of the HLGP is always higher than 100 W m−1 K−1

when the sample was vertically compressed all the way to a 30%compressive strain. The compressive modulus of HLGPestimated from the stress−strain curve (Figure 3f) is ∼0.87MPa, and the compressive stress exhibits a change from 81 to500 kPa as the strain increases from 10% to 30%, which are ingood agreement with the mechanical property requirements ofcommercial TIMs and fall in the range of regular packagingpressure.13 In general, metals usually have satisfactory thermalconductivity (35−420Wm−1 K−1) with the same level of HLGP(Figure 3g), but the corresponding compressive modulus is ashigh as 10−300 GPa, which is far above that of conventionalTIM (<2 MPa), resulting in metals too hard to deform enoughunder a regular packaging pressure in the real TIMapplications.43−49 Soft silicones are typically thermal insulators(0.1−0.3Wm−1 K−1),9,37 and although the incorporation of softsilicone with thermally conductive fillers, such as graphene,CNT, and liquid metal (LM), can achieve an improved thermalconductivity (0.58−23.3 W m−1 K−1), the correspondingcompressive modulus will also increase (3.3−8.5 MPa).50−54

In contrast, based on the hierarchical microstructure, our HLGPhas both a high through-plane thermal conductivity of 143 Wm−1 K−1, exceeding that of many metals, and a low compressivemodulus of 0.87 MPa, which is less than that of polydimethylsi-loxane (PDMS, ∼2 MPa estimated from Figure 3f) and otherreported silicone-based composites (2−10 MPa).9,37,55 Ascompared to the various materials, the HLGP almost has thelowest compressive modulus with a similar level of thermalconductivity, to the best of our knowledge. The low compressivemodulus would give the HLGP excellent deformability foradequately filling the microgaps between the mating surfacesafter applying a moderate contact pressure, anticipating a lowcontact thermal resistance in the actual TIM application.Figure 4a−c present the measurements of thermal resistances,

including total thermal resistance and contact thermalresistance, of an HLGP used as a TIM based on an infraredmicroscope system, which was commonly used to estimate thethermal resistance of vertically aligned CNT arrays.56,57 Themeasurement configuration is schematically presented in Figure4a, and more details can be found in Figure S3 and our previouswork.58 At the steady state, the cross-sectional temperaturedistribution of HLGP packaged between the heater and the heatsink at different compressive strains was recorded (see Figure 4band Figure S3). It is known that the total thermal resistance(Rtotal) is the sum of bulk thermal resistance (RTIM) of theHLGPand contact thermal resistance (Rcontact) of two interfaces, asshown in eq 1:36

κ

= + = −

= −

R R R R R

R BLT/

contact heater/TIM TIM/heatsink total TIM

total TIM (1)

where bulk thermal resistance (RTIM) is the product of thereciprocal of through-plane thermal conductivity of the HLGP(κTIM) multiplied by the bond line thickness (BLT), which is thethickness of compressed TIMs under packaging. As the results inFigure 4c and Table S5 indicate, the total thermal resistance ofthe HLGP decreases from 26.9 K mm2 W−1 to 11.8 K mm2 W−1

when the sample was compressed at a strain from 10% to 30%.Correspondingly, the calculated contact thermal resistance(double sides) decreases from 20.2 K mm2 W−1 to 5.8 K mm2

Figure 4. (a) Configuration for TIM performance measurement. (b) Cross-sectional temperature distribution of the HLGP obtained based onthe system in (a). Scale bar in (b) is 500 μm. (c) Total and contact thermal resistance of HLGP evaluated at different compression ratios.

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W−1, which is far lower than that of silicone-based thermal padsand comparable to conventional metallic solders.59

Besides, as schematically illustrated in Figure 5a, a controlledexperiment was carried out by removing the horizontal graphenelayer at both the top and bottom sides of the HLGP using H2plasma etching to investigate in-depth the relation between thecontact thermal resistance and the hierarchical microstructure ofHLGP. After cap removal, the originally smooth surface of theHLGP was replaced by a closely packed, vertically alignedgraphene architecture, as shown in surface SEM and topo-graphic images (Figure 5b and c). The through-plane thermalconductivity of the sample without the cap is up to 176 W m−1

K−1, with an increase of 23% compared to that of the HLGP(143 W m−1 K−1), due to the removal of horizontal graphenelayers with poor through-plane heat conduction. Interestingly, inFigure 5d, we found that the contact thermal resistance of thecap-removed sample greatly increases ∼3.8 times (from 5.8 Kmm2 W−1 to 22.0 K mm2 W−1) compared to that of the HLGP,when both samples with the same BLT of 630 μmunderwent theTIM test under the vertical pressure of 500 kPa. The highcontact thermal resistance of the cap-removed sample results ina ∼2.3 times increase of total thermal resistance, indicating thedegradation of the corresponding TIM properties by more thanhalf.To further understand the importance of the cap layer

composed of horizontally oriented graphene on the reduction ofthe contact thermal resistance, we conducted nonequilibriummolecular dynamics (NEMD) simulations on the systems of L-shaped graphene and vertical graphene, to mimic the HLGP and

the cap-removed sample, respectively, in contact with the copperheat source, as presented in Figure 5e−h. The details can be seenin the Supporting Information (Figure S4). Figure 5e and f showthat in the L-shaped graphene system the surface temperaturedistribution at the mating surface of the copper heat source ismuch more uniform than the sample without the cap. Thissuggests that the cap layer seamlessly connected to the verticalgraphene can act as a heat spreader, leading to a maximum ofcontact area at the microscale to avoid local overheating and asignificant improvement of effective contact thermal con-ductance by ∼3 times (from 7.8 MW m−2 K−1 to 22.5 MWm−2 K−1) at the mating interface. In addition, considering thegraphene nanoplatelets used in this work are multilayered, oursimulations also indicate that the effective contact thermalconductance in the L-shaped graphene system is essentiallyindependent of graphene layer number, as shown in Figure 5f−h(also see Figure S5f−h). As a result, we demonstrate that thehorizontally oriented graphene layer at both sides of the HLGPis an essential part for achieving a low contact thermal resistance.Table 1 presents the feature parameters of the HLGP and

some reported TIMs, in which it can be found that the heattransport properties of the HLGP are significantly superior tothat of some current advanced TIMs, including vertically alignedCNT (VACNT) arrays and vertically aligned (VA) graphenemonoliths. The higher through-plane thermal conductivity ofthe HLGP can be attributed to its characteristic structurecomposed of mainly VA graphene. The relatively low contactthermal resistance results from two points: (i) the goodcompressibility of the HLGP to adequately fill the microgaps

Figure 5. (a) Surfacemorphology change of HLGP after cap removal and the corresponding (b) SEM and (c) topographic images. Scale bar: (b)10 μm; (c) 200 μm. The variation of total thermal resistance and contact thermal resistance of HLGP after cap removal. Temperaturedistribution at themating surface of a copper heat source in contact with (e) vertical graphene and L-shaped (f) single-layer, (g) bilayer, and (h)trilayer graphene, based onNEMD simulations of the effective contact thermal conductance. The computational area in (e)−(h) is 80 Å× 80 Å.

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between the mating surfaces and (ii) the horizontally orientedgraphene layer at both ends of the HLGP acting as a heatspreader to further improve the interfacial heat transferefficiency. As described in eq 1, TIMs with a larger thickness(BLT) usually have a higher total interface resistance. However,by the combination of the high through-plane conductivity andlow contact thermal resistance, the total thermal resistance of

our HLGP with a BLT of 630 μm is even lower than that ofVACNT arrays with a BLT of 7.5−15 μm and is similar to that oforiented polythiophene arrays with a BLT of 3 μm, as exhibitedin Table 1. Besides, it is important to emphasize that the superiorTIM properties of HLGP were obtained by directly contactingthe sample to the mating surface, without further metallizationor coating, exhibiting the convenient for install and use of

Table 1. Comparison of TIM Properties between HLGP and Reported TIMs

TIMs κ (W m−1 K−1) Rcontact (K mm2 W−1)a Rtotal (K mm2 W−1) BLT μm bonding ref

Reported TIMsoriented polythiophene 4.4 12.1 12.8 3 Au metallization 60VACNT arrays 77 300 Au metallization 27Cu-filled VACNT arrays 25 7.5 polymer coating 61VA graphene monolith 112.2 10.2 30.5 2300 indium coating 36VAMWCNT arrays 40 15 direct contact 59VACNF/rubber 1.21 300 300 direct contact 33graphene hybrid paper 17.6 47 61.0 250 direct contact 62This WorkHLGP 143 5.8 11.8 630 direct contact

aRcontact is the contact thermal resistance of double sides (the sum of Rheater/TIM and RTIM/heat sink).

Figure 6. (a) Schematic configuration of the TIM performancemeasurement system. Temperature evolution of a ceramic heater as a function of(b) the heating time at an applied power of 20 W cm−2 and (c) various power densities at steady state. (d) Calculated total thermal resistance(Rtotal) versus saturated heater temperature in (b). (e) Comparison of heat dissipation capability based on fluid dynamics simulation. (f) Goodthermal shock stability in cyclic heating/cooling tests and (g) good thermal durability of HLGP in a long-term TIM performance test (7 days).

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HLGP. Those excellent performances endow HLGP withpromising potentials to meet the ever-increasing thermalmanagement requirement for next-generation power electrondevices.In order to investigate and compare the cooling performance

between HLGP and state-of-the-art TIMs in real cases, anevaluation system was built to simulate the heat dissipationprocess in electronic components. As schematically illustrated inFigure 6a, our HLGP and a commercial thermal pad (∼17 Wm−1 K−1, Fujipoly XR-m, Japan) with a same BLT of 630 μm(HLGP: compressed at 30%) and a lateral size of 2 cm × 2 cmwere placed, respectively, between the ceramic heater (20 Wcm−2) and the heat sink. The heat sink was connected to a forcedair cooling system to extract the generated heat. In Figure 6b,when the heater was turned on at 200 s, the heater temperaturerose rapidly whether with or without TIMs. When the systemreached a steady state at 800 s, markedly, compared to the heatertemperature without TIMs, the cooling performance of ourHLGP by a decrease of 65 °C is much greater than that of theXR-m thermal pad (38 °C). In Figure 6c, based on the linearvariation of the steady heater temperature versus the powerdensities, the equivalent heat transfer coefficients (equal to thereciprocal of the slope63) of 0.2 (without TIM), 0.5 (XR-m), and1.5 W cm−1 °C−1 (HLGP) can be calculated. It demonstratesthat the system cooling efficiency with the HLGP as TIM is∼7.6times and∼3 times as high as that without TIM and with an XR-m thermal pad, respectively.A computational fluid dynamics software (ANSYS Icepak)

was employed for thermal analysis of our evaluation system (seeFigures 6a and S5) at an applied power density of 20 W cm−2,and the total thermal resistance (Rtotal) of two TIMs at the sametest conditions can be simulated based on the results in Figure6b. As presented in Figure 6d and Figure S5e, we demonstratedthat not only can the total thermal resistance of HLGP (13.2 Kmm2W1−) decrease by 91.0% compared to that of XR-m (146.8K mm2 W−1), more significantly, the contact thermal resistance(Rcontact: double sides) of HLGP (∼7.2 K mm2 W−1) is alsoconsiderably lower than that of the XR-m thermal pad (∼109.7K mm2 W−1). The calculation details can be seen in theSupporting Information. We proposed that the lower contactthermal resistance of HLGP compared to that of the XR-mthermal pad could be attributed to three reasons: (1) the lowcompressive modulus of the HLGP (0.87 MPa) comparable tothat of silicones contributes an adequate gap-filling underpackaging status, as demonstrated in Figure S6a,b; (2) thehorizontal graphene layer with high in-plane thermal con-ductivity (273 W m−1 K−1) at both ends of the HLGP acts as aheat spreader to transfer the heat on the interfacial boundarymuch faster compared to that of silicone (0.2 W m−1 K−1) forXR-m (Figure S6c), contributing to a more efficient heattransmission through the heater/TIM and the TIM/heat sinkinterface; and (3) the graphene/metal interface for the HLGPand heat sink has a lower Kapitza resistance (10−8 K m2 W−1)compared to the silicone/metal interface (10−6−10−7 K m2

W−1) for the XR-m thermal pad and heat sink.64,65 Combinedthe high through-plane thermal conductivity and low thermalcontact resistance, the simulated temperature profiles in Figure6e confirm the outstanding heat transfer properties of HLGP forTIM use.Additionally, a thermal shock test for the HLGP was

performed by alternatively switching the heater power between20 and 70 W cm−2 for 2500 cycles, as shown in Figure 6f, inwhich the HLGP presented excellent thermal shock stability in

the usual operating temperature range of the devices during thecyclic heating/cooling test. Figure 6g exhibits a long-term TIMperformance test of the HLGP by fitting the test system (Figure6a) in real environments and a continuously running system(heater power density: 20 W cm−2) for 7 days. The temperatureof the heater (Theater) and the ambient (Tambient) were recordedby a thermocouple. In Figure 6g, Theater exhibits a fluctuatedchange with the time, due to the ever-changing Tambient, as aresult of the large temperature difference between day (6:00−18:00) and night (18:00−6:00 next morning), as well as thevariability of the weather in this testing period. Still, thetemperature difference between the heater and the ambient(Theater − Tambient) has remained approximately constant duringthis long-term TIM performance test, demonstrating anexcellent thermal durability of HLGP over time. Finally, asshown in Figures S7 and S8 and Movie S3, based on the all-inorganic composition of graphene, it is worth emphasizing thatthe operating temperature of the HLGP in air covers a very widerange from−196 to 500 °C, which is far beyond that of silicone-based thermal pads (−50 to 200 °C),66 indicating potential heattransport applications in extreme environments. As a result, ourHLGP is a high-performance all-inorganic TIM aimed to replacestate-of-the-art thermal pads for electronics cooling applications.

CONCLUSIONSIn summary, using conventional graphene paper as startingmaterial, a graphene monolith could be fabricated by amechanical machining process using a self-developed automa-tion equipment. The resulting HLGP has a hierarchicalmicrostructure composed of mainly vertically aligned grapheneand a thin cap of horizontal graphene layers on both the top andbottom sides. The vertically oriented graphene within theHLGPcontributes to a high through-plane thermal conductivity (143W m−1 K−1), while maintaining excellent deformability. Inaddition, we demonstrated that the horizontal graphene layers atboth surface sides play a crucial role to reduce the contactthermal resistance of the HLGP in the TIM performancemeasurement. As a result, the cooling efficiency of the HLGPused as a TIM is superior than that of a state-of-the-art thermalpad (17 W m−1 K−1) with a decrease of the heater temperatureby 65 °C versus 38 °C, showing a significant improvement in theheat dissipation capacity in electronics. In addition, the HLGPwith excellent compressibility (compressive modulus: 0.87MPa) is a kind of thermal pad, by which, compared to othertypes of TIMs (e.g., thermal greases and phase changematerials),our HLGP as a TIM has some special advantages, such as thecapacity of thermal shock absorption and the possibility forapplying to multiple IC chips with different heights. Moreover,based on the all-inorganic composition of graphene, our HLGPnot only exhibits good stability during TIM operation but alsopossesses a wide operating temperature range (−196 to 500 °C),making the HLGP a promising candidate for next-generationTIM applications. Last but not least, this work aims to study therelation between structure and properties of a graphene-basedmonolith and focuses on modulating the arrangement ofgraphene sheets to achieve an HLGP with the best balancebetween thermal conductivity and mechanical property. There-fore, the HLGP reported in this study was directly fabricated bya mechanical machining process of the graphene paper, withoutany further chemical or physical optimized treatment. Herein,some designed surface modification, such as surface function-alization or the micro/nanostructured surface treatment ofHLGP, may be carried out to modulate the interfacial

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interaction of graphene and its contact surface to further reducethe contact thermal resistance between the HLGP and theheater/heat sink, aiming at different application scenarios.

METHODSMaterials. Graphene nanoplatelets synthesized by the intercalation

and exfoliation of graphite were provided by Ningbo MorshTechnology Co., Ltd. (China), and the characterizations of graphenenanoplatelets can be found in Figure S1. The lateral size and thethickness of graphene nanoplatelets are 5.4 ± 0.3 μm and 10.6 ± 0.3nm, respectively. Anhydrous ethanol (analytical grade) was purchasedfrom SinopharmChemical Reagent Co., Ltd. (China) and used withouta further purification.Preparation of the Graphene Monolith. A uniform graphene

dispersion was obtained by ultrasonication of 0.9 g of graphenenanoplatelets in 2000 mL of ethanol for 0.5 h. Graphene paper with adiameter of 26 cm was fabricated by vacuum filtration of a graphenedispersion using a porous polytetrafluoroethylene (PTFE) membranewith a pore size of 0.22 μm. In order to produce a crumpled structure,graphene paper was first bonded onto a polyacrylate pressure-sensitivetape, which was biaxially prestretched up to 400% × 400%. As theprestretched elastomeric tape was gradually released, the looselyadhered graphene paper could be crumpled up due to the mismatch ofelastic moduli between them. The deformation process from graphenepaper to crumple graphene can be seen inMovie S1. After removing thepolyacrylate tape via an ethanol immersion treatment (2 h), theobtained crumpled graphene paper was then laterally compressed byapplying a compaction pressure to prepare the graphene monolith. Inthis compression step, in order to avoid the expansion of the graphenemonolith after removing the compressive stress, a cyclic compression−decompression operation for five times followed by maintaining thepressure for 5 min was carried out. The monolith presents only a slightexpansion less than 3% as soon as the sample is taken out of thecompression equipment. And all samples used for all measurements andanalysis are the postexpanded monoliths with stable dimensions andshapes. As a result, the final size of the graphene monolith is 3.6 cm ×3.6 cm × 0.9 mm with a density of ∼0.77 g cm−3, and the occupiedvolume of graphene in the monolith is 61.5%, based on the calculationof the density ratio between the HLGP (0.77 g cm−3) and graphenesheets (2.0 g cm−3).Characterizations. The morphology and microstructure of the

samples were observed by optical microscopy (Leica DM-2500, LeicaMicrosystems, Germany), field emission scanning electron microscopy(Quanta FEG250, FEI, USA), and atomic force microscope(Dimension 3100, Veeco, USA). Raman spectra were recorded usinga Reflex Raman system (Renishaw plc, Wotton-under-edge, UK)employing a laser wavelength of 532 nm. X-ray diffraction (XRD)patterns were recorded by a D8 Discover with GADDS (Bruker,Germany) with Cu Kα radiation (λ: 1.54 Å). X-ray photoelectronspectroscopy (XPS) was carried out with an AXIS Ultra DLDspectrometer (Kratos Analytical, UK). The compression modulus ofthe samples with a size of 2 cm × 2 cm was tested using a universaltesting machine (UTM, model 5567A, Instron, USA). The loading ratewas controlled as 50 μm min−1. The compressive modulus of thesamples was obtained by calculating the average value of the tangentmodulus (E = dσ/dε) in the range of 0−10% strain, where σ is thecompressive stress and ε is the corresponding strain. The specific heatcapacity (Cp) of the samples was evaluated using a DSC (PYRISDiamond, PerkinElmer, USA). Thermal conductivity of the sampleswas measured with an LFA 467 NanoFlash apparatus (Netzsch,Germany). The IR photos were captured by using an infrared camera(Fluke, Ti400, USA).

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.9b05163.

Additional information (PDF)

Microstructural variations from graphene paper tocrumpled graphene during mechanical processing(MP4)Comparison of through-plane heat transfer propertybetween the HLGP and pure Mo (MP4)Demonstration of cryogenic availability of the HLGP bybending the sample in a liquid nitrogen environment(MP4)

AUTHOR INFORMATIONCorresponding Authors*E-mail: jiangnan@nimte.ac.cn.*E-mail: yanwang@unr.edu.*E-mail: linzhengde@nimte.ac.cn.ORCIDWen Dai: 0000-0003-0601-5150Qiuping Wei: 0000-0002-8109-5982Jinhong Yu: 0000-0001-9134-7568Jianbo Wu: 0000-0002-3574-5585Shiyu Du: 0000-0001-6707-3915Rong Sun: 0000-0001-9719-3563Jing Kong: 0000-0003-0551-1208Chingping Wong: 0000-0003-3556-8053Shigeo Maruyama: 0000-0003-3694-3070Cheng-Te Lin: 0000-0002-7090-9610Author ContributionsW.D. and C.-T.L. conceived the preliminary experiments andwrote the manuscript at all stages. T.M. and Y.W. provided theNEMD simulation and corresponding data analysis. Q.Y. andL.L. measured partial thermal properties of the HLGP. X.T. andH.H. performed mainly characterizations of the graphene sheetsand the graphene paper including Raman, XRD, and XPS. J.G.prepared the cap removal samples and took the correspondingSEM images in modified version. Q.W. and J.W. providedsuggestions to demonstrate the thermal conductivity of theHLGP. J.Y. and S.D. helped with the mechanism explanation oninterfacial thermal transfer performance of the HLGP. Y.Y., R.S.,and C.-P.W. provided the equipment and measured theresistance of the HLGP. N.J. mainly designed the automationequipment to prepare the crumpled graphene paper, providedsome suggestions about the hydrogen etching of the HLGP, andgave some experimental designs determining the long-termstability of the HLGP’s shape, structure, and performance. J.K.and S.M. provided constructive writing thoughts on themanuscript and supervised this study.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSThe authors are grateful for the financial support by the NationalKey R&D Program of China (2017YFB0406000), the StrategicPriority Research Program of the Chinese Academy of Sciences(XDA22000000), Scientific Instrument Developing Project ofthe Chinese Academy of Sciences (YZ201640), Science andTechnology Major Project of Ningbo (2016S1002 and2016B10038), and International S&T Cooperation Programof Ningbo (2017D10016). We also thank the Chinese Academyof Sciences for Hundred Talents Program, Chinese CentralGovernment for Thousand Young Talents Program, and 3315Program of Ningbo. T.M. and Y.W. would like to acknowledgethe financial support from the NASA EPSCoR program (NASA

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Space Grant NNX15AK48A). S.M. would like to acknowledgethe financial support by JSPS KAKENHI, Grant NumbersJP15H05760 and JP18H05329.

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