Vertically aligned high-quality graphene foams for anisotropically

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Applications of Polymer, Composite, and Coating Materials

Vertically aligned high-quality graphene foams for anisotropically conductive polymer composites with ultrahigh through-plane thermal conductivities Fei An, Xiaofeng Li, Peng Min, Pengfei Liu, Zhi-guo Jiang, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04230 • Publication Date (Web): 28 Apr 2018 Downloaded from http://pubs.acs.org on April 28, 2018

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ACS Applied Materials & Interfaces

Vertically

aligned

high-quality

graphene

foams

for

anisotropically conductive polymer composites with ultrahigh through-plane thermal conductivities Fei An,1 Xiaofeng Li,1* Peng Min,2 Pengfei Liu,1 Zhi-Guo Jiang1, Zhong-Zhen Yu1,2* 1

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China. 2

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China E-mail: [email protected] (X. Li), [email protected] (Z.-Z. Yu) ABSTRACT: Although graphene based thermal interface materials (TIMs) have great potentials in removing excess heat generated during highly efficient running of electronic devices, their practical applications are usually limited by their unsatisfactory thermal conductions, which are mainly caused by unsatisfactory dispersion and distribution, low loading and low quality of graphene sheets, as well as the thermal interfacial resistance between graphene sheets and polymer matrix. Herein, we develop vertically aligned graphene hybrid foams (GHFs) with high densities by hydrothermal reduction of graphene oxide in the presence of high-quality graphene nanoplatelets (GNPs) followed by air-drying. The reduced graphene oxide sheets play an important role in constructing a vertically aligned interconnection network for accommodating GNPs during the hydrothermal reduction process, while the incorporated GNPs not only make the thermal conductance network denser but also prevent excessive shrinkage of the foams during the air-drying. More critically, 1 ACS Paragon Plus Environment

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graphitization of GHF at 2800 oC removes the residual oxygen-containing groups and heals the defects of their reduced graphene oxide component, leading to high-quality graphene foams. The resultant vertically aligned high-quality graphene porous architecture with high density as an ideal thermal conductance network of TIMs is highly efficient in improving thermal conductivity of its epoxy composite, which exhibits an ultrahigh through-plane thermal conductivity of 35.5 W m-1 K-1 at a graphene loading of 19.0 vol%. The excellent thermally conductive performance makes the annealed GHF/epoxy composites suitable for the thermal management. KEYWORDS: vertical alignment; graphene foams; thermal conductivity; graphitization; composites 1. INTRODUCTION With the integration and miniaturization of electronic devices, timely dissipation of excess heat during their highly efficient running becomes a great challenge, which deeply influences the performance and lifetime of electronic.1 Thermal interface materials (TIMs), composed of polymer and thermally conductive fillers, are highly required to solve the problem of excess heat dissipation. Compared to conventional thermally conductive fillers, such as metal nanoparticles, boron nitride platelets and carbon nanotubes, graphene is a highly promising candidate as its ultrahigh in-plane thermal conductivity of ~5300 W m-1 K-1.2-6 However, its polymer based composites usually exhibit less satisfactory thermal conductivities, which are mainly caused not only by the undesired thermal resistance between loosely contacted graphene sheets but also the interfacial resistance between polymer matrix and graphene 2 ACS Paragon Plus Environment

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sheets.7 To solve this problem, construction of continuous graphene networks before compounding with polymers has been confirmed to be an effective way.8-10 Wong et al synthesized a graphene-based carbon aerogel as a thermally conductive porous network for phase change materials.11 The thermal conductivity of 1-hexadecanol was enhanced by 107.9% with only 2 wt% of carbon aerogel. Bai et al. combined high-quality graphene foam with multilayer graphene flakes to enhance the thermal conductivity of polydimethylsiloxane (PDMS).12 Thanks to the more effective thermal conductance paths than those of individuals, the resultant composite presented a much high thermal conductivity than that of neat PDMS. Besides, because of their 2-dimensional layered structure with high aspect ratios, graphene sheets like to be orientated along the flow direction during their compounding with thermoplastic polymers or distributed horizontally during curing process of their thermoset polymer composites, leading to high horizontal (in-plane) thermal conductivity of their polymer composite plates, but their vertical (through-plane) thermal conductivity is rather low,13, 14 which restricts their practical application as TIMs. To fully take advantage of the high in-plane thermal conductivity of graphene, graphene sheets should be vertically aligned to form an efficient thermal conductance network along the vertical direction of TIMs. Recently, directional freezing followed by freeze-drying has been widely used to construct anisotropic 3-dimensional (3D) porous architectures with ceramic platelets and graphene sheets as building blocks.15-21 Kim et al. developed highly aligned graphene aerogels with ultralow densities using a unidirectional freeze-casting of graphene oxide (GO) suspension followed by thermal reduction at 900 oC in an inert 3 ACS Paragon Plus Environment


atmosphere, and their electrically conductive epoxy composites exhibited significantly anisotropic electrical properties with a ultralow percolation threshold.20 Yu et al. fabricated anisotropic graphene aerogels by directional-freezing of graphene hydrogel using anisotropically grown ice crystals as templates followed by freeze-drying, which exhibited ultralow density, excellent compressibility, and strain-sensitive electrical conductivity.21 However, this approach could usually fabricate vertically aligned graphene foams/aerogels with low densities and thus low thermal conductivities, which is quite reasonable by considering the processability of the graphene suspensions or reduced graphene oxide (RGO) hydrogels and the orientability of their graphene and RGO components. The density of graphene based conductive networks could be increased by 3D printing,22 controlling compaction23 and air-drying.24-26 For example, Worsley et al. prepared a high-density graphene-derived carbons with highly electrical and mechanical properties by air-drying.27 Although air-drying is convenient and time-saving, the resulting graphene monolith usually presents an isotropic structure and the shape shrinkage is less controlled, leading to solid-like architecture with rather high density, which may be difficult for backfill infiltration of polymers or monomers. Additionally, the quality of graphene building blocks is also vital for ultimate thermal conductivity of nanomaterials and composites.28,29 Currently, many graphene porous architectures are constructed by chemically and/or thermally reduced graphene sheets, which usually contain residual oxygen-containing groups and lattice defects that could cause serious phonon scattering.30-32 Fortunately, high-temperature annealing is proved to be efficient in 4 ACS Paragon Plus Environment

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removing these functional groups and healing the defects, Lian et al. prepared thermally annealed defect-free graphene sheets by high temperature annealing at 2200 oC for thermally conductive phase change material, which exhibited an high thermal conductivity of 3.55 W m-1 K-1 at a 10 wt% defect-free graphene loading due to the greatly reduced phonon scattering centers for thermal transport.33 Gao et al. fabricated a high-quality graphene film with ultrahigh thermal conductivity and excellent flexibility by high-temperature annealing and mechanical pressing.29 The thermally annealed graphene sheets would be ideal thermally conductive fillers for high-performance TIMs. Recently, we fabricated highly anisotropic graphene/BN hybrid aerogels with high density and long-range ordered architecture by hydrothermally treating the suspension of GO sheets and BN platelets to form RGO/BN hybrid hydrogels followed by air-drying.25 The thermally annealed aerogel at 2000 oC endowed epoxy with a ultrahigh through-plane thermal conductivity of ~11.01 W m-1 K-1 at a high RGO/BN loading of 44 wt%.25 As graphene has a much higher thermal conductivity than that of BN, in the present work high-quality graphene nanoplatelets (GNPs) are chosen to replace BN platelets to further enhance the thermal conductivity of the anisotropic porous architectures. Firstly, vertically aligned RGO/GNP hybrid hydrogels are prepared by hydrothermally assembling the aqueous suspension of GO and GNPs, followed by air-drying. The resultant graphene hybrid foams (GHFs) exhibit an anisotropic structure and high porosity, and their density can be tuned in the range of 0.3-0.5 g cm-3 by varying the initial mass ratio of GO/GNP components. Secondly, high-temperature annealing of GHFs is adopted to improve the quality of their RGO component, leading to 5 ACS Paragon Plus Environment


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high-quality graphene foams with vertically aligned structure and moderate density. The thermally annealed GHF/epoxy composite prepared by vacuum-assisted impregnation of epoxy monomer and curing additives into the porous architecture and subsequent thermal curing exhibits an ultrahigh through-plane (vertical) thermal conductivity of 35.5 W m-1 K-1 ，which is higher than those of stainless steel (16.7 W m-1 K-1) and bronze (26.2 W m-1 K-1),34 with an enhancement efficiency (η) of 884%. 2. EXPERIMENTAL SECTION 2.1. Materials. The following materials were used as received： Pristine natural graphite (100 mesh, Huatai Lubricant and Sealing); commercially available GNPs with lateral sizes of ~25 µm (M25, XG Sciences); KOH (99.5%, Beijing Chemical Factory); polyvinyl pyrrolidone (PVP,

K30,

Sigma-Aldrich);

bisphenol-A

epoxy

resin

(Jiafa

Chemicals);

1,2-bis(2,3-epoxypropoxy) ethane reactive diluent and methyl hexahydrophthalic anhydride curing agent (Adamas Reagent); 2, 4, 6-tris(dimethylaminomethyl) phenol curing accelerator (Aladdin Industrial). 2.2. Fabrication of high-quality graphene hybrid foams. Graphite oxide was synthesized from pristine graphite flakes using a modified Hummers method35 and GO aqueous suspension of 10 mg mL-1 was obtained by ultrasonication of graphite oxide in 50 mL deionized water for 10 min. A certain amount of GNPs and 0.1 g of PVP were dispersed in 50 mL deionized water with ultrasonication for 30 min. After the aqueous suspensions of GNP and GO were mixed and homogenized for 30 min with 0.132 M of KOH, the resulting hybrid suspension was transferred into an autoclave and heated at 160 oC for 3 h, and the 6 ACS Paragon Plus Environment

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resulting RGO/GNP hybrid hydrogel was immersed in deionized water for 3 days to remove residual KOH, followed by air-drying at ambient conditions to obtain a GHF. With the initial mass ratios of GO/GNP were 1/5, 1/8, and 1/10, the resultant GHFs were designated as GHF5, GHF8, and GHF10, respectively. For comparison, the RGO/GNP hybrid hydrogel or RGO hydrogel were also conventionally freeze-dried under vacuum at -50 oC for 3 days to get freeze-dried RGO/GNP hybrid foam (fGHF). To stress the contribution of KOH, a graphene hybrid foam without the use of KOH was also prepared by the similar hydrothermal reduction and subsequent air-drying, exhibiting an isotropic architecture (iGHF). In the absence of GNPs, hydrothermal treatment of GO suspension (5 mg mL-1) followed by freeze-drying or air-drying, result in freeze-dried RGO foam (fGF) and air-dried RGO monolith (GM), respectively. To improve the quality of its RGO component, GHF5 was thermally annealed at 1000, 1500, 2000 and 2800 oC for 2 h under an argon flow protection to obtain thermally annealed GHFs with vertically aligned high-quality graphene sheets and GNPs, named as GHF-1000, GHF-1500, GHF-2000 and GHF-2800, respectively. 2.3 Preparation of thermally conductive epoxy composites. Epoxy resin, reactive diluent, curing agent, and curing accelerator with a mass weight ratio of 8/2/9.48/0.0576 were uniformly mixed at ambient temperature, and the mixture was impregnated into the foams under vacuum followed by curing at 80 oC for 4 h and post-curing at 120 oC for 2 h. For comparison, GHF5 was milled into powders with lateral sizes of less than 50 µm by using an agate mortar, and the resultant was designated as milled GHF5. GNPs or milled GHF5 were mixed with epoxy, and the mixture was then poured into a mold and cured using the same 7 ACS Paragon Plus Environment


thermal curing conditions to produce epoxy composites filled with GNPs or milled GHF5. The loading of milled GHF5 or GNPs in the composites is the same as that of GHF5 (35 wt%). 2.4 Characterization. Scanning electron microscope (SEM, JEOL JSM-7800F) was used to observe the morphologies of foams and determine lateral size of GNPs. The samples for SEM observation were prepared by cutting the cylindrical foams along their radial direction (Figure S1). The lateral size and thickness of GO were measured by a Bruker dimension Fastscan 2-SYS atomic force microscope (AFM). The birefringence of liquid crystal was examined with a Nikon ECLIPSE Ci-POL polarized light optical microscope (POM). Raman spectra were collected on a Renishaw inVia Raman microscope at a laser excitation wavelength of 633 nm. To obtain the Raman mapping of ID/IG, a piezo stage was used to move the sample with a step size of 1 µm in a square area (40×40 µm2) and a Raman spectrum was recorded at every point. The chemical compositions of GO and graphene based foams were measured using a Thermo Scientific ESCALAB 250 XI X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max 2500 X-ray Diffractometer with a Cu Ka radiation of 0.154 nm at a generator voltage of 40 kV. Thermal conductivity (K) was calculated using the formula of K = α×Cp×ρ, where α is the thermal diffusivity, Cp is the specific heat capacity, and ρ is the density of a composite. The in-plane and through-plane thermal diffusivities of composites were measured on a Netzsch LFA467 NanoFlash light flash apparatus at 30 oC. The specimens for in-plane (ϕ25.4 mm × 1 mm) and through-plane (ϕ12.7 mm × 1 mm) thermal conductivity measurements 8 ACS Paragon Plus Environment

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were made by cutting the cylindrical epoxy composites along their radial direction followed by surface polishing. Specific heat capacity was measured by a Perkin Elmer Pyris 1 differential scanning calorimeter (DSC) at a scanning rate of 10 oC min-1. The density of composites was characterized using an electronic Mettler-Toledo balance with a density determination kit 33360. The apparent density of a foam was calculated by dividing its mass by its volume, and its porosity was obtained by dividing its apparent density by the true density of graphite (2.25 g cm-3). The content of graphene-based fillers was determined by using the following equations:

M GHF =

ρGHF ρcom

VGHF = M GHF ×

(1) * ρ com * ρ GHF

(2)

Where M GHF is the weight content of GHF in the composite, ρ GHF and ρcom are the apparent densities of GHF and the composites, respectively; VGHF

is the volume

* * concentration of GHF in the composite, ρ GHF and ρ com are respectively true densities of * * GHF and the composite. For the GHF, ρ GHF = 2.25 g cm-3. For the composite, ρ com is

equal to ρcom . For mechanical compression tests, GHFs were prepared in a cylindrical shape (~ 11 mm in diameter and in height). The compression tests were performed on an Instron 3365 universal testing system using a 5 kN load cell in stress control mode. 3. RESULTS AND DISCUSSION Figure 1 shows a schematic illustrating the fabrications of (1) vertically aligned RGO/GNP hybrid hydrogel, (2) air-dried graphene hybrid foam (GHF), (3) thermally 9 ACS Paragon Plus Environment


annealed GHF and (4) its anisotropically thermally conductive epoxy composite. The GO sheets, made from graphite with a modified Hummers method,35 exhibit a large lateral size of ~80 µm with a thickness of ~2 nm (Figure S2), and like to form liquid crystals at relatively low concentrations.36, 37 Whereas, GNPs has a relatively small lateral dimension of ~10 µm (Figure S3). With the addition of KOH, the GO suspension (5 mg mL-1) presents a distinct Schlieren texture under polarized light (Figure S4), because KOH partially deoxygenates GO and enhances the repulsion between adjacent GO sheets, and thus helps form GO liquid crystals.38 Even after adding the isotropic suspension of GNPs, the mixture is still highly anisotropic. The hydrothermal reduction of the hybrid suspension at 160 oC and subsequent freeze-drying leads to a freeze-dried RGO/GNP hybrid foam (fGHF) with highly ordered structure (Figure 2a), which is inherited from that of GO liquid crystals. GO sheets in the liquid crystals can align along the inner surface of the container.38 As the container we used is cylindrical, GO sheets form an onion-like structure with a parallel alignment to the cylinder axis. During the self-assembly of RGO sheets, GNPs are wrapped and connected by RGO sheets by π-π interactions, resulting in a similar onion-like structure but narrower gaps than freeze-dried RGO foam (fGF) (Figure 2b). With increasing the content of GNPs, the extent of order is weakened (Figure S5), implying that excessive GNPs would affect the ordered structure of GO liquid crystals and cause disorder during the hydrothermal reduction process.

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Figure 1. A schematic illustrating the fabrication of (1) vertically aligned RGO/GNP hybrid hydrogel by hydrothermally assembling aqueous suspension of GO and GNP, (2) Air-dried vertically aligned GHF, (3) thermally annealed GHF and (4) its anisotropically conductive epoxy composite. After air-drying at ambient condition, three GHFs are obtained with different GO/GNP initial mass ratios. It is noticed that the GHFs shrink differently along the axial and radial direction, because of the anisotropic structure of their RGO/GNP hybrid hydrogel precursors. The large gaps of ~10 µm between adjacent layers (Figure 2a) lead to much larger shrinkage along the radial direction than that along the axial direction. The shrinkage of GHFs is also dependent on the mass ratio of GO/GNP components. GHF5 shrinks more than GHF8 and GHF10, resulting in a higher density (0.42 g cm-3) (Table 1). Unlike freeze-dried counterpart 11 ACS Paragon Plus Environment


(fGHF5) with onion-like structure, GHF5 has a less clearly ordered structure (Figure 2c), which is ascribed to the nonhomogeneous shrinkage of adjacent layers during air-drying. However, most graphene sheets are still vertically aligned, namely parallel to the cylinder axis. In the absence of GNPs, air-drying of neat RGO hydrogel leads to a seriously shrunk RGO monolith (GM) with a high density (1.61 g cm-3), due to the capillary-force induced collapse during air-drying.27 As shown in Figure 2d, the top-view image of GM is very smooth without distinct voids, which is consistent with its high density and low porosity. It is now clear that the incorporated GNPs play important roles in preventing excessive shrinkage of RGO hydrogels during air-drying and thus adjusting the density and porosity of GHFs.26 The presence of more GNPs result in less shrinkage and lower density of GHFs (Figure 2e,f). However, excessive GNPs weaken the vertical alignment of the RGO sheets (Figure S5a,b), which would be unfavorable to the thermal conduction improvement along the vertical direction. In addition, KOH is essential for assembling the ordered structure. Without the use of KOH, the resultant graphene hybrid foam (iGHF) exhibits an isotropic structure with randomly distributed RGO sheets and large pores caused by the trapping of CO2 during the hydrothermal reduction (Figure S5f).39 By comparing the vertical alignment extent, density and porosity of all the foams fabricated, GHF5 with moderate density and porosity is chosen as a promising thermal conductance network of TIMs. To enhance the quality of its RGO building blocks, it is subjected to thermal annealing under an inert gas protection at 1000, 1500, 2000 and 2800 oC.


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Figure 2. Top-view SEM images of (a) fGHF5, (b) fGF, (c) GHF5, (d) GM. Digital images of hydrogels and corresponding foams dried at ambient conditions with different GO/GNP mass ratios of (e) 1/0 and (f) 1/5. Table 1. The components, densities and porosities of air-dried graphene-based foams. Samples

GM GHF5 GHF8 GHF10

GO/GNP (w/w) 1/0 1/5 1/8 1/10

Density (g cm-3) 1.61 0.42 0.36 0.33

Porosity (%) 28.4 81.3 84.0 85.3

Figures 3a and S6 show the XRD patterns of GNP, GO and GHFs. The commercially available GNPs show a sharp peak at 26.6o with an interlayer spacing of 0.34 nm similar to that of graphene, suggesting their high crystallinity, which ensures their high thermal conductivity. Differently, GO has a broad diffraction peak at ~12o corresponding to an interlayer spacing of 0.74 nm,40 much larger than that of graphite (0.34 nm) due to the intercalation of abundant oxygen-containing functional groups. However, the hydrothermally reduced and air-dried hybrid foam (GHF) and its thermally annealed counterpart (GHF-2800) exhibits two peaks: the sharp and strong peak at 26.6o is attributed to the highly stacked 13 ACS Paragon Plus Environment


graphene sheets; while the weak peak at ~24o is ascribed to the imperfect restacking caused by the curvature of graphene sheets .41,42

Figure 3. (a) XRD patterns of GO, GNP, GHF5 and GHF-2800 with an inset showing the weak peak ~24o. Raman ID/IG mapping of (b) GHF5, (c) GHF-1000, (d) GHF-1500, (e) GHF-2000 and (f) GHF-2800. Graphene derivatives usually present two D band (~1350 cm-1) and G band (~1580 cm-1), and their intensity ratio (ID/IG) reflects their reduction extent.43 ID/IG mapping is conducted to characterize GHF5 and its thermally annealed counterparts (Figure 3b-f). The narrow 14 ACS Paragon Plus Environment

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spacing of 1µm between data points enables the accurate reflection of the structure of GHFs by the Raman measurements. GHF5 exhibits large blue regions surrounded by yellow and green ones. The blue regions are related with high-quality GNPs, resulting in low ID/IG value of ~0.2; Whereas, the yellow and green regions, reflecting higher ID/IG values (> 0.5), result from RGO sheets that have residual oxygen functional groups and defects. This ID/IG distribution indicates that GNPs are connected by RGO sheets. After the thermal annealing at 1000oC for 2 h, more orange and red regions (ID/IG > 1) appear, ascribed to the newly generated defects due to the removal of the oxygen functional groups.44 As the annealing temperature rises to 1500 oC, most of the red regions fade away and green regions occupy the largest proportion of mapping, suggesting that the lattice defects on RGO begin to be removed or healed.45 With further increases of the annealing temperature, the blue regions become larger and the ID/IG values decrease dramatically. For GHF-2800, a GHF annealed at 2800 oC, its Raman mapping is full of dark blue and most ID/IG values maintain ~0.025, because such a temperature is high enough to remove the residual oxygen functional groups completely and heal the lattice defects fully. Figure S7 shows ID/IG frequency histograms of GHF5 and its thermally annealed counterparts. Before annealing, the distribution of ID/IG is peaked at 0.75, owing to the defects on the RGO component of GHF5. After annealing at 1000 oC, the proportion of ID/IG >1 increases, which is consistent with the emergence of new defects.44 Only when the annealing temperature is higher than 1500 oC, the peak value of ID/IG decreases and its distribution narrows, indicating the removal and healing of the defects on RGO building 15 ACS Paragon Plus Environment


blocks. The efficient removal of residual oxygen-containing groups by the thermal annealing is well confirmed by comparing XPS of GHF5 and its thermally annealed counterparts (Figure S8). Compared to the C/O atomic ratio of GHF5 (8.2), GHF-2800 has an extremely high C/O atomic ratio of 137.9, implying it has negligible oxygen-containing groups. Figure 4a shows the compressive stress-strain curves of GHFs with different initial mass ratios of GO/GNP components. Like other cellular materials, GHFs also have three regions in their stress-strain curves: nearly linear elastic region (ε < 5%), flat plateau region (5% < ε < 60%), and densification region (ε > 60%).46 During the first region, the deformation is dominated by bending of the cell walls. When the strain reaches up to 5%, irreversible plastic deformation occurs with near-zero Poisson’s ratio (Figure 4b). This phenomenon results from the special structure of GHFs. In the GHF architecture, GNPs and RGO sheets are assembled by weak Van der Waals forces, which enable the graphene sheets slide under compression to induce plastic deformation. 27 Besides, the high porosity of GHFs could avoid the contact between the sliding cell walls, which prevents the lateral expansion that results in positive Poisson’s ratio.47 Thus, GHFs exhibit near-zero Poisson’s ratio durting the flat plateau region. Increasing the strain further results in the collapse of the cells and the densification through the foam, leading to a sharply increased compression strength during the third region. With increasing the content of GNPs, the plateau region lengthens, which is due to the higher porosity and looser graphene network. Meanwhile, their modulus decreases, because of the decreased density (Figure 4c). Although the GHFs have different densities, their compressive modulus (E) all follow the relationship of E ~ ρ2, which is expected for 16 ACS Paragon Plus Environment

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open cell porous structures.46 Figure 4d compares the modulus of GHFs, commercial graphite, other porous materials with low densities as a function of density.26,27,48,49 Before the thermal annealing, the scaling of the modulus to a value of GHF is similar with that of low-density cellular materials, but higher than those of graphene monolith and commercial graphite with isotropic structures. Interestingly, the modulus of GHF-2800 (406 MPa) is approximately 3 times higher than that of GHF5 (100 MPa) with similar densities, because of the enhanced inter-sheet interactions within graphene sheets and larger crystalline domains.50,51 These results demonstrate that both the vertically aligned structure and high-temperature annealing are highly beneficial for improving the compression performances of graphene hybrid foams.

Figure 4. (a) Compressive stress-strain curves of GHFs with different ratio of GO/GNP; (b) Digital images of GHF5 during compression test; (c) Modulus of GHFs with different 17 ACS Paragon Plus Environment


densities; and (d) comparison of moduli of GHF5 and GHF-2800 with those of other carbon foams reported in the literature. Figure 5a shows the through-plane thermal conductivities of epoxy and its composites with different loading of graphene. Neat epoxy resin has a low thermal conductivity of ~0.21 W m-1 K-1. After compounding with graphene, the thermal conductivity is improved. Because of the less defects in GNP, the GNP/epoxy composite has a thermal conductivity of 1.91 W m-1 K-1, slightly higher than that of milled GHF5/epoxy composite (1.65 W m-1 K-1) at the same loading. Compared to the filled epoxy composites, the preformed continuous thermal conductance network with RGO sheets is more efficient in endowing epoxy with high thermal conductivities. For example, iGHF5 is an isotropic GHF hydrothermally synthesized in the absence of KOH, and its epoxy composite exhibits a high through-plane thermal conductivity of 3.22 W m-1 K-1. However, the thermal conductivity of fGHF/epoxy composite is only 0.51 W m-1 K-1, due to the low filler loading of 6.4 wt%. Interestingly, GHF5, an anisotropic GHF hydrothermally synthesized in the presence of KOH, has a vertically aligned graphene network, which is an efficient thermal conduction paths. Thanks to the fully taking advantage of the high thermal conductivity of graphene basal plane, the GHF5/epoxy composite presents a fairly high through-plane thermal conductivity of 8.43 W m-1 K-1. As shown in Figure 5b, the thermal conductivity of GHF/epoxy composites are highly anisotropic because of the unique structure of GHF. Along through-plane direction of GHF, the RGO sheets and GNP are tightly contacted during the hydrothermal synthesis in the presence of KOH, which help reduce the thermal contact resistance between RGO sheets and 18 ACS Paragon Plus Environment

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thus form dense conductance paths in the trough-plane direction. However, along the in-plane direction there are many gaps, which would be filled with low thermally conductive epoxy leading to low in-plane thermal conductivity of GHF/epoxy composite.52 Meanwhile, the intrinsic thermal conductivity of RGO sheets perpendicular to their basal planes is also low, which also hinders the in-plane thermal conduction of the composite.53

Figure 5. (a) Comparison of thermal conductivities of epoxy composites with GNP, milled GHF5, iGHF5 and GHF5, and the inset illustrates the through-plane and in-plane directions; (b) Thermal conductivities and (c) their enhancement efficiencies of GHF/epoxy composites 19 ACS Paragon Plus Environment


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with different GO/GNP initial mass ratios; (d) Thermal conductivities and (e) their enhancement efficiencies of annealed GHF/epoxy composites with different annealing temperatures; and (f) Comparison of thermal conductivities of GHF/epoxy composites with those reported in the literature. Figure 5b also shows the influence of initial mass ratio of GO/GNP components on thermal conductivity of GHF/epoxy composites. It is seen that the thermal conductivity increases with the initial content of GO. To eliminate the influence of filler loading, thermal conductivity enhancement efficiency (η) is adopted.54

η=

K - Km × 100% 100VK m

(3)

Where Km and K are thermal conductivities of epoxy and GHF/epoxy composite, respectively, V is the volume loading of fillers in a composite. As shown in Figure 5c, GHF5/epoxy composite has a higher enhancement efficiency than others, which is attributed to the dense thermal conductance paths with reduced thermal contact resistance between the RGO sheets. It is also noted that excessive GNPs enlarge the gaps between GNP and RGO sheets and hence form a loose network, which would cause more phonon scattering.55 As high temperature annealing is an effective way to remove the functional groups and heal the defects of RGO building blocks,56,57 GHF5 is thermally annealed at different high-temperatures under the protection of an inert atmosphere. As shown in Figure 5d and e, the thermal conductivity and corresponding enhancement efficiency of the GHF/epoxy composites increase with the annealing temperature. As shown in Figure 5f, GHF-2800/epoxy composite with 19.0 vol% of graphene loading exhibits an ultrahigh 20 ACS Paragon Plus Environment

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through-plane thermal conductivity of 35.5 W m-1 K-1 with an enhancement efficiency of 884%, which are much higher than those reported in the literature. 10,23,26,33,34,53,58-66 The slope of the dotted line is proportional to the enhancement efficiency. It is seen that GHF5/epoxy composite has a relative high thermal conductivity, but its enhancement efficiency is not high because of residual oxygen functional groups and defects on the RGO component of GHF5. On the contrary, GHF-2800/epoxy composite not only has an outstanding thermal conductivity but also a highly competitive enhancement efficiency, even better than the composites with graphene foam prepared by chemical vapor deposition.23,58 In addition to the enhanced thermal conductivity, the electrical conductivity of GHF/epoxy composites also increases with the annealing temperature (Figure S9). The GHF-2800/epoxy composite exhibits an electrical conductivity of up to 1.5×104 S m-1, much higher than that of GHF5/epoxy composite (33 S m-1). Such an ultrahigh conductivity should be mainly attributed to (1) the vertically aligned graphene network by the hydrothermal assembly in the presence of KOH, (2) the high-quality graphene building blocks resulted from the high-temperature graphitization, and (3) the high loading of conducting fillers by the incorporation of GNPs. 4. CONCLUSION GHFs with vertically aligned network are prepared by hydrothermal assembly of GO/GNP suspension in the presence of KOH followed by air-drying. The resultant RGO sheets construct a vertically aligned interconnection network for accommodating GNPs, while the GNPs make dense thermal conduction paths and prevent excessive shrinkage of the foams 21 ACS Paragon Plus Environment


during air-drying. The density and porosity of GHFs can be tuned by the initial mass ratio of GO/GNP components. When the initial mass ratio of GO/GNP is 1/5, the corresponding GHF foam exhibits a moderate density of 0.42 g cm-3. By annealing at 2800 oC to remove the oxygen-containing groups and heal the lattice defects, a vertically aligned high-quality graphene network is achieved, which exhibits a high modulus of 406 MPa and near-zero Poisson’s ratio during compression measurement. This annealed GHF-2800 endows epoxy with an ultrahigh through-plane thermal conductivity of 35.5 W m-1 K-1 and a thermal conductivity enhancement efficiency of 884% with 19.0 vol% of graphene, which is one of the highest values for all graphene/polymer composites reported in the literature. Acknowledgements Financial support from the National Natural Science Foundation of China (51403016, 51773008, 51533001, 51521062), the National Key Research and Development Program of China (2016YFC0801302), the Fundamental Research Funds for the Central Universities (JD1820) is gratefully acknowledged. REFERENCES 1.

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F.,

Fabrication

of

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and

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thermally

conductive

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TOC

A highly anisotropic graphene hybrid foam with vertically aligned structure and moderate density is prepared by hydrothermal reduction and assembly of a mixture of graphene oxide and graphene nanoplatelets followed by air-drying. The graphitized anisotropic foam is highly efficient in endowing epoxy with an ultrahigh through-plane thermal conductivity.


Vertically aligned high-quality graphene foams for anisotropically

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