Lightweight All Graphene‐Based Two‐Phase Heat Transport Devices

Graphene‐based composites show great potential in the development of lightweight functional devices and systems, and polymers are usually used as binders to enhance the mechanical properties of these composites. Due to the low thermal conductivity of the polymer and the high inherent interfacial thermal resistance between polymer and graphene, however, the developed devices and systems generally possess low thermal conductivity, which seriously limits their further thermal‐related applications. Here, a lightweight two‐phase heat transport device (TPHTD) based on a vapor‐liquid phase transition‐based heat transfer by using a graphene‐based composite material as the casing is generated. The graphene‐based TPHTD has high thermal conductivity of up to 1408 W (mK)−1 and realizes ultra‐high specific thermal conductivity of ≈5600 W (mKg)−1. This graphene‐based device with lightweight and high heat transfer capacity offers new opportunities for efficient thermal management of compact, portable, or wearable electronic and power systems.


DOI: 10.1002/admi.202201687
interactions of weak van der Waals force to form thick graphene-based film, and the films are not strong enough to withstand tensile and compression due to weak connection. [1] Past efforts in solving this problem were focused on improving the strength of graphene-based films through incorporating a polymer as a binder. [2][3][4] While this strategy has helped improve the mechanical performance of graphene-based films, it leads to the deterioration of other properties, such as thermal properties. The use of polymer binders greatly reduces the thermal conductivity of graphene-based film due to the low thermal conductivity of polymers and the high inherent interfacial thermal resistance between graphene and polymers. [5,6] To improve the thermal conductivity of graphene-based films, one effective solution is to incorporate polymers with high intrinsic thermal conductivity, [7] but the improvement is limited to a certain level.
In order to generate graphene-based systems with high thermal conductivity, graphene films can be used as casing materials to prepare a two-phase heat transport device (TPHTD). Such a device mainly relies on the phase change heat transfer process and takes advantage of both the large latent heat and the fast flow of the vapor from the evaporator to the condenser to realize efficient heat transfer, thus achieving high thermal conductivity, enabling its extensive use in thermal management devices. [8][9][10] So far, graphene has been used in two different ways for the TPHTD. In one way, graphene is coated on the inner wall of the metal-based casing in the TPHTD. [11][12][13][14] While this approach improves the thermal conductivity of the TPHTD due to good-wettability and low flow resistance of working medium on the surface of graphene, the resulting device is still heavy due to the metal-based casing, which does not take advantage of the lightweight property of graphene. The other is to use graphene as the casing material and metal frame as the support to prepare TPHTDs. [15] Such an approach still has not yet been realized to lightweight all graphene-based TPHTDs. To achieve all graphene-based TPHTDs with high thermal performance, an alternative approach is needed to form reliable binding between graphene nanosheets at nanoscale and achieve desired geometry of devices at macroscale.
Graphene-based composites show great potential in the development of lightweight functional devices and systems, and polymers are usually used as binders to enhance the mechanical properties of these composites. Due to the low thermal conductivity of the polymer and the high inherent interfacial thermal resistance between polymer and graphene, however, the developed devices and systems generally possess low thermal conductivity, which seriously limits their further thermal-related applications. Here, a lightweight two-phase heat transport device (TPHTD) based on a vapor-liquid phase transition-based heat transfer by using a graphene-based composite material as the casing is generated. The graphene-based TPHTD has high thermal conductivity of up to 1408 W (mK) −1 and realizes ultra-high specific thermal conductivity of ≈5600 W (mKg) −1 . This graphene-based device with lightweight and high heat transfer capacity offers new opportunities for efficient thermal management of compact, portable, or wearable electronic and power systems.

Introduction
For the pursuit of lightness of matter, since the 19th century entrepreneurs and inventors began to replace heavier materials with lighter ones, including light alloys, plastics, and nanostructured materials. Among all nanostructured materials, graphene-based composites show promise for the generation of lightweight devices and systems. In the construction of graphene-based composite, one key issue is how to achieve reliable binding between graphene nanosheets. Typically, graphene nanosheets are connected only by the www.advmatinterfaces.de In this work, we develop a combined self-assembly method for preparing a high-performance lightweight TPHTD. Through the self-assembly process under vacuum filtration with Ni(OH) 2 template, the graphene oxide (GO) nanosheets with polyvinyl alcohol (PVA) are stacked under compressive pressure to form the desired geometry. During the self-assembly process of GO/PVA, the prepared Ni(OH) 2 template with micropores was used as a template and put into the GO/PVA solution to prepare microchannels for flow of fluid and vapor in the TPHTD. After reduction via HI acid (Figure 1a), the resulting reduced graphene oxide (rGO)/PVA film has a high tensile stress of 53.8 MPa at the strain of 2.29%, cross-plane thermal conductivity of 0.59 W (mK) −1 and ultra-low leak rate of 4.16 × 10 −10 Pa×m 3 s −1 (Figure 1b). The final rGO/PVA-based TPHTD utilizes ethanol as the phase change working medium. The rGO/ PVA film demonstrates the well-wettability and low flow resist-ance to ethanol molecules (Figure 1c,d), which is used as a phase change working medium to achieve rapid heat transfer in the rGO/PVA-based TPHTD. All graphene-based TPHTD exhibits a highly effective thermal conductivity and offers promise for the development of TPHTD with lightweight and high heat transfer capacity in high-efficient thermal management of portable devices and wearable electronics.

Methods
GO nanosheets were prepared from nano-graphite powder by using a modified Hummers' method and uniformly dispersed into deionized water (DI water). [16] As shown in Figure 2a, 0.50 g of graphite powder, 0.50 g of NaNO 3 , and 40 mL of concentrated sulfuric acid were added to a 500 mL www.advmatinterfaces.de round-bottom flask in an ice water bath and formed a mixture. The mixture was oxidized by slowly adding 3.00 g of KMnO 4 . Then, the flask was transferred to a water bath of 42 °C with magnetic stirring at a speed of 750 revolution per minute (rpm). After stirring for 90 min, 30 mL of DI water was added into the flask. By stirring for another 30 min, 100 mL of DI water was added to dilute the solution. Next, 3 mL of H 2 O 2 solution was dropped into the flask. Finally, the solution was centrifuged at 1000 rpm for 2 min to extract the supernatant. The extracted supernatant was centrifuged at 8000 rpm for 15 min to obtain the sediment. Through ultrasound treatment, the obtained sediment was re-dispersed in DI water. This centrifuged step at 8000 rpm was repeated for 4 times to obtain the GO solution. In this case, the concentration of GO solution is 10 mg mL −1 . Figure S1, Supporting Information, shows the TEM image of the GO nanosheet.
The Ni(OH) 2 template with micropores was prepared by vacuum filtration and laser-etched. 1.16 g of Ni(NO 3 ) 2 ·6H 2 O was added to 400 mL of DI water, and then was mixed with 0.37 g of 2-aminoethanol by stirring (Figure 2b). The mixture was left for 48 h to obtain the Ni(OH) 2 solution via Qu methods. [17] Ni(OH) 2 solution was vacuumized by using an acetate filter membrane with polymathic methacrylate (PMMA) template for preparing rectangular membrane. On this prepared membrane, a regular arrangement of micropores was generated by laser drilling. During the filtration of GO/PVA solution, these micropores will be filled with GO nanosheets and PVA, which serve as support columns to prevent the collapse of graphenebased TPHTD during vacuum evacuation and filling.

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GO/PVA solution was prepared by mixing 10 mg mL −1 of GO solution and 1 wt.% of PVA solution with a fixed weight ratio of 4:1 into a flask placed in a water bath at 30 °C under stirring ( Figure 2c). After stirring for 60 min, the flask was transferred to a vacuum chamber to remove the gas in the mixed solution. The GO/PVA solution was poured into a suction flask and vacuumized. Subsequently, the prepared Ni(OH) 2 template was placed in the middle of AAO template, and then the GO/ PVA solution was poured again for further vacuum filtration. After the GO/PVA film completely covered the surface of the Ni(OH) 2 template, the filtration was completed and the GO/ PVA film covered Ni(OH) 2 template was removed from AAO template. This GO/PVA casing covered Ni(OH) 2 template was then placed into HI acid and reduced to rGO/PVA casing at a temperature of 30 °C for 1 h. During the reduction process, the Ni(OH) 2 template inside the rGO/PVA casing was also dissolved by HI acid through a previously designed gap. The open space formed after the dissolution was used as the channel for the flow of phase-change working medium. Figure 2d shows the picture of a TPHTD prepared by the self-assembly process. The three-dimensional structure of the TPHTD reconstructed by micro-computed tomography (CT) shows unobstructed channels for flowing of phase-change working medium and support columns with uniform distribution (Figure 2d(ii)). The prepared device was filled with a certain amount of ethanol through the preset charging tube at a vacuum of 5 × 10 −3 torr (Figure 2d(iii)).

Results and Discussion
For a TPHTD, the layered structure formed by self-assembly and the distance between layers affect the gas permeability of the graphene-based casing, which is closely related to proper operation and service life. The cross-section morphology of GO film, rGO film, GO/PVA film, and rGO/PVA film was characterized (Figure 3a-d). The SEM image of cross-section of GO film demonstrates that there are large gaps between the GO nanosheets due to the repulsive interaction of many functional groups on the surface of GO nanosheets, which indicates a loose laminar structure. [18] Through reduction, most of the oxygen-contained functional groups on the surface of GO nanosheets are removed, which enables the compact structure. [19] The GO/PVA solution is vacuum-pumped and filtered into a film, in which GO www.advmatinterfaces.de nanosheets are connected by PVA molecules. PVA molecules enhance the adhesion of GO nanosheets. [20] This compact laminar structure indicates that the PVA molecules improve the interlayer adhesion between GO nanosheets and reduce the interlayer porosity. When HI acid is used to reduce GO/PVA film, the redundant functional groups on the GO nanosheets are removed, resulting in a more compact laminar structure of rGO/PVA film than before reduction. [20] By the loading of PVA and the reduction via HI acid, a compact laminar structure can effectively reduce the penetration of gas molecules, enabling rGO/PVA film as a reliable leak-tight casing.
For TPHTD, high cross-plane thermal conductivity of the casing materials can enable the effective transfer of heat generated by the heat source to the liquid-state working medium. The graphene-based materials transfer heat primarily through phonons, while metals rely on electrons for heat transfer. The cross-plane thermal conductivity of PVA film, GO film, rGO film, GO/PVA film, and rGO/PVA film can be calculated by Equation (1): where, α is the coefficient of cross-plane thermal diffusion ( Figure S2), C p is the specific heat capacity ( Figure S3, Supporting Information), and ρ is the density ( Figure S4, Supporting Information). We have listed the values of the parameters for calculating the thermal conductivity in Table 1.
Due to a large number of hydrogen bonding interactions between PVA molecules, PVA film prepared has a high thermal conductivity of 0.38 W (mK) −1 (Figure 4a), which is consistent with the value reported in previous literature. [21] There exist large amounts of functional groups on the GO nanosheets and holes seen in the SEM figure for the GO film, which increased the phonon scattering, leading to low cross-plane thermal conductivity of GO film. Moreover, as the GO nanosheets interact with each other through van der Waals forces, only the lowfrequency phonon vibration mode can transfer a small amount of heat energy. Thus, GO film has a low cross-plane thermal conductivity of only 0.22 W (mK) −1 . Through the reduction, the functional groups on the GO nanosheets are removed, which not only reduces phonon scattering, but also decreases the layer spacing between them, shortening the distance of heat transfer. By comparison, the cross-plane thermal conductivity of rGO film is higher than that of GO film, reaching 0.3 W (mK) −1 . The effective medium theory was employed to express the association between the thermal conductive phenomenon and the laminated structure of the film and demonstrated the π-π stacking and hydrogen-bond interaction between rGO in responsible for the increase in thermal conductivity. [22] With the loading of PVA, the GO nanosheets and PVA molecules in GO/PVA film are bonded by hydrogen bonding, which helps generate a new channel of heat conduction between GO nanosheets. Thus, compared with GO film, the cross-plane thermal conductivity of GO/PVA film is improved, reaching 0.32 W (mK) −1 . The reduction removes the functional groups on GO nanosheets in GO/PVA film, thus reducing phonon scattering and layer spacing. The reduced layer spacing can decrease the thermal resistance between rGO nanosheets. Adding PVA molecules, which interact with rGO via hydrogen bond, would remove the gaps and bridge the rGO and thus reduce the number of effective phonons scattering centers, leading to the improvement of thermal conductivity. [23] The hydrogen bonding interaction between PVA and rGO brings them into contact with each other (Figure 1a), so that an interconnected and penetrated thermal conductive network is formed in the rGO/PVA films. [24] Thus, the rGO/PVA film generated has a higher thermal conductivity than GO/PVA film, reaching 0.59 W (mK) −1 .
For the casing material, the excellent gas-barrier performance plays an important role in maintaining high heat transfer performance and long service life of TPHTD. The gas-barrier property is mainly used to prevent the escape of vapor-state working medium and the infiltration of external noncondensation gas. Generally, permeability decreases with increasing molecular weight of the gas. Thus, helium (He) was selected as the molecular probe to test the permeability of PVA film, GO film, rGO film, GO/PVA film, and rGO/PVA film, [25] as shown in Figure 4b. The gas permeation testing setup is shown in Figure S5, Supporting Information. It is well known that polymer has poor gas-barrier capacity. For pure PVA film, the leak rate is up to 1.25 × 10 −7 Pa × m 3 s −1 . The leak rate of GO film is as low as 4.91 × 10 −9 Pa × m 3 s −1 , which is a two-order of magnitude lower than that of PVA film. After reduction, the gas-barrier capacity of rGO film is supposed to be better than that of GO film as the lower layer spacing increases the penetration resistance of helium and other gas molecules. While the loading of PVA slightly increases the layer spacing of rGO film, the PVA molecules is embedded into the layer spacing of rGO nanosheets, filling the gap and acting as a barrier. [26,27] Through cross-linking between PVA molecules and rGO nanosheets, the rGO/PVA film archives ultra-low gas-permeability, which is only 4.16 × 10 −10 Pa × m 3 s −1 . This graphene-based casing material have a significantly lower leakage rate than polymers and are capable of meeting the requirements of long-term device use. [28][29][30][31][32] The greater tensile strength and rupture strain, higher cross-plane thermal conductivity, and good gas-barrier capacity enable the use of rGO/PVA film as the casing material for preparing the TPHTD with high reliability.
Additionally, the surface with good wettability and low flow resistance can effectively enhance the heat transfer performance of the TPHTD. When the graphene-based materials are used as the casing of the device, it has been proved that graphene has good wettability and low flow resistance for water. [33][34][35] Ethanol, as another common phase-change working medium, has a very low permeability to graphene-based films, but lack of the research on its wettability and flow resistance on graphene-based films. To gain theoretical insight into the wettability and flow resistance, we simulated the diffusion and slipping of ethanol molecules on the surface of graphenebased material using molecular dynamics simulations. Details of molecular dynamics simulations are explained in the supporting information (Supplementary section 2). As shown in Figure 5a, when a clump of ethanol molecule is placed freely in the center of the graphene surface, it only takes 200 ps for the ethanol molecules to cover the entire graphene surface. This indicates that ethanol has a very fast diffusion rate on the surface of graphene. According to the formula Equation S1, Supporting Information, the diffusion coefficient of ethanol on graphene surface can be calculated to be 0.43 × 10 −3 cm 2 s −1 , which is the same order of magnitude as that of water molecule on graphene surface reported in previous literature. [36,37] In Figure 5c, we compare the diffusion coefficient of ethanol molecule with that of water and dodecane on graphene. The diffusion coefficient is inversely proportional to the molecular weight. Water has the largest diffusion coefficient due to its small molecular weight, while dodecane has the highest molecule weight, resulting in the lowest diffusion coefficient. For graphene-based casing, it is highly desired that the phasechange working medium can have both low permeability and good wettability. Based on the overall consideration, ethanol is one of the most suitable choices because of its low permeability and relatively good wettability. The good wettability enables the ethanol to effectively condense and nucleate on the surface of graphene and facilitate the evaporation of thin film, thus enhancing condensation and evaporation in TPHTD. In addition, according to Young's equation the graphene-based surface with good wettability can generate a large capillary force to promote liquid reflux.
In the simulation of the flow of ethanol molecules on the graphene-based surface, we apply a certain flow rate to the ethanol molecules at the top, which drives the molecules below to flow freely. The simulation shows that the ethanol molecules in the bottom layer move to the right under the dragging of the upper layer molecules, which indicates that the adhesion of the graphene-based surface to ethanol molecules can be negligible (Figure 5b). According to the relationship between velocity distribution and time, when t = 500 ps the flow of ethanol molecules reaches a stable state, and the velocity presents a linear distribution along the thickness direction, which is consistent with the typical Couette flow. With the velocity distribution along the flow direction, the pressure gradient can be calculated to be zero (Section S3, Supporting Information), indicating that the surface viscosity does not affect the flow of ethanol. In addition, it is worth mentioning that the flow rate of ethanol at the bottom layer (y = 0) is not zero, indicating that it is a sliding flow and has low flow resistance on the surface of graphene-based materials. [38] Therefore, graphene-based materials as microchannels facilitate the rapid transport of ethanol molecules.
As shown in Figure 6a, the thickness of the prepared rGO/ PVA-based TPHTD is only 0.35 mm, and its inner microchannel is only ≈0.20 mm in height, which is used for the rapid flow of vapor-state and liquid-state working medium. To characterize the heat transfer capacity of the TPHTD, we built a testing platform shown in Figure 6b to mapping its surface temperature distribution via an IR camera under different heat loads. This homemade testing platform consists of a heater (18 × 2.5 mm 2 ) for heating and a condensate block (18 × 5 mm 2 ) for cooling. The thermal grease is coated between the test device and the heater and condensate block, which can reduce the thermal contact resistance between them. According to the heating area and cooling area, we divide the device into an evaporation section, condensation section, and adiabatic section. The heat generated by the heater is first transferred to the evaporation section of the device via thermal conduction mode, and then rapidly transferred to the condensation section through vapor-liquid phase-change heat transfer mode, and finally conducted to the condensation block. During this process, the phase-change working medium needs to undergo evaporation, flow, and condensation to rapidly transfer the heat. The condensed working medium is driven by the capillary force of the porous medium back to the evaporation section for next cycle without external work. When a certain power is loaded through the heater, the surface temperature distribution of the TPHTD was recorded by IR camera after 5 min. The temperature distribution for each section of such device was averaged to represent a temperature of that specific section.
To demonstrate the advantages of using graphene in twophase heat transfer, we prepared a PVA-based TPHTD with the same size as the rGO/PVA-based TPHTD ( Figure S6, Supporting Information), and compared the differences in heat transfer capacity between them. Thermal resistance and effective thermal conductivity are two parameters that characterize the heat transfer performance of TPHTD.

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The temperature difference: The thermal resistance of heat conduction in the casing material can be expressed as:  For the evaporation section, the inner temperature of the TPHTD was calculated from the equation as below: For the condensation section, the inner temperature of the TPHTD was calculated from the equation as below: The thermal resistance of TPHTD: The effective thermal conductivity of TPHTD: The effective length: The cross-section area: In the above equations, T e , T c is the temperature on out-wall of the evaporation and condensation section; T ei , T ci is the temperature on inner-wall of the evaporation and condensation section; δ is the thickness of wall of TPHTD; K cross − plane is the cross-plane thermal conductivity of casing material; A e is the area of evaporation section, A c is the area of condensation section, and ΔT is the temperature difference of evaporation section and condensation section; L e is the length of evaporation section, L a is the length of adiabatic section, L c is the length of condensation section, δ t is the total thickness of TPHTD, and W is the width of TPHTD ( Table 2).
As shown in Figure 6c,d, with the increasing of heat load the mean temperatures on both evaporation section and condensation sections of rGO/PVA-based TPHTD rise slowly from 29.1 °C to 42.1 °C and 21.4 °C to 22.9 °C, respectively. By comparison, the mean temperature of PVA-based TPHTD at the evaporation section increases greatly as the heat load increases, and finally reaches 68.7 °C at a heat load of 3.0 W, which is 1.6 times that of rGO/PVA-based TPHTD. Through relevant theoretical calculations, the resulted thermal resistance and effective thermal conductivity can be used to compare the heat transfer performance of PVA-based and rGO/ PVA-based TPHTDs. The thermal resistances of TPHTDs can be calculated by Equation 7. Figure 6e shows that the thermal resistance of rGO/PVA-based TPHTD is lower than that of PVA-based TPHTD under the heat load from 1.0 W to 3.0 W, and their thermal resistances decrease with the increase of head load. [39] Such difference is due to the fact that the low heat load cannot make the liquid-state ethanol at the evaporation section undergo a sufficient phase change, leading to the formation of thick liquid-state ethanol film and thus increasing the thermal resistance of heat conduction. As a result, the overall thermal resistance is larger at low heat load. With the increase of heat load, the phase change of liquidstate ethanol at the evaporation section becomes more and more severe and the thinner liquid film will be formed, which not only enhances the evaporation heat transfer coefficient but also reduces the thermal resistance through the ethanol film. And eventually the overall thermal resistance is substantially reduced as the heat load increases. In addition, low flow resistance and good wettability of ethanol on the surface of rGO nanosheet enable lower overall thermal resistance of the rGO/PVA-based TPHTD than the PVA-based TPHTD. For the rGO/PVA-based TPHTD, the thermal resistance is as low as 2.59 K W −1 at 3.0 W of heat load, which is approximately half of that of the PVA-based TPHTD (4.72 K W −1 ). The effective thermal conductivity of the rGO/PVA-based TPHTD gradually increases and reaches 1408 W m −1 K −1 at the heat load of 3.0 W, which approximately doubles that of the PVA-based TPHTD ( Figure S7, Supporting Information). This result confirms that the low flow resistance and good wettability of ethanol molecules on rGO nanosheets can greatly improve the heat transfer capacity of TPHTD. The thermal conductivity will be 780 W (mK) −1 if we consider the thermal resistance of the evaporator and condenser. The resistance of the evaporator and condenser can be further reduced if we use polymer binder with improved thermal conductivity. [40] Furthermore, thermal stability tests have been carried out. After 6 hours of operation, the evaporator temperature of the PVA-based TPHTD was rising rapidly compared to the initial state, and this device eventually fails to dissipate heat effectively due to the high gas-permeability of PVA film ( Figure S8, Supporting Information). In contrast, the rGO/PVA-based TPHTD can always maintain the same heat transfer capacity even after continuous operation for 48 h ( Figure S9, Supporting Information).
In addition, in order to embody the lightweight and high heat transfer capacity of the rGO/PVA-based TPHTD, the specific thermal conductivity is used to describe the overall performance of the device. Figure 6f compared the specific thermal conductivity of the rGO/PVA-based TPHTD with different thermal management materials and devices. [9,15,[41][42][43][44][45][46][47][48][49][50][51][52] By combining the advantages of lightweight of rGO/PVA casing material and high thermal conductivity of two-phase heat transfer, the rGO/PVA-based TPHTD generated achieves the highest specific thermal conductivity, up to 5600 W (mKg) −1 .

Conclusion
We developed a lightweight all graphene-based TPHTD with high reliability by self-assembly of GO/PVA solution and reduction via HI acid. During the self-assembling process, a Ni(OH) 2 template with micropores was used as the template for preparing the flow microchannel of working medium in the TPHTD. Besides the removal of the Ni(OH) 2 template, HI acid also removes functional groups and reduces GO to rGO, which enables the reduced layer spacing between nanosheets. The loading of PVA improves the adhesion between rGO nanosheets due to the hydrogen bonding formed between PVA molecules and rGO. The resulting rGO/PVA casing not only achieves ultra-low gas-permeability of 4.16 × 10 −10 Pa × m 3 s −1 due to the small layer spacing and the barrier of PVA molecular chain, but also has high tensile stress of 53.8 MPa at a strain of 2.29%. In combination with good wettability and low flow-resistance of graphene to ethanol, the rGO/PVA-based TPHTD shows effective thermal conductivity up to ≈1400 W (mK) −1 at a heat load of 3.0 W and ultra-high specific thermal conductivity of up to 5600 W (mKg) −1 . This approach paves the way for the development of two-phase heat transfer devices with lightweight and high performance for the thermal management of flexible and portable electronic.
Characterization of Different Graphene-Based Films and Heat Transfer Capacity of TPHTDs: The surface morphologies and cross-section microstructures of GO film, rGO film, GO/PVA film, and rGO/PVA film were characterized by a field-emission scanning electron microscope (FE-SEM, FEI Sirion 200). Raman scattering spectroscopy (Renishaw inVia Qontor, England) was used to characterize the composition distribution of PVA film, GO film, rGO film, GO/PVA film, and rGO/PVA film. During the process of Raman scanning, a 532 nm laser was selected as the excitation source and the Raman shift range is from 500 to 3000 cm −1 . The XRD patterns with a 2θ range of 5°-40° at a scan rate of 5.0° min −1 were used for characterizing the layer spacing of different films by using X-Ray diffraction (XRD, Rigaku, Ultima IV) with Cu Kα radiation (λ = 1.54178 Å). Microscopic imaging infrared spectroscopy (iN10 MX, America) was used to measure Fourier Transform Infrared spectroscopy (FTIR) of different samples. The tensile strength of different films was measured by Dynamic Thermomechanical Analyzer (Q850, America). The thermal conductivities of PVA film, GO film, rGO film, GO/PVA film, and rGO/PVA film were measured by laser thermal-conductivity testing instrument (LFA 467, Netzsch. Ltd, Germany). The specific heat capacities of different films were measured by differential scanning calorimetry (DSC 204 F1, Germany). The three-dimension structure of the graphene-based TPHTD was reconstructed by Micro-CT (Xradia 520 Versa, Germany). Infrared (IR) camera (T640, FLIR, USA) was used to measure the temperature distribution of different samples.
Preparation of 1wt.% PVA solution: 1 wt.% PVA solution was prepared by dissolving 1 g PVA powder in 99 g deionized water within a 250 ml flask at 90 °C under stirring. After vigorous stirring for 1 h, the solution became clear. The PVA solution was obtained after cooling to room temperature.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.