A Novel Structure for Heat Transfer Enhancement in Phase Change

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A Novel Structure for Heat Transfer Enhancement in Phase Change Composite: Rolled Graphene Film Embedded in Graphene Foam Jia Yu, Li Kong, Haoqing Wang, Hongji Zhu, Qingshan Zhu, and Jiawen Su ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01752 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019

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ACS Applied Energy Materials

A Novel Structure for Heat Transfer Enhancement in Phase Change Composite: Rolled Graphene Film Embedded in Graphene Foam

Jia Yu1, Li Kong1, *, Haoqing Wang1, Hongji Zhu1, Qingshan Zhu1, Jiawen Su1

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College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, P. R. China

* Corresponding authors: [email protected] (Li Kong)

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ACS Paragon Plus Environment

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Abstract Phase change material (PCM) is an excellent heat storage material which can absorb a large amount of heat in the form of latent heat and can be used to improve the power generation effect of thermoelectric generator (TEG) in a fluctuating thermal environment. The thermal conductivity of PCM plays a key role. In this paper, graphene films as thermal conductive fillers were introduced into graphene foams through the ice-templated method. The composite structure can increase the thermal conductivity of paraffin by 44 times to 11.594 W/mK, and the corresponding mass fraction is only 1.14 wt%. On the other hand, the influence of thermal conductivity of PCM on TEG under periodic heat flux condition was analyzed through simulation. The results indicate that increasing thermal conductivity helps the PCM to better play a buffering effect, and significantly reduce the fluctuation of the power generation voltage.

Keywords: Phase change materials; Graphene foams; Graphene films; Thermal conductivity; Thermoelectric generators

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1. Introduction The phase change material (PCM) is a promising heat storage material.1,2 When phase transition occurs, a large amount of heat can be absorbed or released without a significant change in temperature.3 Similar to the storage of electrical energy in the form of chemical energy in batteries or fuel cells,4 the PCM stores thermal energy in a latent heat manner, which can be considered as container of heat. Benefit from the phase transition process, PCM is widely used in various thermal management systems. Phase-change temperature control was first applied in aerospace electronic equipment, such as controlling the battery temperatures in the Mars rovers.5 With the development trend of miniaturization and integration of electronic devices, phase-change temperature control technology has received more and more attention in the field of temperature control of civil electronic devices, such as thermal management of handheld devices use PCMs.6 Low thermal conductivity is the main drawback that limits the performance of PCM in temperature control applications. The introduction of a thermal enhanced porous network skeleton is an effective approach, such as metal foams see as copper foams and aluminum foams,7,8 and graphitized carbon foams.9,10 With the development of nanotechnology, the use of nanomaterials to construct highly conductive, porous three-dimensional network structures has become a research hotspot, such as expanded graphite,11,12 graphene foam,13,14 BN gel15 and so on. Waseem Aftab16 has published a review of the thermo-physical properties and potential applications of nanoconfined phase change materials from the point of view of material design. Although these structures are consecutive, the heat conduction path is abnormally tortuous due to too many nodes, and the effect of enhancing heat conduction is 3



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not significant. In addition, the fillers bring another problem. Higher latent heat is required for thermal energy engineering and management,17 while the filler occupies the space of the PCM, the latent heat of the composite PCM tends to decrease, and the falling ratio is approximately similar to the mass content of the filler. Ling Ziye12 have dispersed 35wt% expanded graphite(EG) into RT44HC to prepare RT44HC/EG composites, which can increase the thermal conductivity by 24 times, but at the same time, the latent heat of composite phase change materials also decreased by 32%. This is disadvantageous for phase change materials who use latent heat for temperature management. The graphene film18-20 is stacked by layers of graphene nanoplatelets, having a micron-thickness, millimeter-scale in-plane size, possessing extremely high in-plane thermal conductivity. Some literatures have used graphene films to enhance the thermal conductivity of polymer-based materials. Zhang and partners has immersed epoxy resin21 and polydimethylsiloxane22 in dense graphene film rolls to prepare Graphene film/epoxy resin (GF/E) composites and VAGF/PDMS composites for high performance thermal interface materials. Referring to the metal fin/PCM structure23,24 is conceivable whether graphene can be embedded as a carbon fin in the graphene foam, which greatly improves the thermal conductivity of the substrate at a very small mass and volume cost. The research on high thermal conductivity phase change composites will promote the further development of PCMs in the energy field. In the field of applied energy, thermoelectric technology is receiving more and more attention. Thermoelectric generators (TEG) use the Seebeck effect to achieve direct conversion between thermal energy and electrical energy without any moving parts.25 Thermoelectric 4


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technology can be used to recover various forms of thermal energy, from the aerodynamic heat of the front edge of the hypersonic aircraft wing26,27 to the exhaust heat of the automobile28. As long as there is a temperature difference, thermoelectric power generation can be considered.29 The performance of the TEG is sensitive to temperature changes, while the actual thermal environment is always unstable. Considering the advantages of phase change temperature control, it is quite reasonable to combine PCM with TEG. This paper uses the simple ice-templated method to prepare the novel thermal network structure. Through one step operation, the graphene film is embedded in the graphene foam to form a composite skeleton, which greatly improves the thermal conductivity of the paraffin, and the quality cost is very small. Furthermore, the influence of PCM and its thermal conductivity on the power generation effect of TEG under periodic large heat flux environment was analyzed through simulation. 2. Experimental Section 2.1. Synthesis of Gfilm-Gfoam structure and composites The graphene film was purchased from Changzhou Fuxi Technology Co., Ltd. and has an in-plane thermal conductivity of 1522 (W/mK). Graphene Oxide Powder was purchased from Chengdu Organic Chemicals Co., Ltd., and has a nanoplatelet diameter section of about 50 μm on average. The narrow strips (210 mm × 10 mm) of graphene film was wound into a roll (2 mm gaps, overall diameter 25 mm), immersed in an aqueous graphene oxide solution (5 mg/mL), and placed on a liquid nitrogen precooled stainless steel block. The graphene oxide nanoplatelets use ice crystal as template to replicate the shape to form a mesoporous foam structure, and a longitudinally arranged graphene film is wrapped therein to form an inlaid 5



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structure. The Gfilm-Gfoam skeleton structure was obtained by vacuum freeze-drying (−60°C, 48 h) and hydrazine hydrate reduction (90°C, 24 h). Further, the paraffin was filled by vacuum impregnation (80°C, 3 h), and cooled to obtain a Gfilm-Gfoam/PW composite. 2.2. Characterization of materials The morphologies and detailed microstructures of the samples were observed using SEM (Hitachi S-4800). The crystal structures of the graphene materials were characterized by XRD (BRUKER D8 ADVANCE) and FT-IR (Nicolet iS50). The heat storage characteristics of the phase change materials are obtained by DSC (Netzsch DSC 204F1) characterization. Finally, the thermal conductivities of the composites was measured by laser flashing method (Netzsch LFA 45) at room temperature. Thermal conductivities were calculated from the equation k      C p , where  is the thermal diffusivity,  is the density, and C p is the specific heat capacity of the samples.

3. Results and Discussion 3.1. Structure of Samples Figure 1 (a) is a photograph of the Gfilm-Gfoam structure, which is the skeleton of the composite material in Figure 1 (d). As can be seen from the figure, the graphene film roll has a spiral shape, and the intervening graphene foam is tightly wrapped around the graphene film to form a unique Gfilm-Gfoam composite structure. The microstructure of graphene foam and graphene film is shown in Figure 1 (b and c). Figure 1 (b) shows a connected network structure with an pore diameter of about 50μm, which can encapsulate the liquid PCM with help of capillary forces. On the other hand, Figure 2 (d) shows that the foam 6


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structure is a mesoporous structure because the BET analysis curve shows a distinct peak in the range of 4-20 nm, and the corresponding pore diameter is 4.27 nm. Micropores and mesopores are associated with the growth of ice crystals during the ice crystallization process and the intertwining of graphene sheets. It can be seen from Figure 1 (c) that the cross-sectional structure of the graphene film is a distinct layered structure, which is formed by stacking graphene nanoplatelets with flat and continuous in-plane, which provides a structural basis for a smooth heat conduction path. The abundant pore structure in the graphene foam provides ample space for the PCM. Figure 1 (d) is a photograph of a Gfilm-Gfoam/PW composite from which the presence of graphene film can be seen. Figure 1 (e) shows the inside of the composite. It can be seen that the paraffin wax tightly wraps the tortuous layer of graphene to fill the pores. However, the paraffin does not completely fill the inside of the foam. This is because the liquid paraffin adheres to the graphene sheet during the solidification process, and because of this, the liquid paraffin can be contained by the skeleton and not overflow when the composite phase change material is melted again. Figure 1 (f) is an electron micrograph of the raw material graphene oxide(GO). It can be seen from the figure that the GO film has a diameter of about 50 μm and the sheet is soft and wrinkled. The AFM height image Figure 1 (g) and the section line Figure 1 (h) show that the thickness of the GO layer is 4.45~6.90 nm.

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Figure 1. (a) Photograph of Gfilm-Gfoam. (b) SEM image of Gfoam, enlarged from the insert image. (c) SEM image of Gfilm, the insert is the photograph of Gfilm which embeded in (a) and (d). (d) Photograph of Gfilm-Gfoam/PW, red dots shows the route of the graphene film. (e) SEM image of Gfilm-Gfoam/PW. (f) SEM and (g) AFM height image of GO. (h) Section line analysis of (g). 3.2. Thermal properties of PCMs Figure 2 (a) is the XRD spectra of GO, Gfoam and Gfilm. The respective characteristic peaks appear at 10.68°, 25.20°, and 26.18°, respectively, indicating that the layer spacing is 0.828, 0.353 and 0.340 nm, respectively. The characteristic peaks of Gfoam are wide while those of Gfilm are sharp. The XRD pattern reflects that the prepared graphene foam wall is formed by stacking multiple layers of graphene nanoplatelets, the graphene structure is repaired, the interlayer spacing is reduced, and the graphene film is closely packed by single layers of carbon atoms. Figure 2 (b) is the FT-IR spectra of GO, Gfoam and Gfilm. The larger absorption peaks of all samples in the range of 3650-3200 cm-1 are caused by the moisture 8


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absorbed by the sample during the test. GO has several distinct diffraction peaks at 1734 and 1300-1000 cm-1, corresponding to C-O and C=O groups, indicating that a large number of oxygen-containing functional groups are attached to GO, which is the reason for the large spacing of GO layers. The diffraction peak of Gfoam at 1632 cm-1 is enhanced, corresponding to C=C, indicating that the graphene microchip is repaired. The important diffraction peak of Gfilm is at 1632 cm-1, indicating that it is mainly composed of graphene which is not oxidized. From Figure 2 (c), the characteristic peak representing the carbon sp2 structure (1580 cm-1) can be read in the Raman spectrum of Gfilm, and the shape of the spectrum is close to that of graphite. It indicates that the graphene film is closely packed by graphene nanoplatelets, which provides a structural basis for its high thermal conductivity.

Figure 2. (a) XRD patterns and (b) FT-IR spectra of GO, Gfilm and Gfoam. (c) Raman spectrum of Gfilm. (d) Pore diameter distribution of Gfoam. 9



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For phase change energy storage materials, the adverse effects on their latent heat should be minimized while improving thermal conductivities. Figure 3 (a) shows the DSC curves for pure PCM, Gfoam/PW and Gfilm-Gfoam/PW composites. Since the presence of the filler occupies the space of the original PCM, the latent heat value of the composite PCM decreases, and the decrease caused by the Gfoam and Gfilm-Gfoam skeleton are only 2.4% and 5.1%, respectively, which are slightly. In contrast, the increase in thermal conductivity is enormous, as can be seen visually from Figure 3 (b). On the basis of pure PCM, the Gfoam and Gfilm-Gfoam skeletons increased the thermal conductivity by 0.75 and 44 times, respectively, of 0.450 W/mK and 11.594 W/mK. Figure 3 (c) compares the thermal conductivity of Gfoam/PW and Gfilm-Gfoam/PW with paraffin-based composite phase change materials in the literature17,19-20,30-33. Although graphene nanoplatelets (GNPs), expanded graphite (EG), boron nitride (BN), carbon nanofiber (CNF), carbon nanotube (CNT), these fillers themselves have extremely high thermal conductivity and are used to greatly improve the thermal conductivity of PCM. It is still difficult because there is often a lack of effective heat transfer paths. As can be seen from Figure 3 (c), a sample containing 35 wt% EG can have a thermal conductivity of 15.2 W/mK, corresponding to a thermal conductivity enhancement (TCE) of 59.8 times, as shown the tallest column in Figure.3 (d). However, when averaged by the addition quality of the filler, the TCE per unit mass fraction filler is only 1.71 times. In contrast, although Gfilm-Gfoam/PW has a thermal conductivity of 11.594 W/mK, the filler mass fraction is only 1.14wt%, so the TCE and TCE per unit mass fraction can reach 44.11 and 38.69 times 10


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respectively. This means that the Gfilm-Gfoam structure of this paper can effectively improve the thermal conductivity of pure PCM, and has little effect on latent heat.

Figure 3. (a) DSC curves of pure PW, Gfoam/PW and Gfilm-Gfoam/PW. (b) Latent heat and thermal conductivity of pure PW, Gfoam/PW and Gfilm-Gfoam/PW. Comparison of the thermal conductivities (c) and thermal conductivity enhancement as well as the enhancement per mass fraction of the filler (d) for Gfoam/PW and Gfilm-Gfilm/PW composites with others in literature. The numbers in (c) and (d) represent the references. TCE means the thermal conductivity enhancement. TCE = [k (composite) /k (pure PCM) –1] * 100%, where k means thermal conductivity. TCE per wt% equals to the value of the TCE divided by the value of mass fraction of the filler, which means the contribution rate of filler per unit mass fraction to thermal conductivity growth.

The heat transfer mechanism is shown in Figure 4. Figure 4 (a) shows the photograph of a composite PCM, a part of which was cut away in order to expose the cross section and longitudinal section. The route of Gfilm can be seen clearly, as highlighted by a series of red dots. Figure 4 (b) displays the schematic of mechanism. Heat transfer is faster along the 11



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direction of the Gflim, because the heat flux depends strongly on the orientation27. There are graphene sheets in Gfoam in various directions, make the heat transfer path twists and turns, resulting in slower heat transfer compared to the Gfilm. Due to the faster heat transfer in the Gfilm, the isothermal line in the PCM bulges near the Gfilm. The Gfilm and Gfoam combined physically to form the Gfilm-Gfoam structure. Based on Gfoam that enhances the internal heat transfer in PCM, Gfilm greatly enhances the longitudinal heat transfer of the PCM, so that the heat can be quickly transferred in the PCM to promote the overall phase change of PCM and speed up the rate of latent heat storage and release.

Figure 4. Structure and heat conduction mechanism of the Gfilm-Gfoam/PW composite. (a) The composite material sample. A small part is cut off, while the cross section and longitudinal section are exposed. The curve formed by red dots shows the route of the graphene film. (b) Enlargement of the red box in (a), intuitively shows the thermal conductivity mechanism of the Gfilm-Gfoam structure. Red color represents high temperature, blue color represents low temperature, and heat transferred from bottom to top. Since heat transfer more quickly in Gfilm, convex curve is generated around Gfilm on the isotherms. (A color version of this figure can be viewed online).

3.3. Heat transfer experiments The heat transfer experimental device is shown in Figure 5(a). The container consists of a Teflon tube with an inner diameter of 25 mm and an aluminum plate, and the outside is wrapped with polystyrene whose thermal conductivity is very low. A ceramic heating plate is 12


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mounted on the bottom of the container to heat the phase change material through the aluminum plate. Thus, the phase change material only gets heat from the bottom, and the other boundaries are approximately adiabatic. Thermocouples are used to obtain temperature values. A heat flux density of 16297 W/m2 was applied to the hot side, the setup information is shown in supporting information. In the case of pure paraffin, the paraffin wax close to the heat source is first melted, the hot edge temperature curve rises rapidly to the phase transition temperature point, and then the temperature platform appears, the paraffin undergoes solid-liquid phase change, and the heat is absorbed in the form of latent heat while the temperature remains substantially unchanged. At the same time, the temperature rise of the cold side (thickness of 10 mm) is extremely slow. The paraffin near the heat source completes the phase change and the temperature continues to rise. Because the thermal conductivity of paraffin wax is extremely low, the heat accumulates on the hot side and cannot be quickly transferred to the cold side, which causes the hot edge temperature to continuously rise. When the experiment is carried out until 120 s, the hot side temperature exceeds the allowable temperature of the insulating material, forcing the experiment to stop. At the same time, the cold side is far from the melting point of the phase change material. Comparing the curves of the Gfilm-Gfoam/PW composites, it can be found that the hot edge temperature rises slowly after the end of the plateau period, and is still slightly lower than 70 °C after 200 s; indicating that the heat is conducted to the inside of the phase change material effectively, relieving the pressure of the hot side and avoids the occurrence of overheating. Pure paraffin wax has a hot edge temperature of less than 70 °C for 115 s, and the 13



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Gfilm-Gfoam/PW composite can reach 74% more time than pure paraffin. On the other hand, the cold edge temperature rises rapidly and begins to melt at 180s, and the temperature rises to 56 °C at 200s; indicating that the phase change material in the middle is completely melted at the end of the experiment (the phase change material has a melting point of 52 °C). However, due to the wrapping of the graphene foam, the whole still appears as a solid shape. The introduction of the Gfilm-Gfoam skeleton can increase the temperature control time by 74% and achieve effective temperature control.

Figure 5. (a) Basic experimental setup schematic. (b) Temperature-time curves of pure PW and Gfilm-Gfoam/PW in experiment as shows in (a). Inset: the photography of PCM in (a).

3.4. Numerical simulation of thermo-electric conversion The effect of PCM on TEG power generation in a periodic large heat flux density heat source environment was considered in this section. The generated voltage of the TEG (has 126 pairs p-n junctions) was used to evaluate the performance. The solid-liquid transition process of PCM was simulated using the solidification & melting model in Fluent 15.0. Figure 6 shows the schematic and boundary conditions. More details such as material parameters and settings, see Support Information. 14


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Figure 6. Schematic of the simulation system The shading in Figure 7(a) shows a periodic heat source with a peak of 40,000 W/m2. The input heat flux is sized to match the PCM volume to achieve a fully melting. Six sets of different thermal conductivities were calculated. The temperature curves are shown in Figure 7(a), and the temperature data of all six groups are shown in Figure S4. There are several observations. First, lower thermal conductivity simulations (see 0.15 and 1 W/mK) showed a significant temperature fluctuation on the hot side and a slowly rising temperature on the cold side, and the temperature plateau cannot be seen. By contrast, the increased thermal conductivity leads to a more gentle temperature curve, and the plateau presented in both cold and hot side temperature curves. Due to the enhanced thermal conductivity, the PCM is fully melted during a cycle, releasing the maximum latent heat storage capacity. It can be seen from the curve of k=10W/mK in Figure7 (b) that the TEG hot side temperature reaches the melting point of the PCM, indicating that the PCM has completely melted, The heat stored as latent heat can continue to supply heat to the TEG 15



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during the interval in which the heat flux is zero. So, the temperature curve fluctuates much less than the temperature fluctuation of the TEG hot side when there is no PCM.

Figure 7. Numerical data. (a) Temperature profiles of the top and bottom side of PCMs with thermal conductivities of 0.15, 1 and 10 W/mK respectively. Pattern of thermal input is shown in shadows. (b) Temperature profiles of the top side of TEG. (c) Temperature differences of the PCMs with different thermal conductivities at 250s. The insert one is the model for numerical calculation. (d) Open circuit voltage profile of the system.

It is also significant to note the temperature magnitude and difference. By increasing thermal conductivity the temperature throughout the main body of the PCM can be greatly reduced, which is evident from Figure7 (a). With the increase of thermal conductivity from 0.15 to 10W/mK，the temperature difference at 250s drops from 750K to 18K. The variation trend of the temperature difference between the hot side and the cold side of PCM at various thermal

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conductivities was shown in Figure7 (c), the temperature values were taken from 250s. These indicate that the capability of the entire PCM to absorb thermal energy has been improved. It can be observed from Figure 7(b) that the temperature fluctuation of the TEG hot side is closely related to the thermal conductivity of the PCM. In lower thermal conductivity simulations(less than 1W/mK), the temperature rises slowly in each heating cycle, and shows a fluctuant increasing process as the cyclic heating proceeds. When the thermal conductivity goes high, in a single cycle, the temperature is approximately symmetrically distributed affecting by the intermittent heat flux, and the variation pattern is almost the same as the cyclic heating proceeds. When thermal conductivity is 10W/mK, the temperature fluctuation range is 20 K, and the fluctuation amplitude is reduced by 66.7% compared to TEG without PCM, has a amplitude of 60 K. In addition, the temperature of the TEG hot side increases as the PCM thermal conductivity increases. In the case of high thermal conductivity, the PCM can complete the overall melting in a single heating cycle, so that a gentle slope occurs when the temperature of the TEG hot side approaches the PCM melting point. The PCM effectively buffers the temperature of the TEG hot side in the fluctuating thermal environment. The higher the thermal conductivity, the more obvious the buffering effect. It is also noted that during the interruption of the heating, temperature in the TEG hot side changes to a convex curve when there is PCM or concave curve without PCM. This means that the heat stored by the PCM plays a role when the heating flux goes zero. When the external heat source disappears, the PCM can release heat to maintain the temperature of TEG hot side, slowing the temperature drop until the heat supply reappears. The PCM's

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reservoir effect compensates for the loss of power generated during heating where the PCM significantly lower the TEG hot end temperature. As shown in Figure7 (d), the PCM is used for the TEG hot side, which effectively reduces the fluctuation range of the TEG power generation voltage due to the buffering effect. When there is no PCM, the amount of TEG power generation drops sharply to zero when the heating is stopped. The presence of the PCM allows the TEG to continue to generate electricity and maintain the open circuit voltage above 0.7V as the thermal conductivity increases. Also note from Figure 7(b) that when the last cycle stopped, the temperature of the TEG hot side with a PCM was above 307 K. Indicating that the heat stored in the PCM has not been completely released, and it can continue to supply heat to the TEG, further extending the power generation time.

4. Conclusions In this paper, a novel Gfilm-Gfoam structure was prepared to improve the thermal conductivity of PCM, and the effect of improving the thermal conductivity of PCM on the performance of TEG in a periodic large heat flux condition was simulated. A composite structure Gfilm-Gfoam composed by rolled graphene film embedded in graphene foam was obtained by a simple ice-templated method. The novel three-dimensional graphene structure improved the thermal conductivity of the phase change material with a very small mass cost. The thermal conductivity enhancement per unit packing mass fraction reached 38.69 times. The simulation results show that PCM can effectively buffer the TEG hot side temperature in the fluctuating thermal environment, reducing the temperature fluctuation. The higher the 18


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thermal conductivity, the more obvious the buffering effect. When thermal conductivity is 10W/mK, the temperature fluctuation range is 20 K, and the fluctuation amplitude is reduced by 66.7% compared with control group of 60 K. In addition, the reservoir effect of the PCM compensates for the power generation voltage that is lost due to a significant decrease in the hot side temperature. Therefore, increasing the thermal conductivity helps the PCM to better exert the buffering effect, and significantly reduce the fluctuation of the power generation voltage.

Supporting Information Photos about the preparation process of Gfilm-Gfoam structure and the Gfilm-Gfoam/PW, porosity of the skeleton, details of experiments and simulations, as well as the performance comparison table.

Notes The authors declare no competing financial interest.

Acknowledgments We gratefully acknowledge Qing Yan for helpful discussions. This research was supported by the National Natural Science Foundation of China (Project No. 51672054).

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For Table of Contents Only

Phase change material with enhanced heat transfer can effectively improve the power generation performance of TEG under periodic large heat flux. Under the condition that the interface has good heat conduction, the increase of thermal conductivity can make the buffering effect of PCM more significant.

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A Novel Structure for Heat Transfer Enhancement in Phase Change

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