Expanded graphite-based high thermal conductivity composite phase change energy storage material and preparation method thereof

Abstract

The present invention relates to an expanded graphite-based high thermal conductivity composite phase change energy storage material, which is prepared by a matrix material expanded graphite EG, a binder polyvinylidene fluoride PVDF, a pore-forming agent sodium chloride NaCl, a thermally conductive filler reduced graphene oxide rGO, and a phase change material n-octadecane OD, and is prepared by a salt template method and a vacuum impregnation method, wherein the role of EG is to provide a porous structure and a thermally conductive skeleton; the role of PDVF is to provide bonding conditions; and the role of NaCl is to construct a porous structure. Its preparation method includes the following steps: 1, preparation of an expanded graphite-based three-dimensional porous sponge; 2, loading and reduction of graphene oxide; 3, vacuum adsorption of a phase change material. Its application simultaneously has high thermal conductivity, phase change heat storage performance, temperature control performance, photothermal conversion performance, and electrothermal conversion performance; thermal conductivity is 2.3244-6.3840W / (m·K); heat storage density is 166.46-168.93J / g; photothermal conversion efficiency is 96.3%, and electrothermal conversion efficiency is 74.4%.

Description

Technical Field

[0001] The present invention relates to the field of thermal management, high thermal conductivity and heat storage, and in particular to an expanded graphite-based high thermal conductivity composite phase change energy storage material and a preparation method thereof. Background Art

[0002] Phase change materials (PCMs) are efficient thermal energy storage methods that can regulate the ambient temperature to achieve thermal management. Among them, new organic phase change materials (OPCMs) have the advantages of chemical thermal stability, large latent heat, low cost, non-toxicity, corrosion resistance, and a suitable phase change temperature range. However, in the field of energy buildings, they cannot be applied due to the problems of low thermal conductivity, liquid phase leakage, weak light absorption capacity for solar energy to heat conversion, and low electrothermal conversion efficiency.

[0003] In order to solve the problems of low thermal conductivity and liquid phase leakage of phase change materials, microcapsule coating technology can be used to encapsulate phase change materials. For example, in prior art 1 (Wang, X., Chen, Z., Xu, W. et al. Capric acid phase change microcapsules modified with graphene oxide for energy storage. J MaterSci 54, 1483414844, 2019), an in-situ polymerization method was used to synthesize phase change microcapsules with capric acid as the core material and urea-formaldehyde resin modified with graphene oxide GO as the shell material. The preparation of microcapsules includes the following processes: 1. Preparation of graphite oxide dispersion; 2. Preparation of prepolymer; 3. Emulsification of phase change material; 4. Synthesis of capric acid phase change microcapsules. This technology proves that the addition of graphene oxide improves the thermal conductivity of capric acid phase change microcapsules as a whole and the microcapsules do not leak during the cycle. However, the problem with this technical solution is that it not only requires four steps, but also some of the steps involve complex processes, which affects the performance effect during large-scale production. This technical problem is determined by the microcapsule preparation technology and can only be overcome by adopting other processes; in addition, this technical solution can only achieve an energy storage density of 60.7%.

[0004] In order to solve the problems of complex process, low energy storage density, weak light absorption capacity of solar energy to heat conversion and low electrothermal conversion efficiency, a conductive and thermal network can be constructed in a three-dimensional network structure to enhance the photothermal / electrothermal conversion efficiency. Prior art 2 (Cheng, H., Xing, L., Zuo, Y. et al. Constructing nickel chain / MXene networks inmelamine foam towards phase change materials for thermal energy management and absorption-dominated electromagnetic interference shielding. Adv Compos Hybrid Mater 5, 755-765, 2022) uses melamine foam as a template to construct a continuous thermal / conductive network by dip-coating magnetized nickel (Ni) / MXene (NiM), and then encapsulates polyethylene glycol (PEG) into a porous NiM / MF hybrid sponge by vacuum impregnation. The three-dimensional enhanced heat conduction path formed by NiM along the MF frame and the surrounding makes the NiM / PCM composite material exhibit a thermal conductivity of 0.39W / mK, an energy storage density of 78%, a phase change enthalpy of 142.2J / g and excellent thermal stability. Due to the addition of Ni / MXene, the excellent light / electric-heat conversion performance of the NiM / PCM composite material enriches its multifunctional applications. However, the problem with this technical solution is that melamine foam, as a matrix frame, has a low thermal conductivity and is suitable for the field of temperature protection. However, for fields with high heat dissipation requirements, the effect of melamine foam as a matrix frame cannot meet the thermal conductivity requirements.

[0005] In order to better apply it to the field of thermal conductivity of energy buildings, for the problem of low thermal conductivity of the matrix material, a porous carbon matrix can be selected for construction. Existing technology 3 (CN116004197A) uses easily decomposable material ammonium bicarbonate and high thermal conductivity material graphite powder to mix according to mass ratio, put the mixture in a mold for pressing and forming, and place the pressed material in a constant temperature drying oven for drying to obtain a porous carbon matrix. Take excess filler material paraffin and vacuum adsorb it into the porous carbon matrix, cool and remove excess material on the surface to obtain a composite phase change material. This technical preparation method is not only simple in steps and low in cost, but also, due to the design of the porous carbon matrix, when the volume fraction of graphite powder increases from 15.40% to 35.72%, the thermal conductivity of the composite phase change material is directly realized from 7.48W / (m·K) to 19.20W / (m·K), and the composite phase change material has high thermal conductivity and anti-leakage characteristics. This technical solution realizes pore formation by thermally decomposing the pore-forming agent ammonium bicarbonate to produce NH3 and CO2. However, the pore size formed by the gas template pore-forming method is mainly mesopores, which can achieve the effect of adsorbing paraffin. However, when modifying the carrier, such as filling modified graphene oxide, the size of the modified graphene oxide is much larger than the mesopores, and the filling cannot be effectively completed. In addition, there is a certain process cost when the gas generated by the gas template pore-forming method is recycled and reused. Summary of the invention

[0006] The purpose of the present invention is to provide an expanded graphite-based high thermal conductivity composite phase change energy storage material and a preparation method thereof. The salt template method is used to adjust the pore structure during pore formation to facilitate the composite of modified graphene oxide. The main technical principles are:

[0007] 1. The expanded graphite-based three-dimensional porous sponge matrix is ​​prepared by the salt template method to obtain a large-pore structure, which is conducive to filling multifunctional materials; at the same time, the raw material pore-forming agent NaCl used is green and pollution-free, low-cost, and easy to recycle;

[0008] 2. By adding modified graphene oxide, a continuous three-dimensional porous carbon network is formed with expanded graphite, which synergistically enhances the thermal conductivity of the composite phase change energy storage material;

[0009] 3. Modified graphene oxide enhances the photothermal conversion performance of the composite phase change energy storage material, and gives the composite phase change energy storage material the electrothermal conversion performance, so that the composite phase change energy storage material can realize the mutual conversion of solar energy, thermal energy and electrical energy.

[0010] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical scheme:

[0011] An expanded graphite-based high thermal conductivity composite phase change energy storage material, which is prepared from expanded graphite EG as a matrix material, polyvinylidene fluoride PVDF as a binder, sodium chloride NaCl as a pore-forming agent, reduced graphene oxide rGO as a thermal conductive filler, and n-octadecane OD as a phase change material, and is prepared by a salt template method and a vacuum impregnation method;

[0012] The function of the matrix material EG is to provide a porous structure and a heat-conducting skeleton for the composite phase change material;

[0013] The function of the binder PDVF is to provide bonding conditions for constructing an expanded graphite-based three-dimensional network;

[0014] The function of the pore-forming agent NaCl is to construct a porous structure for the expanded graphite-based sponge;

[0015] The function of the thermal conductive filler rGO is to enhance the thermal conductivity of the composite phase change material in coordination with the expanded graphite, and at the same time, provide the composite phase change material with light-heat conversion and electric-heat conversion performance;

[0016] The phase change material OD has the function of providing the composite phase change material with phase change temperature control performance and heat storage performance.

[0017] A method for preparing an expanded graphite-based high thermal conductivity composite phase change energy storage material comprises the following steps:

[0018] Step 1, preparation of expanded graphite-based three-dimensional porous sponge, first, with expanded graphite EG, polyvinylidene fluoride PVDF, sodium chloride NaCl satisfying a certain mass ratio, EG, PVDF and NaCl are mixed and ground to obtain a mixture, then, under certain conditions, the mixture is tableted, then, under certain conditions, it is heated, and finally, it is soaked and washed under certain conditions to remove the sodium chloride template to obtain a porous network structure, and then freeze-dried to obtain an expanded graphite-based three-dimensional porous sponge, which is recorded as EG / PVDF;

[0019] In the step 1, the mass ratio of EG, PVDF and NaCl is (2-8): 10:85, the tableting treatment conditions are: pressure of 5 MPa, time of 1 min; the heating treatment conditions are: heating temperature of 200° C., heating time of 10 h; the soaking and washing conditions are: soaking temperature of 90° C., and deionized water is replaced every 1 h;

[0020] Step 2, loading and reducing graphene oxide, EG / PVDF is vacuum impregnated in a graphene oxide solution under certain conditions, the obtained product is dried after the impregnation treatment, and then immersed in a vitamin C solution, thermally reduced under certain conditions, and freeze-dried after the immersion to obtain an expanded graphite-based three-dimensional porous composite material, which is recorded as EG / PVDF / rGO;

[0021] In step 2, the conditions for the immersion treatment are that the concentration of the graphene oxide solution is 9.9 mg / mL, the immersion temperature is 50° C., and the immersion time is 24 h. The conditions for the thermal reduction are that the concentration ratio of graphene oxide to vitamin C satisfies 1:1, the solution volume is 50 mL, the immersion temperature is 100° C., and the immersion time is 6 h.

[0022] Step 3, vacuum adsorption of phase change materials. Under certain conditions, the EG / PVDF / rGO obtained in step 2 is immersed in n-octadecane OD for vacuum adsorption to obtain an expanded graphite-based composite phase change energy storage material, which is recorded as OD@EG / PVDF / rGO.

[0023] In step 3, the vacuum adsorption conditions are as follows: the adsorption temperature is 50° C. and the adsorption time is 36 h.

[0024] An application of an expanded graphite-based high thermal conductivity composite phase change energy storage material, which has high thermal conductivity, phase change heat storage performance, temperature control performance, light-heat conversion performance and electric-heat conversion performance;

[0025] The high thermal conductivity is that the thermal conductivity is 2.3244-6.3840 W / (m·K);

[0026] The phase change heat storage performance is that the heat storage density is 145.99-188.19 J / g;

[0027] The photothermal conversion performance is that the photothermal conversion efficiency is 96.3%, and the electrothermal conversion performance is that the electrothermal conversion efficiency is 74.4%.

[0028] The technical effects of the present invention have been tested and the specific contents are as follows:

[0029] After FTIR performance testing, OD@EG / PVDF / rGO showed a similar FTIR spectrum to that of pure OD, indicating that there was physical adsorption mixing between OD and EG / PVDF / rGO;

[0030] Through XRD performance testing, it can be concluded from the diffraction crystal planes corresponding to different diffraction peaks that OD@EG / PVDF / rGO contains all the characteristic peaks of EG, rGO, PVDF, and OD;

[0031] According to SEM performance test, EG / PVDF presents a three-dimensional porous structure, rGO is embedded in EG / PVDF, forming a continuous thermal conductive network with EG, and OD is adsorbed in the porous structure of EG / PVDF / rGO;

[0032] According to the thermal conductivity test, the thermal conductivity of OD@EG / PVDF / rGO is 2.3244-6.3840W / (m·K);

[0033] According to infrared thermal imaging tests, the heat transfer efficiency of OD@EG / PVDF / rGO is proportional to the thermal conductivity;

[0034] According to DSC performance testing, the heat storage density of OD@EG / PVDF / rGO is 166.46-168.93 J / g.

[0035] After the leakage performance test, no leakage occurred under constant temperature of 50°C for 80 minutes, and OD@EG / PVDF / rGO has good anti-leakage performance.

[0036] After the temperature control performance test, the higher the thermal conductivity of OD@EG / PVDF / rGO, the faster the thermal response rate, and the heat storage performance is proportional to the temperature control time.

[0037] According to the photothermal performance test, the photothermal conversion efficiency of OD@EG / PVDF / rGO is 96.3%;

[0038] According to the electrothermal performance test, the electrothermal conversion efficiency of OD@EG / PVDF / rGO is 74.4%.

[0039] Therefore, the present invention has the following advantages over the prior art:

[0040] 1. The salt template method is used to prepare the expanded graphite-based three-dimensional porous sponge matrix. The process is simple, green and controllable, the macropores can be filled with functional materials, and NaCl is easy to recycle;

[0041] 2. Expanded graphite is used as the matrix skeleton, and modified graphene oxide is used as the thermal conductivity enhancing material. The common adsorption properties of the two are utilized to make them better embedded, ensuring the stable adsorption of the phase change material;

[0042] 3. Add modified graphene oxide to form a continuous three-dimensional porous carbon network with expanded graphite, synergistically enhancing the thermal conductivity of the composite phase change energy storage material;

[0043] 4. Modified graphene oxide enhances the photothermal conversion performance of composite phase change energy storage materials, and endows composite phase change energy storage materials with electrothermal conversion performance, so that composite phase change energy storage materials can realize the mutual conversion of energy between solar energy, thermal energy and electrical energy, greatly increasing the application potential;

[0044] 5. The phase change temperature of the phase change material is controlled at around 20-35°C. The temperature control range of the composite material is consistent with the suitable temperature of the human body and meets the human body's temperature comfort requirements. The selected organic phase change material has a large phase change enthalpy value and a stable phase change temperature, which is suitable for the field of energy buildings. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is the FTIR graph of Example 1;

[0046] Figure 2 is the XRD pattern of Example 1;

[0047] Figure 3 is the SEM image of Example 1; wherein, Figure 3 a and Figure 3 d is the SEM image of EG / PVDF, Figure 3 b and Figure 3 e is the SEM image of EG / PVDF / rGO, Figure 3 c and Figure 3 f is the SEM image of OD@EG / PVDF / rGO;

[0048] Figure 4 The thermal conductivity diagram of pure n-octadecane in Example 1, Example 1, Comparative Example 2, Comparative Example 3 and Example 2;

[0049] Figure 5 Graphs showing thermal conductivity of pure n-octadecane in Example 1, Example 1 and Comparative Example 1;

[0050] Figure 6 The infrared thermal images of Example 1 and Comparative Example 1 are shown;

[0051] Figure 7 The DSC melting curves of Example 1, Comparative Example 2, Comparative Example 3 and Example 2;

[0052] Figure 8 1 is the DSC crystallization curve of Example 1, Comparative Example 2, Comparative Example 3 and Example 2;

[0053] Fig. 9 The DSC melting curves of pure n-octadecane in Example 1, Example 1 and Comparative Example 1;

[0054] Fig.10 : are the DSC crystallization curves of pure n-octadecane in Example 1, Example 1 and Comparative Example 1;

[0055] Fig.11 It is the leakage prevention test diagram of pure n-octadecane in Example 1, Example 1 and Comparative Example 1;

[0056] Fig.12 The temperature control curves of pure n-octadecane in Example 1, Example 1 and Comparative Example 1;

[0057] Fig.13 The photothermal conversion test curves of pure n-octadecane in Example 1, Example 1 and Comparative Example 1;

[0058] Fig.14 This is the electrothermal conversion test curve of Example 1. DETAILED DESCRIPTION

[0059] The present invention is further described in detail through embodiments and in conjunction with the accompanying drawings, but is not intended to limit the present invention.

[0060] Example 1

[0061] A method for preparing an expanded graphite-based high thermal conductivity composite phase change energy storage material specifically comprises the following steps:

[0062] Step 1, preparation of expanded graphite-based three-dimensional porous sponge, first, 0.6g of matrix material expanded graphite EG, 1g of binder polyvinylidene fluoride PVDF and 8.5g of pore-forming agent sodium chloride NaCl are mixed and ground to obtain a mixture, then, under the conditions of a pressure of 5Mpa and a time of 1min, the mixture is tableted, then, heated at a heating temperature of 200°C and a heating time of 10h, finally, soaked and washed at an immersion temperature of 90°C, deionized water is changed every 1h, the sodium chloride template is removed, and a porous network structure is obtained, and the expanded graphite-based three-dimensional porous sponge can be obtained by freeze drying, which is recorded as EG / PVDF;

[0063] Step 2, loading and reduction of graphene oxide, immersing EG / PVDF in a thermal conductive filler graphene oxide GO solution with a concentration of 9.9 mg / mL, performing vacuum impregnation treatment under the conditions of an immersion temperature of 50°C and an immersion time of 24 hours, drying the obtained product after the immersion treatment, and then immersing the product in 50 mL of vitamin C solution, wherein the concentration ratio of rGO to vitamin C satisfies 1:1, the immersion temperature is 100°C, and the immersion time is 6 hours for thermal reduction, and after immersion, freeze-drying is performed to obtain an expanded graphite-based three-dimensional porous composite material, recorded as EG / PVDF / rGO;

[0064] Step 3, vacuum adsorption of the phase change material n-octadecane OD, under the conditions of adsorption temperature of 50°C and adsorption time of 36h, the EG / PVDF / rGO obtained in step 2 is immersed in OD for vacuum adsorption to obtain an expanded graphite-based high thermal conductivity composite phase change energy storage material, recorded as OD@EG / PVDF / rGO.

[0065] In order to prove the composition of OD@EG / PVDF / rGO, FTIR test was performed. The test results are shown in Figure 1 As shown in the figure, the FTIR spectra of OD@EG / PVDF / rGO are similar to those of pure n-octadecane. The test results show that OD is successfully adsorbed by the EG / PVDF / rGO matrix; and the connection between OD and EG / PVDF / rGO is physical adsorption mixing.

[0066] In order to further prove the composition of OD@EG / PVDF / rGO, XRD test was carried out. The test results are shown in Figure 2 As shown, OD@EG / PVDF / rGO contains all the characteristic peaks of EG, PVDF, rGO and OD. The test results show that EG / PVDF / rGO successfully adsorbs OD and has no effect on the crystallization behavior of OD, which ensures the good heat storage capacity of the composite phase change material.

[0067] In order to verify the microstructure of OD@EG / PVDF / rGO, SEM test was performed on OD@EG / PVDF / rGO. At the same time, in order to further confirm the microstructure changes during the preparation process, SEM test was performed on EG / PVDF obtained in step 1 and EG / PVDF / rGO obtained in step 2. The SEM test results are shown in Figure 3 shown.

[0068] The SEM test results of EG / PVDF are as follows Figure 3 a and Figure 3 As shown in d, EG / PVDF presents a three-dimensional porous structure, i.e., a sponge-like skeleton;

[0069] The SEM test results of EG / PVDF / rGO are as follows Figure 3 As shown in b and 3e, rGO is interspersed in the skeleton formed by the three-dimensional porous structure of EG / PVDF, forming a continuous thermal conductive network with EG;

[0070] The SEM test results of OD@EG / PVDF / rGO are as follows Figure 3 As shown in Figures 3c and 3f, OD is filled in the three-dimensional porous structure constructed by EG and rGO, and the EG / PVDF skeleton is also wrapped by the phase change material.

[0071] The test results show that the three-dimensional porous network structure constructed by expanded graphite and reduced graphene oxide can adsorb phase change materials well.

[0072] In order to prove the thermal conductivity of OD@EG / PVDF / rGO, the thermal conductivity of OD@EG / PVDF / rGO was tested. At the same time, in order to further confirm the change of thermal conductivity, the thermal conductivity of OD was tested as a benchmark. The test results are shown in Figure 5 And as shown in Table 1.

[0073] The thermal conductivity of OD is 0.1720 W / (m·K);

[0074] The thermal conductivity of OD@EG / PVDF / rGO is 6.3840 W / (m·K), and the thermal conductivity is improved by 3611.63% compared with pure OD;

[0075] The test results show that the dense thermal conductive network formed by the connection between EG and rGO synergistically enhances thermal conductivity and directly achieves a significant improvement in thermal conductivity.

[0076] Table 1 Comparison of thermal conductivity of expanded graphite-based high thermal conductivity composite phase change energy storage materials

[0077]

[0078]

[0079] In order to prove the thermal response heat transfer rate of the thermal conductivity of OD@EG / PVDF / rGO, infrared thermal imaging test was carried out. The test results are shown in Figure 6 As shown in the figure, on the same constant temperature heating stage, the temperature of OD@EG / PVDF / rGO increased from 24.7°C to 38.9°C within 120s, and the temperature increased by 14.2°C. The test results show that OD@EG / PVDF / rGO has thermal response performance.

[0080] In order to demonstrate the thermal storage performance of OD@EG / PVDF / rGO, differential scanning calorimetry (DSC) tests were performed.

[0081] The test results of the melting curve of OD@EG / PVDF / rGO are as follows: Fig. 9 As shown in Table 2, within the test temperature range of -10-90 °C, the melting process has an endothermic peak at 31.67 °C, and the latent heat value is 168.93 J / g;

[0082] The test results of the crystallization curve of OD@EG / PVDF / rGO are as follows: Fig.10 As shown in Table 2, within the test temperature range of -10-90 °C, the crystallization process has an exothermic peak at 22.23 °C, and the latent heat value is 168.36 J / g;

[0083] From the data in Table 2, it can be seen that with the increase of expanded graphite content, the latent heat value decreases, and the higher the thermal conductivity of the composite phase change material, the earlier the melting and crystallization behavior occurs.

[0084] Therefore, the test results show that the three-dimensional porous composite material composed of EG and rGO does not affect the phase change behavior of pure OD, and OD@EG / PVDF / rGO has good phase change behavior and heat storage capacity.

[0085] Table 2 Phase transition enthalpy and phase transition temperature of OD@EG / PVDF / rGO doped with different EG contents

[0086]

[0087] In order to prove the encapsulation performance of the prepared OD@EG / PVDF / rGO, a leakage test was conducted on OD@EG / PVDF / rGO. At the same time, for comparison, a leakage test was conducted on OD as a benchmark. The specific test method of the leakage test is to heat the sample to be tested to 50°C on a constant temperature heating table and observe and record the material status every 20 minutes.

[0088] The leak test results of OD are as follows: Fig.11 As shown, after 40 min, OD completely melted and flowed to the surrounding areas;

[0089] The leakage test results of OD@EG / PVDF / rGO are shown in Fig.11 As shown, there is no obvious change after 80 minutes of leakage test, that is, there is no leakage;

[0090] The test results show that OD@EG / PVDF / rGO has good encapsulation properties and can effectively prevent the leakage of OD.

[0091] In order to prove the role of OD@EG / PVDF / rGO in the field of temperature control, a temperature control test was conducted. At the same time, for comparison, a temperature control test was conducted on OD as a benchmark. The specific test method is to set the program of the temperature control box to increase the temperature from 0℃ to 50℃ at a constant speed, keep the temperature constant for 10 minutes, and then cool it down to 0℃ at a constant speed. During the same time period of the temperature change, the temperature control curve of the sample is observed.

[0092] The temperature control test results of OD are as follows Fig.12 As shown, the temperature began to rise after 20 s and a platform appeared at 120 s;

[0093] The temperature control test results of OD@EG / PVDF / rGO are as follows Fig.12 As shown, it can immediately transfer heat to sense temperature changes, and a platform appears at 95s; compared with pure OD, it has a faster heating speed and better temperature control performance.

[0094] The test results show that OD@EG / PVDF / rGO has efficient temperature control ability.

[0095] In order to prove the photothermal conversion performance of OD@EG / PVDF / rGO, photothermal conversion test was carried out on OD@EG / PVDF / rGO. Fig.13 As shown, the photoconversion efficiency of OD@EG / PVDF / rGO is calculated to be 96.3%, and the cooling curve shows a crystallization platform that lasts for 30 minutes. The test results show that OD@EG / PVDF / rGO has excellent photothermal conversion efficiency, which is attributed to the synergistic photon collection properties of expanded graphite and reduced graphene oxide.

[0096] In order to prove the electrothermal conversion performance of OD@EG / PVDF / rGO, an electrothermal conversion test was carried out on OD@EG / PVDF / rGO. The test results are shown in Fig.14 As shown, there are melting and crystallization platforms in the time-temperature curve, which indicates that Joule heat can be stored and released by OD@EG / PVDF / rGO; it is calculated that the electrothermal conversion efficiency of OD@EG / PVDF / rGO is 74.4%.

[0097] In order to demonstrate the effect of rGO on the performance of OD@EG / PVDF / rGO, comparative example 1 is provided, which is a composite phase change energy storage material without rGO doping.

[0098] Comparative Example 1

[0099] A composite phase change energy storage material without doping reduced graphene oxide, namely, made of expanded graphite, n-octadecane and polyvinylidene fluoride. The steps not particularly specified are the same as those in Example 1, except that: the composite phase change energy storage material without doping reduced graphene oxide can be obtained, which is recorded as OD@EG / PVDF.

[0100] The thermal conductivity test results of OD@EG / PVDF are as follows: Figure 5 As shown in Table 1, the thermal conductivity of OD@EG / PVDF is 3.6124 W / (m·K). Compared with the test results of Example 1, it can be seen that the addition of rGO can increase the thermal conductivity by 76.72%. The reason is that the addition of rGO can synergistically enhance the thermal conductivity with EG.

[0101] The thermal storage performance test results of OD@EG / PVDF are as follows:

[0102] The test results of the melting curve of OD@EG / PVDF are as follows: Fig. 9 As shown in Table 2, within the test temperature range of -10-90 °C, the melting process has an endothermic peak at 33.16 °C, and the latent heat value is 168.24 J / g;

[0103] The test results of the crystallization curve of OD@EG / PVDF / rGO are as follows: Fig.10 As shown in Table 2, within the test temperature range of -10-90 °C, the crystallization process has an exothermic peak at 19.63 °C, and the latent heat value is 167.88 J / g;

[0104] Compared with the test results of Example 1, it can be seen that adding rGO will not affect the phase change behavior and heat storage capacity of the composite material.

[0105] The temperature control performance test results of OD@EG / PVDF are as follows: Fig.12As shown in the figure, the time-temperature curve of OD@EG / PVDF shows a platform during both the heating and cooling process, indicating that OD@EG / PVDF has temperature control performance. Compared with the test results in Example 1, it can be seen that adding rGO can accelerate the time for the platform to appear. The reason is that adding rGO can enhance thermal conductivity and make the thermal response rate faster.

[0106] The infrared thermal imaging thermal response heat transfer rate test results of OD@EG / PVDF are as follows: Figure 6 As shown in the figure, on the same constant temperature heating table, within 120 seconds, the temperature of OD@EG / PVDF increased from 25.1°C to 36.6°C, and the temperature increased by 11.5°C. Compared with the test results in Example 1, it can be seen that the addition of rGO can accelerate the thermal response rate. The reason is that rGO and EG form a dense thermal conductive network and improve the thermal conductivity.

[0107] The leak-proof test results of OD@EG / PVDF are as follows: Fig.11 As shown, leakage occurs after 60 minutes of leakage test. Compared with Example 1, adding rGO can improve the packaging performance.

[0108] The photothermal conversion test results of OD@EG / PVDF are as follows: Fig.13 As shown, the test results show that compared with Example 1, OD@EG / PVDF cannot stably undergo crystallization exothermic behavior, which corresponds to the leakage test results. Adding rGO makes the crystallization behavior of OD@EG / PVDF / rGO more stable and the exothermic time longer.

[0109] In order to demonstrate the effect of the EG addition ratio on the performance of OD@EG / PVDF / rGO, comparative example 2, comparative example 3 and embodiment 2 are provided, and composite phase change energy storage materials are prepared with the mass ratios of EG and PVDF being 1:5, 2:5 and 4:5 respectively.

[0110] Comparative Example 2

[0111] A method for preparing a composite phase change energy storage material with a mass ratio of 1:5, wherein the steps not specifically described are the same as those in Example 1, except that in step 1, the amount of EG added is 0.2 g, and the obtained material is recorded as OD@EG / PVDF / rGO-2.

[0112] The thermal conductivity test results of OD@EG / PVDF / rGO-2 are as follows: Figure 4As shown in Table 1, the thermal conductivity of OD@EG / PVDF / rGO-2 is 2.3244W / (m·K), and the thermal conductivity of OD@EG / PVDF / rGO is increased by 174.65% compared with OD@EG / PVDF / rGO-2. Compared with Example 1, the increase in EG content can make the three-dimensional thermal conductive network structure of OD@EG / PVDF / rGO more compact and have a higher thermal conductivity.

[0113] The thermal storage performance test results of OD@EG / PVDF / rGO-2 are as follows:

[0114] The test results of the melting curve of OD@EG / PVDF / rGO-2 are as follows: Figure 7 As shown in Table 2, within the test temperature range of -10-90 °C, the melting process has an endothermic peak at 34.22 °C, and the latent heat value is 191.18 J / g;

[0115] The test results of the crystallization curve of OD@EG / PVDF / rGO-2 are as follows: Figure 8 As shown in Table 2, within the test temperature range of -10-90°C, the crystallization process has an exothermic peak at 19.93°C, and the latent heat value is 189.81 J / g. Compared with the test results of Example 1, it can be seen that the increase in EG content can advance the melting and crystallization behavior time; combined with the thermal conductivity test, it can be seen that the reason is that the increase in EG content directly leads to the improvement of thermal conductivity, thereby obtaining the improvement of heat storage performance.

[0116] Comparative Example 3

[0117] A method for preparing a composite phase change energy storage material with a mass ratio of 2:5, wherein the steps not specifically described are the same as those in Example 1, except that: in step 1, the amount of EG added is 0.4 g, and the obtained material is recorded as OD@EG / PVDF / rGO-4.

[0118] The thermal conductivity test results of OD@EG / PVDF / rGO-4 are as follows Figure 4 As shown in Table 1, the thermal conductivity of OD@EG / PVDF / rGO-4 is 3.3595 W / (m·K), and the thermal conductivity of OD@EG / PVDF / rGO is increased by 90.03% compared with that of OD@EG / PVDF / rGO-4.

[0119] The thermal storage performance test results of OD@EG / PVDF / rGO-4 are as follows:

[0120] The test results of the melting curve of OD@EG / PVDF / rGO-4 are as follows: Figure 7 As shown in Table 2, within the test temperature range of -10-90 °C, the melting process has an endothermic peak at 33.95 °C, and the latent heat value is 173.91 J / g;

[0121] The test results of the crystallization curve of OD@EG / PVDF / rGO-4 are as follows: Figure 8 As shown in Table 2, within the test temperature range of -10-90°C, the crystallization process has an exothermic peak at 20.58°C, and the latent heat value is 172.50 J / g.

[0122] The same conclusion as that of Comparative Example 2 can be drawn from Comparative Example 3.

[0123] Example 2

[0124] A method for preparing a composite phase change energy storage material with a mass ratio of 4:5, wherein the steps not specifically described are the same as those in Example 1, except that: in step 1, the amount of EG added is 0.8 g, and the obtained material is recorded as OD@EG / PVDF / rGO-8.

[0125] The thermal conductivity test results of OD@EG / PVDF / rGO-8 are as follows: Figure 4 As shown in Table 1, the thermal conductivity of OD@EG / PVDF / rGO-8 is 6.0035W / (m·K), and the thermal conductivity of OD@EG / PVDF / rGO is slightly higher than that of OD@EG / PVDF / rGO-8, which is at the same level. The test results show that in the preparation process of the three-dimensional porous structure sponge, the EG content reaches the threshold when it is 0.6g, and the addition of excessive EG cannot make rGO better impregnated and doped into the expanded graphite-based porous structure.

[0126] The thermal storage performance test results of OD@EG / PVDF / rGO-8 are as follows:

[0127] The test results of the melting curve of OD@EG / PVDF / rGO-8 are as follows: Figure 7 As shown in Table 2, within the test temperature range of -10-90 °C, the melting process has an endothermic peak at 32.59 °C, and the latent heat value is 166.46 J / g;

[0128] The test results of the crystallization curve of OD@EG / PVDF / rGO-8 are as follows: Figure 8 As shown in Table 2, within the test temperature range of -10-90 °C, the crystallization process has an exothermic peak at 21.74 °C, and the latent heat value is 166.41 J / g;

[0129] Combined with the thermal conductivity test results, it can be seen that adding excessive EG will reduce the heat storage capacity.

[0130] The following conclusions can be drawn from the above Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Example 2:

[0131] Conclusion 1, about thermal conductivity.

[0132] The improvement in thermal conductivity comes from the continuous three-dimensional thermal conductive network constructed by expanded graphite and reduced graphene oxide;

[0133] It can be proved through Example 1 and Comparative Example 1 that the thermal conductivity of the composite phase change energy storage material after adding reduced graphene oxide is higher, indicating that reduced graphene oxide and expanded graphite synergistically enhance thermal conductivity;

[0134] In addition, it can be further proved through Example 1, Comparative Example 2, Comparative Example 3 and Example 2 that the content of expanded graphite affects the thermal conductivity of the composite phase change energy storage material.

[0135] Conclusion 2, about heat storage performance.

[0136] It can be proved through Example 1 and Comparative Example 1 that doping reduced graphene oxide does not affect the phase change heat storage capacity of the composite material, and can better adsorb the phase change material and reduce leakage;

[0137] It can be further proved through Example 1, Comparative Example 2, Comparative Example 3 and Example 2 that as the content of expanded graphite increases, the latent heat value decreases, and the latent heat of the composite phase change material is inversely proportional to the content of expanded graphite.

[0138] Conclusion 3, about photothermal conversion and electrothermal conversion performance.

[0139] The composite phase change energy storage material after adding reduced graphene oxide has a more stable and lasting light-to-heat conversion capability, and also has electrothermal conversion performance, realizing the mutual conversion between multiple energies.

Claims

1. A method for preparing an expanded graphite-based high thermal conductivity composite phase change energy storage material, Features The following steps are involved: Step 1, preparation of expanded graphite-based three-dimensional porous sponge, first, with expanded graphite EG, polyvinylidene fluoride PVDF, sodium chloride NaCl satisfying a certain mass ratio, EG, PVDF and NaCl are mixed and ground to obtain a mixture, then, under certain conditions, the mixture is tableted, then, under certain conditions, it is heated, and finally, it is soaked and washed under certain conditions to remove the sodium chloride template to obtain a porous network structure, and then freeze-dried to obtain an expanded graphite-based three-dimensional porous sponge, which is recorded as EG / PVDF; Step 2, loading and reducing graphene oxide, EG / PVDF is vacuum impregnated in a graphene oxide solution under certain conditions, the obtained product is dried after the impregnation treatment, and then immersed in a vitamin C solution, thermally reduced under certain conditions, and freeze-dried after the immersion to obtain an expanded graphite-based three-dimensional porous composite material, which is recorded as EG / PVDF / rGO; Step 3, vacuum adsorption of phase change materials. Under certain conditions, the EG / PVDF / rGO obtained in step 2 is immersed in n-octadecane OD for vacuum adsorption to obtain an expanded graphite-based composite phase change energy storage material, which is recorded as OD@EG / PVDF / rGO.

2. The preparation method according to claim 1, Features: In the step 1, the mass ratio of EG, PVDF and NaCl is (2-8): 10:85, the conditions for tableting are: pressure of 5 MPa and time of 1 min; the conditions for heating are: heating temperature of 200° C. and heating time of 10 h; the conditions for immersion washing are: immersion temperature of 90° C. and deionized water is replaced every 1 h.

3. The preparation method according to claim 1, Features: In step 2, the conditions for the immersion treatment are that the concentration of the graphene oxide solution is 9.9 mg / mL, the immersion temperature is 50° C., and the immersion time is 24 h. The conditions for the thermal reduction are that the concentration ratio of graphene oxide to vitamin C satisfies 1:1, the solution volume is 50 mL, the immersion temperature is 100° C., and the immersion time is 6 h.

4. The preparation method according to claim 1, Features: In step 3, the vacuum adsorption conditions are as follows: the adsorption temperature is 50° C. and the adsorption time is 36 h.

5. The preparation method according to claim 1, Features: The obtained expanded graphite-based high thermal conductivity composite phase change energy storage material is prepared from expanded graphite EG as a matrix material, polyvinylidene fluoride PVDF as a binder, sodium chloride NaCl as a pore-forming agent, reduced graphene oxide rGO as a thermal conductive filler, and n-octadecane OD as a phase change material, and is prepared by a salt template method and a vacuum impregnation method; The function of the matrix material EG is to provide a porous structure and a heat-conducting skeleton for the composite phase change material; The function of the binder PDVF is to provide bonding conditions for constructing an expanded graphite-based three-dimensional network; The function of the pore-forming agent NaCl is to construct a porous structure for the expanded graphite-based sponge; The function of the thermal conductive filler rGO is to enhance the thermal conductivity of the composite phase change material in coordination with the expanded graphite, and at the same time, provide the composite phase change material with light-heat conversion and electric-heat conversion performance; The phase change material OD has the function of providing the composite phase change material with phase change temperature control performance and heat storage performance.

6. The preparation method according to claim 1, Features: The obtained expanded graphite-based high thermal conductivity composite phase change energy storage material simultaneously has high thermal conductivity, phase change heat storage performance, temperature control performance, photothermal conversion performance and electrothermal conversion performance.

7. The preparation method according to claim 1, Features: The thermal conductivity of the obtained expanded graphite-based high thermal conductivity composite phase change energy storage material is 2.3244-6.3840 W / (m·K); the heat storage density is 145.99-188.19 J / g; the photothermal conversion efficiency is 96.3%, the electrothermal conversion performance is, and the electrothermal conversion efficiency is 74.4%.

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