A guar gum@graphitic carbon nitride aerogel-based composite phase change material and its preparation method and application

Abstract

The present invention discloses a guar gum and graphite carbon nitride carbon aerogel-based composite phase change material. The material uses carbon nitride (g-C3N4), guar gum (GG), diammonium phosphate, and paraffin wax (PW) as raw materials. The flexible long-chain polymer guar gum (GG) is interwoven and wrapped with carbon nitride nanosheets. This is then cross-linked with diammonium phosphate to produce GG@g-C3N4 carbon aerogel. This carbon aerogel skeleton serves as a heat conduction channel and encapsulation carrier for the paraffin wax (PW) phase change material. The carbon aerogel and paraffin wax (PW) are then combined to produce the guar gum and graphite carbon nitride carbon aerogel-based composite phase change material. This composite phase change material exhibits high light-to-heat conversion and heat storage capabilities, strong thermal conductivity, and effectively maintains the phase change latent heat characteristics of the phase change material. Furthermore, it exhibits excellent packaging properties.

Description

Technical Field

[0001] The present invention relates to the fields of phase change energy storage, solar thermal effect and solar thermal power generation, and in particular to a guar gum@graphite carbon nitride carbon aerogel-based composite phase change material and a preparation method and application thereof. Background Art

[0002] With the rapid development of human society, the excessive consumption of fossil energy and serious environmental pollution have become key points affecting the development of the world economy. Renewable energy has gradually become an ideal alternative energy source. Therefore, many countries have begun to encourage the development of energy-saving technologies based on renewable energy. The development and utilization of clean energy and how to improve energy utilization efficiency are of great significance. Thermal energy, as an important way to utilize energy, is closely related to human life and social production. Thermal energy storage (TES) is widely used to solve the timeliness problem of thermal energy supply and demand, thereby further improving the utilization efficiency of thermal energy. It plays a vital role in promoting the development and full utilization of renewable and sustainable energy, and has become an effective way to solve the current global supply and demand problems of traditional energy.

[0003] Phase change materials (PCMs) are excellent latent heat materials that can absorb and release large amounts of thermal energy when their phase transitions occur with changes in ambient temperature. PCMs have the characteristic of absorbing and releasing thermal energy at a nearly constant temperature. Therefore, due to their advantages such as high heat capacity, good thermal stability and low cost, PCMs are one of the most effective materials for TES systems. However, huge application challenges still severely limit the performance of PCMs in thermal energy storage systems, such as their easy leakage and low thermal conductivity.

[0004] In order to prepare composite PCMs with better comprehensive performance, researchers are committed to constructing shape-stable composite PCMs and improving their thermal conductivity through structural design, which is of great significance for the practical application of PCMs. One of the most effective ways to solve the leakage problem and enhance its performance is to use encapsulation technology, that is, to wrap or seal PCMs with tough and performance-enhancing materials. The most widely considered effective encapsulation methods include: (1) chemical branching, (2) microcapsule coating, and (3) porous framework composite shaping.

[0005] Existing literature 1 (Wang B, Shi M, Yao H, et al. Preparation and application of low-temperature binary eutectic lauric acid-stearic acidSiO2 phase change microcapsules[J]. Energy and Buildings. 2023, 279: 112706.) uses lauric acid (LA) and stearic acid (SA) as core materials and silica as shell material to prepare low eutectic temperature microcapsule phase change materials (MPCMs) through interfacial polymerization. The results show that the latent heats of melting and freezing of the microcapsules are 170.3 J / g and 155.7 J / g, respectively, and have good thermal stability.

[0006] Existing literature 2 (Lu X, Fang C, Sheng Xet al. One-step and solvent-free synthesis of polyethylene glycol-based polyurethane as solid-solid phasechange materials for solar thermal energy storage[J].

[0007] Industrial & Engineering Chemistry Research, 2019, 58(8): 024-3032) With polyethylene glycol (PW) as the functional group and hexamethylene diisocyanate trimer (HDIT) as the crosslinker and supporting skeleton, the obtained solid-solid phase change materials (SSPCMs) exhibited excellent light-to-heat conversion performance and good thermal stability. Wang et al. synthesized a series of polyurethane / graphene oxide (PU / GO) SSPCMs. The results showed that: with the increase of PW content, the shape stability of PU / GO decreased, while the phase change enthalpy and light-to-heat conversion efficiency of PU / GO increased. In addition, the introduction of a low content of GO can maintain a considerable energy storage density, and the light absorption capacity is greatly improved by reasonably adjusting the content of the soft segment.

[0008] Although the above two solutions have achieved good results in preventing phase change material leakage and enhancing thermal conductivity, they also bring problems such as non-recyclability, non-machinability or non-ductility. The microencapsulation method shows defects such as low encapsulation efficiency and complex processing and preparation process, which greatly hinders its comprehensive and sustainable development. Summary of the Invention

[0009] The present invention addresses the shortcomings of the prior art by providing a guar gum and graphite carbon nitride carbon aerogel-based composite phase change material, its preparation method, and its application. This composite phase change material exhibits high light-to-heat conversion and heat storage capabilities, strong thermal conductivity, effectively retains the phase change latent heat characteristics of the phase change material, and also exhibits excellent packaging properties.

[0010] The technical solution for achieving the purpose of the present invention is:

[0011] A guar gum@graphite carbon nitride carbon aerogel-based composite phase change material is prepared using carbon nitride g-C3N4, guar gum GG, diammonium hydrogen phosphate, and paraffin wax (PW) as raw materials. The specific preparation process is as follows: flexible long-chain polymer guar gum GG is interwoven and wrapped with carbon nitride nanosheets, and then secondary cross-linked with diammonium hydrogen phosphate to form continuous electron and phonon transmission channels, thereby constructing a GG@g-C3N4 carbon aerogel with orderly stacked carbon layers, tightly intertwined carbon nitride nanosheets and a carbon skeleton, and uniformly loaded. The carbon aerogel skeleton serves as a heat conduction channel and packaging carrier for the phase change material paraffin wax (PW). The carbon aerogel and paraffin wax (PW) are composited to obtain the guar gum@graphite carbon nitride carbon aerogel-based composite phase change material.

[0012] The flexible long-chain polymer guar gum GG is interwoven and wrapped with carbon nitride nanosheets, and then secondary cross-linked with diammonium hydrogen phosphate to obtain GG@g-C3N4 carbon aerogel. Specifically, the carbon nitride nanosheets are hydrogen-bonded with -OH in guar gum GG, and the carbon nitride nanosheets are tightly bound in the guar gum GG network structure. Then, the -NH3 in diammonium hydrogen phosphate is used to cross-link the carbon nitride nanosheets. + The hydrogen bonding effect with guar gum GG and carbon nitride nanosheets further strengthens the cross-linking effect of the system. Under the synergistic effect of guar gum GG, carbon nitride and diammonium hydrogen phosphate, GG@g-C3N4 carbon aerogel is prepared. The g-C3N4 nanosheets in the GG@g-C3N4 carbon aerogel are uniformly and tightly intertwined in the carbon skeleton of the GG@g-C3N4 carbon aerogel, forming a three-dimensional porous network structure with regular and uniform pores.

[0013] The thermal conductivity of the guar gum@graphite carbon nitride carbon aerogel-based composite phase change material is 0.5763-0.6599 W / (m·K). The latent heat of crystallization of the guar gum@graphite carbon nitride carbon aerogel-based composite phase change material is 198.64-204.93 J / g, and the latent heat of melting is 191.12-202.93 J / g.

[0014] The preparation method of the guar gum@graphite carbon nitride carbon aerogel-based composite phase change material comprises the following steps:

[0015] Step 1, preparation of graphitic carbon nitride g-C3N4, dicyandiamide and glucose powder were mixed and ground in a mortar at a ratio of 20:1, and then heated to 900°C in a N2 atmosphere tube furnace at a heating rate of 3°C / min and kept at this temperature for 2 hours to obtain carbon nitride nanosheets;

[0016] Step 2, preparation of guar gum @ graphite carbon nitride carbon aerogel, adding a certain mass of carbon nitride nanosheet powder to 100mL deionized water, ultrasonicating for 1h to evenly disperse the carbon nitride nanosheet powder, to prevent the guar gum GG powder from sticking, adding guar gum GG under stirring after the ultrasonication, mechanically stirring for 10h to allow the guar gum GG to fully dissolve and disperse in the aqueous solution, then adding diamine hydrogen phosphate of the same mass as the guar gum GG and continuing to stir for 1h, ultrasonicating for 10min after the stirring, standing for defoaming, putting it in the refrigerator to freeze after the defoaming, and then freeze-drying, thereby preparing a carbon nitride-containing bioaerogel. The biomass-based aerogel was treated in two steps. First, the microstructure was stabilized by pre-oxidation. The biomass-based aerogel was placed in a muffle furnace and heated to 240°C at 0.5°C / min and kept warm for 4 hours to obtain pre-oxidized aerogel. The pre-oxidized aerogel was then transferred to a tubular furnace and carbonized in three stages under a N2 atmosphere. In the first stage, the temperature was increased to 300°C at 5°C / min. In the second stage, the temperature was increased to 400°C at 0.5°C / min and kept warm for 1 hour. In the final stage, the temperature was increased to 750°C at 5°C / min and kept warm for 2 hours to obtain guar gum @ graphite carbon nitride carbon aerogel, namely GND carbon aerogel.

[0017] Step 3, preparation of guar gum @ graphite carbon nitride carbon aerogel-based composite phase change material, placing the GND carbon aerogel in a 120°C forced air drying oven for 6 hours to ensure sufficient drying, weighing 94wt%, 95wt%, 96wt% and 96.89wt% of paraffin wax PW of the sum of the mass of GND carbon aerogel and composite phase change material respectively, placing them in different beakers and heating to melt, then transferring the GND carbon aerogel to beakers containing different mass percentages of paraffin wax PW and vacuum impregnating them in a vacuum drying oven at 80°C for 2 hours to obtain different guar gum @ graphite carbon nitride carbon aerogel-based composite phase change materials.

[0018] The above-mentioned guar gum@graphite carbon nitride carbon aerogel-based composite phase change material is used as a composite phase change material in heat storage and photothermal applications, with a photothermal conversion efficiency of 92.95% and excellent packaging capability and thermal cycle stability.

[0019] The carbon nitride is a nano-sheet material with a thickness of nanometers, which is made by uniformly mixing and carbonizing dicyandiamide and glucose. It has a novel heterojunction structure and is considered to be a promising visible light responsive material due to its large specific surface area, rich pore structure, moderate band gap, good stability, appropriate electronic band structure, and non-toxicity. Therefore, the addition of carbon nitride will not only greatly improve the leakage problem of the phase change material, but also provide continuous phonon and electron transmission channels for the carbon aerogel, thereby improving the thermal conductivity and photothermal conversion performance of the composite phase change material.

[0020] The guar gum is a low-cost biomass material containing flexible molecular chains. The numerous -OH groups in its molecular units give it good structural design, and it can form a network structure through self-assembly and interweaving between functional groups. At the same time, the mutual support between guar gum and g-C3N4 can effectively prevent the shrinkage and collapse of the aerogel structure, improve the flexibility of the carbon aerogel, and provide volume expansion space for phase change of the phase change material.

[0021] The diammonium hydrogen phosphate is a flame retardant additive. The diammonium hydrogen phosphate can be tightly combined with g-C3N4 through hydrogen bonding to make the aerogel structure more regular and compact. At the same time, the flame retardant effect of the diammonium hydrogen phosphate can also effectively reduce the volume shrinkage and collapse of the aerogel during the carbonization process.

[0022] The paraffin wax is a phase change material and provides a phase change energy storage function.

[0023] Compared with the existing technology, this technical solution has the following advantages:

[0024] 1. This technical solution uses g-C3N4 nanosheets as a photocatalyst-modified carbon aerogel as the matrix, significantly improving the light-to-heat conversion capability of the composite phase change material and expanding its application range;

[0025] 2. This technical solution improves the mechanical strength of the aerogel through the good dispersion of flexible long-chain polymer GG and g-C3N4 nanosheets and nano-reinforcement medium. The addition of diammonium hydrogen phosphate induces strong cross-linking of GG, further increasing the structural compactness of the aerogel. At the same time, diammonium hydrogen phosphate acts as a flame retardant to improve the structural stability of the carbon aerogel during the carbonization process.

[0026] 3. The highly thermally conductive network structure of the carbon skeleton in the carbon aerogel of this technical solution provides a continuous and stable thermal conduction path, effectively accelerating the thermal conduction of PCMs. The thermal conductivity of PCMs is 0.5763-0.6599 W / (m·k), which is 3.62 times higher than that of pure PW.

[0027] 4. A guar gum and graphite carbon nitride carbon aerogel-based composite phase change material prepared by this technical solution has crystallization enthalpy values and melting enthalpy values of 198.64-204.93 J / g and 191.12-202.93 J / g, respectively, for thermal storage and photothermal applications, showing high thermal storage capacity. The composite PCMs also maintained the same thermal performance after 200 thermal cycles, demonstrating good thermal cycling stability.

[0028] 5. The guar gum @ graphite carbon nitride carbon aerogel-based composite phase change material prepared by this technical solution has excellent packaging performance in heat storage and photothermal applications, in which the combination of the matrix and the phase change material is a physical action rather than a chemical action, which effectively maintains the phase change latent heat characteristics of the phase change material; 6. The guar gum @ graphite carbon nitride carbon aerogel-based composite phase change material prepared by this technical solution has excellent packaging performance in heat storage and photothermal applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 FT-IR and XRD patterns of Example 1, Example 2, Example 3, Example 4, and PW, wherein Figure a is an FT-IR pattern and Figure b is an XRD pattern;

[0030] Figure 2 Figure 2 is the SEM image of GND-0, GND-1, GND-2, and GND-3 in the embodiment, wherein Figure a is the SEM image of GND-0, Figure b is the SEM image of GND-1, Figure c is the SEM image of GND-2, and Figure d is the SEM image of GND-3;

[0031] Figure 3 The SEM images of Example 1, Example 2, Example 3, and Example 4 are shown in FIG. a, wherein FIG. b is the SEM image of Example 2, FIG. c is the SEM image of Example 3, and FIG. d is the SEM image of Example 4;

[0032] Figure 4 The DSC curves of Example 1, Example 2, Example 3, Example 4, and Comparative Example 1 are shown in Figure a, wherein Figure a is a DSC curve of the crystallization process, Figure b is a DSC curve of the melting process, Figure c is a DSC curve of the thermal cycling process of Example 3, and Figure d is an FT-IR spectrum of Example 3 before and after the thermal cycling process;

[0033] Figure 5 The leakage test diagrams and thermal conductivity histograms of Example 1, Example 2, Example 3, Example 4, and Comparative Example 1 are shown, wherein Figure a is a leakage test diagram and Figure b is a thermal conductivity histogram;

[0034] Figure 6 These are the experimental setup diagrams and temperature-time curves of Comparative Example 1, Example 3, and GND-1 in the light-to-heat conversion experiment, where Figure a is the experimental setup diagram and Figure b is the temperature-time curve. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the present invention is not limited thereto.

[0036] Example 1:

[0037] A method for preparing a guar gum@graphite carbon nitride carbon aerogel-based composite phase change material comprises the following steps:

[0038] Step 1, preparation of graphitic carbon nitride g-C3N4, 20g of dicyandiamide and 1g of glucose powder were mixed and ground uniformly in a mortar, and then heated to 900°C at 3°C / min in a N2 atmosphere tube furnace and kept at this temperature for 2h to obtain calcined carbon nitride nanosheets;

[0039] Step 2, preparation of guar gum @ graphite carbon nitride carbon aerogel, adding 0.05g of carbon nitride nanosheet powder to 100mL of deionized water, ultrasonicating for 1h to evenly disperse the carbon nitride nanosheet powder, to prevent the guar gum GG powder from sticking, adding 1g of guar gum GG under stirring after the ultrasonication, mechanically stirring for 10h to allow the guar gum GG to fully dissolve and disperse in the aqueous solution, then adding diamine hydrogen phosphate with the same mass as the guar gum GG and continuing to stir for 1h, ultrasonicating for 10min after the stirring is completed, standing for defoaming, putting it in a refrigerator for freezing after the defoaming is completed, and then freeze-drying is performed to obtain a carbon nitride-containing aerogel; the aerogel is subjected to two-step treatment, first, pre-oxidation is performed to stabilize the microstructure The aerogel was placed in a muffle furnace and heated to 240°C at 0.5°C / min and kept warm for 4h to obtain a pre-oxidized aerogel; the pre-oxidized aerogel was then transferred to a tubular furnace and carbonized in three stages under a N2 atmosphere. In the first stage, the temperature was increased to 300°C at 5°C / min, in the second stage, the temperature was increased to 400°C at 0.5°C / min and kept warm for 1h, and in the final stage, the temperature was increased to 750°C at 5°C / min and kept warm for 2h, and the obtained carbon aerogel was GND-1; in this example, the same steps as step 2 were used, and 0g, 0.1g and 0.15g of carbon nitride nanosheet powder were added to 100mL of deionized water, respectively, and the obtained carbon aerogels were GND-0, GND-2 and GND-3, respectively;

[0040] Step 3, preparation of GG@g-C3N4 carbon aerogel-based composite PCMs, place GND-1 in a 120°C forced air drying oven for 6 hours to ensure sufficient drying, weigh 94wt% of the mass percentage of paraffin PW of the sum of the mass of GND-1 and composite PCMs, place it in a beaker and heat to melt, then transfer GND-1 to the beaker of paraffin PW and vacuum impregnate it in a vacuum drying oven at 80°C for 24 hours to obtain guar gum@graphite carbon nitride carbon aerogel-based composite phase change material, i.e., PCMs-1.

[0041] In order to prove the composition of PCMs-1, i.e., its successful preparation, FT-IR and XRD tests were performed on PCMs-1. Figure 1 As shown, from Figure 1 In (a), the CH in -CH3 and -CH2 is observed at 2920 cm -1 , 2850cm -1 The stretching vibration characteristic peak is shown at 1378 cm -1 , 1462cm -1 The characteristic peak of bending vibration is shown at 719cm -1 The characteristic peaks at are CH in-plane rocking vibrations. These peaks confirm the presence of long alkyl chains, which is consistent with the PW structure. Observation of the spectra of pure PW and all PCMs-1 revealed that the characteristic peaks of PCMs-1 and PW are extremely similar. Except for a slight shift in the peak position, there is no disappearance or generation of characteristic peaks, which indicates that PW is successfully adsorbed in the porous network structure of GND-1. Although there is a strong intermolecular hydrogen bond between PW and GND-1, its structural characteristics have not changed. Figure 1 (b) Typical characteristic peaks of PW appeared at 22° and 24° in the XRD pattern. The diffraction peaks of PCMs-1 and PW are very similar, indicating that the PW in it has the same crystal structure as pure PW and still maintains excellent phase transition behavior. In addition, no new diffraction peaks appeared in the XRD spectrum, but the diffraction peak intensity of PCMs-1 was weaker than that of pure PW, indicating that the crystallization behavior of PW was slightly constrained after being wrapped by the carbon aerogel skeleton. This experimental phenomenon shows that PCMs-1 can stabilize the morphology of PW and has no effect on the crystallization behavior of PW.

[0042] In order to verify the microstructure of PCMs-1, SEM test was performed on the obtained PCMs-1. The test results of PCMs-1 are shown in the figure below. Figure 3 As shown in (a), the carbon skeleton of PCMs-1 is not completely filled, and a partially exposed carbon layer appears. Some pores of the carbon aerogel are incompletely filled. Due to the small amount of PW adsorption, the structure inside the composite phase change material is loose, so that the carbon aerogel has weak binding force on the phase change material PW.

[0043] In order to prove the phase change performance of PCMs-1, differential scanning calorimetry (DSC) test was carried out. Figure 4 (a) Figure 4 (b) and Table 1. Test results show that within the test temperature range of 10-70°C, PCMs-1 exhibits an endothermic peak at 58.66°C during melting with a latent heat of 191.12 J / g, and an exothermic peak at 51.33°C during crystallization with a latent heat of 198.64 J / g. These results demonstrate PCMs-1's excellent phase transition behavior and heat storage capacity.

[0044] In order to prove the thermal conductivity of PCMs-1, the thermal conductivity of PCMs-1 was tested. Figure 5 As shown in Figure (b), the thermal conductivity of the phase change material PW is 0.1821W / (m·K), and the thermal conductivity of PCMs-1 is 0.6599W / (m·K), and the thermal conductivity is increased by about 3.62 times.

[0045] In order to prove the packaging performance of the prepared PCMs-1, a leakage test was conducted on PCMs-1. The specific test method is to heat PCMs-1 at a heating temperature of 80°C for 1.5 and observe the material state. The test results are as follows: Figure 5 As shown in Figure (a), after 15 minutes of heating, the PW completely melted and flowed in all directions. However, the PCMs-1 showed no significant change. Even after 90 minutes of heating, the PCMs-1 still showed no leakage. These experimental results demonstrate that the composite phase change material has excellent encapsulation properties and can effectively prevent PW leakage.

[0046] Comparative Example 1:

[0047] The method for preparing pure PW samples using sliced paraffin wax PW phase change material as raw material comprises the following steps:

[0048] Step 1: Take out the paraffin wax PW sample block and cut it into small particles with scissors for later use;

[0049] In step 2, the prepared PW particles are placed in a customized tablet pressing mold and pressed at a pressure of 5 MPa for 30 seconds. The sample is taken out to obtain a pure PW sample.

[0050] In order to prove the phase change performance of pure PW samples, differential scanning calorimetry (DSC) test was carried out. The test results are shown in Figure 2. Figure 4 (a) Figure 4 (b) and shown in Table 1. The test results show that within the test temperature range of 10-70℃, the pure PW sample has an endothermic peak at 58.41℃ during the melting process with a latent heat value of 209.59J / g, and an exothermic peak at 52.16℃ during the crystallization process with a latent heat value of 211.62J / g.

[0051] In order to prove the thermal conductivity of pure PW samples, the thermal conductivity coefficient of pure PW samples was tested. The test results are as follows Figure 5 As shown in Figure (b), the thermal conductivity of the pure PW sample is 0.241W / (m·K).

[0052] In order to prove the packaging performance of the prepared pure PW sample, the pure PW sample was subjected to a leakage test. The specific test method is to heat the pure PW sample at a heating temperature of 80℃ for 1.5 hours and observe the material state. The test results are as follows: Figure 5 As shown in Figure (a), the pure PW sample is completely melted and flows to the surroundings.

[0053] In order to prove the photothermal conversion performance of the prepared pure PW sample, the photothermal conversion performance of the pure PW sample was tested. The test results are as follows Figure 6 As shown in Figure (b), due to the poor light absorption performance of the pure PW sample, the temperature did not reach the phase transition temperature, and no crystallization and melting platform appeared under the same lighting conditions.

[0054] Example 2:

[0055] A method for preparing a guar gum@graphite carbon nitride carbon aerogel-based composite phase change material comprises the following steps:

[0056] In step 3, paraffin wax PW accounting for 95 wt% of the mass percentage of the composite PCMs was weighed and placed in a beaker and heated to melt to obtain PCMs-2. The remaining steps were the same as in Example 1.

[0057] In order to prove the composition of PCMs-2, i.e., its successful preparation, FT-IR and XRD tests were performed. The test results are shown in Figure 2. Figure 1 As shown, from Figure 1 In (a), the CH in -CH3 and -CH2 is observed at 2920 cm -1 , 2850cm -1 The stretching vibration characteristic peak is shown at 1378 cm -1 , 1462cm -1 The characteristic peak of bending vibration is shown at 719cm -1 The characteristic peaks at are CH in-plane rocking vibrations. These peaks confirm the presence of long alkyl chains, which is consistent with the PW structure. Figure 1 (b) The XRD pattern shows characteristic peaks typical of PW at 22° and 24°. The diffraction peaks of PCMs-2 and PW are very similar, indicating that the PW contained in PCMs-2 has the same crystal structure as pure PW and maintains excellent phase transition behavior. This experimental observation demonstrates that PCMs-2 stabilizes the PW morphology and has no effect on its crystallization behavior.

[0058] The SEM results of PCMs-2 are as follows Figure 3 As shown in Figure (b), compared with PCMs-1, the skeleton and pores of the carbon aerogel in PCMs-2 are obviously wrapped and filled, PW has filled most of the internal channels and gaps, the carbon skeleton is smoother, PW is tightly integrated with the carbon aerogel skeleton structure, and there is no separation between PW and GND-1.

[0059] In order to prove the phase change performance of PCMs-2, differential scanning calorimetry (DSC) test was carried out. Figure 4 (a) Figure 4(b) and Table 1. Within the test temperature range of 10-70°C, PCMs-2 exhibited an endothermic peak at 58.95°C during melting with a latent heat of 198.56 J / g, and an exothermic peak at 51.66°C during crystallization with a latent heat of 200.80 J / g. These test results demonstrate PCMs-2's excellent phase transition behavior and heat storage capacity.

[0060] In order to prove the thermal conductivity of the prepared PCMs-2, thermal conductivity test was carried out. Figure 5 As shown in (b), the thermal conductivity of PCMs-2 is 0.6121W / (m·K), and the thermal conductivity is increased by 3.36 times the original one, which shows a significant improvement effect.

[0061] In order to prove the packaging performance of the prepared PCMs-2, leakage tests were carried out on PCMs-2 and PW. Figure 5 As shown in Figure (a), PCMs-2 showed no significant change. Even after heating for 90 minutes, PCMs-2 still did not leak. The experimental results show that PCMs-2 has excellent packaging performance and can effectively prevent PW leakage.

[0062] Example 3:

[0063] A method for preparing a guar gum@graphite carbon nitride carbon aerogel-based composite phase change material comprises the following steps:

[0064] In step 3, paraffin wax PW accounting for 96 wt% of the mass percentage of the composite PCMs was weighed and placed in a beaker and heated to melt to obtain PCMs-3. The remaining steps were the same as in Example 1.

[0065] In order to prove the composition of PCMs-3, i.e., the successful preparation, FT-IR and XRD tests were performed on PCMs-3. The test results are shown in Figure 2. Figure 1 As shown, from Figure 1 In (a), the CH in -CH3 and -CH2 is observed at 2920 cm -1 , 2850cm -1 The stretching vibration characteristic peak is shown at 1378 cm -1 , 1462cm -1 The characteristic peak of bending vibration is shown at 719cm -1 The characteristic peaks at are CH in-plane rocking vibrations. These peaks confirm the presence of long alkyl chains, which is consistent with the PW structure. Figure 1(b) The XRD pattern shows characteristic peaks typical of PW at 22° and 24°. The diffraction peaks of PCMs-3 and PW are very similar, indicating that the PW contained in the PCMs-3 has the same crystal structure as pure PW and maintains excellent phase transition behavior. This experimental observation demonstrates that PCMs-3 stabilizes the PW morphology and has no effect on its crystallization behavior.

[0066] The SEM results of PCMs-3 are as follows Figure 3 As shown in Figure (c), the skeleton and pores of the carbon aerogel in PCMs-3 are further wrapped and filled, the carbon skeleton is smoother, the PW is tightly combined with the carbon aerogel skeleton structure, and there is no separation between PW and GND-1.

[0067] In order to prove the phase change performance of PCMs-3, differential scanning calorimetry (DSC) test was carried out. Figure 4 (a) Figure 4 (b) and Table 1. In the test temperature range of 10-70℃, PCMs-3 has an endothermic peak at 58.62℃ during the melting process with a latent heat value of 201.93J / g, and an exothermic peak at 51.67℃ during the crystallization process with a latent heat value of 203.50J / g. The test results show that PCMs-3 has good phase change behavior and heat storage capacity. PCMs-3 was further tested 200 times with the same melting-crystallization cycle using the DSC test method. The results are shown in the figure. Figure 4 (c) is shown. The figure shows the DSC curves of PCMs-3 after the 1st, 100th, and 200th cycles and the melt value retention rate after every 25 cycles. The DSC curve of PCMs-3 after 200 thermal cycles is basically consistent with the first curve, and the melt value retention rate has almost no change, which shows that the GND carbon aerogel has an excellent encapsulation effect and ensures good cycle stability. In addition, Figure 4 (d) The characteristic peaks of the FT-IR spectrum of PCMs-3 before and after 200 melt-crystallization cycles are basically the same, which indicates that it has good structural durability.

[0068] In order to prove the thermal conductivity of the prepared PCMs-3, thermal conductivity test was carried out. Figure 5 As shown in Figure (b), the thermal conductivity of PCMs-3 is 0.5975W / (m·K), and the thermal conductivity is increased by 3.28 times, which is a significant improvement. In order to prove the packaging performance of the prepared PCMs-3, the leakage test of PCMs and PW was carried out respectively. The test results are shown in Figure 2. Figure 5 As shown in Figure (a), PCMs-3 showed no significant change. Even after heating for 90 minutes, PCMs-3 still did not leak. The experimental results show that PCMs-3 has excellent packaging performance and can effectively prevent PW leakage.

[0069] In order to prove the photothermal conversion performance of the prepared PCMs-3, leakage tests were carried out on PCMs-3, GND-1 and PW. Figure 6 As shown in Figure (a), the laboratory independently built a light-to-heat conversion device consisting of a simulated sunlight illumination system (AM1.5) and a multi-channel data recording system to test pure PW, PCMs-3, and aerogel. The light-to-heat conversion ability of the sample was tested by placing it under simulated sunlight and obtaining a temperature-time curve. The sample was kept at a constant light intensity of 100mW / cm 2 The light-heat conversion experiment was carried out under the following conditions. The test results are as follows Figure 6 As shown in Figure (b), the temperatures of the three samples increased continuously under light irradiation. The heating rate of PCMs-3 was much higher than that of pure PW, but slightly lower than that of GND-1. After 1800 seconds of simulated sunlight irradiation, the temperatures of PW, PCMS-3, and GND-1 increased from room temperature to 59.3°C, 82.3°C, and 85.7°C, respectively. This indicates that the presence of GND-1 carbon aerogel in PCMs-3 significantly enhanced its light absorption. The heating curve of PCMs-3 exhibited a temperature plateau between 48.5°C and 59.8°C, indicating that PW underwent a solid-liquid phase transition under light irradiation in this range. Light energy was absorbed by PCMs-3 and stored as latent heat. After the light irradiation ended, the temperatures of all three samples dropped sharply, with GND-1 showing the fastest cooling rate. Near the crystallization point, PW and PCMS-3 underwent a crystallization phase transition, resulting in the appearance of a new temperature plateau. The test results indicate that the light-to-heat conversion efficiency of PCMs-3 after light irradiation was 92.95%. The above results all indicate that the thermal conductive network structure of carbon nitride nanosheets and GND-1 carbon aerogel can effectively absorb a large amount of light energy, thanks to which the light-to-heat conversion efficiency of PCMs-3 has been significantly improved.

[0070] Example 4:

[0071] A method for preparing a guar gum@graphite carbon nitride carbon aerogel-based composite phase change material comprises the following steps:

[0072] In step 3, paraffin wax PW accounting for 96.89 wt% of the mass percentage of the composite PCMs was weighed and placed in a beaker and heated to melt to obtain PCMs-4. The remaining steps were the same as in Example 1.

[0073] In order to prove the composition of PCMs-4, i.e., its successful preparation, FT-IR and XRD tests were performed. The test results are shown in Figure 2. Figure 1 As shown, from Figure 1 In (a), the CH in -CH3 and -CH2 is observed at 2920 cm -1 , 2850cm-1 The stretching vibration characteristic peak is shown at 1378 cm -1 , 1462cm -1 The characteristic peak of bending vibration is shown at 719cm -1 The characteristic peaks at are CH in-plane rocking vibrations. These peaks confirm the presence of long alkyl chains, which is consistent with the PW structure. Figure 1 (b) The XRD pattern shows characteristic peaks typical of PW at 22° and 24°. The diffraction peaks of PCMs-4 and PW are very similar, indicating that the PW contained in the PCMs-4 has the same crystal structure as pure PW and maintains excellent phase transition behavior. This experimental observation demonstrates that PCMs-4 stabilizes the PW morphology and has no effect on its crystallization behavior.

[0074] The SEM results of PCMs-4 are as follows Figure 3 As shown in Figure (d), the skeleton and pores of the GND-1 carbon aerogel in PCMs-4 are obviously over-wrapped and filled, and there is no obvious interface separation between the two. At the same time, excess PW accumulates on the surface of PCMs-4.

[0075] In order to prove the phase change performance of PCMs-4, differential scanning calorimetry (DSC) test was carried out. Figure 4 (a) Figure 4 (b) and Table 1. Within the test temperature range of 10-70°C, PCMs-4 exhibited an endothermic peak at 58.87°C during melting with a latent heat of 202.93 J / g, and an exothermic peak at 52.04°C during crystallization with a latent heat of 204.93 J / g. These test results demonstrate PCMs-4's excellent phase transition behavior and heat storage capacity.

[0076] Table 1 Phase change enthalpy and temperature of PCMs with different PW contents

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[0079] In order to prove the thermal conductivity of the prepared PCMs-4, the thermal conductivity coefficient test was carried out. The test results are shown in Figure 2. Figure 5 As shown in Figure (b), the thermal conductivity of PCMs-4 is 0.5763W / (m·K), and the thermal conductivity is increased by 3.16 times, which is a significant improvement. In order to prove the packaging performance of the prepared PCMs-4, the leakage test of PCMs and PW was carried out respectively. The test results are shown in Figure 2. Figure 5As shown in Figure (a), PCMs-4 showed no significant change. When the heating time was 90 minutes, only a small amount of liquid leakage occurred when the PW was overdosed with PCMs-4. The experimental results show that PCMs-4 has good encapsulation properties and can effectively prevent PW leakage.

[0080] The following conclusions can be drawn from Example 1, Example 2, Example 3, Example 4 and Comparative Example 1:

[0081] 1. Using carbon nitride nanosheets as a photocatalyst modified carbon aerogel as the matrix, the light-to-heat conversion capacity of the composite phase change material was significantly improved (92.95%), expanding its application range;

[0082] 2. The mechanical strength of the aerogel is enhanced by the good dispersion of flexible long-chain polymer GG and carbon nitride nanosheets and the nano-reinforcement medium. The addition of diammonium hydrogen phosphate induces strong cross-linking of GG, further increasing the structural compactness of the aerogel. At the same time, diammonium hydrogen phosphate, as a flame retardant, improves the structural stability of the carbon aerogel during the carbonization process. The resulting GND carbon aerogel forms a 3D network structure with orderly stacked carbon layers and uniform pore size distribution. It has good compatibility with PW and provides strong support for the preparation of composite phase change materials.

[0083] 3. The high thermal conductivity network structure of the carbon skeleton in GND carbon aerogel provides a continuous and stable thermal conduction path, effectively accelerating the thermal conduction of PCMs. The thermal conductivity of PCMs is 0.5763-0.6599W / (m·k), which is significantly increased by 3.16-3.62 times compared with pure PW.

[0084] 4. A guar gum and graphite carbon nitride carbon aerogel-based composite phase change material with crystallization enthalpy and melting enthalpy values of 198.64-204.93 J / g and 191.12-202.93 J / g, respectively, for thermal storage and photothermal applications. It has high thermal storage capacity and maintains the same thermal performance after 200 thermal cycles, showing good thermal cycling stability.

[0085] 5. A guar gum and graphite carbon nitride carbon aerogel-based composite phase change material and a heat storage and photothermal application substrate. The combination of the phase change material and the substrate is a physical rather than chemical reaction, effectively maintaining the phase change latent heat characteristics of the phase change material.

[0086] 6. A guar gum@graphite carbon nitride carbon aerogel-based composite phase change material and heat storage and photothermal applications have excellent packaging performance and will not leak for at least 1.5 hours at 80°C.

[0087] In this case, XRD, FT-IR, SEM, TEM, DSC and other tests show that:

[0088] like Figure 1 As shown in b, the XRD test results show that with the increase of carbon nitride nanosheets in the carbon aerogel, the half-peak width of the XRD pattern curve gradually narrows, indicating that the carbonization degree of the aerogel gradually increases, proving that the addition of carbon nitride nanosheets can greatly improve the graphitization degree of the carbon aerogel structure. This result shows that the carbon nitride nanosheets are intertwined with the GG molecular chains to successfully prepare GG / g-C3N4 carbon aerogel. In addition, in the XRD spectra of pure PW and composite PCMs with different PW loadings, PW showed typical characteristic peaks of PW at 22° and 24°. The diffraction peaks of the composite PCMs and PW are very similar, indicating that the PW therein has the same crystal structure as pure PW and still maintains excellent phase change behavior. In addition, no new diffraction peaks appear in the XRD spectrum, but the diffraction peak intensity of the composite PCMs with different PW contents is weaker than that of pure PW, which indicates that the crystallization behavior of PW is slightly constrained after being wrapped by the carbon aerogel skeleton;

[0089] like Figure 1 As shown in a, the FT-IR test results show that the 1400-1650 cm -1 The typical stretching vibration mode of the heptazine-derived repeating unit was observed at 1318 cm -1 and 1250cm -1 The stretching vibration mode of the C-NH(-C)-C (complete condensation) and C-NH-C (partial condensation) linker is at 812 cm -1 The strong peak at 2920 cm-1 is the out-of-plane bending vibration characteristic of the heptazine ring. These results indicate that carbon nitride is successfully combined with the aerogel to form an interconnected porous aerogel. In the infrared spectra of pure PW and composite PCMs with different PW loadings, the CH in -CH3 and -CH2 is observed at 2920 cm-1. -1 , 2850cm -1 The stretching vibration characteristic peak is shown at 1378 cm -1 , 1462cm -1 The characteristic peak of bending vibration is shown at 719cm -1 The characteristic peaks at are CH in-plane rocking vibrations. These peaks confirm the presence of long alkyl chains, which is consistent with the PW structure. Observation of the spectra of pure PW and all composite PCMs revealed that the characteristic peaks of the composite PCMs and PW were extremely similar. Except for a slight shift in the peak position, there was no disappearance or generation of characteristic peaks, indicating that PW was successfully adsorbed into the porous network structure of GND-1. Although there was a strong intermolecular hydrogen bond between PW and GND-1, its structural characteristics were not changed.

[0090] The SEM test results show that the GND carbon aerogels all form interconnected porous network structures. The pore diameter gradually decreases with the addition of carbon nitride nanosheets, and the carbon sheet of the carbon aerogel becomes thicker. Figure 2 Figures a, b, c, and d in the figure show that the doped carbon nitride nanosheets uniformly interweave with the surface-active long-chain polymer GG to form the aerogel structure. Furthermore, as the carbon nitride dosage increases, the aerogel surface becomes increasingly rough, which is more conducive to the loading of PCMs. However, when the carbon nitride dosage exceeds 0.05g, the carbon aerogel surface becomes piled up, and adhesion between the sheets occurs, resulting in a decrease in the interlayer spacing, which is not conducive to the adsorption of PCMs.

[0091] The composite PCMs prepared using GND-1 as the loading matrix, with the increase of PW adsorption amount, the pores are gradually filled, and the skeleton structure of the carbon aerogel in the composite PCMs gradually becomes smooth until it is completely filled and disappears, as shown in Figure 2. Figure 3 As shown in a, when the PW loading is 94wt%, the carbon skeleton structure of PCMs-1 is not completely wrapped by PW, and some carbon layers are exposed. Some pore structures of the carbon aerogel are filled but not completely. At this time, due to the low adsorption of PW, the internal structure of the composite PCMs is loose and the gaps are large, so the carbon aerogel has a weak binding force on PW. Figure 3 As shown in Figure b, when the PW loading is 95wt%, the skeleton and pores of the carbon aerogel are obviously wrapped and filled, the PW is tightly combined with the carbon aerogel skeleton structure, and the PW has been filled into most of the internal channels and pores. There is no separation or non-adhesion between the PW and GND-1. Figure 3 As shown in Figure d, when the PW loading reaches 96.89 wt%, indicating excessive loading, the GND-1 structure and pore size are over-coated and filled, with no obvious interface separation between the two. Meanwhile, excess PW accumulates on the surface of the composite PCMSs. This result demonstrates the good compatibility of the carbon aerogel skeleton with PW. The porous network structure provides a large number of loading sites for PW loading, effectively confining the PW and facilitating the preparation of shape-stable composite PCMs.

[0092] like Figure 4 As shown in the DSC test results, the composite PCMs exhibit two peaks during each thermal process, with peak intensity decreasing with decreasing PCM content. The crystallization temperature of the composite PCMs shifts to a lower temperature, while the melting temperature shifts to a higher temperature, relative to pure PW. However, the temperature fluctuations are minor. The crystallization and melting enthalpies of the composite PCMs are 198.64-204.93 J / g and 191.12-202.93 J / g, respectively, close to the theoretical melting values, demonstrating that PW retains its excellent phase transition behavior in the composite PCMs.

[0093] PCMs-3 was subjected to 200 melt-crystallization cycle tests. The DSC curve of PCMs-3 after 200 thermal cycles was essentially the same as the first curve, and the enthalpy retention rate remained almost unchanged, indicating that the GND network has an excellent encapsulation effect and ensures good cycling stability. These results show that GG / g-C3N4 carbon aerogel-based composite PCMs have high energy density and excellent thermal stability, and can be used in thermal energy storage applications.

[0094] The results of thermal conductivity tests using the transient plane heat source method showed that the thermal conductivity of a guar gum / graphite carbon nitride carbon aerogel-based composite phase change material and its applications in heat storage and photothermal applications was 0.5763-0.6599 W / (m·k);

[0095] The leakage performance test results show that a guar gum / graphite carbon nitride carbon aerogel-based composite phase change material and heat storage and photothermal applications have high leakage resistance;

[0096] The results of the light-to-heat conversion performance test show that a guar gum / graphite carbon nitride carbon aerogel-based composite phase change material and heat storage and photothermal applications have excellent light-to-heat conversion capacity (92.95%) and have great potential in the field of thermal energy conversion and storage.

Claims

1. A guar gum@graphite carbon nitride carbon aerogel-based composite phase change material, characterized in that: Carbon nitride g-C3N4, guar gum GG, diammonium phosphate, and paraffin wax (PW) are used as raw materials. The flexible long-chain polymer guar gum GG is interwoven and wrapped around g-C3N4 nanosheets. This is then cross-linked with diammonium phosphate to obtain GG@g-C3N4 carbon aerogel. This carbon aerogel skeleton serves as a heat conduction channel and encapsulation carrier for the phase change material paraffin wax (PW). After the carbon aerogel and paraffin wax are composited, a guar gum@graphite carbon nitride carbon aerogel-based composite phase change material is obtained. The guar gum@graphite carbon nitride carbon aerogel-based composite phase change material has a crystallization latent heat value of 198.64-204.93 J / g and a melting latent heat value of 191.12-202.93 J / g.

2. The guar gum@graphite carbon nitride carbon aerogel-based composite phase change material according to claim 1, characterized in that: The flexible long-chain polymer guar gum GG is interwoven and wrapped with g-C3N4 nanosheets, and then secondary cross-linked with diammonium hydrogen phosphate to obtain GG@g-C3N4 carbon aerogel. Specifically, the carbon nitride nanosheets are hydrogen-bonded with the -OH groups in the guar gum GG, and the carbon nitride nanosheets are tightly bound in the guar gum GG network structure. Then, the -NH3 in the diammonium hydrogen phosphate is used to cross-link the carbon nitride nanosheets. + Through hydrogen bonding with guar gum GG and carbon nitride nanosheets, and under the synergistic effect of guar gum GG, carbon nitride g-C3N4 and diammonium hydrogen phosphate, GG@g-C3N4 carbon aerogel was prepared. The carbon nitride nanosheets in the GG@g-C3N4 carbon aerogel were uniformly and tightly intertwined in the carbon skeleton of the GG@g-C3N4 carbon aerogel, forming a three-dimensional porous network structure with regular and uniform pores.

3. The method for preparing a guar gum@graphite carbon nitride carbon aerogel-based composite phase change material according to claim 1, characterized in that: The steps include: Step 1, preparation of graphite carbon nitride g-C3N4, dicyandiamide and glucose powder were mixed and ground in a mortar at a ratio of 20:1, and then sintered in a N2 atmosphere tube furnace at 3 o C / min and the heating rate was increased to 900 o C and kept warm for 2 h, and calcined to obtain carbon nitride nanosheets; Step 2, preparation of guar gum @ graphite carbon nitride carbon aerogel, adding a certain mass of carbon nitride nanosheet powder to 100 mL of deionized water, ultrasonicating for 1 h to evenly disperse the carbon nitride nanosheet powder, to prevent the guar gum GG powder from sticking, adding guar gum GG under stirring after the ultrasonication, mechanically stirring for 10 h to allow the guar gum GG to be fully dissolved and dispersed in the aqueous solution, then adding diamine hydrogen phosphate of the same mass as the guar gum GG and continuing to stir for 1 h, ultrasonicating for 10 min after the stirring, standing to defoam, putting it in a refrigerator to freeze after defoaming, and then freeze-drying, thereby preparing a carbon nitride-containing aerogel; the aerogel was treated in two steps, first, pre-oxidation to stabilize the microstructure, and the aerogel was placed in a muffle furnace at 0.5 o The preoxidized aerogel was then transferred to a tubular furnace and carbonized in a nitrogen atmosphere in three stages: the first stage was heated to 300 °C at 5 °C / min, the second stage was heated to 400 °C at 0.5 °C / min, and the o C and kept warm for 1 h, and in the final stage, the temperature was raised to 750 ℃ at 5 °C / min and kept warm for 2 h to obtain guar gum@graphite carbon nitride carbon aerogel, namely GND carbon aerogel; Step 3, preparation of guar gum @ graphite carbon nitride carbon aerogel-based composite phase change material, the GND carbon aerogel is fully dried, and paraffin wax PW with a mass percentage of 94 wt%, 95 wt%, 96 wt% and 96.89 wt% of the sum of the mass of GND carbon aerogel and the composite phase change material is weighed respectively and placed in different beakers and heated to melt, and then the GND carbon aerogel is transferred to beakers containing different mass percentages of paraffin wax PW and vacuum impregnated at 80 ° C for 24 h to obtain different guar gum @ graphite carbon nitride carbon aerogel-based composite phase change materials.

4. The guar gum@graphite carbon nitride carbon aerogel-based composite phase change material according to claim 1, characterized in that: The thermal conductivity of the guar gum@graphite carbon nitride carbon aerogel-based composite phase change material is 0.5763-0.6599 W / (m·K).

5. A guar gum@graphite carbon nitride carbon aerogel-based composite phase change material used as a composite phase change material in heat storage and photothermal applications, characterized in that: The photothermal conversion efficiency is 92.95%.

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