Composite phase change material based on porous wood and preparation method thereof

Abstract

The invention provides a porous wood-based composite phase change material, which comprises a porous wood gel frame and a phosphorus-modified flame-retardant phase change material, the phosphorus-modified flame-retardant phase change material is impregnated into the porous wood gel frame, the porous wood gel frame is prepared by impregnating a Mxene two-dimensional material with porous wood gel, and the porous wood gel frame is a porous wood gel-impregnated Mxene two-dimensional material. The adding amount of the Mxene two-dimensional material accounts for 1-20% of the mass of the porous wood gel. The invention further discloses a preparation method of the composite phase change material based on the porous wood. The composite material has the beneficial effects that the problem of leakage of the phase change material is effectively solved by utilizing the porous structure of wood and the photothermal conversion characteristic of MXene, meanwhile, the thermal conductivity and long-term durability of the material are improved, and a new solution is provided for a solar heat utilization and energy-saving system.

Description

Technical Field

[0001] The present invention relates to the field of phase change materials, and in particular to a composite phase change material based on porous wood with excellent light and heat energy storage performance, flame retardancy and thermal conductivity, and a preparation method thereof. Background Art

[0002] With the continuous growth of global energy demand and the increasingly severe environmental problems, it is particularly important to develop energy and improve energy utilization efficiency. As a clean and renewable energy source, the efficient utilization of solar energy is of great significance to alleviating the energy crisis and reducing environmental pollution. Phase change materials (PCMs) have been widely studied for use in solar thermal energy storage and thermal management due to their ability to absorb or release a large amount of latent heat during the phase change process. However, traditional phase change materials usually have problems such as leakage, flammability and poor thermal conductivity, which limit their practical application range. Therefore, the development of composite phase change materials, especially materials with high safety, high energy storage efficiency and good thermal conductivity, is of great significance for the effective utilization of solar thermal energy.

[0003] As a natural porous material, wood shows great application potential in the field of energy storage and conversion due to its good biodegradability and renewability. In particular, porous wood that has undergone chemical treatment and structural modification, such as delignified wood (DW), not only retains the porous structure of wood, but also provides more active sites and a larger specific surface area by removing part of the lignin and hemicellulose, which is beneficial to the encapsulation and performance improvement of phase change materials. In addition, by introducing two-dimensional materials with excellent photothermal conversion performance and flame retardant properties, such as MXene, the thermal stability and flame retardant properties of the composite material can be further improved. However, how to effectively combine these materials to prepare a composite material with both high energy storage efficiency and good flame retardant properties remains a technical challenge that needs to be solved.

[0004] In the existing technology, although the photothermal energy storage, flame retardancy and thermal conductivity have been greatly improved, the composite phase change material is a medium-temperature material and its application in low-temperature and high-temperature environments is subject to certain limitations. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned prior art and provide a composite phase change material based on porous wood, which is prepared by physical impregnation and chemical grafting and has the advantages of easy encapsulation, high energy storage efficiency, high photothermal conversion efficiency, excellent thermal stability, and excellent flame retardant properties.

[0006] Another object of the present invention is to provide a method for preparing the composite phase change material based on porous wood, which is prepared by physical impregnation and chemical grafting, has a wide range of raw material sources, low economic cost and simple synthesis process.

[0007] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is: a composite phase change material based on porous wood, comprising a porous wood gel framework (MW) and a phosphorus-modified flame-retardant phase change material (P-SAL), wherein the phosphorus-modified flame-retardant phase change material (P-SAL) is impregnated into the porous wood gel framework (MW), and the porous wood gel framework (MW) is made of a porous wood gel (LW) impregnated with a MXene two-dimensional material, and the amount of the MXene two-dimensional material added is 1-20% of the mass of the porous wood gel (LW).

[0008] The phosphorus-modified flame-retardant phase change material is prepared by reacting POCl3 and stearyl alcohol in toluene.

[0009] A method for preparing a composite phase change material based on porous wood comprises the following steps: (1) Preparation of porous wood gel (LW); (2) Preparation of MXene dispersion; (3) Preparation of porous wood gel framework (MW): MXene dispersion was added dropwise into the porous wood gel (LW) using a pipette, and the amount of MXene added was controlled based on the mass percentage of MXene in the porous wood gel (LW). (4) Preparation of composite phase change material (MW-P-SAL-x): After the phosphorus-modified flame-retardant phase change material (P-SAL) is melted into liquid, the (P-SAL) molten liquid is impregnated into the porous wood gel framework (MW).

[0010] The porous wood gel (LW) is prepared by ultrasonically treating wood in a 90wt% ethanol aqueous solution and then drying it; immersing the wood in an alkaline mixed solution containing approximately 7wt% sodium hydroxide and 12wt% urea for 30 minutes; freezing the wood at -13°C for 24 hours, then removing the wood and placing it in a 60°C oven for 30 minutes; and then immersing the treated wood in a 1wt%-20wt% tert-butyl alcohol solution four times. The resulting sample is then frozen at -40°C and freeze-dried (-60°C) for at least 12 hours to produce the aerogel wood.

[0011] The wood is preferably balsa wood, which is cut into wood blocks with a size of 40 mm×40 mm×30 mm.

[0012] The preparation method of the porous wood gel framework (MW) includes the following steps: adding a MXene dispersion dropwise into the porous wood gel (LW) using a pipette, and impregnating under vacuum conditions, wherein the amount of MXene added is controlled based on the mass percentage of MXene in the porous wood gel (LW).

[0013] The preparation method of the MXene dispersion includes the following steps: first, 2g of MAX phase powder is slowly added to 40mL of 9mol / L HCl aqueous solution at a constant temperature of 38°C, and the reaction is continued for 48 hours under magnetic stirring to ensure that the MAX phase powder is fully in contact with the HCl and an etching reaction occurs; after the reaction is completed, the precipitated reaction mixture is collected by centrifugation; it is washed three times with 1mol / L HCl solution and 1mol / L LiCl solution to remove impurities and residual chemical reagents generated during the reaction process; then, the reaction mixture particles are repeatedly rinsed with deionized water until the pH of the centrifuged product of the reaction mixture is close to neutral (about 6.0) to ensure the purity and stability of the final product. In order to further improve the dispersibility of the MXene nanosheets, 100mL of deionized water is added to the treated reaction mixture, and the mixture is mechanically shaken using a vibrator, and then manually shaken for 10 minutes to completely disperse the aggregated MXene particles. The reaction mixture solution was then placed in a 250 mL plastic bottle, the bottle mouth was sealed with parafilm and filled with nitrogen to exclude oxygen interference, and then the plastic bottle was placed in an ultrasonic cleaner and sonicated in a 20 ° C water bath for 10 minutes. Ultrasonic treatment effectively broke the van der Waals force between the MXene nanosheets, allowing them to be evenly and stably dispersed in the solvent. Finally, the sonicated dispersion was aliquoted into a centrifuge tube and centrifuged at 3500 rpm at 20 ° C for 1 hour to remove undispersed larger particles and impurities to obtain a high-quality MXene dispersion.

[0014] The centrifuge used was a high-speed centrifuge model TG16B produced by Hunan Kaida Co., Ltd. The ultrasonic cleaning instrument used was a product model SK3300HP produced by Shanghai Kedao Ultrasonic Instrument Co., Ltd.

[0015] The MAX phase (titanium silicon carbide) powder is a product produced by Forsman Technology (Beijing) Co., Ltd. with product model 2202-82-3.

[0016] The preparation method of the composite phase change material (MW-P-SAL-x) includes the following steps: melting a phosphorus-modified flame-retardant phase change material (P-SAL) into a liquid, immersing a porous wood gel frame (MW) in the phosphorus-modified flame-retardant phase change material (P-SAL) molten liquid, and maintaining it under vacuum and a negative pressure of -1 MPa for 1 hour; fully penetrating it in a 95°C forced air oven for 2 hours, and then wiping off excess phosphorus-modified flame-retardant phase change material on the surface with filter paper.

[0017] The preparation method of the phosphorus-modified flame-retardant phase change material (P-SAL) includes the following steps: dissolving 15.33g of POCl3 and 27.05g of stearyl alcohol in 40ml of toluene in the following proportions, stirring at 25°C for 2 hours, then heating to 60°C and stirring for an additional 8 hours; dripping the reactants dropwise into acetonitrile at approximately -10°C, collecting the reactants by vacuum filtration, and washing with glacial acetonitrile; and drying in a vacuum at approximately 25°C for 24 hours. The stearyl alcohol is produced by Shanghai Aladdin Biochemical Technology Co., Ltd., and the POCl3 is sold by Sinopharm Chemical Reagent Co., Ltd.

[0018] The porous wood is natural wood with a nanocellulose network, which provides a carrier for the uniform loading of MXene and can effectively overcome the leakage of phosphorus-modified flame-retardant phase change material (P-SAL).

[0019] The phosphorus-modified flame-retardant phase change material (P-SAL) is prepared by dissolving POCl3 and stearyl alcohol in toluene, with a yield of about 85%.

[0020] The MXene is a 2D stacked layered material composed of transition metal carbides and carbonitrides, obtained by etching and exfoliation of the MAX phase (titanium aluminum carbide).

[0021] The composite phase change material (MW-P-SAL) is prepared by modifying MXene onto a porous wood gel (LW) by a vacuum impregnation method, and then impregnating POCl3-modified stearyl alcohol (PSAL) into the porous wood gel framework (MW) by a vacuum impregnation method.

[0022] The composite phase change material of the present invention utilizes the strong surface tension and capillary force of the wood support material. The composite phase change material (MW-P-SAL) can effectively wrap the molten phosphorus-modified stearyl alcohol and inhibit its leakage. The wood treated by freeze-salting partially removes the lignin in the wood pores, leaving a lignin gel, thereby enhancing the long-term durability of the composite phase change material (MW-P-SAL).

[0023] In the composite phase change material described in the present invention, the temperature at which MW-P-SAL-10 loses 5% of its mass is only 241.4°C, and the carbon residue is 19.72%. The melting enthalpy and solidification enthalpy reach 148.8 kJ / kg and 149.62 kJ / kg respectively, with excellent thermal conversion performance and stability.

[0024] The present invention prepares a wood gel with a large amount of cellulose nano-network in the micropores by salting out and freezing. Then, MXene is modified onto the porous wood gel (LW) by vacuum impregnation to prepare a porous wood gel framework (MW). Subsequently, POCl3-modified stearyl alcohol (PSAL) is impregnated into the porous wood gel framework (MW) by vacuum impregnation to successfully prepare a composite phase change material (MW-P-SAL-x) with excellent photothermal energy storage performance, flame retardancy and thermal conductivity. The porous wood gel framework (MW) has good surface tension and capillary force, which can well support stearyl alcohol (PSAL). The nanocellulose network of the porous wood gel (LW) also provides a carrier for the uniform loading of MXene, which can not only overcome the leakage of stearyl alcohol (PSAL) but also enhance the thermal conductivity to a certain extent. The manufacturing method is simple to operate, does not require high-precision and expensive equipment, uses low-cost materials, and is easy to promote and apply.

[0025] In general, compared with the prior art, the present invention has the following beneficial effects: (1) Encapsulating flame retardant materials and phase change materials in porous materials by physical impregnation and chemical grafting to form a composite material with high flame retardancy and thermal conductivity; (2) The raw materials required for the synthesis of the materials are widely available, the composite materials are environmentally friendly and the operation process is relatively simple; (3) This composite material utilizes the porous structure of wood and the photothermal conversion properties of MXene to effectively solve the leakage problem of phase change materials, while improving the thermal conductivity and long-term durability of the material, providing a new solution for solar thermal utilization and energy-saving systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 X-ray diffraction spectra of composite phase change materials based on porous materials prepared in Examples 1-4 of the present invention;

[0027] Figure 2 Fourier infrared spectra of the composite phase change materials based on porous materials prepared in Examples 1-4 of the present invention;

[0028] Figure 3 DSC test graphs (cooling crystallization curve and heating melting curve) of the composite phase change material based on porous material prepared in Examples 1-4 of the present invention;

[0029] Figure 4 This is a surface scanning electron microscope image of the composite phase change material based on porous material in Example 2;

[0030] Figure 5 This is an axial scanning electron microscope image of the composite phase change material based on porous material in Example 2. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below with reference to the accompanying drawings and specific examples, but the examples given do not constitute a limitation to the scope of protection claimed in the present invention. The following numerical parameters of this application are only the values ​​for verifying the examples. If the numerical ratios published in this application are amplified and implemented, the corresponding target products of this application can also be obtained. Among them, it is worth noting that since the porous wood gel (LW) in different embodiments is immersed in MXene solutions of different concentrations, the test results of each embodiment may be different. In order to show these differences more accurately, the data are displayed in the following arrangement: Example 1: Using a 5% concentration of MXene solution, the corresponding Figure 1 、 Figure 2 、 Figure 3 The curve marked with MW-P-SAL-5. Example 2: Using a 10% concentration of MXene solution, the corresponding Figure 1 、 Figure 2 、 Figure 3 The curve labeled MW-P-SAL-10. Example 3: Using a 15% concentration of MXene solution, the corresponding Figure 1 、 Figure 2 、 Figure 3 The curve labeled MW-P-SAL-15. Example 4: Using a 20% concentration of MXene solution, the corresponding Figure 1 、 Figure 2 、 Figure 3 The curve marked with MW-P-SAL-20. Example 1

[0032] A method for preparing a composite phase change material based on porous wood comprises the following steps: (1) Preparation of porous wood gel (LW): Balsa wood (cut into blocks of 4.0 cm × 4.0 cm × 0.3 cm) was treated with ultrasound at a frequency of 53 kHz for 20 min in a 90 wt% ethanol aqueous solution and then transferred to a fume hood to dry for 2 h. The dried wood block was immersed in an alkaline mixed solution containing 7 wt% sodium hydroxide and 12 wt% urea at room temperature for 30 min until it was completely penetrated by the alkaline salt solution. The entire solution containing the wood was frozen at -13°C for 24 h. The balsa wood was removed and placed at 60°C for 30 min. The treated balsa wood was introduced into a 20 wt% tert-butyl alcohol solution for immersion exchange. After soaking in the tert-butyl alcohol solution for 30 min, the wood block was removed and immersed in deionized water for 30 min. The above operation was repeated four times. The obtained sample was frozen at -40°C and then freeze-dried (-60°C) for 12 h to obtain aerogel wood. The aerogel wood was frozen at -40°C for 24 hours, then placed in a freeze dryer and freeze-dried at -80°C and a pressure of <1 Pa for 12 hours to obtain the first intermediate product 1LW. (2) Preparation of MXene dispersion: First, 2 g of MAX phase powder was slowly added to 40 mL of 9 mol / L HCl aqueous solution at a constant temperature of 38 °C, and the reaction was continued under magnetic stirring for 48 h. After the reaction was completed, the precipitated reaction mixture was collected by centrifugation and washed three times with 1 mol / L HCl solution and 1 mol / L LiCl solution. Subsequently, the reaction mixture particles were repeatedly rinsed with deionized water until the pH value of the centrifuged product of the reaction mixture was 6.0. 100 mL of deionized water was added to the treated reaction mixture, and the mixture was mechanically shaken using a vibrator for 10 min, and then manually shaken for 10 min to completely disperse the aggregated MXene particles. Then, the reaction mixture solution was placed in a 250 mL plastic bottle, the bottle mouth was sealed with a sealing film (parafilm) and filled with nitrogen to eliminate oxygen interference, and then the plastic bottle was placed in an ultrasonic cleaner and ultrasonically treated at a frequency of 53 kHz for 10 minutes in a 20°C water bath; finally, the ultrasonicated dispersion was aliquoted into a centrifuge tube and centrifuged at 3500 rpm for 1 hour at 20°C; the MXene dispersion was diluted to 2 mg / ml; and the second intermediate product 2MXene dispersion was prepared. (3) Preparation of porous wood gel framework (MW): The MXene dispersion was added dropwise into the porous wood gel (LW) using a pipette. The amount of MXene added was 5% of the mass of the porous wood gel (LW), and the third intermediate product MW-5 was obtained. (4) Preparation of phosphorus-modified flame-retardant phase change material (P-SAL): Dissolve 15.33 g of POCl3 and 27.05 g of stearyl alcohol in 40 ml of toluene according to the following proportions, and stir at 25 °C for 2 h; heat to 60 °C and stir for another 8 h; after completion, add the reactants dropwise into acetonitrile at about -10 °C, collect the reactants by vacuum filtration, and wash them three times with acetonitrile to obtain the fourth intermediate product P-SAL. (5) Preparation of composite phase change material (MW-P-SAL-5): Phosphorus modified flame retardant phase change material (P-SAL) was melted into liquid at 95°C, and a porous wood gel frame (MW) with a mass percentage of 5% of MXenen was immersed in the molten liquid of phosphorus modified flame retardant phase change material (P-SAL), and maintained under vacuum and negative pressure of -1MPa for 1h. The whole process can eliminate the air inside the micropores of the porous wood gel frame (MW), which is conducive to the encapsulation of phosphorus modified flame retardant phase change material (P-SAL). The above operation was repeated 6 times and the impregnation was completed; it was fully penetrated in a 95°C blast oven for 2h, and then the excess phosphorus modified flame retardant phase change material (P-SAL) on the surface was wiped off with filter paper to obtain a shape-stable composite phase change material (MW-P-SAL-5).

[0033] In this example, after MXene is modified onto a porous wood gel (LW), phosphorus-modified stearyl alcohol (P-SAL) is impregnated into the porous wood gel framework (MW) by vacuum impregnation to prepare a shape-stable composite phase change material (MW-P-SAL-5). The XRD spectrum of the obtained composite phase change material (MW-P-SAL-5) sample was obtained. Figure 1 ,like Figure 1 As shown in the middle curve, the characteristic peaks of phosphorus-modified stearyl alcohol (P-SAL) and Mxene appear in the composite material, indicating that the synthesis is successful.

[0034] The above composite phase change material (MW-P-SAL-5) sample was subjected to Fourier infrared testing and the following results were obtained: Figure 2 result, Figure 2 Comparison of the middle curve shows a new infrared characteristic absorption peak produced by the reaction, indicating that the synthesis was successful.

[0035] The raw materials and composite phase change material (MW-P-SAL-5) samples were subjected to DSC tests respectively, and the results were as follows Figure 3 As shown, through Figure 3 Comparison of the middle curves shows that the phase transition temperature has almost no change compared to the phase transition temperature of phosphorus-modified stearyl alcohol (P-SAL), and the Mxene content has little effect on the enthalpy value of the composite phase change material (MW-P-SAL-5). Example 2

[0036] A method for preparing a composite phase change material based on porous wood comprises the following steps: (1) Preparation of porous wood gel (LW): Balsa wood (4.0 cm × 4.0 cm × 0.3 cm) was treated with ultrasound at a frequency of 53 kHz in 90 wt% ethanol for 20 min and then transferred to a fume hood to dry for 2 h. The dry wood block was first immersed in an alkaline mixed solution containing 7 wt% sodium hydroxide and 12 wt% urea at room temperature for 30 min. Subsequently, the entire solution containing the wood was frozen at -13 ° C for 24 h, then taken out and placed at 60 ° C for 30 min. Then, the treated balsa wood was introduced into a 20 wt% tert-butyl alcohol solution for immersion exchange. After soaking in the tert-butyl alcohol solution for 30 min, it was taken out and soaked in deionized water for 30 min. This operation was repeated four times. The obtained sample was frozen at -40 ° C and then freeze-dried (-60 ° C) for 12 h to obtain aerogel wood. The replaced wood block was frozen at -40°C, placed in a freeze dryer, and freeze-dried at -80°C and a pressure of <1 Pa for 12 h to obtain the first intermediate product 1LW. (2) Preparation of MXene dispersion: First, 2 g of MAX phase powder was slowly added to 40 mL of 9 mol / L HCl aqueous solution at a constant temperature of 38 °C, and the reaction was continued under magnetic stirring for 48 h. After the reaction was completed, the precipitated reaction mixture was collected by centrifugation and washed three times with 1 mol / L HCl solution and 1 mol / L LiCl solution. Subsequently, the reaction mixture particles were repeatedly rinsed with deionized water until the pH value of the centrifuged product of the reaction mixture was 6.0. 100 mL of deionized water was added to the treated reaction mixture, and the mixture was mechanically shaken using a vibrator for 10 min, and then manually shaken for 10 min to completely disperse the aggregated MXene particles. Then, the reaction mixture solution was placed in a 250 mL plastic bottle, the bottle mouth was sealed with a sealing film (parafilm) and filled with nitrogen to eliminate oxygen interference, and then the plastic bottle was placed in an ultrasonic cleaner and ultrasonically treated at a frequency of 53 kHz for 10 minutes in a 20°C water bath; finally, the ultrasonicated dispersion was aliquoted into a centrifuge tube and centrifuged at 3500 rpm for 1 hour at 20°C; the MXene dispersion was diluted to 2 mg / ml; and the second intermediate product 2MXene dispersion was prepared. (3) Preparation of MW framework: The MXene dispersion was added dropwise into the porous wood gel (LW) using a pipette. The amount of MXene added was 10% of the mass of the porous wood gel (LW) to obtain the third intermediate product MW-10. (4) Preparation of phosphorus-modified flame-retardant phase change material (P-SAL): Preparation of phosphorus-modified stearyl alcohol (P-SAL): Dissolve 15.33g POCl3 and 27.05g stearyl alcohol in 40ml toluene and stir at 25°C for 2h; heat to 60°C and stir for another 8h; after completion, add the reactants dropwise into acetonitrile at about -10°C, collect the reactants by vacuum filtration, and wash them with acetonitrile for 3 times to obtain the fourth intermediate product P-SAL. (5) Preparation of composite phase change material (MW-P-SAL-10): The phosphorus-modified flame-retardant phase change material (P-SAL) was melted into a liquid at 95°C, and a porous wood gel frame (MW) with a mass percentage of MXenen of 10% was immersed in the molten liquid of the phosphorus-modified flame-retardant phase change material (P-SAL). The mixture was kept in a vacuum and at a negative pressure of -1 MPa for 1 hour. The whole process can eliminate the air inside the micropores of the porous wood gel frame (MW), which is beneficial to the encapsulation of the phosphorus-modified flame-retardant phase change material (P-SAL). The above operation was repeated 6 times and the impregnation was completed. The mixture was fully infiltrated in a 95°C blast oven for 2 hours, and then the excess phosphorus-modified flame-retardant phase change material (P-SAL) on the surface was wiped off with filter paper to obtain a shape-stable composite phase change material (MW-P-SAL-10).

[0037] In this example, after modifying the porous wood gel (LW) with Mxene, phosphorus-modified stearyl alcohol (P-SAL) was impregnated into the porous wood gel framework (MW) by vacuum impregnation to prepare a shape-stable composite phase change material (MW-P-SAL-10). The XRD spectrum of the obtained composite phase change material (MW-P-SAL-10) sample was obtained. Figure 1 ,like Figure 1 As shown in the middle curve, the characteristic peaks of phosphorus-modified stearyl alcohol (P-SAL) and Mxene appear in the composite material, indicating that the synthesis is successful.

[0038] The above composite phase change material (MW-P-SAL-10) sample was subjected to Fourier infrared testing and the following results were obtained: Figure 2 result, Figure 2 Comparison of the middle curve shows a new infrared characteristic absorption peak produced by the reaction, indicating that the synthesis was successful.

[0039] The raw materials and composite phase change material (MW-P-SAL-10) samples were subjected to DSC tests respectively, and the results were as follows Figure 3 As shown, through Figure 3 Comparison of the middle curves shows that the phase transition temperature has almost no change compared to that of phosphorus-modified stearyl alcohol (P-SAL), and the enthalpy value is higher, with the melting enthalpy reaching 148.8 kJ / kg and the solidification enthalpy reaching 149.62 kJ / kg.

[0040] The above composite phase change material (MW-P-SAL-10) sample was tested by scanning electron microscopy and the following results were obtained: Figure 4 and Figure 5 As shown, it shows that phosphorus-modified stearyl alcohol (P-SAL) is successfully encapsulated in the porous wood gel framework (MW), and phosphorus-modified stearyl alcohol (P-SAL) can uniformly fill the pores. Example 3

[0041] A method for preparing a composite phase change material based on porous wood comprises the following steps: (1) Preparation of porous wood gel (LW): Balsa wood (cut into blocks of 4.0 cm × 4.0 cm × 0.3 cm) was treated with ultrasound at a frequency of 53 kHz for 20 min in a 90 wt% ethanol aqueous solution and then transferred to a fume hood to dry for 2 h. The dried wood block was immersed in an alkaline mixed solution containing 7 wt% sodium hydroxide and 12 wt% urea at room temperature for 30 min until it was completely penetrated by the alkaline salt solution. The entire solution containing the wood was frozen at -13°C for 24 h. The balsa wood was removed and placed at 60°C for 30 min. The treated balsa wood was introduced into a 20 wt% tert-butyl alcohol solution for immersion exchange. After soaking in the tert-butyl alcohol solution for 30 min, the wood block was removed and immersed in deionized water for 30 min. The above operation was repeated four times. The obtained sample was frozen at -40°C and then freeze-dried (-60°C) for 12 h to obtain aerogel wood. The aerogel wood was frozen at -40 °C for 24 h, then placed in a freeze dryer and freeze-dried at -80 °C and pressure <1 Pa for 12 h to obtain the first intermediate product 1LW. (2) Preparation of MXene dispersion: First, 2 g of MAX phase powder was slowly added to 40 mL of 9 mol / L HCl aqueous solution at a constant temperature of 38°C, and the reaction was continued under magnetic stirring for 48 hours. After the reaction was completed, the precipitated reaction mixture was collected by centrifugation and washed three times with 1 mol / L HCl solution and 1 mol / L LiCl solution. Subsequently, the reaction mixture particles were repeatedly rinsed with deionized water until the pH value of the centrifuged product of the reaction mixture was 6.0. 100 mL of deionized water was added to the treated reaction mixture, and the mixture was mechanically shaken using a vibrator for 10 minutes, and then manually shaken for 10 minutes to completely disperse the aggregated MXene particles. Then, the reaction mixture solution was placed in a 250 mL plastic bottle, the bottle mouth was sealed with a sealing film (parafilm) and filled with nitrogen to eliminate oxygen interference, and then the plastic bottle was placed in an ultrasonic cleaner and ultrasonically treated at a frequency of 53 kHz for 10 minutes in a 20°C water bath; finally, the ultrasonicated dispersion was aliquoted into a centrifuge tube and centrifuged at 3500 rpm for 1 hour at 20°C; the MXene dispersion was diluted to 2 mg / ml; and the second intermediate product 2MXene dispersion was prepared. (3) Preparation of MW framework: The MXene dispersion was added dropwise into the porous wood gel (LW) using a pipette. The amount of MXene added was 15% of the mass of the porous wood gel (LW) to obtain the third intermediate product MW-15. (4) Preparation of phosphorus-modified flame-retardant phase change material (P-SAL): Dissolve 15.33 g of POCl3 and 27.05 g of stearyl alcohol in 40 ml of toluene according to the following proportions, and stir at 25 °C for 2 h; heat to 60 °C and stir for another 8 h; after completion, add the reactants dropwise into acetonitrile at about -10 °C, collect the reactants by vacuum filtration, and wash them three times with acetonitrile to obtain the fourth intermediate product P-SAL. (5) Preparation of composite phase change material (MW-P-SAL-10): Phosphorus-modified flame-retardant phase change material (P-SAL) was melted into liquid at 95°C, and a porous wood gel frame (MW) with a mass percentage of 5% of MXenen was immersed in the molten liquid of phosphorus-modified flame-retardant phase change material (P-SAL). The mixture was kept in vacuum and at a negative pressure of -1 MPa for 1 hour. The whole process can eliminate the air inside the micropores of the porous wood gel frame (MW), which is conducive to the encapsulation of the phosphorus-modified flame-retardant phase change material (P-SAL). The above operation was repeated 6 times and the impregnation was completed. The mixture was fully infiltrated in a 95°C blast oven for 2 hours, and then the excess phosphorus-modified flame-retardant phase change material (P-SAL) on the surface was wiped off with filter paper to obtain a shape-stable composite phase change material (MW-P-SAL-15).

[0042] In this example, after MXene is modified onto a porous wood gel (DW), phosphorus-modified stearyl alcohol (P-SAL) is impregnated into the porous wood gel framework (MW) by vacuum impregnation to prepare a shape-stable composite phase change material (MW-P-SAL-15). The XRD spectrum of the obtained composite phase change material (MW-P-SAL-15) sample was obtained. Figure 1 ,like Figure 1 As shown in the middle curve, the characteristic peaks of phosphorus-modified stearyl alcohol (P-SAL) and Mxene appear in the composite material, indicating that the synthesis is successful.

[0043] The above composite phase change material (MW-P-SAL-15) sample was subjected to Fourier infrared testing and the following results were obtained: Figure 2 result, Figure 2 Comparison of the middle curve shows a new infrared characteristic absorption peak produced by the reaction, indicating that the synthesis was successful.

[0044] The raw materials and composite phase change material (MW-P-SAL-15) samples were subjected to DSC tests respectively, and the results were as follows Figure 3 As shown, through Figure 3Comparison of the middle curves shows that the phase transition temperature has almost no change compared with that of phosphorus-modified stearyl alcohol (P-SAL), and the Mxene content has little effect on the enthalpy value of MW-P-SAL-15. Example 4

[0045] A method for preparing a composite phase change material based on porous wood comprises the following steps: (1) Preparation of porous wood gel (LW): Balsa wood (cut into blocks of 4.0 cm × 4.0 cm × 0.3 cm) was treated with ultrasound at a frequency of 53 kHz for 20 min in a 90 wt% ethanol aqueous solution and then transferred to a fume hood to dry for 2 h. The dried wood blocks were immersed in an alkaline mixed solution containing 7 wt% sodium hydroxide and 12 wt% urea at room temperature for 30 min until they were completely penetrated by the alkaline salt solution. The entire solution containing the wood was frozen at -13°C for 24 h. The balsa wood was removed and placed at 60°C for 30 min. The treated balsa wood was introduced into a 20 wt% tert-butyl alcohol solution for immersion exchange. After soaking in the tert-butyl alcohol solution for 30 min, the wood blocks were removed and immersed in deionized water for 30 min. The above operation was repeated four times. The obtained sample was frozen at -40°C and then freeze-dried (-60°C) for 12 h to obtain aerogel wood. The aerogel wood was frozen at -40°C for 24 hours, then placed in a freeze dryer and freeze-dried at -80°C and a pressure of <1 Pa for 12 hours to obtain the first intermediate product 1LW. (2) Preparation of MXene dispersion: First, 2 g of MAX phase powder was slowly added to 40 mL of 9 mol / L HCl aqueous solution at a constant temperature of 38 °C, and the reaction was continued under magnetic stirring for 48 h. After the reaction was completed, the precipitated reaction mixture was collected by centrifugation and washed three times with 1 mol / L HCl solution and 1 mol / L LiCl solution. Subsequently, the reaction mixture particles were repeatedly rinsed with deionized water until the pH value of the centrifuged product of the reaction mixture was 6.0. 100 mL of deionized water was added to the treated reaction mixture, and the mixture was mechanically shaken using a vibrator for 10 min, and then manually shaken for 10 min to completely disperse the aggregated MXene particles. Then, the reaction mixture solution was placed in a 250 mL plastic bottle, the bottle mouth was sealed with a sealing film (parafilm) and filled with nitrogen to eliminate oxygen interference, and then the plastic bottle was placed in an ultrasonic cleaner and ultrasonically treated at a frequency of 53 kHz for 10 minutes in a 20°C water bath; finally, the ultrasonicated dispersion was aliquoted into a centrifuge tube and centrifuged at 3500 rpm for 1 hour at 20°C; the MXene dispersion was diluted to 2 mg / ml; and the second intermediate product 2MXene dispersion was prepared. (3) Preparation of porous wood gel framework (MW): The MXene dispersion was added dropwise into the porous wood gel (LW) using a pipette. The amount of MXene added was 20% of the mass of the porous wood gel (LW), and the third intermediate product MW-20 was obtained. (4) Preparation of phosphorus-modified flame-retardant phase change material (P-SAL): Dissolve 15.33 g of POCl3 and 27.05 g of stearyl alcohol in 40 ml of toluene according to the following proportions, and stir at 25 °C for 2 h; heat to 60 °C and stir for another 8 h; after completion, add the reactants dropwise into acetonitrile at about -10 °C, collect the reactants by vacuum filtration, and wash them three times with acetonitrile to obtain the fourth intermediate product P-SAL. (5) Preparation of composite phase change material (MW-P-SAL-20): Phosphorus-modified flame-retardant phase change material (P-SAL) was melted into liquid at 95°C, and a porous wood gel frame (MW) with a mass percentage of 5% of MXenen was immersed in the molten liquid of phosphorus-modified flame-retardant phase change material (P-SAL). The mixture was kept in vacuum and at a negative pressure of -1 MPa for 1 hour. The whole process can eliminate the air inside the micropores of the porous wood gel frame (MW), which is conducive to the encapsulation of the phosphorus-modified flame-retardant phase change material (P-SAL). The above operation was repeated 6 times and the impregnation was completed. The mixture was fully infiltrated in a 95°C blast oven for 2 hours, and then the excess phosphorus-modified flame-retardant phase change material (P-SAL) on the surface was wiped off with filter paper to obtain a shape-stable composite phase change material (MW-P-SAL-20).

[0046] In this example, after MXene is modified onto a porous wood gel (LW), phosphorus-modified stearyl alcohol (P-SAL) is impregnated into the porous wood gel framework (MW) by vacuum impregnation to prepare a shape-stable composite phase change material (MW-P-SAL-20). The XRD spectrum of the obtained composite phase change material (MW-P-SAL-20) sample was obtained. Figure 1 ,like Figure 1 As shown in the middle curve, the characteristic peaks of phosphorus-modified stearyl alcohol (P-SAL) and Mxene appear in the composite material, indicating that the synthesis is successful.

[0047] The above composite phase change material (MW-P-SAL-20) sample was subjected to Fourier infrared testing and the following results were obtained: Figure 2 result, Figure 2 Comparison of the middle curve shows a new infrared characteristic absorption peak produced by the reaction, indicating that the synthesis was successful.

[0048] The raw materials and composite phase change material (MW-P-SAL-20) samples were subjected to DSC tests respectively, and the results were as follows Figure 3 As shown, through Figure 3Comparison of the middle curves shows that the phase transition temperature has almost no change compared with that of phosphorus-modified stearyl alcohol (P-SAL), and the Mxene content has little effect on the enthalpy value of MW-P-SAL-20.

[0049] By measuring the thermal stability of the multi-composite phase change materials prepared in Examples 1-4, it can be seen that different concentrations of MXene solutions may lead to differences in the thermal stability, thermal conductivity and other indicators of the materials. It can be seen that as the concentration of the MXene solution increases, the thermal stability of the material continues to improve. The specific values ​​are shown in the following table: index Example 1 Example 2 Example 3 Example 4 <![CDATA[T -5％ (℃)]]> 241.4 254.5 251.8 259.0 <![CDATA[T max (℃)]]> 317 335 330.8 310 <![CDATA[R max (% / min)]]> -50.34 -118.77 -138.48 -143.15 Char residues at 800℃(%) 18.14％ 19.72％ 13.31％ 17.59％

[0050] The above is only a preferred embodiment of the present invention and should not be used to limit the scope of implementation of the present invention. In other words, any simple equivalent changes and modifications made according to the scope of the patent application and the content of the invention description are still within the scope of the patent of the present invention.

Claims

1. A composite phase change material based on porous wood, characterized in that: The invention comprises a porous wood gel frame and a phosphorus-modified flame-retardant phase-change material, wherein the phosphorus-modified flame-retardant phase-change material is impregnated into the porous wood gel frame, and the porous wood gel frame is made of porous wood gel impregnated with MXene two-dimensional material, and the addition amount of MXene two-dimensional material accounts for 1-20% of the mass of the porous wood gel.

2. The porous wood-based composite phase change material according to claim 1, characterized in that: The phosphorus-modified flame-retardant phase change material is prepared by reacting POCl3 and stearyl alcohol in toluene.

3. The method for preparing a composite phase change material based on porous wood according to claim 1 or 2, characterized in that: The steps involved are as follows: (1) Preparation of porous wood gel; (2) Preparation of MXene dispersion; (3) Preparation of porous wood gel framework: immersing MXene dispersion into porous wood gel; (4) Preparation of composite phase change material: The phosphorus-modified flame-retardant phase change material is melted into a liquid and then impregnated into the porous wood gel framework.

4. The method for preparing a composite phase change material based on porous wood according to claim 3, characterized in that: The preparation of the porous wood gel comprises the following steps: placing wood in an ethanol aqueous solution with a concentration of 90wt% for ultrasonic treatment, and then drying; immersing the wood in an alkaline mixed solution containing a concentration of about 7wt% sodium hydroxide and 12wt% urea for 30 minutes; freezing the wood at -13°C for 24 hours, then removing the wood and placing it in a 60°C oven for 30 minutes; introducing a tert-butyl alcohol solution with a concentration of 1-20wt% into the wood for immersion exchange, then freezing it at -40°C, and then freeze-drying it for 12 hours.

5. The method for preparing a composite phase change material based on porous wood according to claim 4, characterized in that: The wood is balsa wood, which is cut into wood blocks with a size of 40 mm×40 mm×30 mm using a wood saw.

6. The method for preparing a composite phase change material based on porous wood according to claim 3, characterized in that: The preparation method of the porous wood gel framework includes the following steps: adding MXene dispersion dropwise into the porous wood gel using a pipette, and impregnating under vacuum conditions.

7. The method for preparing a composite phase change material based on porous wood according to claim 6, characterized in that: The preparation method of the MXene dispersion includes the following steps: slowly adding 2 g of MAX phase powder to 40 mL of a 9 mol / L HCl aqueous solution and reacting for 48 hours, and collecting the precipitated reaction mixture by centrifugation; washing the mixture three times with a 1 mol / L HCl solution and a 1 mol / L LiCl solution, and repeatedly rinsing the reaction mixture particles with deionized water until the pH is close to neutral; adding 100 mL of deionized water to the treated reaction mixture, and then placing the mixture in a 250 mL plastic bottle, filling it with nitrogen, and performing ultrasonic treatment, and then centrifuging it.

8. The method for preparing a composite phase change material based on porous wood according to claim 3, characterized in that: The preparation method of the composite phase change material includes the following steps: melting the phosphorus-modified flame-retardant phase change material into a liquid, immersing the porous wood gel frame in the phosphorus-modified flame-retardant phase change material molten liquid, maintaining it under vacuum and a negative pressure of -1 MPa for 1 hour; and fully penetrating it in a 95°C forced air oven for 2 hours.

9. The method for preparing a composite phase change material based on porous wood according to claim 8, characterized in that: The preparation method of the phosphorus-modified flame-retardant phase change material includes the following steps: dissolving 15.33 g of POCl3 and 27.05 g of stearyl alcohol in 40 ml of toluene in the following proportions, stirring the mixture at 25°C for 2 hours, then heating the mixture to 60°C and stirring for another 8 hours; dripping the reactants dropwise into acetonitrile at about -10°C, collecting the formed sample by vacuum filtration, and washing it with acetonitrile; and drying it in a vacuum at about 25°C for 24 hours.

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