Porosity-adjustable phase-change composite aerogel based on quantitative and uniform coating and dipping and preparation method of porosity-adjustable phase-change composite aerogel

Abstract

The invention discloses a porosity-adjustable phase-change composite aerogel based on quantitative uniform coating and impregnation and a preparation method thereof.The phase-change composite aerogel comprises carbon aerogel and a phase-change material loaded in pores, and the carbon aerogel is a three-dimensional porous carrier formed by directionally arranging carbon nanotubes along a starch aerogel template skeleton; the phase change material is stearic acid loaded by a quantitative uniform coating impregnation method; according to the invention, quantitative regulation and control on the stearic acid loading capacity and the aerogel porosity are realized through an innovative quantitative uniform coating and dipping method, the phase-change composite aerogel has excellent photo-thermal conversion performance and heat storage and heat preservation performance, and the performance of the material in photo-thermal energy conversion, storage and heat insulation is improved.

Description

Technical Field

[0001] This invention relates to the technical field of photothermal energy conversion and storage, specifically to a phase change composite aerogel prepared by a quantitative uniform coating and impregnation method, and its application in photothermal energy conversion, storage and thermal insulation. Background Technology

[0002] From the perspective of reducing energy consumption and alleviating environmental problems, converting solar energy into thermal energy and storing it is an attractive technology for utilizing abundant solar energy. Photothermal conversion and thermal storage are two indispensable parts of this technology. By loading phase change thermal storage materials with porous materials that have photothermal conversion properties, phase change composite photothermal materials can be manufactured, which can realize the direct conversion and storage of photothermal energy driven by sunlight and have the potential to be used as building materials.

[0003] Vacuum impregnation is often used when loading phase change thermal storage materials onto porous materials, allowing the phase change material to occupy all the pore spaces of the porous material (porosity approaching 0%), thus maximizing the thermal storage capacity. Simultaneously, to promote the homogenization of the temperature field throughout the porous material, high thermal conductivity fillers are often added to improve thermal conductivity. However, the thermal conductivity of porous materials loaded with phase change materials is difficult to exceed 1 W / m². -1 K -1 It still falls into the category of low thermal conductivity materials; graphene is an excellent thermally conductive and photothermal conversion material; Tang et al. (Advanced Functional Materials 2024, 34, 48, 2408693) developed a composite material using graphene aerogel as the framework and lauric acid as the phase change material, achieving effective encapsulation of lauric acid (encapsulation rate of 93.1%) and increasing the thermal conductivity to 1.164 W / m. -1 K -1 The photothermal conversion efficiency reaches 90.5%; patent CN112724936A discloses a method for preparing a new energy storage material, which uses graphene aerogel as a carrier and paraffin as a phase change material. The phase change energy storage material obtained by vacuum adsorption for 2 hours has a strength of 0.967 W / m³. -1 K -1 The thermal conductivity is 172.6 J g. -1 The latent heat of phase change; the thermal conductivity of the above graphene composite material is still much lower than that of well thermally conductive composites formed by compression molding (thermal conductivity reaches 30-200 W / m). -1 K -1 ).

[0004] Abandoning the traditional goal of enhancing thermal conductivity, this study directly utilizes the low thermal conductivity of porous photothermal materials and combines them with phase change materials to create photothermal storage / insulation materials. These materials can meet the needs of both heat preservation and storage, while resisting significant temperature fluctuations. Wang et al. (Materials Today Communications 2022, 32, 104011) immersed nanofiber Kevlar aerogel in excess molten polyethylene glycol and maintained it in an oven for 8 hours to achieve saturated adsorption, achieving a polyethylene glycol loading of over 97%. The melting enthalpy and crystallization enthalpy of this composite material were 163.10 J g. -1 and 162.33 J g -1 The thermal conductivity is 0.1349 W / m. -1 K -1 However, excessively high phase change material loading increases the solid-state heat transfer path, which is detrimental to excellent thermal insulation. Patent CN118422526A discloses a thermal insulation composite filler for corrugated cardboard and its preparation method. By changing the content of the thermal insulation composite filler containing tetradecane phase change microcapsules and hollow glass microspheres, the final surface temperature of the final material is reduced, and the surface temperature fluctuation is reduced by the heat storage properties of the phase change material, thus obtaining a composite corrugated cardboard material with heat storage / insulation coupling temperature control function.

[0005] The above solutions cannot achieve a balance in terms of photothermal energy conversion, heat storage, and insulation, thus affecting the efficiency of solar energy conversion and utilization. Therefore, developing a method that can precisely control the loading of phase change materials and the porosity of porous materials to achieve synergistic optimization of photothermal conversion, heat storage, and insulation is of great practical significance. Summary of the Invention

[0006] The purpose of this invention is to solve the problems in the prior art and propose a phase change composite aerogel with adjustable porosity based on quantitative uniform coating and impregnation, and its preparation method, which can improve the performance of materials in photothermal energy conversion, storage and thermal insulation.

[0007] To achieve the above objectives, this invention proposes a phase change composite aerogel with adjustable porosity based on quantitative uniform coating and impregnation. The phase change composite aerogel includes carbon aerogel and phase change material loaded in the pores. The carbon aerogel is a three-dimensional porous carrier in which carbon nanotubes are oriented along the starch aerogel template framework. The phase change material is stearic acid loaded by quantitative uniform coating and impregnation.

[0008] Preferably, the carbon aerogel has a multi-level pore structure with both nanopores and micropores. The nanopores originate from the carbon nanotubes and the gaps within the carbon aerogel skeleton formed by the oriented arrangement of multiple carbon nanotubes. The micropores originate from the pores between the carbon aerogel skeleton formed by the oriented arrangement of multiple carbon nanotubes. The porosity of the carbon aerogel is 90-100%.

[0009] Preferably, the stearic acid is in the form of a mixture of worm-like and blocky shapes, and the stearic acid type is a mixture of type B stearic acid, free type C stearic acid, and associated type C stearic acid.

[0010] Preferably, the B-type stearic acid is formed by the nano-confinement effect of the microporous and mesoporous structures, the free C-type stearic acid and associated C-type stearic acid are mainly present in the macroporous structure, and the associated C-type stearic acid has hydrogen bonding with the carbon aerogel.

[0011] Preferably, the quantitative uniform coating and impregnation method involves setting the theoretical volume loading of stearic acid, calculating the required stearic acid mass based on the porosity of the carbon aerogel, and using ethanol as an aid to achieve quantitative control of the stearic acid loading and the porosity of the phase change composite aerogel. The theoretical volume loading of stearic acid is 10-70%, and the corresponding theoretical porosity of the phase change composite aerogel is 85.6-26.7%.

[0012] Preferably, the quantitative uniform coating and impregnation method utilizes the phase change of the ethanol / stearic acid mixture under different temperature and pressure conditions. Specifically, the ethanol / stearic acid mixture is liquid at 70°C / normal pressure, and transforms into a white solid ethanol / stearic acid mixture during vacuum impregnation, uniformly coating the carbon aerogel. As the temperature of the solid ethanol / stearic acid mixture rises to 85°C, the solid ethanol / stearic acid mixture transforms into a liquid, stearic acid is drawn into the pores of the carbon aerogel by capillary force, and ethanol is transformed into a gas.

[0013] Preferably, the phase change composite aerogel uses carbon nanotubes as the photothermal material and stearic acid as the heat storage material.

[0014] Preferably, the phase change composite aerogel has a carbon aerogel framework and a phase change material. The carbon aerogel framework, which has photothermal conversion properties, absorbs sunlight and converts it into heat energy. The loading rate and porosity of the phase change material in the phase change composite aerogel are controlled and balanced by a quantitative and uniform coating and impregnation method. The phase change material is used to store heat, and the porous properties provide thermal insulation properties, thus achieving the dual utilization of heat storage and thermal insulation.

[0015] To achieve the above objectives, this invention also proposes a method for preparing phase change composite aerogels with tunable porosity based on quantitative and uniform coating and impregnation, comprising the following steps: a. Preparation of starch / konjac glucomannan / carbon nanotube hydrogel: Multi-walled carbon nanotubes were added to sodium hydroxide solution and sonicated until completely and uniformly dispersed. Potato starch was then added and stirred evenly. Konjac glucomannan was then added and the solution was stirred rapidly and vigorously until a semi-solid gel was formed. The solution was then transferred to a cylindrical mold and placed in a water bath at 85-95°C. After 8-12 minutes, the mold was removed and allowed to cool naturally to obtain the hydrogel. b. Molding: After the hydrogel in step a is cooled to room temperature, it is frozen and then freeze-dried to obtain an aerogel; c. Carbonization: The aerogel from step b is carbonized to obtain carbon aerogel; d. Impregnation: The carbon aerogel from step c is quantitatively and uniformly coated and impregnated with stearic acid to obtain a phase change composite aerogel.

[0016] Preferably, in step a, the concentration of multi-walled carbon nanotubes is 10 g / L. -1 The concentration of the sodium hydroxide solution is 0.1 mol / L. -1 The ultrasonic treatment time was 30 minutes, and the concentration of potato starch was 32 g / L. -1 The stirring time after adding potato starch was 30 minutes, and the mass ratio of konjac glucomannan to potato starch was 1:2. The molecules of potato starch and konjac glucomannan were linked by hydrogen bonds. After the addition of konjac glucomannan, the solution quickly turned into a hydrogel state, stabilizing the three-dimensional structure and increasing the viscosity of the system. The semi-solid gel was poured into a cylindrical polytetrafluoroethylene mold, and the mold was heated in a water bath at 90℃ for 10 minutes. During the heating process to 90℃, some hydrogen bonds in the starch system broke. The broken starch and KGM molecules absorbed water and swelled, forming intertwined macromolecular chains, which increased the viscosity of the system. The cooling method was natural cooling. During natural cooling, the broken potato starch molecules rearranged and crystallized, resulting in irreversible aging. Therefore, the hydrogel has an irreversible gelation structure.

[0017] In step b, the freezing method is liquid nitrogen freezing, the freezing time is 20-40 minutes, the freeze-drying time is 48-72 hours, the freeze-drying temperature is -60℃, and the vacuum degree is 1Pa; the aerogel skeleton is a uniform and dense three-dimensional cross-linked sheet structure.

[0018] In step c, carbonization is performed at a nitrogen flow rate of 300 ml / min. -1 The experiment was conducted in a tubular furnace at 5°C for 5 min. -1The temperature was increased to 500°C and held for two hours, then naturally cooled to room temperature to obtain carbon aerogel. More than 80% of the network structure of starch / konjac glucomannan in the aerogel was lost through pyrolysis, and the carbon nanotubes were oriented along the starch aerogel skeleton structure. The carbon aerogel has a hierarchical pore structure composed of nanopores and micropores, wherein the nanopores contain mesopores of 11-50 nm, and the micropores provide the phase transition space for the phase change material. The porosity of the carbon aerogel is 90-100%. The carbon aerogel has a black appearance.

[0019] In step d, the volume and mass of the three-dimensional porous carrier are first measured. The total pore volume is obtained based on the porosity of the carbon aerogel. The theoretical loading of four types of stearic acid with a volume ratio of 10%-70% is set. The mass of stearic acid required for each loading is calculated using the total pore volume of C500 and the density of stearic acid.

[0020] In step d, stearic acid is quantitatively and uniformly coated and impregnated with ethanol. The stearic acid is placed in an oven until it melts, and then poured into 10 ml of hot ethanol at 70°C and stirred rapidly for 5 minutes until completely mixed.

[0021] In step d, a uniform stearic acid / ethanol solution is placed in an 85°C oven. The carbon aerogel is quickly immersed in the solution, and air is uniformly evacuated to create a vacuum inside the oven. Vacuuming is then repeated at intervals of 10 minutes, 30 minutes, and 1 hour. After 3 hours, the sample is removed to obtain the phase change composite gel. During vacuuming, the 70°C stearic acid / ethanol mixture gradually transforms into a white solid, coating the carbon aerogel. Ethanol vapor is then removed at intervals of 10 minutes, 30 minutes, and 1 hour. After 3 hours, the sample is removed. A phase change composite gel was obtained. Under vacuum, the temperature of a white solid substance containing a mixture of stearic acid and ethanol was increased from 70°C to 85°C. The solid substance gradually melted, the ethanol evaporated, and the stearic acid was absorbed into the pores of the carbon aerogel by capillary force. After the sample was removed, the stearic acid underwent a recrystallization process. The types of stearic acid in the phase change composite aerogel were B-type stearic acid, free C-type stearic acid, and hydrogen-bonded C-type stearic acid. The stearic acid morphology was worm-like near the carbon nanotubes and blocky away from the carbon nanotubes. The actual porosity of the phase change composite aerogel was 85.6%-26.7%.

[0022] The beneficial effects of this invention are: 1. This invention achieves quantitative control of stearic acid loading and aerogel porosity through an innovative quantitative and uniform coating and impregnation method. The phase change composite aerogel exhibits excellent photothermal conversion and heat storage properties, improving the material's performance in photothermal energy conversion, storage, and thermal insulation. This method utilizes the phase change (liquid-solid-liquid) of the stearic acid / ethanol mixture under different temperature and pressure conditions to achieve quantitative and uniform coating and impregnation of the aerogel with stearic acid. Compared with traditional vacuum impregnation methods, it eliminates the need for excessive impregnation and subsequent complex surface treatment. The process allows for precise control of stearic acid loading and aerogel porosity by directly adjusting the mixing ratio of stearic acid and ethanol and the vacuum impregnation conditions. This solves the problems of difficult precise control of phase change material loading and difficulty in balancing thermal insulation and heat storage capacity in existing technologies. At the same time, the nano-confinence effect provided by the nanopores of carbon aerogel kinetically restricts stearic acid during impregnation, which helps metastable and bulk stable crystal forms coexist, thereby expanding the phase change temperature range and improving the material's ability to resist temperature fluctuations over a wider temperature range. 2. Precise performance control: Through quantitative and uniform coating and impregnation methods, the stearic acid loading can be precisely controlled according to actual needs, thereby obtaining phase change composite aerogels with different porosities, achieving a balance between porosity and heat storage capacity, and meeting the dual requirements of heat preservation and heat storage. 3. Highly efficient photothermal conversion: Carbon aerogel is mainly composed of a framework of carbon nanotubes oriented along a starch aerogel template. Carbon nanotubes have excellent light absorption characteristics and photothermal effects, exhibiting strong absorption in the ultraviolet-visible-near-infrared bands, and can generate heat through non-radiative relaxation. Simultaneously, when incident light enters the pores of the carbon aerogel, the pores can reflect the light beam multiple times, improving the absorption efficiency of the incident light. Therefore, the phase change composite aerogel has a full-spectrum absorption rate of over 95%. Under one solar intensity, the upper surface temperature reaches 60 ℃ in 2.6 minutes and begins to store heat, rising to 80 ℃ within 15 minutes, and finally reaching equilibrium at 86 ℃, thus efficiently converting solar energy into thermal energy. 4. Excellent thermal storage performance: The high porosity of carbon aerogel provides the loading space for the phase change material. The abundant pores of carbon aerogel provide capillary forces, and the hydrogen bonds formed between carbon aerogel and stearic acid ensure that stearic acid is stably loaded inside the pores, avoiding leakage of stearic acid during multiple phase changes. Therefore, the phase change composite aerogel has a high thermal storage capacity. For example, when the volume loading of stearic acid is 42%, the resulting PCCA50 has a porosity of 50%, and the enthalpy of melting and the enthalpy of crystallization are 182.1 J g. -1 and 174.5 J g -1 The actual packaging efficiency reaches 89.2%; 5. Thermal Insulation: The "quantitative and uniform coating and impregnation" method can quantitatively control the stearic acid loading and the porosity of the phase change composite aerogel. For example, when the stearic acid volume loading is 42%, the resulting PCCA50 has a porosity of 50%, containing nanopores including mesopores with a size of 5-70 nm, which is lower than the mean free path of air molecules (approximately 70 nm). This not only reduces the solid-state heat conduction path but also reduces heat transfer caused by airflow. Therefore, the phase change composite aerogel has excellent thermal insulation properties; at 25 ℃, the thermal conductivity of PCCA50 is as low as 0.112 W / m². -1 K -1 Compared to stearic acid (0.22 W m), -1 K -1 The temperature was reduced by 49.1%; under 1 solar intensity, the temperature difference between the upper and lower surfaces of PCCA50 reached 25-30 ℃. 6. Effective resistance to temperature fluctuations: The mesopores with a size of 11-50 nm in the carbon aerogel provide a nano-confinement effect. During the recrystallization process of stearic acid entering the pores of the carbon aerogel, the reduced diffusion rate within the nano-confined space leads to slow kinetics, promoting the emergence of the metastable B-type stearic acid and its slow transformation into the stable phase. The intermolecular interactions of B-type stearic acid are weaker than those of C-type stearic acid, and the bond energy is lower, causing the lower limit of the phase transition temperature of the phase change composite aerogel to decrease to 60 °C. Hydrogen bonds are formed between stearic acid and carbon aerogel, which restricts the thermal motion of stearic acid molecules, thus increasing the upper limit of the phase transition temperature to 85 °C. The broadened phase transition temperature range of the phase change composite aerogel enhances its resistance to temperature fluctuations. When the light intensity decreases from 1 solar unit to 0.8 solar units, the heat stored in the phase change composite aerogel can quickly compensate for part of the temperature drop caused by the decrease in light intensity. Therefore, the temperature drop on the upper surface is within 5 °C, and the temperature drop at the bottom is within 1 °C. 7. Stable and reliable structure: Stearic acid is uniformly loaded in the pores of the aerogel, forming a stable composite structure through capillary forces and hydrogen bonds. After multiple cycles of light irradiation tests, the equilibrium temperature of the phase change composite aerogel remains unchanged, and there is no leakage of phase change material, demonstrating good cycle stability and structural reliability. 8. Real-world application potential: Phase change composite aerogels have application potential under real sunlight. Under real sunlight intensity equivalent to one sun, the surface of PCCA50 phase change composite aerogel with a porosity of 50% undergoes rapid photothermal conversion, reaching 60°C within 1 minute to begin heat storage, and reaching an equilibrium temperature of 76°C within 10 minutes, with a heat output of 182.1 J / g. -1 It has the capacity to store heat; and at the same time, it has low thermal conductivity (0.112 W / m²). -1 K -1This gives it excellent heat retention performance, maintaining the bottom temperature at 53℃; when there are fluctuations in light intensity (light intensity decreases from 1 sun to 0.57 suns) or strong winds, the PCCA50 quickly releases heat to limit the bottom temperature drop to within 5℃.

[0023] The features and advantages of the present invention will be described in detail through embodiments and in conjunction with the accompanying drawings. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the process of a phase change composite aerogel according to the present invention; Figure 2 This is a diagram showing the morphological changes of a sample during the quantitative and uniform coating and impregnation process of a phase change composite aerogel according to the present invention. Figure 3 This is a morphological image of PCCA50-SA, a phase change composite aerogel prepared by direct vacuum impregnation of stearic acid without ethanol assistance, according to the technical solution of Comparative Example 2 of this invention. Figure 4 These are X-ray diffraction patterns of phase change composite aerogel PCCA50 and carbon aerogel C500 prepared by the technical solutions of Embodiment 3 and Comparative Example 1 of this invention. Figure 5 This is a scanning electron microscope image of carbon aerogel C500 prepared by the technical solution of Comparative Example 1 of the present invention; Figure 6 These are scanning electron microscope images of the phase change composite aerogels PCCA10, PCCA30, PCCA50 and PCCA70 prepared by the technical solutions of Examples 1-4 of this invention; Figure 7 The thermal conductivity of the phase change composite aerogels PCCA10, PCCA30, PCCA50, PCCA70 and carbon aerogel C500 prepared by the technical solutions of Examples 1-4 and Comparative Example 1 of this invention. Figure 8 These are DSC curves of the endothermic process of the phase change composite aerogels PCCA10, PCCA30, PCCA50 and PCCA70 prepared by the technical solutions of Examples 1-4 of this invention; Figure 9 These are DSC curves of the exothermic process of the phase change composite aerogels PCCA10, PCCA30, PCCA50 and PCCA70 prepared by the technical solutions of Examples 1-4 of this invention; Figure 10 The absorption spectra of phase change composite aerogels PCCA10, PCCA30, PCCA50, PCCA70 and carbon aerogel C500 prepared by the technical solutions of Examples 1-4 and Comparative Example 1 in the 200-2500 nm wavelength range are shown. Figure 11The temperature change curves of phase change composite aerogels PCCA30, PCCA50, PCCA70 and carbon aerogel C500 prepared by the technical solutions of Examples 2-4 and Comparative Example 1 under a simulated solar radiation intensity are shown. Figure 12 This is a graph showing the average temperature, maximum temperature, and temperature difference between the upper and lower surfaces of the phase change composite aerogel PCCA50 prepared by the technical solution of Embodiment 3 of the present invention under simulated different solar light intensities. Figure 13 This is a graph showing the thermal leakage performance test results of the phase change composite aerogel PCCA50 prepared by the technical solution of Embodiment 3 of the present invention; Figure 14 This is a temperature change curve of the phase change composite aerogel PCCA50 prepared by the technical solution of Embodiment 3 of the present invention under natural light. Figure 15 This is a temperature fluctuation diagram of the phase change composite aerogel PCCA50 prepared by the technical solution of Embodiment 3 of the present invention under natural light. Detailed Implementation

[0025] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0026] Example 1: In this embodiment, a phase change composite aerogel PCCA10 with a theoretical stearic acid volume loading of 10% was prepared. 1. Add 0.5g of multi-walled carbon nanotubes to 50ml of a 0.1mol / L solution. -1 In a sodium hydroxide solution, sonicate for 30 minutes until completely and uniformly dispersed; add 1.6g of potato starch to the solution and stir evenly, then add 0.8g of konjac glucomannan and stir rapidly and vigorously until the solution transforms into a semi-solid gel; 2. Transfer the semi-solid gel to a cylindrical mold, place it horizontally until the gel surface is smooth, then place it in a 90 ℃ water bath for 10 min, remove it and let it cool naturally to room temperature to obtain the hydrogel; 3. The hydrogel was placed in an incubator containing liquid nitrogen to freeze and solidify, and then dried in a freeze dryer for 3 days to obtain an aerogel; 4. Place the aerogel in a tube furnace and incubate at 5 °C for 5 min. -1 The temperature was increased to 500 °C at a rate of 1 and held for 2 hours. After natural cooling, carbon aerogel was obtained. 5. Based on the porosity (92.7%) and volume (14.4 cm³) of carbon aerogel. 3 The density of stearic acid (0.84 g cm⁻¹) -3Based on the theoretical volume loading of stearic acid (10%), the required mass of stearic acid was calculated to be 1.12 g. 1.12 g of stearic acid was weighed and placed in an oven at 85 ℃ to melt. It was then poured into 10 ml of hot ethanol at 70 ℃ and stirred rapidly for 5 minutes until completely mixed to obtain a stearic acid / ethanol liquid mixture. 6. Place the carbon aerogel in a liquid mixture of stearic acid / ethanol and put it in an oven at 85 ℃. Evacuate the air at a uniform speed to make the inside of the oven a vacuum state. After maintaining the vacuum environment for 3 hours, take out the sample to obtain the phase change composite aerogel PCCA10.

[0027] Example 2: In this embodiment, a phase change composite aerogel PCCA30 with a theoretical stearic acid volume loading of 30% was prepared. 1. Add 0.5 g of multi-walled carbon nanotubes to 50 ml of a 0.1 mol / L solution. -1 In a sodium hydroxide solution, sonicate for 30 minutes until completely and uniformly dispersed; add 1.6 g of potato starch to the solution and stir evenly, then add 0.8 g of konjac glucomannan and stir rapidly and vigorously until the solution transforms into a semi-solid gel; 2. Transfer the semi-solid gel to a cylindrical mold, place it horizontally until the gel surface is smooth, then place it in a 90 ℃ water bath for 10 min, remove it and let it cool naturally to room temperature to obtain the hydrogel; 3. The hydrogel was placed in an incubator containing liquid nitrogen to freeze and solidify, and then dried in a freeze dryer for 3 days to obtain an aerogel; 4. Place the aerogel in a tube furnace and incubate at 5 °C for 5 min. -1 The temperature was increased to 500 °C at a rate of 1 and held for 2 hours. After natural cooling, carbon aerogel was obtained. 5. Based on the porosity (92.7%) and volume (14.4 cm³) of carbon aerogel. 3 The density of stearic acid (0.84 g cm⁻¹) -3 Based on the theoretical volume loading of stearic acid (30%), the required mass of stearic acid was calculated to be 3.36 g. 3.36 g of stearic acid was weighed and placed in an oven at 85 ℃ to melt. It was then poured into 10 ml of hot ethanol at 70 ℃ and stirred rapidly for 5 minutes until completely mixed to obtain a stearic acid / ethanol liquid mixture. 6. Place the carbon aerogel in a liquid mixture of stearic acid / ethanol and put it in an oven at 85 ℃. Evacuate the air at a uniform speed to make the inside of the oven a vacuum state. After maintaining the vacuum environment for 3 hours, take out the sample to obtain the phase change composite aerogel PCCA30.

[0028] Example 3: In this embodiment, a phase change composite aerogel PCCA50 with a theoretical stearic acid volume loading of 50% was prepared. 1. Add 0.5 g of multi-walled carbon nanotubes to 50 ml of a 0.1 mol / L solution. -1 In a sodium hydroxide solution, sonicate for 30 minutes until completely and uniformly dispersed; add 1.6 g of potato starch to the solution and stir evenly, then add 0.8 g of konjac glucomannan and stir rapidly and vigorously until the solution transforms into a semi-solid gel; 2. Transfer the semi-solid gel to a cylindrical mold, place it horizontally until the gel surface is smooth, then place it in a 90 ℃ water bath for 10 min, remove it and let it cool naturally to room temperature to obtain the hydrogel; 3. The hydrogel was placed in an incubator containing liquid nitrogen to freeze and solidify, and then dried in a freeze dryer for 3 days to obtain an aerogel; 4. Place the aerogel in a tube furnace and incubate at 5 °C for 5 min. -1 The temperature was increased to 500 °C at a rate of 1 and held for 2 hours. After natural cooling, carbon aerogel was obtained. 5. Based on the porosity (92.7%) and volume (14.4 cm³) of carbon aerogel. 3 The density of stearic acid (0.84 g cm⁻¹) -3 Based on the theoretical volume loading of stearic acid (50%), the required mass of stearic acid was calculated to be 5.61 g. 5.61 g of stearic acid was weighed and placed in an oven at 85 ℃ to melt. It was then poured into 10 ml of hot ethanol at 70 ℃ and stirred rapidly for 5 minutes until completely mixed to obtain a stearic acid / ethanol liquid mixture. 6. Place the carbon aerogel in a liquid mixture of stearic acid / ethanol and put it in an oven at 85 ℃. Evacuate the air at a uniform speed to make the inside of the oven a vacuum state. After maintaining the vacuum environment for 3 hours, take out the sample to obtain the phase change composite aerogel PCCA50.

[0029] Example 4: In this embodiment, a phase change composite aerogel PCCA70 with a theoretical stearic acid volume loading of 70% was prepared. 1. Add 0.5 g of multi-walled carbon nanotubes to 50 ml of a 0.1 mol / L solution. -1 In a sodium hydroxide solution, sonicate for 30 minutes until completely and uniformly dispersed; add 1.6 g of potato starch to the solution and stir evenly, then add 0.8 g of konjac glucomannan and stir rapidly and vigorously until the solution transforms into a semi-solid gel; 2. Transfer the semi-solid gel to a cylindrical mold, place it horizontally until the gel surface is smooth, then place it in a 90 ℃ water bath for 10 min, remove it and let it cool naturally to room temperature to obtain the hydrogel; 3. The hydrogel was placed in an incubator containing liquid nitrogen to freeze and solidify, and then dried in a freeze dryer for 3 days to obtain an aerogel; 4. Place the aerogel in a tube furnace and incubate at 5 °C for 5 min. -1 The temperature was increased to 500 °C at a rate of 1 and held for 2 hours. After natural cooling, carbon aerogel was obtained. 5. Based on the porosity (92.7%) and volume (14.4 cm³) of carbon aerogel. 3 The density of stearic acid (0.84 g cm⁻¹) -3 Based on the theoretical volume loading of stearic acid (70%), the required mass of stearic acid was calculated to be 7.85 g. 7.85 g of stearic acid was weighed and placed in an oven at 85 ℃ to melt. It was then poured into 10 ml of hot ethanol at 70 ℃ and stirred rapidly for 5 minutes until completely mixed to obtain a stearic acid / ethanol liquid mixture. 6. Place the carbon aerogel in a liquid mixture of stearic acid / ethanol and put it in an oven at 85 ℃. Evacuate the air at a uniform speed to make the inside of the oven a vacuum state. After maintaining the vacuum environment for 3 hours, take out the sample to obtain the phase change composite aerogel PCCA70.

[0030] Comparative Example 1: In this embodiment, carbon aerogel C500 was prepared. 1. Add 0.5 g of multi-walled carbon nanotubes to 50 ml of a 0.1 mol / L solution. -1 In a sodium hydroxide solution, sonicate for 30 minutes until completely and uniformly dispersed; add 1.6 g of potato starch to the solution and stir evenly, then add 0.8 g of konjac glucomannan and stir rapidly and vigorously until the solution transforms into a semi-solid gel; 2. Transfer the semi-solid gel to a cylindrical mold, place it horizontally until the gel surface is smooth, then place it in a 90 ℃ water bath for 10 min, remove it and let it cool naturally to room temperature to obtain the hydrogel; 3. The hydrogel was placed in an incubator containing liquid nitrogen to freeze and solidify, and then dried in a freeze dryer for 3 days to obtain an aerogel; 4. Place the aerogel in a tube furnace and incubate at 5 °C for 5 min. -1 The temperature was increased to 500 °C at a rate of 100 °C and held for 2 hours. After natural cooling, carbon aerogel was obtained.

[0031] Comparative Example 2: This embodiment prepares phase change composite aerogel PCCA50-SA; 1. Add 0.5 g of multi-walled carbon nanotubes to 50 ml of a 0.1 mol / L solution. -1 In a sodium hydroxide solution, sonicate for 30 minutes until completely and uniformly dispersed; add 1.6 g of potato starch to the solution and stir evenly, then add 0.8 g of konjac glucomannan and stir rapidly and vigorously until the solution transforms into a semi-solid gel; 2. Transfer the semi-solid gel to a cylindrical mold, place it horizontally until the gel surface is smooth, then place it in a 90 ℃ water bath for 10 min, remove it and let it cool naturally to room temperature to obtain the hydrogel; 3. The hydrogel was placed in an incubator containing liquid nitrogen to freeze and solidify, and then dried in a freeze dryer for 3 days to obtain an aerogel; 4. Place the aerogel in a tube furnace and incubate at 5 °C for 5 min. -1 The temperature was increased to 500 °C at a rate of 1 and held for 2 hours. After natural cooling, carbon aerogel was obtained. 5. Based on the porosity (92.7%) and volume (14.4 cm³) of carbon aerogel. 3 The density of stearic acid (0.84 g cm⁻¹) -3 Based on the theoretical volumetric loading of stearic acid (50%), the required mass of stearic acid was calculated to be 5.61 g. 5.61 g of stearic acid was weighed and placed in an oven at 85 ℃ to melt. The carbon aerogel was placed in the molten stearic acid, and the air was uniformly extracted to make the inside of the oven a vacuum state. After maintaining the vacuum environment for 72 hours, the sample was taken out to obtain the phase change composite aerogel PCCA50-SA.

[0032] See Figure 1 This is a schematic diagram of the process of preparing phase change composite aerogels according to the technical solutions of Examples 1-4. The aerogel skeleton after freeze-drying exhibits a cross-linked sheet structure; the skeleton of carbon aerogel C500 after carbonization treatment is formed by multiple carbon nanotubes arranged along the starch aerogel template; stearic acid is coated around the carbon nanotubes in the phase change composite aerogel.

[0033] See Figure 2 The above describes the morphological changes of the sample during the quantitative and uniform coating and impregnation process when preparing phase change composite aerogels using the technical solutions in Examples 1-4. The stearic acid / ethanol solid mixture is uniformly coated around the carbon aerogel C500 during the vacuum process. After vacuum impregnation for 3 hours, the ethanol evaporates, and the stearic acid is absorbed into the pores of the carbon aerogel through capillary force. The resulting phase change composite aerogel is a black cylinder, which is beneficial for the absorption of incident light. The cross-section of the phase change composite aerogel shows a uniform gray color, which is caused by the coexistence of white stearic acid and black carbon aerogel.

[0034] See Figure 3The image shows the morphology of PCCA50-SA obtained by directly vacuum impregnating stearic acid without ethanol assistance when preparing phase change composite aerogel using the technical scheme of Comparative Example 2. When carbon aerogel is placed in the same mass of molten stearic acid required for the preparation of PCCA50, after vacuum impregnation for 72 hours, the white stearic acid still cannot be uniformly distributed throughout the carbon aerogel. This indicates that the "liquid-solid-liquid" phase change of the stearic acid / ethanol mixture is beneficial for accurately achieving quantitative and uniform impregnation of stearic acid in the pores of carbon aerogel, thereby controlling the porosity of the phase change composite aerogel and achieving a balance between heat storage and heat insulation properties.

[0035] See Figure 4 The figures show the X-ray diffraction patterns of phase change composite aerogels PCCA50 and C500 prepared by the technical solutions of Example 3 and Comparative Example 1 of this invention; the stearic acid-ethanol / C500 mixture and PCCA50 are shown in X-ray diffraction patterns at 2... θ The presence of characteristic peaks associated with type B stearic acid at 5.6° and 9.4° indicates that the nanoconfining effect provided by the nanopores in the carbon aerogel caused a crystal transformation of stearic acid during recrystallization, thus broadening the lower limit of phase transition temperature from 69 °C for stearic acid to 60 °C.

[0036] See Figure 5 , Figure 6 Table 1 shows scanning electron microscope images and pore structure parameters of carbon aerogels and phase change composite aerogels prepared by the technical solutions of Comparative Example 1 and Examples 1-4 of the present invention, respectively. Figure 5 In the image, the framework of C500 is composed of multiple carbon nanotubes arranged along the starch aerogel template, and the orange box contains the remaining 20% ​​of the starch aerogel template. Figure 6 In the figures, a, b, c, and d represent the phase change composite aerogels PCCA10, PCCA30, PCCA50, and PCCA70 prepared according to the technical schemes of Examples 1, 2, 3, and 4, respectively. Stearic acid is coated on the surface of carbon nanotubes and biomass carbon skeletons in two shapes: the layer of stearic acid close to the skeleton appears as a worm-like structure, while the part farther from the skeleton is a large layered or blocky structure; as the stearic acid loading increases, the proportion of blocky stearic acid increases; after entering the pores, stearic acid first exists in the form of hydrogen bonding with carbon aerogel, exhibiting the following characteristics: Worm-like; as the stearic acid loading increases, the free stearic acid increases, resulting in a blocky morphology; Table 1 shows the pore structure parameters of the phase change composite aerogels PCCA10, PCCA30, PCCA50, PCCA70 and carbon aerogel C500 prepared by the technical solutions of Examples 1-4 and Comparative Example 1 of the present invention. The porosity of the carbon aerogel and phase change composite aerogel prepared by Comparative Example 1 and Examples 1-4 proves that the "quantitative coating and uniform impregnation" method can accurately adjust the stearic acid loading with an error within 10%.

[0037] Table 1

[0038]

[0039] See Figure 7 The values ​​represent the thermal conductivity of the phase change composite aerogels PCCA10, PCCA30, PCCA50, PCCA70, and carbon aerogel C500 prepared using the techniques described in Examples 1-4 and Comparative Example 1. The thermal conductivity of PCCA50 at 25 °C is 0.112 W / m². -1 K -1 Compared to stearic acid (0.22 W m), -1 K -1 The temperature was reduced by 49.1%, which is beneficial to thermal insulation properties.

[0040] See Figure 8 , Figure 9 Tables 2 and 3 show the DSC curves and thermophysical parameters of the endothermic and exothermic processes of the phase change composite aerogels PCCA10, PCCA30, PCCA50, and PCCA70 prepared by the technical schemes in Examples 1-4, respectively. The phase change peak of PCCA10 appears at 80℃, indicating that most of the stearic acid forms hydrogen bonds with the carbon aerogel, restricting the thermal motion of stearic acid. PCCA30, PCCA50, and PCCA70 show a phase change peak at 60℃. Phase transitions begin at ℃, with melting peaks occurring between 69-72℃ and 80-85℃, respectively, demonstrating the existence of three stearic acid forms: B-type stearic acid, free C-type stearic acid, and associated C-type stearic acid. This reflects that the confined kinetics within the nanoscale confinement space facilitate the gradual stabilization of the compressible phase crystal form, thereby broadening the phase transition temperature range to 60-85℃. Table 2 shows the thermophysical parameters of the phase change composite aerogels PCCA10, PCCA30, PCCA50, PCCA70, and stearic acid SA prepared by the technical solutions of Examples 1-4 of this invention. Table 3 shows the thermal storage performance parameters of the phase change composite aerogels PCCA10, PCCA30, PCCA50, and PCCA70 prepared by the technical solutions of Examples 1-4 of this invention. It can be seen that PCCA50 has an encapsulation efficiency of 89.2%, and melting enthalpy and crystallization enthalpy are 182.1 and 174.5 J g, respectively. -1 This indicates that phase change composite aerogels have excellent heat storage capacity.

[0041] Table 2

[0042]

[0043] Table 3

[0044]

[0045] See Figure 10The images show the absorption spectra of phase change composite aerogels PCCA10, PCCA30, PCCA50, PCCA70, and carbon aerogel C500 prepared using the techniques described in Examples 1-4 and Comparative Example 1, in the 200-2500 nm wavelength range. In the ultraviolet band (200-400 nm), both the phase change composite aerogels and carbon aerogels exhibit light absorption rates exceeding 85%. In the visible light band (400-800 nm), both phase change composite aerogels and carbon aerogels exhibit light absorption rates of 90%. In the near-infrared band (800-2500 nm), both phase change composite aerogels and carbon aerogels exhibit light absorption rates of 95%. This excellent light absorption capability is due, on the one hand, to the fact that the pores of the porous materials can reflect incident light multiple times, and on the other hand, to the fact that carbon nanotubes enhance the absorption rate of incident light.

[0046] See Figure 11 The graphs show the temperature changes of phase change composite aerogels PCCA30, PCCA50, PCCA70, and carbon aerogel C500 prepared under simulated solar intensity and in the absence of air flow. The upper surface temperatures of PCCA30 and PCCA50 both reach 60 °C in 2.6 minutes and begin storing heat, rising to 80 °C within 10 and 15 minutes, respectively. The upper surface temperature of PCCA70 rises to 78 °C after 60 minutes. After reaching equilibrium, the upper surface temperature of C500 fluctuates significantly due to light intensity fluctuations, approximately 5 °C, while the fluctuation of PCCA is within 2 °C. During the cooling process after turning off the simulated solar light source, the temperatures of PCCA50 and PCCA70 range from 78 to 60 °C. There is a clear slowdown phase in the cooling rate between ℃, which is due to stearic acid releasing stored heat to resist the temperature drop; while PCCA30 and C500 do not have this phase, and their cooling rate is faster than PCCA50 and PCCA70; the time taken for C500, PCCA30, PCCA50 and PCCA70 to drop from the equilibrium temperature to 60 ℃ is 1, 2, 8 and 8 minutes, respectively; thus, the carbon aerogel framework composed of carbon nanotubes has excellent light absorption and rapid photothermal conversion characteristics, the presence of stearic acid can weaken the temperature change caused by light intensity fluctuations, and can prolong the duration of the phase transition temperature by releasing heat.

[0047] See Figure 12 The figure shows the average temperature, maximum temperature, and temperature difference between the upper and lower surfaces of the phase change composite aerogel PCCA50 prepared by the technical solution in Example 3 under simulated different solar light intensities and in an environment with air convection. The presence of pores in the phase change composite aerogel endows it with thermal insulation properties. Therefore, under one solar light intensity, the temperature difference between the upper and lower surfaces of PCCA50 reaches 25°C. The temperature difference between the upper and lower surfaces increases with the increase of light intensity.

[0048] See Figure 13 The figure shows the thermal leakage performance test results of the phase change composite aerogel PCCA50 prepared by the technical solution in Example 3. After 12 hours of light exposure, the mass of PCCA50 remained unchanged, and no stearic acid leakage was observed on the filter paper. This demonstrates that the capillary force, nano-constraint, and hydrogen bonds of carbon aerogel can stably adsorb stearic acid, thus enabling it to be used for long-term stable photothermal conversion and heat storage.

[0049] See Figure 14 Figure 1 shows the temperature change curves of phase change composite aerogels PCCA30, PCCA50, PCCA70, and carbon aerogel C500 prepared under natural light. In the natural light environment, there are conditions of reduced light intensity (from 1 solar intensity to 0.57 solar intensity) and strong winds. The gray area represents the situation with strong winds, and the dashed line perpendicular to the x-axis indicates that the light intensity has decreased to 0.57 solar intensity. C500, PCCA30, and PCCA50 reach equilibrium temperatures of 90, 80, and 76 °C, respectively, within 10 minutes, while PCCA70 reaches an equilibrium temperature of 75 °C within 20 minutes. Under conditions of strong air convection and reduced light intensity, the temperature fluctuation of C500 is much greater than that of PCCA. Especially at 0.57 sun, the upper surface temperature of C500, 59 °C, is the lowest among the four samples, demonstrating that the heat storage / release properties of stearic acid effectively resist temperature fluctuations.

[0050] See Figure 15 Figure 1 shows the temperature fluctuation of phase change composite aerogels PCCA30, PCCA50, PCCA70 and carbon aerogel C500 prepared by the technical solutions of Examples 2-4 and Comparative Example 1 under natural light. C500 has the largest temperature fluctuation with environmental changes. PCCA70 has the smallest temperature fluctuation, with the temperature fluctuation controlled below 5°C under all conditions. The highest temperature value of PCCA50 fluctuates to 10°C when the light intensity is reduced to 0.57sun. Apart from this, the temperature fluctuation under other conditions is controlled below 5°C.

[0051] The above embodiments are illustrative of the present invention and are not intended to limit the present invention. Any simple modifications to the present invention are within the scope of protection of the present invention.

Claims

1. A phase change composite aerogel with tunable porosity based on quantitative and uniform coating and impregnation, characterized in that: The phase change composite aerogel includes carbon aerogel and phase change material loaded in the pores. The carbon aerogel is a three-dimensional porous carrier in which carbon nanotubes are oriented along the starch aerogel template skeleton, and the phase change material is stearic acid loaded by a quantitative and uniform coating impregnation method.

2. The phase change composite aerogel with adjustable porosity based on quantitative and uniform coating and impregnation according to claim 1, characterized in that: The carbon aerogel has a multi-level pore structure with both nanopores and micropores. The nanopores originate from carbon nanotubes and the gaps within the carbon aerogel skeleton formed by the directional arrangement of multiple carbon nanotubes. The micropores originate from the pores between the carbon aerogel skeleton formed by the directional arrangement of multiple carbon nanotubes. The porosity of the carbon aerogel is 90-100%.

3. The phase change composite aerogel with adjustable porosity based on quantitative and uniform coating and impregnation according to claim 1, characterized in that: The stearic acid is in the form of a mixture of worm-like and block-like shapes, and the stearic acid type is a mixture of type B stearic acid, free type C stearic acid, and associated type C stearic acid.

4. The phase change composite aerogel with adjustable porosity based on quantitative and uniform coating and impregnation according to claim 3, characterized in that: The B-type stearic acid is induced by the nano-confinement effect of the microporous and mesoporous structures. The free C-type stearic acid and associated C-type stearic acid are mainly present in the macroporous structure. The associated C-type stearic acid has hydrogen bonding with the carbon aerogel.

5. The phase change composite aerogel with adjustable porosity based on quantitative and uniform coating and impregnation according to claim 1, characterized in that: The quantitative uniform coating and impregnation method involves setting the theoretical volume loading of stearic acid, calculating the required stearic acid mass based on the porosity of the carbon aerogel, and using an ethanol / stearic acid mixture with the assistance of ethanol to achieve quantitative control of the stearic acid loading and the porosity of the phase change composite aerogel. The theoretical volume loading of stearic acid is 10-70%, and the corresponding theoretical porosity of the phase change composite aerogel is 85.6-26.7%.

6. The phase change composite aerogel with adjustable porosity based on quantitative and uniform coating impregnation according to claim 5, characterized in that: The quantitative uniform coating and impregnation method utilizes the phase change of the ethanol / stearic acid mixture under different temperature and pressure conditions. Specifically, the ethanol / stearic acid mixture is liquid at 70°C / normal pressure. During the vacuum impregnation process, it transforms into a white solid ethanol / stearic acid mixture, which uniformly coats the carbon aerogel. As the temperature of the solid ethanol / stearic acid mixture rises to 85°C, it transforms into a liquid state. Stearic acid is drawn into the pores of the carbon aerogel by capillary force, and ethanol is transformed into a gaseous state.

7. The phase change composite aerogel with adjustable porosity based on quantitative and uniform coating and impregnation according to claim 1, characterized in that: The phase change composite aerogel uses carbon nanotubes as the photothermal material and stearic acid as the heat storage material.

8. The method for preparing phase change composite aerogels with adjustable porosity based on quantitative and uniform coating and impregnation according to claim 1, characterized in that: The phase change composite aerogel has a carbon aerogel framework and a phase change material. The carbon aerogel framework, which has photothermal conversion properties, absorbs sunlight and converts it into heat energy. The loading rate and porosity of the phase change material in the phase change composite aerogel are controlled and balanced by a quantitative and uniform coating and impregnation method. The phase change material is used to store heat, and the porous properties provide thermal insulation properties, thus achieving the dual utilization of heat storage and thermal insulation.

9. A method for preparing phase change composite aerogels with adjustable porosity based on quantitative and uniform coating and impregnation as described in any one of claims 1 to 8, characterized in that: Includes the following steps: a. Preparation of starch / konjac glucomannan / carbon nanotube hydrogel: Multi-walled carbon nanotubes were added to sodium hydroxide solution and sonicated until completely and uniformly dispersed. Potato starch was then added and stirred evenly. Konjac glucomannan was then added and the solution was stirred rapidly and vigorously until a semi-solid gel was formed. The solution was then transferred to a cylindrical mold and placed in a water bath at 85-95°C. After 8-12 minutes, the mold was removed and allowed to cool naturally to obtain the hydrogel. b. Molding: After the hydrogel in step a is cooled to room temperature, it is frozen and then freeze-dried to obtain an aerogel; c. Carbonization: The aerogel from step b is carbonized to obtain carbon aerogel; d. Impregnation: The carbon aerogel from step c is quantitatively and uniformly coated and impregnated with stearic acid to obtain a phase change composite aerogel.

10. The method for preparing phase change composite aerogels with adjustable porosity based on quantitative and uniform coating impregnation according to claim 9, characterized in that: In step a, the concentration of multi-walled carbon nanotubes is 10 g / L. -1 The concentration of the sodium hydroxide solution is 0.1 mol / L. -1 The ultrasonic treatment time was 30 minutes, and the concentration of potato starch was 32 g / L. -1 The stirring time after adding potato starch is 30 minutes, and the mass ratio of konjac glucomannan to potato starch is 1:

2. In step b, the freezing method is liquid nitrogen freezing, the freezing time is 20-40 minutes, the freeze-drying time is 48-72 hours, the freeze-drying temperature is -60℃, and the vacuum degree is 1Pa; In step c, carbonization is performed at a nitrogen flow rate of 300 ml / min. -1 The experiment was conducted in a tubular furnace at 5°C for 5 minutes. -1 The temperature was increased to 500°C at a rate of 100°C and held for two hours, then naturally cooled to room temperature to obtain carbon aerogel. In step d, the volume and mass of the three-dimensional porous carrier are first measured, the total pore volume is obtained based on the porosity of the carbon aerogel, the volume ratio is set to 10%-70% of the theoretical stearic acid loading, and the mass of stearic acid required for each loading is calculated using the total pore volume of C500 and the density of stearic acid. In step d, stearic acid is quantitatively and uniformly coated and impregnated with the help of ethanol. The stearic acid is placed in an oven until it melts, and then poured into 10 ml of hot ethanol at 70°C and stirred rapidly for 5 minutes until completely mixed. In step d, a uniform stearic acid / ethanol solution is placed in an oven at 85°C. The carbon aerogel is then quickly placed into the solution, and air is uniformly extracted to create a vacuum inside the oven. The vacuum is then evacuated again at intervals of 10 minutes, 30 minutes, and 1 hour. After 3 hours, the sample is removed to obtain a phase change composite gel with a porosity of 85.6%-26.7%.

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