Preparation method of double-layer dynamic thermal management material

Abstract

The invention discloses a preparation method of a double-layer dynamic thermal management material, which comprises the following steps: step 1, preparing a radiation cooling layer: dissolving cellulose acetate in an organic solvent, and uniformly dispersing nano hexagonal boron nitride to form a mixed solution, a substrate is coated with the mixed solution, and then a radiation cooling layer of a porous structure is formed through treatment of a non-solvent induced phase separation method; 2, a phase change heat storage layer is prepared, specifically, the wood subjected to delignification treatment is soaked in a mixed solution containing a silane coupling agent and a phase change material, and the phase change heat storage layer is formed through vacuum treatment and drying; and 3, compounding the radiation cooling layer and the phase change heat storage layer to form the double-layer dynamic heat management material. The double-layer dynamic heat management material prepared by the invention realizes efficient heat radiation and dynamic temperature regulation and control through the synergistic effect of the radiation cooling layer and the phase change heat storage layer.

Description

Technical Field

[0001] The present invention relates to the technical field of new materials, and particularly relates to a preparation method of a double-layer dynamic thermal management material. Background Art

[0002] Energy is the key to social and economic development. At present, building energy consumption has accounted for half of the total energy consumption, and the energy consumption during the building operation stage is the largest. It is crucial to use zero-carbon energy for green upgrading in the building thermal management link and establish a low-carbon transformation of energy utilization. Passive radiative cooling technology uses the cold universe (3K) as a cold source to achieve heat radiation, with the characteristic of achieving cooling without external energy input, and it is a low-carbon representative in the field of thermal management. The key to this technology lies in two aspects. On the one hand, it is to maximize the reflection of the material in the ultraviolet-visible-near-infrared band (0.3-2.5 μm) of the solar band, so as to avoid more heat being generated due to the material absorbing more light. On the other hand, it is to maximize the emission of the material in the mid-infrared band (8-13 μm), that is, the atmospheric window band, where the atmospheric transmittance is relatively high, and the material can radiate its own heat in the form of electromagnetic waves into the universe.

[0003] Currently, radiative cooling has been developed for building materials such as coatings, wood, and insulation foams. However, due to the passive and continuous heat radiation effect of radiative cooling, and the cooling effect at night is much greater than that during the day, this brings the problem of overcooling at night. Especially when the radiative cooling material is used for human thermal management or building thermal management, people expect its thermal management ability to be dynamically adjusted according to the actual outdoor temperature to achieve all-weather dynamic radiative cooling. In addition, limited by the structural characteristics of the material for radiative cooling, its cooling power has a certain limit. Therefore, improving the cooling performance of the radiative cooling material also needs to be focused on. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of a double-layer dynamic thermal management material. The double-layer dynamic thermal management material prepared by the present invention realizes efficient heat radiation and dynamic temperature regulation through the synergistic effect of a radiative cooling layer and a phase change heat storage layer.

[0005] The technical solution of the present invention: A preparation method of a double-layer dynamic thermal management material includes the following steps:

[0006] Step 1, preparing a radiative cooling layer: Dissolve cellulose acetate in an organic solvent and uniformly disperse hexagonal boron nitride nanoparticles to form a mixed solution. After coating the mixed solution on a substrate, treat it by non-solvent induced phase separation method to form a radiative cooling layer with a porous structure;

[0007] Step 2: Prepare the phase change heat storage layer: Immerse the delignified wood in a mixed solution containing a silane coupling agent and a phase change material, and form the phase change heat storage layer through vacuum treatment and drying.

[0008] Step 3: Composite the radiative cooling layer with the phase change heat storage layer to form a double-layer dynamic thermal management material.

[0009] In the preparation method of the above double-layer dynamic thermal management material, in Step 1, the organic solvent is composed of a compound of N,N-dimethylformamide and acetone; in the mixed solution, the mass ratio of cellulose acetate, nano-hexagonal boron nitride, N,N-dimethylformamide, and acetone is 1:0.1-0.3:4-6:4-6.

[0010] In the preparation method of the aforementioned double-layer dynamic thermal management material, in Step 1, the acetyl content in the cellulose acetate is 32.0 wt%, and the hydroxyl content is 8.7 wt%; the particle size of the nano-hexagonal boron nitride is 500 nm.

[0011] In the preparation method of the aforementioned double-layer dynamic thermal management material, in Step 1, the non-solvent induced phase separation method includes: The solution coated on the substrate is naturally air-dried for 3-8 minutes and then immersed in pure water for 15-25 minutes.

[0012] In the preparation method of the aforementioned double-layer dynamic thermal management material, in Step 2, the delignification treatment of the wood includes: Immerse the wood in a sodium chlorite solution with a concentration of 1-2 g / mL, adjust the pH to 4-5, and heat it in a water bath at 80-90 °C for 4-8 hours.

[0013] In the preparation method of the aforementioned double-layer dynamic thermal management material, in Step 2, the silane coupling agent is methyltrimethoxysilane, the phase change material is octadecane, and the mass ratio of the silane coupling agent, the phase change material, and water is 1:0.8-1.2:1.5-2.5.

[0014] In the preparation method of the aforementioned double-layer dynamic thermal management material, in Step 2, the preparation process of the mixed solution containing the silane coupling agent and the phase change material is as follows:

[0015] First, add hydrochloric acid to water to adjust the pH value to 3.5-4.5, where the concentration of hydrochloric acid is 0.1 mol / L, and then add methyltrimethoxysilane and octadecane for blending to make the mass ratio of the silane coupling agent, the phase change material, and water 1:1:2.

[0016] In the preparation method of the aforementioned double-layer dynamic thermal management material, in Step 2, the pressure of the vacuum treatment is 0.1 MPa, and the impregnation time is 1-3 hours.

[0017] In the preparation method of the aforementioned double-layer dynamic thermal management material, in step 3, the radiative cooling layer is located in the upper layer, the phase change heat storage layer is located in the lower layer, and the radiative cooling layer and the phase change heat storage layer are bonded through the viscosity of the phase change heat storage layer.

[0018] Compared with the prior art, on the one hand, the radiative cooling layer of the present invention is prepared by the non-solvent induced phase separation method, so it has a very high porosity, and most of the pore diameters are distributed in the range of 0.2 μm - 2.5 μm. In addition, the nano-hexagonal boron nitride with a particle size of 500 nm greatly improves the reflectivity at about 500 nm, so it has a relatively high reflectivity in the entire ultraviolet-visible-near-infrared spectral band. At the same time, cellulose acetate and nano-hexagonal boron nitride have a certain mid-infrared emissivity in the atmospheric window band of 8 μm - 13 μm. On the other hand, the phase change heat storage layer is impregnated into the delignified wood through the hydrolysis and condensation of methyltrimethoxysilane and blending with octadecane. The impregnation of octadecane enables the phase change heat storage layer to have a better phase change heat storage capacity, and the impregnation after the hydrolysis and polymerization of methyltrimethoxysilane enables the phase change heat storage layer to have a relatively high emissivity in the atmospheric window to supplement the insufficient emissivity of the radiative cooling layer. Therefore, the double-layer dynamic thermal management material of the present invention not only significantly improves the overall emissivity of the material to enhance the radiative cooling ability, but also enables the material to have a dynamic temperature control effect before and after the phase change temperature of octadecane. Description of the Drawings

[0019] Figure 1 It is a scanning electron microscope photograph of the radiative cooling layer of the double-layer dynamic thermal management material in the embodiment of the present application;

[0020] Figure 2 It is a scanning electron microscope photograph of the phase change heat storage layer of the double-layer dynamic thermal management material in the embodiment of the present application;

[0021] Figure 3 It is a solar spectral reflectivity curve graph of the double-layer dynamic thermal management material in the embodiment of the present application in the 0.3 μm - 2.5 μm band;

[0022] Figure 4 It is an emissivity curve graph of the double-layer dynamic thermal management material in the embodiment of the present application in the 8 μm - 13 μm atmospheric window band;

[0023] Figure 5 It is a differential scanning calorimeter test curve graph of the double-layer dynamic thermal management material in the embodiment of the present application. Detailed Embodiments

[0024] The following embodiments further illustrate the present invention, but do not serve as the basis for limiting the present invention.

[0025] Embodiment 1: A preparation method of a double-layer dynamic thermal management material, comprising the following steps:

[0026] Step 1: Prepare the radiative cooling layer: Dissolve cellulose acetate in an organic solvent, and uniformly disperse hexagonal boron nitride nanoparticles to form a mixed solution. After coating the mixed solution on a substrate, treat it by the non-solvent induced phase separation method to form a radiative cooling layer with a porous structure.

[0027] Step 2: Prepare the phase change heat storage layer: Immerse delignified wood in a mixed solution containing a silane coupling agent and a phase change material, and form the phase change heat storage layer through vacuum treatment and drying.

[0028] Step 3: Composite the radiative cooling layer and the phase change heat storage layer to form a double-layer dynamic thermal management material.

[0029] Among them, the radiative cooling layer refers to a porous structure layer formed by cellulose acetate and hexagonal boron nitride nanoparticles through non-solvent induced phase separation. Specifically, cellulose acetate can be dissolved in a mixed solvent of N,N-dimethylformamide and acetone, disperse hexagonal boron nitride nanoparticles, coat the substrate, and achieve phase separation through immersion treatment in pure water. This structure enhances the solar band reflection and mid-infrared emission capabilities through the porous morphology, reduces heat absorption, and improves the radiative heat dissipation efficiency.

[0030] Among them, the phase change heat storage layer refers to a composite material layer formed by delignified wood loaded with a silane coupling agent and a phase change material. Specifically, after removing the lignin of wood with sodium chlorite solution, immerse it in a mixed solution of methyltrimethoxysilane and octadecane, and vacuum treat it to fill the wood pores with the phase change material. This layer regulates the temperature fluctuation through the latent heat storage and release of the phase change material, and the silane coupling agent enhances the interfacial bonding stability between the wood and the phase change material.

[0031] Among them, the non-solvent induced phase separation method refers to naturally drying the substrate coated with the mixed solution and then immersing it in pure water, and forming a porous structure by the exchange of solvent and non-solvent. This method adjusts the pore size and distribution by controlling the air drying time and immersion duration, and optimizes the optical properties of the radiative cooling layer.

[0032] Among them, the delignification treatment refers to removing lignin from wood by high-temperature treatment with sodium chlorite solution under acidic conditions. Specifically, it can be treated at pH 4-5 and 80-90 °C for 4-8 hours. This treatment expands the internal pore volume of the wood, and improves the subsequent loading amount and heat storage capacity of the phase change material.

[0033] Among them, the vacuum treatment refers to placing the wood impregnated with the mixed solution in a negative pressure environment of 0.1 MPa, and driving the mixed solution to penetrate into the internal pores of the wood through the pressure difference. This process ensures that the phase change material fully fills the three-dimensional pore structure of the delignified wood, and improves the heat capacity of the heat storage layer.

[0034] The core innovation of the present invention lies in achieving dynamic thermal management through the dual-layer collaborative effect of a radiative cooling layer and a phase change heat storage layer. The radiative cooling layer utilizes the high reflectivity of the porous structure and nano-hexagonal boron nitride in the solar band and the high emissivity in the atmospheric window band to continuously dissipate heat. The phase change heat storage layer actively regulates the diurnal temperature difference by means of the latent heat of phase change of the phase change material. After the two are combined, the phase change material absorbs excess heat during the day to reduce the temperature of the radiation layer, enhancing the heat dissipation efficiency, and releases heat at night to alleviate excessive cooling, forming a temperature self-regulation mechanism. This structure breaks through the passive working limitation of a single radiative cooling material and realizes all-weather thermal management by combining active heat storage and release with passive heat dissipation.

[0035] The working process and principle of the present invention are as follows: The dual-layer dynamic thermal management material realizes all-weather dynamic temperature balance through the composite structure of the radiative cooling layer and the phase change heat storage layer. The radiative cooling layer is composed of cellulose acetate and nano-hexagonal boron nitride, and a porous structure is formed by the non-solvent induced phase separation method. The porous structure enhances the light scattering and heat radiation efficiency, and nano-hexagonal boron nitride optimizes the reflection and emission characteristics of the material in specific bands. The radiative cooling layer reflects ultraviolet-visible-near-infrared light in the solar band to reduce heat absorption, and at the same time radiates heat outward through the atmospheric window in the mid-infrared band, realizing continuous passive heat dissipation. The phase change heat storage layer uses delignified wood as a carrier to load the phase change material. The delignified wood has a rich pore structure, and the vacuum impregnation process is used to ensure a high loading amount of the phase change material. The silane coupling agent enhances the interfacial bonding stability between the wood and the phase change material. The phase change heat storage layer absorbs excess heat during the day through the latent heat of phase change and releases it at night, and actively adjusts the heat storage and release rhythm according to the ambient temperature change. After the two-layer structure is combined, a synergistic effect is formed: the radiative cooling layer continuously conducts passive heat dissipation, and the phase change heat storage layer actively adjusts the heat storage and release rhythm according to the ambient temperature change, jointly realizing all-weather dynamic temperature balance.

[0036] As a preferred embodiment, the solution of the present application is specifically implemented as follows: First, prepare the radiative cooling layer. Dissolve cellulose acetate in a mixed solvent of N,N-dimethylformamide and acetone, add nano-hexagonal boron nitride powder, and form a uniform mixed solution by ultrasonic dispersion. Coat the mixed solution on a glass substrate, air-dry it naturally, and then immerse it in pure water for non-solvent induced phase separation treatment to form a radiative cooling layer with a porous structure. Secondly, prepare the phase change heat storage layer. Select poplar wood as the raw material and conduct delignification treatment in sodium chlorite solution. Immerse the treated wood in a mixed solution containing methyltrimethoxysilane, octadecane, and water, conduct impregnation treatment in a vacuum environment, and then dry to form the phase change heat storage layer. Finally, place the radiative cooling layer above the phase change heat storage layer and bond them using the viscosity of the phase change heat storage layer to form a dual-layer dynamic thermal management material.

[0037] Through the above solutions, the present invention solves the problems that the radiative cooling material cannot dynamically adjust its heat management ability according to the actual temperature and has excessive cooling at night, while improving the cooling performance of the radiative cooling material. The radiative cooling layer continuously conducts passive heat dissipation, and the phase change heat storage layer actively adjusts the heat storage and heat release rhythm according to the ambient temperature change. The synergistic effect of the two achieves all-weather dynamic temperature balance. The radiative cooling layer with a porous structure enhances the light scattering and thermal radiation efficiency. The introduction of nano hexagonal boron nitride further optimizes the reflection and emission characteristics of the material in a specific wavelength band, improving the cooling performance. The phase change heat storage layer uses delignified wood as a carrier, which has a rich pore structure. The vacuum impregnation process ensures a high loading amount of the phase change material, enhancing the temperature regulation ability. The use of silane coupling agent improves the interfacial bonding stability between the wood and the phase change material, extending the service life of the material.

[0038] Example 2. In some of the above solutions of the present application, if the type, ratio of the organic solvent, and the non-solvent induced phase separation method are not properly selected, problems such as insufficient dissolution of cellulose acetate and uneven dispersion of nanomaterials may occur, which will in turn affect the formation of the porous structure of the radiative cooling layer and the final heat management performance.

[0039] In response to this, the present invention further proposes that the organic solvent is composed of a mixture of N,N-dimethylformamide and acetone; in the mixed solution, the mass ratio of cellulose acetate, nano hexagonal boron nitride, N,N-dimethylformamide, and acetone is 1:0.1-0.3:4-6:4-6.

[0040] Among them, N,N-dimethylformamide, as a strongly polar solvent, its dissolving ability can ensure the full stretching of the cellulose acetate polymer chains. Acetone, as a low-boiling solvent, preferentially volatilizes during the subsequent natural air-drying process to trigger the initial stage of phase separation. The mass ratio of the two solvents is limited to the balanced range of 4-6:4-6, so that the solution viscosity not only meets the leveling requirement of the coating process but also avoids the collapse of the pore structure caused by solvent residue. When the addition amount of nano hexagonal boron nitride is controlled within 10%-30% of the mass of cellulose acetate, its solid content in the mixed solution reaches 5%-12wt%. This concentration range not only ensures the formation of a continuous reflection network of nanosheets in the matrix but also prevents the aggregation phenomenon caused by excessive addition.

[0041] Specifically, in the coating process, when the mass ratio of N,N-dimethylformamide to acetone is 1:1, the solvent evaporation gradient in the natural air-drying stage promotes the orderly arrangement of cellulose acetate molecular chains at the solvent-non-solvent interface, forming through pores with a pore diameter of 50-200nm. In the subsequent pure water immersion stage, the residual N,N-dimethylformamide diffuses into the water, and finally a three-dimensional network structure with a porosity of 85%-92% is formed.

[0042] As a preferred embodiment, the solution of the present invention is specifically implemented as follows: When preparing the radiative cooling layer, N,N-dimethylformamide and acetone are selected as organic solvents. First, cellulose acetate is dissolved in N,N-dimethylformamide, then nano-hexagonal boron nitride powder is added, and finally acetone is added for mixing. The mass ratio of cellulose acetate, nano-hexagonal boron nitride, N,N-dimethylformamide, and acetone in the mixed solution can be 1:0.2:5:5. After the mixed solution is stirred evenly, it is coated on the substrate by the doctor blade method and subjected to non-solvent induced phase separation treatment to form a radiative cooling layer with a porous structure. Through the above technical solution, the present application can optimize the dissolution of cellulose acetate and the dispersion process of nano-hexagonal boron nitride. The compounding of N,N-dimethylformamide and acetone can synergistically adjust the polarity and volatility of the solvent, promote the full dissolution of cellulose acetate, and accelerate the solvent evaporation in the subsequent phase separation stage, thereby more efficiently forming a uniform porous structure. Controlling the mass ratio of cellulose acetate, nano-hexagonal boron nitride, and the two solvents within a specific range can ensure that the dispersion concentration of the nano-material can not only enhance the reflection performance of the radiative cooling layer but also not damage the stability of the porous structure due to excessive aggregation. By precisely controlling the solvent ratio, the solution viscosity and the rate of non-solvent induced phase separation can be balanced, avoiding problems such as poor leveling or pore structure collapse during the coating process, and ultimately achieving the efficient preparation and performance improvement of the radiative cooling layer.

[0043] As a preferred embodiment, when preparing the radiative cooling layer, cellulose acetate with an acetyl content of 32.0 wt% and a hydroxyl content of 8.7 wt% is selected, and nano-hexagonal boron nitride with a particle size of 500 nm is used. Among them, the acetyl content of cellulose acetate realizes chemical stability by balancing the solubility and film-forming property of the material in organic solvents. For example, when the acetyl content is lower than 32.0 wt%, the insufficient rigidity of the material may cause the collapse of the porous structure, while higher than this value reduces the hydrophilicity and affects the phase separation effect. The hydroxyl content enhances the interfacial binding ability by providing polar groups. For example, when the hydroxyl content is 8.7 wt%, the intermolecular force between cellulose acetate and nano-hexagonal boron nitride is optimized. The particle size of nano-hexagonal boron nitride controls the matching relationship between dispersion stability and optical properties. For example, particles with a size of 500 nm can improve the scattering efficiency while avoiding agglomeration and sedimentation, and their size matches the wavelength of the solar band.

[0044] As a preferred embodiment, the substrate after coating in the present invention is first air-dried naturally at room temperature for 5 minutes to form a preliminary phase separation interface on the surface of the solution. Subsequently, the substrate is immersed in pure water for 20 minutes. During this process, water, as a non-solvent, exchanges with the remaining solvent, promoting the final phase separation and curing of the mixed solution of cellulose acetate and nano-hexagonal boron nitride to form a radiation cooling layer with a porous structure. Through the above technical solution, the present invention realizes the precise control of the non-solvent induced phase separation process. Thereby, it avoids the problem that excessive solvent residue caused by too short air-drying time destroys the pore morphology, or the problems of structural collapse or interlayer delamination caused by too long soaking time. Further, this method ensures the uniformity and stability of the porous structure, and improves the mechanical properties and optical properties of the radiation cooling layer.

[0045] Example 3, in some of the above solutions of the present application, if the reaction conditions for the delignification treatment are not precisely controlled, and the types, ratios of the mixed solution containing the silane coupling agent and the phase change material, and the process parameters of the vacuum treatment are improper, it may have an adverse impact on the phase change heat storage layer.

[0046] For this, the delignification treatment of the wood in the present invention includes the following steps: First, prepare a sodium chlorite solution with a concentration of 1.5 g / mL, and immerse the wood (balsa wood) sample in the solution. Subsequently, use dilute hydrochloric acid to adjust the pH of the solution to 4.5. Place the solution soaked with the wood sample in a constant temperature water bath, set the temperature to 85 °C, and heat-treat for 6 hours. After the treatment is completed, take out the wood sample and rinse it repeatedly with deionized water until neutral, and finally dry it in an oven at 60 °C for 24 hours. Through the above technical solution, the present invention realizes the balance between the lignin removal efficiency and the wood structure integrity during the wood delignification process. The precise control of the sodium chlorite solution concentration, pH value, reaction temperature and time ensures the efficient removal of lignin while retaining the porous structure and mechanical strength of the wood. This treatment method forms a uniform porous structure, providing sufficient voids and interface binding sites for the subsequent impregnation of the phase change material, thereby improving the heat storage performance and heat conduction efficiency of the phase change heat storage layer.

[0047] As a preferred embodiment, when preparing the phase change heat storage layer, methyltrimethoxysilane is selected as the silane coupling agent and octadecane is selected as the phase change material. First, a mixed solution is prepared by mixing methyltrimethoxysilane, octadecane, and water in a mass ratio of 1:1:2. Specifically, hydrochloric acid with a concentration of 0.1 mol / L is added to 100 g of water to adjust the pH value to 4.0, and then 50 g of methyltrimethoxysilane and 50 g of octadecane are added and stirred evenly to form a mixed solution. The wood treated by delignification is impregnated in the above mixed solution, treated under a vacuum condition of 0.1 MPa for 2 hours, and then the wood is taken out and dried at 60 °C for 24 hours to finally form the phase change heat storage layer. Through the above technical solution, the present invention realizes the precise control of the composition and ratio of the mixed solution. Methyltrimethoxysilane, as the silane coupling agent, reacts with the hydroxyl groups on the wood surface through the methoxy groups in its molecular structure to form stable chemical bonds, enhancing the interfacial bonding force between the phase change heat storage layer and the delignified wood matrix, and avoiding the shedding of the phase change material during use due to thermal stress or external force. Octadecane, as the phase change material, has a suitable phase change temperature and can efficiently store and release heat through solid-liquid phase change when the ambient temperature changes. By limiting the mass ratio of the silane coupling agent, the phase change material, and water to 1:1:2, it not only ensures the full progress of the hydrolysis reaction of the silane coupling agent to form a uniform coupling agent solution, but also enables the phase change material to be fully infiltrated with the coupling agent solution, avoiding stratification of the mixed solution or uneven dispersion of the phase change material due to unbalanced ratios, thereby ensuring that the finally formed phase change heat storage layer has stable heat storage capacity and lasting interfacial bonding performance. By controlling the pH value and component ratio of the mixed solution, the hydrolysis reaction of the silane coupling agent is promoted, and a three-dimensional network structure that uniformly coats the phase change material is formed. This structure not only increases the loading amount of the phase change material in the wood, but also improves the bonding strength between the phase change heat storage layer and the wood matrix.

[0048] As a preferred embodiment, the mixed solution and the wood are placed in a vacuum treatment device, the vacuum pump is started to reduce the ambient pressure to 0.1 MPa, and the impregnation treatment is continuously carried out while maintaining this vacuum degree. The impregnation time is controlled within the range of 1 hour to 3 hours. After the impregnation is completed, the wood is taken out and placed in a drying oven for curing treatment to enable the silane coupling agent to be fully bonded to the wood surface, and at the same time enable octadecane to be stably loaded inside the wood pores. Through the above technical solution, the present invention effectively balances the contradiction between the penetration depth of the mixed solution in the wood pores and the material structure stability. Under the vacuum condition of 0.1 MPa, the mixed solution fully enters the internal pore structure of the wood driven by the pressure difference, avoiding the collapse of the wood fiber structure caused by excessive pressure. The impregnation time of 1 - 3 hours not only ensures the full reaction of the silane coupling agent with the hydroxyl groups on the wood surface, but also enables the phase change material to be evenly distributed in the pores. The thus formed phase change heat storage layer has stable loading capacity, providing a structural basis for the dynamic thermal management performance of the subsequent composite material.

[0049] Example 4: In some of the above solutions of this application, a double-layer dynamic thermal management material is proposed by combining a radiative cooling layer and a phase change heat storage layer. However, in the composite process, if an external adhesive or mechanical fixing method is used, it may lead to an increase in the interfacial thermal resistance between layers, affecting the heat conduction efficiency. In addition, if the bonding between layers is not stable, it is prone to peeling under thermal stress or environmental changes, resulting in the failure of the material structure.

[0050] In response to this, the present invention further proposes that the radiative cooling layer is located in the upper layer and the phase change heat storage layer is located in the lower layer. The radiative cooling layer and the phase change heat storage layer are bonded through the viscosity of the phase change heat storage layer. Among them, the upper layer of the radiative cooling layer is positioned so that it directly faces the incident direction of solar radiation. This structural arrangement can effectively cover the ultraviolet-visible-near infrared band of the solar spectrum. For example, when the particle size of nano hexagonal boron nitride in the radiative cooling layer is controlled at 500 nm, its scattering characteristics can achieve efficient reflection in the 0.3-2.5 μm band. The setting of the phase change heat storage layer in the lower layer is adapted to its thermal buffering function. When the chemical bonding between the silane coupling agent and the delignified wood fiber in this layer reaches a stable state, the three-dimensional network structure formed inside can effectively load the phase change material octadecane. Through the above technical solutions, the present invention solves the problem of increased interfacial thermal resistance between composite layers caused by external adhesives, and at the same time avoids the risk of structural peeling caused by mechanical fixing. The bonding interface forms a stable connection through the chemical bonding between the silane coupling agent and the wood fiber, ensuring efficient heat conduction between layers. The viscous surface of the phase change heat storage layer is in direct contact with the radiative cooling layer to form a continuous heat conduction path, so that the heat reflected during the day can be quickly transferred to the heat storage layer for storage, and the heat released by the heat storage layer at night can be reversely conducted through the interface to the radiative cooling layer to achieve thermal equilibrium.

[0051] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings of the embodiments will be briefly introduced below. Figure 1 Scanning electron microscope photograph of the radiative cooling layer of the double-layer dynamic thermal management material in the embodiment of this application; Figure 2 Scanning electron microscope photograph of the phase change heat storage layer of the double-layer dynamic thermal management material in the embodiment of this application; Figure 3 Solar spectrum reflectivity curve of the double-layer dynamic thermal management material in the 0.3 μm - 2.5 μm band in the embodiment of this application; Figure 4 Emissivity curve of the double-layer dynamic thermal management material in the 8 μm - 13 μm atmospheric window band in the embodiment of this application; Figure 5 Differential scanning calorimeter test curve of the double-layer dynamic thermal management material in the embodiment of this application, where the double-layer dynamic thermal management materials tested are all prepared by the process of the preferred embodiment. Comprehensive Figures 1 to 5In the attached drawings of the present invention, the double-layer dynamic thermal management material of the present invention has a solar spectral weighted average reflectivity of 96.2% in the solar spectral band of 0.3 μm - 2.5 μm, a solar spectral weighted average reflectivity of 98.4% in the visible-near infrared band of 0.36 μm - 1.16 μm, a weighted average reflectivity of 93.8% in the atmospheric window band of 8 μm - 13 μm, and a crystallization enthalpy of 124.578 J / g and a melting enthalpy of 126.23 J / g.

[0052] In summary, on the one hand, the radiation cooling layer of the present invention is prepared by the non-solvent induced phase separation method, so it has a very high porosity, and most of the pore diameters are distributed in the range of 0.2 μm - 2.5 μm. In addition, the nano-hexagonal boron nitride with a particle size of 500 nm greatly improves the reflectivity around 500 nm, so it has a relatively high reflectivity in the entire ultraviolet-visible-near infrared spectral band. At the same time, cellulose acetate and nano-hexagonal boron nitride have a certain mid-infrared emissivity in the atmospheric window band of 8 μm - 13 μm. On the other hand, the phase change heat storage layer is impregnated into the interior of the delignified wood by hydrolyzing and condensing methyltrimethoxysilane and blending with octadecane. The impregnation of octadecane enables the phase change heat storage layer to have better phase change heat storage capacity, and the impregnation after hydrolysis and polymerization of methyltrimethoxysilane enables the phase change heat storage layer to have a higher emissivity in the atmospheric window to supplement the insufficient emissivity of the radiation cooling layer. Thus, the double-layer dynamic thermal management material of the present invention not only significantly improves the overall emissivity of the material to enhance the radiation cooling ability, but also enables the material to have a dynamic temperature control effect before and after the phase change temperature of octadecane.

Claims

1. A method for preparing a double-layer dynamic thermal management material, characterized in that: The following steps are involved: Step 1, preparing a radiation cooling layer: dissolving cellulose acetate in an organic solvent and uniformly dispersing nano hexagonal boron nitride to form a mixed solution, coating the mixed solution on a substrate and then treating the mixed solution by a non-solvent induced phase separation method to form a radiation cooling layer with a porous structure; Step 2, preparing a phase change heat storage layer: immersing the delignified wood in a mixed solution containing a silane coupling agent and a phase change material, and forming a phase change heat storage layer through vacuum treatment and drying; Step 3: Compound the radiation cooling layer and the phase change heat storage layer to form a double-layer dynamic thermal management material.

2. The method for preparing a double-layer dynamic thermal management material according to claim 1, characterized in that: In step 1, the organic solvent is composed of N,N-dimethylformamide and acetone; in the mixed solution, the mass ratio of cellulose acetate, nano hexagonal boron nitride, N,N-dimethylformamide and acetone is 1:0.1-0.3:4-6:4-6.

3. The method for preparing the double-layer dynamic thermal management material according to claim 2, characterized in that: In step 1, the acetyl content of the cellulose acetate is 32.0 wt %, and the hydroxyl content is 8.7 wt %; the particle size of the nano hexagonal boron nitride is 500 nm.

4. The method for preparing a double-layer dynamic thermal management material according to claim 1, characterized in that: In step 1, the non-solvent induced phase separation method comprises: the solution coated on the substrate is naturally air-dried for 3-8 minutes and then immersed in pure water for 15-25 minutes.

5. The method for preparing a double-layer dynamic thermal management material according to claim 1, characterized in that: In step 2, the delignification treatment of the wood includes: immersing the wood in a 1-2 g / mL sodium chlorite solution, adjusting the pH to 4-5, and heating the wood in a water bath at 80-90° C. for 4-8 hours.

6. The method for preparing a double-layer dynamic thermal management material according to claim 1, characterized in that: In step 2, the silane coupling agent is methyltrimethoxysilane, the phase change material is octadecane, and the mass ratio of the silane coupling agent, the phase change material and water is 1:0.8-1.2:1.5-2.

5.

7. The method for preparing the double-layer dynamic thermal management material according to claim 6, characterized in that: In step 2, the preparation process of the mixed solution containing the silane coupling agent and the phase change material is as follows: First, hydrochloric acid is added to water to adjust the pH value to 3.5-4.5, wherein the concentration of the hydrochloric acid is 0.1 mol / L, and then methyltrimethoxysilane and octadecane are added and blended to make the mass ratio of the silane coupling agent, the phase change material and the water be 1:1:

2.

8. The method for preparing a double-layer dynamic thermal management material according to claim 1, characterized in that: In step 2, the pressure of the vacuum treatment is 0.1 MPa, and the immersion time is 1-3 hours.

9. The method for preparing a double-layer dynamic thermal management material according to claim 1, characterized in that: In step 3, the radiation cooling layer is located at the upper layer, and the phase change heat storage layer is located at the lower layer, and the radiation cooling layer and the phase change heat storage layer are bonded together by the viscosity of the phase change heat storage layer.

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