Composite coating based on bismuth-doped selenate refrigeration material as well as preparation method and application of composite coating

Abstract

The invention provides a composite coating based on a bismuth-doped selenate refrigeration material as well as a preparation method and application, aims to solve the problems that an existing passive radiation refrigeration material lacks regulation and control in a static state and is difficult to self-adapt, and belongs to the technical field of functional materials and building energy conservation. The chemical composition of the refrigeration material comprises four elements of Sr, Bi, Se and O. The refrigeration material has the photochromic characteristic, and under the action of strong ultraviolet radiation, the body color reversibly changes from white to pink purple; the refrigeration material is prepared by taking SrCO3, SeO2 and Bi2O3 as raw materials and carrying out high-temperature solid-phase reaction; wherein the molar ratio of Sr to Se to Bi in the raw materials is (0.4-0.6): 1: (0.001-0.01); the high-temperature solid-phase reaction comprises the step of heating the mixed raw materials to 900-1000 DEG C in an air atmosphere for sintering. According to the composite coating, the efficient all-weather dynamic thermal management effect can be achieved.

Description

Technical Field

[0001] This application belongs to the field of functional materials and building energy conservation technology, specifically relating to a composite coating based on bismuth-doped selenate refrigeration material, its preparation method, and its application. Background Technology

[0002] Against the backdrop of global warming, the frequent occurrence of extreme heat events poses an unprecedented challenge to power supply systems, exacerbating the imbalance between energy supply and demand. In particular, the large-scale operation of energy-intensive devices such as air conditioners puts immense pressure on power supply systems, leading to power outages or supply restrictions. In this context, the development of efficient and green thermal management technologies is particularly urgent. Improving the efficiency of refrigeration systems is a key aspect of addressing the energy challenge. Passive cooling systems, which can achieve temperature reductions of several degrees Celsius without additional energy consumption, have great potential in assisting air conditioning cooling. Currently, radiative cooling technology has demonstrated unique advantages in dealing with extreme heat, especially in the building sector. This technology is based on the principle of thermal radiation, utilizing the high emissivity of specific materials in the atmospheric infrared transparency window band (8-13 μm) to dissipate heat into the low-temperature outer space in the form of infrared radiation, achieving energy-free cooling.

[0003] However, relying solely on traditional static radiative cooling materials to achieve 24-hour uninterrupted passive heat dissipation faces a core bottleneck: a lack of dynamic spectral control capabilities. Their fixed high reflectivity and high emissivity prevent the materials from differentiating between seasonal and diurnal needs. In winter or on cold nights, they continue to radiate heat into outer space, causing indoor "overcooling" and significantly increasing heating energy consumption, failing to achieve true year-round energy savings. Furthermore, when faced with extreme heat waves, static materials cannot dynamically increase their cooling power based on light intensity, and their limited net cooling capacity per unit area is insufficient to handle peak heat loads, resulting in inadequate cooling. In addition, these materials lack adaptability to environmental changes and cannot automatically adjust their operating mode under cloudy, low-light, or different solar incidence angles, missing opportunities to utilize weak solar radiation for heating, leading to energy efficiency losses due to "blind operation."

[0004] Therefore, there is an urgent need for a passive cooling coating that can adaptively adjust the cooling intensity according to specific lighting conditions. Summary of the Invention

[0005] In view of this, this application provides a composite coating based on bismuth-doped selenate refrigeration material, its preparation method, and its application, in order to solve the problem that existing passive radiation refrigeration materials lack static control and are difficult to adapt.

[0006] To solve the above problems, the technical solution adopted in this application is as follows: In one aspect, this application proposes a bismuth-doped selenate refrigeration material, the chemical composition of which includes four elements: Sr, Bi, Se, and O. The refrigeration material exhibits photochromic properties, undergoing a reversible color change from white to pinkish-purple under strong ultraviolet irradiation. The refrigeration material is prepared from SrCO3, SeO2, and Bi2O3 as raw materials via a high-temperature solid-state reaction. The molar ratio of Sr, Se, and Bi in the raw materials is 0.4-0.6:1:0.001-0.01. The high-temperature solid-state reaction includes sintering the mixed raw materials at 900-1000°C in an air atmosphere.

[0007] Secondly, this application also proposes a composite coating based on the bismuth-doped selenate cooling material described in the first aspect, the composite coating comprising: a radiation cooling layer comprising a first resin matrix and a radiation cooling filler dispersed in the first resin matrix; and a photochromic layer located on at least a portion of the surface of the radiation cooling layer, the photochromic layer comprising a second resin matrix and the bismuth-doped selenate cooling material dispersed in the second resin matrix.

[0008] Preferably, the radiation cooling packing includes inorganic oxide packing, wherein the inorganic oxide packing is at least one of silicon dioxide and titanium dioxide.

[0009] Preferably, the radiation cooling filler comprises silicon dioxide and titanium dioxide in a mass ratio of 1:0.5-2.

[0010] Preferably, the composite coating further includes a substrate and a base layer, the radiation cooling layer is formed on the surface of the base layer, and the base layer is formed on the surface of the substrate; the base layer is composed of a third resin matrix; the thickness of the radiation cooling layer and the photochromic layer is 10-1000 μm.

[0011] Preferably, the mass ratio of the radiation cooling filler to the first resin matrix is ​​1:2-4; and / or, the mass ratio of the bismuth-doped selenate cooling material to the second resin matrix is ​​1:2-4.

[0012] Preferably, the first resin matrix, the second resin matrix, and the third resin matrix are each independently selected from one or more of silicone resin, fluororesin, epoxy resin, polyurethane resin, or hybrid systems thereof.

[0013] Preferably, when the first resin matrix, the second resin matrix, and the third resin matrix are a two-component system, they include a resin component and a curing agent component, and the mass ratio of the resin component to the curing agent component is 1:0.8-1.2.

[0014] Thirdly, this application also proposes a method for preparing a composite coating based on bismuth-doped selenate refrigeration material as described in the second aspect, comprising the following steps: Step 1: Preparing bismuth-doped selenate photochromic refrigeration material, weighing SrCO3, SeO2 and Bi2O3 in a molar ratio of 0.4-0.6:1:0.001-0.01, grinding and mixing, sintering in an air atmosphere, then cooling and grinding and sieving; Step 2: Mixing the precursor of a first resin matrix with a radiation refrigeration filler to form a first mixture; mixing the precursor of a second resin matrix with the bismuth-doped selenate refrigeration material to form a second mixture; Step 3: Applying the first mixture to a substrate layer, curing it to form a radiation refrigeration layer; then applying the second mixture to at least a portion of the surface of the radiation refrigeration layer, curing it to form a photochromic layer, thus obtaining the composite coating.

[0015] Preferably, the specific sintering process in step 1 is as follows: the temperature is raised to 500°C at a rate of 10°C / min, then raised to 900-1000°C at a rate of 5°C / min and held for 5 hours, then cooled to 500°C at a rate of 5°C / min, and finally cooled naturally to room temperature with the furnace.

[0016] Preferably, in step 3, before preparing the radiation cooling layer, the preparation of the substrate layer is also included: placing the substrate on a spin coater, adding the precursor of the third resin matrix, rotating it at 900 rpm for 1 minute, then at 3000 rpm for 2 minutes, and then curing it.

[0017] Preferably, in step 2, the mixing method of the first mixture and / or the second mixture includes grinding and / or stirring.

[0018] Preferably, the curing process of the substrate layer, the radiation cooling layer and / or the photochromic layer in step 3 is as follows: heating to 80-100°C at a heating rate of 5-10°C / min, holding at that temperature for 20-60 minutes, and then cooling to room temperature.

[0019] Preferably, the application method is a blade coating method, in which an adjustable wet film coater is used to control the distance between the blade and the substrate to be 0-1000 μm for uniform coating.

[0020] Fourthly, this application also proposes the application of a composite coating as described in the first aspect or a composite coating prepared by the preparation method as described in the third aspect in buildings, outdoor equipment, automobiles, textiles, machinery and electronic devices.

[0021] In summary, due to the adoption of the above technical solution, the beneficial effects of this application are: This application constructs a two-layer composite structure with an upper photochromic layer and a lower radiative cooling layer. Strong light excites the upper material to darken from white, absorbing solar radiation energy. This, in conjunction with the lower radiative cooling layer, ultimately results in a lower temperature near the cooling component (the bottom surface of the lower layer) compared to a single white radiative cooling layer, achieving highly efficient cooling under extreme daytime heat loads. At night or in low-light environments, when ultraviolet light disappears, the upper photochromic material automatically reverts to its white state, ceasing to absorb solar heat. The presence of the upper material physically provides a certain degree of coverage to the lower layer. This coverage, especially at night and on cloudy days, slightly obstructs the channel for the lower layer to radiate heat directly into the cold night sky, resulting in a slightly weaker overall cooling effect at night compared to an unobstructed pure radiative cooling layer. This reduction effectively avoids the risk of structural cooling or condensation due to excessive cooling at night, acting as an adaptive insulation valve. The system thus achieves intelligent adjustment based on light intensity: maximizing heat dissipation during the day and in strong sunlight to combat high temperatures, and moderately suppressing heat dissipation at night and on cloudy days to prevent overcooling. This achieves highly efficient all-weather dynamic thermal management. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 The X-ray diffraction (XRD) spectra of the cooling materials in Examples 1-2 of this application are shown in comparison with those in Comparative Example 1. Figure 2 The diagram shows the color change test schematics of Embodiments 1-2 and Comparative Example 1 of this application; Figure 3 The illustration shows the composite coating prepared by Example 1 and the radiation cooling material in Example 3 of this application, and the color change diagram after simulated sunlight test. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The components of the embodiments of this application described and shown in the accompanying drawings can be arranged and designed in various different configurations.

[0024] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0025] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0026] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0027] In this application, unless otherwise expressly specified and limited, "above" or "below" a second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of a second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" a second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature. The terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of this application can be implemented in orders other than those illustrated or described herein, and the objects distinguished by "first," "second," etc., are generally of the same class and do not limit the number of objects; for example, a first object can be one or more. Furthermore, "and / or" in the specification and claims indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0028] Existing radiative cooling technology has demonstrated unique advantages in coping with extreme high temperatures, particularly with significant potential applications in the building sector. Based on the principle of thermal radiation, this technology utilizes the high emissivity of specific materials in the atmospheric infrared transparency window band (8-13μm) to dissipate heat into the low-temperature outer space via infrared radiation, achieving energy-free cooling. However, current reliance on traditional static radiative cooling materials for 24-hour passive heat dissipation faces a core bottleneck: a lack of dynamic spectral control capabilities. Their fixed high reflectivity and high emissivity prevent the materials from differentiating between seasonal and diurnal needs, continuing to radiate heat into outer space during winter or cold nights, causing indoor "overcooling" and significantly increasing heating energy consumption, thus failing to achieve true year-round energy savings. Furthermore, when faced with extreme heat waves, static materials cannot dynamically increase their cooling power according to light intensity; their limited net cooling capacity per unit area is insufficient to handle peak heat loads, resulting in inadequate cooling effects. In addition, these materials lack the ability to adapt to environmental changes and cannot automatically adjust their working mode under cloudy, low light or different solar incidence angles, thus missing the opportunity to use weak solar radiation for heating and resulting in energy efficiency loss due to "blind operation".

[0029] This application constructs a two-layer composite structure with an upper photochromic layer and a lower radiative cooling layer. Strong light excites the upper material to darken from white, absorbing solar radiation energy. This, in conjunction with the lower radiative cooling layer, ultimately results in a lower temperature near the cooling component (the bottom surface of the lower layer) compared to a single white radiative cooling layer, achieving highly efficient cooling under extreme daytime heat loads. At night or in low-light environments, when ultraviolet light disappears, the upper photochromic material automatically reverts to its white state, ceasing to absorb solar heat. The presence of the upper material physically provides a certain degree of coverage to the lower layer. This coverage, especially at night and on cloudy days, slightly obstructs the channel for the lower layer to radiate heat directly into the cold night sky, resulting in a slightly weaker overall cooling effect at night compared to an unobstructed pure radiative cooling layer. This reduction effectively avoids the risk of structural cooling or condensation due to excessive cooling at night, acting as an adaptive insulation valve. The system thus achieves intelligent adjustment based on light intensity: maximizing heat dissipation during the day and in strong sunlight to combat high temperatures, and moderately suppressing heat dissipation at night and on cloudy days to prevent overcooling. This achieves highly efficient all-weather dynamic thermal management.

[0030] The following is in conjunction with the appendix Figures 1 to 3 The technical solutions provided in this application will be described in detail through specific embodiments and application scenarios.

[0031] Firstly, such as Figure 1-2As shown, this application proposes a bismuth-doped selenate refrigeration material. The chemical composition of the refrigeration material includes four elements: Sr, Bi, Se, and O. The refrigeration material exhibits photochromic properties, undergoing a reversible color change from white to pinkish-purple under strong ultraviolet irradiation. The refrigeration material is prepared by a high-temperature solid-state reaction using SrCO3, SeO2, and Bi2O3 as raw materials. The molar ratio of Sr, Se, and Bi in the raw materials is 0.4-0.6:1:0.001-0.01. The high-temperature solid-state reaction includes sintering the mixed raw materials at 900-1000℃ in an air atmosphere.

[0032] Specifically, within the temperature range of 900-1000℃, the raw materials undergo decomposition and solid-phase diffusion reactions: SrCO3 decomposes into SrO and CO2, SeO2 partially volatilizes at high temperatures or participates in the formation of a selenate framework, and Bi2O3 acts as a dopant source to provide Bi. 3+ Ions. Due to Bi 3+ The ionic radius of Sr 2+ There are differences, Bi 3+ Partially replaces Sr 2+ Se enters the crystal lattice, simultaneously introducing oxygen vacancies or defect energy levels to maintain charge balance. Se exists in different oxygen coordination forms (e.g., [SeO3]). 2- (e.g., ) participate in the construction of structural units, forming layered or three-dimensional network structures with Sr-O and Bi-O layers. Under ultraviolet light irradiation, the material absorbs photon energy, exciting electrons to transition from the valence band to defect energy levels or intermediate energy levels related to different Bi valence states, forming color centers or metal-oxygen charge transfer states, causing the material's body color to change from white to pinkish-purple; after the ultraviolet irradiation is removed or thermal excitation is applied, the electrons return to the ground state, the defects are restored, and the color reversibly returns to white. This material has a reversible photochromic response, good chemical stability, and tunable color-changing sensitivity, making it suitable for intelligent temperature-controlled coatings, adaptive radiative cooling, and other applications. It can dynamically respond to the solar spectrum by controlling the optical absorption and emission characteristics of the material through light irradiation without the need for external power supply.

[0033] Secondly, such as Figure 3 As shown, this application also proposes a composite coating based on the bismuth-doped selenate cooling material described in the first aspect. The composite coating includes: a radiation cooling layer comprising a first resin matrix and a radiation cooling filler dispersed in the first resin matrix; and a photochromic layer located on at least a portion of the surface of the radiation cooling layer, the photochromic layer comprising a second resin matrix and the bismuth-doped selenate cooling material dispersed in the second resin matrix.

[0034] Specifically, the radiation-cooling filler is typically an inorganic material with high emissivity in the 8-13 μm atmospheric window band. This material can directly scatter heat from the matrix and environment into outer space through the atmospheric window in the form of thermal radiation, achieving passive cooling without consuming energy. The photochromic layer sits atop the radiation-cooling layer; this material undergoes a reversible color change from white to pinkish-purple under strong ultraviolet irradiation.

[0035] This technical solution constructs a two-layer composite structure with an upper photochromic layer and a lower radiative cooling layer. By coupling thermodynamic principles with material properties, it achieves a counterintuitive yet highly efficient all-weather dynamic thermal management effect. Its working principle breaks away from the traditional understanding that radiative cooling must maintain low temperature and low absorption throughout. Instead, it cleverly utilizes the characteristic that photochromic materials become darker, absorb more heat, and reach higher temperatures with stronger light, actively adjusting the system's thermal state. This overcomes the performance limitations of single materials even under strong daylight conditions.

[0036] Under strong daylight, the lower radiative cooling material remains white, absorbing very little sunlight. Cooling is primarily achieved through its high emissivity. If exposed alone, the temperature of the material and the surrounding environment requiring cooling remains low. However, according to the Stefan-Boltzmann law, an object's radiative power is proportional to the fourth power of its absolute temperature. A lower temperature means a relatively weak ability to radiate heat (i.e., instantaneous cooling power), making it difficult to completely offset a large heat load. The radiative cooling layer itself experiences a limited rate of temperature increase, and the overflowing heat load is easily conducted to the surrounding environment. This solution introduces a photochromic layer on top. Strong light excites the upper material to darken, absorbing solar radiation and causing a significant temperature increase. This high temperature is rapidly transferred to the lower radiative cooling layer via heat conduction, causing its temperature to rise accordingly. While temperature increases are generally considered the opposite of cooling, in the radiative cooling mechanism, the increased temperature of the lower layer exponentially increases its thermal radiation power, greatly enhancing its ability to release heat into outer space through the atmospheric window. Although the dark material on top partially blocks the radiation channels and heats up, this strategy of exchanging heat for high radiative flux results in the lower radiative cooling layer dissipating heat per unit time far exceeding the increased heat load due to its temperature rise. Ultimately, this leads to a lower temperature near the cooling body (such as the bottom surface of the lower layer) compared to the single white radiative cooling layer, achieving efficient cooling under extreme heat loads during the day.

[0037] At night or in low-light environments, when ultraviolet light disappears, the upper photochromic material automatically reverts to a white state and stops absorbing solar heat. At this time, the upper material no longer inputs additional heat energy into the system, and the temperature of the lower radiative cooling layer is primarily determined by heat exchange with the environment. It is worth noting that the presence of the upper material physically always provides some coverage to the lower layer. This coverage, especially at night and on cloudy days, slightly obstructs the channel for the lower layer to radiate heat directly into the cold night sky, resulting in a slightly weaker overall cooling effect at night compared to an unobstructed pure radiative cooling layer. However, this reduction effectively avoids the risk of structural cooling or condensation due to excessive cooling at night, acting as an adaptive "thermal valve." The system thus achieves intelligent adjustment based on light intensity: during the day and in strong sunlight, it maximizes heat dissipation to combat high temperatures, while at night and on cloudy days, it moderately suppresses heat dissipation to prevent overcooling.

[0038] This solution overcomes long-standing technical biases. In traditional engineering experience, photochromic materials, due to their darkening under light and heat absorption properties, are often considered the enemy of radiative cooling systems. This is because the goal of radiative cooling is usually to lower the temperature, and people habitually believe that any heat absorption or temperature increase will weaken the cooling effect. However, this technical solution takes a reverse approach, not only not ignoring the temperature-increasing characteristic of photochromic materials, but also using it as a key driving force to improve the overall system performance. Through precise thermal design, the high emissivity of the photochromic layer at high temperatures is transformed into a cooling advantage for the system, successfully coupling two seemingly contradictory material properties (one absorbing heat and increasing temperature, the other radiating heat and decreasing temperature) into a synergistic whole. The photochromic layer not only enhances the cooling efficiency of the radiative cooling layer and improves the overall cooling performance during the day and under strong light, but also weakens the cooling efficiency of the radiative cooling layer at night and on cloudy days, preventing overcooling.

[0039] Furthermore, the radiation cooling packing includes inorganic oxide packing, wherein the inorganic oxide packing is at least one of silicon dioxide and titanium dioxide.

[0040] Specifically, these two materials possess intrinsic high phonon polarization resonance characteristics in the 8-13 μm atmospheric window band, enabling them to generate extremely high thermal emissivity, ensuring the physical basis for the core function of the lower-layer radiative cooling. First, silicon dioxide and titanium dioxide possess excellent thermal stability and chemical inertness, able to withstand the high-temperature shock conducted down from the upper photochromic layer after absorbing heat under strong light. When the upper layer heats up due to photochromic changes and heats the lower layer, the radiative cooling layer composed of inorganic oxide fillers does not undergo phase transitions, decomposition, or performance degradation. Instead, it utilizes its high heat resistance to stably maintain a high emissivity state under high-temperature conditions. This allows the lower-layer material to safely enter a high-power radiation mode driven by the high temperature transferred from the upper layer, fully utilizing the fourth-power gain effect of temperature in the Stefan-Boltzmann law to efficiently convert absorbed heat into radiant energy directed into space, without worrying about thermal damage to the material itself. Second, these two inorganic fillers typically have extremely high solar reflectivity, providing a safety margin for the lower-layer radiative cooling layer. Even when the shading efficiency of the upper photochromic layer fluctuates due to extreme light exposure or aging, the lower white inorganic oxide substrate can still reflect residual direct sunlight to the maximum extent, preventing uncontrolled heat accumulation within the system. This material selection solves the problem that organic radiative cooling materials often have poor heat resistance and difficulty in maintaining high emissivity at high temperatures, making this adaptive cooling solution more valuable for engineering applications and longer in harsh outdoor environments.

[0041] Furthermore, the radiation cooling filler comprises silicon dioxide and titanium dioxide in a mass ratio of 1:0.5-2.

[0042] The mass ratio of silica (SiO2) to titanium dioxide (TiO2) is limited to 1:0.5-2. Excessive TiO2 (exceeding the upper limit) while increasing sunlight reflection may dilute the SiO concentration in the infrared window, leading to a decrease in the maximum radiant power of the lower layer under high-temperature conditions. Conversely, excessive SiO2 (below the lower limit) results in strong infrared emission but insufficient visible light reflectivity, causing the lower layer to absorb excessive solar heat when there is no upper layer to block it or when the upper layer fails, increasing the system's thermal load. The 1:0.5-2 ratio ensures that the lower layer material possesses both sufficient SiO2 for explosive heat radiation upon heating and sufficient TiO2 to maintain its white, low-absorption characteristics, serving as a thermal safety barrier for the system. Furthermore, this mass ratio optimizes the filler packing density and interfacial compatibility within the matrix. Since silica and titanium dioxide particles typically have different particle sizes, the 1:0.5-2 ratio helps form a denser microstructure, reducing heat convection losses due to pores and enhancing the internal thermal conductivity network of the coating. This allows the heat transferred from the upper layer to be distributed more evenly and quickly throughout the entire radiative cooling layer, avoiding material failure caused by local overheating, and also avoiding radiation dead zones caused by uneven heat conduction.

[0043] Furthermore, the composite coating also includes a substrate and a base layer, the radiation cooling layer is formed on the surface of the base layer, and the base layer is formed on the surface of the substrate; the base layer is composed of a third resin matrix; the thickness of the radiation cooling layer and the photochromic layer is 10-1000 μm.

[0044] First, the base layer (third resin matrix), as the key transition interface between the radiative cooling layer and the substrate, solves the problems of poor adhesion and mismatched thermal expansion coefficients between inorganic fillers (silica / titanium dioxide) and different substrate materials (such as metal, concrete, or polymer). Under strong sunlight during the day, the upper photochromic layer absorbs heat and heats up, which is conducted to the lower layer, causing the entire coating system to experience drastic temperature fluctuations. Without a dedicated base layer buffer, the high content of inorganic fillers in the radiative cooling layer is prone to peeling off from the substrate or generating microcracks due to thermal stress concentration, leading to failure of the cooling function. The third resin matrix, through its excellent flexibility and adhesion properties, not only firmly anchors the filler-rich radiative cooling layer, but also effectively absorbs the periodic thermal expansion stress driven by photochromic heating, ensuring the integrity of the coating structure under extreme high-temperature and high-radiation conditions. Second, the minimum thickness limit (10 μm) of the radiative cooling layer and the photochromic layer ensures that the radiative cooling layer has sufficient physical thickness to form a continuous light scattering network. For micron- or nano-sized silica / titanium dioxide fillers, a certain thickness is necessary to achieve sufficient reflection of sunlight (especially in the near-infrared band) (maintaining the low absorption characteristics of the lower layer) and sufficient emission (high emissivity) in the 8-13 μm infrared band. If the thickness is too thin, the substrate will be exposed or the filler packing density will be insufficient, weakening the upper limit of the lower layer's radiative power at high temperatures and preventing full utilization of the heat energy transferred from the upper layer. The upper limit of the thickness of the radiative cooling layer and the photochromic layer (1000 μm) avoids excessive thermal resistance due to excessive coating thickness. If the total coating thickness is too large, the heat conduction path will be prolonged, preventing the high temperature of the upper layer from being transferred to the depths of the radiative cooling layer in time, causing an imbalance in the temperature gradient between the upper and lower layers. This prevents the lower layer from entering a highly efficient radiative state, thus delaying or even negating the gain effect brought by the photochromic layer. A thickness range of 10-1000 μm ensures that heat can penetrate the coating within seconds, allowing the entire radiative cooling layer to respond rapidly to temperature changes in the upper layer and achieve synchronous and efficient heat dissipation. Finally, the presence of the substrate layer allows this composite coating to be widely applied to various substrates with vastly different thermal conductivity (such as fast-conducting metal roofs or slow-conducting concrete walls) without affecting the coating's internal thermal management logic due to differences in substrate thermal properties. Simultaneously, the wide allowable thickness range (two orders of magnitude) reduces the difficulty of large-scale spraying or troweling applications, ensuring that it falls within the optimal thermal response range under various construction conditions. This enables the technical solution to overcome complex site conditions and achieve standardized mass production in practical engineering applications.

[0045] Furthermore, the mass ratio of the radiation cooling filler to the first resin matrix is ​​1:2-4; and / or, the mass ratio of the bismuth-doped selenate cooling material to the second resin matrix is ​​1:2-4.

[0046] First, if the filler ratio is too high (i.e., a ratio less than 1:2, such as 1:1), although theoretically the number of infrared emission sources increases, the excessive inorganic particle content will prevent the resin matrix from completely encapsulating the filler, forming a large number of micropores and interfacial thermal resistance, significantly reducing the overall thermal conductivity of the coating. This will hinder the rapid and uniform transfer of heat generated by the upper photochromic layer to the depths of the radiative cooling layer, resulting in a "hot surface, cold interior" temperature stratification in the lower layer, preventing the entire layer from entering a high-power radiation state. If the filler ratio is too low (i.e., a ratio greater than 1:4, such as 1:5), the excessive resin content will dilute the concentration of the functional filler, resulting in insufficient effective emission thickness in the 8-13μm band. Even if the temperature of the lower layer is raised by the upper layer, it will not be able to explosively dissipate heat due to the lack of sufficient radiation centers. Second, in this scheme, the coating needs to frequently experience drastic temperature fluctuations, such as absorbing heat and heating up under strong sunlight during the day and cooling down at night. For novel refrigeration materials such as bismuth-doped selenates, the interfacial bonding force between them and the resin is particularly critical. A 1:2-4 ratio allows the resin matrix to fully wet each functional particle, forming a robust interfacial coating layer. This effectively buffers internal stress caused by differences in thermal expansion coefficients, preventing particle shedding, microcrack propagation, or coating chalking during repeated heating. For traditional silica / titanium dioxide systems, this ratio avoids increased brittleness due to overly dense filler packing, maintaining the necessary flexibility of the coating. This enables the bilayer structure to withstand additional thermal shocks caused by the photochromic layer during long-term outdoor service. Finally, this specific mass ratio optimizes film formation processability and surface smoothness.

[0047] Furthermore, the first resin matrix, the second resin matrix, and the third resin matrix are each independently selected from one or more of silicone resin, fluororesin, epoxy resin, polyurethane resin, or hybrid systems thereof.

[0048] Because the upper photochromic layer absorbs a large amount of sunlight during the day, the coating surface is subjected to a high photon flux and high oxidative stress environment for a long time. Ordinary acrylic or polyester resins are prone to chain breakage, yellowing, or chalking under this environment, leading to photochromic failure or surface peeling. Fluoropolymers, with their extremely high CF bond energy, provide excellent UV resistance and chemical corrosion resistance, ensuring that the upper layer does not degrade under strong light and heat absorption conditions. Silicone resins possess inorganic Si-O backbones, excellent heat resistance, and good UV transmittance, protecting the internal structure without hindering the isomerization reaction of photochromic molecules. Epoxy / polyurethane and their hybrid systems provide excellent crosslinking density and mechanical strength. The selected silicone and fluoropolymers typically have high thermal stability (resistant to high temperatures without decomposition) and moderate thermal conductivity, maintaining the integrity of the solid structure when the upper layer heats up, while allowing heat to be rapidly conducted downwards, preventing heat accumulation at the interface and causing delamination. The high crosslinking density of epoxy resin further locks in the filler position, preventing sedimentation or agglomeration during thermal cycling. Hybrid systems (such as silicone-modified epoxy and fluorocarbon-modified polyurethane) combine the advantages of different resins, retaining the heat and weather resistance of inorganic components while possessing the flexibility and adhesion of organic components. This is crucial for balancing the stress between rigid inorganic fillers and flexible polymer matrices, especially under conditions of large diurnal temperature variations and frequent thermal expansion and contraction. It effectively prevents microcrack formation, blocks moisture penetration paths, and avoids moisture entering the coating to reduce infrared emissivity or cause substrate corrosion.

[0049] Furthermore, when the first resin matrix, the second resin matrix, and the third resin matrix are a two-component system, they include a resin component and a curing agent component, and the mass ratio of the resin component to the curing agent component is 1:0.8-1.2.

[0050] In this design, the heat absorption of the upper photochromic layer leads to a significant increase in the local temperature of the coating (potentially far exceeding the ambient temperature). If the curing agent is insufficient (ratio < 1:0.8), the cross-linking reaction will be incomplete, resulting in a large number of unreacted functional groups and linear segments within the network. Under high temperatures, these insufficiently cross-linked regions are prone to segment slippage, softening, and even creep, causing displacement or aggregation of internal inorganic fillers (such as silica and bismuth-doped selenates), thus disrupting the microscopic spectral structure. If the curing agent is excessive (ratio > 1:1.2), the excess curing agent not only fails to participate in the reaction to form an effective network but also remains in the matrix as small molecules, forming microscopic defects (such as bubbles and stress concentration points). This lowers the glass transition temperature of the coating, causing it to soften prematurely upon heating.

[0051] Thirdly, this application also proposes a method for preparing a composite coating based on bismuth-doped selenate refrigeration material as described in the second aspect, comprising the following steps: Step 1: Preparing bismuth-doped selenate photochromic refrigeration material, weighing SrCO3, SeO2 and Bi2O3 in a molar ratio of 0.4-0.6:1:0.001-0.01, grinding and mixing, sintering in an air atmosphere, heating to 500℃ at a rate of 10℃ / min, then heating to 900-1000℃ at a rate of 5℃ / min and holding for 5 hours, then cooling to 500℃ at a rate of 5℃ / min, and finally naturally cooling to room temperature with the furnace and grinding and sieving; Step 2: Grinding and / or stirring the precursor of the first resin matrix with the radiation refrigeration filler to form a first mixture; mixing the precursor of the second resin matrix with the bismuth-doped selenate... Step 3: The salt-cooling material is ground and / or stirred to form a second mixture; Step 4: The substrate is placed on a spin coater, and the precursor of the third resin matrix is ​​dropped on it. It is first rotated at 900 rpm for 1 minute, then at 3000 rpm for 2 minutes, and then cured. The first mixture is scraped onto the surface of the substrate layer and cured to form a radiation-cooling layer; then the second mixture is scraped onto at least a portion of the surface of the radiation-cooling layer and cured to form a photochromic layer, thus obtaining the composite coating; The curing procedure of the substrate layer, the radiation-cooling layer and / or the photochromic layer is as follows: the temperature is raised to 80-100℃ at a heating rate of 5-10℃ / min, held for 20-60 minutes, and then cooled to room temperature; During the scraping process, an adjustable wet film coater is used to control the distance between the scraper and the substrate to be 0-1000 μm for uniform coating.

[0052] Specifically, firstly, the specific molar ratio and air-atmosphere sintering process in step 1 are crucial for obtaining high-performance bismuth-doped selenate crystal phases, directly determining the sensitivity and cooling efficiency of the photochromic response. The strontium source ratio (0.4-0.6) precisely controls the stoichiometric balance of the matrix lattice. If the Sr content is too low, it leads to excessive defects in the selenate lattice, affecting crystal integrity and reducing the mobility of photochromic ions; if it is too high, it easily generates impurity phases (such as unreacted SrO or carbonates), diluting the effective functional phase. This ratio ensures the high purity of the main crystal phase, providing a perfect lattice environment for the photochromic effect. The bismuth doping amount (0.001-0.01) is within the trace doping range. Bismuth ions, as the active center for photochromism, will result in insignificant color changes and insufficient endothermic driving force if the concentration is too low; if the concentration is too high, it will cause a concentration quenching effect, leading to lattice distortion and even the formation of non-radiative recombination centers, which not only weakens the color-changing effect but may also destroy the low absorption characteristics required by the underlying layer at night due to the introduction of too many light absorption centers. This ratio ensures that the material rapidly and significantly darkens from white under light (maximizing heat absorption), while fully recovering its white color in the absence of light (maximizing reflection). Crucially for selenate systems, air sintering ensures that selenium is in the correct oxidation state (Se). 4+ / Se 6+ This process avoids the potential formation of elemental selenium (red / black, interfering with color change) or volatile loss under a reducing atmosphere, ensuring batch-to-batch consistency and long-term stability of the material. The segmented heating in step 1 (10℃ / min to 500℃, then 5℃ / min to 900-1000℃) removes CO2 and volatile impurities from strontium carbonate decomposition in the initial rapid heating stage, preventing premature sintering and agglomeration of particles due to prolonged low-temperature holding. The subsequent slow heating (5℃ / min) avoids lattice stress concentration and cracking near the high-temperature phase transition point caused by excessively rapid heating, ensuring that bismuth ions diffuse uniformly and slowly into the selenate lattice substitution sites, forming a uniform solid solution rather than surface enrichment or impurity precipitation. The 5-hour holding at 900-1000℃ provides sufficient thermodynamic time for crystal growth, eliminating internal micropores and dislocations. High crystallinity means fewer non-radiative recombination centers, maximizing photochromic electronic transition efficiency and resulting in a faster endothermic response. The controlled cooling step (5°C / min to 500°C followed by furnace cooling) is crucial to prevent thermal cracking and residual stress in the crystal. Rapid cooling leads to uneven lattice shrinkage, generating numerous microcracks that not only reduce mechanical strength but also become channels for moisture intrusion, causing material failure.

[0053] Secondly, the stepwise mixing in step 2 prevents flocculation or chemical reactions of different functional fillers (radiocooling filler and photochromic material) in the liquid phase, ensuring the independence of their respective functions. In step 2, for inorganic fillers (radiocooling filler) with high filling amounts and functional powders (bismuth-doped selenate), simple stirring is insufficient to break up nano / micron-level agglomerates. The introduction of a grinding step utilizes shear force to forcibly deagglomerate, ensuring that each functional particle is fully coated by the resin precursor. This not only eliminates scattering losses caused by agglomeration (affecting reflectivity / emissivity) but also prevents stress concentration points formed by large particles in the coating, improving the density of the coating. The preparation order of curing the radiocooling layer first and then the photochromic layer in step 3 avoids interfacial missolution and contamination. If a reverse order or mixed coating is used, the slurry rich in highly thermally conductive inorganic fillers (silica / titanium dioxide) is easily washed away or dissolved during coating, or the two solvent layers permeate each other, causing interface blurring. This not only disrupts the distribution of photochromic molecules but may also cause inorganic fillers to float to the surface, blocking the photochromic layer's light response. First, curing the lower layer to form a hard, dense substrate, and then coating the upper layer on its surface, ensures a tight physical contact between the two layers rather than a simple mechanical stacking. This sequence allows heat absorbed by the upper layer to be directly conducted to the high-emissivity filler network of the lower layer through the interface with minimal thermal resistance. If the sequence is reversed, the shrinkage stress during the curing of the lower layer may pull on the already cured upper layer, causing micro-voids (thermal resistance layer) at the interface, hindering heat transfer and preventing the lower layer from sensing the temperature rise of the upper layer in time, thus delaying the increase in radiant power. In step 3, spin coating the substrate layer (900 rpm for 1 minute + 3000 rpm for 2 minutes), the low-speed stage (900 rpm) allows the resin precursor to fully spread and penetrate into the micropores of the substrate surface, forming a strong mechanical interlock (anchoring effect) to solve the coating adhesion problem; the high-speed stage (3000 rpm) uses centrifugal force to remove excess resin and form an extremely thin and uniform transition layer. This ultra-thin transition layer not only smooths out the roughness of the substrate surface, providing an ideal smooth base for subsequent thick film coating, but also acts as a stress buffer layer, effectively releasing the shear stress caused by the difference in thermal expansion coefficients between the upper and lower layers, and preventing delamination.

[0054] Finally, the gentle curing process (heating at 5-10℃ / min, holding at 80-100℃) is tailored to the characteristics of photochromic materials and organic resins, avoiding solvent bursts (pinholes) or thermal degradation of photosensitive molecules caused by rapid high-temperature curing. Slow heating allows the solvent to evaporate in an orderly manner, and the resin network gradually cross-links, forming a defect-free, dense film. The holding temperature of 80-100℃ ensures complete resin curing while remaining well below the thermal quenching temperature of bismuth-doped selenates, perfectly protecting functional activity.

[0055] In summary, this preparation method transforms laboratory-level material properties into engineering-level product reliability through precise thermodynamic control of crystal growth, physicochemical optimization of interfacial bonding, and engineering-precise control of film thickness. It not only solves the common problems of functional materials such as easy agglomeration, cracking, and poor interfacial bonding, but also ensures seamless spatiotemporal integration of the two processes of "photochromic heat absorption" and "radiative cooling heat dissipation" through synergistic matching of process parameters. This is a core technological solution for the industrialization of efficient, long-lasting, and intelligent temperature-controlled coatings.

[0056] Fourthly, this application also proposes the application of a composite coating as described in the first aspect or a composite coating prepared by the preparation method as described in the third aspect in buildings, outdoor equipment, automobiles, textiles, machinery and electronic devices.

[0057] Example 1

[0058] Using a 0.01 mol sample as a standard, the mass of the corresponding raw materials was calculated according to the stoichiometric ratio of each element in the target product, and then accurately weighed using an electronic balance. According to the chemical composition Sr-Se-O-Bi, Sr₂CO₃, SeO₂, and Bi₂O₃ were weighed out in a molar ratio of 0.5:1:0.005, i.e., 0.7382 g of Sr₂CO₃, 1.1096 g of SeO₂, and 0.0233 g of Bi₂O₃ were weighed. To reduce experimental error, the error between the actual weighing result and the calculated target result was controlled within ±0.3 mg. The weighed raw materials were placed in an agate mortar and ground for 30 minutes to ensure thorough and uniform mixing of the powder. After grinding, the mixture was transferred to an alumina crucible, which was then placed in a high-temperature box furnace for sintering. The sintering parameters are as follows: In an air atmosphere, the temperature is raised to 500 ℃ at a heating rate of 10 ℃ / min, then raised to 950 ℃ at a heating rate of 5 ℃ / min, held at that temperature for 5 hours, and then cooled to 500 ℃ at a cooling rate of 5 ℃ / min. After cooling to room temperature, the material is removed and ground until there are no obvious particles, thus obtaining the required cooling material.

[0059] Example 2

[0060] Using a 0.01 mol sample as a standard, the mass of the corresponding raw materials was calculated according to the stoichiometric ratio of each element in the target product, and then accurately weighed using an electronic balance. Sr₂CO₃, SeO₂, and Bi₂O₃ were weighed according to the chemical composition Sr-Se-O-Bi, with a molar ratio of 0.5:1:0.0025. That is, 0.7382 g of Sr₂CO₃, 1.1096 g of SeO₂, and 0.0116 g of Bi₂O₃ were weighed. The weighed raw materials were placed in an agate mortar and ground for 30 minutes to ensure thorough and uniform mixing of the powders. After grinding, the mixture was transferred to an alumina crucible, which was then placed in a high-temperature box furnace for sintering. The sintering parameters were the same as in Example 1.

[0061] Example 3

[0062] Weigh a certain mass of the photochromic material and resin from Example 1, ensuring a sample mass: resin mass ratio of 1:3. The resin is a transparent film-forming matrix material, specifically methylphenyl vinyl silicone resin (organosilicon resin). This resin is a two-component system, including component A (containing vinyl methylphenyl polysiloxane) and component B (containing hydrosilicone oil and platinum catalyst). Component A resin and component B curing agent need to be weighed separately, ensuring the mass ratio of component A to component B is 1:1 (meeting the optimal stoichiometric ratio range of 1:0.8-1.2). After weighing, mix the two for 10-15 minutes. The radiation cooling materials selected are nano-silica (SiO2) and nano-titanium dioxide (TiO2), weighed and mixed uniformly at a mass ratio of 1:1, to ensure uniform mixing with the selected resin. The mass ratio of radiation cooling material to resin is 1:3. Subsequently, a composite coating of photochromic cooling material and radiation cooling material is prepared. The specific method for coating preparation is as follows: First, select a clean and dry glass slide, place it on a slide holder, and drop a drop of the mixed pure resin (i.e., the third resin matrix precursor, composed of components A and B in a 1:1 ratio). Set the spin coater speed program: Level I, 900 rpm, rotation time 1 minute; Level II, 3000 rpm, rotation time 2 minutes. After the glass slide is evenly coated with a layer of resin, place it in a high-temperature box oven for curing. The curing procedure is as follows: heating rate 10 ℃ / min, holding temperature 80-100 ℃, holding time 30 minutes. After cooling to room temperature, remove it to form an interface transition layer. Mix the weighed radiative cooling material with the evenly mixed first resin matrix precursor (composed of components A and B in a 1:1 ratio), and stir for 10-15 minutes until there is no obvious material agglomeration. Then, use a scraping method to evenly coat it onto the glass slide with the cured interface transition layer. Using an adjustable wet film coater, the distance between the doctor blade and the substrate can be adjusted, controlled at 10-1000 μm, and coated at a uniform speed to ensure the slurry spreads evenly on the substrate surface. After coating, the coating is cured using the curing procedure described above to form a radiation-cooling layer. The cured glass slide is then removed, and the weighed bismuth-doped selenate cooling material prepared in Example 1 (raw material molar ratio SrCO3:SeO2:Bi2O3 = 0.5:1:0.005, after segmented sintering and grinding and sieving) is mixed with a uniformly mixed second resin matrix precursor (composed of components A and B in a 1:1 ratio). The mixture is stirred for 10-15 minutes until no obvious material agglomeration occurs. This mixture is then uniformly coated onto the cured radiation-cooling coating using a doctor blade coating method, following the aforementioned method. After coating, the coating is cured using the curing procedure described above to form a photochromic layer. Once cured, the target composite coating is obtained.

[0063] Comparative Example 1

[0064] Using a 0.01 mol sample as a standard, the mass of the corresponding raw materials was calculated according to the stoichiometric ratio of each element in the target product, and then accurately weighed using an electronic balance. Strontium carbonate (SrCO3) and selenium dioxide (SeO2) were weighed according to the chemical composition Sr-Se-O, with a molar ratio of 0.5:1. That is, 0.7382 g of SrCO3 and 1.1096 g of SeO2 were weighed. Bismuth oxide (Bi2O3) was not added in this comparative example. The weighed raw materials were placed in an agate mortar and ground for 30 minutes to ensure thorough and uniform mixing of the powders. After grinding, the mixture was transferred to an alumina crucible, which was then placed in a high-temperature box furnace for sintering. The sintering parameters were the same as in Example 1 (i.e., in an air atmosphere, the temperature was increased to 500°C at a rate of 10°C / min, then increased to 950°C at a rate of 5°C / min and held for 5 hours, then decreased to 500°C at a rate of 5°C / min, and finally cooled naturally in the furnace).

[0065] Comparative Example 2

[0066] Weigh out a radiation-cooling material composed of nano-silica (SiO2) and nano-titanium dioxide (TiO2) in a mass ratio of 1:1, and mix it with resin in a mass ratio of 1:3. The resin is selected as described in Example 1 (i.e., methylphenyl vinyl silicone resin, with component A and component B mixed in a mass ratio of 1:1). Prepare a single radiation-cooling coating. The preparation method is as follows: First, select a clean and dry glass slide, place it on a slide holder, and drop a drop of the mixed pure resin (the third resin matrix precursor). Set the spin coater speed program: Level I speed setting is 900 rpm for 1 minute; Level II speed setting is 3000 rpm for 2 minutes. After the glass slide is evenly coated with a layer of resin, place it in a high-temperature box oven for curing. The curing program is as follows: heating rate is 10℃ / min, holding temperature is 80-100℃, holding time is 30 minutes, and then cool to room temperature before removing it. The weighed radiative cooling material is mixed with the homogeneously mixed first resin matrix precursor and stirred for 10-15 minutes until no obvious material agglomeration occurs. Then, it is evenly coated onto a glass slide with a pre-cured resin layer using a doctor blade. An adjustable wet film coater is used, with the distance between the doctor blade and the substrate adjustable from 0-1000 μm, and the coating is applied at a uniform speed to ensure even spread of the slurry on the substrate surface. After coating, the coating is cured using the curing procedure described above. Once cured, the target radiative cooling coating (excluding the photochromic layer) is obtained.

[0067] Comparative Example 3

[0068] Weigh a certain mass of the bismuth-doped selenate refrigeration material (containing Bi doping) prepared in Example 1, and mix it uniformly with resin at a mass ratio of 1:3. The resin is selected as described in Example 1 (i.e., methylphenyl vinyl silicone resin, with component A and component B mixed at a mass ratio of 1:1). Prepare a single photochromic refrigeration coating. The preparation method is as follows: First, select a clean and dry glass slide, place it on a slide holder, and drop a drop of the mixed pure resin (the third resin matrix precursor). Set the spin coater speed program: Level I speed setting is 900 rpm, rotation time is 1 minute; Level II speed setting is 3000 rpm, rotation time is 2 minutes. After a layer of resin is uniformly coated on the surface of the glass slide, place it in a high-temperature box furnace for curing. The curing program is as follows: heating rate is 10℃ / min, holding temperature is 80-100℃, holding time is 30 minutes, and then it is cooled to room temperature and removed. The weighed cooling material from Example 1 is mixed with the uniformly mixed second resin matrix precursor and stirred for 10-15 minutes until no obvious material agglomeration occurs. Then, it is uniformly coated onto a glass slide with a pre-cured resin layer using a blade coating method. An adjustable wet film coater is used, with the distance between the blade and the substrate adjustable from 0-1000 μm, and coating is performed at a uniform speed to ensure the slurry spreads evenly on the substrate surface. After coating, the coating is cured using the curing procedure described above. Upon curing, the target photochromic cooling coating (excluding the radiation cooling layer) is obtained.

[0069] The phosphor samples prepared above were subjected to X-ray diffraction tests using a DX-2700 BH X-ray diffractometer from Dandong Haoyuan Instrument Co., Ltd. After ensuring the samples were pure, photochromic tests were first performed on Example 1, Comparative Example 1, and Comparative Example 2, respectively. The test methods are as follows: 1. Pour a small amount of powder sample onto a weighing paper and take a picture of the powder before it is exposed to ultraviolet light.

[0070] 2. Place each powder sample separately under a UV lamp for 6 minutes, using a composite light source with wavelengths including 254 nm and 365 nm.

[0071] 3. After irradiation, take a photo of the powder and compare it with the photo before irradiation to confirm whether the refrigeration material of Example 1 can change color and whether the color change effect meets the requirements of the refrigeration design described above.

[0072] 4. Then, take the samples from Comparative Examples 1-2 again and place them under a UV lamp for 10-15 minutes. After irradiation, observe whether a color change occurs.

[0073] The glass slide used to prepare the cooling coating is 26 mm in size. 76 mm, the spin coater used was a KW-4B spin coater from Beijing Saidekais Electronic Co., Ltd.

[0074] The method for testing the cooling effect is as follows.

[0075] 1. After preparation, the color-changing effect of the coating was first tested. The coating was placed under a sun lamp to simulate the sunlight of a sunny summer day and exposed for 30 minutes to confirm that the coating prepared in Example 1 could change color and meet the requirements of the above-mentioned cooling design.

[0076] 2. Subsequently, Examples 2, 3, and 4, and an untreated glass slide were placed on the same open space and tested on a sunny morning from 11:00 AM to 12:00 PM. Five points were marked on the surface of the coating and the glass slide, and the temperature of the upper and lower surfaces of the coating was measured with a temperature gun at regular intervals to verify the cooling effect of the coating.

[0077] Characterization results

[0078] Figure 1 The X-ray diffraction (XRD) spectra of the cooling materials in Examples 1-2 and Comparative Example 1 are compared. The diffraction peak positions of the main samples (Examples 1-2 and Comparative Example 1) match well with the Sr-Se-O standard PDF card, and the crystal phase composition is clear. Among them, the intensity of the peaks observed at 26.7° and 28.4° in Examples 1 and 2 is inconsistent with the intensity of the standard PDF card, but it does not affect the phase identification. In addition, the angle of the diffraction peak representing the (400) crystal plane in the undoped sample of Comparative Example 1 is shifted to a smaller angle compared with the 28.4° of the (400) crystal plane represented by the Sr-Se-O standard PDF card. After doping with bismuth ions, the angle of the diffraction peak representing the (400) crystal plane in Examples 1 and 2 is shifted to a larger angle compared with Comparative Example 1, and matches the Sr-Se-O standard PDF card. According to Bragg's law 2dsinθ = nλ (where n is the reflection order, λ is the wavelength of the incident X-ray, d is the interplanar spacing, and θ is the angle between the incident ray, the reflected ray, and the reflecting crystal plane), introducing ions with different effective ionic radii will cause an angular shift in the diffraction peak position. Therefore, the shift of the 28.4° diffraction peak relative to Comparative Example 1 in Examples 1 and 2 indicates successful bismuth ion doping, and the bismuth ion doping also promotes the relative diffraction intensity of the (400) crystal plane. With the spectra of Examples 1 and Comparative Examples 1-2 showing good matching with the standard PDF card and the absence of obvious impurity phases, it is confirmed that the target refrigeration material with high crystallinity was successfully prepared under the above conditions.

[0079] Figure 2Schematic diagrams of color change tests for Examples 1-2 and Comparative Example 1 are shown. The results indicate that Example 1 exhibits a powder color change after 6 minutes of UV irradiation, while Examples 2 and Comparative Example 1 do not. Further UV irradiation of Examples 2 and Comparative Example 1 revealed that Example 2 exhibits a powder color change after 10-15 minutes of UV irradiation, while Comparative Example 1 consistently fails to show a powder color change under UV irradiation. This indicates that Comparative Example 1 cannot achieve a relatively rapid powder color change under UV irradiation. Comparative Example 2 does exhibit a powder color change under UV irradiation, but the change rate is slow and inferior to the performance demonstrated by Example 1.

[0080] Figure 3 The diagram shows the coating prepared in Example 3 using the material from Example 1 and a radiation-cooling material, and a schematic of its color change after simulated sunlight testing. The results indicate that the coating prepared in Example 1 using the resin still possesses the intended color-changing capability.

[0081] Table 1 presents four sets of test results: the first set of composite cooling coatings, the second set of single photochromic cooling coatings, the third set of single radiation cooling coatings, and the fourth set of glass slides, all tested in a natural environment. The data obtained are the average values ​​of five selected points. The results show that the composite coating represented by Example 2 has a better cooling effect compared to Comparative Example 3, Comparative Example 4, and the glass slides.

[0082] Table 1. Cooling effect tests of glass slides, Example 2, Comparative Example 2, and Comparative Example 3

[0083] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between the various embodiments can be referred to interchangeably.

[0084] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0085] Furthermore, it should be noted that the scope of the methods and apparatus in the embodiments of this application is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. In addition, features described with reference to certain examples may be combined in other examples.

Claims

1. A bismuth-doped selenate refrigeration material, characterized in that, The chemical composition of the refrigeration material contains four elements: Sr, Bi, Se, and O. The refrigeration material has photochromic properties and undergoes a reversible color change from white to pinkish-purple under strong ultraviolet irradiation. The refrigeration material is prepared by high-temperature solid-state reaction using SrCO3, SeO2 and Bi2O3 as raw materials; wherein the molar ratio of Sr, Se and Bi in the raw materials is 0.4-0.6:1:0.001-0.01; the high-temperature solid-state reaction includes sintering the mixed raw materials at 900-1000℃ in an air atmosphere.

2. A composite coating based on the bismuth-doped selenate refrigeration material of claim 1, characterized in that, The composite coating includes: A radiation cooling layer comprising a first resin matrix and radiation cooling filler dispersed in the first resin matrix; A photochromic layer, the photochromic layer being located on at least a portion of the surface of the radiation-cooling layer, the photochromic layer comprising a second resin matrix and the bismuth-doped selenate cooling material dispersed in the second resin matrix.

3. The composite coating based on bismuth-doped selenate refrigeration material according to claim 2, characterized in that, The radiation cooling packing includes inorganic oxide packing, wherein the inorganic oxide packing is at least one of silicon dioxide and titanium dioxide; The radiation cooling filler comprises silicon dioxide and titanium dioxide in a mass ratio of 1:0.5-2.

4. The composite coating based on bismuth-doped selenate refrigeration material according to claim 3, characterized in that, The composite coating further includes a substrate and a base layer, wherein the radiation cooling layer is formed on the surface of the base layer, and the base layer is formed on the surface of the substrate; the base layer is composed of a third resin matrix; the thickness of the radiation cooling layer and the photochromic layer is 10-1000 μm; And / or, the mass ratio of the radiation cooling filler to the first resin matrix is ​​1:2-4; And / or, the mass ratio of the bismuth-doped selenate refrigeration material to the second resin matrix is ​​1:2-4.

5. The composite coating based on bismuth-doped selenate refrigeration material according to any one of claims 2-4, characterized in that, The first resin matrix, the second resin matrix, and the third resin matrix are each independently selected from one or more of silicone resin, fluororesin, epoxy resin, polyurethane resin, or hybrid systems thereof; When the first resin matrix, the second resin matrix, and the third resin matrix are a two-component system, they include a resin component and a curing agent component, and the mass ratio of the resin component to the curing agent component is 1:0.8-1.

2.

6. A method for preparing a composite coating based on a bismuth-doped selenate refrigeration material as described in any one of claims 2-5, characterized in that, Includes the following steps: Step 1: Prepare bismuth-doped selenate photochromic refrigeration material. Weigh SrCO3, SeO2 and Bi2O3 in a molar ratio of 0.4-0.6:1:0.001-0.01, grind and mix them, sinter them in air atmosphere, then cool them down and grind and sieve them. Step 2: Mix the precursor of the first resin matrix with the radiation-cooling filler to form a first mixture; mix the precursor of the second resin matrix with the bismuth-doped selenate cooling material to form a second mixture; Step 3: Apply the first mixture to the base layer and cure it to form a radiation cooling layer; then apply the second mixture to at least a portion of the surface of the radiation cooling layer and cure it to form a photochromic layer, thus obtaining the composite coating.

7. The method for preparing a composite coating based on bismuth-doped selenate refrigeration material according to claim 6, characterized in that, The specific sintering process in step 1 is as follows: heat up to 500℃ at a rate of 10℃ / min, then heat up to 900-1000℃ at a rate of 5℃ / min and hold for 5 hours, then cool down to 500℃ at a rate of 5℃ / min, and finally cool naturally to room temperature with the furnace.

8. The method for preparing a composite coating based on bismuth-doped selenate refrigeration material according to claim 6, characterized in that, In step 3, before preparing the radiation cooling layer, the preparation of the substrate layer is also included: the substrate is placed on a spin coater, the precursor of the third resin matrix is ​​dropped on, first rotated at 900 rpm for 1 minute, then rotated at 3000 rpm for 2 minutes, and then cured. And / or, in step 2, the mixing method of the first mixture and / or the second mixture includes grinding and / or stirring.

9. The method for preparing a composite coating based on bismuth-doped selenate refrigeration material according to claim 8, characterized in that, The curing procedure for the substrate layer, the radiation cooling layer and / or the photochromic layer in step 3 is as follows: heat to 80-100°C at a heating rate of 5-10°C / min, hold at that temperature for 20-60 minutes, and then cool to room temperature; And / or, the application method is a blade coating method, in which an adjustable wet film coater is used to control the distance between the blade and the substrate to be 0-1000 μm for uniform coating.

10. The application of a composite coating as described in any one of claims 2-5 or a composite coating prepared by the preparation method as described in any one of claims 6-9 in buildings, outdoor equipment, automobiles, textiles, machinery and electronic devices.

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