Radiative cooling film and method of making the same

Abstract

The application relates to a radiation refrigeration film and a preparation method thereof, and the material of the radiation refrigeration film is a composite material of a high-entropy alloy and a carbon-coated metal oxide; wherein the high-entropy alloy has a polarization-nonpolarization reversible structure; when the external temperature is greater than 300K, the high-entropy alloy forms a polarization state; when the external temperature is less than 300K, the high-entropy alloy forms a nonpolarization state; the carbon-coated metal oxide has a nano-micron composite pore structure; the porosity of micropores in the radiation refrigeration film is 15%-25%, and the porosity of nanopores is 8%-15%. The radiation refrigeration film has a dynamic adjustable emissivity characteristic, can adapt to day-night or seasonal temperature differences, and significantly improves refrigeration efficiency.

Description

Technical Field

[0001] This application relates to the field of radiation cooling technology, and in particular to a radiation cooling film and its preparation method. Background Technology

[0002] Traditional radiative cooling materials typically have structures such as photonic crystals, metasurfaces, or multilayer dielectric films. These structures directly define the peak position, bandwidth, and intensity of the spectral response. Since emissivity and reflectivity follow Kirchhoff's thermal radiation law and Fresnel's equations, the fixed structural design constrains spectral selectivity, making it impossible to dynamically adjust during use. It also lacks the ability to respond to changes in external environments such as temperature, irradiance, and humidity, thus limiting its application in the field of intelligent temperature control.

[0003] This static design flaw is particularly prominent in the field of intelligent thermal management. Ideally, radiant cooling materials should be able to actively switch between "cooling" and "insulating" modes according to environmental needs. However, traditional structures cannot suppress radiative heat loss under low-temperature conditions, especially in winter or at night in frigid regions. Due to their fixed structure, traditional radiant cooling materials maintain a high mid-infrared emissivity, continuously radiating heat into outer space, leading to "overcooling." This not only further lowers the indoor temperature but also requires the heating system to operate at high loads to compensate for excessive radiative loss, significantly increasing the total energy consumption of the heating system and thus reducing the overall energy-saving benefits of radiant cooling technology throughout the year.

[0004] Therefore, when traditional radiative cooling materials are applied in environments with large temperature differences or regions with distinct seasons, their static emissivity characteristics limit their application prospects in the field of intelligent temperature control. There is an urgent need to develop dynamic radiative cooling materials with adjustable emissivity and environmental adaptability. Summary of the Invention

[0005] Based on this, it is necessary to provide a radiation cooling film and its preparation method to address the above problems. The radiation cooling film provided in this application has dynamically adjustable emissivity, which can adapt to day and night or seasonal temperature differences and significantly improve cooling efficiency.

[0006] A radiation-cooling film, wherein the radiation-cooling film is a composite material of a high-entropy alloy and a carbon-coated metal oxide;

[0007] The high-entropy alloy has a polarized-nonpolarized reversible structure. When the external temperature is >300K, the high-entropy alloy forms a polarized state; when the external temperature is <300K, the high-entropy alloy forms a nonpolarized state.

[0008] The carbon-coated metal oxide has a nano-micro composite pore structure;

[0009] The porosity of the micron-sized pores in the radiation-cooling film is 15%~25%, and the porosity of the nano-sized pores is 8%~15%.

[0010] In one embodiment, the carbon-coated metal oxide is 4 wt% to 6 wt% of the high-entropy alloy.

[0011] In one embodiment, the carbon-coated metal oxide is selected from at least one of carbon-coated MgO, carbon-coated ZnO, carbon-coated Al2O3, carbon-coated ZrO2, and carbon-coated CoO;

[0012] And / or, the particle size of the carbon-coated metal oxide is 300 nm to 800 nm;

[0013] And / or, the chemical formula of the high-entropy alloy is Ti a Cr b Hf c Nb d Al e O 2-x , and a:b:c:d:e=(0.18~0.22):(0.18~0.22):(0.18~0.22):(0.38~0.42):(0.18~0.22), x=0.05~0.15.

[0014] The radiation-cooling film described in this application employs a composite material of a specific high-entropy alloy and carbon-coated metal oxide. The high-entropy alloy possesses a polarization-nonpolarization reversible structure, and its responsiveness to external temperature or electric field stimuli can trigger reversible lattice distortion, thereby dynamically controlling the material's optical properties and achieving dynamic emissivity adjustment. Simultaneously, the carbon-coated metal oxide has a nano-micron composite pore structure, enabling the radiation-cooling film to possess specific micron and nanopore porosities. Combined with the high-entropy alloy, this enhances Mie scattering and photonic crystal effects. Therefore, the radiation-cooling film described in this application not only possesses broad-spectrum and highly efficient optical control capabilities but also suppresses the "overcooling" phenomenon, systematically solving the core problem of the inability of traditional radiation-cooling films to dynamically control their properties.

[0015] A method for preparing a radiation-cooling film includes the following steps:

[0016] A high-entropy alloy is prepared by mixing metal oxide raw materials in a specified ratio, followed by reduction sintering and annealing. The chemical formula of the high-entropy alloy is Ti. a Cr b Hf c Nb d Al e O 2-x, and a:b:c:d:e=(0.18~0.22):(0.18~0.22):(0.18~0.22):(0.38~0.42):(0.18~0.22), x=0.05~0.15;

[0017] Carbon-coated metal oxides were obtained by heat treatment of organometallic salt gels.

[0018] A composite material was prepared by mixing and pressing a high-entropy alloy with a carbon-coated metal oxide and then subjecting it to spark plasma sintering.

[0019] A radiation cooling film is prepared on the substrate surface by depositing a composite material as the target material and then etching it with a wet method.

[0020] In one embodiment, the reduction sintering temperature is 1400℃~1800℃, and the time is 1h~3h;

[0021] And / or, the annealing temperature is 350℃~450℃, and the time is 1h~3h.

[0022] In one embodiment, the organometallic salt gel includes at least one of magnesium citrate gel, zinc citrate gel, aluminum citrate gel, zirconium citrate gel, and cobalt citrate gel.

[0023] In one embodiment, the heat treatment includes: first, heating to 350°C to 450°C at a rate of 4°C / min to 6°C / min, holding in air for 1 hour to 3 hours, and then heating to 550°C to 650°C at a rate of 8°C / min to 12°C / min, holding in a protective gas for 1 hour to 3 hours.

[0024] In one embodiment, when the high-entropy alloy is mixed with carbon-coated metal oxide, the carbon-coated metal oxide accounts for 4 wt% to 6 wt% of the high-entropy alloy.

[0025] In one embodiment, the temperature of the spark plasma sintering is 1000℃~1400℃, the heating rate is 80℃ / min~120℃ / min, and the holding time is 4min~6min.

[0026] In one embodiment, the deposition temperature is 600°C to 800°C, the time is 40 min to 80 min, and the laser frequency is 4 Hz to 6 Hz.

[0027] The preparation method described in this application prepares a high-entropy alloy with polarized and non-polarized regions through a solid-state reaction-reduction sintering method. Oxygen vacancies are arranged on a specific crystal plane (110) to form reversible polarization channels, achieving reversible polarization. The response to external temperature or electric field stimulation can trigger reversible lattice distortion, thereby dynamically controlling the optical properties of the material and achieving dynamic adjustment of emissivity. At the same time, carbon-coated metal oxides are combined with high-entropy alloys containing oxygen vacancies, and a spark plasma sintering process is used to promote particle rearrangement, forming a dense and crack-resistant target material for deposition and film formation. A hierarchical porous structure is generated through etching, enhancing Mie scattering and photonic crystal effects.

[0028] Therefore, the radiation-cooling film prepared by the method provided in this application has dynamically adjustable emissivity, which can adapt to day-night or seasonal temperature differences, significantly improving cooling efficiency. Furthermore, it exhibits broadband phonon scattering, enabling high emission across the entire atmospheric window, with a radiation power density as high as 120 W / m². 2 . Attached Figure Description

[0029] 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 recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 The image shows a SEM image of the radiation cooling film prepared in Example 1. Detailed Implementation

[0031] To facilitate understanding of this application, it will be described in more detail below. However, it should be understood that this application can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments or implementations only and is not intended to be limiting of the application. In this application, when numerical ranges are mentioned, unless otherwise specified, such ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Furthermore, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0033] This application provides a radiation cooling film, wherein the radiation cooling film is a composite material of a high-entropy alloy and a carbon-coated metal oxide;

[0034] The high-entropy alloy has a polarized-nonpolarized reversible structure. When the external temperature is >300K, the high-entropy alloy forms a polarized state; when the external temperature is <300K, the high-entropy alloy forms a nonpolarized state.

[0035] The carbon-coated metal oxide has a nano-micro composite pore structure;

[0036] The porosity of the micron-sized pores in the radiation-cooling film is 15%~25%, and the porosity of the nano-sized pores is 8%~15%.

[0037] The radiation-cooling film described in this application employs a composite material of a specific high-entropy alloy and carbon-coated metal oxide. The high-entropy alloy possesses a polarization-nonpolarization reversible structure, and its responsiveness to external temperature or electric field stimuli can trigger reversible lattice distortion, thereby dynamically controlling the material's optical properties and achieving dynamic emissivity adjustment. Simultaneously, the carbon-coated metal oxide has a nano-micron composite pore structure, enabling the radiation-cooling film to possess specific micron and nanopore porosities. Combined with the high-entropy alloy, this enhances Mie scattering and photonic crystal effects. Therefore, the radiation-cooling film described in this application not only possesses broad-spectrum and highly efficient optical control capabilities but also suppresses the "overcooling" phenomenon, systematically solving the core problem of the inability of traditional radiation-cooling films to dynamically control their properties.

[0038] In one embodiment of this application, the chemical formula of the high-entropy alloy is Ti. a Cr b Hf c Nb d Al e O 2-xAnd a:b:c:d:e=(0.18~0.22):(0.18~0.22):(0.18~0.22):(0.38~0.42):(0.18~0.22), x=0.05~0.15. Among them, the difference in high electronegativity of Ti, Cr, and Hf (Ti 4+ Cr 3+ Hf 4+ Al, with its affinity for oxygen, promotes the formation of polar regions in the localized polar bond (Ti-O-Cr). Simultaneously, the d0 electrons of Ti-Cr-Hf hybridize with the O2p orbitals, forming a local dipole moment. As a p-block element, Al has a small atomic radius (143 pm) and readily forms Al-O bonds (high bond energy). During acid etching, Al oxides are relatively easy to etch, which can help form a richer hierarchical porous structure. Furthermore, Al… 3+ The introduction of Al can adjust the electron concentration of the system, indirectly affecting the valence equilibrium of Nb, and the low atomic weight of Al helps maintain the lightweight properties of high-entropy alloys; Nb provides metallic bonding and electron delocalization (Nb 5+ 、Nb 4+ Mixed valence states) form a nonpolar region, stabilizing the high-entropy matrix, while Nb 4+ / Nb 5+ Valence variation induces dynamic migration of oxygen vacancies, driving metal-insulator phase transitions, and the Jahn-Teller effect causes Nb-O polyhedral distortion (Nb 4+ ,4d1) to form a reversible polarized-nonpolarized nanostructure.

[0039] Specifically, when the ambient temperature is >300K, Nb 5+ Occupying vacancies, forming a chemically disordered high configurational entropy, forming a polarized state, releasing entropy (ΔS>0) and absorbing heat; when the external temperature is <300K, oxygen vacancies are arranged in an orderly manner along a specific crystal plane (110), the non-polarized state is reconstructed, entropy is absorbed (ΔS<0), and infrared radiation is enhanced, thereby adapting to day and night or seasonal temperature differences and realizing dynamic adjustment of emissivity.

[0040] Furthermore, the high-entropy alloy described in this application not only has reversible phase transformation, but the Cr and Ti in the alloy also help to form a dense passivation surface oxide layer. On the one hand, it can prevent the high-entropy alloy from being corroded by acid during subsequent processing. On the other hand, it can give the radiation cooling film excellent environmental stability and a long service life, with performance degradation of less than 22% within three years.

[0041] In one embodiment of this application, the carbon-coated metal oxide is 4wt% to 6wt% of the high-entropy alloy, including but not limited to any one of 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, or any range between two of them.

[0042] In one embodiment of this application, the carbon-coated metal oxide is selected from at least one of carbon-coated MgO, carbon-coated ZnO, carbon-coated Al2O3, carbon-coated ZrO2, and carbon-coated CoO, preferably carbon-coated MgO or carbon-coated ZnO.

[0043] In one embodiment of this application, the particle size of the carbon-coated metal oxide is preferably 300nm~800nm, including but not limited to any one of 300nm, 400nm, 500nm, 600nm, 700nm, and 800nm ​​or any range between two of them.

[0044] This application provides a method for preparing a radiation-cooling film, comprising the following steps:

[0045] Step 1: Mix the metal oxide raw materials according to the specified ratio, and obtain a high-entropy alloy through reduction sintering and annealing. The chemical formula of the high-entropy alloy is Ti. a Cr b Hf c Nb d Al e O 2-x , and a:b:c:d:e=(0.18~0.22):(0.18~0.22):(0.18~0.22):(0.38~0.42):(0.18~0.22), x=0.05~0.15;

[0046] Step 2: Heat-treat the organometal salt gel to obtain carbon-coated metal oxides;

[0047] Step 3: After mixing and pressing the high-entropy alloy with carbon-coated metal oxide, the mixture is subjected to spark plasma sintering to obtain the composite material.

[0048] Step 4: Using the composite material as the target material, a radiation cooling film is deposited on the surface of the substrate and then wet-etched to obtain the radiation cooling film on the surface of the substrate.

[0049] In step one of this application, a specific ratio of metal oxide raw materials is subjected to solid-state reaction-reduction sintering and synergistic annealing treatment to arrange oxygen vacancies on a specific crystal plane (110), forming a reversible polarization channel to achieve reversible polarization. Among them, the high electronegativity difference of Ti, Cr, and Hf (Ti 4+ Cr 3+ Hf 4+Al, with its affinity for oxygen, promotes the formation of polar regions in the localized polar bond (Ti-O-Cr). Simultaneously, the d0 electrons of Ti-Cr-Hf hybridize with the O2p orbitals, forming a local dipole moment. As a p-block element, Al has a small atomic radius (143 pm) and readily forms Al-O bonds (high bond energy). During acid etching, Al oxides are relatively easy to etch, which can help form a richer hierarchical porous structure. Furthermore, Al… 3+ The introduction of Al can adjust the electron concentration of the system, indirectly affecting the valence equilibrium of Nb, and the low atomic weight of Al helps maintain the lightweight properties of high-entropy alloys; Nb provides metallic bonding and electron delocalization (Nb 5+ 、Nb 4+ Mixed valence states) form a nonpolar region, stabilizing the high-entropy matrix, while Nb 4+ / Nb 5+ Valence variation induces dynamic migration of oxygen vacancies, driving metal-insulator phase transitions, and the Jahn-Teller effect causes Nb-O polyhedral distortion (Nb 4+ ,4d1) to form a reversible polarized-nonpolarized nanostructure.

[0050] Specifically, when the ambient temperature is >300K, Nb 5+ Occupying vacancies, forming a chemically disordered high configurational entropy, forming a polarized state, releasing entropy (ΔS>0) and absorbing heat; when the external temperature is <300K, oxygen vacancies are arranged in an orderly manner along a specific crystal plane (110), the non-polarized state is reconstructed, entropy is absorbed (ΔS<0), and infrared radiation is enhanced, thereby adapting to day and night or seasonal temperature differences and realizing dynamic adjustment of emissivity.

[0051] Furthermore, the high-entropy alloy prepared in this application not only has reversible phase transformation, but the Cr and Ti in the alloy also help to form a dense passivation surface oxide layer. On the one hand, it can prevent the high-entropy alloy from being corroded by acid during subsequent processing. On the other hand, it can give the radiation cooling film excellent environmental stability and a long service life, with performance degradation of less than 22% within three years.

[0052] In one embodiment of this application, the metal oxide raw materials are mixed in a proportion including but not limited to wet ball milling. Specifically, the wet ball milling process includes: using zirconia balls with a diameter of 3 mm to 5 mm as grinding balls, with a ball-to-material ratio of (8 to 12):1; using organic solvents such as ethanol as a medium, with the liquid surface covering 75% to 85% of the grinding balls; setting the ball mill speed to 200 rpm to 400 rpm; and setting the ball milling time to 22 h to 26 h; after ball milling, vacuum drying at 40°C to 80°C for 10 h to 14 h, and sieving to obtain precursor powder.

[0053] It is understandable that, considering the oxidation and other problems that may occur during the mixing process, reducing agents and binders can be added to the metal oxide raw materials. Specifically, the reducing agent includes, but is not limited to, carbon powder; the amount of reducing agent is 0.1wt% to 2wt% of the total amount of metal oxide raw materials; the binder includes, but is not limited to, polyvinyl alcohol; the amount of binder is 0.1wt% to 1wt% of the total amount of metal oxide raw materials.

[0054] In one embodiment of this application, the reduction sintering temperature is preferably 1400℃~1800℃, including but not limited to any one of 1400℃, 1500℃, 1600℃, 1700℃, and 1800℃, or a range between any two; the time is preferably 1h~3h, including but not limited to any one of 1h, 1.5h, 2h, 2.5h, and 3h, or a range between any two. It is understood that the reducing atmosphere includes, but is not limited to, a mixture of hydrogen and an inert gas, and is preferably a mixture of hydrogen and argon.

[0055] In one embodiment of this application, the annealing temperature is preferably 350℃~450℃, including but not limited to any one of 350℃, 380℃, 400℃, 420℃, and 450℃, or a range between any two; the annealing time is preferably 1h~3h, including but not limited to any one of 1h, 1.5h, 2h, 2.5h, and 3h, or a range between any two. It is understood that the reducing atmosphere includes but is not limited to a protective gas, preferably a mixture of hydrogen and argon.

[0056] Ti, Cr, Hf, Al, and Nb oxides are dissolved in hot reducing sintering to form a high-entropy alloy. Furthermore, some lattice oxygen is converted into gaseous H2O through the reduction action of H2.

[0057] This creates oxygen vacancies; subsequently, through low-temperature annealing, VO-Nb is formed. 4+ Defect pair.

[0058] It should be noted that this application does not limit the apparatus for reduction sintering and annealing, including but not limited to tubular furnaces; a heating rate of 1℃ / min to 5℃ / min is preferred during reduction sintering and annealing; before sintering, the temperature is preferably raised to 800℃ to 1200℃ at a heating rate of 15℃ / min to 25℃ / min, and calcined in air atmosphere for 1h to 2h to remove residual organic matter in the apparatus.

[0059] High-entropy alloys typically exist in powder form. If they are directly sintered or pressed into targets, insufficient bonding between powder particles leads to numerous pores within the target, resulting in a significantly lower overall density. This structural defect not only weakens the mechanical strength of the target, making it prone to cracking along particle interfaces or even overall cracking during subsequent processing or use, but also causes unstable directionality and compositional fluctuations in the plasma plume generated during pulsed laser deposition due to the uneven density distribution of the target. This results in uneven film thickness, deviations from the original composition ratio, and severely reduced uniformity and repeatability of the coating.

[0060] Based on this, in step two, the organometal salt gel is heat-treated to obtain carbon-coated metal oxides. Then, in step three, the contact time between the carbon-coated metal oxides and the high-entropy alloy is reduced by rapidly heating through spark plasma sintering. This avoids the introduction of additional metal elements or the formation of composite metal oxides in the high-entropy alloy. Specifically, taking carbon-coated MgO as an example, avoiding prolonged contact between carbon-coated MgO and the high-entropy alloy can prevent Mg elements from combining with the high-entropy alloy or forming MgTiO3 under high-temperature sintering conditions.

[0061] It is understood that this application does not restrict the preparation order of step one and step two. Step one can be prepared first and then step two, or step two can be prepared first and then step one, or steps one and two can be prepared simultaneously.

[0062] In one embodiment of this application, the organometallic salt gel includes, but is not limited to, at least one of magnesium citrate gel, zinc citrate gel, aluminum citrate gel, zirconium citrate gel, and cobalt citrate gel, preferably magnesium citrate gel.

[0063] It is understood that this application does not restrict the preparation method of organometallic salt gels, and conventional preparation processes can be used. For example, taking magnesium citrate gel as an example, Mg(NO3)2·6H2O can be dissolved in an aqueous solution of ethanol, citric acid can be added as a carbon source, stirred to form a transparent sol, and dried in a vacuum oven at 80°C to form a gel.

[0064] In one embodiment of this application, the heat treatment includes: first, heating to 350°C to 450°C at a rate of 4°C / min to 6°C / min, and holding in air for 1 hour to 3 hours to decompose citric acid into amorphous carbon; then heating to 550°C to 650°C at a rate of 8°C / min to 12°C / min, and holding in a protective gas for 1 hour to 3 hours to decompose the metal inorganic salt into metal oxide, while the carbon layer stably coats the surface of the metal oxide.

[0065] In one embodiment of this application, when the high-entropy alloy is mixed with carbon-coated metal oxide, the carbon-coated metal oxide accounts for 4wt% to 6wt% of the high-entropy alloy, which is beneficial to further optimize the coating structure.

[0066] It is understood that this application does not restrict the mixing method of high-entropy alloys and carbon-coated metal oxides, but ball milling is preferred.

[0067] In one embodiment of this application, the temperature of the spark plasma sintering is preferably 1000℃~1400℃, including but not limited to any one of 1000℃, 1100℃, 1200℃, 1300℃, 1400℃ or any range between two; the heating rate is preferably 80℃ / min~120℃ / min, including but not limited to any one of 80℃ / min, 90℃ / min, 100℃ / min, 110℃ / min, 120℃ / min or any range between two; the holding time is preferably 4min~6min, including but not limited to any one of 4min, 4.5min, 5min, 5.5min, 6min or any range between two.

[0068] In one embodiment of this application, the deposition temperature is preferably 600℃~800℃, including but not limited to any one of 600℃, 650℃, 700℃, 750℃, 800℃ or any range between two; the deposition time is preferably 40min~80min, including but not limited to any one of 40min, 50min, 60min, 70min, 80min or any range between two; the laser frequency is preferably 4Hz~6Hz, including but not limited to any one of 4Hz, 5Hz, 6Hz or any range between two.

[0069] The etching process oxidizes the carbon layer and dissolves the exposed metal oxides, thereby forming a hierarchical porous structure, which is beneficial for enhancing Mie scattering and photonic crystal effects.

[0070] In one embodiment of this application, the etching solution used in wet etching is preferably a mixed solution of peroxide and acid. It is understood that the wet etching process also includes neutralization and cleaning post-treatment to neutralize and remove residual acid.

[0071] The following specific embodiments will further illustrate the radiation-cooling film and its preparation method. However, those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0072] Example 1

[0073] High-purity oxides TiO2, Cr3O4, HfO2, Al2O3, and Nb2O5 were weighed according to the molar ratio of Ti:Cr:Hf:Al:Nb = 2:2:2:2:4. 1 wt% carbon powder was added as a reducing agent, and 0.5 wt% polyvinyl alcohol as a binder. Zirconia grinding balls (a mixture of 5mm and 3mm diameter zirconia balls in a 3:2 ratio) were used, with a ball-to-material ratio of 10:1. Ethanol was used as the medium, covering 80% of the grinding balls. The ball mill was set to 300 rpm and milled for 24 hours. After milling, the mixture was vacuum dried at 60℃ for 12 hours and passed through a 200-mesh sieve to obtain the precursor powder.

[0074] A tube furnace was heated to 1000℃ at a heating rate of 20℃ / min and calcined in air for 2 hours to remove residual organic matter. The precursor powder was placed in a ceramic boat and calcined in a tube furnace to 1600℃ at a heating rate of 5℃ / min in a 5% H₂ / Ar atmosphere for 2 hours; then heated to 400℃ at a heating rate of 5℃ / min and annealed in a 5% H₂ / Ar atmosphere for 2 hours to obtain a high-entropy alloy. The elemental molar ratio was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES), and the oxygen vacancy content was analyzed by X-ray photoelectron spectroscopy (XPS). The chemical formula of the high-entropy alloy obtained in this example was confirmed to be Ti. 0.2 Cr 0.21 Hf 0.19 Nb 0.3 8Al 0.2 O 1.92 (x=0.08).

[0075] Dissolve Mg(NO3)2·6H2O in 50% ethanol / water, and add citric acid as a carbon source (Mg). 2+ (The mass ratio of citric acid to MgO is 1:2). Stir for 1 hour to form a transparent sol, then dry in a vacuum oven at 80°C to form a gel. Place the gel in a ceramic boat and heat it to 400°C in a tube furnace at a heating rate of 5°C / min. Hold the temperature in air for 2 hours, then heat it to 600°C at a heating rate of 10°C / min and hold it in Ar atmosphere for 2 hours to obtain carbon-coated MgO.

[0076] 5 wt% carbon-coated MgO (particle size approximately 550 nm) was added to a high-entropy alloy and ball-milled using anhydrous ethanol as the medium at 300 rpm with ZrO2 as the grinding balls at a ball-to-material ratio of 10:1 for 6 hours. After ball milling, the mixture was vacuum-dried at 60℃ for 12 hours and passed through a 200-mesh sieve to obtain a mixed powder. The mixed powder was then subjected to cold isostatic pressing at 200 MPa for 5 minutes to form a Φ30 mm × 5 mm blank, which was then subjected to spark plasma sintering to obtain the composite material. The spark plasma sintering parameters were set as follows: heating rate of 100℃ / min, sintering temperature of 1200℃, holding time of 5 minutes, pressure of 50 MPa, atmosphere of high-purity Ar (flow rate 20 sccm), and pulse mode of 12 ms ON / 2 ms OFF.

[0077] The glass substrate was ultrasonically cleaned sequentially with ethanol, acetone, ethanol, and ultrapure water for 30 minutes to remove grease and contaminants from the glass surface. Al2O3 material with a 0001 crystal plane was selected and ultrasonically cleaned with ethanol, acetone, ethanol, and ultrapure water for 30 minutes to remove surface contaminants. It was then treated with Piranha solution (H2SO4:H2O2 = 3:1, soaked for 5 minutes) to thoroughly remove carbon residue. The composite material was cut into Φ25mm×3mm discs using a diamond wire cutter, and then progressively polished with diamond polishing paste until the surface roughness Ra < 50nm to serve as the target material. The distance between the target and the substrate was 50mm, the laser frequency was set to 5Hz, the deposition temperature to 700℃, the oxygen partial pressure to 0.01mbar, and the deposition time to 60 minutes. Finally, use an etching solution with a volume ratio of H2O2:HCl:H2O = 5:5:90, heat in a 50°C water bath for 10 minutes, soak in 5% NaHCO3 solution for 2 minutes to neutralize residual acid, rinse with deionized water 3 times, and dry with nitrogen.

[0078] Surface observations were performed on the radiation-cooling film prepared in Example 1, and the results are as follows: Figure 1 As shown.

[0079] Example 2

[0080] High-purity oxides TiO2, Cr3O4, HfO2, Al2O3, and Nb2O5 were weighed according to the molar ratio of Ti:Cr:Hf:Al:Nb = 2:2:2:1.5:5. 1 wt% carbon powder was added as a reducing agent, and 0.5 wt% polyvinyl alcohol as a binder. Zirconia grinding balls (a mixture of 5 mm and 3 mm diameter zirconia balls in a 3:2 ratio) were used, with a ball-to-material ratio of 10:1. Ethanol was used as the medium, covering 80% of the grinding balls. The ball mill was set to 300 rpm and milled for 24 hours. After milling, the mixture was vacuum dried at 60℃ for 12 hours and passed through a 200-mesh sieve to obtain the precursor powder.

[0081] The temperature was increased to 1000℃ in a tube furnace at a heating rate of 20℃ / min, and calcined for 2 hours in air to remove residual organic matter. The precursor powder was placed in a ceramic boat and calcined in a tube furnace at a heating rate of 5℃ / min to 1500℃ in a 5% H₂ / Ar atmosphere for 1.5 hours; then annealed at a heating rate of 5℃ / min to 350℃ in a 5% H₂ / Ar atmosphere for 2 hours to obtain a high-entropy alloy. ICP-AES and XPS tests confirmed that the chemical formula of the high-entropy alloy obtained in this example was Ti. 0.19 Cr 0.2 Hf 0.2 Nb 0.41 Al 0.18 O 1.90 .

[0082] Dissolve Mg(NO3)2·6H2O in 50% ethanol / water, and add citric acid as a carbon source (Mg). 2+ The mixture was stirred for 1 hour to form a transparent sol (citric acid mass ratio 1:2), and then dried in a vacuum oven at 80°C to form a gel. The gel was placed in a ceramic boat and heated to 350°C in a tube furnace at a heating rate of 4°C / min. The temperature was then maintained in air for 2.5 hours. Next, the temperature was increased to 550°C at a heating rate of 8°C / min and maintained in Ar atmosphere for 2.5 hours to obtain carbon-coated MgO.

[0083] 4 wt% carbon-coated MgO (particle size approximately 400 nm) was added to a high-entropy alloy and ball-milled at 300 rpm using anhydrous ethanol as the medium, with ZrO2 as the grinding balls and a ball-to-material ratio of 10:1 for 6 hours. After ball milling, the mixture was vacuum-dried at 60℃ for 12 hours and passed through a 200-mesh sieve to obtain a mixed powder. The mixed powder was then subjected to cold isostatic pressing at 200 MPa for 5 minutes to form a Φ30 mm × 5 mm blank, which was then subjected to spark plasma sintering to obtain the composite material. The spark plasma sintering parameters were set as follows: heating rate of 80℃ / min, sintering temperature of 1100℃, holding time of 4 minutes, pressure of 50 MPa, atmosphere of high-purity Ar (flow rate 20 sccm), and pulse mode of 12 ms ON / 2 ms OFF.

[0084] The glass substrate was ultrasonically cleaned sequentially with ethanol, acetone, ethanol, and ultrapure water for 30 minutes to remove grease and contaminants from the glass surface. Al2O3 material with a 0001 crystal plane was selected and ultrasonically cleaned with ethanol, acetone, ethanol, and ultrapure water for 30 minutes to remove surface contaminants. It was then treated with Piranha solution (H2SO4:H2O2 = 3:1, soaked for 5 minutes) to thoroughly remove carbon residue. The composite material was cut into Φ25mm×3mm discs using a diamond wire cutter, and then progressively polished with diamond polishing paste until the surface roughness Ra < 50nm to serve as the target material. The distance between the target and the substrate was 50mm, the laser frequency was set to 4Hz, the deposition temperature to 650℃, the oxygen partial pressure to 0.01mbar, and the deposition time to 50 minutes. Finally, use an etching solution with a volume ratio of H2O2:HCl:H2O = 5:5:90, heat in a 50°C water bath for 10 minutes, soak in 5% NaHCO3 solution for 2 minutes to neutralize residual acid, rinse with deionized water 3 times, and dry with nitrogen.

[0085] Example 3

[0086] High-purity oxides TiO2, Cr3O4, HfO2, Al2O3, and Nb2O5 were weighed according to the molar ratio of Ti:Cr:Hf:Al:Nb = 2:2:1.5:2:5. 1 wt% carbon powder was added as a reducing agent, and 0.5 wt% polyvinyl alcohol as a binder. Zirconia grinding balls (a mixture of 5 mm and 3 mm diameter zirconia balls in a 3:2 ratio) were used, with a ball-to-material ratio of 10:1. Ethanol was used as the medium, covering 80% of the grinding balls. The ball mill was set to 300 rpm and milled for 24 hours. After milling, the mixture was vacuum dried at 60℃ for 12 hours and passed through a 200-mesh sieve to obtain the precursor powder.

[0087] The temperature was increased to 1000℃ in a tube furnace at a heating rate of 20℃ / min, and calcined for 2 hours in air to remove residual organic matter. The precursor powder was placed in a ceramic boat and calcined in a tube furnace at a heating rate of 5℃ / min to 1700℃ in a 5% H₂ / Ar atmosphere for 2.5 hours; then annealed at a heating rate of 5℃ / min to 450℃ in a 5% H₂ / Ar atmosphere for 1.5 hours to obtain a high-entropy alloy. ICP-AES and XPS tests confirmed that the chemical formula of the high-entropy alloy obtained in this example was Ti. 0.21 Cr 0.2 0Hf 0.18 Nb 0.40 Al 0.21 O 1.93 .

[0088] Dissolve Mg(NO3)2·6H2O in 50% ethanol / water, and add citric acid as a carbon source (Mg). 2+The mixture was stirred for 1 hour to form a transparent sol (citric acid mass ratio 1:2), and then dried in a vacuum oven at 80°C to form a gel. The gel was placed in a ceramic boat and heated to 450°C in a tube furnace at a heating rate of 6°C / min. It was then held at this temperature in air for 1.5 hours, and then heated to 650°C at a heating rate of 12°C / min. It was then held at this temperature in an Ar atmosphere for 1.5 hours to obtain carbon-coated MgO.

[0089] 6 wt% carbon-coated MgO (particle size approximately 600 nm) was added to a high-entropy alloy and ball-milled using anhydrous ethanol as the medium at 300 rpm with ZrO2 as the grinding balls at a ball-to-material ratio of 10:1 for 6 hours. After ball milling, the mixture was vacuum-dried at 60℃ for 12 hours and passed through a 200-mesh sieve to obtain a mixed powder. The mixed powder was then subjected to cold isostatic pressing at 200 MPa for 5 minutes to form a Φ30 mm × 5 mm blank, which was then subjected to spark plasma sintering to obtain the composite material. The spark plasma sintering parameters were set as follows: heating rate of 120℃ / min, sintering temperature of 1300℃, holding time of 6 minutes, pressure of 50 MPa, atmosphere of high-purity Ar (flow rate 20 sccm), and pulse mode of 12 ms ON / 2 ms OFF.

[0090] The glass substrate was ultrasonically cleaned sequentially with ethanol, acetone, ethanol, and ultrapure water for 30 minutes to remove grease and contaminants from the glass surface. Al2O3 material with a 0001 crystal plane was selected and ultrasonically cleaned with ethanol, acetone, ethanol, and ultrapure water for 30 minutes to remove surface contaminants. It was then treated with Piranha solution (H2SO4:H2O2 = 3:1, soaked for 5 minutes) to thoroughly remove carbon residue. The composite material was cut into Φ25mm×3mm discs using a diamond wire cutter, and then progressively polished with diamond polishing paste until the surface roughness Ra < 50nm, serving as the target material. The distance between the target and the substrate was 50mm, the laser frequency was set to 6Hz, the deposition temperature to 750℃, the oxygen partial pressure to 0.01mbar, and the deposition time to 70 minutes. Finally, use an etching solution with a volume ratio of H2O2:HCl:H2O = 5:5:90, heat in a 50°C water bath for 10 minutes, soak in 5% NaHCO3 solution for 2 minutes to neutralize residual acid, rinse with deionized water 3 times, and dry with nitrogen.

[0091] Example 4

[0092] The difference between Example 4 and Example 1 is that carbon-coated ZnO was prepared by using zinc citrate gel and undergoing the same heat treatment.

[0093] Comparative Example 1

[0094] The difference between Comparative Example 1 and Example 1 is that the molar ratio Ti:Cr:Hf:Al:Nb = 0.15:0.15:0.15:0.45:0.10, and the chemical formula of the resulting high-entropy alloy is Ti. 0.15 Cr 0.15 Hf 0.15 Nb 0.45 Al 0.10 O 1.75 .

[0095] Comparative Example 2

[0096] The difference between Comparative Example 2 and Example 1 is that the molar ratio Ti:Cr:Hf:Al:Nb = 0.25:0.25:0.25:0.35:0.25, and the chemical formula of the resulting high-entropy alloy is Ti. 0.25 Cr 0.25 Hf 0.25 Nb 0.35 Al 0.25 O 1.68 .

[0097] Comparative Example 3

[0098] The difference between Comparative Example 3 and Example 1 is that an equimolar amount of Si is used instead of Hf.

[0099] Comparative Example 4

[0100] The difference between Comparative Example 4 and Example 1 is that an equimolar amount of V is used instead of Nb.

[0101] Comparative Example 5

[0102] The difference between Comparative Example 5 and Example 1 is that conventional sintering was used instead of spark plasma sintering in the preparation of the composite material. Specifically, the sintering conditions were as follows: heating to 1200°C at a heating rate of 5°C / min, holding at that temperature for 60 min in a high-purity Ar atmosphere (flow rate 20 sccm), and then cooling to room temperature at a rate of 3°C / min. The remaining steps (preparation of high-entropy alloy, mixing of carbon-coated MgO, preparation and deposition of target material) were the same as in Example 1.

[0103] Comparative Example 6

[0104] The difference between Comparative Example 6 and Example 1 is that the high-entropy alloy and carbon-coated MgO were directly pressed together and then subjected to spark plasma sintering to obtain two targets for simultaneous deposition.

[0105] The performance of the radiation-cooling films prepared in all embodiments and comparative examples was tested, and the specific tests are as follows:

[0106] (1) Cooling efficiency: The ratio of the net cooling capacity of the membrane layer to the theoretical maximum cooling capacity was tested in a closed environment simulation chamber (temperature 30℃, relative humidity 50%, no direct light).

[0107] (2) Emissivity of the atmospheric window 8-13 μm: The average emissivity of the film in the 8-13 μm band was measured using a Fourier transform infrared spectrometer (FTIR);

[0108] (3) Radiative power density: The radiative heat dissipation power of the film layer under standard conditions was tested by using an infrared thermal imager combined with blackbody radiation calibration;

[0109] (4) Stability: The membrane was placed in an environment of 50℃ and 90% relative humidity for 1000h, and the rate of decrease in cooling efficiency before and after was tested;

[0110] (5) Cooling performance: Simulate a high temperature environment, maintain 40℃ for 6 hours, and record the surface temperature change of the radiation cooling film;

[0111] (6) Heating performance: Simulate a low temperature environment, maintain at -10℃ for 6 hours, and record the surface temperature change of the radiation cooling film;

[0112] (7) Reliability: Simulating a large temperature difference environment, one cycle is defined as holding at -20℃ for 2 hours and then at 60℃ for 2 hours, and 100 consecutive cycles are performed. The rate of change of heating / cooling efficiency before and after the cycle is tested as ΔE%=[(E 循环前 -E 循环后 ) / E 循环前 ]×100%.

[0113] The results are shown in Table 1.

[0114] Table 1

[0115]

[0116] According to Table 1:

[0117] (1) Examples 1-4 showed the best performance: cooling efficiency ≥79%, emissivity ≥0.92 in 8-13μm, and radiation power density ≥950W / m 2 The attenuation rate over 1000 hours is ≤3.5%, the cooling temperature difference is ≥-7.8℃, the heating temperature difference is ≥+4.8℃, and the efficiency change rate over 100 cycles is ≤2.1%. This indicates that the technical solution provided in this application has dynamically adjustable emissivity, can adapt to day and night or seasonal temperature differences, and has excellent cooling efficiency and stability.

[0118] (2) The performance of Comparative Examples 1 and 2 decreased significantly: due to the excessively high or low molar ratio of elements and oxygen vacancies, the single-phase structure of the high-entropy alloy was destroyed and the oxygen vacancy was unbalanced, resulting in a cooling efficiency of only 52% to 57%, an emissivity of 0.75 to 0.78, and a decay rate of 12.5% ​​to 15.3% after 1000 hours.

[0119] (3) Performance degradation of Comparative Examples 3-4: After replacing Hf / Si and Nb / V, the infrared absorption and oxygen vacancy stability in the film layer were impaired, the cooling efficiency dropped to 63%~65%, and the emissivity was ≤0.83. This fully verifies the key role of Hf and Nb elements in performance.

[0120] (4) Performance degradation of Comparative Example 5: Conventional sintering leads to a decrease in the density of the composite material, a decrease in refrigeration efficiency to 70.3%, and an increase in stability degradation rate to 6.5%.

[0121] (5) Performance degradation in Comparative Example 6: Co-deposition led to uneven film composition, cooling efficiency dropped to 68.7%, and reliability cycle efficiency change rate increased to 4.7%.

[0122] In summary, the technical solution protected in this application can effectively improve the overall performance of the radiation cooling film and has significant technical advantages.

[0123] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0124] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A radiation-cooling film, characterized in that, The material of the radiation cooling film is a composite material of high entropy alloy and carbon-coated metal oxide; The high-entropy alloy has a polarized-nonpolarized reversible structure. When the external temperature is >300K, the high-entropy alloy forms a polarized state; when the external temperature is <300K, the high-entropy alloy forms a nonpolarized state. The carbon-coated metal oxide has a nano-micro composite pore structure; The porosity of the micron-sized pores in the radiation-cooling film is 15%~25%, and the porosity of the nano-sized pores is 8%~15%.

2. The radiation-cooling film according to claim 1, characterized in that, The carbon-coated metal oxide is 4 wt% to 6 wt% of the high-entropy alloy.

3. The radiation-cooling film according to claim 1, characterized in that, The carbon-coated metal oxide is selected from at least one of carbon-coated MgO, carbon-coated ZnO, carbon-coated Al2O3, carbon-coated ZrO2, and carbon-coated CoO; And / or, the particle size of the carbon-coated metal oxide is 300 nm to 800 nm; And / or, the chemical formula of the high-entropy alloy is Ti a Cr b Hf c Nb d Al e O 2-x , and a:b:c:d:e=(0.18~0.22):(0.18~0.22):(0.18~0.22):(0.38~0.42):(0.18~0.22), x=0.05~0.

15.

4. A method for preparing a radiation-cooling film, characterized in that, Includes the following steps: A high-entropy alloy is prepared by mixing metal oxide raw materials in a specified ratio, followed by reduction sintering and annealing. The chemical formula of the high-entropy alloy is Ti. a Cr b Hf c Nb d Al e O 2-x , and a:b:c:d:e=(0.18~0.22):(0.18~0.22):(0.18~0.22):(0.38~0.42):(0.18~0.22), x=0.05~0.15; Carbon-coated metal oxides were obtained by heat treatment of organometallic salt gels. A composite material was prepared by mixing and pressing a high-entropy alloy with a carbon-coated metal oxide and then subjecting it to spark plasma sintering. A radiation cooling film is prepared on the substrate surface by depositing a composite material as the target material and then etching it with a wet method.

5. The method for preparing the radiation-cooling film according to claim 4, characterized in that, The reduction sintering temperature is 1400℃~1800℃, and the time is 1h~3h; And / or, the annealing temperature is 350℃~450℃, and the time is 1h~3h.

6. The method for preparing the radiation-cooling film according to claim 4, characterized in that, Organometallic salt gels include at least one of magnesium citrate gel, zinc citrate gel, aluminum citrate gel, zirconium citrate gel, and cobalt citrate gel.

7. The method for preparing the radiation-cooling film according to claim 4, characterized in that, The heat treatment includes: first, heating to 350℃~450℃ at 4℃ / min~6℃ / min, holding in air for 1h~3h, then heating to 550℃~650℃ at 8℃ / min~12℃ / min, and holding in a protective gas for 1h~3h.

8. The method for preparing the radiation-cooling film according to claim 4, characterized in that, When high-entropy alloys are mixed with carbon-coated metal oxides, the carbon-coated metal oxides account for 4 wt% to 6 wt% of the high-entropy alloy.

9. The method for preparing the radiation-cooling film according to claim 4, characterized in that, The temperature for spark plasma sintering is 1000℃~1400℃, the heating rate is 80℃ / min~120℃ / min, and the holding time is 4min~6min.

10. The method for preparing the radiation-cooling film according to claim 4, characterized in that, The deposition temperature was 600℃~800℃, the time was 40min~80min, and the laser frequency was 4Hz~6Hz.

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