Transparent directional radiation refrigeration material and preparation method thereof

Abstract

The invention relates to a transparent directional radiation refrigeration material and a preparation method thereof. The transparent directional radiation refrigeration material is formed by assembling a sawtooth grating film deposited with an infrared reflecting layer and an impedance matching layer filled with a sawtooth structure. The infrared reflecting layer is a transparent metal oxide layer. The transparent directional radiation refrigeration material satisfies the following conditions: the average transmittance of a visible light wave band is greater than or equal to 65%; the average emissivity difference value between the high emission side and the low emission side of the middle infrared band is larger than or equal to 40%. Compared with the prior art, the radiation cooling material disclosed by the invention has high transmittance in a visible light wave band and asymmetric directional high emissivity in a middle infrared wave band, so that the radiation cooling effect of the building facade transparent enclosure structure can be improved; the problem that the radiation refrigeration performance is limited due to the omni-directional high emissivity of traditional glass of a building facade is solved, and the energy-saving effect of a facade window can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of radiation refrigeration, in particular to a transparent directional radiation refrigeration material and a preparation method thereof. BACKGROUND

[0002] In recent years, with global warming, the emission of a large amount of greenhouse gases and a large amount of waste heat in human production and life has intensified this phenomenon, and high-temperature weather occurs frequently, which leads to an increasing building air conditioning energy consumption, and windows consume a large amount of building air conditioning energy consumption as the part with the lowest energy efficiency in buildings.

[0003] Radiative cooling is a zero-energy consumption refrigeration technology that radiates heat to outer space through thermal radiation to reduce the surface temperature of an object. This process does not have any energy input. Existing transparent radiative cooling materials usually have high emissivity at any angle, however, most of the windows of buildings are in the facade, therefore, when the window combines with the traditional transparent radiative cooling material, only the view angle towards the sky can effectively radiate heat to outer space, while the direction towards the ground and the environment will absorb thermal radiation from the ground and the environment, which inhibits its refrigeration effect.

[0004] CN113527740A discloses a radiative cooling film with surface periodic micro-nano structure and a preparation method thereof, which includes a periodic micro-nano structure layer, a polymer film layer and a reflective coating. However, the technical solution in this patent document still has certain limitations. First, the film has omnidirectional high emissivity regardless of the angle, which makes it only suitable for horizontal radiative cooling, limiting its application range. Second, the patent document does not explicitly mention how to improve the transmittance of the film in the visible light band, which is crucial for realizing radiative cooling on transparent surfaces.

[0005] WO2023284350A1 improves the preparation method of the radiative cooling film by adding a periodic micro-nano structure on the surface of the polymer film layer to control the absorption and radiation characteristics of the radiative cooling film in the visible light and infrared light bands. The film proposed in this patent document also has the characteristic of omnidirectional high emissivity, which limits its radiative cooling performance when used on building facades.

[0006] At present, radiative cooling technology has been widely applied in the fields of building energy saving, automobiles, solar cell cooling, outdoor equipment heat dissipation, agricultural greenhouse, tent, umbrella, textile, etc. Among the radiative cooling products that have been commercialized, the radiative cooling film has a higher market share. When used, the radiative cooling film is usually attached to the outer surface of the object to be cooled, such as the outer wall or roof of a building, to achieve the refrigeration effect. However, since most of the surfaces of buildings are facades, the radiative cooling effect of the existing radiative cooling film used on building facades still needs to be further improved.

[0007] At present, the transparent radiation refrigeration material designed towards the sky direction has very excellent refrigeration performance, and it is still difficult to develop a directional transparent radiation refrigeration material suitable for building facade windows. Therefore, developing a transparent directional radiation refrigeration material is crucial to improve the energy saving effect of the window. SUMMARY

[0008] In view of the poor refrigeration effect of the existing transparent radiation refrigeration material for building facade window refrigeration, the present application provides a transparent directional radiation refrigeration material preparation method, which has the advantages of improving the radiation refrigeration effect of building facade glass.

[0009] The object of the present application can be achieved by the following technical solutions:

[0010] The present application provides a transparent directional radiation refrigeration material, which is assembled by a sawtooth grating film with an infrared reflection layer deposited thereon and an impedance matching layer filled in the sawtooth structure.

[0011] The transparent directional radiation refrigeration material satisfies:

[0012] The infrared reflection layer is a transparent metal oxide layer.

[0013] The average transmittance in the visible light waveband is greater than or equal to 65%.

[0014] The average emissivity difference between the high emissivity side and the low emissivity side in the mid-infrared waveband is greater than or equal to 40%.

[0015] Further, the transparent directional radiation refrigeration material can reflect the thermal radiation energy of high-temperature ground or environment when placed vertically, effectively improving the radiation refrigeration performance of the facade window.

[0016] Further, the thickness of the transparent directional radiation refrigeration material is 0.05-3mm.

[0017] Further, the thickness of the infrared reflection layer is 50-500nm, preferably 300nm.

[0018] The sawtooth period of the polymer sawtooth grating film is 10-300μm, the bottom angle alpha is 10°-60°, and the bottom angle beta is 10°-60°.

[0019] The transparent metal oxide layer is indium tin oxide or aluminum-doped zinc oxide.

[0020] Further, the infrared reflection layer is prepared by a directional deposition process, wherein the transparent metal oxide layer forms a continuous covering layer on the inclined surface of the sawtooth grating in the sawtooth grating film, and presents an intermittent distribution on the vertical surface of the sawtooth grating film.

[0021] When the sawtooth grating film is vertically placed, the vertical surface of the sawtooth in the sawtooth grating film faces the sky, so that the direction facing the sky presents high emission characteristics; the inclined surface of the sawtooth of the sawtooth grating film faces the ground, so that the direction facing the ground presents mid-infrared low-emissivity characteristics.

[0022] That is, when the film is vertically placed, the mid-infrared high-emission polymer side of the sawtooth faces the sky, so that the direction facing the sky presents high emission characteristics, and the transparent metal oxide surface of the sawtooth faces the ground, so that the direction facing the ground presents mid-infrared low-emissivity characteristics.

[0023] Further, the material of the sawtooth grating is selected from at least one of polydimethylsiloxane, polymethyl methacrylate, polyvinyl alcohol, polyurethane, polycarbonate, polyethylene terephthalate, and polypropylene.

[0024] Further, the material of the impedance matching layer is selected from at least one of polyethylene, polypropylene, butylene-styrene thermoplastic elastomer.

[0025] The second aspect of the present application provides a preparation method of the transparent directional radiation refrigeration material, comprising the following steps:

[0026] S1, dissolving a visible light transparent mid-infrared high-emission polymer in a first solvent to form a solution A;

[0027] S2, casting the solution A to a sawtooth template for solidification, and demolding to obtain a sawtooth grating film;

[0028] S3, placing the sawtooth grating film obliquely, and directionally depositing an infrared reflective layer on the sawtooth grating film;

[0029] S4, dissolving an infrared transparent polymer in a second solvent to form a solution B;

[0030] S5, casting the solution B on the sawtooth grating film covered with the reflective layer and solidifying to form an impedance matching layer, to obtain the transparent directional radiation refrigeration material.

[0031] Further, in S1, the mass ratio of the visible light transparent mid-infrared high-emission polymer and the first solvent forming the solution A is (0.1-1):1, and one embodiment in the present application is preferably 0.175:1;

[0032] The visible light transparent mid-infrared high-emission polymer is selected from at least one of polydimethylsiloxane, polymethyl methacrylate, polyvinyl alcohol, polyurethane, polycarbonate, polyethylene terephthalate, and polypropylene, and is preferably polyvinyl alcohol (PVA);

[0033] The first solvent is selected from at least one of water, tetrahydrofuran, chloroform, toluene, cyclohexane, n-hexane, toluene, xylene, and an embodiment of the present application is preferably water.

[0034] Further, in S3, the method of directional deposition of the infrared reflective layer is selected from one of magnetron sputtering, electron beam evaporation, and thermal evaporation.

[0035] Further, the directional deposition process refers to the deposition of transparent metal oxides in a specific direction of the sawtooth grating film during the preparation of the infrared reflective layer by controlling the deposition conditions (such as angle, gas pressure, etc.). Specifically, the polymer sawtooth grating film is fixed on a sample stage and inclined at a certain angle, and then the transparent metal oxide is sputtered on the polymer sawtooth grating film in a specific direction by using a vapor deposition method (such as magnetron sputtering, electron beam evaporation, or thermal evaporation, etc.). Due to the angle control and physical shielding effect during the deposition process, the transparent metal oxide forms a continuous covering layer on the inclined surface of the sawtooth grating, while it is discontinuously distributed on the vertical surface of the sawtooth, so that the film has different reflection and transmission characteristics in different directions for infrared light, thereby achieving the effect of directional radiative cooling.

[0036] Further, in S4, the second solvent is selected from at least one of cyclohexane, tetrahydrofuran, chloroform, toluene, cyclohexane, n-hexane, ethyl acetate, butyl acetate, xylene, toluene, and carbon tetrachloride.

[0037] Compared with the prior art, the present application has the following beneficial effects:

[0038] 1) The present application proposes a preparation method of a transparent directional radiative cooling material. The radiative cooling material has high transmittance in the visible light band and asymmetric directional high emissivity in the infrared band. It can improve the radiative cooling effect of the transparent enclosure structure of the building, make up for the problem of insufficient cooling performance caused by the omnidirectional high emissivity of the traditional glass building facade, and effectively reduce the increased cooling energy consumption due to the absorption of ambient thermal radiation in hot seasons. The present application provides a new method for the design of radiative cooling windows, which has potential economic value and social benefits.

[0039] 2) The present application solves the defects in the prior art and prepares a transparent infrared asymmetric directional radiative cooling material by casting and thin film directional deposition. After the transparent directional radiative cooling material is used in the window of the building facade, the cooling effect of the window can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 The physical picture of the transparent directional radiative cooling material;

[0041] Figure 2SEM surface and cross-section images of the transparent directional radiation refrigeration material;

[0042] Figure 3 Visible light transmittance spectrum of the transparent directional radiation refrigeration material;

[0043] Figure 4 Average emissivity of the transparent directional radiation refrigeration material at different incident angles;

[0044] Figure 5 Temperature of the tested transparent directional radiation refrigeration material and glass when vertically placed outdoors during the noon period;

[0045] Figure 6 Schematic diagram of the transparent directional radiation refrigeration material. DETAILED DESCRIPTION

[0046] Overall, the present application relates to a transparent directional radiation refrigeration material and a preparation method thereof. The radiation refrigeration material is assembled by a polymer sawtooth grating with an infrared reflection layer deposited in a specific direction and an impedance matching layer filling the sawtooth structure. Among them, the transparent directional radiation refrigeration material has a transmittance of more than 65% in the visible light wave and an asymmetric directional high emissivity in the mid-infrared wave band. The transparent radiation refrigeration material can reflect the thermal radiation energy of high-temperature ground or environment when vertically placed, which can effectively improve the radiation refrigeration performance of the facade window. In view of the problem that the traditional transparent radiation refrigeration material has insufficient refrigeration performance when used in the facade window, the present application provides a method for controlling the direction of thermal radiation of the transparent radiation refrigeration material, which can improve the energy-saving effect of the facade window

[0047] In specific implementation, a transparent directional radiation refrigeration material in the present application is assembled by a sawtooth grating with an infrared reflection layer deposited in a specific direction and an impedance matching layer filling the sawtooth structure.

[0048] In specific implementation, the thickness of the transparent directional radiation refrigeration material is 0.05-3mm.

[0049] In specific implementation, the visible light transmittance of the transparent directional radiation refrigeration material is not less than 65%, and the average emissivity difference between the high emissivity side and the low emissivity side in the mid-infrared wave band is greater than or equal to 40%.

[0050] Another object of the present application is to disclose the above-mentioned transparent directional radiation refrigeration material and a preparation method thereof, which comprises the following steps:

[0051] a) mixing the visible light transparent mid-infrared high emissivity polymer and the solvent A according to a certain mass ratio, and fully stirring until the visible light transparent mid-infrared high emissivity polymer is completely dissolved to form a solution A;

[0052] b) Casting the solution A on a sawtooth grating template and solidifying the solution A under certain conditions; after waiting for the solution A to be completely solidified, peeling off the solidified and demolded polymer film from the sawtooth grating template to obtain the polymer sawtooth grating film.

[0053] c) Fixing the polymer sawtooth grating film on a sample stage and tilting at a certain angle, sputtering a certain thickness of transparent metal oxide in a specific direction of the polymer sawtooth grating film by a vapor deposition method. After the deposition is completed, a transparent metal oxide / polymer sawtooth grating film is obtained.

[0054] d) Mixing the visible-infrared transparent polymer for the impedance matching layer and the solvent B at a certain mass ratio, and stirring sufficiently until the visible-infrared transparent polymer is completely dissolved to form a solution B;

[0055] e) Casting the solution B on the transparent metal oxide / polymer sawtooth grating film obtained in step c) and solidifying the solution B under certain conditions; after waiting for the solution B to be completely solidified, the transparent directional radiation refrigeration material is obtained.

[0056] In specific implementation, the mass ratio of the visible light transparent mid-infrared high-emission polymer and the solvent A in step a) is (0.1-1): 1.

[0057] In specific implementation, the visible light transparent mid-infrared high-emission polymer in step a) can be polydimethylsiloxane, polymethyl methacrylate, polyvinyl alcohol, polyurethane, polycarbonate, polyethylene terephthalate, and polypropylene, etc.

[0058] In specific implementation, the solvent A in step a) is used to dissolve the visible light transparent mid-infrared high-emission polymer, wherein the solvent can be one of water, tetrahydrofuran, chloroform, toluene, cyclohexane, n-hexane, toluene and xylene, or a combination of multiple solvents.

[0059] In specific implementation, the sawtooth period of the sawtooth grating template in step b) is 10-300 μm, see Figure 6 , the sawtooth base angle α is 10°-60°, and the base angle β is 10°-60°.

[0060] In specific implementation, the vapor deposition method in step c) can be a magnetron sputtering, electron beam evaporation and thermal evaporation, etc.

[0061] In specific implementation, the transparent metal oxide in step c) can be indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO), etc.

[0062] In specific implementation, the certain thickness in step c) is 50-500 nm.

[0063] In particular implementation, the visible-infrared transparent polymer used as the impedance matching layer in step d) can be polyethylene, polypropylene, butylene-styrene thermoplastic elastomer, and the like.

[0064] In particular implementation, the solvent B used for dissolving the visible-infrared transparent polymer in step e) can be one of cyclohexane, tetrahydrofuran, chloroform, toluene, cyclohexane, n-hexane, ethyl acetate, butyl acetate, dimethylbenzene, toluene, carbon tetrachloride, and the like, or a combination of multiple solvents.

[0065] The present application will be described in detail below with reference to the accompanying drawings and specific examples. In the present technical solution, if the preparation means, materials, structures, or component ratios of the features that are not explicitly described are considered as common technical features disclosed in the prior art.

[0066] Example 1

[0067] In this embodiment, the optical properties of the radiation cooling film in the visible and infrared wave bands are regulated by designing a periodic sawtooth micro-nano structure on the surface of the polymer film layer. Specifically, the periodic sawtooth structure interacts with light waves in the visible and infrared wave bands by virtue of its unique geometric form, thereby achieving modulation of the light waves. The design of this structure is based on the scattering and geometric optics principles, which can ensure high transmittance of visible light while presenting directional high emissivity characteristics in the mid-infrared. This enables the film to more effectively dissipate heat to outer space through thermal radiation when used for facade radiation cooling, thereby improving the efficiency of facade radiation cooling.

[0068] In this embodiment, a transparent directional radiation cooling material is prepared, including the following steps:

[0069] a) Dissolve polyvinyl alcohol (PVA) completely in water to obtain a PVA aqueous solution A with a concentration of 15 wt% after stirring at a temperature of 80°C for 2 h;

[0070] b) Pour 2 mL of the PVA aqueous solution A in step a) onto a sawtooth grating template with a size of 2.5*5 cm, wherein the sawtooth period of the template is 31 um, and the two base angles of the sawtooth are 69° and 21°, respectively; after drying at room temperature for 24 hours, the PVA is peeled off to obtain a PVA sawtooth grating film A;

[0071] c) Fix the PVA sawtooth grating film A obtained in step b) on a sample table of a magnetron sputtering device and place it at an inclination of 30°. Deposit 300 nm thick ITO on the surface of the sample by magnetron sputtering, and the sputtering process is carried out at a radio frequency of 60 W and a gas pressure of 0.4 Pa. After sputtering, an ITO / PVA sawtooth grating film A is obtained;

[0072] d) SEBS was completely dissolved in cyclohexane, and after stirring for 2 h, SEBS solution A with a concentration of 5 wt% was obtained;

[0073] e) 1 mL of SEBS solution A obtained in step d) was cast onto the ITO / PVA sawtooth grating film A obtained in step c), and dried at room temperature without forced convection for 24 h to wait for the complete evaporation of cyclohexane in SEBS solution A, to obtain a transparent directional radiative cooling material;

[0074] The actual appearance of the transparent directional radiative cooling material is shown in Figure 1

[0075] In the present application, the infrared reflection layer is prepared by a directional deposition process, in which the transparent metal oxide layer forms a continuous covering layer on the inclined surface of the sawtooth grating in the sawtooth grating film, and presents an intermittent distribution on the vertical surface of the sawtooth of the sawtooth grating film.

[0076] Referring to Figure 6 When the film is placed vertically, the side of the sawtooth with the mid-infrared high-emission polymer faces the sky, and thus presents a high-emission characteristic in the direction facing the sky; and the side of the sawtooth with the transparent metal oxide faces the ground, and thus presents a mid-infrared low-emissivity characteristic in the direction facing the ground.

[0077] Traditional planar radiative cooling films usually adopt a single polymer material or a uniform coating on the surface. Such a structure cannot control the direction of emitted thermal radiation, and neither can it effectively reflect the ground thermal radiation from below to reduce heat absorption, nor can it emit thermal radiation in the direction facing the sky only to increase heat radiation as in the present application. In contrast, the transparent metal oxide layer prepared by the directional deposition process on the sawtooth grating film in the present application forms a continuous covering layer on the inclined surface and presents an intermittent distribution on the vertical surface, and utilizes the difference in the mid-infrared optical characteristics of the materials on both sides of the sawtooth structure to realize the control of the directionality of thermal radiation, exhibiting unique optical performance and more superior facade cooling effect, and effectively solving the problem of limited cooling performance of traditional planar films when used in facades.

[0078] The periodic micro-nano structure proposed in the present application can change the propagation path of mid-infrared light in the film, while maintaining the ability of visible light to pass through the interior of the film without being reflected or absorbed. When placed vertically, the high visible light transmittance of the film can ensure the lighting inside the building, and at the same time, transfer heat through thermal radiation in the direction facing the sky, thereby reducing the surface temperature. This light control effect is based on the scattering and geometric optics principles of light, and by controlling the size, shape and arrangement of the micro-nano structure, precise modulation of multiple waveband light can be achieved.

[0079] ​At the level of the preparation process, part of the prior art uses complex three-dimensional micro-nano structures to enhance the radiation refrigeration effect, but these structures often rely on precise photolithography or etching process, resulting in high preparation cost, complex process and difficult large-scale production. The present application combines the sawtooth grating design with the directional deposition process, simplifies the preparation process, reduces the production cost, and is more conducive to large-scale industrial application while achieving efficient regulation of infrared light. The innovation of the present application is that with relatively simple structure and operable process, the refrigeration performance of the complex micro-nano structure material is achieved or even surpassed, providing a more advantageous solution for the popularization and application of radiation refrigeration technology.

[0080] Test Example 1

[0081] This test example tests the micro-morphology and optical performance of the transparent directional radiation refrigeration material prepared in Example 1.

[0082] a) Characterization of the micro-morphology of the transparent directional radiation refrigeration material:

[0083] The micro-morphology of the transparent directional radiation refrigeration material was observed by field emission scanning electron microscope. As shown in FIG. 1, the surface of the transparent directional radiation refrigeration material is very smooth, and the sawtooth structure can be clearly seen in the cross section. Figure 2

[0084] b) Test the transmittance of the transparent directional radiation refrigeration material in the visible light band:

[0085] The transmittance spectrum of the transparent directional radiation refrigeration material in the wavelength range of 300-800 nm in the visible light band was measured by ultraviolet-visible-near infrared spectrophotometer (Lambda 1050, PerkinElmer). As shown in FIG. 2, the average transmittance of the transparent directional radiation refrigeration material in the visible light band is 83%. Figure 3

[0086] c) Test the average emissivity of the transparent directional radiation refrigeration material in different incident angle directions:

[0087] The average emissivity of the transparent directional radiation refrigeration material in the wavelength range of 6-20 μm was measured by integral sphere Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo Scientific). As shown in FIG. 3, the average emissivity of the transparent directional radiation refrigeration material in the normal incident angle direction is 0.86, and the average emissivity in the negative incident angle direction is 0.36. Figure 4

[0088] ​​​From the above test, it can be seen that the transparent directional radiative cooling material has high transmittance in the visible light band and an asymmetric directional high emissivity in the mid-infrared band, indicating that the transparent directional radiative cooling material can be used for facade windows to perform radiative cooling in the direction facing the sky and reduce heat radiation from the ground and the environment.

[0089] d) Test the radiative cooling performance of the transparent directional radiative cooling material:

[0090] The temperature test of the radiative cooling performance is carried out by two acrylic measuring chambers with a size of 10x10x5 cm, and a solar light reflection film is attached outside the measuring chambers. The transparent directional radiative cooling material and glass are respectively placed in the windows of the two measuring chambers. The glass is used as a control sample. Both are placed on a polystyrene foam support inside the measuring chamber, and the distance between them and the window is 1 cm. The measuring chambers are vertically placed. The temperature of the sample during the test is recorded by a thermocouple with a temperature uncertainty of 0.1℃. The test results show that the average temperature of the ground during the noon period is as high as 58.3℃, and the temperature of the transparent directional radiative cooling material is 1.4℃ lower than that of the glass on average. It shows that the prepared transparent directional radiative cooling material has excellent radiative cooling effect when placed vertically.

[0091] The directional radiative cooling principle of the present application is based on the scattering and interference effect of light. The periodic micro-nano structure enhances the emissivity of the thin film in the infrared band, so that the thin film can more effectively absorb environmental heat and dissipate it to outer space in the form of infrared radiation, thereby achieving the cooling effect. At the same time, this structure can also reduce the reflectivity of the thin film in the visible light band and increase the transmittance, so that the thin film can absorb more solar radiation heat under sunlight during the day, further improving the cooling power. This innovative design enables the radiative cooling film of the present application to significantly improve the cooling efficiency while maintaining high light transmittance, solving the problem of poor cooling effect in the prior art.

[0092] The above description of the embodiments is for the convenience of those skilled in the art to understand and use the invention. Those skilled in the art can easily make various modifications to these embodiments, and apply the general principles described herein to other embodiments without having to go through creative labor. Therefore, the present application is not limited to the above embodiments, and those skilled in the art can make improvements and modifications within the scope of the present application without departing from the scope of the present application.

Claims

1. A transparent directional radiation cooling material, characterized in that: The transparent directional radiation cooling material is assembled from a sawtooth grating film deposited with an infrared reflection layer and an impedance matching layer filling the sawtooth structure; The infrared reflection layer is a transparent metal oxide layer; The transparent directional radiation refrigeration material meets the following requirements: The average transmittance in the visible light band is ≥65%; The average emissivity difference between the high emission side and the low emission side in the mid-infrared band is ≥40%.

2. The transparent directional radiation cooling material according to claim 1, characterized in that: The thickness of the transparent directional radiation cooling material is 0.05-3 mm.

3. The transparent directional radiation cooling material according to claim 1, characterized in that: The thickness of the infrared reflection layer is 50-500 nm; The sawtooth period of the sawtooth grating film is 10-300 μm, the sawtooth base angle α is 10°-60°, and the base angle β is 10°-60°; The transparent metal oxide layer is indium tin oxide or aluminum-doped zinc oxide.

4. The transparent directional radiation cooling material according to claim 3, characterized in that: The infrared reflective layer is prepared by a directional deposition process, wherein the transparent metal oxide layer forms a continuous covering layer on the sawtooth inclined surface of the sawtooth grating in the sawtooth grating film, and is discontinuously distributed on the sawtooth vertical surface of the sawtooth grating film; When the sawtooth grating film is placed vertically, the sawtooth surface without the infrared reflective layer in the sawtooth grating film faces the sky, so that the direction toward the sky exhibits high emissivity characteristics, and the sawtooth surface with the infrared reflective layer deposited in the sawtooth grating film faces the ground, so that the direction toward the ground exhibits mid-infrared low emissivity characteristics.

5. The transparent directional radiation cooling material according to claim 1, characterized in that: The sawtooth grating is made of at least one material selected from polydimethylsiloxane, polymethyl methacrylate, polyvinyl alcohol, polyurethane, polycarbonate, polyethylene terephthalate, and polypropylene.

6. The transparent directional radiation cooling material according to claim 1, characterized in that: The material of the impedance matching layer is selected from at least one of polyethylene, polypropylene, and butylene-styrene thermoplastic elastomer.

7. A method for preparing the transparent directional radiation cooling material according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1, dissolving a visible light transparent mid-infrared high-emissive polymer in a first solvent to form a solution A; S2, casting solution A until the sawtooth template is solidified, and demoulding to obtain a sawtooth grating film; S3, tilting the sawtooth grating film, and directionally depositing an infrared reflection layer on the sawtooth grating film; S4, dissolving the infrared transparent polymer in a second solvent to form a solution B; S5. Casting solution B on the sawtooth grating film covered with a reflective layer and solidifying it to form an impedance matching layer, thereby obtaining a transparent directional radiation cooling material.

8. The method for preparing a transparent directional radiation cooling material according to claim 7, characterized in that: In S1, the mass ratio of the visible light transparent mid-infrared high-emission polymer and the first solvent to form solution A is (0.1-1):1; The visible light transparent mid-infrared high-emission polymer is selected from at least one of polydimethylsiloxane, polymethyl methacrylate, polyvinyl alcohol, polyurethane, polycarbonate, polyethylene terephthalate, and polypropylene; The first solvent is selected from at least one of water, tetrahydrofuran, chloroform, toluene, cyclohexane, n-hexane, toluene, and xylene.

9. The method for preparing a transparent directional radiation cooling material according to claim 7, characterized in that: In S3, the method of directionally depositing the infrared reflective layer is selected from magnetron sputtering, electron beam evaporation, and thermal evaporation.

10. The method for preparing a transparent directional radiation cooling material according to claim 7, characterized in that: In S4, the second solvent is selected from at least one of cyclohexane, tetrahydrofuran, chloroform, toluene, cyclohexane, n-hexane, ethyl acetate, butyl acetate, xylene, toluene, and carbon tetrachloride.

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