A high diurnal radiation cooling power double-layer film, a preparation method and application thereof

Abstract

This application relates to the field of radiation cooling materials technology, and discloses a bilayer thin film with high day and night radiation cooling power, its preparation method, and its application. The preparation method includes: uniformly mixing a first inorganic particle, polyethersulfone, and a first organic solvent to obtain a casting solution; coating the casting solution onto a glass substrate, and then immersing it in a non-solvent coagulation bath for curing; removing and drying the cured film to obtain a porous substrate film; uniformly mixing a second inorganic particle, polyvinylidene fluoride, and a second organic solvent to obtain a spraying solution; spraying the spraying solution onto the surface of the porous substrate film, and drying it to obtain a bilayer thin film with high day and night radiation cooling power. This application prepares a PES / inorganic particle porous substrate through non-solvent-induced phase separation, and then sprays a PVDF / inorganic particle layer onto its surface to construct a bilayer radiation cooling film, which can maintain high radiation cooling power simultaneously during the day and night. Furthermore, the preparation process is simple and can be mass-produced.

Description

Technical Field

[0001] This application relates to the field of radiation cooling materials technology, specifically to a bilayer thin film with high day and night radiation cooling power, its preparation method, and its application. Background Technology

[0002] Passive radiative cooling (PRC) achieves energy regulation through spectral selectivity. It uses high reflectivity in the solar wavelength range (0.3–2.5 μm) to avoid heat absorption and strong radiation in the atmospheric transparency window (8–13 μm) to achieve heat dissipation in deep space, thereby reducing temperature without energy consumption. It is considered an important strategy for alleviating energy contradictions and coping with extreme climates.

[0003] Porous polymer films have attracted widespread attention due to their ease of preparation, structural stability, and tunable performance. Their cooling performance relies on the synergistic effect of multi-scale optical mechanisms and chemical bonding characteristics. The refractive index mismatch between pores and the polymer can induce strong Mie scattering, enhancing solar reflection. Chemical bonds such as COC, C–F, and S=O in polymer molecules exhibit characteristic vibrations in the mid-infrared region, generating selective emission through atmospheric windows. The superposition of surface micro- and nano-roughness and hierarchical channels further improves light scattering and reflection efficiency. However, most existing porous films employ a single-layer design, leading to a coupling of reflection and emission performance, making it difficult to balance cooling power between day and night. For example, patent CN117659496A uses a solvent-inducible phase separation process to prepare a porous cooling film, which is structurally stable but struggles to simultaneously achieve high reflectivity and high emissivity, resulting in significant differences in performance between day and night. Patent CN115028885A obtains a hierarchical porous structure through freeze-drying a blend of polycarbonate and polydimethylsiloxane, improving reflection somewhat, but exhibiting a low average emissivity through atmospheric transparency windows, limiting heat dissipation at night. Traditional single-layer porous films cannot simultaneously meet the requirements of all-weather radiative cooling.

[0004] Therefore, there is an urgent need to develop a double-layer thin film with high day and night radiative cooling power. Summary of the Invention

[0005] This application provides a bilayer thin film with high day and night radiative cooling power, its preparation method and application, aiming to solve the technical problem that existing porous polymer radiative cooling materials are difficult to maintain high radiative cooling power during both day and night.

[0006] To achieve the above objectives, the present application adopts the following technical solution.

[0007] A first aspect of this application provides a method for preparing a bilayer thin film with high day and night radiative cooling power, comprising:

[0008] S1, the first inorganic particles, polyethersulfone and the first organic solvent are mixed evenly to obtain a casting solution; the solution is coated onto a glass substrate and then immersed in a non-solvent coagulation bath for curing; after curing, it is taken out and dried to obtain a porous substrate film;

[0009] S2, the second inorganic particles, polyvinylidene fluoride, and the second organic solvent are mixed evenly to obtain a spraying liquid; the spraying liquid is sprayed onto the surface of the porous substrate film and dried to obtain a double-layer film with high day and night radiative cooling power.

[0010] Preferably, in the casting solution, the mass fraction of the first inorganic particles is 3-18 wt%, and the mass fraction of polyethersulfone is 10-30 wt%.

[0011] Preferably, in the spraying liquid, the mass fraction of the second inorganic particles is 5-10 wt%, and the mass fraction of polyvinylidene fluoride is 2-5 wt%.

[0012] Preferably, the first inorganic particles include at least one of SiO2, BN, TiO2, BaSO4, Al2O3 or CaO; the particle size of the first inorganic particles is 200 nm to 3 μm.

[0013] The first organic solvent includes at least one of DMF, DMAc, NMP, or DMSO;

[0014] The non-solvent coagulation bath is at least one of anhydrous ethanol or deionized water.

[0015] Preferably, the second inorganic particle includes at least one of CaCO3, ZnO, SiO2, Y2O3, MgO or BN; the particle size of the second inorganic particle is 50 nm to 1 μm.

[0016] The second organic solvent includes at least one of DMF, DMAc, or acetone.

[0017] Preferably, in step S1, the soaking time in the non-solvent coagulation bath is 3 to 4 hours;

[0018] The drying temperature is 60–70℃, and the time is 12–24 hours;

[0019] The thickness of the porous substrate film is 200–600 μm.

[0020] Preferably, in step S2, the drying temperature is 30–50°C and the drying time is 24–48 hours.

[0021] Preferably, in step S2, the spraying process parameters are:

[0022] The nozzle diameter is 0.2–0.5 mm, the spraying distance is 5–15 cm, the spraying time is 1–30 min, and the spraying thickness is 1–50 μm.

[0023] A second aspect of this application provides a bilayer thin film with high day and night radiative cooling power prepared by the above-described preparation method.

[0024] A third aspect of this application provides the application of the aforementioned high day and night radiative cooling power double-layer film in radiative cooling of vehicle body surfaces and building exteriors.

[0025] Compared with the prior art, the beneficial effects of this application are as follows:

[0026] This application prepares a PES / inorganic particle porous substrate by solvent-inducing phase separation and then sprays a PVDF / inorganic particle layer on its surface to construct a double-layer radiation cooling film. This film can maintain high radiation cooling power during both day and night, and the preparation process is simple and can be mass-produced.

[0027] This application presents a bilayer thin film that achieves continuous and efficient radiative cooling day and night through multi-scale scattering enhancement, synergistic resonance of molecular vibrations and phonon polaritons, and effective medium optimization. Specifically, the hierarchical channels of the bottom film form an aperture distribution matching the wavelength of sunlight, generating Mie scattering and superimposed Rayleigh scattering, thereby achieving broad-spectrum high reflectivity in the 0.3–2.5 μm range and significantly suppressing absorption. The top PVDF / inorganic particle film increases the scattering interface and refractive index gradient, extending the light transport path within the film and further enhancing reflectivity. In the mid-infrared band, polymer molecular bonds (C–F, COC, S=O, etc.) and inorganic particle lattice vibrations synergistically form, optimizing the radiative emission efficiency within the 8–13 μm atmospheric window. Effective medium modulation further ensures a high degree of matching between the local dielectric environment and the atmospheric transparency window. This layered structure achieves quasi-decoupling of reflection and emission, overcoming the limitations of single-layer film spectral modulation.

[0028] In addition, PES and PVDF possess excellent thermal stability, mechanical strength, and weather resistance, enabling the film to maintain structural and optical stability under high temperature, humid heat, and friction conditions. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying 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 graph shows the test results of the solar reflectance and mid-infrared emissivity of the double-layer thin film in Example 1.

[0031] Figure 2 The graph shows the test results of the solar reflectance and mid-infrared emissivity of the single-layer thin film in Comparative Example 1.

[0032] Figure 3 The graph shows the daytime cooling power test results of the bilayer thin film prepared in Example 1;

[0033] Figure 4 The graph shows the nighttime cooling power test results of the bilayer thin film prepared in Example 1. Detailed Implementation

[0034] 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 some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0035] In the following description of this embodiment, the terms "including", "comprising", "having", and "containing" are all open-ended terms, meaning that they include but are not limited to.

[0036] In the following description of this embodiment, the term "and / or" is used to describe the association relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, B existing alone, and A and B existing simultaneously. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0037] In the following description of this embodiment, the term "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0038] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0039] Those skilled in the art should understand that, in the following description of the embodiments of this application, the sequence of numbers does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0040] Those skilled in the art will understand that the numerical ranges in the embodiments of this application should be understood as each intermediate value between the upper and lower limits of the specifically disclosed range. Each smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this application. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0041] Unless otherwise stated, the technical / scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. While this application describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this application. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0042] Firstly, this application provides a method for preparing a bilayer thin film with high day and night radiative cooling power. The method involves preparing a PES / inorganic particle porous substrate through solvent-induced phase separation, and then spraying a PVDF / inorganic particle layer onto its surface to construct a bilayer radiative cooling thin film. This film can maintain high radiative cooling power simultaneously during the day and night, and the preparation process is simple and can be mass-produced. The specific preparation method includes:

[0043] S1, the first inorganic particles, polyethersulfone and the first organic solvent are mixed evenly to obtain a casting solution; the solution is coated onto a glass substrate and then immersed in a non-solvent coagulation bath for curing; after curing, it is taken out and dried to obtain a porous substrate film;

[0044] In this application, the first inorganic particles are selected from at least one of SiO2, BN, TiO2, BaSO4, Al2O3, or CaO, and the particle size of the first inorganic particles is preferably 200 nm to 3 μm. The first organic solvent includes at least one of DMF, DMAc, NMP, or DMSO.

[0045] In the casting solution, the mass fraction of the first inorganic particles is preferably 3-18 wt%, and the mass fraction of polyethersulfone is preferably 10-30 wt%.

[0046] In this application, a glass substrate coated with casting solution is immersed in a non-solvent coagulation bath for 3-4 hours; then the glass substrate is removed and dried at 60-70°C for 12-24 hours to obtain a porous substrate film; the thickness of the porous substrate film is preferably 200-600 μm.

[0047] The non-solvent coagulation bath is anhydrous ethanol, deionized water, or a mixture of anhydrous ethanol and deionized water.

[0048] In this application, the porosity of the porous substrate film is 50-70%, and the pore size distribution is 50nm-3μm.

[0049] S2, the second inorganic particles, polyvinylidene fluoride, and the second organic solvent are mixed evenly to obtain a spraying liquid; the spraying liquid is sprayed onto the surface of the porous substrate film and dried to obtain a double-layer film with high day and night radiative cooling power.

[0050] In this application, the second inorganic particle is selected from at least one of CaCO3, ZnO, SiO2, Y2O3, MgO, or BN, and the particle size of the second inorganic particle is 50 nm to 1 μm. The second organic solvent is selected from at least one of DMF, DMAc, or acetone.

[0051] In the spraying liquid, the mass fraction of the second inorganic particles is 5-10 wt%, and the mass fraction of polyvinylidene fluoride is 2-5 wt%.

[0052] In this application, a double-layer film with high day and night radiative cooling power is obtained by drying a porous substrate film coated with a spraying liquid at 30–50°C for 24–48 hours. The preferred spraying process parameters are: nozzle diameter of 0.2–0.5 mm, spraying distance of 5–15 cm, spraying time of 1–30 min, and spraying thickness of 1–50 μm.

[0053] The bilayer thin film of this application achieves continuous and efficient radiative cooling day and night through multi-scale scattering enhancement, synergistic resonance of molecular vibration and phonon polaritons, and effective medium optimization.

[0054] The bottom layer film, with its hierarchical channels forming an aperture distribution matching the wavelength of sunlight, generates Mie scattering and is superimposed with Rayleigh scattering, thus achieving broad-spectrum high reflectivity in the 0.3–2.5 μm range and significantly suppressing absorption. The top layer PVDF / inorganic particle film increases the scattering interface and refractive index gradient, extending the light transport path within the film and further enhancing reflectivity. In the mid-infrared band, polymer molecular bonds (C–F, COC, S=O, etc.) and inorganic particle lattice vibrations work synergistically to optimize the radiative emission efficiency within the 8–13 μm atmospheric window. Effective dielectric modulation further ensures a high degree of matching between the local dielectric environment and the atmospheric transparency window. This layered structure achieves quasi-decoupling of reflection and emission, overcoming the limitations of single-layer film spectral modulation.

[0055] In addition, PES and PVDF possess excellent thermal stability, mechanical strength, and weather resistance, enabling the film to maintain structural and optical stability under high temperature, humid heat, and friction conditions.

[0056] The high-power day-and-night radiative cooling double-layer film of this application can be used for radiative cooling. For example, it can cool the surface of a vehicle or the exterior of a building through radiative cooling, thereby achieving energy conservation and emission reduction.

[0057] The present application will be further illustrated by the following examples.

[0058] Example 1

[0059] BN and polyethersulfone with a particle size of 2 μm were mixed uniformly in NMP to obtain a PES / BN casting solution; wherein the mass fraction of BN in the casting solution was 6 wt% and the mass fraction of polyethersulfone was 25 wt%. The casting solution was coated onto a glass substrate and then immersed in an anhydrous ethanol coagulation bath for 3 h; after removal, it was dried in a forced-air drying oven at 60 °C for 24 h to obtain a PES / BN porous substrate film with a thickness of 429 μm.

[0060] SiO2 and PVDF with a particle size of 1 μm were mixed evenly in DMF to obtain a spraying solution; wherein the mass fraction of SiO2 in the spraying solution was 5 wt% and the mass fraction of PVDF was 3 wt%. 3 ml of the spraying solution was taken and sprayed onto the surface of a PES / BN porous substrate film at a distance of 15 cm using a spray gun with a 0.3 mm nozzle for 5 min. Then, it was dried at 50 °C for 24 h to obtain a bilayer film with a thickness of 434 μm and high day and night radiative cooling power.

[0061] Example 2

[0062] SiO2 and polyethersulfone with a particle size of 1 μm were mixed uniformly in DMAc to obtain a PES / SiO2 casting solution; wherein the mass fraction of SiO2 in the casting solution was 9 wt% and the mass fraction of polyethersulfone was 30 wt%. The casting solution was coated onto a glass substrate and then immersed in an anhydrous ethanol and deionized water coagulation bath for 3.5 h; after removal, it was dried in a forced-air drying oven at 60 °C for 24 h to obtain a PES / SiO2 porous substrate film with a thickness of 300 μm.

[0063] ZnO and PVDF with a particle size of 500 nm were mixed evenly in DMAc to obtain a spraying solution; wherein the mass fraction of ZnO in the spraying solution was 5 wt% and the mass fraction of PVDF was 3 wt%. 2 ml of the spraying solution was taken and sprayed onto the surface of a PES / SiO2 porous substrate film at a distance of 10 cm using a spray gun with a 0.2 mm nozzle for 30 min. Then, it was dried at 30 °C for 48 h to obtain a bilayer film with a thickness of 323 μm and high day and night radiative cooling power.

[0064] Example 3

[0065] BaSO4 and polyethersulfone with a particle size of 2 μm were mixed uniformly in NMP to obtain a PES / BaSO4 casting solution; wherein the mass fraction of BN in the casting solution was 12 wt% and the mass fraction of polyethersulfone was 25 wt%. The above casting solution was coated onto a glass substrate and then immersed in an anhydrous ethanol and deionized water coagulation bath for 3 h; after removal, it was dried in a forced-air drying oven at 70 °C for 12 h to obtain a PES / BaSO4 porous substrate film with a thickness of 432 μm.

[0066] TiO2 and PVDF with a particle size of 50 nm were mixed evenly in a mixed solution of DMAc and acetone to obtain a spraying solution; wherein the mass fraction of TiO2 in the spraying solution was 10 wt% and the mass fraction of PVDF was 5 wt%. 1 ml of the spraying solution was taken and sprayed onto the surface of a PES / BaSO4 porous substrate film at a distance of 5 cm using a spray gun with a 0.3 mm nozzle for 5 min. Then, it was dried at 35 °C for 48 h to obtain a bilayer film with a thickness of 436 μm and high day and night radiative cooling power.

[0067] Example 4

[0068] BN and polyethersulfone with a particle size of 500 nm were uniformly mixed in DMF to obtain a PES / BN casting solution; wherein the mass fraction of BN in the casting solution was 12 wt% and the mass fraction of polyethersulfone was 30 wt%. The casting solution was coated onto a glass substrate and then immersed in an anhydrous ethanol and deionized water coagulation bath for 4 h; after removal, it was dried in a forced-air drying oven at 65 °C for 12 h to obtain a PES / BN porous substrate film with a thickness of 320 μm.

[0069] MgO and PVDF with a particle size of 200 nm were mixed evenly in a mixed solution of DMF and acetone to obtain a spraying solution; wherein the mass fraction of MgO in the spraying solution was 7 wt% and the mass fraction of PVDF was 4 wt%. 5 ml of the spraying solution was taken and sprayed onto the surface of a PES / BN porous substrate film at a distance of 15 cm using a spray gun with a 0.5 mm nozzle for 30 min. Then, it was dried at 40 °C for 24 h to obtain a bilayer film with a thickness of 366 μm and high day and night radiative cooling power.

[0070] Example 5

[0071] TiO2 and polyethersulfone with a particle size of 1 μm were mixed uniformly in NMP to obtain a PES / TiO2 casting solution; wherein the mass fraction of TiO2 in the casting solution was 5 wt% and the mass fraction of polyethersulfone was 15 wt%. The casting solution was coated onto a glass substrate and then immersed in a deionized water coagulation bath for 4 h; after removal, it was dried in a forced-air drying oven at 65 °C for 24 h to obtain a PES / TiO2 porous substrate film with a thickness of 233 μm.

[0072] SiO2 and PVDF with a particle size of 1 μm were mixed evenly in a mixed solution of DMAc and acetone to obtain a spraying solution; wherein the mass fraction of SiO2 in the spraying solution was 6 wt% and the mass fraction of PVDF was 2 wt%. 3 ml of the spraying solution was taken and sprayed onto the surface of a PES / TiO2 porous substrate film at a distance of 10 cm using a spray gun with a 0.5 mm nozzle for 10 min. Then, it was dried at 30 °C for 48 h to obtain a bilayer film with a thickness of 249 μm and high day and night radiative cooling power.

[0073] Example 6

[0074] Al₂O₃ with a particle size of 200 nm, CaO with a particle size of 2 μm, and polyethersulfone were uniformly mixed in DMF to obtain a PES / Al₂O₃ / CaO casting solution; wherein the mass fraction of Al₂O₃ and CaO in the casting solution was 12 wt%, and the mass fraction of polyethersulfone was 30 wt%. The casting solution was coated onto a glass substrate and then immersed in a deionized water coagulation bath for 3 h; after removal, it was dried in a forced-air drying oven at 70 °C for 12 h to obtain a PES / Al₂O₃ / CaO porous substrate film with a thickness of 512 μm.

[0075] Y₂O₃ and PVDF with a particle size of 100 nm were mixed evenly in DMAc to obtain a spraying solution; wherein the mass fraction of Y₂O₃ in the spraying solution was 8 wt% and the mass fraction of PVDF was 3 wt%. 4 ml of the spraying solution was taken and sprayed onto the surface of a PES / Al₂O₃ / CaO porous substrate film at a distance of 15 cm using a spray gun with a 0.3 mm nozzle for 25 min. Then, it was dried at 45 °C for 36 h to obtain a bilayer film with a thickness of 535 μm and high day and night radiative cooling power.

[0076] Comparative Example 1

[0077] BN and polyethersulfone with a particle size of 2 μm were mixed uniformly in NMP to obtain a PES / BN casting solution; wherein the mass fraction of BN in the casting solution was 6 wt% and the mass fraction of polyethersulfone was 25 wt%. The casting solution was coated onto a glass substrate and then immersed in an anhydrous ethanol coagulation bath for 3 h; after removal, it was dried in a forced-air drying oven at 60 °C for 24 h to obtain a PES / BN porous substrate film with a thickness of 429 μm.

[0078] The solar reflectance and mid-infrared emissivity of the bilayer film prepared in Example 1 and the comparative monolayer film were tested, and the results are as follows: Figure 1 and Figure 2 As shown.

[0079] in, Figure 1 The results show the solar reflectance and mid-infrared emissivity of the bilayer thin film prepared in Example 1. Figure 2 The results show the solar reflectance and mid-infrared emissivity of the single-layer thin film in Comparative Example 1. Figure 1 and Figure 2 In the diagram, the left axis represents the reflectivity in the solar radiation band, and the right axis represents the emissivity in the mid-infrared band.

[0080] from Figure 1 and Figure 2 It can be seen that, compared with the single-layer film of Comparative Example 1, the double-layer film in Example 1 has better radiative cooling performance; its key parameters, namely solar spectral reflectance and atmospheric window emissivity, were simultaneously optimized, which confirms the effectiveness of the double-layer structure design in overcoming the performance coupling limitations of single-layer materials.

[0081] The cooling power of the bilayer thin film prepared in Example 1 was tested, wherein Figure 3 The graph shows the daytime cooling power test results. Figure 4 The graph shows the results of its nighttime cooling power test.

[0082] from Figure 3 It can be seen that when the solar power is 850W / m 2Under an ambient temperature of 35℃, the daytime cooling power reached 101.26 W / m² under different non-radiative heat transfer coefficients. 2 .

[0083] from Figure 4 It can be seen that when the solar power is 0W / m 2 Under an ambient temperature of 35℃, the nighttime cooling power reached 123.07 W / m² under different non-radiative heat transfer coefficients. 2 .

[0084] from Figure 3 and Figure 4 The test results show that the bilayer film prepared in this application maintains high radiative cooling power during both day and night by relying on optical coupling effect, micro-nano structure scattering enhancement mechanism and polymer-dielectric particle synergistic effect.

[0085] Although this application has been described in detail in this specification with general descriptions and specific embodiments, some modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, such modifications or improvements made without departing from the spirit of this application are all within the scope of protection claimed in this application.

Claims

1. A method for preparing a bilayer thin film with high day and night radiative cooling power, characterized in that, include: S1, the first inorganic particles, polyethersulfone and the first organic solvent are mixed evenly to obtain the casting solution; The coating was applied to a glass substrate and then immersed in a solvent-free coagulation bath for curing. After curing, the substrate was removed and dried to obtain a porous substrate film. The first inorganic particle includes at least one of SiO2, BN, TiO2, BaSO4, or Al2O3; S2, the second inorganic particles, polyvinylidene fluoride, and the second organic solvent are mixed evenly to obtain a spraying liquid; the spraying liquid is sprayed onto the surface of the porous substrate film and dried to obtain a double-layer film with high day and night radiative cooling power; The second inorganic particle includes at least one of CaCO3, ZnO, SiO2, Y2O3, MgO, or BN.

2. The preparation method according to claim 1, characterized in that, In the casting solution, the mass fraction of the first inorganic particles is 3~18wt%, and the mass fraction of polyethersulfone is 10~30wt%.

3. The preparation method according to claim 1, characterized in that, In the spraying liquid, the mass fraction of the second inorganic particles is 5-10 wt%, and the mass fraction of polyvinylidene fluoride is 2-5 wt%.

4. The preparation method according to claim 1, characterized in that, The particle size of the first inorganic particle is 200 nm to 3 μm; The first organic solvent includes at least one of DMF, DMAc, NMP, or DMSO; The non-solvent coagulation bath is at least one of anhydrous ethanol or deionized water.

5. The preparation method according to claim 1, characterized in that, The particle size of the second inorganic particle is 50 nm to 1 μm; The second organic solvent includes at least one of DMF, DMAc, or acetone.

6. The preparation method according to claim 1, characterized in that, In step S1, the soaking time in the non-solvent coagulation bath is 3-4 hours; The drying temperature is 60~70℃, and the time is 12~24h; The thickness of the porous substrate film is 200~600μm.

7. The preparation method according to claim 1, characterized in that, In step S2, the drying temperature is 30~50℃ and the time is 24~48h.

8. The preparation method according to claim 1, characterized in that, In step S2, the spraying process parameters are as follows: The nozzle diameter is 0.2~0.5mm, the spraying distance is 5~15cm, the spraying time is 1~30min, and the spraying thickness is 1~50μm.

9. A bilayer thin film with high day and night radiative cooling power prepared by the preparation method according to any one of claims 1-8.

10. The application of the high day and night radiative cooling power double-layer film as described in claim 9 in radiative cooling of vehicle body surface and building exterior.

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