Daytime radiation refrigeration coating and preparation method thereof

Abstract

The invention relates to the technical field of radiation refrigeration coatings, and discloses a daytime radiation refrigeration coating and a preparation method thereof.The daytime radiation refrigeration coating is attached to the surface of a base material and comprises an interface thermal buffer layer, a directional scattering reflection layer and an infrared selective emission layer from inside to outside; the interface thermal buffer layer comprises zinc oxide arrays grown on the surface of the base material in situ through a hydrothermal reaction, and graphene oxide distributed among the zinc oxide arrays; the directional scattering reflection layer comprises barium sulfate particles and titanium dioxide particles; the infrared selective emission layer comprises a core-shell structure formed by coating silicon dioxide with titanium dioxide, and polydimethylsiloxane; the base material is an aluminum plate, a stainless steel plate or glass. By constructing the interface thermal buffer layer containing the zinc oxide array and the graphene oxide, heat conduction and interface thermal stress between the scattering layer and the infrared selective emission layer are effectively relieved, and the thermal stability and long-term refrigeration performance of the coating are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of radiation cooling coatings, and in particular to a daytime radiation cooling coating and a preparation method thereof. Background Art

[0002] Daytime radiative cooling is a cutting-edge technology for achieving zero-energy passive cooling. Its core goal is to keep the surface temperature of an object below the ambient temperature by efficiently reflecting the solar spectrum (0.3-2.5μm) and strongly transmitting infrared radiation in the atmospheric window band (8-13μm). To achieve excellent cooling effects, radiative cooling coatings are typically designed with a multilayer structure, each with high solar reflectivity and high infrared emission.

[0003] However, existing multilayer coatings often encounter heat conduction issues between the scattering layer and the infrared selective emission layer in practical applications. This rapid heat transfer causes the infrared emission layer to heat up, reducing infrared emission efficiency and, in turn, weakening overall cooling performance. Furthermore, interlayer heat conduction exacerbates thermal stress concentration at the interface, making the coating susceptible to cracking, delamination, and even peeling, seriously affecting the coating's stability and service life.

[0004] There is an urgent need to develop a multi-layer radiative cooling coating structure that can effectively buffer interface thermal stress, inhibit heat conduction, and enhance interlayer bonding strength to ensure its long-term stability and efficient cooling performance. Summary of the Invention

[0005] In view of this, the present invention proposes a daytime radiation cooling coating and a preparation method thereof to solve the problem of lack of effective interface thermal buffering measures in the prior art to alleviate such heat conduction and stress concentration.

[0006] On the one hand, the present invention provides a daytime radiation cooling coating, which is attached to the surface of a substrate and comprises, from the inside to the outside, an interface thermal buffer layer, a directional scattering reflection layer, and an infrared selective emission layer;

[0007] The interface thermal buffer layer comprises: a zinc oxide array in-situ grown on the surface of the substrate by a hydrothermal reaction, and graphene oxide distributed between the zinc oxide arrays;

[0008] The directional scattering reflection layer includes: barium sulfate particles and titanium dioxide particles;

[0009] The infrared selective emission layer comprises: a core-shell structure of titanium dioxide coated with silicon dioxide, and polydimethylsiloxane;

[0010] The substrate is an aluminum plate, a stainless steel plate or glass.

[0011] Preferably, the C / O atomic ratio in the graphene oxide is 2-3, and the mass ratio of zinc oxide to graphene oxide in the interface thermal buffer layer is 1-2:10.

[0012] Preferably, the particle size of the barium sulfate particles in the directional scattering reflection layer is 400-700 nm, the particle size of the titanium dioxide particles is 50-150 nm, and the mass ratio of the barium sulfate particles to the titanium dioxide particles is 3-5:1.

[0013] Preferably, in the infrared selective emission layer, the crystal form of titanium dioxide is anatase, the shell thickness of titanium dioxide is 10-50 nm, the core diameter of silicon dioxide is 50-300 nm, and the core-shell structure is 3-15% of the mass of polydimethylsiloxane.

[0014] Preferably, the titanium dioxide as the shell structure is doped with Mn 2+ , Mn 2+ The molar number of titanium dioxide is 1-5%.

[0015] Preferably, the thickness of the interface thermal buffer layer is 1-3 μm, the thickness of the directional scattering reflection layer is 5-20 μm, and the thickness of the infrared selective emission layer is 3-10 μm.

[0016] On the other hand, the present invention also provides a method for preparing a daytime radiant cooling coating, comprising the following steps:

[0017] S1: After cleaning and surface roughening the substrate, place it in a closed reactor;

[0018] S2: Add Zn-containing 2+ A mixed solution of the precursor and hexamethylenetetramine is subjected to a hydrothermal reaction at 90-95°C for 4-6 hours to in situ grow a zinc oxide array;

[0019] S3: drop coating or dipping a graphene oxide aqueous dispersion on the surface of the zinc oxide array, and obtaining an interfacial thermal buffer layer after drying;

[0020] S4: mixing barium sulfate particles and titanium dioxide particles in a mass ratio of 3-5:1, dispersing the mixture in a 70% ethanol solution containing 0.5-0.05 wt% polyvinyl alcohol, and dispersing the mixture by ultrasonic treatment to obtain a dispersion;

[0021] S5: applying the dispersion on the interface thermal buffer layer by spraying, dipping or doctor blade coating, drying until a film is formed to obtain a directional scattering reflective layer, and applying the directional scattering reflective layer on the surface of the interface thermal buffer layer;

[0022] S6: Synthesis of silica spheres with a diameter of 50-300 nm based on the Stober method;

[0023] S7: adding silica spheres to a NaOH solution, stirring at 60-80° C. for 1-2 hours, and then adding tetrabutyl titanate to control the shell thickness to be 10-50 nm to synthesize the core-shell structure;

[0024] S8: Dispersing the core-shell structure in polydimethylsiloxane and mixing evenly, coating the core-shell structure on the surface of the directional scattering reflective layer by spin coating or dip coating, and curing the core-shell structure at room temperature or by heat curing at 80-120° C. to obtain an infrared selective emission layer.

[0025] Preferably, in step S1, the cleaning is to place the substrate in an ultrasonic cleaning instrument and ultrasonically clean it with ethanol and deionized water respectively;

[0026] The surface roughening treatment comprises: adding 1-2% by volume of 3-aminopropyltriethoxysilane to an ethanol solution and immersing the substrate therein for 30 minutes;

[0027] Equimolar amounts of Zn(CH3COO)3·2H3O and TEA were added to ethanol to a concentration of 5-10 mM. The mixture was stirred and heated to 60-80°C for 20-30 minutes to obtain a seed solution.

[0028] Remove the substrate, dry it, and apply the seed solution to the surface of the substrate by spin coating or dip coating;

[0029] The substrate is annealed at 80-100°C for 10 minutes, repeated 2-3 times; and annealed at 250-300°C for 30 minutes.

[0030] Preferably, in step S3, the graphene oxide water concentration is 0.5-2 mg / mL.

[0031] Preferably, in step S8, after the core-shell structure is synthesized, Mn(NO3)2 is added to make Mn 2+ The doping amount is 1-5% of the molar amount of Ti.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The interfacial thermal buffer layer consists of a hydrothermally in-situ grown zinc oxide array and graphene oxide distributed therein. The nanopillar structure of the zinc oxide array provides a high specific surface area and a porous structure, which can hinder the rapid transfer of heat and form a physical heat diffusion barrier. Graphene oxide, with its two-dimensional layered structure and excellent thermal conductivity, plays a role in uniformly dispersing and alleviating thermal stress. At the same time, by regulating the mass ratio of zinc oxide to graphene oxide, the interfacial thermal conductivity is rationally controlled, reducing the speed of heat directly transferred to the upper infrared selective emission layer, reducing the temperature rise of the infrared layer, and ensuring its stable radiation performance. At the same time, the thermal expansion properties of the buffer layer are matched, reducing the interfacial stress caused by temperature changes, avoiding cracking and peeling of the coating, and improving the mechanical stability and durability of the coating.

[0034] The directional scattering reflective layer utilizes a rationally designed particle size ratio of barium sulfate and titanium dioxide particles, controlled within the 400-700nm and 50-150nm ranges, respectively, and mixed in a mass ratio of 3-5:1. This combination effectively enhances sunlight scattering, increasing the coating's overall solar reflectivity while reducing the heat generated by sunlight absorption and the coating's thermal load. Furthermore, the efficient scattering reduces heat input to the infrared selective emission layer, helping to maintain its low-temperature operating state.

[0035] The infrared selective emission layer is composed of a core-shell structure of titanium dioxide coated with silicon dioxide and polydimethylsiloxane. The shell thickness of the core-shell structure is 10-50nm, the core diameter is 50-300nm and Mn 2+ The introduction of doping not only improves the anatase crystal quality of the material but also enhances the infrared radiation selectivity in the 8-13μm band, achieving efficient thermal radiation heat dissipation. Furthermore, polydimethylsiloxane, as an organic polymer material, imparts excellent flexibility and environmental stability to the coating, ensuring the durability of the infrared emission layer under various environmental conditions.

[0036] The present invention organically combines a zinc oxide-graphene oxide interface thermal buffer layer, a directional scattering reflection layer, and an infrared selective emission layer multilayer structure to form a set of functionally complementary and synergistically optimized daytime radiation cooling system. The interface thermal buffer layer effectively suppresses the rapid transfer of heat and the concentration of thermal stress at the interface, ensuring that the infrared selective emission layer maintains a low-temperature working state; the directional scattering reflection layer greatly improves the scattering and reflection ability of sunlight, reducing the heat load generated by light absorption; the infrared selective emission layer achieves efficient mid-infrared radiation, effectively releasing heat into outer space through the atmospheric window band. The thickness and composition of the three-layer structure have been optimized and matched, which not only improves the cooling efficiency, but also enhances the mechanical stability and environmental durability of the coating, solves the problems of excessive heat conduction, stress concentration, and coating failure in existing multilayer coatings, and realizes a high-performance, long-life daytime radiation cooling coating. DETAILED DESCRIPTION

[0037] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0038] Example 1

[0039] Preparation of substrate: an aluminum plate (thickness 1 mm) was selected, ultrasonically cleaned with ethanol and deionized water for 15 minutes each, the surface was immersed in an ethanol solution containing 1.5% volume fraction of 3-aminopropyltriethoxysilane for 30 minutes, and dried at room temperature for use.

[0040] Preparation of interface thermal buffer layer:

[0041] Prepare seed solutions of Zn(CH3COO)2·2H2O and TEA with a concentration of 8 mM each, and heat to 70°C with stirring for 25 minutes.

[0042] The seed solution was spin-coated on the surface of the aluminum plate, and annealed at 80°C for 10 minutes. This was repeated three times, and then annealed at 250°C for 30 minutes.

[0043] The treated aluminum plate was placed in a closed reactor and 10 mM Zn 2+ The ZnO nanorod arrays were formed by reacting with a hydrothermal solution of 15 mM hexamethylenetetramine at 90 °C for 5 hours.

[0044] A graphene oxide aqueous dispersion (concentration 1.5 mg / mL, C / O ratio 2.5) was drop-coated and dried to obtain an interfacial thermal buffer layer with a thickness of 2 μm. The mass ratio of zinc oxide to graphene oxide was 1.5:10.

[0045] Preparation of directional scattering reflection layer:

[0046] Barium sulfate particles (particle size 600 nm) and titanium dioxide particles (particle size 100 nm) were mixed in a mass ratio of 3:1, dispersed in a 70% ethanol solution containing 0.1 wt % polyvinyl alcohol, and ultrasonically treated for 30 minutes.

[0047] The dispersion was applied to the buffer layer by spraying and dried to form a film with a thickness of 15 μm.

[0048] Preparation of infrared selective emission layer:

[0049] Silica spheres with a diameter of 200 nm were synthesized using the Stober method.

[0050] The silica spheres were added to NaOH solution (0.1 M) and stirred at 70 °C for 1.5 hours. Tetrabutyl titanate was then added to control the shell thickness to 40 nm. Mn was doped2+ The concentration is 3 mol% of Ti.

[0051] The core-shell structure is dispersed in a solution containing 10 wt% polydimethylsiloxane, coated on the surface of the directional scattering and reflecting layer by spin coating, and cured at room temperature. The layer thickness is 7 μm.

[0052] The end result: a multi-layer coating with a total thickness of 24 μm, exhibiting excellent interfacial thermal buffering, high solar reflectivity, and strong 8-13 μm infrared emission.

[0053] Example 2

[0054] Preparation of substrate: A stainless steel plate (thickness 0.5 mm) was selected and ultrasonically cleaned with ethanol and deionized water for 10 minutes each. The surface was roughened by immersing it in a 1% volume fraction 3-aminopropyltriethoxysilane solution for 30 minutes and air-dried.

[0055] Preparation of interface thermal buffer layer:

[0056] Prepare a seed solution of 10 mM each of Zn(CH3COO)2·2H2O and TEA, and react at 70°C for 30 minutes.

[0057] Three layers of seed solution were spin-coated, each layer was annealed at 80°C for 10 minutes, and finally annealed at 250°C for 30 minutes.

[0058] The hydrothermal reaction temperature was set at 95°C and the time was 4 hours, and the growth of the zinc oxide nanocolumn array was completed.

[0059] The concentration of the graphene oxide aqueous dispersion was 1 mg / mL, and the coating was carried out and dried. The thickness of the interfacial thermal buffer layer was 1.5 μm, and the mass ratio of zinc oxide to graphene oxide was 1:10.

[0060] Preparation of directional scattering reflection layer:

[0061] The particle size of barium sulfate particles is 450 nm, the particle size of titanium dioxide particles is 70 nm, and the mass ratio is 4:1.

[0062] After the dispersion was ultrasonically treated for 20 minutes, it was evenly coated by dip coating and dried to form a film with a layer thickness of 10 μm.

[0063] Preparation of infrared selective emission layer:

[0064] The diameter of the synthesized silica sphere is 120nm, the shell thickness is 30nm, and the Mn 2+ Doping amount 1mol%.

[0065] The core-shell structure is dispersed in a polydimethylsiloxane solution (content 8 wt%), dip-coated on the directional scattering reflective layer, and thermally cured at 80° C. for 2 hours. The layer thickness is 5 μm.

[0066] Example 3

[0067] Preparation of substrate: A glass substrate (3 mm thick) was ultrasonically cleaned with ethanol and deionized water for 15 minutes each, immersed in a 2% volume fraction 3-aminopropyltriethoxysilane solution for 30 minutes, and air-dried at room temperature.

[0068] Preparation of interface thermal buffer layer:

[0069] The seed solution Zn(CH3COO)2·2H2O and TEA were both 5mM, and the reaction was carried out at 70℃ for 20 minutes.

[0070] The seed solution was spin-coated twice, and low-temperature annealing was performed at 80°C for 10 minutes each time and at 250°C for 30 minutes.

[0071] The ZnO nanorod arrays were grown by hydrothermal reaction at 95°C for 5 hours.

[0072] Graphene oxide dispersion (2 mg / mL, C / O=3) was drop-coated and dried to form an interfacial thermal buffer layer with a thickness of 3 μm and a mass ratio of zinc oxide to graphene oxide of 2:10.

[0073] Preparation of directional scattering reflection layer:

[0074] Barium sulfate particles with a particle size of 700 nm and titanium dioxide particles with a particle size of 150 nm were selected at a mass ratio of 5:1, dispersed in a 0.05 wt % polyvinyl alcohol 70% ethanol solution, and ultrasonically treated for 40 minutes.

[0075] The coating is done by knife coating and dried to form a film with a layer thickness of 20 μm.

[0076] Preparation of infrared selective emission layer:

[0077] The diameter of the synthesized silica sphere is 300nm, the shell thickness is 10nm, and the Mn 2+ Doping amount 5mol%.

[0078] The core-shell structure was dispersed in polydimethylsiloxane solution (15 wt %), spin-coated, and thermally cured at 120° C. for 1 hour, with a layer thickness of 10 μm.

[0079] Comparative Example A1

[0080] The difference between this comparative example and Example 1 is that the interface thermal buffer layer only uses hydrothermally grown zinc oxide nanorod arrays, and no graphene oxide is added.

[0081] Comparative Example A2

[0082] The difference between this comparative example and Example 1 is that: there is no interface thermal buffer layer, the directional scattering reflection layer is directly sprayed on the surface of the substrate, and there is no zinc oxide array and graphene oxide buffer layer.

[0083] Comparative Example B1

[0084] The difference between this comparative example and Example 1 is that the directional scattering reflection layer contains only barium sulfate particles and no titanium dioxide particles.

[0085] Comparative Example B2

[0086] The difference between this comparative example and Example 1 is that the directional scattering reflection layer contains only titanium dioxide particles and no barium sulfate particles.

[0087] Comparative Example C1

[0088] The difference between this comparative example and Example 1 is that the infrared selective emission layer is only a single titanium dioxide coating without a silicon dioxide core-shell structure.

[0089] Comparative Example C2

[0090] The difference between this comparative example and Example 1 is that the infrared selective emission layer is a titanium dioxide-coated silicon dioxide core-shell structure, but is not doped with Mn. 2+ .

[0091] Comparative Example C3

[0092] The difference between this comparative example and Example 1 is that the infrared selective emission layer contains Mn 2+ The doped core-shell structure, but without polydimethylsiloxane organic polymer, lacks flexibility and environmental stability improvements.

[0093] Comparative Example D1

[0094] The difference between this comparative example and Example 1 is that the coating only contains a directional scattering reflection layer, and no interface thermal buffer layer and infrared selective emission layer are provided.

[0095] Comparative Example D2

[0096] The difference between this comparative example and Example 1 is that the coating contains a directional scattering reflection layer and an infrared selective emission layer, but lacks an interface thermal buffer layer.

[0097] Comparative Example D3

[0098] The difference between this comparative example and Example 1 is that the coating contains an interface thermal buffer layer and a directional scattering reflection layer, but no infrared selective emission layer.

[0099] Examples 1-3 and all comparative examples were tested, and the test items are shown in Table 1, and the test results are shown in Table 2.

[0100] Table 1

[0101]

[0102]

[0103] Table 2

[0104]

[0105] The following conclusions can be drawn from the table:

[0106] 1. Solar reflectivity

[0107] The reflectivity of Examples 1-3 all exceeded 94%, reaching a maximum of 95.5%, significantly outperforming all comparative examples, particularly Comparative Examples A2 (88.5%) and B2 (89.8%). This demonstrates that the optimal combination of barium sulfate and titanium dioxide particle sizes and ratios (400-700 nm and 50-150 nm, respectively, with a mass ratio of 3-5:1) in the directional scattering reflective layer significantly enhances scattering capability. Comparative Examples B1 and B2 exhibited lower scattering efficiencies, with reflectivities below 91%.

[0108] 2. Mid-infrared emissivity

[0109] The emissivity of the embodiment is 91.8%-93.0%, which is significantly better than that of comparative examples C1 (80.5%) and C2 (88.0%). This shows that the core-shell structure of TiO2@SiO2 and Mn 2+ Doping is the key to effectively improve the selective emission performance in the 8-13μm band. Comparative Example C1 uses only TiO2, which lacks photon localization structure and crystal optimization, resulting in a significant decrease in emission rate. C2 is not doped with Mn 2+ , although there is an improvement, the effect is still not as good as the complete structure.

[0110] 3. Surface temperature drop

[0111] The cooling range of the examples reached 8.3-8.7°C, while the maximum cooling range of the comparative examples (C3) was only 7.0°C, demonstrating that the synergistic effect of the three-layer structure significantly enhances cooling efficiency. Comparative examples A1 and A2, lacking only the interface thermal buffer layer, saw cooling drop to 4.8-5.2°C, demonstrating the critical role of the thermal buffer layer in maintaining the low temperature of the infrared layer.

[0112] 4. Thermal conductivity

[0113] The thermal conductivity of the example was significantly reduced to 0.11-0.13 W / m·K, significantly better than that of A1 (0.30 W / m·K) and A2 (0.35 W / m·K). This indicates that the composite structure of zinc oxide nanoarrays and graphene oxide in the interfacial thermal buffer layer effectively blocks the heat diffusion path, providing strong thermal resistance. A1 lacks graphene, which exacerbates thermal diffusion, while A2 completely fails without the buffer layer.

[0114] 5. Interface bonding strength

[0115] The bonding strength of the examples was 4.3-4.6 MPa, which was better than that of the comparative examples (mostly between 1.8-3.5 MPa). This indicates that the interfacial thermal buffer layer improves thermal expansion mismatch; the mechanical intercalation of the porous zinc oxide array and other layers improves overall adhesion strength; and the polydimethylsiloxane provides flexibility protection in the outermost layer.

[0116] 6. Thermal shock stability

[0117] Examples 1-3 showed no cracking during the hot-cold cycling test, while Comparative Examples A2 and D1 exhibited mild to severe cracking, demonstrating the critical importance of interface buffering and structural matching in thermally stressed environments. In particular, D1, with its simplified structure retaining only the scattering layer, was highly susceptible to failure in thermal environments, demonstrating the necessity of a coordinated multilayer structure.

[0118] 7. Aging performance

[0119] The performance degradation of the examples after aging was less than 5%, while that of the comparative examples was mostly 10-25%, indicating that the introduction of polydimethylsiloxane and core-shell structure not only improved the functionality but also gave the coating good environmental weather resistance and structural stability.

[0120] From the data of Examples 1-3, it can be seen that the interface thermal buffer layer controls the problems of vertical heat conduction and stress concentration, and is the basis for maintaining infrared emission efficiency and structural integrity; the directional scattering reflection layer greatly reduces solar energy absorption through strong scattering, and is the main factor in suppressing external heat input; the infrared emission layer is the energy outlet for active heat dissipation, and the core-shell structure and doping regulation are the core mechanisms for achieving high emissivity.

[0121] The improvement of many performance indicators (such as thermal resistance, emissivity, and temperature reduction) depends on the coordinated work of the three layers. The absence of any single layer will cause a significant decline in coating performance (Comparative Examples D1-D3).

[0122] Through the comparative examples A1, A2, and D1 settings, it can be seen that: without an interfacial thermal buffer layer or graphene filling, the thermal conductivity increases significantly, causing the temperature of the infrared layer to rise, the emissivity to decrease, and the cooling amplitude to decrease; without an infrared selective layer (D3) or a simplified infrared layer (C1), the emissivity is insufficient and effective cooling cannot be achieved; without a flexible polymer (C3), the long-term stability and aging resistance are seriously reduced.

[0123] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A daytime radiation cooling coating, characterized in that: The daytime radiation cooling coating is attached to the surface of the substrate, and the daytime radiation cooling coating comprises, from the inside to the outside, an interface thermal buffer layer, a directional scattering reflection layer, and an infrared selective emission layer; The interface thermal buffer layer comprises: a zinc oxide array in-situ grown on the surface of the substrate by a hydrothermal reaction, and graphene oxide distributed between the zinc oxide arrays; The directional scattering reflection layer includes: barium sulfate particles and titanium dioxide particles; The infrared selective emission layer comprises: a core-shell structure of titanium dioxide coated with silicon dioxide, and polydimethylsiloxane; The substrate is an aluminum plate, a stainless steel plate or glass.

2. The daytime radiation cooling coating according to claim 1, characterized in that: The C / O atomic ratio in the graphene oxide is 2-3, and the mass ratio of zinc oxide to graphene oxide in the interface thermal buffer layer is 1-2:

10.

3. The daytime radiation cooling coating according to claim 1, characterized in that: The particle size of the barium sulfate particles in the directional scattering reflection layer is 400-700 nm, the particle size of the titanium dioxide particles is 50-150 nm, and the mass ratio of the barium sulfate particles to the titanium dioxide particles is 3-5:

1.

4. The daytime radiation cooling coating according to claim 1, characterized in that: In the infrared selective emission layer, the crystal form of titanium dioxide is anatase, the shell thickness of titanium dioxide is 10-50 nm, the core diameter of silicon dioxide is 50-300 nm, and the core-shell structure is 3-15% of the mass of polydimethylsiloxane.

5. The daytime radiation cooling coating according to claim 1, characterized in that: Titanium dioxide as a shell structure is doped with Mn 2+ , Mn 2+ The molar number of titanium dioxide is 1-5%.

6. The daytime radiation cooling coating according to claim 1, characterized in that: The thickness of the interface thermal buffer layer is 1-3 μm, the thickness of the directional scattering reflection layer is 5-20 μm, and the thickness of the infrared selective emission layer is 3-10 μm.

7. A method for preparing a daytime radiation cooling coating, characterized in that: The following steps are involved: S1: After cleaning and surface roughening the substrate, place it in a closed reactor; S2: Add Zn-containing 2+ A mixed solution of the precursor and hexamethylenetetramine is subjected to a hydrothermal reaction at 90-95°C for 4-6 hours to in situ grow a zinc oxide array; S3: drop coating or dipping a graphene oxide aqueous dispersion on the surface of the zinc oxide array, and obtaining an interfacial thermal buffer layer after drying; S4: mixing barium sulfate particles and titanium dioxide particles in a mass ratio of 3-5:1, dispersing the mixture in a 70% ethanol solution containing 0.5-0.05 wt% polyvinyl alcohol, and dispersing the mixture by ultrasonic treatment to obtain a dispersion; S5: applying the dispersion on the interface thermal buffer layer by spraying, dipping or doctor blade coating, drying until a film is formed to obtain a directional scattering reflective layer, and applying the directional scattering reflective layer on the surface of the interface thermal buffer layer; S6: Synthesis of silica spheres with a diameter of 50-300 nm based on the Stober method; S7: Add silica spheres to NaOH solution, stir at 60-80°C for 1-2 hours, then add tetrabutyl titanate to control the shell thickness to 10-50 nm to synthesize a core-shell structure; S8: Dispersing the core-shell structure in polydimethylsiloxane and mixing evenly, coating the core-shell structure on the surface of the directional scattering reflective layer by spin coating or dip coating, and curing the core-shell structure at room temperature or by heat curing at 80-120° C. to obtain an infrared selective emission layer.

8. The method for preparing a daytime radiation cooling coating according to claim 7, characterized in that: In step S1, the cleaning is to place the substrate in an ultrasonic cleaning instrument and ultrasonically clean it with ethanol and deionized water respectively; The surface roughening treatment comprises: adding 1-2% by volume of 3-aminopropyltriethoxysilane to an ethanol solution and immersing the substrate therein for 30 minutes; Equimolar amounts of Zn(CH3COO)3·2H3O and TEA were added to ethanol to a concentration of 5-10 mM. The mixture was stirred and heated to 60-80°C for 20-30 minutes to obtain a seed solution. Remove the substrate, dry it, and apply the seed solution to the surface of the substrate by spin coating or dip coating; The substrate is annealed at 80-100°C for 10 minutes, repeated 2-3 times; and annealed at 250-300°C for 30 minutes.

9. The method for preparing a daytime radiation cooling coating according to claim 7, characterized in that: In step S3, the graphene oxide water concentration is 0.5-2 mg / mL.

10. The method for preparing a daytime radiation cooling coating according to claim 7, characterized in that: In step S8, after the core-shell structure is synthesized, Mn(NO3)2 is added to make Mn 2+ The doping amount is 1-5% of the molar amount of Ti.

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