Daytime passive radiation refrigeration ceramic metamaterial capable of regulating and controlling emissivity wave band and preparation method of daytime passive radiation refrigeration ceramic metamaterial

Abstract

The invention belongs to the technical field of refrigeration materials, and particularly discloses a daytime passive radiation refrigeration ceramic metamaterial capable of regulating and controlling the emissivity wave band and a preparation method thereof.The daytime passive radiation refrigeration ceramic metamaterial comprises a base material and a filling material, and the base material is a ceramic composite material which has a broadband radiation refrigeration function and is of a hierarchical pore structure; and the filling material is a high-molecular polymer material with a selective radiation refrigeration function. By controlling the filling proportion of the polymer to the ceramic pore structure, a controllable adjusting method for the thermal radiation emission wave band of the ceramic metamaterial is provided, and conversion of the material between broadband radiation refrigeration and selective radiation refrigeration is achieved. The preparation method disclosed by the invention is simple and easy to implement, and the hard ceramic and polymer composite metamaterial has the characteristics of high toughness and high durability; and meanwhile, conversion between broadband radiation refrigeration and selective radiation refrigeration can directionally meet different scene requirements, and wide application prospects are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of refrigeration materials, and particularly relates to a daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band and a preparation method thereof. Background Art

[0002] According to statistics, about 10% of the global energy is used for refrigeration every year. With the rapid growth of the population, the intensive development of industry, and the improvement of people's living standards, it is expected that by 2050, the energy consumption for refrigeration will increase by more than 10 times. This trend will greatly exacerbate the energy crisis and climate warming problems. Therefore, the development of energy-saving and environment-friendly refrigeration technologies has become an urgent task.

[0003] As a zero-energy refrigeration material, daytime radiative cooling materials have significant energy-saving potential. It reflects more than 90% of sunlight through its surface structure, inhibits solar radiation heating, and at the same time transfers its own heat to the extremely cold outer space (3K) in the form of long-wave infrared radiation through the atmospheric transparent window, thereby achieving the refrigeration effect. However, the existing radiative cooling materials have the following limitations:

[0004] The band where the high thermal radiation emissivity is located cannot be dynamically adjusted: According to the actual engineering needs, the existing radiative cooling materials are mainly divided into two categories: selective and broadband. Selective radiative cooling materials usually have high emissivity only in the main atmospheric window band of 8μm - 13μm, while broadband radiative cooling materials show high emissivity within the entire atmospheric window of 5μm - 25μm. However, there is still an obvious gap in how to achieve the dynamic conversion between broadband radiative cooling and selective radiative cooling. This limitation restricts the application effect of radiative cooling materials under diverse environmental conditions and is also difficult to meet the demand for flexible regulation of multi-band emissivity in practical engineering.

[0005] The mechanical properties of radiative cooling ceramics are insufficient: In order to enhance the solar reflectivity and long-wave infrared emissivity, the existing radiative cooling ceramic materials usually have a high porosity. Although this high porosity helps to improve the radiative cooling performance, it also results in low strength and toughness of the materials, restricting their widespread use in practical applications. Summary of the Invention

[0006] To solve the problems existing in the prior art, the present invention provides a daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band and a preparation method thereof. By optimizing the microstructure and components of the material, the dynamic regulation of the band where the high thermal radiation emissivity is located is achieved, and at the same time, the mechanical properties and environmental stability of the material are significantly improved, thereby meeting the requirements of complex application scenarios and solving the problems mentioned in the above background art.

[0007] To achieve the above object, the present invention provides the following technical solution: A daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band, the daytime passive radiative cooling ceramic metamaterial includes a matrix material and a filling material, the matrix material is a composite ceramic material with a multi-porous structure having broadband radiative cooling function, and the filling material is a polymer with selective radiative cooling function; by controlling the filling ratio of the polymer to the composite ceramic material with multi-porous structure, the conversion between broadband radiative cooling and selective radiative cooling of the daytime passive radiative cooling ceramic metamaterial is realized;

[0008] When the volume filling ratio of the polymer to the composite ceramic material reaches the maximum ratio of 0.5:1 - 0.8:1, the daytime passive radiative cooling ceramic metamaterial realizes selective radiative cooling;

[0009] When the filling ratio of the polymer to the composite ceramic material is less than the maximum volume filling ratio, the conversion between broadband radiative cooling and selective radiative cooling of the daytime passive radiative cooling ceramic metamaterial is realized.

[0010] Preferably, the solar reflectivity of the daytime passive radiative cooling ceramic metamaterial is not less than 95%; the thermal emissivity of the daytime passive radiative cooling ceramic metamaterial in the broadband infrared band of 5μm - 25μm or the selective infrared band of 8μm - 13μm is not less than 93%; the flexural strength of the daytime passive radiative cooling ceramic metamaterial is not less than 300MPa, the compressive strength is not less than 600Mpa, and the porosity is 50% - 80%.

[0011] On the other hand, to achieve the above object, the present invention also provides the following technical solution: A preparation method of a daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band, including the following steps:

[0012] Step 1, preparing the matrix material: adding a polymer solution to metal oxides and micron hollow glass microspheres to form a mud; placing the mud in a mold; applying pressure to the mud in the mold to form a green body; drying the green body; sintering the dried green body to obtain a composite ceramic matrix material with a multi-porous structure having broadband radiative cooling function;

[0013] Step 2, preparing the filling material: dissolving a polymer with selective radiative cooling function as the filling material to form a filling solution;

[0014] Step 3, preparing the daytime passive radiative cooling ceramic metamaterial: ultrasonically cleaning the composite ceramic matrix material with a multi-porous structure having broadband radiative cooling function; drying the cleaned composite ceramic matrix material; preparing the composite ceramic matrix material and the filling solution by mechanical pressing method and / or vacuum assisted impregnation method to obtain the daytime passive radiative cooling ceramic metamaterial.

[0015] Preferably, adding a polymer solution to the metal oxide and the micron hollow glass microspheres to form a mud material specifically includes the following steps:

[0016] 1.1. Dry the metal oxide powder and the micron hollow glass microspheres;

[0017] 1.2. Mix the dried metal oxide powder and the micron hollow glass microspheres to obtain a mixed dry powder;

[0018] 1.3. Add the polymer solution to the mixed dry powder and stir to obtain a mud material.

[0019] Preferably, the mass ratio of the micron hollow glass microspheres to the metal oxide powder is 0.25 - 1; the mass ratio of the polymer solution to the mixed dry powder is 0.2 - 1.

[0020] Preferably, the sintering of the dried green body specifically includes: placing the dried green body in a sintering space for sintering, the temperature of the sintering space rising from room temperature to 400 - 600 °C at a rate of 2 - 4 °C / min and maintaining for 2 - 4 hours; then the temperature of the sintering space rising again at a rate of 2 - 4 °C / min to 800 - 1200 °C and maintaining for 3 - 6 hours; then naturally cooling the fired ceramic to room temperature; the air supply speed of the sintering space is set to 100 - 300 ml / min.

[0021] Preferably, in step one, one or more of the micron hollow glass microspheres with a particle size of 2 μm - 50 μm are selected; the metal oxide is one or more of powdery Al2O3, TiO2, MgO, CaCO3, ZnO, ZrO2; the polymer solution is one or more of polyvinylidene fluoride solution, polydimethylsiloxane solution, polymethyl methacrylate solution, polyvinyl alcohol solution.

[0022] Preferably, in step two, the high molecular polymer is one or more of polyoxymethylene POM and polyvinylidene fluoride PVDF.

[0023] Preferably, in step three, the mechanical pressing method is specifically: uniformly placing the dried composite ceramic matrix material and the filling solution in a mold, applying pressure to the material in the mold to obtain a daytime passive radiative cooling ceramic metamaterial;

[0024] Applying pressure to the material in the mold includes: uniformly applying pressure to the material in the mold by a press, the pressure is set to 2 - 15 MPa, maintaining the pressure for more than 30 minutes, and fully filling the filling material into the matrix material.

[0025] Preferably, in step three, the vacuum-assisted impregnation method specifically comprises: vacuumizing the dried composite ceramic matrix material, and filling the filling solution into an impregnation tank to obtain a daytime passive radiation cooling ceramic metamaterial;

[0026] Filling the filling solution into the impregnation tank includes opening the valve of the liquid storage tank, injecting the filling material into the impregnation tank by gravity or external air pressure of 2 to 15 MPa, and controlling the injection speed to be 2 to 10 mL / min.

[0027] The beneficial effects of the present invention are:

[0028] 1) By controlling the type and ratio of polymers filled in porous ceramics, the present invention enables the synthesized ceramic metamaterial to achieve dynamic regulation between broadband radiative cooling and selective radiative cooling, so as to meet practical engineering needs in a targeted manner.

[0029] 2) The present invention uses high molecular polymer to fill the internal pores of ceramics, which can significantly enhance the original mechanical strength and toughness of the base ceramics.

[0030] 3) The internal pores of the porous ceramic polymer metamaterial of the present invention are mainly filled with polymer, which can effectively prevent pollutants such as dust from penetrating into the material along the pore structure, thereby preventing the reduction of daytime radiant cooling power. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of laying a daytime passive radiation cooling ceramic metamaterial with adjustable emissivity band on a surface to be cooled in an embodiment of the present invention;

[0032] Figure 2 Schematic diagram of the process of mechanical pressing and preparing a daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band according to an embodiment of the present invention;

[0033] Figure 3 Schematic diagram of the process of vacuum-assisted impregnation preparation of daytime passive radiative cooling ceramic metamaterials with adjustable emissivity bands according to an embodiment of the present invention;

[0034] Figure 4 Schematic diagram of the preparation of mud in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] Example 1

[0037] The present invention provides a technical solution: a daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band, comprising a matrix material and a filling material. The matrix material is a ceramic composite material with a multi-porous structure having broadband radiative cooling function; the filling material is a polymer material with selective radiative cooling function. By controlling the filling ratio of the polymer to the ceramic pore structure, a controllable adjustment method for the thermal radiation emission band of the ceramic metamaterial is proposed to achieve the conversion of the material between broadband radiative cooling and selective radiative cooling. As Figure 1 shown, this material can be laid on the surface to be cooled. The surface to be cooled can be wood, concrete, tile, glass, metal, and plastic; or, the surface of the main body of the surface to be cooled, where the main body is a building, an automobile, a train, a ship, a road, an oil tank, or a pipeline.

[0038] Further, the ceramic composite material with a multi-porous structure of the matrix material is composed of metal oxides and micron-sized hollow glass microspheres.

[0039] Further, the filling material is selected from one or more of the polymer polyoxymethylene (POM), polyvinylidene fluoride (PVDF), etc.

[0040] Further, the filling pressure range of the matrix material and the filling material is 2 MPa to 15 MPa.

[0041] Further, when the volume filling ratio of the polymer to the composite ceramic material reaches the maximum ratio of 0.5:1 - 0.8:1, the daytime passive radiative cooling ceramic metamaterial realizes selective radiative cooling; when the filling ratio of the polymer to the composite ceramic material is less than the maximum volume filling ratio, the daytime passive radiative cooling ceramic metamaterial realizes the conversion between broadband radiative cooling and selective radiative cooling.

[0042] The daytime passive radiative cooling ceramic metamaterial of the present invention is formed by the penetration of a polymer with selective radiative cooling function into the interior of a porous ceramic with broadband radiative cooling function under high pressure. By changing the polymer type and filling ratio, the dynamic regulation of the band where the metamaterial has a high thermal radiation emissivity is realized, while maintaining the original high solar reflectivity, and having excellent radiative cooling effect.

[0043] The solar reflectivity of the daytime passive radiative cooling ceramic metamaterial is not less than 95%; the thermal emissivity of the daytime passive radiative cooling ceramic metamaterial in the broadband infrared band of 5 μm - 25 μm or the selective infrared band of 8 μm - 13 μm is not less than 93%; the flexural strength of the daytime passive radiative cooling ceramic metamaterial is not less than 300 MPa, the compressive strength is not less than 600 Mpa, and the porosity is 50% - 80%.

[0044] Example 2

[0045] As shown in Figure 2 the figure Figure 2 is a process schematic diagram of a mechanical pressing preparation method for a day passive radiative cooling ceramic metamaterial with an adjustable emissivity band in the present invention. The method includes:

[0046] S1: Add a polymer solution to the metal oxide powder and micron hollow glass microspheres to form a mud material.

[0047] Among them, the metal oxide powder of the matrix material can be any one powder or a mixture of multiple powders of Al2O3, TiO2, MgO, CaCO3, ZnO, ZrO2.

[0048] Among them, the micron hollow glass microspheres are selected from one or more with a particle size of 2μm - 50μm.

[0049] Among them, the mass ratio of the micron hollow glass microspheres to the metal oxide powder is between 0.25 and 1.

[0050] Among them, the polymer solution is a solution formed by dissolving a polymer in an organic solution. The polymer can be any one or any combination of polyvinylidene fluoride (PVDF), polydimethylsiloxane (PDMS), polymethyl methacrylate (PMMA), and polyvinyl alcohol (PVA). Optionally, in one embodiment, the polymer solution is a 3% polyvinyl alcohol solution.

[0051] Further, as shown in Figure 4 the figure Figure 4 is a process schematic diagram of step S1 of the preparation method. Step S1 can specifically include:

[0052] S1.1: Dry the metal oxide powder and micron hollow glass microspheres.

[0053] Specifically, put the metal oxide powder and micron hollow glass microspheres into an oven for drying. The specific drying time can be determined according to requirements. For example, it can be dried for 12 hours. This step is mainly to remove the moisture in the metal oxide powder and micron hollow glass microspheres.

[0054] S1.2: Mix the dried metal oxide powder and micron hollow glass microspheres.

[0055] Specifically, mix the dried micron hollow glass microspheres and the metal oxide powder. The mass ratio of the micron hollow glass microspheres to the metal oxide powder is between 0.25 and 1. Use a magnetic stirrer to stir the micron hollow glass microspheres and the metal oxide powder for 5 - 10 minutes to make the two dry powders fully and evenly mixed.

[0056] S1.3. Add the polymer solution to the mixed dry powder and stir to obtain a mud material.

[0057] Specifically, during the stirring process, add the 3% PVA solution to the dry powder in 3 - 5 portions within 3 - 5 minutes. Set the mass ratio of the polymer solution to the dry powder to be between 0.2 and 1. After adding, continue stirring for 5 - 10 minutes until the powder is evenly dispersed without obvious agglomeration to obtain a uniformly mixed mud material.

[0058] S2. Place the mud material in a mold.

[0059] It can be understood that the mold can be set to any shape according to requirements to form a ceramic composite material with a multi - level pore structure of a specific shape.

[0060] S3. Apply pressure to the mud material in the mold to form a green body.

[0061] Specifically, use a press to uniformly apply pressure to the mud material in the mold. Set the pressure to 2MPa - 15MPa and maintain the pressure for more than 30 minutes to degas the mud material and form a green body.

[0062] S4. Dry the green body.

[0063] Specifically, dry the degassed green body. The drying time can be set according to requirements. For example, it can be dried at room temperature for 24 hours.

[0064] Optionally, in one embodiment, the dried green body can be cut into any shape according to needs.

[0065] S5. Sinter the dried green body to obtain a ceramic composite material with a multi - level pore structure.

[0066] The sintering process can be carried out in a furnace. In an optional embodiment, step S5 can specifically include: placing the dried green body in a sintering space for sintering. The temperature of the sintering space rises from room temperature to 400 - 600℃ at a rate of 2 - 4℃ / min and is maintained for 2 - 4 hours; then the temperature of the sintering space rises again to 800 - 1200℃ at a rate of 2 - 4℃ / min and is maintained for 3 - 6 hours; then the fired ceramic is naturally cooled to room temperature; the air supply speed of the sintering space is set to 100 - 300ml / min.

[0067] S6. Use ultrasonic cleaning on the obtained matrix material;

[0068] Specifically, perform ultrasonic cleaning on the matrix material. The cleaning time can be set according to the size of the matrix material and the cleaning power, and generally needs to be repeated 3 - 5 times.

[0069] S7. Dry the cleaned substrate material to avoid residual moisture affecting the filling.

[0070] Specifically, vacuum drying or oven drying can be selected for drying. The drying time can be set according to requirements. Generally, for oven drying, the set temperature is 80 - 200 °C and the time is 2 - 6 hours.

[0071] S8. Dissolve the filling material of the polymer with selective radiative cooling function to form a filling solution.

[0072] Specifically, select appropriate organic solvents, such as toluene, benzene, carbon tetrachloride and other organic solvents to dissolve the polymer with selective radiative cooling function.

[0073] Specifically, stir the polymer solution until the micron-sized silica particles are evenly dispersed without obvious agglomeration. Then remove the bubbles by slow stirring to degas the suspension to obtain the filling solution.

[0074] Optionally, the mixed solution of the polymer and the organic solvent can be heated to promote the dissolution of the polymer.

[0075] S9. Place the dried substrate material and the filling solution evenly in a mold.

[0076] S10. Apply pressure to the material in the mold to obtain the target ceramic metamaterial.

[0077] Specifically, use a press to apply pressure evenly to the material in the mold. First, perform a pre-press of (0.5 - 1 MPa), then gradually increase the pressure to the target pressure (2 - 15 MPa), and maintain the pressure in stages for more than 30 minutes in each stage to fully squeeze out the bubbles in the mud. After the pressure application is completed, the material is slowly depressurized to prevent the material from rebounding and cracking.

[0078] Example 3

[0079] As Figure 3 shown, Figure 3 is a flow chart of the vacuum-assisted impregnation preparation method of the day-time passive radiative cooling ceramic metamaterial with adjustable emissivity band in the present invention. The method includes:

[0080] S1: Add a polymer solution to the metal oxide powder and micron-sized hollow glass microspheres to form a mud.

[0081] Among them, the metal oxide powder of the substrate material can be any one powder or a mixture of multiple powders of Al2O3, TiO2, MgO, CaCO3, ZnO, ZrO2.

[0082] Among them, the micron-sized hollow glass microspheres are selected from one or more with a particle size of 2 μm - 50 μm.

[0083] The mass ratio of the micron hollow glass microspheres to the metal oxide powder is between 0.25 and 1.

[0084] The polymer solution is a solution formed by dissolving a polymer in an organic solvent. The polymer may be any one or more of polyvinylidene fluoride (PVDF), polydimethylsiloxane (PDMS), polymethyl methacrylate (PMMA), and polyvinyl alcohol (PVA). Optionally, in one embodiment, the polymer solution is a 3% polyvinyl alcohol solution.

[0085] Further, such as Figure 4 As shown, Figure 4 is a flow chart of step S1 of the preparation method, and step S1 may specifically include:

[0086] S1.1. Dry the metal oxide powder and micron hollow glass microspheres.

[0087] Specifically, the metal oxide powder and micron hollow glass microspheres are placed in an oven for drying. The specific drying time can be determined according to demand, for example, it can be dried for 12 hours. This step is mainly to remove moisture from the metal oxide powder and micron hollow glass microspheres.

[0088] S1.2. Mix the dried metal oxide powder and micron hollow glass microspheres.

[0089] Specifically, dry micron hollow glass microspheres and metal oxide powder are mixed, with a mass ratio of the micron hollow glass microspheres to the metal oxide powder being between 0.25 and 1, and the micron hollow glass microspheres and the metal oxide powder are stirred for 5-10 minutes using a magnetic stirrer to ensure that the two dry powders are fully and evenly mixed.

[0090] S1.3. Add the polymer solution to the mixed dry powder and stir to obtain a slurry.

[0091] Specifically, during the stirring process, 3% PVA solution is added to the dry powder in 3-5 times within 3 to 5 minutes, and the mass ratio of polymer solution to dry powder is set between 0.2 and 1. After adding, stirring is continued for 5-10 minutes until the powder is evenly dispersed and there is no obvious agglomeration, so as to obtain a uniformly mixed mud.

[0092] S2. Place the clay in the mold.

[0093] It is understandable that the mold can be set to any shape according to requirements, so as to form a ceramic composite material with a multi-level pore structure of a specific shape.

[0094] S3. Apply pressure to the clay in the mold to form a green body.

[0095] Specifically, a press is used to uniformly apply pressure to the mud in the mold, and the pressure is set to 2 MPa - 15 MPa and maintained for more than 30 minutes to degas the mud to form a green body.

[0096] S4. Dry the green body.

[0097] Specifically, dry the degassed green body, and the drying time can be set according to requirements. For example, it can be dried at room temperature for 24 hours.

[0098] Optionally, in one embodiment, the dried green body can be cut into any shape according to needs.

[0099] S5. Sinter the dried green body to obtain a ceramic composite material with a hierarchical pore structure.

[0100] The sintering process can be carried out in a furnace. In an optional embodiment, step S5 can specifically include: placing the dried green body in a sintering space for sintering. The temperature of the sintering space rises from room temperature to 400 - 600 °C at a rate of 2 - 4 °C / min and is maintained for 2 - 4 hours; then the temperature of the sintering space rises again to 800 - 1200 °C at a rate of 2 - 4 °C / min and is maintained for 3 - 6 hours; then the fired ceramic is naturally cooled to room temperature; the air supply speed of the sintering space is set to 100 - 300 ml / min.

[0101] S6. Use ultrasonic cleaning on the obtained matrix material;

[0102] Specifically, perform ultrasonic cleaning on the matrix material, and the cleaning time can be set according to the size of the matrix material and the cleaning power. Generally, it needs to be cleaned 3 - 5 times repeatedly.

[0103] S7. Dry the cleaned matrix material to avoid residual moisture affecting filling;

[0104] Specifically, vacuum drying or oven drying can be selected for drying, and the drying time can be set according to requirements. Generally, for oven drying, the temperature is set to 80 - 200 °C and the time is 2 - 6 hours.

[0105] S8. Dissolve the filling material of the polymer with selective radiative cooling function to form a filling solution;

[0106] Specifically, select a suitable organic solvent, such as toluene, benzene, carbon tetrachloride and other organic solvents to dissolve the polymer with selective radiative cooling.

[0107] Specifically, stir the polymer solution until the micron silica particles are evenly dispersed without obvious agglomeration. Then, remove the bubbles by slow stirring to degas the suspension and obtain the filling solution.

[0108] Optionally, the mixed solution of the polymer and the organic solvent can be heated to promote the dissolution of the polymer.

[0109] S9. Vacuumize the treated ceramic matrix material;

[0110] Specifically, when vacuumizing the ceramic matrix material, it is necessary to gradually pump to the target vacuum degree and maintain it for 20 min after the vacuum degree is stable to ensure the complete discharge of the gas in the matrix. The target vacuum degree is determined according to specific experimental requirements.

[0111] S10. Fill the treated polymer material into the impregnation tank to obtain the target ceramic metamaterial.

[0112] Specifically, impregnate the polymer solution, which specifically includes opening the valve of the liquid storage tank, injecting the filling material into the impregnation tank by gravity or external air pressure (2 - 15 MPa), and controlling the injection speed (such as 2 - 10 mL / min) to avoid the mixing of bubbles caused by turbulence; after the injection is completed, the impregnation tank needs to be kept stationary in a vacuum environment (10 - 60 minutes), and the penetration is completed by relying on the pressure difference and capillary action.

[0113] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band, characterized in that, The daytime passive radiative cooling ceramic metamaterial includes a matrix material and a filling material. The matrix material is a composite ceramic material with a hierarchical pore structure having broadband radiative cooling function, and the filling material is a polymer with selective radiative cooling function. By controlling the filling ratio of the polymer to the composite ceramic material with hierarchical pore structure, the conversion between broadband radiative cooling and selective radiative cooling of the daytime passive radiative cooling ceramic metamaterial is achieved; When the volume filling ratio of the polymer to the composite ceramic material reaches the maximum ratio of 0.5:1 - 0.8:1, selective radiative cooling of the daytime passive radiative cooling ceramic metamaterial is achieved; When the filling ratio of the polymer to the composite ceramic material is less than the maximum volume filling ratio, the conversion between broadband radiative cooling and selective radiative cooling of the daytime passive radiative cooling ceramic metamaterial is achieved.

2. The adjustable emissivity band daytime passive radiative cooling ceramic metamaterial according to claim 1, wherein: The solar reflectivity of the daytime passive radiative cooling ceramic metamaterial is not less than 95%; the thermal emissivity of the daytime passive radiative cooling ceramic metamaterial in the broadband infrared band of 5 μm - 25 μm or the selective infrared band of 8 μm - 13 μm is not less than 93%; the flexural strength of the daytime passive radiative cooling ceramic metamaterial is not less than 300 MPa, the compressive strength is not less than 600 Mpa, and the porosity is 50% - 80%.

3. A preparation method of a tunable emissivity band daytime passive radiative cooling ceramic metamaterial according to any one of claims 1-2, characterized in that: It includes the following steps: Step 1, preparing the matrix material: adding a polymer solution to metal oxide and micron hollow glass microspheres to form a mud; placing the mud in a mold; Applying pressure to the mud in the mold to form a green body; Drying the green body; sintering the dried green body to obtain a composite ceramic matrix material with a hierarchical pore structure having broadband radiative cooling function; Step 2, preparing the filling material: dissolving a polymer with selective radiative cooling function as the filling material to form a filling solution; Step 3, preparing the daytime passive radiative cooling ceramic metamaterial: ultrasonically cleaning the composite ceramic matrix material with hierarchical pore structure; drying the cleaned composite ceramic matrix material; preparing the composite ceramic matrix material and the filling solution by mechanical pressing method and / or vacuum assisted impregnation method to obtain the daytime passive radiative cooling ceramic metamaterial.

4. The preparation method of the daytime passive radiative cooling ceramic metamaterial with an adjustable emissivity band according to claim 3, wherein: The adding of the polymer solution to metal oxide and micron hollow glass microspheres to form a mud specifically includes the following steps: 1.1, drying the metal oxide powder and micron hollow glass microspheres; 1.2, mixing the dried metal oxide powder and micron hollow glass microspheres to obtain a mixed dry powder; 1.3, adding the polymer solution to the mixed dry powder and stirring to obtain a mud.

5. The preparation method of the day-time passive radiative cooling ceramic metamaterial with adjustable emissivity band according to claim 4, wherein: The mass ratio of the micron hollow glass microspheres to the metal oxide powder is 0.25 - 1; the mass ratio of the polymer solution to the mixed dry powder is 0.2 - 1.

6. The preparation method of the daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band according to claim 3, characterized in that: The sintering of the dried green body specifically includes: placing the dried green body in a sintering space for sintering. The temperature of the sintering space rises from room temperature to 400 - 600 °C at a rate of 2 - 4 °C / min and is maintained for 2 - 4 hours; then the temperature of the sintering space rises again to 800 - 1200 °C at a rate of 2 - 4 °C / min and is maintained for 3 - 6 hours; then the fired ceramic is naturally cooled to room temperature; the air supply speed of the sintering space is set to 100 - 300 ml / min.

7. The preparation method of the daytime passive radiative cooling ceramic metamaterial with adjustable emissivity band according to claim 3, characterized in that: In step one, the micron hollow glass microspheres are selected from one or more with a particle size of 2 μm - 50 μm; the metal oxide is one or more of powdery Al2O3, TiO2, MgO, CaCO3, ZnO, ZrO2; the polymer solution is one or more of polyvinylidene fluoride solution, polydimethylsiloxane solution, polymethyl methacrylate solution, polyvinyl alcohol solution.

8. The preparation method of the day-time passive radiative cooling ceramic metamaterial with adjustable emissivity band according to claim 3, characterized in that: In step two, the high molecular polymer is one or more of polyoxymethylene POM, polyvinylidene fluoride PVDF.

9. The preparation method of the adjustable emissivity band daytime passive radiative cooling ceramic metamaterial according to claim 3, wherein: In step three, the mechanical pressing method specifically is: evenly placing the dried composite ceramic matrix material and the filling solution in a mold, applying pressure to the materials in the mold to obtain a daytime passive radiative cooling ceramic metamaterial. Applying pressure to the materials in the mold includes: using a press to evenly apply pressure to the materials in the mold, setting the pressure to 2 - 15 MPa, maintaining the pressure for more than 30 minutes, and fully filling the filling material into the interior of the matrix material.

10. The preparation method of the daytime passive radiative cooling ceramic metamaterial with an adjustable emissivity band according to claim 3, wherein: In step three, the vacuum-assisted impregnation method specifically is: evacuating the dried composite ceramic matrix material, filling the filling solution into an impregnation tank to obtain a daytime passive radiative cooling ceramic metamaterial. Filling the filling solution into the impregnation tank includes opening the valve of the liquid storage tank, injecting the filling material into the impregnation tank using gravity or an external air pressure of 2 - 15 MPa, and controlling the injection speed to be 2 - 10 mL / min.

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