Radiative cooling sheet and method for producing the same

Abstract

To provide a radiative cooling sheet which achieves a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more, and a method for producing the same.SOLUTION: According to one aspect of the present invention, a radiative cooling sheet 10 having a laminated structure is provided. The radiative cooling sheet 10 comprises: a boron nitride-containing layer 11, containing boron nitride powder and a matrix resin; and a silica-containing layer 12, containing silica and laminated on the boron nitride-containing layer 11.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a radiative cooling sheet and a method for manufacturing the same.

Background Art

[0002] In recent years, measures to combat global warming and decarbonization have been demanded. For this reason, passive cooling methods that do not use electricity have attracted attention. Such cooling methods include water spraying and dry mist, but water spraying and dry mist require the use of precious resources such as water.

[0003] For this reason, currently, radiative cooling sheets using radiative cooling have attracted attention. With a radiative cooling sheet, a cooling effect can be obtained without using resources such as water or electricity even during the day. As radiative cooling sheets, those using hexagonal boron nitride (see, for example, Non-Patent Documents 1 and 2) and those using barium sulfate (see, for example, Non-Patent Document 3) are known. Non-Patent Document 1 describes a radiative cooling sheet having a solar reflectance of 97.9% (0.979) and an average emissivity in the atmospheric window of 0.83. Non-Patent Document 2 describes a radiative cooling sheet having a solar reflectance of 93.6% (0.936) and an average emissivity in the atmospheric window of 0.89. Non-Patent Document 3 describes a radiative cooling sheet having a solar reflectance of 98.1% (0.981) and an average emissivity in the atmospheric window of 0.95. Since the radiative cooling sheet of Non-Patent Document 3 is excellent in solar reflectance and average emissivity, it has been certified as a Guinness record.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

[0005] However, at present, higher solar reflectance and higher average mid-infrared emissivity (average emissivity in the atmospheric window) are desired.

[0006] The present invention has been made to solve the above problems. That is, an object of the present invention is to provide a radiative cooling sheet capable of obtaining a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more, and a method for manufacturing the same. [Means for Solving the Problems]

[0007] [1] A radiation cooling sheet having a laminated structure, comprising a boron nitride-containing layer containing boron nitride powder and a matrix resin, and a silica-containing layer laminated on the boron nitride-containing layer and containing silica.

[0008] [2] The radiation cooling sheet according to [1] above, wherein the silica-containing layer contains silica powder and a matrix resin.

[0009] [3] The radiation cooling sheet according to [1] or [2] above, wherein the thickness of the boron nitride-containing layer is 10 μm or more and 10000 μm or less.

[0010] [4] The radiation cooling sheet according to any one of [1] to [3] above, wherein the thickness of the silica-containing layer is 10 μm or more and 1000 μm or less.

[0011] [5] The radiation cooling sheet according to any one of [1] to [4] above, wherein the average particle diameter of the boron nitride powder is 0.1 μm or more and 500 μm or less.

[0012] [6] The radiation cooling sheet according to any one of [2] to [5] above, wherein the average particle diameter of the silica powder is 0.1 μm or more and 500 μm or less.

[0013] [7] The radiation cooling sheet according to any one of [1] to [6] above, wherein the refractive index difference between the boron nitride powder and the matrix resin is 0.1 or more.

[0014] [8] The radiation cooling sheet according to any one of [1] to [7] above, wherein the surface of the radiation cooling sheet is the surface of the silica-containing layer.

[0015] [9] A method for manufacturing a radiation cooling sheet, comprising a step of forming a boron nitride-containing layer containing boron nitride powder and a matrix resin, and a step of forming a silica-containing layer containing silica on the surface of the boron nitride-containing layer.

[0016]

[10] The method for manufacturing a radiative cooling sheet according to [9] above, wherein the step of forming the boron nitride-containing layer is a step of forming the boron nitride-containing layer on a substrate.

[0017]

[11] The method for manufacturing a radiative cooling sheet according to

[10] above, further comprising a step of peeling the substrate after the formation of the silica-containing layer.

[0018]

[12] The step of forming the silica-containing layer is a step of forming a silica-containing layer containing silica on a release film, the step of forming the boron nitride-containing layer is a step of forming a boron nitride-containing layer containing boron nitride powder and a matrix resin on the surface of the silica-containing layer, and the method for manufacturing a radiative cooling sheet according to [9] above, further comprising a step of peeling the release film from the silica-containing layer after the formation of the boron nitride-containing layer.

[0019]

[13] The method for manufacturing a radiative cooling sheet according to any one of [9] to

[12] above, wherein the formation of the boron nitride-containing layer and the formation of the silica-containing layer are performed by a roll-to-roll method. [Advantages of the Invention]

[0020] According to the radiative cooling sheet and the method for manufacturing the same according to the present invention, it is possible to provide a radiative cooling sheet capable of obtaining a solar reflectance of 0.98 or more and an average mid-infrared light emissivity of 0.90 or more. [Brief Description of the Drawings]

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0022] Hereinafter, the radiative cooling sheet and its manufacturing method according to the embodiment of the present invention will be described. Fig. 1 is a schematic configuration diagram of the radiative cooling sheet according to the present embodiment, and Fig. 2 is a diagram showing an application example of the radiative cooling sheet according to the present embodiment. Figs. 3A to 3C and Figs. 4A to 4D are schematic diagrams showing the manufacturing process of the radiative cooling sheet according to the embodiment, and Figs. 5A to 5C and Figs. 6A to 6D are schematic diagrams showing other manufacturing processes of the radiative cooling sheet according to the present embodiment.

[0023] <<<Radiative cooling sheet>>> The radiative cooling sheet 10 shown in FIG. 1 has a laminated structure and includes a boron nitride-containing layer 11 containing boron nitride powder and a matrix resin, and a silica-containing layer 12 laminated on the boron nitride-containing layer 11 and containing silica. The surface 10A of the radiative cooling sheet 10 is the surface 12A of the silica-containing layer 12. In the present invention, the "surface of the radiative cooling sheet" means the surface on the side where sunlight is incident.

[0024] The radiative cooling sheet 10 has a solar reflectance of 0.98 (98%) or more and an average mid-infrared emissivity of 0.90 (90%) or more. Such a radiative cooling sheet 10 can exhibit high radiative cooling performance. The wavelength range of sunlight is 300 nm to 2500 nm, and the wavelength range of mid-infrared light is 3 μm to 16 μm. Here, mid-infrared light with a wavelength of 8 μm to 13 μm is called the "atmospheric window" and reaches outer space with little influence of absorption by the atmosphere. Therefore, when the mid-infrared light radiated from the radiative cooling sheet has a wavelength of 8 μm to 13 μm, heat can be efficiently dissipated by thermal radiation. The solar reflectance is more preferably 0.982 (98.2%) or more, 0.984 (98.4%) or more, or 0.986 (98.6%) or more. The average mid-infrared reflectance is more preferably 0.91 (91%) or more, or 0.92 (92%) or more.

[0025] The solar reflectance (R solar ) is obtained by the following formula (1).

Equation

[0026] The average mid-infrared emissivity (εsky (λ) is obtained as follows. First, measure the mid-infrared light reflectance (R(λ)) of the radiative cooling sheet. Then, using the measured mid-infrared light reflectance (R(λ)), calculate the mid-infrared light absorptance (A(λ)) from the following formula (2). A(λ)=1 - R(λ) - T(λ) … Formula (2) In the above formula (2), T(λ) is the mid-infrared light transmittance. However, since the thickness of the radiative cooling sheet is sufficiently thick, it is set to zero.

[0027] The mid-infrared light emissivity (ε(λ)) and the obtained mid-infrared light absorptance (A(λ)) satisfy the following formula (3) according to Kirchhoff's law of thermodynamics. ε(λ)=A(λ) … Formula (3)

[0028] Therefore, using the mid-infrared light absorptance (A(λ)), the mid-infrared light emissivity (ε(λ)) can be obtained from formula (3). Then, find the average value of the emissivity in the wavelength range of 8 μm to 13 μm to obtain the mid-infrared light average emissivity (ε sky (λ)).

[0029] It is known that the radiative cooling performance of the radiative cooling sheet 10 can be represented by a figure of merit (FoM) (see Non-Patent Document 3). Since the larger the value of the figure of merit (FoM), the better the radiative cooling performance, the figure of merit (FoM) of the radiative cooling sheet 10 is preferably 0.77 or more, 0.78 or more, 0.80 or more, or 0.81 or more.

[0030] The figure of merit (FoM) is obtained by the following formula (4) using the solar reflectance (R solar ) and the mid-infrared light average emissivity (ε sky (λ)). FoM = ε sky (λ)-r(1 - R solar ) … Formula (4) In the above formula (4), r is the ratio when assuming that the thermal radiation intensity radiated by a blackbody with a solar light intensity of 1000 / m 2 and 300 K in the wavelength range of 8 μm to 13 μm is about 100 W / m 2 and is set to 10.

[0031] <<Boron nitride-containing layer>> The boron nitride-containing layer 11 contains boron nitride powder and a matrix resin as described above. The boron nitride-containing layer 11 substantially does not contain silica powder.

[0032] The thickness of the boron nitride-containing layer 11 is preferably 10 μm or more. If this thickness is 10 μm or more, the number of times of sunlight scattering in the boron nitride-containing layer increases and the sunlight reflectance increases, so that a higher sunlight reflectance can be obtained. The lower limit of the above thickness is more preferably 100 μm or more, 200 μm or more, or 500 μm or more. Also, if this thickness is above a certain level, the sunlight reflectance hardly changes, so from the viewpoint of cost reduction, the upper limit of the above thickness may be 10000 μm or less, 2000 μm or less, or 1500 μm or less.

[0033] The thermal conductivity of the boron nitride-containing layer 11 is preferably 0.2 W / m / K or more. If this thermal conductivity is 0.2 W / m / K or more, the heat on the back surface of the radiation cooling sheet 10 can be dissipated. The lower limit of the above thermal conductivity is more preferably 0.3 W / m / K or more, 0.4 W / m / K or more, or 0.5 W / m / K or more.

[0034] <Boron nitride powder> The average particle size of the boron nitride powder is preferably 0.1 μm or more and 500 μm or less. If the average particle size of the boron nitride powder is within this range, a higher sunlight reflectance can be obtained. The lower limit of the above average particle size is preferably 0.2 μm or more, 0.5 μm or more, 1.0 μm or more, or 10.0 μm or more, and the upper limit of the above average particle size is preferably 80 μm or less, 60 μm or less, or 40 μm or less. The "average particle size" in this specification means the 50% diameter in the volume-based integrated fraction measured in accordance with JIS R 1629:1997 "Method for Measuring Particle Size Distribution of Fine Ceramics Raw Materials by Laser Diffraction / Scattering Method".

[0035] The crystal structure of boron nitride constituting the boron nitride powder is not particularly limited and may be any of hexagonal, cubic, and cubic wurtzite types. However, from the perspective of production, it is preferably hexagonal.

[0036] The refractive index difference between the boron nitride powder and the matrix resin is preferably 0.1 or more. If the refractive index is 0.1 or more, sunlight is likely to be scattered in the boron nitride-containing layer 11, so a higher sunlight reflectance can be obtained. The lower limit of the refractive index difference is preferably 0.3 or more, 0.4 or more, or 0.5 or more. The refractive index of the boron nitride powder and the refractive index of the matrix resin can be measured in accordance with JIS K 7142:2014 "Plastics - Method for Measuring Refractive Index".

[0037] The content of the boron nitride powder in the boron nitride-containing layer 11 is preferably 1% by weight or more and 90% by weight or less. If the content of the boron nitride powder is 1% by weight or more, the number of times of sunlight scattering in the boron nitride-containing layer increases and the sunlight reflectance increases, so a higher sunlight reflectance can be obtained. If it is 90% by weight or less, the boron nitride-containing layer 11 can be formed by coating. The lower limit of the content is preferably 2.5% by weight or more, 5% by weight or more, 10% by weight or more, 50% by weight or more, or 70% by weight or more. The upper limit of the content may be 85% by weight or less, or 83% by weight or less. The content of the boron nitride powder in the boron nitride-containing layer 11 can be calculated by the composite rule from the density of each raw material and the density of the mixture according to, for example, the loss on ignition and the residue on ignition by JIS K 0067:1992 "Test Methods for Loss in Weight and Residue of Chemical Products", the thermogravimetric differential thermal analyzer (TG-DTA), and JIS K 0061:2001 "Methods for Measuring the Density and Specific Gravity of Chemical Products".

[0038] The solid weight ratio of the matrix resin to the boron nitride powder (matrix resin / boron nitride powder) is preferably 0.01 or more, 0.10 or more, 0.15 or more, 0.20 or more as the lower limit, and preferably 0.90 or less, 0.50 or less, 0.30 or less, or 0.25 or less as the upper limit. The volume fraction of the boron nitride powder in the boron nitride-containing layer 11 is preferably 0.10 or more, 0.30 or more, 0.50 or more, 0.60 or more, or 0.70 or more as the lower limit, and preferably 0.90 or less, 0.85 or less, or 0.80 or less as the upper limit.

[0039] <matrix resin> The matrix resin is not particularly limited, but it is preferably non-absorbent or has low absorption in the wavelength range of sunlight (300 nm to 2500 nm), and has absorption in the mid-infrared region (2.5 μm to 25 μm). Specifically, for example, when a matrix resin layer of about 50 μm is formed using the matrix resin, the absorption rate of the matrix resin layer in the wavelength range of 300 nm to 2500 nm is preferably 20% or less, and the absorption rate in the wavelength range of 8 μm to 13 μm is preferably 80% or more. In this case, it is more preferable that the absorption rate of the matrix resin layer is 5% or less particularly in the wavelength range of 400 nm to 2000 nm.

[0040] The matrix resin is not particularly limited, and examples thereof include thermoplastic resins. Examples of the thermoplastic resin include polymethyl methacrylate resin (PMMA), fluorine-containing resin, polyvinyl alcohol resin, polypropylene resin, polystyrene resin, polyurethane resin, polyamide resin, polycarbonate resin, polyethylene terephthalate resin, polyvinyl chloride resin, ABS resin, and the like. As the matrix resin used for the boron nitride-containing layer 11, polymethyl methacrylate resin and fluorine-containing resin are preferable, and fluorine-containing resin is more preferable. As the fluorine-containing resin, amorphous fluorine-containing resin is preferable.

[0041] <<silica-containing layer>> The silica-containing layer 12 contains silica as described above. An example of a preferred embodiment of the silica-containing layer 12 is an embodiment containing silica powder and a matrix resin. On the other hand, the silica-containing layer 12 only needs to contain silica and does not necessarily need to contain a matrix resin. Also, the silica does not necessarily need to be contained as silica powder. For example, the silica-containing layer may be a silica vapor deposition layer formed by sputtering or the like.

[0042] The thickness of the silica-containing layer 12 is preferably 10 μm or more and 1000 μm or less. If the thickness of the silica-containing layer 12 is 10 μm or more, a higher average mid-infrared light emissivity can be obtained. Also, if it is 1000 μm or less, sunlight can reach the boron nitride-containing layer 11, so a higher sunlight reflectivity can be obtained by the boron nitride-containing layer 11. The lower limit of the above thickness is preferably 20 μm or more, 50 μm or more, or 80 μm or more, and the upper limit may be 300 μm or less, 200 μm or less, or 150 μm or less.

[0043] The content of the silica powder in the silica-containing layer 12 is preferably 1% by weight or more and 90% by weight or less. If the content of the silica powder is 1% by weight or more, the silica powder is appropriately present in the silica-containing layer 12, so a higher average mid-infrared light emissivity can be obtained. Also, if it is 90% by weight or less, the silica-containing layer 12 can be formed by coating. The lower limit of the above content is preferably 3% by weight or more, 5% by weight or more, or 6% by weight or more. The upper limit of the above content may be 80% by weight or less, 50% by weight or less, 20% by weight or less, or 10% by weight or less. The content of the silica powder in the silica-containing layer 12 can be calculated by the same method as the content of the above boron nitride powder.

[0044] The solid weight ratio of the matrix resin to the silica powder (matrix resin / silica powder) is preferably at least 0.01, at least 0.03, at least 0.05, or at least 0.07, and preferably at most 0.90, at most 0.50, at most 0.30, or at most 0.10. The volume fraction of the silica powder in the silica-containing layer 12 is preferably at least 0.10, at least 0.30, at least 0.50, at least 0.70, or at least 0.80, and preferably at most 0.95, at most 0.93, or at most 0.90.

[0045] The thermal conductivity of the silica-containing layer 12 is preferably at least 0.1 W / m / K. If the thermal conductivity is at least 0.1 W / m / K, the heat conduction in the radiation cooling sheet 10 is not significantly inhibited, and for example, the heat of a building covering a heat-generating device can be radiated to the outside from the surface 10A of the radiation cooling sheet 10 through the radiation cooling sheet 10. The lower limit of the thermal conductivity is more preferably at least 0.2 W / m / K, at least 0.3 W / m / K, or at least 0.4 W / m / K.

[0046] <Silica powder> The average particle size of the silica powder is preferably from 0.1 μm to 500 μm. If the average particle size of the silica powder is within this range, a higher average mid-infrared light emissivity can be obtained. The lower limit of the average particle size is preferably at least 0.2 μm, at least 0.5 μm, or at least 1.0 μm, and the upper limit of the average particle size is preferably at most 200 μm, at most 150 μm, or at most 100 μm.

[0047] <Matrix resin> The matrix resin used in the silica-containing layer 12 is not particularly limited, and examples thereof include the same resins as those used in the matrix resin for the boron nitride-containing layer 11. As the matrix resin used for the silica-containing layer 12, a polymethyl methacrylate resin is preferable. The matrix resin used for the silica-containing layer 12 may be the same resin as the matrix resin used for the boron nitride-containing layer 11, or may be a different resin. For example, when the matrix resin used for the silica-containing layer 12 is a polymethyl methacrylate resin, the matrix resin used for the boron nitride-containing layer 11 may be a polymethyl methacrylate resin or a fluorine-containing resin, and it is preferably a fluorine-containing resin.

[0048] <<<Application Examples of Radiative Cooling Sheet>>> The location where the radiative cooling sheet 10 is disposed is not particularly limited. For example, it is preferably disposed outdoors. By disposing the radiative cooling sheet 10 outdoors, it serves as a countermeasure against heat in summer and the like. Further, it may be applied to a building that covers a heat-generating device such as a server or a distribution board. In this case, it functions as an auxiliary heat dissipation. FIG. 2 is a view showing the radiative cooling sheet 10 installed on the roof 20A of the building 20.

[0049] <<<Manufacturing Method of Radiative Cooling Sheet>>> Such a radiative cooling sheet 10 can be manufactured as follows. First, as shown in FIG. 3A, a boron nitride-containing composition is applied to a base material 31 to form a coating layer 32. The boron nitride-containing composition contains boron nitride powder and a matrix resin.

[0050] After forming the coating layer 32 of the boron nitride-containing composition, for example, as shown in FIG. 3B, the coating layer 32 is heated to 100° C. or higher to form a boron nitride-containing layer. This procedure is repeated to increase the thickness of the boron nitride-containing layer, and a boron nitride-containing layer 11 as shown in FIG. 3C is obtained.

[0051] Next, after forming the boron nitride-containing layer 11, a silica-containing composition is applied to the surface 11A of the boron nitride-containing layer 11 to form a coating layer 33 as shown in FIG. 4A. The silica-containing composition includes silica powder and a matrix resin.

[0052] After forming the coating layer 33 of the silica-containing composition, for example, as shown in FIG. 4B, the coating layer 33 is heated to 100° C. or higher to form a silica-containing layer. This procedure is repeated to increase the thickness of the silica-containing layer, and a silica-containing layer 12 is obtained as shown in FIG. 4C. Thereby, the radiative cooling sheet 10 is obtained. These steps are preferably performed by a roll-to-roll method. By performing these steps in a roll-to-roll method, it is possible to form the radiative cooling sheet 10 in a continuous process.

[0053] When the base material 31 is the surface of a cooling object such as the roof of a building, the base material 31 may be used in the form in which the radiative cooling sheet 10 is formed. When the base material 31 is not a cooling object, the base material 31 is peeled off from the radiative cooling sheet 10 as shown in FIG. 4D.

[0054] The radiative cooling sheet 10 can also be manufactured by the following method. First, a silica-containing composition is applied to a release film 41 to form a coating layer 42 as shown in FIG. 5A. The silica-containing composition includes silica powder and a matrix resin.

[0055] After forming the coating layer 42 of the silica-containing composition, for example, as shown in FIG. 5B, the coating layer 42 is heated to 100° C. or higher to form a silica-containing layer. This procedure is repeated to increase the thickness of the silica-containing layer, and a silica-containing layer 12 is obtained as shown in FIG. 5C.

[0056] Next, after forming the silica-containing layer 12, a boron nitride-containing composition is applied to the exposed surface of the silica-containing layer as shown in FIG. 6A to form a coating layer 43. The boron nitride-containing composition includes boron nitride powder and a matrix resin.

[0057] After forming the coating layer 43 of the boron nitride-containing composition, for example, as shown in FIG. 6B, the coating layer 43 is heated to 100° C. or higher to form a boron nitride-containing layer. This procedure is repeated to increase the thickness of the boron nitride-containing layer and form the boron nitride-containing layer 11 as shown in FIG. 6C. These steps are preferably carried out by a roll-to-roll method. By carrying out these steps by the roll-to-roll method, it is possible to form the radiative cooling sheet 10 in a continuous process.

[0058] Thereafter, for example, the radiative cooling sheet 10 is attached to the object to be cooled, such as the roof of a building, via an adhesive layer or an adhesive layer (not shown) so that the object to be cooled side, such as the roof of a building, becomes the boron nitride-containing layer 11. When the release film 41 is attached to the radiative cooling sheet 10, the release film 41 is peeled off from the radiative cooling sheet 10 as shown in FIG. 6D.

[0059] As a result of intensive studies to obtain a high solar reflectance and a high average mid-infrared emissivity, the inventors of the present invention surprisingly found that by laminating a silica-containing layer on a boron nitride-containing layer, a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more can be obtained. This effect utilizes the excellent solar reflectance of the boron nitride-containing layer and the excellent average mid-infrared emissivity of the silica-containing layer, and thus is obtained by combining the boron nitride-containing layer and the silica-containing layer. That is, it is an effect obtained by a specific combination of the boron nitride-containing layer and the silica-containing layer. According to the present embodiment, since the boron nitride-containing layer 11 containing boron nitride powder and a matrix resin and the silica-containing layer 12 laminated on the boron nitride-containing layer 11 are provided, a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more can be obtained. For example, as shown in the comparative example described later, in a radiative cooling sheet containing boron nitride powder and silica powder in one layer, a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more cannot be obtained, so it is necessary to laminate the boron nitride-containing layer and the silica-containing layer.

Example

[0060] To describe the present invention in detail, examples will be given below for illustration, but the present invention is not limited to these descriptions. FIG. 7A is a graph showing the reflectance of the radiative cooling sheet according to Example 1 and Comparative Example 5 with respect to wavelengths from 300 nm to 2500 nm, and FIG. 7B is a graph showing the emissivity of the radiative cooling sheet according to Example 1 and Comparative Example 5 with respect to mid-infrared light. FIG. 8A is a graph showing the reflectance of the radiative cooling sheet according to Example 2 and Comparative Example 8 with respect to wavelengths from 300 nm to 2500 nm, and FIG. 8B is a graph showing the emissivity of the radiative cooling sheet according to Example 2 and Comparative Example 8 with respect to mid-infrared light. FIG. 9 is a schematic configuration diagram of the measuring device used in the outdoor test. FIG. 10A is a graph showing the cooling performance of the radiative cooling sheet according to Example 1 and Comparative Example 5, and FIG. 10B shows the air temperature and solar irradiance on the day when the cooling performance of the radiative cooling sheet according to Example 1 and Comparative Example 5 was measured. FIG. 11A is a graph showing the cooling performance of the radiative cooling sheet according to Example 2 and Comparative Example 8, and FIG. 11B shows the air temperature and solar irradiance on the day when the cooling performance of the radiative cooling sheet according to Example 2 and Comparative Example 8 was measured.

[0061] <Preparation of Boron Nitride-Containing Composition> (Boron Nitride-Containing Composition 1) First, 1 g of granular polymethyl methacrylate resin (PMMA, product code "138-02735", manufactured by Fujifilm Wako Pure Chemical Corporation) was added to 10 mL of anisole (product code "016-15895", manufactured by Fujifilm Wako Pure Chemical Corporation), and stirred in an oil bath at 50 °C for 1 hour or more to obtain a PMMA solution. Next, boron nitride powder with an average particle size of 24.8 μm (product code "028-02281", manufactured by Fujifilm Wako Pure Chemical Corporation) was dispersed in the PMMA solution so that the solid weight ratio of PMMA to boron nitride powder (PMMA solid weight / boron nitride powder solid weight) was 0.18, and boron nitride-containing composition 1 was obtained.

[0062] (Boron Nitride-Containing Composition 2) An amorphous fluorine-containing resin in liquid form (product name "CYTOP (registered trademark) CLT-107MK", manufactured by AGC Inc.) and boron nitride powder with an average particle size of 24.8 μm (product code "028-02281", manufactured by Fujifilm Wako Pure Chemical Corporation) were dispersed so that the solid weight ratio of the amorphous fluorine-containing resin to the boron nitride powder (fluorine-containing resin solid weight / boron nitride powder solid weight) was 0.22, to obtain a boron nitride-containing composition 2.

[0063] (Boron nitride-containing composition 3) Instead of the boron nitride powder with an average particle size of 24.8 μm, hexagonal boron nitride powder with an average particle size of 6.1 μm (product number "BN GP", manufactured by Denka Co., Ltd.) was used, and the boron nitride powder was dispersed in a PMMA solution so that the solid weight ratio of PMMA to the boron nitride powder (PMMA solid weight / boron nitride powder solid weight) was 0.22. Otherwise, in the same manner as the boron nitride-containing composition 1, a boron nitride-containing composition 3 was obtained.

[0064] (Preparation of silica-containing composition) (Silica-containing composition 1) First, 1 g of granular polymethyl methacrylate resin (PMMA, product code "138-02735", manufactured by Fujifilm Wako Pure Chemical Corporation) was added to 10 mL of anisole (product code "016-15895", manufactured by Fujifilm Wako Pure Chemical Corporation), and stirred in an oil bath at 50 °C for 1 hour or more to obtain a PMMA solution. Next, silica powder with an average particle size of 6.6 μm (product number "FB-5D", manufactured by Denka Co., Ltd.) was dispersed in the PMMA solution so that the solid weight ratio of PMMA to the silica powder (PMMA solid weight / silica powder solid weight) was 0.075, to obtain a silica-containing composition 1.

[0065] (Silica-containing composition 2) A silica-containing composition 2 was obtained in the same manner as the silica-containing composition 1, except that silica powder with an average particle size of 45.2 μm (product number "FB-40R", manufactured by Denka Co., Ltd.) was used instead of the silica powder with an average particle size of 6.6 μm.

[0066] <Preparation of Alumina-Containing Composition> (Alumina-Containing Composition 1) First, 1 g of granular polymethyl methacrylate resin (PMMA, product code "138-02735", manufactured by Fujifilm Wako Pure Chemical Corporation) was added to 10 mL of anisole (product code "016-15895", manufactured by Fujifilm Wako Pure Chemical Corporation), and stirred in an oil bath at 50 °C for 1 hour or more to obtain a PMMA solution. Next, alumina powder with an average particle size of 5.1 μm (product number "DAM-03", manufactured by Denka Co., Ltd.) was dispersed in the PMMA solution so that the solid weight ratio of PMMA to alumina powder (PMMA solid weight / alumina powder solid weight) was 0.075, and Alumina-Containing Composition 1 was obtained.

[0067] (Alumina-Containing Composition 2) Alumina-Containing Composition 2 was obtained in the same manner as Alumina-Containing Composition 1, except that alumina powder with an average particle size of 96.3 μm (product number "DAM-90", manufactured by Denka Co., Ltd.) was used instead of the alumina powder with an average particle size of 5.1 μm.

[0068] <Preparation of Aluminum Nitride-Containing Composition> (Aluminum Nitride-Containing Composition 1) First, 1 g of granular polymethyl methacrylate resin (PMMA, product code "138-02735", manufactured by Fujifilm Wako Pure Chemical Corporation) was added to 10 mL of anisole (product code "016-15895", manufactured by Fujifilm Wako Pure Chemical Corporation), and stirred in an oil bath at 50 °C for 1 hour or more to obtain a PMMA solution. Next, aluminum nitride powder with an average particle size of 3.7 μm (product number "FAN-f05-A1", manufactured by Furukawa Electric Co., Ltd.) was dispersed in the PMMA solution so that the solid weight ratio of PMMA to aluminum nitride powder (PMMA solid weight / aluminum nitride powder solid weight) was 0.075, and Aluminum Nitride-Containing Composition 1 was obtained.

[0069] (Aluminum Nitride-Containing Composition 2) Except for using aluminum nitride (product number "FAN-f80-A1", manufactured by Furukawa Electric Co., Ltd.) with an average particle size of 94.1 μm instead of aluminum nitride powder with an average particle size of 3.7 μm, aluminum nitride-containing composition 2 was obtained in the same manner as aluminum nitride-containing composition 1.

[0070] <Preparation of Boron Nitride and Silica Mixed Composition> (Boron Nitride and Silica Mixed Composition 1) First, 1 g of granular polymethyl methacrylate resin (PMMA, product code "138-02735", manufactured by Fujifilm Wako Pure Chemical Corporation) was added to 10 mL of anisole (product code "016-15895", manufactured by Fujifilm Wako Pure Chemical Corporation), and stirred in an oil bath at 50 °C for 1 hour or more to obtain a PMMA solution. Next, boron nitride powder with an average particle size of 24.8 μm (product code "028-02281", manufactured by Fujifilm Wako Pure Chemical Corporation) and silica powder with an average particle size of 6.6 μm (product number "FB-5D", manufactured by Denka Co., Ltd.) were added to the PMMA solution at a weight ratio of 4:1, and dispersed so that the solid weight ratio of PMMA to the total solid weight of boron nitride powder and silica powder (PMMA solid weight / total solid weight of boron nitride powder and silica powder) was 0.18 to obtain boron nitride and silica mixed composition 1.

[0071] (Boron Nitride and Silica Mixed Composition 2) Except for changing the addition weight ratio of boron nitride powder and silica powder to 3:2, boron nitride and silica mixed composition 2 was obtained in the same manner as boron nitride and silica-containing composition 1.

[0072] (Boron Nitride and Silica Mixed Composition 3) Except for changing the addition weight ratio of boron nitride powder and silica powder to 2:3, boron nitride and silica mixed composition 3 was obtained in the same manner as boron nitride and silica-containing composition 1.

[0073] (Boron Nitride and Silica Mixed Composition 4) A boron nitride and silica mixed composition 4 was obtained in the same manner as the boron nitride and silica-containing composition 1, except that the added weight ratio of the boron nitride powder and the silica powder was changed to 1:4.

[0074] <Example 1> The boron nitride-containing composition 1 was applied to the surface of an aluminum plate with a thickness of 0.2 mm to form a coating layer. Subsequently, this coating layer was heated to 100 °C or higher on a hot plate. Then, in order to increase the thickness, the formation and heating of the coating layer were repeated to form a boron nitride-containing layer with a thickness of 2 mm.

[0075] After forming the boron nitride-containing layer, the silica-containing composition 1 was applied to the surface of the boron nitride-containing layer to form a coating layer. Subsequently, this coating layer was heated to 100 °C or higher on a hot plate. Then, in order to increase the thickness, the formation and heating of the coating layer were repeated to form a silica-containing layer with a thickness of 0.3 mm.

[0076] Thereafter, for reflectance measurement, the surface of the silica-containing layer was rubbed with a metal file to flatten the unevenness on the surface of the silica-containing layer in the submillimeter order. Thereby, a radiative cooling sheet according to Example 1 was obtained.

[0077] <Example 2> In Example 2, a radiative cooling sheet was obtained in the same manner as in Example 1, except that the boron nitride-containing composition 2 was used instead of the boron nitride-containing composition 1 to form a boron nitride-containing layer with a thickness of 0.8 mm, and the thickness of the silica-containing layer was 0.1 mm.

[0078] <Example 3> In Example 3, a radiative cooling sheet was obtained in the same manner as in Example 1, except that the boron nitride-containing composition 2 was used instead of the boron nitride-containing composition 1 to form a boron nitride-containing layer with a thickness of 0.8 mm, and the thickness of the silica-containing layer was 0.2 mm.

[0079] <Comparative Example 1> A boron nitride-containing composition 3 was applied to the surface of an aluminum plate with a thickness of 0.2 mm to form a coating layer. Then, this coating layer was heated to 100 °C or higher on a hot plate. Thereafter, in order to increase the thickness, the formation and heating of the coating layer were repeated to form a boron nitride-containing layer with a thickness of 0.5 mm.

[0080] Thereafter, for reflectance measurement, the surface of the boron nitride-containing layer was polished with a metal file to flatten the unevenness on the order of submillimeters present on the surface of the boron nitride-containing layer. Thereby, a radiative cooling sheet according to Comparative Example 1 was obtained.

[0081] <Comparative Example 2> In Comparative Example 2, a radiative cooling sheet was obtained in the same manner as in Comparative Example 1, except that a boron nitride-containing composition 1 was used instead of the boron nitride-containing composition 3.

[0082] <Comparative Example 3> In Comparative Example 3, a radiative cooling sheet was obtained in the same manner as in Comparative Example 2, except that the thickness of the boron nitride-containing layer was set to 1.0 mm.

[0083] <Comparative Example 4> In Comparative Example 4, a radiative cooling sheet was obtained in the same manner as in Comparative Example 2, except that the thickness of the boron nitride-containing layer was set to 1.5 mm.

[0084] <Comparative Example 5> In Comparative Example 5, a radiative cooling sheet was obtained in the same manner as in Comparative Example 2, except that the thickness of the boron nitride-containing layer was set to 2.0 mm.

[0085] <Comparative Example 6> In Comparative Example 6, a radiative cooling sheet was obtained in the same manner as in Comparative Example 2, except that the thickness of the boron nitride-containing layer was set to 2.5 mm.

[0086] <Comparative Example 7> In Comparative Example 7, a radiative cooling sheet was obtained in the same manner as in Comparative Example 1, except that a boron nitride-containing composition 2 was used instead of the boron nitride-containing composition 3.

[0087] <Comparative Example 8> In Comparative Example 8, a radiation cooling sheet was obtained in the same manner as in Comparative Example 7, except that the thickness of the boron nitride-containing layer was 0.8 mm.

[0088] <Comparative Example 9> In Comparative Example 9, a radiation cooling sheet was obtained in the same manner as in Comparative Example 7, except that the thickness of the boron nitride-containing layer was 1.0 mm.

[0089] <Comparative Example 10> Composition 1 containing silica was applied to the surface of an aluminum plate with a thickness of 0.2 mm to form a coating layer. Next, this coating layer was heated to 100 °C or higher on a hot plate. Then, in order to increase the thickness, the formation and heating of the coating layer were repeated to form a silica-containing layer with a thickness of 0.3 mm.

[0090] Thereafter, for reflectance measurement, the surface of the silica-containing layer was polished with a metal file to flatten the unevenness on the order of submillimeters present on the surface of the silica-containing layer. Thereby, a radiation cooling sheet according to Comparative Example 10 was obtained.

[0091] <Comparative Example 11> In Comparative Example 11, a radiation cooling sheet was obtained in the same manner as in Comparative Example 10, except that the thickness of the silica-containing layer was 0.5 mm.

[0092] <Comparative Example 12> In Comparative Example 12, a radiation cooling sheet was obtained in the same manner as in Comparative Example 10, except that the thickness of the silica-containing layer was 0.9 mm.

[0093] <Comparative Example 13> In Comparative Example 13, a radiation cooling sheet was obtained in the same manner as in Comparative Example 10, except that Composition 2 containing silica was used instead of Composition 1 containing silica, and the thickness of the silica-containing layer was 0.5 mm.

[0094] <Comparative Example 14> The alumina-containing composition 1 was applied to the surface of an aluminum plate with a thickness of 0.2 mm to form a coating layer. Subsequently, this coating layer was heated to 100 °C or higher on a hot plate. Then, in order to increase the thickness, the formation and heating of the coating layer were repeated to form an alumina-containing layer with a thickness of 0.5 mm.

[0095] Thereafter, for reflectance measurement, the surface of the alumina-containing layer was polished with a metal file to flatten the sub-millimeter-order irregularities present on the surface of the alumina-containing layer. Thereby, a radiative cooling sheet according to Comparative Example 14 was obtained.

[0096] <Comparative Example 15> In Comparative Example 15, a radiative cooling sheet was obtained in the same manner as in Comparative Example 14, except that the alumina-containing composition 2 was used instead of the alumina-containing composition 1.

[0097] <Comparative Example 16> The aluminum nitride-containing composition 1 was applied to the surface of an aluminum plate with a thickness of 0.2 mm to form a coating layer. Subsequently, this coating layer was heated to 100 °C or higher on a hot plate. Then, in order to increase the thickness, the formation and heating of the coating layer were repeated to form an aluminum nitride-containing layer with a thickness of 0.5 mm.

[0098] Thereafter, for reflectance measurement, the surface of the aluminum nitride-containing layer was polished with a metal file to flatten the sub-millimeter-order irregularities present on the surface of the aluminum nitride-containing layer. Thereby, a radiative cooling sheet according to Comparative Example 16 was obtained.

[0099] <Comparative Example 17> In Comparative Example 17, a radiative cooling sheet was obtained in the same manner as in Comparative Example 16, except that the aluminum nitride-containing composition 2 was used instead of the aluminum nitride-containing composition 1.

[0100] <Comparative Example 18> In Comparative Example 18, a radiation cooling sheet was obtained in the same manner as in Example 1, except that an alumina-containing layer having a thickness of 0.3 mm was formed using an alumina-containing composition 1 instead of the silica-containing layer.

[0101] <Comparative Example 19> In Comparative Example 19, a radiation cooling sheet was obtained in the same manner as in Example 1, except that an alumina-containing layer having a thickness of 0.3 mm was formed using an alumina-containing composition 2 instead of the silica-containing layer.

[0102] <Comparative Example 20> In Comparative Example 20, a radiation cooling sheet was obtained in the same manner as in Example 1, except that an aluminum nitride-containing layer having a thickness of 0.3 mm was formed using an aluminum nitride-containing composition 1 instead of the silica-containing layer.

[0103] <Comparative Example 21> In Comparative Example 21, a radiation cooling sheet was obtained in the same manner as in Example 1, except that an aluminum nitride-containing layer having a thickness of 0.3 mm was formed using an aluminum nitride-containing composition 2 instead of the silica-containing layer.

[0104] <Comparative Example 22> A boron nitride and silica mixed composition 1 was applied to the surface of an aluminum plate having a thickness of 0.2 mm to form a coating layer. Next, this coating layer was heated to 100°C or higher on a hot plate. Thereafter, in order to increase the thickness, the formation and heating of the coating layer were repeated to form a boron nitride and silica-containing layer having a thickness of 0.5 mm.

[0105] Thereafter, for reflectance measurement, the surface of the boron nitride and silica-containing layer was polished with a metal file to flatten the submicron irregularities present on the surface of the boron nitride and silica-containing layer. Thereby, a radiation cooling sheet according to Comparative Example 22 was obtained.

[0106] <Comparative Example 23> In Comparative Example 23, a radiation cooling sheet was obtained in the same manner as in Comparative Example 22, except that a boron nitride and silica-containing composition 2 was used instead of the boron nitride and silica mixed composition 1.

[0107] <Comparative Example 24> In Comparative Example 24, a radiation cooling sheet was obtained in the same manner as in Comparative Example 22, except that a boron nitride and silica mixed composition 3 was used instead of the boron nitride and silica mixed composition 1.

[0108] <Comparative Example 25> In Comparative Example 25, a radiation cooling sheet was obtained in the same manner as in Comparative Example 22, except that a boron nitride and silica mixed composition 4 was used instead of the boron nitride and silica mixed composition 1.

[0109] <Measurement of reflectance in the ultraviolet, visible, and near-infrared regions (wavelength 300 nm to 2500 nm) and calculation of solar reflectance (R solar ) First, the reflectance of the radiation cooling sheets according to the Examples and Comparative Examples at wavelengths from 300 nm to 2500 nm was measured. Specifically, a diffuse reflectance measurement unit (manufactured by JASCO Corporation) was attached to a spectrophotometer (product name "V-770", manufactured by JASCO Corporation), and the radiation cooling sheet was placed inside the spectrophotometer to measure the reflectance of the radiation cooling sheet at wavelengths from 300 nm to 2500 nm. The reflectance was calculated based on the reflectance of a diffuse reflectance standard plate (manufactured by Labsphere, Spectralon (registered trademark)). Then, using the calculated reflectance (R(λ)), the solar reflectance (R solar ) was determined from the above formula (1).

[0110] <Measurement of reflectance in the mid-infrared region (wavelength 3 μm to 16 μm) and calculation of mid-infrared light average emissivity (ε sky ) The reflectance of the radiative cooling sheet according to the examples and comparative examples at wavelengths from 3 μm to 16 μm was measured. Specifically, an integrating sphere made of gold (product name "integratIR", manufactured by PIKE) was attached to an infrared spectrophotometer (product name "Nicolet iS50R FT-IR", manufactured by Thermo Fisher Scientific), and the reflectance of the radiative cooling sheet at wavelengths from 3 μm to 16 μm was measured. The reflectance was calculated based on the reflectance of the gold diffusion reference of PIKE. Then, using the calculated reflectance (R(λ)), the absorptance A(λ) was calculated from the above formula (2). Here, since the thickness of the radiative cooling sheet according to the examples and comparative examples was sufficiently thick, the mid-infrared light transmittance was set to zero. Next, using the absorptance A(λ), the mid-infrared light emissivity ε(λ) was calculated from the above formula (3). Then, the average value of the mid-infrared light emissivity at wavelengths from 8 μm to 13 μm was obtained, and the mid-infrared light average emissivity (ε sky ) was obtained.

[0111] <Calculation of the performance index (FoM) of daytime radiative cooling> Using the solar reflectance (R solar ) and the mid-infrared light average emissivity (ε sky ) calculated above, the performance index (FoM) of daytime radiative cooling was calculated from formula (4).

[0112] The results are shown in Tables 1 to 3 below. Note that the volume fraction of the powder in Tables 1 and 2 indicates the ratio of the volume of the powder in the layer containing the powder.

Table 1

[0113]

Table 2

[0114]

Table 3

[0115] The results are described below. As shown in Table 1, since the radiative cooling sheets according to Comparative Examples 1 to 17 have a single-layer structure, as shown in Table 3, it was not possible to achieve a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more. Further, as shown in Table 1, although the radiative cooling sheets according to Comparative Examples 18 to 21 have a two-layer structure, since the second layer is not a silica-containing layer, it was not possible to achieve a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more. Further, as shown in Table 2, although the radiative cooling sheets according to Comparative Examples 22 to 25 contain boron nitride powder and silica powder, since they have a single-layer structure, as shown in Table 3, it was not possible to achieve a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more. On the other hand, as shown in Table 1, since the radiative cooling sheets according to Examples 1 to 3 have a two-layer structure of a boron nitride-containing layer and a silica-containing layer, as shown in Table 3, it was possible to achieve a solar reflectance of 0.98 or more and an average mid-infrared emissivity of 0.90 or more.

[0116] From FIGS. 7A and 7B, it can be understood that the reflectance of the radiative cooling sheet according to Example 1 in the wavelength range of 300 nm to 2500 nm is almost the same as the reflectance of the radiative cooling sheet according to Comparative Example 5 in the wavelength range of 300 nm to 2500 nm, but the mid-infrared emissivity of the radiative cooling sheet according to Example 1 is higher than the mid-infrared emissivity of the radiative cooling sheet according to Comparative Example 5. Further, from FIGS. 8A and 8B, it can be understood that the reflectance of the radiative cooling sheet according to Example 2 in the wavelength range of 300 nm to 2500 nm is almost the same as the reflectance of the radiative cooling sheet according to Comparative Example 8 in the wavelength range of 300 nm to 2500 nm, but the mid-infrared emissivity of the radiative cooling sheet according to Example 2 is higher than the mid-infrared emissivity of the radiative cooling sheet according to Comparative Example 8.

[0117] The FoM of the radiative cooling sheet according to Non-Patent Document 1, which has a solar reflectance of 0.979 and an average emissivity in the atmospheric window of 0.83, is 0.62. The FoM of the radiative cooling sheet according to Non-Patent Document 2, which has a solar reflectance of 0.936 and an average emissivity in the atmospheric window of 0.89, is 0.25. The FoM of the radiative cooling sheet according to Non-Patent Document 3, which is recognized as a Guinness record, has a solar reflectance of 0.981 and an average emissivity in the atmospheric window of 0.95, is 0.76. On the other hand, the FoM of the radiative cooling sheets according to Examples 1 to 3 is 0.781 to 0.817. Therefore, the radiative cooling sheets according to Examples 1 to 3 have a higher FoM than the radiative cooling sheets according to Non-Patent Documents 1 to 3, particularly Non-Patent Document 3 recognized as a Guinness record.

[0118] <Outdoor Test> The radiative cooling sheets according to Examples 1 and 2 and Comparative Examples 5 and 8 were placed outdoors for testing the radiative cooling performance. Specifically, first, as shown in FIG. 9, the four corners of the sample S of the radiative cooling sheet of about 2 cm square were fixed to the edges of the holes made in the plastic case 51. The plastic case 51 was covered with an aluminum foil 52 to reduce the influence of solar heat. The plastic case 51 was fixed to a styrofoam board 53 with a thickness of about 3 cm and installed on the roof of a five-story building in the Namiki area of the National Institute for Materials Science (1-1 Namiki, Tsukuba City, Ibaraki Prefecture, 36°04′07.8″ N latitude, 140°07′58.7″ E longitude). Then, the temperature of the sample S, the air temperature on the measurement day, and the solar irradiance were measured. The temperature of the sample S was recorded by a thermocouple 54 attached to the back surface of the aluminum plate. The air temperature and solar irradiance on the measurement day were recorded at a location within 1.0 m of the sample by a temperature sensor and a solar irradiance meter shielded from direct sunlight with an aluminum foil. The measurements of the radiative cooling sheets according to Example 1 and Comparative Example 5 were conducted on October 11, 2022, and the measurements of the radiative cooling sheets according to Example 2 and Comparative Example 8 were conducted on October 21, 2022.

[0119] As a result of outdoor tests, the graphs shown in FIGS. 10A, 10B, 11A, and 11B were obtained. In the graphs of FIGS. 10A and 11A, since the vertical axis represents the difference between the sample of the radiative cooling sheet and the air temperature, if the value on the vertical axis is negative, the temperature of the sample is lower than the air temperature, and the greater the negative value, the higher the cooling performance. From the graph shown in FIG. 10A, it can be understood that the radiative cooling sheet according to Example 1 was superior in cooling performance to the radiative cooling sheet according to Comparative Example 5. Similarly, from the graph shown in FIG. 11A, it can be understood that the radiative cooling sheet according to Example 2 was superior in cooling performance to the radiative cooling sheet according to Comparative Example 8.

[0120] <Thermal conductivity measurement> An aluminum plate was peeled off from the radiative cooling sheets according to Comparative Examples 2, 7, and 11, and the thermal diffusivity of the radiative cooling sheet was determined by the temperature wave method, and the density of the radiative cooling sheet was determined by the Archimedes method, respectively. Also, the specific heat was calculated from the mixing ratio of the powder and the matrix resin. The thermal conductivity was calculated using these three values. Also, the thermal conductivity was calculated for a sheet made of methyl methacrylate resin without powder and a sheet made of CYTOP.

[0121] The results are shown in Table 4.

Table 4

[0122] From Table 4, it can be understood that by dispersing the powder, the thermal conductivity becomes higher than that of the resin alone, and the higher the thermal conductivity of the powder or the matrix resin, the higher the thermal conductivity of the radiative cooling sheet.

Explanation of reference numerals

[0123] 10…Radiative cooling sheet 11…Boron nitride-containing layer 12…Silica-containing layer 31…Substrate 32, 33, 42, 43…Coating layer 41…Release film

Claims

1. A radiative cooling sheet having a laminated structure, a boron nitride-containing layer including boron nitride powder and a matrix resin; a silica-containing layer laminated on the boron nitride-containing layer and containing silica; A radiative cooling sheet comprising:

2. 2. The radiative cooling sheet of claim 1, wherein the silica-containing layer comprises silica powder and a matrix resin.

3. The radiative cooling sheet according to claim 1 or 2, wherein the thickness of the boron nitride-containing layer is 10 μm or more and 10,000 μm or less.

4. The radiative cooling sheet according to claim 1 or 2, wherein the thickness of the silica-containing layer is 10 μm or more and 1000 μm or less.

5. The radiative cooling sheet according to claim 1 or 2, wherein the average particle size of the boron nitride powder is 0.1 μm or more and 500 μm or less.

6. The radiative cooling sheet according to claim 2, wherein the average particle size of the silica powder is 0.1 μm or more and 500 μm or less.

7. The radiative cooling sheet according to claim 1 or 2, wherein a refractive index difference between the boron nitride powder and the matrix resin is 0.1 or more.

8. The radiative cooling sheet according to claim 1 or 2, wherein the surface of the radiative cooling sheet is a surface of a silica-containing layer.

9. forming a boron nitride-containing layer comprising boron nitride powder and a matrix resin; forming a silica-containing layer containing silica on a surface of the boron nitride-containing layer; A method for producing a radiative cooling sheet comprising:

10. 10. The method for producing a radiative cooling sheet according to claim 9, wherein the step of forming the boron nitride containing layer is a step of forming the boron nitride containing layer on a substrate.

11. The method for producing a radiative cooling sheet according to claim 10, further comprising the step of peeling off the substrate after forming the silica-containing layer.

12. The step of forming the silica-containing layer is a step of forming a silica-containing layer containing silica on a release film, the step of forming the boron nitride-containing layer is a step of forming a boron nitride-containing layer containing boron nitride powder and a matrix resin on a surface of the silica-containing layer, 10. The method of claim 9, further comprising peeling the release film from the silica-containing layer after forming the boron nitride-containing layer.

13. 13. The method for producing a radiative cooling sheet according to claim 9 or 12, wherein the formation of the boron nitride-containing layer and the formation of the silica-containing layer are performed by a roll-to-roll method.

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