Preparation method of h-BN / BMSC daytime radiation refrigeration material

Abstract

The invention discloses a preparation method of an h-BN / BMSC daytime radiation refrigeration material, which comprises the following steps: (1) blending 39.9-66.5 parts by mass of few-layer h-BN nanosheets and 133 parts by mass of light calcined magnesia, and slowly stirring to obtain a mixed material; (2) dispersing an additive into 61 parts by mass of a magnesium sulfate heptahydrate solution, uniformly mixing, adding into the mixed material in the step (1), and stirring at a high speed to obtain slurry; and (3) putting the slurry obtained in the step (2) into a mold, and curing at room temperature to obtain the few-layer h-BN / BMSC composite material. The prepared gelling-based composite material has good radiation cooling performance and heat dissipation performance in an outdoor high-temperature environment, and can be used as a radiation cooling heat dissipation material with the temperature higher than the environment temperature. The passive cooling heat dissipation plate has important significance in improving the working efficiency of outdoor electronic equipment in a high-temperature environment, prolonging the service life and guaranteeing the reliability of a system.

Description

Technical Field

[0001] This invention relates to a method for preparing h-BN / BMSC daytime radiation cooling material. Background Technology

[0002] Radiation cooling (RC) materials refer to materials whose surfaces can spontaneously exchange heat with the cooler outer space. According to thermodynamics, heat spontaneously flows from high-temperature objects to low-temperature objects. The Earth's surface temperature is approximately 15-30°C, allowing direct heat exchange with outer space (approximately -270°C) through thermal radiation. For cooling above ambient temperature, outdoor electronic devices such as high-temperature electronic equipment, 5G base stations, new energy batteries, and solar cells generate significant heat during operation, primarily due to high power consumption and direct sunlight. The lifespan of electronic devices typically decreases by 10-20% for every 10°C increase in temperature. For example, in crystalline silicon photovoltaic panels, every 1°C increase in temperature leads to a 0.45% decrease in photoelectric conversion efficiency.

[0003] Materials used for radiative cooling above ambient temperature need to maintain good broadband emissivity within the blackbody radiation spectrum. Furthermore, due to the harshness of the working environment, they must also meet specific requirements such as heat resistance, heat dissipation, flame retardancy, high thermal stability, low coefficient of thermal expansion, radiation resistance, anti-icing properties, and self-cleaning. Currently, there are no cementitious materials that combine both radiative cooling and flame-retardant heat dissipation functions. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a gel-based composite material that has both good radiative cooling and heat dissipation performance in outdoor high-temperature environments.

[0005] Technical solution: The preparation method of the h-BN / BMSC composite material of the present invention includes the following steps: (1) 39.9~46.55 parts by weight of few-layer h-BN nanosheets were mixed with 133 parts by weight of lightly calcined magnesium oxide and stirred slowly to obtain a mixture. (2) Disperse the additive in 61 parts by mass of magnesium sulfate heptahydrate solution, mix well and add it to the mixture in step (1), stir at high speed to obtain slurry; (3) Place the slurry from step (2) into a mold and cure at room temperature to obtain a few-layer h-BN / BMSC composite material.

[0006] In step (1), the few-layer h-BN nanosheets are prepared by the following method, the specific steps of which are as follows: (1.1) Take 2~4g of h-BN nanosheets and disperse them in 200~400mL of DMF (N,N-dimethylformamide), stir overnight to obtain a uniform h-BN nanosheet dispersion; (1.2) Centrifuge the h-BN nanosheet (hexagonal boron nitride) dispersion at a speed of not less than 8000 rpm for at least 30 min; take the supernatant (the supernatant contains few-layer h-BN nanosheets); this step is used to peel off the multilayer nanosheets into few-layer h-BN nanosheets; (1.3) After filtering the supernatant, wash with water and dry overnight to obtain few-layer h-BN nanosheets.

[0007] In step (1.3), the thickness of the few-layer h-BN nanosheets is 120~200nm, and the diameter of the h-BN nanosheets is 0.8~1.4µm.

[0008] In step (1), the speed of slow stirring is 300~350 rpm and the stirring time is 2~3 min.

[0009] In step (2), the stirring speed is 500~550 rpm and the stirring time is 2~3 min. The additives are citric acid and naphthalene-based water-reducing agent; the amount of citric acid added is 0.5~1% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2~4% of the mass of light-burned magnesium oxide.

[0010] In step (3), the slurry is placed in the mold and allowed to flow naturally to a thickness of 3-5 mm. Self-leveling can ensure the in-plane orientation of h-BN and enhance the backscattering component.

[0011] This invention utilizes the intrinsic orientation properties of self-leveling few-layer h-BN nanosheets to achieve parallel oriented arrangement of few-layer h-BN nanosheets within a BMSC (basic magnesium sulfate cementitious material) matrix (parallel oriented arrangement of nanosheets can be achieved through shear force field and gravity induction). This self-assembled few-layer h-BN nanosheet structure not only enhances the backscattering component of sunlight in BMSC materials, significantly improving their solar reflectivity, but also eliminates the inherent high-density internal interlayer interfaces in multilayer h-BN. The few-layer h-BN significantly reduces the internal thermal resistance of the cementitious material, thus providing a more efficient heat flow path and achieving better heat dissipation. Therefore, the few-layer h-BN / BMSC composite material obtained by this invention exhibits excellent reflectivity and heat dissipation.

[0012] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The gel-based composite material prepared by the present invention has both good radiative cooling performance and heat dissipation performance in outdoor high temperature environment, and can be used as a radiative cooling material with a temperature higher than the ambient temperature. This is of great significance for improving the working efficiency of electronic equipment, extending its service life and ensuring the reliability of the system. Attached Figure Description

[0013] Figure 1 Electron micrographs of multilayer h-BN / BMSC composites (thickness direction) (a); multilayer h-BN / BMSC composites (planar direction) (b); and few-layer h-BN / BMSC composites (c); Figure 2 The solar spectral reflectance comparison diagram shows the few-layer h-BN / BMSC composite material prepared in Example 1 and the multilayer h-BN / BMSC composite material prepared in Comparative Example 3 under the same preparation conditions. Detailed Implementation

[0014] Example 1 The preparation method of the h-BN / BMSC radiation cooling material of the present invention includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 8000 rpm for 30 min to separate the multilayer h-BN nanosheets into few-layer h-BN nanosheets, and take the supernatant (the supernatant contains few-layer h-BN nanosheets). (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 120~200nm. (4) 39.9 parts by weight of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 30% of the mass of light-burned magnesium oxide) are mixed with 133 parts by weight of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and let it flow naturally to a thickness of 5 mm; (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0015] The few-layer h-BN / BMSC composite material prepared in Example 1 has a visible spectrum reflectance of 75.4%, a weighted emissivity of 87.4%, and an average thermal conductivity of 6.75 W / mK.

[0016] Example 2 The preparation method of the h-BN / BMSC radiation cooling material of the present invention includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 8000 rpm for 30 min to separate the multilayer h-BN nanosheets into a few-layer h-BN nanosheets and take the supernatant. (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 120~200nm. (4) 39.9 parts by weight of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 30% of the mass of light-burned magnesium oxide) are mixed with 133 parts by weight of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and let it flow naturally to a thickness of 3 mm; (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0017] The few-layer h-BN / BMSC composite material prepared in Example 2 has a visible spectrum reflectance of 73.2% and a weighted emissivity of 84.5%. The thickness variation has little effect on the thermal conductivity of the material, and its average thermal conductivity is 6.72 W / mK.

[0018] Example 3 The preparation method of the h-BN / BMSC radiation cooling material of the present invention includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 8000 rpm for 30 min to separate the multilayer h-BN nanosheets into few-layer h-BN nanosheets, and take the supernatant (the supernatant contains few-layer h-BN nanosheets). (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 120~200nm. (4) 46.55 parts by weight of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 35% of the mass of light-burned magnesium oxide) are mixed with 133 parts by weight of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and let it flow naturally to a thickness of 5 mm; (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0019] The few-layer h-BN / BMSC composite material prepared in Example 3 has a visible spectrum reflectance of 71.8%, a weighted emissivity of 84.5%, and an average thermal conductivity of 6.76 W / mK.

[0020] Comparative Example 1 A method for preparing an h-BN / BMSC daytime radiation cooling material includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 3000 rpm for 30 min to separate the multilayer h-BN nanosheets into few-layer h-BN nanosheets, and take the supernatant (the supernatant contains few-layer h-BN nanosheets). (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 800~1000 nm. (4) 39.9 parts by weight of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 30% of the mass of light-burned magnesium oxide) are mixed with 133 parts by weight of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and let it flow naturally to a thickness of 5 mm; (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0021] The few-layer h-BN / BMSC composite material prepared in Comparative Example 1 has a visible spectrum reflectance of 54.3%, a weighted emissivity of 86.9%, and an average thermal conductivity of 5.22 W / mK.

[0022] Comparative Example 2 A method for preparing an h-BN / BMSC daytime radiation cooling material includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 8000 rpm for 15 min to separate the multilayer h-BN nanosheets into few-layer h-BN nanosheets, and take the supernatant (the supernatant contains few-layer h-BN nanosheets). (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 600~800 nm. (4) 39.9 parts by weight of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 30% of the mass of light-burned magnesium oxide) are mixed with 133 parts by weight of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and let it flow naturally to a thickness of 5 mm; (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0023] The few-layer h-BN / BMSC composite material prepared in Comparative Example 2 has a visible spectrum reflectance of 61.1%, a weighted emissivity of 85.7%, and an average thermal conductivity of 5.46 W / mK.

[0024] Comparative Example 3 A method for preparing an h-BN / BMSC daytime radiation cooling material includes the following steps: (1) Disperse 0.665 parts by mass of citric acid and 2.66 parts by mass of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by mass of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28) to obtain a mixture; (2) Pour the mixture from step (1) into 133 parts by mass of lightly calcined magnesium oxide, and add 39.9 parts by mass of multilayer h-BN nanosheets (the amount of h-BN nanosheets added is 30% of the mass of lightly calcined magnesium oxide). The diameter of the multilayer h-BN nanosheets (thickness is about 2~3µm) is 0.8~1.4µm. Stir the obtained mixture in a mortar mixer at 300rpm for 2min, and then at 500rpm for 2min to obtain a slurry. (3) Place the slurry from step (2) into the mold and let it flow naturally to a thickness of 5 mm; (4) After curing at room temperature, h-BN / BMSC composite material was obtained.

[0025] The h-BN / BMSC composite material prepared in Comparative Example 3 has a visible spectrum reflectance of 52.2%, a weighted emissivity of 86.1%, and an average thermal conductivity of 5.31 W / mK.

[0026] Comparative Example 4 A method for preparing an h-BN / BMSC daytime radiation cooling material includes the following steps: (1) Disperse 0.665 parts by mass of citric acid and 2.66 parts by mass of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by mass of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28) to obtain a mixture; (2) Pour the mixture from step (1) into 133 parts by mass of lightly calcined magnesium oxide, and add 39.9 parts by mass of multilayer h-BN nanosheets (the amount of h-BN nanosheets added is 30% of the mass of lightly calcined magnesium oxide). The diameter of the multilayer h-BN nanosheets is 0.8~1.4µm. Stir the obtained mixture in a mortar mixer at 300rpm for 2min, and then at 500rpm for 2min to obtain a slurry. (3) Place the slurry from step (2) into the mold and let it flow naturally to a thickness of 3 mm; (4) After curing at room temperature, h-BN / BMSC composite material was obtained.

[0027] The h-BN / BMSC composite material prepared in Comparative Example 4 has a visible spectrum reflectance of 51.7%, a weighted emissivity of 84.0%, and an average thermal conductivity of 5.30 W / mK.

[0028] Comparative Example 5 A method for preparing an h-BN / BMSC daytime radiation cooling material includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 8000 rpm for 30 min to separate the multilayer h-BN nanosheets into few-layer h-BN nanosheets, and take the supernatant (the supernatant contains few-layer h-BN nanosheets). (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 120~200nm. (4) 39.9 parts by weight of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 30% of the mass of light-burned magnesium oxide) are mixed with 133 parts by weight of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and pour it to a thickness of 5 mm. Place the slurry on the vibrating table and vibrate it violently. This operation will destroy the in-plane orientation of the few-layer nanosheets and increase the random orientation of the few-layer nanosheets. (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0029] The few-layer h-BN / BMSC composite material prepared in Comparative Example 5 has a visible spectrum reflectance of 50.0%, a weighted emissivity of 85.1%, and an average thermal conductivity of 3.84 W / mK.

[0030] Comparative Example 6 A method for preparing an h-BN / BMSC daytime radiation cooling material includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 8000 rpm for 30 min to separate the multilayer h-BN nanosheets into few-layer h-BN nanosheets, and take the supernatant (the supernatant contains few-layer h-BN nanosheets). (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 120~200nm. (4) 13.3 parts by weight of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 10% of the mass of light-burned magnesium oxide) are mixed with 133 parts by weight of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and let it flow naturally to a thickness of 5 mm; (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0031] The few-layer h-BN / BMSC composite material prepared in Comparative Example 6 has a visible spectrum reflectance of 52.9% and a weighted emissivity of 85.7%. The average thermal conductivity is 3.12 W / mK.

[0032] Comparative Example 7 A method for preparing an h-BN / BMSC daytime radiation cooling material includes the following steps: (1) Take 4g of h-BN nanosheets (diameter 0.8~1.4µm) and disperse them in 400mL of DMF. Stir overnight to obtain a uniform h-BN nanosheet dispersion. (2) Centrifuge the h-BN nanosheet dispersion from step (1) at 8000 rpm for 30 min to separate the multilayer h-BN nanosheets into few-layer h-BN nanosheets, and take the supernatant (the supernatant contains few-layer h-BN nanosheets). (3) After filtering the supernatant, wash it with water three times and dry it overnight to obtain a few-layer h-BN nanosheets with a thickness of 120~200nm. (4) 66.5 parts by mass of few-layer h-BN nanosheets (the amount of few-layer h-BN nanosheets added is 50% of the mass of light-burned magnesium oxide) are mixed with 133 parts by mass of light-burned magnesium oxide and stirred at 300 rpm for 2 min to obtain a mixture. (5) Disperse 0.665 parts by weight of citric acid and 2.66 parts by weight of naphthalene-based water-reducing agent (the amount of citric acid added is 0.5% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2% of the mass of light-burned magnesium oxide) in 61 parts by weight of magnesium sulfate heptahydrate aqueous solution (the Baumé degree of magnesium sulfate solution is 28), mix well, pour it into the mixture in step (4), stir at 500 rpm for 2 min to obtain slurry; (6) Place the slurry from step (5) into the mold and let it flow naturally to a thickness of 5 mm; (7) After curing at room temperature, a few-layer h-BN / BMSC composite material was obtained.

[0033] The few-layer h-BN / BMSC composite material prepared in Comparative Example 7 has a visible spectrum reflectance of 60.3% and a weighted emissivity of 86.4%. The average thermal conductivity is 6.68 W / mK.

[0034] Radiation cooling performance and thermal conductivity of Examples 1-3 and Comparative Examples 1-7 were tested, and the results are shown in Table 1 (Note: Different thicknesses of the same sample have a potential impact on radiation cooling performance, but the impact on thermal conductivity is negligible).

[0035] Table 1

[0036] As shown in Table 1, the difference between Example 1 and Example 2 lies in the thickness of the cementitious material. Increasing the thickness is beneficial for enhancing radiative cooling performance without significantly affecting the thermal conductivity. The difference between Example 1 and Example 3 lies in the increased content of the few-layer h-BN. Under the same dispersion process conditions, the increased content of nanofillers enhances the van der Waals interactions between particles, resulting in varying degrees of agglomeration and a slight decrease in uniformity.

[0037] The difference between Comparative Examples 1 and 2 and Example 1 lies in the slower centrifugation speed and shorter centrifugation time during the preparation of few-layer h-BN, resulting in a lower yield of few-layer h-BN and an inability to achieve high backscattering efficiency. Simultaneously, the thermal conductivity is also slightly reduced. The difference between Comparative Examples 3 and 4 and Examples 1 and 2 lies in the use of multilayer h-BN as an additive. Its reflection efficiency increases with increasing slurry thickness, but is still lower overall than that of few-layer h-BN doping. Regarding thermal conductivity, the high-density internal interlayer interfaces of multilayer h-BN increase its internal thermal resistance, resulting in relatively poor heat dissipation performance in the composite material.

[0038] The difference between Comparative Example 5 and Example 1 lies in the disruption of the in-plane orientation of the few-layer h-BN, resulting in high orientation randomness and thus reducing its backscattering efficiency. Due to the random orientation of its internal nanosheets, this structure forces frequent changes in the heat flow direction during transmission. Phonons must repeatedly traverse crystal planes and interfaces with different orientations, leading to strong phonon scattering. This disordered path significantly increases the overall thermal resistance of the material, limiting the improvement of thermal conductivity. The difference between Comparative Example 6 and Example 1 lies in the reduced doping amount of few-layer h-BN, resulting in decreased reflection efficiency and a corresponding decrease in thermal conductivity. The difference between Comparative Example 7 and Example 1 lies in the increased doping amount of few-layer h-BN; however, excessive h-BN nanomaterials are prone to agglomeration within the basic magnesium sulfate matrix, resulting in poor performance.

[0039] pass Figure 1 It can be seen that the diameter of the multilayer nanosheets (a~b) is between 0.8 and 1.4 µm, and the thickness is between 2 and 3 µm; the thickness of the few-layer h-BN nanosheets after exfoliation is about 120~200 nm.

[0040] pass Figure 2 It is evident that the few-layer h-BN / BMSC exhibits higher reflectivity. This is because the few-layer h-BN nanosheets achieve near-parallel orientation during composite material preparation through shear force and gravity induction. This ordered structure significantly enhances the backscattering probability of photons along their internal propagation path, resulting in a substantial increase in overall solar reflectivity. In contrast, multilayer h-BN nanosheets or their randomly stacked structures lead to more frequent diffuse scattering and disordered reflection of photons within them. The photon propagation path is more complex and tortuous, and some energy is ultimately absorbed by the material and converted into heat during multiple scatterings, resulting in greater energy loss and thus relatively lower overall solar reflectivity. From a thermal conductivity perspective, multilayer h-BN has a high-density, complex internal interlayer interface and relatively high internal thermal resistance, thus performing worse than few-layer h-BN nanosheets in terms of heat dissipation performance in composite materials.

Claims

1. A method for preparing an h-BN / BMSC daytime radiation cooling material, characterized in that, Includes the following steps: (1) 39.9~46.55 parts by weight of few-layer h-BN nanosheets were mixed with 133 parts by weight of lightly calcined magnesium oxide and stirred slowly to obtain a mixture. (2) Disperse the additive in 61 parts by mass of magnesium sulfate heptahydrate solution, mix well and add it to the mixture in step (1), stir at high speed to obtain slurry; (3) Place the slurry from step (2) into a mold and cure at room temperature to obtain a few-layer h-BN / BMSC composite material.

2. The preparation method according to claim 1, characterized in that: In step (1), the few-layer h-BN nanosheets are prepared by the following method, the specific steps of which are as follows: (1.1) Disperse h-BN nanosheets in DMF and stir overnight to obtain a uniform h-BN nanosheet dispersion; (1.2) Centrifuge the h-BN nanosheet dispersion at a speed of not less than 8000 rpm for at least 30 min; take the supernatant; (1.3) After filtering the supernatant, wash with water and dry overnight to obtain few-layer h-BN nanosheets.

3. The preparation method according to claim 2, characterized in that: In step (1.1), the mass-to-volume ratio of the h-BN nanosheets to DMF is 2~4g:200~400mL.

4. The preparation method according to claim 2, characterized in that: In step (1.3), the thickness of the few-layer h-BN nanosheets is 120~200nm and the diameter is 0.8~1.4µm.

5. The preparation method according to claim 1, characterized in that: In step (1), the speed of slow stirring is 300~350 rpm and the stirring time is 2~3 min.

6. The preparation method according to claim 1, characterized in that: In step (2), the speed of rapid stirring is 500~550 rpm, and the stirring time is 2~3 min.

7. The preparation method according to claim 1, characterized in that: In step (2), the additive is citric acid and naphthalene-based water-reducing agent.

8. The preparation method according to claim 7, characterized in that: The amount of citric acid added is 0.5-1% of the mass of light-burned magnesium oxide, and the amount of naphthalene-based water-reducing agent added is 2-4% of the mass of light-burned magnesium oxide.

9. The preparation method according to claim 1, characterized in that: In step (3), the slurry is placed in the mold and allowed to flow naturally.

10. The preparation method according to claim 9, characterized in that: The slurry should be allowed to flow naturally to a thickness of 3-5 mm.

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