A method for the preparation of a magnesium aerogel material for daytime radiative cooling

Abstract

The application discloses a preparation method of magnesium aerogel material for daytime radiation cooling, and comprises the following steps: (1) fully dissolving magnesium sulfate heptahydrate in water under heating condition, then adding chemical additive into the solution and stirring uniformly to obtain a solution; (2) adding light-burned magnesium oxide into the solution in step (1) in batches, and stirring uniformly to obtain a freezing precursor solution; (3) pouring the freezing precursor solution into a mold, placing the mold on a cooling table, carrying out heat preservation treatment on the top of the mold, and then carrying out unidirectional freezing at a power of 6W-12W, and obtaining a frozen embryo after freezing for 1-1.5h; (4) carrying out demolding and freeze-drying treatment on the frozen embryo to obtain the magnesium aerogel material, and carrying out humidity curing on the obtained magnesium aerogel material to obtain a target product. The magnesium aerogel material prepared by the method can reach a reflectivity of more than 90% in the 0.3-2.5mu m solar spectrum range without adding high-refractive nano materials such as TiO2 and BaSO4, and meanwhile, the high infrared radiation capacity of the magnesium aerogel material in the 8-13mu m atmospheric window is reserved.

Description

Technical Field

[0001] This invention relates to a method for preparing a material for daytime radiation cooling. Background Technology

[0002] Daytime radiative cooling is a passive thermal management strategy that cools the body by selectively radiating heat into outer space without requiring external energy. Ideal daytime radiative cooling materials typically need to simultaneously meet two key spectral characteristics: first, high infrared emissivity in the atmospheric window band (approximately 8–13 μm) to radiate heat into outer space; and second, high reflectivity in the solar spectrum (approximately 0.3–2.5 μm) to suppress heat absorption caused by solar irradiation. Conventional basic magnesium sulfate cementitious materials (BMSCs), due to their intrinsic lattice vibrations and multiphase hydration product characteristics, generally exhibit high infrared emissivity in the atmospheric window band, making them potential matrix materials for radiative cooling. However, their reflectivity in the solar spectrum is relatively insufficient, especially exhibiting low scattering in the visible and near-infrared regions. This is a common problem with cementitious materials, limiting their net cooling capacity under solar irradiation conditions.

[0003] To address the aforementioned issues, existing technologies introduce white inorganic particles with high refractive index or high scattering rate into BMSCs to enhance solar reflectivity, such as TiO2, BaSO4, and Al2O3. However, this method suffers from significant agglomeration effects due to high nanoparticle doping, resulting in uneven particle dispersion, local defects, and decreased optical scattering efficiency. Furthermore, agglomeration can induce pores, cracks, or interface weakening, thereby affecting the material's mechanical properties and durability. Additionally, the reflectivity of magnesium cementitious materials using the aforementioned methods is limited to approximately 80% at high nanoparticle doping levels, failing to reach 90%. This results in insufficient cooling power and poor refrigeration performance during application. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for preparing magnesium aerogel materials. The magnesium aerogel materials prepared by this method can achieve a reflectivity of more than 90% in the 0.3~2.5μm solar spectrum range without the addition of high refractive index nanomaterials such as TiO2 and BaSO4, while retaining their high infrared radiation capability in the 8~13μm atmospheric window.

[0005] Technical solution: The preparation method of the magnesium aerogel material for daytime radiation cooling according to the present invention includes the following steps:

[0006] (1) Dissolve magnesium sulfate heptahydrate fully in water under heating conditions; then add chemical additives and stir until homogeneous to obtain a solution;

[0007] (2) Add the lightly calcined magnesium oxide in two batches (each batch is 1 / 2 of the total amount of lightly calcined magnesium oxide) to the solution in step (1) to avoid agglomeration. Stir well to obtain the freezing precursor solution.

[0008] (3) Pour the frozen precursor solution into the mold, then place the mold on the cold table, keep the top of the mold warm, and then freeze it unidirectionally with a power of 6W~12W (corresponding to a freezing temperature of -20℃~-30℃) for 1~1.5h to obtain the frozen embryo.

[0009] (4) After demolding the frozen preform, place it in a freeze dryer to sublimate the ice template and obtain magnesium aerogel material. Finally, perform humidity curing on the obtained magnesium aerogel material (humidity curing is used to promote the growth of some hydration products during the curing process) to obtain magnesium aerogel.

[0010] In step (1), the molar ratio of magnesium sulfate heptahydrate to water is 1:20~30; the heating temperature is 35~45℃; the chemical additive is citric acid, and its dosage is no more than 0.5% of the mass of lightly calcined magnesium oxide.

[0011] In step (2), the molar ratio of the lightly calcined magnesium oxide to magnesium sulfate heptahydrate is 6~8:1; the particle size of the lightly calcined magnesium oxide is 325 mesh, and the activity is about 60%.

[0012] In step (3), the mold is a mold with a copper plate bottom and polytetrafluoroethylene sides. The heat preservation treatment refers to wrapping the top of the mold with foam to achieve heat preservation. The purpose is to ensure the unidirectional transmission of temperature, thereby obtaining a unidirectional ice template structure.

[0013] In step (4), the freeze-drying pressure is 0.01~0.02 mbar, the temperature is -65~-80℃, and the processing time is 12~14h. Humidity curing is carried out at 80%~90% air humidity and 25℃ for no less than 7 days.

[0014] This invention employs a unidirectional freezing ice template method to construct a monolithic material in a BMSC matrix that combines the characteristics of layered microstructures and equiaxed honeycomb pores. This method can induce the formation of honeycomb-like equiaxed pores and secondary layered microstructures with pore walls arranged along the freezing direction. The formation of these microstructures is mainly attributed to the use of lightly calcined magnesium oxide (no layered framework was found in magnesium aerogels made of magnesium oxide nanoparticles). In the application of radiation cooling: the approximately 10 μm equiaxed pore structure is conducive to forming a three-dimensional interconnected scattering channel with high porosity, significantly extending the propagation path of incident sunlight, thus serving as the framework of a high reflectivity material; while the approximately 1 μm layered pore wall structure satisfies the characteristic scale comparable to the visible-near infrared wavelength, allowing the scattering parameters to enter the efficient range of Mie scattering, while increasing the number of solid-gas interfaces on the pore wall, utilizing the refractive index difference between the solid phase and air to enhance scattering, thereby improving the diffuse reflection / backscattering ratio; the composite scattering mechanism of the synergistic effect of the two microstructures enables magnesium cementitious materials to achieve the highest reflectivity in the 0.3~2.5 μm solar spectrum range without the addition of high refractive index nanomaterials such as TiO2 and BaSO4, while retaining its high infrared radiation capability in the 8~13 μm atmospheric window.

[0015] Beneficial effects: Compared with existing technologies, the present invention has the following significant advantages: The magnesium aerogel material prepared by the method of the present invention has ultra-high solar spectral reflectivity and emissivity under conditions without nanoparticle doping, thereby significantly improving daytime radiative cooling performance at 800 W / m 2 Under solar irradiation, its radiative cooling power is approximately 86.4 W / m². 2 Because the resulting material has high porosity and low density, it not only has the advantage of being lightweight, but also helps to reduce thermal conductivity and environmental heat regeneration. Attached Figure Description

[0016] Figure 1 SEM images of the approximately 10 μm pore structure (a) and approximately 1 μm layered pore wall structure (b) in the magnesium aerogel material prepared in Example 1;

[0017] Figure 2 SEM images of the magnesium aerogel material prepared in Comparative Example 2 and the magnesium cementitious material prepared in Comparative Example 5 are shown. Among them, (a) is the SEM image of the through-pore structure and 1μm layered pore wall structure in the magnesium aerogel material prepared in Comparative Example 2; (b) is the SEM image of the magnesium aerogel material prepared in Comparative Example 5 using 200~300nm MgO.

[0018] Figure 3 This is a schematic diagram showing the reflectivity and emissivity of the magnesium aerogel material prepared in Example 1. Detailed Implementation

[0019] Example 1

[0020] The present invention discloses a method for preparing magnesium aerogel materials for daytime radiative cooling, comprising the following steps:

[0021] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid (0.5% of the mass of lightly calcined MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution;

[0022] (2) Add 21g of 325 mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the freezing precursor solution;

[0023] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 12W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0024] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0025] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25°C for 7 days to obtain the magnesium aerogel material of the present invention.

[0026] The magnesium aerogel material prepared in Example 1 has a visible spectrum reflectance of 92% and an atmospheric window emissivity of 95%.

[0027] pass Figure 1 As can be seen, the magnesium aerogel material prepared in Example 1 has a pore structure of approximately 6-10 μm, with pore walls composed of a layered structure of approximately 1 μm. The pore structure originates from multi-point instantaneous nucleation under deep supercooling conditions, while the layered structure originates from the low activity and size characteristics of lightly calcined magnesium oxide. Furthermore, MIP testing showed that the internal porosity of this material is approximately 60%. The 1 μm layered interface primarily generates strong scattering of visible and near-infrared light; while the 6-10 μm macropore structure, by increasing the effective optical thickness, further enhances the material's multiple scattering of mid- and far-infrared light. This multi-level scattering mechanism collectively achieves its extremely high reflectivity within the 0.3-2.5 μm solar spectrum range.

[0028] Example 2

[0029] The present invention discloses a method for preparing magnesium aerogel materials for daytime radiative cooling, comprising the following steps:

[0030] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid (0.5% of the mass of lightly calcined MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution;

[0031] (2) Add 21g of 325 mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the freezing precursor solution;

[0032] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 6W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0033] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0034] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25°C for 7 days to obtain the magnesium aerogel material of the present invention.

[0035] The magnesium aerogel material prepared in Example 2 has a visible spectrum reflectance of 91% and an atmospheric window emissivity of 94%.

[0036] Example 3

[0037] The present invention discloses a method for preparing magnesium aerogel materials for daytime radiative cooling, comprising the following steps:

[0038] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid (0.5% of the mass of lightly calcined MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution;

[0039] (2) Add 21g of 325 mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the freezing precursor solution;

[0040] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 6W and freeze it unidirectionally for 1.5h to obtain the frozen embryo.

[0041] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0042] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25°C for 7 days to obtain the magnesium aerogel material of the present invention.

[0043] The magnesium aerogel material prepared in Example 3 has a visible spectrum reflectance of 87% and an atmospheric window emissivity of 93%.

[0044] Example 4

[0045] The present invention discloses a method for preparing magnesium aerogel materials for daytime radiative cooling, comprising the following steps:

[0046] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid to the magnesium sulfate solution and stir until homogeneous to obtain a solution.

[0047] (2) Add 21g of 325-mesh lightly calcined magnesium oxide to the solution and stir evenly to obtain a freezing precursor solution;

[0048] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 10W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0049] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0050] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25℃ for 7 days to obtain magnesium aerogel material.

[0051] The magnesium aerogel material prepared in Example 4 has a visible spectrum reflectance of 90% and an atmospheric window emissivity of 93%.

[0052] Comparative Example 1

[0053] A method for preparing a magnesium aerogel material includes the following steps:

[0054] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid to the magnesium sulfate solution and stir until homogeneous to obtain a solution.

[0055] (2) Add 21g of 325 mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the freezing precursor solution;

[0056] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the four sides are made of polytetrafluoroethylene. The top and four sides of the mold are insulated. The mold is placed on a cold table with a power of 12W and frozen unidirectionally for 1 hour to obtain the frozen embryo.

[0057] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0058] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25℃ for 7 days to obtain magnesium aerogel material.

[0059] The magnesium aerogel material prepared in Comparative Example 1 has a visible spectrum reflectance of 76% and an atmospheric window emissivity of 94%.

[0060] In Comparative Example 1, foam was wrapped around the four sides of the mold for insulation, in order to enhance the unidirectionality of temperature and reduce the formation of dendrites. However, this would lead to a decrease in the pore structure and ultimately a significant reduction in reflectivity.

[0061] Comparative Example 2

[0062] A method for preparing a magnesium aerogel material includes the following steps:

[0063] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.08g of citric acid (0.5% of the mass of lightly calcined MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution;

[0064] (2) Add 16g of 325 mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the freezing precursor solution;

[0065] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 12W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0066] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0067] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25℃ for 7 days to obtain magnesium aerogel material.

[0068] The magnesium aerogel material prepared in Comparative Example 2 has a visible spectrum reflectance of 80% and an atmospheric window emissivity of 94%.

[0069] Comparative Example 2 reduced the content of lightly calcined magnesium oxide in the system, and its microstructure was as follows: Figure 2 As shown in (a), reducing the solid content enhances the connectivity of the material's pore structure and reduces its regularity, resulting in numerous irregular, large-sized through-holes, while the layered structure of the pore walls remains unchanged. This degradation of the microstructure leads to severe optical mismatch, weakening the cavity resonance scattering effect of the material on the atmospheric window band and causing a decrease in reflectivity.

[0070] Comparative Example 3

[0071] A method for preparing a magnesium aerogel material includes the following steps:

[0072] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid (0.5% of the mass of lightly calcined MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution;

[0073] (2) Add 21g of 325 mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the freezing precursor solution;

[0074] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold in liquid nitrogen and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0075] (4) After the frozen preform is demolded, it is placed in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65°C and freeze-dried for 12 hours. However, it is impossible to obtain a shaped magnesium aerogel material.

[0076] Comparative Example 3 used liquid nitrogen as a cold source (freezing point -196℃). Due to the excessively rapid cooling rate, the ice crystal structure of the ice template became too large. During the freeze-drying process, the magnesium gel cracked along the direction perpendicular to the ice crystal growth.

[0077] Comparative Example 4

[0078] A method for preparing a magnesium aerogel material includes the following steps:

[0079] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid (0.5% of the mass of lightly calcined MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution;

[0080] (2) Add 21g of 325 mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the freezing precursor solution;

[0081] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold in liquid nitrogen containing 99% ethanol and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0082] (4) After the frozen preform is demolded, it is placed in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65°C and freeze-dried for 12 hours. However, it is impossible to obtain a shaped magnesium aerogel material.

[0083] Comparative Example 4 used liquid nitrogen to cool high-purity (99%) ethanol to its freezing point (-114℃) as a cold source. Due to the excessively fast cooling rate, the ice crystal structure of the ice template was too large; the magnesium gel cracked along the direction perpendicular to the ice crystal growth during the freeze-drying process.

[0084] Comparative Example 5

[0085] A method for preparing a magnesium aerogel material includes the following steps:

[0086] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.063g of citric acid (0.5% of the mass of MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution.

[0087] (2) Add 12.6g of magnesium oxide nanoparticles with a particle size of 200~300nm into the solution in portions to avoid agglomeration, stir evenly, and obtain a freezing precursor solution;

[0088] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 12W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0089] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0090] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25℃ for 7 days to obtain magnesium aerogel material.

[0091] The magnesium aerogel material prepared in Comparative Example 5 has a visible spectrum reflectance of 74% and an atmospheric window emissivity of 96%.

[0092] Comparative Example 5 uses MgO nanoparticles instead of 325-mesh lightly calcined MgO to prepare a magnesium aerogel with the following microstructure: Figure 2 As shown in (b), the layered structure of its pore walls disappears, and it consists of a single-pore system with macropores of 6-10 μm (only the 10 μm pore structure formed by multi-point nucleation under deep supercooling conditions is retained, and the layered pore wall structure is lacking). This change leads to a significant decrease in the material's scattering ability, weakening its diffuse reflection of visible and near-infrared light.

[0093] Comparative Example 6

[0094] A method for preparing a magnesium gel material includes the following steps:

[0095] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid (0.5% of the mass of lightly calcined MgO) to the magnesium sulfate solution and stir until homogeneous to obtain a solution;

[0096] (2) Add 21g of 325-mesh lightly calcined magnesium oxide to the solution in portions to avoid agglomeration, stir evenly, and obtain the precursor solution;

[0097] (3) Pour the precursor solution into a plastic mold and cure it under standard air conditions for 1 day before demolding. Continue curing for 7 days to obtain magnesium gel material.

[0098] The magnesium gel material prepared in Comparative Example 5 has a visible spectrum reflectance of 45% and an atmospheric window emissivity of 89%.

[0099] Comparative Example 6 is a basic magnesium sulfate cementitious material without nanoparticle doping. It has a high intrinsic emissivity but poor reflectivity and cannot achieve daytime radiative cooling.

[0100] Comparative Example 7

[0101] A method for preparing a magnesium aerogel material includes the following steps:

[0102] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid to the magnesium sulfate solution and stir until homogeneous to obtain a solution.

[0103] (2) Add 21g of 325-mesh lightly calcined magnesium oxide to the solution in portions and stir until homogeneous to obtain a freezing precursor solution;

[0104] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate, the side walls are made of plastic material, and the top is not insulated. Place the mold on a cold table with a power of 12W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0105] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0106] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25℃ for 7 days to obtain magnesium aerogel material.

[0107] Comparative Example 7 uses a bottom copper plate and a plastic sidewall mold. Its poor sidewall insulation caused severe heat leakage perpendicular to the temperature gradient direction. Furthermore, the lack of top insulation disrupted the unidirectional temperature field, leading to disordered ice crystal growth and ultimately forming a microstructure with disordered orientation and low porosity. This structure makes the material extremely prone to brittle fracture and pulverization under unidirectional pressure, resulting in a significant deterioration in mechanical properties.

[0108] Comparative Example 8

[0109] A method for preparing a magnesium aerogel material includes the following steps:

[0110] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid to the magnesium sulfate solution and stir until homogeneous to obtain a solution.

[0111] (2) Add 21g of 325-mesh lightly calcined magnesium oxide to the solution and stir evenly to obtain a freezing precursor solution;

[0112] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 12W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0113] (4) After demolding the frozen embryo, it was placed in a freeze dryer at a temperature of -30℃ and freeze-dried for 12 hours to obtain magnesium aerogel;

[0114] (5) Place the magnesium aerogel in a curing chamber with an air humidity of 80%~90% and a temperature of 25℃ for 7 days to obtain magnesium aerogel material.

[0115] The magnesium aerogel material prepared in Comparative Example 8 has a visible spectrum reflectance of 70% and an atmospheric window emissivity of 89%.

[0116] In Comparative Example 8, the temperature of the freeze dryer was increased to -30°C. An unavoidable temperature difference existed between the ambient temperature and the set temperature, which caused local melting at the edge of the frozen preform. The resulting huge capillary force caused partial irreversible densification of the wet gel network, and the fine hierarchical structure on the surface was partially destroyed, thus resulting in poor radiative cooling performance.

[0117] Comparative Example 9

[0118] A method for preparing a magnesium aerogel material includes the following steps:

[0119] (1) Dissolve 10g of magnesium sulfate heptahydrate in 21.6g of water at 35℃ to obtain a magnesium sulfate solution; then add 0.105g of citric acid to the magnesium sulfate solution and stir until homogeneous to obtain a solution.

[0120] (2) Add 21g of 325-mesh lightly calcined magnesium oxide to the solution and stir evenly to obtain a freezing precursor solution;

[0121] (3) Pour the frozen precursor solution into the mold. The bottom plate of the mold is made of copper plate and the side walls are made of polytetrafluoroethylene. Only the top of the mold is insulated. Place the mold on a cold table with a power of 12W and freeze it unidirectionally for 1 hour to obtain the frozen embryo.

[0122] (4) After demolding the frozen embryo, place it in a freeze dryer with a pressure of 0.01 mbar and a temperature of -65℃ and freeze dry for 12 h to sublimate the ice template and obtain magnesium aerogel;

[0123] (5) The magnesium aerogel was cured in standard air for 7 days to obtain magnesium aerogel material.

[0124] The magnesium aerogel material prepared in Comparative Example 9 has a visible spectrum reflectance of 89% and an atmospheric window emissivity of 91%.

[0125] In Comparative Example 9, when the magnesium aerogel was placed in standard air (low humidity environment) for 7 days, its internal hydration reaction was severely inhibited due to insufficient moisture, and it could not effectively generate a large network of hydration products. This would result in a serious lack of mechanical strength development of the aerogel, with the structure still mainly consisting of a fragile initial gel skeleton, exhibiting low mechanical strength, easy pulverization, and unsuitability for practical applications.

[0126] The radiative cooling performance of Examples 1-4 and Comparative Examples 1-9 was tested, and the results are shown in Table 1.

[0127] Table 1

[0128] 。

Claims

1. A method for preparing a magnesium aerogel material for daytime radiative cooling, characterized in that, Includes the following steps: (1) Dissolve magnesium sulfate heptahydrate fully in water under heating conditions; then add chemical additives and stir until homogeneous to obtain a solution; (2) Add the lightly calcined magnesium oxide to the solution in step (1) in portions and stir until homogeneous to obtain the freezing precursor solution; (3) Pour the frozen precursor solution into the mold, then place the mold on the cold table, keep the top of the mold warm, and then freeze it unidirectionally with a power of 6W~12W for 1~1.5h to obtain the frozen embryo. (4) Demold the frozen preform and freeze-dry it to obtain magnesium aerogel material. Then, cure the obtained magnesium aerogel material with humidity to obtain magnesium aerogel.

2. The preparation method according to claim 1, characterized in that: In step (1), the molar ratio of magnesium sulfate heptahydrate to water is 1:20~30.

3. The preparation method according to claim 1, characterized in that: In step (1), the heating temperature is 35~45℃.

4. The preparation method according to claim 1, characterized in that: In step (1), the chemical additive is citric acid, and its dosage is no more than 0.5% of the mass of lightly calcined magnesium oxide.

5. The preparation method according to claim 1, characterized in that: In step (2), the molar ratio of the lightly calcined magnesium oxide to magnesium sulfate heptahydrate is 6~8:

1.

6. The preparation method according to claim 1, characterized in that: In step (3), the mold is a mold with a copper plate bottom and polytetrafluoroethylene sides.

7. The preparation method according to claim 6, characterized in that: The heat insulation treatment refers to wrapping foam around the top of the mold.

8. The preparation method according to claim 1, characterized in that: In step (4), the freeze-drying pressure is 0.01~0.02 mbar, the temperature is -65~-80℃, and the processing time is 12~14h.

9. The preparation method according to claim 1, characterized in that: In step (4), humidity maintenance is carried out at an air humidity of 80%~90% and a temperature of 25~30℃.

10. The preparation method according to claim 9, characterized in that: The humidity curing time is no less than 7 days.

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