Radiative cooling ceramic composite material with gradient surface and preparation method thereof

Abstract

This invention discloses a gradient-surface radiation-cooling ceramic composite material and its preparation method. The porous aluminum nitride base layer effectively conducts heat, and the intermediate layer achieves spectral modulation of specific wavelengths. Through the thickness gradient design of the SiO2 and TiO2 layers, the Bragg reflection bandwidth is expanded to cover the non-atmospheric window band of 5-25μm (reflectivity >90%), while allowing high transmission in the 8-13μm band. The protective layer endows the material with functions such as anti-pollution and self-cleaning. The material can automatically adjust its radiation characteristics according to changes in ambient temperature, thereby maintaining the best cooling effect in different environments. The protective layer endows the material with functions such as anti-pollution, self-cleaning, and anti-corrosion, which helps to extend the life of the material.

Description

Technical Field

[0001] This invention relates to the field of materials technology, specifically to a gradient surface radiation-cooled ceramic composite material and its preparation method. Background Technology

[0002] With the rapid development of science and technology, the demand for material performance is also increasing. In particular, in the pursuit of efficient energy utilization and intelligent temperature control, the design and invention of materials that can significantly reduce temperature has become a research hotspot.

[0003] Currently, these materials are designed to regulate the temperature of objects or human body surfaces, bringing innovations to fields such as energy conservation, thermal management, and human comfort. However, while the radiative cooling materials developed so far have achieved cooling effects to some extent, they mostly employ relatively simple structures, such as porous silica (SiO2) or silicon carbide (SiC) coatings.

[0004] The defects of this type of cooling material are as follows:

[0005] First, there is insufficient spectral selectivity. Ideally, radiative cooling materials should possess highly selective emission characteristics, meaning they should efficiently radiate heat into outer space within the atmospheric transparency window (5–8 μm and 14–25 μm) while minimizing emission in other non-transparent bands (such as the mid-infrared region) to prevent heat from being reabsorbed by the atmosphere. However, existing single-structure materials often struggle to achieve this precise spectral control, resulting in higher emissivity in non-atmospheric window bands. Consequently, a large amount of radiative heat is trapped by the atmosphere, reducing overall cooling efficiency.

[0006] Secondly, poor environmental stability is a major issue. The long-term stability of a material is crucial for its continued effectiveness in practical applications. However, many current radiative cooling materials are susceptible to surface contamination or oxidation from environmental dust and other pollutants. This damages their radiative performance, severely impacting their lifespan and effectiveness.

[0007] Third, dynamic regulation is lacking. Temperature changes in nature are complex and varied, including rapid fluctuations between day and night as well as long-term seasonal variations. An ideal radiative cooling material should have the ability to automatically adjust its radiative properties according to the ambient temperature to adapt to these changes and ensure optimal cooling performance under any conditions. However, current single-structure materials often lack this dynamic regulation mechanism and cannot flexibly respond to changes in the external environment, thus limiting their application potential in a wider range of scenarios and requiring improvement. Summary of the Invention

[0008] To address at least one of the aforementioned technical deficiencies, the present invention provides the following technical solution:

[0009] The first aspect of this application discloses a gradient surface radiation-cooled ceramic composite material, comprising:

[0010] Substrate layer: formed of highly thermally conductive porous aluminum nitride ceramic with a porosity of 50-70%;

[0011] Intermediate layer: A gradient photonic crystal layer composed of periodically alternating SiO2 / TiO2 stacks, with 6 layers and a gradient thickness distribution for each layer from bottom to top;

[0012] Protective layer: A hydrophobic-photocatalytic composite coating consisting of fluorinated SiO2 nanoparticles as the outer layer and TiO2 nanowire array as the inner layer.

[0013] The porous aluminum nitride substrate effectively conducts heat, while the intermediate layer enables spectral modulation of specific wavelengths. Through the thickness gradient design of the SiO2 and TiO2 layers, the Bragg reflection bandwidth is expanded to cover the non-atmospheric window band of 5-25μm (reflectivity >90%), while allowing high transmission in the 8-13μm band.

[0014] The protective layer endows the material with functions such as anti-pollution and self-cleaning, effectively preventing moisture or other liquids from corroding the material and helping to extend the material's service life.

[0015] The gradient photonic crystal structure in the middle layer and the nano-metasurface work together to achieve the dual functions of high reflection of sunlight and high infrared emission in the atmospheric window band, effectively reducing the surface temperature of objects without the need for external energy input.

[0016] Furthermore, it also includes a functional layer disposed on the intermediate layer. The functional layer is a Mg1-xZnxAl2O4 thin film, wherein the X value of Zn is between 0.1 and 0.5. Dynamic thermal management is achieved by utilizing its phase transition characteristics. By adjusting the X value of Zn, the infrared emissivity of the 8-13μm atmospheric window is enhanced, thereby achieving additional cooling at high temperatures.

[0017] Furthermore, the substrate layer is pre-embedded with VO2 nanowires with a diameter of 10-40 nm and a volume fraction of 5%-10%, which can improve its thermal conductivity and utilize its unique optical properties to adjust the transmittance or reflectance of light of different wavelengths in response to environmental changes.

[0018] Furthermore, in the hydrophobic-photocatalytic composite coating of the protective layer, the particle size of the fluorinated SiO2 nanoparticles is 10-50 nm, and the diameter of the TiO2 nanowires is 5-20 nm, with a length of 100-500 nm.

[0019] The second aspect of this application discloses a method for preparing the above-mentioned composite material, comprising the following steps:

[0020] Substrate preparation: AlN powder is mixed with a pore-forming agent and shaped, and then sintered under nitrogen protection to form a porous structure;

[0021] Intermediate layer preparation: Gradient photonic crystal deposition, SiO2 and TiO2 layers are alternately deposited on the surface of the base layer to form a gradient photonic crystal structure. The thickness of the SiO2 layer gradually increases from 50nm to 200nm, and the thickness of the TiO2 layer gradually decreases from 100nm to 50nm, with a total of 6 layers.

[0022] Protective layer preparation: A hydrophobic-photocatalytic composite coating is formed on the surface of the intermediate functional layer, with the outer layer being fluorinated SiO2 nanoparticles and the inner layer being a TiO2 nanowire array.

[0023] Furthermore, VO2 nanowires with a diameter of 10-40 nm are pre-embedded before or during sintering, with a volume fraction of 5%-10%.

[0024] Furthermore, a functional layer is prepared on the intermediate layer, and a Mg1-xZnxAl2O4 thin film with a thickness of 10-40 nm is deposited on the surface of the gradient photonic crystal, wherein the X value of Zn is between 0.1 and 0.5.

[0025] Furthermore, the functional layer is deposited and shaped by electron beam evaporation at an evaporation rate of 0.05-0.5 nm / s.

[0026] Furthermore, the sintering temperature of the base layer is 1700-1900℃, and the sintering time is 3-5 hours; the deposition of the intermediate layer is carried out by magnetron sputtering, with a deposition rate of 0.1-1.0 nm / s.

[0027] Furthermore, the coating thickness of the fluorinated SiO2 nanoparticles is 10-50 nm, and the thickness of the TiO2 nanowire array is 50-200 nm.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. This invention, through structural design and composition optimization of the material, achieves excellent reflectivity and emissivity, enabling effective cooling under solar radiation and maintaining a good low temperature at night. Its strong environmental stability ensures that the material maintains its performance over long-term use. The material can automatically adjust its radiation characteristics according to changes in ambient temperature, thus maintaining optimal cooling effects in various environments. Furthermore, the protective layer endows the material with anti-fouling, self-cleaning, and anti-corrosion functions, helping to extend its lifespan. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is an electron microscope image of the basal layer;

[0032] Figure 2 This is an electron microscope image of the alternating layers in the middle layer. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1

[0035] The preparation steps of a gradient surface radiation-cooled ceramic composite material are as follows:

[0036] Substrate preparation

[0037] Mixing and Shaping: First, AlN (aluminum nitride) powder is uniformly mixed with 15% by weight of a pore-forming agent (polymethyl methacrylate microspheres). This pore-forming agent will burn and volatilize during sintering, thereby forming pores inside the ceramic. Subsequently, the mixture is pressed into a green body of the desired shape using a dry pressing technique.

[0038] Sintering: The formed green body is placed in a nitrogen protective atmosphere and sintered at a high temperature of 1800℃ for 4 hours. This allows the AlN powder to bond tightly, forming a dense ceramic structure. Simultaneously, the spaces left after the pore-forming agent burns create a porous structure with a porosity reaching 60%. After sintering, the green body exhibits... Figure 1 As shown.

[0039] Pre-embedded VO2 nanowires: VO2 (vanadium dioxide) nanowires with a diameter of 20 nm are pre-embedded in the green body before sintering, and their volume fraction is controlled at 6%.

[0040] Intermediate layer preparation

[0041] SiO2 (silicon dioxide) and TiO2 (titanium dioxide) layers were alternately deposited on the surface of a pre-prepared substrate using magnetron sputtering. The thickness of the SiO2 layer increased from 50 nm to 200 nm from bottom to top, while the thickness of the TiO2 layer decreased from 100 nm to 50 nm from bottom to top. A total of 6 layers were deposited at a deposition rate of 0.5 nm / s. The alternating coating configuration is as follows: Figure 2 As shown.

[0042] Preparation of functional layers

[0043] Raw material preparation: High-purity magnesium nitrate (Mg(NO3)2·6H2O), zinc nitrate (Zn(NO3)2·6H2O), and aluminum nitrate (Al(NO3)3·9H2O) are selected as metal sources, citric acid (C6H8O7) is used as a complexing agent, and deionized water is used as a solvent.

[0044] Solution preparation: Accurately weigh the nitrate according to the molar ratio of Mg 0.9 Zn 0.1 Al2O4 and dissolve it in deionized water. Simultaneously, add citric acid at a molar ratio of metal ions to citric acid of 1:1.5 to form a stable metal complex solution.

[0045] Sol formation: Place the solution in an 80°C water bath and stir until a transparent sol is formed.

[0046] Gelation and drying: The sol was dried at 120°C for 12 hours to obtain a fluffy precursor gel.

[0047] Calcination: The precursor gel is placed in an air atmosphere and heated to 600°C at a rate of 5°C / min, and held at that temperature for 2 hours to complete the calcination process.

[0048] Using electron beam evaporation technology, a 30 nm thick Mg1-xZnxAl2O4 (magnesium zinc aluminate) film was deposited on the surface of the formed intermediate layer after calcination of the sol, with an evaporation rate of 0.2 nm / s.

[0049] Preparation of protective layer

[0050] A protective layer is formed on the surface of the already formed functional layer. Concentrated hydrochloric acid and deionized water are mixed at a volume ratio of 1:1, and then tetrabutyl titanate is added to a concentration of 0.3 mol / L. The prepared base layer, intermediate layer and other layer materials are added to the tetrabutyl titanate solution and reacted at 180℃ for 4 hours. The product is then removed and dried. The surface of the functional layer has a TiO2 nanowire array with an average thickness of about 100 nm. The diameter of the TiO2 nanowires is 5-20 nm and the length is 100-500 nm.

[0051] Fluorinated SiO2 nanoparticles, polydimethylsiloxane, polyamide, curing agent (184 curing agent), and solvent were mixed to form a coating paste. The mass ratio of fluorinated SiO2 nanoparticles, polydimethylsiloxane, polyamide, and curing agent (184 curing agent) was 18:20:1.0:1.2. The concentration of fluorinated SiO2 nanoparticles in the mixture was 80 mg / ml. The solvent was tetrahydrofuran, ethanol, and dichloromethane mixed in a volume ratio of 6:1:1. The coating paste was spin-coated onto a TiO2 nanowire array to a thickness of 30 nm and cured at 60 °C for 5 h.

[0052] Example 2

[0053] The preparation steps of a gradient surface radiation-cooled ceramic composite material are as follows:

[0054] Substrate preparation

[0055] Mixing and Shaping: First, AlN (aluminum nitride) powder is uniformly mixed with 16% by weight of a pore-forming agent (polymethyl methacrylate microspheres). This pore-forming agent will burn and volatilize during sintering, thereby forming pores inside the ceramic. Subsequently, the mixture is pressed into a green body of the desired shape using a dry pressing technique.

[0056] Sintering: The formed green body is placed in a nitrogen protective atmosphere and sintered at a high temperature of 1850℃ for 4 hours. This allows the AlN powder to bond tightly, forming a dense ceramic structure. Simultaneously, the spaces left after the pore-forming agent burns create a porous structure with a porosity of 65%. After sintering, the green body exhibits... Figure 1 As shown.

[0057] Pre-embedded VO2 nanowires: VO2 (vanadium dioxide) nanowires with a diameter of 25 nm are pre-embedded in the green body before sintering, and their volume fraction is controlled at 7%.

[0058] Intermediate layer preparation

[0059] SiO2 (silicon dioxide) and TiO2 (titanium dioxide) layers were alternately deposited on the prepared substrate surface using magnetron sputtering. The thickness of the SiO2 layer increased from 50 nm to 200 nm from bottom to top, while the thickness of the TiO2 layer decreased from 100 nm to 50 nm from bottom to top. A total of 6 layers were deposited at a deposition rate of 0.6 nm / s. The alternating coating configuration is as follows: Figure 2 As shown.

[0060] Preparation of functional layers

[0061] Raw material preparation: High-purity magnesium nitrate (Mg(NO3)2·6H2O), zinc nitrate (Zn(NO3)2·6H2O), and aluminum nitrate (Al(NO3)3·9H2O) are selected as metal sources, citric acid (C6H8O7) is used as a complexing agent, and deionized water is used as a solvent.

[0062] Solution preparation: Accurately weigh the nitrate according to the molar ratio of Mg 0.9 Zn 0.1 Al2O4 and dissolve it in deionized water. Simultaneously, add citric acid at a molar ratio of metal ions to citric acid of 1:1.5 to form a stable metal complex solution.

[0063] Sol formation: Place the solution in an 80°C water bath and stir until a transparent sol is formed.

[0064] Gelation and drying: The sol was dried at 120°C for 12 hours to obtain a fluffy precursor gel.

[0065] Calcination: The precursor gel is placed in an air atmosphere and heated to 600°C at a rate of 8°C / min, and held at that temperature for 2 hours to complete the calcination process.

[0066] Using electron beam evaporation technology, a 30 nm thick Mg1-xZnxAl2O4 (magnesium zinc aluminate) film was deposited on the surface of the formed intermediate layer after calcination of the sol, with an evaporation rate of 0.3 nm / s.

[0067] Preparation of protective layer

[0068] A protective layer is formed on the surface of the already formed functional layer. Concentrated hydrochloric acid and deionized water are mixed at a volume ratio of 1:1, and then tetrabutyl titanate is added to a concentration of 0.2 mol / L. The prepared base layer, intermediate layer and other layer materials are added to the tetrabutyl titanate solution and reacted at 180℃ for 4 hours. The product is then removed and dried. The surface of the functional layer has a TiO2 nanowire array with an average thickness of about 100 nm. The diameter of the TiO2 nanowires is 5-20 nm and the length is 100-500 nm.

[0069] Fluorinated SiO2 nanoparticles, polydimethylsiloxane, polyamide, curing agent (184 curing agent), and solvent were mixed to form a coating paste. The mass ratio of fluorinated SiO2 nanoparticles, polydimethylsiloxane, polyamide, and curing agent (184 curing agent) was 18:20:0.9:1.1. The concentration of fluorinated SiO2 nanoparticles in the mixture was 90 mg / ml. The solvent was tetrahydrofuran, ethanol, and dichloromethane mixed in a volume ratio of 6:1:1.2. The coating paste was spin-coated onto a TiO2 nanowire array to a thickness of 30 nm and cured at 60 °C for 5 h.

[0070] The properties of the materials prepared in Examples 1 and 2 were tested, and the properties of the two were basically the same. The specific properties of the material prepared in Example 1 are as follows.

[0071] Spectral performance

[0072] Visible-near-infrared reflectance: The material has a reflectance of up to 96% in the wavelength range of 0.3 to 2.5 μm, a figure based on AM1.5 solar spectrum weighting.

[0073] AM1.5 solar spectrum is a standard spectrum that accurately simulates solar radiation at midday on a clear day. Its high reflectivity means the material can effectively reflect the visible and near-infrared portions of sunlight, significantly reducing heat absorption and laying a solid foundation for subsequent cooling effects.

[0074] Atmospheric window emissivity: The emissivity of this material is 0.98 in the wavelength range of 8 to 13 micrometers, and less than 0.3 in the non-window bands (5-8 μm, 14-25 μm).

[0075] This high emissivity indicates that the material can radiate absorbed heat with extremely high efficiency within this wavelength range. This helps reduce heat loss in these wavelength ranges, further optimizing the material's thermal management performance and enabling it to maintain stable performance even under complex thermal environments.

[0076] Cooling effect

[0077] Under midday solar irradiance: When the solar irradiance intensity reaches 1000 W / m 2 When the solar radiation intensity is equivalent to that at noon on a clear summer day, the surface temperature of the material is 15°C lower than the ambient temperature.

[0078] This significant cooling effect is mainly due to the material's high reflectivity and high emissivity in the atmospheric window band. High reflectivity allows the material to effectively reflect most of the sunlight, reducing heat absorption; while high emissivity allows the material to quickly radiate the small amount of absorbed heat, thus achieving a significant cooling effect and providing strong performance assurance for applications in high-temperature environments.

[0079] Nighttime temperature reduction: At night, without the heating effect of solar radiation, the material can further reduce the surface temperature by up to 20°C thanks to its efficient radiation characteristics.

[0080] This characteristic helps maintain the material's low temperature at night, reducing heat accumulation and thus ensuring good performance in all-weather use scenarios, further expanding the material's application range.

[0081] Environmental stability

[0082] Damp heat aging test: After 1000 hours of damp heat aging test (test conditions: 85℃ / 85% relative humidity), the emissivity of the material decreased by less than 3%.

[0083] This result fully demonstrates that the material has excellent stability and durability in harsh humid and hot environments, can maintain its efficient radiation characteristics for a long time, and will not experience a significant decline in performance due to environmental factors, thus ensuring the reliability and stability of the material during long-term use.

[0084] Hydrophobic coating performance: The surface of this material is coated with a hydrophobic coating with a contact angle greater than 150°, exhibiting extremely strong hydrophobic properties.

[0085] This hydrophobic property helps prevent moisture accumulation on the material surface, thereby reducing heat loss due to moisture evaporation. Simultaneously, the material surface is coated with a TiO2 photocatalytic layer, which can decompose 90% of surface contaminants. Under ultraviolet light irradiation, the TiO2 photocatalytic layer generates highly oxidizing free radicals. These free radicals efficiently decompose organic and inorganic contaminants, maintaining the cleanliness and efficient radiation properties of the material surface, further enhancing the material's self-cleaning ability and long-term performance stability in complex environments.

[0086] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A gradient surface radiation-cooled ceramic composite material, characterized in that, include: Substrate layer: formed of highly thermally conductive porous aluminum nitride ceramic with a porosity of 50-70%; Intermediate layer: A gradient photonic crystal layer is formed by periodically alternating stacks of SiO2 and TiO2. There are 6 layers, and the thickness of each layer is gradient from bottom to top. The thickness of the SiO2 layer gradually increases from 50nm to 200nm, and the thickness of the TiO2 layer gradually decreases from 100nm to 50nm. Protective layer: A hydrophobic-photocatalytic composite coating consisting of fluorinated SiO2 nanoparticles as the outer layer and TiO2 nanowire array as the inner layer.

2. The gradient surface radiation-cooled ceramic composite material as described in claim 1, characterized in that: It also includes a functional layer disposed on the intermediate layer, wherein the functional layer is a Mg1-xZnxAl2O4 thin film, wherein the x value of Zn is between 0.1 and 0.

5.

3. The gradient surface radiation-cooled ceramic composite material as described in claim 2, characterized in that: The substrate layer contains embedded VO2 nanowires with a diameter of 10-40 nm, and their volume fraction is 5%-10%.

4. The gradient surface radiation-cooled ceramic composite material as described in claim 1, characterized in that: In the hydrophobic-photocatalytic composite coating of the protective layer, the particle size of fluorinated SiO2 nanoparticles is 10-50 nm, and the diameter of TiO2 nanowires is 5-20 nm, with a length of 100-500 nm.

5. A method for preparing the composite material according to any one of claims 1-4, characterized in that: Includes the following steps: Substrate preparation: AlN powder is mixed with a pore-forming agent and shaped, and then sintered under nitrogen protection to form a porous structure; Intermediate layer preparation: Gradient photonic crystal deposition, SiO2 and TiO2 layers are alternately deposited on the surface of the base layer to form a gradient photonic crystal structure. The thickness of the SiO2 layer gradually increases from 50nm to 200nm, and the thickness of the TiO2 layer gradually decreases from 100nm to 50nm, with a total of 6 layers. Protective layer preparation: A hydrophobic-photocatalytic composite coating is formed on the surface of the intermediate functional layer, with the outer layer being fluorinated SiO2 nanoparticles and the inner layer being a TiO2 nanowire array.

6. The preparation method according to claim 5, characterized in that: Before or during sintering, VO2 nanowires with a diameter of 20 nm are pre-embedded, with a volume fraction of 5%-10%.

7. The preparation method according to claim 5, characterized in that: A functional layer is prepared on the intermediate layer, and a Mg1-xZnxAl2O4 thin film with a thickness of 10-40 nm is deposited on the surface of the gradient photonic crystal, wherein the X value of Zn is between 0.1 and 0.

5.

8. The preparation method according to claim 7, characterized in that: Functional layers are deposited and shaped by electron beam evaporation at an evaporation rate of 0.05-0.5 nm / s.

9. The preparation method according to claim 5, characterized in that: The sintering temperature of the base layer is 1700-1900℃, and the sintering time is 3-5 hours; the deposition of the intermediate layer is carried out by magnetron sputtering, and the deposition rate is 0.1-1.0 nm / s.

10. The preparation method according to claim 5, characterized in that: The coating thickness of the fluorinated SiO2 nanoparticles is 10-50 nm, and the thickness of the TiO2 nanowire array is 50-200 nm.

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