Radiative cooling ceramic material with spectral selectivity and method of making the same

Abstract

The application discloses a kind of radiation refrigeration ceramic materials with spectral selectivity and preparation method thereof, specifically a kind of gradient porous MgO ceramic material, and the pore size and porosity of ceramic such as parameter are optimized by calculation, finally the preparation of ceramic is completed using pressureless sintering process.The application solves the problem that the selectivity of the existing spectral selective radiation refrigeration material is not outstanding, the structure process is complex and the cost is high, and promotes the practical application of spectral selective radiation refrigeration material in the field of construction.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of radiation refrigeration ceramic materials, and particularly relates to a radiation refrigeration ceramic material with spectral selectivity and a preparation method thereof. BACKGROUND

[0002] Radiation refrigeration technology can achieve refrigeration effect without consuming any energy, and it realizes spontaneous cooling by strongly reflecting sunlight (0.28-2.5 μm) and radiating heat to cold outer space through the atmospheric long-wave infrared transmission window (8-13 μm), which provides a sustainable means to solve the greenhouse effect and energy crisis. Compared with broadband radiation refrigeration materials, selective radiation refrigeration materials usually have low emissivity in the remaining infrared waveband (2.5-8, 13-25 μm), which can effectively avoid atmospheric radiation in the non-window waveband and radiation from the ground and surrounding buildings, and can achieve lower refrigeration temperature and better all-weather refrigeration effect.

[0003] The selective radiation refrigeration materials reported in the literature mainly include nanofiber films, photonic multilayer films and inorganic hybrid structures, wherein:

[0004] The spectral selectivity (the ratio of the emissivity in the 8-13 μm waveband to the emissivity in the 2.5-25 μm waveband) of the polymeric nanofiber film (Kyuin Park * Kyuin Park Department of Human Centered Design, College of Human Ecology, Cornell University, Ithaca, New York, United States * Email: emailprotected More by Kyuin Park https: / / orcid.org / ---, Park K, Emailprotected E, et al. Designing an Effective and Scalable UV-Protective Cooling Textile with Nanoporous Fibers [J]. [2024-01-02].) is as high as 1.67, but it has high transmission characteristics in the non-atmospheric window waveband, and cannot effectively reflect the radiation of the surrounding environment, so it is not suitable for practical application in buildings and other scenarios;

[0005] Metal-dielectric photonic multilayer films (Raman, Aaswath P, Rephaeli, et al. Passive radiative cooling below ambient air temperature under direct sunlight. [J]. Nature, 2014. DOI: 10.1038 / nature13883.) and inorganic hybrid structures (Lin Y, Qin C, Fang L, et al. Colored Polymeric Films with a Bilayer Porous Design for Efficient Subambient Radiative Cooling[J]. [2024-01-02].) usually require a metal reflective layer to achieve high reflectivity in the solar or infrared waveband, which is usually composed of multiple layers, and the structure and process are relatively complex;

[0006] In addition, the spectral selectivity of metal-dielectric photonic multilayer films and inorganic hybrid structures is relatively limited (1.48 and 1.46, respectively), and the cooling potential needs to be further improved. SUMMARY

[0007] In view of the problems that the spectral selective radiation cooling materials in the prior art are not prominent in spectral selectivity, the structure and process are complex, and the cost is high, the present application provides a radiation cooling ceramic material with spectral selectivity and a preparation method thereof, specifically a gradient porous magnesium oxide (MgO) ceramic material, and the parameters such as the pore size and porosity of the ceramic are optimized by calculation, and finally the ceramic is prepared by pressureless sintering process. The present application solves the problems of the prior art that the spectral selective radiation cooling material is not prominent in selectivity, the structure and process are complex, and the cost is high, and promotes the practical application of the spectral selective radiation cooling material in the field of building.

[0008] The working principle of the present application is as follows:

[0009] The surface structure of the gradient porous MgO ceramic is dense, and contains many randomly distributed air pores inside, the porosity increases from the surface to the inside in a gradient manner, and remains relatively stable at a certain depth.

[0010] In the 0.28-8 μm waveband, the ceramic shows high reflectivity due to the strong scattering effect of the internal pores;

[0011] In the 8-13 μm waveband, the intrinsic multi-phonon absorption effect of the MgO ceramic provides strong emission ability;

[0012] In the 13-25 μm band, the residual reflection effect of the dense MgO ceramic surface makes the ceramic show strong reflection again.

[0013] In the pressureless sintering process, due to the difference between the surface and internal surface energy of the ceramic sheet, the internal surface of the ceramic has more pores after shrinkage and densification, and the preparation of a gradient porous structure can be realized.

[0014] The structure design process of the present application is as follows:

[0015] 1) The scattering efficiency of pores with different pore sizes in the MgO medium is calculated using the Mie scattering theory, and the scattering efficiency of the pores increases with the increase of the pore size, and when the pore size increases to 0.1 μm, a strong scattering effect can be maintained in the whole 0.28-8 μm;

[0016] 2) Combined with the Chandrasekhar radiation transfer theory, the spectral reflection curve of the ceramic under different pore volume fractions and pore sizes is calculated, and the reflectivity R solar and the emissivity E LWIR of the ceramic in the solar band and the infrared atmospheric window band are calculated according to the spectral curve respectively; when the thickness is constant, the R solar and E LWIR of the ceramic have opposite corresponding trends with the change of the pore size and the porosity, and when the pore size is 0.1-1 μm and the porosity is 5%-15%, a ceramic structure with strong solar reflection and infrared window radiation can be obtained;

[0017] 3) For the spectrum of the ceramic at the position of the residual reflection band, since the pore size (0.1-1 μm) calculated in the previous step is smaller than the reference wavelength, the influence of the pore size is ignored, and the equivalent medium theory (EMT) is used for approximate calculation; with the increase of the pore volume fraction, the intensity of the residual reflection band of the ceramic gradually decreases from 13 μm; in addition, for the dense MgO structure, the maximum penetration depth of the infrared radiation in the 13-25 μm band does not exceed 20 μm.

[0018] In order to obtain the best spectral selection characteristics, the calculation results show that:

[0019] When the gradient porous ceramic material has a dense surface with a depth of at least 20 μm, and contains air pores with a porosity of 5%-15% and a pore size distribution of 0.1-1 μm in the interior, the best spectral selection characteristics can be obtained under the premise of maintaining high solar reflection and high infrared atmospheric window emission.

[0020] The purpose of the present application is achieved by the following technical scheme:

[0021] A preparation method of a radiation refrigeration ceramic material with spectral selectivity, comprising the following steps:

[0022] 1) Grinding: weigh the light MgO into a mortar and grind, make sure there is no lump, then pour the ground powder into a mold and vibrate to make the surface relatively flat;

[0023] 2) Tabletting: put the mold of the above step into a tabletting machine, apply a pressure of 60-80 MPa and keep for 1-3 min, then demold to obtain the MgO green body;

[0024] 3) Sintering: put the MgO green body obtained in the above step into a muffle furnace for pressureless sintering, first heat to 150℃ at a rate of 10℃ / min, keep for 20-30 min to remove water, then heat to 800℃ at a rate of 10℃ / min, pre-sinter for 20-30 min, finally heat to 1500℃ at a rate of 5℃ / min, keep for 30-60 min for densification sintering, to obtain a spectral selective radiation cooling ceramic material, which is a gradient porous MgO ceramic material, the gradient porous ceramic material has a dense surface of at least 20μm deep, contains air pores with a porosity of 5%-15% and a pore size distribution of 0.1-1μm in the interior, has a solar reflectivity of more than 0.90 and an infrared window emissivity, at the same time, the spectral selectivity is more than 1.65, and the average contact angle is 43°.

[0025] In the present application:

[0026] Further, the weighing in step 1) is performed by using a balance.

[0027] Further, the light MgO in step 1) is analytical pure (AR) with a purity of ≥98.5%.

[0028] Further, the ground powder is poured into a mold with a diameter of 40mm and a powder mass of 6-10g for vibration in step 1).

[0029] The present application also relates to a spectral selective radiation cooling ceramic material obtained by the preparation method of the above spectral selective radiation cooling ceramic material, which is a gradient porous MgO ceramic material, the gradient porous ceramic material has a dense surface of at least 20μm deep, contains air pores with a porosity of 5%-15% and a pore size distribution of 0.1-1μm in the interior, has a solar reflectivity of more than 0.90 and an infrared window emissivity, at the same time, the spectral selectivity is more than 1.65, and the average contact angle is 43°, which is the best spectral selective radiation cooling material in terms of comprehensive spectral performance at present, and has the strongest cooling potential.

[0030] Compared with the prior art, the present application has the following advantages:

[0031] 1. The radiation cooling ceramic material with spectral selectivity has a gradient porous MgO ceramic prepared by a pressureless sintering process, the material structure is simple, the process is simple, the cost of the spectral selective radiation material is greatly reduced, and the practical application of the spectral selective radiation cooling material in the building field is promoted.

[0032] 2. The radiation cooling ceramic material with spectral selectivity has a gradient porous MgO ceramic prepared by a pressureless sintering process, the material structure is simple, the process is simple, the cost of the spectral selective radiation material is greatly reduced, and the practical application of the spectral selective radiation cooling material in the building field is promoted.

[0033] 3. The radiation cooling ceramic material with spectral selectivity has a gradient porous MgO ceramic prepared by a pressureless sintering process, the material structure is simple, the process is simple, the cost of the spectral selective radiation material is greatly reduced, and the practical application of the spectral selective radiation cooling material in the building field is promoted. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a structural schematic diagram of the radiation cooling ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application;

[0035] Figure 2 is a photograph of the radiation cooling ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application;

[0036] Figure 3 is a CT scanning porosity change diagram with depth of the radiation cooling ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application;

[0037] Figure 4 is an EBSD test result diagram of the radiation cooling ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application;

[0038] Figure 5 is a cross-section SEM photograph diagram of the radiation cooling ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application;

[0039] Figure 6 is a spectral property curve diagram of the radiation cooling ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application;

[0040] Figure 7is a mechanical property test result graph of the radiation refrigeration ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application;

[0041] Figure 8 is a water contact angle graph of the radiation refrigeration ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1 of the present application. DETAILED DESCRIPTION

[0042] The present application is further described in detail by the following examples, but these examples should not be considered as limiting the present application.

[0043] Example 1:

[0044] A method for preparing a radiation refrigeration ceramic material with spectral selectivity, comprising the following steps:

[0045] 1) Grinding: 8g of light MgO (AR, ≥98.5%) was weighed into a mortar and ground, and the ground powder was poured into a mold with a diameter of 40mm and vibrated to ensure the surface of the powder was relatively flat;

[0046] 2) Tabletting: The mold was placed in a tabletting machine, a pressure of 60MPa was applied and held for 1min, and then demolded to obtain a MgO green body;

[0047] 3) Sintering: The MgO green body was placed in a muffle furnace for pressureless sintering, first heated to 150℃ at a rate of 10℃ / min for 20min to remove water, then pre-sintered at 800℃ for 20min at a rate of 10℃ / min, and finally densified sintered at 1500℃ for 45min at a rate of 5℃ / min, finally obtaining a gradient porous MgO ceramic.

[0048] The optical photograph of the gradient porous MgO ceramic obtained by pressureless sintering in this example is shown in Figure 2 The diameter of the ceramic sheet is 30mm and the thickness is 3mm.

[0049] Figure 1 is a structural schematic diagram of the radiation refrigeration ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1;

[0050] Figure 2 is an optical photograph of the radiation refrigeration ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1;

[0051] Figure 3 is a graph of the change of porosity of the CT scanned ceramic with depth of the radiation refrigeration ceramic material with spectral selectivity (gradient porous MgO ceramic) in experimental example 1;

[0052] from Figure 3The CT scan results show that the porosity of the ceramic increases gradually from the surface to the interior, and reaches a relative level after a depth of about 50 μm.

[0053] Figure 4 The image shows the EBSD test results of the spectrally selective radiation-cooled ceramic material (gradient porous MgO ceramic) in Experiment Example 1 [(A) Surface; (B) Cross section (the colored area in the figure represents MgO grains, and the white area represents pores];

[0054] Depend on Figure 4 The EBSD quantitative test results show that the porosity of the surface and interior of the gradient porous ceramic is 4.1% and 12.0%, respectively.

[0055] Figure 5 This is a cross-sectional SEM image of the spectrally selective radiation-cooled ceramic material (gradient porous MgO ceramic) in Experiment Example 1.

[0056] right Figure 5 SEM scan images of the ceramic cross-section were used to statistically analyze the pore size distribution inside the ceramic. The results showed that the average pore diameter was 0.5 μm, with a single peak distribution centered on this pore.

[0057] Figure 6 This is a spectral characteristic curve of the radiation-cooled ceramic material (gradient porous MgO ceramic) with spectral selectivity in Experiment Example 1 (the yellow shaded area in the figure is the AM1.5 solar radiation spectrum, and the blue shaded area is the atmospheric transmittance spectrum).

[0058] Figure 6 The spectral characteristic curve of the ceramic is shown, with a reflectivity of up to 0.96 in the solar band and an emissivity of up to 0.95 in the long-wave infrared atmospheric window band. At the same time, the selectivity index is as high as 1.69, making it the radiation cooling material with the most outstanding spectral selectivity performance to date.

[0059] Figure 7 The mechanical property test results of the spectrally selective radiation-cooled ceramic material (gradient porous MgO ceramic) in Experiment Example 1 are shown in the figure [(A) flexural strength curve; (B) compressive strength curve].

[0060] Figure 7 The mechanical properties of the ceramic were tested, showing a flexural strength of 340 MPa, an elastic modulus of 107 GPa, and a hardness as high as 5.35 GPa, which meets the basic mechanical performance requirements for building roof or exterior wall materials.

[0061] Figure 8 This is a water contact angle diagram of the spectrally selective radiation-cooled ceramic material (gradient porous MgO ceramic) in Experiment Example 1.

[0062] Figure 8The average contact angle of the refrigeration ceramic is 43°, and the ceramic has good hydrophilicity. At the same time, due to the high surface density of the ceramic, the water absorption rate is almost zero, which means that water can easily spread on the surface of the ceramic. The completely spread water film can easily evaporate under the action of wind and sunlight, and the impurities attached to the surface can be removed, so as to realize the self-cleaning function.

[0063] Example 2:

[0064] A preparation method of a radiation refrigeration ceramic material with spectral selectivity, comprising the following steps:

[0065] 1) Grinding: weigh 6g of light MgO (AR, ≥98.5%) and put it into a mortar for grinding, and then pour the ground powder into a mold with a diameter of 40mm and vibrate it to ensure that the surface of the powder is relatively flat;

[0066] 2) Tabletting: put the mold of the above step into a tabletting machine, apply a pressure of 60MPa and keep it for 1min, and then demold to obtain a MgO green body;

[0067] 3) Sintering: put the MgO green body obtained in the above step into a muffle furnace for pressureless sintering, first heat to 150℃ at a rate of 10℃ / min, keep for 30min to remove water, then heat to 800℃ at a rate of 10℃ / min, pre-sinter for 30min, finally heat to 1500℃ at a rate of 5℃ / min, keep for 60min for densification sintering, and finally obtain a radiation refrigeration ceramic material with spectral selectivity.

[0068] Results:

[0069] The radiation refrigeration ceramic material with spectral selectivity obtained in Example 2 has a reflectivity of 0.93 in the solar wave band and an emissivity of 0.91 in the long-wave infrared atmospheric window wave band, and the selectivity index is 1.77;

[0070] The mechanical property test results of the ceramic material show that the bending strength is 352MPa, the elastic modulus is 112GPa, and the hardness is as high as 5.38GPa;

[0071] The average contact angle of the refrigeration ceramic is 42°.

[0072] Example 3:

[0073] A preparation method of a radiation refrigeration ceramic material with spectral selectivity, comprising the following steps:

[0074] 1) Grinding: weigh 6g of light MgO (AR, ≥98.5%) and put it into a mortar for grinding, and then pour the ground powder into a mold with a diameter of 40mm and vibrate it to ensure that the surface of the powder is relatively flat;

[0075] 2) tabletting: the mold of the previous step is put into a tablet press, a pressure of 80 MPa is applied and the pressure is maintained for 1 min, and then the MgO green body is obtained by demolding;

[0076] 3) sintering: the MgO green body obtained in the previous step is placed in a muffle furnace for pressureless sintering, first heated to 150℃ at a rate of 10℃ / min, and then heated to 800℃ at a rate of 10℃ / min for 25 min of pre-sintering, and finally heated to 1500℃ at a rate of 5℃ / min for 45 min of densification sintering, and finally a spectral selective radiation cooling ceramic material is obtained.

[0077] Results:

[0078] The spectral selective radiation cooling ceramic material obtained in Example 3 has a reflectivity as high as 0.95 in the solar waveband, an emissivity as high as 0.94 in the long-wave infrared atmospheric window waveband, and a selectivity index of 1.72.

[0079] The mechanical properties test results of the ceramic material show that the bending strength is 332 MPa, the elastic modulus is 103 GPa, and the hardness is as high as 5.28 GPa.

[0080] The average contact angle of the cooling ceramic is 44°.

[0081] Comparative Example 1:

[0082] Comparative Example 1 and Example 1 differ in that the densification sintering temperature of step 3) is 1400℃, and the others are the same as Example 1.

[0083] Results:

[0084] The spectral selective radiation cooling ceramic material obtained in Example 3 has a reflectivity as high as 0.97 in the solar waveband, an emissivity as high as 0.93 in the long-wave infrared atmospheric window waveband, and a selectivity index of 1.38.

[0085] The mechanical properties test results of the ceramic material show that the bending strength is 113 MPa, the elastic modulus is 33 GPa, and the hardness is 1.34 GPa.

[0086] The average contact angle of the cooling ceramic is 48°.

[0087] Comparative Example 2:

[0088] Comparative Example 2 and Example 1 differ in that the mass of the MgO powder in step 3) is 5g, and the others are the same as Example 1.

[0089] Results:

[0090] The radiation cooling ceramic material with spectral selectivity obtained in Embodiment 3 has a reflectivity of 0.93 in the solar wave band, an emissivity of 0.89 in the long-wave infrared atmospheric window wave band, and a selectivity index of 1.79;

[0091] The mechanical property test result of the ceramic material shows that the bending strength is 347 MPa, the elastic modulus is 112 GPa, and the hardness is as high as 5.41 GPa;

[0092] The average contact angle of the cooling ceramic is 43°.

[0093] The results show that:

[0094] 1. Through the comparison of Embodiments 1-3 and Comparative Example 1, it is shown that the densification sintering temperature has an important influence on the spectral selectivity and mechanical property of the cooling ceramic. In order to achieve a spectral selectivity of more than 1.65 and improve the mechanical property of the ceramic, the densification sintering temperature must be higher than 1400℃.

[0095] 2. Through the comparison of Embodiments 1-3 and Comparative Example 2, it is shown that the mass of the MgO powder has an obvious influence on the emissivity of the cooling ceramic in the infrared window wave band. In order to achieve an infrared window emissivity of more than 0.90, the mass of the MgO powder must be more than 5g.

[0096] The above is only a preferred embodiment of the present application, and the protection scope of the present application is not limited to the above-mentioned embodiments. Various process schemes without substantial difference from the concept of the present application are within the protection scope of the present application.

Claims

1. A method for preparing a spectrally selective radiation-cooled ceramic material, characterized in that: Includes the following steps: 1) Grinding: Weigh out light MgO and grind it in a mortar to ensure there are no lumps. Then pour the ground powder into a mold and compact it to ensure that the powder surface is relatively flat. 2) Tableting: Place the mold from the previous step into the tablet press, apply a pressure of 60-80MPa and hold the pressure for 1-3 minutes, then demold to obtain MgO green embryo; 3) Sintering: The MgO green obtained in the previous step is placed in a muffle furnace for pressureless sintering. First, the temperature is raised to 150℃ at a rate of 10℃ / min and held for 20-30 min to remove moisture. Then, the temperature is raised to 800℃ at a rate of 10℃ / min for pre-firing for 20-30 min. Finally, the temperature is raised to 1500℃ at a rate of 5℃ / min and held for 30-60 min for densification sintering, resulting in a spectrally selective radiation-cooled ceramic material, which is a gradient porous MgO ceramic material. The gradient porous ceramic material has a dense surface at least 20μm deep, contains air pores with a porosity of 5%-15% and a pore size distribution of 0.1-1μm, has a solar reflectance and infrared window emissivity of ≥0.90, and a spectral selectivity of ≥1.65, with an average contact angle of 43°.

2. The method for preparing a spectrally selective radiation-cooled ceramic material according to claim 1, characterized in that: The weighing described in step 1) is done using a balance.

3. The method for preparing a spectrally selective radiation-cooled ceramic material according to claim 1, characterized in that: The light MgO mentioned in step 1) is of analytical grade with a purity ≥ 98.5%.

4. The method for preparing a spectrally selective radiation-cooled ceramic material according to claim 1, characterized in that: Step 1) involves pouring the ground powder into a mold and compacting it. The mold has a diameter of 40mm, and the powder weighs 6-10g.

5. A radiation-cooled ceramic material with spectral selectivity, characterized in that: The method described in any one of claims 1-4 for preparing a spectrally selective radiation-cooled ceramic material results in a gradient porous MgO ceramic material. This gradient porous ceramic material has a dense surface at least 20 μm deep, contains air pores with a porosity of 5%-15% and a pore size distribution of 0.1-1 μm, exhibits a solar reflectance and infrared window emissivity of 0.90 or higher, a spectral selectivity of 1.65 or higher, and an average contact angle of 43°.

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