Preparation Of YAlO₃ Red Cool Pigments Based On Defect Engineering And Their Research On Solar Thermal Regulation Performance

With the development of society,mankind has to face the problem of energy shortage due to the drastic increase in energy consumption.In China,building energy consumption accounts for more than 23%of total social energy consumption,of which cooling energy consumption for air conditioning accounts for about 50%.Therefore,reducing the energy consumption of building cooling is of great significance to social energy conservation and solving the problem of energy shortage.Both cool roof technology and radiative cooling technology can effectively mitigate the urban heat island effect and can also become effective tools to reduce building energy consumption.The materials used in these two technologies should have high solar reflectance and infrared emissivity（atmospheric window 8-14 μm）,and should also have rich colors to meet the aesthetic requirements of architectural appearance.Color inorganic cool pigments with excellent solar reflectance performance and high infrared emissivity are very suitable for use as the substrate of these two building refrigeration technologies,and the development of such inorganic pigments will also play a positive role in promoting the practical application and development of building refrigeration energy-saving technology.In addition,rich colors can be obtained by ion doping and other methods,and its solar thermal control performance can be improved.On this basis,the mechanism of ion doping on the structure and properties of pigments is studied in depth,which can provide a theoretical basis for further directional regulation of pigment structure and properties.Perovskite-type oxides have a special structure,and there is a large selection space for dopant ions,so it is convenient to control their chemical defects.Cr-doped YAlO₃ has been developed as red ceramic pigments,and its chemical defects can be regulated by ion doping,so as to gradually improve the solar thermal regulation performance of this series of pigments,which has a very positive significance for the development of red inorganic cool pigments.In this paper,perovskite YAlO₃ was selected as the matrix,and the chemical defects in YAlO₃-based powder pigments were effectively regulated by doping the Y3+or Al3+positions with equivalent or non-equivalent elements,so as to optimize the color and solar reflectance properties of the pigments.By co-precipitation and hightemperature solid-state method,a series of YAlO₃-based cool red pigments with different chemical defects and different optical properties were obtained.The influence mechanism of chemical defects on pigment color properties and solar reflectance properties were explored,and the optical properties of pigments were gradually optimized on this basis.The feasibility of the prepared pigment as cool pigments in the practical applications on building roofs or facades is verified.The main research results of this paper are as follows:（1）CaCO₃ and Na2B4O7 assist the preparation of YAl1-xCrxO3 red pigment and the regulation of chemical defects.When CaCO₃ is introduced,Ca2+replaces Y3+to form a negatively charged Ca’Y defect,which leads to an increase in high-valent chromium[CrAl`（Cr4+）,CrAl…（Cr6+）]and oxygen vacancies（VO¨）.The high-valent chromium promotes the absorption of visible light below 600nm,changing the pigment from light pink to bright red.Due to the tailing absorption between 600～1000nm of the oxygen vacancies（VO¨）,the brightness value（L*）and near-infrared reflectivity（RNIR）of the pigment are reduced.The addition of mineralizer Na2B4O7 further improves the color properties of powder pigments.By regulating the addition of CaCO₃,Na2B4O7 mineralizer and the doping amount of Cr,the color rendering and solar reflectance properties of YAlO₃-based powder pigments were comprehensively improved.In addition,in cooling simulation experiments,pigment-based coatings showed much better cooling performance than cement coatings and commercial Fe2O3-based pigment coatings.And the pigment coating has a high infrared emissivity（0.952）in the 8-14 μm band.（2）Mg/Cr co-doped YAlO₃ red pigment was prepared by double-doping of Y3+and Al3+by Mg2+,and the regulation of the chemical defects.Mg2+enters the crystal lattice to form a chemical defect Mg’Al/Mg’Y,which further promotes the formation of highvalence Cr ions,thereby inhibiting the generation of oxygen vacancies（VO¨）to a certain extent and reducing the negative impact of VO¨ on the optical properties.By further regulating the chemical defects in the YAlO₃ matrix,the color properties and solar reflectance properties of the pigments were successfully optimized.By further regulating the doping amount of Cr and Mg in the YAlO₃ matrix,the near-infrared reflectivity of the pigment can reach 89.9%,and the color rendering effect can be improved.In the outdoor direct sunlight simulation test,the temperature of the prepared red coating was 10.5℃and 7.9℃ lower than that of blank cement and commercial Fe2O3 coating,respectively.The prepared pigment coating also has an infrared emissivity greater than 0.97 in the 814 μm band,which allows the temperature of the pigment coating surface to be 2.1℃lower than the ambient temperature at night.（3）Preparation of Ce ion synergistic Mg ion doped YAl1-xCrxO3 red cool pigments and their optical performance optimization.After the Ce ions enter the YAlO₃ lattice,CeY（Ce3+）and CeY·（Ce4+）defects coexist in the system.The formation of CeY（Ce4+）can regulate the content of CrAl·（Cr4+）and CrAl…（Cr6+）in the system and inhibit the formation of oxygen vacancies（VO¨）,which greatly increases the near-infrared reflectivity and solar reflectivity of pigments（RNIR>90%,Rsol>70%）,and the powder pigments can still maintain a good red hue（a*>20）.With the addition of the mineralizer Na2B4O7,the a*value of the pigments can be increased to 35.86,while maintaining excellent RNIT（>77%）and Rsol（>53%）,and the comprehensive optical properties of this series of pigments are significantly better than the red pigments prepared in the previous two systems.In outdoor simulation test,the single-layer coating made of this series of powder pigments showed excellent reflectance performance and night cooling effect under direct outdoor sunlight.In addition,the prepared pigment-TiO2 double-layer red coating showed much better RNIR and Rsol than the single-layer pigment coating on the basis of maintaining a certain color rendering effect.In the 8-14 μm band,the infrared emissivity of all prepared coatings is above 0.95.In summary,the effective regulation of chemical defects is realized by doping Cr,Ca,Mg,and Ce ions in Y3+/A13+positions of YAlO₃ lattice,and the influence mechanism of ion doping on the optical properties of pigments is established based on defect engineering,and the color properties of powder pigments and solar heat reflectance performance are gradually optimized.The prepared red cool coatings have certain feasibility in the actual application of the building roof or the exterior wall.