Study On The Regulation Of Magnesium Phosphate Emission Performance For Daytime Radiative Cooling

Objects always loss their heat through continuous infrared radiation to the surroundings.This is a passive cooling method with no energy consumption.Especially through the atmospheric window of 8-13?m,net heat loss from objects to outer space(?3K)can be realized.This is of great interest in saving energy for cooling,alleviating urban heat island effect,and thermal management in industrial and civil applications.Nighttime radiative cooling is easy,only requiring the material to have high infrared emissivity.However,daytime radiative cooling is more challenging,requiring not only high infrared emissivity,but also high reflectivity to the solar radiation.This places special requirements on the spectral characteristics of materials.Previous studies reported daytime radiative cooling suing super-structured materials to achieve the spectral control of the target band,but the complex and expensive preparation methods greatly limit the application of such novel materials.In this thesis propose using one single material to achieve daytime radiative cooling.We synthesized two new radiative cooling materials:magnesium hydrogen phosphate and magnesium pyrophosphate.The absorption of water molecules in the 2.5-8?m band is regulated so that the entire infrared band of the material exhibits broad-spectrum high emission(>2.5?m)and atmospheric window band(8-13?m)selective high emission.The cooling test device is designed to test the actual cooling effect,and the radiative cooling power is analyzed by theoretical evaluation and actual measurement of the two materials.The main research work is as follows:(1)According to the vibration absorption of water molecules in the range of 2.5-8?m and HPO₄^2-in the range of 8-13?m,these spectral characteristics are designed to synthesize magnesium hydrogen phosphate radiative cooling powder with crystal water.Three preparation methods were used to prepare hydrogen phosphate powder with broad-spectrum emission characteristics.The study found that magnesium hydrogen phosphate(Mg HPO₄·1.2H₂O)synthesized by hydrothermal method is a porous nano-level high emission radiative cooling material with broad spectrum characteristics.The emissivity in the atmospheric window is 0.94 and the reflectivity to the solar spectrum is 92.20%.In the infrared region,multiple P-O splitting peaks and P-O-H vibration absorption peaks in the range of 769-1250 cm^-1result in high emissivity of Mg HPO₄·1.2H₂O in the atmospheric window.(2)In order to obtain the cooling effect in the actual environment,the Mg HPO₄·1.2H₂O powder is prepared into a radiative cooling coating.The average reflectivity of the solar spectrum(0.3-2.5?m)of the Mg HPO₄·1.2H₂O coating is 96.13%,and the average emissivity of the atmospheric window(8-13?m)is 0.92.A 100?m thick Mg HPO₄·1.2H₂O coating was coated on the ceramic tiles and placed in a radiative cooling device.The coating achieve cooling below ambient temperature throughout the year.In addition,under direct sunlight,the Mg HPO₄·1.2H₂O coating is 8-9°C and 2-5°C lower than the Ca CO₃and Ti O₂coatings,respectively.After 500h UV aging and outdoor exposure,the Mg HPO₄·1.2H₂O coating has good optical indicators.(3)The water molecules in Mg HPO₄·1.2H₂O are removed by heat treatment at 500°C,700°C and 900°C,and amorphous and crystalline Mg₂P₂O₇with selective emission characteristics are synthesized.The Mg₂P₂O₇calcined at 700°C has obvious spectral selectivity,with an average reflectivity of 98.04%for the solar spectrum(0.3-2.5?m)and an average infrared emissivity of 0.92 in the atmospheric window(8-13?m).The increase in the lattice density causes the increase in the dielectric constant,which leads to the increase in the reflectivity,and the decrease in the emissivity may be due to the better symmetry of the PO₄tetrahedron,the reduction of the P-O-P splitting peaks and the disappearance of the P-O-H bond in the crystal structure.The Y-doped amorphous Mg₂P₂O₇has a gap-like regular morphology.This morphology increases the backscattering of sunlight through the Mie scattering effect,effectively increasing the solar spectral reflectance to 99.63%,and the Y-doped amorphous Mg₂P₂O₇powder appears selective.(4)The Mg₂P₂O₇radiative cooling coating with spectral selectivity was prepared by the coating machine and the electrostatic spinning method.Under the condition of low thickness,the coating prepared greatly reduces the amount of radiative cooling powder incorporated by the electrospinning method,and the reflectivity of the coating is increased by 4.74%compared with the coating machine method.The temperature measurement of the radiative cooling device shows that the Mg₂P₂O₇(700°C)coating is superior to the blank sample,the different calcination temperatures coatings and the functional coatings.However,after 500hours of outdoor exposure,multiple days of heavy rain caused a significant decrease in the reflectivity of the coating.(5)The theoretical power calculation and actual measurement of Mg HPO₄·1.2H₂O with broad-spectrum emission characteristics and Mg₂P₂O₇with selective emission characteristics are carried out.The theoretical cooling power of Mg HPO₄·1.2H₂O powder under direct sunlight and at night is 83.82 W/m²and 115.17 W/m²,respectively,and the theoretical cooling power of Mg₂P₂O₇powder is 85.69 W/m²and 83.74 W/m²,respectively.The power measurement device is designed by the feedback heating method,and the measured power is close to the theoretical power.However,the actual real power also needs to consider the influence of thermal mass,which results in the real power being less than the measured power.