Research On Magnesium Phosphite Crystal For Radiative Cooling Material

The proportion that China’s building energy consumption accounts for the world’s total building energy consumptiona has been increasing over the last decades.It has become a new research hotspot to realize building cooling and energy saving by radiative cooling.In this thesis,a variety of phosphite crystal materials were synthesized by hydrothermal method,Mg₁₁（HPO₃）₈（OH）₆ was found to have high UV-Vis-NIR reflectivity and high infrared emission properties at the same time.The crystal structure,chemical composition,microstructure and related properties of samples were characterized through a variety of testing methods such as XRD,SEM,FT-IR and so on.The explanations of Mg₁₁（HPO₃）₈（OH）₆ having high reflectivity and emissivity were proposed.The radiation cooling coating with Mg₁₁（HPO₃）₈（OH）₆ as the functional filler was fabricated and its actual cooling effect has been tested.The main results are as follows:（1）The solar reflectivity of magnesium phosphite mixed phase in the solar spectrum（200²500nm）is as high as 94.60%,the emissivity in the infrared region（1²2?m）is as high as 0.93,which means magnesium phosphite is an ideal functional filler for radiation cooling coating.（2）The pH value is the main factor for the generation of two kinds of crystal structure of magnesium phosphite.Mg₁₁（HPO₃）₈（OH）₆ phase appears when 7<pH<8;Mg（HPO₃）（H₂O）phase disappears when 8<pH<9.As pH value increased from 5 to 10,the particle morphology changed gradually,from the composed of nanowires into similar flower shape with nanowires along the radial direction,and into a sphere with nanowires structure covered on surface,and then into the dumbbell shape with the nanowires structure in both ends,and then into similar stub shape with one smooth end and nanowires distributed at the other end,When pH increased to 10,acicular structure almost disappeared,the particles were short rod,the solar spectrum reflectivity and infrared emissivity would reach their maximum,respectively,95.19%,0.933.（3）The optical energy band gap of Mg₁₁（HPO₃）₈（OH）₆ crystal is 6.06 eV,which is greater than the visible and near infrared light absorption line 3.1 eV,and particles with nanowire structure meet the conditions of Rayleigh scattering,multilayer rule arrangement nanowires structure increases the effective scattering area,causing Mg₁₁（HPO₃）₈（OH）₆ crystal to have high reflectivity performance;Mg₁₁（HPO₃）₈（OH）₆ crystal contains a variety of molecular bonds which have IR absorption activity,and[HPO₃]tetrahedron asymmetry is better than[PO₄]tetrahedron,lattice vacancy and a variety of molecular bond length lead to high lattice stress,results in the high emissivity of Mg₁₁（HPO₃）₈（OH）₆ crystal.（4）The radiative cooling coating was fabricated using Mg₁₁（HPO₃）₈（OH）₆ crystal as the functional filler.The cooling test in the strong sunlight showed that the temperatures in space below Mg₁₁（HPO₃）₈（OH）₆ coating were 2⁴°C lower than those below the coatings based on commercial solar heat reflection material TiO₂ and CaCO₃,and 6⁸°C lower than that below Al-foil.Mg₁₁（HPO₃）₈（OH）₆ crystal coating possesses radiative cooling power of 45.36 W/m²in the sunlight in an ideal condition.