Study On Preparation And Radiation-cooling Properties Of SiO_ Coatings

Radiation cooling technology is a new cooling technology which uses infrared radiation to transfer heat from heat source to cold source into outer space through the atmospheric window of infrared radiation.Silica has significant phonon-polarized phonon resonance at 9.6μm.When monodispersed silica microspheres are randomly distributed in the medium to form optical metamaterials,and their electromagnetic resonance is stimulated intensively,they can produce nearly perfect absorption in the resonance region,which has great potential in the field of radiation refrigeration.Firstly,the controllable preparation methods of silica microspheres with different particle sizes were studied,and the influence of reactants and reaction conditions on the micromorphology of silica microspheres was revealed.Then the Si O₂/PU coating was prepared by spraying and scraping,and the effects of silica particle size,content,coating thickness,and reflective properties of the substrate on the performance of radiation cooling were studied.The radiation cooling coating was applied on the surface of cans to evaluate the cooling effect.The main research work and conclusions are as follows:（1）Silica microspheres of different nanometer and micron sizes（450nm～1200nm）with good monodispersity and regularity were prepared by modified Stober method,and the effects of reactants and reaction conditions on the particle size,monodispersity and regularity were studied.The experimental results showed that when the reactant ratio was distilled water:ethanol:ammonia:ethyl silicate=1:6:1:1,the reaction temperature was 35℃,and the reaction time was 5h,the silica particle size reached about 550nm,and the monodispersity and regularity were good.On this basis,appropriately increasing the amount of ethyl silicate and ammonia,increasing the reaction time,decreasing the reaction temperature and increasing the stirring rate can all increase the particle size of silica microspheres to different degrees,but also damage the monodispersity and regularity of microspheres to different degrees.When the reaction temperature was increased and the content of ethyl silicate was increased,the reaction temperature was increased and the ammonia concentration was increased at the same time,the two influence mechanisms worked in coordination.The increase of silica particle size was higher than that of the single condition,and the monodispersity and regularity were maintained.（2）A series of Si O₂/PU radiation coatings with different particle sizes,contents and thicknesses were prepared by spraying and scraping.The composite refrigeration coatings were constructed by coating Si O₂/PU radiation layer on Al/PU high reflection substrate,and the spectral reflection properties of different coatings were studied.Experimental results show that when the particle size of the silica microspheres is 300nm to 10μm microns,the content is 5wt.%～30wt.%,the radiation layer thickness is 5 microns～20 microns and the aluminum content is 30wt.%～70wt.%,the composite cooling coatings have low reflectivity（<10%）in 8～14μm band and high reflectivity（60%～75%）in the sun radiation zone.Among them,when the silica particle size of 1μm～10μm microns,the silica content was 10wt.%～15wt.%,the radiation layer thickness is 5 microns,aluminum powder content is30%,the coating will show better radiation cooling properties.（3）The actual temperature control performance of the coating was tested by some soda cans.The silicon dioxide radiation coating showed obvious radiation cooling effect.The maximum temperature drop of the coating was 2.5℃at 12:00 noon compared with the can without coating.For the composite coating of aluminum reflection layer and silicon dioxide radiation layer,the maximum temperature drop is 5.5℃ at 12:00 noon in autumn sunny days.