| The“Urban Heat Island Effect?UHIE?”is one of the most obvious characteristics of urban climate.One of the reasons for such effect is that dense artificial structures absorb a large amount of solar radiation,causing the surface temperature too high to radiate heat to the atmosphere.In order to reduce the impact of high temperature,cooling methods such as air conditioning are widely used,and the surging energy consumption in turn further aggravates“UHIE”.Passive cooling materials that reduce heat absorption by high reflection of solar radiation and radiate heat through atmospheric window?813mm?into outer space?3 K?effectively cool the material,opening up the possibility of mitigating the“UHIE”.In this paper,hydrothermal method was used to synthesize modified magnesium phosphite crystal by adding fluorine and calcium ions.By comparing the solar spectrum reflectance and infrared emissivity of crystal powders,the new crystal Ca0.5Mg10.5?HPO3?8?OH?3F3?labed CFM?with the best comprehensive performance was selected.The phase and crystal structure of CFM were characterized by XRD,FTIR and TG-DSC.The theoretical analysis of the high solar spectral reflectance of CFM samples was carried out by energy band theory,micromorphology and particle size analysis.Rietveld refinement,FTIR absorption spectrum described the microstructure of CFM.The high emission mechanism of the sample was explained from the aspects of infrared absorption of molecular bonds and lattice defects.The CFM sample was used as the functional filler,and the single variable method was used to determine the type of film former,the optimum filler particle size and the coating thickness of the CFM functional coating.Through the evaluation of the cooling effect of CFM coating under three types of cooling devices,the most reasonable test device and condition were selected,and valuable cooling evaluation indicators were obtained.The conclusions of the full text study are as follows:?1?When studying the influence of the addition amount of F-and Ca2+on the reflectivity and emissivity of the modified sample,it is concluded that the CFM crystal obtained under the ratio of x=3,y=0.5.CFM has the best comprehensive performance with solar spectrum reflectance of above 0.98 and broad-spectrum infrared emissivity of 0.920.?2?The CFM sample has a forbidden band width of 5.75 eV,which indicates that the ultra-wide band gap makes it absorb sunlight as little as possible.SEM photograph shows that the special micro-nano structure and size of CFM powder particles satisfy the condition of Rayleigh and Mie double scattering.The strong scattering between sunlight and particles further enhances the reflection ability of CFM crystal.The high infrared emissivity of CFM samples is mainly due to the infrared absorption of the abundant molecular bonds.In addition,structure distortion caused by doping and defects like vacancies cause lattice stress,thereby enhancing the infrared emission performance of the sample.?3?When investigating the influence of the film former type,the particle size and the coating thickness on the reflection and emission properties of the coating,it is obtained that?a?the overall performance of the CFM coating is the best obtained by PVDF;?b?the larger the filler particle size is,the higher the reflectivity of the coating is,and but the effect of particle size on emissivity is not obvious;?c?the thicker the coating is,the higher the reflectivity and the emissivity of the coating are.Under the same conditions,the reflection and emission properties of CFM coating are better than those of commercial white coating and TiO2 coating.?4?When the solar irradiance is close to 900 W/m2 at noon time in summer in real environment,the surface temperature of CFM coating is 5.1? lower than the ambient temperature,and the maximum cooling power is close to 86 W/m2.The maximum temperature difference is over 4? at night.The maximum temperature difference between the surface temperature of CFM coating and the ambient temperature can reach 2.3? during winter daytime,and the temperature is nearly 2? lower than the ambient temperature at night. |
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