| Radiative cooling is a new,clean cooling method that allows for spontaneous cooling without any additional energy input.Radiative cooling includes daytime cooling and nighttime cooling,while daytime radiative cooling is a hot spot and difficulty to overcome.In order to achieve daytime radiative cooling,the cooler needs to have both high solar spectral reflectivity and strong emissivity in the mid-infrared band.The usual radiative cooling devices are complex and costly,requiring sophisticated instrument processing and complex optical design.Most of them are limited to small-scale research in the laboratory.Therefore,we have developed and prepared aluminophosphate with a single material,low cost and green environmental protection as a new type of radiative cooling material.Aluminium phosphate is an inorganic non-metallic material with stable chemical properties,high temperature resistance,good thermal shock resistance and oxidation resistance.It has special spectral properties and has the potential to achieve radiative cooling with a single material.In this paper,various preparation methods of new aluminum phosphate radiative cooling materials are used.And the crystal structure,micromorphology,chemical environment and spectral properties of different crystal forms of aluminum phosphate were also studied.Besides,the mechanism of radiative cooling was discussed in detail.The following specific researches are carried out:（1）Amorphous aluminum phosphate is prepared by polyesterification of P2O5 and Al（NO3）3·9H2O in C2H5OH.It can maintain its amorphous structure heat treatment to 900℃,and the infrared emissivity can be maintained above 0.91.It has the potential to be applied in the field of high infrared radiation.This is because the amorphous state reduces the symmetry of material structure,making molecular vibration and rotation easier,thereby increasing infrared absorption and radiation.However,since the prepared sample is black,its solar reflectivity is not high and it can not be used as a radiative cooling material.（2）Using hydrothermal synthesis method,through the selection of template agent and the adjustment of HF content of mineralizer,three kinds of aluminum phosphate crystals with different molecular sieve structure can be obtained,and the difference in skeleton structure is relatively large:AlPO4-12 is a four-membered ring surrounded by four six-membered rings;AlPO4-5 is a huge twelve-ring channel composed of six six-membered rings and six four-membered rings;AlPO4-11 is a ten-ring tunnel composed of six six-membered rings and four four-membered rings.For the spectral properties,AlPO4-12 has the highest emissivity,and the emissivity in 813μm is 0.92,but its spectral selectivity is poor and the low reflectivity of 0.92.The reflectivity of AlPO4-11 is 0.97,but its emissivity is low,only 0.87.The reflectivity and the emissivity are in the antagonistic relationship.（3）Using the homogeneous precipitation method,after heat treatment at 1000℃,C-type aluminum phosphate can be obtained,the crystal structure of which belongs to the orthorhombic system,the solar reflectivity is higher,but emissivity is low,only 0.88.Therefore,the infrared radiation performance is improved by doping yttrium.The doping of yttrium converts the AlPO4 phase from orthorhombic to monoclinic system,and its emissivity is significantly increased to 0.93.The doping of yttrium leads to an improvement in emissivity for two reasons.One is that the doping of yttrium reduces the symmetry of the lattice vibration and obtains having a large cavity channel formed by six-membered ring.Another reason is that the reduction in grain size can enhance the infrared emissivity.（4）Direct precipitation method is used to prepare a T-type aluminum phosphate sample.The crystal structure of the sample is monoclinic system.The solar reflectivity is up to 0.97,and the highest emissivity in the atmospheric window is 0.90.Detailed mechanism analysis is also been carried out.It is found that T-AlPO4,with six-membered rings stacked on each other to form a huge cavity channel,these nondense cavities will affect the infrared absorption and radiation of the molecule vibration,thus having a higher emissivity.Moreover,the T-AlPO4 is a selective radiator,which has great application prospects in daytime cooling.（5）Aluminum phosphate coating is prerared and cooling test is also carried out.The best daytime cooling effect is:under direct sunlight,its maximum temperature can be lower than the ambient temperature by6℃,below 4.8℃than commercial paints.The daytime cooling power is 76 W/m2,which is similar to the theoretical calculation of 78 W/m2.Secondly,the influence of ambient temperature on the calculation of cooling power is discussed.The results show that the higher the ambient temperature,the greater the cooling power and the maximum cooling temperature difference.In addition,the coating is also applied to the tile surface,achieving a cooling effect of3℃lower than the blank tile and ambient temperature,and the measured daytime cooling power is 68 W/m2,also close to the theoretically calculated value of 65 W/m2. |
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