Preparation Of Spectral Selective Materials And Study On Daytime Radiative Cooling Performance

Radiative cooling,a current research hotspot,can reduce the energy consumption of active cooling methods and alleviate the urban heat island effect and greenhouse effect.The efficiency of radiative cooling is limited by the spectral properties of the materials,and there is still a certain gap compared with active cooling methods.In order to solve the problem of low mid-wave infrared reflectance of the disordered porous cooling film and improve the radiative cooling performance of the material,this paper carried out the preparation of bismuth oxychloride（BiOCl）mid-infrared reflective filler and the study of its hybrid performance with the Poly(vinylidene fluoride-cohexafluoropropene（PVDF-HFP）porous film.The main work and conclusions are as follows:（1）BiOCl was synthesized by hydrothermal method.By studying the feed ratio,reactant concentration,hydrothermal reaction temperature,hydrothermal reaction time,amount of additive,calcination temperature and calcination time,the synthesis mechanism of hydrothermal reaction was discussed,and different morphologies were obtained.The relationship between morphology and spectral characteristics were studied.Excessive bismuth source promotes nucleation,and excessive chlorine source promotes growth.The role of additive is to promote nucleation,and reduces the thickness of the nanosheets,then induces the self-assembly of thin nanosheets into spherical powders,which has the most significant impact on the morphology.When the feed ratio of bismuth source and chlorine source is（4～5）:5,chlorine source concentration is 5 mmol/30 m L,concentration of additive ascorbic acid is 0.6 g/30 m L,hydrothermal reaction at 200°C for 3 h,calcination at 400°C for 3 hours,the R2.5?8μm of the BiOCl microspheres obtained reaches 70.17%,and it has excellent sunlight reflection ability（R240?2500 nm=89.09%）.Reducing the proportion of bismuth source,increasing the concentration of reactants,extending the hydrothermal reaction time or calcination time will cause the powder particle size to become coarser,the sphericity becomes worse.These changes are not good for reflecting sunlight,but can further improve the mid-wave infrared reflectance,R2.5?8μm can reach up to 78.16%.Without additive,the flake-shaped BiOCl can be obtained directly,with R2.5?8μ_m=84.21%.（2）The controllable preparation of BiOCl reflective fillers with specific morphology is achieved by changing the amount of VC and the concentration of reactants.Finally,the maximum particle size of the flake powder increased from 2μm to 8μm,and the mid-wave infrared reflectance of 84.73%is obtained.The morphology has a great influence on the band gap.The transition of the powder from flake to spherical is the main reason for the increase in the band gap.The band gap increases from 2.83 e V to 2.91 e V,but there is still a gap from the theoretical band gap（3.4 e V）,which leads to UV ray absorption.Increasing the amount of VC can obtain earth-colored fillers.R₇₆₀?_{2500 nm} of Color?2 reaches 67.90%,showing the potential for radiant cooling,and also has the high reflectivity of mid-wave infrared（R2.5?8μm=59.25%）.It is a visible light-infrared stealth that takes into account thermal management.（3）The preparation of the BiOCl/PVDF-HFP porous hybrid membrane was studied,and a better structure scheme was determined.On this basis,the influence of filler morphologies on the spectral behavior and radiation cooling performance of the hybrid film was investigated.The hybrid membrane doped with 60%flake BiOCl filler and the pure PVDF-HFP porous film are superimposed according to the thickness ratio of 1:4.Compared with porous film,hybrid membrane retains the sunlight reflection ability and the infrared radiation ability in the window,and the mid-wave infrared reflectance increases from 48.36%to 58.10%.The radiative cooling temperature at night in a high-humidity environment reaches 3.2°C,which is better than spherical powder hybrid membrane,and better than pure porous membrane（2.6°C）.It proves that BiOCl,acted as mid-wave infrared reflective filler,can optimize spectral selectivity and the importance of selective radiation in radiative cooling materials.