Controllable Preparation And Optical Properties Of Aerogel Hybrid Thick Films For Daytime Radiative Cooling

Radiative cooling refers to a new type passive cooling method that uses the infrared emission performance of the material itself to discharge the heat on the earth into outer space through the atmospheric window.If it can be applied to building cooling to replace the traditional air-conditioning refrigeration with heavy energy consumption,it will be of great significance to alleviate the current energy shortage situation.The infrared emittance of the material in the atmospheric window is an important indicator to measure its radiative cooling performance.The emittance of the ideal selective radiator can reach 1 in the atmospheric window and 0 in other bands.SiO₂ aerogel has been proven to have strong absorption in the infrared band and high infrared emittance within 8?13?m,which is a good choice of radiator materials.Based on this,this paper creatively composites nanoporous SiO₂ microspheres with highly transparent organic polymer polymethylpentene?TPX?to prepare SiO₂-TPX hybird thick films.The effects of the thickness of the hybrid film,the amount of SiO₂,and the particle size of the SiO₂ on the optical properties of the materials were studied and the preparation process corresponding to the highest infrared emittance was determined.Moreover,a simple and reliable equipment was built to test the radiative cooling performance of the hybrid thick films.On the basis of the SiO₂-TPX hybrid thick film,SiO₂-PDMS hybrid thick film was prepared,and methyl modified nanoporous SiO₂microspheres were used to effectively improve the radiative cooling performance.The main research contents and results are described as follows:?1?Preparation and optical properties of SiO₂-TPX hybrid thick film:SiO₂-TPX hybrid thick film was successfully prepared on the FTO glass substrate which is almost opaque in the atmospheric window by tape casting method.By ultrasonic dispersion and sealing drying,hybrid films with good quality and controllable thickness were obtained.Due to the crystalline properties of TPX,the morphology of the surface of the hybrid film shows a crystalline loose pore structure.The optical transmittance in the visible-near infrared band decreases with the increase of the film thickness and the SiO₂/TPX volumetric ratio,and with the decrease of the microsphere size.The emittance at 8-13?m increases with the increase of the film thickness and the SiO₂/TPX ratio and with the decrease of microsphere size.The film with 50?m thickness,5%SiO₂/TPX ratio and 8?m microspheres exhibits the highest emittance?up to 0.91?in the atmospheric window and the reflectance of 0.96 after silvering.The hybrid film demonstrates the lowest temperature in the diurnal test,about 20 ^oC lower than the black surface,12 ^oC lower than the Ag-coated glass and 8 ^oC lower than the FTO sample.In the nocturnal test,the maximum 4.5 ^oC reduction relative to the ambient temperature was observed.?2?Preparation and optical properties of SiO₂-PDMS hybrid thick film:a series of SiO₂-PDMS hybrid thick films were prepared on the glass substrate by tape casting.By ultrasonic dispersion and step-heating curing,the aggregation of SiO₂ microspheres and the generation of air bubbles in the film were avoided.The effect of the amount of SiO₂ on the infrared emittance of hybrid films is shown as follows:when the content is small,the infrared emittance of the hybrid film decreases compared to pure PDMS films,and when the content is large enough?20 wt%?,the infrared emittance of the hybrid film can reach 0.94,which is higher than that of the pure PDMS film.The surface of the methyl-modified SiO₂ microspheres is smoother and the number of pore structures has reduced.The methyl-modified SiO₂-PDMS hybrid film has a higher infrared emittance?0.95?.In the diurnal temperature test,the methyl-modified SiO₂-PDMS composite film showed the best cooling performance,which was 5?lower than that of the pure PDMS composite film and nearly 10?lower than that of silver-plated glass.In this paper,the controllable preparation and radiative cooling performance of SiO₂-TPX hybrid thick films and SiO₂-PDMS hybrid thick films were studied, deepening the understanding of microstructure and optical properties of the composite system.A new material system of nanoporous SiO₂ microspheres for radiative cooling was developed,which laid the foundation for its practical application in the field of building cooling.