Doping Of SiO₂ Nanoparticles,Modulation Of Infrared Radiation Properties And Their Application In Radiative Cooling Coatings

Radiation cooling can transfer the heat of an object to extra-terrestrial space in the form of infrared radiation through the atmospheric window,and it is a passive and green cooling method that does not consume energy,and its research has received a lot of attention.The current research focuses on the controllable preparation and application of metamaterial,including thin film structured metamaterials,nanophotonic structured metamaterials,random structured metamaterials and other types.From the material point of view,SiO₂ is widely used for the preparation of thin film structured metamaterials and random structured metamaterials based on polymers and highly reflective nanoparticles because of its excellent atmospheric window selective radiation properties and unique micro/nanosphere Mie resonance effect.However,the majority of studies have focused on the preparation of SiO₂ micro and nano-metamaterials and their optical thermal properties when compounded with polymeric matrices,and no researcher has yet tuned their intrinsic IR emission properties.The intrinsic infrared emissivity of SiO₂ in the atmospheric window band is still somewhat different from the ideal choice of emitter.Based on this,this paper explores the regulation of the infrared emissivity of SiO₂nanoparticles in the atmospheric window band from the doping perspective,selects Mg and B as doping sources,and prepares Mg-doped and B-doped SiO₂nanoparticles by a modified St（?）ber method combined with high-temperature sintering,systematically investigates the effects of parameters such as elemental doping,sintering temperature and silicon source addition on the morphology,crystallinity,chemical composition and infrared emissivity of SiO₂nanoparticles.In addition,pure Mg-doped SiO₂ inorganic porous coatings and BSNs@PVDF flexible composite coatings were prepared by tape casting method,and the effects on morphology,solar reflectivity and radiation cooling performance of Mg doping content on the inorganic coatings and BSNs content on the flexible composite coatings were investigated.The main study contents and results are as follows:（1）Infrared radiation performance of Mg-doped SiO₂ nanoparticles and al-inorganic radiation cooling coatingsMg-doped SiO₂ nanoparticles were prepared by modified St（?）ber method combined with a high-temperature sintering,and pure Mg-doped SiO₂ coatings were prepared on FTO glass substrates by tape casting and high temperature heat treatment.It was found that trace Mg doping（0.003-0.03 mol）induced the formation of cristobalite nanocrystals of SiO₂nanoparticles at 1000°C,transforming their morphology from regular spheres to irregular nanoporous particles and increasing their average IR emissivity in the atmospheric window band to 0.96.In addition,the effective doping of Mg in SiO₂ structure was confirmed by XPS analysis,and the mechanism of the effect of Mg doping on the morphology and crystal structure of SiO ₂ nanoparticles was analyzed from the perspectives of precursor chemistry and nucleation-crystal growth.The Mg-doped SiO₂ inorganic coating has a thickness of about 90μm,a rough,porous morphology,and a solar reflectivity of 86.19%.Its radiation cooling performance is better than that of the the pure SiO₂ coating and commercial SiO₂ coating,and the temperature can be reduced by 3～5°C compared with the pure SiO₂ coating,and the maximum temperature drop is 4.5°C compared with the commercial SiO₂ coating.（2）Infrared radiation performance of boron-doped SiO₂nanoparticles and flexible radiation cooling coatingsB-doped SiO₂ nanoparticles were prepared by a modified St（?）ber method combined with high-temperature sintering,and BSNs@PVDF flexible composite coatings were prepared by tape casting method and immersion precipitation phase separation methods.It was found that B-doping also induced the crystallization of SiO₂ nanoparticles at a sintering temperature of 1000°C and the formation of B-O-Si bonds at 550°C,which effectively enhanced the radiative properties of SiO₂ in the mid-infrared band with an average emissivity of up to 0.982 in the atmospheric window band.SEM analysis showed that B doping increased the particle size,average pore size and specific surface area of SiO₂ nanoparticles;the effect of the silicon source addition method on the nanoparticle morphology and pore structure was investigated,and it was found that the particle size of the sample prepared by the continuous dr ips method was larger,with a uniform and regular spherical shape and larger average pore size and specific surface area,while the particle size of the sample prepared by the rapid pour-in method was smaller,with an irregular shape and smaller average pore size and specific surface area.BSNs@PVDF flexible composite coating is porous structure,BSNs are uniformly distributed on the surface and pore wall of PVDF skeleton,the reflectivity of composite coating in UV-Vis-NIR band increases with the increase of BSNs content,the average reflectivity of 50 wt%BSNs@PVDF composite coating in solar band reaches 80.12%,its radiation cooling performance is better than pure PVDF coating The average temperature of the corresponding test cavity is 34.2°C,which is～15.8°C lower than that of the cavity without samples and～3.9°C lower than that of the pure PVDF coating.The thesis demonstrates the great potential of elemental doping technology in regulating the microscopic morphology,high-temperature crystallographic properties,and infrared radiation properties of SiO₂ nanoparticles,as well as the good effect of doped SiO₂ nanoparticles in regulating the properties of radiation cooling coatings,and its basic principles and methods are expected to be extended to other doped elemental systems and other oxide-based infrared devices,thus promoting the development of radiation cooling materials and other infrared materials and devices to a certain extent.