Microstructure Control Of SiO₂ Aerogel And Application Of Phase Change Thermal Storage Composites

In recent years,with the deepening of the concept of sustainable development,to improve the energy efficiency and develop renewable energy appears increasingly important.In this context,organic solid-liquid phase change materials have drawn wide attention from all walks of life due to their cleanness,low cost,good thermal cycling performance,and large latent thermal of phase change,especially in the fields of thermal storage,waste thermal energy recovery and temperature control,and have developed rapidly in the past decades.However,in practical applications,solid-liquid phase change materials are prone to leakage and have large volume changes during melting,which greatly limits their application scope.In order to solve the problems in practical applications,the preparation of shape-stabilized phase change composites by filling phase change core materials into porous materials has become a research focus in recent years.Among them,SiO2 aerogel has high specific surface area,high porosity,low density,good chemical compatibility,especially the controllability of microstructure,making it one of the best choices for preparing the supporting framework of formalized phase change thermal storage materials.SiO2 aerogel was prepared by using sol-gel method and supercritical drying technology with ethyl orthosilicate as silicon source and hydrochloric acid and ammonia as catalyst.The influence of the distribution ratio of each group in the gel-gel reaction process,the parameters of the supercritical drying process and the surface modification process on the microstructure of the SiO2 aerogel were investigated.The results showed that by changing the molar ratio between ethyl orthosilicate,ethanol,water and acid-base catalysts in the sol-gel reaction,adjusting the pressure holding time,pressure relief rate and and ethanol concentration of the supercritical drying medium during supercritical drying,and selecting the modification method,the micro physical and chemical structure of SiO2 aerogel could be adjusted freely.SiO2 aerogels with different specific surface areas(200?1000m2/g),different nano-particle sizes(6?19nm),different average mesopore size(7?22nm),different mesoporous volume(2?4cm3/g),different pore shapes,different mesoporous pore size distributions and different surface chemical structures were successfully prepared.Based on the research on the microstructure control of SiO2 aerogel in Chapter 2,SiO2 aerogels with different specific surface area,pore size,pore shape and surface chemical structure were used as porous support materials,and paraffin was used as phase change core material,a series of SiO2 aerogel/paraffin phase change thermal storage composite materials were prepared by a vacuum impregnation process to research the effect of SiO2 aerogel microstructure on phase change behavior,thermal storage capacity,thermal conductivity and shape-stabilized capability of phase change thermal storage composites.Revealing that the pore size has a significant impact on achieving good phase transformation and three kinds of phase transition behavior of paraffin limited to different pore size ranges are obtained.The phase transition behavior cannot occur in the paraffin which is confined in the small pore size.Although phase transition behavior can also occur in paraffin with biggish pore size,phase transition behavior is completely different from that of pure paraffin,which is usually manifested as severe supercooling.The phase transition behavior of paraffin confined to larger pore size is not affected.On the storage capacity,the larger the pore size,the smaller the specific surface area,and the more the pore shape is biased toward the narrow slit type,the higher the energy density,and the obtained maximum thermal storage density is 200.9 J/g.As far as shape stability is concerned,SiO2 aerogels with alkylated surface chemical structure,small pore size and diverse and complex pore structure has the best shape stability performance.In addition,by increasing the specific surface area and reducing the pore size,the thermal energy storage-release rate of SiO2 aerogel/paraffin phase change composites can be effectively improved.