Preparation And Performance Study Of Aerogels With Both Compressibility And High-temperature Resistance

With the rapid development of high techonology and high-end devices,the demand for materials exhibiting both high-temperature resistance and compressive property is becoming more and more urgent.Except the basic thermal stability and mechanical property,materials also need to possess other physical properties such as low density and low thermal conductivity.Aerogels are highly porous materials,which exhibit many kinds of unique properties like low density,low thermal conductiveity and low dielectric constant.However,the high porosity also confers aerogels with inferior mechanical properties.Aerogels are easy to be cracked under external force.Although increasing the content of organic component in aerogels can effectively improve the mechanical properties,the thermal stability of aerogels will be degraded simultaneously.Aerogels will exhibit drastic shrinkage and even be cracked under high temperature.Therefore,it is very significant to prepare aerogels combining both high-temperature resistance and compressive property.In this dissertation,we deeply studied the relationship between organic-inorganic crosslinking degree and the properties of polyorganosilsesquioxane aerogels at first.On this basis,the skeleton of polyorganosilsesquioxane exhibiting outstanding mechanical properties was chosen to combine with the high-temperature resistant polyimide(PI).The polyorganosilsesquioxane-PI aerogels combining both high-temperature resistance and compressive property were finally prepared.We detailedly demonstrated the chemical reaction mechanism involved in the preparation of composite aerogels.And we systematically studied the effect of various sol-gel parameters,technological parameters on the microstructure and performance of composite aerogels.Based on the results,we chose alumina aerogel as basic skeleton,polyamic acid ionic salt(PAS)as template to prepare alumina nanobelt(ANB)aerogel by ethanol supercritical fluid drying(ESFD)and calcination.The obtained ANB aerogel exhibited both ultrahigh-temperature resistance and compressive property.The effect of building block of microstructure on mechanical and thermal properties of aerogels was studied deeply.The research of this dissertation specifically included the following aspects:(1)A series of polyorganosilsesquioxane aerogels with different organic-inorganic crosslinking degree were prepared by introducing methyl groups or hydrocarbon chains into the skeleton of silica aerogels.The elastic property of aerogels can be effectively improved by decreasing the organic-inorganic crosslinking degree of skeleton.Meanwhile,the thermal stability of aerogels was degraded with the increase of the introduced organic component.This part of research provided the therotical guidance and experimental basis for preparation of aerogel with both high-temperature resistance and compressive property.(2)Polyvinylpolymethylsiloxane(PVPMS)-PI aerogels with both high mechanical strength and high-temperature resistance were prepared by stepwise chemical liquid deposition(SCLD)and chemical imidization process.We detailedly demonstrated the chemical reaction mechanism involved in the SCLD processs,such as the combination between PI monomers and basic skeleton,and the chemical imidization process.And we systematically studied the effect of experimental parameters such as chemical liquid deposition way,the content of silane coupling agent,the rigid of PI monomers and deposition time on the mechanical and thermal properties of PVPMS-PI aerogels.The prepared samples can be compressed to 80%of their own height without fracture,and they can be processed to any shapes.Meanwhile,PVPMS-PI aerogels also exhibited high thermal stability.The initial decomposition temperature is up to 400 ~oC,and the aerogel can maintain high structural integrity after 30 minutes of flame burning.(3)Polymethylsilsesquioxane(PMSQ)-PI aerogel with both high-temperature resistance and flexibility was prepared by co-gel stragegy and ESFD process.We detailedly demonstrated the chemical reaction mechanism involved in the col-gel process and ESFD process.And we systematically studied the effect of the two key sol-gel parameters,the concentration and dosage of PAS,on the mechanical and thermal properties of PMSQ-PI aerogel.The prepared samples exhibited porous network structure constructed by neutron-like building block.Increasing the concentration or the dosage of PAS was beneficial to forming neutron-like nanostructure,which endowed the aerogels with outstanding compressive performance and excellent high-temperature resistance.The compressive ratio of PMSQ-PI aerogel is up to 90%,and the aerogel can still maintain complete shape after undergoing 1050 ~oC of thermal treatment.(4)ANB aerogel with both ultrahigh-temperature resistance and compressive property was prepared via the method of epoxide addition,ESFD process and calcination.We detailedly demonstrated the chemical reaction mechanism related to the method of epoxide addition and formation of alumina nanobelt.The unique nanobelt structure endowed ANB aerogels with mechanical and thermal properties that were not possessed for traditional alumina aerogels.The compressive ratio of ANB aerogel is up to 80%,and the specific surface area is still up to 228 m~2/g after 1300 ~oC of calcination.In addition,the aerogel can still maintain original shape after being burnt by 1300 ~oC of butane flame for 300 s,and the backside temperature is only 69 ~oC after 300 s,which indicate its excellent high-temperature thermal insulation property.This dissertation develops new methods to prepare polyimide composite aerogels and synthesis of alumina aerogels by polyimide template.These related works contribute to preparation of aerogels with both high-temperature resistance and compressibility,which promote the development of aerogels.