The Preparation,Performance And Application Of Silica Aerogel And Its Composites

Silica aerogels is a nanoporous material with various excellent properties,such as high porosity,large specific surface area,low thermal conductivity and low dielectric constant.It has lots of potential application in many fields,such as aerospace,military,chemical industry,metallurgy,energy-saving buildings,etc.However,the high cost?organosiloxane,supercritical drying?,low strength,high brittleness and poor mechanical properties are inhibiting the broader application of silica aerogel.In this study,we successfully synthetized monolithic hydrophobic silica aerogels by a new co-precursor?tetraethoxysilane?TEOS?and alkaline silica sol?system based on sol-gel method and ambient pressure drying process.The mechanism of the sol-gel reaction of TEOS and alkaline silica sol was explained,while the influence of the molar ratio of two precursors,the dilution of the silica sol and the surface modification procedure on the sol-gel process or the properties of the SiO₂ aerogel samples were discussed in detail.The structure and properties of the SiO₂ aerogels were investigated by means of XRD,SEM,TEM,FTIR,TG-DTA and BET method.The monolithic hydrophobic silica aerogel was obtained by optimized preparing conditions,which exhibited typical properties of 0.119 g?cm^-33 density,749.6 m²?g^-11 surface area,3.892cm³?g^-11 pore volume,11.2nm average pore size and 0.0281W/?m?K?thermal conductivity.Based on this,we prepare rod-like cellulose whiskers?CWs?with length of 15-20?m and lateral width of 1-1.5?m to fabricate cellulose whiskers-silica aerogel composites.We also prepare xonotlite fibers?XFs?with length of 5-10?m and lateral width of 150-200 nm to fabricate xonotlite fibers-silica aerogel composites.The results show that,the CWs and XFs which are prepared respectively by acid hydrolysis method and dynamic hydrothermal method have good wettability and produce a hydrogen bond with the Si-O bond on the surface of wet gel.With good interfacial bond between the CWs?XFs?and aerogel,the microcracks are inhibited during the ambient pressure drying process,that is advantageous to the preparation of monolithic silica aerogel composites.The CWs/SiO2 aerogels and XFs/SiO2 aerogels exhibit basic properties of the aerogel matrix,such as nanoporous morphology,high hydrophobicity and good thermal insulation.The thermal degradation of CWs at high temperature affects the thermal stability of CWs/SiO₂ aerogel,while XFs can maintain stability and reduce the densification or volume shrinkage of the aerogel skeleton at high temperature.The specific surface area and pore volume of XFs/SiO₂ aerogel remained 365.6 m²/g and2.78 cm³/g comparing with the blank samples which were 215.6 m²/g and 1.41 cm³/g.Due to the size effect of whiskers or fibers reinforced SiO₂ aerogel during the ambient pressure drying process,we use the polyurethane spong to fabricate sponge-reinforced silica aerogel composites.The mechanical properties of sponge-reinforced SiO₂ aerogel composites are effectively improved compared with pure aerogels.The TEOS-based and TEOS/silica sol-based sponge-reinforced SiO₂ aerogel composites exhibit brittle failure under external force,while the failure process includes the initiation,expansion and penetration of microcracks in composites before the macrocracks are formed.When the compression strain is 28.3%and 32.5%,the two composites have the maximum stress of 0.349MPa and 0.313MPa,respectively.The MTMS-based composites exhibit excellent flexibility and can return to the original size when unloaded.The compressive strength of MTMS-based composite is only0.073MPa with 55%strain.Since the pores and surfaces of the sponge are filled or covered by hydrophobic SiO₂ aerogels,sponge-reinforced SiO₂ aerogel composites exihbit good thermal insulation performace,high hydrophobicity and large oil adsorption capacity.Finally,the fractal feature of the SiO₂ aerogels and aerogel composites were studied by small angle X-ray scattering and gas adsorption-desorption method.The microporous structure of the SiO₂ aerogel was characterized by using fractal geometry theory.The size of the silica particles and aerogel clusters,the mass distribution uniformity,and the surface roughness of aerogel skeleton were obtained,which provides a theoretical basis for better understanding of the macroscopical properties of SiO₂ aerogels,such as thermal insulation,oil absorbability and catalytic performance.