The Synthesis And Performance Of Silica Aerogels And Their Composites

Silica aerogels are regarded as a novel class of porous materials.Much attention has been attracted because of their outstanding properties,such as low density,high surface area,and low thermal conductivity.Silica aerogels have prospects of thermal insulation,catalysis and so on.The main reason that hinders the large-scale production of silica aerogels is that the supercritical drying method,as the traditional drying method,not only consumes large amounts of energy and money,but also is dangerous.Thus,the surface modification and organic-inorganic hybrid methods were used in this paper,resulting in silica aerogels dried at ambient pressure.And the thermal and catalytic properties of the silica aerogels were also studied.In this paper,the silica aerogels dried at ambient pressure were obtained by using tetraethoxysilane?TEOS?as the silicon source,ethanol as the solvent through a acid-base catalyzed two-step method after surface modification with trimethylchlorosilane?TMCS?.The effects of various process parameters on the properties of the aerogels during the sol-gel process and gel post-treatment process,were studied.On this basis,the second silicon source vinyl triethoxysilane?TEVS?was introduced to co-hydrolyze with TEOS in order to introduce vinyl groups in situ.At the same time,the effect of different contents of TEVS on the properties of aerogel composites was studied.The morphology,structure and properties of silica aerogels and their composites were characterized by SEM,TEM,FTIR,TG-DSC and BET,etc.The results showed that silica aerogels with low density?0.101 g/cm³?,high specific surface aera?1144.01 m²/g?,high porosity?95.3%?,and super hydrophobicity?the water contact angle reached 154°?were obtained when the molar ratio of TEOS:H₂O:EtOH:HCl:NH₄OH=1:5:10:10^-3:3×10^-3,after aging,solvent replacement,surface modification and ambient pressure drying.The process conditions were applied to composite with glass fibers.The specific surface area of the obtained composite was 1056.22 m²/g,the thermal conductivity was 0.0164 W/?m?K?,the contact angle was 152°,and the superhydrophobicity was maintained below 270?.Through experiments,it was found that the obtained aerogels exhibited good elasticity when the molar ratio of TEVS:TEOS=5:5.The copolymerization of TEVS and TEOS?organic-inorganic hybrid method?can omit the surface modification step and shorten the preparation cycle.And-CH=CH₂ in TEVS was a flexible group,which can effectively improve the mechanical properties of the silica aerogels.The aerogels were used as carriers.The hydrosilylation reaction of n-octene with triethoxysilane was carried out as a template to study the effects of the reaction temperature,reaction time,and the molar ratio of n-octene/triethoxysilane on the conversions of n-octene and the yields of n-octyltriethoxysilane.The results showed that the conversions of n-octene reached97.09%,and the yields of n-octyltriethoxysilane reached 87.00%when the reaction temperature was 100?,the reaction time was 60 min,and the molar ratio of n-octene:triethoxysilane was 1:1.5.