Study On Process And Properties Of Low Density Silica Aerogels Prepared By One-step And Two-step Method

In this thesis, silica aerogels with low density were made from water glass by ambientdrying, using two different sol-gel process, one-step and two-step. At the same time, FT-IR,XRD, SEM and BET surface area have been studied to characterize phase, specific surfacearea, partical size, morphology and hydrophobic, heat insulation performance of silicaaerogels. The effects of sol-gel process, aging process and heat-treated process on themicrostructure and performance of silica aerogels have been studied.In the preparation of silica aerogels by the acid and alkali two step catalytic process, theeffect of the type of the alkali catalyst, gelling temperature and the height to diameter ratio ofwet gels on the gel time, aerogel density and pore diameter have been studied within sol-gelprocess, and the process of sol-gel was confirmed as:1mol/L ammonia as the alkalinecatalyst, the wet gel height to diameter ratio of0.4, and subjected to gel in20℃. In the agingprocess, by studying the aging temperature-time, step-by-step aging process, the solventexchange temperature-time, step-by-step solvent exchange process, surface modificationtime, modified solvent exchange time and other factors of the prepared aerogels performance,determined the optimal process parameters for each factor. While silica aerogels with a tapdensity of0.123g/cm~3were prepared by one-step solvent exchange-surface modification.Theverification test were proceeded by synthesizing the optimal process parameters of variousinfluencing factors in two-step catalyzed sol-gel process, and the microstructure andperformance of the typical sample was characterized. The result shows that: silica aerogelswas a kind of amorphous material with density0.057g/cm~3, porosity97.41%, pore size9.35nm, specific surface area695m~2/g, thermal conductivity0.02088W/(m·K). Themicro-morphology shows that the silica aerogels was composited by nanometer-grade grainwith a loose structure which was homogeneous diameter, and the FT-IR shows that there was-CH3in surface, contact angle141°and well hydrophobicity.In the preparation of silica aerogels by one step acid catalytic process, the effect of thevolume ratio of waterglass and water, the type and concentration of the acid catalyst andgelling system pH value on the gel time, aerogel density have been studied within sol-gel process, and the process of sol-gel was confirmed as: Vsodium silicate/Vwater=1:4, adjusting pHvalue to8~9with2mol/L HCl. The typical sample was with density0.1211g/cm~3,porosity94.5%, pore size20.20nm, specific surface area571m~2/g, thermal conductivity0.02403W/(m·K), contact angle131°. The micro-morphology shows that the silica aerogelswas composited by a large number of nanoscale particles which have agglomerations, thenumber of holes is less and single hole size is larger. The effects of heat-treated process on tapdensity, hydrophobic property and specific area of the silica aerogels have been studied andthe result shows that heat treatment at200℃can reduce the density while maintaining itsgood water repellency and high specific surface area.By comparing two kinds of sol-gel process, the result shows that silica aerogels preparedby two-step process is better on performance, but one-step process is simple and easy torealize industrialized production.