Study On Preparation Of SiO₂ Aerogels By Ambient Drying

Crack-free and hydrophobic aerogels with low density have been made from p-TEOS by ambient drying, using two different modification process, hydrophobic modification and self-hydrophobic modification. The process of sol-gel, surface modification, the effects of various process parameters on the density and microstructure and drying process have been studied to reduce or avoid shrinkage and crack during gel dried at ambient pressure.In the process of hydrophobic modification, silica alcogels are prepared at 60℃ by keeping the optimized molar ratio of p-TEOS, EtOH, H2O and HF. Then the wet gels are modified by different modification agents(TMCS, HMKD, HMDSO). After ambient drying, we can get hydrophobic SiO2 aerogels, whose density and specific surface area are at the range of 130~250kg/cm3 and 450~600m2/g.In the process of self-hydrophobic modification, the modified silica alcogels are made at 60 ℃ by keeping the optimized molar ratio of p-TEOS , EtOH, H2O, NH3H2O and MTES. After solvent exchange process and ambient drying, we can get hydrophobic SiO2 aerogels, whose density and specific surface area are at the range of 300~450kg/cm3 and 900~1000m2/g.With heat-treated process, the aerogels made from self-hydrophobic modification become hydrophobic aerogels with the density of 250~400kg/cm3 and specific surface area of 900-1000m2/g. And we study the effects of heat-treated process on the aerogels' density, specific area, microstructure, hydrophobic property and so on.After comparing the two process, we think that the self-hydrophobic modification process are the appropriate processe for the industrialization of preparing silica aerogels at ambient pressure, for the lower cost and shoter time.According to the power relation between the bulk modulus of a gel and the density, we have studied the gel' shrinkage rule during drying with the surface-chemical theory. A model is provided for predicting gel shrinkage during drying in case of m=4 and t=1 nm (or t=0).