Thermal and temporal aging of two step acid-base catalyzed silica gels in water/ethanol solutions

Dissolution and reprecipitation of silica during aging in water improve the wet gels' mechanical stiffness and strength, and hence shrinkage during supercritical drying is reduced. We have investigated how the strength and stiffness of a 2-step TEOS acid-base catalyzed wet gel can be improved by aging in a solution of water/ethanol (20-40 vol%) at various temperatures (20-70 degrees C) and time (2 h and 24 h) and how this influences the aerogels' properties. The linear shrinkage during supercritical drying was reduced from 29% to 2% by introducing the aging step in the 20 vol% water/ethanol solution for 24 h at 60 degrees C. We have also in previous works introduced the idea of preparing ambient pressure dried silica aerogels by increasing the wet gels' stiffness by aging in a TEOS solution until shrinkage during drying is almost eliminated. The gels aged in the water/ethanol solutions were further aged in a TEOS/ethanol solution and the effect of the increasing water content in the pore liquid was studied. A xerogel density of 0.20 g/cm(3) is reported for gels with a shear modulus (G) of 30 MPa.