THERMAL AND TEMPORAL AGING OF 2-STEP ACID-BASE CATALYZED SILICA-GELS IN WATER ETHANOL SOLUTIONS/

Dissolution and reprecipitation of silica during aging in water improv e 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 ca n 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 ag ing 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 amb ient pressure dried silica aerogels by increasing the wet gels' stiffn ess by aging in a TEOS solution until shrinkage during drying is almos t eliminated. The gels aged in the water/ethanol solutions were furthe r aged in a TEOS/ethanol solution and the effect of the increasing wat er 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.