SAXS and BET studies of aging and densification of silica aerogels

Abstract

Aerogels prepared by hypercritical drying of gels obtained by hydrolysis of TMOS/methanol 50% solution in acid and basic conditions were studied combining SAXS and BET methods. Diffraction results do not seem to reveal a fractal character as the POROD's limit I(q) q4 → Cte was obtained in all cases without well characterised portions in log I(q) vs log q graphs. Aging produces a small increase in the density of the matrix. Densification studies indicate that for moderate temperatures 530°-810°C the poresshow a shrinkage due to a diffusion controlled mechanism with an activation energy ΔE ~ 9 Kcal/mole. For higher temperatures expansion due to bloating was observed. The density of the matrix undergoes only a slight increase.