Impedance spectroscopic studies of sol-gel derived subcritically dried silica aerogels

The surface-hydration, dehydration and dehydroxylation and their dependence on the electrical properties of the silica aerogel have been investigated using electrochemical impedance spectroscopy. In equilibrium with the atmosphere, the aerogel exhibits strong sorption of water from the environment, but was found to be mainly due to physisorption. The mobile species responsible for electrical conduction was traced out as protons and the proton conduction was highly augmented by the presence of adsorbed water. An S-shaped dependence of conductivity with relative humidity was observed, with largest increase between 58% (8.412 x 10(-7) and 7.658 x 10(-7) S/cm for aerogel samples calcined at 250 and 400 degreesC, respectively) and 81% relative humidity (3.924 x 10(-3) and 2.68 x 10(-3) S/cm for identical samples calcined at 250 and 400 degreesC, respectively). The mechanism of water sorption was found to obey Dubinin-Serpinsky theory. The in situ impedance measurements of the aerogel samples indicated that, the gel precalcined at 250 degreesC and saturated at 97% relative humidity, could retain the physisorbed water up to 100 degreesC, compared to 80 degreesC for the same precalcined at 400 degreesC under identical conditions. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.