Influence of Sol-Gel Parameters on Transparency and Monolithicity of Silica Aerogels

Abstract

Considering the importance of silica aerogels for various scientific and industrial applications, we present here the experimental results on the effect of sol-gel parameters on transparency and monolithicity of silica aerogels. The aerogels were prepared using different precursors, acidic and basic catalysts, and solvents followed by supercritical drying. The alcogels were aged for various periods ranging from a few hours to several days. It has been observed that TEOS gels with acidic catalysts and TMOS gels with basic catalysts yielded monolithic and transparent silica aerogels. Higher catalyst concentrations (> 0.1N) resulted in cracked aerogels, whereas monolithic and transparent aerogels have been obtained for lower catalyst concentrations (0.01N). Very low catalyst concentrations (< 0.001N) yielded monolithic but opaque aerogels. While solvents with longer chain lengths and more branching gave cracked and semi-transparent aerogels,whereas highly transparent and monolithic aerogels were obtained with solvents of shorter chain lengths and lesser branching. To obtain good quality aerogels, in the cases of TMOS precursor, stoichiometric ratio of water and TMOS is sufficient, whereas in the case of TEOS precursor, the molar ratio of water to TEOS of more than the stoichiometric ratio is needed. The results have been discussed by taking into account the hydrolysis and condensation reaction mechanisms along with the drying stresses.