Chemical and structural evolution of silica alcogels during their formation: Acoustical study

Silica aerogels are materials of very high porosity obtained through a sol- gel process. Since their structural properties greatly influence their acou stical and mechanical properties, it is important to follow how the differe nt chemical reactions occur during this process. We present a study of thes e evolutions for two different types of alcogels through acoustical measure ments in the ultrasonic range. These samples have been chosen because of th e significantly different behavior of the derived aerogels. The data obtain ed during gelation show fluctuations of the velocity that can be interprete d in terms of chemical changes. The absence of major changes in the velocit y before and after gelation is explained with the aid of Blot's theory of a coustic propagation in porous media.