Structural characterization of SiO2 aerogels

Abstract

Abstract SiO2 aerogels with densities between 0.131 and 0.182 g/cm3 were prepared by CO2 supercritical drying of wet gels formed from differently concentrated TMOS solutions by hydrolytic polycondensation under basic conditions (pH ≈ 8). The formation and growth of SiO2 particles in the originally homogeneous starting solutions has been followed in situ by SAXS (small-angle X-ray scattering). A chemical limited particle growth takes place until a reduced time of t/tg ≈ 0.3, which slowed to the point of gelation and produced, on the average, ⋍ 11 nm sized particles for all alcogels. These partices form branched and interconnected chains determining the fractal structure of the aerogels. The internal structure of these particles is disturbed by incorporated alcoholic groups caused by the basic hydrolysis and the relatively soft regime of CO2 supercritical drying. In particular, the reduced height of the first sharp diffraction peak suggests that the middle-range order is less pronounced than in the corresponding SiO2 xerogel.