STRUCTURAL CHARACTERIZATION OF SIO2 AEROGELS

SiO2 aerogels with densities between 0.131 and 0.182 g/cm(3) were prepared by CO2 supercritical drying of wet gels formed from differently concentrated TMOS solutions by hydrolytic polycondensation under basic conditions (pH approximate to 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/t(g) approximate to 0.3, which slowed to the point of gelation and produced, on the average, similar or equal to 11 nm sized particles for all alcogels. These particles 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.