Structural studies of fractal silica aerogels

The structural properties of silica aerogels of various compositions and prepared in acidic, neutral and basic conditions are studied by SAXS, BET, density and TEM techniques. All structures can be described as mass fractals (D < 3). Observation of cross over points in the scattering curves confirms the existence of structural units forming the SiO 2 matrix having average size of - 13.5 Å (basic), - 8.5 Å (neutral) and < 7 Å(acidic). The hypercritical drying process smoothes slightly the structural units surfaces but does not alter the overall structure. Results are consistent with an aerogel model formed by a light amorphous matrix having a porosity occupying the lower volume fraction.