Production of Fine Amorphous Silica Particles by Rapid Expansion of Supercritical Water

The rapid expansion of supercritical fluid solutions proposed by Matson et al. was applied to the system of silica and water. A filmwise silica with cracks was obtained when water was evaporated slowly from the mixture received on a plate. However, fine amorphous silica spheres were collected on a hot plate by impinging the mixture on the plate at a temperature higher than 423K, i.e., when water was rapidly separated from the silica particles. The amorphous silica spheres collected on the hot plate were of median diameter in the range from 0.050.15 μm, and the diameter increased with increasing solubility of silica in water.