Sintering of foamed silica gels

Foamed silica gels prepared from a colloidal silica containing sodium were leached to avoid α-cristobalite crystallization during sintering. The sodium content in the foamed gels was decreased from 1.5 to 0.1 wt% by first drying at 80°C and then leaching in 5M nitric acid for 2 days at 40°C. When sintered at 1000°C for 4 h or 1100°C for 8 min, the leached foams did not crystallize, had a matrix density of 92%, and a foam bulk density of about 30%. Prior to densification, the foam consists of spherical cells of 100-150 μm diameter uniformly distributed in a matrix of 12 nm silica particles. The colloidal silica matrix densifies according to Scherer's viscous sintering model. Due to their low surface curvature, cellular voids do not contribute to the sintering driving force and thus, bulk foam densification and cell size shrinkage can be calculated by applying Scherer's viscous sintering model.