Very large-scale structures in sintered silica aerogels as evidenced by atomic force microscopy and ultra-small angle X-ray scattering experiments

During the last few years the bulk structure of silica aerogels has been ex tensively studied mainly by scattering techniques (neutrons, X-rays, light) . It has been shown that small silica particles aggregate to constitute a f ractal network. Its spatial extension and fractal dimension are strongly de pendent on the synthesis conditions (e.g., pH of gelifying solutions). Thes e typical lengths range from 1 to 10 nm. Ultra-small angle X-ray scattering (USAXS) and atomic force microscopy (AFM) experiments have been carried ou t on aerogels at different steps of densification. The results presented in this payer reveal the existence of a spatial arrangement of the solid part at a very large length scale. The evolution of this very large-scale struc ture during the densification process has been studied and reveals a contra ction of this macro-structure made of aggregates of clusters. (C) 2001 Else vier Science B.V. All rights reserved.