ELASTIC PROPERTIES OF XEROGELS AND SINTERED AEROGELS PROBED BY INELASTIC LIGHT-SCATTERING

Elastic properties of silica xerogels and sintered aerogels of densiti es from rho = 0.5 g/cm(3) to 2.2 g/cm(3) were investigated by Brilloui n light scattering. Sound velocities and acoustic attenuation at hyper sonic frequencies was measured as a function of density. The elastic m odulus C follows a scaling power law C = rho(alpha) with alpha = 3.3 /- 0.1 and confirms previous data at low frequencies. For xerogels, th e comparison between as prepared and oxidized samples after heat treat ments, shows an increase of the elastic constant greater than 50% with out significant change of the alpha exponent. This alpha value is clos e to that observed for densified aerogels emphasizing that this expone nt seems to be independent on the sample treatment and preparation. Th erefore, the fractal behavior of the material is not essential to expl ain power-law dependence. The acoustic attenuation has also been deter mined as a function of the density. A relaxational process coupled to acoustic vibrations disappears after heat treatment. This effect is al so related to large inelastic light contribution in the very low-frequ ency part of the Raman spectra. (C) 1998 Elsevier Science B.V. All rig hts reserved.