Acoustic and thermal properties of silica aerogels and xerogels

Comparative measurements of Brillouin light scattering and ultrasounds in a wide class of silica aerogels and xerogels show the existence of distinct mechanisms governing the temperature behaviors of the acoustic attenuation in the different frequency ranges. In the MHz range the attenuation is mainly regulated by dynamical mechanisms due (i) to thermally activated local motions of structural defects typical of vitreous silica at low temperatures and (ii) to relaxations of "extrinsic" defects at high temperatures, i.e., the hydroxyl groups covering the inner surface of the pores in aerogels. In the MHz range, instead, the attenuation is dominated by a temperature independent, or "static," process due to the scattering of phonons by pores. By growing the density of gels up to values close to that of dense vitreous silica, the acoustic attenuation shows a strongly temperature dependent behavior. The sound velocity scales with the density of the system following a power law, which is in good agreement with the predictions of a model describing silica gels in terms of a disordered network of microrods or microplates. The same law also permits to account for the temperature dependence of the sound velocity which reflects very closely the behavior observed in dense vitreous SiO2. Finally the analysis of the low temperature specific heat (1.5-20 K) reveals that, for densities larger than about 1000 kg m(-3), the vibrational dynamics of these porous systems tends to reproduce the one of vitreous SiO2.