Phonon attenuation in vitreous silica and silica porous systems

The mechanisms responsible for phonon attenuation in glasses and in porous systems have been investigated. The acoustic attenuation has been measured by Brillouin light scattering using a Fabry-Pérot apparatus and a new ultraviolet spectrometer. In particular, melt-quenched vitreous silica (ρ=2200 kgm-3), silica xerogels with different densities (ρ=510±50, 770±80, 1380±140 and 2190±200 kgm-3), and silica aerogel (ρ=670±80 kgm-3) have been studied. The porosities of the samples have been measured by nitrogen adsorption–desorption techniques. The Brillouin linewidth is found to increase with increasing pore size, being related to the effect of the growing structural disorder. Comparison between the acoustic attenuation obtained by the two wavelengths shows the existence of a crossover length a* which identifies the wave-vector values above which the disorder induced by the pore size starts to be the dominant cause of absorption. The position of the crossover is found to be frequency dependent.