Acoustic attenuation in 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. Melt-quenched vitreous SiO2 (ρ=2200 kg/m3), silica xerogels with different densities (ρ=510±50, 770 ± 80, 1380 ± 140 and 2190 ± 200 kg/m3), and silica aerogel (ρ=760±80 kg/m3) have been investigated. The porosity of the samples was measured by N2 adsorption/desorption techniques. For pores sizes smaller than ∼8 nm the acoustical attenuation, at the Brillouin frequency, at room temperature, is the same as in v-SiO2 and consequently is attributed to dynamical processes. For larger pore sizes, the Brillouin line width increases and its variation is related to the effect of phonons scattering from the growing structural disorder. On the basis of the present experiment, in porous systems, static and dynamical attenuation mechanisms have been found.