Low-temperature energy excitations and thermal properties of silica aerogels

We present measurements of specific heat, thermal conductivity, internal friction, and relative change in sound velocity in the temperature range 0.1–10 K on several base-catalyzed and neutral-reacted low-density silica aerogels (ρ: 0.87–0.12 g/${\mathrm{cm}}^3$). The results show important differences in the behavior of classical disordered dense solids (a-${\mathrm{SiO}}_2$). They are analyzed within the framework of an elastic model of rods assembled in a disordered hierarchical network.