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/cm³). The results show important differences in the behavior of classical disordered dense solids (a-SiO₂). They are analyzed within the framework of an elastic model of rods assembled in a disordered hierarchical network.