Acoustic properties of silica aerogels between 400 mK and 400 K

Using acoustic resonant spectroscopy, we have studied the low frequency elastic properties of silica aerogels with densities of 0.045–0.267 g/cm 3 over a temperature range of 400 mK to 400 K. We observe a large drop in the elastic moduli between 20 and 100 K accompanied by a damping peak and attribute this to a thermally activated relaxation process. At higher temperatures the elastic moduli increase linearly with temperature, a phenomenon normally associated with elastomers like rubber and indicative of a significant entropic contribution to the stiffness of the aerogels.