Acoustic properties of silica aerogels from 400 mK to 400 K

Using acoustic resonant spectroscopy, we have studied the low-frequency elastic properties of silica aerogels with densities between 0.045 and 0.267 g/cm3 over a temperature range of 400 mK to 400 K. We observe a large change in the elastic moduli between 20 and 100 K, accompanied by a damping peak, and attribute this to a thermally activated relaxation process. Above 100 K, the elastic moduli increase linearly with temperature, a phenomenon normally associated with elastomers such as rubber, and indicative of a significant entropic contribution to the stiffness of the aerogels