ULTRASONIC PROPERTIES OF SILICA AEROGELS AT LOW-TEMPERATURES

We have made ultrasonic measurements on several silica aerogels in the previously unstudied region between room temperature and 15 K. We pre sent detailed results for two samples with densities of 360 and 290 kg /m(3) (porosities of 84 and 89%) and sound velocities of a few hundred meters per second. Below about 120 K, both longitudinal and transvers e sound velocities increased substantially (by up to 16%, much more th an in bulk glasses). These velocity changes were almost frequency inde pendent and were accompanied by attenuation peaks whose magnitude incr eased linearly with frequency. The maximum attenuation was so large, n early 400 dB/cm at 10 MHz in the lower-density sample, that measuremen ts could only be made below about 15 MHz. Below about 40 K, the attenu ation was proportional to the cube of the temperature. The velocity ch anges and attenuation peaks resemble, but are much larger than, those seen in bulk dielectric glasses where they are associated with a relax ation process but the origin of the T-3 attenuation is unclear.