Mechanical properties of SiO₂aerogels

Ultrasonic and static compression experiments were performed in the frequency range 50 kHz to 1 MHz in order to investigate the mechanical behaviour of silica aerogels as a function of internal gas pressure p G and external stress P ext . The measurement of longitudinal and transverse sound velocities allows the Young's modulus and the Poisson's ratio for aerogels of different densities to be determined upon variation of p G and P ext . For low-density aerogels ( rho approximately=100 kgm -3 ) the authors found a decrease in sound velocity upon evacuation. Surprisingly, the sound velocity decreases upon uniaxial compression with small loads. The most important finding from the static compression experiments is that low-density aerogels display creeping with a time constant of about 45 min.