SOUND-PROPAGATION IN SIO2 AEROGELS

The longitudinal sound velocity upsilon(L) of pure SiO2 aerogels with densities in the range of 5 to 500 kg m-3, either made from TEOS (tetraethoxysilane), TMOS (tetramethoxysilane), or waterglass was measured. The variation of upsilon(L) with density rho for evacuated aerogels is more complex than described earlier: The simple scaling law upsilon(L) is-proportional-to rho(alpha), with alpha = 1.3, only holds for rho > 100 kg m-3; below this density alpha almost-equal-to 0.8 is valid. Data are also collected for sintered SiO2 aerogels. For specimens with densities around 100 kg m-3 or below, sound velocity measurements under variation of air pressure have also been performed. The dependence of the sound velocity on air pressure can be correctly described by a simple theory involving quasi-isothermal compression of the gas within the pores. From these findings a reliable method to determine the macroscopic aerogel density is derived.