ULTRASONIC EVALUATION OF ELASTIC PROPERTIES OF SILICA AEROGELS

The velocity and attenuation of ultrasonic waves were measured in nano structured silica aerogels as a function of frequency, density, gas pr essure within the pores and elastic strain. The sound velocity was fou nd to be independent of frequency. For the density dependence of the e lastic modulus a scaling law was observed for each class of specimens with exponents ranging from 2.0 to 3.6. The gas pressure strongly infl uences the sound velocity only for aerogels with densities below 30 kg m-3. Aerogels exhibit elastic non-linearity of an unexpected kind: th e sound velocity decreases with increasing static compression. This is attributed to structural properties of these materials. The ultrasoni c attenuation of silica aerogels is found to be lower than that of oth er highly porous materials. The main contributions under ambient condi tions are the interaction between the pore gas and skeleton, and the w ater content, which is typically about 5 wt.%. Evacuated and heat trea ted specimens show elastic hysteresis with a loss tangent of about 10( -3). This value is compared with preliminary data derived from creep m easurements.