ULTRASONIC VELOCITY-MEASUREMENTS IN SILICA, CARBON AND ORGANIC AEROGELS

Sound propagation in SiO2-aerogels and three types of organic aerogels - resorcinol-formaldehyde, melamine-formaldehyde and vitreous carbon - as well as of the corresponding xerogels was investigated. The main parameter determining the sound velocity is the density; however, a scaling law for the entire density range investigated cannot be found. The influence of the constituent material and the production process on the sound velocity is comparably weak. Three types of low density aerogels were observed to become less stiff and the sound velocity was lowered when the specimens are uniaxially compressed. This is explained by a 'knee model' for the internal structure. For very low density aerogels, the sound velocity is mainly determined by the elastic properties of the gas within the tenuous structure instead of the skeletal stiffness.