Viscoelasticity of silica aerogels at ultrasonic frequencies

In this letter, the possibility to use a technique based on the analysis of thickness resonances of air-surrounded aerogel plates at ultrasonic frequencies to obtain viscoelastic properties is investigated. These resonances were excited and sensed by airborne ultrasonic waves. Toward this purpose, specially designed air-coupled, high-sensitivity, and broadband piezoelectric transducers were used. Precise and simultaneous measurements of the velocity and attenuation of longitudinal and shear waves at different frequencies, as well as aerogel density, were obtained. It allowed us to afford a full characterization of the viscoelastic properties of these materials at ultrasonic frequencies.