Density dependence of acoustic characteristics of silica nanofoam

Abstract

In this study, we experimentally estimated the sound speed of silica nanofoam as a function of the density through acousto-optic measurements. He–Ne laser light at the wavelength of 632.8 nm was emitted through the sample in the direction perpendicular to the propagation of the ultrasound. The diffraction of the light wave by the ultrasonic longitudinal waves with frequency in the range of 500–2,000 kHz agreed with the Raman-Nath theory. The sound speed estimated from the Raman-Nath diffraction angle varied in the range of 55–178 m/s for the sample density of 100–300 kg/m³. The measured sound speed almost agreed with the sound speed calculated from the density, bulk Young’s modulus and Poisson’s ratio. This implies that the low sound speed might be attributed to the reduced bulk elastic constant owing to the structure and might not be affected much by air confined in the porous structure.