The Speed of Sound in Nanopowder Created by High-Energy Electron Beam

The work is devoted to a technique for measuring the speed of sound when passing through a thick layer of “poured” nanopowder in an immersed state. A simple experimental setup is proposed consisting of two speakers and one microphone immersed in a container with a nanosized powder. The setup does not require calibration. Two indirect methods for determining speed in the nanopowder using two speakers and a microphone located at different distances from the speakers are shown. Experimental measurements are carried out in a silicon dioxide nanopowder with an average particle size of about 50 nm. It has been established that the speed of sound in this medium is less than that in gas and solid matter and is equal to 35 m/s. It has been shown that the speed of sound does not depend on the frequency of sound for frequencies up to 1600 Hz. A new hypothesis has been proposed that the nanopowder behaves like a new type of continuous medium, “heavy gas,” when a sound wave passes through it. The hypothesis allows one to apply formulas for determining the speed of sound in gas for this case. Based on experimental data, the adiabatic constant for the “heavy gas” is estimated. The proposed application of the effective medium approximation, such as the Hertz–Mindlin contact theory, to estimate the speed of sound in nanopowder shows that the nanoparticles are not in close contact with each other. This can be explained by the inapplicability of this theory to the case of “poured” nanopowder due to the packing of nanoparticles that differs from the theory.