Levitation using intense acoustic fields at high temperatures

Abstract

Intense acoustic fields can be used to position objects without mechanical contact. This phenomenon finds application in high-temperature materials research by enabling a specimen to be heated, melted, reacted, cooled, and solidified in a containerless state. The acoustic force vectors capable of levitating an object are given by the gradient of the acoustic potential energy density. By suitable shaping of the acoustic field, a closed energy well can be created and a small specimen captured therein. Some recent experiments were carried out in microgravity aboard the Space Shuttle in order to reduce the requirements for intense acoustic fields, typically from 145 to 165 dB, at 15 kHz. Preliminary results are presented showing successful containerless processing of glass specimens of density 5 g/cm3 at 1550 °C. Ground-based experiments have levitated densities of 20 g/cm3 at STP and densities up to 4 g/cm3 at 1000 °C. Various results from these experiments will be presented along with a discussion of the effects of acoustic cooling, thermal perturbations of the acoustic field, and the presence of harmonics. [Work supported by NASA.]