No abstract

Acoustic levitation of a small specimen is obtained in the energy well produced by a single axis arrangement consisting of a sound source and a small acoustic reflector. At high temperatures, the acoustic forces are generally insufficient to levitate a specimen except in the microgravity environment available in space. A single axis acoustic levitator (SAAL) has been built to levitate a specimen inside a high-temperature furnace where it may be heated, melted, cooled, and solidified while being positioned without physical contact. The acoustic field configuration in such a containerless processing device has been analyzed and the expected levitation or positioning forces are calculated for the specific case of the NASA-SAAL experimental hardware as flown on the Space Shuttle on the STS-61A mission. This experiment successfully levitated and processed three samples at temperatures from 600 °C to 1550 °C. Experimental data are presented and the results compared with those predicted. [Work supported by NASA.]

A new technique for acoustically levitating a specimen is described. The technique deviates from a previous single-axis acoustic levitation method that utilizes a single-acoustic radiator and a passive reflector to generate the interference produced sound field. The new method involves the use of two opposing focusing acoustic radiators that produce an interference field that has capabilities for the levitation and positioning of small objects or liquid drops. The theoretical predictions regarding this technique are examined and those results are compared with laboratory measurements. The nature of the apparatus and its capabilities is also addressed. [Work supported by NASA.]

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.]

An improved piezoelectric driven, focusing acoustic radiator for acoustic levitation has been designed, tested, and evaluated. This device displays a significant improvement in focusing to produce better specimen stability and energy well shaping as well as increased transducer power bandwidth and electroacoustic conversion efficiency. Corresponding effects resulting from the horn loading of this acoustic radiator are considered as well as the benefits of using the radiator in conjunction with hot wall high-temperature furnaces. In order to aid in the evaluation of these acoustic transducer improvements, a simple and useful technique for acoustic field mapping will also be discussed. [Work supported by NASA.]