Acoustic levitation technique for containerless processing at high temperatures in space
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I.
INTRODUCTION
A C O U S T I C levitation is a new, experimental technique which uses acoustic fields in order to levitate or control the position of a liquid or solid object without physical contact. m This technique mal 4 > 12
EXPERIMENTAL RESULTS
A. Levitator Performance The results of the most recent acoustic levitation experiments were achieved using the SAAL aboard the U.S. METALLURGICAL TRANSACTIONS A
Space Shuttle flight (61-A) of the Challenger in October/ November of 1985. The SAAL is shown in Figure 3 without the outer gas-tight container, and is carried in the cargo bay on the Materials Experiment Assembly. The gas is dry air at one atmosphere pressure. Although the SAAL has the capability of processing eight specimens, only three experiments were actually performed because of a lack of coolant flow. In these three experiments a number of objectives were obtained. Table II summarizes these experiments, the maximum processing temperatures, and the duration of the soak time of the specimen.
Fig. 3--Picture of single axis acousticlevitatorwithoutouter container.
Table II.
Experiment Number
Sample alumina gallia-calcia-silica hollow glass
METALLURGICALTRANSACTIONSA
B. Positioning during Heating and Cooling of Specimens The primary objective of the first experiment was to demonstrate stability of containerless positioning on a nonmelting specimen and to provide engineering data about the equipment operation. Data on specimen residual motions and stability, as a function of time, and on the effects of temperature changes on the energy well positions were obtained. Similar data were also obtained in the other two experiments, and the photographic record indicates that all three experiments successfully obtained injection and capture of the specimen and containerless positioning, throughout the heating, melting, and soaking processes. In the first experiment the specimen was cooled slowly by removing power from the furnace heaters. Containerless positioning was also maintained in the first and third experiments throughout the entire cooling period of the specimen. In both the second and third experiments' period, more rapid cooling rates were attempted by opening one entire side of the furnace at the end of the soak period. This permitted the specimen to radiate heat through the large opening to the cooler surroundings. However, the influx of cold air drawn through this large opening produced a sudden shift of the energy well position, producing a subsequent perturbation of the specimen. In the case of the second experiment, the specimen displacement was so large that it came in contact with the constraining cage wires, to which it adhered during the remainder of the cooling period. The photographic data have provided information about the strength of the acoustic restoring forces along two axes: k a parallel to the acoustic axis, and Kr perpendicular to it (along a radial direction). In each experiment it is clear that the simple harmonic motion displayed by the specimen in oscillation around the
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