SOLIDIFICATION BEHAVIOR OF UNDERCOOLED LIQUID ALUMINUM-OXIDE

Solidification of aluminum oxide from undercooled melts was investigat ed in containerless experiments, Specimens were levitated in a gas jet , stabilized with an acoustic positioning device, and melted with cw C O2 laser beams, Cooling curves were obtained by optical pyrometry when the laser intensity was reduced, The materials examined were high-pur ity Verneuil sapphire, 99.5% polycrystalline alumina, and oxide materi als recovered from the effluent of an aluminum-fueled rocket motor, Th e degree of undercooling, the apparent temperature behavior during the thermal arrest on solidification, and the structure of the materials formed were different in argon and oxygen atmospheres, Undercooling of the sapphire and alumina materials was 360 +/- 10 K in an oxygen atmo sphere and approximately 450 K in argon, Melting and solidification of high-purity sapphire resulted in a dendritic and porous polycrystalli ne material in oxygen, Dense, larger crystals were obtained in argon, Products formed from 99.5% alumina were discolored and the cores were white, indicating impurity segregation effects, More reproducible beha vior was observed for the sapphire and 99.5% alumina than for the tung sten-contaminated rocket motor effluent materials.