Growth kinetics of highly undercooled Al2O3 melts

Abstract

An aeroacoustic levitator was employed to solidify the Al2O3 melt in a containerless condition at different melt undercoolings when a laser beam heating system was utilized. The sample was simulated to crystallize at well-defined temperatures and the recalescence front was imaged using a high-speed video. Both the observation of solidified microstructures and the theoretical calculation of the hypercooling limit of the undercooled melt indicated that the final microstructure should consist of the primary dendrite formed during rapid recalescence and the subsequent product yielded after recalescence near the melting temperature. Microstructure analysis showed that the advancement rate of the recalescence front should be viewed as the growth velocity of the undercooled melt. The accurate relationship of growth velocity versus melt undercooling was acquired, which was essential in characterizing the growth kinetics of the undercooled melts. Further analysis indicated that the linear kinetic coefficient for the free growth of Al2O3 was about 0.05 m/sK, which was much higher than that of some other compounds with complicated crystalline structures. The growth kinetics of the undercooled melt can be well clarified when considering the complexity level of the crystalline phase and the structure of coordination in the undercooled melt in comparison with some other oxides.