SHORTENED FLOATING ZONE CRYSTAL-GROWTH UNDER NORMAL GRAVITY

A shortened melt zone is highly desirable in floating zone crystal growth under normal gravity, because of its significantly greater stability. A simple method for shortening a stationary floating melt zone has been discovered recently and demonstrated with NaNO3 as the test material. In the present study, this method was applied to floating zone crystal growth. NaNO3 single crystals were grown both upward and downward, with fast feed/crystal counterrotation and with fast feed or crystal single rotation. First, single crystals of 0.6 cm diameter were grown with both 200 and 400 rpm counterrotation. Second, single crystals of 1.0 cm were grown with 200 rpm counterrotation. Third, single crystals of 0.6 cm diameter were grown with 400 rpm single rotation. In each case, the melt zone was significantly shortened and very stable, and the length/diameter ratio of the melt zone was significantly less than one. Calculated results from a computer model were used to explain the effect of counterrotation on the length/diameter ratio, the shapes of the melt/solid interfaces and the formation of gas porosity.