Morphological instability and supersaturation threshold pull rates in float-zone traveling solvent crystal growth of LaB6

In this article we analyze the problem of the onset of morphological instability and increasing supersaturation gradient in float zone traveling solvent crystal growth of LaB6 in excess of La with mass transfer dominated by Marangoni convection. A parametric study reveals how threshold pull rates marking the onset of these phenomena and resulting in various solid microstructures depend on other operational parameters, First, an increase of the initial mass of La solvent leads to a strong decrease in the threshold pull rates, Second, an increase in the crystal radius leads to a decrease in the threshold pull rates for a given solute composition. Third, an increase in the heating source width does not alter the threshold pull rates provided that this width is smaller than the zone height. However, when this width becomes larger than the zone height a further increase results in a very sharp decrease in the threshold pull rates due to a sharp decrease in the temperature gradient in the growing crystal because the heating Bur distribution overlaps the crystal surface. Fourth, an increase in the zone height (for a given initial mass of La solvent and heating flux width) may lead to a very strong increase in the threshold pull rates due to the combined effects of solvent content decrease and cessation of overlapping of the heating source distribution with the crystal surface. Finally, explicit analytical expressions are proposed to evaluate these threshold pull rates.