Investigation of thermosolutally caused convection during BRIDGMAN-growth of BiSbTe3-mixed crystals at normal and reduced gravity.

BiSbTe3-mixed crystals have been grown at normal and reduced gravity (durin g the MIR'97-mission) using a BRIDGMAN-configuration of the TITUS facility. The distribution of the components in the melt, and so the homogeneity of the growing crystal, is strongly influenced by the flow in the melt even in the case of weak convection. The flow configuration in the melt especially in front of the solid-liquid phase boundary can be investigated by means o f a segregation analysis of the system components and an additional Pb-dopa nt. The BiSbTe3-system is because of its hydro-dynamic properties a typical representative of semiconductor melts (low PRANDTL number, high SCHMIDT nu mber) but there are also some special properties relating to the segregatio nally caused enrichment of the lighter tellurium at the phase boundary and the resulting solutal destabilities. Experimental experiences from segregat ion analysis have shown that the mass transport in the melt at normal gravi ty is mainly influenced by convective mixing determined by thermally and so lutally caused buoyancy forces. Numerical simulations have been performed f or the real experimentally used configurations. These simulations have show n that a strong coupling of thermal and solutal effects exists and have giv en axial as well as radial segregation profiles being in excellent agreemen t with the experimental results for the vertical normal gravity grown cryst als. For micro gravity conditions a reduction of the flow velocity of more than two orders of magnitude (depending on the micro gravity level and the direction of the residual acceleration) resulting in diffusion controlled c omponent segregation has been predicted. The results of the two micro gravi ty grown crystals, especially the axial and radial segregation profiles as a sensitive indicator for the flow configuration in front of the phase boun dary will be given and discussed in the paper. They will be compared with t he results of numerical simulations of the melt flow for the real processin g parameters measured during the TITUS growth processes and with experiment al as well as numerical results for vertical normal gravity grown reference samples. (C) 1998 International Astronautical Federation Published by Else vier Science Ltd. All rights reserved.