ZONE-MELTING OF BI0,5SB1,5TE3 CRYSTALS UNDER MICROGRAVITY

Two samples of the thermoelectrical material Bi0.5Sb1.5Te3 were grown by zone melting technique during a space flight of the russian space s tation ''MIR'' in 1994. By comparing the flight samples with the refer ence ground samples relevant conclusions regarding the influence of di fferent mass transport situations in the melt on micro-and macrosegreg ation are possible. The samples were analyzed metallographically, by l ocal resolved scanning of the SEEBECK-coefficient and by WDX measureme nts. Contrary to the ground samples the night samples showed no convec tion-induced striations in the bulk. Consequently under microgravity u nsteady flow in the melt has been avoided. The axial distribution of t he Te component in the night samples showed an unexpected PFANN-like b ehaviour, which points to a convection-controlled growth. Contrary to this the axial distribution of the metal components was diffusion-cont rolled The axial macrosegregation of tellurium found in the flight sam ples can be explained by the high sensitivity of components with a low distribution coefficient to weakest convective flows. The radial dist ribution of the Te component in the flight samples is more homogeneous compared to the ground samples. The explanation of these differences succeeds only partly by the curvature of the interface and by the vari ation of the Te concentration boundary layer across the interface.