Zone Melting of Bi0,5Sb1,5Te3 Crystals under Microgravity

Abstract

Two samples of the thermoelectrical material Bi0.5Sb1.5Te3 were grown by zone melting technique during a space flight of the russian space station “MIR” in 1994. By comparing the flight samples with the reference ground samples relevant conclusions regarding the influence of different mass transport situations in the melt on micro- and macrosegregation are possible. The samples were analyzed metallographically, by local resolved scanning of the SEEBECK-coefficient and by WDX measurements. Contrary to the ground samples the flight samples showed no convection-induced striations in the bulk. Consequently under microgravity unsteady flow in the melt has been avoided. The axial distribution of the Te component in the flight samples showed an unexpected Pfann-like behaviour, which points to a convection-controlled growth. Contrary to this the axial distribution of the metal components was diffusion-controlled. The axial macrosegregation of tellurium found in the flight samples can be explained by the high sensitivity of components with a low distribution coefficient to weakest convective flows. The radial distribution 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 variation of the Te concentration boundary layer across the interface.