EXPERIMENTAL INVESTIGATIONS OF BUOYANCY-DRIVEN CONVECTION IN VERTICAL(BIXSB1-X)2TE3 MOLTEN ZONES

Experimental results for various states of buoyancy driven flow in ver tical (Bi0.23Sb0.75)2Te3 molten zones with covered surface are present ed. Critical thermal wall Rayleigh numbers Ra(w)c2 for the onset of ti me-dependent convection have been determined by means of temperature m easurements. The stability diagram obtained for the existing buoyancy driven convection shows the increase of Ra(w)c2 with increasing aspect ratio. This relation is also known from other crystal growth configur ations and is due to the damping influence of container walls. At the beginning in the oscillatory region of convection extremely long perio ds of oscillation (maximum 850 s) were observed, which are caused by a nother mechanism than periods (25 ... 37 s) registered at increasing m elt heights. Furthermore, Bi0.5Sb1.5Te3 crystals were grown by using t he vertical zone melting technique. The microscopic striations observe d in the grown crystals correlate exactly with the temperature signals caused by time dependent convection. However, the fluctuations of the tellurium distribution in axial direction measured by scanning the Se ebeck coefficient are presumably generated by unsteady solutal convect ion during growth.