Abstract
A comparative experimental study of thermal and thermosolutal convection is carried out by solidifying a concentrated Ge-Si binary system and a dilute Ge-Ga system in a new Bridgman furnace where radial thermal gradients have been minimized and well characterized. Very different solutal boundary layer extents (δ GeSi > 3 cm, δ Gega ≦ 2 mm) are found for the two systems solidified under the same thermal conditions. Simple analytical hydrodynamic models are then used to explain these results. It is demonstrated that the larger boundary layer extent observed for Ge-Si and the corresponding pure diffusive transport regime are the result of the stabilizing effect of the longitudinal solutal gradient in this system. It is also shown that Hart's analytical model [J. Fluid Mech, 49 (1971) 279] can be adapted to describe the thermal/solutal coupling of the fluid flow in crystal growth configurations and therefore to predict, in a first approximation, the segregation behaviour in the crystals through a boundary layer model [Favier, Acta Met. 29 (1981) 197, 205].
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