Oscillatory convection and control by rotation in a large Prandtl number melt during a Czochralski process

摘要

This paper reports on a numerical investigation of melt convection in a model of a Czochralski process. The study is related to an actual growth experiment carried out on BGO crystals. The analysis focuses on the time-dependent disturbances that arise in the melt and on their control by crystal rotation. These unsteady motions are found to be important because they result in inhomogeneities in the crystal composition. The results emphasize the strong correlation that exists between the occurrence of striations in a grown Bi_(12)GeO_(20) crystal, and the computed time-dependent bulk flow at specific melt heights that corrrespond to the beginning of the growth process. The effect of varying the rotation Reynolds number on the flow and on the temperature distribution in the melt is explored over a range of rotation rates. The frequency of the disturbance is shown to depend on the rotation rate (at a given Grashof number) in agreement with measurements performed, for large Pr melt, during the actual Czochralski growth or in experimental set-ups from other authors (Iliev et al, 1991), (Ozoe et al, 1991).