The distribution of minute impurities in the molten metal near the solid-melt interface that determines the concentration of impurities in the solidified metal may be affected largely by the convective flow of the melt.C.Wagner has treated the same subject, with the assumption that such a type of flow is caused by the difference of local concentrations in the melt. However, in the case of ordinary zone melting where the travelling resistance furnace is used, it is expected that natural convection caused by the local temperature difference will rather predominate. Therefore, with the model of natural convective flow due to thermal gradient perpendicular to the vertical solid-melt interface, the author discusses the distribution of impurity concentration near the interface and derives the effective distribution coefficient as a function of the solidification rate and the distance measured downward from the free surface. The results are in good agreement with the values measured with Sb-doped germanium single crystals grown parallel to the < 111 > axis by zone leveling method.
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