Purification of Metals by Zone Melting
It has been stated  that metal purification by zone melting is dependent only on the ratio of the solubilities for the impurities in the solid and liquid phases as well as concentration ratio in the solid and in the adjacent liquid layer. However, we have found  that the impurity uptake by a growing crystal is determined also by the number of subindividuals present; the impurity uptake increases with this number for nonisomorphous substances, and none of the latter may be taken up if there are no subindividuals. The latter tend to displace impurities from one towards another, and so the impurities accumulate between the subindividuals in relatively larger amounts. We have shown  that subindividual formation is dependent on the supercooling differences between parts of the surface, which arise from concentration or convection currents. If the differences are substantial, a part in contact with the more supercooled melt starts a new layer before the previous one has spread entirely over the face, and so subindividuals can arise there. The name critical is given to the supercooling difference needed to initiate sub individual formation; it is less for fast-growing faces.
KeywordsCrystallization Starch Convection Iodine Supersaturation
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