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Why can the impurity-rich end of a zone-refined rod be removed by cutting it off?
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zone melting
Figure 1: Schematic representation of (A) normal freezing, (B) zone refining
In the zone refining process, the impurity-rich end of a rod can be removed by cutting it off because the technique relies on the redistribution of impurities during the liquid-solid phase transition.12
As a molten zone travels along the rod, it carries impurities with it because the freezing point of the substance is depressed by those impurities. Consequently, the impurities prefer the liquid phase over the solid phase and are pushed toward the end of the rod as the material behind the moving zone crystallizes in a purer state. By the end of the operation, the impurities are concentrated and solidified at the final section of the rod, allowing for their simple physical removal.12