Definition of zone refining

1) A method of purifying a bar of metal by passing it through an induction heater.

2) Developed by Bill Pfann in Bell Telephone Laboratories as a method to prepare high purity materials for manufacturing transistors. Its early use was on germanium for this purpose, but it can be extended to virtually any solute-solvent system having an appreciable concentration difference between solid and liquid phases at equilibrium.

In zone refining, solutes are segregated at one end of the ingot in order to purify the remainder, or to concentrate the impurities for analytical or other purposes. In zone leveling, the objective is to distribute solute evenly throughout the purified material, which may be sought in the form of a single crystal. For example, in the preparation of a transistor or diode semiconductor, an ingot of germanium is first purified by zone refining. Then a small amount of antimony is placed in the molten zone, which is passed through the pure germanium. With the proper choice of rate of heating and other variables, the antimony can be spread evenly through the germanium. This technique is also used for the preparation of silicon for use in computer chips.

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