1952

Zone Refining

William Gardner Pfann (1917–1982)

Impurities lower a substance’s melting point. That’s why salts of various kinds are used to melt ice—salty water has a freezing point well below that of pure water. It’s also the reason that carefully determined melting points are considered an excellent test of purity. In the old days of organic synthesis, before modern instrumentation, the final test of whether a natural product compound had truly been synthesized was a “mixed melting point.” That involved mixing together the newly synthesized sample with a small amount of an authentic sample and checking the melting point before and after. The melting point of this mixture would stay the same only if its two components were identical.

Lowering the melting point is also the basis for an ingenious technique that’s been used to purify metals and other substances. If a long rodlike sample is pulled slowly through a narrow heating zone, any impurities in the sample are swept along in the molten band (whose melting point goes down more and more as the impurities accumulate), and much cleaner (purer) material is left behind. At the end of the process, the impurities have concentrated at one end of the sample, which can be sawn off. In 1952, American chemist and materials scientist William Gardner Pfann developed the technique, known as zone refining, at Bell Labs to purify semiconductor materials such as germanium and (later on) higher-melting silicon, producing material that was one thousand times cleaner than had ever been tested.

Zone refining is still an excellent method for producing extremely pure samples of high-value materials, such as metals and semiconductors, although some materials are much better candidates for it than others. The best are substances whose solid and liquid phases have quite different properties. Even in these cases, the impurities have to be able to move through the melt zone at decent rates, and those rates need to be similar, but zone refining is still one of the best purification techniques available when those conditions are met.

Ultra-pure silicon and material with precisely known amounts of the right impurities are essential for computer chip production.