ZONE MELTING

any of a group of techniques used to purify an element or a compound or control its composition by melting a short region (i.e., zone) and causing this liquid zone to travel slowly through a relatively long ingot, or charge, of the solid. As the zone travels, it redistributes impurities along the charge. The final distribution of the impurity depends on its distribution in the starting charge of material; its distribution between the liquid and solid phase of the material (called its distribution coefficient, k, which is a characteristic of the particular impurity); and on the size, number, and travel direction of the zones. Zone melting is a means of using the freezing process to manipulate impurities. It combines the fact that a freezing crystal differs in composition from the liquid from which it crystallizes with the idea of passing a short liquid zone along a lengthy solid. Zone refining is the most important of the zone-melting techniques. In zone refining, a solid is refined by passing a number of molten zones through it in one direction. Each zone carries a fraction of the impurities to the end of the solid charge, thereby purifying the remainder. Zone refining was first described by the U.S. scientist W.G. Pfann and was first used in the early 1950s to purify germanium for transistors. The purity achieved was hitherto unheard ofless than one part of detectable impurity in 10,000,000,000 parts of germanium. The method was adopted in transistor manufacture around the world. The principles of zone refining are quite general, and so the method has been applied to many substances. More than one-third of the elements and hundreds of inorganic and organic compounds have been raised to their highest purity by zone refining. Many of these were, for the first time, made pure enough for their intrinsic properties to be determined.