zone melting
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What is zone melting?
Zone melting is a set of techniques used to purify or control the composition of an element or compound. It involves melting a short region (a zone) and moving this liquid zone slowly through a relatively long piece of solid material.
As the zone travels, it redistributes impurities along the material, with the final impurity distribution depending on factors such as the initial impurity distribution, the distribution coefficient (a characteristic of the impurity), and the size, number, and direction of the zones.
Zone refining, the most important of these techniques, refines a solid by passing multiple molten zones through it in one direction, moving a fraction of the impurities to the end, thus purifying the remainder. This method is especially important in the manufacture of semiconductors.
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Why is zone melting used in industry?
Zone melting is used in industry for purification or to control the composition of a substance. In this technique, a short molten zone travels slowly through a long solid ingot, redistributing impurities.
Zone refining, the most important of these techniques, involves passing several molten zones through a solid in one direction, moving impurities to the end and purifying the remainder. This method is especially important in the manufacture of semiconductors and has been used to purify more than a third of the elements and hundreds of inorganic and organic compounds to their highest purity. While relatively slow, zone refining offers simplicity and reduces contamination. Float zoning, a variation, is used to produce ultrapure silicon.
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How does the zone melting process work?
Zone melting is a technique used to purify or control the composition of elements or compounds. It involves melting a short region (the zone) and moving it slowly through a long piece of solid material. As the zone travels, it redistributes impurities. The process relies on the fact that a freezing crystal has a different composition than the liquid it came from.
Zone refining, the most important of these techniques, refines a solid by passing several molten zones through it in one direction. Each zone carries impurities to the end of the solid, purifying the rest. This method is especially important in the manufacture of semiconductors.
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What types of materials are purified using zone melting?
Zone melting is used to purify various materials, including more than a third of the elements, and hundreds of inorganic and organic compounds. It has been utilized as the ultimate purification technique for hundreds of substances. Zone refining is especially important in the manufacture of semiconductors. The principles of zone refining are quite general, so the method has been applied to many substances. Purities as high as 99.999 percent are often obtained by application of this technique. Many of these materials were, for the first time, made pure enough for their intrinsic properties to be determined.
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How does zone melting improve the properties of materials like semiconductors?
Zone melting enhances semiconductor properties by purifying materials. In this method, a short molten zone travels through a solid ingot, redistributing impurities. As the zone moves, it picks up impurities at its advancing edge, allowing a purer material to recrystallize at its opposite edge.
The process leverages the fact that a freezing crystal's composition differs from the liquid it crystallizes from. Impurities concentrate in the liquid zone and are moved to the end of the material, which can then be removed. This technique is especially useful when the separation in boiling points is not favorable. Float zoning, a variation, further reduces contamination by eliminating the need for a container. By repeating the process, materials can achieve ultrahigh purities, reaching levels as high as 99.999%.
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What industries rely most on zone melting, and why is it important to them?
Zone melting is most crucial to industries requiring ultrapure materials, especially semiconductors. It is a method of purifying a substance 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.
The semiconductor industry adopted zone refining in the early 1950s to purify germanium for transistors. A U.S. scientist invented float zoning to produce ultrapure silicon, which is even more useful than germanium for most transistor applications. Purities as high as 99.999 percent are often obtained by application of this technique. More than a third of the elements and hundreds of inorganic and organic compounds have been raised to their highest purity using zone refining.
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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 of—less than one part of detectable impurity in 10,000,000,000 parts of germanium. The method was adopted in transistor manufacture around the world.
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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.
Principles of zone refining.
When a cylinder of a substance A containing an impurity B is melted and then slowly frozen from one end to the other, as in , the impurity is usually concentrated in the last-to-freeze region of the cylinder. This procedure is normal freezing. Component B is redistributed in this example because the atoms (or molecules) of B at the liquid-solid interface prefer the liquid phase to the solid phase. A measure of this preference is the distribution coefficient, k, defined as the ratio of the concentration of B in the just-forming solid A to that in liquid A. At very slow freezing rates an equilibrium exists; the distribution coefficient under these equilibrium conditions is termed k0. At moderate freezing rates, about 1 to 30 centimetres per hour (0.4 to 12 inches per hour), the effective distribution coefficient, k, will lie somewhere between k0 and unity. This is because, for k less than unity, the rejected impurity B accumulates in the liquid just ahead of the advancing solid, so that the just-forming solid “sees” a liquid more impure than the bulk liquid. If freezing is rapid enough, k0 may approach unity; that is, the impurity concentration would be the same in the liquid and solid phases. Under these conditions, there would be no zone refining, and the interface probably would become dendritic or branching in shape.
The normal freezing operation is the basis of the long-known technique of repeated fractional crystallizations. Although this technique was employed by the Curies to isolate radium it never became widely used because it entailed a lengthy and troublesome sequence of operations: partial freezing, separation of the crystals from the unfrozen liquid, remelting, and recombining with other fractions.
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Zone refining achieves the same result very simply. A series of molten zones traverse the ingot in the same direction, usually through a series of heaters, as suggested in . Each zone takes in impurity at its melting interface and freezes out solid purer than the liquid at its freezing interface. There is no need to separate and recombine fractions, or even to touch or move the charge at all.
The distribution of impurity B after successive zone passes for an ingot 10 zones long and for a distribution coefficient k equal to 0.5 (a value neither especially favourable nor especially unfavourable) is shown in .
As more zone passes are made the impurity concentration at the beginning of the ingot drops lower and lower until it eventually reaches a limit called the ultimate distribution. The lowest concentration of impurity B is extremely small, less than 0.0001.