Zone refining method

Atmospheric sensitivity renders the preparation of ultrapure samples difficult. Nevertheless, vacuum distillation ", ultra-high-vacuum reactive distillation " and crystal growth purification methods " are described zone-refining methods have been applied on a limited scale only - , presumably because of the high volatility of the metals and the unfavorable distribution coefficients. 

Figure 2 Schematic diagram of float zone or zone-refining method of Si purification...

To circumvent the various limitations of the basic technique, several continuous zone refining methods have been developed in which feed enters at one point in the sample while the product and waste leave at other points (Figure 4). The effect of countercurrent movement of solid and liquid phases is achieved by the movement of molten zones. In addition to the horizontal continuous refiner, the zone void vertical refiner and zone transport refiner are other modifications of this class. Cross-flow zone refiners and rotating drum multistage crystallizers based on the above principle are mainly used in growing single crystals rather than in purification of materials. 

Zone refining methods can produce materials that are about 99.999999 percent pure, containing impurities of only a few parts per billion. 

A zone refining method is used to produce single crystals of silicon that are typically 30 centimeters in diameter and more than 1 meter in length. 

Purification presented a much greater problem. In the early 1950 s The Siemens Company developed a method based on reaction of Si with HCl to produce dichlorosilane, SiH2Cl2, an easily evaporated liquid. The dichlorosilane is fractionally distilled and subsequently reduced in a reverse reaction to produce pure Si. (See also Chapter 4.) In 1952, W.G. Pfann created the zone refining method for further reduction of impurities, which was subsequently improved to reduce handling of the material between process steps. This method was used through the 1970 s. Around that time, improvements in the initial purification process made zone refining largely unnecessary. 

TJItrahigh (99.999 + %) purity tellurium is prepared by zone refining in a hydrogen or inert-gas atmosphere. Single crystals of tellurium, tellurium alloys, and metal teUurides are grown by the Bridgman and Czochralski methods (see Semiconductors). 

Crystallization from the liquid metal has been assessed theoretically and experimentally as a purification method for both Sr and Ba After five recrystallizations, metals of 99.9999% purity can be produced. The purification of Ba by zone refining is inefficient, many passes being required for the removal of the other alkaline-earth metals. The application of combined methods (i.e., vacuum distillation plus crystal growthor vacuum distillation plus zone refining ) to the purification of both metals has also been described. 

It is for these reasons that zone-refining has found limited application, since actual purification by this method does not produce a crystal with a completely uniform impurity distribution. 

The method of zone refining which was first used in the production of very pure germanium depends for its success on the difference between the thermodynamic properties of an impurity, present as a dilute constituent dissolved in... 

Magnesium chloride and excess magnesium are removed by distillation at reduced pressure. Pure zirconium may be prepared by several methods that include iodide decomposition process, zone refining, and electron beam melting. Also, Zr metal may be electrorefined in a molten salt bath of potassium zirconium fluoride, K2ZrFe... 

Although all the physical methods of purification, which comprise the various forms of distillation and of crystallisation, as well as adsorption, can be done in a high vacuum system, the only ones for which this is generally worthwhile are crystallisation without solvent and its sophisticated version zone-refining, and adsorption. Of course, degassing cannot be done otherwise than on a vacuum system. 

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