A fast method of zone melting as an aid in analytical chemistry

Bollen, N.J.G.; Essen, Marius J. van; Smit, W. M.

doi:10.1016/s0003-2670(01)80588-2

Cited by 18 publications

(3 citation statements)

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“…With vertical zone melting and horizontal zone melting without a gas bubble, simple tube rotation at a constant moderate velocity does not significantly influence δ. In those cases, accelerated crucible rotation or spin up-spin down could be used (72)(73)(74)(75). The tube is spun more rapidly than described above, but not at constant velocity.…”

Section: Stirringmentioning

confidence: 99%

Zone Refining

Wilcox¹

2000

Kirk-Othmer Encyclopedia of Chemical Technology

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Zone refining is one of a class of techniques known as fractional solidification, in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1-8). Solid-liquid phase equilibria are utilized, but the phenomena are much more complex than in separation processes utilizing vaporliquid equilibria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive solid is formed slowly and a sizable temperature gradient is imposed at the solid-liquid interface.Zone refining was developed in the early 1950s at Bell Telephone Laboratories in response to the need for extremely pure germanium. It was an essential step for the development of the transistor and thus for the entire electronics revolution. Before that time, purification was carried out by normal freezing, also called progressive freezing. A melt was slowly frozen, causing more impurities to be concentrated in the last melt to freeze. In progressive freezing, however, a single solidification did not give sufficient purification. Simply remelting the entire ingot would redistribute the impurities, thereby eliminating the purification of the first solidification. the impure end could be removed, but handling and cutting introduce new impurities. The problem was solved by melting only a small part of the material and passing this molten zone down the ingot. Subsequent zone passes increased purification without requiring handling or cutting or permitting excessive back-mixing.Zone refining can be applied to the purification of almost every type of substance that can be melted and solidified, eg, elements, organic compounds, and inorganic compounds. Because the solid-liquid phase equilibria are not favorable for all impurities, zone refining often is combined with other techniques to achieve ultrahigh purity.The high cost of zone refining has thus far limited its application to laboratory reagents and valuable chemicals such as electronic materials. The cost arises primarily from the low processing rates, handling, and high energy consumption owing to the large temperature gradients needed.Actually, zone refining is only one of a class of techniques known as zone melting in which a molten zone is passed down a solid rod. Zone melting is used routinely to collect impurities in high purity materials, eg, silicon, in preparation for chemical analysis. Growth of bulk single crystals is an important application that includes commercial float-zoning of silicon crystals for the semiconductor industry (7). Floating-zone melting is being considered for use aboard the U.S. Space Shuttle because the absence of gravity might permit larger crystals to be grown with greater homogeneity.Addition of solvent to the zone allows crystals to be grown that either decompose before melting (incongruent melting) or have a very high vapor pressure at the melting point. This t...

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Section: Stirringmentioning

confidence: 99%

Zone Refining

Wilcox¹

2000

Kirk-Othmer Encyclopedia of Chemical Technology

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“…It was tested on benzene doped with anthracene and was found to remove the anthracene effectively. It was also applied to batch and continuous purification of 1 -nitronaphthalene.…”

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“…One of the most troublesome of these problems is their high viscosity, which makes difficult the removal of impurity rejected from the advancing solid/liquid interface. Stirring of the liquid at the interface has been used to create shear at the interface, to achieve higher separation efficiency (1). Another problem, especially in attempts to purify multikilogram quantities, is the low thermal conductivity of organic compounds, which limits the rate at which the latent heat of fusion may be applied to the sample and removed from it.…”

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confidence: 99%