Chemistry of inorganic compounds – Silicon or compound thereof – Elemental silicon
Patent
1986-03-07
1989-05-09
Doll, John
Chemistry of inorganic compounds
Silicon or compound thereof
Elemental silicon
75 05R, 75 05B, 148 1, 148 131, 148 15, 423290, 423299, 423344, 423345, 423349, 423350, 423440, 423508, 423DIG12, C01B 3302, C22C 104, C21D 144, C21D 148
Patent
active
048288144
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION 1. Field of the Invention
This invention relates to a process for the purification of a solid material. More particularly, this process relates to a process for purifying a solid material such as a metal, a metalloid, or a metal compound which has been heated to a temperature approaching the melting point of the material to be purified while contacting the heated material with a purifying agent which is substantially non-reactive with the solid material.
2. Description of the Prior Art
There is an increasing demand for high purity materials such as silicon, titanium, boron, gallium arsenide, silicon carbide, etc. for diverse applications such as solar cells, rocket fuel, high purity alloys, semiconductors, and nuclear fuel applications.
For example, an increasing demand for silicon of sufficiently high purity to be suitable for use in the semiconductor and solar cell industries has lead to investigation of many processes to achieve such purity levels. Such processes typically involve some sort of treatment of molten silicon. Purification of a material such as silicon in a molten state is, however, not new. For example, Allen U.S. Pat. No. 1,037,713 describes the purification of silicon by treating molten silicon with metals, such as alkali metals and alkaline earth metals including magnesium.
Brockbank U.S. Pat. No. 1,180,968 describes melting silicon under a slag of natural or artificial silica to eliminate impurities while Pacz U.S. Pat. No. 1,518,872 describes silicon as a valuable byproduct of a reaction between aluminum powder and a metallic fluorosilicate, such as magnesium fluorosilicate.
Pruvot et al U.S. Pat. No. 3,034,886 describes the purification of silicon or ferrosilicons by the injection of silicon fluoride gas into the liquid bath to react with aluminum and calcium impurities to form aluminum and calcium fluorides.
The use of molten metal fluorides for purification of silicon at a temperature of 1000.degree.-1600.degree. C. has been proposed by Coursier et al U.S. Pat. No. 3,148,131. The patentees, however, propose the use of metal fluorides which, in the main, either represent costly materials or materials known to react with silicon to form silicon fluoride and inject impurities in the silicon that are detrimental to its electronic properties.
Boulos U.S. Pat. No. 4,379,777 teaches passing powdered silicon through a plasma which apparently causes migration of the impurities to the surface of the molten silicon particles. After quenching, the particles are acid-leached to remove the surface impurities.
Kapur et al U.S. Pat. No. 4,388,286 combines vacuum refining of silicon with mixing the silicon with an effective fluxing agent, such as a fluoride of an alkali metal or an alkaline earth metal, to form a molten silicon phase and a slag phase.
One of us has also authored or coauthored papers which refer to the purification of molten silicon in contact with NaF in "Silicon Sheet for Solar Cells", by A. Sanjurjo published in the Journal of the Electrochemical Society, Volume 128, pp. 2244-2247 (1981) and "Fluxing Action of NaF on Oxidized Silicon", by L. Nanis, A. Sanjurjo, and S. Westphal published in Metallurgical Transactions B, Volume 12B, pp. 535-573 of the American Society for Metals and the Metallurgical Society of AIME (1981).
Not all prior silicon purification processes, however, involve the melting of silicon. Ingle U.S. Pat. No. 4,172,883 discloses a process for purifying metallurgical grade silicon by heating it to 800.degree. to 1350.degree. C. and contacting it with silicon fluoride gas which is said to react with the impurities causing them to deposit out. The aforementioned Coursier et al patent also speaks of purification temperatures below the melting point of silicon.
It is also known to purify materials such as silicon by acid-leaching of the material in powder form as well as by unidirectional solidification of the material. In the case of silicon, some of these processes may be less expensive than the conventional method for obtaining high pur
REFERENCES:
patent: 1037713 (1912-09-01), Allen
patent: 1180968 (1916-04-01), Brockbank
patent: 1386227 (1921-08-01), Becket
patent: 1518872 (1924-12-01), Pacz
patent: 3034886 (1962-05-01), Pruvot et al.
patent: 3148131 (1964-09-01), Coursier et al.
patent: 3933981 (1976-01-01), Wakefield et al.
patent: 4172883 (1979-10-01), Ingle et al.
patent: 4379777 (1983-04-01), Boulos
patent: 4388286 (1983-06-01), Kapur et al.
patent: 4612179 (1986-09-01), Sanjurjo et al.
A. Sanjurjo, "Silicon Sheet for Solar Cells," Journal of Electrochemical Society, vol. 128, No. 10, Oct., 1981.
L. Nanis, A. Sanjurjo, and S. Westphal, "Fluxing Action of NaF on Oxidized Silicon," Metallurgical Transactions B, vol. 12B, Sep., 1981.
Pressacco Sylvia
Sanjurjo Angel
Doll John
Freeman Lori S.
SRI - International
Taylor John P.
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