Chemistry of inorganic compounds – Silicon or compound thereof – Binary compound
Patent
1996-12-06
2000-02-08
Langel, Wayne
Chemistry of inorganic compounds
Silicon or compound thereof
Binary compound
C01B 3136
Patent
active
060225157
DESCRIPTION:
BRIEF SUMMARY
The present invention refers to a process for producing silicon carbide. In particular the invention refers to a method for a two-step process for producing silicon carbide.
The process for industrial production of silicon carbide commonly used at the present comprises reacting a silicon source with a carbon source in an electrically heated furnace. For industrial production the so-called "Acheson"-process is used, patented in 1892 and named after the inventor of said process.
A general account re silicon carbide, properties and production thereof can i.a. be found in Kirk & Othner, Encyclopedia Chemical Technology, 2nd edition, volume 4, the pages 114-132.
From GB patent application No. 2.076.385 it is known to produce .beta.-silicon carbide and it is stated that in order to obtain a good yield of .beta.-silicon carbide the starting material must be present as a powder with a particle size less than 150 .mu.m.
From GB patent application No. 2.162.504 A is known to produce .beta.-silicon carbide by packing the reactants to a bulk-density of not more than 0,23 g/cm.sup.3 and heating the mixture for reaction at a temperature of 1500-2000.degree. C. under an inert atmosphere. Said application also refers to a vertical continuous furnace for carrying out the process.
GB patent application No. 2.220.409 A refers to a method for preparing .beta.-silicon carbide powder suitable as a sintering material by mixing a silicon powder of high purity and not larger than 50 .mu.m in particle size with a carbon powder not larger than 1 .mu.m in particle size and heating the mixture in a non-oxidizing atmosphere at a temperature of 1700-2000.degree. C. for a period not exceeding 2 hours. Particular care should be taken not to use a temperature above 2000.degree. C., in which case there may be a risk of phase transition from .beta.-silicon carbide to .alpha.-silicon carbide.
GB patent No. 1.208.001 refers to a method for producing crystalline silicon carbide by heating a system comprising silicon and carbon and/or compounds of said elements together with lanthanum or a lanthanum compound to a temperature exceeding 1000.degree. C., whereby silicon carbide crystals are deposited from a gaseous or vapour phase.
In spite of the extensive research described in the literature, the patent literature included, .alpha.-silicon carbide is still produced in accordance with the Acheson's process with comprises premixing of the starting material consisting of a carbon source and a silicon source, the latter in the form of quarts sand, and as a carbon source petrol coke is commonly used.
The premixed starting materials are introduced into large rectangular electrical resistant furnaces, the end wall of which are fitted with electrodes for electrical power supply and the side walls consisting of movable concrete sections. The oven is half filled with the mixed starting materials, and subsequently a graphite core, forming and electrical heating element is supplied, whereafter the remaining charge of the starting material is added. The reaction between the starting materials takes place at a temperature above 1500.degree. C. and silicon carbide is formed in accordance with the following overall equation
This general reaction equation is simplified in that the end product, i.e. .alpha.-SiC, is formed via a series of intermediates, thus complicating the reactions.
In general the conversion into silicon carbide takes place via the formation of .beta.-SiC (cubic modification) at a temperature of 1500-1800.degree. C., whereas the transition of .beta.-SiC into the desired end product, i.e. the hexagonal .alpha.-SiC modification, takes place in a recrystallization step at a temperature above 1800.degree. C.
The heat is supplied in the centre of the charge of starting material, consequently a temperature gradient will develop throughout the starting material with the hottest part in the centre of the starting material with the temperature and falling towards the outer parts of the material. As a result after a appropriate reaction time, which for a
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patent: 4162167 (1979-07-01), Enomoto et al.
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patent: 5021230 (1991-06-01), Krstic
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patent: 5258170 (1993-11-01), Parent
Gmelins Handbuch Der Anorganischen Chemie--Silicum, 1959, p. 761-789 Verlag Chemie, GMBH, Weinheim/Bergstrasse.
St.o slashed.le Kjell Arnulf
Velken Sjur Vidar
Cahoon Colin P.
Langel Wayne
Norton AS
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