GB867871A - Improvements in or relating to the preparation of elemental silicon - Google Patents

Abstract

Silicon halides are purified by contacting the halide in the vapour state with silicon at 850 DEG C. to 925 DEG C. The silicon should be as pure as possible, and the halide preferably remains in contact with the silicon for 0.1 to 10 seconds. The silicon halide may be prepurified by passing the vapour through a bed of fused silica rings at about 1000 DEG C., condensing the vapour, adding water, preferably 0.01 to 10% mols, allowing the mixture to stand, distilling it, and again vaporizing the halide prior to passing it over the silicon by dropping it on to fused silica chips maintained at 600-700 DEG C. All apparatus is preferably constructed of fused silica with welded joints, Silicon halides referred to include disilicon hexachloride, silicon tetrabromide, silicon tetraiodide, and especially silicon tetrachloride. The silicon halide thus purified may be converted to elemental silicon by reducing it with elemental zinc or cadmium in the vapour state. The zinc or cadmium vapour is preferably purified prior to the reduction by passing it through a bed of heated silicon at 800-1100 DEG C. in a similar fashion to that employed for purifying the halide. Preferably the reactants are passed p directly from the purifying steps to an elongated reactor, preferably maintained at a temperature above the boiling points of the reactants and by-products, and below the melting point of the silicon, for example about 950 or 1000 DEG C. A slight excess of the halide may be maintained. Crystalline silicon is deposited, and the by-product metal halide vapours and excess silicon halide vapours are removed and recovered. When the reactor is full of silicon the flow of metal vapour is stopped and the flow of halide continued for some time. The flow of halide is then stopped and the silicon product is then cooled with pinging by an inert gas, for example nitrogen.ALSO:Zinc or cadmium of high purity may be obtained by contacting the metal vapour with silicon at 800-1100 DEG C., preferably for a period of 0.1 to 10 seconds. The silicon should be as pure as possible. The thus purified metal vapour is used for the reduction of purified silicon halides to give pure silicon (see Group III).