Method of enhancing fused quartz body purity

Glass manufacturing – Processes – With chemically reactive treatment of glass preform

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Details

65 3013, 65 184, 65111, 65136, 156602, C03C 2300, C03B 2000

Patent

active

050964798

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a method of enhancing the purity of a body of fused quartz, e.g. vitreous silica, having opposed boundary surfaces, the method being of the kind, which includes the steps of comprising maintaining the body at a temperature above 1000.degree. C. and at the same time applying a polarizing potential across the boundary surfaces by electrodes in contact with the boundary surfaces so that at least some of the residual impurity ions are made to migrate away from one boundary surface towards the opposite boundary surface thereof and are subsequently discharged into the gaseous phase at the latter boundary surface. In particular the invention is concerned with an electrical purification method and articles purified by the method.
High purity vitreous silica (fused quartz) articles are commonly made by flame or electrical fusion of carefully refined powders derived from natural quartz crystal. With increasing attention being paid to residual impurities by certain industries, e.g. semiconductor and optical fibre manufacture, the level of purities sought is at times beyond that which can be achieved by conventional chemical and physical refinement techniques. There may even be an undesirable concentration of alkali ions, in articles manufactured from synthetic quartz or amorphous silica powders, either derived from the starting material or introduced accidentally as contamination in the manufacturing process.
For achieving very low levels of certain mobile metal ion impurities, notably ions of the alkali metals, e.g. lithium, sodium, potassium and copper, and the application of an electrical polarizing potential across the wall of a heated quartz article has been proposed (GB-A-2166434). This can effect migration of the impurity ions away from one of the wall surfaces, and toward the opposite wall surface. The article may subsequently be cooled to ambient temperature while the polarizing potential is still applied, in which case the impurities accumulating near the cathodic wall surface may be removed by machining or etching off a thin layer at that surface to leave the remaining mass of vitreous silica of higher average purity.
In one example included in the above-noted patent application, a method was described in which a vitreous silica crucible was inverted over an internal graphite mould and the crucible was rotated slowly while being heated wtih an oxy-propane burner. An electric potential of 4.5 kV was applied with the mould serving as negative electrode and the burner flame as positive electrode. Significant depletion of alkali metal ions was observed in the inner layers of the crucible, but while some coloration was observed in the flame due to the release of ions electrolyzed from the crucible into the gas flame, significant amounts of alkali metal remained in the outer layers of glass.


SUMMARY OF THE INVENTION

The present invention represents an advance on the prior art and while it has several applications, it is particularly suited to the manufacture of fused quartz tubing, crucibles, and other hollow ware. The method of the invention may be applied to a finished tube or hollow ware, or in the course of drawing of the tube, or manufacture of the hollow ware or both during and after manufacture.
According to the present invention a method of the kind referred to of enhancing the purity of a body of fused quartz having opposed boundary surfaces, is characterised in that each of the electrodes is gaseous and at least partially ionized.
Whereas in the past one or both of the electrodes used for electrolysing fused quartz has been a solid in contact with a surface of the fused quartz, it has now been found that it is possible to effect purification of a mass of heated fused quartz, for example a tube, by non-contact means using hot conductive gas flows to serve as both positive and negative electrodes. This is particularly important in the purification of tubing for fibre optic applications, where any solid contact with the hot tube could cause unaccep

REFERENCES:
patent: 3485613 (1969-12-01), Herczog et al.
patent: 4654065 (1987-03-01), Naumann et al.
patent: 4759787 (1988-07-01), Winterburn
patent: 4824458 (1989-04-01), Ettori

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