Patent GB-1430681-A (GB)
Title: Method for producing high quality fuese silica
IPC: C03B19/14
Filed: 1973-03-29
Abstract: 1430681 Fused silica CORNING GLASS WORKS 29 March 1973 [30 March 1972] 15262/73 Heading C1M A method of making articles of fused silica of high optical quality which are optically transparent, water-free and free of gaseous occlusions, in which a silicon-containing soot is deposited upon a mandrel, comprises the steps of depositing on to a non-metallic mandrel a soot formed by flame hydrolysis of a silicon compound so as to form a preform, the burner used in said flame hydrolysis being sufficiently close to the mandrel that the said mandrel is within the outer cone portion of the flame from said burner, and said mandrel being translated and rotated for depositing a uniform dense soot, and gradually inserting said mandrel with said soot deposited thereon into a furnace at a rate sufficient to sinter the soot and at a rate which permits entrapped gas to escape through portions of the soot not in the hot zone of the furnace as the soot is sintered and shrinks around the mandrel. The soot may be undoped or may be doped with zirconium, tin, germanium, tantalum, titanium, aluminium, tungsten, molybdenum, beryllium and niobium Suitably the SiO 2  soot is deposited by flame hydrolysis of SiCl 4  optionally with a dopant halide. Layers up to 1 inch or more may be deposited. The mandrel may be of graphite, refractory or crystalline ceramics, e.g. Al 2 O 3 , mullite, BN or SiC, glass or glass ceramic. An induction furnace having a temperature of 1400-1700 C. is particularly suitable and the preform is preferably passed into the furnace at 0À1-1À0 inches per minute. An inert dry atmosphere, e.g. nitrogen, helium, neon or argon, is also preferably maintained in the furnace to avoid oxidation of the graphite mandrel. As the preform is gradually inserted into the furnace it generally has air entrapped in it. As it moves into the furnace this is replaced by the inert gas which is then easily dissolved in the glass during its sintering. In the embodiment specifically described the mandrel is inserted downwardly into the furnace. After the preform is consolidated it is quickly removed from the furnace. A schematic diagram of a flame hydrolysis unit for producing doped or undoped soots on a mandrel is illustrated in Fig. 5. This comprises a dopant vapour generator A for generating vapours of doping materials whose halide forms are solids at temperatures where their vapour pressure is # 40 mm. Hg, which is connected to a vapour delivery system B into which passes the dopant vapour and SiCl 4  vapour which is generated from a flask D. The dopant vapour generator A consists of two main parts: a tube furnace 30 and placed therein a generating chamber 31 to which is attached an inlet tube 37 through which the carrier gas enters the vapour chamber. The halide compound powder is placed on a series of stainless steel screens 39. The uppermost screen is left uncovered. A specific example refers to the production of a glass soot comprising 96% by weight SiO 2  and 4% by weight ZrO 2 . The blanks produced can be used in optical waveguides, lenses, prisms, filters, high-temperature lamp envelopes, crucibles and tubing.