Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Patent
1994-11-10
1998-02-03
Cano, Milton
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
65426, 65427, 65519, 65530, 65540, 6537413, 6537415, C03B 3200, C03B 37014, C03B 37029
Patent
active
057139792
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an improved heat treatment furnace and facility for the manufacture of large synthetic vitreous silica bodies.
2. Description of the Related Art
The manufacture of high purity vitreous silica bodies is of growing commercial importance due, in particular, to the needs of the optical fibre and semiconductor industries. Initially the requirements of these industries could be met with fused quartz glasses, made by melting highly refined quartz crystal powders by using electrical or flame fusion techniques. As purity requirements have advanced, however, it has been increasingly difficult, using naturally occurring crystal feedstocks, to achieve the purities sought and attention has turned to alternative synthetic sources of vitreous silica. Much work has been published concerning such alternative routes, the most important of which have been vapour deposition and sol-gel or related techniques.
For example, large bodies of porous synthetic vitreous silica can now be made by deposition of silica soot from a flame, in which silicon tetrachloride or an alternative volatile silicon-containing compound is hydrolysed or oxidised in a hydrogen-oxygen, or methane-oxygen flame. The flux of silica micro-particles is typically directed at a rotating cylindrical substrate, where it deposits as a porous body of pure vitreous silica, frequently known as a silica soot body. This soot body is subsequently converted to glass by a process of sintering, (also referred to as consolidation or vitrification). By adding appropriate dopant reagents to the flame, the soot body may be doped with either metallic or non-metallic species. Thus germanium, phosphorus etc., may be added as dopants to raise the refractive index of the product glass, and fluorine or boron may be added to reduce refractive index.
Such porous vitreous silica bodies may be alternatively made by casting shapes made using slurries of ultra-fine synthetic vitreous silica powders (e.g. P. K. Bachmann et al, Shape Forming of Synthetic Silica Tubes by Layerwise Centrifugal Particle Deposition, Ceramic Bulletin, 68(10), 1989), or by dry-pressing such powders (e.g. U.S. Pat. No. 4,620,862), or even by casting, from precipitated silica sols, though this last method presents problems for making large bodies.
In this specification it should be understood that where reference is made to a soot body, should the context allow, a body of porous synthetic silica derived from any alternative process is alternatively possible.
The above methods commonly give rise to solid or hollow cylindrical bodies of porous silica glass, typically of density 0.4 to 1.0 g/ml, which may subsequently be purified, for example by heating in a chlorine-containing gas. This treatment converts undesirable impurities in the glass into volatile chlorides which may thus be removed. Typical chlorine compounds include Cl.sub.2, SiCl.sub.4, SOCl.sub.2, COCl.sub.2 and CCl.sub.4. Removal of transition metals, e.g., iron, is illustrated in the following equation: +2FeCl.sub.3(g) +11/2O.sub.2 1
Another gas-phase process commonly undertaken to produce doped glass products for the optical fibre industry is treatment of the porous body in a fluorine-containing gas which, as noted above, yields a glass of lower refractive index. Typical dopant gases include SiF.sub.4, SF.sub.6, CF.sub.4 and other fluorocarbons, or chlorofluorocarbons.
The above gaseous treatment processes are undertaken typically at temperatures of between 800.degree. and 1200.degree. C.
Following any such purification or doping treatment, the porous vitreous silica body is sintered, generally in an atmosphere of helium, or optionally helium plus some chlorine- or fluorine-containing gas. Helium comprises the bulk of the atmosphere since it is somewhat soluble in vitreous silica, and diffuses rapidly through the glass. Thus if, as sintering proceeds, some gas is trapped in a closed pore or bubble, provided that gas is comprised substantially of helium, it may escape by di
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Nicholson Robert
Poullain Bernard Phillipe Robert
Sayce Ian George
Cano Milton
TSL Group PLC
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