Glass manufacturing – Fiber making apparatus – Having coating or treating means
Patent
1995-12-05
1998-04-07
Bennett, Henry A.
Glass manufacturing
Fiber making apparatus
Having coating or treating means
239422, 239423, C03B 37018
Patent
active
057359288
DESCRIPTION:
BRIEF SUMMARY
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a 371 of PCT/GB94/01304, filed Jun. 16, 1994, which claims priority of UK patent application 9312634.0, filed Jun. 18, 1993.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The manufacture of synthetic vitreous silica by soot deposition has been known for several decades. In the last 20 years, much progress has been made, stimulated by the need to prepare ultrapure and doped synthetic silica glasses for the manufacture of optical fibre. Two of the major processes which have emerged are;
The OVD process involves the formation of a porous synthetic silica glass body by deposition of silica soot produced by one or more synthesis burners fed with fuel gas, oxygen and a source of silica vapour species, most commonly silicon tetrachloride but sometimes other halosilanes have been used. The plume of silica fume issuing from the burner or burners is directed to deposit silica soot on a mandrel or substrate, typically rotating about a horizontal or vertical axis. In some variants of the process, additional burners are used for soot consolidation, and for maintaining the temperature at each end of the soot body to prevent cracking.
Although the use of such a volatile silicon compound permits precise control of the purity of the deposited silica, care must be taken to avoid deposition of silica on the surface of the burner, as this may distort the flame, or block the exit ports of the burner. In the design of silica deposition burners it is essential to overcome this problem.
2. Description of the Related Art
A typical prior art burner is shown schematically at 10 in FIG. 1 of the accompanying drawings. The burner is made of metal with a central port 11 through which is projected a stream of silicon tetrachloride vapour, in an oxygen carrier gas stream. This central port is surrounded by one or more circular arrays of further ports 12, 13 which typically carry oxygen or premixed oxygen and methane, directed parallel to, or convergently with, the axis of the central port. The silicon tetrachloride is hydrolysed in the flame to form a plume of silica fume which is projected onto a mandrel or substrate made from alumina, graphite or silica to deposit as silica soot. The substrate may comprise a doped vitreous silica core rod which is to form the central component of an optical fibre preform.
In a typical prior art VAD process, the deposition occurs on the lower end of a vertically suspended and rotating silica soot body. Typically one burner deposits core material to form the axial region of a cylindrical soot body, and several further burners are used for depositing the cladding soot. The general design of one such burner comprises a series of co-annular quartz glass tubes defining a series of concentric annular slots through which the feed materials for the burner are supplied is shown at 20 in FIG. 2. Typically a central tube 21 is fed with silicon tetrachloride and oxygen, and successive annular slots (three of which are shown in FIG. 2 at 22, 23 and 24) are fed with, for example, argon, hydrogen or oxygen. A simple two- or three-slot burner may be surrounded by further annular slots creating additional hydrogen and oxygen sheaths, increasing the thermal output, lengthening the flame and promoting consolidation of the deposited soot.
Optionally, these annular slots may be separated by intermediate annular slots fed with inert gas e.g., argon, helium or nitrogen, to provide some cooling, for example to reduce the thermal load on the burner. Again, optionally, the burner may be supplied with an alternative fuel gas to hydrogen, such as methane.
The burners shown in FIGS. 1 and 2 produce an approximately cylindrical plume of silica particles which is impacted upon the substrate. The particles move towards the substrate under the influence of thermophoresis (motion of particles through a gas towards a region of lower temperature), and a proportion of the particles strike, and adhere to the substrate. The soot collection rate and collection efficiency of these kno
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Nicholson Robert
Sayce Ian George
Turnbull Paul Willikam
Wells Peter John
Bennett Henry A.
TSL Group PLC
Wilson Gregory A.
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