Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2001-02-01
2004-05-25
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S414000, C065S415000, C065S416000, C065S017400, C065S027000
Reexamination Certificate
active
06739156
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the delivery of liquid reactants to a combustion zone formed adjacent a burner assembly to create soot used in the manufacture of glass. More particularly, the present invention relates to a system and method of delivering liquid reactants to a combustion zone that avoids the premature solidification of the liquid reactants within the burner assembly.
While the invention is subject to a wide range of glass soot deposition applications, it is especially suited for use in producing soot for glass preforms used in the manufacture of optical waveguides, and will be particularly described in that connection.
BACKGROUND OF THE INVENTION
Various processes are known in the art that involve the production of metal oxides from vaporous reactants. Such processes require a feedstock solution or precursor, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site (also known as a soot reaction zone or combustion zone to those skilled in the art), and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected in any number of ways, ranging from capture in a collection chamber to deposition on a rotating mandrel. The collected soot may be simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. This process is usually carried out with specialized equipment having a unique arrangement of nozzles, injectors, burners and/or burner assemblies.
Much of the initial research that led to the development of such processes focused on the production of bulk silica. Selection of the appropriate feedstock was an important aspect of that work. Consequently, it was at that time determined that a material capable of generating a vapor pressure of between 200-300 millimeters of mercury (mm Hg) at temperatures below approximately 100° C. would be useful for making such bulk silica. The high vapor pressure of silicon tetrachloride (SiCl
4
) suggested its usefulness as a convenient vapor source for soot generation and launched the discovery and use of a series of similar chloride-based feedstocks. This factor, more than any other is responsible for the presently accepted use of SiCl
4
, GeCl
4
, POCl
3
, and BCl
3
as feedstock vapor sources.
Use of these and other halide-based feedstocks as vapor sources, however, does have its drawbacks. The predominate drawback being the formation of hydrochloric acid (HCl) as a by-product of oxidation. HCl is not only detrimental to the deposition substrates and the reaction equipment, but to the environment as well. Overcoming this drawback, amongst others, led to the use of halide-free compounds as precursors or feedstocks for the production of soot for optical waveguides.
Although use of halide-free silicon compounds as feedstocks for fused silica glass production, as described in U.S. Pat. Nos. 5,043,002 and 5,152,819, for example, avoids the formation of HCl, other problems remain, particularly when the soot is intended for the formation of optical waveguides. It has been found that, in the course of delivering a vaporized polyalkylsiloxane to the burner, high molecular weight species can be deposited as gels in the lines carrying the vaporous reactants to the burner, or within the burner itself. This leads to a reduction in the deposition rate of the soot that is subsequently consolidated to a blank from which an optical waveguide fiber is drawn. It also leads to imperfections in the blank that often produce defective and/or unusable optical waveguide fiber from the effected portions of the blank. An additional problem encountered while forming silica soot using siloxane feedstocks is the deposition of particulates having high molecular weights and high boiling points on the optical waveguide fiber blank. The build-up of these particulates results in “defect” or “clustered defect” imperfections that adversely affect the optical and structural quality of optical waveguides formed using the silica soot.
Other feedstocks, some of which are, and others of which may be useful in forming soot for the manufacture of optical waveguides are not currently acceptable alternatives to the halide-based and halide-free feedstocks for delivery via vapor deposition. Materials such as salts and those known as rare-earth elements, for example, are extremely unstable as vapors and often decompose before they can be delivered in their vapor phase, or do not have sufficiently high vapor pressures to be vaporized at accessible temperatures.
Although it is often possible to deliver at least a percentage of these elements to the combustion zone as a vapor, it is technically very difficult. Elaborate systems incorporating expensive equipment are necessary to convert these elements to the vapor phase, and further, to deliver them to the combustion zone without leaving deposits in the lines leading to the burners and in the burners themselves. Moreover, if multiple elements are being delivered as vapors and a specific percentage of each is needed for the desired composition, it is difficult to control the delivery since different elements have different vapor pressures.
U.S. patent application Ser. No. 08/767,653, discloses that these and other limitations can be overcome by delivering a feedstock to an injector or burner in liquid form, atomizing the feedstock to form an aerosol containing fine droplets of the liquid feedstock, and converting the atomized liquid feedstock into soot at the combustion zone. Because the feedstock is delivered directly into the burner flame as a liquid rather than a vapor, the vapor pressure of tie feedstock is no longer a limiting factor in the formation of soot for use in the manufacture of optical waveguides The injectors, burners, and burner assemblies disclosed in U.S. patent application Ser. No. 08/767,653 and other currently pending applications rely on very small orifices to deliver the liquid in a fine stream for proper atomization. Because the orifices are so small, they are extremely susceptible to plugging. Even a small solid particle in the liquid being delivered can partially clog the orifice, which in turn adversely effects the soot deposition rate, and the homogeneity of the soot collected.
Although materials never before delivered to a combustion zone to form soot for the manufacture of glass can now be delivered in a liquid solution, many of these materials have inherent short-comings while in a liquid form. Most problematic is that many of these liquid materials quickly form solids when exposed to oxygen and/or water. Thus, any exposure to the air during liquid delivery of these reactants likely will result in the formation of solids, which clog the lines leading to the burners and the small orifices of the burners and the burner assemblies themselves. When the orifices become partially clogged, the flame, and thus the soot stream becomes non-uniform and the soot deposition rate suffers. As a result, the liquid delivery system must be shut down so that it can be cleaned. Such cleaning operations typically require partial disassembly of the burner assembly, which results in significant production down time.
In liquid delivery systems, plugging or clogging of the burner assembly orifices is particularly problematic during the start up and shut down stages of the liquid delivery cycle. During these periods, the liquid reactant tends to trickle or sputter out of the injector orifice. This occurs during the start up stage of the liquid delivery cycle before steady state pressure is available, and at the shut down stage of the liquid delivery cycle after steady state pressure is no longer available. These limited pressure stages result in significantly reduced liquid flow rates, which in turn can provide the exposure time necessary for the slow moving liquid to react with the air to form solids. Alternatively, these liquid feedstocks can leak and solidify on the
Hawtof Daniel W.
Stone, III John
Whalen Joseph M.
Able Kevin M.
Corning Incorporated
Hoffmann John
Krogh Timothy R.
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