Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1997-07-30
2001-11-06
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C065S017400
Reexamination Certificate
active
06312656
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the formation of silica and silica preforms and, more particularly, to a method and apparatus for forming silica and silica preforms from liquid silicon-containing compounds.
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, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a conversion reaction site, and a means of catalyzing oxidation and combustion coincidentally to produce finely divided, spherical aggregates, called soot. This soot can be collected on any deposition receptor in any number of ways ranging from a collection chamber to a rotating mandrel. It 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 and burners.
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 200-300 millimeters of mercury (mm Hg) at temperatures below 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 vapor sources, even though these materials have certain chemically undesirable properties.
Silicon, germanium, zirconium, and titanium are metals often used in halide form as vaporous reactants in forming metal oxide glasses. However, SiCl
4
has been the industry standard over the years for the production of high purity silica glasses. As disclosed in U.S. Pat. No. 3,698,936, one of several reactions may be employed to produce high purity silica via oxidation of SiCl
4
; namely:
SiCl
4
+O
2
→SiO
2
+2Cl
2
, (1)
SiCl
4
+2/3O
3
→SiO
2
+2Cl
2
, or, (2)
SiCl
4
+2H
2
O→SiO
2
+4HCl, (3)
whereby burners or jet assemblies are utilized in feeding the reactant gases and vapors to a reaction space. It should be noted that reaction (2) rarely occurs or is used. There are inherent economic disadvantages to each of these reactions. Moreover, these reactions, which oxidize SiCl
4
through pyrolysis and hydrolysis, have the disadvantage of producing chlorine or a very strong acid by-product.
While the first two reactions occur theoretically, an auxiliary fuel is generally needed to achieve pyrolytic temperature. The hydrolysis of SiCl
4
results in the formation of hydrochloric acid (HCl), a by-product that is detrimental not only to many deposition substrates and to reaction equipment but also is harmful to the environment. Emission abatement systems have proven to be very expensive due to down-time, loss, and maintenance of equipment caused by the corrosiveness of HCl.
Notwithstanding the problems with handling and disposal of the HCl by-product, the third reaction, hydrolysis of SiCl
4
, tends to be the preferred commercial method of producing silica for economic reasons.
Though hydrolysis of SiCl
4
has been the preference of industry for producing high purity silica over the years, the enhanced global sensitivity to environmental protection has led to more strict government regulation of point source emissions, prompting a search for less environmentally pernicious feedstocks. Point source emission regulations require that HCl, the by-product of hydrolyzing SiCl
4
, as well as many particulate pollutants be cleansed from exhaust gases prior to their release into the atmosphere. The economic consequences of meeting these regulations have made commercial production of silica from halide-based feedstocks less attractive to industry.
As an alternative, high purity quartz or silica has also been produced by thermal decomposition and oxidation of silanes. However, this requires taking safety measures in handling because of the violent reaction that results from the introduction of air into a closed container of silanes. Silanes react with carbon dioxide, nitrous oxide, oxygen, or water to produce high purity materials that are potentially useful in producing, among other things, semiconductor devices. However, silanes have proven to be much too expensive and reactive to be considered for commercial use except possibly for small scale applications requiring extremely high purity.
A number of-patents describe the production of high purity metal oxides, particularly silica, from a chloride-based feedstock. These patents disclose equipment with a number of burner arrangements and feedstock delivery systems to achieve oxidation of a metal chloride through flame hydrolysis or pyrolysis. Illustrative of this is U.S. Pat. No. 4,491,604 to Lesk et al., where trichlorosilane, dichlorosilane, and silicon tetrachloride are flame hydrolyzed to form soot, and U.S. Pat. No. 3,666,414 to Bayer, where silicon halides such as trichlorosilane or chloroform are flame hydrolyzed. In similar processes, U.S. Pat. Nos. 3,486,913 to Zirngibl (“Zirngibl”) and 2,269,059 to McLachlan (“McLachlan”) teach oxidation of halides. Volatilized inorganic halide components such as TiCl
4
, CrCl
3
, CrO
2
Cl
2
, SiCl
4
, AlCl
3
, ZrCl
4
, FeCl
2
, FeCl
3
, ZnCl
2
, or SnCl
4
that are oxidized with air, steam, or oxygen are employed in Zirngibl, while silicon halides, ethyl silicate, methyl borate, TiCl
4
, AlCl
3
, and ZrCl
4
are used by McLachlan.
U.S. Pat. No. 3,416,890 to Best et al. discloses a process for preparing finely-divided metal or metalloid oxides by the decomposition of a metal or metalloid perhalide in a flame produced by the combustion of an oxidizing gas and an auxiliary fuel, such as carbon disulfide, carbon selenide sulfide, thiophosgene, or other hydrogen-free compounds containing sulfur bonded directly to carbon.
U.S. Pat. No. 2,239,551 to Dalton discloses a method of making glass by decomposing a gaseous mixture of glass-forming compounds in a flame of combustible gas. The mixture is used in the formation of anhydrous oxides of silicon, aluminum, and boron. Decomposable compounds such as ethyl or methyl silicate, trichlorosilane, and silicon tetrafluoride may be substituted for silicon tetrachloride; methyl borate or boron hydride may be substituted for boron fluoride, etc.
U.S. Pat. No. 2,326,059 to Nordberg details a technique for making silica-rich ultra-low expansion glass by vaporizing tetrachlorides of Si and Ti into the gas stream of an oxy-gas burner, depositing the resultant mixture to make a preform, vitrifying the preform at 1500° C. to make an opal glass, and firing the opal preform at a higher temperature to cause it to become transparent.
U.S. Pat. No. 2,272,342 to Hyde discloses a method of producing glass articles containing vitreous silica by vaporizing a hydrolyzable compound of silicon such as silicon chloride, trichlorosilane, methyl silicate, ethyl silicate, silicon fluoride, or mixtures thereof, using a water bath. The silicon compound vapor is hydrolyzed by water vapor in the flame of a burner, and the resulting amorphous oxide is collected and subsequently sintered until a transparent glass results.
U.S. Pat. No. 4,501,602 to Miller et al. describes the production of particulate metal oxide soot through the vapor phase deposition of &bgr;-diketonate complexes of metals from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, and the rare earth series of the Periodic Table.
Also cited in the art are several patents where silane compounds have been used in producing high purity silica.
Japanese Patent Application No. 90838-1985 to Okam
Blackwell Jeffery Lynn
Fu Xiaodong
Hawtof Daniel Warren
Powers Dale Robert
Carlson Robert L.
Corning Incorporated
Hendrickson Stuart L.
Servilla Scott S.
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