Method of making high purity, non-porous fused silica bodies

Abstract: This invention relates to the production of high purity fused silica glass through oxidation or flame hydrolysis of a vaporizable silicon-containing compound. More particularly, this invention is directed to the use of vaporizable, halide-free compounds in said production. In the preferred practice, a polymethylsiloxane comprises said vaporizable, halide-free compound.

Description: Various processes are known in the art that involve the production of metal oxides from vaporous reactants. The most basic requirements of such processes necessitate a feedstock solution, a means of generating and transporting vapors of the feedstock solution (hereafter called vaporous reactants) and an oxidant to a reaction site, and a means of catalyzing oxidation and combustion coincidentally, producing finely divided, spherical aggregates, called soot. This soot may be collected in any number of ways, ranging from a collection chamber to a rotating mandrel, and simultaneously or subsequently heat treated to form a non-porous, transparent, high purity glass article. The means for executing these reactions is usually a specialized piece of equipment with a unique arrangement of nozzles and burners. Much of the initial research that led to the development, and thus patent protection, of a plethora of such processes focused on the production of fused silica. Selection of the appropriate feedstock was found to be as important in the production of high purity fused silica as the equipment used in its production. Consequently, a material was identified that could generate the needed vapor pressure of 200 - 300 mm at temperatures below 100°C; the high vapor pressure of silicon tetrachloride (SiCl 4 ) isolated it as a convenient vapor source for soot generation, thus launching 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 for forming high purity metal oxide glasses. However, SiCl 4 has been the industry standard among metal-source vaporous reactants used over the years for the production of high purity silica glasses. As disclosed in U.S. Patent 3,698,936, one of several reactions may be employed to produce high purity fused silica via oxidation of SiCl 4 ; namely: SiCl 4 + O 2 → SiO 2 + Cl 2 , SiCl 4 + O 3 → SiO 2 + Cl 2 , or SiCl 4 + H 2 O → SiO 2 + HCl, whereby burners or jet assemblies are utilized in feeding the reactant gases and vapors to a reaction space. There are inherent economic disadvantages to each of these reactions. These reactions, which oxidize SiCl 4 through pyrolysis and hydrolysis, have the disadvantage of producing a very strong acid by-product. While the first two reactions occur theoretically, it is likely that an auxiliary fuel is needed to achieve pyrolytic temperature, thus leading to hydrolysis of the silicon tetrachloride and formation of hydrochloric acid (HCl). Such a by-product is not only a detriment to many deposition substrates and the reaction equipment, but also is a detriment 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. The first reaction, which utilizes oxygen as it occurs naturally, requires elevated reaction temperatures which, generally, are difficult to maintain without using specialized equipment. The second reaction requires ozone, an unstable form of molecular oxygen that not only warrants special handling, but also must be manufactured on site due to a lack of commercial availability. Notwithstanding the handling and disposal of the HCl by-product necessitated by the hydrolysis and pyrolysis of SiCl 4 , the third reaction, also hydrolysis of SiCl 4 , tends to be the preferred commercial method of producing fused silica for economic reasons. Though hydrolysis of SiCl 4 has been the preference of industry for producing high purity fused 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. In new point source emission regulations, HCl, the by-product of hydrolyzing SiCl 4 , as well as many particular pollutants, has to be cleansed from exhaust gases prior to their release into the atmosphere. The economic consequences of meeting these regulations have made commercial production of fused silica by downstream removal of HCl and other metal oxides from halide-based feedstocks less attractive to industry. As an alternative, high purity fused quartz or silica may also be produced by thermal decomposition and oxidation of silane, a compound that requires taking safety measures in handling due to the violent reaction caused when air is introduced into a closed container of silane. Silane is commonly reacted with carbon dioxide, nitrous oxide, oxygen, or water to produce a high purity material that is useful in producing, among other things, semiconductor devices. However, silane has proven to be much too expensive and reactive to be considered for commercial use except possibly for extremely high purity applications. WO-A-90/10596, which has an international filing date of 14th March 1990, which claims a priority date of 15th March 1989, and which was published on 20th September 1990 describes inter alia , a method of making a non-porous body of high purity fused silica glass comprising the steps of: (a) producing a gas stream containing a vaporizable, halide-free, silicon-containing compound capable of being converted through oxidation or flame hydrolysis and decomposition to Si0 2 ; (b) passing said gas stream into the flame of a combustion burner to form amorphous particles of fused Si0 2 ; (c) depositing said amorphous particles onto a support; and (d) either essentially simultaneously with said deposition or subsequently thereto consolidating said deposit of amorphous particles into a non-porous body; wherein said vaporizable, silicon-containing compound comprises a cyclopolysiloxane, for example hexamethyldisiloxane or octamethylcyclotetrasiloxane, or a straight chain volatile silicon compound of the general formula R 3 Si-0(SiR 2 0) n - SiR 3 , mixed with an inert gas. Attention is also drawn to the following prior art documents: (1) CHEMICAL ABSTRACTS, vol. 103, no. 14, 7 October 1985, Columbus, Ohio, US; abstract no. 109041k, page 268; & JP-A-60 090 938. (2) CHEMICAL ABSTRACTS, vol. 103, no. 14, 7 October 1985, Columbus, Ohio, US; abstract no. 109042m, page 269; & JP-A-60 090 837. (3) CHEMICAL ABSTRACTS, vol. 103, no. 14, 7 October 1985, Columbus, Ohio, US; abstract no. 109043n, page 269; & JP-A-60 090 836. These describe a method having the features (a) to (d), set out above in relation to WO-A-90/10596, but wherein the vaporizable, silicon-containing compound is a silane ester, preferably MeSi(OMe) 3 or Si(OMe) 4 . The present invention provides, in one aspect, a method having the features set out in claim 1 hereof. In another aspect it provides a method having the features set out in claim 2 hereof. The teachings of the instant invention are easily adapted to known methods of producing high purity fused silica by flame pyrolysis or hydrolysis, such as those disclosed in the early patents by Nordberg (U.S. Patent 2,239,551) in 1941 and Hyde (U.S Patent 2,272,342) in 1942. It is anticipated that this process alteration may be adapted to a variety of deposition/collection techniques as well. The method of this invention, by utilizing alternative silicon-source compounds, greatly reduces, if not eliminates, the need for elaborate pollution abatement equipment. While it is recognized that the primary application of the instant invention relates to the production of fused silica, the technology applied herein is generally applicable in instances where a high purity metal oxide glass is desired. It is a further object of this invention to provide an improved method of making high purity metal oxide glasses through the use of alternative metal oxide source compounds, thereby greatly reducing the need for expensive pollution abatement systems. Summary of the Invention As set out above, the instant invention utilizes halide-free, silicon-containing compounds as a replacement for the halide-based source feedstocks that are often oxidized by flame hydrolysis or pyrolysis, to produce transparent, high-purity silica glass articles. Fused silica glass produced through the use of silicon-containing compounds as the feedstock components results in carbon dioxide and water as by-products. We have found that polymethylsiloxanes are particularly useful as substitutes for halide-based, silicon-containing compounds, and of that family of siloxanes the polymethylcyclosiloxanes perform exceptionally well. The present invention envisages the use of certain polymethylcyclosiloxanes, namely hexamethylcyclotrisiloxane (HMCTS), octamethylcyclotetrasiloxane (OMCTS), and decamethylcyclopentasiloxane (DMCPS), and mixtures of such polymethylcyclosiloxanes. OMCTS and DMCPS have been found to be the most preferred. It will be appreciated that, similarly to the current commercial processes for doping fused SiO 2 articles produced via the hydrolysis/oxidation of SiCl 4 with various metals in order to modify the chemical and/or physical properties thereof, the fused SiO 2 articles prepared in accordance with the present invention can likewise be doped with metals. For example, fused SiO 2 articles have been doped commercially with Al 2 O 3 , B 2 O 3 , GeO 2 , P 2 O 5 , and TiO 2 utilizing halide-containing compounds of aluminum, boron, germanium, phosphorous, and titanium, respectively. Like dopants can be utilized in the present inventive process but would, of course, provide a source of halide emissions. Consequently, to eliminate point source emissions of halides, organometallic compounds of the dopant metals will be employed. For example, isopropyl titanate and titanium ethoxide can be used as sources of titanium and methylborate can furnish the dopant source of boron. Further examples of operable organometallic dopants are found in U.S. Patent No. 4,501,602 (Miller et al.). That patent describes the production of glass and glass/ceramic articles via a vapor phase oxidation process wherein β-diketonate complexes of metals selected from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, and the rare earth series of the Periodic Table are vaporized, the vapor is transported to an oxidation site, such as a burner or a hot plasma zone which is adjacent to a deposition substrate or within a deposition tube, and oxidized in the vapor phase to form particulate metal oxide soot. β-diketonate complexes are also available of metals in Group VA of the Periodic Table, notably vanadium and tantalum. Accordingly, the use of β-diketonate complexes provides a vaporizable source for a wide variety of dopant metals. In summary, our invention comprehends doping of fused SiO 2 articles with P 2 O 5 and/or at least one metal oxide selected from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, and the rare earth series of the Periodic Table. Prior Art A plethora of patents have issued that describe the production of high purity metal oxides, and particularly fused silica, from a halide-based feedstock encompassed in or feeding into a specialized piece of equipment. Such equipment has featured a number of burner arrangements and feedstock delivery systems, all based on the oxidation of a metal halide through flame hydrolysis or pyrolysis. With respect to the known processes, in addition to the prior art documents discussed above, reference is made to U.S. Patents 4,491,604, 3,666,414,3,486,913, 2,269,059, 3,416,890, 2,239,551, 2,326,059, 2,272,342, 4,501,602, 3,117,838, 4,810,673, and 4,242,487. Brief Description of the Drawings FIGURE 1 comprises a schematic representation of the apparatus and process for forming large masses of fused silica. FIGURES 2 and 2A comprise schematic representations of the apparatus and process for depositing silica soot on a rotating mandrel to form a porous blank or preform. FIGURE 3 comprises a schematic representation of a heating chamber wherein the porous blank is fired in an atmosphere of helium and chlorine to full consolidation to a non-porous body. FIGURE 4 graphically records the deposition efficiencies measured utilizing SiCl 4 and OMCTS as the silicon-containing source materials. Description of Preferred Embodiments In the most preferred embodiment of the instant invention octamethylcyclotetrasiloxane (OMCTS), represented by the chemical formula -[SiO(CH 3 ) 2 ] 4 -, is the halide-free, cyclosiloxane compound used as the feedstock in the fused silica boule process, wherein large boules of high purity fused silica are produced, or in the vapor deposition processes utilized in making high purity fused silica for optical waveguide applications. Fused silica produced by oxidation of OMCTS results in the production of carbon dioxide and water as by-products. The conventional boule process used in making fused silica is a one-step process, whereas the conventional vapor deposition process used in making silica glass for optical waveguide applications is a three-step process. In the conventional boule process, a carrier gas is bubbled through a SiCl 4 feedstock that is maintained at a specified low temperature. The vaporous SiCl 4 is entrained in the carrier gas and is thereby transported to the reaction site. The reaction site is comprised of a number of burners that combust and oxidize the vaporous SiCl 4 at a temperature greater than 1700°C. Example 1 The aforementioned system is illustrated in Figure 1 wherein SiCl 4 was replaced with an OMCTS feedstock 1 in a commercial furnace to produce boules of high purity fused silica. An inert gas, nitrogen, was used as the carrier gas and a bypass stream of nitrogen 2 was introduced to prevent saturation of the vaporous stream. The vaporous reactant was passed through a distribution mechanism 3 to the reaction site wherein a number of burners 4 are present in close proximity to a furnace crown 5. The reactant was combined with a fuel/oxygen mixture 0 at these burners and combusted and oxidized at a temperature greater than 1700°C, directing high purity metal oxide soot and heat downward through the refractory furnace crown 5 where it is immediately deposited and consolidated to a non-porous mass on a hot bait 6. In the production of relatively large boules, the maximum soot collection efficiencies measured using SiCl 4 as the feedstock have ranged about 60-70%. Extensive trials have indicated that the average deposition efficiency for boule process utilizing OMCTS as the source material is at least 10% higher than those processes using SiCl 4 . Therefore, in addition to eliminating halide emissions, the quantity of particulate emissions is likewise reduced. It is well recognized in the art that processing of the feedstock requires apparatus and transfer system capable of vaporizing the feedstock and delivering it to the burner in the vapor state. Somewhat higher temperatures (≈104°-150°C) are necessary with OMCTS due to its lower vapor pressure when compared to SiCl 4 . Example 2 DMCPS and HMCTS, were tested using Outside Vapor Deposition (OVD) technology. Proficiency was demonstrated in producing fused silica glass with each compound. Example 3 A bench-scale furnace, modeled after a commercial furnace was constructed. MTMS, OMCTS, and TEOS (tetraethylorthosilicate) were tested. Existing commercial burners were used for deposition. These tests demonstrated again that MTMS and OMCTS can be used to successfully produce high purity fused silica at deposition rates and efficiencies comparable to those of SiCl 4 . Contrariwise, TEOS, which is not within the scope of this invention, proved to be too difficult to control to be a satisfactory starting material. Most of the processes being developed by industry today for the manufacture of optical waveguides employ the chemical vapor deposition (CVD) concept or a modified version thereof. In a CVD experiment, each of the component liquids is heated to a constant temperature at which enough vapor pressure is generated to produce a reasonable rate of deposition. The individual vapors are entrained in a carrier gas stream, mixed together prior to combustion to ensure homogeneous output, and then passed through a burner flame, usually a natural gas/oxygen mixture and frequently containing excess oxygen. The vapors in the mixture are converted to their respective oxides upon exiting the burner orifice to form a stream of volatile gases and finely-divided, amorphous, spherical aggregates, called soot. The soot is collected on a mandrel (OVD) or bait tube [Axial Vapor Deposition (AVD)] and deposited in thin layers. The final product of soot collection, the porous preform, is then subjected to high temperature in which the preform consolidates to a non-porous monolithic glassy body. In usual practice, the optical waveguide process is a three-step process. In the first stage of optical fiber fabrication, as depicted in FIGURE 2, oxygen, the carrier gas, is bubbled through a liquid feedstock of SiCl 4 that is maintained at a constant temperature. The resulting vaporous reactant is transported to a reaction site, such as a burner, via a carrier gas, wherein the vaporous gas streams are combusted in a burner flame. The presence of oxygen serves to convert the vaporous reactants to their respective oxides, exiting the burner orifice to form a stream of volatile gases and finely-divided, amorphous, spherical particles of soot that are deposited onto a substrate, forming a porous blank or preform of opaque, white silica soot. Water, HCl, and carbon dioxide are emitted as by-products of this reaction. In the second stage, represented in FIGURE 3, the blank or preform is subsequently heat treated in a helium/chlorine atmosphere to full consolidation. In the third and final stage, conventional fiber-draw technology is utilized in extracting optical waveguide fiber from the preform. Example 4 As indicated in FIGURE 2, SiCl 4 was replaced with an OMCTS feedstock 7 in the standard OVD process used in making optical waveguides. An inert gas, nitrogen, was employed as the carrier gas 11 and a methane/oxygen mixture 12 was employed as the burner flame fuel, whereby combustion and oxidation was induced at the burner 8. The resulting soot was deposited on a rotating rod 9, thus forming a preform or blank 10 of silica soot shown at FIGURE 2A. The preform was then heat treated in a consolidation furnace 13, in a He/Cl 2 atmosphere 14 to full consolidation. Conventional fiber draw techniques can then be employed in making optical waveguide fiber. No additional equipment was required, but the delivery system had to be capable of vaporizing the material and delivering it to a standard OVD burner in the vapor state. The observed deposition efficiency was an added benefit that may be specific to OMCTS. OMCTS-based soot was found to deposit more efficiently than SiCl 4 -based soot. Initial deposition efficiencies were increased by about 20%. FIGURE 4 shows this difference as a function of the total amount of SiO 2 produced at the burner for a specific blank size. Therefore, in addition to eliminating HCl emissions, OMCTS reduces the quantity of particulate emissions with accompanying increased production rates. Although the cost of OMCTS by weight is higher than that of SiCl 4 , when the amount of SiO 2 deposited from each of the two sources is compared, the cost per unit amount of SiO 2 deposited is approximately the same. To illustrate: In optical waveguide production, deposition efficiency increases with increasing blank size. As deposition begins, collection deficiencies for SiCl 4 frequently are less than 10%, whereas the use of OMCTS can yield initial deposition efficiencies up to 25%. This factor of greater than twofold efficiency in deposition results in a corresponding increase in preform growth rate for equivalent rates of SiO 2 particles exiting the burner, and about a 20% by weight or more decrease in soot that has to be cleaned from the exhaust gases. (And in addition, of course, the use of OMCTS eliminates the costs involved in removing HCl from the exhaust gases.) Examples 5 and 6 describe two other compounds which were investigated to produce high purity fused silica. These compounds, silane and methyltrichlorosilane (MTCS), are not the halide-free, silicon-containing compounds comprising the basis of the instant invention. Example 5 SiCl 4 was replaced with silane in the production of optical waveguide blanks. Though blanks were successfully produced, silane proved to be much too expensive and reactive to be considered for commercial use, except possibly for extremely high purity applications. Example 6 SiCl 4 was replaced in the production of optical waveguide blanks with MTCS. High purity fused silica glass preforms were successfully produced. Testing showed an estimated 25% reduction in chloride emissions when compared to the conventional use of SiCl 4 .

Claims: A method of making a non-porous body of high purity fused silica glass comprising the steps of: (a) producing a gas stream containing a vaporizable, halide-free, silicon-containing compound capable of being converted through oxidation or flame hydrolysis and decomposition to SiO 2 ; (b) passing said gas stream into the flame of a combustion burner to form amorphous particles of fused Si0 2 ; (c) depositing said amorphous particles onto a support; and (d) either essentially simultaneously with said deposition or subsequently thereto consolidating said deposit of amorphous particles into a non-porous body; wherein said vaporizable, silicon-containing compound consists of a polymethylcyclosiloxane selected from the group consisting of octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, hexamethylcyclotrisiloxane, and mixtures thereof, and further wherein consolidation of said deposit of amorphous particles into said non-porous body is carried out in an atmosphere containing chlorine. A method of making a non-porous body of high purity silica doped with P 2 0 5 and/or at least one metal oxide selected from Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, and the rare earth series of the Periodic Table comprising the steps of: (a) producing a gas stream containing a vaporizable, halide-free, silicon-containing compound capable of being converted through oxidation of flame hydrolysis and decomposition to SiO 2 and a vaporizable compound capable of being converted through oxidation or flame hydrolysis to P 2 O 5 and/or at least one metal oxide selected from the Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, and the rare earth series of the Periodic Table; (b) passing said gas stream into the flame of a combustion burner to form amorphous particles of fused SiO 2 doped with P 2 O 5 and/or at least one metal oxide selected from the Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, and the rare earth series of the Periodic Table; (c) depositing said amorphous particles onto a support; and (d) either essentially simultaneously with said deposition or subsequently thereto consolidating said deposit of amorphous particles into a non-porous body; wherein said vaporizable, silicon-containing compound consists of a polymethylcyclosiloxane selected from the group consisting of octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, hexamethylcyclotrisiloxane, and mixtures thereof, and further wherein consolidation of said deposit of amorphous particles into said non-porous body is carried out in an atmosphere containing chlorine. A method according to claim 1 or 2, wherein said polymethylcyclosiloxane is octamethylcyclotetrasiloxane, and said gas stream containing said octamethylcyclotetrasiloxane is produced and passed at a temperature between 104° and 150°C. A method according to claim 2 or 3, wherein said metal oxide is TiO 2 , and said vaporizable compound capable of being converted through oxidation or flame hydrolysis to TiO 2 is isopropyl titanate and/or titanium ethoxide. A method according to any preceding claim, wherein said atmosphere also contains helium.

Inventors: DOBBINS MICHAEL SEAN, MCLAY ROBERT ERNEST, DOBBINS, MICHAEL SEAN, MCLAY, ROBERT ERNEST
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1991
Main classification: C03B8
IPC codes: C03B20, C03B37, C03C13, C03B19, C03B8, G02B6