MXPA00001101A - Method for forming silica by combustion of liquid reactants using oxygen - Google Patents

Abstract

The present invention is directed to a method for making silica. A liquid siloxane-containing feedstock capable of being converted by thermal oxidative decomposition to SiO2 is provied and introduced directly into the flame of a combustion burner, which converts the compound to silica, thereby forming finely divided amorphous soot. The soot is vaporized at the conversion and/or deposition site where the liquid is converted into silica by atomizing the liquid with a stream of oxygen gas, or a mixture of oxygen gas and other gas, such as nitrogen. The amorphous soot is deposited on a receptor surface where, either substantially simultaneously with or subsequently to its deposition, the soot is consolidated into a body of fused silica glass, such as an optical fiber preform.

Description

METHOD FOR FORMING SILICA BY COMBUSTION OF LIQUID REAGENTS USING OXYGEN RELATED REQUESTS This application is a continuation in part of the patent application of E.U.A. series No. 08 / 767,653, filed on December 17, 1996 whose content is based on and incorporated by reference in its entirety.

FIELD OF THE INVENTION The present invention relates to the formation of silica and silica preforms and very particularly to a method and apparatus for forming silica and silica preforms from liquid compounds containing silicon.

BACKGROUND OF THE INVENTION Various methods are known in the art involving the production of metal oxides from gaseous reactants. Such processes require a solution of supply material, a means to generate and transport vapors from the supply material solution (hereinafter referred to as gaseous reactants) and an oxidant to a conversion reaction site, and a means to catalyze oxidation. and combustion and at the same time to produce finely divided spherical aggregates, called soot. This soot can be collected at any deposition receiver in any number of ways ranging from a collection chamber to a rotating mandrel. They can be treated by heat simultaneously or subsequently to form a non-porous, transparent and high purity glass article. This procedure is usually carried out with specialized equipment that has a special arrangement of nozzles and burners. Most of the first research that led to the development of such procedures focused on the production of bulk silica. The selection of suitable supply material was an important aspect of that work. As a consequence, at that time it was 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 to obtain such silica in bulk. The high vapor pressure of silicon tetrachloride (SiCl) indicated its usefulness as a convenient steam source for soot generation and launched the discovery and use of a series of chlorine-based supply materials. This factor, more than any other, is responsible for the currently accepted use of SiCU, GeCU, POCI3, and BCI3 as vapor sources, although these materials have certain chemically undesirable properties.

Although the use of halogenide-free silicon compounds as sourcing materials for the production of fused silica glass, as described in the U.S. Patents. Nos. 5,043,002 and 5,152,819, the formation of HCl is avoided, some problems remain, particularly when the intention is that the glass is used for the formation of optical waveguides and high purity silica soot. Applicants have discovered that when a vaporized polyalkylsiloxane supply material is supplied to the burner, high molecular weight species such as a gel can be deposited in the line that carries the gaseous reactants to the burner or inside the burner itself. This leads to a reduction in the deposition rate of the soot preform that subsequently consolidates to another preform from which an optical waveguide fiber is extracted. In addition, this leads to imperfections in the preform that will produce a defective or useless optical waveguide fiber for the affected portions of the same preform. In copending application series No. 08 / 767,653 it was described that accumulated defects could be reduced by supplying a liquid siloxane supply material to a conversion site, atomizing the supply material at the conversion site, and converting the atomized supply material. on silica, precisely at the conversion site. One way of atomizing the supply material at the conversion site includes pneumatically or air-jetting the siloxane liquid supply material at the conversion site with a supply gas such as an inert gas. By atomization "pneumatic" or "by air jet", is not intended to imply that the air should be used as the atomizing gas, and the gas can also be an inert gas such as argon, nitrogen, or helium, a combustible gas such as methane, oxygen or a mixture of these gases . Although by atomizing the liquid supply material the accumulated defects are reduced, said liquid supply system presents various challenges. For example, by increasing the velocity of the supply gas desirably, smaller liquid droplets are produced, which can be easily vaporized and burned in the flame of the burner. Smaller droplets are desirable because larger droplets cause wart-like defects ("warts") on the surface of the soot preform. In addition, the smaller droplets can be more easily concentrated with the surrounding gases to produce a more concentrated deposition stream. On the other hand, by increasing the speed of the atomizing gas turbulence is added to the burner flame, which can reduce the speed of soot capture and seems to be a cause of a physical soot defect known as "lizard skin". The term lizard skin is a term used for a rough surface in the soot preform. Accordingly, it would be desirable to provide a method in which a liquid supply system could produce a concentrated deposition stream having small droplets without high gas velocity, in which there is low turbulence of the burner flame.

BRIEF DESCRIPTION OF THE INVENTION The present invention focuses on a method for making silica. In one embodiment, a liquid compound, preferably a silicon-containing halide-free liquid compound, capable of being converted by thermal oxidant decomposition to SiO 2, is directly provided in the flame of a combustion burner, forming soot at that site amorphous finely divided. The amorphous soot can be deposited on a receiving surface where, whether substantial, simultaneous with or subsequent to its deposition, the soot can be consolidated into a fused silica glass body. The fused silica glass body can then be used to either produce products directly from the molten body, or the molten body can be further treated, for example, by forming an optical waveguide such as that shown in FIGS. drawings, to make optical waveguide fiber, see later for example, the US patent application No. 08 / 574,961, entitled "Method for Purifying Polyalkylsiloxanes and the Resulting Products", where their end uses are described and whose content is incorporated herein by reference. The invention further comprises an apparatus for forming silica from liquid reagents preferably free of silicon-containing halide, comprising: a combustion burner which, in operation, generates a flame of conversion site; an injector for supplying a liquid compound containing silicon to the flame to convert the compound by means of thermal oxidant decomposition to a finely divided amorphous soot; and a receiving surface that is positioned with respect to said combustion burner to allow deposition of soot on the receiving surface. Applicants have now discovered that the problem described above is inhibited by supplying the siloxane supply material in its liquid form to the conversion site during the silica manufacturing process. By supplying the siloxane supply material in liquid form instead of vapor, gelation of the siloxane supply material is prevented, since the exposure of the siloxane supply material to the high temperature means of a vaporizer and of the vaporizer is avoided. steam supply system. This improves the yield and the quality of silica obtained and also reduces the maintenance requirements of the production system. Therefore, the invention provides a method for inhibiting the gelation of a siloxane sourcing material in the silica manufacturing process by supplying the siloxane sourcing material to the conversion site in liquid form. As the high energy means of the vaporizer and the steam supply system, which promote the formation of heavy gel, are avoided, the silica manufacturing process is improved. The siloxane supply material is supplied to the conversion site as a liquid and does not vaporize until just before or simultaneous with the moment it is converted to amorphous silica soot. The amorphous silica soot is then deposited on a receiving surface. Either substantially simultaneously with or subsequent to its deposition, the soot can be consolidated into a fused silica glass body, from which, for example, an optical waveguiding fiber can be formed by means of extraction. Another aspect of the invention further comprises a silica manufacturing apparatus which includes a tank of liquid siloxane supply material for containing the liquid siloxane supply material and a conveyor line for the liquid siloxane supply material for supplying the supply material. siloxane liquid to an injector, which injects the liquid supply material to the conversion site where it decomposes in a flame of the combustion burner to form a finely divided amorphous silica soot which is deposited on a receiving surface. The body of the fused silica glass can be impurified with an oxide dopant where, in addition to the liquid siloxane supply material, a dopant compound is introduced into the flame of the burner which is capable of being converted by oxidation or by flame hydrolysis to an element of the group consisting of P2O5 and metal oxide with a component of Metal selected from groups IA, IB, NA, IIB, IIIA, IIIB, IVA, VA, and from the series of rare earths of the Periodic Table of Elements. The fused silica glass impurified with oxide obtained by this means, for example, can be poured into an optical wave guide fiber.

In yet another aspect of the present invention, applicants have discovered that by atomically atomizing the siloxane liquid supply material at the conversion site with a fluid oxygen stream, defects in the preform are greatly reduced. Therefore, by using oxygen as the supply gas in a pneumatic atomizer, softer and better quality cast silica preforms can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a reagent delivery system according to the present invention. Figure 2 is a schematic representation of a liquid reagent provided to the flame of a burner by means of a syringe according to the present invention. Figure 3 is a schematic representation of particles of the liquid reagent that has been provided to the flame of a burner by means of a transducer according to the present invention. Figure 4 is a schematic representation of an atomizer incorporated in the structure of a burner according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 schematically describes a system for supplying a liquid supply material and, optionally, compounds that provide a dopant to the burner 10. A liquid siloxane supply material such as a polymethylcyclosiloxane is stored in the supply material tank 11. The tank of supply material 11 is connected to the injector of liquid supply material 15 at the site of introduction of the reagent by means of a system of conveyor conduits of liquid supply material which may include, if desired, a metering pump 12, a filter optional 13 and a preheater 14. Said conveying conduit of the liquid supply material has a first end of terminal 51 and a second end of terminal 52. The liquid of the siloxane supply material is transferred from tank 11 through the material conveying line of liquid supply by means of the pump 12 at through the filter 13 to the optional preheater 14. The liquid supplied by means of the filter 13 is under sufficient pressure to prevent and substantially inhibit its volatilization in the preheater 14, which is optionally used to heat the liquid reagent prior to its introduction to the burner 10 and avoids the high temperatures of a vaporizer that normally promotes gel formation. The burner is preferably provided in a conventional manner with internal protective gas, external protective gas, and a mixture of methane and oxygen for the flame, as described, for example, in US Pat. No. 4,165,223 to D. R. Powers, which is incorporated herein by reference. The liquid reagent is transported from the optional filter 13 or from the optional preheater 14 through the second end of the terminal 52 to the liquid injector 15, which supplies the liquid to the burner 10. The injector includes a device for supplying the liquid reagent, either as a liquid stream or as atomized liquid particles, directly to the flame of the burner 10. The reagent is generally referred to in the description as "liquid" form. What is meant by this expression is that the reagent is in a substantially liquid state. Some small portion of the reagent may be in the form of vapor, particularly when the preheater 14 is employed, or where a blanket of nitrogen is employed on the liquid. A small portion of the reagent may be in the form of vapor as supplied to the combustion site without having an adverse effect on the operation of the invention. A liquid injector 15 can include, for example, a syringe provided with a fine needle, by means of which a liquid stream can be injected at a high velocity towards the flame of the burner. Although you can use a syringe on a smaller scale, commercial operations will require a larger scale equivalent, for example, an atomizer. Various types of atomization media capable of forming very small particles of liquid are known in the spray technique as described in Atomization and Spravs. by Arthur H. Lefebure, Hemisphere Publishing Co., 1989, which is incorporated herein by reference. The atomizers can be operated by various energy sources such as liquid, gaseous, mechanical, electrical and vibratory energy, and can be classified as, for example, jet, swirl, jet-swirl, pneumatic, rotating, acoustic, ultrasonic and electrostatics. Preferably, a propulsion atomizer is used; very preferably still the propellant atomizer is a jet-swirl propeller atomizer, which swirls the liquid and subsequently as the atomizers generally do, extract the liquid at a high speed out of a small orifice. Various types of atomizers are described in Liquid Atomization, by L. Bayvel and Z. Orzechowski, Taylor & Francis, (1993), which are incorporated herein by reference. Another preferred type is a pneumatic atomizer operated by means of nitrogen or by means of air pressure. In preferred embodiments, in particular, the atomizer can be incorporated into the structure of the combustion burner. The atomized particles of the siloxane reactive compound are combusted in a burner that has been provided with fuel by means of, preferably a combination of methane and oxygen. The atomized particles of the reagent can be transported from the atomizer to the flame of the burner by means of a carrier gas such as nitrogen, which preferably is the atomizing gas. Most preferably, a mixture of nitrogen and oxygen is used as the atomizing gas. Still more preferably, oxygen is used as the atomizing gas to reduce the formation of defects in the soot preform. Figure 2 schematically describes an apparatus according to the present invention, wherein the syringe 21 injects a liquid stream of the reagent 22 into the flame of the conversion site 23 produced by the burner 24. The thermal oxidant decomposition of the reagent produces soot finely divided amorphous 25, which is deposited on a rotating mandrel 26. Figure 3 is a schematic representation of another embodiment of the apparatus of the present invention, wherein the atomizer 31 injects small liquid particles of the reagent 32 into the flame 23 produced by the burner 24. The combustion of the reagent produces soot 25, which is deposited on the rotating mandrel 26. Figure 4 is a cross-sectional view of a preferred embodiment of the apparatus of the present invention. Here, the burner 40 incorporates within its structure the atomizer 41, which injects very finely atomized liquid reagent particles into the flame 23. As with the previously described embodiments, the amorphous soot produced by combustion of the liquid reagent is collected in the rotary mandrel 26. As shown in Figure 4, the burner 40 comprises a series of concentric channels surrounding the atomizer 41. The liquid siloxane is supplied by means of the atomizer 41. A stream of an inert gas such as nitrogen gas, a mixture of gases, such as oxygen and nitrogen, or oxygen only supplied through the channel 43 atomizes the liquid supply material by the kinetic energy of the flowing gas to create liquid projections 42 which are converted into soot reactive particles in the burner flame 23. The area near the face of the burner 53 and the flame 23 therefore acts as a conversion site to convert the liquid projections 42 into reagent particles from the soot. Oxygen can be supplied to the flame 23 through channels 45 and 46, an inert gas, such as nitrogen, argon or helium is supplied through channel 44 to inhibit the reaction of the liquid supply material and the accumulation of soot on the face of the burner 53. Applicants have discovered that when oxygen or a mixture of oxygen and inert gas is used as the atomizing gas, better results are obtained by supplying an inert gas via the channel 44. A premix of oxygen and a fuel such as methane to the flame through the outer channel 47. A burner equipped with an atomizer injector, such as the embodiment described in Figure 4, produces a broad stream of soot, which achieves an improved concentricity of the core and coating regions of an optical waveguide fiber formed subsequently. The most preferred burner 40 of the invention as shown in Figure 4, is constituted by a pneumatic atomizer. With said pneumatic atomizer, the liquid siloxane supply material supplied through the atomizer 41 is atomized by the kinetic energy of a gas stream flowing through the internal channel 43. The high velocity gas is used to atomize the material of catering. This produces atomized liquid projections 42 with a speed on the scale of 0.5 to 50.0 m / sec. The use of an inert gas such as N2 gas with the pneumatic atomizer is preferred. The use of N2 gas as the pneumatic gas helps to cover the oxygen supply material in the flame and prevents accumulation in the burner. To reduce the velocity of the atomizing gas and prevent the effects on the surface on the soot preform, the O2 gas is the most preferred gas for use with the pneumatic atomizer. Previously it was believed that O2 would not help to prevent the combustion of the supply material prior to the complete vaporization of the liquid supply material. However, applicants have discovered that using oxygen as the atomizing gas allows better mixing of the siloxane with oxygen before the conversion to soot. It is believed that the use of this atomizing gas results in a faster heating of liquid and helps provide the O2 that is required for the reaction. Therefore, the velocity of the oxygen atomizing gas can be significantly reduced, by at least about 50% of the velocity of the nitrogen atomizing gas. This reduction in gas velocity consequently reduces the turbulence of the burner flame and the defects of the soot preform.

In addition, it was also observed that using a mixture of oxygen and nitrogen as the atomizing gas allowed reducing the speed of the atomizing gas and also reducing the defects of the preform. For example, a mixture of 75% oxygen and 25% nitrogen by volume as the atomizing gas allowed a significant reduction in the atomizer gas velocity and a reduction in the soot preform defects. However, compared to this example, using only oxygen provided a lower atomizer gas velocity. With the use of the pneumatic atomizer in the invention, the high-velocity exhaust gas is effectively lowered to achieve a beneficial level of atomization of the siloxane in the burner and in the flame. In practicing the invention, although it is preferred to have a sprayer unit 41 as an integral part of the burner unit 40, it is possible to use a pneumatic atomizer that is specially separated from the burner, as with the sprays 21 and 31 in FIGS. and Figure 3. The apparatus can also be provided with a doping supply tank 16, which is shown in Figure 1, which contains a compound capable of being converted by oxidation or flame hydrolysis to P2O5 or a metal oxide whose metallic component is selected from groups IA, IB, NA, IIB, IIIA, IIIB, IVA, IVB, VA, and the rare earth series of the periodic table. These oxide dopants combine with the silica that was generated in the burner to provide a doped silica glass, which can subsequently be formed into optical waveguide fibers.

The compound supplying the silica glass dope can be supplied to feed the tank 11 from the doping supply 16 of figure 1. Alternatively, the doping agent can be supplied from the supply 16 to the liquid injector 15 by means of a pump separate dispenser and optionally a filter (not shown) analogous to the delivery system used for the silicon-containing compound. According to the invention, the reactive compound containing silicon preferably free of halide, preferably includes a polyalkylsiloxane, for example hexamethyldisiloxane. Most preferably the polyalkylsiloxane comprises a polymethylcyclosiloxane. Most preferably still the polymethylcyclosiloxane is selected from the group consisting of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane and mixtures thereof. As described in the co-pending patent application E.U.A.

No. 08 / 574,961, the use of such siloxane sourcing materials as octamethylcyclotetrasiloxane presents problems with the conventional silica manufacturing process in such a way that the siloxane sourcing material is prone to polymerize and form gels, which clog and impede the vaporizer of the supply material and the vaporized supply system of supply material. The following examples further illustrate the invention.

EXAMPLE 1 Generation of soot by means of injecting a liquid reagent into a flame with a syringe A stream of liquid octamethylcyclotetrasiloxane (OMCTS) was injected into a burner flame of a lathe equipped with a glass rod using a syringe provided with a 0.0254 cm diameter needle. The resulting porous soot particles containing Si02 were collected on the rotating glass rod 2.54 cm in diameter. This procedure demonstrates the feasibility of obtaining S0O2 by means of combustion of the compound of siloxane supply material in liquid form.

EXAMPLE 2 Generation of soot using an ultrasonic transducer atomizer as an injector Liquid octamethylcyclotetrasiloxane (OMCTS) was supplied to a combustion burner by means of a 20-khz Vibra-Cell® ultrasonic transducer atomizer (available from Sonics &Materials, Inc., Danbury, CT) which was inserted downstream of the central line of the burner. The atomizer was surrounded by two internal rings of oxygen supply and an outer ring of premixed CH O2. The following flow rates were used: Octamethylcyclotetrasiloxane (OMCTS), 11 grams per minute; oxygen, 10 standard liters per minute (SLPM); premix, 10 SLPM CH and 8.4 SLPM O2. The combustion was continued for approximately 10 minutes. A considerable deposit of SO22 soot was collected in the mandrel, further demonstrating the practical ability to produce S0O2 from a siloxane supply material that is introduced as small liquid particles in a burner flame.

EXAMPLE 3 Generation of soot by means of an atomizing combustion burner An atomizing burner was constructed as described in Figure 4. Various dimensions of the atomizer 41 and surrounding channels were tested, according to the following: Internal diameter of the atomizer 41: 0.017 cm at 0.038 cm Internal diameter of the channel 43: 0.091 cm at 0.127 cm Outside diameter of channel 43: 0.121 cm to 0.160 cm Using an atomizer 41 having an internal diameter of 0. 038 cm, the soot particles were generated from octamethylcyclotetrasiloxane (OMCTS) for 65 minutes. The flow rates were the following: Premixture through channel 46: 10 SLPM CH4 and 8 SLPM 02. O2 through channels 44 and 45: 26 SLPM. N2 through channel 43: 5.6 SLPM. Octamethylcyclotetrasiloxane (OMCTS) by means of the atomizer 41: 6 milliliters per minute (ml / min) for the first five minutes, then 10 ml / min for the next 60 minutes. The objective or attractant, a glass rod with a diameter of 2.54 cm, was placed to give between 1 and 5 rotations per second, going back and forth at a speed of approximately 15 meters per minute. The distance of the receiving surface from burner to attractant was approximately 16.51 cm. The complete combustion of octamethylcyclotetrasiloxane was achieved (OMCTS) of the reagent, and the target weight was increased by 247 grams over the 65 minute deposition period (3.8 grams / minute). Subsequently, the soot was consolidated in an oven, producing clear glass free of defects. The invention has been described in detail for the purpose of the illustration, but it is understood that said detail is only for that purpose and variations may be made therein by those skilled in the art without departing from the spirit and scope of the invention, which they are defined by the following claims.

Claims (12)

NOVELTY OF THE INVENTION CLAIMS

1. A method for manufacturing silica constituted by the steps of: a) supplying a liquid siloxane supply material in liquid form to a conversion site; b) atomizing said liquid siloxane supply material near the conversion site by supplying the siloxane sourcing material with a gas including oxygen; c) converting said atomized siloxane supply material into silica.

2. A method according to claim 1, further characterized in that said siloxane is octamethylcyclotetrasiloxane.

3. A method according to claim 1, further characterized in that said atomizing gas includes nitrogen.

4. A method according to claim 3, further characterized in that said atomizing gas consists essentially of oxygen and nitrogen.

5. A method according to claim 4, further characterized in that said atomizing gas contains at least about 50% oxygen by volume.

6. A method according to claim 5, further characterized in that said method includes the step of impurifying said silica with at least one element of the group consisting of P2O5 or a metal oxide having a metal component selected from groups IA , IB, HA, IIB, IIIA, IIIB, IVA, IVB, VA, and the series of rare earths of the Periodic Table of the Elements.

7. A method for manufacturing a silica preform consisting of the following steps: a) supplying a liquid siloxane supply material in the liquid form to a conversion site; b) atomizing said liquid siloxane supply material near the conversion site by supplying the siloxane sourcing material with a gas including oxygen; c) converting said atomized siloxane supply material into silica.

8. A method according to claim 7, further characterized in that said siloxane is octamethylcyclotetrasiloxane.

9. A method according to claim 7, further characterized in that said atomizing gas includes nitrogen.

10. A method according to claim 9, further characterized in that said atomizing gas consists essentially of oxygen and nitrogen.

11. A method according to claim 10, further characterized in that said atomizing gas contains at least about 50% oxygen by volume.

12. A method according to claim 11, further characterized in that said method includes the step of impurifying said silica with at least one element of the group consisting of P2O5 and a metal oxide having a metal component selected from the groups IA , IB, HA, IIB, IIIA, IHB, IVA, IVB, VA, and the series of rare earths of the Periodic Table of the Elements.