MXPA00000586A - Method and apparatus for forming silica by combustion of liquid reactants using a heater - Google Patents

Abstract

The present invention is directed to a method for making silica glass and silica glass preforms. A liquid, preferably halide-free, silicon-containing compound capable of being converted by thermal oxidative decomposition to SiO2 is provided 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 proximate thesite where the liquid is converted into silica glass by atomizing the liquid, preferably with a stream of atomizing gas. A heater proximate the burner face and around the burner flame increases soot capture rate and allows for a reduction of the velocity of the atomizing gas. 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.

Description

METHOD AND APPARATUS FOR FORMING SILICA BY COMBUSTION OF LIQUID REAGENTS USING A HEATER 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 compounds containing silicon.

BACKGROUND OF THE INVENTION Metal halides of silicon, germanium, zirconium, and titanium are often used as vapor reagents in the formation of metal oxide crystals. For example, the hydrolysis of SiCI4 has been the industry's preference to produce high purity silica for years. The oxidation of SÍCI4, through pyrolysis and hydrolysis, however, has the disadvantage of producing chloride or a very strong acid by-product, hydrochloric acid (HCl). Hydrochloric acid is harmful not only to many deposition substrates and to the reaction equipment but is also harmful to the environment. Emission abatement systems have proven to be very expensive due to the loss and maintenance of equipment caused by the corrosivity of HCl.

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 due to 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 for producing, among other things, semiconductor devices. However, silanes have shown that they are very expensive and reactive to be considered for commercial use except possibly for small scale applications that require extremely high purity. The Patent of E.U.A. No. 5,043,002 to Dobbinis et al, which serves as a base and is incorporated by reference, proposes alternative silica precursor materials. This patent describes bubbling a carrier gas through a silicon-containing reactive compound, preferably a halide-free compound such as polymethylsiloxanes, in particular, polymethylcyclosiloxanes such as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane ("OMCTS"), and decamethylcyclopentasiloxane. A mixture of the reactive compound vapor and nitrogen is transported to the burner at the reaction site, where the reagent is combined with a gaseous / oxygen fuel mixture and consumed. The Patent of E.U.A. No. 5,152,819 to Blackwell et al, the description of which serves as a base and is incorporated by reference, discloses additional halide-free silicon compounds, in particular, organo-nitrogen compounds having a basic Si-N-Si structure, siloxasilazones having a Si-N-Si-O-Si basic structure, and mixtures thereof, which can be used to produce high purity silica crystals without the concomitant generation of corrosive, polluting byproducts. Although the use of halogenide-free silicon compounds as a supply material for production of silica crystals prevents the formation of HCl, some problems remain, particularly when the crystals are designed for the formation of high purity fused bulk silica and optical products of high quality as optical waveguides. For example, as described in the patent application of E.U.A. co-pending No. 08 / 574,961 entitled "Method for Purifying Polyalkylsiloxanes and the Resulting Products", which serves as a base and is incorporated by reference, the presence of high-boiling impurities in, for example, a polyalkylsiloxane sourcing material, It can result in the formation of gel deposits in the vaporization and supply systems that carry the vaporous reagents to the burner or inside the burner itself. Such polymerization and gelling of the siloxane supply material inhibits the controllability and consistency of the silica manufacturing process. This problem is more prevalent when an oxidizing carrier gas such as oxygen is included in the reactive vapor stream, because the oxidants appear to catalyze the polymerization of the siloxane sourcing material. Said polymerization and gelation reduce the deposition laying speed of the bulk silica soot or soot preform that can be consolidated subsequently to a preform from which an optical waveguide is manufactured. An additional problem encountered when silica soot or silica preforms are formed using siloxane starting material is that particles of high boiling, high molecular weight impurities can be deposited in the bulk silica soot or on the preform of the optical waveguide fiber resulting in "defect" or "agglomeration defect". Defects or agglomerated defects are imperfections that adversely affect the optical and structural quality of the optical waveguides formed using silica soot. In the co-pending application serial number 08/767/653, the content of which serves as a base and is incorporated by reference, it is disclosed that the agglomerated defects could be reduced by supplying a liquid siloxane sourcing material to a conversion site. , atomizing the supply material in the conversion site, and converting the atomized supply material into the silica conversion site. One way to atomize the supply material in the conversion site involves atomizing pneumatically or "by air jet" the liquid siloxane supply material by supplying the liquid supply material to the conversion site with an inert gas. Although atomizing the liquid siloxane supply material near the conversion site reduces the agglomeration defects, such a liquid delivery system presents several additional challenges. For example, increasing the rate of atomization gas desirably produces smaller liquid droplets, which evaporate and burn more rapidly in the burning flame. Smaller droplets are desirable because larger droplets cause wart-like defects ("warts") on the surface of the soot preform. In addition, smaller droplets can be more easily focused with surrounding gases to produce a more focused deposition current. On the other hand, increasing the atomization gas velocity adds turbulence to the burning flame, which reduces the rate of soot capture and is a cause of a physical soot defect known as "lizard skin". Lizard skin is a term for a rough soot preform surface. Accordingly, it would be desirable to provide a method in which a liquid supply system could produce a focused deposition stream containing small droplets without a high gas velocity, and in which there is low turbulence of the burner flame.

BRIEF DESCRIPTION OF THE INVENTION Accordingly, the present invention generally provides a method and apparatus for making silica and silica preforms in which a system supplies a precursor of liquid supply material to a conversion site, typically a flame burner, in the which the liquid supply material is converted to silica particles. Applicants have now discovered that using a heater near the conversion site provides for combustion of larger liquid droplets, thereby reducing the need for high velocity gas to more fully vaporize the liquid droplets, and thus having turbulence. of lower burner flame. The advantages and additional features of the invention will be set forth in the description that follows, and in part will be apparent from the description, as well as from the drawings appended thereto. To achieve these and other advantages, the invention includes a method of and an apparatus for making silica, which involves supplying a liquid supply material, preferably siloxane, in liquid form to a conversion site. The supply material is atomized close to the conversion site and converted to silica in a combustion burner. Advantageously, the invention includes a heater close to the atomized supply material. The heater may consist of a cover such as a refractory brick or other heat retaining material around the conversion site to retain the heat of the combustion burner. The heater may consist of a ring type heater around the conversion site with an additional heat source, such as an electric resistance heating element. The invention additionally comprises an apparatus for forming silica from silicon-containing reagents, preferably free of halide, liquids, consisting of a tank containing a liquid supply material and a supply conduit of liquid supply material having first and second terminal ends. The apparatus also consists of a combustion burner which, in operation, generates a conversion site flame and an injector of liquid supply material near the second terminal end of the liquid supply material supply conduit to supply a compound containing silicon to the flame. Advantageously, a heater is located near the conversion site, which allows the apparatus to produce a more focused soot stream having finer particles divided finer than without a heater. The heater may consist of a cover around the conversion site to retain the heat of the combustion burner, such as a refractory brick or other material that retains heat, or the heater may be an auxiliary heater, such as a ceramic ring heater. The flame of the combustion burner converts the compound by thermal oxidative decomposition to a finely divided amorphous soot and a receiving surface positioned with respect to the combustion burner to allow deposition of the soot on the receiving surface. Applicants have discovered that providing a heater near the conversion site improves the performance and quality of the silica produced by reducing the aforementioned lizard skin defect on the surface of the soot preforms. The heater allows the combustion of larger liquid droplets. Therefore, when a pneumatic atomizer is used to atomize the supply material, the heater significantly reduces the flow of gas required to atomize the supply material. It should be understood that the aforementioned general description and the following detailed description are illustrative and explanatory and are designed to provide additional explanation of the invention as claimed. The appended drawings are included to provide a further understanding of the invention illustrating various embodiments of the invention, and together with the description, serve to explain the principles of the invention. Wherever possible, the same reference numbers will be used throughout the drawings to denote similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a reagent delivery system used in accordance with the present invention. Figure 2 is a schematic representation of a cross-sectional view of a burner structure having an atomizer incorporated therein which would be used in accordance with the present invention.

Figure 3 is a schematic representation of a perspective view of a burner structure having a heater around the burner. Figure 4 is a schematic representation of a perspective view of a burner structure surrounded by refractory bricks.

DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the preferred embodiments of the invention which are illustrated in the appended drawings. According to the invention, the present invention for a silica manufacturing apparatus, Figure 1, schematically describes a system for supplying liquid siloxane supply material and, optionally, compounds that provide a dopant to a conversion site wherein the material of Liquid siloxane supply is converted to silica. The conversion of the liquid supply material to silica preferably occurs in an area close to the burner 10, which is preferably a combustion burner. A liquid siloxane supply material such as a polymethylcyclosiloxane is stored in the supply material tank 1 1. The supply material tank 1 1 is connected to the liquid supply material injector 15 at the reagent introduction site through a liquid supply material conveying system which can, if desired, include a metering pump 12, an optional filter 13, and a preheater 14. Said liquid supply material transport conduit has a first terminal end 17 connected to the liquid supply material tank 11, and a second terminal end 18 near the liquid supply injector. liquid supply material 15. The siloxane liquid supply material from tank 11 is transferred through the liquid supply material transport conduit via pump 12 through filter 13 to optional preheater 14. Liquid supplied through the filter 13 is under sufficient pressure to substantially prevent and inhibit its volatilization in the preheater 14, which is optionally used to heat the liquid reagent before it is introduced into the burner 10 and avoids the high temperatures of a vaporizer that normally promote gel formation. The burner is preferably provided in a conventional manner with inner cover gas, outer cover gas, and a mixture of methane and oxygen for the flame, as described, for example, in U.S. Patent No. 4,165,223 to D.R. Powers, which serves as a basis and is incorporated herein by reference. The liquid reagent is conveyed from the optional filter 13 or optional preheater 14 through the second terminal end 18 to a liquid injector 15, which supplies the liquid to the burner 10. The injector 15 consists of a device for supplying the liquid reagent, either as a liquid stream or as atomized liquid particles, directly towards the flame of the burner 10. Normally we refer to the reagent in the description as being in "liquid" form. What we mean by this expression is that the reagent is substantially in the liquid state. Some small portion of the reagent may be in the form of vapor, particularly where the preheater 14 is used, or where a blanket of nitrogen is used on the liquid. A small portion of the reagent may be in the form of vapor as supplied to the combustion site without adversely affecting the operation of the invention. The liquid injector 15 can consist, for example, of a syringe provided with a fine needle, by which a liquid stream can be injected at high speed towards the flame burner. Although a syringe can be used on a small scale, commercial operations will require a reasonably large scale equivalent, eg, an atomizer. Various types of atomizing media capable of forming very small liquid particles are known in the art of atomization and are described in Atomization and Sprays, 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, mechanical, electrical and vibratory gas, and can be categorized as, for example, current, swirl, current-swirl, pneumatic, rotating, acoustic, ultrasonic, and electrostatic. Preferably a jet atomizer is used; even more preferably, the jet atomizer is a swirl-jet atomizer, which swirls the liquid and then, as the atomizers do in general, shoots the liquid at high speed out of a small orifice. Various types of atomizers are discussed in Liquid Atomization, by L. Bayvel and Z. Orzechowski, Taylor & Francis, (1993), which is incorporated herein by reference. Another preferred type is a pneumatic atomizer operated by inert gas, oxygen gas, a mixture of oxygen gas and an inert, air, or fuel gas pressure. By inert gas, we mean a gaseous element such as nitrogen, argon, or helium, which is non-reactive under normal conditions. By combustible gas, we mean a gas that is typically used in the combustion of silica precursors, such as methane, or a mixture of methane and oxygen. In particularly preferred embodiments, the atomizer may be incorporated in the structure of the combustion burner. The atomized particles of the siloxane reactive compound are consumed in a burner fed by a fuel gas, preferably a combination of methane and oxygen. The atomized reagent particles can be transported from the atomizer to the burning flame by a gas carrier such as an inert gas, oxygen, a mixture of an inert gas and oxygen, or a combustible gas. Figure 2 is a cross-sectional view of one embodiment of the burner and atomizing apparatus used with the present invention. Herein, the burner 40 incorporates within its structure the atomizer 41, which injects finely atomized liquid reagent particles 42 toward the flame 23. The amorphous soot 25 produced by combustion of the liquid reagent can be collected on a rotary spindle 26 for form a soot target or preform that can be used to manufacture an optical waveguide. In an alternate embodiment, the silica soot may be pooled for subsequent consolidation, or the silica soot may be collected in a collection chamber in which the soot is immediately consolidated into its desired shape (not shown). As shown in Figure 2, the burner 40 consists of a series of concentric channels surrounding the atomizer 41. The liquid siloxane is supplied through the atomizer 41. A stream of an inert gas such as nitrogen gas, a mixture of oxygen gas and nitrogen gas, or oxygen gas only supplied through 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 reagent particles in the flame burner 23. The area near the face of the burner 53 and the flame 23 thus acts as a conversion site for converting the liquid projections 42 into soot reagent particles. Oxygen gas 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 through channel 44. A premix of oxygen and a fuel as methane is conducted to the flame through the outermost channel 47. In the embodiments in which the silica soot preform is used to form optical waveguide fibers, a burner equipped with an atomizer injector, such as that of the embodiment described in figure 2, produces a broad soot stream, which achieves a core concentricity and improved coating regions of the optical waveguide fiber ica formed subsequently. A preferred atomizing unit 40 of the invention as shown in Figure 2 consists of a pneumatic atomizer. With such pneumatic atomizer, the liquid siloxane supply material is atomized by the kinetic energy of a gas stream flowing through the channel 43. The high velocity gas is used to atomize the supply material. This produces atomized liquid projections 42 with a gas velocity in the range of 0.5 to 50.0 m / sec. An inert gas, such as nitrogen or argon gas, oxygen gas, a fuel gas, or a mixture of oxygen and an inert gas can be used as the atomizing gas. With the use of the pneumatic atomizer in the invention, the high velocity gas jet is spread in an effective manner to achieve an effective level of siloxane atomization close to the face of the burner and in the flame. In practicing this invention, even though it is preferred to have the atomizing unit 40 as an integral part of the burner unit, it is possible to use a pneumatic atomizer which is spatially separated from the burner, as with the atomizers shown in FIGS. 2 and 3 of FIG. copending application serial number 08 / 767,653. Referring now to Figure 3, a preferred embodiment of the apparatus of the present invention includes a burner 40, with the details of the burner similar to the burner shown in Figure 2, which produces a flame of deposition 23 to decompose the liquid projections 42. produced by the atomizer (not shown) in soot particles. A heater assembly 60 supports a heater 62, which helps vaporize the liquid projections 42, thereby allowing the velocity of the atomizing gas to be reduced. The additional heat provided by the heater 62 will help vaporize the larger liquid projections in the spray size distribution produced by the atomizer. The heater 62 may be a ring or ceramic tube surrounding the flame 23, and may additionally have an auxiliary heating source such as an electric heater or a gas heated heater. Heating tube temperatures of 800 ° C to 1500 ° C are preferred, with a more preferred scale of 950 ° C to 1250 ° C. Using a ceramic heating tube or ring with the heater temperature from 1000 ° C to 1200 ° C allows the spray gas velocity to be reduced by approximately 50% compared to a burner in which a heater is not used.

Figure 4 shows an alternative embodiment of the present invention. A burner 40 produces a flame of deposition 23, which will decompose the atomized liquid projections 42 leaving the atomizer (not shown). In this embodiment, the heater assembly 60 supports a cover 64, which retains heat from the flame of the burner 23, and thus the cover 64 functions as a heater. Annex 64 can be made of any suitable material that retains heat as a ceramic or refractory brick. In yet another embodiment (not shown), the flame of the burner could be surrounded by a ring or heater tube having an additional heating source, and the entire structure could be enclosed by a cover made from a material that retains the hot. In addition to effectively reducing the size of the liquid spray in the spray, the additional heat provided by the heater 62 can increase the rate of soot capture by increased thermophoresis. Thermophoresis is the procedure by which soot is attracted to the preform. In fact, it produces the driving force that moves the particles towards the chilling preform. The hot gases from the burner pass around the preform during the run; the soot particles do not have enough momentum by combustion alone to attack the preform. Thermophoresis moves particles at a temperature gradient from hot regions to colder regions. The gases burned from a burner are hotter than the preform. As these gases pass around the preform, a temperature gradient is produced. The hot gas molecules have a higher velocity than the cold gas molecules. When the hot gas molecules hit a particle, they transmit more momentum to the whole particle than a cold gas molecule does. Therefore, the particles are driven to the colder gas molecules and, in turn, to the preform. It is believed that the soot particles, after being provided with additional heat from the heater, will have a larger driving force to attract them to a relatively cooler target. The heater also reduces the lizard skin defect on the surface of a soot preform or preform used for optical waveguides. Using a heater with a liquid burner in accordance with the present invention allows a reduced atomizing gas velocity which reduces the turbulence of flame which provides a better quality of soot and soot preforms. The apparatus may also be provided with a doping supply tank 16, shown in Figure 1, which contains a compound capable of being converted by oxidation or flame hydrolysis to P205 or to a metal oxide whose metal component is selected from the groups IA, IB, HA, IIB, IIIA, IIIB, IVA, IVB, VA, and the series of strange lands of the periodic table. These oxide dopants combine with the silica generated in the burner to provide impurified silica crystals, which can subsequently be formed into optical waveguide fibers. The compound supplying the silica glass dope can be provided to supply the tank 11 from the dopant supply 16 of figure 1. Alternatively, the doping agent can be supplied from the supply 16 to the liquid injector 15 through a separate metering pump and optionally a filter (not shown) analogous to the delivery system used for the silicon-containing compound. According to the invention, the silicon-containing reactive compound, preferably free of alidide, preferably comprises a polyalkylsiloxane, for example hexamethyldisiloxane. More preferably, the polyalkylsiloxane consists of a polymethylcyclosiloxane. More preferably, the polymethylcyclosiloxane is selected from the group consisting of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, and mixtures thereof. The invention has been described in detail for purposes of illustration, but it should be understood that such detail is only for that object and variations may be made thereto by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the following claims.

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. A method for manufacturing silica that consists of the steps of: a) supplying a liquid supply material in the liquid form to a conversion site; b) atomizing said liquid supply material near the conversion site; c) converting said atomized supply material into silica near the conversion site in a combustion burner; d) provide a heater near the conversion site.

2. A method according to claim 1 characterized in that the liquid supply material is a liquid siloxane.

3. A method according to claim 2, characterized in that the step of atomizing said liquid siloxane supply material additionally consists of the step of atomizing the liquid siloxane supply material with the kinetic energy of a flowing gas stream.

4. A method according to claim 2, characterized in that the heater consists of a cover around the conversion site to retain the heat provided by the combustion burner.

5. - A method according to claim 2, characterized in that the cover comprises a ring heater having an additional heat source.

6. An apparatus for manufacturing silica that consists of: a tank that contains a liquid supply material; a liquid supply material supply conduit having a first terminal end and a second terminal end, said first terminal end connected to the liquid supply material tank; an injector of liquid supply material, said injector near the second terminal end of the liquid supply material supply conduit; a site for converting supply material to silica close to said injector, in which the supply material projected from the injector is converted to silica; and a heater near the conversion site.

7. An apparatus for manufacturing silica according to claim 6, characterized in that the liquid supply material is liquid siloxane.

8. An apparatus according to claim 7, characterized in that the heater comprises a cover around the conversion site to retain the heat provided by the combustion burner.

9. An apparatus according to claim 8, characterized in that the cover additionally includes an additional heat source.

10. - An apparatus according to claim 7, characterized in that the atomizer consists of a pneumatic atomizer that is close to the combustion burner.