METHOD OF MAKING A TITANIA-DOPED FUSED SILICA PREFORM

Abstract: A method of vaporizing a liquid TiO2 precursor utilized in making a titania-doped fused silica preform includes passing the liquid TiO2 precursor through a packed-bed vaporizer with a carrier gas under conditions sufficient to vaporize the TiO2 precursor without substantial thermal degradation thereof. Further, the method includes commingling the vaporized TiO2 precursor with a vaporized SiO2 precursor and delivering the mixture to a vapor utilization site to form the preform. In one embodiment of the present invention, liquid TiO2 and SiO2 precursors are contemporaneously vaporized in the vaporizer and delivered to the vaporization utilization site to form the preform.

Description: METHOD OF MAKING A TITANIA-DOPED FUSED SILICA PREFORM FIELD OF THE INVENTION This invention relates to a method of making a titania-doped silica preform which can be used to produce optical or acoustic waveguide fibers as well as ultralow expansion glass. BACKGROUND OF THE INVENTION Halide-containing SiO 2 precursors have been used in the manufacture of preforms by vapor phase deposition techniques, such as, the modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), and outside vapor deposition (OVD) techniques. In the MCVD technique, the SiO 2 precursors are vaporized and reacted with oxygen to form oxide particles which are deposited on the inside of a fused-silica tube. In the VAD and OVD procedures, vaporized SiO 2 precursors are hydrolyzed in a burner to produce soot particles which are collected on a rotating starting rod (bait tube) in the case of VAD or a rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously- formed core preform, rather than on a mandrel. Various vaporizers utilized in such processes are found in U.S. Patent. No. 4,212,663 to Aslami, U.S. Patent No. 4,314,837 to Blankenship, U.S. Patent. No. 4,529,427 to French, U.S. Patent No. 4,938,789 to Tsuchiya et al., U.S. Patent No. 5,078,092 to Antos et al., Japanese Patent Publication No. 58-125633, and U.K. Patent Publication No. 1,559,987. U.S. Patent No. 5,090,985 to Soubeyrand et al. discloses the use of a horizontal thin film evaporator for vaporizing various raw materials employed in the preparation of coated glass articles. None of these systems utilize a vaporization system of the present invention in which specific flow patterns are used to vaporize a thermally degradable TiO 2 precursor. It is known that optical fibers having one or more outer layers doped with titania exhibit superior strength and ultra-low expansion qualities, as compared to homogeneous silica clad fibers. For example, U.S. Patent No. 2,326,059 to Nordberg discloses a process for making silica-rich, ultra-low expansion glass by vaporizing tetrachlorides of Si and Ti into the gas stream of an oxy-gas burner. U.S. Patent No. 4,501,602 to Miller et al. describes the production of glass and glass/ceramic articles utilizing a vapor phase oxidation process of β-diketonate complexes of metals selected from Groups LA, IB, IIA, IIB, IIIA, IIIB, IV A, IVB, VA, and the rare earth series of the Periodic Table. However, halide-containing SiO 2 and TiO 2 precursors produce halide- containing by-products, such as, halide acids (e.g., hydrochloric acid). Such halide- containing by-products are a source of environmental pollution, which requires collection and environmentally-safe disposal of the by-products. This, in turn, causes an overall increase in the cost of preform production. As a solution to the environmental problem, halide-free SiO 2 and TiO 2 precursors are utilized as starting materials for preform production. For example, U.S. Patent No. 5,043,002 to Dobbins et al. discloses the production of high purity fused silica glass through oxidation or flame hydrolysis of a halide-free SiO precursor, such as polymethylsiloxanes, with the polymethylcyclosiloxanes being particularly preferred, and with octamethylcyclotetrasiloxane (OMCTS) being especially preferred. Further, U.S. Patent No. 5,154,744 to Blackwell et al. discloses the production of high purity fused silica glass doped with titania through oxidation or flame hydrolysis of a gaseous mixture of OMCTS and a halide-free organometalhc compound (i.e. a TiO 2 precursor), preferably titanium isopropoxide (Ti(OC 3 H ) 4 ), titanium ethoxide (Ti(OC 2 H 5 ) 4 ), titanium-2-ethylhexyloxide (Ti(OCH 2 (C 2 H 5 )CHCH H 9 ) 4 ), titanium cyclopentyloxide (Ti(OC H ) 4 ), a titanium amide (Ti(NR 2 ) 4 ), wherein R 2 is a methyl or ethyl group, or a combination thereof. Both liquid OMCTS and the liquid TiO 2 precursor are respectively flash vaporized in heated flash tanks at 175 °C, carried through heated fume lines by nitrogen to an oven, mixed, and passed through a flame of a combustion burner to form amorphous particles of fused silica doped with titania. The amorphous particles are thereafter consolidated into a glass body. These same halide-free, SiO 2 and TiO 2 precursors are preferred for use with the present invention. U.S. Patent Nos. 5,558,687 and 5,707,415 to Cain et al. disclose a vertical, packed-bed film vaporizer (evaporator) specifically developed for use with halide-free, SiO 2 precursors, such as OMCTS. The vaporizer includes a plurality of packed-bed columns surrounding a central tube. A mixture of the liquid SiO 2 precursor and oxygen is sprayed onto the top surfaces of the columns by a set of spray nozzles. The precursor and oxygen flow downward together through the heated columns. The liquid precursor evaporates into the oxygen until the dew point temperature is reached, and thereafter exits the bottom surfaces of the columns. At that point, the direction of flow changes from downward to upward, causing any higher molecular weight species to separate from the vapor stream. The vapor stream exits the evaporator through the central tube and is thereafter supplied to soot producing burners. The same vaporizer is preferred for use with the present invention. Both patents are silent regarding TiO 2 precursors. Further, there is no indication that the vaporizer is capable of vaporizing a TiO 2 precursor at a temperature low enough to substantially prevent thermal degradation thereof, particularly while contemporaneously vaporizing a SiO 2 precursor. WO 99/15468 to Maxon et al. describes a method of producing a titania-doped fused silica glass by flame hydrolysis. The process comprises delivering a mixture of a SiO 2 precursor, such as OMCTS, and a TiO 2 precursor, such as titanium alkoxide (e.g. titanium isopropoxide), in vapor form to a flame, passing the vapor mixture through the flame to form SiO -TiO particles, and depositing the particles within a furnace where they melt to form a solid glass body. The liquid SiO 2 and TiO 2 precursors are disposed within separate tanks and vaporized by passing a carrier gas, such as nitrogen, through the respective precursor to entrain vapors thereof. Respective by-pass streams of carrier gas are introduced to the entrained vapor streams to prevent precursor saturation. These two separate vapor streams are fed to and mixed within a manifold. From the manifold, the mixed vapors pass through fume lines to a furnace wherein they pass through the flames of burners. Maxon et al. state that titanium alkoxides degrade to higher order polymers, oxidation products, and trace elements, thereby changing the precursor's vapor pressure. The change in vapor pressure ultimately leads to turbulence in the furnace, which aggravates buildup of glass condensate. Importantly, Maxon et al. advocate using higher temperatures with titanium alkoxides to increase the vapor pressure of the alkoxides. This results in a lower carrier vapor flow rate, which contributes to less turbulence in the vapor delivery system and minimal glass buildup in the furnace. Decomposition of the TiO 2 precursor, according to Maxon et al., can be avoided by controlling the temperature in the fume lines. Unfortunately, halide-free TiO 2 precursors, particularly titanium isopropoxide, continue to present preform production difficulties. Even though titanium isopropoxide has a boiling point of 232°C, it is thermally unstable at 175 °C and produces higher molecular weight gels. Further, halide-free TiO 2 precursors are generally reactive with oxygen to form contaminants. These gels and contaminants cannot be utilized in preform production and must be discarded. This waste results in higher preform production cost due to lower production efficiency and additional waste disposal cost. Despite the use of halide-free, SiO 2 and TiO 2 precursors in producing titania- doped fused silica preforms, the need for a method of producing such preforms substantially without thermal degradation of a liquid TiO precursor remains. Further, there remains a need for a method of producing a titania-doped fused silica preform wherein a TiO precursor is vaporized absent flash vaporization and at a temperature low enough to substantially prevent thermal degradation thereof, particularly while contemporaneously vaporizing a SiO 2 precursor. The present invention is directed to overcoming these deficiencies in the art. SUMMARY OF THE INVENTION One aspect of the present invention relates to a method of vaporizing a liquid TiO 2 precursor which is capable of being converted through thermal decomposition with oxidation or flame hydrolysis to TiO 2 . The liquid TiO 2 precursor is passed through a packed-bed column along with a carrier gas under conditions effective to vaporize the liquid TiO 2 precursor without substantially thermally degrading the TiO 2 precursor. Another aspect of the present invention relates to a method of providing reactant vapors comprising vaporized SiO 2 and TiO 2 precursors which are respectively capable of being converted through thermal decomposition with oxidation or flame hydrolysis to SiO 2 and TiO 2 . A liquid TiO 2 precursor is passed through a packed-bed column along with a carrier gas under conditions effective to vaporize the liquid TiO precursor without substantially thermally degrading the TiO 2 precursor. The vaporized TiO 2 precursor and carrier gas are co-mingled with a vaporized SiO precursor to form a reactant vapor/gas mixture. Still another aspect of the present invention relates to a method of vaporizing liquid SiO 2 and TiO 2 precursors to provide reactant vapors. Liquid SiO 2 and TiO 2 precursors which are respectively capable of being converted through thermal decomposition with oxidation or flame hydrolysis to SiO 2 and TiO 2 are contemporaneously passed through a packed-bed column along with a carrier gas under conditions effective to vaporize the respective precursors. This is accomplished without substantially thermally degrading either precursor. The present invention is an improvement over the prior art in at least four respects. First, the vaporizer minimizes thermal degradation of the temperature sensitive titania precursors and eliminates the need for periodic disposal and waste thereof due to contaminants from thermal degradation. Second, the process is simpler with the implementation of one vaporizer as compared to the current need for two flash tanks. Third, the process is more robust than prior art systems, because the carrier gas flow in the vaporizer is essentially independent of the respective titania and silica precursor flow rates. Fourth, no current ultra-low expansion glass delivery system can account for between 2-5 ppb/°C variance in coefficient of thermal expansion (CTE) without compensating for changes in barometric pressure. Consistent precursor flow within the prior art systems is dependent on atmospheric pressure, tight control of flash tank temperature, and carrier gas flow rate. The process of the present invention is independent of all of these variables and maintains consistent CTE control with accurate delivery of the silica and titania precursors to the vaporizer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of one embodiment of a vapor delivery system constructed in accordance with the present invention. Figure 2 is a schematic, horizontal cross-sectional view of a vaporizer for use in a vapor delivery system in accordance with the present invention. Figure 3 is a schematic, vertical cross-sectional view taken along lines 3-3 in Figure 2. Figure 4 is a block diagram of another embodiment of a vapor delivery system constructed in accordance with the present invention. Figure 5 is a graph, indicating theoretical predictions for vaporization of titanium isopropoxide at a flow rate of 41 g/minute, wherein "GOOD" indicates a regime of operating conditions predicting complete vaporization, and "BAD" indicates a regime of operation conditions predicting incomplete vaporization. Figure 6 is a graph, indicating theoretical predictions for vaporization of titanium isopropoxide at flow rates between 10 and 80 g/minute. Figure 7 is a graph, indicating theoretical values for vaporizing a mixture of titanium isopropoxide and octamethylcyclotetrasiloxane. Figure 8 is an expanded image of the graph of Figure 7. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of vaporizing a liquid TiO 2 precursor which is utilized in making either a titania-doped fused silica preform a titania-doped glass, such as an ultralow expansion glass. Such TiO 2 precursor is capable of being converted through thermal decomposition with oxidation or flame hydrolysis to TiO 2 (titania). Referring to Figure 1, a liquid TiO 2 precursor is stored in liquid feed tank 10 and supplied to a vaporizer 13 by liquid metering pump 12. Pump 12 establishes a liquid TiO 2 precursor flow rate. The carrier gas is stored in gas feed tank 14 and is transferred to the vaporizer 13 at its own predetermined flow rate by gas mass flow controller 16. Within the vaporizer 13, the liquid TiO 2 precursor is vaporized to produce a vapor/gas mixture of liquid TiO 2 precursor and carrier gas. For example, if the liquid reactant is TPT and the carrier gas is nitrogen, a preferred flow rate for the carrier gas is in the range from about 2.0 to about 9.0 standard liters per minute (slpm) per gram minute of TiO 2 precursor, with a flow rate of about 5.0 slpm per gram/minute of TiO precursor being particularly preferred. From the vaporizer 13, the vapor/gas mixture is directed to the vapor utilization site 20 where it is introduced to a conventionally vaporized SiO 2 precursor to form the titania-doped fused silica preform. The liquid TiO 2 precursor is passed through a packed-bed column (e.g., a vertically- oriented packed-bed column 22 as shown in Figs. 2 and 3) along with a carrier gas under conditions effective to vaporize the liquid TiO 2 precursor without substantial thermal degradation of the TiO 2 precursor. A gas stream containing a mixture of the vaporized TiO 2 precursor and the carrier gas exits the packed-bed column 22 and is delivered to a vapor utilization site 20 capable of forming the titania-doped fused silica preform. Another embodiment of the present invention relates to a method of providing reactant vapors to make a titania-doped fused silica preform. As shown in Figs. 1-3, the liquid TiO 2 precursor is passed through the packed-bed column 22 along with the carrier gas under conditions effective to vaporize the liquid TiO 2 precursor without substantial thermal degradation thereof. A gas stream containing a mixture of the vaporized TiO 2 precursor and the carrier gas exits the packed-bed column 22 and is co- mingled with a second gas stream containing a vaporized SiO 2 precursor to form a reactant vapor/gas mixture. The reactant vapor/gas mixture is delivered to the vapor utilization site 20 to form the titania-doped fused silica preform. Still another embodiment of the present invention relates to another method of providing reactant vapors to make a titania-doped fused silica preform. Referring to Figs. 2-4, a liquid SiO 2 (silica) precursor and a liquid TiO 2 (titania) precursor are contemporaneously passed through the packed-bed column 22 along with a carrier gas under conditions effective to vaporize the respective precursors. This is accomplished without substantial thermal degradation of either precursor. A reactant vapor/gas comprising a mixture of the vaporized SiO 2 precursor, the vaporized TiO 2 precursor, and the carrier gas exits the packed-bed column 22 and is delivered to the vapor utilization site 20 to form the titania-doped fused silica preform. At the vapor utilization site 20, the reactant vapor/gas mixture is passed into a flame of a combustion burner to form amorphous particles of fused SiO 2 doped with TiO 2 . A burner construction for use with the present invention is disclosed in U.S. Patent No. 5,599,371 to Cain et al., which is incorporated herein by reference. In addition to the vapor/gas mixture, these burners are also supplied with a combustible gas, such as methane, and oxygen. The amorphous particles are deposited onto a support (not shown) to form the preform. Thermal decomposition with oxidation or flame hydrolysis is discussed in detail in U.S. Patent No. 3,806,570 to Flamenbaum et al., U.S. Patent No. 3, 864,113 to Dumbaugh, Jr. et al., U.S. Patent No. 3,923,484 to Randall, and U.S. Patent No. 5,152,819 to Blackwell et al., all of which are incorporated herein by reference. Either essentially simultaneously with deposition or subsequently thereto, the amorphous particles of the preform are consolidated into a non-porous body. Alternatively, glass may be made directly without formation of a preform. A waveguide fiber can be conventionally drawn from the non-porous body, particularly when the non-porous body is a non-porous, transparent glass body. Methods of drawing a waveguide fiber are disclosed in U.S. Patent No. 3,859,073 to Schultz and U.S. Patent No. 3,933,453 to Burke et al., both of which are incorporated herein by reference. As mentioned above, TiO precursors utilized in the present invention are capable of being converted through thermal decomposition with oxidation or flame hydrolysis to TiO . Preferably, such TiO 2 precursors are halide-free titanium alkoxides, titanium amides, or a combination thereof. Examples of suitable TiO 2 precursors are titanium-2-ethylhexyloxide, titanium cyclopentyloxide, titanium isopropoxide, titanium methoxide, a titanium amide having the structure Ti(NR 2 ) 4 , wherein R 2 is a methyl or ethyl group, or a combination thereof. Titanium isopropoxide (TPT) is the most preferred TiO 2 precursor. The SiO 2 precursors utilized in the present invention are likewise capable of being converted through thermal decomposition with oxidation or flame hydrolysis to SiO 2 . Generally, as disclosed in U.S. Patent No. 5,152,819 to Blackwell et al., which is incorporated herein by reference, the SiO 2 precursors are an organosilicon-Y compounds, wherein Y is an element of the Periodic Table, the Si-Y bond dissociation energy is no higher than the dissociation energy of the Si-O bond, and the compound has a boiling point no higher than 350°C. Such organosilicon-Y compounds include organosilicon-oxygen compounds having a basic Si-O-Si structure, organosilicon- nitrogen compounds having a basic Si-N-Si structure, and siloxasilazanes having a basic Si-N-Si-O-Si structure, or a combination thereof. Preferably, the SiO 2 precursors are a halide-free polymethylsiloxane, a polymethylsilane, a polyethylsilane, a polyethylsilicate, an aminosilane, a linear silazane, a cyclosilazane, or a combination thereof. Examples of such SiO 2 precursors are octamethylcyclotetrasiloxane, hexamethyldisiloxane, octamethyltrisiloxane, hexamethylcyclotrisiloxane, decamethylcyclopentasiloxane, methyltrimethoxysilane, tetraethylorthosilicate, dimethyldimethoxysilane, trimethylmethoxysilane, tetramethoxysilane, methyltriethoxysilane, tetraethoxysilane, tris ketenimine, nonamethyltrisilazane, octamethylcyclotetrasilazane, or a combination thereof. Octamethylcyclotetrasiloxane (OMCTS) is the most preferred SiO 2 precursor. As discussed in WO 99/15468 to Maxon et al., which is incorporated herein by reference, the alkoxides of the transitions metals are known to be sensitive to light, heat, and moisture. Further, the metal alkoxides readily hydrolyze with moisture to form the hydroxide and oxide of the metal. Accordingly, the SiO 2 precursor, e.g., OMCTS, should be dehydrated or dried to have a water content preferably less than 2 ppm to inhibit formation of white deposits of TiO 2 in the system. The carrier gas is preferably any gas that is inert with respect to the SiO 2 or TiO 2 precursors. Such gases include nitrogen, helium, neon, argon, krypton, xenon, or combinations thereof, with nitrogen being most preferred. Although oxygen may be utilized as the carrier gas as well, it is not preferred because of its tendency to react with the TiO 2 precursors. Now, referring again to Figure 4, liquid TiO 2 precursor and liquid SiO 2 precursor are respectively stored in liquid feed tanks 10 and 64. The liquid TiO 2 and liquid SiO 2 precursors are contemporaneously supplied to the vaporizer 13 by respective liquid metering pumps 12 and 66. Pump 12 establishes a liquid TiO 2 precursor flow rate. Pump 66 establishes a liquid SiO 2 precursor flow rate. The carrier gas is stored in gas feed tank 14 and is transferred to the vaporizer 13 at its own predetermined flow rate by gas mass flow controller 16. Within the vaporizer 13, the liquid TiO 2 and SiO 2 precursors are contemporaneously vaporized to produce a reactant vapor/gas mixture of TiO 2 precursor, SiO 2 precursor, and carrier gas. For example, if the TiO 2 precursor is TPT, the SiO 2 precursor is OMCTS, and the carrier gas is nitrogen, a preferred flow rate for the carrier gas is in the range from about 2.0 to about 9.0 slpm per gram/minute of TiO 2 precursor, with a flow rate of about 5.0 slpm per gram minute of TiO precursor being particularly preferred. From the vaporizer 13, the vapor/gas mixture is directed to the vapor utilization site 20 where it is formed into the titania-doped fused silica preform. Figs. 2 and 3 show a preferred construction for the vaporizer 13 utilized with the embodiments of present invention, which is described in detail in U.S. Patent Nos. 5,558,687 and 5,707,415 to Cain et al., which are incorporated herein by reference. The vaporizer 13 employs a plurality of packed-bed columns 22 surrounding a central, vertically-oriented, vapor/gas receiving tube 24 to vaporize liquid SiO and/or TiO 2 precursors. The packed-bed columns 22 and central tube 24 are preferably made of stainless steel, have a diameter of about 25 mm, and a length of about 75 cm. Other materials and component dimensions can be used, if desired. Various packing materials can be used in the packed-bed columns 22, including beads, fibers, rings, saddles, and the like, which can be composed of glass, metal, ceramics, or other materials that are substantially inert to the reactant. A preferred packing material comprises ceramic elements having a saddle shape and a length of about 6 mm. This packing provides a large surface area to packed volume ratio. A bottom surface 56 of the packed-bed column is equipped with a grate, screen, or the like to retain the packing material within the column. As indicated in Figure 1, the liquid TiO 2 precursor and the carrier gas are metered to a series of spray nozzles 32 located at the top of the vaporizer 13 using liquid metering pump 12 and the gas mass flow controller 16. Similarly shown in Figure 4, the liquid precursors and the carrier gas are metered to the spray nozzles 32 using liquid metering pumps 12 and 66 and the gas mass flow controller 16. Distribution of any liquid precursor or combination thereof across top surfaces 54 of the packed-bed columns 22 is important since the liquid precursor will tend to flow in a narrow stream through the column if supplied to only a portion of the top surface 54. Such a narrow stream will reduce the effective surface area available for evaporation of the liquid precursor and thus reduce the efficiency of the vaporizer 13. Other means of supplying the liquid precursors and the carrier gas to the top of the packed-bed columns 22, such as a manifold of tubes for each packed-bed column 22 for distributing one or more liquid precursors across a top surface 54 of the column and a single tube for each column for the carrier gas, can be used if desired, although the use of spray nozzles 32 is preferred. Gas atomization nozzles 32 which disperse the liquid precursor into a cone 58 of droplets carried by the carrier gas are particularly preferred. Such nozzles 32 ensure uniform contact between the liquid precursor and the carrier gas and uniform wetting of the bed packing. Nozzles 32 of this type normally have separate inlets (not shown) for the liquid precursor and the carrier gas. To ensure uniform operation of the plurality of packed-bed columns 22 of Figs. 2 and 3, the liquid precursor inlets of the nozzles 32 are connected to a gas pressure equalizing manifold (not shown). These manifolds, in turn, are connected to liquid metering pump 12 and gas mass flow controller 16, respectively. In the embodiment where both the SiO 2 and TiO precursors are contemporaneously vaporized by the vaporizer 13 , the manifolds are connected to liquid metering pumps 12 and 66 and gas mass flow controller 16, respectively. Nozzle 32 selection is based upon three primary factors, including the liquid precursor throughput, the spray pattern, and the carrier gas flow rate. The nozzle 32 should allow enough liquid precursor or precursors flow to supply the needs of the system. Also, the nozzle 32 should spray uniformly to cover the packed-bed. Finally, the flow rate of the carrier gas needed to atomize the liquid precursor should be sufficiently low to prevent diluting of the liquid precursor flow below a desired operating level. The carrier gas flow rate should be maintained at a sufficient rate to produce the desired conically-shaped spray 58 of droplets. Different nozzles 32 are available for various liquid precursor flow rates. In practice, gas atomization nozzles 32 having an orifice size of about 3 mm, a spray cone 58 angle of about 60°, and a pressure drop across the nozzle 32 of about 10 psig work successfully with the embodiments of the present invention and are preferred. Arrows 38, 40, and 42 in Figure 3 illustrate the flow pattern through the vaporizer 13. In the following discussion, the reference to liquid precursor includes the TiO 2 precursor and the mixture of TiO 2 and SiO precursors. Arrows 38 represent the concurrent downward flow of the liquid precursor and the carrier gas through the packed-bed columns 22, with the liquid precursor or precursors vaporizing prior to reaching the bottom surfaces 56 of the columns. Anow 40 represents the upward flow of the vapor/gas mixture through the central tube 24, and arrows 42 represent the change in the direction of flow of the vapor/gas mixture from downward to upward in a phase separator 44. This change in direction effectively removes undesirable gels and gums from the vapor/gas stream; Other approaches for cleaning the vapor/gas stream include turning the stream through an angle of less than 180°, e.g., by 90°, and placing an impingement plate and/or filtration material such as glass wool, in the region of the stream which carries the impurities after the turn has been negotiated, i.e., in the outer portions of the stream as seen from the center of the turn. As noted above, the stream exiting the packed columns can be filtered without a change in direction, if desired. Port 48 is provided for removing the higher molecular weight species 46 from the separator 44. Arrows 60 and 62 in Figure 1 illustrate the flow of hot oil (e.g., silicone oil) from a hot oil transfer system 18 to the vaporizer 13. Hot oil 28 serves to heat the packed-bed columns 22 by flowing around the outside walls 50 of the columns. It also serves to heat the vapor/gas mixture flowing in the central tube 24 by heating the outside wall 52 of that tube. The vaporizer 13 is heated to a temperature between about 110°C and about 175°C, preferably between about 110°C and 140°C. Heated fluids other than hot oil, e.g., steam, can be used to heat the packed-bed columns 22 and the central tube 24, if desired. Also, other heating means, such as electrical heat tape, can be used, although the heated fluid approach is preferred since it minimizes the chances for hot spots in the vaporizer 13 which may lead to polymerization or thermal degradation of the liquid precursors. Hot oil 28 enters the vaporizer 13 through inlet port 34 and leaves through outlet port 36. While in the vaporizer 13, the hot oil 28 flows through passages 30 defined by the vaporizer's outer shell 26 and the outside walls 50 and 52 of the packed- bed columns 22 and the central tube 24. Baffles (not shown) can be incorporated in passages 30 to ensure uniform distribution of hot oil 28 around the circumferences of the outside walls 50 and 52. An outer shell 26 extends above nozzles 32 and above the pressure equalizing manifolds discussed above to which those nozzles 32 are connected so that the inside of the vaporizer 13 is a closed space. Passing through the outer shell 26 are oil ports 34 and 36, the outlet of central tube 24, and inlet ports (not shown) which provide the liquid TiO 2 precursor and the carrier gas to the pressure equalizing manifolds. The carrier gas and the liquid precursor are heated simultaneously as they pass downward through the packed-bed column 22. As their temperature increases, more liquid precursor vaporizes into the carrier gas. The process is controlled by the flow of heat and diffusion of the vapor through the carrier gas at the interface between the carrier gas and the liquid TiO 2 precursor. The process continues as the mixture flows downward through the columns 22 until all of the liquid precursor is converted into vapor, except for the undesirable higher molecular weigh species of the liquid precursor. Thereafter, the vapor/gas mixture continues to increase in temperature as it flows downward towards the column's bottom surface 56. The packed-bed column 22 equalizes the residence time of the entire stream by maintaining a uniform velocity, profile throughout the diameter of the packed-bed 22. A uniform residence time minimizes thermal degradation of the precursors. The residence time of the TiO 2 precursor in the packed-bed column is between about 0.5 sec. to about 10.0 sec. The temperature of the hot oil 28 is set at a value that will ensure that the liquid precursor and the carrier gas are heated to a temperature sufficient to vaporize the liquid precursor before reaching the bottom surface 56 of the column. However, the selected temperature should be below the thermal degradation temperature of the TiO 2 precursor. For example, it is prefened that the temperature of the vaporizer 13 should not exceed 140°C when TPT is the TiO 2 precursor. The amount of heating needed for various precursors, mole ratios, operating pressure values, and other vaporizer dimensions can be readily determined by persons skilled in the art from the disclosure herein. Temperature within the vaporizer varies as atomized liquid precursor and carrier gas absorb energy for both evaporation and temperature rise. Accordingly, temperature distribution within the packed-beds 22 of the vaporizer 13 is dependent on the temperature of the incoming liquid precursor and carrier gas. After the liquid precursor and the carrier gas enter the vaporizer 13, the rate of temperature increase (i.e., the temperature gradient) along the length of the heated packed-beds 22 is higher for components entering at ambient temperature versus preheated components. For example, preheating liquid precursors and the carrier gas to 100°C significantly reduces the temperature gradient within the vaporizer 13 as compared to such components entering at 25°C. Preheating is not necessary, but reducing the temperature gradient also reduces the length of the vaporizer 13 needed to completely vaporize a given quantity of liquid precursor. Reducing the vaporizer 13 length necessarily decreases the residence time of the liquid precursor within the vaporizer 13. This, in turn, reduces thermal degradation of the TiO precursor by reducing the amount of time the TiO 2 precursor is exposed to high temperatures (e.g., 140°C). Importantly, it has been discovered that interaction of TPT and OMCTS at high temperatures (e.g., 140°C) for short durations of 2 to 5 seconds results in no observable reactions between or thermal degradation of these precursors. A typical residence time of the precursors within the vaporizer 13 is about 1 second. After leaving phase separator 44, the clean vapor/gas mixture travels upward through central tube 24. From the vaporizer 13, the vapor/gas mixture is directed to a vapor utilization site 20 where it is introduced to a conventionally vaporized SiO 2 precursor to form the titania-doped fused silica preform. The mass flow of the TiO 2 and SiO 2 precursors into the vaporizer 13 are controlled by respective pumps 13 and 66 for the TiO and SiO 2 precursors, or a rotameter (not shown) for the higher flows of TiO 2 and SiO 2 precursor mixtures. Alternatively, the flow rates can be measured by timing the weight loss of the respective precursors from their containers per unit time. Referring now to Figs. 5-8, theoretical predictions on the required operating temperatures were made for TPT using Equation 1 combined with the vapor pressure versus temperature relation for TPT as expressed in Equation 2, below. Flow Rate of TiO 2 Precursor = CG (VP/(P atm - VP)) (1) wherein the TiO 2 precursor flow rate is grams/minute, CG is the slpm flow rate of the carrier gas (nitrogen), VP is TiO 2 precursor vapor pressure in units of Torr, and Patm s the atmospheric pressure (Torr). Log (P) = 8.972-2997.96/T (2) wherein P is pressure in units of torr, and T is the absolute temperature in degrees Kelvin. Equation 2 was experimentally verified by measuring the vapor pressure of TPT at various temperatures between 120°C and 140°C in accordance with ASTM Standard E 1719-95, entitled "Standard Test Method for Vapor Pressure of Liquids by Ebulliometry." The theoretical predictions at 41 g/minute are shown in Figure 5, which separates operating conditions into two regimes. The "Good" regime includes the operating conditions, i.e., temperature and gas flow rates, where complete vaporization should theoretically take place. The "Bad" regime refers to operating conditions where incomplete vaporization occurs. Figure 6 is the same plot for TPT flow rates between 10 and 80 g/min. Three conclusions can be drawn from Figs. 5 and 6. First, no carrier gas is required for any TiO 2 precursor (i.e., TPT) flow rate at a temperature above the precursor boiling point (232°C), because the pressure is sufficient to carry any level of TiO 2 precursor downstream. Second, increasing the flow rate of the carrier gas decreases the operating temperature of the vaporizer 13. Finally, either the vaporizer 13 operating temperature or the carrier gas flow rate must increase in order to vaporize increasing amounts of the TiO 2 precursor. Experiments were carried out within the vaporizer at 170°C for nitrogen flow rates between 10 and 15 slpm and for mass flow rates of TPT between 30 and 50 g/min. The results are summarized in Table 1 and show that complete vaporization took place at temperatures 5 ± 2°C above the theoretical values. This indicates excellent heat transfer within the vaporizer 13 and predictable vaporization behavior. A liquid mixture of TPT and OMCTS was vaporized within a vaporizer 13 to high levels. Theoretical predictions for mixtures of TPT and OMCTS flows are shown in Figs. 7 and 8. As also indicated in Table 1, the vaporizer 13 behaved in a manner closely predicted by theory. The mixture was pumped at 105 g/minute through the vaporizer at 161.5°C with a nitrogen carrier gas flow rate of 16.5 slpm. Theory predicts a required temperature of 154.5°C for these conditions, a difference of only 7°C, which indicates good heat transfer even at these relatively high flow rates. The capability to vaporize a mixture of TiO 2 and SiO 2 precursors through a single vaporizer 13 as opposed to two separate flash tanks simplifies the process of making preforms. Surprisingly, a greatly reduced carrier gas flow is possible. Typically, prior art systems utilizing flash tanks require about .8-1.2 slpm for about .8 to 1.2 g/minute of liquid precursors. The method of the present invention requires only about 0.2 to 0.3 slpm of nitrogen for the same flow rate of precursors. The vaporizer 13 therefore allows much more independent control of the carrier gas, TiO 2 precursor, and SiO 2 precursor flow rates than flash tank systems. The method of the present invention is more robust than the prior art flash tank processes because nitrogen flow rates are surprisingly independent of liquid precursor flow rates within the vaporizer 13. Also unexpected, vaporization occurs at values closely predicted by theory, indicating minimal interaction between TPT and OMCTS. Interactions were expected to alter the dew point of these precursors and/or cause gel formation, which was not observed. EXAMPLES Example 1 A series of experiments were conducted involving passing either liquid TPT or a liquid mixture of TPT and OMCTS in a 1.0:5.0 weight ratio through a vaporizer. The results of these experiments are set forth in Table I. The vaporizer temperature is taken at the vaporizer outlet with the term "preheat" indicating that the carrier gas (i.e., nitrogen) is preheated to about 120°C and the liquid is preheated to about 100°C. The term "Good" means complete vaporization, and the term "Bad" refers to incomplete vaporization. The TPT utilized had some small level of degradation, as noted by its brownish-orange color. This suggests that higher molecular weight (MW) components are soluble in TPT. The higher MW components did not vaporize and acted as a dye. As TPT passed through the vaporizer, the brownish-orange material collected in a separator. When vaporization was successful, the output material appeared clear. However, when complete vaporization was unsuccessful, a brownish-orange liquid and TPT would build up in the separator and then exit the vaporizer, as indicated by a brownish-orange color in the vapor stream. This same method was used for a mixture of OMCTS and TPT, with a less intense color due to the dilution from OMCTS. TABLE 1 Although the invention has been described in detail for the purpose of illustration, it is understood that such detail is solely for that purpose, and variations can be made therein by those skilled in the art without departing form the spirit and scope of the invention which is defined by the following claims.

Claims: WHAT IS CLAIMED: 1. A method of vaporizing a liquid TiO 2 precursor comprising: providing a TiO 2 precursor in liquid form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to TiO ; providing a carrier gas; providing a packed-bed column; and passing the liquid TiO 2 precursor and the carrier gas through the packed-bed column under conditions effective to vaporize the liquid TiO 2 precursor substantially without thermal degradation thereof and produce a gas stream containing a mixture of the vaporized TiO 2 precursor and the carrier gas. 2. A method according to claim 1, wherein the TiO 2 precursor is a titanium alkoxide, a titanium amide, or a combination thereof. 3. A method according to claim 1, wherein the carrier gas is selected from the group consisting of nitrogen, helium, neon, argon, krypton, xenon, and combinations thereof. 4. A method according to claim 1, further comprising: heating the packed-bed column. 5. A method according to claim 1, further comprising: heating the TiO 2 precursor and the carrier gas prior to said passing the liquid TiO 2 precursor and the carrier gas through the packed-bed column. 6. A method of providing reactant vapors comprising: providing a first gas stream containing a SiO precursor in vapor form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to SiO ; providing a TiO 2 precursor in liquid form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to TiO 2 ; providing a carrier gas; providing a packed-bed column; passing the liquid TiO 2 precursor and the carrier gas through the packed-bed column under conditions effective to vaporize the liquid TiO precursor substantially without thermal degradation thereof and produce a second gas stream containing a mixture of the vaporized TiO 2 precursor and the carrier gas; and co-mingling the second gas stream with the first gas to form a reactant vapor/gas mixture. 7. A method according to claim 6, wherein the TiO 2 precursor is a titanium alkoxide, a titanium amide, or a combination thereof. 8. A method according to claim 6, wherein the TiO 2 precursor is selected from the group consisting of titanium-2-ethylhexyloxide, titanium cyclopentyloxide, titanium isopropoxide, titanium methoxide, a titanium amide having the structure Ti(NR 2 ) 4 , wherein R 2 is a methyl or ethyl group, or a combination thereof. 9. A method according to claim 6, wherein the SiO 2 precursor is a polymethylsiloxane, a polymethylsilane, a polyethylsilane, a polyethylsihcate, an aminosilane, a linear silazane, a cyclosilazane, or a combination thereof. 10. A method according to claim 6, wherein the SiO 2 precursor is selected from the group consisting of octamethylcyclotetrasiloxane, hexamethyldisiloxane, octamethyltrisiloxane, hexamethylcyclotrisiloxane, decamethylcyclopentasiloxane, methyltrimethoxysilane, tetraethylorthosilicate, dimethyldimethoxysilane, trimethylmethoxysilane, teframethoxysilane, methyltriethoxysilane, tefraethoxysilane, tris ketenimine, nonamethyltrisilazane, octamethylcyclotetrasilazane, and combinations thereof. 11. A method according to claim 6, wherein the carrier gas is selected from the group consisting of nitrogen, helium, neon, argon, krypton, xenon, and combinations thereof. 12. A method according to claim 6, further comprising: heating the packed-bed column. 13. A method according to claim 6, further comprising: heating the TiO 2 precursor and the carrier gas prior to said passing the liquid TiO 2 precursor and the carrier gas through the packed-bed column. 14. A method according to claim 12, wherein the packed-bed column is heated to a temperature between about 110°C and about 175°C. 15. A method according to claim 6, wherein the TiO 2 precursor has a residence time in the packed-bed column of between about 0.5 to about 10.0 seconds. 16. A method according to claim 6, further comprising: separating higher molecular weight impurities from the second gas stream. 17. A method according to claim 16, wherein said separating comprises changing a direction of flow of the second gas stream. 18. A method according to claim 6, further comprising: passing the reactant vapor/gas mixture into a flame of a combustion burner to form amorphous particles of fused SiO doped with TiO 2 ; depositing the amorphous particles onto a support to form the preform; and either essentially simultaneously with deposition or subsequently thereto consolidating the preform comprising amorphous particles into a non- porous body. 19. A method according to claim 18, wherein the non-porous body is a non- porous, transparent glass body, said method further comprising: drawing waveguide fiber from the non-porous body. 20. A method of providing reactant vapors comprising: providing a SiO 2 precursor in liquid form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to SiO 2 ; providing a TiO 2 precursor in liquid form capable of being converted through thermal decomposition with oxidation or flame hydrolysis to TiO 2 ; providing a carrier gas; providing a packed-bed column; and passing the liquid SiO 2 precursor, the liquid TiO 2 precursor, and the carrier gas through the packed-bed column under conditions effective to vaporize the liquid SiO 2 and TiO 2 precursors substantially without thermal degradation to either precursor and produce a reactant vapor/gas containing a mixture of the vaporized SiO precursor, the vaporized TiO 2 precursor, and the carrier gas. 21. A method according to claim 20, wherein the TiO 2 precursor is a titanium alkoxide, a titanium amide, or a combination thereof. 22. A method according to claim 20, wherein the TiO 2 precursor is selected from the group consisting of titanium-2-ethylhexyloxide, titanium cyclopentyloxide, titanium isopropoxide, titanium methoxide, a titanium amide having the structure Ti(NR 2 ) 4 , wherein R 2 is a methyl or ethyl group, and combinations thereof. 23. A method according to claim 20, wherein the SiO 2 precursor is a polymethylsiloxane, a polymethylsilane, a polyethylsilane, a polyethylsihcate, an aminosilane, a linear silazane, a cyclosilazane, or a combination thereof. 24. A method according to claim 20, wherein the SiO 2 precursor is selected from the group consisting of octamethylcyclotetrasiloxane, hexamethyldisiloxane, octamethyltrisiloxane, hexamethylcyclotrisiloxane, decamethylcyclopentasiloxane, methyltrimethoxysilane, tetraethylorthosilicate, dimethyldimethoxysilane, trimethylmethoxysilane, teframethoxysilane, methyltriethoxysilane, tefraethoxysilane, tris ketenimine, nonamethyltrisilazane, octamethylcyclotetrasilazane, and combinations thereof. 25. A method according to claim 20, wherein the carrier gas is selected from the group consisting of nitrogen, helium, neon, argon, krypton, xenon, and combinations thereof. 26. A method according to claim 20, further comprising: heating the packed-bed column. 27. A method according to claim 26, wherein the packed-bed column is heated to a temperature between about 110°C and about 175°C. 28. A method according to claim 20, further comprising: heating at least one of the liquid SiO 2 precursor, the liquid TiO 2 precursor, and the carrier gas prior to said passing the liquid SiO 2 and TiO 2 precursors and the carrier gas through the packed-bed column. 29. A method according to claim 20, wherein the TiO 2 precursor has a residence time in the packed-bed column of between about 0.5 to about 10.0 seconds. 30. A method according to claim 20, further comprising: separating higher molecular weight impurities from the reactant vapor/gas mixture. 31. A method according to claim 30, wherein said separating comprises changing a direction of flow of the reactant vapor/gas mixture. 32. A method according to claim 20, further comprising: passing the reactant vapor/gas mixture into a flame of a combustion burner to form amorphous particles of fused SiO 2 doped with TiO 2 ; depositing the amorphous particles onto a support to form the preform; and either essentially simultaneously with deposition or subsequently thereto consolidating the deposit of amorphous particles into a non-porous body. 33. A method according to claim 32, wherein the non-porous body is a non- porous, transparent glass body, said method further comprising: drawing waveguide fiber from the body.

Inventors: CAIN MICHAE B, HRDINA KENNETH E, ROSPLOCK CYNTHIA K, CAIN, MICHAE, B., HRDINA, KENNETH, E., ROSPLOCK, CYNTHIA, K.
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2001
Main classification: C03B37
IPC codes: C03B19, C03B37, C23C16