Abstract:
A crucible for melting a silica for fusion of said silica into a desired shape. The crucible having a main body with inner and outer surfaces comprised of a refractory material. In addition, the inner surface adjacent at least a melting zone of the furnace includes a substantially gas-tight barrier layer comprised of a material selected from rhenium, osmium, iridium, and mixtures thereof.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for the production of tubing, rods and the like from crystalline quartz or other glass like materials. Particularly, this invention relates to an apparatus for use in the production of elongated quartz members from a silica melt. The present invention is particularly directed to the manufacture of fused silica tubes for use in the manufacture of optical fibers. 
     Various types of elongated members have been formed continuously by melting of quartz crystal or sand in an electrically heated furnace whereby the desired shape is drawn from the furnace through a suitable orifice or die in the bottom of the furnace as the raw material is melted. One apparatus for continuous production of fused quartz tubing, for example, is a tungsten-lined molybdenum crucible supported vertically and having a suitable orifice or die in the bottom to draw cane, rods, or tubing. The crucible is surrounded by an arrangement of tungsten heating elements or rods which heat the crucible. The crucible, together with its heating unit, is encased in a refractory chamber supported by a water-cooled metal jacket. The crucible is heated in a reducing atmosphere of nitrogen and hydrogen. 
     An alternative apparatus provides fused quartz tubing by feeding natural quartz crystal into a refractory metal crucible heated by electrical resistance under a particular gas atmosphere to reduce the bubble content. The bubbles formed by gas entrapment between crystals and the molten viscous mass of fused quartz do not readily escape from the molten glass and, hence, remain as bubbles or lines in the product drawn from the fused quartz melt. By substituting a melting atmosphere gas which readily diffuses through the molten material (such as pure helium, pure hydrogen or mixtures of these gases) the gas pressure in the bubbles is reduced and thereby the bubble size is reduced. This process uses a mixture of 80% helium and 20% hydrogen by volume. 
     In a further alternative method, a product is obtained by continuously feeding a raw material of essentially pure silicon dioxide in particulate form into the top section of an induction-heated crucible, fusing the raw material continuously in an upper-induction heat zone of the crucible in an atmosphere of hydrogen and helium while maintaining a fusion temperature not below approximately 2050° C. The fused material in the lower zone of the crucible is heated by separate induction heating means to produce independent regulation of the temperature in the fused material. The fused material is continuously drawn from the lower zone of the crucible through forming means in the presence of an atmosphere of hydrogen containing a non-oxidizing carrier gas. 
     Unfortunately, most of the refractory metal and non-metal materials used in the crucibles of the above-described apparatus are undesirable impurities if present in the drawn silica article. Such refractory material contamination causes discoloration and occlusions in the silica glass. Furthermore, the presence of refractory material particles can degrade the strength of the resultant silica article. Moreover, the particles become a flaw in the drawn article that can cause the strand to break. 
     Accordingly, there is a need in the art to reduce contamination of fused glass occurring from the refractory materials used in constructing the furnace. This need has increased recently as semiconductor and fiber optics manufacturing processes, a primary use for the glass products obtained from the subject process, have required higher levels of purity and performance. 
     Unfortunately, because the furnace is typically constructed of refractory materials, the manufacturing plant is usually contaminated therewith. Accordingly, even a furnace having melting and drawing zones insulated from refractory materials cannot fully prevent contamination. It would therefore be desirable to have available an apparatus which facilitates removing and/or reducing the effect of refractory materials contamination on the resultant silica article. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a furnace for melting of the silica and subsequent drawing into a desired shape is comprised of a body having an outer surface constructed of a refractory metal and including a substantially gas impermeable inner lining extending from at least slightly below a melt line throughout the melt zone of the furnace. The lining is comprised of a non-reactive barrier material. The lining is preferably formed of rhenium, osmium, iridium, platinum or mixtures thereof. Preferably, the furnace will include an inlet tube for introduction of a gas to the melt zone, the outer circumference of the inlet tube forming a gas-tight seal with the lining. In addition, a silica raw material feeder tube, or a protection sheath circumferentially disposed around the tube is provided, the outer circumference of the feeder tube forming a gas tight seal with the lining. 
     The present crucible construction provides a number of advantages. Particularly, furnaces constructed with a substantially gas-tight rhenium, iridium, platinum and/or osmium lined melting zone can produce products with much lower levels of refractory metal in the solution. For example, the metal dissolved in the silica can be reduced to below about 1 ppb and preferably below the current level of detection via NAA. This reduced amount of refractory metal contamination in the silica melt improves the chemical composition of the silica glass allowing for a decrease in discoloration and surface haze. Furthermore, utilization of a furnace equipped with a crucible including the non-reactive lining in the melt zone allows operation at optimum temperature ranges and in the presence of a desired atmosphere to reduce or eliminate contaminants. 
     It should be noted that the terms “quartz” and “silica” are used interchangeably throughout this application, both being directed generally to the compound SiO 2 . Nonetheless, the present invention encompasses the use of any raw material introduced to the melting furnace, including but not limited to natural silica/quartz and synthetic silica. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure, operation and advantages of the present preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a longitudinal sectional view of a furnace of the present invention; and 
     FIG. 2 is a schematic view of a furnace demonstrating the present inventive construction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In one of its preferred embodiments, the fused quartz product of the present invention can be formed in a furnace configuration having the features shown in FIG,  1 . Moreover, FIG. 1 demonstrates the suitability of the present inventive construction technique in the rebuild of an existing furnace design. More particularly, the furnace has a general cylindrical shape. Preferably, an elongated cylindrical melting crucible  10  constructed of a refractory metal layer  11 , such as tungsten or molybdenum as well as combinations thereof, is used. The melting crucible  10  further includes a continuous lining of rhenium  13  over the refractory metal layer  11  throughout an upper melting zone  14  and into the melt  18 . 
     A purified sand raw material is fed through a top opening tube  12  into the upper melting zone  14  of the crucible  10 . The top opening tube  12  is provided with movable closure means  16 , such as a trapdoor which can be kept closed except for observing the level of the melt  18  and during feeding of the raw material  19  into the crucible  10 . Automatic feeder means  20  is provided at the top opening of the crucible  10  to maintain a predetermined level of the raw material in the crucible  10 . The feeder includes a discharge tube  22  having its outlet opening located in the crucible  10  so as to provide the raw material in the upper melt zone  14  where melting takes place, a purge gas inlet tube  24  and reservoir means  26  which contains supply of the raw material being fed automatically to the discharge tube. To provide a suitably controlled melting zone gas atmosphere, top tube opening  12  is preferably welded around its periphery  27  to form a gas-tight seal with rhenium lining  13 . 
     Simple gravity flow of the raw material to the melting zone of the crucible member takes place as the melt level in the crucible drops with fusion of the sand particles. The purge gas being supplied to the feeder helps eliminate gases contained in the raw material which could form bubbles in the fused quartz melt. The composition of the purge gas is selected to reduce bubbles and ridges in the final product and which may consist of a gas mixture of hydrogen and helium in the volume ratios 40-100% hydrogen and 60-0% helium. In addition, the substantially gas-tight environment created by non-reactive rhenium facilitates the introduction of reactive gases if desired. 
     The lower portion  28  (a drawing zone) of the crucible  10  includes an annular ring  30  having central opening  32  through which the elongated fused quartz member is continuously formed by drawing the viscous material through the opening. A core  34  is centrally disposed in the opening  32  and extends below the annular ring as the means of forming tubing from the viscous material being drawn from the melt. As known by the skilled artisan, the position of the core can be shifted as necessary to produce the desired size of extrudate. Support element  35  is affixed to the wall of the crucible and provides rigid support of the core which helps to maintain a constant size opening from which the product is being drawn. The core is fabricated with a hollow interior  36  which is connected to inlet pipe  38  so that a supply of non-oxidizing gas having a different composition than supplied to the melting zone of the crucible can be furnished as a forming atmosphere while the tubing  40  is being drawn. In accordance with the invention, inlet pipe  38  is welded (or otherwise sealed) to rhenium lining  13  at  41  to maintain a sealed upper melting zone  14 . 
     A second inlet pipe  42  supplies a forming atmosphere which can be a mixture containing hydrogen in a non-oxidizing carrier gas such as nitrogen in volume ratios 1-20% hydrogen and 99-80% carrier gas as a protective atmosphere which surrounds the exterior wall of the crucible. This supply of forming gas is provided to annular space  44  which provides a housing means for the crucible and includes a central bottom opening  46  providing exhaust from said cavity for the forming gas in a manner which envelops the exterior surface of the elongated fused quartz member being drawn from the furnace. The exterior wall of the annular space comprises a refractory cylinder  48  which in combination with exterior housing  50  of the furnace construction serves as the container means for the induction heating coils of the apparatus. More particularly, a concentric passageway  52  is defined between the exterior wall of the refractory cylinder  48  and the interior wall of housing  50  in which is disposed two helical-shaped induction heating coils  54  and  56  supplying separate heating sources for the upper and lower zones of the crucible, respectively. Of course, additional coils may be employed as governed by the size of the furnace, for example, it may be beneficial to include additional coil(s) in fining zone  57 . 
     The heating sources and the power supplies thereto can be of conventional construction which include electrical conductors that are hollow for water cooling and electrically connected to separate A.C. power supplies for the independent heating utilized in the practice of the present invention. The remainder of the passageway occupied by the coils is preferably packed with a stable refractory insulation such as zirconia in order to conserve heat in the furnace. 
     A third supply pipe  58  is located in the top section of exterior housing  50  and supplies a purge gas mixture (or a reactive gas if desired) to the melting zone  14  of the crucible. Of course, a weld is created at  59  to form a gas-tight seal between pipe  58  and rhenium lining  14 . The above-described furnace is operated in connection with conventional tube or rod drawing machinery which has been omitted from the drawing as forming no part of the present invention. 
     Of course, the present inventive use of a non-reactive crucible lining is not limited to the furnace or crucible shown in FIG.  1 . In fact, the use of the non-reactive lining is suitable for use in any furnace/crucible embodiment known to the skilled artisan (e.g. FIG.  2 ). 
     In accordance carrying out the process of the present invention in the above-described apparatus, a natural silica sand having a nominal particle size of −50 mesh U.S. screen size which has been purified by chemical treatment to the nominal impurity content below is supplied to the top opening of the crucible member in the apparatus. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 RAW MATERIAL 
               
             
          
           
               
                   
                 Impurity 
                 Natural (p.p.m.) 
                 Synthetic (p.p.m.) 
               
               
                   
                   
               
             
          
           
               
                   
                 Fe 2 O 3   
                 1 
                 0.07 
               
               
                   
                 TiO 2   
                 2 
                 &lt;.02 
               
               
                   
                 Al 2 O 3   
                 20 
                 100 
               
               
                   
                 CaO 
                 0.4 
                 &lt;.01 
               
               
                   
                 MgO 
                 0.1 
                 &lt;.05 
               
               
                   
                 K 2 O 
                 0.6 
                 0.1 
               
               
                   
                 Na 2 O 
                 0.7 
                 0.1 
               
               
                   
                 Li 2 O 
                 0.6 
                 &lt;.05 
               
               
                   
                 B 
                 &lt;0.2 
                 — 
               
               
                   
                 ZrO 2   
                 &lt;1.0 
                 &lt;.02 
               
               
                   
                   
               
             
          
         
       
     
     The above raw material is provided to the crucible member which has been heated in excess of 2050° C., preferably in excess of about 2350° C. After a predetermined melt level of fused quartz has been established in the crucible and the molten material caused to flow by gravity through central bottom opening  32  in the crucible member, tubing or rod is then drawn continuously by the drawing machine (not shown) in the presence of a forming gas atmosphere. In any continuous drawing of tubing/rod in the foregoing described manner, the electrical power being supplied to the lower heating coil  56  is typically maintained at a lower level than the electrical power being supplied to the upper heating coil  54  in order to lower the temperature of the material as it is being drawn to below a temperature of 2050° C. 
     As stated above, the internal surface of the melting zone  14  furnace crucible  10  includes a rhenium, osmium or iridium sheet or coating  13 . The coating  13  may be applied to the refractory metal layer  11  by chemical vapor deposition, electrolysis, plasma spray or any other technique known to the skilled artisan (hereinafter referred to as “chemical bonding”). The non-reactive layer  13  may also be physically attached to the refractory metal layer  11  by attaching a sheet directly to the wall of the crucible with rivets, bolts, screws, etc., preferably constructed from the same or similar material as the non-reactive lining itself Alternatively, a properly shaped rhenium sleeve can be inserted into the crucible. In fact, a combination of coating or lining methods may be used depending on the geometric complexity of the segments comprising the crucible assembly. In accordance with the present invention, it should be noted that these methods of attachment may require further sealing of the fastening members which is necessary to prevent vapor phase transport from outside the lining (i.e. a nongas-tight lining) 
     Referring now to FIG. 2, the application of the present inventive coating applied to the entire crucible is demonstrated. Moreover, a coating of rhenium  113  is applied on the inner wall  117  of the crucible  119 . In this manner, the entire system is protected from tungsten/molybdenum contamination by the barrier layer. This protection is supplemented by feeding a dry hydrogen gas through tube  121  to the space  123  between cup  113  and walls  117 . A tube  125  is provided to feed wet hydrogen into the melt zone  115 , and a tube  126  is provided to exhaust wet hydrogen gas. Of course, proper seals are provided between tube  125  and sand feed tube  127  to create a gas barrier within cup  113 . As is conventional in the art, a layer of insulation  129  is disposed between tungsten walls  117  and the induction heating coils  131 . As shown in this embodiment, feed sand  133  is beneficially in a wet hydrogen environment  119  as it fuses into a molten state  135  for eventual product forming. 
     While the invention has been described by reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements without departing from the scope of the invention. In addition, any modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but the invention will include all embodiments falling within the scope of appended claims.