Patent Document

FIELD OF THE INVENTION 
     The present invention relates to a method for manufacturing glass, with which molten glass is enclosed at least partially by precious metal walls and/or refractory metal walls, and with which the oxygen partial pressure of the molten glass is influenced by a treatment means to prevent disturbances, a device for this purpose, and the use of the glass obtained as a result. 
     BACKGROUND OF THE INVENTION 
     In this context, the term “precious metal” includes platinum, gold, rhenium, all other metals in the platinum group, and the alloys of the aforementioned metals, and the stated metals and alloys in oxide dispersion-strengthened form. Molybdenum is used in particular as the refractory metal, as are tungsten, niobium and tantalum. 
     It is known that, at high temperatures such as those that occur in a glass melt, for example, a small portion of the water that is present breaks down into hydrogen and oxygen. When the glass melt comes in contact with components made of precious metals, in particular platinum and its alloys, the hydrogen that is formed can pass through the platinum part. As a result, the oxygen in the melt is enriched and oxygen bubbles form that—provided no further action is carried out—remain in the finished glass product and lower its quality in a critical manner. Particularly problematic in this regard is the fact that precious metal components are used primarily directly after the refining area, making it very difficult to remove the oxygen bubbles that form on the precious metal components from the glass melt. 
     Publication EP 1 101 740 A1 therefore proposes that bubbles be prevented from forming by applying a reverse voltage via electrochemical means. A variant is proposed, among others, with which the one molybdenum electrode is located in the glass melt upstream of the precious metal components, the electrode being connected in a conductive manner with the precious metal components. An adequate reverse voltage is generated as a result without an external power supply. A method is described in publication U.S. Pat. No. 5,785,726, with which oxygen bubbles are prevented from forming in the glass melt by preventing hydrogen from the glass melt from escaping through the precious metal walls by establishing an atmosphere on the side of the precious metal walls facing away from the glass melt that contains a high percentage of water vapour. The purpose of this is to prevent hydrogen gas from the glass melt from diffusing to the outside through the precious metal walls made permeable by the high temperatures. With the two known methods, oxygen bubbles can indeed be largely prevented from forming in the glass melt on precious metal components, but, as a result, the oxygen partial pressure is held at the level that existed before the melt flowed into the platinum system. 
     It has been shown that bubbles still form with the known procedures described above, however, and defective glasses are produced as a result. 
     The problem addressed by the present invention is to provide a method and a device for producing glass, with which the formation of bubbles on precious metal components—and other disturbances—can be reliably prevented. 
     SUMMARY OF THE INVENTION 
     The problem is solved, with the method of the type described initially, by determining the particular oxygen partial pressure on a precious metal part using a voltages between the precious metal part and a reference electrode located in the melt and calaulating it from the voltage. The precious metal part, particular the walls of the tank and/or the pipe, is preferably subdivided into segments that are electrically insulated from each other. Insulation of this type is composed, for example, of parts of fire-resistant material such as mullite, sillimanite, quartzal and/or corundum. This makes it possible to regulate the oxygen partial pressure in the safe range in an individualized manner for each individual insulated precious metal part via separate regulation of the treatment means. 
     According to the present invention, it was discovered that, with the known method described above for suppressing the formation of oxygen bubbles, the oxygen partial pressure in the melt in the region of contact with the precious metal components is reduced to the extent that at least partially reducing conditions exist. It has been shown according to the present invention that, surprisingly, at an oxygen partial pressure that is too low, N 2 , CO 2  and SO 2  bubbles start to form and/or alloy damage occurs. The present invention is therefore based on the finding that extreme reducing conditions must be prevented to ensure good glass quality. According to the present invention, these extreme reducing conditions are prevented by adjusting and/or regulating the oxygen partial pressure that exists at the interface of the glass melt and the precious metal part in particular within a safe range. This can take place at the interface itself, for example, or by using an electrode located in the melt. The electrode is preferably located in the region close to the interface. The oxygen partial pressure can be raised or lowered by applying a reverse voltage with the appropriate polarity. In addition, the oxygen content can be lowered by using a reducing hydrogen or water vapour atmosphere, and it can be increased by rinsing with oxygen in pure or diluted form. 
     This safe range can be determined for the particular glass being used in a first method step, for example. In this step, the upper limiting value of the safe range is limited by the formation and/or prevention of O 2  bubbles, and the lower limiting value is limited by the formation and/or prevention of N 2 , CO 2  and/or SO 2  bubbles and/or alloy damage. It was also discovered that various glasses have different safe ranges. The determination of the particular safe ranges takes place in laboratory tests carried out in advance, for example. Surprisingly, it has also been shown that even the same glasses can have deviating safe ranges depending on the raw materials used and, in particular, on the water content of the melt. In addition, the water content of the atmosphere above and around the melt also has a strong influence on the oxygen partial pressure in the melt. 
     A further development of the method according to the present invention is characterized by the fact that a lower threshold value of the oxygen partial pressure of the safe range is above 10 −7  bar, preferably 10 −4  bar, and, in particular, above 10 −3  bar. Depending on the particular glass that is used, it has been shown that, with certain glasses, disturbances do not occur until the pressure drops below 10 −7  bar. With other glasses, a disturbance does not occur until the pressure falls below 10 −4  bar. Above 10 −3  bar, formation of N 2 , CO 2  and SO 2  bubbles, and the occurrence of alloy damage is ruled out in nearly every case. 
     With another further development, an upper threshold value of the oxygen partial pressure of the safe range is 0.4 bar. When the pressure falls below 0.4 bar and pressure is being regulated in a safe range, disturbance caused by the formation of oxygen bubbles on precious metal components is reliably prevented in most cases. By way of this regulation in the safe range, it is not necessary to increase a reverse voltage for a period long enough to prevent bubbles from forming, for example. Rather, this regulation makes it possible to adjust to conditions in the safe range in advance, so that defect-free production is ensured. 
     The basis of the present invention is to measure the oxygen partial pressure, for which an electrode pair composed of a reference electrode and a measuring electrode is used as the probe. In particular, a zircon oxide reference electrode or a molybdenum reference electrode may be used as the reference electrode. A platinum measuring electrode can be used as the measuring electrode. It is also possible to use the precious metal walls themselves as the measuring electrode, however. 
     It has proven advantageous to use the oxygen partial pressure in the melt as the setpoint value for the regulation and, in fact, in its state when it leaves the melting area and, in particular, before the oxygen partial pressure has changed due to contact with a precious metal wall. The oxygen partial pressure of the setpoint value is measured in the melt at a precious metal electrode in particular, whereas the actual value is the oxygen partial pressure at the respective precious metal wall. The same reference electrode can then be used, preferably, to determine the setpoint value and the actual value. The oxygen partial pressure in the unchanged melt is compared with the oxygen partial pressure at the interface with the precious metal wall in the manner described. Good results are obtained as a result. It can also be necessary, however, to select another setpoint value based on the known safe range. 
     With the procedure according to the present invention, the oxygen partial pressure is determined using the electromotive force, EMF, between the measuring electrode and the reference electrode of the electrode pair. The electromotive force, EMF, is linked with oxygen partial pressure in a known manner, via the Nernst equation, by way of which the oxygen partial pressure can be easily determined. A reference electrode for performing an electrochemical determination of the oxygen partial pressure is known from DE 43 24 922 A1, in particular. In general, when a “ZrO 2  reference electrode” composed of yttrium-stabilized zircon oxide is used, the oxygen partial pressure PO 2  (2) of the melt can be determined from the electromotive force, EMF, of the isothermal electrochemical measuring chain
 
Pt, O 2  (1)/Zr O 2   +x  Y 2 O 3 /melt, O 2  (2)/Pt
 
as follows:
 
 p O 2 (2)= p O 2 (1)* e   −E4F/(RT)  
 
whereby F is the Faraday constant
         R is the gas constant   T is the temperature in the glass melt in °K.
 
With non-isothermal melts, the temperatures of measuring and reference electrodes must be measured separately. To calculate the oxygen partial pressure, an expanded formula is therefore necessary:
 
PO 2 (S) =exp ((4*F/R*T (S) )*(E−(−0.0004739)* (T (S) −T (R) ))+T (R) /T (S) * In  p O 2(R)  
       

     The following apply:
         T (S)  is the temperature of the melt measured at the site of the platinum measuring electrode.   T (R)  is the temperature at the site of the ZrO 2  reference electrode.   PO 2(R)  is the oxygen partial pressure of the rinse gas of the reference electrode.   −0.0004739 V/K is the Seebeck coefficient of melt and/or zircon oxide.   For more information, refer to Baucke, “Electrochemistry of Glasses and Glass Melts, Including Glass Electrodes”, Springer Verlag Berlin Heidelberg New York 2000
 
A prerequisite for the use of the equation described is that a defined oxygen partial pressure pO 2 (1) exists at the three-phase limit Pt (1), O 2  (1) ZrO 2 +x Y 2 O 3 .
       

     To influence the oxygen partial pressure, a counter electrode can be located upstream in the glass melt. It can be located so far upstream, for example, that any gas bubbles that form on this counter electrode do not result in damage to the glass production. To influence the oxygen partial pressure, a reverse voltage can then be applied between the counter electrode and the precious metal wall. It is also possible to use a counter electrode composed of refractory metal, in particular molybdenum, however, and to connect it in a conductive manner with the precious metal wall. This molybdenum electrode can then discharge positively charged metal ions into the glass melt and thereby provide a suitable reverse voltage at the precious metal wall. 
     With another embodiment, to influence the oxygen partial pressure, mixtures of water vapour and air or water vapour and nitrogen with a specified portion of vapour or hydrogen gas at a specified partial pressure are directed at the side of the precious metal wall facing away from the glass melt. The former have the lowest reduction effect and the latter have the strongest reduction effect. Oxygen-nitrogen mixtures have an oxiding effect and can prevent drifting into the reducing region with subsequent formation of N 2 , CO 2 , SO 2  bubbles. For this purpose, the precious metal wall can be provided with a double-wall configuration, for example, so that the hydrogen gas or water vapour need be applied only to a cavity located between the double wall. 
     The device according to the present invention includes at least one precious metal wall or refractory metal wall that at least partially encloses the molten glass, and is characterized by a regulating system for regulating the oxygen partial pressure, in particular in the region near the interface of the melt and the tank wall. 
     In particular, the device includes a first electrode pair for determining the oxygen partial pressure in the glass melt as a setpoint value and a second electrode pair for determining the oxygen partial pressure as an actual value for the regulation, whereby the precious metal wall is preferably the measuring electrode of the electrode pair. It is also possible, in principle, to locate the measuring electrode in a region close to the wall such that it is insulated from the wall. Preferably, all electrode pairs should have a common reference electrode; less outlay is required and the accuracy of the measurement is increased as a result. 
     The glass obtained with the method or the device having the features of the present invention is suited in particular for the production of LCDs, TFTs, monitors, television screens, optical lenses, cooking utensils, microwave utensils, electronic devices, cook tops, window glasses, lamp glasses and display glasses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of a device with the features according to the present invention, 
         FIG. 2  shows a stirring crucible with a regulating system from  FIG. 1  for adjusting the oxygen partial pressure using reverse voltage, 
         FIG. 3  shows a stirring crucible with regulation of the oxygen partial pressure using vapour or gas rinsing, and 
         FIG. 4  shows a diagram of safe ranges for various types of glass. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a device having the features of the present invention, as a first exemplary embodiment. The figure shows a melting area  10  with glass melt  11  located therein. Melting area  10  is shown as a schematic illustration only. In particular, various areas for melting and refining are not shown in the figure. Melting area  10  is connected with a stirring crucible  13  via a feeder channel  12 . In the exemplary embodiment shown, feeder channel  12  and stirring crucible  13  are composed of platinum or a platinum alloy. In addition, a counter electrode  14  is located in the melting area  10  in the region of the glass melt  11 , the counter electrode being connected with a control system  16  by a line  15 . Counter electrode  14  can also be located at another point. It is important that counter electrode  14  be located upstream of stirring crucible  13  and, in particular, upstream of a refining area. 
       FIG. 2  shows a schematic illustration of stirring crucible  13  and control system  16  in  FIG. 1 . As shown in the figure, molten glass is forwarded to stirring crucible  13  from feeder channel  12 , as indicated by arrow A. An outlet  17  is located on the side of stirring crucible  13  opposite feeder channel  12 , through which said outlet glass is forwarded for further production, as indicated by arrow B. In addition, a stirring mechanism  18  with a drive  19  is located in stirring crucible  13 . In the exemplary embodiment shown, stirring mechanism  18  is composed of platinum. 
     In addition, a reference electrode  20  and a measuring electrode  21  are located in the region of stirring crucible  13 , the electrodes being immersed in the glass melt located in stirring crucible  13 . In the exemplary embodiment shown, reference electrode  20  is a zircon oxide reference electrode. A molybdenum rod can also be used as the reference electrode, if it is ensured that the molybdenum rod does not alloy on the surface in the melt, and that it is not alloyed. Reference electrode  20  includes a supply line  22  and a discharge line  23 . Supply line  22  and discharge line  23  serve to supply and carry away, respectively, gas with a defined oxygen partial pressure, as indicated by arrows C and D. Reference electrode  20  is connected via a line  24  with a first evaluation unit  25 . Reference electrode  20  is also connected via two lines  26  with a temperature measuring device  27 . In the exemplary embodiment shown, reference electrode  20  includes a thermoelement, the two ends of which are connected via lines  26  with temperature measuring device  27 , which is a thermoelement measuring device  27  in this case. Thermoelement measuring device  27  is also connected via a line  28  with first evaluation unit  25 . 
     Electrode  21  is also connected via a line  29  with first evaluation unit  25 . In addition, electrode  21  also includes a thermoelement, which is connected via lines  30  with a temperature measuring device  31  similar to temperature measuring device  27 . The temperature of measuring electrode  21  measured by temperature measuring device  31  is forwarded to measuring device  25  via line  37 . 
     Lines  24  and  28  are connected via lines  32  and  33  with a second evaluation unit  34  similar to first evaluation unit  25 . A third input to second evaluation unit  34  is connected via a line  35  with feeder channel  12  in the region of the transition of feeder channel  12  to stirring crucible  13 . The outputs of first evaluation unit  25  and second evaluation unit  34  are connected via lines  36 ,  38  with the inputs of a regulating unit  39 . The regulating unit  39  is also connected via a line  40  with feeder channel  12  in the region of the transition to stirring crucible  13 . In addition, regulating unit  39  is connected via line  15  with counter electrode  14 . 
     The electromotive force, EMF, between reference electrode  20  and electrode  21  is determined using first evaluation unit  25  and, based also on the temperatures determined by thermoelement measuring devices  27  and  31 , is converted to an oxygen partial pressure. This oxygen partial pressure is forwarded via line  36  to regulating unit  39  as the setpoint value for the regulation. Second evaluation unit  34  determines the electromotive force between reference electrode  20  and the wall of stirring crucible  13  and/or feeder channel  12  in the region of stirring crucible  13  and converts it, based on the temperature determined by thermoelement measuring device  31 , to an oxygen partial pressure, which is forwarded to the regulating unit  39  via line  38  as the actual value for the regulation. Regulating unit  39  compares the actual value of the oxygen partial pressure transferred from evaluation unit  34  with the setpoint value for the oxygen partial pressure transferred from evaluation unit  25  and regulates a reverse voltage between counter electrode  14  and feeder channel  12  in the region of the transition to stirring crucible  13 . The oxygen partial pressure in the region of the wall of stirring crucible  13  and feeder channel  12  in the region of the wall of stirring crucible  13  can be adjusted to a desired range in the manner described by reliably preventing disturbances to the glass that exits at B. 
       FIG. 3  shows a schematic illustration of a further exemplary embodiment of a stirring crucible  41  and a control system having the features of the present invention. Identical elements are labelled with the same reference numerals. Stirring crucible  41  is connected with melting area via a feeder channel  43  similar to feeder channel  12 . In contrast to stirring crucible  13  and feeder channel  12 , stirring crucible  41  and feeder channel  43  have a double-wall configuration, so that gas can be directed through between the walls. An inlet in the region of feeder channel  43  is connected with a regulating unit  45  using a gas line  44 . Similar to regulating unit  39 , regulating unit  45  is connected with first evaluation unit  25  and second evaluation unit  34 . Furthermore, regulating unit  45  includes an inlet for a gas line  46  and a further inlet for a gas line  47 . Gas line  46  serves to supply a carrier gas such as nitrogen, as indicated by arrow E. Gas line  47  serves to supply a reactive gas, as indicated by arrow F. Water vapour and/or a hydrogen/nitrogen mixture can be used as the reactive gas. 
     An outlet of stirring crucible  41  is connected via a gas line  48  with a washing bottle  49 , the outlet  50  of which leads to a waste gas purification system or an exhaust air line. 
     Similar to the exemplary embodiment in  FIG. 2 , regulating unit  45  performs regulation by comparing the actual value of the oxygen partial pressure at feeder channel  43  obtained from line  38  with the setpoint value of the oxygen partial pressure at electrode  21  obtained via line  36 . 
     The mixing ratio of reactive gas F with carrier gas E is adjusted as a function of this setpoint/actual value comparison. The gas mixture obtained as a result is directed into the double wall of stirring crucible  41  and/or feeder channel  43 . By adjusting a suitable hydrogen or water vapour partial pressure in the double wall of stirring crucible  41  and/or feeder channel  43 , equilibrium is established between the diffusion of hydrogen from the glass melt through the wall of stirring crucible  41  and hydrogen from the double wall back through the wall and into the glass melt, so that a desired oxygen partial pressure at the interface of the metal and glass melt can be obtained. The gas then passes through gas line  48  and into washing bottle  49 , and can then be carried away via outlet  50 . 
       FIG. 4  shows a diagram that indicates safe ranges for various types of glass for various oxygen partial pressures in the glass melt. For example, a safe range for a glass AF  37  extends from 10 −3  to 0.4 bar. Above 0.4 bar, O 2  bubbles start to form. Below 10 −3  bar, N 2 , CO 2  and SO 2  bubbles start to form. In an oxygen partial pressure range between 10 −6  and 10 −5 , no O 2 , N 2 , CO 2  or SO 2  bubbles form. Alloy damage can occur at oxygen partial pressures below 10 −6  bar. 
     For Duran 8330 glass (from Schott Glas, Mainz, Germany), a safe range without disturbances from 10 −7  to 0.4 bar was determined for oxygen partial pressures. At oxygen partial pressures above 0.4 bar, O 2  bubbles can be expected to form, as is the case with AF37 glass (Schoft Glas). When oxygen partial pressures are below 10 −7  bar, there is a risk that alloy damage will occur. 
     The third glass shown in the diagram is Fiolax 8412 (Schott Glas). A safe range for oxygen partial pressure from 10 −4  to 0.4 bar was determined for this glass. In this case, O 2  bubbles can be expected to form at oxygen partial pressures above 0.4 bar. And, at oxygen partial pressures below 10 −4  bar, there is a risk of alloy damage. 
     According to the present invention, damage and disturbance to the glass that is produced are prevented by determining a safe range, depending on the type of glass used, and regulating to oxygen partial pressures in the particular safe range using control systems  16 ,  42  and, in particular, regulating units  39 ,  45 .

Technology Category: 8