GLASS PRODUCT AND METHOD FOR PRODUCING SAME

Abstract

A method for producing a glass product, preferably a sheet-like glass product, is provided that includes conveying a molten silicate glass through a conduit system from one area of a glass product producing installation to another area of the glass product producing installation. The conduit system includes noble metal and is configured to conduct an electric current through the noble metal so as to generates Joule heat in the conduit system. The current is an alternating current for which the time integral over a positive and a negative half-wave results in a zero value.

Claims

1. A method for producing a glass product, comprising: conveying a molten silicate glass through a conduit system from one area of a glass product producing installation to another area of the glass product producing installation, wherein the conduit system comprises a noble metal; and conducting an alternating electric current through the noble metal while conveying the molten silicate glass through the conduit system, the alternating electric current generating Joule heat in the noble metal, wherein the alternating current has a time integral over a positive and a negative half-wave that results in a zero value.

2. The method of claim 1, wherein the conduit system comprises a tubular conduit element and wherein the noble metal is a coating on an inner surface of the tubular conduit element, the alternating current being conducted in a longitudinal direction of the tubular conduit element.

3. The method of claim 1, wherein the alternating current is sinusoidal and has a basic frequency ω.sub.0.

4. The method of claim 3, wherein the basic frequency ω.sub.0 is between at least 2*10.sup.2 Hz and at most 2*10.sup.4 Hz.

5. The method of claim 3, wherein the basic frequency ω.sub.0 is between at least 5*10.sup.2 Hz and at most 1.5*10.sup.4 Hz.

6. The method of claim 1, wherein the time integral has a deviation over a full wave from an ideal sinusoidal pulse signal curve of less than 10%.

7. The method of claim 1, wherein the time integral has a deviation over a full wave from an ideal sinusoidal pulse signal curve of less than 2%.

8. The method of claim 1, further comprising measuring a phase angle θ.sub.0 between current and voltage at a basic frequency ω.sub.0 at least once.

9. The method of claim 8, further comprising adjusting the basic frequency ω.sub.0 based on the phase angle θ.sub.0 between current and voltage.

10. The method of claim 8, further comprising adjusting the basic frequency ω.sub.0 such that the phase angle θ.sub.0 between current and voltage as a function of frequency is at a local minimum at which a local derivative of the phase angle θ with respect to frequency assumes a zero value.

11. The method of claim 8, wherein the phase angle θ.sub.0 between current and voltage is smaller than ±10°.

12. The method of claim 8, wherein the phase angle θ.sub.0 between current and voltage is smaller than ±2°.

13. The method of claim 1, further comprising generating the alternating electric current I(ω) with a time-dependent profile of a voltage curve U(ω) having signal components with a plurality of discrete frequencies ω.sub.1, ω.sub.2, ω.sub.3, . . . ω.sub.n, wherein n is a non-zero natural number, and wherein the overall voltage curve U(ω) resulting from the superposition of the individual signal components results as follows:
U(ω)=U.sub.1(ω.sub.1)+U.sub.2(ω.sub.2)+U.sub.3(ω.sub.3)+ . . . U.sub.n(ω.sub.n), wherein each of U.sub.1(ω.sub.1), U.sub.2(ω.sub.2), U.sub.3(ω.sub.3) . . . U.sub.n(ω.sub.n) is a respective voltage signal with a sinusoidal or cosinusoidal shape with a respective frequency ω.sub.1, ω.sub.2, ω.sub.3, . . . ω.sub.n; wherein, each of the discrete frequency components with ω.sub.1, ω.sub.2, ω.sub.3, . . . ω.sub.n meet the condition that for each of these frequency components with ω.sub.1, ω.sub.2, ω.sub.3, . . . ω.sub.n the phase angle θ.sub.1(ω.sub.1), θ.sub.2(ω.sub.2), θ.sub.3(ω.sub.3), . . . θ.sub.n(ω.sub.n) between current and voltage at the respective frequency is less than ±10°.

14. The method of claim 1, further comprising generating the alternating electric current I(ω) with a time-dependent profile of a voltage curve U(ω) having signal components with a continuous spectrum of sinusoidal or cosinusoidal signal components Ui(ω.sub.i) with different frequencies ω.sub.i from the spectral range or frequency interval from ω.sub.x to ω.sub.y, wherein the following applies for the frequency ω.sub.i of each of these signal components:
ω.sub.x<ω.sub.i<ω.sub.y wherein ω.sub.x is the frequency at which a phase angle θ between current and voltage is −10°, and wherein ω.sub.y the frequency at which a phase angle θ between current and voltage is +10°.

15. The method of claim 1, wherein, during the conveying step, the molten silicate glass has a temperature of between 1000° C. and 1650° C.

16. A glass product, comprising: a sheet-like glass product of a silicate glass having a thickness of at most 1000 μm and at least 15 μm; and less than four particles of a noble metal comprising material per kilogram of glass, wherein the less than four particles have a size of less than 200 μm.

17. The glass product of claim 16, further comprising less than three 3 bubbles per kilogram of glass, wherein the less than three bubbles have a size of less than 200 μm.

18. The glass product of claim 16, wherein the silicate glass comprises in wt %: SiO.sub.2 50-87; and Al.sub.2O.sub.3 0-25 and/or B.sub.2O.sub.3 5-25.

19. The glass product of claim 16, wherein the silicate glass comprises at most 2500 ppm of SnO.sub.2 based on the weight and/or at least 100 ppm of chloride based on the weight.

20. The glass product of claim 16, wherein the silicate glass comprises at most 2500 ppm of SnO.sub.2 based on the weight and/or at most 2500 ppm of chloride based on the weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0129] The invention will now be further explained with reference to drawings, in which

[0130] FIG. 1 is a schematic diagram of an experimental setup;

[0131] FIGS. 2a-2c and 3a-3c show photographs of silicate molten glass from an experimental setup according to FIG. 1;

[0132] FIG. 4 shows a schematic diagram of a further experimental setup for electrochemical impedance spectroscopy; and

[0133] FIG. 5 shows an impedance spectrum from an experimental setup according to FIG. 4, showing the absolute value of complex impedance Z as a function of frequency ω;

[0134] FIG. 6 shows an impedance spectrum from an experimental setup according to FIG. 4, showing the phase angle θ as a function of frequency ω;

[0135] FIG. 7 shows a substantially tubular conduit element of a conduit system, which has a coating comprising at least one noble metal on its inner surface and in which an alternating current is passed through the noble metal using a generator G;

[0136] FIG. 8 shows an oscilloscope image displaying a periodic voltage curve as a function of time, this voltage curve exhibiting a strong deviation from a sinusoidal shape, which is essentially caused by phase cutting;

[0137] FIG. 9 shows an oscilloscope image displaying a periodic voltage curve as a function of time, this voltage curve exhibiting only a very small deviation from a sinusoidal shape;

[0138] FIG. 10 illustrates the introduction of particulate matter into a molten glass under various forms of alternating current which is used for heating a molten glass located in a noble metal comprising conduit element;

[0139] FIG. 11 shows an oscilloscope image displaying a voltage curve for explaining the current flow during time T.sub.1 of FIG. 10;

[0140] FIG. 12 shows an oscilloscope image displaying a periodic voltage curve for explaining the current flow during time T.sub.3 of FIG. 10;

[0141] FIG. 13 shows a basic circuit diagram of an exemplary circuit arrangement; and

[0142] FIGS. 14 and 15 are exemplary scanning electron micrographs of noble metal comprising particles;

[0143] FIG. 16 shows a further, essentially tubular conduit element of a conduit system, which has a coating comprising at least one noble metal on its inner surface and in which a generator G passes an alternating current through the noble metal of a respective section out of three sections which are designated overflow 0 (OF0), overflow 1 (OF1), overflow 2 (OF2).

DETAILED DESCRIPTION

[0144] FIG. 1 shows a schematic diagram of an experimental set-up, not drawn to scale, for determining the influence of pulse modulation in the generation of the alternating current I(ω) in a silicate molten glass. A silicate molten glass 2 is melted in a crucible made of a refractory material comprising SiO.sub.2, for example a so-called QUARZAL® crucible.

[0145] Two noble metal comprising electrodes 31, 32 of the same size, with a surface area of 0.5 cm by 1 cm, were each embedded in a respective half of the crucible 1. The crucible halves are connected via a bridge of molten glass, which means that the current I(ω) flowing between electrodes 31, 32 is entirely conducted through the molten glass 2. The respective electrode 31, 32 is made of a noble metal alloy, by way of example, namely an alloy of platinum and rhodium, which may also be referred to as “PtRh10”, that is 10 wt % of rhodium and 90 wt % of platinum. The molten glass 2 was a molten silicate glass.

[0146] The space surrounding the crucible 1 is flushed with inert gas (here argon) in order to prevent a gas-phase transport reaction with respect to the noble metal comprising electrodes 31, 32.

[0147] The crucible 1 is brought to a temperature of 1450° C., for example, in a furnace.

[0148] Then, between the electrodes 31, 32, the signal shape of the current I(ω) flowing between the two electrodes 31, 32 was varied using different modulators within the generator G which represents an alternating current source, under the boundary condition to have a geometric time-averaged current density of 25 mA/cm.sup.2 flowing between the electrodes 31, 32 in each of the tests.

[0149] Three tests were conducted, as will be described in more detail below, during which the two electrodes 31, 32 were exposed to the modulation and to molten glass contact for 24 hours.

[0150] After the holding time of 24 hours, one of the electrodes 31, 32 was removed from the crucible half and quickly frozen with the glass attached. Photographs thereof can be seen in FIGS. 2a to 2c.

[0151] In FIG. 2a it can be seen that with currentless heating and with an at least approximately sinusoidal signal curve in FIG. 2b, the noble metal of the electrode and the structure of the respective electrodes do not exhibit changes in grain structure.

[0152] FIG. 9 shows an exemplary oscilloscope image with a periodic voltage curve U(ω) displayed thereon, as a function of time at a basic frequency ω.sub.0, and this voltage curve only exhibits a very small deviation from a sinusoidal shape and represents the shape of the alternating current I(ω). Here, an exemplary sinusoidal full wave is denoted as interval Vω.sub.1. The basic frequency ω.sub.0was 50 Hz, by way of example.

[0153] However, when phase cutting is employed for generating the alternating current I(ω), for example using a thyristor as in FIG. 2c, a clear change in the reflection properties of the coarse-grain noble metal crystals can be seen, so that it can be concluded that a chemical reaction has occurred.

[0154] FIG. 8 shows an exemplary oscilloscope image with a periodic voltage curve U(ω) displayed thereon, as a function of time at a basic frequency ω.sub.0, and this voltage curve shows a strong deviation from a sinusoidal shape, which is essentially caused by phase cutting and represents the shape of the alternating current I(ω) used here. The basic frequency ω.sub.0was 50 Hz, by way of example. Here, an exemplary first, non-sinusoidal half-wave generated by phase cutting is denoted as interval Hω.sub.1, and a second non-sinusoidal half-wave generated by phase cutting is denoted as interval Hω.sub.2.

[0155] Once the entire crucible 1 had been tempered down, the glass body of the crucible half, from which the corresponding electrode was previously removed, was drilled out and the base was polished. The images of the samples taken in transmitted light are shown in FIGS. 3a to 3c.

[0156] It can be clearly seen that no bubbles are visible in the case of a currentless signal curve in FIG. 3a, and that only very few bubbles have arisen with an at least approximately sinusoidal signal curve in FIG. 3b.

[0157] However, if phase cutting by a thyristor as in FIG. 3c is employed, not only significant bubble formation can be observed, but also darkening of the glass around the bubbles formed, which can be attributed to the formation of noble metal particles.

[0158] In the further processes, the inventors used electrochemical impedance spectroscopy in order to be able to identify properties of the respective employed glass in more detail.

[0159] A schematic experimental setup for electrochemical impedance spectroscopy is shown in FIG. 4. Here, glass was melted in a platinum crucible 50 with a diameter of about 10 cm, and the filling height F of the silicate molten glass 51 was about 10 cm. The crucible 51 was kept at temperature in an oven, and the electrode was introduced into the molten glass 51 to be examined, in the present case a rectangular platinum electrode 53 with a size of approximately 2×4 cm.

[0160] Both the crucible 51 and the electrodes 52, 53 are electrically addressable, through a respective platinum wire 54. Furthermore, an O.sub.2|Pt|ZrO.sub.2 reference electrode 52 (rinsed with 1 bar of O.sub.2 as a reference) was introduced into the molten glass 51 in order to have an independent reference potential for the electrochemical measurements.

[0161] The electrochemical impedance spectrometer was connected in the following configuration:

[0162] The working electrode 53 is the platinum electrode under test, the reference electrode 52 is the introduced O.sub.2|Pt|ZrO.sub.2 reference electrode, the counter electrode is defined by the crucible 51.

[0163] The impedance spectra were recorded by potentiostatic electrochemical impedance spectroscopy, and an excitation potential of 25 mV was selected.

[0164] The following impedance spectra were recorded of a molten glass 51 of a composition corresponding to AS87 glass, at frequencies from 10.sup.6 Hz to 5*10.sup.−3 Hz at melting temperatures 1200° C., 1300° C., 1400° C., 1500° C.

[0165] Merely by way of example, the current generated in this case is designated as I(ω), and the voltage occurring here as U(ω). The complex impedance is resulting here as a function of frequency, as Z(ω)=U(ω)/I(ω), the absolute value |Z| of which is shown in the impedance spectrogram of FIG. 5 for different temperatures.

[0166] The frequency-dependent phase angle θ(ω) between current I(ω) and voltage U(ω), which is denoted by “theta” in FIG. 6, showed a clear frequency dependency with a pronounced minimum, and the exploitation thereof with respect to the method will be described in more detail below.

[0167] These tests are intended for simulating an arrangement such as that shown in FIG. 7 and in particular the interaction of the noble metal, in particular of a noble metal comprising conduit system, with the molten silicate.

[0168] Surprisingly it has been found that the test results obtained with the arrangements shown in FIGS. 1 and 4 were substantially also transferable to other embodiments, such as, for example, to the embodiment shown in FIG. 7 in which substantially no current was passed directly through the molten silicate or molten glass 2, rather it was passed substantially through the noble metal comprising zone, that is through the coating or lining 62 that will be described in more detail below. Although this positive effect does not seem to be fully understood, one reason for the transferability of the present results may be the skin effect of an alternating-frequency current in a conductor, according to which higher current densities occur near the surface of a conductor than in the interior thereof in the case of alternating-frequency currents, since the conductor tries to remain free of fields and voltage inside. These higher current densities occurring near the surface of the respective conductor are therefore in direct contact with the molten glass 2 adjoining the conductor 62.

[0169] FIG. 7 shows a substantially tubular conduit element 60 of a conduit system for conveying a molten glass. This conduit system may extend between a melting unit and a device for hot forming, for example.

[0170] The conduit element 60 comprises a tubular section 61 made of a refractory material and has, on its inner surface, a coating 62 comprising at least one noble metal, or a noble metal comprising lining 62.

[0171] As mentioned above, this noble metal may for example comprise platinum or platinum alloys. For example, platinum may be alloyed with rhodium, iridium and gold, and/or may additionally comprise zirconium dioxide and/or yttrium oxide for fine-grain stabilization.

[0172] The generator G is used to pass the alternating current I(ω) through the noble metal, whereby the alternating voltage U(ω) is generated at the generator, as shown in FIGS. 8 and 9.

[0173] The basic frequency ω.sub.0 was set based on the phase angle θ.sub.0 between current and voltage.

[0174] The basic frequency ω.sub.0 was in particular set such that the phase angle θ.sub.0 between current and voltage as a function of frequency ω is at a local minimum where the local derivative of the phase angle θ with respect to frequency ω assumes a zero value.

[0175] Such a minimum can be seen in the graph of FIG. 6 for the value of frequency ω.sub.0, by way of example.

[0176] However, depending on how the process was conducted, this minimum was not sharply localized, with a pronounced peak, but rather was within a range with a low slope. For the presently disclosed embodiments, an angular range with such a low slope, in which the phase angle θ.sub.0 between current and voltage is less than ±10°, preferably less than ±5°, and most preferably less than ±2° has proven to be advantageous as well.

[0177] Generally, as can be seen from the view of FIG. 6, for example, for the glasses disclosed in the present invention, in a temperature range from 1000° C. to 1650° C. and for a phase angle θ.sub.0 between current and voltage of less than ±10°, the basic frequency ω.sub.0 was preferably at least about 2*10.sup.2 Hz to 5*10.sup.2 Hz at a phase angle θ.sub.0 of −10° between current and voltage, corresponding to ω.sub.x, and ranged to at most about 1.5*10.sup.4 Hz to 2*10.sup.4 Hz, corresponding to ω.sub.y, at a phase angle θ.sub.0 of +10° between current and voltage.

[0178] Although the arrangement shown in FIG. 7 essentially only comprises currents I(ω) which flow in the direction of arrow P within the molten glass 2, it has been found, as already stated above, that the results obtained experimentally with the setup shown in FIG. 1 were surprisingly well transferable to the conduit element 60 illustrated in FIG. 7 and that the method with minimized phase angle allowed to achieve a strong reduction both in the formation of bubbles and in particulate introduction.

[0179] FIG. 5 and FIG. 6 show two graphs illustrating the results of impedance spectroscopy. In FIG. 5, the absolute value of the complex impedance Z is plotted as a function of frequency. Curve 101 was measured at a melting temperature of 1500° C., curve 102 at a melting temperature of 1400° C., curve 103 at a melting temperature of 1300° C., and curve 104 at a melting temperature of 1200° C.

[0180] It can be clearly seen that the absolute value of the impedance passes through a minimum at frequencies between about at least about 2*10.sup.2 Hz to 5*10.sup.2 Hz and at most about 1.5*10.sup.4 Hz to 2*10.sup.4 Hz, as a function of temperature.

[0181] In FIG. 6, the phase angle θ is plotted as a function of frequency. Curve 105 was measured for the same glass at a melting temperature of 1500° C., curve 106 at a melting temperature of 1400° C., curve 107 at a melting temperature of 1300° C., and curve 108 at a melting temperature of 1200° C. Here, too, it can be seen that at these temperatures the phase angle assumes a minimum at frequencies of at least 5*10.sup.2 Hz to at most 2*10.sup.4 Hz, i.e. very low values ranging between not more than ±10°, for example at most ±5°, or even at most ±2°.

[0182] The results that can be achieved with the method according to the invention are shown in FIG. 10, merely by way of example.

[0183] FIG. 10 shows the results of the production of an alkali-free alkaline earth silicate glass with an exemplary composition as specified above, in an exemplary device for making glass products, which is also referred to as a tank, for short.

[0184] In this tank, there is a connection between a refining tube and a crucible of the device upstream of or constituting part of the hot forming process, which connection comprises a transfer tube, i.e. the conduit element 60 shown in FIG. 7 and in a further embodiment in FIG. 16. This conduit element 60 was initially heated by three heating circuits referred to as overflow 0 (OF0), overflow 1 (OF1), overflow 2 (OF2). Although FIG. 16 shows heating circuits of overflow 0 (OF0), overflow 1 (OF1) and overflow 2 (OF2) that are arranged one behind the other by way of example, these heating circuits may also be arranged in parallel in the embodiment shown in FIG. 7.

[0185] All 3 heating circuits were initially operated using transformers with a tap of 10 V, as substantially corresponding to the diagram in FIG. 7, although only one heating circuit is shown in FIG. 7, by way of example and for the sake of clarity, which provides the voltage U(ω) and the current I(ω), by generator G. This situation is again shown in FIG. 16, in more detail.

[0186] The effect of the heating circuits is shown by the corresponding current measurement curves 701, 703, 705, with measurement curve 701 being associated with overflow 2, measurement curve 703 with overflow 1, and measurement curve 705 with overflow 0, and by measurement curves 702, 704, 706 for the electrode potential E (plotted as voltage U), with measurement curve 702 being associated with overflow 2, measurement curve 704 with overflow 1, and measurement curve 706 with overflow 0.

[0187] Also by way of example, the number 8 of noble metal comprising particles that were introduced into the molten glass during this time is plotted, namely in the form of square symbols which are not all labeled, for the sake of clarity.

[0188] Now, 3 different states can be described:

[0189] Time period T1 was about six and a half days.

[0190] All three heating circuits were operated using a transformer with a tap of 10 V.

[0191] Heating circuit OF0 was operated at an RMS voltage of about 8.2 V, at an RMS current of about 1700 A, and with relatively low phase cutting, however still generated harmonics with frequencies above ω.sub.y.

[0192] Heating circuit OF1 was operated at an RMS voltage of about 2.9 V, at an RMS current of about 700 A, and with strong phase cutting.

[0193] Heating circuit OF2 was operated at an RMS voltage of about 3.1 V, at an RMS current of about 500 A, and with strong phase cutting, which generated harmonics with frequencies above ω.sub.y in each case.

[0194] FIG. 9 shows an oscilloscope image displaying a voltage curve for overflow 1 during time period T2. Phase cutting is relatively low here.

[0195] FIG. 11 shows an oscilloscope image displaying a voltage curve for overflow 1 during time period T1. Phase cutting is very pronounced here and therefore has a high proportion of frequencies above ω.sub.y. These frequencies arise within a respective full wave of U(ω) at the strongly pronounced voltage jumps Sp1, Sp2, Sp3, and Sp4, which are easily recognizable in FIG. 11. It has also been found that exceeding the frequencies that has been specified as preferred, i.e. ω.sub.y, had more detrimental effects than undershooting them.

[0196] With the above procedure, the average number of noble metal particles, in particular platinum particles, introduced into the molten glass 2 was approx. 7.0 particles per kg.

[0197] Time period T.sub.2 was about 15 days and was consecutive to time period T.sub.1.

[0198] Heating circuit OF1 and heating circuit OF2 were merged, so that a new heating circuit (OF1) was obtained.

[0199] Both heating circuits were operated using a transformer with a tap with an RMS voltage of 10 V.

[0200] Heating circuit OF0 was operated at an RMS voltage of approx. 8.2 V, at an RMS current of approx. 1650 A, and with relatively small phase cutting.

[0201] Heating circuit OF1 was operated at an RMS voltage of approx. 4.7 V, at an RMS current of approx. 640 A, and with reduced phase cutting compared to the view of FIG. 11.

[0202] With this procedure just described, the average number of noble metal particles introduced into the molten glass 2, in particular platinum particles, was approx. 3.8 particles per kg.

[0203] Time period T.sub.3 was about nine and a half days and was consecutive to time period T.sub.2.

[0204] Heating circuit OF0 was operated using a variable transformer with an RMS voltage tap of 8 V.

[0205] Heating circuit OF1 was operated using a transformer with an RMS voltage tap of 10 V.

[0206] Heating circuit OF0 was operated at an RMS voltage of approx. 7.6 V, at an RMS current of approx. 1550 A, and with phase cutting optimized as best as possible, which means that it was smoothed.

[0207] The overflow OF1 was operated at an RMS voltage of approx. 4.7 V, at an RMS current of approx. 640 A, and with reduced phase cutting compared to the view of FIG. 11.

[0208] The fact that in the operation described above the RMS voltage values were lower than the RMS voltage tap values during time periods T.sub.1 to T.sub.3 represents the normal case of a current-loaded transformer, which can exhibit a decrease in the RMS voltage value as the RMS current value increases.

[0209] FIG. 12 shows an oscilloscope image displaying a voltage curve for overflow 1 during time period T.sub.3. As can be seen, phase cutting is significantly reduced here compared to the voltage curve shown in FIG. 11, as has been already mentioned above for voltage curves with reduced phase cutting.

[0210] With this procedure just described, the average number of noble metal particles introduced into the molten glass 2, in particular platinum particles, was approx. 2.5 particles per kg.

[0211] These examples show that a reduced influence of the phase cutting and a more sinusoidal alternating current I(ω) lead to a minimization in particulate introduction into the molten glass 2.

[0212] FIG. 13 shows a greatly simplified basic circuit diagram of an exemplary circuit arrangement. Lines L1, L2, L3, and N are lines which in particular carry the phases of a power supply network 70 which may either be part of an internal or of an external power supply network. This power supply network 70 may, for example, provide an alternating voltage with an RMS voltage of 230 V between two respective lines that include the phases L1, L2, L3, at a network frequency of 50 Hz or even higher in the case of an internal power supply network. With this arrangement in which the basic frequency ω.sub.0 was not yet optimally selected, it was already possible to show that the avoidance of harmonics with frequencies ω outside, in particular above the preferred frequency range, had a positive impact in the sense of the stated object of the invention.

[0213] Via a fused contactor or protection switch 71, the lines of phases L1 and L3 are routed to the further circuit as will be described in more detail below.

[0214] When the contactor 71 is closed, phase L3 is supplied to a parallel circuit comprising the thyristors T1 and T2, and the thyristors T1 and T2 are selectively driven, in particular ignited, by a control circuit 72.

[0215] Thyristors T1 and T2 are usually connected between the potentials labeled U1 and U2 in order to generate the phase cutting and to jointly power the variable transformer 73, with the phase-cut phase L3 and with phase L1.

[0216] Variable transformer 73 is adapted to transform the voltage generated by thyristors T1 and T2 with phase cutting to a defined low voltage.

[0217] The use of such a variable transformer 73 is moreover also an expedient option to equal out, i.e. to smooth, the phase cutting as generated by thyristors T1 and T2.

[0218] Variable transformer 73 supplies the voltages and currents described above for the electrodes 31 and 32 also described above, at its connections U and V. The connection denoted PE may be at ground potential E for the grounding of respective assemblies, for example the conduit element or conduit system which is also known as a channel.

[0219] The generator G mentioned above is essentially provided by the internal or external power supply network 70, fused contactor or protection switch 71, control circuit 72 and thyristors T1 and T2, and variable transformer 73.

[0220] If the power supply network 70 is in the form of an internal power supply network, it may also be operated at other RMS voltages and other basic frequencies ω.sub.0 other than the RMS voltage of 220 V given as an example and other than the alternating voltage with basic frequency ω.sub.0 of 50 Hz given as an example.

[0221] These basic frequencies ω.sub.0 can then correspond to the frequencies as shown in FIGS. 5 and 6, for example, in particular in the case of an internal power supply network.

[0222] FIG. 14 shows a scanning electron micrograph of an exemplary needle-shaped particle comprising at least one noble metal, which may also be referred to as a noble metal comprising needle. Here, this needle has a maximum lateral dimension of approx. 100 μm, and thus a size G.sub.p in the sense of the present disclosure of approx. 100 μm, and the aspect ratio of such needles is typically 100. This means that with a length of about 100 μm, the needle has a width and a depth of only about 1 μm. The scale 9 given in the lower part of FIG. 14 stands for a length of 60 μm.

[0223] FIG. 15 shows a further scanning electron microscope image of an exemplary particle comprising at least one noble metal with a size G.sub.p in the sense of the present disclosure of about 32 μm, which in comparison to the needle of FIG. 14 has a clearly smaller aspect ratio. Despite the deviation of the particle shape from an ideal circular or spherical shape, such particles are still referred to as spherical. The scale given in the lower part of FIG. 15 stands for a length of 10 μm.

LIST OF REFERENCE SYMBOLS

[0224] 1 Crucible [0225] 2 Molten glass [0226] 8 Number of noble metal comprising particles [0227] 9 Scale [0228] 31, 32 Electrodes [0229] 41, 42 Conductors [0230] 50 Noble metal comprising crucible [0231] 51 Molten glass [0232] 52 Reference electrode [0233] 53 Working electrode [0234] 54 Conductor [0235] 60 Conduit element as part of a conduit system [0236] 61 Tubular section of 60, made of a refractory material [0237] 62 Coating or lining of conduit element 60 comprising at least one noble metal [0238] 70 Internal or external power supply network, e.g. with 220 V RMS voltage and an exemplary basic frequency ω.sub.0 of 50 Hz of the alternating voltage [0239] 71 Fused contactor or protection switch [0240] 72 Control circuit for thyristors T1 and T2 [0241] 73 Variable transformer [0242] 81 Particle in the form of a needle comprising noble metal [0243] 82 Spherical particle comprising noble metal [0244] 101, 105 Measurement curves for a melting temperature of 1500° C. [0245] 102, 106 Measurement curves for a melting temperature of 1400° C. [0246] 103, 107 Measurement curves for a melting temperature of 1300° C. [0247] 104, 108 Measurement curves for a melting temperature of 1200° C. [0248] 701, 703, 705 Current measurement curves [0249] 702, 704, 706 Electrode potential measurement curves [0250] F Glass fill level during impedance measurement [0251] G Generator G.sub.p Size of noble metal comprising particle [0252] P Direction of currents I(ω) in molten glass 2 [0253] Sp1 Voltage jump in a full wave of U(ω) [0254] Sp2 Voltage jump in a full wave of U(ω) [0255] Sp3 Voltage jump in a full wave of U(ω) [0256] Sp4 Voltage jump in a full wave of U(ω) [0257] T1 Thyristor [0258] T2 Thyristor [0259] U1 First potential to which thyristors T1 and T2 are applied [0260] U2 Second potential to which thyristors T1 and T2 are applied [0261] U Connection of the variable transformer to electrode 31 [0262] OF0 Heating circuit of overflow 0 [0263] OF1 Heating circuit of overflow 1 [0264] OF2 Heating circuit of overflow 2 [0265] V Connection of the variable transformer to electrode 32 [0266] PE Connection to ground potential [0267] E Ground potential for grounding respective assemblies, e.g. the conduit element or conduit system, which is also referred to as a channel [0268] Vw.sub.1 Full wave of a substantially sinusoidal current I(ω) [0269] Hw.sub.1 First half-wave of a substantially non-sinusoidal current I(ω) [0270] Hw.sub.2 Second half-wave of a substantially non-sinusoidal current I(ω)