METHOD FOR MANUFACTURING A GLASS PRODUCT

Description

Field of invention

[0001] The present invention relates generally to a glass product and a method for its manufacture. Background of the invention

[0002] In glass manufacturing, particularly in the production of products made of or comprising glass, molten glass is transported from the melting furnace to the shaping area via a piping system. This piping system must be maintained at a specific temperature by means of appropriately designed heat-dissipating components to ensure that the appropriate temperature for the specific glass melt and shaping process is available at the relevant point. Consequently, the piping system typically requires heating, especially to provide the necessary viscosity for transporting the molten glass and to guarantee the required production reliability.

[0003] Examples include indirect heating systems using belt heaters or other types of radiant heaters, which indirectly maintain the temperature of the glass-carrying pipe system through a heat conduction process.

[0004] Direct heating by heating the walls of the glass-carrying pipe system through resistance heating is also known, in which Joule heat is usually transferred to the glass.

[0005] Australian patent AU 473 784 B discloses a method for producing flat glass in which the viscosity of hot-formable glass is adjusted by electrical heating before it is formed into a glass ribbon. An electric current is passed through the glass to control its temperature and flow. A disadvantage of such methods is that bubble formation and electrochemical reactions can occur.

[0006] DE 10 2016 107 577 A1 describes a device and a method for producing glass products from molten glass, in which the device has a crucible, such as a stirring crucible, and a component located therein, such as a rotatably mounted stirring element, for processing the molten glass, and wherein the device has an alternating current generator for heating the molten glass, which supplies current to the crucible or stirring crucible via electrical connection elements.

[0007] DE 10 2005 015 651 A1 discloses generally a method and a circuit arrangement for determining the impedance of an electrically heated glass melting tank, as well as the use of the method and the device for glass production. This publication also describes how the heating current is conducted through the glass itself. The impedance measurement is used to detect the energy consumption of heating electrodes or the palisade stones of the melting tank, and to determine whether a platinum-coated stirrer is exhibiting an eccentric stirring motion. Furthermore, it is intended to detect unwanted ground faults in or on the glass melting tank; to calculate the currents flowing between all electrodes of the glass melting tank; and to calculate or determine direct current paths that can cause unwanted blistering and corrosion.

[0008] International patent application WO 2020 / 023218 A1 describes a method for directly heating a metallic vessel in a glass manufacturing process. Several electrical heating circuits can be selected for heating, which, for example, have different phase angles.

[0009] JP 2018002539 A discloses a method and a device for producing a glass substrate. When transferring molten glass using a transfer tube, the glass is heated by heating the transfer tube using an alternating current with a frequency higher than 60 Hz.

[0010] JP H0920521 A discusses a method for preventing bubble formation in a glass melt and cracking in a conduit by standardizing the current density across the entire length of a four-way conduit and heating the entire length of the conduit. For this purpose, single-phase alternating currents with a phase difference of 60° are injected at various points along the four-way conduit, thus standardizing the current density across the entire length of the conduit.

[0011] JP 2017030987 A describes a device for heating molten glass, a device for glass production, and a method for producing a glass article. In the device for heating the glass, temperature differences in the molten glass are avoided in order to suppress heterogeneity in the molten glass.

[0012] CN102001825 A discloses an alkali-free glass for flat panel displays and a process for its manufacture. The glass comprises the following components in mass percent: 54–68 percent SiO₂, 10.8–17.1 percent Al₂O₃, 7.6–12.5 percent B₂O₃, 0.2–1.8 percent MgO, 4.2–15 percent CaO, 0.6–7.1 percent SrO, 0.1–5 percent BaO, 0.2–1 percent ZnO, 0.1–1.54 percent ZrO₂, and 0.1–1.3 percent SnO+SnO₂. The aluminum oxide borosilicate glass contains no arsenic or antimony, which can lead to environmental pollution.

[0013] Typically, when heating current-carrying conductor elements, the applied voltage and the current flowing through the conductor system are controlled to regulate the heat generated, as is, if necessary, the modulation of the alternating current. Modulation can be achieved by transformation or by pulse modulation with pulse groups, which can also be implemented using phase-angle control. In circuit design, this is usually realized using transformers, transducers, or thyristors.

[0014] The disadvantage of direct heating is generally that the presence of electrical power, precious metal and glass leads to electrochemical reactions, especially at the interface, which can result in glass defects in the product such as bubbles and / or metallic particles and / or a reduction in optical transparency.

[0015] The disadvantage of indirect heating is that the heat conduction process creates a time delay in the temperature control for the temperature of the glass in the piping system.

[0016] During the transport of molten glass from the melting furnace to the forming area, defects can occur during direct heating. These defects can result, for example, from interactions between the molten glass and the refractory materials. Typically, the molten glass is conveyed from the melting furnace to the forming area through a piping system made of or containing precious metals, such as platinum or platinum alloys. For example, platinum may be alloyed with rhodium, iridium, and / or gold, and / or may additionally include zirconium dioxide and / or yttrium oxide for fine-grain stabilization. The advantage of using components containing precious metals as piping materials is that these components are electrically conductive. This allows them to be electrically heated, preferably by passing an alternating current through the component, generating Joule heating.

[0017] However, it has been shown that interactions can occur between the components of the piping system containing precious metals and the molten glass during transport, particularly at the point of contact. These interactions manifest as defects such as bubbles or the introduction of particles, such as precious metal particles. This is detrimental because bubbles and / or particles can generally be problematic for the product being processed and can lead to increased rejects.

[0018] This is particularly critical for specialty glasses with specific, often very high, product quality requirements. Especially in the production of very thin glass products, i.e., so-called thin glass or thin glass sheets, only a very small number of defects is permissible. It is not only the absolute number of defects that is significant, but also their type and size, depending on the specific product requirements. For example, very small particles may just be acceptable, whereas larger particles, regardless of their number, can always lead to rejection.

[0019] There is therefore a need for glass products, especially thin glass or thin glass sheets, which contain only a few defects such as bubbles and / or particles. Furthermore, there is a need for a process for manufacturing such products. Object of the invention

[0020] The object of the invention is to provide a method for manufacturing a glass product which at least mitigates the weaknesses of known methods. Summary of the invention

[0021] The problem is solved by the subject matter of claim 1. Preferred and / or specific embodiments are found in the dependent claims. The description and drawings provide background information and explain the invention as defined in the claims.

[0022] A glass product, in particular a disc-shaped glass product, preferably with a thickness of at most 1100 µm and at least 15 µm, comprising a silicate glass, is described herein according to a first aspect outside the invention. The glass product has fewer than 4 particles per kilogram of glass comprising a material containing a precious metal, preferably fewer than 3 particles per kilogram of glass comprising a material comprising a precious metal, preferably with a particle size of less than 200 µm, wherein the particle size Gp is ​​understood to be the greatest distance in any spatial direction between constituents of the particle (atoms or molecules). Average diameters of particles can thus be smaller than their size defined above.

[0023] For the purposes of this disclosure, silicate glass is understood to be a non-metallic glass with a high SiO2 content, which, for example, comprises a SiO2 content of at least 50 wt.%, preferably at least 55 wt.% and preferably at most 87 wt.%.

[0024] A silicate glass melt is understood to be a glass melt which contains a silicate glass as defined in the preceding paragraph.

[0025] Glasses for the manufacture of the glass products disclosed herein include, for example, the groups of borosilicate (BS), aluminosilicate (AS) or boro-aluminosilicate glasses or lithium-aluminosilicate glass ceramics (LAS), which are mentioned here by way of example without losing the claim to generality.

[0026] The glass product according to one embodiment comprises a glass comprising at least 50 wt. % SiO 2 and preferably at most 87 wt. % SiO 2 .

[0027] According to one variant of the glass product, in addition to the component SiO 2, the glass also comprises the component Al 2 O 3, preferably up to a content of at most 25 wt.% and particularly preferably at least 3 wt.%, and may also include B 2 O 3 in the glass.

[0028] According to another variant of the glass product, in addition to the component SiO 2, the glass also comprises the component B 2 O 3, preferably to at least 5 wt.% and particularly preferably to at most 25 wt.%, and may also include Al 2 O 3 in the glass.

[0029] In particular, a Li-Al-Si glass with a Li 2 O content of 4.6 wt.% to 5.4 wt.% and a Na 2 O content of 8.1 wt.% to 9.7 wt.% and an Al 2 O 3 content of 16 wt.% to 20 wt.% can be used.

[0030] For example, a Li-Al-Si glass with a composition comprising Li 2 O 3.0 - 4.2; Al 2 O 3 19 - 23, SiO 2 60 - 69 wt.% as well as TiO 2 and ZrO 2 can be used as a glass that can be ceramicized to a glass-ceramic or also as green glass.

[0031] Borosilicate glass can be any glass containing the following components (in wt.%): SiO2 70 - 87 B2O3 7 - 25 Na₂O + K₂O 0,5 - 9 Al2O3 0 - 7 CaO 0 - 3 includes.

[0032] Borosilicate glass can also be made from glass with the following composition: SiO2 70-86 wt.% Al2O3 0-5 wt.% B2O3 9.0-25 wt.% Na₂O 0.5-5.0 wt.% K2O 0-1.0 wt.% Li 2 O 0-1.0 wt.%, or a glass, in particular an alkali borosilicate glass, may be used, which contains SiO2 78.3-81.0 wt.% B2O3 9.0-13.0 wt.% Al2O3 3.5-5.3 wt.% Na₂O 3.5-6.5 wt.% K2O 0.3-2.0 wt.% CaO 0.0-2.0 wt.% or also a glass, in particular an alkali borosilicate glass, comprising the following components in wt.%: SiO2 55 to 85 B2O3 3 to 20 Al2O3 0 to 15 Na₂O 3 to 15 K2O 3 to 15 ZnO 0 to 12 TiO2 0.5 to 10 CaO 0 to 0.1.

[0033] For example, an alkali-free alkaline earth silicate glass can be used with the following composition in wt.%: SiO2 58 to 65 B2O3 6 to 10.5 Al2O3 14 to 25 MgO 0 to 3 CaO 0 to 9 BaO 3 to 8 ZnO 0 to 2, where the sum of the content of MgO, CaO and BaO is characterized by being in the range of 8 to 18 wt.%.

[0034] A silicate glass for the production of the glass products disclosed herein may further comprise the following oxide-based components in wt.%: SiO2 50 to 65, preferably 55 to 65 Al2O3 15 to 20 B2O3 0 to 6 Li 2 O 0 to 6 Na₂O 8 to 16 K2O 0 to 5 MgO 0 to 5 CaO 0 to 7, preferably 0 to 1 ZnO 0 to 4, preferably 0 to 1 ZrO 2 0 to 4 TiO2 0 to 1, preferably essentially TiO2-free

[0035] The glass may also contain, at 0 to 1 wt%: P 2 O 5 , SrO , BaO; as well as fining agents at 0 to 1 wt%: SnO 2 , CeO 2 or As 2 O 3 or other fining agents, and possibly other components, for example fluorine.

[0036] According to a second aspect outside the invention, a glass product is hereby described, in particular a disc-shaped glass product, preferably with a thickness of at most 1100 µm and at least 15 µm, comprising a silicate glass, wherein the glass product has fewer than 3 bubbles per kilogram of glass, preferably with a bubble size of less than 200 µm, where the bubble size is understood to be the greatest distance within the bubble in any spatial direction. Average diameters of bubbles can therefore be smaller than their size defined above.

[0037] This is advantageous because particles and / or bubbles, especially particles containing precious metals, are glass defects that can lead to rejection. Whether a glass product containing a defect, such as a particle or bubble, is rejected or still acceptable for a specific application depends on both the defect frequency (usually specified per unit weight of glass) and the defect size. While defects above a certain size always lead to rejection, smaller defects may still be acceptable for a particular application, provided they are small enough and not too numerous.

[0038] Especially for specialty glasses, the requirements are constantly increasing. Therefore, there is a continuous need to provide glass products with very low defect rates, also to ensure continued economic viability, particularly in highly demanding product areas.

[0039] Such glass products with improved product quality, namely reduced particle and / or bubble frequency and / or only small glass defects, such as particles and / or bubbles, can be produced in a surprisingly simple way in a method for producing a glass product according to claim 1.

[0040] It has been shown that the type, quantity and / or size of the defects occurring can be influenced by the type of current flow in the component(s) containing the precious metal that are in contact with a molten glass.

[0041] Furthermore, it has proven advantageous that the method according to the present disclosure also makes it possible to dispense with components in the glass composition that are critical with regard to the stability and durability of a component comprising a precious metal.

[0042] Thus, it is advantageously possible with the method according to embodiments to melt glasses without using SnO₂ as a refining agent. In particular, it is possible, for example, to refine with table salt. Generally, without being limited to the aforementioned embodiments of a glass product, it is therefore possible for the glass product to comprise a glass containing at most 2500 ppm, preferably 2000 ppm, particularly preferably at most 1000 ppm, more preferably at most 500 ppm, and preferably even at most 100 ppm, of SnO₂, based on weight. In other words, the glass product can generally comprise a glass containing SnO₂ only in the form of unavoidable impurities. The glass product can further generally comprise a glass containing chloride, Cl⁻, preferably at least 100 ppm and up to 2500 ppm, based on weight.

[0043] Such a design of the glass product is advantageous because, in other words, the glass product in this way comprises a glass which can be melted with a gentler refining agent, which is significantly less aggressive to components containing precious metals and can thus also advantageously contribute to the reduction of particle formation and / or bubble formation.

[0044] Electrochemical reactions are generally dependent on the current density at the location of the reaction.

[0045] Accordingly, the invention discloses a method for manufacturing a glass product according to claim 1.

[0046] The piping system according to the invention preferably serves only for the transport and, if necessary, the tempering of the silicate glass melt during this transport, but not for further functions such as refining or homogenization.

[0047] For the purposes of this disclosure, a piping system comprising a precious metal is understood to mean that the piping system may consist, for example, predominantly (i.e., at least 50 wt.%), substantially (i.e., at least 90 wt.%), or even entirely of a precious metal or an alloy comprising at least one precious metal, for example, also a precious metal alloy. However, other configurations are also conceivable. For example, for the purposes of this disclosure, a piping system comprising a precious metal may also be configured such that the piping system has, for example, in a piping element, such as a tubular piping system, a coating arranged on its inner surface which comprises at least one precious metal.

[0048] In contrast to the state of the art, not only the time-averaged current densities are considered, but essentially all current densities flowing at any given time. This is surprising, and not documented in any publication, because pulse modulation has an influence on defect formation.

[0049] This is particularly surprising at first if the conducting system has an essentially tubular conducting element which has a coating of precious metal on its inside and in which the alternating current is essentially guided in the longitudinal direction of the tubular conducting element, because then it would also be assumed that the alternating current is completely guided in the precious metal and the space outside the precious metal is voltage-free, so that the shape of the voltage and current distribution should have only a minor influence on defects in the glass.

[0050] In a preferred embodiment, the alternating current is essentially sinusoidal and has only a single fundamental frequency ω 0 and essentially no other frequency components.

[0051] In preferred embodiments, the deviation of the time integral of the alternating current signal over a full wave from the time integral of an ideal sinusoidal pulse signal curve is less than 10%, preferably less than 5% and particularly preferably less than 2%.

[0052] In a further, particularly preferred method for producing a glass product, preferably a disc-shaped glass product, a silicate glass melt is transferred through a conductor system comprising a precious metal from one area of ​​an aggregate for producing a glass product to another area of ​​the aggregate for producing a glass product, and the conductor system comprising the precious metal is current-carrying in such a way that Joule heating is generated in the conductor system comprising the precious metal, in particular in the precious metal, by an electric current passed through the precious metal, wherein the phase angle θ 0 between current and voltage is measured at the fundamental frequency ω 0.

[0053] This measurement of the fundamental frequency ω 0, at which the phase angle θ 0 between current and voltage is measured, is preferably carried out at least once for each glass of a silicate glass melt used for the present disclosed process, specifically before or at the start of the process.Although it is in principle sufficient to measure the fundamental frequency ω 0 only at the value or infinitesimally close to the value at which the phase angle θ 0 between current and voltage as a function of the frequency ω 0 is at a local minimum, or to measure at those points where the phase angle θ 0 between current and voltage is less than ±10°, preferably less than ± 5° and particularly preferably less than ± 2°, it has nevertheless proven advantageous to measure or tune the fundamental frequency preferably in a range of about 4 * 10 -2< Hz to about 10 6< Hz in order to be able to identify the corresponding, previously mentioned ranges of the phase angle with higher process reliability.

[0054] From this, the angle θ is obtained for the respective glass at which the phase angle θ 0 between current and voltage as a function of frequency lies at a local minimum, at which the local derivative of the phase angle θ with respect to the frequency ω takes on the value zero, and those regions are also obtained within which the phase angle θ 0 between current and voltage is less than ±10°, preferably less than ± 5° and particularly preferably less than ± 2°.

[0055] The term "at least once the phase angle θ0 between current and voltage is measured for the silicate glass melt at the fundamental frequency ω0" also indicates that the measured values ​​of the phase angle θ between current and voltage as a function of the frequency ω are then available for every silicate glass melt used in the disclosed process. Provided the composition of the glass melt remains unchanged, this measurement can then be maintained for the fundamental frequency ω0 settings described below, and in particular for further iterations of the process, without the need to measure this phase angle θ0 again.

[0056] However, if the composition of the silicate melt is changed, for example, if its components are altered, the phase angle θ0 between current and voltage is preferably measured again at least once at the fundamental frequency ω0 for the silicate glass melt with the changed composition, as described above. The measured values ​​obtained in this way can then continue to be used as long as the changed composition of the silicate melt remains unchanged. A change in the composition of the glass of the silicate melt is defined as a change in composition in which at least one component of the glass of the silicate melt is altered by more than ±0.5 wt%.

[0057] Based on the measurements described above, the fundamental frequency ω 0 is set based on the measured phase angle θ 0 between current and voltage for the further execution of the procedure.

[0058] It is particularly preferred that the fundamental frequency ω 0 is set such that the phase angle θ 0 between current and voltage as a function of frequency lies at a local minimum at which the local derivative of the phase angle θ with respect to the frequency ω takes on the value zero.

[0059] In addition to this optimal and preferred setting, the phase angle θ₀ between current and voltage during the execution of the method is less than ±10°, preferably less than ±5°, and particularly preferably less than ±2°. In the context of the preceding statement, the term phase angle θ₀ means that the subscript "0" indicates that this phase angle θ₀ lies not only at the frequency for which the minimum of the derivative of the phase angle θ with respect to the frequency ω occurs, but also within the range of phase angles θ between current and voltage of less than ±10°, preferably less than ±5°, and particularly preferably less than ±2°. Accordingly, within the scope of this disclosure, these phase angles θ₀ are also referred to as minimized phase angles.

[0060] Similarly, the subscript "0" when specifying the frequency ω means that the frequency ω 0 is a frequency at which a minimized phase angle θ 0 exists in accordance with the above definition.

[0061] In the embodiments described herein, time-dependent, in particular periodic, voltages with a voltage curve U(ω) can also be used, which generates the alternating current used in the method disclosed herein, whose signal components have more than one discrete frequency ω, thus, for example, the discrete frequencies ω 1 , ω 2 , ω 3 , ... up to ω n, where n is a natural number not equal to zero, and where their overall voltage curve U(ω) resulting from the superposition of the individual voltage signals is as follows: U ω = U 1 ω 1 + U 2 ω 2 + U 3 ω 3 + … U n ω n

[0062] Here, U1 (ω1), U2 (ω2), U3 (ω3) ... Un (ωn) is each a voltage signal with a sinusoidal or cosinusoidal shape and the respective frequency ω1, ω2, ω3, ... ωn. Such signals can each be generated with a sine wave generator, superimposed accordingly, and then amplified as required depending on the application.

[0063] Even in the case of voltage curves with several discrete frequency components, the following condition, specified above for the fundamental frequency ω 0, is fulfilled for each of the discrete frequency components with ω 1 , ω 2 , ω 3 , ... to ω n, namely that for each of these frequency components with ω 1 , ω 2 , ω 3 , ... to ω n, the phase angle at the respective frequency θ 1 (ω 1 ), θ 2 (ω 2 ), θ 3 (ω 3 ), ... to θ n (ω n ) is less than ±10°, preferably less than ± 5° and particularly preferably less than ± 2°.

[0064] In a further embodiment, time-dependent, in particular periodic, voltages with a voltage curve U(ω), which generates the alternating current used in the present disclosed method, can also be used, the signal components of which have a continuous spectrum of sinusoidal or cosinusoidal signal components Ui(ω i ) with different frequency components ω i from the spectral range or frequency interval from ωx to ω y, wherein the following applies to the frequency ω i of each of these signal components: ωx < ω i < ω y where ωx represents the frequency at which there is a phase angle θ between current and voltage of -10°, and ωy represents the frequency at which there is a phase angle θ between current and voltage of +10°.

[0065] Signals with such frequency components can be generated, for example, with a noise generator that essentially provides white noise as an output voltage signal. This output voltage signal is then filtered with a bandpass filter whose passband allows frequencies within an interval of approximately ωx to approximately ωy to pass through. A signal obtained in this way can then be further amplified as required for the specific application.

[0066] In the glasses disclosed herein, however, for the temperature ranges of the silicate melt disclosed herein without limitation of generality, the fundamental frequency ω 0 is at least 5 * 10 2 Hz, preferably at least 1 * 10 3 Hz, and extends to a maximum of 1.5 * 10 4 Hz, preferably 2 * 10 4 Hz. Similarly, the frequencies ω 1 , ω 2 , ω 3 , ... up to ω n as well as ω i lie within the interval of at least 5 * 10 2 Hz, preferably at least 1 * 10 3 Hz, each up to a maximum of 1.5 * 10 4 Hz, preferably up to a maximum of 2 * 10 4 Hz.

[0067] It is preferred that further components of the voltage curve U(ω), which have frequency components that are smaller than ω x, have a time-averaged magnitude of these frequency components that is less than 15%, preferably less than 5%, and particularly preferably less than 3% of the time-averaged magnitude of the voltage curve U(ω).

[0068] It is also preferred that further components of the voltage curve U(ω), which have frequency components larger than ω y, such as harmonics, are less than 15%, preferably less than 5% and particularly preferably less than 3% of the time-averaged value of the magnitude of the voltage curve U(ω).

[0069] Surprisingly, it has been shown that with such a process, which is also referred to as process with minimized phase angle, glass products with significantly lower particle and / or bubble numbers are obtained than with conventional resistance heating of the precious metal-containing component.

[0070] The inventors do not know the cause of this effect. However, it is suspected that minimizing the phase angle allows the charge carriers in the precious metal component to follow the alternating current signal more effectively, or that the movement of positive charge carriers balances out the movement of negative charge carriers, resulting in lower stress on the precious metal component and consequently improving its mechanical stability. This then leads to the observed reduction in particle ingress into the glass product.

[0071] In the process disclosed herein, the temperature of the molten glass was between 1200°C and 1500°C. Under production conditions, temperatures of the molten glass between 1000°C and 1650°C are conceivable.

[0072] The method disclosed herein is used to produce, or to produce, a glass product, in particular a disc-shaped glass product, with a thickness of at most 1100 µm and at least 15 µm, comprising a silicate glass, wherein the glass product has fewer than four particles per kilogram of glass of a material comprising a precious metal, preferably fewer than three particles per kilogram of glass of a material comprising a precious metal, preferably with a particle size of less than 200 µm.

[0073] The method disclosed herein is used to produce, or to produce, a glass product, in particular a disc-shaped glass product, with a thickness of at most 1100 µm and at least 15 µm, comprising a silicate glass, wherein the glass product has fewer than 3 bubbles per kilogram of glass, preferably with a bubble size of less than 200 µm.

[0074] Within the scope of this disclosure, the following definitions apply: Within the scope of this disclosure, a precious metal is defined as a metal from the following list: platinum, rhodium, iridium, osmium, rhenium, ruthenium, palladium, gold, silver and alloys of these metals.

[0075] For the purposes of this disclosure, a component is referred to as comprising a precious metal if it comprises at least one metal from the above list to a significant extent, i.e., in a content exceeding unavoidable traces, in particular at least 0.1 wt.%, preferably at least 1 wt.%, and more preferably at least 5 wt.%. In particular, this also includes a component that consists predominantly (i.e., to more than 50 wt.%), substantially (i.e., to more than 90 wt.%), or even entirely of at least one precious metal, a mixture of precious metals, or an alloy of one or more precious metals. A typical alloy used is, for example, Ptlr1 and / or Ptlr5, i.e., a platinum alloy with 1 wt.% iridium or 5 wt.% iridium, respectively.

[0076] The glass melts of the present invention comprise oxide melts, in particular silicon-containing oxide melts, thus silicate melts.

[0077] For the purposes of this disclosure, "glass" is understood to mean an amorphous material obtainable by a melting process. "Glass product" is understood to mean a product (or article or article) that comprises the material glass, in particular consisting predominantly (i.e., more than 50% by weight), substantially (i.e., more than 90% by weight), or even entirely of glass.

[0078] For the purposes of this disclosure, a disk-shaped product is understood to be a product in which the lateral dimension in a first spatial direction of a Cartesian coordinate system is at least one order of magnitude smaller than in the two other spatial directions perpendicular to the first spatial direction. This first spatial direction can also be understood as the thickness of the product, and the two other spatial directions as the length and width of the product. In other words, the thickness of a disk-shaped product is at least one order of magnitude smaller than its length and width.

[0079] For the purposes of this disclosure, a bubble is understood to be a fluid-filled, usually gas-filled, cavity in a material and / or in a product. A bubble can be closed, i.e., enclosed in all directions by the material, for example, the material of a product consisting of that material, or open, for example, if the bubble is located at the edge of the product and is then not completely enclosed by the material forming or encompassed by the product.

[0080] For the purposes of this disclosure, a particle is understood to be, in particular, a particle made of, or at least comprising, a precious metal. Specifically, particles may comprise platinum or a platinum alloy, or be formed from platinum or a platinum alloy. The morphology of the particles can vary. For example, spherical particles are possible, i.e., particles that are at least approximately spherical, but needle-like or needle-shaped particles or rods are also possible. The dimensions of the particles can be up to 100 µm; typical dimensions are up to approximately 30 µm. As defined above, the dimensions specified in this disclosure refer to the maximum lateral dimension of the respective particle or bubble.For a needle-shaped particle, the specified size is therefore the length in the direction of the particle's longest extension.

[0081] A glass product manufacturing unit is understood to be a device in which the process steps customary for the manufacture of glass and glass products are carried out or can be carried out. These customary process steps include the feeding and melting of a batch, refining, conditioning, and hot forming. A section of such a unit is understood to be a part of the device in which specific process steps are carried out. These sections may be spatially separated from other sections of the device, such that, for example, transfer devices from one section of the device to another may be present. Such transfer devices, in which the molten glass is transferred from one section of the unit to another, are also referred to in this disclosure as a conduit element or conduit system.Such a conduit element or system can also be called a trough. Typical components of a glass production unit include, for example, the refining chamber or the working tank. Specifically, the glass production apparatus can comprise a melting tank, in which, for example, the batch is melted; a refining tank, in which the molten glass is purified; and a settling or working tank, in which conditioning takes place. Homogenization is generally carried out in a stirring section, in which the molten glass is homogenized by a stirring rod.

[0082] Such optimized process control with a minimized phase angle is possible, for example, via amplitude modulation. Typically, thyristor controllers are used to generate the alternating current for directly heating a conductor system carrying molten glass. If these are retained, a pulse signal waveform that is as sinusoidal as possible, or at least closely resembling a sinusoid, can be achieved with an additional circuit that smooths the phase angles to create a signal waveform that is at least partially sinusoidal.

[0083] The circuit can, for example, include an additional variable transformer on the primary side, in addition to the antiparallel thyristors. This makes it possible to reduce the primary-side voltage to the operating point as needed, so that further phase angles are minimal and the signal waveform no longer exhibits discontinuities, or at least only very minor ones, and is therefore significantly more sinusoidal.

[0084] In one embodiment of the method, it is further preferred if the harmonic content of the time-averaged magnitude of the pulse signal curve is less than 15%, preferably less than 5%, and particularly preferably less than 3%. Description of the drawings

[0085] The invention will be further explained below with reference to the drawings. They show Fig. 1 a schematic representation of an experimental setup, Figs. 2 and 3 photographic images of silicate glass melts from an experimental setup according to Fig. 1 Fig. 4 shows a schematic representation of another experimental setup for electrochemical impedance spectroscopy, and Fig. 5 shows an impedance spectrum from an experimental setup according to Fig. 4 , which shows the magnitude of the complex impedance Z as a function of the frequency ω, Fig. 6 an impedance spectrum from an experimental setup according to Fig. 4, which shows the phase angle Θ as a function of the frequency ω, Fig. 7 a substantially tubular conductor element of a conductor system, which has a coating comprising at least one noble metal on its inside, and in which an alternating current is passed through the noble metal by means of a generator G, Fig. 8 the image of an oscilloscope with a periodic voltage curve displayed thereon as a function of time, wherein this voltage curve shows a strong deviation from a sinusoidal shape, which is essentially caused by phase-angle control, Fig. 9 the image of an oscilloscope with a periodic voltage curve displayed thereon as a function of time, wherein this voltage curve shows only a very small deviation from a sinusoidal shape, Fig.10 the particulate entry into a glass melt under different forms of alternating current, which is used to heat a glass melt arranged in a conducting element with precious metal, and Fig. 11 a picture of an oscilloscope with a voltage curve shown on it to illustrate the current flow in the period T 1 in . Fig. 10 , Fig. 12 an image of an oscilloscope with a voltage curve shown on it to illustrate the current flow in the period T 3 in Fig. 10Fig. 13 a schematic diagram of an exemplary circuit arrangement, and Figs. 14 and 15 exemplary scanning electron microscope images of particles comprising precious metal. Fig. 16 another, essentially tubular, conductor element of a conductor system, which has a coating comprising at least one precious metal on its inside, and in which an alternating current is passed through the precious metal of each of three sections, which are represented as overflow 0 (ÜL0), overflow 1 (ÜL1), overflow 2 (ÜL2), by means of a generator G. Detailed description of the embodiments disclosed herein

[0086] Fig. 1Figure 1 shows a schematic and not-to-scale representation of an experimental setup for determining the influence of pulse modulation on the generation of alternating current I(ω) in a silicate glass melt. A silicate glass melt 2 is melted in a crucible made of a SiO₂-containing refractory material, for example, a so-called QUARZAL® crucible.

[0087] Two equally sized electrodes 31, 32, each containing a precious metal and measuring 0.5 cm² by 1 cm², were embedded in one half of the crucible 1. The crucible halves are connected by a glass melt bridge, meaning that the current I(ω) flowing between the electrodes 31, 32 is conducted entirely through the glass melt 2. Each electrode 31, 32 is, by way of example, made of a precious metal alloy, namely a platinum-rhodium alloy, also known as "PtRh10", i.e., 10 wt.% rhodium and 90 wt.% platinum. The glass melt 2 was a silicate glass melt.

[0088] The space surrounding the crucible 1 is purged with inert gas (here argon) to prevent a gas phase transport reaction with respect to the precious metal-containing electrodes 31, 32.

[0089] The crucible 1 is heated to a temperature of, for example, 1450°C in an oven.

[0090] The waveform of the current I(ω) flowing between the two electrodes 31, 32 was then varied by means of different modulators within the generator G, which represents an alternating current source, under the boundary condition that in each of the experiments a geometrically time-averaged current density of 25mA / cm 2< flows between the electrodes 31, 32.

[0091] The following three experiments, described in more detail below, were carried out, in which electrodes 31 and 32 were each exposed to modulation and glass melting contact for 24 hours.

[0092] After a waiting period of 24 hours, one of the electrodes 31, 32 was removed from the crucible half and rapidly frozen with the glass still attached. Photographic images of this are shown in Figs. 2a to 2c to see.

[0093] In Fig. 2It can be seen that the precious metal encompassed by the electrode and the structure of the respective electrodes change upon heating without current (Figure a of Fig. 2 ) and an at least approximately sinusoidal signal curve (Figure b of Fig. 2 ) show no changes in grain structure.

[0094] Figure 9 The image shows an example of an oscilloscope displaying a periodic voltage curve U(ω) as a function of time at a fundamental frequency ω₀. This voltage curve exhibits only a very slight deviation from a sinusoidal shape and represents the form of the alternating current I(ω). An example sinusoidal full wave with the interval Vw₁ is marked. The fundamental frequency ω₀ was, for example, 50 Hz.

[0095] When using phase control in the generation of alternating current I(ω) by means of, for example, a thyristor (Figure c of Fig. 2However, a clear change in the reflection properties of the coarse-grained precious metal crystals can be observed, suggesting that a chemical reaction has taken place.

[0096] Figure 8 Figure 1 shows an example of an oscilloscope displaying a periodic voltage curve U(ω) as a function of time at a fundamental frequency ω₀. This voltage curve exhibits a significant deviation from a sinusoidal waveform, primarily due to phase-angle control, and represents the shape of the alternating current I(ω) used. The fundamental frequency ω₀ was, for example, 50 Hz. An example of a first non-sinusoidal half-wave generated by phase-angle control is marked with the interval Hw₁, and a second non-sinusoidal half-wave generated by phase-angle control is marked with the interval Hw₂.

[0097] Following the annealing of the entire crucible 1, the glass body of the crucible half, from which the corresponding electrode had previously been removed, was drilled out and the base polished. The transmitted light images of the samples are in Figs. 3a to 3c depicted.

[0098] It is clearly visible that with a currentless signal curve (Figure a of Fig. 3 ) no bubbles are visible, and the signal curve is at least approximately sinusoidal (Figure b of Fig. 3 ) very few bubbles have formed.

[0099] However, when using phase control via a thyristor (Figure c of Fig. 3 ) not only can a distinct formation of bubbles be observed, but also a darkening of the glass around the formed bubbles, which is due to the formation of precious metal particles.

[0100] In their further processes, the inventors used electrochemical impedance spectroscopy to be able to identify the properties of the respective glass used in more detail.

[0101] A schematic experimental setup for electrochemical impedance spectroscopy is Fig. 4 to be extracted. Glass was melted in a platinum crucible 50 with a diameter of approximately 10 cm, with the fill level F of the silicate glass melt 51 being approximately 10 cm. The crucible 51 was kept at temperature in a furnace, and the electrode to be examined, in this case a rectangular platinum electrode 53 measuring approximately 2 x 4 cm, was inserted into the glass melt 51.

[0102] Both the crucible 51 and the electrodes 52, 53 are electrically responsive via a platinum wire 54. Furthermore, an O₂|Pt|ZrO₂ reference electrode 52 (which is purged with 1 bar of O₂ as a reference) was introduced into the glass melt 51 to provide an independent reference potential for the electrochemical measurements.

[0103] The electrochemical impedance spectrometer was connected in the following configuration: The working electrode 53 is the platinum electrode under test, the reference electrode 52 is the inserted O 2 |Pt|ZrO 2 reference electrode, the counter electrode is the crucible 51.

[0104] The impedance spectra were recorded using potentiostatic electrochemical impedance spectroscopy and an excitation potential of 25mV was chosen.

[0105] The following impedance spectra were recorded from a glass melt 51 of a composition corresponding to glass AS87 at frequencies from 10 6< Hz to 5*10 -3< Hz at the melting temperatures 1200°C, 1300°C, 1400°C, 1500°C.

[0106] For illustrative purposes only, the current generated here is denoted as I(ω) and the resulting voltage as U(ω). The complex impedance is also frequency-dependent and is given by Z(ω) = U(ω) / I(ω), the magnitude of which |Z| for different temperatures is shown in the impedance spectrogram of the Figure 5 shown.

[0107] The phase angle Θ(ω) between current I(ω) and voltage U(ω), shown here as a frequency dependency, which in Figure 6 The value denoted by "theta" showed a clear frequency dependence with a pronounced minimum, the procedural use of which will be described in more detail below.

[0108] These experiments aim to replicate an arrangement like the one in Figure 7 The process is depicted, simulated, and in particular the interaction of the precious metal, especially a precious metal-containing conduit system, with the silicate melt is investigated.

[0109] It has surprisingly turned out that those involved in the Figure 1 and 4 The test results obtained with the arrangements shown were also essentially transferable to other embodiments, such as those in Figure 7The illustrated embodiment, in which essentially no current is passed directly through the silicate melt or glass melt 2, but rather is guided essentially within the area encompassing the precious metal, thus within the coating or layer 62 described in more detail below. Although this positive effect does not yet appear to be fully understood, one reason for the transferability of the present results may lie in the skin effect of an alternating current in a conductor, where higher current densities occur on the surface of a conductor than in its interior, since the conductor attempts to remain field- and voltage-free in its interior. These higher current densities occurring on the surface of the respective conductor are therefore in direct contact with the glass melt 2, which borders the conductor 62.

[0110] In Figure 7Figure 60 shows a substantially tubular conduit element of a conduit system for transporting molten glass. This conduit system can, for example, extend between a melting unit and a hot forming device.

[0111] The conductor element 60 comprises a tubular section 61 made of a fire-resistant refractory material and has on its inside a coating 62 comprising at least one precious metal or a covering 62 with precious metal.

[0112] As mentioned above, this precious metal can include, for example, platinum or alloys containing platinum. For example, platinum can be alloyed with rhodium, iridium, and gold, and / or may additionally include zirconium dioxide and / or yttrium oxide for fine-grain stabilization. Using generator G, the alternating current I(ω) is passed through the precious metal, generating the alternating voltage U(ω) at the generator, as described in the... Figures 8 and 9 is shown.

[0113] Here, the fundamental frequency ω 0 was set based on the phase angle θ 0 between current and voltage.

[0114] In particular, the fundamental frequency ω 0 was set such that the phase angle θ 0 between current and voltage as a function of the frequency ω lies at a local minimum at which the local derivative of the phase angle θ with respect to the frequency ω takes on the value zero.

[0115] Such a minimum is exemplified by the representation from Figure 6 to be taken from the value of the frequency ω 0.

[0116] Depending on the method, this was not sharply localized with a pronounced peak, but rather within a range with a low slope. For the embodiments disclosed here, an angular range with this low slope has also proven advantageous, in which the phase angle θ 0 between current and voltage is less than ±10°, preferably less than ± 5° and particularly preferably less than ± 2°.

[0117] In general, the fundamental frequency ω 0 was preferred, as shown, for example, in the representation from Figure 6It can be seen that, with a phase angle θ 0 between current and voltage of less than ±10° for the glasses disclosed herein in a temperature range of 1000 °C to 1650 °C at a frequency of at least approximately 2 * 10 2< Hz to 5 * 10 2< Hz with a phase angle θ 0 between current and voltage of -10°, corresponding to ω x , and extended to a maximum of approximately 1.5 * 10 4< Hz to 2 * 10 4< Hz with a phase angle θ 0 between current and voltage of +10°, corresponding to ω y .

[0118] Although the in Figure 7 Since the arrangement shown essentially only exhibits currents I(ω) which flow within the glass melt 2 in the direction of arrow P, it becomes apparent, as already explained above, that the experimentally compared with the in Figure 1 The results obtained in the presented arrangement are applied to the in Figure 7The depicted conductor element 60 was surprisingly well transferable and a strong reduction in both bubble formation and particulate entry could be achieved through the method with minimized phase angle.

[0119] In Fig. 5 and Figure 6 The results of the impedance spectroscopy are presented in two diagrams. Fig. 5 The graph shows the magnitude of the complex impedance Z 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.

[0120] It is clearly evident that the magnitude of the impedance, which is temperature-dependent, passes through a minimum at frequencies between approximately at least 2 * 10 2< Hz and 5 * 10 2< Hz, and at most approximately 1.5 * 10 4< Hz and 2 * 10 4< Hz.

[0121] In Figure 6The phase angle θ is plotted as a function of frequency. Curve 105 was measured for the same type of 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 is evident that at these temperatures, the phase angle exhibits a minimum at frequencies of at least 5 × 10² Hz to at most 2 × 10⁴ Hz, i.e., very small values ​​of at most ±10°, for example, at most ±5° or even at most ±2°.

[0122] The results obtainable with the method according to the invention are merely examples. Figure 10 depicted.

[0123] Figure 10shows the results of the production of an alkali-free alkaline earth silicate glass, whose exemplary composition has already been given above, on an exemplary apparatus for the manufacture of glass products, which is also referred to as a tank for short.

[0124] On this tank, between a lauter tube and a crucible of the device that is upstream of or part of the hot forming process, there exists a connection to a transfer tube, which in Figure 7 and in a further embodiment in Figure 16 The illustrated conduit element 60. This conduit element 60 was originally heated via three heating circuits, which are designated as overflow 0 (ÜL0), overflow 1 (ÜL1), overflow 2 (ÜL2). Figure 16 Although the diagram shows exemplary heating circuits of overflow 0 (ÜL0), overflow 1 (ÜL1) and overflow 2 (ÜL2) arranged one after the other, these can also be connected in parallel to the circuit shown in the diagram. Figure 7as shown in the illustrated embodiment.

[0125] Initially, all 3 heating circuits were operated via transformers with a 10 V tap, as essentially shown in the diagram. Figure 7 corresponds, whereby in Figure 7 For the sake of clarity, only one heating circuit is shown as an example, which provides the voltage U(ω) and the current I(ω) by means of the generator G. This matter is explained in more detail in Figure 16 shown.

[0126] The heating circuits are represented in their effect by the corresponding measurement curves 701, 703, 705 of the current, where measurement curve 701 is assigned to overflow 2, measurement curve 703 to overflow 1 and measurement curve 705 to overflow 0, as well as the measurement curves 702, 704, 706 for the electrode potential E (plotted as voltage U), where measurement curve 702 is assigned to overflow 2, measurement curve 704 to overflow 1 and measurement curve 706 to overflow 0.

[0127] Another example is the number 8 of precious metal particles that were added to the glass melt during this period, shown in the form of square symbols, not all of which are labeled for the sake of clarity.

[0128] Now, three different states can be described: period T 1, which was about six and a half days.

[0129] All three heating circuits were operated via a transformer with a 10 V tap.

[0130] The heating circuit ÜL0 was operated with a voltage with an RMS value of approximately 8.2 V, with a current with an RMS value of approximately 1700 A and a phase angle which was relatively low, but still generated harmonics with frequencies above ω y.

[0131] Heating circuit ÜL1 was operated with an RMS voltage of approximately 2.9 V, a current of approximately 700 A, and a strong phase angle. Heating circuit ÜL2 was operated with an RMS voltage of approximately 3.1 V, a current of approximately 500 A, and a strong phase angle, each generating harmonics with frequencies above ωy.

[0132] In Fig. 12 The image shows an oscilloscope displaying a voltage curve for overflow 1 during period T3. The phase angle is relatively small here.

[0133] In Fig. 11The image shows an oscilloscope displaying a voltage curve for overflow 1 during time period T1. The phase angle is pronounced here, exhibiting a high proportion of frequencies above ωy. These frequencies arise within each full cycle of U(ω) at the points shown in Fig. 11 Clearly recognizable and pronounced voltage jumps Sp1, Sp2, Sp3 and Sp4. It has also been shown that exceeding the preferred frequencies, i.e., ω y, had more detrimental effects than falling below them.

[0134] In the above procedure, the average number of precious metal particles, especially platinum particles, introduced into the glass melt 2 was approximately 7.0 particles per kg.

[0135] Period T 2, which lasted approximately 15 days and followed period T 1.

[0136] Heating circuit ÜL1 and heating circuit ÜL2 were combined, resulting in a new heating circuit (ÜL1).

[0137] Both heating circuits were operated via a transformer with a tap providing an RMS voltage of 10 V.

[0138] The heating circuit ÜL0 was operated with a voltage with an RMS value of approximately 8.2 V, with a current with an RMS value of approximately 1650 A and a phase angle that was relatively low.

[0139] The heating circuit ÜL1 was operated with a voltage of approximately 4.7 V RMS, a current of approximately 640 A RMS, and a [missing information] compared to the representation in Figure 11 operated with reduced phase angle.

[0140] In the process described above, the average number of precious metal particles, especially platinum particles, introduced into the glass melt 2 was approximately 3.8 particles per kg.

[0141] The period T3, which lasted approximately nine and a half days, followed the period T2. The heating circuit ÜL0 was operated via a variable transformer with a tap at an RMS voltage of 8 V.

[0142] The heating circuit ÜL1 was operated via a transformer with a tap at an RMS voltage of 10 V.

[0143] The heating circuit ÜL0 was operated with a voltage with an RMS value of approximately 7.6 V, with a current with an RMS value of approximately 1550 A and a phase angle that was optimized as much as possible, meaning smoothed.

[0144] The overflow ÜL1 was operated with an RMS voltage of approximately 4.7 V, a current of approximately 640 A, and a phase angle that differs from the representation in Figure 11 was reduced, operated.

[0145] The fact that the RMS values ​​of the voltage in the operation described above were lower than the RMS values ​​of the tap voltage during the periods T 1 to T 3 represents the normal case of a current-loaded transformer, which can show a decrease in the RMS value of the voltage as the RMS value of the current increases.

[0146] In Fig. 12 The image shows an oscilloscope displaying a voltage curve for overflow 1 during period T3. As can be seen, the phase angle here is different compared to the one in Fig. 11 The voltage curve shown is significantly reduced, as already mentioned above for voltage curves with reduced phase control.

[0147] In the process described above, the average number of precious metal particles, especially platinum particles, introduced into the glass melt 2 was approximately 2.5 particles per kg.

[0148] These examples show that a reduced influence of the phase angle and a more sinusoidal alternating current I(ω) leads to a minimization of the particulate entry into the melt 2.

[0149] Fig. 13Figure 1 shows a highly simplified schematic diagram of an exemplary circuit arrangement. Lines L1, L2, L3, and N carry the phases of a power supply network 70, which can be part of either an internal or an external power supply network. This power supply network 70 can, for example, provide an alternating voltage with an RMS voltage of 230 V between any two lines with phases L1, L2, and L3, at a network frequency of 50 Hz or higher in the case of an internal power supply network. Even with this arrangement, in which the fundamental frequency ω₀ was not yet optimally chosen, it could be shown that avoiding harmonics with frequencies ω₀ that lie outside, and especially above, the preferred frequency range had a positive effect in terms of the stated objective.

[0150] By means of a contactor or circuit protection device 71 equipped with fuses, the lines with phases L1 and L3 are supplied to the further circuit, which is described in more detail below.

[0151] With the contactor 71 closed, phase L3 is supplied to a parallel circuit comprising the thyristors T1 and T2, wherein the thyristors T1 and T2 are controlled in a defined manner by a control circuit 72, in particular each is triggered.

[0152] The thyristor T1 and T2 are usually located between the potentials designated U1 and U2 in order to generate the phase angle and, together with the phase L3, which has the phase angle, and the phase L1, to feed the variable transformer 73.

[0153] The variable transformer 73 is designed to transform the voltage generated by the thyristors T1 and T2 to a defined low voltage using phase control.

[0154] Furthermore, the use of such a variable transformer 73 is also a useful way to equalize, i.e. smooth, the phase angle generated by the thyristors T1 and T2.

[0155] The variable transformer 73 supplies the voltages and currents described above at its terminals U and V for the electrodes 31 and 32 also described above. The terminal marked PE can include an earth potential E for grounding corresponding assemblies, for example the conductor element or conductor system, which is also referred to as a trough.

[0156] The internal or external supply network 70, the fuse-equipped contactor or switching contactor 71, the control circuit 72, as well as the thyristors T1 and T2 and the variable transformer 73, essentially provide the aforementioned generator G.

[0157] If the supply network 70 is designed as an internal supply network, it can be operated with a fundamental frequency w 0 of 50 Hz or with other effective voltages and other fundamental frequencies ω 0 instead of the exemplary RMS voltage specified as 220 V and the exemplary AC voltage specified.

[0158] These fundamental frequencies ω 0 can then, for example, be used in an internal supply network to... Figures 5 and 6 The frequencies shown correspond to those shown.

[0159] Fig. 14Figure 1 shows a scanning electron microscope image of an exemplary needle-shaped particle comprising at least one precious metal, which can also be described as a precious metal-comprising needle. This needle has a maximum lateral dimension of approximately 100 µm, and thus a size Gp, as defined in the present disclosure, of approximately 100 µm, with the aspect ratio of such needles typically being 100. This means that the needle, with a length of approximately 100 µm, has a width and depth of only approximately 1 µm. The lower region of Fig. 14 Scale bar 9 shown represents a length of 60 µm.

[0160] Fig. 15 shows another scanning electron microscope image of an exemplary particle comprising at least one precious metal with a size Gp of approximately 32 µm as defined in the present disclosure, which, compared to the needle of the Fig. 14exhibits a significantly low aspect ratio. Despite the deviation of the particle shape from an ideal circle or sphere, such particles are still described as spherical. The lower range of Fig. 15 The scale shown represents a length of 10 µm. Reference symbol list

[0161] 1 Crucible 2 Molten glass 8 Number of particles containing precious metal 9 Scale 31, 32 Electrodes 41, 42 Arresters 50 Crucible containing precious metal 51 Molten glass 52 Reference electrode 53 Working electrode 54 Arrester 60 Conductor element as part of a conduction system 61 Tubular section made of a refractory refractory material 60 62 Coating or covering of the conductor element containing at least one precious metal 60 70 Internal or external power supply network, for example with 220 V RMS voltage and an example fundamental frequency ω 0 of the AC voltage of 50 Hz 71 Contactor or switching contactor equipped with fuses 72 Control circuit of the thyristors T1 and T2 73 Variable transformer 81 Needle-shaped particle containing precious metal 82 Spherical particle containing precious metal 101, 105 Measurement curves Measurement curves at 1500°C melting point 102, 106; at 1400°C melting point 103, 107; at 1300°C melting point 104.108 Measurement curves at 1200°C melting temperature 701, 703, 705 Current measurement curves 702, 704, 706 Electrode potential measurement curves F Glass fill level during impedance measurement G Generator G p Size of the precious metal particle PR Direction of currents I(ω) within the glass melt 2 Sp1 Voltage step within a full wave of U(ω) Sp2 Voltage step within a full wave of U(ω) Sp3 Voltage step within a full wave of U(ω) Sp4 Voltage step within a full wave of U(ω) T1 Thyristor T2 Thyristor U1 First potential at which thyristors T1 and T2 are connected U2 Second potential at which thyristors T1 and T2 are connected U Connection of the variable transformer to the electrode 31 ÜL0 Overflow heating circuit 0 ÜL1 Heating circuit of overflow 1 ÜL2 Heating circuit of overflow 2 V Connection of the control transformer to the electrode 32 PE Connection with earth potential E Earth potential for grounding corresponding assemblies, for example the conductor element or conductor system,which is also called a channel Vw 1 full wave of an essentially sinusoidal current I(ω) Hw 1 first half wave of an essentially non-sinusoidal current I(ω) Hw 2 second half wave of an essentially non-sinusoidal current I(ω),

Claims

1. A method for making a glass product, preferably a sheet-like glass product, wherein a silicate glass melt is transferred through a precious metal-comprising conveyance system from one area of an aggregate for making a glass product to another area of the aggregate for making a glass product, and wherein the precious metal-comprising conveyance system is current-carrying in a way so that an electric current passed through the precious metal generates Joule heating in said precious metal-comprising conveyance system, in particular in the precious metal, characterized in that at the fundamental frequency ω0, the phase angle θ0 between current and voltage is measured, preferably at least once, wherein the fundamental frequency ω0 is adjusted based on the phase angle θ0 between current and voltage, wherein the phase angle θ0 between current and voltage is less than ± 10°, preferably less than ± 5°, and most preferably less than ± 2°.

2. The method for making a glass product according to the preceding claim, wherein the fundamental frequency ω0 is adjusted such that the phase angle θ0 between current and voltage, as a function of frequency, is at a local minimum where the local derivative of the phase angle θ0 with respect to frequency ω assumes the value zero.

3. The method for making a glass product according to any one of the preceding claims, wherein the time-dependent course of the voltage curve U(ω) that generates the alternating current I(ω) comprises signal components having more than one discrete frequency ω, thus preferably comprising the discrete frequencies ω1, ω2, ω3, ... up to ωn, where n is a non-zero natural number, and wherein the total voltage curve U(ω) resulting from the superposition of the individual signal components is obtained as follows: U ω = U 1 ω 1 + U 2 ω 2 + U 3 ω 3 + … U n ω n , where each of U1(ω1), U2(ω2), U3(ω3) ... Un(ωn) is a voltage signal having a sinusoidal or cosinusoidal waveform with the respective frequency ω1, ω2, ω3, ... ωn, wherein, preferably, for each of the discrete frequency components with ω1, ω2, ω3, ... up to ωn, the condition is satisfied that for each of these frequency components with ω1, ω2, ω3, ... up to ωn the phase angle between current and voltage at the respective frequency, θ1(ω1), θ2(ω2), θ3(ω3) , ... up to θn(ωn), is less than ±10°, preferably less than ±5° and particularly preferably less than ±2°.

4. The method for making a glass product according to any one of the preceding claims, wherein the time-dependent course of the voltage waveform U(ω) that generates the alternating current I(ω) comprises signal components having a continuous spectrum of sinusoidal or cosinusoidal signal components Ui(ωi) with different frequencies ωi from the spectral range or frequency interval from ωx to ωy, wherein, for the frequency ωi of each of these signal components, the following applies: ω x < ω i < ω y wherein ωx represents the frequency at which a phase angle θ between current and voltage is -10°, and ωy represents the frequency at which a phase angle θ between current and voltage is +10°.

5. The method according to any one of the preceding claims, wherein the fundamental frequency ω0 is at least 2 * 102 Hz, preferably 5 * 102 Hz, up to at most 2 * 104, preferably 1.5 * 104 Hz.

6. The method for making a glass product according to any one of the preceding claims, wherein the current is an alternating current I(ω) in which the time integral over a positive and a negative half-wave essentially yields the value zero.

7. The method for making a glass product according to claim 6, wherein the conveyance system comprises a substantially tubular conduit element which has a precious metal-comprising coating on its inner surface, and wherein the alternating current is carried substantially longitudinally of the tubular conduit element.

8. The method for making a glass product according to claim 6 or 7, wherein the alternating current I(ω) is substantially sinusoidal and has a fundamental frequency ω0.

9. The method for making a glass product according to claim 6, 7 or 8, wherein the time integral of the alternating current signal over a full wave deviates from the time integral of an ideal sinusoidal pulse signal waveform by less than 10 %, preferably less than 5 %, and particularly preferably less than 2 %.

10. The method according to any one of the preceding claims, wherein the temperature of the glass melt is between 1000 °C and 1650 °C.