Glass, glass producing method and glass melting furnace

Abstract

To provide a glass element with a reduced number of inclusions in a micrometer range, a method of producing the same, a glass melting furnace therefor, and an operation method thereof.SOLUTION: Provided is a glass element, preferably a glass element made of borosilicate glass, the glass element containing no more than 50 inclusions of sizes ranging from 2 μm to 10 μm per kilogram of glass. In a method of producing the same, during a step of heating a raw material mixture to form a glass melt and / or a step of conditioning the glass melt, at least one of the following conditions is satisfied: (a) applying an alternating current with a frequency of 1 kHz to 200 kHz to an electrode to electrically resistively heat the glass melt; (b) a contact surface in contact with the glass melt includes 30% or more of a contact material in the form of a melt-cast zirconium oxide material, with a ratio of ZrO2 exceeding 70 wt.%; and (c) extracting 1% to 50% of a volume flow rate of a bottom material of the glass melt in a bottom region of the glass melt.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a glass element, preferably a glass element made of borosilicate glass.

[0002] The present invention further relates to a method for manufacturing a glass element, preferably a glass element made of borosilicate glass.

[0003] The present invention further relates to a glass melting furnace, a method for operating the glass melting furnace, and a glass element manufactured using the method and / or the glass melting furnace, preferably made of borosilicate glass, preferably in the form of a borosilicate sheet glass.

[0004] The present invention is generally applicable to all glass elements, and will be described below with respect to glass elements in the form of sheet glass.

[0005] A glass melting furnace for manufacturing glass generally includes a melting or heating section that forms a glass melt from raw materials called a so-called glass batch, a subsequent clarification section that clarifies the glass melt to remove residual bubbles that still remain after the melting or heating process from the glass melt, and a downstream conditioning section that serves to further condition the clarified glass melt. The temperature required for the entire melting process greatly depends on the type of glass. For example, soda-lime glass used for manufacturing window glass and glass containers is melted at a considerably lower temperature compared to special glass for displays and glass ceramics. The conditioning section of such a glass melting furnace can be designed as a working tank or as a flow path system or a distribution device system. Usually, refractory materials are used for the wall material of the glass melting furnace, and many of them are made of aluminum-zirconium-silicate materials. However, it is known that when such materials are used as the wall material or the melting contact material, bubbles and / or veins may occur in the glass melt, which can ultimately result in defective glass products.

[0006] In addition, for example, further defects such as particles peeled off from the wall material of the melting tank also cause corresponding inclusions in the micrometer range in, for example, float borosilicate glass, and this glass becomes unsuitable for various uses. In samples of sheet glass made of float borosilicate glass manufactured by known methods, there are many obstacles such as metallic and non-metallic inclusions and bubbles with a wide size distribution. Inclusions with a size exceeding 50 μm can be sorted by conventional optical detection methods. On the other hand, inclusions with a size of 50 μm or less are very difficult to detect, and usually, only a trained person can identify, quantify, and judge them with great effort.

[0007] Therefore, an object of the present invention is to provide a glass element, preferably sheet glass, with a reduced number of inclusions in the micrometer range, and to present a method for manufacturing the glass element, preferably sheet glass, a glass melting furnace, and a method for operating the glass melting furnace that significantly reduces the number of inclusions in the micrometer range.

[0008] A further object of the present invention is to present an alternative glass element, preferably sheet glass, an alternative method for manufacturing the glass element, preferably sheet glass, an alternative glass melting furnace, and an alternative method for operating the glass melting furnace.

[0009] The present invention is a glass element, preferably a glass element made of borosilicate glass, wherein the glass element solves the above problems by having 50 or fewer inclusions with a size of 2 μm or more and 10 μm or less per 1 kg of glass.

[0010] The present invention is a method for manufacturing a glass element, preferably a glass element made of borosilicate glass as described in any one of claims 1 to 12, and the method comprises: i) In weight% units, as follows: SiO 2 60 to 90%, preferably 76% to 90%; B 2 O 3 0 to 20%; Al 2 O3 0 to 20%; Li 2 O 0 to 10%; Na 2 O 0 to 10%; K 2 O 0 to 10%; MgO 0 to 10%; CaO 0 to 10%; SrO 0 to 10%; and BaO 0 to 10%; providing a mixture containing; ii) heating the mixture to form a glass melt; iii) conditioning the glass melt; iv) cooling the glass melt to provide a glass element including, wherein during step ii) and / or iii), the following conditions: a) applying a heating current to the electrodes to at least partially heat and / or warm the glass melt by electrical resistance heating, and using an alternating current with a frequency of 1 kHz to 200 kHz as the heating current; b) the contact surface in contact with the glass melt contains at least 30% of a contact material in the form of a fused-cast zirconia material with a ZrO 2 proportion of more than 70% by weight, preferably a fused-cast zirconia material with a ZrO 2 proportion of more than 85% by weight; and c) withdrawing 1% to 50% of the volume flow rate from the bottom material of the glass melt in the bottom region of the glass melt at least one, preferably all, of which are satisfied to solve the above problems.

[0011] The present invention is a glass melting furnace for implementing the method according to any one of claims 13 to 17, wherein the glass melting furnace comprises a heating unit for heating the mixture to form a glass melt; a conditioning unit for conditioning the glass melt; A bottom drawing unit for drawing out the bottom material of the glass melt, and comprising a heating unit and / or a conditioning unit, the heating unit having at least two electrodes for resistance heating, A) The heating unit is designed or controlled such that heating current in the form of alternating current with a frequency of 1 kHz to 200 kHz is applied to at least two electrodes, and / or B) The contact surface in contact with the glass melt contains more than 30% of the contact material, and the contact material is a fused-cast zirconia material with a ZrO 2 proportion of more than 70% by weight, preferably a fused-cast zirconia material with a ZrO 2 proportion of more than 85% by weight, and / or C) The bottom drawing unit is designed to draw 1% to 50% of the volume flow rate from the bottom material of the glass melt in the bottom region of the heating unit and / or the conditioning unit. The glass melting furnace solves the above problems.

[0012] The present invention also relates to an operating method of the glass melting furnace according to claim 16, comprising the following: I) A step of applying heating current to the electrodes to operate the heating unit, using alternating current with a frequency of 1 kHz to 200 kHz as the heating current; II) A step of bringing the glass melt into contact with the contact surface, the contact surface being in contact with the glass melt and containing more than 30% of a fused-cast zirconia material with a ZrO 2 proportion of more than 70% by weight, preferably a fused-cast zirconia material with a ZrO 2 proportion of more than 85% by weight, and III) A step of operating the bottom drawing unit so that 1% to 50% of the volume flow rate is drawn from the bottom material of the glass melt in the bottom region. The above problems are solved by a method of implementing at least one, preferably all, of the above steps.

[0013] Furthermore, the present invention provides a glass element produced by using the method according to any one of claims 13 to 17 and / or the glass melting furnace according to claim 18, preferably according to any one of claims 1 to 12, preferably made of borosilicate glass, preferably in the form of borosilicate sheet glass, which solves the above problems.

[0014] In this specification, the term zirconium oxide means ZrO 2 and means.

[0015] In this specification, unless otherwise specified, % is equal to wt%.

[0016] One of the advantages obtained thereby is that inclusions, namely metallic inclusions, non-metallic inclusions, and bubbles can be significantly reduced. Preferably, when steps ii) to iii) are carried out and all conditions a) to c) or A) to C) are satisfied, or when steps I) to III) are carried out, in addition to the number of metallic inclusions, the number of non-metallic inclusions, particularly the recrystallized products of corrosion products melted in refractories and glass, can also be significantly reduced. Here, according to the present invention, it has been found that preferably, when condition a) or A) is satisfied, or when step I) is carried out, the number of metallic inclusions is reduced. Below the frequency range according to the present invention, surprisingly, the number of metallic inclusions increases, while an increase in frequency beyond the aforementioned frequency range has been found to be extremely complicated and thus can only be carried out at high cost. When condition b) or B) is satisfied, or when step II) is carried out, according to the present invention, it has been found that this makes it possible to reduce the non-metallic inclusions of the refractory. When condition c) or C) is satisfied, or when step III) is carried out, according to the present invention, it has been found that the inclusions of the recrystallized products of the corrosion products melted in the glass are reduced.

[0017] Preferred embodiments are embodiments in which condition or steps a, A or I; b, B or II; c, C or III; a, A or I and b, B or II; a, A or I and c, C or III; b, B or II and c, C or III; or a, A or I, b, B or II and c, C or III are satisfied or implemented. According to a preferred embodiment, the glass element is sheet glass; a primary pharmaceutical packaging made of glass, preferably a glass vial, a glass syringe, a glass ampoule and / or a glass cartridge; or a glass tube, preferably made of borosilicate glass.

[0018] The term "bottom material" associated with the term "glass melt" means the material portion of the glass melt that can be withdrawn through the opening at the bottom, for example, through the opening of a heating unit and / or a conditioning unit, preferably through the opening of the tank of the conditioning unit. In this specification, generally, the withdrawal of a predetermined percentage of the volumetric flow rate from the bottom material of the glass melt in the bottom region of the glass melt means that a predetermined volume percentage of the glass melt in the bottom region is withdrawn per unit time. Preferably, only the supernatant portion of the glass melt is used for forming the glass element in the form of sheet glass. The sum of the withdrawal from the opening at the bottom and the withdrawal of the supernatant portion of the glass melt is 100% of the volumetric flow rate. For example, if 25 liters are withdrawn from the bottom material of the glass melt in the bottom region of the glass melt within a certain time period and 75 liters of the glass melt are withdrawn to form a glass element, preferably sheet glass, within the same time period, this corresponds to the withdrawal of 25% of the volumetric flow rate from the bottom material of the glass melt in the bottom region of the glass melt.

[0019] The term "contact surface" associated with the term "molten glass" means a surface that is in contact with or touches the molten glass. This includes, for example, the inner wall at the normal filling level of the melting tank, also called the working tank, stirrers, electrode holders and electrodes, the inlets and outlets of each tank (section), the flow paths in the melting tank, etc. In particular, the contact surface does not include the surface of the mixture supply unit to the melting tank, or the contact surface downstream of the direct outlet from the melting tank, such as the float bath and / or the cooling flow path.

[0020] The term "glass element" means a three-dimensional object that is at least partially, preferably mostly, and advantageously entirely made of glass, more preferably borosilicate glass, and is optionally shaped, preferably as sheet glass; as a primary packaging for pharmaceuticals made of glass, preferably as a glass vial, as a glass syringe, as a glass ampoule, and / or as a glass cartridge; or may be designed as a glass tube.

[0021] The term "sheet glass" means a three-dimensional object made of glass, which in the simplest case, for example, is designed as a rectangular parallelepiped having substantially thickness, length, and width. The smallest dimension is the thickness, and the other two larger dimensions represent the surfaces having length and width. The observation surface is along the normal of this surface.

[0022] The thickness, length, and width can be freely selected. Preferably, the thickness is from 0.01 cm to 10 cm, more preferably from 0.1 cm to 5 cm, more preferably from 0.15 cm to 2.5 cm, and most preferably from 0.2 cm to 0.4 cm. Preferably, the length and width are each from 1 cm to 500 cm, more preferably from 3 cm to 400 cm, more preferably from 5 cm to 100 cm, more preferably from 20 cm to 70 cm, and most preferably from 40 cm to 60 cm. This has the advantage that thin transparent sheet glass can be provided, for example, as a transparent cover.

[0023] The term "glass tube" means a three-dimensional object made of glass, which, in the simplest case, is designed as an elongated hollow body having, for example, an outer diameter, a length, and a wall thickness. The smallest dimension is, for example, the wall thickness, and the two larger dimensions are the outer diameter and the length. The observation plane is along the normal of the outer peripheral surface of the tube.

[0024] The outer diameter, the length, and the wall thickness can be freely selected. Preferably, the length of the glass tube is 2 cm or more, preferably 10 cm or more, more preferably 20 cm or more, more preferably 30 cm or more, more preferably 40 cm or more, more preferably 50 cm or more, more preferably 110 cm or more and / or 500 cm or less, preferably 400 cm or less, more preferably 300 cm or less, more preferably 200 cm or less, more preferably 100 cm or less, more preferably 50 cm or less. Preferably, the outer diameter is 3 mm or more, more preferably 4 mm or more, more preferably 5 mm or more, more preferably 6 mm or more, more preferably 7 mm or more, more preferably 8 mm or more, more preferably 9 mm or more, more preferably 10 mm or more, more preferably 15 mm or more, more preferably 20 mm or more and / or 20 cm or less, preferably 15 cm or less, more preferably 10 cm or less, more preferably 5 cm or less, more preferably 4 cm or less, more preferably 3 cm or less, more preferably 2 cm or less. Preferably, the wall thickness is 0.1 mm, preferably 0.5 mm, more preferably 0.8 mm, more preferably 1.0 mm, more preferably 1.5 mm, more preferably 2.0 mm, more preferably 3.0 mm and / or 10.0 mm or less, preferably 5.0 mm or less, more preferably 4.0 mm or less, more preferably 3.0 mm or less, more preferably 2.0 mm or less, more preferably 1.0 mm or less.

[0025] The transmittance of the glass element is not particularly limited. Preferably, the transmittance of the glass element, preferably a sheet glass or a glass tube with a thickness or wall thickness of 6.5 mm, at a wavelength of 400 nm to 800 nm normalized by the thickness is 70% or more, more preferably 80% or more, still more preferably 90% or more, and most preferably 95% or more. The transmittance is usually measured using an ultraviolet-visible spectrophotometer, for example, the spectrophotometer Specord 250 Plus (Analytik Jena). The transmittance of the sample is measured in the wavelength range of 250 to 1050 nm, and the evaluation of the measured values is usually carried out in accordance with DIN EN 410:2011-04 (Glass for building - Determination of photometric and radiometric properties of glazing; German version EN 410:2011).

[0026] The measurement of the number and size of the inclusions according to the embodiment of the present invention was carried out by the method described later.

[0027] The glass element to be inspected, here a sheet glass, has visible light coupled into it in a dark room perpendicularly to the extension line of the thickness of the sheet glass, preferably with a linear cut, and parallel to the plane formed by two dimensions (length and width) larger than this. When the light is reflected by inclusions in the sheet glass, the inclusions are identified. For distinguishing dust of small particles in the sheet glass in a size range of less than, for example, 20 μm from inclusions, a portable microscope, for example, the portable microscope "Wide Stand Microscope" manufactured by PEAK, can be used. The particles thus optically identified are marked in a visible form. The sheet glass marked in this way is observed along the normal of the plane under an optical microscope, for example, under the Axio Imager M2m manufactured by Zeiss equipped with an objective lens LD EC Epiplan 50x / 0.55 HD DIC and an eyepiece PI 10x / 2, to classify the inclusions and measure the size of the inclusions. The size of the inclusions and particles in this specification refers to the longest of the dimensions visible on the observation plane. In this type of measurement, a three-dimensionally formed inclusion may extend along its maximum longitudinal extension in the direction of the optical axis of the microscope during measurement, that is, along the normal of the observation plane. In this case, it is recognized and accepted that the measured value of the obtained inclusion is smaller than the actual value of the current longitudinal extension of the inclusion, for example, a crystal or crystallite. The minimum size of the inclusions measurable with the Axio Imager M2m manufactured by Zeiss is about 2 μm. Therefore, inclusions smaller than 2 μm in size are not considered when counting the inclusions.

[0028] Even without detailed explanation, those skilled in the art will have no problem applying the above procedure to a glass element such as a glass tube.

[0029] Smaller inclusions ranging in size from less than 2 μm to 50 nm can be measured, for example, with the X-ray microscope "Xradia 800 Ultra" manufactured by Zeiss. Therefore, even smaller inclusions smaller than 50 nm in size are not counted or considered in terms of inclusions.

[0030] When the weight of the glass element is less than 1 kg, the inclusions are calculated according to the ratio. When the weight of the glass element exceeds 1 kg, the number of inclusions also increases proportionally. The weight of the glass element is not particularly limited. The weight of the glass element is preferably 5 g to 400 kg, more preferably 0.01 kg to 350 kg, more preferably 0.1 kg to 20 kg, more preferably 0.2 kg to 18 kg, more preferably 0.5 kg to 15 kg, and most preferably 1.0 kg to 10 kg.

[0031] Unless otherwise specified, all percentage values in this specification mean weight percentage and are abbreviated as wt%.

[0032] The term "inclusion" is construed broadly in this specification and includes, in particular, inclusions in the form of metallic and non-metallic inclusions, also called particles, and inclusions in the form of gases such as air, i.e., bubbles.

[0033] The term "metallic inclusion" is construed broadly in this specification. A metallic inclusion refers, in particular, to inclusions of particles mainly composed of one or more metals in elemental form in this specification. The metal of the metallic inclusion has an oxidation number of 0. Precious metals and refractory metals are particularly included in the metallic inclusions.

[0034] The term "non-metallic inclusion" is understood broadly. A non-metallic inclusion is an inclusion composed of a cation (oxidation number > 0) and an anion (oxidation number < 0) in this specification and is also called a salt. In particular, the non-metallic inclusion contains particles in the form of crystals of glass components and one or more salts of refractory metals and precious metals, and in particular, particles in the form of crystals of glass components and one or more silicates or oxides of refractory metals and precious metals.

[0035] In this specification, when the inclusion consists substantially of a single metal, that is, when about 100% consists of a single metal (oxidation number 0), it is counted as a metal inclusion; otherwise, it is counted as a non-metal inclusion. The distinction between metal and non-metal can be made optically, for example, as described above. Refractory metals are metals such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten.

[0036] Noble metals include semi-noble metals in this specification, for example, ruthenium, rhodium, palladium, osmium, iridium, platinum, silver, gold, copper, technetium, rhenium, antimony, bismuth, and polonium.

[0037] Other examples of metals that can be included in inclusions are magnesium, calcium, aluminum, silicon, tin (metal inclusion) and its salts (non-metal inclusion).

[0038] The term "bubble" shall be understood in a broad sense. Bubbles are formed by the presence of gas in the glass. The gas may be gaseous at room temperature (20 °C) or may condense after cooling, and bubbles with negative pressure are formed. Examples of gases that form inclusions include oxygen, nitrogen, nitrogen oxides, carbon dioxide, sulfur dioxide, etc.

[0039] Here, unless otherwise defined, all preferred parameters and embodiments of the glass element are similarly applicable to the two methods and the glass melting furnace, and vice versa.

[0040] Further features, advantages, and further embodiments of the present invention will be described below or will become apparent therefrom.

[0041] According to a preferred further embodiment, the glass element has 50 or fewer inclusions having a size of 500 nm or more and 10 μm or less, preferably 50 nm or more and 10 μm or less, more preferably 10 μm or less, per kg of glass. This has the advantage that higher quality glass is preferably provided for high energy laser applications.

[0042] According to a preferred embodiment, the glass element has 2000 or fewer, preferably 1000 or fewer, more preferably 500 or fewer, more preferably 250 or fewer, more preferably 100 or fewer, more preferably 50 or fewer inclusions having a size of 20 μm or less, preferably 30 μm or less, more preferably 40 μm or less, most preferably 50 μm or less and a size of 2 μm or more, preferably 50 nm or more, per kg of glass. This has the advantage that higher quality glass is preferably provided for high energy laser applications. Even if the number of inclusions is the same, if there is variation within a wide size range, the adverse effect on the use of high energy lasers is reduced.

[0043] Here, the following points should be noted: For example, when the number of inclusions having a size of 10 μm or less per kg of glass is limited to 50 or less, a glass element according to an embodiment of the present invention can include 50 inclusions having a size of 5 μm and 50 inclusions having a size of 40 μm per kg of glass. However, when the number of inclusions having a size of 50 μm or less per kg of glass is limited to 50 or less, for example, a glass element having 50 inclusions having a size of 5 μm and 50 inclusions having a size of 40 μm per kg of glass does not satisfy this condition, but for example, a glass element having 10 inclusions having a size of 5 μm, 20 inclusions having a size of 25 μm, and 15 inclusions having a size of 45 μm per kg of glass satisfies that condition.

[0044] The glass element may optionally contain a greater number of additional inclusions. Larger inclusions can be easily detected by conventional measurement techniques, for example, immediately after the production of the glass element and / or before further processing, and the whole or part of the glass element can be sorted. In the embodiments of the invention described herein, larger inclusions can also be significantly reduced as well. Thus, the glass element preferably does not contain inclusions with a size exceeding 50 μm, more preferably exceeding 40 μm, more preferably exceeding 30 μm, more preferably exceeding 20 μm, and most preferably exceeding 10 μm.

[0045] According to a preferred embodiment, the glass element contains 50 or fewer inclusions per kg of glass, preferably 40 or fewer, preferably 30 or fewer, more preferably 20 or fewer, and most preferably 10 or fewer inclusions, where these inclusions can have a size of 10 μm or less, preferably 20 μm or less, preferably 30 μm or less, more preferably 40 μm or less, and most preferably 50 μm or less. This preferably has the advantage of improving the quality of the glass element for high-energy laser applications. The fewer the inclusions per kg of glass, the less the adverse effect on high-energy laser applications. The quality of the glass element is improved.

[0046] Variations in parameters such as the frequency of the alternating current, the percentage of the volume flow rate withdrawn, and the proportion of zirconium oxide on the contact surface can affect the number and size of the inclusions. Even when only one of conditions a) or A), b) or B), and c) or C) is satisfied, or when only one of steps I), II), or III) is carried out, at least one of the parameters of the size of the inclusions and / or the number of inclusions per kg of glass is reduced.

[0047] According to a preferred embodiment, the inclusions include metal inclusions, and the glass element has 40 or fewer, preferably 30 or fewer, more preferably 20 or fewer, more preferably 10 or fewer, most preferably 5 or fewer metal inclusions with a size of 5 μm or less, preferably 10 μm or less, more preferably 20 μm or less, more preferably 30 μm or less, more preferably 40 μm or less, most preferably 50 μm or less per 1 kg of the glass. Thus, the glass element has overall fewer inclusions due to metal particles, and the quality of the glass element is further improved. Frequently occurring metal inclusions include, in particular, metals such as tungsten, zirconium, platinum, rhodium, iridium, molybdenum, tin, and copper, because these metals come into contact with the glass during the manufacture of the glass element.

[0048] According to a preferred embodiment, the inclusions include non-metal inclusions, and the glass element has 25 or fewer, preferably 15 or fewer, more preferably 8 or fewer, more preferably 1 or fewer, more preferably 0.1 or fewer, more preferably 0.05 or fewer, most preferably 0.00 non-metal inclusions with a size of 2 μm or more and 10 μm or less, preferably 2 μm or more and 20 μm or less, more preferably 2 μm or more and 30 μm or less, more preferably 2 μm or more and 40 μm or less, most preferably 2 μm or more and 50 μm or less per 1 kg of the glass. Thereby, the number and size of the non-metal inclusions are reduced, and the quality of the glass element is also improved. Frequently occurring non-metal inclusions include, in particular, crystals of the glass components and / or one or more silicates and / or oxides of refractory metals and / or noble metals. The crystals crystallize, for example, from the glass melt, while the silicates and / or oxides of refractory metals and / or noble metals are formed from the contact material in contact with the glass melt.

[0049] According to a preferred embodiment, the inclusion contains bubbles, and the glass element has 25 or less, preferably 15 or less, more preferably 8 or less, more preferably 1 or less, more preferably 0.1 or less, more preferably 0.05 or less, and most preferably 0.00 bubbles with a size of 10 μm or less, more preferably 20 μm or less, more preferably 30 μm or less, more preferably 40 μm or less, and most preferably 50 μm or less and / or 2 μm or more, preferably 50 nm or more per 1 kg of the glass. Therefore, regarding the inclusion in the form of bubbles, the quality of the glass element is overall improved.

[0050] According to a preferred embodiment, the composition of the glass element is as follows in weight% units: SiO 2 60 - 90%, more preferably 76% - 90%; B 2 O 3 0 - 20%; Al 2 O 3 0 - 20%; Li 2 O 0 - 10%; Na 2 O 0 - 10%; K 2 O 0 - 10%; MgO 0 - 10%; CaO 0 - 10%; SrO 0 - 10%; and BaO 0 - 10% has.

[0051] In a further preferred embodiment, the glass element has the following composition in weight% units: SiO 2 More than 76%; B 2 O 3 0 - 15%; Al 2 O 3 0 - 5%; Li 2 O 0 - 4%; Na 2 O 0 - 4%; K 2O 0 to 4%; MgO 0 to 4%; CaO 0 to 4%; SrO 0 to 4%; and BaO 0 to 4%; and inevitable impurities, namely less than 0.01% having.

[0052] In a further preferred embodiment, the glass element has the following composition in weight %: SiO 2 76% to 85%; B 2 O 3 0 to 15%; Al 2 O 3 0 to 5%; Li 2 O 0 to 4%; Na 2 O 0 to 4%; K 2 O 0 to 4%; MgO 0 to 4%; CaO 0 to 4%; SrO 0 to 4%; and BaO 0 to 4%, and inevitable impurities, namely less than 0.01% having.

[0053] Preferably, the glass element contains iron oxide, arsenic oxide and / or antimony oxide in a weight ratio of less than 0.05 wt%, more preferably less than 0.01 wt%. More preferably, the glass element does not contain iron oxide, arsenic oxide and antimony oxide. A glass element that does not contain arsenic oxide and antimony oxide provides an environmentally friendly glass element. Iron oxide may be generated as an impurity, which may adversely affect the color of the glass element. This can be prevented by appropriately selecting the starting materials.

[0054] According to a preferred embodiment, the glass element has one or more of the following characteristics: i) The weight of the glass element is from 0.01 kg to 350 kg, preferably from 0.1 kg to 20 kg, more preferably from 0.5 kg to 15 kg, and most preferably from 1.0 kg to 10 kg; ii) When the glass element is provided in the form of sheet glass, the thickness of the glass element is from 0.01 cm to 10 cm, preferably from 0.1 cm to 5 cm, more preferably from 0.15 cm to 2.5 cm, and most preferably from 0.2 cm to 0.4 cm; iii) When the glass element is provided in the form of sheet glass, the length and width of the glass element are each from 1 cm to 500 cm, preferably from 3 cm to 400 cm, more preferably from 5 cm to 100 cm, more preferably from 20 cm to 70 cm, and most preferably from 40 cm to 60 cm; iv) The transmittance of the glass element normalized with a glass element in the form of sheet glass with a thickness of 6.5 mm at a wavelength of 400 nm to 800 nm is 70% or more, more preferably 80% or more, more preferably 90% or more, and most preferably 95% or more; and v) The glass element does not contain inclusions with a size exceeding 50 μm, preferably exceeding 40 μm, more preferably exceeding 30 μm, more preferably exceeding 20 μm, and most preferably exceeding 10 μm.

[0055] More preferably, the glass element has the above characteristics i; ii; iii; iv; v; i + ii; i + iii; i + iv; i + v; ii + iii; ii + iv; ii + v; iii + iv; iii + v; iv + v; i + ii + iii; i + ii + iv; i + ii + v; i + iii + iv; i + iii + v; i + iv + v; ii + iii + iv; ii + iii + v; ii + iv + v; iii + iv + v; i + ii + iii + iv; i + ii + iii + v; i + ii + iv + v; i + iii + iv + v; or i + ii + iii + iv + v.

[0056] According to a preferred embodiment of the present method, the viscosity of the glass melt is made to be, and / or the value is maintained at, a value of 30 Pa·s to 450 Pa·s, preferably 33 Pa·s to 400 Pa·s, more preferably 35 Pa·s to 265 Pa·s, at least in steps ii) and iii). According to a preferred embodiment of the present method, the viscosity of the glass melt is made to be, and / or the value is maintained at, a value of 33 Pa·s to 265 Pa·s, preferably 35 Pa·s to 200 Pa·s, at least in step ii). According to a preferred embodiment of the present method, the viscosity of the glass melt is made to be, and / or the value is maintained at, a value of 70 Pa·s to 450 Pa·s, preferably 100 Pa·s to 400 Pa·s, at least in step iii). One of the advantages obtained in this way is that the present method can be applied to a large number of various glasses to be manufactured, which have different temperatures during manufacturing.

[0057] According to a preferred embodiment, the glass element is provided in the form of a sheet glass or a glass tube. This enables easy manufacturing of the glass element or flexible use for a wide range of applications.

[0058] According to a preferred embodiment, the glass element has 2000 or less, preferably 1000 or less, more preferably 500 or less, more preferably 250 or less, more preferably 100 or less, more preferably 50 or less inclusions having a size of 50 nm or more and 500 nm or less per 1 kg of the glass. This has the advantage that higher-quality glass is preferably provided for high-energy laser applications.

[0059] According to a preferred embodiment, the glass element has 2000 or less, preferably 1000 or less, more preferably 500 or less, more preferably 250 or less, more preferably 100 or less, more preferably 50 or less inclusions having a size of 500 nm or more and 2 μm or less per 1 kg of the glass. This has the advantage that the quality of the glass element is further improved.

[0060] According to a preferred embodiment of the present method, an alternating current with a frequency of 1 kHz to 100 kHz, preferably 5 kHz to 50 kHz, more preferably 8 kHz to 15 kHz is used as the heating current. If the frequency is 1 kHz or more, preferably 5 kHz or more, more preferably 8 kHz or more, the number of inclusions, particularly metal inclusions, is reduced. However, if the frequency is 200 kHz or less, preferably 100 kHz or less, more preferably 50 kHz or less, more preferably 15 kHz or less, this frequency is easier to achieve and unexpected problems such as the generation of bubbles due to local overheating are less likely to occur.

[0061] According to a preferred embodiment of the present method, the contact surface in contact with the glass melt contains 50% or more, more preferably 60% or more, more preferably 70% or more, more preferably 80% or more, more preferably 90% or more, more preferably 95% or more of a contact material in the form of a fused-cast zirconium oxide material. Thereby, the number of inclusions such as refractories is reduced, and the quality of the glass and glass elements is improved.

[0062] According to a preferred embodiment of the present method, the contact material contains a fused-cast zirconium oxide material in which the proportion of ZrO 2 is more than 90% by weight, preferably more than 95% by weight. Thereby, the number of inclusions in the refractory is further reduced, and the quality of the glass element is further improved.

[0063] According to a preferred embodiment of the present method, 5% to 40% of the volume flow rate, preferably 20% to 40%, more preferably 25% to 35% is withdrawn from the bottom material of the glass melt in the bottom region of the glass melt. This has the advantage of improving the quality of the glass because the number of inclusions of corrosion products, i.e., non-metallic inclusions, is reduced.

[0064] In a preferred embodiment, in a method for manufacturing a glass element in the form of sheet glass, preferably borosilicate sheet glass, the following steps S1 to S4 are carried out, preferably S1 is carried out first, then S2, then S3, and finally S4.

[0065] According to one embodiment of the present invention, in the first step S1, a mixture is provided. In a further step S2, the mixture is heated to form a glass melt. In a further step S3, the glass melt is conditioned, and in a further step S4, the glass melt is cooled to provide a glass element.

[0066] During the heating in step S2 and the conditioning in step S3, at least one, preferably all, of the following conditions are satisfied: a) By applying a heating current to the electrodes for electrical resistance heating to at least partially heat and / or warm the glass melt, and using an alternating current with a frequency of 1 kHz to 200 kHz as the heating current; b) The contact surface in contact with the glass melt contains 30% or more of a contact material, and the contact material is in the form of a fused-cast zirconia material with a ZrO 2 proportion of more than 70% by weight, and c) In the bottom region of the glass melt, 1% to 50% by volume is withdrawn from the bottom material of the glass melt.

[0067] Further preferred features and advantages of the present invention are apparent from the dependent claims, the drawings, and the description of the corresponding drawings.

[0068] Of course, the features mentioned above and described below can be used not only in the indicated combinations but also in other combinations or alone without departing from the scope of the present invention.

[0069] Preferred implementations and embodiments of the present invention are shown in the drawings and will be described in more detail in the following description. The same reference numerals refer to the same or similar or functionally identical components or elements.

Brief Description of the Drawings

[0070]

Figure 1

Figure 2

[0071] FIG. 1 shows a glass melting furnace according to an embodiment of the present invention.

[0072] Specifically, FIG. 1 shows a glass melting furnace 1. The glass melting furnace 1 includes a heating unit 2 in which starting materials called so-called batches are heated and melted. For this reason, the heating unit 2 includes a tank 2a provided with a bottom extraction unit 6a for extracting the bottom material 11 of the glass melt 10.

[0073] In order to ensure sufficient homogeneity and the absence of bubbles, after melting, the glass melt 10 is clarified in a fining unit 23 arranged downstream. This unit can also be used as a transport region for transporting the glass melt 10 to the downstream part. The essential purpose of fining the glass melt 10 is to remove the gas physically and chemically bonded to the glass melt 10 from the glass melt 10. After the fining is completed, it is necessary to at least reduce or prevent the generation of new bubbles in the melt. The fining may, in principle, be carried out in the same tank 2a or in a separate tank. After the fining unit 23, a conditioning unit 3 follows in which further conditioning and / or homogenization of the glass melt 10 is carried out. At the bottom of the conditioning unit 3, a further bottom extraction unit 6b is provided. The walls 20 of the aforementioned units 2, 23, 3 of the glass melting furnace 1 are made of a refractory material such as the aforementioned fused-cast zirconia material. According to FIG. 1, the heating unit 2 is separated from the fining unit 23 by a flow-influencing element 15. This element 15 extends transversely to the flow direction over the entire width of the glass melting furnace 1 in this region and substantially completely blocks this region except for a slight protrusion to the glass liquid level line. The flow-influencing element 15 may alternatively or additionally be arranged in the transition region between the fining unit 23 and the conditioning unit 3.

[0074] Furthermore, the glass melting furnace 1 includes a heating unit 4 having electrodes 5 for resistance heating. The heating unit 4 is connected to an AC power supply 8 that provides an alternating current with a frequency of 1 kHz to 200 kHz as the heating current. Further, for example, the glass melt can be heated with a gas burner (not shown). The control unit 30 can, on the one hand, withdraw 1% to 50%, for example 25%, of the volume flow rate of the glass melt 10 from the bottom material of the glass melt and supply this to the heating unit 2 as required, and on the other hand, control the conditioning unit 3, the heating unit 2, the AC power supply 8, the fining unit 23 and the bottom withdrawal units 6a, 6b as required so that the resistance heating of the glass melt 10 is carried out at least in the conditioning unit 3. Further, the contact surface 7 in contact with the glass melt 10 is made of a fused-cast zirconia material with a ZrO 2 proportion of more than 70 wt%, preferably a contact material 7a in the form of a fused-cast zirconia material with a ZrO 2 proportion of more than 85 wt%.

[0075] Figure 2 shows the steps of a method for manufacturing a glass element according to an embodiment of the present invention.

[0076] The method includes the following steps: According to an embodiment of the present invention, in a first step S1, a mixture is provided. In a further step S2, the mixture is heated to form a glass melt. In a further step S3, the glass melt is conditioned, and in a further step S4, the glass melt is cooled to provide a glass element.

[0077] In a preferred embodiment, in the method according to any one of claims 9 to 13 for manufacturing a glass element according to any one of claims 1 to 12, preferably in the form of sheet glass, preferably borosilicate sheet glass, the following steps S1 to S4 are carried out, preferably first S1, then S2, then S3, and finally S4.

[0078] Examples The following table shows, in different categories separated into bubbles, non-metallic inclusions, and metallic inclusions, the total number of inclusions in the comparative examples and the examples according to the embodiments of the present invention:

Table 1

[0079] The size distribution in the range of 0 to 50 μm of metallic inclusions per 1 kg of glass in Examples 3 to 6 is shown in the following table. In this case, only inclusions with a size of 2 μm or more were considered in the size interval of 0 μm to 5 μm shown in the following table.

[0080]

Table 2

[0081] In Comparative Example 1 and Example 2, the size distribution of the inclusions was not accurately grasped. In Example 2 and Example 3, although the total number of inclusions per 1 kg of glass was more than 50 (for example, Example 2: 0.3 + 0.2 + 53.2 = 53.7 / kg), since the size of some inclusions exceeded 50 μm, the number of inclusions with a size of 50 μm or less, and thus also the number of inclusions with a size of 10 μm or less, was less than 50 per 1 kg of glass (see Table 2, Example 3: 10.5 + 10.5 + 10.5 + 7.0 + 3.5 + 3.5 = 45.5 / kg). In Example 3, in addition to the inclusions shown in Table 2, inclusions with a size exceeding 100 μm were confirmed. In Examples 3 to 6, the number of inclusions with a size of 50 μm or less was less than 50 per kg (see Table 2), and in Examples 4 to 6, it was even less than 10 per kg. Furthermore, there were no further inclusions with a size exceeding 50 μm in the sheet glass of Examples 4 to 6 (obvious from the comparison between Table 1 and Table 2).

[0082] Comparative Example 1 is an example of a glass element in the form of sheet glass using no improvement measures for the glass melting furnace described in this specification, that is, using a conventionally well-known glass melting furnace or a conventionally well-known manufacturing method. In this case, the total number of inclusions far exceeded 50, and the number of inclusions in each section was also very large. Even when the exact size of each inclusion in Comparative Example 1 was not grasped, since the number of inclusions was large even in the range of less than 50 μm, it is presumed that the number of inclusions per kilogram far exceeded 50. Based on this, the improvement of the test glass melting furnace was sequentially advanced.

[0083] First, in the experiment in the glass melting furnace, a contact material composed of a cast fused zirconium oxide material with a ZrO 2 proportion exceeding 70 wt% was provided on a part (about 70% or more) of the contact surface in contact with the glass melt. At the same time, the glass melting furnace was operated so that 1% to 50% of the volume flow rate was drawn from the bottom material of the glass melt in the bottom region of the glass melt. As a result, bubbles and non-metallic inclusions were significantly reduced (see, for example, Example 2). In Example 2, the number of metal inclusions exceeded 50 (number = 53.2). Among them, as described above, most of the inclusions have a size of 50 μm or less, but there are also some inclusions with a size exceeding 50 μm. The sheet glass manufactured in this way has 50 or fewer inclusions with a size of 10 μm or less per kilogram of glass. Even when the proportion of the contact surface provided with a specific contact material was small (30% or more of the contact surface), bubbles and non-metallic inclusions were reduced. When most (90% or more, or even 95% or more) of the contact surface in contact with the glass melt is a cast fused zirconium oxide material with a high ZrO 2 proportion (85 wt% or more, or even 95 wt% or more), and furthermore, a large proportion (25 vol% to 35 vol%) is drawn from the bottom material of the glass melt in the bottom region of the glass melt, in the sheet glass manufactured in the glass melting furnace, the number of bubbles is 0, and the number of non-metallic inclusions with a size of 50 μm or less is 0 pieces / kg (see Examples 3 to 6).

[0084] In addition, the optimization of electric resistance heating was also carried out. For this purpose, for test purposes, a novel electric resistance heating element according to an embodiment of the present invention was attached to a glass melting furnace. In Comparative Example 1, conventional resistance heating was used, and it was found that a large number of metal inclusions were present in the sheet glass produced using this. In Examples 2 to 6, a heating current was applied to the electrodes to at least partially heat and / or warm the glass melt by electric resistance heating. At this time, as the heating current, an alternating current with a frequency of 1 kHz to 200 kHz was used, and the number of metal inclusions could be reduced. By adjusting the frequency to 1 kHz to 100 kHz, the number of metal inclusions could be further significantly reduced, and finally, it was found that the number of metal inclusions was particularly small at a frequency of 8 kHz to 15 kHz (see Examples 4 to 6).

[0085] Also, as is clear from the examples, when all three improvement steps described in this specification are applied, the best results can be obtained.

[0086] In summary, at least one of the embodiments of the present invention has at least one of the following advantages: - Reduction of inclusions with a size of 50 μm or less, preferably inclusions of metal particles - Improvement of the flexibility of glass for various applications - Improvement of glass quality - Reduction of defective products - Reduction of the number of bulk defects or the bulk defect density

[0087] The glass or glass element produced according to the embodiment of the present invention can preferably be used in the following applications: - High energy density laser applications - Large-sized, homogeneous and / or low-defect photomasks for lithography applications - Nanoimprint lithography, transfer of precision lenses and nanostructures in the nm range such as hard disk magnetic layers by imprint molds - Substrates for flat metasurface concepts and metasurface optical systems - Super glass wafer, and - Precision window for a display, preferably for OLED

[0088] The present invention has been described with reference to preferred embodiments, but the present invention is not limited thereto, and various modifications are possible.

Explanation of Reference Numerals

[0089] 1 Glass melting furnace 2 Heating unit 2a Tank 3 Conditioning unit 4 Heating unit 5 Electrode 6a, 6b Bottom drawing unit 7 Contact surface 7a Contact material 8 AC power supply 10 Glass melt 11 Bottom material 20 Wall 23 Clarifying unit 30 Control unit

Claims

1. A glass element, preferably made of borosilicate glass, said glass element containing no more than 50 inclusions per kg of glass with a size between 2 μm and 10 μm.

2. 2. The glass element according to claim 1, wherein the glass element contains 2000 or less inclusions, preferably 1000 or less, more preferably 500 or less, more preferably 250 or less, more preferably 100 or less, and more preferably 50 or less, with a size of 500 nm or more and 10 μm or less, preferably 50 or less, with a size of 500 nm or more and 10 μm or less, more preferably 10 μm or less, per 1 kg of glass.

3. 3. The glass element according to claim 1, wherein the glass element contains 50 or less inclusions per kg of glass having a size of 20 μm or less, preferably 30 μm or less, more preferably 40 μm or less, most preferably 50 μm or less and / or a size of 2 μm or more, preferably 50 nm or more.

4. 4. The glass element according to claim 1, wherein the glass element contains at most 40 inclusions per kg of glass, preferably at most 30 inclusions, more preferably at most 20 inclusions, and most preferably at most 10 inclusions per kg of glass.

5. 5. The glass element according to claim 1 , wherein the inclusions include metal inclusions, and the glass element contains 40 or less, preferably 30 or less, more preferably 20 or less, more preferably 10 or less, and most preferably 5 or less metal inclusions with a size of 5 μm or less, preferably 10 μm or less, more preferably 20 μm or less, more preferably 30 μm or less, more preferably 40 μm or less, and most preferably 50 μm or less per kg of glass.

6. 6. The glass element according to claim 1, wherein the inclusions include non-metallic inclusions, and the glass element contains 25 or less, preferably 15 or less, more preferably 8 or less, more preferably 1 or less, more preferably 0.1 or less, more preferably 0.05 or less, and most preferably 0.00 non-metallic inclusions having a size of 2 μm or more and 10 μm or less, preferably 2 μm or more and 20 μm or less, more preferably 2 μm or more and 30 μm or less, more preferably 2 μm or more and 40 μm or less, and most preferably 2 μm or more and 50 μm or less per kg of glass.

7. 7. The glass element according to claim 1 , wherein the inclusions include bubbles, and the glass element contains 25 or less, preferably 15 or less, more preferably 8 or less, more preferably 1 or less, more preferably 0.1 or less, more preferably 0.05 or less, and most preferably 0.00 bubbles with a size of 10 μm or less, more preferably 20 μm or less, more preferably 30 μm or less, more preferably 40 μm or less, most preferably 50 μm or less, and / or 2 μm or more, preferably 50 nm or more, per kg of glass.

8. The composition of the glass element is as follows, in weight percent: SiO ２ 60-90%, preferably 76%-90%; B ２ O ３ 0~20%; <h2 style=";text-align:left;direction:ltr">Al<h2 style=";text-align:left;direction:ltr"> ２ <h2 style=";text-align:left;direction:ltr"> O<h2 style=";text-align:left;direction:ltr"> ３ <h2 style=";text-align:left;direction:ltr"> 0~220%; Li ２ O 0~10%; Na ２ O0~10%; K ２ O 0~10%; MgO 0-10%; CaO 0-10%; SrO 0-10%; and BaO 0-10% 8. The glass element according to claim 1 , wherein

9. The glass element has the following characteristics: i) the weight of said glass element is between 0.01 kg and 350 kg, preferably between 0.1 kg and 20 kg, more preferably between 0.5 kg and 15 kg, and most preferably between 1.0 kg and 10 kg; ii) when the glass elements are provided in the form of glass sheets, the thickness of the glass elements is from 0.01 cm to 10 cm, preferably from 0.1 cm to 5 cm, more preferably from 0.15 cm to 2.5 cm, and most preferably from 0.2 cm to 0.4 cm; iii) when the glass elements are provided in the form of glass sheets, the length and width of the glass elements are respectively from 1 cm to 500 cm, preferably from 3 cm to 400 cm, more preferably from 5 cm to 100 cm, more preferably from 20 cm to 70 cm, and most preferably from 40 cm to 60 cm; iv) the transmittance of said glass element in the form of a glass plate having a thickness of 6.5 mm at wavelengths of 400 nm to 800 nm is 70% or more, more preferably 80% or more, more preferably 90% or more, and most preferably 95% or more; and v) said glass element is free of inclusions with a size greater than 50 μm, preferably greater than 40 μm, more preferably greater than 30 μm, more preferably greater than 20 μm, most preferably greater than 10 μm; 9. The glass element according to claim 1, further comprising one or more of the following:

10. 10. The glass element according to claim 1, wherein the glass element is provided in the form of a glass sheet or a glass tube.

11. 11. The glass element according to claim 1, wherein the glass element contains 2000 or less inclusions with a size of 50 nm or more and 500 nm or less per kg of glass, preferably 1000 or less, more preferably 500 or less, more preferably 250 or less, more preferably 100 or less, more preferably 50 or less.

12. 12. The glass element according to claim 1, wherein the glass element contains 2000 or less inclusions with a size of 500 nm or more and 2 μm or less per kg of glass, preferably 1000 or less, more preferably 500 or less, more preferably 250 or less, more preferably 100 or less, and more preferably 50 or less.

13. A method for manufacturing a glass element, preferably made of borosilicate glass, according to any one of claims 1 to 12, said method comprising the steps of: i) in weight percent, SiO ２ 60-90%, preferably 76%-90%; B ２ O ３ 0~20%; <h2 style=";text-align:left;direction:ltr">Al<h2 style=";text-align:left;direction:ltr"> ２ <h2 style=";text-align:left;direction:ltr"> O<h2 style=";text-align:left;direction:ltr"> ３ <h2 style=";text-align:left;direction:ltr"> 0~220%; Li ２ O 0~10%; Na ２ O0~10%; K ２ O 0~10%; MgO 0-10%; CaO 0-10%; SrO 0-10%; and BaO 0-10%; providing a mixture (S1) comprising: ii) heating the mixture to form a molten glass (10) (S2); iii) a step (S3) of conditioning the glass melt (10); iv) cooling the glass melt (10) to provide a glass element (S4); wherein during said steps ii) and / or iii) the following conditions are met: a) applying a heating current to the electrodes (5) to at least partially heat and / or warm the molten glass (10) by electrical resistance heating, and using an alternating current having a frequency of 1 kHz to 200 kHz as the heating current; b) The contact surface (7) in contact with the glass melt (10) is made of ZrO ２ and c) withdrawing from the bottom region of the glass melt (10) between 1% and 50% of the volume flow rate of the bottom material (11) of the glass melt (10); At least one, and preferably all, of the following are satisfied:

14. 14. The method according to claim 13, further comprising: bringing the viscosity of the glass melt (10) to and / or maintaining said value at least in steps ii) and / or iii) between 30 Pa.s and 450 Pa.s, preferably between 33 Pa.s and 400 Pa.s, more preferably between 35 Pa.s and 265 Pa.s.

15. 15. The method according to claim 13 or 14, wherein as heating current an alternating current with a frequency of 1 kHz to 100 kHz, preferably 5 kHz to 50 kHz, more preferably 8 kHz to 15 kHz is used.

16. The contact surface (7) in contact with the glass melt (10) is made of ZrO ２ 16. The method according to claim 13, wherein the contact material (7a) is in the form of a fused cast zirconium oxide material having a proportion of at least 75% by weight, preferably at least 80% by weight, more preferably at least 90% by weight, most preferably at least 95% by weight.

17. The method according to any one of claims 13 to 16, wherein 5% to 40%, preferably 20% to 40%, more preferably 25% to 35% of the volume flow rate of the bottom material (11) of the glass melt (10) in the bottom region is withdrawn.

18. A glass melting furnace (1) for carrying out the method according to any one of claims 13 to 17, said glass melting furnace (1) comprising: a heating unit (2) for heating the mixture to form a glass melt (10); a conditioning unit (3) for conditioning the glass melt (10); a bottom drawing unit (6a, 6b) for drawing the bottom material (11) of the glass melt (10); wherein the warming unit (2) and / or the conditioning unit (3) comprises a heating unit (4) having at least two electrodes for resistive heating, A) said heating unit (4) is designed or controlled in such a way that a heating current in the form of an alternating current having a frequency between 1 kHz and 200 kHz is applied to said at least two electrodes (5); and / or B) The contact surface (7) in contact with the glass melt (10) contains 30% or more of a contact material (7a), and the contact material (7a) is ZrO ２ % by weight of the fused cast zirconium oxide material, preferably ZrO ２ and / or is designed to be in the form of a fused cast zirconium oxide material having a proportion of greater than 85% by weight. C) The bottom withdrawal unit (6a, 6b) is designed such that 1% to 50% of the volume flow rate of the bottom material (11) of the glass melt (10) is withdrawn in the bottom region of the warming unit (2) and / or the conditioning unit (3).

19. 19. A method for operating a glass melting furnace (1) according to claim 18, comprising the steps of: I) applying a heating current to the electrode (5) to operate the heating unit (4), wherein the heating current is an alternating current having a frequency of 1 kHz to 200 kHz; II) contacting the glass melt (10) with a contact surface (7), the contact surface (7) being in contact with the glass melt (10) and the contact surface (7) being made of ZrO ２ % by weight of the fused cast zirconium oxide material, preferably ZrO ２ and comprising at least 30% of a fused cast zirconium oxide material having a proportion of greater than 85% by weight of III) operating the bottom withdrawal units (6a, 6b) such that 1% to 50% of the volume flow rate of the bottom material (11) of the glass melt (10) is withdrawn in the bottom region; The method of claim 1, further comprising:

20. 19. A glass element, preferably made of borosilicate glass, preferably in the form of a borosilicate sheet glass, preferably produced using the method according to any one of claims 1 to 12, the method according to any one of claims 13 to 17 and / or the glassmelting furnace (1) according to claim 18.

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