Lithium aluminosilicate glass ceramics

Abstract

The invention relates to a lithium aluminosilicate glass ceramic having a coefficient of thermal expansion in the range of 20 DEG C to 700 DEG C of-0.5 to 1.9 ppm / K and to the use thereof. The glass ceramic contains the following components in percentage by weight on the basis of oxides: 60-70% of SiO2, 17-25% of Al2O3, 2.0-lt of Li2O, 0.1-0.5% of MgO, 0.1-0.5% of MgO, 0.1-0.5% of Na2O, 0.1-0.5% of Na2O, 0.1-0.5 3.2, 0 to 1t of MgO; 0.5% of ZnO, 1.5 to 1t of ZnO; 3, 0.2 to 3 parts of BaO, and 2gt of NaO2; 0.05 to 1t; 0.6, and K2Ogt; 0.05 to 1t; 0.5, and the condition MgOlt; k2O, and MgOlt; and Na2O.

Description

Technical Field

[0001] The present invention relates to a lithium aluminosilicate glass-ceramic suitable for use as a cooking surface of a cooking appliance, and to its use. Background Art

[0002] Glass-ceramic cooking surfaces and the glass-ceramics used therefor have been known for many years. For this purpose, lithium aluminosilicate (LAS) glass-ceramics are used, which contain high quartz mixed crystals (HQMK) especially for transparent materials or hydrothermal quartz mixed crystals (KMK) especially for translucent or opaque materials as the main crystal phase. To manufacture such glass-ceramics, the raw glass, i.e., the so-called green glass, is first manufactured by a conventional method for glass manufacture. The green glass is converted into a glass-ceramic by a heat treatment process, i.e., ceramization.

[0003] The basic property of these materials used as cooking surfaces is that in the temperature range from room temperature to 700 °C, their thermal expansion is very low. The low thermal expansion in turn results in high thermal shock resistance. The thermal expansion is composed of a crystal phase with negative thermal expansion and an amorphous residual glass phase with positive thermal expansion. In the glass-ceramics used so far, the lithium content is usually more than 3.6% to 5.0% by weight.

[0004] Due to the rising raw material price of lithium, it is economically advantageous to minimize the lithium content in the glass-ceramic. However, since lithium is one of the three main components in lithium aluminosilicate glass-ceramics, the lithium cannot be reduced arbitrarily. The content of Li2O directly affects the key properties of the glass-ceramic, such as the viscosity important for manufacturability in the melt or the thermal expansion important for use as a cooking surface.

[0005] The price trend of lithium is not a new issue. The price of lithium has been rising continuously in the past 20 years. Nevertheless, due to the long-term demand, most of the glass-ceramics used for cooking surfaces on the market contain about 3.8% by weight of Li2O. So far, in practice, no glass-ceramic containing less than 3.5% of Li2O and capable of competing with the glass-ceramics on the current market has been found.

[0006] Glass-ceramics with an Li2O content of less than 3.5% by weight are known from the following documents: WO 2012 / 010341A1, EP3502069 A1, US2017050880, US2020189965, US2020140322, US2021387899, WO2021 / 224412A1. However, these glass-ceramics have various disadvantages, such as reduced thermal shock resistance or poor fusibility of the green glass.

[0007] Here, transparent glass ceramics are generally understood as glass ceramics with low light scattering. The transmittance of transparent glass ceramics can be adjusted within a wide range by absorption characteristics (i.e., coloring properties) components.

[0008] When transparent glass ceramics are used for cooking surfaces, they are either volume-colored by adding colored oxides or a bottom coating is provided to visually hide the technical devices located below the cooking surface. Various colored oxides can be used for the volume coloring of glass ceramics. These include in particular V2O5, CoO, Fe2O3, Cr2O3, Nd2O3, NiO, CuO, MnO, and MoO3. Each of these colored oxides has a different effect on the absorption of glass ceramics in the visible and infrared spectral ranges. The coloring of glass ceramics is described in particular in the following documents: WO 11089220 A1, US 8765619, DE 102008050263B4, DE 102009013127B4. Summary of the Invention

[0009] The object of the present invention is to provide a lithium aluminosilicate glass ceramic that has good green glass melting properties and is cost-effective without imposing limitations on the use characteristics.

[0010] Good melting performance mainly includes here: a processing point at a temperature below 1340 °C, preferably below 1330 °C, and particularly preferably below 1320 °C. The processing point is the temperature at which the green glass has a viscosity of 10 4 dPa*s. The thermoforming of the green glass occurs near this temperature. The higher the temperature during thermoforming, the more complicated it is to discharge the heat introduced into the forming machine through the glass. In the case of temperatures above 1340 °C, this can only be achieved by reducing the glass production volume to reduce the heat. However, this is economically disadvantageous.

[0011] During the thermoforming process, when the devitrification temperature is below the upper limit, undesired spontaneous crystallization may occur. To prevent this, the upper limit of the devitrification temperature should be at least 15 K, preferably at least 20 K, and particularly preferably at least 30 K lower than the processing point.

[0012] In particular, the glass ceramic should meet all the requirements for use in combination with all types of heating elements as a cooking surface. These heating elements include in particular radiant heating elements, induction heating elements, and gas heating elements. This particularly requires a sufficiently high thermal shock resistance and high long-term temperature resistance characteristics.

[0013] The object of the present invention is achieved by the subject matter of the independent claims. Preferred embodiments and improvements are derived from the dependent claims.

[0014] The lithium aluminosilicate glass-ceramic according to the invention has a coefficient of thermal expansion in the range from 20 °C to 700 °C of from -0.5 to 1.9 ppm / K. The glass-ceramic contains the following components in oxide form in the indicated amounts in % by weight:

[0015]

[0016]

[0017] and the condition

[0018] MgO < K2O

[0019] MgO < Na2O.

[0020] The glass-ceramic having the corresponding coefficient of thermal expansion has both high thermal shock resistance and high long-term thermal stability. Thereby it is suitable as a cooking surface for use in conjunction with all types of heating elements. The coefficient of expansion is at least -0.5 ppm / K. Here, "ppm" means "parts per million", i.e. the relative change in size per 1 K change in temperature is 10 -6 . A more negative coefficient of thermal expansion is also to be avoided. In the case of negative expansion, i.e. shrinkage, tensile stresses are generated in the surface of the glass-ceramic during heating. When the value is less than -0.5 ppm / K, the stress reduces the mechanical strength of the cooking surface at the typical operating temperatures of cooking appliances. A coefficient of thermal expansion above 1.9 ppm / K is also to be avoided. When the expansion exceeds 1.9 ppm / K, a sufficiently high thermal shock resistance cannot be ensured, so that the glass-ceramic cannot be used in cooking appliances with radiant heating elements.

[0021] In a further embodiment of the invention, the coefficient of thermal expansion is at least -0.4 ppm / K, -0.2 ppm / K, 0.0 ppm / K, 0.2 ppm / K, 0.4 ppm / K, 0.6 ppm / K, 0.8 ppm / K, or even 0.9 ppm / K. Additionally, the coefficient of thermal expansion is preferably at most 1.7 ppm / K, 1.5 ppm / K, 1.3 ppm / K, 1.1 ppm / K, 1.0 ppm / K, 0.8 ppm / K, or even at most only 0.6 ppm / K.

[0022] In a preferred embodiment of the invention, the coefficient of thermal expansion of the glass-ceramic is from -0.5 to 1.0 ppm / K, preferably from -0.1 to 0.8 ppm / K, particularly preferably from 0 to 0.6 ppm / K. This glass-ceramic is particularly suitable for the cooking surface of cooking appliances with radiant heating elements.

[0023] In a further preferred embodiment of the present invention, the glass-ceramic has a coefficient of thermal expansion of 0.5 to 1.9 ppm / K, preferably -0.7 to 1.7 ppm / K, particularly preferably 0.9 to 1.5 ppm / K. Such glass-ceramics are suitable, for example, for cooking surfaces in cooking appliances having induction heating elements.

[0024] The glass-ceramic according to the invention contains the following components in % by weight:

[0025] SiO2 60 - 70,

[0026] Al2O3 17 - 25, and

[0027] Li2O 2.0 - <3.2.

[0028] The components SiO2, Al2O3 together with Li2O in the glass-ceramic form the main constituents of the crystalline phases. At the same time, they essentially determine the glass-forming properties and the viscosity of the green glass.

[0029] The SiO2 content of the glass-ceramic according to the invention should be at most 70% by weight, since this component greatly increases the viscosity of the glass, in particular the processing point. For good glass melting and low forming temperatures, a higher SiO2 content is uneconomical. The minimum content of SiO2 should be 60% by weight, since this is advantageous for the required properties, such as chemical resistance and temperature resistance. When the SiO2 fraction is very high, above 70% by weight, deep quartz crystals are formed during the ceramization process. This leads to a sharp increase in thermal expansion.

[0030] Preferably, the glass-ceramic contains at least 61% by weight, 62% by weight, 63% by weight, 64% by weight, or even 65% by weight of SiO2. The more SiO2 the glass-ceramic contains, the better its temperature resistance and chemical resistance. Furthermore, it preferably contains at most 69% by weight, 68% by weight, 67% by weight, or even only 66% by weight of SiO2. The less SiO2 the glass-ceramic contains, the better the fusibility and processability of the green glass in hot forming.

[0031] The Al2O3 content of the glass-ceramic according to the invention is in the range of 17 to 25 weight percent. A higher Al2O3 fraction leads to devitrification problems and the formation of undesired mullite. Therefore, it should not exceed 25% by weight. An Al2O3 content below 17% by weight is disadvantageous for the formation of high quartz mixed crystals and promotes the formation of undesired crystalline phases.

[0032] Preferably, the glass-ceramic contains at least 18 wt%, 19 wt%, or even 20 wt% of Al2O3. The more Al2O3 the glass-ceramic contains, the better its heat resistance. In addition, it preferably contains at most 24 wt%, 23 wt%, 22 wt%, or even only 21 wt% of Al2O3. The less Al2O3 the glass-ceramic contains, the better the fusibility and processability of the green glass during thermoforming.

[0033] It has been shown that if the glass-ceramic contains 17 - <19.0 wt%, preferably 17.5 - 18.9 wt%, particularly preferably 18 - 18.8 wt% of Al2O3, it is particularly advantageous for the fusibility of the green glass.

[0034] It has been shown that if the glass-ceramic contains >21.0 - 25 wt%, preferably 21.5 - 24 wt%, particularly preferably 22.0 - 23 wt% of Al2O3, it is particularly advantageous for the heat resistance of the glass-ceramic.

[0035] The Li2O content of the glass-ceramic according to the present invention is in the range of 2.0 - <3.2 wt%. Surprisingly, it has been shown that when the Li2O content is within this range in combination with the remaining components within the above ranges, a glass-ceramic with high thermal shock resistance and good fusibility can be achieved. Since Li2O has a great influence on the thermal expansion of the glass-ceramic, Li2O is selected within the above range to be combined with the remaining components of the glass-ceramic of the present invention to achieve the required thermal shock resistance of the present invention. In addition, an Li2O fraction higher than 2.0 wt% has a positive effect on the manufacturability of the glass-ceramic because it reduces the electrical resistance of the glass melt, reduces the viscosity, and thus also reduces the processing point. By reducing the viscosity of the glass melt, the clarification efficiency can also be improved. The improved clarification results in fewer production rejects due to the formation of bubbles in the green glass.

[0036] In a preferred embodiment, the glass-ceramic contains at least 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, 2.5 wt%, 2.6 wt%, >2.7 wt%, or even >2.9 wt% of Li2O. As an upper limit, the glass-ceramic preferably contains at most 3.1 wt%, 3.0 wt%, or even 2.95 wt% of Li2O. Particularly preferably, the glass-ceramic contains 2.2 - <3.2 wt% or 2.4 - <3.2 wt% or >2.7 - <3.2 wt% or >2.9 - <3.2 wt% of Li2O. Glass-ceramics with particularly high thermal shock resistance can be obtained within these narrower ranges.

[0037] For cost reasons, natural mineral raw materials (such as spodumene or petalite) or alternatively Li2CO3 produced synthetically are usually used as the source of lithium. However, natural mineral raw materials contain impurities which may, for example, have an undesirable effect on the optical properties of the glass-ceramic. In addition, the amount of impurities in the natural raw materials may vary with each delivery, making it difficult to adjust the desired properties of the glass-ceramic. For this reason too, it is advantageous to minimize the amount of Li2O in the glass-ceramic.

[0038] In a preferred embodiment, the glass-ceramic contains high-quartz mixed crystals as the main crystal phase. "Main crystal phase" means that the glass-ceramic contains a greater volume fraction of high-quartz mixed crystals than of hydrothermal quartz mixed crystals. In a further embodiment of this embodiment, the glass-ceramic contains <10% by volume, preferably <5% by volume, particularly preferably <3% by volume of hydrothermal quartz mixed crystals. Here, % by volume refers to the volume of the glass-ceramic, preferably the volume of the crystal phase. The volume fraction is determined from the X-ray diffraction spectrum using Rietveld analysis.

[0039] Hydrothermal quartz mixed crystals generally have a higher thermal expansion than high-quartz mixed crystals. The coexistence of a high fraction of high-quartz mixed crystals and a low fraction of hydrothermal quartz mixed crystals is therefore particularly advantageous for the coefficient of thermal expansion of the glass-ceramic. It thus improves the thermal shock resistance of the glass-ceramic.

[0040] In addition to the above amounts of SiO2, Al2O3 and Li2O, the glass-ceramic according to the invention also contains 0 - <0.5% by weight of MgO. Since MgO causes an increase in the thermal expansion of the glass-ceramic, the amount of MgO in the glass-ceramic is limited to a maximum of <0.5% by weight. Preferably, the glass-ceramic contains a maximum of 0.4% by weight, 0.3% by weight or even 0.2% by weight of MgO.

[0041] In an advantageous design of the invention, it is preferred that the glass-ceramic contains a small amount of MgO. A small amount of MgO can be used to lower the processing point and the upper devitrification temperature. The glass-ceramic preferably may contain at least 0.05% by weight, and particularly preferably at least 0.1% by weight of MgO. MgO can also be introduced into the glass-ceramic as an impurity of the raw materials.

[0042] In addition, the glass-ceramic contains 1.5 - 3% by weight of ZnO. ZnO causes the formation of undesirable zinc spinel crystals, especially in combination with a large amount of Al2O3. Therefore, the amount in the glass-ceramic according to the invention is limited to <3% by weight. Additionally, experience has shown that glass-ceramics with a very high amount of ZnO tend to form undesirable crystals on the surface of the glass-ceramic. Therefore, the amount of ZnO is preferably limited to a maximum of 2.8% by weight, 2.6% by weight, 2.4% by weight, or even 2.2% by weight.

[0043] In the glass-ceramics according to the invention, ZnO reduces the processing point as well as the upper devitrification temperature. Thus, the glass-ceramics preferably contain at least 1.6 wt%, 1.7 wt%, 1.8 wt% or even at least 1.9 wt% of ZnO. The thermal shock resistance of the glass-ceramics is improved in these ranges.

[0044] In a preferred embodiment, for the above reasons, the glass-ceramics contain 1.6 - 2.8 wt%, preferably 1.7 - 2.6 wt%, particularly preferably 1.8 - 2.4 wt% of ZnO.

[0045] The glass-ceramics according to the invention contain 0.2 - 3 wt% of BaO. Like Li2O, BaO reduces the viscosity of the glass melt and thus the processing point. In order to improve the fusibility of the green glass, it is advantageous for the glass-ceramics to contain at least 0.3 wt%, preferably at least 0.4 wt%, 0.6 wt%, 0.8 wt% or even 1 wt% of BaO when combined with the specified amount of Li2O according to the invention. In the glass-ceramics, BaO also makes a significant contribution to improving the devitrification characteristics during the hot forming of the green glass.

[0046] However, it has been shown that BaO has a negative impact on the formation of the crystalline phase during the ceramization process. In order to avoid the need for a long ceramization time, the amount of BaO is therefore preferably limited to a maximum of 3 wt%, preferably a maximum of 2.9 wt%, particularly preferably 2.8 wt% or even a maximum of 2.7 wt%. The less BaO the glass-ceramics contain, the faster the ceramization proceeds.

[0047] The addition of the alkalis Na2O and K2O improves the fusibility and devitrification characteristics during the glass forming process. These two components can increase the electrical conductivity of the melt. This contributes to the energy coupling through the electrical heaters in the melting tank. However, since these components are not incorporated into the crystalline phase but remain essentially in the residual glass phase of the glass-ceramics, the content is limited. Excessive content will damage the crystallization behavior during the conversion of the crystallizable starting glass into glass-ceramics, particularly damaging the rapid ceramization rate here. In addition, a higher content has an adverse effect on the time / temperature tolerance of the glass-ceramics. Thus, the glass-ceramics according to the invention contain >0.05 - <0.6 wt% of Na2O and >0.05 - <0.5 wt% of K2O.

[0048] In a preferred further embodiment, the glass-ceramics contain 0.1–0.5 wt%, preferably 0.2–0.4 wt% of Na2O. In a further preferred further embodiment, the glass-ceramics contain 0.1–0.4 wt%, preferably 0.2–0.3 wt% of K2O.

[0049] The sum of the alkalis Na2O + K2O combined with the remaining components in the glass-ceramic is preferably at least 0.2% by weight and at most 1% by weight. Particularly preferably, the sum is at least 0.3% by weight, or at least 0.4% by weight or at least 0.5% by weight, and at most 1.0% by weight or at most 0.9% by weight or at most 0.8% by weight or at most 0.7% by weight or even 0.6% by weight. In these amounts, for the glass-ceramic according to the invention, a particularly good compromise between fusibility and devitrification is achieved without the ceramization rate deteriorating.

[0050] Furthermore, the glass-ceramic according to the invention meets the following conditions: MgO < K2O and MgO < Na2O.

[0051] Not only MgO but also K2O and Na2O act positively on the electrical conductivity of the melt. Since potassium and sodium are more mobile as ions than magnesium, they have a stronger influence on the conductivity. At the same time, MgO has a stronger influence on the thermal expansion of the glass-ceramic than K2O and Na2O. Therefore, it is advantageous for the glass-ceramic to contain more K2O and more Na2O than MgO.

[0052] Via the ratio of K2O to Na2O, the fusibility and thermal expansion can be finely adjusted. Na2O improves melting and reduces the viscosity of the glass melt more strongly than K2O in the present composition, but also increases the thermal expansion of the glass-ceramic more strongly.

[0053] In a first advantageous embodiment of the invention, the ratio of K2O to Na2O (in % by weight) is in the range of 0.1 - 2, preferably 0.5 - 1.5, particularly preferably 0.7 - 1.3. In this range, the above-mentioned properties are particularly compromised.

[0054] In a second advantageous embodiment of the invention, the ratio of K2O to Na2O (in % by weight) is in the range of 0.1 - <1, preferably 0.2 - 0.9, particularly preferably 0.3 - 0.8. This embodiment has improved fusibility and viscosity. This can be particularly advantageous when the remaining components contained in the glass-ceramic tend to be more refractory or have a higher viscosity in combination.

[0055] In a third advantageous embodiment of the invention, the ratio of K2O to Na2O (in % by weight) is in the range of 1 - 2, preferably 1.1 - 1.9, particularly preferably 1.2 - 1.8. This embodiment has improved thermal expansion. This can be particularly advantageous when the remaining components contained in the glass-ceramic tend to have a higher thermal expansion in combination.

[0056] The glass-ceramics according to the invention contain 0.1 - <1.0 wt% of SnO₂. 0.1 wt% of SnO₂ in combination with the other components of the glass-ceramics according to the invention is advantageous to ensure sufficient nucleation for the properties according to the invention. However, an amount of <1.0 wt% should not be exceeded. Higher contents lead to the precipitation of Sn-containing crystalline phases at the contact materials (e.g., Pt / Rh) during shaping and should be avoided. Preferably, the glass-ceramics contain at most 0.8 wt%, 0.6 wt% or even only 0.4 wt% of SnO₂.

[0057] In a further embodiment of the invention, the glass-ceramics can contain 0.1 - 0.8 wt%, preferably 0.2 - 0.7, particularly preferably 0.3 - 0.6 wt% of SnO₂. SnO₂ in these amounts can support the clarification of green glass. The glass-ceramics with SnO₂ in these amounts are remarkable for particularly few defects caused by trapped air bubbles.

[0058] In a further additional embodiment of the invention, the glass-ceramics can contain 0 - 0.8 wt%, preferably 0.1 - 0.6 wt%, particularly preferably 0.2 - 0.4 wt% of CeO₂. CeO₂ in combination with SnO₂ can also support clarification and improve the bubble quality.

[0059] In a preferred embodiment, the glass-ceramics contain TiO₂. TiO₂ together with SnO₂ contributes to nucleation. The amount of TiO₂ is limited to a maximum value of 5 wt%. Larger amounts of TiO₂ would lead to devitrification during thermoforming. Additionally, it would lead to an undesired increase in the refractive index of the residual glass phase. Preferably, the glass-ceramics contain at least 1 wt%, 1.5 wt%, 2.0 wt%, >2.5 wt% or even 3.0 wt% of TiO₂. At the same time, it contains preferably at most 4.5 wt%, 4.2 wt%, 4.0 wt%, 3.8 wt%, 3.6 wt% or even only 3.4 wt% of TiO₂. In the case of a higher TiO₂ fraction, nucleation proceeds faster. Thereby, the ceramization time of the glass-ceramics can be reduced. A lower TiO₂ fraction can stabilize the ceramization process and prevent accidental devitrification of the green glass during thermoforming.

[0060] In a preferred embodiment, the glass-ceramics can contain, for example, 1 - 5 wt%, preferably 2 - 4.5 wt%, particularly preferably >2.5 - 4.0% of TiO₂ for the above reasons.

[0061] Furthermore, the glass-ceramics preferably contain 1.0 - 4.0 wt% of ZrO₂. ZrO₂ and SnO₂ mainly act as nucleating agents in the glass-ceramics and interact closely as nucleating agents. A content of 1.0 wt% of ZrO₂ in combination with the above amounts of SnO₂ and TiO₂ is advantageous for improving nucleation.

[0062] The amount of ZrO2 is limited to a value of 4.0 wt%, since ZrO2 increases the viscosity of the glass melt and thus also the processing point. Additionally, ZrO2 can cause devitrification during the thermoforming process. Here, this can lead to the formation of undesired baddeleyite. Preferably, the glass-ceramic contains at least > 1.3 wt%, particularly preferably > 1.7 wt%, of ZrO2. Additionally, it preferably contains at most 3.9 wt%, 3.8 wt%, 3.2 wt%, 3.0 wt% or even only 2.0 wt% of ZrO2. In these amounts, a particularly good compromise can be achieved between the positive contribution to nucleation and the acceptable deterioration of fusibility and thermoforming.

[0063] In a further embodiment of the invention, for the above reasons, the glass-ceramic contains 1.0 - 4.0 wt%, preferably > 1.3 - 3.9 wt%, particularly preferably > 1.7 - 3.8 wt%, of ZrO2.

[0064] In the production of glass-ceramics, As2O3 and Sb2O3 are usually used as clarifying agents. However, in the glass-ceramics according to the invention, these components have surprisingly proven to be disadvantageous for devitrification stability. Therefore, the amounts of As2O3 and Sb2O3 are preferably each limited to less than 0.1 wt%. Particularly preferably, the glass-ceramics each contain less than 0.09 wt%, 0.08 wt%, 0.07 wt%, 0.06 wt% or even less than 0.05 wt% of As2O3 and Sb2. Particularly preferably, apart from unavoidable traces, the glass-ceramics do not contain As2O3 and Sb2O3.

[0065] However, As2O3 and Sb2O3 can occur as impurities in the glass-ceramics, especially when using batches containing As2O3 and Sb2O3 for manufacturing the glass-ceramics. This is especially the case when using batches from the recycling cycle for cooking surfaces. For reasons of environmental protection and sustainability, it is advantageous to use batches from the recycling cycle as raw materials. Therefore, the glass-ceramics preferably contain at least 0.01 wt%, 0.02 wt%, 0.03 wt% or even at least 0.04 wt% of As2O3 and / or Sb2O3, respectively. If both As2O3 and Sb2O3 are present, they are each present in said amounts.

[0066] The addition of alkaline earth metals CaO, SrO, and B2O3 improves the fusibility and devitrification characteristics during glass forming. CaO can especially be contained in the glass-ceramic to lower the processing point and the upper devitrification temperature. However, since these components do not incorporate into the crystal phase but remain essentially in the residual glass phase of the glass-ceramic, their content is limited. Excessive content impairs the crystallization characteristics when the starting glass that can be crystallized transforms into the glass-ceramic, especially the rapid ceramization rate here. In addition, higher content acts unfavorably on the time / temperature tolerance of the glass-ceramic. Therefore, the glass-ceramic contains each of these components in an amount of 0 - 2 wt%.

[0067] In a further embodiment of the invention, the glass-ceramic contains 0 - <1 wt% of P2O5. P2O5 acts positively on the devitrification stability of the green glass. However, a larger amount will reduce the ceramization rate and act negatively on the acid resistance of the glass-ceramic. Therefore, the amount of P2O5 is preferably limited to a maximum of <1 wt%, preferably a maximum of 0.9 wt%, particularly preferably a maximum of 0.8 wt%. To improve the devitrification stability, it is advantageous for the glass-ceramic to contain at least 0.01 wt%, preferably at least 0.05 wt%, particularly preferably at least 0.1 wt% of P2O5.

[0068] In a further embodiment of the invention, it may be advantageous for the glass-ceramic to contain Cl - . The addition of a certain amount of Cl - results in better bubble quality of the green glass and thus better bubble quality of the glass-ceramic. It has been proven that, in combination with the remaining components, the glass-ceramic contains 0.003 - 0.1 wt%, preferably 0.005 - 0.03 wt%, particularly preferably 0.007 - 0.02 wt% of Cl - is particularly advantageous. An amount below 30 ppm does not have a sufficient effect on the bubble quality. Amounts above 1000 ppm should be avoided because the added chloride part may react with other components in the mixture as well as with the process exhaust gas. For example, HCl may be formed here, which can cause corrosive damage to the tank. In addition, the evaporation of alkali metal chlorides and alkaline earth metal chlorides is also undesirable. The amount of Cl in the glass-ceramic can be adjusted, for example, by adding NaCl to the mixture - .

[0069] In addition to these components, in a further embodiment of the invention, the glass-ceramic can also contain coloring components. As coloring components, for example, V2O5, CoO, Fe2O3, Cr2O3, Nd2O3, NiO, CuO, MnO, or MoO3 can be included singly or in combination. The specific selection of the type and amount of the coloring component depends on the optical characteristics to be achieved.

[0070] The coloring of glass ceramics is a complex and non-linear process. Many of the components contained in glass ceramics can affect the degree to which the coloring components absorb light. Therefore, those skilled in the art will adjust the amount of the coloring component according to the respective basic compositions of the glass ceramics to obtain the desired optical properties.

[0071] The coloring of the glass ceramics according to the invention by means of V2O5 as the main coloring agent is shown here by way of example as follows. In order to reduce the transmittance to the desired value, more V2O5 will be used compared to comparable glass ceramics with a higher Li2O fraction. Thus, the reduction of Li2O results in a weaker absorption of V2O5 in the glass ceramics. Similar, and sometimes even opposite, correlations also exist with other components of the basic composition.

[0072] Even small amounts of V2O5 can generally cause very intense coloring in glass ceramics. Glass ceramics colored with V2O5 have a relatively low transmittance in the blue and green spectral ranges, while having a relatively high transmittance in the red spectral range. The glass ceramics preferably contain 0 to 0.1% by weight of V2O5. Particularly preferably, they contain >0.002 to 0.08% by weight, >0.003 to 0.07% by weight, >0.004 to 0.06% by weight, >0.005 to 0.05% by weight, or even >0.01 - 0.04% by weight of V2O5. With these amounts of V2O5, for a thickness of 4 mm, the light transmittance of the glass ceramics can be adjusted within the range of 0.1% to 80%.

[0073] In a particularly preferred further embodiment of the above-described embodiment, the ratio V2O5 / Li2O is 0.005 - 0.06, preferably 0.007 - 0.055, particularly preferably 0.01 - 0.05. Without limitation of generality, it is assumed that the coloring effect of V2O5 depends on the microstructure of the glass ceramics. The glass ceramics according to the invention have a relatively low crystalline phase fraction due to the low Li2O fraction, and at the same time, the grain size is small. It has been shown that particularly effective coloring can be achieved when the ratio of V2O5 to Li2O is adjusted within the above range. When this ratio is set within this range, for a thickness of 4 mm, the spectral transmittance at a wavelength of 630 nm can reach within the range of 0.5 - 15%, preferably within the range of 1 - 13%, particularly preferably within the range of 2 - 10%. In the case of these transmittances, commercially available red light-emitting displays can be applied when using the glass ceramics as a cooking surface.

[0074] With the help of MoO₃, the glass-ceramic can be colored to be particularly neutral. The advantage of this is that a light-emitting display with a white light color can be applied in a cooking appliance without changing the color of the light of the display when passing through the glass-ceramic. The glass-ceramic preferably contains 0 to 0.5% by weight of MoO₃. Particularly preferably, it contains >0.002 to 0.4% by weight, >0.003 to 0.3% by weight, >0.004 to 0.2% by weight, >0.005 to 0.15% by weight, or even >0.01 to 0.1% by weight of MoO₃. With these amounts of MoO₃, for a thickness of 4 mm, the light transmittance of the glass-ceramic can be adjusted in the range of 0.1% to 80%. At the same time, a white light-emitting display can achieve a colorless-distortion presentation.

[0075] In a particularly preferred further embodiment of the above-described embodiment, the ratio of MoO₃ / Li₂O is 0.015 - 0.1, preferably 0.02 - 0.08, particularly preferably 0.025 - 0.07. By setting this ratio within this range, for a thickness of 4 mm, a light transmittance in the range of 0.5 - 4%, preferably 0.8 - 3.5%, particularly preferably 0.7 - 3.3%, more particularly preferably 1.0 - 3.0% can be achieved. With these transmittances, a white light-emitting display can be used when the glass-ceramic is used as a cooking surface. At the same time, the visibility of the components present inside the cooking appliance is also greatly reduced.

[0076] Nd₂O₃ can also be used for coloring. It is different from the other colorants in that it produces relatively narrow absorption bands in the glass-ceramic. These absorption bands are mainly located in the green spectral range. With a small amount of Nd₂O₃, the color coordinates of the transmitted light passing through the glass-ceramic can be finely adjusted. For example, a glass-ceramic containing only a small amount of Fe₂O₃ as a coloring component usually has a yellowish tint. This may occur, for example, when the glass-ceramic contains both TiO₂ and Fe₂O₃ introduced through raw material impurities at the same time. If such a glass-ceramic has, for example, a white bottom coating, these bottom coatings have a clearly perceptible yellowish tint. In such a glass-ceramic, the addition of Nd₂O₃ can be used to reduce or eliminate the yellowish tint without significantly reducing the light transmittance. This makes it possible to produce a cooking surface with a white appearance.

[0077] Preferably, the amount of Nd₂O₃ contained in the glass-ceramic is 0 - 0.6% by weight. Since Nd₂O₃ is relatively expensive, its amount should be limited to 0.6% by weight. Particularly preferably, the glass-ceramic contains 0.005 - 0.5% by weight, 0.01 - 0.4% by weight, 0.02 - 0.3% by weight, 0.03 - 0.2% by weight, or even 0.04 - 0.1% by weight of Nd₂O₃.

[0078] Fe2O3 affects not only the transmittance in the visible spectral range but also the transmittance of near-infrared light with wavelengths up to about 3 μm. Therefore, Fe2O3 not only affects the achievability of a specific color or the presentability of a color display. The absorption of near-infrared light determines how much thermal energy can be absorbed by the glass melt in the trough. This determines how much of the heating power of the radiant heating element can pass through the glass-ceramic. This also determines whether and which infrared sensors can be used in the stove. For example, these sensors can be designed as optical touch sensors or infrared receivers for wireless data transmission. At the same time, Fe2O3 is often contained as an impurity in the raw materials used for production. A higher content of Fe2O3 in the glass-ceramic makes it possible to use raw materials with a higher impurity content but lower cost. All of these must be considered when selecting an appropriate amount of Fe2O3. The amount of Fe2O3 should preferably be 0 - 0.4 wt%. Glass-ceramics containing more than 0.4 wt% of Fe2O3 are incompatible with commercially available radiant heating elements for cooking appliances due to the low transmittance of near-infrared light. Preferably, the glass-ceramic contains 0.005 - 0.3 wt%, 0.01 - 0.25 wt%, 0.02 - 0.2 wt%, or even 0.04 - 0.18 wt% of Fe2O3. Such glass-ceramics can be produced at low cost and are at the same time compatible with radiant heating elements and optical sensors for cooking appliances. Fe2O3 is usually contained as an impurity in the raw materials used for glass production, for example, spodumene.

[0079] CoO can be contained in the glass-ceramic, for example, in an amount of 0 - 0.5 wt%. Preferably, it is contained in an amount of 0.01 - 0.2 wt%, more preferably 0.02 - 0.08 wt%, and particularly preferably 0.04 - 0.06 wt%.

[0080] Preferably, the glass-ceramic colored with 0.02 - 0.1 wt% of CoO additionally contains 0.02 - 0.1 wt% of Cr2O3. The glass-ceramic particularly preferably also contains 0.05 - 0.25 wt% of Fe2O3 and especially <30 ppm of V2O5. With these amounts of CoO and preferably other colorants, the light transmittance of the glass-ceramic can be adjusted in the range of 0.1% to 80% for a thickness of 4 mm. Thus, a white display can also be achieved in the warm white spectral range.

[0081] Different from V2O5, MoO3 or CoO, Cr2O3, NiO, CuO and MnO are usually used for auxiliary coloring, but they are rarely used as main coloring agents. Herein, the main coloring agent refers to the coloring component that has the most important influence on the transmittance of the glass-ceramic in the visible spectral range. They usually appear as impurities in the raw materials. These components are preferably included in the glass-ceramic in an amount of 0 to 0.5% by weight respectively. Particularly preferably, they are included in the glass-ceramic in an amount of 0.001 - 0.4% by weight, 0.002 - 0.3% by weight, 0.004 - 0.2% by weight, 0.006 - 0.1% by weight, 0.008 - 0.08% by weight, or even 0.01 - 0.05% by weight.

[0082] In a preferred embodiment, the glass-ceramic contains 0 to 0.1% by weight of V2O5 or 0 to 0.5% by weight of MoO3 or 0 to 0.6% by weight of Nd2O3 or 0 to 0.4% by weight of Fe2O3 or 0 to 0.5% by weight of CoO or 0 to 0.5% by weight of Cr2O3 or 0 to 0.5% by weight of NiO or 0 to 0.5% by weight of CuO or 0 to 0.5% by weight of MnO or a combination of these components.

[0083] In addition to the coloring effect, these components can also have a positive impact on the glass quality. This is especially the case for components that absorb in the infrared spectral range in the glass melt. Due to the absorption of infrared light, the heat introduced into the melting tank by the heating device can be more effectively absorbed by the glass melt. This will cause the temperature of the glass melt to rise under the same energy input. This has a positive impact on both the melting of raw materials that are difficult to melt and the reduction of bubbles during clarification. This is especially the case for the above amounts of Fe2O3, CoO and NiO.

[0084] In a further aspect of the present invention, for a thickness of 4 mm, the glass-ceramic has a light transmittance of 80 - 90% or 81 - 89% or 82 - 88% or even 83 - 87%. For a thickness of 4 mm, the glass-ceramic having such a light transmittance preferably has a transmission chromaticity C* in the range of 0 - 6, preferably 1.5 - 5, particularly preferably 3.0 - 4.6.

[0085] Herein, "for a thickness of 4 mm" means that the corresponding property is either determined on a sample with a material thickness of 4 mm or determined for different material thicknesses and converted to a material thickness of 4 mm. For the transmittance data, the Lambert-Beer law can be used for conversion.

[0086] According to the provisions of DIN 5033, the light transmittance is determined using the standard light type D65 in the wavelength range of 380 - 780 nm. This value corresponds to the luminance Y in the CIExyY color space.

[0087] The chromaticity C* is determined from the L*a*b* color coordinates according to the following formula:

[0088]

[0089] The color coordinates a* and b* are determined from the transmission spectrum of the glass-ceramic in a known manner using standard light of standard light type D65.

[0090] Glass-ceramics having a light transmittance of 80 - 90% for a thickness of 4 mm are particularly suitable for use as fireplace viewing windows or cooking plates. In a fireplace, this transmittance makes the flame particularly clearly visible. For example, in a cooking appliance, this transmittance makes light-emitting displays with a relatively low light-emitting density (such as LCD or OLED displays) particularly clearly visible.

[0091] A chromaticity C* of 0 - 6 causes only a very slight change in the color of light when passing through the glass-ceramic. For example, this enables the glass-ceramic to have a white coating, which still produces a white impression when viewed through the glass-ceramic. This is particularly important, for example, when used as a cooking surface or a fireplace viewing window. The glass-ceramic in these applications typically has a thickness of 4 mm. The light reflected by the rear coating thus travels an optical path of 8 mm, making the color shift caused by the inherent color of the glass-ceramic have a greater effect compared to when the path is shorter. Therefore, a correspondingly low chromaticity is particularly advantageous for a cooking surface or a fireplace viewing window with a white coating on the rear side.

[0092] In a further embodiment of the present invention, a glass-ceramic having a light transmittance in the range of 80 - 90% or a correspondingly preferred range and having a chromaticity C* in the range of 2 - 6 or a correspondingly preferred range, in addition to the composition according to the present invention, further comprises one or more of the following components (in wt%):

[0093] Nd2O3 0.005 - 0.1, preferably 0.01 - 0.08, particularly preferably 0.03 - 0.065,

[0094] Fe2O3 0 - 0.02, preferably 0.0025 - 0.018, particularly preferably 0.005 - 0.016,

[0095] V2O5 0 - 0.0015, preferably 0 - 0.001, particularly preferably 0 - 0.0005,

[0096] Cr2O3 0 - 0.001, preferably 0 - 0.0005, particularly preferably 0 - 0.0003,

[0097] MoO3 0 - 0.001, preferably 0 - 0.0008, particularly preferably 0 - 0.0006,

[0098] CoO 0 - 0.001, preferably 0 - 0.0005, particularly preferably 0 - 0.0001,

[0099] NiO 0 - 0.001, preferably 0 - 0.0005, particularly preferably 0 - 0.0001,

[0100] CuO 0 - 0.001, preferably 0 - 0.0007, particularly preferably 0 - 0.0002,

[0101] MnO 0 - 0.02, preferably 0 - 0.01, particularly preferably 0 - 0.006,

[0102] TiO2 1.6 - 2.5, preferably 2.0 - 2.4, particularly preferably 2.2 - 2.3,

[0103] ZrO2 0 - 2.2 or 0.1 - 2.0 or 0.2 - 1.8, or even 0.3 - 1.6,

[0104] SnO2 0.1 - 0.2, preferably 0.1 - 0.18, particularly preferably 0.1 - 0.15.

[0105] In a particularly preferred further embodiment, the glass - ceramic contains all these components present in these amounts. If these components are included in the glass - ceramic in the amounts mentioned here, then it is further preferred that the sum of Fe2O3 + V2O5 + Cr2O3 is 0 - 0.0225 wt%, preferably 0.0005 - 0.0175 wt%, particularly preferably 0.0010 - 0.0170 wt%.

[0106] These components, either alone or in combination with each other, affect the light transmittance and chromaticity of the glass - ceramic. If the above - mentioned amounts are adhered to, the light transmittance and chromaticity can be finely adjusted within the above - mentioned ranges.

[0107] The following table contains three embodiments of the glass - ceramic according to the invention, based on oxides and in wt%:

[0108]

[0109]

[0110] The uses of the glass - ceramic according to the invention are as a cooking surface, a fireplace viewing window, a grill or frying surface, a lid for a combustion element in a gas grill, an oven viewing window (especially a viewing window of a pyrolysis furnace), a workbench or tabletop in a kitchen or laboratory, a lid for a lighting device, a refractory glass and a safety glass, optionally for laminated composites, a carrier plate in heat treatment or an oven lining or the back cover of a mobile electronic device.

[0111] The glass-ceramics according to the invention can in particular be used as a cooking surface. The cooking surface can here be provided, in whole or in part, with a decorative or functional coating on the top and / or bottom surface. A touch sensor for operating the cooking surface can also be provided on the bottom surface. This can be, for example, a printed, bonded or pressed capacitive sensor.

[0112] Furthermore, the glass-ceramics can also be present in the form of a three-dimensionally shaped plate. This means that the plate can be angled or curved, or for example contain regions shaped in the form of a wok. A cut-out for operating a gas burner, for example, is also possible. Detailed description

[0113] The invention will now be explained in more detail on the basis of embodiments.

[0114] The crystallizable green glass of the example was melted from technical batch raw materials commonly used in the glass industry at a temperature of 1680 °C for 4 hours. This choice can meet the requirements for economical raw materials and for undesired impurities with a low impurity content. After melting the batch in a crucible made of sintered quartz glass, the melt was poured into a Pt / Rh crucible with an inner crucible made of quartz glass and homogenized by stirring at a temperature of 1600 °C for 90 minutes. After homogenization, the glass was clarified at 1640 °C for 3 hours. Subsequently, blocks with dimensions of approximately 120×140×30 mm 3 were cast and the cooling in the cooling furnace was started at 30 K / h from 640 - 670 °C to room temperature (depending on the viscosity of the glass) in order to reduce stress. The cast blocks were divided into the dimensions required for research and ceramization.

[0115] The ceramization of the samples in the green glass state was carried out in a continuous furnace by a ceramization process in the following steps:

[0116] a) Heating from room temperature to 740 °C at a heating rate of 30 K / min,

[0117] b) Holding at 740 °C for 3 minutes and 20 seconds,

[0118] c) Increasing the temperature from 740 °C to 810 °C at a heating rate of 28 K / min,

[0119] d) Holding at 810 °C for 9 minutes and 20 seconds,

[0120] e) Increasing the temperature from 810 °C to 930 °C at a heating rate of 21 K / min,

[0121] f) Holding at 930 °C for 6 minutes,

[0122] g) Cooling to room temperature at a cooling rate of 15 K / min.

[0123] The following table contains the compositions and material properties of examples according to the present invention.

[0124] In the case of a heating rate of 2 K / min, the coefficient of thermal expansion (CTE) of the rod-shaped sample is determined dynamically with a push-rod dilatometer.

[0125] To measure the upper devitrification temperature (OEG), the green glass is melted in a Pt / Rh10 crucible. The crucible is then held for 5 hours at different temperatures within the processing temperature range. The highest temperature at which the first crystals appear at the contact surface of the glass melt with the crucible wall determines the OEG.

[0126] According to the DIN ISO 7884-2 standard, a stirrup viscometer is used to determine the processing point (T4) of the green glass.

[0127] When the green glass is transformed into glass-ceramics, the density increases because the density of the crystalline phase is higher than that of the amorphous glass. The shrinkage rate represents the linear change in length when the green glass transforms into glass-ceramics. It is calculated based on the density of the green glass and the density of the glass-ceramics:

[0128]

[0129] Tg represents the transformation temperature of the green glass, also known as the glass transition temperature. It is determined by the dilatometric method.

[0130] In accordance with DIN 5033, the light transmittance is determined in the wavelength range of 380 - 780 nm using light of the standard light type D65. This value corresponds to the luminance Y in the CIExyY color space. This value is a measure of the human eye's brightness perception.

[0131] According to the xy color coordinates of the CIExyY color space, the distance d to the color coordinates (0.3127 / 0.3290) of the standard light of the standard light type D65 is determined as follows:

[0132]

[0133] The transmission spectrum is determined according to ISO 15368:2021. Table 2 exemplarily contains the spectral transmittances "T@..." for wavelengths of 470 nm, 600 nm, 630 nm, 700 nm, 950 nm, and 1600 nm.

[0134] According to the provisions of CIE 1932, the color coordinates in the CIExyY color space and the Lab color space are determined in transmission with an 8° observer angle and using light of the standard light source D65.

[0135] All transmission measurements are performed on samples with a thickness of 4 mm and smooth surfaces on both sides.

[0136] Use Rietveld analysis to determine the volume fraction "XRD fraction HQMK" or "KMK" and the grain size of the crystalline phase "XRD grain size HQMK" or "KMK" based on the X-ray diffraction spectrum.

[0137] Example 1 2 3 4 5 6 7 8 <![CDATA[Li2O]]> 3.010 2.780 2.600 2.610 2.700 2.710 3.000 3.040 <![CDATA[Na2O]]> 0.392 0.274 0.384 0.410 0.284 0.272 0.383 0.310 <![CDATA[K2O]]> 0.408 0.452 0.279 0.276 0.333 0.314 0.420 0.250 MgO 0.199 0.198 0.100 0.099 0.099 0.096 0.206 0.340 CaO 0.814 0.813 0.775 0.916 0.841 0.897 1.110 0.910 SrO 0.017 0.017 0.019 0.000 0.012 0.013 0.013 0.020 BaO 1.790 1.720 2.020 2.050 1.270 1.360 1.410 2.550 ZnO 2.210 2.510 2.240 2.510 2.490 2.390 2.430 2.630 <![CDATA[Al2O3]]> 21.420 21.300 21.462 21.400 21.550 21.400 20.662 18.720 <![CDATA[SiO2]]> 64.700 64.799 65.006 64.600 65.300 65.400 65.205 66.140 <![CDATA[TiO2]]> 3.180 3.180 3.190 3.180 3.190 3.170 3.170 3.140 <![CDATA[ZrO2]]> 1.392 1.396 1.399 1.390 1.400 1.392 1.392 1.400 <![CDATA[SnO2]]> 0.282 0.279 0.281 0.282 0.277 0.278 0.275 0.280 <![CDATA[Fe2O3]]> 0.093 0.091 0.090 0.094 0.090 0.115 0.118 0.116 <![CDATA[V2O5]]> 0.027 0.026 0.027 0.029 0.024 0.050 0.058 0.021 <![CDATA[Cr2O3]]> 0.006 0.004 0.003 0.005 0.000 0.000 0.005 0.005 <![CDATA[MnO2]]> 0.020 0.017 0.016 0.018 0.000 0.000 0.020 0.017 <![CDATA[P2O5]]> 0.074 0.072 0.067 0.070 0.005 0.005 0.076 0.053

[0138] Table 1: Composition of examples according to the present invention

[0139] Example 9 10 11 12 13 <![CDATA[Li2O]]> 2.590 2.870 2.780 2.860 2.180 <![CDATA[Na2O]]> 0.289 0.394 0.288 0.288 0.395 <![CDATA[K2O]]> 0.359 0.287 0.314 0.319 0.491 MgO 0.052 0.102 0.098 0.094 0.190 CaO 0.810 0.710 0.769 0.709 0.910 SrO 0.018 0.023 0.018 0.023 0.024 BaO 1.900 2.500 1.940 2.500 2.520 ZnO 2.010 2.030 2.030 2.000 2.000 <![CDATA[Al2O3]]> 21.400 21.440 21.340 21.180 21.370 <![CDATA[SiO2]]> 65.500 64.600 65.300 64.900 64.800 <![CDATA[TiO2]]> 3.180 3.180 3.160 3.170 3.180 <![CDATA[ZrO2]]> 1.390 1.390 1.401 1.392 1.403 <![CDATA[SnO2]]> 0.278 0.285 0.280 0.275 0.275 <![CDATA[Fe2O3]]> 0.094 0.093 0.116 0.120 0.090 <![CDATA[V2O5]]> 0.026 0.026 0.052 0.056 0.029 <![CDATA[Cr2O3]]> 0.005 0.004 0.005 0.005 0.005 <![CDATA[MnO2]]> 0.017 0.019 0.018 0.019 0.014 <![CDATA[P2O5]]> 0.073 0.077 0.073 0.074 0.065

[0140] Continued Table 1

[0141]

[0142] Table 2: Material properties of examples according to the present invention

[0143]

[0144] Continued Table 2.

Claims

1. A lithium aluminosilicate glass-ceramic, having a coefficient of thermal expansion in the range of -0.5 to 1.9 ppm / K from 20 °C to 700 °C, and comprising, on an oxide basis, a composition by weight %: and the condition MgO < K2O MgO < Na2O.

2. The lithium aluminosilicate glass ceramic according to claim 1, wherein The lithium aluminosilicate glass-ceramic contains 2.2 - < 3.2 wt% or 2.4 - < 3.2 wt% or > 2.7 - < 3.2 wt% or > 2.9 - < 3.2 wt% of Li2O.

3. The lithium aluminosilicate glass-ceramic according to claim 1, wherein The lithium aluminosilicate glass-ceramic contains 1.6 - 2.8 wt%, preferably 1.7 - 2.6 wt%, particularly preferably 1.8 - 2.4 wt% of ZnO.

4. The lithium aluminosilicate glass-ceramic according to claim 1, wherein The lithium aluminosilicate glass-ceramic contains 1 - 5 wt%, preferably 2 - 4.5 wt%, particularly preferably > 2.5 - 4.0% of TiO2.

5. The lithium aluminosilicate glass-ceramic according to any one of the preceding claims, characterized in that, The lithium aluminosilicate glass-ceramic contains 1.0 - 4.0 wt%, preferably > 1.3 - 3.9 wt%, particularly preferably > 1.7 - 3.8 wt% of ZrO2.

6. The lithium aluminosilicate glass-ceramic according to any one of the preceding claims, characterized in that, The amount of Na2O + K2O is at least 0.2 wt% and at most 1 wt%, preferably at least 0.3 wt%, or at least 0.4 wt% or at least 0.5 wt%, and at most 1.0 wt% or at most 0.9 wt% or at most 0.8 wt% or at most 0.7 wt% or even 0.6 wt%.

7. The lithium aluminosilicate glass-ceramic according to any one of the preceding claims, characterized in that, Less than 0.1 wt% of As2O3 and less than 0.1 wt% of Sb2O3.

8. The lithium aluminosilicate glass-ceramic according to any one of the preceding claims, characterized in that, The ratio K2O / Na2O (by weight %) is in the range of 0.1 - 2, preferably 0.5 - 1.5, particularly preferably 0.7 - 1.3, or in the range of 0.1 - < 1, preferably 0.2 - 0.9, particularly preferably 0.3 - 0.8, or in the range of 1 - 2, preferably 1.1 - 1.9, particularly preferably 1.2 - 1.

8.

9. The lithium aluminosilicate glass-ceramic according to any one of the preceding claims, characterized in that, The lithium aluminosilicate glass-ceramic contains 0 to 0.1 wt% of V2O5 or 0 to 0.5 wt% of MoO3 or 0 to 0.6 wt% of Nd2O3 or 0 to 0.4 wt% of Fe2O3 or 0 to 0.5 wt% of CoO or 0 to 0.5 wt% of Cr2O3 or 0 to 0.5 wt% of NiO or 0 to 0.5 wt% of CuO or 0 to 0.5 wt% of MnO or a combination of these components.

10. Use of the glass-ceramic according to any one of the preceding claims as: a cooking surface; a fireplace viewing window; a grill or frying surface; a lid for a combustion element in a gas grill; an oven viewing window, in particular a viewing window of a pyrolysis oven; a workbench or tabletop in a kitchen or laboratory; a lid for a lighting device; a refractory glass and a safety glass; an optional laminated composite material; a carrier plate or an oven lining in heat treatment or a rear cover of a mobile electronic device.

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