LITHIUM ALUMINUM SILICATE GLASS CERAMIC

Description

Field of the invention

[0001] The invention relates to a lithium aluminum silicate glass ceramic which is suitable for use as a cooking surface in cooking appliances and to the use thereof. Background of the invention

[0002] Glass-ceramic cooktops and the glass-ceramics used for them have been known for many years. Lithium aluminum silicate (LAS) glass-ceramics are used for this purpose, which contain either high-quartz solid solution (HQMK), particularly for transparent materials, or keatite solid solution (KMK), particularly for translucent or opaque materials, as the main crystal phase. To produce such glass-ceramics, starting glasses, so-called green glasses, are first produced using processes common to glass production. These green glasses are then converted into glass-ceramics through a thermal treatment known as ceramization.

[0003] The key property of these materials for use as cooking surfaces is their very low thermal expansion over a temperature range from room temperature to 700°C. This low thermal expansion, in turn, results in high resistance to thermal shock. The thermal expansion is achieved by combining crystal phases with negative thermal expansion and an amorphous residual glass phase with positive thermal expansion. The glass ceramics used to date typically contain a lithium content of more than 3.6 to 5.0 weight percent.

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

[0005] The price trend for lithium is not a new problem. Over the past 20 years, the price of lithium has risen continuously. Despite this long-standing need, most commercially available glass-ceramics for cooktops contain approximately 3.8 wt.% Li2O. To date, it has not been possible to find a glass-ceramic containing less than 3.5% Li2O that would be competitive with the glass-ceramics currently available on the market.

[0006] Glass ceramics with a Li 2 O content of less than 3.5 wt.% are known from the following documents: WO 2012 / 010341 A1, EP 3502069 A1, US 2017050880, US 2020189965, US2020140322, US2021387899, WO2021 / 224412 A1. However, these glass ceramics have various disadvantages, such as reduced thermal shock resistance or poor meltability of the green glass.

[0007] In this context, a transparent glass-ceramic is generally understood to be a glass-ceramic with low light scattering. The transmission of a transparent glass-ceramic can be adjusted over a wide range using absorbing, i.e., coloring, components.

[0008] When transparent glass ceramics are used for cooking surfaces, they are either colored by adding color oxides or provided with an underside coating to visually conceal the technical installations beneath the cooking surface. Various color oxides can be used for the volume coloring of the glass ceramic. These include, in particular, V 2 O 5 , CoO, Fe 2 O 3 , Cr 2 O 3 , Nd 2 O 3 , NiO, CuO, MnO and MoO 3 . Each of these color oxides has a different effect on the absorption of the glass ceramics in the visible and infrared spectral ranges. The coloring of glass ceramics is described in the following documents, among others: WO 11089220 A1, US 8765619, DE 102008050263 B4, DE 102009013127 B4. Object of the invention

[0009] The object of the invention is to provide a lithium aluminum silicate glass ceramic which has good melting properties of the green glass and is cost-effective without resulting in restrictions in the performance properties.

[0010] Good melting properties include, among other things, a processing point of less than 1340 °C, preferably less than 1330 °C, and particularly preferably less than 1320 °C. The processing point is the temperature at which the green glass has a viscosity of 10 4 < dPa*s. Hot forming of the green glass takes place near this temperature. The higher the hot forming temperature, the more difficult it is to dissipate the heat introduced into the forming machines by the glass. At temperatures above 1340 °C, this can only be achieved by reducing the amount of heat by reducing the throughput of the glass. However, this is economically disadvantageous.

[0011] If the upper devitrification temperature is exceeded during hot forming, undesirable spontaneous crystallization may occur. To prevent this, the upper 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] The glass ceramic should meet all requirements for use as a cooking surface with all types of heating elements, including radiant, induction, and gas heating elements. This requires, in particular, sufficiently high thermal shock resistance as well as high long-term temperature stability. Summary of the invention

[0013] The object of the invention is achieved by the subject matter of the independent claims. Preferred embodiments and further developments can be found in the dependent claims.

[0014] The lithium aluminum silicate glass ceramic according to the invention has a thermal expansion coefficient in the range from 20°C to 700°C of -0.5 to 1.9 ppm / K. The glass ceramic contains the following components in the specified amounts in wt.% on an oxide basis: SiO 2 60 - 70 Al 2 O 3 17 - 25 Li 2 O 2.0 - <3.2 MgO 0 - <0.5 ZnO 1.5 - <3 BaO 0.2 - 3 Na2O >0.05 - <0.6 K2O >0.05 - <0.5 SnO 2 0.1 - <1.0 and the conditions MgO < K 2 O MgO < Na 2 O.

[0015] A glass-ceramic with an appropriate thermal expansion coefficient exhibits both high resistance to thermal shock and high long-term thermal stability. This makes it suitable for use as a cooking surface with all types of heating elements. The expansion coefficient is at least -0.5 ppm / K. "ppm" stands for "parts per million," meaning a relative change in size of 10-6< for a temperature change of 1 K. More negative values of thermal expansion are avoided. Negative expansion, i.e., contraction, creates tensile stresses in the surface of the glass-ceramic during heating. At a value of less than -0.5 ppm / K, these stresses can reduce the mechanical strength of the cooking surface at typical cooking appliance operating temperatures. Thermal expansion of more than 1.9 ppm / K is also avoided.If the expansion is more than 1.9 ppm / K, it cannot be guaranteed that the thermal shock resistance is sufficiently high to allow the glass ceramic to be used in cooking appliances with radiant heating elements.

[0016] In a further development of the invention, the thermal expansion coefficient 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. In addition, the thermal expansion coefficient is preferably a maximum of 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 a maximum of only 0.6 ppm / K.

[0017] In a preferred embodiment of the invention, the thermal expansion coefficient of the glass ceramic is -0.5 to 1.0 ppm / K, preferably -0.1 to 0.8 ppm / K, particularly preferably 0 to 0.6 ppm / K. Such glass ceramics are particularly suitable for cooking surfaces in cooking appliances with radiant heating elements.

[0018] In a further preferred embodiment of the invention, the thermal expansion coefficient of the glass ceramic is 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 with induction heating elements.

[0019] The glass ceramic according to the invention contains the following components in wt.%: SiO 2 60 - 70, Al 2 O 3 17 - 25 and Li 2 O 2.0 - <3.2.

[0020] The components SiO 2 and Al 2 O 3, together with Li 2 O, form the main components of the crystal phase in glass-ceramics. At the same time, they significantly determine the glass formation properties and the viscosity of the green glass.

[0021] The SiO 2 content of the glass-ceramic according to the invention should not exceed 70 wt.%, because this component greatly increases the viscosity of the glass, particularly the processing point. Higher SiO 2 contents are uneconomical for efficient glass melting and low forming temperatures. The minimum SiO 2 content should be 60 wt.%, as this is advantageous for the required properties, such as chemical resistance and thermal stability. At very high SiO 2 contents of more than 70 wt.%, deep quartz crystals can form during ceramization. This leads to a significant increase in thermal expansion.

[0022] The glass-ceramic preferably contains at least 61 wt.%, 62 wt.%, 63 wt.%, 64 wt.%, or even 65 wt.% SiO 2 . The more SiO 2 the glass-ceramic contains, the better its thermal stability and chemical resistance. Furthermore, it preferably contains at most 69 wt.%, 68 wt.%, 67 wt.%, or even only 66 wt.% SiO 2 . The less SiO 2 the glass-ceramic contains, the better the meltability and processability of the green glass during hot forming.

[0023] The Al 2 O 3 content of the glass-ceramic according to the invention is in the range of 17 to 25 weight percent. A higher Al 2 O 3 content leads to problems with devitrification and the undesirable formation of mullite. Therefore, 25 wt.% should not be exceeded. Amounts of Al 2 O 3 less than 17 wt.% are unfavorable for the formation of high-quartz solid solutions and promote the formation of undesirable crystal phases.

[0024] The glass-ceramic preferably contains at least 18 wt.%, 19 wt.%, or even 20 wt.% Al 2 O 3 . The more Al 2 O 3 the glass-ceramic contains, the better its thermal stability. Furthermore, it preferably contains at most 24 wt.%, 23 wt.%, 22 wt.%, or even only 21 wt.% Al 2 O 3 . The less Al 2 O 3 the glass-ceramic contains, the better the meltability and processability of the green glass during hot forming.

[0025] It has proven particularly advantageous for the meltability of the green glass if the glass ceramic contains 17 - <19.0 wt.%, preferably 17.5 - 18.9 wt.%, particularly preferably 18 - 18.8 wt.%.

[0026] It has proven particularly advantageous for the temperature resistance of the glass ceramic if the glass ceramic contains >21.0 - 25 wt.%, preferably 21.5 - 24 wt.%, particularly preferably 22.0 - 23 wt.% Al 2 O 3.

[0027] The Li 2 O content of the glass ceramic according to the invention is in the range 2.0 - <3.2 wt.%. It has surprisingly been found that with a Li 2 O content in this range, in combination with the other components within the stated limits, a glass ceramic with high thermal shock resistance and good meltability can be achieved. Since Li 2 O has a strong influence on the thermal expansion of the glass ceramic, Li 2 O is selected in combination with the other components of the glass ceramic according to the invention within the above-mentioned limits in order to be able to achieve the thermal shock resistance required for the invention. In addition, a proportion of more than 2.0 wt.% Li 2 O has a positive effect on the manufacturability of the glass ceramic, since it reduces the electrical resistance of the glass melt, reduces the viscosity and thus also lowers the processing point.Reducing the viscosity of the glass melt can also improve the efficiency of refining. Improved refining leads to less production waste due to bubble formation in the green glass.

[0028] 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.% Li2O. As an upper limit, the glass ceramic preferably contains at most 3.1 wt.%, 3.0 wt.% or even 2.95 wt.% Li2O. More 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.% Li2O. Within these narrower limits, a glass ceramic with particularly high thermal shock resistance can be obtained.

[0029] For cost reasons, natural mineral raw materials such as spodumene or petalite, or alternatively synthetically produced Li 2 CO 3 , are typically used as the source of lithium. However, these natural mineral raw materials contain impurities that can have an undesirable influence, for example, on the optical properties of the glass-ceramic. Furthermore, the amount of impurities in natural raw materials can vary between deliveries, making it difficult to achieve the desired properties of the glass-ceramic. For this reason, it is also advantageous to reduce the amount of Li 2 O in the glass-ceramic as much as possible.

[0030] In a preferred embodiment, the glass-ceramic contains high-quartz solid solution as the main crystal phase. "Main crystal phase" means that the glass-ceramic contains more high-quartz solid solution than keatite solid solution by volume. In a further development of this embodiment, the glass-ceramic contains <10 vol.%, preferably <5 vol.%, particularly preferably <3 vol.% keatite solid solution. The vol.% refers to the volume of the glass-ceramic, preferably to the volume of the crystal phase. The volume fractions are determined using Rietveld analysis from X-ray diffraction spectra.

[0031] Keatite solid solutions generally have higher thermal expansion than high-quartz solid solutions. Therefore, a high proportion of high-quartz solid solutions combined with a low proportion of keatite solid solutions is particularly beneficial for the thermal expansion coefficient of the glass-ceramic. It thus improves the glass-ceramic's resistance to thermal shock.

[0032] In addition to the aforementioned amounts of SiO 2 , Al 2 O 3 , and Li 2 O, the glass-ceramic according to the invention contains 0 - <0.5 wt.% MgO. Since MgO leads to 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 wt.%. Preferably, the glass-ceramic contains a maximum of 0.4 wt.%, 0.3 wt.%, or even only 0.2 wt.% MgO.

[0033] In advantageous embodiments of the invention, it may be preferred for the glass-ceramic to contain small amounts of MgO. Small amounts of MgO can be used to reduce the processing point and the upper devitrification temperature. The glass-ceramic can preferably contain at least 0.05 wt.% and particularly preferably at least 0.1 wt.% MgO. MgO can also be introduced into the glass-ceramic as an impurity of raw materials.

[0034] Furthermore, the glass-ceramic contains 1.5 - <3 wt.% ZnO. ZnO, particularly in combination with high amounts of Al2O3, can lead to the undesirable formation of gahnite crystals. Therefore, the amount in the glass-ceramic according to the invention is limited to <3 wt.%. In addition, it has been empirically shown that glass-ceramics with very high amounts of ZnO tend to form undesirable crystals on the surface of the glass-ceramic. Therefore, the amount of ZnO is preferably limited to amounts of at most 2.8 wt.%, 2.6 wt.%, 2.4 wt.%, or even 2.2 wt.%.

[0035] ZnO reduces the processing point and the upper devitrification temperature in the glass-ceramics according to the invention. Therefore, the glass-ceramic preferably contains at least 1.6 wt.%, 1.7 wt.%, 1.8 wt.%, or even at least 1.9 wt.% ZnO. In these ranges, the thermal shock resistance of the glass-ceramic is particularly improved.

[0036] In a preferred embodiment, the glass ceramic contains 1.6 - 2.8 wt.%, preferably 1.7 - 2.6 wt.%, particularly preferably 1.8 - 2.4 wt.% ZnO for the reasons stated above.

[0037] The glass ceramic according to the invention contains 0.2-3 wt.% BaO. BaO, like Li2O, lowers the viscosity of the glass melt and thus reduces the processing point. To improve the meltability of the green glass, it is advantageous if the glass ceramic, in combination with the above-mentioned amounts of Li2O, contains at least 0.3 wt.%, preferably at least 0.4 wt.%, 0.6 wt.%, 0.8 wt.%, or even 1 wt.% BaO. In the glass ceramic, BaO also contributes significantly to improving the devitrification behavior during hot forming of the green glass.

[0038] However, it has been shown that BaO can have a negative impact on the formation of the crystal phase during ceramization. To avoid the need for long ceramization times, the amount of BaO is 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-ceramic contains, the faster the ceramization process.

[0039] The addition of the alkalis Na2O and K2O improves meltability and devitrification behavior during glass forming. Both components can increase the electrical conductivity of the melt. This facilitates the coupling of energy by electrical heaters in the melting tank. However, their contents are limited because these components are not incorporated into the crystal phases, but essentially remain in the residual glass phase of the glass-ceramic. Excessively high contents impair the crystallization behavior during the conversion of the crystallizable starting glass into the glass-ceramic, particularly at the expense of faster ceramization rates. In addition, higher contents have an adverse effect on the time / temperature resistance of the glass-ceramic. Therefore, the glass-ceramic according to the invention contains >0.05 - <0.6 wt.% Na2O and >0.05 - <0.5 wt.% K2O.

[0040] In a preferred development, the glass ceramic contains 0.1 - 0.5 wt.%, preferably 0.2 - 0.4 wt.% Na 2 O. In a further preferred development, the glass ceramic contains 0.1 - 0.4 wt.%, preferably 0.2 - 0.3 wt.% K 2 O.

[0041] The sum of the alkalis Na 2 O + K 2 O in combination with the other constituents of the glass ceramic is preferably at least 0.2 wt.% and at most 1 wt.%. More preferably, the sum is 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 at most 0.6 wt.%. In these amounts, a particularly good compromise between improved meltability and devitrification without impairing the ceramization rate can be achieved for the glass ceramics according to the invention.

[0042] Furthermore, the glass ceramic according to the invention meets the following conditions: MgO < K2O and MgO < Na2O.

[0043] Both MgO, K2O, and Na2O have a positive effect on the electrical conductivity of the melt. However, since potassium and sodium ions are more mobile than magnesium, they have a stronger influence on conductivity. At the same time, MgO has a stronger increasing effect on the thermal expansion of the glass-ceramic than K2O and Na2O. Therefore, it is advantageous if the glass-ceramic contains more K2O and more Na2O than MgO.

[0044] The ratio of K2O to Na2O allows for fine-tuning of meltability and thermal expansion. Na2O improves melting and, in the present compositions, reduces the viscosity of the glass melt somewhat more than K2O, but increases the thermal expansion of the glass-ceramic somewhat more.

[0045] In a first preferred embodiment of the invention, the ratio of K 2 O to Na 2 O in wt. % is in the range 0.1-2, preferably 0.5-1.5, particularly preferably 0.7-1.3. In this range, the above-mentioned properties are particularly balanced.

[0046] In a second preferred embodiment of the invention, the ratio of K2O to Na2O in wt. % is in the range 0.1 - <1, preferably 0.2 - 0.9, particularly preferably 0.3 - 0.8. This embodiment exhibits improved meltability and viscosity. This can be particularly advantageous when the glass-ceramic contains the remaining components in a combination that tends to be somewhat harder to melt or has a somewhat higher viscosity.

[0047] In a third preferred embodiment of the invention, the ratio of K2O to Na2O in wt. % is in the range 1-2, preferably 1.1-1.9, particularly preferably 1.2-1.8. This embodiment exhibits improved thermal expansion. This can be particularly advantageous if the glass-ceramic contains the remaining components in a combination that tends to exhibit somewhat higher thermal expansion.

[0048] The glass ceramic according to the invention contains 0.1 - <1.0 wt.% SnO 2 . 0.1 wt.% SnO 2 is advantageous in combination with the other constituents of the glass ceramic according to the invention in order to ensure sufficient nucleation for the properties according to the invention. However, the amount of <1.0 wt.% should not be exceeded. Higher contents lead to the crystallization of Sn-containing crystal phases on the contact materials (e.g. Pt / Rh) during shaping and should be avoided. The glass ceramic preferably contains at most 0.8 wt.%, 0.6 wt.%, or even only 0.4 wt.% SnO 2 .

[0049] In a further development of the invention, the glass-ceramic can contain 0.1-0.8 wt.%, preferably 0.2-0.7, particularly preferably 0.3-0.6 wt.% SnO 2 . In these amounts, the SnO 2 can support the refining of the green glass. A glass-ceramic with these amounts of SnO 2 is characterized by particularly few defects due to trapped gas bubbles.

[0050] In a further development of the invention, the glass ceramic can contain 0-0.8 wt.%, preferably 0.1-0.6 wt.%, particularly preferably 0.2-0.4 wt.% CeO 2. The CeO 2 can also support the refining and improve bubble quality in combination with SnO 2.

[0051] In a preferred embodiment, the glass-ceramic contains TiO 2 . TiO 2 contributes to nucleation together with SnO 2 . The amount of TiO 2 is limited to values of at most 5 wt.%. In larger amounts, TiO 2 can lead to devitrification during hot forming. In addition, it can lead to an undesirable increase in the refractive index of the residual glass phase. The glass-ceramic preferably contains at least 1 wt.%, 1.5 wt.%, 2.0 wt.%, >2.5 wt.% or even >3.0 wt.% TiO 2 . At the same time, it preferably contains at most 4.5 wt.%, 4.2 wt.%, 4.0 wt.%, 3.8 wt.%, 3.6 wt.% or even only 3.4 wt.% TiO 2 . With higher proportions of TiO 2, nucleation proceeds more quickly. This can reduce the ceramization time of the glass-ceramic. Lower TiO 2 contents stabilize the ceramization process and prevent unintentional devitrification during hot forming of the green glass.

[0052] In a preferred embodiment, the glass ceramic may contain, for example, 1 - 5 wt.%, preferably 2 - 4.5 wt.%, particularly preferably >2.5 - 4.0% TiO 2 for the reasons mentioned above.

[0053] Furthermore, the glass-ceramic preferably contains 1.0–4.0 wt.% ZrO 2 . ZrO 2 and SnO 2 act, among other things, as nucleating agents in the glass-ceramic and, as such, interact closely. A content of 1.0 wt.% ZrO 2 is advantageous in combination with the above-mentioned amounts of SnO 2 and TiO 2 to improve nucleation.

[0054] The amount of ZrO 2 is limited to values of 4.0 wt.%, since ZrO 2 increases the viscosity of the glass melt and thus also the processing point. In addition, ZrO 2 can lead to devitrification during hot forming. This can lead to the undesirable formation of baddeleyite. The glass-ceramic preferably contains at least >1.3 wt.%, particularly preferably >1.7 wt.% ZrO 2 . Furthermore, it preferably contains at most 3.9 wt.%, 3.8 wt.%, 3.2 wt.%, 3.0 wt.% or even only 2.0 wt.% ZrO 2 . In these amounts, a particularly good compromise can be achieved between a positive contribution to nucleation and an acceptable deterioration in meltability and hot forming.

[0055] In a further development of the invention, the glass ceramic contains 1.0 - 4.0 wt.%, preferably >1.3 - 3.9 wt.%, particularly preferably >1.7 - 3.8 wt.% ZrO 2 for the reasons stated above.

[0056] In the production of glass ceramics, As 2 O 3 and Sb 2 O 3 are frequently used as refining agents. However, in the glass ceramics according to the invention, these components have surprisingly proven to be detrimental to devitrification stability. Therefore, the amount of As 2 O 3 and Sb 2 O 3 is preferably limited to less than 0.1 wt.% in each case. More preferably, the glass ceramic contains less than 0.09 wt.%, 0.08 wt.%, 0.07 wt.%, 0.06 wt.% or even less than 0.05 wt.% of As 2 O 3 and Sb 2 . More preferably, the glass ceramic is free of As 2 O 3 and Sb 2 O 3 except for unavoidable traces.

[0057] However, As 2 O 3 and Sb 2 O 3 can occur as impurities in the glass ceramic, especially if cullet containing As 2 O 3 and Sb 2 O 3 is used to manufacture the glass ceramic. This is particularly the case when cullet from cooking surfaces from a recycling cycle is used. For reasons of environmental protection and sustainability, it is advantageous to use cullet from a recycling cycle as raw material. Therefore, the glass ceramics preferably each contain at least 0.01 wt.%, 0.02 wt.%, 0.03 wt.% or even at least 0.04 wt.% As 2 O 3 and / or Sb 2 O 3. If As 2 O 3 and Sb 2 O 3 are present together, they can each be present in the amounts stated.

[0058] The addition of the alkaline earth metals CaO and SrO, as well as B2O3, improves meltability and devitrification behavior during glass forming. In particular, CaO can be included in the glass-ceramic to reduce the processing point and the upper devitrification temperature. However, the contents are limited because these components are not incorporated into the crystal phases, but essentially remain in the residual glass phase of the glass-ceramic. Excessively high contents impair the crystallization behavior during the conversion of the crystallizable starting glass into the glass-ceramic, particularly at the expense of faster ceramization rates. Furthermore, higher contents have an adverse effect on the time / temperature resistance of the glass-ceramic. Therefore, the glass-ceramic can contain each of these components in amounts of 0–2 wt.%.

[0059] In a further development of the invention, the glass ceramic contains 0 - <1 wt.% P 2 O 5. The P 2 O 5 has a positive effect on the devitrification stability of the green glass. Larger amounts, however, reduce the ceramization rate and have a negative effect on the acid resistance of the glass ceramic. Therefore, the amount of P 2 O 5 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 can be advantageous if the glass ceramic contains at least 0.01 wt.%, preferably at least 0.05 wt.%, particularly preferably at least 0.1 wt.% P 2 O 5.

[0060] In a further development of the invention, it can be advantageous if the glass ceramic contains Cl -<. It has been shown that the addition of Cl -< in certain amounts leads to improved bubble quality in the green glass and thus also in the glass ceramic. In combination with the other components, it has proven particularly advantageous if the glass ceramic contains 0.003 - 0.1 wt. %, preferably 0.005 - 0.03 wt. %, particularly preferably 0.007 - 0.02 wt. % Cl -<. Amounts smaller than 30 ppm do not have a sufficient influence on bubble quality. Amounts exceeding 1000 ppm should be avoided, as some of the added chloride can react with other components of the batch and with process exhaust gases. This can lead to the formation of HCl, for example, which can cause corrosive damage to the tank. Furthermore, the evaporation of alkali chlorides and alkaline earth chlorides is undesirable.The amount of Cl -< in the glass ceramic can be adjusted, for example, by adding NaCl to the mixture.

[0061] In addition to these components, in a further development of the invention, the glass-ceramic can also contain coloring components. Examples of coloring components that can be present are V2O5, CoO, Fe2O3, Cr2O3, Nd2O3, NiO, CuO, MnO, or MoO3, individually or in combination. The precise choice of the type and amount of coloring components depends on the optical properties to be achieved.

[0062] The coloring of glass-ceramics is a complex, nonlinear process. Many of the components contained in the glass-ceramic can influence how strongly the coloring components absorb light. Therefore, the expert will adjust the amount of coloring components to the respective basic composition of the glass-ceramic in order to achieve the desired optical properties.

[0063] With regard to the coloring of glass-ceramics according to the invention using V 2 O 5 as the main colorant, the following has been shown, for example. To reduce the transmission to a desired value, more V 2 O 5 is used than with comparable glass-ceramics with a higher Li 2 O content. Reducing the Li 2 O content therefore results in the V 2 O 5 absorbing less strongly in the glass-ceramic. Similar, sometimes even opposing, correlations also exist with other components of the basic composition.

[0064] V 2 O 5 generally colors glass ceramics very intensively, even in small quantities. Glass ceramics colored using V 2 O 5 have a relatively low transmission in the blue and green spectral range and a relatively high transmission in the red spectral range. The glass ceramic preferably contains 0 to 0.1 wt.% V 2 O 5 . It particularly preferably contains >0.002 to 0.08 wt.%, >0.003 to 0.07 wt.%, >0.004 to 0.06 wt.%, >0.005 to 0.05 wt.% or even >0.01 - 0.04 wt.% V 2 O 5 . With these amounts of V 2 O 5 it is possible to adjust the light transmittance of the glass ceramic, based on a thickness of 4 mm, in the range 0.1 to 80%.

[0065] In a particularly preferred development of the aforementioned embodiment, the ratio V 2 O 5 / Li 2 O is 0.005 - 0.06, preferably 0.007 - 0.055, particularly preferably 0.01 - 0.05. Without limiting the generality, it is assumed that the coloring effect of the V 2 O 5 depends on the microstructure of the glass ceramic. Due to the low Li 2 O content, the glass ceramics according to the invention have a relatively low crystal phase content and, at the same time, a small crystallite size. It has been shown that particularly effective coloring is possible when the ratio of V 2 O 5 to Li 2 O is set within the above-mentioned limits. When the ratio is set in this range, a spectral transmittance at a wavelength of 630 nm based on a thickness of 4 mm in the range 0.5 - 15%, preferably 1-13%, particularly preferably 2-10% can be achieved.At these transmission levels, commercially available red light indicators can be used when using the glass ceramic as a cooking surface.

[0066] Using MoO 3 , it is possible to give glass ceramics a particularly color-neutral color. This has the advantage that illuminated displays with a white light color can be used in cooking appliances without the color of the light from the display being changed when passing through the glass ceramic. The glass ceramic preferably contains 0 to 0.5 wt.% MoO 3 . It is particularly preferred that it contains >0.002 to 0.4 wt.%, >0.003 to 0.3 wt.%, >0.004 to 0.2 wt.%, >0.005 to 0.15 wt.% or even >0.01 - 0.1 wt.% MoO 3 . With these amounts of MoO 3, it is possible to adjust the light transmittance of the glass ceramic in the range 0.1 to 80% based on a thickness of 4 mm. At the same time, a color-unadulterated display of white illuminated displays is possible.

[0067] In a particularly preferred development of the aforementioned embodiment, the ratio of MoO 3 / Li 2 O is 0.015 - 0.1, preferably 0.02 - 0.08, particularly preferably 0.025 - 0.07. By setting the ratio within this range, a light transmittance based on a thickness of 4 mm in the range of 0.5 - 4%, preferably 0.8 - 3.5%, particularly preferably 0.7 - 3.3%, and most preferably 1.0 - 3.0% can be achieved. With these transmittances, white illuminated displays can be used when using the glass ceramic as a cooking surface. At the same time, the visibility of the components inside the cooking appliance is greatly reduced.

[0068] Nd 2 O 3 can also be used for coloring. It differs from other colorants in that it creates relatively narrow absorption bands in the glass ceramic. These absorption bands are predominantly in the green spectral range. Using small amounts of Nd 2 O 3, it is possible to finely adjust the color location of light transmitted through the glass ceramic. For example, glass ceramics that contain only small amounts of Fe 2 O 3 as a coloring component often have a yellow tint. This can be the case, for example, with glass ceramics that contain both TiO 2 and Fe 2 O 3 that has been introduced via contamination of the raw materials. If such glass ceramics are provided with white underside coatings, these underside coatings have a clearly noticeable yellow tint.In such glass-ceramics, the addition of Nd 2 O 3 can be used to reduce or eliminate the yellow tint without significantly reducing the light transmittance. This enables the production of cooktops with a white appearance.

[0069] Preferably, Nd 2 O 3 is present in the glass-ceramic in amounts of 0-0.6 wt.%. Since Nd 2 O 3 is relatively expensive, the amount should be limited to 0.6 wt.%. Particularly preferably, the glass-ceramic contains 0.005-0.5 wt.%, 0.01-0.4 wt.%, 0.02-0.3 wt.%, 0.03-0.2 wt.%, or even 0.04-0.1 wt.% Nd 2 O 3 .

[0070] Fe2O3 not only affects transmission in the visible spectral range, but also in the near-infrared up to a wavelength of approximately 3 µm. This means that Fe2O3 not only influences the achievability of certain colors or the representability of color displays. Absorption in the near-infrared determines how much thermal energy the molten glass in the tank can absorb. It determines how much heat output from radiant heating elements can pass through the glass ceramic. Furthermore, it determines whether and which infrared sensors can be used in a cooktop. Such sensors can be designed, for example, as optical touch sensors or as infrared receivers for wireless data transmission. At the same time, Fe2O3 is often present as an impurity in the raw materials used for production. A higher amount of Fe2O3 in the glass ceramic makes it possible to use cheaper raw materials with higher amounts of impurities.All of this must be taken into account when selecting the appropriate amount of Fe2O3. The amount of Fe2O3 should preferably be 0–0.4 wt.%. Glass-ceramics containing more than 0.4 wt.% Fe2O3 are not compatible with commercially available radiant heating elements for cooking appliances due to their low near-infrared transmission. Preferably, the glass-ceramics contain 0.005–0.3 wt.%, 0.01–0.25 wt.%, 0.02–0.2 wt.%, or even 0.04–0.18 wt.% Fe2O3. Such glass-ceramics can be produced cost-effectively and are compatible with radiant heating elements and optical sensors for cooking appliances. Fe2O3 is often present as an impurity in raw materials for glass production, for example, in spodumene.

[0071] CoO can, for example, be present in the glass-ceramic in amounts of 0-0.5 wt.%. It is preferably present in amounts of 0.01-0.2 wt.%, preferably 0.02-0.08 wt.%, and particularly preferably 0.04-0.06 wt.%.

[0072] Glass-ceramics colored with 0.02-0.1 wt.% CoO preferably additionally contain 0.02-0.1 wt.% Cr 2 O 3 . They particularly preferably additionally contain 0.05-0.25 wt.% Fe 2 O 3 and, in particular, <30 ppm V 2 O 5 . With these amounts of CoO and preferred additional colorants, it is possible to adjust the light transmittance of the glass-ceramic, based on a thickness of 4 mm, in the range of 0.1 to 80%. Furthermore, it is thus possible to enable white displays in the warm-white spectral range.

[0073] Cr2O3, NiO, CuO and MnO are generally used for supporting coloration, but unlike V2O5, MoO3 or CoO they are rarely used as the main colorant. A main colorant is understood to be the coloring component that has the greatest influence on the transmission of the glass-ceramic in the visible spectral range. They often occur as impurities in raw materials. These components are preferably contained in the glass-ceramic in amounts of 0 to 0.5 wt.%. They are particularly preferably contained in the glass-ceramic in amounts of 0.001 - 0.4 wt.%, 0.002 - 0.3 wt.%, 0.004 - 0.2 wt.%, 0.006 - 0.1 wt.%, 0.008 - 0.08 wt.% or even 0.01 - 0.05 wt.%.

[0074] In a preferred embodiment, the glass-ceramic contains 0 to 0.1 wt.% V 2 O 5 , or 0 to 0.5 wt.% MoO 3 or 0 to 0.6 wt.% Nd 2 O 3 or 0 to 0.4 wt.% Fe 2 O 3 or 0 to 0.5 wt.% CoO or 0 to 0.5 wt.% Cr 2 O 3 or 0 to 0.5 wt.% NiO or 0 to 0.5 wt.% CuO or 0 to 0.5 wt.% MnO or combinations of these components.

[0075] In addition to their coloring effect, these components can also have a positive influence on glass quality. This is particularly the case for components that absorb in the infrared spectral range in the glass melt. Due to absorption in the infrared, the heat introduced into the melting tank by heating devices can be absorbed more efficiently by the glass melt. With the same energy input, this can lead to an increase in the temperature of the glass melt. This can have a positive effect both on the melting of poorly melting raw materials and on the reduction of bubbles during refining. This is particularly the case for Fe2O3, CoO and NiO in the quantities mentioned above.

[0076] In a further development of the invention, the glass-ceramic has a light transmittance of 80-90% or 81-89% or 82-88% or even 83-87%, based on a thickness of 4 mm. Glass-ceramics with such a light transmittance preferably have a chroma C* in transmission based on a thickness of 4 mm in the range of 0-6, preferably 1.5-5, particularly preferably 3.0-4.6.

[0077] "Related to a thickness of 4 mm" means that the corresponding properties are either determined on a sample with a material thickness of 4 mm or determined at a different material thickness and converted to a material thickness of 4 mm. For transmission data, the conversion can be performed using the Beer-Lambert law.

[0078] The light transmittance is determined in the wavelength range 380 - 780 nm using light of standard illuminant D65 according to the specifications of DIN 5033. This value corresponds to the brightness Y in the CIExyY color space.

[0079] The chroma C* is determined from the L*a*b* color coordinates according to the following formula: C * = a * 2 + b * 2 .

[0080] The color coordinates a* and b* are determined in a known manner from the transmission spectrum of the glass ceramic using standard light of standard illuminant D65.

[0081] Glass ceramics with a light transmittance of 80-90% relative to a thickness of 4 mm are particularly suitable for use as fireplace viewing panels or cooking plates. In fireplaces, this transmission makes the fire particularly visible. In cooking appliances, for example, this transmission makes relatively low-luminance displays, such as LCD or OLED displays, particularly visible.

[0082] The chroma C* of 0 - 6 means that the color of light changes only very slightly when passing through the glass ceramic. This makes it possible, for example, to provide the glass ceramic with a white coating that still creates a white color impression even when viewed through the glass ceramic. This is particularly important when used as a cooking surface or fireplace viewing panel. Glass ceramics in these applications are often 4 mm thick. Light reflected by a rear coating therefore travels an optical path of 8 mm, meaning that color shifts caused by the inherent color of the glass ceramic have a greater impact than over shorter paths. For cooking surfaces or fireplace viewing panels with a white coating on the rear, a correspondingly low chroma is therefore particularly advantageous.

[0083] In a further development of the invention, a glass ceramic with a light transmittance of 80-90% or a correspondingly preferred range and a chroma C* of 2-6 or a correspondingly preferred range contains, in addition to the composition according to the invention, one or more of the following components in wt.%: Nd 2 O 3 0.005 - 0.1, preferably 0.01 - 0.08, particularly preferably 0.03 - 0.065, Fe 2 O 3 0 - 0.02, preferably 0.0025 - 0.018, particularly preferably 0.005 - 0.016, V 2 O 5 0 - 0.0015, preferably 0 - 0.001, particularly preferably 0 - 0.0005, Cr 2 O 3 0 - 0.001, preferably 0 - 0.0005, particularly preferably 0 - 0.0003, MoO 3 0 - 0.001, preferably 0 - 0.0008, particularly preferably 0 - 0.0006, CoO 0 - 0.001, preferably 0 - 0.0005, particularly preferably 0 - 0.0001, NiO 0 - 0.001, preferably 0 - 0.0005, particularly preferably 0 - 0.0001 CuO 0 - 0.001, preferably 0 - 0.0007, particularly preferably 0 - 0.0002 MnO 0 - 0.02, preferably 0 - 0.01, particularly preferably 0 - 0.006 TiO 2 1.6 - 2.5, preferably 2.0 - 2.4, particularly preferably 2.1 - 2.3, ZrO 2 0 - 2.2 or 0.1 - 2.0 or 0.2 - 1.8 or even 0.3 - 1.6, SnO 2 0.1 - 0.2, preferably 0.1 - 0.18, particularly preferably 0.1 - 0.15.

[0084] In a particularly preferred embodiment, the glass-ceramic contains all of these components in these amounts. If these components are present in the glass-ceramic in the amounts mentioned here, it may be further preferred if the sum of Fe 2 O 3 + V 2 O 5 + Cr 2 O 3 is 0-0.0225 wt.%, preferably 0.0005-0.0175 wt.%, particularly preferably 0.0010-0.0170 wt.%.

[0085] These components, both individually and in combination, influence the light transmittance and chroma of the glass-ceramic. If the above-mentioned quantities are adhered to, it is possible to finely adjust the light transmittance and chroma within the above-mentioned limits.

[0086] The following table contains three further developments of the glass ceramic according to the invention in wt.% on an oxide basis: provided especially provided most preferred The 2 O 2,0 - <3,2 2,2 - <3,2 2,4 - <3,2 Na 2 O >0,05 - <0,6 0,1 - 0,5 0,2 - 0,4 K 2 O >0,05 - <0,5 0,1 - 0,5 0,2 - 0,4 MgO 0 - <0,5 0,05 - 0,4 0,1 - 0,3 CaO 0 - 2 0,2 - 1,6 0,3 - 1,4 SrO 0 - 2 0,2 - 1,6 0,3 - 1,4 BaO 0,2 - 3 0,6 - 2,9 1 - 2,8 ZnO 1,5 - <3 1,6 - 2,8 1,7 2,6 Al 2 O 3 17-25 17 - 23 17 - 21 SiO 60-70 61 - 69 62 - 68 TiO 1 - 5 2 - 4,5 > 2,5 - 4,0 ZrO 1,0 - 4,0 >1,3 - 3,9 >1,7 - 3,8 SnO 0,1 - <1,0 0,1 - 0,8 0,2-0,7 Fe 2 O 3 0-0,4 0,005 - 0,3 0,01 - 0,25 V 2 O 5 0-0,1 0-0,1 0-0,1 MoO 3 0-0,5 0-0,5 0-0,5 Cr 2 O 3 0-0,5 0-0,5 0-0,5 Nd 2 O 3 0-0,6 0-0,6 0-0,6 CoO 0-0,5 0-0,5 0-0,5 NiO 0-0,5 0-0,5 0-0,5 CuO 0-0,5 0-0,5 0-0,5 MnO 0-0,5 0-0,5 0-0,5 P 2 O 5 0-5 0,01-4 0,05-3 MgO < K 2 O MgO < K 2 O MgO < K 2 O MgO < Na 2 O MgO < Na2O MgO < Na 2 O

[0087] The glass ceramic according to the invention is used as a cooking surface, fireplace viewing panel, grill or frying surface, cover for fuel elements in gas grills, oven viewing panel, in particular for pyrolysis ovens, worktop or table top in kitchens or laboratories, cover for lighting devices, in fire-resistant glazing and as safety glass, optionally in laminate composite, as a carrier plate or as oven lining in thermal processes or as a rear cover for mobile electronic devices.

[0088] The glass ceramic according to the invention can be used in particular as a cooking surface. The cooking surface can be provided with a decorative coating or functional coating on the top and / or bottom, either entirely or partially. Touch sensors for operating the cooking surface can also be provided on the bottom. These can be, for example, printed, glued, or pressed-on capacitive sensors.

[0089] Furthermore, the glass ceramic can also be available in the form of three-dimensionally shaped plates. This means that the plates can be angled or curved, or, for example, contain a wok-shaped section. Cutouts, for example, for the operation of gas burners, are also possible.

[0090] In the following, the invention is further illustrated by means of exemplary embodiments.

[0091] The crystallizable green glasses used in the examples were melted from technical batch raw materials commonly used in the glass industry at temperatures of 1680°C for 4 hours. This choice reconciles the requirements for economical raw materials and a low level of undesirable impurities. After melting the batch in sintered silica crucibles, the melts were poured into Pt / Rh crucibles with silica inner crucibles and homogenized by stirring at temperatures of 1600°C for 90 minutes. Following this homogenization, the glasses were refined for 3 hours at 1640°C. Subsequently, pieces measuring approximately 120 × 140 × 30 mm 3< were cast and cooled in a lehr, starting at 640–670°C at 30 K / h to room temperature, depending on the glass viscosity, to relieve stresses.The castings were divided into the sizes required for the investigations and for ceramization.

[0092] The ceramization of the samples in the green glass state was carried out using a ceramization process in a continuous furnace with the following steps: a) Heating from room temperature to 740 °C at a heating rate of 30 K / min., b) Holding at 740 °C for 3 min. and 20 s, c) Temperature increase from 740 to 810 °C at a heating rate of 28 K / min., d) Holding at 810 °C for 9 min. and 20 s, e) Temperature increase from 810 °C to 930 °C at a heating rate of 21 K / min., f) Holding at 930 °C for 6 min., g) Cooling to room temperature at a cooling rate of 15 K / min.

[0093] The following tables contain the composition and material properties of the examples according to the invention.

[0094] The thermal expansion coefficient CTE was determined dynamically on rod-shaped samples using a push-rod dilatometer at a heating rate of 2 K / min.

[0095] To measure the upper devitrification temperature (UET), the green glasses were melted in Pt / Rh10 crucibles. The crucibles were then held for 5 hours at various temperatures within the processing temperature range. The upper temperature at which the first crystals appear at the contact surface of the glass melt and the crucible wall determines the UET.

[0096] The processing point (T4) of the green glasses was determined using a stirring viscometer according to DIN ISO 7884-2.

[0097] During the conversion of green glass into glass-ceramic, the density increases because the crystal phase has a higher density than the amorphous glass. Shrinkage indicates the linear change in length during the conversion of green glass into glass-ceramic. It is calculated from the density of the green glass and the density of the glass-ceramic as follows: Schrumpf = 1 − Dichte Gr ü nglas Dichte Glaskeramik 1 3 .

[0098] Tg indicates the transformation temperature, also known as the glass transition temperature, of green glasses. It is determined dilatometrically.

[0099] The light transmittance is determined in the wavelength range 380–780 nm using standard illuminant D65 according to the specifications of DIN 5033. This value corresponds to the brightness Y in the CIExyY color space. This value is a measure of the brightness perception of the human eye.

[0100] From the xy color coordinates of the CIExyY color space, the distance d to the color location of standard light of standard illuminant D65 (0.3127 / 0.3290) was determined as follows: d = x − 0 , 3127 2 + y − 0 , 3290 2 .

[0101] Transmission spectra were determined according to ISO 15368:2021. Table 2 shows the spectral transmittances "T@ ..." for the wavelengths 470 nm, 600 nm, 630 nm, 700 nm, 950 nm, and 1600 nm.

[0102] The color coordinates in the CIExyY color space and the Lab color space were determined according to the specifications of CIE 1932 using an 8° observer and light of standard illuminant D65 in transmission.

[0103] All transmission measurements were performed on 4 mm thick, smooth samples on both sides.

[0104] The volume fraction "XRD fraction HQMK" or "KMK" and the crystallite size of the crystalline phases "XRD crystallite size HQMK" or "KMK" were determined by Rietveld analysis from X-ray diffraction spectra. Table 1: Compositions of examples according to the invention Bsp 1 2 3 4 5 6 7 8 The 2 O 3,010 2,780 2,600 2,610 2,700 2,710 3,000 3,040 Na 2 O 0,392 0,274 0,384 0,410 0,284 0,272 0,383 0,310 K 2 O 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 Al 2 O 3 21,420 21,330 21,462 21,400 21,550 21,400 20,662 18,720 SiO 64,700 64,799 65,006 64,600 65,300 65,400 65,205 66,140 TiO 3,180 3,180 3,190 3,180 3,190 3,170 3,170 3,140 ZrO 1,392 1,396 1,399 1,390 1,400 1,392 1,392 1,400 SnO 0,282 0,279 0,281 0,282 0,277 0,278 0,275 0,280 Fe 2 O 3 0,093 0,091 0,090 0,094 0,090 0,115 0,118 0,116 V 2 O 5 0,027 0,026 0,027 0,029 0,024 0,050 0,058 0,021 Cr 2 O 3 0,006 0,004 0,003 0,005 0,000 0,000 0,005 0,005 MnO 0,020 0,017 0,016 0,018 0,000 0,000 0,020 0,017 P 2 O 5 0,074 0,072 0,067 0,070 0,005 0,005 0,076 0,053 Continued Table 1 Bsp 9 10 11 12 13 The 2 O 2,590 2,870 2,780 2,860 2,180 Na 2 O 0,289 0,394 0,288 0,288 0,395 K 2 O 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 Al 2 O 3 21,400 21,440 21,340 21,180 21,370 SiO 65,500 64,600 65,300 64,900 64,800 TiO 3,180 3,180 3,160 3,170 3,180 ZrO 1,390 1,390 1,401 1,392 1,403 SnO 0,278 0,285 0,280 0,275 0,275 Fe 2 O 3 0,094 0,093 0,116 0,120 0,090 V 2 O 5 0,026 0,026 0,052 0,056 0,029 Cr 2 O 3 0,005 0,004 0,005 0,005 0,005 MnO 0,017 0,019 0,018 0,019 0,014 P 2 O 5 0,073 0,077 0,073 0,074 0,065 Table 2: Material properties of inventive examples Bsp 1 2 3 4 5 6 7 T4[°C] 1312 1312 OEG [°C] 1310 1295 Dense Glass [g / cm 3< ] 2,489 2,492 2,490 2,499 2,481 2,483 2,487 CTE_Glasceramic [ppm / K] 0,26 0,30 0,42 0,40 0,22 0,33 0,23 Schrumpf [%] 1,04 1,14 1,22 1,16 1,31 1,34 1,01 Dense GC [g / cm 3< 2,569 2,580 2,584 2,588 2,582 2,585 2,563 XRD Anteil HQMK 60 59 57 56 63 60 45 XRD Crystallite size of HQMK 42 37 37 38 36 34 61 XRD Anteil KMK 0 0 0 0 0 0 0 4mm Transmission x (CIE) 0,538 0,532 0,524 0,539 0,529 0,613 0,650 and (CIE) 0,389 0,389 0,395 0,392 0,395 0,368 0,344 Y(CIE) / twis 4,182 4,765 6,648 4,709 6,527 1,507 0,315 d=sqrt((x-0.3127)^2 + (y-0.3290)^2) 0,233 0,227 0,221 0,235 0,226 0,303 0,337 T@ 470 nm 0,008 0,010 0,014 0,008 0,001 0,013 0,000 T@ 600 nm 0,071 0,080 0,111 0,080 0,013 0,110 0,005 T@ 630 nm 0,118 0,132 0,173 0,132 0,028 0,173 0,015 T@ 700 nm 0,294 0,316 0,372 0,315 0,124 0,377 0,089 T@ 950 nm 0,734 0,751 0,759 0,733 0,691 0,765 0,608 T@ 1600 nm 0,738 0,746 0,744 0,729 0,753 0,743 0,694 Continued Table 2 Bsp 8 9 10 11 12 13 T4[°C] 1319 OEG [°C] <1240 Dense Glass [g / cm 3< ] 2,492 2,494 2,4837 2,484 2,494 2,497 CTE_Glasceramic [ppm / K] 0,23 0,38 0,28 0,24 0,25 1,01 Schrumpf [%] 1,12 1,08 1,27 1,23 1,12 1,15 Dense GC [g / cm 3< 2,578 2,577 2,580 2,579 2,579 2,585 XRD Anteil HQMK 57 58 61 59 48 XRD Crystallite size of HQMK 40 39 36 37 46 XRD Anteil KMK 0 0 0 0 4mm Transmission x (CIE) 0,525 0,516 0,604 0,608 0,540 and (CIE) 0,399 0,392 0,373 0,369 0,403 Y(CIE) / twice 7,287 5,885 1,745 1,384 6,180 d=sqrt((x-0.3127)^2 + (y-0.3290)^2) 0,223 0,213 0,294 0,298 0,239 T@ 470 nm 0,0017 0,014 0,014 0,001 0,001 0,009 T@ 600 nm 0,0153 0,122 0,097 0,032 0,025 0,106 T@ 630 nm 0,0301 0,187 0,151 0,066 0,053 0,166 T@ 700 nm 0,1178 0,389 0,333 0,218 0,191 0,351 T@ 950 nm 0,6297 0,755 0,727 0,682 0,666 0,716 T@ 1600 nm 0,7061 0,736 0,721 0,689 0,687 0,738

Claims

1. Lithium aluminium silicate glass-ceramic having a coefficient of thermal expansion in the range from 20°C to 700°C of -0.5 to 1.9 ppm / K and having a composition comprising in % by weight based on oxides SiO2 60 - 70 Al2O3 17 - 25 Li2O 2.0 - <3.2 MgO 0 - <0.5 ZnO 1.5 - <3 BaO 0.2 - 3 Na2O >0.05 - <0.6 K2O >0.05 - <0.5 SnO2 0.1 - <1.0 and the conditions MgO< K2O MgO < Na2O.

2. Lithium aluminium silicate glass-ceramic according to Claim 1, characterized in that it comprises 2.2%-<3.2% by weight or 2.4% - <3.2% by weight or >2.7%-<3.2% by weight or >2.9% - <3.2% by weight of Li2O.

3. Lithium aluminium silicate glass-ceramic according to Claim 1, characterized in that it comprises 1.6%-2.8% by weight, preferably 1.7% - 2.6% by weight, particularly preferably 1.8% - 2.4% by weight of ZnO.

4. Lithium aluminium silicate glass-ceramic according to Claim 1, characterized in that it comprises 1% - 5% by weight, preferably 2% - 4.5% by weight, particularly preferably >2.5% - 4.0% of TiO2.

5. Lithium aluminium silicate glass-ceramic according to any of the preceding claims, characterized in that it comprises 1.0% - 4.0% by weight, preferably >1.3% - 3.9% by weight, particularly preferably >1.7% - 3.8% by weight of ZrO2.

6. Lithium aluminium silicate glass-ceramic according to any of the preceding claims, characterized in that the amount of Na2O + K2O is at least 0.2% by weight and at most 1% by weight, preferably 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 at most 0.6% by weight.

7. Lithium aluminium silicate glass-ceramic according to any of the preceding claims, characterized in that it contains less than 0.1% by weight of As2O3 and less than 0.1% by weight of Sb2O3.

8. Lithium aluminium silicate glass-ceramic according to any of the preceding claims, characterized in that the ratio of K2O to Na2O, K2O / Na2O, in % by weight is in the range 0.1 - 2, preferably 0.5 - 1.5, particularly preferably 0.7 - 1.3 or in the range 0.1 - <1, preferably 0.2 - 0.9, particularly preferably 0.3 - 0.8 or in the range 1 - 2, preferably 1.1 - 1.9, particularly preferably 1.2 - 1.8.

9. Lithium aluminium silicate glass-ceramic according to any of the preceding claims, characterized in that it 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 combinations of these constituents.

10. Use of a glass-ceramic according to any of the preceding claims as a cooking surface, fireplace viewing pane, grilling or roasting surface, covering of fuel elements in gas grills, oven viewing pane, in particular for pyrolysis cookers, worktop or tabletop in kitchens or laboratories, covering for lighting devices, in fire-resistant glazing and as safety glass, optionally in composite laminate form, as a support plate or as a furnace lining in thermal processes or as a back cover for mobile electronic devices.