Lithium aluminum silicate glass ceramics

Abstract

The present invention relates to lithium aluminum silicate glass-ceramics having a thermal expansion coefficient of -0.5 to 1.9 ppm / K in the temperature range of 20°C to 700°C, and uses thereof. The glass-ceramics contain the following components in the indicated amounts, expressed in weight percent on an oxide basis: SiO2 60-70, Al2O3 17-25, Li2O 2.0-3.4, MgO 0-1.9, ZrO2 0.8-4.0, ZnO >2.2-6.0, BaO 0.2-<2.9, TiO2 >1.8-5.0, and SnO2 0.1-<1.0.

Description

[Technical Field]

[0001] The present invention relates to lithium aluminum silicate glass-ceramics suitable for use as cooktops in cooking appliances and their uses.

[0002] Background of the Invention Glass-ceramic cooktops and the glass-ceramics used therein have been known for a long time. These include lithium aluminum silicate (LAS) glass-ceramics, which contain either high-quartz solid solution (HQMK) for transparent materials or keatite solid solution (KMK) for translucent or opaque materials as the predominant crystalline phase. To produce such glass-ceramics, a starting glass, known as a green glass, is first prepared by methods conventionally used in glassmaking. This green glass is then converted into a glass-ceramic by a heat treatment known as ceramization.

[0003] An essential characteristic of these materials used as cooktops is their very low coefficient of thermal expansion in the temperature range from room temperature to 700°C. This low coefficient of thermal expansion results in high thermal shock resistance. The coefficient of thermal expansion is set by the combination of a crystalline phase with a negative coefficient of thermal expansion and a residual amorphous glass phase with a positive coefficient of thermal expansion. For this reason, conventional glass-ceramics typically use lithium percentages of more than 3.6% by weight and up to 5.0% by weight.

[0004] Due to the rising cost of lithium as a raw material, it is advantageous for economic reasons to minimize the proportion of lithium in glass-ceramics. However, lithium is one of the three main components of lithium aluminum silicate glass-ceramics, so it cannot simply be reduced at will. The proportion of Li2O directly affects key properties of glass-ceramics, such as their viscosity when melted, which is important for manufacturability, and their coefficient of thermal expansion, which is important for use as cooktops.

[0005] The trend in lithium prices is not a new issue. For the past 20 years, the price of lithium has been steadily increasing. Despite this long-standing need, most commercially available glass-ceramics for cooktops contain approximately 3.8 wt% Li2O. To date, it is virtually impossible to find glass-ceramics containing less than 3.5 wt% Li2O that can compete with currently available glass-ceramics.

[0006] Glass-ceramics with a proportion of LiO of less than 3.5% by weight are known from WO 2012 / 010341, EP 3502069, US 2017050880, US 2020189965, US 2020140322, US 2021387899, and WO 2021 / 224412. However, these glass-ceramics have various drawbacks, such as reduced thermal shock resistance and poor melting properties of the green glass.

[0007] In this context, transparent glass-ceramics are understood, as is customary, to mean glass-ceramics with low light scattering, the transmittance of which can be adjusted over a wide range by the addition of absorbing, i.e. coloring, components.

[0008] When transparent glass-ceramics are used for cooktops, they are volume-colored by adding coloring oxides or by applying an undercoat to visually conceal the technical equipment underneath. Various coloring oxides can be used for volume-coloring glass-ceramics. These include, among others, V2O5, CoO, Fe2O3, Cr2O3, Nd2O3, NiO, CuO, MnO, and MoO3. Each of these coloring 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, inter alia, in WO 11089220, U.S. Pat. No. 8765619, DE 102008050263, and DE 102009013127.

[0009] Problem to be solved by the invention The object of the present invention is to provide a lithium aluminum silicate glass ceramic which has the good melting properties of green glass without any limitations in terms of use properties and is inexpensive.

[0010] Good melting properties include, in particular, a processing point of less than 1340° C., advantageously less than 1330° C., particularly preferably less than 1320° C. The processing point is when the green glass is 10 4 dPa * This is the temperature at which the glass has a viscosity of 1000 psi. Hot forming of green glass is carried out around this temperature. The higher the temperature during hot forming, the more complicated it becomes to dissipate the heat introduced into the forming machine by the glass. At temperatures above 1340 °C, this can only be achieved by reducing the heat output by reducing the glass throughput, which is economically unfavorable.

[0011] If the temperature during hot forming falls below the upper devitrification temperature, undesirable spontaneous crystallization may occur. To avoid this, the upper devitrification temperature should be at least 10 K, preferably at least 20 K, and particularly preferably at least 30 K lower than the processing point.

[0012] Glass-ceramics are particularly desirable for use as cooktops with all types of heating elements, including radiant, induction, and gas heating elements, which require a sufficiently high thermal shock resistance as well as a high long-term heat resistance.

[0013] Summary of the Invention The problem of the present invention is solved by the subject matter of the independent claims. Preferred embodiments and developments can be found in the dependent claims.

[0014] The lithium aluminum silicate glass ceramic according to the present invention has a thermal expansion coefficient of -0.5 to 1.9 ppm / K in the range of 20° C. to 700° C. The glass ceramic contains the following components in the indicated amounts, expressed in weight percent on an oxide basis: SiO260~70 Al2O317~25 Li2O 2.0~3.4 MgO 0~1.9 ZrO20.8~4.0 ZnO >2.2~6.0 BaO 0.2~<2.9 TiO2>1.8~5.0 SnO20.1~<1.0.

[0015] Glass-ceramics with an appropriate thermal expansion coefficient combine high thermal shock resistance with high long-term thermal stability, making them suitable for use as cooktops with any type of heating element. The thermal expansion coefficient is at least -0.5 ppm / K, where "ppm" stands for "parts per million," i.e., 10 parts per million for a temperature change of 1 K. -6This refers to the relative magnitude change in the coefficient of thermal expansion. Shifting the coefficient of thermal expansion towards more negative values ​​must be avoided. A negative coefficient of thermal expansion, i.e., contraction, generates tensile stresses on the surface of the glass-ceramic during heating. At values ​​below -0.5 ppm / K, these stresses can reduce the mechanical strength of the cooktop at typical operating temperatures of the cooking appliance. A coefficient of thermal expansion above 1.9 ppm / K must also be avoided. A coefficient of thermal expansion above 1.9 ppm / K does not ensure sufficient thermal shock resistance for the use of glass-ceramics in cooking appliances with radiant heating elements.

[0016] In one 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. Furthermore, the thermal expansion coefficient is advantageously 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.

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

[0018] In a further preferred embodiment of the invention, the glass-ceramics have a thermal expansion coefficient of 0.5 to 1.9 ppm / K, advantageously -0.7 to 1.7 ppm / K, particularly preferably 0.9 to 1.5 ppm / K. Such glass-ceramics are suitable, for example, for cooktops of cooking appliances equipped with induction heating elements.

[0019] The glass ceramic according to the invention comprises the following components in % by weight: SiO260~70, Al2O317~25 and Li2O 2.0~3.4.

[0020] The elements SiO2 and Al2O3 form the main crystalline phase together with Li2O in glass-ceramics. At the same time, they largely determine the glass-forming properties and viscosity of the green glass.

[0021] The SiO2 content of the glass ceramics according to the present invention should be a maximum of 70% by weight, since this component significantly increases the viscosity of the glass, especially the processing point. Higher SiO2 contents are uneconomical for achieving good glass melting and low forming temperatures. The minimum SiO2 content should be 60% by weight, since this is advantageous for desired properties, such as chemical and heat resistance. If the SiO2 proportion is too high, exceeding 70% by weight, low quartz crystals may form during ceramization, which leads to a significant increase in the thermal expansion coefficient.

[0022] Advantageously, the glass-ceramic contains at least 61%, 62%, 63%, 64%, or even 65% by weight of SiO2. The more SiO2 contained in the glass-ceramic, the better its heat and chemical resistance. Furthermore, the glass-ceramic advantageously contains 69%, 68%, 67%, or even only 66% or less by weight of SiO2. The less SiO2 contained in the glass-ceramic, the better the meltability and processability of the green glass in hot forming.

[0023] The Al2O3 content of the glass-ceramics according to the present invention is in the range of 17-25 wt. Higher Al2O3 proportions lead to problems of devitrification and undesired mullite formation. Therefore, it should not exceed 25 wt. An Al2O3 content below 17 wt. % is detrimental to the formation of a high-quartz solid solution and promotes the formation of undesired crystalline phases.

[0024] Advantageously, the glass-ceramic contains at least 18%, 19%, or even 20% Al2O3 by weight. The more Al2O3 contained in the glass-ceramic, the better its heat resistance. Furthermore, the glass-ceramic advantageously contains 24%, 23%, 22%, or even only 21% Al2O3 by weight or less. The less Al2O3 contained in the glass-ceramic, the better the meltability and processability of the green glass in hot forming.

[0025] It has proven particularly advantageous for the meltability of the green glass if the glass ceramic comprises 17 to <19.0% by weight, advantageously 17.5 to 18.9% by weight, particularly preferably 18 to 18.8% by weight, of Al2O3.

[0026] It has proven particularly advantageous for the heat resistance of the glass-ceramics if they contain >21.0-25% by weight, advantageously 21.5-24% by weight, particularly preferably 22.0-23% by weight, of Al2O3.

[0027] The Li2O content of the glass ceramics according to the present invention is in the range of 2.0 to 3.4% by weight. Surprisingly, it has been found that a Li2O content in this range, combined with the remaining components within the stated limits, allows for the production of glass ceramics with high thermal shock resistance and good melting properties. Because Li2O significantly influences the thermal expansion coefficient of the glass ceramic, Li2O is selected within the above-mentioned limits in combination with the remaining components of the glass ceramics according to the present invention. This allows for the thermal shock resistance required for the present invention to be achieved. Furthermore, a Li2O content of at least 2.0% by weight reduces the electrical resistance and viscosity of the glass melt, thereby lowering the processing point, thereby favorably affecting the manufacturability of the glass ceramics. The reduced viscosity of the glass melt also improves the efficiency of the fining process. This improved fining process reduces production waste due to bubble formation in the green glass.

[0028] Advantageously, the glass-ceramics contain at least 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, 2.5 wt%, 2.6 wt%, 2.7 wt%, >2.8 wt%, >2.9 wt%, or even 3.0 wt% LiO. Advantageously, the glass-ceramics contain no more than 3.3 wt%, 3.2 wt%, 3.1 wt%, <3.0 wt%, 2.9 wt%, 2.8 wt%, 2.7 wt%, 2.6 wt%, or even 2.5 wt% LiO.

[0029] In one development of the invention, the proportion of LiO is 2.0 to 2.8% by weight, 2.1 to 2.7% by weight, 2.2 to 2.6% by weight, or even 2.3 to 2.5% by weight. Within these narrow limits, glass-ceramics can be obtained that are particularly inexpensive to produce.

[0030] In one development of the invention, the proportion of LiO is >2.8-3.4 wt.-%, >2.9-3.3 wt.-%, or even 3.0-3.2 wt.-%. Within these narrow limits, glass-ceramics with particularly high thermal shock resistance can be obtained.

[0031] In a further development of the invention, the proportion of LiO is 2.1-3.3 wt.-%, 2.2-3.2 wt.-%, 2.3-3.1 wt.-%, 2.4-<3.0 wt.-%, 2.5-2.9 wt.-%, or even 2.6-2.8 wt.-%. Within these narrow limits, a particularly balanced combination of thermal stability and low production costs is possible.

[0032] Lithium sources are typically natural mineral sources such as spodumene and petalite, or synthetically produced Li2CO3. However, natural mineral sources contain impurities that can have undesirable effects on, for example, the optical properties of glass-ceramics. Furthermore, the amount of impurities in natural materials can vary from source to source, making it difficult to determine the desired properties of the glass-ceramics. For these reasons, it is advantageous to minimize the amount of Li2O in glass-ceramics.

[0033] In a preferred embodiment, the glass-ceramic comprises a high-quartz solid solution as the predominant crystalline phase. "Predominant crystalline phase" means that the glass-ceramic comprises more high-quartz solid solution than keatite solid solution by volume. In one development of this embodiment, the glass-ceramic comprises <10 vol.%, preferably <5 vol.%, particularly preferably <3 vol.% keatite solid solution. Here, these volume % data are relative to the volume of the glass-ceramic, advantageously the volume of the crystalline phase. The volume fraction is determined by Rietveld refinement from the X-ray diffraction spectrum.

[0034] Keatite solid solutions generally have a higher thermal expansion coefficient than high-quartz solid solutions. Therefore, a high proportion of high-quartz solid solution and a low proportion of keatite solid solution are particularly advantageous for the thermal expansion coefficient of the glass-ceramic, thereby improving the thermal shock resistance of the glass-ceramic.

[0035] In addition to the aforementioned amounts of SiO2, Al2O3 and Li2O, the glass-ceramics according to the invention contain 0-1.9 wt. % MgO and 0.2-<2.9 wt. % BaO.

[0036] Since MgO leads to an increase in the thermal expansion coefficient of the glass-ceramic, the amount of MgO in the glass-ceramic is limited to a maximum of 1.9 wt. Advantageously, the glass-ceramic contains a maximum of 1.7 wt. %, 1.4 wt. %, <1.4 wt. %, 1.2 wt. %, 0.8 wt. %, or even 0.6 wt. % MgO.

[0037] In an advantageous embodiment of the present invention, it may be preferable for the glass ceramic to contain a small amount of MgO. This small amount can be used to lower the processing point and the upper devitrification temperature. The glass ceramic preferably contains at least 0.1 wt. %, 0.2 wt. %, 0.3 wt. %, 0.4 wt. %, or even 0.5 wt. % MgO. MgO may also be introduced into the glass ceramic as an impurity in the raw materials.

[0038] In one development of the invention, the glass-ceramic comprises >0-1.7 wt. %, 0.1-1.4 wt. %, 0.2-1.2 wt. %, 0.3-0.8 wt. %, or even 0.4-0.6 wt. % MgO.

[0039] Like LiO, BaO reduces the viscosity of the glass melt and thus the working point. Therefore, to ensure good meltability of the green glass, the glass ceramic contains at least 0.2 wt. %, advantageously at least 0.4 wt. %, 0.6 wt. %, 0.8 wt. %, or even 1 wt. % BaO in combination with the above-mentioned amount of LiO. In the glass ceramic according to the present invention, BaO also significantly contributes to improving the devitrification behavior of the green glass during hot forming. BaO lowers the upper devitrification limit of the green glass, thereby contributing to widening the distance between the working point and the upper devitrification limit. This widens the temperature window for stable hot forming without devitrification problems.

[0040] However, it has been found that BaO can adversely affect crystalline phase formation during ceramization. To avoid the need for long ceramization times, the amount of BaO is limited to <2.9 wt. %. Advantageously, therefore, the glass-ceramic contains less than 2.7 wt. %, 2.5 wt. %, 2.0 wt. %, 1.8 wt. %, 1.6 wt. %, 1.4 wt. %, or even 1.2 wt. The less BaO contained in the glass-ceramic, the faster the ceramization.

[0041] Furthermore, the glass-ceramics according to the present invention contain 0.8 to 4.0 wt.% ZrO2, >1.8 to 5.0 wt.% TiO2, and 0.1 to <1.0 wt.% SnO2. TiO2, ZrO2, and SnO2 act particularly as nucleating agents in the glass-ceramics and interact closely with each other. A content of 1 wt.% ZrO2, >1.8 wt.% TiO2, and 0.1 wt.% SnO2 is advantageous in combination with the other components of the glass-ceramics according to the present invention to ensure sufficient nucleation for the properties according to the present invention.

[0042] Because ZrO2 also increases the viscosity of the glass melt, i.e., the processing point, the amount of ZrO2 is limited to 4.0 wt.%. Furthermore, ZrO2 can cause devitrification during hot forming, which can lead to the undesirable formation of baddeleyite. Advantageously, the glass-ceramic contains at least 1.0 wt.%, 1.2 wt.%, 1.3 wt.%, 1.4 wt.%, or even 1.5 wt.% ZrO2. Furthermore, it advantageously contains 3.8 wt.%, 3.5 wt.%, 3.2 wt.%, 3.0 wt.%, or even only 2.5 wt.% or less ZrO2. At these amounts, a particularly good compromise can be achieved between a positive contribution to nucleation and still acceptable deterioration in meltability and hot forming.

[0043] The amount of TiO2 is limited to a value of 5.0 wt.% or less. Higher amounts of TiO2 can cause devitrification during hot forming. This can also lead to an undesirable increase in the refractive index of the residual glass phase. Preferably, the glass-ceramic contains at least 2 wt.%, 2.3 wt.%, 2.6 wt.%, or even 2.9 wt.% TiO2. At the same time, it preferably contains 4.5 wt.%, 4.2 wt.%, 4.0 wt.%, 3.8 wt.%, 3.6 wt.%, or even only 3.4 wt.% or less TiO2. A higher TiO2 content accelerates nucleation. As a result, the ceramization time of the glass-ceramic can be shortened. A lower TiO2 content stabilizes the ceramization process and prevents unintended devitrification during hot forming of the green glass.

[0044] Amounts of SnO2 of <1.0 wt% should not be exceeded. Higher contents lead to the crystallization of Sn-containing crystalline phases on contact materials (e.g., Pt / Rh) during molding and should be avoided. Advantageously, the glass-ceramics contain no more than 0.8 wt%, 0.6 wt%, or even only 0.4 wt% SnO2.

[0045] In one development of the invention, the glass-ceramics can contain 0.1 to 0.8 wt. %, preferably 0.2 to 0.7 wt. %, and particularly preferably 0.3 to 0.6 wt. % SnO2. In these amounts, SnO2 can assist in the fining process of green glass. Glass-ceramics with these amounts of SnO2 are characterized by a particularly low number of defects due to trapped gas bubbles.

[0046] In a further development of the invention, the glass-ceramic may contain 0 to 0.8 wt. %, preferably 0.1 to 0.6 wt. %, particularly preferably 0.2 to 0.4 wt. % CeO2, which in combination with SnO2 can likewise aid the fining process and improve bubble quality.

[0047] Furthermore, the glass ceramics according to the present invention contain >2.2 to 6.0 wt. % ZnO. ZnO, especially in combination with large amounts of Al2O3, can lead to the undesirable formation of gahnite crystals. Therefore, the amount in the glass ceramics according to the present invention is limited to 6.0 wt. %. Furthermore, it has been empirically found that glass ceramics with very large amounts of ZnO tend to form undesirable crystals on the surface of the glass ceramic. Therefore, advantageously, the amount of ZnO is limited to an amount equal to or less than 5.5 wt. %, 5.0 wt. %, 4.5 wt. %, or even 4.0 wt. %.

[0048] However, it has been found that ZnO in the glass ceramics according to the present invention can significantly reduce the thermal expansion coefficient of the glass ceramics. Furthermore, ZnO lowers the processing point and upper devitrification temperature within the ranges defined herein. Therefore, advantageously, the glass ceramics contain at least >2.5 wt%, 2.7 wt%, 2.9 wt%, 3.1 wt%, 3.3 wt%, or even 3.5 wt% ZnO. The thermal shock resistance of the glass ceramics is particularly improved within these ranges.

[0049] It is particularly advantageous if the ZnO content is in the range of >2.2 to 3.5 wt. % or >3.5 to 6.0 wt. A ZnO content of >2.2 to 3.5 wt. % results in glass-ceramics with particularly few undesirable surface crystals or gahnite crystals in the crystalline phase. Such glass-ceramics are preferred for applications where thermal shock resistance is not very important. This is the case, for example, in induction cooking appliances. In this case, excellent surface quality, free of crystallites, is more important. A ZnO content of >3.5 to 6.0 wt. % results in glass-ceramics with a particularly low coefficient of thermal expansion and therefore particularly high thermal shock resistance. Such glass-ceramics are preferred for applications requiring particularly high thermal shock resistance. This is the case, for example, in cooking appliances with radiant heating elements.

[0050] It has been found to be particularly advantageous if the glass-ceramics contain more ZnO than LiO, i.e., if the condition ZnO > LiO is met. By observing this condition within the composition range required for the glass-ceramics according to the invention, it is possible to obtain glass-ceramics with particularly high thermal shock resistance.

[0051] Additions of alkali metals Na2O and K2O and alkaline earth metals CaO, SrO, and B2O3 improve meltability and devitrification behavior during glass forming. CaO, in particular, can be included in glass-ceramics to lower the processing temperature and upper devitrification temperature limits. However, there are limits to their content, since these elements are not incorporated into the crystalline phase but remain essentially in the residual glass phase of the glass-ceramic. Excessive content impairs the crystallization behavior during the conversion of a crystallizable starting glass to a glass-ceramic, particularly at the expense of a high ceramization rate. Furthermore, high content negatively impacts the long-term durability / heat resistance of the glass-ceramic.

[0052] Therefore, the glass ceramics may contain each of these components in an amount of 0 to 2% by weight, and more preferably, these components may be contained in an amount of 0.1 to 1.8% by weight, 0.2 to 1.6% by weight, or 0.3 to 1.4% by weight.

[0053] In a preferred development, the glass ceramic contains 0-2 wt. % Na2O and 0-2 wt. % K2O. The addition of K2O lowers the upper devitrification temperature without significantly changing the processing point. Therefore, K2O can be used to set the temperature interval during hot forming without having to increase the temperature. This is particularly advantageous for hot forming.

[0054] The sum of the alkali metals Na2O+K2O in combination with the above-mentioned amount of ZrO2 is advantageously more than 0.2% by weight and not more than 3% by weight. Particularly preferably, the sum is at least 0.4% by weight, or at least 0.6% by weight, or at least 0.9% by weight, or at least 1% by weight, or at least 1.1% by weight, or at least 1.2% by weight, or even at least 1.3% by weight, and not more than 2.5% by weight, or not more than 2.0% by weight, or not more than 1.9% by weight, or not more than 1.8% by weight, or even not more than 1.7% by weight. These amounts make it possible to achieve a particularly good compromise between improved melting properties and devitrification without impairing the ceramization rate.

[0055] When both MgO and KO are present in the glass-ceramic, it is particularly advantageous if the glass-ceramic contains less MgO than KO. In addition to other components, such glass-ceramics contain: MgO >0~<1.9wt%, K2O >0-2 wt% and MgO <K2O。

[0056] Both MgO and K2O have a positive effect on the electrical conductivity of the melt. However, potassium as an ion is more mobile than magnesium and therefore has a stronger effect on conductivity. At the same time, MgO has a stronger effect on the thermal expansion coefficient of the glass-ceramic than K2O. Therefore, it is advantageous for the glass-ceramic to contain less MgO than K2O.

[0057] Glass-ceramics containing less MgO than KO offer a particularly good compromise between meltability, especially viscosity control of the melt, electrical resistivity and process stability during hot forming. At the same time, the glass-ceramics have a particularly low coefficient of thermal expansion and good thermal shock resistance.

[0058] In one particularly preferred development of this embodiment, the ratio of MgO to K2O, i.e. the quotient MgO / K2O, for the reasons mentioned above, is a value in the range 0><1, advantageously 0.01 to 0.9, particularly preferably 0.05 to 0.8, or even 0.1 to 0.5.

[0059] As2O3 and Sb2O3 are often used as fining agents in the production of glass ceramics. However, in the glass ceramics according to the present invention, these components have surprisingly been found to be detrimental to devitrification stability. Therefore, the amount of As2O3 and Sb2O3 is advantageously limited to less than 0.1 wt. % each. Particularly preferably, the glass ceramic contains less than 0.09 wt. %, less than 0.08 wt. %, less than 0.07 wt. %, less than 0.06 wt. %, or even less than 0.05 wt. It is particularly preferred that the glass ceramic does not contain As2O3 and Sb2O3, except in unavoidable trace amounts.

[0060] However, As2O3 and Sb2O3 can occur as impurities in glass ceramics, especially when cullet containing As2O3 and Sb2O3 is used in the production of glass ceramics. This is particularly true when cullet from cooktops from the recycling cycle is used. For environmental protection and sustainability reasons, it is advantageous to use cullet from the recycling cycle as raw material. Therefore, advantageously, the glass ceramic contains at least 0.01 wt.%, 0.02 wt.%, 0.03 wt.%, or even at least 0.04 wt.% of As2O3 and / or Sb2O3, respectively. When As2O3 and Sb2O3 are included together, they can each be present in the amounts mentioned.

[0061] In one development of the invention, the glass ceramic can contain 0 to 5 wt. % P2O5. P2O5 has a positive effect on the devitrification stability of the green glass. However, higher amounts reduce the ceramization rate and adversely affect the acid resistance of the glass ceramic. Therefore, the amount of P2O5 is limited to a maximum of 5 wt. %, advantageously 4 wt. %, particularly preferably 3 wt. %, 2 wt. %, or even <1 wt. In a particularly preferred embodiment, the amount of P2O5 can even be limited to a maximum of 0.9 wt. %, particularly preferably a maximum of 0.8 wt. To improve devitrification stability, it can be 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. % P2O5.

[0062] In one development of the invention, the glass ceramic is - It may be advantageous to include Cl. - It has been found that the addition of a certain amount of Cl improves the bubble quality of the green glass and thus the glass-ceramics. In combination with other components, the glass-ceramics contain 0.003 to 0.1 wt. %, preferably 0.005 to 0.03 wt. %, and particularly preferably 0.007 to 0.02 wt. % Cl. -It has been found to be particularly advantageous to include chlorides. Amounts below 30 ppm do not have a significant effect on foam quality. Amounts above 1000 ppm should be avoided, since some of the added chlorides may react with other components of the batch and with process waste gases. This can lead, for example, to the formation of HCl, which can cause corrosive damage to the bath. Furthermore, evaporation of alkali metal chlorides and alkaline earth metal chlorides is undesirable. Cl in glass ceramics - The amount of can be adjusted, for example, by adding NaCl to the batch.

[0063] In addition to these components, the glass-ceramics can also contain coloring components in one development of the invention, such as V2O5, CoO, Fe2O3, Cr2O3, Nd2O3, NiO, CuO, MnO, or MoO3, either individually or in combination. The exact choice of type and amount of coloring component depends on the optical properties to be achieved.

[0064] The coloring of glass-ceramics is a complex and nonlinear process. Many components in glass-ceramics can affect the intensity of the coloring component's light absorption. Therefore, a skilled artisan can adjust the amount of coloring component to the respective base composition of the glass-ceramic to achieve the desired optical properties.

[0065] Regarding the coloring of glass-ceramics according to the invention using V2O5 as the main colorant, for example, the following can be shown: To reduce the transmittance to the desired value, more V2O5 is used than in comparable glass-ceramics with a higher Li2O content. Thus, the reduction in Li2O results in less V2O5 absorption in the glass-ceramics. Similar, and sometimes opposite, correlations exist with other components of the base composition.

[0066] V2O5 generally colors glass-ceramics very strongly even in small amounts. Glass-ceramics colored with V2O5 have relatively low transmittance in the blue and green spectral ranges and relatively high transmittance in the red spectral range. Advantageously, the glass-ceramics contain 0 to 0.1 wt.% V2O5. Particularly preferred are >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 to 0.04 wt.% V2O5. These V2O5 amounts allow the light transmittance of the glass-ceramics at a thickness of 4 mm to be set in the range of 0.1 to 80%.

[0067] In a particularly preferred development of the aforementioned embodiment, the ratio V2O5 / Li2O is between 0.005 and 0.06, preferably between 0.007 and 0.055, and particularly preferably between 0.01 and 0.05. Without limiting generality, it is assumed that the coloring effect of V2O5 depends on the microstructure of the glass-ceramic. Due to the low Li2O content, the glass-ceramic according to the present invention has a relatively low proportion of crystalline phases and simultaneously small crystallite sizes. It has been found that particularly effective coloring is possible when the ratio of V2O5 to Li2O falls within the above-mentioned limits. With this ratio, a spectral transmittance at a wavelength of 630 nm at a thickness of 4 mm can be achieved in the range of 0.5 to 15%, advantageously between 1 and 13%, and particularly preferably between 2 and 10%. These transmittances allow the use of commercially available red-light displays when the glass-ceramic is used as a cooktop.

[0068] MoO3 can be used to color glass ceramics, particularly to achieve neutral color tones. This has the advantage that white light displays can be used in cooking appliances without the display light changing color as it passes through the glass ceramic. Advantageously, the glass ceramic contains 0 to 0.5 wt.% MoO3. Particularly preferred are >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 to 0.1 wt.% MoO3. These amounts of MoO3 allow the light transmittance of the glass ceramic at a thickness of 4 mm to be set in the range of 0.1 to 80%. At the same time, accurate color display of the white light display is possible.

[0069] In a particularly preferred development of the aforementioned embodiment, the MoO3 / Li2O ratio is 0.015 to 0.1, advantageously 0.02 to 0.08, and particularly preferably 0.025 to 0.07. This ratio range allows for a light transmittance at a thickness of 4 mm of 0.5 to 4%, advantageously 0.8 to 3.5%, particularly preferably 0.7 to 3.3%, and very particularly preferably 1.0 to 3.0%. These transmittances allow for the use of white light displays when the glass ceramic is used as a cooktop. At the same time, the visibility of the interior components of the cooking appliance is significantly reduced.

[0070] Nd2O3 can also be used for coloring. Unlike other colorants, it produces relatively narrow absorption bands in glass-ceramics. These absorption bands are primarily in the green spectral range. Small amounts of Nd2O3 can be used to precisely control the color of light transmitted through the glass-ceramics. For example, glass-ceramics containing only small amounts of Fe2O3 as a coloring component often have a yellowish tint. This can occur, for example, in glass-ceramics containing both TiO2 and Fe2O3 impurities. If such glass-ceramics are coated with a white undercoat, the undercoat will have a noticeable yellowish tint. Adding Nd2O3 to such glass-ceramics can reduce or eliminate the yellow tint without significantly reducing light transmittance. As a result, it is possible to produce cooktops with a white appearance.

[0071] Advantageously, Nd2O3 is contained in the glass-ceramics in an amount of 0 to 0.6 wt. %. Since Nd2O3 is relatively expensive, it is desirable to limit its amount to 0.6 wt. %. Particularly preferably, the glass-ceramics contain 0.005 to 0.5 wt. %, 0.01 to 0.4 wt. %, 0.02 to 0.3 wt. %, 0.03 to 0.2 wt. %, or even 0.04 to 0.1 wt. % Nd2O3.

[0072] Fe2O3 affects transmittance not only in the visible spectral range but also in the near-infrared range up to wavelengths of approximately 3 μm. Therefore, Fe2O3 not only influences the realization of specific colors and the displayability of color displays. Absorption in the near-infrared range determines how much heat energy the glass melt can absorb in the vessel, which in turn determines how much heat output from radiant heating elements can pass through the glass-ceramic. This also determines whether and what infrared sensors can be used in stovetops. Such sensors can be designed, for example, as optical touch sensors or infrared receivers for wireless data transmission. At the same time, Fe2O3 is often present as an impurity in the raw materials used in production. A higher amount of Fe2O3 in glass-ceramics allows the use of less expensive raw materials with higher impurity concentrations. All of these factors must be taken into consideration when selecting the appropriate amount of Fe2O3. The amount of Fe2O3 is preferably between 0 and 0.4 wt.%. Glass-ceramics containing more than 0.4 wt.% Fe2O3 have low transmittance in the near-infrared range and are therefore not compatible with radiant heating elements used in commercial cooking appliances. Advantageously, 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 are inexpensive to produce and are compatible with radiant heating elements used in cooking appliances and optical sensors. Fe2O3 is often present as an impurity in raw materials used in glass production, such as spodumene.

[0073] CoO may be present in the glass ceramic in an amount of, for example, 0 to 0.5% by weight, advantageously in an amount of 0.01 to 0.2% by weight, preferably 0.02 to 0.08% by weight, particularly preferably 0.04 to 0.06% by weight.

[0074] Advantageously, glass-ceramics colored with 0.02-0.1 wt. % CoO further contain 0.02-0.1 wt. % Cr2O3. Particularly preferred are 0.05-0.25 wt. % Fe2O3, especially <30 ppm V2O5. These amounts of CoO, preferably in combination with other colorants, allow the light transmittance of the glass-ceramics at a thickness of 4 mm to be set in the range of 0.1-80%. Furthermore, this allows for the realization of white displays in the warm white spectral range.

[0075] Unlike V2O5, MoO3, or CoO, Cr2O3, NiO, CuO, and MnO are usually used for secondary coloring and rarely as primary colorants. Primary colorants are understood to be coloring components that have the greatest influence on the transmittance of the glass-ceramics in the visible spectral range. They often occur as impurities in the raw materials. These components are advantageously contained in the glass-ceramics in amounts of 0 to 0.5% by weight each. Particularly preferred are amounts of 0.001 to 0.4%, 0.002 to 0.3%, 0.004 to 0.2%, 0.006 to 0.1%, 0.008 to 0.08%, or even 0.01 to 0.05% by weight.

[0076] In a preferred embodiment, the glass-ceramic comprises 0-0.1 wt.% V2O5 or 0-0.5 wt.% MoO3 or 0-0.6 wt.% Nd2O3 or 0-0.4 wt.% Fe2O3 or 0-0.5 wt.% CoO or 0-0.5 wt.% Cr2O3 or 0-0.5 wt.% NiO or 0-0.5 wt.% CuO or 0-0.5 wt.% MnO or a combination of these components.

[0077] In addition to their coloring effect, these components can also have a positive effect on the quality of the glass. This is especially true for components that absorb in the infrared spectral range in the glass melt. Absorption in the infrared range allows the heat introduced into the melting vessel by the heating device to be absorbed more efficiently by the glass melt. This allows the temperature of the glass melt to be increased with the same energy input. This can have a positive effect on the melting of difficult-to-melt raw materials and on the reduction of bubbles during the fining process. This is especially true for Fe2O3, CoO, and NiO in the amounts mentioned above.

[0078] In one development of the invention, the glass ceramic has a light transmittance of 80 to 90%, or 81 to 89%, or 82 to 88%, or even 83 to 87% at a thickness of 4 mm. Glass ceramics with such a light transmittance advantageously have a chroma C in the range of 0 to 6, preferably 1.5 to 5, particularly preferably 3.0 to 4.6 at a thickness of 4 mm. * It has.

[0079] "At 4 mm thickness" means that the corresponding property is measured on a 4 mm material sample, or measured at another material thickness and converted to a 4 mm material thickness. For transmittance data, the conversion can be performed using the Beer-Lambert law.

[0080] The light transmittance is measured in the wavelength range of 380 to 780 nm using light of standard illuminant D65 in accordance with the specifications of DIN 5033. This value corresponds to the luminance Y in the CIExyY color space.

[0081] Saturation C * L * a * b * It is determined from the color coordinates according to the following formula:

number

[0082] Color coordinates a * and b *is determined in a known manner from the transmission spectrum of the glass-ceramic using standard illuminant D65.

[0083] With a light transmission of 80-90% at a thickness of 4 mm, glass-ceramics are particularly suitable for use as fireplace viewing windows or cooktops. In fireplaces, this transmittance allows for particularly good visibility of the fire. In cooking appliances, this transmittance allows for particularly good visibility of relatively low-brightness optical displays, such as LCD or OLED displays.

[0084] Saturation C * A chromaticity of 0 to 6 ensures that the color of light changes only slightly when passing through the glass-ceramic. As a result, it is possible to apply a white coating to the glass-ceramic, which still gives the impression of white when viewed through the glass-ceramic. This is particularly important when used as a cooktop or fireplace viewing window, for example. Glass-ceramics for such applications are often 4 mm thick. Therefore, light reflected by the backside coating travels an 8 mm optical path, meaning that color shifts due to the inherent color of the glass-ceramic have a greater effect than if the optical path were shorter. Therefore, a correspondingly low chromaticity is particularly advantageous for cooktops or fireplace viewing windows with a white backside coating.

[0085] In one development of the invention, the light transmittance is in the range of 80 to 90% or a correspondingly preferred range and the saturation C is in the range of 2 to 6 or a correspondingly preferred range. * and the glass ceramics having the composition according to the invention contain, in addition to the composition according to the invention, one or more of the following components, expressed in % by weight: Nd2O3 0.005 to 0.1, preferably 0.01 to 0.08, particularly preferably 0.03 to 0.065, Fe2O3 0 to 0.02, preferably 0.0025 to 0.018, particularly preferably 0.005 to 0.016, V2O50 to 0.0015, preferably 0 to 0.001, particularly preferably 0 to 0.0005, Cr2O3 0 to 0.001, preferably 0 to 0.0005, particularly preferably 0 to 0.0003, MoO3 0 to 0.001, preferably 0 to 0.0008, particularly preferably 0 to 0.0006, CoO 0 to 0.001, preferably 0 to 0.0005, particularly preferably 0 to 0.0001, NiO 0 to 0.001, preferably 0 to 0.0005, particularly preferably 0 to 0.0001, CuO 0 to 0.001, preferably 0 to 0.0007, particularly preferably 0 to 0.0002, MnO 0 to 0.02, preferably 0 to 0.01, particularly preferably 0 to 0.006, TiO2>1.8 to 2.5, preferably 2.0 to 2.4, particularly preferably 2.1 to 2.3; ZrO2 0.8-2.2, or 0.9-2.0, or 1.0-1.8, or even 1.1-1.6, SnO2 is 0.1 to 0.2, preferably 0.1 to 0.18, and particularly preferably 0.1 to 0.15.

[0086] In a particularly preferred development, the glass-ceramic comprises all of these components in these amounts. If these components are present in the glass-ceramic in the amounts stated here, it may further be preferred if the sum of Fe2O3 + V2O5 + Cr2O3 is 0 to 0.0225 wt.-%, preferably 0.0005 to 0.0175 wt.-%, particularly preferably 0.0010 to 0.0170 wt.-%.

[0087] These components, either alone or in combination with one another, affect the light transmittance and color saturation of the glass-ceramics, which can be fine-tuned within the above limits if the above amounts are observed.

[0088] The following table contains three further developments of the glass-ceramics according to the invention in % by weight on an oxide basis: [Table 1]

[0089] The glass ceramics according to the invention are used as cooktops, fireplace sight glasses, grill or roasting tops, covers for combustion elements of gas grills, oven sight glasses, in particular oven sight glasses for pyrolysis furnaces, kitchen or laboratory workbenches or table tops, covers for lighting devices, in fire glazing and as safety glass, optionally in laminated composites, as carrier plates or as linings for furnaces in thermal processes or as back covers for portable electronic devices.

[0090] The glass ceramics according to the invention can be used in particular as cooktops. In this case, the cooktops can be provided with a decorative or functional coating on all or part of the top and / or bottom surface. Touch sensors for operating the cooktop can also be provided on the bottom surface. These can be, for example, printed, glued or pressure-bonded capacitive sensors.

[0091] Furthermore, glass-ceramics can be in the form of three-dimensionally shaped plates, i.e., they can be angled, curved, or contain areas shaped like a wok, for example, or recesses for operating a gas burner.

[0092] The present invention will now be further illustrated by the following examples.

[0093] The crystallizable green glass of the example was melted at 1680°C for 4 hours from industrial batch raw materials commonly used in the glass industry. This choice allows for both economical raw materials and a low content of undesirable impurities. After melting the batch in a sintered silica glass crucible, the melt was poured into a Pt / Rh crucible with a silica glass inner crucible and homogenized by stirring at 1600°C for 90 minutes. After this homogenization, the glass was fined at 1640°C for 3 hours. It was then poured into a glass tube measuring approximately 120 x 140 x 30 mm. 3Pieces of this size were produced and cooled to room temperature in a cooling furnace at 30 K / h, starting from 640-670 °C depending on the viscosity of the glass, to relieve stress. The cast pieces were divided into those required for testing and those required for ceramization.

[0094] Ceramization of the samples in the green glass state was carried out in a continuous furnace in a ceramization process having 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 minutes and 20 seconds; c) heating from 740°C to 810°C at a heating rate of 28 K / min; d) holding at 810°C for 9 minutes and 20 seconds; e) heating from 810°C to 930°C at a heating rate of 21 K / min; f) holding at 930°C for 6 minutes; g) Cooling to room temperature at a cooling rate of 15 K / min.

[0095] The following table contains the compositions and material properties of examples according to the invention. Examples 11 and 13, unlike the other examples, were heated to a temperature of 945°C in step e) and held at this temperature in step f). Examples 33, 42 and 48, unlike the other examples, were heated to a temperature of 915°C in step e) and held at this temperature in step f). Examples 34, 43 and 49, unlike the other examples, were heated to a temperature of 905°C in step e) and held at this temperature in step f).

[0096] The coefficient of thermal expansion CTE was measured dynamically on bar samples using a shear rod dilatometer at a heating rate of 2 K / min.

[0097] To measure the upper limit of devitrification temperature (OEG), green glass was melted in a Pt / Rh10 crucible. The crucible was then held at various temperatures within the processing temperature range for 5 hours. The highest temperature at which the first crystals appear at the interface between the glass melt and the crucible wall determines the OEG.

[0098] The working point (T4) of the green glass was measured using a stirring viscometer in accordance with DIN ISO 7884-2.

[0099] When green glass transforms into glass-ceramics, the density increases because the crystalline phase is denser than the amorphous glass. The shrinkage ratio indicates the linear change in length as the green glass transforms into glass-ceramics. The shrinkage ratio is calculated from the density of the green glass and the density of the glass-ceramic as follows:

number

[0100] Tg denotes the transformation temperature of the green glass, also known as the glass transition temperature, which is measured by dilatometry.

[0101] Light transmittance is measured in the wavelength range of 380-780 nm using standard illuminant D65 according to DIN 5033. This value corresponds to the luminance Y in the CIExyY color space. This value is a measure of the luminance perception of the human eye.

[0102] The distance d from the xy color coordinates in the CIExyY color space to the color position of the standard illuminant D65 (0.3127 / 0.3290) was calculated as follows:

number

[0103] The transmission spectra were measured in accordance with ISO 15368:2021. Table 2 lists examples of the spectral transmittance "T@···" at wavelengths of 470 nm, 600 nm, 630 nm, 700 nm, 950 nm, and 1600 nm.

[0104] The color coordinates in the CIExyY color space and Lab color space were determined according to the specifications of CIE 1932 using an 8° observer and transmitted light of standard illuminant D65.

[0105] All transmittance measurements were performed on 4 mm thick samples that were smooth on both sides.

[0106] The volume fraction "XRD fraction HQMK" or "KMK" and the crystallite diameter of the crystalline phase "XRD crystallite diameter HQMK" or "KMK" were determined from the X-ray diffraction spectrum by Rietveld analysis.

[0107] [Table 2-1] [Table 2-2] [Table 2-3] [Table 2-4] [Table 2-5] [Table 2-6] [Table 2-7] [Table 2-8]

[0108] [Table 3-1] [Table 3-2] [Table 3-3] [Table 3-4] [Table 3-5] Table 3-6 Table 3-7

Claims

1. Lithium aluminum silicate glass ceramics, wherein the lithium aluminum silicate glass ceramics are The coefficient of thermal expansion in the range of 20°C to 700°C is -0.5 to 1.9 ppm / K. In terms of oxide-based weight percentages, the following: Yes ２ 60~70 Al ２ O ３ 17-25 Li ２ O 2.0~3.4 MgO 0-1.9 ZrO ２ 0.8~4.0 ZnO >2.2~6.0 BaO 0.2~<2.9 TO ２ >1.8~5.0 SnO ２ 0.1~<1.0 Having a composition including, Lithium aluminum silicate glass ceramics.

2. Li in amounts of 2.0–2.8 wt%, 2.1–2.7 wt%, 2.2–2.6 wt%, or even 2.3–2.5 wt%, or >2.8–3.4 wt%, >2.9–3.3 wt%, or even 3.0–3.2 wt% ２ The lithium aluminum silicate glass ceramic according to claim 1, characterized by containing O.

3. The lithium aluminum silicate glass ceramic according to claim 1, characterized by containing 2.2 to 3.5% by weight of ZnO.

4. The lithium aluminum silicate glass ceramic according to claim 1, characterized by containing 3.5 to 6% by weight of ZnO.

5. The lithium aluminum silicate glass ceramic according to claim 1, characterized by containing 0-1.7% by weight, 0.1-1.4% by weight, 0.2-1.2% by weight, 0.3-0.8%, or further 0.4-0.6% by weight of MgO.

6. At least 1.0 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, or even 1.5 wt%, and 3.8 wt%, 3.5 wt%, 3.2 wt%, 3.0 wt%, or even only 2.5 wt% or less of ZrO ２ The lithium aluminum silicate glass ceramics according to claim 1, characterized by containing the same.

7. Li ２ The lithium aluminum silicate glass ceramic according to claim 1, characterized in that it contains more ZnO than O.

8. Na ２ O+K ２ The lithium aluminum silicate glass ceramic according to claim 1, characterized in that the amount of O is greater than 0.2% by weight and 3% by weight or less, preferably at least 0.4% by weight, or at least 0.6% by weight, or at least 0.9% by weight, or at least 1% by weight, or at least 1.1% by weight, or at least 1.2% by weight, or even more preferably at least 1.3% by weight, and 2.5% by weight or less, or 2.0% by weight or less, or 1.9% by weight or less, or 1.8% by weight or less, or even more preferably just 1.7% by weight.

9. K ２ It contains less MgO than O, and favorably MgO > 0 to < 1.7% by weight, K ２ O > 0 to 2% by weight, and MgO < K ２ The lithium aluminum silicate glass ceramic according to claim 1, characterized in that it is O.

10. Less than 0.1% by weight of As ２ O ３ and less than 0.1% by weight of Sb ２ O ３ The lithium aluminum silicate glass ceramic according to claim 1, characterized by containing the following.

11. 0-0.1% by weight of V ２ O ５ Or 0-0.5% by weight of MoO ３ , or 0-0.6% by weight of Nd ２ O ３ , or 0-0.4% by weight of Fe ２ O ３ , or 0-0.5% by weight of CoO, or 0-0.5% by weight of Cr ２ O ３ The lithium aluminum silicate glass ceramic according to claim 1, characterized by comprising 0 to 0.5 wt% NiO, 0 to 0.5 wt% CuO, 0 to 0.5 wt% MnO, or a combination thereof.

12. Use of the glass ceramics according to any one of claims 1 to 11 as a cooktop, fireplace viewing window, grill top or roast top, cover for combustion element of gas grill, oven viewing window, especially oven viewing window of pyrolysis furnace, kitchen or laboratory workbench or tabletop, cover for lighting fixture, in fire glazing, and as safety glass, optionally in laminated composite materials, as a carrier plate, or as a furnace lining in a thermal process, or as a back cover for portable electronic devices.