DE202023104159U1 - Transparent lithium aluminum silicate glass ceramic - Google Patents

Abstract

und wobei die Glaskeramik bis auf unvermeidbare Verunreinigungen frei von Cr₂O₃, V₂O₅, MnO₂ und MoO₃ ist.Transparent lithium aluminum silicate glass ceramic
with a thermal expansion coefficient in the range from 20 °C to 700 °C of -0.8 to 1.5 ppm/K,
with a chroma C* of less than 6 based on a thickness of 4 mm, with a crystallite size in the high quartz mixed crystal phase of less than 80 nm,
with a composition containing in % by weight based on oxide: _SiO2 62 - 70 _{Al2O3 _} 19 - 23 _Li2O 2.4 - 3.4 Well ₂ O 0 - 1 ZnO 0 - 5 _TiO2 1.5 - 2.5 _ZrO2 1.5 - 2.5 _SnO2 0 - 0.15
and wherein the glass ceramic is free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃ except for unavoidable impurities.

Description

Field of invention

The invention relates to a transparent lithium aluminum silicate glass ceramic, which is particularly suitable for use as a cooking surface in cooking appliances, and to its use.

Background of the invention

Glass ceramics, especially transparent glass ceramics, have been known for many years. A variant of such a glass ceramic that is also known in the prior art is the lithium aluminum silicate (LAS) glass ceramics, which are either high quartz mixed crystals (HQMK), especially for transparent materials, or keatite mixed crystals (KMK), especially for translucent or opaque materials. included as the main crystal phase. To produce such glass ceramics, initial glasses, so-called green glasses, are first produced using processes common for glass production. These green glasses are converted into glass ceramics through a thermal treatment, ceramization. Transparent glass ceramics produced in this way can be used in a variety of areas. Such glass ceramics are used in particular as cooking surfaces. Such glass ceramics are also used as viewing panels for fireplaces, ovens, as worktops or as safety glass.

The essential property of these materials for use as a cooking surface is that they have very low thermal expansion in a temperature range from room temperature to 700 °C. The low thermal expansion in turn results in a high resistance to temperature changes. The thermal expansion is adjusted by the combination of crystal phases with negative thermal expansion and an amorphous residual glass phase with positive thermal expansion. In the glass ceramics used to date, a lithium oxide content of usually more than 3.6 to 5.0% by weight is used.

Due to rising raw material prices for lithium, it is economically advantageous to minimize the proportion of lithium in the glass ceramic. However, since lithium is one of the three main components in lithium aluminum silicate glass ceramics, it cannot easily be reduced at will. The proportion of Li ₂ O has a direct effect on central properties of the glass ceramic, such as the viscosity in the melt and in hot forming, which is important for manufacturability, or on the thermal expansion, which is important for use as a cooking surface.

The price development for lithium is not a new problem. The price of lithium has increased continuously over the past 20 years. Despite this long-standing need, most glass ceramics available on the market for cooking surfaces have approximately 3.8 or more wt.% Li ₂ O.

Glass ceramics with a Li ₂ O content of less than 3.5% by weight 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, for example reduced resistance to thermal shock or poor meltability of the green glass.

Basic approaches with which lithium aluminum silicate glass ceramics can be produced, which avoid the above-mentioned disadvantages despite a low Li ₂ O content, have been described by the applicant, for example in DE 20 2022 106 826 U1 described. Approaches are also described as to how a transparent glass ceramic with a low Li ₂ O content can be produced.

In addition to the temperature stability described above, there are further special requirements for transparent glass ceramics. When using a transparent glass ceramic as a cooking surface, the glass ceramic is usually provided with a colored underside coating so that heating elements arranged under the glass ceramic and the associated electronics are no longer visible from above the glass ceramic. The general requirement is that the color perception of such an underside coating by an end user is not distorted by the glass ceramic. Similar requirements for the glass ceramic also arise when using a transparent glass ceramic as a viewing pane, since in this case too, color distortion when viewed through the viewing pane must be avoided.

In the present case, a transparent glass ceramic is, as usual, a glass ceramic with low light scattering. The transmission of such a transparent glass ceramic can be adjusted over a wide range using absorbent, i.e. coloring, components.

Task of the invention

The object of the invention is to provide a transparent lithium aluminum silicate glass ceramic that is cost-effective without resulting in any restrictions in the usage properties. Furthermore, the glass ceramic according to the invention should have the least possible distortion of color perception by the glass ceramic.

The glass ceramic should in particular meet all requirements for use as a cooking surface with all types of heating elements. These include, in particular, radiation, induction and gas heating elements. This requires in particular a sufficiently high resistance to temperature changes as well as a high long-term temperature resistance.

Summary of the invention

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

The transparent lithium aluminum silicate glass ceramic according to the invention has a thermal expansion coefficient in the range from 20 ° C to 700 ° C of -0.8 to 1.5 ppm / K, a chroma C * of less than 6, preferably less than 5.5 to a thickness of 4 mm and a crystallite size in the high quartz mixed crystal phase of less than 80 nm. The glass ceramic contains the following components in the specified amounts in % by weight based on oxide: _SiO2 62 - 70 _{Al2O3 _} 19-23 _Li2O 2.4-3.4 Well ₂ O 0-1 ZnO 0-5 _TiO2 1.5 - 2.5 _ZrO2 1.5 - 2.5 _SnO2 0 - 0.15.

Apart from unavoidable impurities, the glass ceramic is also free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃ .

A glass ceramic with a corresponding coefficient of thermal expansion has both high thermal shock resistance 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 -0.8 ppm/K. “ppm” means “parts per million”, i.e. a relative size change of 10 ^-6 with a temperature change of 1 K. More negative values of thermal expansion are avoided. In the event of negative expansion, i.e. contraction, tensile stresses arise in the surface of the glass ceramic during heating. At a value of less than -0.8 ppm/K, these stresses can reduce the mechanical strength of the cooking surface at typical operating temperatures of cooking appliances. Thermal expansion of more than 1.5 ppm/K is also avoided. With an expansion of more than 1.5 ppm/K, it is not guaranteed that the thermal shock resistance is sufficiently high to be able to use the glass ceramic in cooking appliances with radiant heating elements.

In a further development of the invention, the thermal expansion coefficient is at least -0.6 ppm/K, -0.4 ppm/K, -0.2 ppm/K, 0.0 ppm/K, 0.2 ppm/K, 0, 4 ppm/K, 0.6 ppm/K or even 0.8 ppm/K. In addition, the thermal expansion coefficient is preferably a maximum of 1.4 ppm/K, 1.3 ppm/K, 1.2 ppm/K, 1.1 ppm/K, 1.0 ppm/K, 0.8 ppm/K or even only a maximum of 0.6 ppm/K.

In a preferred embodiment of the invention, the thermal expansion coefficient of the glass ceramic is -0.8 to 0.8 ppm/K, preferably -0.5 to 0.5 ppm/K, particularly preferably -0.3 to 0.3 ppm/K . Such glass ceramics are particularly suitable for cooking surfaces in cooking appliances with radiant heating elements or induction heating elements.

With regard to the stated optical properties with regard to the transmission and the chroma of the glass ceramic according to the invention, “based on 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 on a different material thickness and on a Material thickness of 4 mm can be converted.

The chroma C* is determined from the L*a*b* color coordinates according to the following formula: $$C*=\sqrt{{\left(a*\right)}^2+{\left(b*\right)}^2}.$$

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 and a 2° observer.

The chroma C* of less than 6, preferably less than 5.5, means that the color of light is only changed very slightly when it passes through the glass ceramic. This is of particular importance, for example, when used as a cooking surface or fireplace viewing panel.

The colorfulness of a glass ceramic is particularly noticeable, for example, when the glass ceramic is provided with a white or generally light underside coating for use as a cooking surface. Glass ceramics in these applications are often 4 mm thick. Light that is reflected on a back coating therefore travels an optical distance of 8 mm, so that color shifts due to the glass ceramic's own color have a greater impact than with shorter distances. For cooking surfaces or chimney windows with a white coating on the back, a correspondingly low level of color is particularly advantageous. The glass ceramic according to the invention preferably has a chroma C* of less than 5.5, in particular less than 5.

The glass ceramic according to the invention has a crystallite size in the high quartz mixed crystal phase of less than 80 nm, preferably less than 70 nm, particularly preferably less than 60 nm and most preferably less than 50 nm. The crystallite size has a direct influence on the optical properties of the glass ceramic. A crystallite size that is too large leads to increased light scattering in the glass ceramic, which manifests itself as clouding of the glass ceramic.

The glass ceramic according to the invention contains the following components in% by weight: _SiO2 62 - 70, _{Al2O3 _} 19 - 23 and _Li2O 2.4 - 3.4.

The components SiO ₂ and Al ₂ O ₃ together with Li ₂ O form the main components of the crystal phase in the glass ceramic. At the same time, they essentially determine the glass formation properties and the viscosity of the green glass.

The SiO ₂ content of the glass ceramic according to the invention should be a maximum of 70% by weight because this component greatly increases the viscosity of the glass, in particular the processing point. For good glass melting and for low shaping temperatures, higher SiO ₂ contents are uneconomical. With very high SiO ₂ contents of more than 70% by weight, keatite can be formed during ceramization. This leads to a sharp increase in thermal expansion and a deterioration in the optical properties of the material, particularly in the form of haze. The minimum SiO ₂ content should be 62% by weight because this is necessary for the required properties, such as. B. chemical resistance and temperature resistance is advantageous.

The glass ceramic preferably contains at least 63% by weight, or even 64% by weight, of SiO ₂ . The more SiO ₂ the glass ceramic contains, the better its temperature resistance and chemical resistance. Furthermore, it preferably contains at most 69% by weight, at most 68% by weight, at most 67% by weight, at most 66% by weight, or even at most 65% by weight, SiO ₂ . The less SiO ₂ the glass ceramic contains, the better the meltability and processability of the green glass in hot forming.

The Al ₂ O ₃ content of the glass ceramic according to the invention is in the range from 19 to 23% by weight. A higher Al ₂ O ₃ content leads to problems with devitrification and the undesirable formation of mullite. Therefore, 23% by weight should not be exceeded. Amounts of Al ₂ O ₃ smaller than 19% by weight are unfavorable for the formation of high quartz mixed crystals and promote the formation of undesirable crystal phases. It has also been shown that low levels of Al ₂ O ₃ have a detrimental effect on the color of the glass ceramic.

The glass ceramic preferably contains at least 20% by weight of Al ₂ O ₃ . The more Al ₂ O ₃ the glass ceramic contains, the better its temperature resistance. Furthermore, it preferably contains at most 22% by weight, preferably at most 21.5% by weight, of Al ₂ O ₃ . The less Al ₂ O ₃ the glass ceramic contains, the better the meltability and processability of the green glass in hot forming.

The Li ₂ O content of the glass ceramic according to the invention is in the range 2.4 - 3.4% by weight. It has been shown that with a Li ₂ O content in this range in combination with the other components within the limits mentioned, 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 limits mentioned above in order to be able to achieve the temperature change resistance required for the invention. In addition, a proportion of more than 2.4% by weight of Li ₂ O has a positive effect on the producibility of the glass ceramic, as this reduces the electrical resistance of the glass melt, reduces the viscosity and thereby also lowers the processing point. The efficiency of refining can also be improved by reducing the viscosity of the glass melt. Improved refining results in less production waste due to bubble formation in the green glass. Furthermore, Li ₂ O also reduces the green glass's tendency to devitrify.

In a preferred embodiment, the glass ceramic contains at least 2.5% by weight, 2.6% by weight, or even 2.7% by weight of Li ₂ O.

As a source of lithium for the production of glass ceramics, natural mineral raw materials such as spodumene or petalite are generally used, or alternatively synthetically produced Li ₂ CO ₃ .

In a preferred embodiment, the glass ceramic contains high quartz mixed crystals as the main crystal phase. “Main crystal phase” means that the glass ceramic contains more high quartz mixed crystal by volume than keatite mixed crystal and other crystal phases. In a further development of this embodiment, the glass ceramic contains <10% by volume, preferably <5% by volume, particularly preferably <3% by volume of keatite mixed crystal. The vol.% refers to the volume of the glass ceramic. The volume fractions are determined using Rietveld analysis from X-ray diffraction spectra.

Keatite mixed crystals generally have a higher thermal expansion than high quartz mixed crystals. Therefore, a high proportion of high quartz mixed crystals with a simultaneously low proportion of keatite mixed crystals is particularly advantageous for a low thermal expansion coefficient of the glass ceramic. A low thermal expansion coefficient also improves the glass ceramic's resistance to temperature changes. At the same time, keatite mixed crystals cause greater scattering of light due to their usually large crystallite size, so that too high a proportion of keatite negatively affects the optical properties of the glass ceramic. For this reason too, the content of keatite in the crystal phase should be kept as low as possible.

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 - 1% by weight of Na ₂ O. A Na ₂ O content of 0 - 1% by weight in the glass ceramic according to the invention additionally improves the meltability and devitrification behavior when shaping the glass. Furthermore, Na ₂ O can increase the electrical conductivity of the melt. This facilitates the coupling of energy through electrical heaters in the melting tank. However, the contents are limited because it is not incorporated into the crystal phases, but essentially remains in the residual glass phase of the glass ceramic. Contents that are too high impair the crystallization behavior when converting the crystallizable starting glass into the glass ceramic, here in particular at the expense of faster ceramization speeds. A consequence of this can be increased crystallite sizes with preferred rapid ceramization in the glass ceramic, which in turn has an unfavorable effect on the scattering behavior of the glass ceramic. In addition, higher contents have an unfavorable effect on the time/temperature resistance of the glass ceramic. Of half, the glass ceramic preferably contains >0.1 - 1% by weight or 0.2 - 0.8% by weight or even 0.3 - 0.6% by weight of Na ₂ O.

The glass ceramic according to the invention also contains 0 - 5% by weight of ZnO, preferably 1.2 - 4.0% by weight of ZnO, preferably 1.4 - 3.6% by weight, particularly preferably 1.4 - 3.0 % by weight. It was previously stated that Li ₂ O contributes to reducing the thermal expansion coefficient of the glass ceramic. A reduction in the Li ₂ O content compared to the proportion usually used in the prior art for transparent glass ceramics therefore leads to a smaller reduction in the thermal expansion coefficient of the high quartz mixed crystal in the glass ceramic, so that the expansion coefficients required for use as a cooking surface may not be achieved can. However, similar to Li ₂ O, ZnO also contributes to reducing the thermal expansion coefficient, so that the effect of reducing the Li ₂ O content can be at least partially offset by adjusting the ZnO content.

Furthermore, ZnO reduces the processing point and the upper devitrification temperature of the glass ceramic. However, too high a ZnO content leads to an increased difference in refractive index between the crystal and residual glass phases in the glass ceramic, so that a corresponding glass ceramic scatters incident light to an increased extent. Consequently, an excessive increase in the ZnO content can negatively affect the transmission and chroma of a glass ceramic.

In addition to the aforementioned components, the glass ceramic according to the invention also contains 1.5 - 2.5% by weight of TiO ₂ . TiO _2, together with ZrO ₂ , contributes significantly to nucleation. The amount of TiO ₂ is limited to values of at most 2.5% by weight, preferably at most 2.4% by weight. Large amounts of 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. At the same time, the glass ceramic contains at least 1.5% by weight, preferably at least 1.6% by weight, at least 1.7% by weight, at least 1.8% by weight, at least 1.9% by weight, at least 2.0% by weight, or even at least 2.1% by weight of TiO ₂ . With higher proportions of TiO ₂ nucleation occurs more quickly. This allows the ceramization time of the glass ceramic to be reduced. Lower proportions stabilize the ceramization process and prevent unintentional devitrification during hot forming of the green glass. In summary, the TiO ₂ content is limited in order to keep the coloring effect of TiO ₂ on the optical properties of the glass ceramic as low as possible. At the same time, however, the TiO ₂ content should not be too low, so that sufficient nucleation and short ceramization times are still guaranteed.

Furthermore, the glass ceramic according to the invention contains 1.5 - 2.5% by weight of ZrO ₂ . ZrO ₂ functions, among other things, as a nucleating agent in glass ceramics and as such can interact with other components of a glass ceramic, such as SnO ₂ and TiO ₂ . A content of more than 1.5% by weight of ZrO ₂ is advantageous in order to improve nucleation. At the same time, it is advantageous to limit the ZrO ₂ content to 2.5% by weight, since ZrO ₂ impairs the melting behavior of the mixture. In addition, ZrO ₂ can lead to devitrification during hot forming.

This can lead to the undesirable formation of Zr-containing crystal phases, for example baddeleyite. The glass ceramic preferably contains at least >1.6% by weight, particularly preferably >1.7% by weight of ZrO ₂ , very particularly preferably >1.8% by weight. Furthermore, it preferably contains at most 2.4% by weight, 2.3% by weight, 2.2% by weight, 2.1% by weight, 2.0% by weight, or even at most 1 .9% by weight of _ZrO2 . In these quantities, a particularly good compromise can be achieved between a positive contribution to nucleation with an acceptable deterioration in meltability and hot forming. By optimizing the ZrO ₂ content, particularly with regard to the TiO ₂ content of the glass ceramic, faster ceramization of the green glass can also be made possible. A faster ceramization with sufficient nucleation beforehand leads to a smaller crystallite size in the glass ceramic and therefore to a smaller scattering of light. Furthermore, by using ZrO ₂ , the proportion of TiO ₂ as a further nucleating agent can be reduced and thus a glass ceramic with reduced chroma can be produced.

Furthermore, the glass ceramic according to the invention contains 0 - 0.15% by weight of SnO ₂ . SnO ₂ supports the refining of the green glass. A glass ceramic with the above-mentioned amounts of SnO ₂ can therefore be characterized by particularly few defects due to trapped gas bubbles. SnO ₂ is also advantageous in combination with the other components 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 0.15% by weight should not be exceeded, since SnO ₂ has a negative effect on the optical properties of the glass ceramic and, in particular, can cause an increase in the color of the glass ceramic. This is due, among other things, to the fact that SnO ₂ can form an additional absorption band for incident light with TiO ₂ , which results in a red shift of the transmitted light. Higher SnO ₂ contents also lead to undesirable crystallization of Sn-containing crystal phases, for example cassiterite, on the contact materials (e.g. Pt/Rh) during shaping and this leads to a further deterioration in the optical properties of the glass ceramic. It is therefore particularly advantageous for the optical properties of the glass ceramic to limit the SnO ₂ content as much as possible.

According to a further embodiment, it is therefore provided that the transparent lithium aluminum silicate glass ceramic contains at most 0.13% by weight of SnO ₂ , preferably at most 0.12% by weight of SnO ₂ , preferably at most 0.11% by weight of SnO ₂ . In this way, the negative influence of SnO ₂ on the optical properties of the glass ceramic can be further reduced, while at the same time sufficient refining of the green glass is ensured by the SnO ₂ content.

The glass ceramic according to the invention is also free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃ , apart from unavoidable impurities. V ₂ O ₅ and MoO ₃ are commonly used as main colorants for coloring bulk-colored glass ceramics, while Cr ₂ O ₃ and MnO ₂ are often used for supporting coloring. The above-mentioned colorants and in particular V ₂ O ₅ generally color glass ceramics very intensively even in small amounts, which is why the contents of the above-mentioned components in the glass ceramic according to the invention should be kept as low as possible in order to ensure the colorfulness of the glass ceramic according to the invention. These components occur, among other things, as impurities in the raw materials of the mixture or as components in shards that are added to the mixture to improve the melting properties. “Unavoidable impurities” is understood to mean a content of less than 20 ppm, in particular less than 10 ppm, in particular less than 5 ppm of the respective components in the composition. Furthermore, the glass ceramic is preferably free of Ce, Ni, Cu, Co, W and/or S or their oxides, apart from unavoidable impurities.

In a further development of the invention it is further provided that the transparent lithium aluminum silicate glass ceramic has a light transmittance τ _vis of more than 80% based on a thickness of 4 mm.

Glass ceramics with a light transmittance of more than 80% based on a thickness of 4 mm are particularly suitable for use as a fireplace viewing panel or as a cooking plate. In fireplaces, this transmission means that the fire is particularly visible. In cooking appliances, for example, this transmission enables light displays with relatively low luminance, such as LCD or OLED displays, to be particularly clearly visible. The glass ceramic according to the invention preferably has a light transmittance τ _vis of more than 82%, particularly preferably more than 85%, based on a thickness of 4 mm.

The above transmission information for the glass ceramic refers to a thickness of 4 mm and can be converted to other substrate thicknesses using the Lambert-Beer law. The light transmittance τ _vis is determined in the wavelength range 380 - 780 nm using light of the standard illuminant D65 and a 2° observer in accordance with the specifications of DIN 5033. This value corresponds to the brightness Y in the CIExyY color space.

To support the refining of the green glass, in addition to the above-mentioned SnO ₂ content, a proportion of up to 1.5% by weight of As ₂ O ₃ and/or Sb ₂ O ₃ can be found to be advantageous, the SnO ₂ content as follows to be reduced as much as possible, since SnO ₂ , as already explained above, in addition to its positive effect on the refining of the green glass, can have a negative effect on the optical properties of the glass ceramic. “As ₂ O ₃ and/or Sb ₂ O ₃ ” means here and below that these two components can be contained individually or simultaneously in the glass ceramic, with both components independently accounting for a proportion of up to 1.5 weight. -% can make up.

Accordingly, according to a further development of the invention, it is provided that the glass ceramic is free of SnO ₂ except for unavoidable impurities and contains 0.5% by weight to 1.5% by weight of As ₂ O ₃ and/or Sb ₂ O ₃ . Such a refining, which takes place completely without SnO ₂ and therefore avoids the above-mentioned influence of SnO ₂ on the optical properties of the glass ceramic, is special This is also advantageous for producing a transparent and color-neutral glass ceramic. Unlike SnO ₂ , As ₂ O ₃ and Sb ₂ O ₃ have no influence on the color of the glass ceramic because, among other things, it does not lead to increased coloring in connection with TiO ₂ .

In summary, the glass ceramic can be refined by adding SnO ₂ alone, by a combination of SnO ₂ and As ₂ O ₃ and/or Sb ₂ O ₃ , or by refining using As ₂ O ₃ and/or Sb ₂ O ₃ in its absence of SnO ₂ .

In a further development of the invention, the glass ceramic contains less than 0.05% by weight of Fe ₂ O ₃ , preferably less than 0.04% by weight, particularly preferably less than 0.03% by weight, most preferably less than 0.02% by weight. Similar to V ₂ O ₅ , Cr ₂ O ₃ , MnO ₂ and MoO _{3 ,} Fe ₂ O ₃ also has a coloring effect in glass ceramics. For this reason, it is advantageous for a transparent glass ceramic if the Fe ₂ O ₃ content is limited to the values mentioned above. For example, glass ceramics that contain Fe ₂ O ₃ as a coloring agent often have a yellow tinge. This can be the case, for example, with glass ceramics that simultaneously contain TiO ₂ and Fe ₂ O ₃ introduced through contamination of the raw materials. This effect occurs in particular with a glass ceramic that is refined with SnO ₂ , since SnO ₂ can contribute to increasing the proportion of Fe ²⁺ in the Fe ₂ O ₃ , which leads to increased coloring in connection with TiO ₂ . For example, if such glass ceramics are provided with white underside coatings, these underside coatings have a clearly noticeable yellow tint.

Furthermore, Fe ₂ O ₃ affects the transmission not only in the visible spectral range, but also in the near-infrared up to a wavelength of approx. 3 µm. Fe ₂ O ₃ therefore not only has an influence on the ability to achieve certain colors or the ability to display color displays. The absorption in the near-infrared determines how much heat energy the glass melt can absorb in the tub. It determines how much heating power from radiant heating elements can pass through the glass ceramic. It also determines whether and which infrared sensors can be used in a hob. Such sensors can be designed, for example, as optical touch sensors or as infrared receivers for wireless data transmission.

It should be noted at this point that, within the scope of the present disclosure, the components of the glass are given in the form as they are usually obtained by analysis. The component in the stable, usually highest, oxidation state is usually specified. However, it should be noted that this does not necessarily correspond to the form in which the component actually exists.

For example, the iron in an oxidic glass can be present in both divalent and trivalent form, i.e. as Fe ²⁺ or FeO or as Fe ³⁺ or as Fe ₂ O ₃ . The information on the contents of corresponding components in the context of the present disclosure therefore refers to usual information from the analysis, although it is understood that the component can also be present in a form that deviates from the “ideal”, usually specified oxidation state.

In a further development of the invention it is further provided that the transparent lithium aluminum silicate glass ceramic contains 0.01 - 0.07 wt.% Nd ₂ O ₃ , preferably 0.03 - 0.06 wt.% Nd ₂ O ₃ . Nd ₂ O ₃ can help to compensate for the colorfulness of the glass ceramic caused by the SnO ₂ , particularly when refining with SnO ₂ . The addition of Nd ₂ O ₃ is particularly advantageous when the SnO ₂ content is more than 0.1% by weight for producing a largely color-neutral glass ceramic.

In glass ceramics with a content of Fe ₂ O ₃ and TiO ₂ within the previously described limits, addition of Nd ₂ O ₃ can further help to reduce or eliminate a yellow tint caused by Fe ₂ O ₃ and TiO ₂ , thereby increasing the light transmittance is only slightly influenced. This effect is based on narrow absorption bands of Nd ₂ O ₃ in the visible spectral range at wavelengths of 513, 526, 575, 585 and 745 nm and is particularly advantageous for producing a transparent, color-neutral glass ceramic.

In a further development of the invention, the glass ceramic also contains less than 1% by weight of K ₂ O. K ₂ O in amounts of 0 - <1% by weight can improve the meltability and devitrification behavior in the glass ceramic when shaping the glass cause. K ₂ O can also increase the electrical conductivity of the melt. This facilitates the coupling of energy through electrical heaters in the melting tank. However, the contents are limited because it is not incorporated into the crystal phases, but essentially remains in the residual glass phase of the glass ceramic. Contents that are too high have an impact affect the crystallization behavior when converting the crystallizable starting glass into the glass ceramic, here in particular at the expense of faster ceramization speeds. Analogous to the above statements regarding Na ₂ O, too high a ceramization speed and therefore an increased crystallite size in the glass ceramic have a disadvantageous effect on the scattering behavior of the glass ceramic. In addition, higher contents have an unfavorable effect on the time/temperature resistance of the glass ceramic.

These properties can be further improved if K ₂ O is present in amounts of at most 0.8 wt%, 0.6 wt%, 0.5 wt%, 0.4 wt%, or even 0 .3% by weight is contained in the glass ceramic. In a special embodiment, it can also be provided that the glass ceramic is free of K ₂ O except for unavoidable impurities.

In a further development of the invention, the glass ceramic contains less than 0.6% by weight of MgO, preferably less than 0.5% by weight, particularly preferably less than 0.4% by weight. MgO has a positive influence on the meltability and devitrification behavior of the glass ceramic. Limiting the MgO content to the aforementioned limits can help to optimize the thermal expansion of the glass ceramic and reduce the chroma.

According to a further development of the invention, it is further provided that the transparent lithium aluminum silicate glass ceramic contains less than 1% by weight of CaO, preferably less than 0.6% by weight, particularly preferably less than 0.5% by weight. CaO has an advantageous effect on the devitrification behavior of the glass ceramic. However, for a colorless, transparent glass ceramic, it has proven to be advantageous to limit the CaO content to the above values, since CaO contributes to an increase in the crystallite size in the glass ceramic. An increase in the size of the crystallites is accompanied by increased scattering and therefore poorer optical properties of the glass ceramic.

It has already been stated previously that the described ZnO content in the glass ceramic according to the invention can contribute to increased scattering, since too high a ZnO content contributes to an increased difference in refractive index between the crystal and residual glass phases in the glass ceramic. In order to counteract this effect, the glass ceramic according to a further embodiment preferably also contains more than 1.8% by weight of BaO, preferably more than 2.5% by weight, particularly preferably more than 2.8% by weight. BaO. An increased proportion of BaO reduces the tendency of the glass ceramic to scatter incident light, since the refractive index of the residual glass phase can be adjusted to the refractive index of the crystalline phase of the glass ceramic with the same crystallite sizes with the addition of BaO. This effect is evident for proportions of Li ₂ O and ZnO within the aforementioned limits, especially when the content of SrO and CaO in the glass ceramic is low.

BaO, similar to Li ₂ O, also causes a reduction in the viscosity of the glass melt and thus a reduction in the processing point. In order to improve the meltability of the green glass, it is advantageous if the glass ceramic in combination with the above-mentioned amounts of Li ₂ O more than 2.6% by weight, more than 2.7% by weight, more than 2, Contains 8% by weight, more than 2.9% by weight, or even more than 3.0% by weight of BaO. In glass ceramics, BaO also makes a significant contribution to improving the devitrification behavior when hot forming the green glass.

However, it has been shown that BaO can have a negative effect on the formation of the crystal phase during ceramization. In order to prevent long ceramization times from being required, the amount of BaO is therefore preferably at a maximum of 5.0% by weight, 4.5% by weight, 4.0% by weight, 3.9% by weight. , 3.8% by weight, 3.7% by weight, 3.6% by weight or even 3.5% by weight.

According to a further development of the invention, the ratio of Al ₂ O ₃ to TiO ₂ is in the range 7 to 12, preferably in the range 8 to 11, particularly preferably in the range 8.5 to 10.5. It has been shown that the ratio of Al ₂ O ₃ to TiO ₂ in this range has a particularly advantageous effect on the color of the glass ceramic and at the same time enables particularly stable ceramization.

In a further development of the invention, the glass ceramic contains the following components in % by weight based on oxide: _SiO2 63 - 70 _{Al2O3 _} 19-23 _Li2O 2.4-3.4 Well ₂ O 0.1-1 _K2O 0-0.8 MgO 0 - 0.6 CaO 0-1 BaO > 1.8 ZnO 1-4 _TiO2 1.5 - 2.5 _ZrO2 1.5 - 2.5 _SnO2 0 - 0.15.

Apart from unavoidable impurities, the glass ceramics are free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃

Furthermore, such a glass ceramic can also contain SrO, the SrO content mainly coming from impurities in the raw materials for producing the glass ceramic. Among other things, SrO contributes to the scattering of light through the glass ceramic. It is therefore advantageous to keep the SrO content in the glass ceramic as low as possible.

The following table contains three further developments of a composition of the glass ceramic according to the invention in% by weight based on oxide: preferred particularly preferred most preferred Si2O _{_} _ 63 - 70 64 - 70 65 - 70 Al2O3 _{_} _ _{_} 19 - 23 19 - 22.5 19 - 22 Li2O _{_} _ 2.5 - 3.4 2.7 - 3.4 2.9 - 3.4 Well ₂ O 0.1 - 0.8 0.3 - 0.7 0.4 - 0.6 K2O _{_} _ <0.8 <0.5 <0.3 MgO 0-0.6 0.1 - 0.5 0.1 - 0.4 CaO <0.7 <0.4 <0.1 BaO 1.8 - 3.0 1.8 - 2.5 1.8 - 2.0 ZnO 1-3 1.2 - 2.5 1.3 - 2.2 TiO2 _{_} 2.0 - 2.5 2.1 - 2.4 2.2 - 2.4 ZrO2 _{_} 1.6 - 2.0 1.7-2.0 1.7-1.9 SnO2 _{_} 0-0.15 0-0.13 0.03 - 0.13 Nd2O3 _{_} _ _{_} 0-0.15 0-0.1 0-0.07

Apart from unavoidable impurities, the glass ceramics are free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃

Furthermore, such a glass ceramic can be free of K ₂ O and As ₂ O ₃ and Sb ₂ O ₃ , in particular apart from unavoidable impurities.

Three alternative developments of a composition of the glass ceramic according to the invention contain the following components in% by weight based on oxide: preferred particularly preferred most preferred Si2O _{_} _ 63 - 69 63 - 68 63 - 67 Al2O3 _{_} _ _{_} 19 - 23 19 - 22.5 19.5 - 22.5 Li2O _{_} _ 2.4 - 3.1 2.4 - 2.9 2.45 - 2.85 Well ₂ O 0.1 - 0.8 0.2 - 0.8 0.25 - 0.8 K2O _{_} _ <0.7 <0.4 <0.1 MgO 0 - 0.6 0 - 0.6 0.1 - 0.6 CaO 0 - 0.82 0 - 0.6 0 - 0.4 BaO 1.8 - 4.5 1.8-4 1.8 - 3.9 ZnO 1.8 - 3.8 2.2 - 3.7 2.7 - 3.6 TiO2 _{_} 2.1 - 2.5 2.1 - 2.4 2.2 - 2.4 ZrO2 _{_} 1.6 - 2.0 1.6 - 1.9 1.7 - 1.9 SnO2 _{_} 0.02 - 0.15 0.05 - 0.15 0.08 - 0.14 Nd2O3 _{_} _ _{_} 0 - 0.15 0 - 0.1 0 - 0.07

Apart from unavoidable impurities, the glass ceramics are free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃

Furthermore, such a glass ceramic can be free of K ₂ O and As ₂ O ₃ and Sb ₂ O ₃ , in particular apart from unavoidable impurities.

Three further developments of a composition of the glass ceramic according to the invention contain the following components in % by weight based on oxide: preferred particularly preferred most preferred Si2O _{_} _ 63 - 69 63 - 68 63 - 66 Al2O3 _{_} _ _{_} 19 - 22 20-22 20.5 - 22 Li2O _{_} _ 2.6 - 3.3 2.8 - 3.2 2.9 - 3.2 Well ₂ O 0.1 - 0.8 0.1 - 0.6 0.2 - 0.5 K2O _{_} _ 0.1 - 0.7 0.1 - 0.5 0.1 - 0.3 MgO 0 - 0.5 0.1 - 0.5 0.3 - 0.45 CaO 0.1 - 0.8 0.2 - 0.6 0.3 - 0.5 BaO > 2.2 2.5 - 3.3 2.9 - 3.2 ZnO 1.5 - 3.5 2.0 - 3.2 2.5 - 2.8 TiO2 _{_} 2.0 - 2.4 2.1 - 2.4 2.1 - 2.3 ZrO2 _{_} 1.6 - 2.0 1.6 - 1.9 1.7 - 1.9 SnO2 _{_} 0 - 0.15 0 - 0.13 0 - 0.12 Nd2O3 _{_} _ _{_} 0 - 0.15 0 - 0.1 0 - 0.07

Apart from unavoidable impurities, the glass ceramics are free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃

In these developments, the refining of green glasses based on the above-mentioned compositions can be done either using SnO ₂ , using a combination of SnO ₂ with As ₂ O ₃ and/or Sb ₂ O ₃ or only using As ₂ O ₃ and/or Sb ₂ O ₃ , i.e. in the absence of SnO ₂ . The previously mentioned limit values can be set for the content of the respective components in the composition.

Three further developments of a composition of the glass ceramic according to the invention contain the following components in % by weight based on oxide: preferred particularly preferred most preferred Si2O _{_} _ 63 - 69 64 - 68 65 - 67 Al2O3 _{_} _ _{_} 19-22 20-22 21 - 22 Li2O _{_} _ 2.5 - 3.4 2.8 - 3.4 3.1 - 3.4 Well ₂ O 0.1 - 0.8 0.3 - 0.8 0.5 - 0.7 K2O _{_} _ 0 - 0.6 0 - 0.4 0 - 0.2 MgO 0 - 0.6 0.1 - 0.5 0.3 - 0.5 CaO 0.1 - 0.8 0.2 - 0.7 0.3 - 0.5 BaO 0.2 - 1.5 0.4 - 1.2 0.7 - 1.1 ZnO 1.1 - 3.0 1.4 - 2.5 1.7 - 2.1 TiO2 _{_} 2.0 - 2.4 2.1 - 2.3 2.1 - 2.3 ZrO2 _{_} 1.6 - 2.0 1.7 - 2.0 1.8 - 2.0 SnO2 _{_} 0 - 0.15 0.05 - 0.15 0.1 - 0.15 Nd2O3 _{_} _ _{_} 0 - 0.07 0.01 - 0.07 0.02 - 0.07

Such a glass ceramic can, in particular, be free of As ₂ O ₃ and Sb ₂ O ₃ except for unavoidable impurities.

Three further developments of a composition of the glass ceramic according to the invention contain the following components in % by weight based on oxide: preferred particularly preferred most preferred Si2O _{_} _ 63 - 68 63 - 67 63 - 66 Al2O3 _{_} _ _{_} 19 - 22 20-22 20.5 - 22 Li2O _{_} _ 2.4 - 3.3 2.4 - 3.2 2.4 - 3.2 Well ₂ O 0.1 - 0.8 0.2 - 0.7 0.25 - 0.4 K2O _{_} _ 0.1 - 0.8 0.1 - 0.6 0.15 - 0.6 MgO 0 - 0.6 0 - 0.5 0 - 0.45 CaO 0 - 1 0.4 - 1 0.8 - 1 BaO 2.0 - 3.0 2.2 - 2.8 2.4 - 2.7 ZnO 1.5 - 4.0 2.0 - 3.8 2.5 - 3.4 TiO2 _{_} 2.0 - 2.4 2.1 - 2.3 2.1 - 2.3 ZrO2 _{_} 1.6 - 2.0 1.7 - 2.0 1.8 - 2.0 SnO2 _{_} 0 - 0.15 0 - 0.12 0 - 0.11 Nd2O3 _{_} _ _{_} 0 - 0.15 0 - 0.1 0 - 0.07

Apart from unavoidable impurities, the glass ceramics are free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃

Furthermore, such a glass ceramic can be free of As ₂ O ₃ and Sb ₂ O ₃ , in particular apart from unavoidable impurities.

The glass ceramic according to the invention is used as a cooking surface, fireplace viewing panel, grill or roasting surface, covering fuel elements in gas grills, oven viewing panel, especially for pyrolysis stoves, worktops or table tops in kitchens or laboratories, covers for lighting devices, in fire protection glazing and as safety glass, optionally in laminate composite, as a carrier plate or as an oven lining in thermal processes or as a back cover for mobile electronic devices.

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

The invention is further illustrated below using exemplary embodiments.

The crystallizable green glasses of the exemplary embodiments were melted from technical batch raw materials common in the glass industry at temperatures of 1620 ° C to 1680 ° C for 4 hours. By choosing such raw materials, the demands for economical raw materials and a low impurity content of undesirable impurities can be reconciled. After melting the mixture in crucibles made of sintered silica glass, the melts were poured into Pt/Rh crucibles with an inner crucible made of silica glass and homogenized by stirring at temperatures of 1600 ° C to 1640 ° C for 60 minutes. After this homogenization, the glasses were refined for 3 to 3.5 hours at 1640 °C to 1680 °C. Blocks measuring approximately 120 × 120 × 30 mm ³ were then cast and, depending on the respective viscosity of the glass, cooled to room temperature at 30 K/h from 650 - 680 °C to room temperature in a cooling oven in order to reduce stress. The castings were divided into the sizes required for the examinations and for ceramization.

1. a) Aufheizen von Raumtemperatur auf 740 °C innerhalb von 25 Minuten,
2. b) Halten bei 740°C für 20 Minuten,
3. c) Erhöhung der Temperatur von 740 °C auf 780 °C innerhalb von 6,5 Minuten,
4. d) Erhöhung der Temperatur von 780 °C auf 825 °C innerhalb von 20 Minuten,
5. e) Erhöhung der Temperatur von 825 °C auf 910 °C innerhalb von 8,5 Minuten,
6. f) Halten der Temperatur bei 910 °C für 10 Minuten,
7. g) Abkühlen auf Raumtemperatur innerhalb von 125 Minuten.

The ceramization of the samples in the green glass state was carried out using a ceramization process with the following steps:

1. a) heating from room temperature to 740 °C within 25 minutes,
2. b) holding at 740°C for 20 minutes,
3. c) increasing the temperature from 740 °C to 780 °C within 6.5 minutes,
4. d) increasing the temperature from 780 °C to 825 °C within 20 minutes,
5. e) increasing the temperature from 825 °C to 910 °C within 8.5 minutes,
6. f) maintaining the temperature at 910 °C for 10 minutes,
7. g) Cool to room temperature within 125 minutes.

This ceramization process is to be viewed as exemplary. It is possible to fine-tune the properties of the glass ceramic by varying the temperatures and durations of the individual steps or by adding further steps, for example additional holding times.

1. a) Aufheizen von Raumtemperatur auf 680 °C bis 760 °C innerhalb von 3 bis 60 Minuten,
2. b) Temperaturerhöhung des kristallisierbaren Grünglases auf den Temperaturbereich der Keimbildung T_a bis 800 °C über einen Zeitraum von 10 bis 100 Minuten,
3. c) Temperaturerhöhung des Kristallisationskeime enthaltenden Glases innerhalb von 5 bis 80 Minuten in den Temperaturbereich hoher Kristallwachstumsgeschwindigkeit von 810 °C bis 930 °C,
4. d) Halten der Temperatur bei 810 bis 930 °C für 2 bis 60 min.,
5. e) Abkühlen auf Raumtemperatur innerhalb weniger als 150 Minuten.

For example, ceramization can also take place with the following parameters:

1. a) Heating from room temperature to 680 °C to 760 °C within 3 to 60 minutes,
2. b) increasing the temperature of the crystallizable green glass to the temperature range of nucleation T _a to 800 ° C over a period of 10 to 100 minutes,
3. c) increasing the temperature of the glass containing crystallization nuclei within 5 to 80 minutes into the temperature range of high crystal growth rates of 810 ° C to 930 ° C,
4. d) maintaining the temperature at 810 to 930 °C for 2 to 60 min.,
5. e) Cool to room temperature within less than 150 minutes.

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

The thermal expansion coefficient CTE was determined using a method that is essentially based on the DIN ISO 7991 from 1998.

To measure the upper devitrification temperature (OEG), the green glasses were melted in PURh10 crucibles. The crucibles were then kept at various temperatures in the processing temperature range for 5 hours. The highest temperature at which the first crystals appear on the contact surface of the glass melt with the crucible wall is determined by the UEG.

The processing point (T ₄ ) of the green glasses was determined using a stirring viscometer according to DIN ISO 7884-2.

T_g gibt die Transformationstemperatur, auch als Glasübergangstemperatur bekannt, der Grüngläser an. Sie wird dilatometrisch bestimmt.When the green glass is converted into the glass ceramic, the density increases because the crystal phase has a higher density than the amorphous glass. The so-called shrinkage indicates the linear change in length when the green glass is converted into the glass ceramic. The shrinkage in percent is calculated as follows from the density of the green glass and the density of the glass ceramic: $$ScHrumpf=\left(1-{\left(\frac{DicHt{e}_{Gr\ddot{u}nGlas}}{DicHt{e}_{Glasskeramik}}\right)}^{\frac{1}{3}}\right)\cdot 100.$$

T _g indicates the transformation temperature, also known as the glass transition temperature, of the green glasses. It is determined dilatometrically.

The light transmittance τ _vis is determined in the wavelength range 380 - 780 nm using light of the standard illuminant D65 and a 2° observer in accordance with the specifications of DIN 5033. This value corresponds to the brightness Y in the CIExyY color space. The value is a measure of the human eye's perception of brightness.

Transmission spectra were determined according to ISO 15368:2021. Table 2 contains examples of the spectral transmittances “T @ ...” for the wavelengths 470 nm, 600 nm, 630 nm, 700 nm, 950 nm and 1600 nm.

The color coordinates in the CIExyY color space and in the CIELab color space were determined in accordance with the specifications of the CIE 1932 with a 2° observer and light of the standard illuminant D65 in transmission.

All transmission measurements were carried out on 4 mm thick samples that were smooth on both sides.

The volume fraction “HQMK phase content” and the crystallite size of the crystalline phase “HQMK crystallite size” were determined from X-ray diffraction spectra using Rietveld analysis. Table 1: Compositions of exemplary glass ceramics in wt.% based on oxide Example No. 1 2 3 4 5 6 7 _SiO2 68,100 67,900 67,900 66,300 67,600 69,300 67,700 _{Al2O3 _} 20,660 20,660 20,550 22,020 20,600 19,020 20,590 _Li2O 2,870 3,190 3,200 3,270 3,250 3,270 3,220 Well ₂ O 0.543 0.516 0.522 0.534 0.536 0.538 0.520 _K2O MgO 0.132 0.130 0.129 0.132 0.133 0.128 0.131 CaO 0.010 0.011 0.011 0.011 SrO 0.018 0.017 0.017 0.018 0.019 0.019 0.017 BaO 1,920 1,900 1,900 1,930 1,930 1,930 1,910 ZnO 1,490 1,460 1,470 1,510 1,490 1,480 1,550 _TiO2 2,320 2,310 2,330 2,330 2,310 2,310 2,320 _ZrO2 1,830 1,820 1,830 1,840 1,840 1,850 1,820 _{P2O5 _} 0.030 0.029 0.029 0.028 0.028 0.032 _{As2O3 _ SnO2} 0.075 0.070 0.075 0.087 0.115 0.069 0.119 _{Fe2O3 _} 0.014 0.015 0.015 0.014 0.015 0.014 0.015 _{Nd2O3 _} 0.055 0.054 0.064 _{MnO2 HfO2} 0.034 0.034 0.034 0.035 0.034 0.035 0.034 Table 1 continued Example No. 8th 9 10 11 12 13 14 _SiO2 67,200 66,100 64,400 64,400 64,500 66,000 64,500 _{Al2O3 _} 20,580 20,670 21,170 21,170 21,170 20,160 21,150 _Li2O 3,040 2,530 3,060 3,060 2,490 2,730 3,090 Well ₂ O 0.530 0.720 0.316 0.321 0.321 0.546 0.299 _K2O 0.235 0.227 0.517 0.227 MgO 0.315 0.315 0.375 0.377 0.076 0.142 0.371 CaO 0.013 0.310 0.952 0.440 0.947 0.012 0.435 SrO 0.017 0.017 0.014 0.017 0.013 0.014 0.017 BaO 1,910 1,920 2,550 3,030 2,540 2,600 3,070 ZnO 2,030 3,000 2,720 2,720 3,260 3,470 2,700 _TiO2 2,330 2,350 2,231 2,233 2,228 2,342 2,231 _ZrO2 1,820 1,820 1,840 1,850 1,840 1,820 1,850 _{P2O5 _} 0.033 0.034 0.030 _{As2O3 _ SnO2} 0.119 0.119 0.098 0.109 0.101 0.105 0.047 _{Fe2O3 _} 0.015 0.015 0.015 0.015 0.015 0.015 0.015 _{Nd2O3 _} 0.064 0.061 _MnO2 0.001 _HfO2 0.033 0.034 0.033 0.033 0.033 0.033 0.033 Table 1 continued Example No. 15 16 17 18 19 20 21 _SiO2 63,900 66,400 66,400 65,900 64,500 64,200 64,200 _{Al2O3 _} 21,080 21,670 21,700 19,920 21,060 21,940 21,950 _Li2O 3,020 3,380 3,170 2,740 3,070 2,740 2,710 Well ₂ O 0.303 0.570 0.570 0.336 0.306 0.507 0.506 _K2O 0.227 0.089 0.091 0.230 MgO 0.377 0.375 0.439 0.136 0.365 0.516 0.140 CaO 0.432 0.373 0.373 0.013 0.434 0.020 0.012 SrO 0.017 0.077 0.076 0.020 0.017 0.014 0.016 BaO 3,020 0.910 0.910 3,770 3,050 2,620 3,010 ZnO 2,700 1,850 1,920 2,830 2,700 3,110 3,090 _TiO2 2,222 2,200 2,203 2,352 2,231 2,340 2,335 _ZrO2 1,830 1,880 1,880 1,840 1,850 1,830 1,830 _{P2O5 _ As2O3 _} 0.830 _SnO2 0.122 0.126 0.106 0.103 0.112 0.113 _{Fe2O3 _} 0.014 0.017 0.017 0.015 0.014 0.014 0.015 _{Nd2O3 _} 0.048 0.055 0.051 _{MnO2 HfO2} 0.032 0.036 0.036 0.033 0.033 0.032 0.032 Table 1 continued Example No. 22 _SiO2 64,900 _{Al2O3 _} 21,310 _Li2O 2,690 Well ₂ O 0.503 _K2O MgO 0.140 CaO 0.012 SrO 0.014 BaO 2,640 ZnO 3,470 _TiO2 2,350 _ZrO2 1,830 _{P2O5 _ As2O3 _ SnO2} 0.109 _{Fe2O3 _} 0.014 _{Nd2O3 _ MnO2 HfO2} 0.032 Table 2: Material properties of the exemplary glass ceramics Example No. 1 2 3 4 5 6 _Tg °C 709 701 Dense glass g/ ^cm3 2.4597 2.4523 HQMK phase content Vol.-% 63 65 66 64 65 HQMK crystallite size nm 38 34 39 35 36 Dense glass ceramic g/ ^cm3 shrink % CTE(20;700°C) ppm/K -0.20 -0.38 -0.36 -0.28 -0.37 -0.46 τ _vis % 84.7 87.1 86.4 86.1 86.2 84.9 L* 93.7 94.8 94.5 94.3 94.4 93.9 a* 0.0 0.0 0.0 0.0 -0.1 -0.2 b* 4.0 3.0 2.1 3.3 2.4 4.1 C* 4.0 3.0 2.1 3.3 2.4 4.1 T @ 400nm % 73.0 77.0 77.6 75.9 75.4 69.2 T @ 470 nm % 80.6 84.0 84.5 82.7 84.1 81.1 T @ 600nm % 86.2 88.4 87.9 87.5 87.6 86.4 T @ 630nm % 87.0 88.8 89.3 88.1 89.2 87.1 T @ 700nm % 88.1 89.7 90.1 89.1 90.0 88.1 T @ 950 nm % 88.8 89.9 90.1 89.6 90.1 88.5 T@ 1600 nm % 88.5 89.4 89.3 88.9 89.3 87.8 Table 2 continued Example No. 7 8th 9 10 11 12 _Tg °C 701 693 693 685 685 686 Dense glass g/ ^cm3 2.4537 2.4656 2.4924 2.5121 2.5143 2.5174 HQMK phase content Vol.-% 62 59 54 HQMK crystallite size nm 47 41 42 Dense glass ceramic g/ ^cm3 2.5759 2.5796 2.5815 shrink % 0.833 0.851 0.835 CTE(20;700°C) ppm/K -0.37 -0.19 0.16 0.16 0.12 0.31 τ _vis % 83.8 83.6 83.8 85.2 86.3 84.6 L* 93.4 93.3 93.4 94.0 94.4 93.7 a* -0.2 -0.2 -0.1 -0.1 -0.2 0.0 b* 2.6 2.5 3.7 5.0 4.6 5.1 C* 2.6 2.5 3.7 5.0 4.6 5.1 T @ 400nm % 72.3 71.6 69.7 67.4 69.0 69.5 T @ 470 nm % 81.6 81.5 80.4 80.4 81.9 79.5 T @ 600nm % 85.4 85.1 85.8 87.1 88.0 86.7 T @ 630nm % 87.0 86.8 87.6 87.9 88.7 87.6 T @ 700nm % 87.8 87.7 88.9 89.2 89.9 89.0 T @ 950 nm % 88.2 88.1 89.7 90.0 90.1 89.5 T @ 1600nm % 87.7 87.3 89.1 89.3 89.3 89.0 Table 2 continued Example No. 13 14 15 16 17 18 _Tg °C 689 683 681 690 695 697 Dense glass g/ ^cm3 2.5036 2.5124 2.5199 2.4605 2.4631 2.5106 HQMK phase content Vol.-% 61 60 61 66 64 58 HQMK crystallite size nm 36 45 53 43 43 39 Dense glass ceramic g/ ^cm3 2.5820 2.5780 2.5859 2.5461 2.5518 2.5893 shrink % 1,023 0.856 0.858 1,133 1,172 1,024 CTE(20;700°C) ppm/K -0.26 0.10 0.11 -0.19 -0.06 -0.10 τ _vis % 85.9 86.3 88.7 85.4 84.6 86.8 L* 94.3 94.4 95.5 94.0 93.7 94.7 a* -0.3 0.1 -0.3 0.0 0.0 -0.3 b* 4.9 3.8 3.2 3.2 3.2 4.2 C* 4.9 3.8 3.2 3.2 3.2 4.2 T @ 400nm % 67.0 74.4 77.0 72.9 72.5 69.8 T @ 470 nm % 81.2 82.4 85.6 82.4 81.5 82.9 T @ 600nm % 87.6 87.9 89.8 87.1 86.4 88.4 T @ 630nm % 88.3 88.7 90.3 88.6 88.1 88.9 T @ 700nm % 89.5 89.7 90.7 89.7 89.2 89.9 T @ 950 nm % 89.9 89.8 90.1 89.9 89.6 90.0 T @ 1600nm % 89.1 89.5 89.8 88.7 88.7 89.6 Table 2 continued Example No. 19 20 21 22 _Tg °C 682 692 Dense glass g/cm3 2.5133 2.5165 HQMK phase content Vol.-% 61 61 60 62 HQMK crystallite size nm 44 43 40 40 Dense glass ceramic g/cm3 2.5795 2.5943 shrink % 0.863 1,010 CTE(20;700°C) ppm/K 0.11 0.22 τ _vis % 84.9 85.1 85.5 84.7 L* 93.8 93.9 94.1 93.8 a* -0.2 0.0 0.0 -0.1 b* 3.7 4.1 4.0 4.6 C* 3.7 4.1 4.0 4.6 T @ 400nm % 69.3 71.7 72.9 69.2 T @ 470 nm % 81.5 81.0 81.4 80.2 T @ 600nm % 86.8 86.8 87.1 86.5 T @ 630nm % 88.5 87.6 87.8 87.3 T @ 700nm % 89.6 88.7 88.9 88.6 T @ 950 nm % 89.9 89.8 89.9 89.6 T @ 1600nm % 89.6 89.0 89.2 89.2

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2012010341 A1 [0006]
• EP 3502069 A1 [0006]
• US 2017050880 [0006]
• US 2020189965 [0006]
• US 2020140322 [0006]
• US 2021387899 [0006]
• WO 2021224412 A1 [0006]
• DE 202022106826 U1 [0007]

Non-patent literature cited

• DIN ISO 7991 [0090]

Claims (13)

Transparent lithium aluminum silicate glass ceramic with a thermal expansion coefficient in the range from 20 ° C to 700 ° C of -0.8 to 1.5 ppm / K, with a chroma C * of less than 6 based on a thickness of 4 mm, with a Crystallite size in the high quartz mixed crystal phase of less than 80 nm, with a composition containing, in% by weight, an oxide basis: _SiO2 62 - 70 _{Al2O3 _} 19 - 23 _Li2O 2.4 - 3.4 Well ₂ O 0 - 1 ZnO 0 - 5 _TiO2 1.5 - 2.5 _ZrO2 1.5 - 2.5 _SnO2 0 - 0.15 and wherein the glass ceramic is free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃ except for unavoidable impurities. Transparent lithium aluminum silicate glass ceramic Claim 1 , characterized in that the transparent lithium aluminum silicate glass ceramic has a light transmittance τ _vis of more than 80% based on a thickness of 4 mm. Transparent lithium aluminum silicate glass ceramic according to one of the Claims 1 or 2 , characterized in that the transparent lithium aluminum silicate glass ceramic contains at most 0.13% by weight, preferably at most 0.12% by weight, particularly preferably at most 0.11% by weight of SnO ₂ . Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the transparent lithium aluminum silicate glass ceramic is free of SnO ₂ except for unavoidable impurities and 0.5% by weight to 1.5% by weight of As ₂ O ₃ and/or or Sb ₂ O ₃ contains. Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the transparent lithium aluminum silicate glass ceramic contains less than 0.05% by weight of Fe ₂ O ₃ , preferably less than 0.04% by weight, particularly preferably less than 0 .03% by weight, most preferably less than 0.02% by weight. Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the transparent lithium aluminum silicate glass ceramic contains 0.01 - 0.07 wt.% Nd ₂ O ₃ . Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the transparent lithium aluminum silicate glass ceramic contains less than 1% by weight of K ₂ O, preferably less than 0.7% by weight, particularly preferably less than 0.5% by weight .-%, most preferably less than 0.3% by weight. Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the transparent lithium aluminum silicate glass ceramic contains less than 0.6% by weight of MgO, preferably less than 0.5% by weight, particularly preferably less than 0.4% by weight .-%. Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the transparent lithium aluminum silicate glass ceramic contains less than 1.0% by weight of CaO, preferably less than 0.6% by weight, particularly preferably less than 0.5% by weight .-%. Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the transparent lithium aluminum silicate glass ceramic contains more than 1.8% by weight of BaO, preferably more than 2.5% by weight, particularly preferably more than 2.8% by weight .-%. Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the ratio of Al ₂ O ₃ to TiO ₂ is in the range 7 to 12, preferably in the range 8 to 11, particularly preferably in the range 8.5 to 10.5. Transparent lithium aluminum silicate glass ceramic according to one of the preceding claims, characterized in that the glass ceramic contains the following components in % by weight based on oxide. _SiO2 63 - 70 _{Al2O3 _} 19 - 23 _Li2O 2.4 - 3.4 Well ₂ O 0.1 - 1 _K2O 0 - 0.8 MgO 0 - 0.6 CaO 0 - 1 BaO > 1.8 ZnO 1-4 _TiO2 1.5 - 2.5 _ZrO2 1.5 - 2.5 _SnO2 0 - 0.15 and wherein the glass ceramic is free of Cr ₂ O ₃ , V ₂ O ₅ , MnO ₂ and MoO ₃ except for unavoidable impurities. Use of a glass ceramic according to one of the preceding claims as a cooking surface, fireplace viewing panel, grill or roasting surface, cover for fuel elements in gas grills, oven viewing panel, in particular for pyrolysis stoves, work or table tops in kitchens or laboratories, cover for lighting devices, in fire protection glazing and as safety glass, optional in a laminate composite, as a carrier board or as an oven lining in thermal processes or as a back cover for mobile electronic devices.