DE102016208300B3 - Crystallizable lithium aluminum silicate glass and transparent glass ceramic produced therefrom, and also methods for producing the glass and the glass ceramic and use of the glass ceramic - Google Patents

Abstract

The invention relates to a crystallisable lithium aluminum silicate glass for the production of transparent glass-ceramics containing the following components (in% by weight based on oxide): Li 2 O 3.4-4.1 Al 2 O 3 20-22.5 SiO 2 64-68 Na 2 O + K 2 O 0 , 4-1.2 MgO 0.05- <0.4 CaO 0-1 CaO + BaO 0.7-1.8 CaO + SrO + BaO 0.8-2 ZnO 0.3-2.2 TiO 2 1, 8- <2.5 ZrO2 1.6- <2.1 SnO2 0.2-0.35 TiO2 + ZrO2 + SnO2 3.8-4.8 P2O5 0-3 Fe2O3> 0.01-0.02. The invention also relates to the glass ceramic produced from this glass and their uses.

Description

The invention relates to a crystallizable lithium aluminum silicate glass, which is convertible into transparent LAS glass ceramics, according to the preamble of claim 1. The invention also relates to the glass ceramic produced therefrom, a process for their preparation and the use of such a LAS glass ceramic.

It is generally known that glasses of the system Li ₂ O-Al ₂ O ₃ -SiO _{2 can be converted} into glass ceramics with high quartz mixed crystals or keatite mixed crystals as main crystal phases. For the first type of glass-ceramics, the synonyms "β-quartz" or "β-eukryptite" are also found in the literature, and for the second type "β-spodumene" as the term for the crystal phases.

Key features of these LAS glass ceramics are their transparency, good chemical resistance to acids and alkalis, high mechanical strength and their low thermal expansion coefficient in a temperature range from room temperature to application temperature of α _20/700 <1.5 · 10 ^-6 / K. As a rule, the thermal expansion behavior is set so that the materials have a very low expansion in the range of their application temperatures, usually 0 ± 0.3 · 10 ^-6 / K. So z. B. when used as a substrate material, such as color filters, displays, sensor, precision wafers, also referred to as wafer stages, or mirror support for telescopes, the thermal expansion in the range of room temperature minimized.

For applications as fire-resistant glazing, transparent chimney panes, as a furnace window outside or inside the oven and as a cooking surface with colored underside coating, the thermal zero expansion in a temperature range between room temperature and about 700 ° C to as low as possible values of <1.5 · 10 ^-6 / K , preferably 0 ± 0.3 × 10 ^-6 / K. Due to the low thermal expansion at their application temperatures, combined with high mechanical strength, these glass ceramics have excellent temperature differential resistance and thermal shock resistance. Requirements for use at high temperatures include that the glass-ceramics retain the required properties (such as thermal expansion, transmission, stress build-up) during their lifetime. In terms of temperature resistance, the low shrinkage of the glass ceramic (compaction) at high temperatures is the critical factor. Since the glass-ceramic articles are usually heated unevenly during use, stresses build up in the article over time due to the different degrees of shrinkage.

Transparency associated with low scattering is the essential feature that distinguishes the transparent glass-ceramics from the colored transparent glass-ceramics. In the case of the latter, V ₂ O _{5 is} added, in particular for cooking surfaces, for coloring in the volume in order to reduce the light transmission and to achieve a black appearance. In the colored transparent glass-ceramics, the light transmittance is usually less than 30, and usually less than 10%, compared to more than 80% of those of the transparent type (at 4 mm thickness).

When used as a cooking surface, the transparent glass ceramic plates are usually provided on the underside with an opaque colored coating to prevent the review of the technical installations and to provide the color appearance. Recesses in the underside coating allow the attachment of colored and white displays, mostly light emitting diodes. High transparency combined with low scattering is also essential in this application, so that the colors of the bottom coating and the ads are not distorted and the displays are clearly visible.

High transparency means high light transmission and low color and thus low absorption, since the absorption bands, depending on the location in the visually visible spectrum, both reduce the light transmission and increase the color. The light transmission is described by the Y-value (brightness) according to the CIE standard color system. The German implementation of the international CIE standard is specified in DIN 5033. The required spectrophotometric measurements are carried out in the context of the invention for 4 mm thick polished samples in a spectral range between 380 and 780 nm. From the measured spectral values in the range representing the visible light spectrum, the light transmission is selected with choice of Normlichtart and observer angle , For glass ceramics it is natural to use as a measure of the color the size c * (chroma) from the CIELAB color system with the coordinates L *, a *, b *, according to the calculation:

The color model is standardized in DIN EN ISO 11664-4 "Colorimetry - Part 4: CIE 1976 L * a * b * Color space". The coordinates of the CIELAB color system can be calculated in a known manner from the color coordinates and the light transmission Y of the CIE color system. The c * value of the samples is determined from the spectrophotometric measurements of the transmission with the selected parameters for standard illuminant and observer angle.

The scattering of the glass ceramics is determined by measuring the haze. Haze is according to ASTM D1003-13 the percentage of transmitted light, which deviates from the incident light beam on average by more than 2.5 °.

The large-scale production of transparent glass ceramics takes place in several stages. First, the crystallizable starting glass is melted and refined from a mixture of shards and powdery batch raw materials. The molten glass reaches temperatures of 1550 ° C to a maximum of 1750 ° C, usually up to 1700 ° C. Occasionally, a high-temperature refining above 1700 ° C, usually used at temperatures around 1900 ° C. For transparent glass-ceramics, arsenic and antimony oxide are technically and economically proven refining agents at conventional refining temperatures below 1700 ° C. with regard to good blistering qualities. Arsenic oxide is particularly advantageous for the transparency (high light transmission and low color) of the glass ceramic. Even though these refining agents are firmly bound in the glass framework, they are disadvantageous in terms of safety and environmental protection. For example, special precautionary measures have to be taken in the production of raw materials, the preparation of raw materials and the evaporation from the melt during glass production. There are therefore numerous development efforts to replace these substances, but come across technical and economic disadvantages.

Increasingly, as an alternative, the environmentally friendly refining agent SnO ₂ is propagated alone or in combination with one or more refining additives such as halides (F, Cl, Br), CeO ₂ , sulfur compounds. However, the use of SnO ₂ has disadvantages. SnO ₂ itself is technically less effective as a refining agent and requires higher temperatures to deliver the purifying oxygen. However, higher concentrations of SnO ₂ on the order of the As ₂ O _{3 used} by about 0.6 to 1 wt .-% are disadvantageous because of the devitrification of Sn-containing crystals in the hot forming. The second significant disadvantage for transparent glass ceramics in the substitution of arsenic by tin oxide as refining agent is that SnO ₂ leads to an additional absorption and increases the color. At higher refining temperatures, the absorption increases, as this increases with the higher proportion of Sn ²⁺ .

In the scriptures JP H11-228 180 A and JP H11-228 181 A describes the environmentally friendly refining of LAS glasses without the use of arsenic oxide by a combination of the refining agents 0.1-2 wt .-% SnO ₂ and Cl. The tan color associated with the use of SnO ₂ is extremely detrimental to transparent glass-ceramics. The references provide no indication of how to limit the SnO ₂ content to ensure low absorption and sufficient resistance to devitrification.

The combination explanation of SnO ₂ in connection with fluorine and / or bromine connections becomes in the writings EP 1 899 276 A1 , respectively. EP 1 901 999 A1 disclosed. However, these halides are problematic because of their evaporation from an environmental point of view and disadvantageous in terms of the corrosion of the internals in the melt and molding.

After melting and refining, the glass usually undergoes hot forming by rolling, casting, pressing or floating. For most applications of transparent glass ceramics, these are required in a flat form, for example in the form of disks. The rolling and more recently also floating is used for the production of the plates. For the economic production of these LAS glasses, on the one hand, a low melting temperature and a low processing temperature V _{A are} desired in the hot forming. On the other hand, the glass must not show any devitrification during the shaping, that is to say that it can not form any disturbing crystals which reduce the strength in the glass-ceramic articles or are visually disturbing. The coldest range critical in roll forming is the contact of the molten glass with the noble metal die (typically a Pt / Rh alloy) before the glass is shaped and cooled by the rolls.

The crystallizable LAS glass is converted into the glass ceramic in a subsequent temperature process by controlled crystallization (ceramization). This ceramization takes place in one Two-stage temperature process, in which first by nucleation at a temperature between 680 and 800 ° C germs, usually from ZrO ₂ / TiO ₂ mixed crystals, are generated. SnO ₂ is also involved in nucleation. The high quartz mixed crystals grow at elevated temperature on these germs. At the maximum production temperature of about 900 ° C, the microstructure of the glass ceramic is homogenized and adjusted the optical, physical and chemical properties. For an economical production short ceramization times are advantageous.

The further conversion into keatite mixed crystals takes place at elevated temperature in a temperature range of about 950 ° C to 1250 ° C. With the conversion, the thermal expansion coefficient of the glass-ceramic increases, and by further crystal growth and associated light scattering, the transparent transforms into translucent to opaque appearance.

A major hurdle for the use of SnO ₂ as an environmentally friendly refining agent is the additional absorption by the resulting Sn / Ti complexes. The Sn / Ti color complexes stain more strongly than the known Fe / Ti color complexes and this disadvantage has hitherto made it difficult for transparent glass ceramics to replace the refining agent arsenic oxide by SnO ₂ . The Sn / Ti color complexes have been found to be disadvantageous, especially by increased color c *. The Fe / Ti color complexes lead to a red-brown, the Sn / Ti color complexes to a yellow-brown color. The absorption mechanism of both color complexes is believed to be due to electronic transfer (charge transfer) between the two adjacent polyvalent cations. The formation of said charge-transfer color complexes takes place significantly in the crystallization. In order to reduce the concentrations of the color complexes, it is advantageous to shorten the nucleation and crystallization times. This is contradicted by the fact that the shortening of the nucleation time lead to increased light scattering and the shortening of the crystallization time to unevenness of the article.

The technical batch raw materials for the melts contain further coloring elements such as Cr, Mn, Ni, V and especially Fe as impurities. The Fe also dyes ionically as Fe ²⁺ or Fe ³⁺ beyond the Fe / Ti color ^complexes . Because of the high cost of low-iron raw materials, it is uneconomical to reduce the Fe ₂ O ₃ content to values below 100 ppm.

Likewise, the approach of avoiding the active nucleating agent responsible for the color complexes TiO ₂ ( WO 2008/065167 A1 ) or limit ( WO 2008/065166 A1 ) so far not led to a technical implementation. The required higher levels of the alternative nucleating agents ZrO ₂ and / or SnO ₂ lead to disadvantages in the melt and shaping, such as higher melting and shaping temperatures and inadequate devitrification resistance during shaping.

WO 2013/124373 A1 describes the physical decolorization of transparent glass ceramics with high quartz mixed crystals as the main crystal phase, which are free from arsenic and antimony except for unavoidable raw material impurities, by additions of 0.005-0.15 wt.% Nd ₂ O ₃ . The principle of discoloration is based on the fact that the existing absorption bands are neutralized by complementary absorption bands of the decolorizing agent. This naturally leads to stronger absorption of the light and thus lowers the light transmission. In order to achieve favorable production conditions, ie low melting and low molding temperatures, the example glasses of this document contain 0.44 to 0.93% by weight of the viscosity-reducing component MgO. Also, the sum ratio of the divalent components MgO + ZnO> CaO + SrO + BaO described for the adjustment of crystal composition and composition of the residual glass phase means higher MgO contents.

The font WO 2013/171288 A1 discloses transparent, substantially colorless and non-diffusing glass-ceramic articles with high-quartz mixed crystals, the glass-ceramics and the precursor glasses. The compositions of the LAS glass ceramics are free of arsenic, antimony oxide and rare earth oxides such as Nd ₂ O ₃ , except for unavoidable trace levels. The disclosed composition contains 62-72 wt.% SiO ₂ , 20-23 wt.% Al ₂ O ₃ , 2.8-5 wt.% Li ₂ O, 0.1-0.6 wt. SnO ₂ , 1.9-4 wt.% TiO ₂ , 1.6-3 wt.% ZrO ₂ , less than 0.4 wt.% MgO, less than 250 ppm Fe ₂ O ₃ and 2.5 -6% by weight. ZnO + BaO + SrO. The average crystallite size is less than 35 nm. A disadvantage for the claimed compositions, in addition to the high melting temperatures, especially the increased processing temperatures V _A , which means a narrow process window and lower service life of the system for the shaping with rollers due to the higher temperatures.

US 2013/0130887 A1 discloses a LAS glass-ceramic of selected composition 55-75 wt.% SiO ₂ , 20.5-27 wt.% Al ₂ O ₃ , ≥ 2 wt.% Li ₂ O, 1.5-3 wt. % TiO ₂ , 3.8-5 wt% TiO ₂ + ZrO ₂ , 0.1-0.5 wt% SnO ₂ , and the component relationships 3.7 ≤ Li ₂ O + 0.741 MgO + 0.367 ZnO ≤ 4,5 and SrO + 1.847 CaO ≤ 0.5. The two component relationships are intended to reduce the color of the transparent glass-ceramics. This document provides no indication as to how the optimization of scattering and manufacturing properties can be accomplished.

The font WO 2016/038319 A discloses transparent, colorless and non-light scattering glass ceramic plates containing crystals of high quartz mixed crystal whose chemical composition does not contain oxides of As, Sb and Nd having a selected composition, 55-75 wt% SiO ₂ , 12-25 wt% Al ₂ O ₃ , 2-5 wt.% Li ₂ O, 0- <2 wt.% Na ₂ O + K ₂ O, 0- <7 wt.% Li ₂ O + Na ₂ O + K ₂ O, 0.3-5 wt% CaO, 0-5 wt% MgO, 0-5 wt% SrO, 0.5-10 wt% BaO,> 1 wt% CaO + BaO, 0-5 wt .-% ZnO, ≤ 1.9 wt .-% TiO ₂ , ≤ 3 wt .-% ZrO ₂ ,> 3.8 wt .-% TiO ₂ + ZrO ₂ , ≥ 0.1 wt % SnO ₂ and the component relationship SnO ₂ / (SnO ₂ + ZrO ₂ + TiO ₂ ) <0.1. The minimum content of the nucleating agents TiO ₂ and ZrO ₂ should avoid scattering of the glass ceramic article and the maximum content of TiO ₂ is limited to avoid yellowing of the article.

There is therefore a need to agree a whole bundle of contradictory demands on glass and glass ceramic, such as environmental friendliness and economical production of the glass without disadvantages in the product quality of the glass ceramic, such as light transmission, color and scattering with short ceramization times.

• – das über eine umweltfreundliche Zusammensetzung verfügt,
• – das günstige Fertigungseigenschaften für eine wirtschaftliche Herstellung besitzt,
• – das sich mit Keramisierungszeiten von unter 300 Minuten in eine transparente LAS-Glaskeramik mit Hochquarz-Mischkristallen als vorherrschende Kristallphase umwandeln lässt,
• – wobei die Glaskeramik geringe Farbe, hohe Lichttransmission und eine geringe Streuung aufweist,

The invention has for its object to find a crystallizable lithium aluminum silicate glass,

• - has an eco-friendly composition,
• - Has the favorable manufacturing properties for economical production,
• Which, with ceramization times of less than 300 minutes, can be converted into a transparent LAS glass ceramic with high-quartz mixed crystals as the predominant crystal phase,
• Wherein the glass ceramic has low color, high light transmission and a low dispersion,

This object is achieved by the crystallizable lithium aluminum silicate glass described in claim 1 and the glass ceramic produced therefrom.

It is also an object of the invention to find a method for producing the glass and the glass ceramic and their use. These objects are achieved by the further independent claims.

Here, the glass ceramic meets the requirements of applications such. As chemical resistance, mechanical strength, transparency (low color, high light transmission), low scattering, temperature resistance and long-term stability in terms of changes in their properties (such as thermal expansion, transmission, build-up of stresses). Under environmentally friendly composition is understood that the crystallizable glass is technically free of arsenic and antimony oxide as usual refining agent to unavoidable raw material impurities. As an impurity, both components are present at levels of less than 1000 ppm, preferably at levels of less than 500 ppm, more preferably less than 200 ppm. Except in the exceptional case when added in the melt Fremdscherben a transparent glass ceramic with arsenic oxide as refining agent, is dispensed with the addition.

In elaborate test series, a narrowly defined composition range was found, which combines favorable production properties with low color, high light transmission and low scattering of the LAS glass ceramic.

The favorable manufacturing properties for economical production include low cost batch raw materials, low melting and forming temperatures, a sufficient amount of refining agent SnO ₂ for conventional refining, resistance to devitrification, and short ceramization times. With the short ceramization times, a high light transmission without visually disturbing light scattering (turbidity) or color is achieved.

For an economical production, a low melting temperature is advantageous and is ensured by a lower viscosity of the molten glass at high temperatures. For this purpose, the temperature at which the viscosity of the glass melt 10 ² dPas is a characteristic size. This so-called 10 ² temperature for the glasses according to the invention is preferably less than 1770 ° C., preferably less than 1765 ° C. and particularly preferably less than 1760 ° C. The low viscosity of the glass melt at high temperatures makes it possible to lower the temperature in the melting bath and thus extends the life of the melting furnace. The energy consumption based on the amount of produced Glass ceramic is reduced. Since a low glass viscosity also promotes the rise of bubbles and thus the refining, a low glass viscosity is also advantageous for the blister quality.

It is economically advantageous to lower the temperature during shaping. The service lives of the forming tools are increased, and there is less dissipated loss of heat. The shaping, usually by rolling or floating, takes place at a viscosity of the glass melt of 10 ⁴ dPas. This temperature is also referred to as processing temperature V _A and is for the glasses according to the invention preferably at most 1330 ° C, preferably at most 1325 ° C.

The crystallizable glass has sufficient resistance to devitrification when molded from the melt. During shaping in contact with the shaping material (eg precious metal in the die in the rolling process), crystals which are not critical for the strength of the glass-ceramic and form visually conspicuous crystals do not form in the glass. The limit temperature below which critical devitrification occurs, ie the upper devitrification limit (OEG), is preferably at least 10 ° C. below the processing temperature V _A. With this minimum difference, a sufficient process window is defined for the forming process. Particularly advantageous is a process window V _A -OEG, which is at least 20 ° C. V _A -OEG is therefore a measure of the devitrification resistance.

In addition, the transparent LAS glass-ceramic produced from the crystallizable glass fulfills certain properties. The light transmission Y should be high, and the color c * should be low, so that the viewing of objects or on a bottom coating and the view of ads is not darkened or falsified in the color. The transparent glass ceramic should be without visually disturbing light scattering, so that transparency and the displays are clear and are not disturbed. This should also be ensured in the desired short Keramisierungszeiten.

The contents of the components responsible for the formation of crystal phases and residual glass are optimized so that the values of light transmission, color and scattering are achieved with short ceramization times. A short ceramization time is understood to mean that the glass ceramic is produced from the crystallisable LAS glass in a time of less than 300 minutes, preferably less than 200 minutes and particularly preferably less than 150 minutes.

In order to realize all these properties, both the crystallisable lithium aluminum silicate glass according to the invention and the glass ceramic according to the invention which can be produced or produced from the glass contain the following components (in% by weight based on oxide):
Li ₂ O 3,4-4,1
Al ₂ O ₃ 20-22.5
SiO ₂ 64-68
Na ₂ O + K ₂ O 0.4-1.2
MgO 0.05- <0.4
CaO 0-1
CaO + BaO 0.7-1.8
CaO + SrO + BaO 0.8-2
ZnO 0.3-2.2
TiO ₂ 1.8- <2.5
ZrO ₂ 1.6- <2.1
SnO ₂ 0.2-0.35
TiO ₂ + ZrO ₂ + SnO ₂ 3.8-4.8
P ₂ O ₅ 0-3
Fe ₂ O ₃ > 0.01-0.02,
with the condition (both in% by weight) B1:
0.15 <MgO / SnO ₂ <1.7

Preferably, the condition applies (also both in wt .-%) B2:
0.4 <MgO / SnO ₂ <1.7.

The oxides Li ₂ O, Al ₂ O ₃ and SiO ₂ are or will be within the specified limits necessary components of the high quartz and / or keatite mixed crystal phases.

The content of Li ₂ O should be 3.4 to 4.1 wt .-%. The minimum content is required to achieve the desired low processing temperature. It has been shown that it is at higher levels when 4.1% by weight is difficult to achieve the desired low color c * of the glass-ceramic with short ceramization times. Preferably, the Li ₂ O content is less than 4 wt .-%.

The selected content of Al ₂ O ₃ is 20-22.5 wt .-%. Higher levels are disadvantageous because of the tendency for devitrification of mullite in molding. It has also been shown that with higher contents, the scatter increases with short ceramization times. The minimum content is 20% by weight, because this is advantageous for the formation of sufficient quantities of the mixed crystals and the low color c * of the glass-ceramic. The Al ₂ O ₃ minimum content is preferably at least 20.5 wt .-%.

The content of SiO ₂ should be at least 64 wt .-%, because this is advantageous for the required properties of the glass-ceramic, such as low thermal expansion and chemical resistance. In addition, the scattering is reduced with short Keramisierungszeiten. Particularly advantageous is a minimum content of 65 wt .-%. The SiO ₂ content should be at most 68 wt .-%, because this component increases the processing temperature of the glass and the melting temperature. The SiO ₂ content is preferably at most 67.5% by weight.

The alkalis Na ₂ O and K ₂ O are components of the residual glass phase of the glass-ceramic and improve the meltability and the devitrification resistance during shaping. In addition, they are advantageous for lowering the processing temperature. The sum of the alkalis Na ₂ O + K ₂ O should be at least 0.4 wt .-%. Too high contents impair the crystallization behavior during the conversion of the crystallizable starting glass into the glass ceramic and lead to scattering with short ceramization times. In addition, higher contents have an unfavorable effect on the color of the transparent glass-ceramics. The sum of the alkalis Na ₂ O + K ₂ O is preferably at most 1.2 wt .-% and particularly preferably at most 1.0 wt .-%.

It has been shown that the component Na ₂ O is more favorable for low values of the color c * with short ceramization times. Therefore, the content of K ₂ O is preferably less than 0.2% by weight. It is thus preferred that the Na ₂ O content is higher than the K ₂ O content (both in% by weight). It is particularly advantageous if (for both in wt .-%) a condition applies: Na ₂ O / K ₂ O> 2. Particularly preferred is (for both in wt .-%), the condition> 3.

In order to achieve the listed desired properties of the glass ceramic, the selection of the MgO content is of particular importance. MgO is both part of the mixed crystals and the residual glass phase and therefore has a strong influence on many properties. The component lowers the melting and processing temperature of the glass and therefore favors economical production. In the glass-ceramic, the component increases the thermal expansion and leads to an adverse increase in color. This is attributed to favoring the formation of Fe / Ti and Sn / Ti color species. The MgO content is 0.05-0.4 wt%. Preferably, the minimum content is greater than 0.1 wt .-%. The minimum content of MgO is particularly preferably 0.15% by weight. In an MgO value range from 0.2 wt.% To 0.35 wt.%, The requirements for color of the glass ceramic and processing temperature can be combined particularly well with one another.

The alkaline earths CaO, SrO and BaO are not incorporated into the mixed crystal phases, but remain in the residual glass phase of the glass ceramic. They are advantageous for lowering the melting and processing temperatures. Excessively high contents impair the nucleation and crystallization behavior during the conversion of the crystallizable glass into the glass ceramic. In addition, higher contents have an unfavorable effect on the time / temperature resistance of the glass ceramic.

The component CaO has proved to be advantageous in order to improve the devitrification resistance and to adjust the refractive index of the residual glass phase. It is contained in contents of 0 to 1 wt .-%. The content is preferably more than 0.01% by weight and also preferably up to 0.8% by weight.

In particular, in order to agree particularly good values of devitrification resistance and low color of the glass-ceramic, it is advantageous if the content of CaO is at least 0.28% by weight. Particularly preferably, the upper limit for the CaO content is less than 0.5 wt .-%, especially when short ceramization particularly low scattering is desired.

In order to reduce the scattering with short ceramization times, it is important to match the refractive indices of residual glass phase and mixed crystal phase. This is determined by the choice of alkaline earths and by achieved special component relationships with each other. The sum of alkaline earths CaO + SrO + BaO is 0.8 to 2 wt .-%.

In order to minimize the color in addition to the scattering in the case of short ceramization times, it is advantageous if the components CaO and BaO are selected to have a rather higher content and the component SrO to have a rather low content. Therefore, the relationship that the sum of the CaO + BaO contents is 0.7-1.8 wt% is applicable. Higher levels are detrimental to devitrification resistance. The ratio of alkaline earths is preferably chosen to have a relationship (all in wt .-%): (CaO + BaO) / SrO> 3. Preferably, the SrO content is chosen below 0.5 wt .-%.

The BaO content is preferably 0 to 1.5% by weight and the SrO content is preferably 0 to 0.5% by weight. Particularly preferred is a BaO content of at least 0.3 wt .-%. Particularly preferred is a BaO content of at most 1.2 wt .-%. Particularly preferred is a SrO content of at least 0.005 wt .-%. Particularly preferred is a SrO content of at most 0.3 wt .-%.

The selected ranges and relationships for the alkalis Na ₂ O, K ₂ O and the alkaline earths CaO, SrO, BaO, which are incorporated into the residual glass phase of the glass-ceramic, are essential to the desired low production properties of the glass with low color, high light transmission and to agree low scattering of the glass ceramic, especially when working with short Keramisierungszeiten.

The alkalis Na ₂ O, K ₂ O and the alkaline earths CaO, SrO, BaO also accumulate on the surface of the glass ceramic, except in the residual glass phase between the crystals. When ceramizing forms an approximately 100 to 1000 nm thick glassy surface layer, which is almost free of crystals and which is enriched with these elements, wherein Li ₂ O is depleted. This glassy surface layer has a favorable effect on the acid resistance of the glass ceramic surface. For this, the sum of alkalis and alkaline earths with their limits are important. For a sufficient thickness of the glassy layer of at least 50 nm, the minimum levels of both classes of components are required. Higher total contents than the upper limits in one or both classes lead to higher thicknesses of the glassy layer, which are detrimental to the strength of the glass ceramic.

The component ZnO is incorporated into the mixed crystal phases, and a part also remains in the residual glass phase. The component is advantageous for lowering the processing temperature of the glass and for reduced scattering with short ceramification times. The ZnO content should be at least 0.3 wt .-%. The ZnO content is limited to values of at most 2.2% by weight during the ceramization because of the tendency to evaporate from the glass melt and because of the problem of the formation of undesired crystal phases such as Zn spinel (Gahnit). Preferred is a content of at least 1.1 wt .-% and a maximum of 2.1 wt .-%.

The component TiO ₂ is indispensable as an effective nucleating agent for the transparency of the glass ceramic. It leads to high nucleation rates and thus even with short Keramisierungszeiten to sufficient formation of germs and thus small average crystallite size. This makes it possible to obtain glass ceramics without visually disturbing scattering even with short ceramization times. However, higher TiO ₂ contents are critical due to the formation of Fe / Ti color complexes and Sn / Ti color complexes. Therefore, the TiO ₂ content should be 1.8 to <2.5 wt .-%. A minimum TiO ₂ content of> 1.9% by weight is preferred. Particularly preferred is a TiO ₂ content of at least 2 wt .-%.

As further nucleating ZrO ₂ and SnO _{2 are} provided. The ZrO ₂ content is limited to less than 2.1% by weight, preferably to less than 2% by weight, since higher contents may worsen the melting behavior of the mixture during glass production and may lead to relicts in the glass at economical throughput rates. Further, devitrification may occur during formation by formation of Zr-containing crystals. As the component helps to avoid higher levels of the alternative nucleating agent TiO ₂ , it is advantageous for providing a low c c * glass ceramics. Their minimum content is 1.6% by weight. The selection of the SnO ₂ content is of particular importance in the context of the invention. SnO ₂ is both a nucleating agent and a refining agent and therefore has a strong influence on many properties, in particular it is critical for the color of the glass-ceramic because of the formation of the Sn / Ti color species in the crystallization. In order to meet the demand for both sufficient refining effect and low color, the SnO ₂ content in a narrow range is 0.2 to 0.35 wt%. Preferably, the SnO ₂ content is at most 0.33 wt .-% and particularly preferably at most 0.31 wt .-%, in order to further reduce the risk of devitrification of Sn-containing crystals. For a good refining, a minimum content of more than 0.25% by weight is particularly advantageous. After the refining are considered as good bladder qualities in particular those with bubble numbers of less than 5, preferably less than 2 bubbles / kg in the crystallizable starting glass or in the glass ceramic (measured from bubble sizes greater than 0.1 mm in one dimension).

The sum of the nucleating agents TiO ₂ + ZrO ₂ + SnO ₂ should be 3.8 to 4.8% by weight. The minimum content is required for a sufficiently rapid nucleation. The minimum content is preferably 3.9% by weight in order to further reduce the scattering with rapid ceramization. The upper limit of 4.8% by weight results from the requirement for devitrification resistance.

To improve the meltability and devitrification resistance during shaping, up to 3% by weight of P ₂ O ₅ may be present. Higher levels are detrimental to chemical resistance and cause scattering of the glass-ceramic with short ceramification times. A preferred lower limit is 0.01 weight percent, a preferred upper limit is 1.5 weight percent P ₂ O ₅ . Particularly preferred is an upper limit of P ₂ O ₅ content of 0.8 wt .-%. A particularly preferred lower limit is 0.02 wt .-%.

The addition of up to 1 wt .-% B ₂ O ₃ improves the meltability and devitrification, but is disadvantageous for the time / temperature resistance of the glass-ceramic. Preferably, the glass ceramic is technically free of B ₂ O ₃ , that is, the contents are below 500 ppm.

Because of the high cost of low-iron batch raw materials, it is uneconomical to limit the Fe ₂ O ₃ content of the crystallizable glass to values below 0.01 wt .-%, ie less than 100 ppm. Since an entry of iron takes place via the comminution in the recycling of shards, the Fe ₂ O ₃ content is more than 0.01 wt .-%. On the other hand, the Fe ₂ O ₃ content also increases the concentration of the Fe / Ti color complexes in the glass ceramic. The color (chroma) c * is increased and absorption reduces the light transmission Y (brightness). The crystallizable glass and the glass ceramic produced therefrom should therefore contain at most 0.02 wt .-%, preferably up to 0.018 wt .-% of Fe ₂ O ₃ . For the same reason, because of the deterioration of light transmission and color, the glass preferably contains no further coloring compounds such as those of Cr, Ni, Cu, V, Mo, S, except for unavoidable impurities. The contents are usually between 2 to less than 20 ppm.

The component ratio MgO / SnO ₂ is of decisive importance in order to reconcile purifiability, transparency and favorable production properties. For the alignment of the refractive indices of crystal phase and residual glass phase to avoid scattering with short ceramification times new solutions are found in the present invention. Thus, color c * and light transmission Y can be further improved without disadvantages in economic production, or the properties are not unduly deteriorated at higher SnO ₂ contents. The glass according to the invention has a ratio of the components MgO to SnO ₂ (both in% by weight) of greater than 0.15 to less than 1.7 (condition B1). This is an essential condition to agree the desired favorable production properties of the glass with low color, high light transmission and low scattering of the glass ceramic even with short Keramisierungszeiten. Advantageously, the ratio of the components MgO to SnO _{2 is} greater than 0.4 and less than 1.7 (condition B2), more preferably at least 0.7. Advantageously, the ratio is less than 1.5, in particular from 0.7 to less than 1.3. In these preferred ranges in combination with the specified compositions, the desired effect of minimizing color and light scattering with rapid ceramification associated with low processing temperatures V _{A is} particularly advantageous.

In a preferred embodiment, both the crystallizable lithium aluminum silicate glass according to the invention and the glass ceramic according to the invention that can be produced or produced from the glass contain the following components as main constituents (in% by weight based on oxide):
Li ₂ O 3.4- <4
Al ₂ O ₃ 20.5-22.5
SiO ₂ 65-67.5
Na ₂ O + K ₂ O 0.4-1.0
MgO> 0.1- <0.4,
CaO> 0.01-0.8
CaO + BaO 0.7-1.8
SrO 0-0.5
BaO 0-1.5
CaO + SrO + BaO 0.8-2
ZnO 1,1-2,1
TiO ₂ > 1.9- <2.5
ZrO ₂ 1.6- <2.0
SnO ₂ 0.2-0.33
TiO ₂ + ZrO ₂ + SnO ₂ 3.9-4.8
P ₂ O ₅ 0-3, preferably 0.01-1.5
Fe ₂ O ₃ > 0.01-0.018
with condition B2:
0.4 <MgO / SnO ₂ <1.7

In the case of the transparent glass ceramics produced from the lithium aluminum silicate glasses according to the invention, in a preferred embodiment the interfering color based on Fe / Ti and / or Sn / Ti color complexes is obtained by additions of Nd ₂ O ₃ in amounts of 0.005% by weight (50 ppm) to 0.2% by weight (2000 ppm). Below 50 ppm of Nd ₂ O ₃ , the decolorizing effect is low, and above 2000 ppm, the light transmission is undesirably deteriorated by the absorption of Nd bands in the visible light range. In order to optimize these two opposing effects, the content of Nd ₂ O _{3 is} preferably more than 100 ppm and preferably up to 1500 ppm. Further preferred lower limits for Nd ₂ O ₃ are 0.03% by weight, in particular 0.05% by weight.

For the optimization with regard to low color of the glass ceramic, it has proved to be advantageous for the contents of the components Nd ₂ O ₃ and Fe ₂ O ₃ (both in% by weight) to satisfy the condition Nd ₂ O ₃ / Fe ₂ O ₃ 2 is met. In order not to impair the light transmission too much, it is further preferred if the contents Nd ₂ O ₃ / Fe ₂ O ₃ <9.

Additions of CoO in amounts up to 30 ppm can aid in decolorization. A CoO content of 0.1 ppm to 20 ppm CoO is preferred. The upper limits for the CoO are based on the fact that it has only partial decolorizing effect and also causes a reddish color cast in transparent glass ceramics at higher levels.

Preferably, the addition of halide compounds is omitted as Läuterhilfsmittel. These evaporate in the melt and enter the melting tank atmosphere. This forms corrosive compounds such as HF, HCl and HBr. These are disadvantageous because of the corrosion of the refractory bricks in the melting tank and in the exhaust system. The glasses and glass ceramics are therefore free from the inevitable traces of F, Cl, Br, and their individual contents are less than 500 ppm.

According to a further embodiment, the crystallizable lithium aluminum silicate Glass or article made therefrom of green glass or after conversion from glass-ceramic preferably prefers a composition which contains in wt .-% based on oxide:
Li ₂ O 3.4- <4
Al ₂ O ₃ 20.5-22.5
SiO ₂ 65-67.5
Na ₂ O + K ₂ O 0.4-1.0
MgO> 0.1- <0.4
CaO> 0.1-0.8
CaO + BaO 0.7-1.8
SrO 0-0.5
BaO 0-1.5
CaO + SrO + BaO 0.8-2
ZnO 1,1-2,1
B ₂ O ₃ 0-1
TiO ₂ > 1.9- <2.5
ZrO ₂ 1.6- <2.0
SnO ₂ 0.2-0.33
TiO ₂ + ZrO ₂ + SnO ₂ 3.9-4.8
P ₂ O ₅ 0.01-0.8
Fe ₂ O ₃ > 0.01-0.018
Nd ₂ O ₃ 0.005-0.2
with condition B2:
0.4 <MgO / SnO ₂ <1.7

In order to further improve economical production with high light transmission, low color and low scattering with short ceramification times, the crystallisable lithium aluminum silicate glass or the article produced therefrom has a particularly preferred composition, which contains in% by weight on an oxide basis:
Li ₂ O 3.4- <4
Al ₂ O ₃ 20.5-22.5
SiO ₂ 65-67.5
Na ₂ O + K ₂ O 0.4-1.0
MgO> 0.1- <0.4,
CaO 0.28-0.8
CaO + BaO 0.7-1.8
SrO 0-0.5
BaO 0-1.5
CaO + SrO + BaO 0.8-2
ZnO 1,1-2,1
B ₂ O ₃ 0-1
TiO ₂ > 1.9- <2.5
ZrO ₂ 1.6- <2.0
SnO ₂ 0.2-0.33
TiO ₂ + ZrO ₂ + SnO ₂ 3.9-4.8
P ₂ O ₅ 0.01-0.8
Fe ₂ O ₃ > 0.01-0.018
Nd ₂ O ₃ > 0.01-0.15
with condition B2:
0.4 <MgO / SnO ₂ <1.7

The abovementioned compositions are to be understood such that the listed components amount to at least 98% by weight, generally 99% by weight, of the total composition. Compounds of a variety of elements such. As F, Cl, the alkalis Rb, Cs or elements such as Mn, Hf are common impurities in the bulk used raw materials. Other compounds, e.g. those of the elements W, Nb, Ta, Y, Mo, rare earths, Bi, V, Cr, Ni may be present in minor proportions, typically in the ppm range.

The water content of the crystallizable glasses for producing the glass ceramics is preferably between 0.015 and 0.06 mol / l, depending on the choice of the batch raw materials and the process conditions in the melt. This corresponds to β-OH values of 0.16 to 0.64 mm ^-1 . In the conversion into the glass ceramic, the IR band changes, which is used to determine the water content. As a result, the β-OH values for the glass ceramic changes without the water content changing. This and the method for determining the β-OH values is z. B. in the EP 1 074 520 A1 described.

The crystallizable glasses are converted into the glass-ceramic by the multistage temperature process described below.

In a first embodiment, the glass-ceramic is transparent and contains high-quartz mixed crystals as the main crystal phase.

In order to minimize scattering, it is advantageous to minimize the crystallite sizes. However, hitherto economically disadvantageous longer nucleation and ceramization times have been required. With the composition according to the invention glass ceramics are obtained in which the refractive indices of residual glass and crystal phase are aligned. Therefore, higher average crystallite sizes for the high quartz mixed crystals are possible than with known compositions. After ceramization, the high-quartz mixed crystals of the glass-ceramic preferably have an average crystallite size of at least 30 nm. The average crystallite size is more preferably greater than 35 nm. The upper limit preferred because of increasing scattering is an average crystallite size of less than 50 nm.

The crystal phase content of the high-quartz mixed crystals of the glass-ceramic is preferably at least 65% by weight and preferably at most 80% by weight. This range is advantageous in order to obtain the desired mechanical and thermal properties of the glass-ceramic.

For a transparent glass ceramic 4 mm thick, the light transmission (brightness) Y is greater than 82%, preferably greater than 83% and particularly preferably greater than 84%. The color c * is at most 5.5, preferably less than 4.5, and more preferably less than 4.

The measurement of the haze according to ASTM D1003-13 gives Haze values for 4 mm thick polished samples of the LAS glass ceramic preferably less than 2.5, preferably less than 2% and particularly preferably less than 1.5%. The transparent glass ceramic according to the invention thus has no visually disturbing light scattering; This will not falsify your view of items and glowing ads. The displays of displays under the glass ceramic plate can thus be clear and the contours sharp and without turbidity visible.

The features of the glass-ceramic according to the invention are preferably based on the combination of a defined composition, which corresponds to that of the green glass from which it originated, with an adapted fast ceramization with a total duration of less than 300 minutes, as with the method according to the invention and in the examples is described.

The thermal expansion of the glass ceramic is usually adjusted to values around 0 ± 0.3 · 10 ^-6 / K in a temperature range between room temperature and about 700 ° C. However, it has been found that negative values are beneficial for color optimization because they allow for an extended composition range with low MgO levels and condition B1. The thermal expansion is preferably α _20/700 <-0.25 × 10 ^-6 / K and particularly preferably <-0.32 × 10 ^-6 / K. The thermal expansion α _20/700 is greater than -0.6, preferably greater than -0.5 · 10 ^-6 / K, since otherwise the thermally induced stresses between cold and hot areas become too high when the glass-ceramic article is heated. The negative expansion opens up additional applications in the athermalization of optical components, without endangering the use in the usual applications, because the deviation from zero expansion is small.

In another embodiment, the glass-ceramic contains keatite mixed crystals as the main crystal phase. For economic reasons, it is advantageous if both transparent glass ceramics with high quartz mixed crystals as the main crystal phase and glass ceramics with keatite mixed crystals as the main crystal phase can be produced from the same composition of the crystallizable lithium aluminum silicate glass. Due to the configuration of the ceramizing program, in particular by the choice of maximum temperature and holding time, the appearance of the latter can be adjusted from transparent to translucent to opaque in the latter case. The average crystallite size is preferably greater than 60 nm. The crystal phase fraction is more than 60% by weight and preferably more than 80% by weight. Due to the different combinations of properties of the two glass ceramic versions, a variety of applications is economically advantageous.

The preferred geometry for the glass ceramics according to the invention or the articles produced therefrom is in the form of plates. The plate preferably has a thickness of 2 mm to 20 mm, because it opens up important applications. With lower thicknesses, the strength is impaired, higher thicknesses are less economical due to the higher material requirements. Apart from the use as safety glass, which depends on high strength, the thickness is therefore preferably chosen below 6 mm.

Suitable shaping methods for the plate-shaped geometry are in particular rollers and floats. The preferred method of forming glass melt is via two rolls, since this process has advantages because of the faster cooling when the compositions tend to surface crystallize. The preferred article of the invention is a rolled glass ceramic plate.

The glass-ceramic plate and the articles preferably produced therefrom can not only be formed flat, but also be deformed three-dimensionally. For example, beveled, angled or curved plates can be used. The plates may be rectangular or in other shapes, and in addition to planar regions, three-dimensionally deformed regions such as e.g. Woks or rolled webs or surfaces as surveys or depressions included. The geometric deformations of the plates are in the hot forming, z. B. by structured shaping rolls, or by subsequent hot forming of the starting glasses, for example by burners, infrared radiators or by gravity sinks made. When ceramifying is supported with supporting ceramic forms, eg. B. flat documents worked to avoid uncontrolled changes in the geometric shape. A subsequent polish of one or both sides is optionally possible if the application requires it.

• a) Bereitstellen eine Gemengesatzes aus technischen Rohstoffen enthaltend 20 bis 80 Gew.-% Scherben;
• b) Schmelze des Gemengesatzes und Läutern in konventionellem Aggregat bei Temperaturen unter 1750 °C;
• c) Abkühlen der Glasschmelze und Formgebung bei Temperaturen in der Nähe der Verarbeitungstemperatur V_A, wobei die gewünschte Form des Artikels erzeugt wird;
• d) Kühlen in einem Entspannungsofen auf Raumtemperatur, wobei unerwünschte Spannungen im Glas entfernt werden.

The process according to the invention for producing the crystallisable lithium aluminum silicate glass is characterized by the steps:

• a) providing a batch of technical raw materials containing 20 to 80 wt .-% shards;
• b) Melting of the batch and refining in conventional aggregate at temperatures below 1750 ° C;
• c) cooling the molten glass and molding at temperatures near the processing temperature V _A , thereby producing the desired shape of the article;
• d) cooling to room temperature in a flash oven, removing unwanted stresses in the glass.

The batch composition is designed so that, after the melt, a glass having the composition and properties according to the invention is formed. The addition of the shard favors melting, and higher throughputs can be obtained. It is dispensed with an additional high-temperature refining unit and the maximum temperature of the glass melt in the melting tank is less than 1750 ° C, preferably less than 1700 ° C, as is compatible with conventional technology. During shaping, a glass sheet with a plate-shaped geometry is preferably produced by means of rolls and cooled to room temperature to avoid stresses in a cooling furnace. After ensuring the quality, volume sheets and surface defects are used to produce plates of the desired size from this glass ribbon.

The next process step is the ceramization on flat or three-dimensional, mostly ceramic substrates, which is preferably carried out in a roller furnace. In the method according to the invention for ceramization, the thermally relaxed crystallizable starting glass is heated to the temperature range of about 680 ° C. within 3 minutes to 60 minutes. The required high heating rates can be realized on an industrial scale in roller kilns. This temperature range of about 680 ° C corresponds approximately to the transformation temperature of the glass. Above this temperature to about 800 ° C is the range with high nucleation rates. The temperature range of nucleation is traversed over a period of 10 minutes to 100 minutes. Thereafter, the temperature of the crystallization nuclei-containing glass is increased within 5 minutes to 80 minutes to a temperature of 850 ° C to 950 ° C, which is characterized by high crystal growth rates of the high-quartz solid solution phase. This maximum temperature is maintained for up to 60 minutes. The structure of the glass-ceramic is homogenized and the optical, physical and chemical properties are adjusted. The resulting glass-ceramic is cooled to room temperature in less than 150 minutes. Preferably, the cooling is carried out to about 700 ° C at a slower rate. Overall, the ceramization time is less than 300 minutes, preferably less than 200 minutes, and more preferably less than 150 minutes.

Preferably, the transparent lithium aluminum silicate glass ceramic with high quartz mixed crystals as the main crystal phase is used as Kaminsichtscheibe, fire glass, oven vision (especially for pyrolysis), cooking surface possibly with underside coating, coverage in the lighting sector, as safety glass optionally in the laminate composite, as a support plate or furnace lining in thermal processes.

For fireplace viewing windows, a good view of the combustion chamber and the flames is desired. For cooking surfaces with colored underside coating, the color of the underside coating should not be distorted by the color of the glass ceramic. For the uses mentioned above, the high levels of light transmission (brightness) Y according to the invention and the low color c * in combination with the low inconspicuous light scattering are preferred.

These uses are alternatively also served by a transparent lithium aluminum silicate glass ceramic with keatite mixed crystals as the main crystal phase.

By applying an opaque coating on the top and / or bottom can be made of the transparent glass ceramic, a colored cooking surface with the required coverage to prevent the insight into the technical installations under the cooking surface. The cooking surface is heated as usual with gas burners, radiant heaters or inductive.

Recesses in the coating allow the mounting of sensor areas, colored and white display devices and displays.

It is possible to combine coatings on top and bottom of the transparent glass ceramic plate and thereby also include partially transparent layers. Likewise, marks, z. B. for cooking zones, are applied. Here are the known different types of coatings such as z. B. with organic or inorganic decorative paints, chandelier colors, silicone and sol-gel-based paints, sputtered layers, metallic, oxynitride, oxicarbide layers and so on combined. Layers can also be applied one above the other. Even with fireplace or furnace windows coatings such. B. to opaque cover on the edge of the discs may be desired. The arrangement of the coatings on top or bottom of the glass ceramic plate is made according to the aesthetic and the specific requirements for chemical and physical properties.

The display devices consist of light-emitting electronic components, such. B. from light emitting diodes, OLEDs, LCDs, fluorescence displays. All forms of displays, both point-to-point and area, including 7-segment displays, are possible. The emission spectra of the radiating displays may have one or more maxima and wide areas such that the displays appear colored (such as blue, purple, red, green, yellow, orange) or white. Due to the low color c * of the glass ceramic also black and white and colored displays or screens can be produced without disturbing color distortion. The color of the displays is optionally changed by color filters or layers applied to the underside. The color of the displays can be influenced and possibly also corrected if it is changed by the glass ceramic. Likewise, control, sensor and control / operating elements, such. As those of the capacitive or inductive type, are attached to the glass ceramic plate.

The glass ceramic with keatite mixed crystals as main crystal phase finds in translucent or opaque form preferably used as a cooking surface, support plate for thermal processes (setterplate), cover plate in microwave ovens or lining of combustion chambers. The light transmission is preferably less than 15%.

Further preferred uses for both glass-ceramics, be they with high-quartz or with keatite mixed crystals as the main crystal phase, are as a carrier plate or furnace lining. In the ceramic, solar or pharmaceutical industry or medical technology, they are particularly suitable for production processes under high-purity conditions, as a lining of furnaces in which chemical or physical coating processes are carried out or as chemically resistant laboratory equipment. They are also used as a glass-ceramic article for high or extreme low temperature applications, as furnace windows for incinerators, as a heat shield for shielding hot environments, as a cover for reflectors, floodlights, projectors, projectors, photocopiers, for applications with thermo-mechanical stress, for example in night vision devices or as a cover for heating elements, in particular as a cooking, grilling or roasting surface, as white goods, as a radiator cover, as a wafer substrate, as an object with UV protection, as a facade panel or as a material for housing components, for example, of electronic devices and / or covers for IT such as mobile phones, laptops, scanner glasses or as a component for ballistic protection.

Further preferred uses for articles comprising glass ceramics with keatite mixed crystals as the main crystal phase and whose thermal expansion from 25 ° C to 50 ° C is less than 0 ± 0.5 x 10 ^-6 / K are those as precision components at room temperature, such as z. B. as distance standards or wafer stages, as a mirror substrate for reflective optical components such as in astronomy and in LCD or EUV lithography, or as a laser gyroscope. By choosing the ceramizing conditions, it is possible to produce glass ceramics with low thermal expansion at room temperature from the compositions according to the invention.

The transparent glass ceramics of the present invention meet the requirements of transparency, which includes both low color and high light transmission, low scattering, high chemical resistance, mechanical strength, temperature resistance and long-term stability with respect to changes in their properties (such as thermal expansion, Transmission, build-up of stresses) sufficiently good.

The present invention will be further clarified by the following examples.

In the single figure, the transmission curve of the glass-ceramic of Example 10 is shown.

The crystallizable glasses were melted from common in the glass industry vegetable raw materials at temperatures of 1620 ° C, 4 hours. The basic raw materials used were: lithium carbonate, aluminum oxide, aluminum hydroxide, aluminum metaphosphate, sand, soda and potassium nitrate, magnesium oxide, lime, strontium carbonate, barium carbonate, zinc oxide, tin oxide, titanium oxide, zirconium silicate and neodymium oxide Pollution content of coloring compounds of Fe, Ni, Cr, V, Cu, Mo, S agree. After melting of the mixture in crucibles made of sintered silica glass, the melts were recast in Pt / Rh crucible with inner crucible made of silica glass and homogenized by stirring at temperatures of 1600 ° C, 60 minutes. After this homogenization, the glasses were refined for 2 hours at 1640 ° C. Then pieces of about 120 × 140 × 30 mm ³ size were poured and cooled in a cooling oven starting from 660 ° C to room temperature to reduce stresses. The castings were subdivided into the sizes required for the investigations and for the ceramization.

For some examples, Table 1 shows compositions and properties of the crystallizable glasses. In addition to the contents of the components, the relevant component relationships are also shown. Glasses 1 to 11 are glasses according to the invention, that is to say exemplary embodiments, and glasses 12 to 14 are comparison glasses outside the scope of the present invention, that is to say comparative examples. The compositions of the comparative glasses are outside the invention and show the disadvantages described in terms of manufacturing properties (melting, processing temperature, devitrification resistance) and / or color after conversion into the glass ceramic. Due to typical impurities in the large-scale batch raw materials used, the compositions do not exactly add up to 100% by weight. Typical impurities, although not intentionally introduced into the composition, are F, Cl, B, Mn, Rb, Cs, Hf, which are usually less than 0.2% by weight. They are often introduced via the raw materials for the related components, e.g. Rb and Cs via the Na or K raw materials, or Hf over the Zr raw material.

The water content of the glasses measured by IR spectroscopy is shown in Table 1.

In Table 1, the properties are also in the glassy state transformation temperature Tg [° C], processing temperature V _A [° C], 10 ² temperature [° C], upper devitrification limit OEG [° C], devitrification strength V _A -OEG and the density [g / cm ³ ]. To measure the OEG, the glasses are melted in Pt / Rh10 crucibles. Subsequently, the crucibles are kept at different temperatures within the range of the processing temperature for 5 hours. The uppermost temperature, at which the first crystals appear at the contact surface of the molten glass to the crucible wall, determines the OEG.

Table 2 relates to the glass ceramics produced from the glasses of Table 1 with high quartz mixed crystals as the main crystal phase. Examples 1 to 14 are exemplary embodiments and examples 15 to 18 are comparative examples. Table 2 shows the properties of the glass ceramics for the respective ceramization programs specified, ie program 1 or program 2, namely spectral transmission (standard light C, 2 °, 4 mm thickness) [%] at 400 nm, infrared transmission (standard light C, 2 °, 4 mm thickness) [%] at 1600 nm, thermal expansion between 20 ° C. and 700 ° C. [10 ^-6 / K], and the phase content [wt%] of the main crystal phase, which is determined by X-ray diffraction, consisting of high quartz mixed crystals, and the average crystallite size [nm] shown. Table 2 also shows the light transmission Y and the color coordinates L *, a *, b * from the CIELAB system and the size c * as a measure of the color (chroma) and the Haze value as a measure of the scattering.

Table 3 shows Examples 19 to 21 for the glass ceramics with keatite mixed crystals produced from the glass 8 of Table 1 as the main crystal phase. In addition to the quantities from Table 2, the color values for the measurement in remission are additionally shown. Next is qualitatively the appearance of the examples described.

In the Keramisierungsprogramm 1 is heated to a temperature of 600 ° C in the ceramizing oven in 20 min. It will continue to heat up. The total time from room temperature to 680 ° C is 36 min. The temperature range of 680 ° C to 800 ° C is important for nucleation. Therefore, it will continue to heat up. The total time between 680 ° C to 800 ° C is 64 min. In the case of this ceramization program, a retention time of 30 minutes is introduced at the temperature of 750 ° C. in the area of nucleation. Above about 800 ° C, the crystallization of the desired high quartz mixed crystal phase takes place. In this area, the formation of the interfering Fe / Ti and Sn / Ti color complexes is also enhanced. The total time from 800 ° C to reach the maximum temperature 903 ° C is 41 min. At the maximum temperature of 903 ° C, holding time 10 min, the composition of crystals and residual glass is adjusted and homogenized the microstructure. The chemical and physical properties of the glass ceramic are adjusted. The cooling is controlled to 700 ° C, then the sample is quenched to room temperature by opening the oven door; So summarized:

Ceramisation program 1 (ceramization time 185 min):

• a) rapid heating from room temperature to 600 ° C in 20 minutes,
• b) temperature increase from 600 ° C to nucleation temperature 750 ° C within 30 min (heating rate 5 ° C / min), holding time 30 min,
• c) temperature increase from 750 ° C to maximum temperature 903 ° C within 61 min (heating rate 2.5 ° C / min), holding time 10 min at maximum temperature,
• d) cooling from 903 ° C to 700 ° C (at 6 ° C / min) within 34 minutes, then rapid cooling to room temperature.

The ceramization program 2 is optimized for the compositions according to the invention in the direction of short ceramization times and lower color c *. The nucleation times may be shortened compared to program 1 only so far that the scattering is not visually conspicuous. In a laboratory oven, which allows high heating rates, is heated to 720 ° C in 25 min. The total time up to 680 ° C is 23.6 min. The temperature range of 680 to 800 ° C is important for nucleation. The total time between 680 ° C to 800 ° C is 21.4 min. The time in this temperature range is chosen so that no visually disturbing scattering occurs. Above 800 ° C up to the maximum temperature of 890 ° C, the heating rate is reduced because crystallization takes place in this area. The concomitant shrinkage process must not run too fast, otherwise it may lead to unevenness of the article. The total time of 800 ° C until reaching the maximum temperature of 890 ° C is 53 min, followed by a holding time of 10 min. The composition of crystals and residual glass is adjusted and the microstructure is homogenized. The time in this temperature range is optimized for a good flatness of the glass ceramic plates. The cooling is controlled to 600 ° C, then the sample is quenched to room temperature by opening the oven door; So summarized:

Ceramization program 2, (ceramization time 137 min):

• a) rapid heating from room temperature to 720 ° C in 25 minutes,
• b) temperature increase from 720 to 800 ° C in 20 min (heating rate 4 ° C / min),
• c) temperature increase from 800 ° C to 890 ° C in 53 min (heating rate 1.7 ° C / min), holding time of 10 min at maximum temperature,
• d) cooling from 890 ° C to 600 ° C within 29 minutes (at 10 ° C / min), then rapid cooling to room temperature.

With an additional Keramisierungsprogramm 3, the conversion of the crystallizable glass no. 8 (Table 1) was carried out in glass ceramics with Keatit mixed crystals as the main crystal phase. In this program, the procedure was 890 ° C as in program 2, then, unlike program 2 without holding time at 890 ° C at a heating rate of 10 ° C / min heated to a maximum temperature T _max with holding time t _max (for details see Table 3 ). From the maximum temperature was cooled at 5 ° C / min to 800 ° C and then cooled rapidly to room temperature.

The transmission measurements were carried out on polished plates of thickness 4 mm with standard light C, 2 ° with the Perkin-Elmer Lambda 900 device. From the measured spectral values in the range between 380 nm and 780 nm, which represents the visible light spectrum, the light transmission Y according to DIN 5033 is calculated for the selected standard illuminant C and observer angle 2 °. From these measurements, the color coordinates L *, a *, b * from the CIELAB system and the size c * are also calculated. The remission was also measured according to DIN 5033 with these parameters.

Tabelle 2: Keramisierungsbedingungen und Eigenschaften der Glaskeramiken mit Hochquarz-Mischkristallen als Hauptkristallphase

*nicht gemessen Tabelle 2 (Fortsetzung): Keramisierungsbedingungen und Eigenschaften der Glaskeramiken mit Hochquarz-Mischkristallen als Hauptkristallphase

*nicht gemessen Tabelle 3: Keramisierungsbedingungen und Eigenschaften der Glaskeramiken mit Keatit-Mischkristallen als Hauptkristallphase Beispiel Nr. 19 20 21 Glas Nr. 8 8 8 Aussehen tranparent bistransluzent transluzent opak Keramisierungsprogramm 3 3 3 Tmax °C 970 1080 1150 tmax min 60 30 30 Eigenschaften keramisiert Transmission Normlicht C, 2° 4 mm Dicke 400 nm % 7,2 < 0,01 < 0,01 1600 nm % 86,8 76,9 19,3 Lichttransmission Y % 56,1 5,2 0,1 L* 79,6 27,4 0,6 a* –1,8 9,6 2 b* 27,4 38,3 1,0 c* 27,5 39,5 2,2 Remission Normlicht C, 2° 4 mm Dicke 400 nm % 10,1 40,1 62,4 1600 nm % 8,9 11,1 46,0 Lichttransmission Y % 11,1 54,4 79,0 L* 39,7 78,7 91,2 a* –1,7 –4,1 –0,7 b* –6,3 0,3 2,5 c* 6,5 4,1 2,6 therm Ausdehnung α20/700 10^–6/K 1,09 1,09 1,05 Röntgenbeugung Keatit-Phasengehalt % 85 85 87 mittlere Kristallitgröße Keatit nm > 120 > 120 > 120 The scattering of the glass ceramics is determined by measuring the haze. In this case, 4 mm thick, both sides polished samples with a spectrophotometer (Method B of ASTM D1003-13) with standard light C for the spectral range 400-800 nm measured. The scattering is characterized by the Haze value in Table 2. Table 1: Compositions and properties of crystallizable glasses Glass no. 1 2 3 4 5 6 composition Wt.% Li ₂ O 3.72 3.76 3.74 3.57 3.70 3.74 Na ₂ O 0.50 0.50 0.51 0.51 0.55 0.55 K ₂ O 0.12 0.12 0.12 0.11 0.11 0.11 MgO 0.21 0.21 0.21 0.22 0.35 0.35 CaO 0.24 0.25 0.39 0.39 0.39 0.39 SrO 0.52 0.01 0.07 BaO 0.53 0.55 0.89 0.89 0.89 0.82 ZnO 1.72 1.71 1.70 1.69 1.69 1.65 Al ₂ O ₃ 21.3 21.4 21.3 21.3 21.4 21.4 SiO ₂ 66.7 67.0 66.7 66.9 66.5 66.4 TiO ₂ 2.22 2.23 2.22 2.22 2.23 2.26 ZrO ₂ 1.83 1.83 1.82 1.82 1.82 1.79 P ₂ O ₅ 0.02 0.03 0.03 0.03 SnO ₂ 0.29 0.30 0.29 0.29 0.29 0.28 Fe ₂ O ₃ 0,015 0,015 0,015 0,015 0,015 0,015 Nd ₂ O ₃ 0,006 0,044 H ₂ O content (β-OH) mm ^-1 0.41 0.42 0.41 0.41 0.45 * Na ₂ O + K ₂ O 0.62 0.62 0.63 0.62 0.66 0.66 CaO + BaO 0.77 0.8 1.28 1.28 1.28 1.21 CaO + SrO + BaO 1.29 0.80 1.29 1.28 1.28 1.28 TiO ₂ + ZrO ₂ + SnO ₂ 4.34 4.36 4.33 4.33 4.34 4.33 MgO / SnO ₂ 0.72 0.70 0.72 0.76 1.21 1.25 Nd ₂ O ₃ / Fe ₂ O ₃ 0.00 0.00 0.00 0.40 0.00 2.93 Na ₂ O / K ₂ O 4.17 4.17 4.25 4.64 5.00 5.00 (CaO + BaO) / SrO 1.48 128.00 17.79 Properties glassy Transformation temperature Tg ° C 685 693 686 697 690 685 10 ² temperature ° C 1760 1766 1767 1765 1756 1754 Processing temperature VA ° C 1324 1327 1322 1330 1317 1318 OEG temperature ° C 1315 1315 1255 1320 1280 1305 Devitrification resistance VA-OEG ° C 9 12 67 10 37 13 density g / cm ³ 2,454 2,445 2,452 2,453 2,457 2,456 *not measured Table 1 (continued): Compositions and properties of crystallizable glasses

Table 2: Ceramization conditions and properties of glass ceramics with high quartz mixed crystals as the main crystal phase

*not measured Table 2 (continued): Ceramization conditions and properties of glass ceramics with high quartz mixed crystals as the main crystal phase

* not measured Table 3: Ceramization conditions and properties of glass ceramics with keatite mixed crystals as the main crystal phase Example no. 19 20 21 Glass no. 8th 8th 8th Appearance tranparent bistranslucent translucent opaque ceramization 3 3 3 Tmax ° C 970 1080 1150 tmax min 60 30 30 Properties ceramized Transmission standard light C, 2 ° 4 mm thickness 400 nm % 7.2 <0.01 <0.01 1600 nm % 86.8 76.9 19.3 Light transmission Y % 56.1 5.2 0.1 L * 79.6 27.4 0.6 a * -1.8 9.6 2 b * 27.4 38.3 1.0 c * 27.5 39.5 2.2 Remission standard light C, 2 ° 4 mm thickness 400 nm % 10.1 40.1 62.4 1600 nm % 8.9 11.1 46.0 Light transmission Y % 11.1 54.4 79.0 L * 39.7 78.7 91.2 a * -1.7 -4.1 -0.7 b * -6.3 0.3 2.5 c * 6.5 4.1 2.6 therm expansion α20 / 700 10 ^-6 / K 1.09 1.09 1.05 X-ray diffraction Keatite phase content % 85 85 87 average crystallite size keatite nm > 120 > 120 > 120

Claims (20)

Crystallizable lithium aluminum silicate glass for the production of transparent glass ceramics, characterized in that it is arsenic- and antimony-free, except for unavoidable raw material impurities, that it contains the following components (in% by weight based on oxide): Li ₂ O 3,4-4,1 Al ₂ O ₃ 20-22.5 SiO ₂ 64-68 Na ₂ O + K ₂ O 0.4-1.2 MgO 0.05- <0.4 CaO 0-1 CaO + BaO 0.7-1, 8 CaO + SrO + BaO 0.8-2 ZnO 0.3-2.2 TiO ₂ 1.8- <2.5 ZrO ₂ 1.6- <2.1 SnO ₂ 0.2-0.35 TiO ₂ + ZrO ₂ + SnO ₂ 3.8-4.8 P ₂ O ₅ 0-3 Fe ₂ O ₃ > 0.01-0.02, with the condition B1: 0.15 <MgO / SnO ₂ <1.7 Crystallizable lithium aluminum silicate glass according to claim 1, characterized in that it contains the following components (in wt .-% based on oxide): Li ₂ O 3.4- <4 Al ₂ O ₃ 20.5-22.5 SiO ₂ 65- 67.5 Na ₂ O + K ₂ O 0.4-1.0 MgO> 0.1- <0.4 CaO 0.01-0.8 CaO + BaO 0.7-1.8 SrO 0-0, 5 BaO 0-1.5 CaO + SrO + BaO 0.8-2 ZnO 1.1-2.1 TiO ₂ > 1.9- <2.5 ZrO ₂ 1.6- <2.0 SnO ₂ O 2-0.33 TiO ₂ + ZrO ₂ + SnO ₂ 3.9-4.8 P ₂ O ₅ 0-3 Fe ₂ O ₃ > 0.01-0.018 with the condition B2: 0.4 <MgO / SnO ₂ <1.7 Crystallizable lithium aluminum silicate glass according to one of the preceding claims, characterized in that it contains 0.005-0.2 wt .-% Nd ₂ O ₃ Crystallizable lithium aluminum silicate glass according to one of the preceding claims, characterized in that for the components Nd ₂ O ₃ and Fe ₂ O ₃ the condition Nd ₂ O ₃ / Fe ₂ O ₃ > 2 applies Crystallizable lithium aluminum silicate glass according to one of the preceding claims, characterized in that for the components Na ₂ O and K ₂ O the condition Na ₂ O / K ₂ O> 2 applies Crystallizable lithium aluminum silicate glass according to any one of the preceding claims, characterized in that the CaO content is at least 0.28 wt .-%. Crystallizable lithium aluminum silicate glass according to one of the preceding claims, characterized in that for the components CaO, BaO and SrO the condition (CaO + BaO) / SrO> 3 applies Crystallizable lithium aluminum silicate glass according to any one of the preceding claims, characterized in that the P ₂ O ₅ content is 0.01 to 1.5 wt .-%. Crystallizable lithium aluminum silicate glass according to one of the preceding claims, characterized in that it contains the following components (in% by weight based on oxide): Li ₂ O 3.4- <4 Al ₂ O ₃ 20.5-22.5 SiO ₂ 65-67.5 Na ₂ O + K ₂ O 0.4-1.0 MgO> 0.1- <0.4, CaO> 0.1-0.8 CaO + BaO 0.7-1.8 SrO 0-0.5 BaO 0-1.5 CaO + SrO + BaO 0.8-2 ZnO 1.1-2.1 B ₂ O ₃ 0-1 TiO ₂ > 1.9- <2.5 ZrO ₂ 1.6- <2.0 SnO ₂ 0.2-0.33 TiO ₂ + ZrO ₂ + SnO ₂ 3.9-4.8 P ₂ O ₅ 0.01- 0.8 Fe ₂ O ₃ > 0.01-0.018 Nd ₂ O ₃ 0.005-0.2 with the condition B2: 0.4 <MgO / SnO ₂ <1.7 Crystallizable lithium aluminum silicate glass according to one of the preceding claims, characterized in that it contains the following components (in% by weight based on oxide): Li ₂ O 3.4- <4 Al ₂ O ₃ 20.5-22.5 SiO ₂ 65-67.5 Na ₂ O + K ₂ O 0.4-1.0 MgO> 0.1- <0.4, CaO 0.28-0.8 CaO + BaO 0.7-1.8 SrO 0 -0.5 BaO 0-1.5 CaO + SrO + BaO 0.8-2 ZnO 1.1-2.1 B ₂ O ₃ 0-1 TiO ₂ > 1.9- <2.5 ZrO ₂ 1, 6- <2.0 SnO ₂ 0.2-0.33 TiO ₂ + ZrO ₂ + SnO ₂ 3.9-4.8 P ₂ O ₅ 0.01-0.8 Fe ₂ O ₃ > 0.01 0.018 Nd ₂ O ₃ > 0.01-0.15 with the condition B2: 0.4 <MgO / SnO ₂ <1.7 Crystallizable lithium aluminum silicate glass according to one of the preceding claims, characterized by • a 10 ² temperature of less than 1770 ° C and / or • a processing temperature V _A of at most 1330 ° C and / or • an upper devitrification limit OEG, the at least 10 ° C is below the processing temperature V _A.  Glass-ceramic, converted from a lithium aluminum silicate glass according to one of claims 1 to 11.  Glass-ceramic according to claim 12, characterized in that it contains high-quartz mixed crystals as main crystal phase. Glass-ceramic according to claim 13, characterized in that it has a light transmission Y of greater than 82% and / or a color c * of not more than 5.5 for a thickness of 4 mm. Glass ceramic according to claim 13, characterized in that for 4 mm thickness, the light transmission Y is greater than 82% and the color c * is at most 5.5 visually inconspicuous scattering with Haze value <2%.  Glass-ceramic according to claim 12, characterized in that it contains keatite mixed crystals as the main crystal phase. Glass ceramic according to one of the preceding claims 12 to 16, characterized in that it is in the form of a plate with a thickness of 2 mm to 20 mm. A process for preparing a crystallizable lithium aluminum silicate glass according to any one of claims 1 to 11, characterized by the steps: a) providing a batch of technical raw materials containing 20 to 80 wt .-% shards; b) Melting of the batch and refining in conventional aggregate at temperatures below 1750 ° C; c) cooling the glass melt and shaping at temperatures close to the processing temperature V _A , and d) cooling in a flash oven to room temperature. Process for producing a glass ceramic according to one of Claims 13 to 15, characterized in that the ceramisation is carried out with the following program: a) raising the temperature of the crystallizable glass to the temperature range of about 680 ° C within 3 to 60 minutes; b) increasing the temperature of the crystallizable glass within the temperature range of nucleation from 680 to 800 ° C over a period of about 10 to 100 minutes; c) increasing the temperature of the glass containing nucleation within 5 to 80 minutes in the temperature range high crystal growth rate of 850 to 950 ° C; d) keep within the temperature range at the maximum temperature of 850 to 950 ° C up to 60 minutes, wherein crystals of the type of high quartz mixed crystals grow on the crystallization nuclei, and thereafter; e) rapidly cooling the resulting glass-ceramic to room temperature in less than 150 minutes, the ceramization of the glass having a total duration of less than 300 minutes. Use of an article comprising a glass-ceramic plate according to claim 17, as a fire-resistant glass, fireplace visor, oven visor, cooking surface optionally with underside coating, cover in the lighting sector, as safety glass optionally in the laminate composite, as a support plate or furnace lining in thermal processes.

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