FR3051185A1 - CRYSTALLIZABLE LITHIUM ALUMINOSILICATE GLASS AND VITROCERAMIC MADE FROM THIS GLASS, PROCESS FOR MANUFACTURING GLASS AND VITROCERAMIC AND USE OF VITROCERAMIC - Google Patents

Abstract

The invention relates to a crystallizable lithium aluminosilicate glass, intended for the manufacture of transparent glass ceramics, which contains the following components (in % by weight on an oxide basis): Li2O 3.4 - 4.1 Al2O3 20 - 22.5 SiO2 64 - 68 Na2O + K2O 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 TiO2 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. On the other hand, the invention relates to the glass-ceramic manufactured from this glass, and the uses thereof.

Description

Crystallizable lithium aluminosilicate glass and glass ceramic made from this glass, manufacturing process of glass and glass ceramic and use of glass ceramic

The present invention relates to a crystallizable lithium aluminosilicate glass which can be processed into transparent LAS glass-ceramics. The invention also relates to the glass-ceramic made from this glass, as well as to a method of making and the use of such a LAS glass-ceramic.

It is generally known that glasses of the LiiO-AhCL-SiCh system can be made into glass-ceramics comprising mixed crystals of quartz-β or mixed crystals of keatite as the main crystal phases. For the first type of glass-ceramics, the synonyms “quartz-β” or “eucryptite-β” are also found in the literature, and for the second type the term “spodumene-β” as a designation for the crystalline phases.

Key characteristics of these LAS glass-ceramics are their transparency, good chemical resistance to acids and bases, high mechanical strength and low coefficient of thermal expansion, in a temperature range from room temperature to application temperature. of 0120/700 <1.5 · 10 "6 / Κ. As a rule, the thermal expansion behavior is regulated in such a way that in the range of their application temperatures the materials exhibit very low expansion, most time of 0 + 0.3 · 10 "6 / Κ. Thus, during the application as a substrate material, for example for color filters, display screens, sensors, precision stages for semiconductor wafers, also called wafer stages, or mirror stands for telescopes, thermal expansion is minimized in the ambient temperature range.

For applications as fire resistant glazing, transparent chimney panes, oven panes, outside or inside the oven, and as a hob with colored underside coating, zero thermal expansion is set to values as low as possible, <1.5 10 "6 / Κ, preferably 0 + 0.3 10" 6 / Κ, in a temperature range from room temperature to about 700 ° vs. Due to the low thermal expansion in the presence of their application temperatures, in combination with high mechanical strength, these glass-ceramics exhibit excellent resistance to temperature variations and thermal shock. Requirements for use at high temperatures include that glass ceramics retain the required properties (eg thermal expansion, transmission, stress formation) during their service life. As regards the resistance to high temperatures, the shrinkage, although small, of the glass ceramic (compaction) in the presence of high temperatures constitutes the critical quantity. Since glass ceramic articles are most often unevenly heated during use, differences in shrinkage over time cause stresses to build up in the article.

Transparency, together with low diffusion, is the essential characteristic which distinguishes transparent glass ceramics from transparent tinted glass ceramics. For the latter, one adds, typically for cooking surfaces, in particular V2O5 for the dyeing in the volume, in order to reduce the transmission of light and to obtain a black appearance. With tinted transparent glass-ceramics, the light transmission is usually less than 30 and most of the time less than 10%, compared to more than 80% for those of the transparent type (for a thickness of 4 mm).

When used as a hob, transparent ceramic hobs are usually provided on their underside with an opaque colored coating to prevent the integrated technical elements from being seen transparently and to give them a pleasant colored appearance. Recesses in the coating of the underside make it possible to attach colored and white display elements, namely light-emitting diodes most of the time. High transparency as well as low scattering are also essential for this application, to prevent the colors of the coating of the underside and display elements from being faded and to ensure that the displays are clearly visible.

High transparency means high light transmission and low color and therefore low absorption, because depending on the position in the visible spectrum, absorption bands both decrease light transmission and increase color.

Light transmission is described by the value Y (brightness) according to the CIE chromaticity system. This international CIE standard is transposed in Germany by DIN 5033. The spectrophotometric measurements necessary for this purpose are carried out within the framework of the invention for polished samples with a thickness of 4 mm, in a spectral range between 380 and 780 nm . From the spectral values measured in the domain that represents the visible light spectrum, the light transmission is calculated by choosing the standard light and the angle of observation.

For glass-ceramics, we have become accustomed to using, as a measure of color, the quantity c * (chromatic intensity) of the CIELAB color system, with the coordinates L *, a *, b *, in accordance with the following 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 chromaticity system can be calculated in a known manner from the colorimetric coordinates and the Y light transmission of the CIE chromaticity system. The c * value of the samples is determined from the spectrophotometric measurements made for the transmission, with the parameters selected for the standard light and the angle of observation.

The diffusion of glass-ceramics is determined by measuring the turbidity (in English: haze). According to ASTM D1003-13, turbidity is the percentage of transmitted light that deviates on average more than 2.5 ° from the incoming light beam.

The industrial manufacture of transparent glass-ceramics takes place in several stages. First, the crystallizable starting glass is melted from a mixture of cullet and batch batch raw materials into powder form, then refined. The glass bath then reaches temperatures between 1550 ° C and a maximum of 1750 ° C, most of the time 1700 ° C. In some cases, high temperature refining is also used, above 1700 ° C, generally at temperatures around 1900 ° C. For transparent glass-ceramics, arsenic oxide and antimony oxide are technically and economically proven refining agents for good bubble qualities at refining temperatures. below 1700 ° C. Arsenic oxide is particularly advantageous for the transparency (high light transmission and low color) of glass ceramic. Even when these refining agents are firmly incorporated into the structure of the glass, they represent a disadvantage from the point of view of safety and environmental protection. Thus, during the production and preparation of raw materials and due to evaporation from the molten bath during the manufacture of glass, special precautionary measures should be taken. For this reason, there are many development attempts to replace these materials, but they have technical and economic drawbacks.

As a replacement, it is increasingly recommended the SnCh refining agent which is environmentally friendly, to be used alone or in combination with one or more refining agents such as halides (F, Cl, Br), CeCh and sulfur compounds.

However, there are drawbacks to using SnCh. SnCh itself is technically less effective as a refining agent and requires higher temperatures to release oxygen to activate refining. However, higher concentrations of SnCh on the order of magnitude of the AS2O3 used, around about 0.6 to 1% by weight, have drawbacks due to the devitrification of the Sn-containing crystals during hot forming. The second essential disadvantage for transparent glass ceramics, when replacing arsenic oxide with tin oxide as a refining agent, is that SnCh causes additional absorption and increases color. . In the presence of higher refining temperatures, the absorption increases because it increases due to the higher proportion of Sn2 +.

The documents JP 11-228180 A2 and JP 11-225181 A2 describe the environmentally friendly refining of LAS glasses, without the use of arsenic oxide, by a combination of the refining agents SnCh and Cl, at 0, 1 to 2% by weight. The yellow-brown tint associated with the use of SnCh has extremely negative effects for transparent glass-ceramics. The cited documents do not provide any guidance on how to limit the SnCh content in order to ensure low absorption and sufficient resistance to devitrification. Combined refining, with SnCh in connection with fluorine or bromine compounds, is disclosed in documents EP 1 899 276 A1 and EP 1 901 999 A1. However, these halides pose problems in terms of the environment, in terms of due to their evaporation, and have drawbacks with regard to the corrosion of the integrated elements, during melting and shaping.

After melting and refining, the glass is usually subjected to hot forming by rolling, casting, pressing or floating. For most applications of transparent glass ceramics, these must be in a planar form, for example in the form of plates. Rolling and, more recently, also floating, are used for the manufacture of these plates. For the economical manufacture of these LAS glasses, we want on the one hand a low melting temperature and a low processing temperature Va during hot forming. On the other hand, during forming, the glass must not exhibit devitrification, that is to say that there must not be formation of undesirable crystals which decrease the resistance in the glass ceramic articles or are troublesome. visually. The coldest zone, which is critical when forming using rolls, is where the glass bath contacts the precious metal die (usually made of a Pt / Rh alloy), before the glass is shaped by the cylinders and cooled.

In a subsequent thermal process, the crystallizable LAS glass is transformed into a glass-ceramic by controlled crystallization (ceramization). This ceramization takes place in a two-stage thermal process, in which seeds are first produced by nucleation, usually from mixed crystals of ZrCh / TiCh, at a temperature between 680 and 800 ° C. SnC> 2 also participates in nucleation. Mixed quartz-β crystals grow on these seeds, in the presence of high temperatures. At the maximum manufacturing temperature, which is around 900 ° C, the structure of the glass ceramic is homogenized, and the optical, physical and chemical properties are adjusted. For economical production, short ceramization times are advantageous.

Subsequent transformation into mixed keatite crystals takes place with an increase in temperature over a range of about 950 ° C to 1250 ° C. With processing, the coefficient of thermal expansion of the glass ceramic increases, and due to continued crystal growth and accompanying light scattering, the transparent appearance changes to a translucent or even opaque appearance.

A considerable barrier to the use of SnCh as an environmentally friendly refining agent is the additional absorption due to the Sn / Ti complexes that form. These Sn / Ti color complexes tint more strongly than the known Fe / Ti color complexes, and so far this disadvantage has meant that the replacement of arsenic oxide, as a refining agent, by SnCh was difficult for transparent glass ceramics. The Sn / Ti color complexes have drawbacks in particular due to the increased c * color. Fe / Ti color complexes create a reddish brown color and Sn / Ti color complexes create a yellow brown color. The absorption mechanism of the two color complexes is probably based on electronic charge transfers between the two neighboring polyvalent cations. The formation of the mentioned electron transfer color complexes takes place to a large extent during crystallization. In order to reduce the concentrations of the color complexes, it is advantageous to shorten the nucleation and crystallization times. In contrast, reducing nucleation time results in greater light scattering, and crystallization time results in article flatness defects.

The technical raw materials of the vitrifiable mixture for molten baths contain other coloring elements, such as Cr, Mn, Ni, V and in particular Fe, as impurities. Fe has a coloring effect not only through Fe / Ti color complexes, but also ionically, as Fe2 + or Fe3 +. Due to the high cost of low iron raw materials, it is uneconomical to reduce the Fe223 content to values below 100 ppm.

Likewise, the approach consisting in avoiding (WO 2008/065167 A1) or limiting (WO 2008/065166 A1) the use of the active T1O2 nucleating agent, responsible for the color complexes, has only present not resulted in a technical implementation. The required higher contents of ZrÜ2 and / or SnÜ2 nucleating agents which can also be used lead to disadvantages during melting and forming, for example higher melting and shaping temperatures, as well as lower melting and shaping temperatures. insufficient resistance to devitrification during shaping.

WO 2013/124373 A1 describes the physical discoloration of transparent glass-ceramics, comprising mixed crystals of quartz-β as the main crystalline phase, which, except for the inevitable impurities of raw materials, are free of arsenic and arsenic. antimony, with additions of 0.005 to 0.15% by weight of Nd2O3. The principle of bleaching is based on the fact that the absorption bands present are neutralized by additional absorption bands of the bleach. This by nature results in a higher absorption of light and thus reduces light transmission. To obtain advantageous manufacturing conditions, that is to say low melting and shaping temperatures, the glasses used as an example of this specification contain from 0.44 to 0.93% by weight of the MgO component reducing viscosity. On the other hand, the ratio of the sums of the bivalent components MgO + ZnO> CaO + SrO + BaO, which is described for the adjustment of the crystal composition and the composition of the residual glass phase, also implies MgO contents higher.

Document WO 2013/171288 A1 discloses transparent glass-ceramic articles, substantially poor in color and non-diffusing, comprising mixed crystals of quartz-β, glass-ceramics and precursor glasses. With the exception of unavoidable traces, the compositions of LAS glass ceramics are free from arsenic oxide, antimony oxide and rare earth oxides, such as Nd2O3. The disclosed composition contains 62 to 72 wt% S1O2, 20 to 23 wt% Al2O3, 2.8 to 5 wt% L12O, 0.1 to 0.6 wt% SnCh, 1.9 to 4 wt% T1O2, 1.6 to 3 wt% ZrCh, less than 0.4 wt% MgO, less than 250 ppm Fe203 and 2.5 to 6 wt% ZnO + BaO + SrO . The average crystallite size is less than 35 nm. For the compositions, for which protection is sought, the disadvantage lies not only in the high melting temperatures, but above all in the increased processing temperatures Va which imply a narrow process window and shorter lifetimes. for forming with cylinders, due to the higher temperatures.

Document US 2013/0130887 A1 discloses a glass ceramic LAS with the selected composition from 55 to 75% by weight of S1O2, 20.5 to 27% by weight of Al2O3,> 2% by weight of L12O, 1.5 to 3% by weight of T1O2, 3.8 to 5% by weight of T1O2 + ZrCh, 0.1 to 0.5% by weight of SnCh, and with component ratios of 3.7 <L12O + 0.741 MgO + 0.367 ZnO < 4.5 and SrO + 1.847 CaO <0.5. The purpose of both component ratios is to reduce the color of transparent glass ceramics. This document does not indicate how the optimization of diffusion and manufacturing properties can be implemented.

Document WO 2016/038319 A discloses transparent, colorless and light-diffusing glass-ceramic plates which contain crystals of the mixed quartz-β type, the chemical composition of which does not contain As, Sb and Nd oxides, with a selected composition of 55 to 75 wt% S1O2, 12 to 25 wt% Al2O3, 2 to 5 wt% L12O, 0 to <2 wt% Na2O + K2O, 0 to <7 wt% Na2O + K2O, 0 to <7 wt% weight L12O + Na2Ü + K2O, 0.3 to 5 wt% CaO, 0 to 5 wt% MgO, 0 to 5 wt% SrO, 0.5 to 10 wt% BaO,> 1 % by weight of CaO + BaO, 0 to 5% by weight of ZnO, <1.9% by weight of T1O2, <3% by weight of ZrCh,> 3.8% by weight of T1O2 + ZrCh,> 0, 1% by weight of SnCh, and with the component ratio SnCh / (SnCh + ZrCh + TiCh) <0.1. The minimum content of the nucleating agents TiCh and ZrCh is intended to prevent diffusion of the glass ceramic article, and the maximum content of T1O2 is limited to prevent yellowing of the article.

Consequently, there is a need to reconcile a whole set of contrary requirements concerning glass and glass-ceramic, such as respect for the environment as well as the economical manufacture of glass without disadvantages for the product quality of glass-ceramics, such as light transmission, color and diffusion for short ceramization times. The object of the invention is to find a crystallizable lithium aluminosilicate glass - which has an environmentally friendly composition, - which has advantageous manufacturing properties for economical production, - which can be transformed into transparent LAS glass ceramic, comprising mixed quartz-β crystals as the main crystalline phase, with ceramization times of less than 300 minutes, - the glass-ceramic exhibiting low color, high light transmission and low diffusion.

This goal is achieved by a crystallizable lithium aluminosilicate glass, intended for the manufacture of transparent glass-ceramics, which, except for inevitable impurities due to raw materials, is free of arsenic and antimony, contains the following components (in% by weight on an oxide basis):

with condition B1:

and by the glass ceramic made from this glass. On the other hand, the invention aims to find a method of manufacturing glass and glass-ceramic and their use.

These objectives are achieved by a process for manufacturing a crystallizable lithium aluminosilicate glass which comprises the following steps: a) providing a vitrifiable mixture composition of technical raw materials, containing 20 to 80% by weight cullet; b) melting and refining of the batch mixture in a conventional unit, at temperatures below 1750 ° C; c) cooling the glass bath and shaping, to temperatures close to the processing temperature Va, and d) cooling in an annealing furnace, to room temperature.

These objectives are moreover achieved by the use of an object which comprises a ceramic hob as fireproof glass, fireplace glass, oven glass, hob, if necessary with a coating on the underside. , covering in the field of lighting, as safety glass, optionally in a laminated composite structure, as a support plate or lining of furnaces in thermal processes.

Glass-ceramic meets the requirements of applications such as chemical resistance, mechanical strength, transparency (little color, high light transmission), low diffusion, resistance to high temperatures and long-term stability with regard to changes in its properties (such as thermal expansion, transmission, stress formation).

By environmentally friendly composition is meant that, except for inevitable raw material impurities, crystallizable glass is technically free from arsenic and antimony oxide as conventional refining agents. As impurities, these two components are present in contents of less than 1000 ppm, preferably contents of less than 500 ppm, and particularly advantageously of less than 200 ppm. Except in exceptional cases where foreign cullet from a transparent glass ceramic, containing arsenic oxide, is added as a refining agent during the melting, this addition is not made.

In complex test runs, a narrowly limited composition range was found which combines advantageous manufacturing characteristics with low color, high light transmission and low scattering of the LAS glass ceramic.

Advantageous manufacturing properties for economical production include inexpensive batch batch raw materials, low melting and forming temperatures, sufficient amount of SnCh refiner for conventional refining, strength. to devitrification and short ceramization times. With the short ceramization times, high light transmission is achieved without light scattering (turbidity) or visually disturbing color.

For economical manufacture, it is advantageous to obtain a low melting temperature which is guaranteed by a lower viscosity of the glass bath at high temperatures. For this purpose, the temperature at which the viscosity of the glass bath is 102 dPas is a characteristic quantity. For the glasses according to the invention, this so-called "102" temperature is preferably less than 1,770 ° C, in particular less than 1,765 ° C and particularly advantageously less than 1,760 ° C. The low viscosity of the bath at high temperatures allows the temperature in the melting basin to be set to a lower value and thus prolongs the life of the basin. In relation to the quantity of glass-ceramic produced, energy consumption is thus reduced. Considering the fact that a low viscosity of the glass also favors the rise of bubbles and therefore the refining, a low viscosity of the glass is also advantageous for the quality in terms of bubbles.

Economically, it is advantageous to lower the temperature during shaping. This increases the lifespan of the shaping tools, and there is less dissipated heat to dissipate. The shaping, carried out most of the time by rolling or floating, is carried out with a viscosity of 104dPas of the glass bath. This temperature is also called "processing temperature Va" and is preferably a maximum of 1330 ° C, in particular 1325 ° C for the glasses according to the invention.

When forming from the molten bath, the crystallizable glass exhibits sufficient resistance to devitrification. When forming in contact with the forming material (e.g. a precious metal for the die for the rolling process), crystals that are critical for the strength of the glass ceramic and noticeable are not formed in the glass. visually. The limit temperature, below which there are critical devitrifications, i.e. the upper devitrification limit (OEG), is preferably at least 10 ° C lower than the processing temperature Va . With this minimum difference, a sufficient process window is defined for the formatting process. It is particularly advantageous to have a Va-OEG process window that is at least 20 ° C. Va-OEG is therefore a measure for resistance to devitrification.

In addition, the transparent LAS glass ceramic made from the crystallizable glass satisfies certain properties. The light transmission Y must be high, and the color c * must be low so that the transparent view on objects or on an underside coating as well as the view on displays are not obscured or impaired at the tint. The transparent glass ceramic should not exhibit any visually disturbing light scattering, so that the transparency and the displays are crisp and not disturbed. This must also be guaranteed with the desired short ceramization times.

The contents of the components responsible for the formation of crystalline phases and residual glass are optimized in such a way that the values of light transmission, color and diffusion are achieved with short ceramization times. By “short ceramization time” is meant that the glass ceramic is produced from crystallizable LAS glass in a time of less than 300 minutes, preferably less than 200 minutes and particularly advantageously less than 150 minutes.

In order to achieve all of these properties, both the crystallizable lithium aluminosilicate glass according to the invention and the glass ceramic according to the invention which can be manufactured or manufactured from the glass contain the following components (in% of weight on an oxide basis):

with the condition (both in% by weight) B1:

Preferably, the condition (also both in% by weight) B2 applies:

The oxides L12O, AI2O3 and S1O2 are or become necessary constituents of mixed crystalline phases of quartz-β and / or keatite, within the limits indicated.

The L12O content should be 3.4-4.1% by weight. This minimum content is necessary to reach the desired low processing temperature. It has been found that with contents greater than 4.1% by weight, it is difficult to obtain, with short ceramization times, the desired low color c * of the glass ceramic. The L12O content is preferably less than 4% by weight.

The selected AI2O3 content is 20 to 22.5% by weight. Higher contents have drawbacks due to the tendency for the mullite to devitrify during forming. On the other hand, it has been found that with higher contents, the diffusion increases in the presence of short ceramization times. The minimum content is 20% by weight, as this is advantageous for the formation of sufficient quantities of the mixed crystals and the low color c * of the glass ceramic. The minimum AI2O3 content is preferably at least 20.5% by weight

The S1O2 content should be at least 64% by weight, as this is advantageous for the required properties of the glass ceramic, such as low thermal expansion and chemical resistance. In addition, the diffusion is reduced for short ceramization times. A minimum content of 65% by weight is particularly advantageous. The S1O2 content should be a maximum of 68% by weight, as this component increases the processing temperature of the glass and the melting temperature. Preferably, the content of S1O2 is at most 67.5% by weight.

The Na2O and K2O alkalis are or become constituents of the residual glass phase of the glass ceramic and improve the meltability and resistance to devitrification during shaping. On the other hand, they are advantageous for lowering the processing temperature. The sum of the Na2O + K2O alkalis should be at least 0.4% by weight. Too high contents adversely affect the crystallization behavior during the transformation of the crystallizable starting glass into glass-ceramic and cause diffusion during short ceramization times. In addition, the higher contents have negative effects on the color of transparent glass ceramics. The sum of the Na2O + K2O alkalis is preferably at most 1.2% by weight and particularly preferably at most 1.0% by weight.

The Na2O component has been found to be more advantageous for low values of the color c *, for short ceramization times. For this reason, the K2O content is preferably less than 0.2% by weight. Therefore, it is preferable that the content of Na2O is greater than the content of K2O (both in% by weight). It is particularly advantageous when an Na2O / K2O condition> 2 applies (for both by weight%). It is especially preferred that the condition is> 3 (for both by weight%).

In order to obtain the mentioned desired properties of the glass ceramic, the choice of the MgO content is of particular importance. MgO is both a constituent of mixed crystals and of the residual glassy phase and therefore exerts an important influence on many properties. This component lowers the melting and processing temperature of the glass and thus promotes economical manufacture. In glass ceramic, the component increases thermal expansion and causes negative color enhancement. This is attributed to the promotion of the formation of Fe / Ti and Sn / Ti colored species. The MgO content is 0.05 - <0.4% by weight. Preferably, the minimum content is greater than 0.1% by weight.

Particularly advantageously, the minimum MgO content is 0.15% by weight. In a range of MgO values from 0.2% by weight to 0.35% by weight, the requirements regarding the color of the glass ceramic and the processing temperature can be combined particularly well.

The alkaline earth metals CaO, SrO and BaO are not inserted in the mixed crystalline phases but remain in the residual glassy phase of the glass ceramic. They are advantageous for lowering the melting and processing temperature. Too high contents adversely affect the nucleation and crystallization behavior when converting crystallizable glass into ceramic glass. In addition, higher contents have negative effects on the time / temperature resistance of the glass ceramic.

The CaO component has been shown to be advantageous in improving resistance to devitrification and in controlling the refractive index of the residual glassy phase. It is present with contents ranging from 0 to 1% by weight. Preferably, the content is greater than 0.01% by weight and is also preferably up to 0.8% by weight.

In particular to combine particularly advantageous values for the resistance to devitrification and a low color of the glass ceramic, it is advantageous that the CaO content is at least 0.28% by weight. Particularly advantageously, the upper limit for the CaO content is less than 0.5% by weight, in particular when a particularly low diffusion is desired for short ceramization times.

In order to reduce the diffusion for short ceramization times, it is important to adapt the refractive indices of the residual glassy phase and of the mixed crystalline phase. This is achieved by the choice of alkaline earths and by the particular ratios of the components between them. The sum of the alkaline earth metals CaO + SrO + BaO is 0.8 to 2% by weight.

In order to minimize not only the diffusion but also the color, with short ceramization times, it is advantageous to choose the CaO and BaO components with rather high contents and the SrO component with rather low contents. Therefore, the relationship is applied that the sum of the contents of CaO + BaO is 0.7 to 1.8% by weight. Higher contents have drawbacks for resistance to devitrification. The alkaline earths ratio is preferably chosen so as to obtain the following relationship: (CaO + BaO) / SrO> 3 (all in% by weight). Preferably, the SrO content is chosen to be less than 0.5% by weight.

Preferably, the BaO content is 0 to 1.5% by weight and the SrO content is 0 to 0.5% by weight. Particularly advantageously, the BaO content is at least 0.3% by weight. Preferably, the BaO content is a maximum of 1.2% by weight. It is particularly advantageous that the SrO content is at least 0.005% by weight and at most 0.3% by weight.

The ranges and relationships chosen for the alkalis Na2O, K2O and the alkaline earth metals CaO, SrO, BaO, which are inserted into the residual glassy phase of the glass-ceramic, are decisive in reconciling the desired advantageous manufacturing properties of the glass with a color. low, high light transmission and low diffusion of the glass ceramic, especially when working with short ceramization times.

The alkalis Na2O, K2O and the alkaline earth metals CaO, SrO, BaO are enriched not only in the residual vitreous phase between the crystals but also on the surface of the glass ceramic. During ceramization, a vitreous surface layer is formed, approximately 100 to 1000 nm thick, which is virtually free from crystals and which is enriched with these elements and depleted in L12O. This vitreous surface layer has beneficial effects on the acid resistance of the glass ceramic surface. To this end, the cumulative contents of alkalis and alkaline earths, with their limits, play an important role. For a sufficient thickness of the glass layer of at least 50 nm, the minimum contents of the two categories of components are required. Higher cumulative contents above the upper limits, for one of the two categories, give greater thicknesses of the vitreous layer, which presents disadvantages for the strength of the glass ceramic.

The ZnO component is inserted into the mixed crystal phases, and a part also remains in the residual glassy phase. This component is advantageous for lowering the processing temperature of the glass and for reduced diffusion for short ceramization times. The ZnO content should be at least 0.3% by weight. Due to the tendency of evaporation from molten glass and because of the problem of formation of unwanted crystal phases, such as Zn spinel (gahnite), during ceramization, the ZnO content is limited to values of up to 2.2% by weight. Preferably, the content is at least 1.1% by weight and at most 2.1% by weight.

The TiCh component, as an effective nucleating agent, is essential for the transparency of the glass ceramic. It gives high nucleation rates and therefore sufficient seed formation and thus a low average crystallite size, even for short ceramization times. This makes it possible to obtain glass-ceramics without visually disturbing diffusion, even with short ceramization times. However, higher TiCh contents are critical due to the formation of Fe / Ti color complexes and Sn / Ti color complexes. Therefore, the proportion of TiCh should be 1.8 to <2.5% by weight. Preferably, the minimum content of TiCh is> 1.9% by weight. It is particularly advantageous to have a TiCh content of at least 2% by weight. Other predicted nucleating agents are ZrCh and SnCh. The ZrCh content is limited to less than 2.1% by weight, preferably less than 2% by weight, since higher contents deteriorate the melting behavior of the batch mixture during the manufacture of glass and, with flow rates basin, can lead to residues in the glass. On the other hand, during shaping, there may be devitrification due to the formation of crystals containing Zr. Since this component avoids higher contents of the other TiCh nucleating agent, it is advantageous for the preparation of a low color c * glass ceramic. Its minimum content is 1.6% by weight.

In the context of the invention, the choice of the SnCh content is of particular importance. SnCh is both a nucleating agent and a refining agent and therefore exerts a great influence on many properties; it is particularly critical for the color of the glass ceramic, due to the formation of the Sn / Ti color species during crystallization. To accommodate the requirement of both sufficient refining effect and low color, the SnCh content is within a narrowly limited range and is 0.2-0.35% by weight. Preferably, the SnCh content is at most 0.33 wt%, and most advantageously it is at most 0.31 wt% to further reduce the risk of devitrification of Sn-containing crystals. For good ripening, a minimum content of more than 0.25% by weight is particularly advantageous. After refining, it is considered that good bubble qualities are in particular those having a bubble number of less than 5, preferably less than 2 bubbles / kg in the starting crystallizable glass or in the glass ceramic (measured from sizes of bubbles greater than 0.1 mm in one dimension).

The sum of the TiCh + ZrCh + SnCh nucleating agents should be 3.8 to 4.8% by weight. The minimum content is necessary for sufficiently rapid nucleation. Preferably, the minimum content is 3.9% by weight to further reduce diffusion during rapid ceramization. The upper limit of 4.8% by weight results from the requirement for resistance to devitrification.

To improve meltability and resistance to devitrification during forming, there may be a P2O5 content of up to 3% by weight. Higher contents have disadvantages for chemical resistance and cause diffusion of the glass ceramic during short ceramization times. A preferred lower limit is 0.01 wt%, and a preferred upper limit is 1.5 wt% P2O5. It is particularly advantageous to have an upper limit of 0.8% by weight for the content of P2O5 P2O5 and a lower limit of 0.02% by weight.

Likewise, the addition of up to 1% by weight of B2O3 improves the meltability and resistance to devitrification, but it has drawbacks for the resistance to temperature and time of the glass ceramic. Preferably, the glass ceramic is technically free from B2O3, ie the contents are less than 500 ppm.

Due to the high costs of iron-poor batch batch raw materials, it is uneconomical to limit the Fe203 content of crystallizable glass to values below 0.01% by weight, i.e. less than 100 ppm. Since in the recycling of cullet, there is input of iron through grinding, the Fe203 content is greater than 0.01% by weight. On the other hand, along with the Fe2Ü3 content, the concentration of Fe / Ti color complexes in the glass ceramic also increases. The color (chromatic intensity) c * is increased, and the light transmission Y (brightness) is reduced by absorption. For this reason, the crystallizable glass and the glass ceramic made from it should contain a maximum of 0.02% by weight, preferably up to 0.018% by weight of Fe2O3. For the same reason, due to the deterioration of light transmission and color, the glass preferably does not contain other coloring compounds, such as those of Cr, Ni, Cu, V, Mo, S, to except for inevitable impurities. The contents are generally between 2 and less than 20 ppm.

The ratio of MgO / SnCh components is of particular importance for reconciling refinability, transparency and good manufacturing properties. For the adaptation of the refractive indices of the crystalline phase and of the residual vitreous phase, in order to avoid diffusion for short ceramization times, the present invention makes it possible to find new solutions. Thus, it is possible to further improve the color c * and the light transmission Y, without any disadvantages for economical manufacture, or, to formulate it otherwise, higher SnCh contents do not. do not impermissibly deteriorate the properties.

The glass according to the invention has a ratio between the components MgO and SnCh (both in% by weight) which ranges from greater than 0.15 to less than 1.7 (condition B1). This is an essential condition for reconciling the desired good manufacturing properties of the glass with a low color, high light transmission and low diffusion of the glass ceramic, even for short ceramization times. Advantageously, the ratio between the MgO and SnCh components is greater than 0.4 and less than 1.7 (condition B2), and particularly advantageously at least 0.7. It is advantageous when the ratio is less than 1.5 and in particular between 0.7 and a value less than 1.3. Within these preferred ranges, in connection with the compositions specified, the desired effect of reducing color and light transmission to a minimum is particularly advantageously obtained with rapid ceramization combined with low temperatures of. implementation Va.

According to a preferred embodiment, both the crystallizable lithium aluminosilicate glass according to the invention and the glass-ceramic which can be manufactured or manufactured from the glass contain the following components as main constituents (in% by weight on a oxide base):

with condition B2:

In transparent glass-ceramics made from lithium aluminosilicate glasses according to the invention, the undesirable color, originating from Fe / Ti and / or Sn / Ti color complexes, is reduced, according to a preferred embodiment, by additions of Nd203 with contents ranging from 0.005% by weight (50 ppm) to 0.2% by weight (2000 ppm). Below 50 ppm Nd20, the bleaching effect is weak, and above 2000 ppm, light transmission is undesirably degraded by absorption of the Nd bands in the visible light range. To optimize these two opposite effects, the Nd203 content is preferably greater than 100 ppm and in particular ranges up to 1500 ppm. Other preferred lower limits for Nd203 are 0.03% by weight and especially 0.05% by weight.

For the optimization concerning the reduced color of the glass ceramic, it has been found to be advantageous when the condition Nd2Ü3 / Fe2Ü 3> 2 is fulfilled for the contents of the components Nd2O3 and Fe203 (both in% by weight). In order not to interfere too much with the light transmission, it is moreover advantageous that the contents Nd203 / Fe2O3 <9.

Additions of CoO, in amounts up to 30 ppm, can promote discoloration. Preferably, the CoO content ranges from 0.1 ppm to 20 ppm CoO. The upper limits for CoO are based on the fact that it has only partial bleaching action and, in higher proportions, also causes a reddish tint in transparent glass ceramics.

Preferably, halide compounds are not added as refining aids. These evaporate during melting and enter the atmosphere of the melting tank and lead to the formation of corrosive compounds, such as HF, HCl and HBr. These have drawbacks due to the corrosion of the refractory stones in the melting tank and in the off-gas discharge duct. Therefore, with the exception of unavoidable traces, glasses and glass-ceramics are free from F, Cl, Br, and the individual contents of these are below 500 ppm.

According to another embodiment, the crystallizable lithium aluminosilicate glass and the article made therefrom, of green glass or, after processing, of glass-ceramic, preferably has a composition which contains in% by weight , on an oxide basis:

with condition B2:

To further improve the economical manufacture with high light transmission, reduced color and low diffusion, with short ceramization times, the crystallizable lithium aluminosilicate glass and the article made therefrom have a composition particularly advantageous which contains in% by weight, on an oxide basis:

with condition B2:

The above compositions are to be understood in the sense that the listed components represent at least 98% by weight, in general 99% by weight, of the total composition. Compounds of a multitude of elements, such as F, Cl, alkalis Rb, Cs or elements like Mn, Hf, constitute impurities which are common with vitrifiable raw materials, used on an industrial scale. Other compounds, for example those of the elements W, Nb, Ta, Y, Mo, rare earths, Bi, V, Cr, Ni, may be contained in low proportions, typically of the order of ppm.

Depending on the choice of raw materials for the vitrifiable mixture and the conditions of the process during melting, the water content of the crystallizable glasses intended for the manufacture of glass-ceramics is preferably between 0.015 and 0.06 mol / l. This corresponds to β-ΟΗ values ranging from 0.16 to 0.64 mm "1. During processing into glass ceramic, the IR band, which is used for the determination of the water content, changes. This changes the value. β-ΟΗ for glass ceramic, without changing the water content This process and the method for determining β-ΟΗ values are described in EP 1 074 520 A1.

Crystallizable glasses are transformed into glass-ceramic by the multi-step thermal process described below.

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

For minimizing diffusion, it is advantageous to reduce crystallite sizes to a minimum. However, this has heretofore required relatively long nucleation and ceramization times which have been economically disadvantageous. With the composition in accordance with the invention, glass-ceramics are obtained in which the refraction indices of the residual glass and of the crystalline phase are suitable.

This makes it possible to have higher average crystallite sizes for mixed quartz-β crystals than with known compositions. Preferably, the mixed quartz-β crystals of the glass ceramic have, after ceramization, an average crystallite size of at least 30 nm. Particularly advantageously, the average size of the crystallites is greater than 35 nm. The upper limit, which is preferred due to increasing diffusion, is an average crystallite size of less than 50 nm.

The proportion of crystalline phase of the mixed quartz-β crystals of the glass ceramic is preferably at least 65% by weight and preferably at most 80% by weight. This range is advantageous for obtaining the desired mechanical and thermal properties of the glass ceramic.

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

The measurement of the diffusion (in English: haze) according to ASTM D1003-13 gives, for polished samples with a thickness of 4 mm of the LAS glass ceramic, turbidity values which are preferably less than 2.5, in particular less at 2% and particularly advantageously less than 1.5%. Thus, the transparent glass-ceramic according to the invention does not exhibit visually disturbing light scattering, and therefore the transparent view of objects and light displays is not impaired. The indications of the displays located under the ceramic hob can thus be clear and the contours can be seen clearly and without turbidity.

The characteristics 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 was obtained, with a suitable rapid ceramization, with a total duration of less of 300 minutes, as described with the process according to the invention and in the examples.

The thermal expansion of the glass ceramic is generally regulated in a temperature range between room temperature and approximately 700 ° C, to values around 0 + 0.3 · 10 "6 / K. However, it has been found that negative values are advantageous for color optimization, as they allow a wider range of compositions with low MgO contents and condition B1. Preferably the thermal expansion is 0120/700 <-0.25 · 10 " 6 / K and very particularly <-0.32 · 10 "6 / K. The thermal expansion 0120/700 is greater than -0.6, preferably greater than -0.5 · 10" 6 / K, because in the otherwise, the thermally induced stresses between cold and hot zones become too great during the heating of the glass-ceramic article. Negative expansion provides access to additional applications in the athermalization of optical elements, without questioning the use with conventional applications, because the deviation from zero expansion is small.

According to another embodiment, the glass ceramic contains mixed crystals of keatite as the main crystalline phase. For economic reasons, it is advantageous when the same composition of the crystallizable lithium aluminosilicate glass makes it possible to produce both transparent glass-ceramics, with mixed quartz-β crystals as the main crystalline phase, and glass-ceramics with crystals. mixed keatite as the main crystal phase. The development of the ceramization program, in particular the choice of the maximum temperature and the holding time, makes it possible to adjust the appearance for the latter, which can range from transparent to opaque, including translucent. The average crystallite size is preferably greater than 60 nm. The proportion of crystalline phase is greater than 60% by weight and preferably greater than 80% by weight. The combinations of different properties of the two types of glass-ceramic make it possible to cover a large number of applications, in an economical manner.

The preferred geometry for the glass-ceramics according to the invention and the articles made from them is in the form of plates. The plate preferably has a thickness ranging from 2 mm to 20 mm, as this gives access to important applications. Smaller thicknesses are detrimental to strength, and higher thicknesses are less economical due to the greater material requirements. Therefore, except for the application as a safety glass, where strength plays an important role, the thickness is preferably chosen less than 6 mm.

Suitable forming methods for the plate geometry include rolling and floating. The preferred forming process from molten glass is the two-roll one, as this process has advantages due to the faster cooling, when the compositions have a tendency for surface crystallization. The preferred article according to the invention is a laminated ceramic hob.

The glass ceramic plate and articles preferably made therefrom can not only be formed in a planar manner but also in three dimensions. For example, chamfered, angled or domed plates can be used. The plates may be at right angles or in other shapes and include not only flat areas but also three-dimensional shaped areas, for example woks or ribs or surfaces produced by rolling, as protrusions. or hollow. The geometric deformations of the plates are produced during hot forming, for example by structured forming rolls or by hot forming downstream on the starting glasses, for example using burners, ray emitters infrared or gravity sinking. During ceramization, one works with auxiliary ceramic forms, for example flat supports, to avoid uncontrolled modifications of the geometric form. Subsequent polishing of one or both sides is possible, if the application requires.

The process according to the invention for the manufacture of crystallizable lithium aluminosilicate glass is characterized by the following steps: a) provision of a vitrifiable mixture of technical raw materials, containing from 20 to 80% by weight of cullet ; b) melting and refining of the batch mixture in a conventional unit, at temperatures below 1750 ° C; c) cooling of the molten glass and shaping, to temperatures close to the processing temperature Va, achieving the desired shape of the article, d) cooling in an annealing furnace, to room temperature , by eliminating unwanted stresses in the glass.

The vitrifiable mixture is produced in such a way that after melting, a glass is obtained having the composition and the properties in accordance with the invention. The addition of cullet promotes melting, and it is possible to achieve higher pond flow rates. An additional high temperature refining unit is not used, and the maximum temperature of the molten glass in the melting basin is less than 1750 ° C, preferably less than 1700 ° C, which is compatible with conventional technology. During shaping, a ribbon of glass with a plate-shaped geometry is preferably produced using cylinders, and to avoid stress, it is cooled to room temperature in an annealing oven. After ensuring the quality in terms of volume and surface defects, plates are made from this ribbon of glass with the desired dimensions. The next step in the process is ceramization on flat or three-dimensional, mostly ceramic supports, which is preferably performed in a roller kiln.

With the process according to the invention for ceramization, the crystallizable, thermally expanded starting glass is heated over a period of 3 min to 60 min, up to the temperature range of approximately 680 ° C. The high heating rates required can be achieved on an industrial scale in roller furnaces. This temperature range of around 680 ° C roughly corresponds to the processing temperature of glass. Above this temperature, up to about 800 ° C, is the range with high nucleation rates. The nucleation temperature range is covered for a period of 10 minutes to 100 minutes. Then the temperature of the glass, which contains crystallization seeds, is increased in 5 to 80 minutes to a temperature ranging from 850 ° C to 950 ° C, which is characterized by high crystal growth rates of the crystal phase mixed quartz-β. This maximum temperature is maintained for up to 60 minutes. Thus, the structure of the glass ceramic is homogenized, and the optical, physical and chemical properties are adjusted. The glass ceramic obtained is cooled in total in less than 150 minutes to room temperature. Preferably, the cooling to about 700 ° C is carried out at a slow rate. In total, the ceramization time is less than 300 minutes, preferably less than 200 minutes and particularly advantageously less than 150 minutes.

Preferably, the transparent lithium aluminosilicate glass-ceramic, with mixed crystals of quartz-β as the main crystalline phase, is used as a fireplace glass, fireproof glass, oven glass (in particular for pyrolysis furnaces), plan baking, optionally provided with an underside coating, as a covering in the field of lighting, as a safety glass, optionally in a laminated composite structure, as a support plate or an oven coating in thermal processes.

For fireplace windows, we want a good view of the combustion chamber and the flames. For hobs with colored coating on the underside, the color of the coating must not be affected by the color of the glass ceramic. For the uses mentioned, it is preferable to obtain the high values, in accordance with the invention, of the light transmission (brightness) Y and the low color c *, in relation to the low light scattering, barely perceptible. .

These uses are also possible with a lithium aluminosilicate glass ceramic having mixed crystals of keatite as the main crystalline phase. The application of an opaque coating on the upper and / or lower face makes it possible to produce, from the transparent glass ceramic, a colored hob with the required covering to prevent the technical elements integrated under the hob from being seen. The heating of the hob is carried out in a conventional manner with gas burners, radiant heating elements or by induction.

Recesses in the coating allow for the installation of sensor areas, colored and white displays, and display screens.

It is possible to combine coatings on the upper side and on the lower side of the glass ceramic hob and also integrate semi-transparent layers. Likewise, it is possible to apply markers, for example for households. For this purpose, the various types of known coatings can for example be combined with organic or inorganic decorative colors, luster colors, silicone and sol-gel-based colors, layers produced by cathode sputtering, metallic layers. , oxynitride, oxycarbide and so on. It is also possible to overlay layers.

For windows of chimneys or ovens, it may also be desirable to provide coatings, for example for an opaque covering on the edge of the panes. The arrangement of the coatings on the upper or lower face of the glass-ceramic plate is carried out according to aesthetic and specific requirements with regard to chemical and physical properties.

Displays are made up of light-emitting electronic components, such as light emitting diodes, OLEDs, liquid crystal displays, or fluorescent displays. All forms of display are possible, for example as points, areas or 7 segments. The emission spectra of radiant displays can have one or more maximums and large ranges, so that the displays appear in color (eg, blue, purple, red, green, yellow, orange) or white. Due to the low color c * of glass ceramic, it is also possible to manufacture display devices or screens in black and white and in color, without disturbing color alterations. The color of visualization elements is optionally changed by color filters or color channels. This makes it possible to influence and possibly correct the shade of the displays, if it is modified by the glass ceramic. Likewise, it is possible to install control, sensor and control elements, for example of the capacitive or inductive type, in the glass-ceramic hob.

The glass-ceramic comprising mixed crystals of keatite as the main crystalline phase is preferably used, in a translucent or opaque form, as a hob, support plate for thermal processes (setterplate), cover plate in oven. microwave or lining of combustion chambers. The light transmission is then preferably less than 15%. Other preferred uses for the two glass ceramics, whether mixed crystal quartz-β or mixed crystal keatite, are as a backing plate or as a furnace liner. In the ceramic, solar or pharmaceutical industry or in medical technology, they are suitable in particular for production processes under very clean conditions, as linings for furnaces in which chemical or physical coating processes are carried out, or as chemical resistant laboratory equipment. On the other hand, they are used as glass ceramic object for high temperature or extremely low temperature applications, as glass for incinerators, as heat shield for hot environments, as cover element for reflectors, projectors, video projectors. , photocopiers, for applications with thermomechanical constraints, for example in night vision devices or as a cover element for heating elements, in particular as a cooking or grill surface, as household appliances, as a cover element for radiators, as a wafer substrate, as a UV protected object, as a faceplate or as a material for housing components, e.g. electronic devices and / or cover glasses for IT, e.g. mobile phones, laptops glass, scanners etc., or as a component of ballistic protection. Other preferred uses for articles which include glass-ceramics with mixed crystals of keatite as the main crystalline phase and whose thermal expansion is less than 0 ± 0.5 10 "6 / K between 25 ° C to 50 ° C, are those as a precision component at room temperature, for example as a spacing standard or as a wafer holder, as a mirror support material for reflective optical components, for example in astronomy and in liquid crystal lithography or EUV, or as a laser gyroscope The choice of ceramization conditions makes it possible to manufacture, from the compositions in accordance with the invention, glass-ceramics with low thermal expansion at room temperature.

The transparent glass-ceramics of the present invention sufficiently satisfy the requirements in terms of transparency, both with regard to reduced color and high light transmission, in terms of low diffusion, high chemical resistance, mechanical resistance, resistance to water. temperature and long-term stability, with respect to modification of their properties (eg thermal expansion, transmission, stress formation).

The present invention will be illustrated in more detail with the aid of the following examples. The only figure represents the transmission curve of the glass ceramic of Example 10.

The crystallizable glasses were melted from batch batch raw materials, commonly used in the glass industry, for 4 hours at temperatures of 1,620 ° C. As raw materials were used lithium carbonate, aluminum oxide, aluminum hydroxide, aluminum metaphosphate, sand, sodium nitrate and potassium nitrate, magnesium oxide, lime, strontium carbonate, barium carbonate, zinc oxide, tin oxide, titanium oxide, zirconium silicate and oxide of neodymium. This choice makes it possible to reconcile the requirements concerning economical raw materials and a low content of impurities of coloring compounds of Fe, Ni, Cr, V, Cu, Mo, S. After the melting of the vitrifiable mixture in glass crucibles of sintered silica, the baths were poured into Pt / Rh crucibles with an internal crucible and homogenized by stirring at temperatures of 1600 ° C. for 60 minutes. After this homogenization, the glasses were refined for 2 hours at 1640 ° C. Then, elements of a size of about 120 x 140 x 30 mm2 were cast and cooled in an annealing furnace, starting at 660 ° C and up to room temperature, with a view to releasing them. constraints. The castings were separated to have the dimensions necessary for testing and ceramization.

For certain examples, Table 1 indicates the compositions and the properties of the crystallizable glasses. In addition to the component contents, they also contain the component ratios that are of interest to us here. The glasses 11 are glasses according to the invention, that is to say exemplary embodiments, and the glasses 12 to 14 are comparison glasses which do not fall within the scope of the present invention, c that is, they are examples of comparison. The compositions of the comparison glasses do not form part of the invention and have the drawbacks described in terms of the manufacturing properties (melting and processing temperature, resistance to devitrification) and / or of the color, after processing. in glass ceramic. Due to typical impurities in batch batch raw materials used on an industrial scale, the compositions do not add exactly to 100% by weight. Typical impurities, although not intentionally introduced into the composition, are F, Cl, B, Mn, Rb, Cs, Hf, which are generally less than 0.2% by weight. They are frequently introduced unintentionally by the raw materials for related components, for example Rb and Cs by the raw materials Na and K, or Hf by the raw material Zr.

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

Table 1 also indicates the properties in the glassy state, namely the transformation temperature Tg [° C], the processing temperature Va [° C], the temperature 102 [° C], the upper limit of devitrification. OEG [° C], resistance to devitrification Va -OEG and density [g / cm3]. To measure OEG, the glasses are melted in Pt / Rh 10 crucibles. Then, the crucibles are kept for 5 hours at different temperatures in the range of the processing temperature. The highest temperature at which the first crystals form on the contact surface of the molten glass with the crucible wall determines OEG.

Table 2 relates to the glass-ceramics comprising the mixed crystals of quartz-β as the main crystalline phase, which are made from the glasses of Table 1. Examples 1 to 14 are embodiments, and Examples 15 to 18 are comparison examples.

Table 2 contains properties of the glass-ceramics for the respective ceramization programs indicated, i.e. program 1 or program 2, i.e. spectral transmission (standard light C, 2 °, 4 mm thick) [%], at 400 nm, infrared transmission (standard light C, 2 °, 4 mm thick) [%], at 1600 nm, thermal expansion between 20 ° C and 700 ° C [10 "6 / K ] and the phase content [% by weight], measured by X-ray diffraction, of the main crystalline phase, consisting of mixed quartz-β crystals, as well as the average crystallite size [nm]. Table 2 also indicates the light transmission Y and the colorimetric coordinates L *, a *, b * of the CIELAB system, as well as the quantity c * as a measure of the color (chromatic intensity) and the turbidity value as a measure of the scattering.

Table 3 shows Examples 19 to 21 for glass-ceramics comprising mixed crystals of keatite as the main crystalline phase, made from glass 8 of Table 1. In addition to the quantities in Table 2, the chromatic values for the color are also presented. reflectance measurement. On the other hand, the appearance of the examples is described qualitatively.

According to the ceramization program 1, it is heated up to a temperature of 600 ° C in the ceramization oven, in 20 minutes. Then we continue to heat. Total time from room temperature up to 680 ° C is 36 min. The temperature range of 680 ° C to 800 ° C is important for nucleation. It is for this reason that the heating is continued. The total time between 680 ° C and 800 ° C is 64 minutes. With this ceramization program, a holding time of 30 min is set up in the nucleation range, at a temperature of 750 ° C. Above about 800 ° C crystallization of the desired quartz-β mixed crystal phase occurs. In this range, the formation of unwanted Fe / Ti and Sn / Ti color complexes is also enhanced. The total time from 800 ° C until the maximum temperature of 903 ° C is reached is 41 min. At the maximum temperature of 903 ° C, with a holding time of 10 min, the composition of crystals and residual glass is set, and the microstructure is homogenized. Thus, the chemical and physical properties of the glass ceramic are regulated. Cooling takes place in a controlled manner to 700 ° C, and then the sample is quenched to room temperature, opening the oven door; So, in summary:

Ceramization program 1 (ceramization time 185 min): a) rapid heating of the ambient temperature up to 600 ° C, in 20 min, b) temperature increase from 600 ° C up to the nucleation temperature of 750 ° C, within 30 mins (heating rate 5 ° C / min), holding time 30 min, c) temperature rise from 750 ° C up to the maximum temperature of 903 ° C, within 61 min (heating rate 2.5 ° C / min), holding time of 10 min at maximum temperature, d) cooling within 34 min, from 903 ° C to 700 ° C (at 6 ° C / min), then rapid cooling to room temperature.

The ceramization program 2 is optimized for the compositions according to the invention, aiming for short ceramization times and a lower c * color. Compared to program 1, the nucleation times should only be shortened to such an extent that the diffusion is not visually perceptible. In a laboratory oven that allows rapid heating rates, heating to 720 ° C takes place in 25 minutes. 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 and 800 ° C is 21.4 min. The duration in this temperature range is chosen such that there is no visually disturbing diffusion.

Above 800 ° C and up to the maximum temperature of 890 ° C, the heating rate is reduced, because crystallization takes place in this range. The accompanying shrinkage process should not take place too quickly, as this can lead to defects in the flatness of the article. The total time from 800 ° C until the maximum temperature of 890 ° C is reached is 53 min, with a subsequent holding time of 10 min. The composition of the crystals and the residual glass is adjusted, and the microstructure is homogenized. The duration in this temperature range is optimized for good flatness of the glass ceramic plates. Cooling takes place in a controlled manner to 600 ° C, then the sample is quenched until it reaches room temperature, opening the oven door; So, in summary:

Ceramization program 2 (ceramization time 137 min): a) rapid heating of the ambient temperature up to 720 ° C, in 25 min, b) temperature increase, from 720 ° C to 800 ° C, in 20 min ( heating rate of 4 ° C / min), c) temperature increase, from 800 ° C to 890 ° C, in 53 min (heating rate of 1.7 ° C / min), holding time from 10 min to maximum temperature, d) cooling over 29 min from 890 ° C to 600 ° C (at 10 ° C / min), then rapid cooling to room temperature.

With an additional ceramization program 3, the transformation of crystallizable glass No. 8 (Table 1) into glass-ceramics was carried out with mixed crystals of keatite as the main crystalline phase. With this program, we proceeded as with program 2, up to 890 ° C. Then, deviating from program 2, it was heated, without holding time at 890 ° C, at a heating rate of 10 ° C / min, up to a maximum temperature T max clVCC a holding time tmax ( see the indications in table 3). From the maximum temperature, it was cooled at a rate of 5 ° C / min, to 800 ° C, then rapidly to room temperature.

The transmission measurements were carried out on polished plates with a thickness of 4 mm, with a standard light C, 2 °, using the Perkin-Elmer Lambda 900 apparatus. From the spectral values measured in the range between 380 nm and 780 nm, which represents the visible light spectrum, the Y light transmission is calculated according to DIN 5033 for the type of standard light C chosen and the viewing angle of 2 °. From these measurements, the colorimetric coordinates L *, a *, b * of the CIELAB system and the quantity c * are also calculated. The reflectance measurement was carried out according to DIN 5033, also with these parameters.

The diffusion of glass-ceramics is determined by measuring the turbidity (in English: haze). For this purpose, samples with a thickness of 4 mm, polished on both sides, are measured with a spectrophotometer (method B of standard ASTM D 1003-13) with standard light C, for the spectral range of 400 at 800 nm. Diffusion is characterized by the turbidity value in Table 2.

Table 1: Compositions and properties of crystallizable glasses * not measured

Table 1 (continued): compositions and properties of crystallizable glasses

Table 2: Ceramization conditions and properties of glass-ceramics comprising mixed crystals of quartz-β as the main crystalline phase

* not measured

Table 2 (continued): Ceramization conditions and properties of glass-ceramics comprising mixed quartz-β crystals as the main crystalline phase

* not measured

Table 3: Ceramization conditions and properties of glass-ceramics comprising mixed crystals of keatite as the main crystalline phase

Claims (19)

1. Crystallizable lithium aluminosilicate glass, intended for the manufacture of transparent glass-ceramics, characterized in that, except for inevitable impurities due to raw materials, it is free of arsenic and antimony, which it contains the following components (in% by weight on an oxide basis): with condition B1: 2. Crystallizable lithium aluminosilicate glass according to claim 1, characterized in that it contains the following components (in% by weight on an oxide basis): with condition B2: 3. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that it contains 0.005 to 0.2% by weight of Nd2Ü3. 4. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that for the components Nd2Ü3 and Fe2Ü3, condition 5 is applied. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that for the components Na2Ü and K2O, the condition is applied 6. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that the proportion of CaO is at least 0.28% by weight. 7. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that for the components CaO, BaO and SrO, the condition is applied 8. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that the proportion of P2O5 is from 0.01 to 1.5% by weight. 9. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that it contains the following components (in% by weight on an oxide basis): with condition B2: 10. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that it contains the following components (in% by weight on an oxide basis): with condition B2: 11. Crystallizable lithium aluminosilicate glass according to one of the preceding claims, characterized in that it has • a temperature 102 of less than 1770 ° C and / or • an implementation temperature Va of at most 1330 ° C and / or • an upper limit of OEG devitrification which is at least 10 ° C lower than the processing temperature Va. 12. Glass-ceramic transformed from a lithium aluminosilicate glass according to one of claims 1 to 11. 13. Glass-ceramic according to claim 12, characterized in that it contains mixed crystals of quartz-β as the main crystalline phase. 14. Glass-ceramic according to claim 13, characterized in that, for a thickness of 4 mm, it has a Y light transmission of more than 82% and / or a color c * of at most 5.5. 15. Glass-ceramic according to claim 13, characterized in that, for a thickness of 4 mm, the light transmission Y is greater than 82% and the color c * is at most 5.5, for a visually barely perceptible diffusion. with a turbidity value <2%. 16. Glass-ceramic according to claim 12, characterized in that it contains mixed crystals of keatite as the main crystalline phase. 17. Glass ceramic according to one of claims 12 to 16, characterized in that it is in the form of a plate with a thickness ranging from 2 mm to 20 mm. 18. A method of manufacturing a crystallizable lithium aluminosilicate glass according to one of claims 1 to 11, characterized in that it comprises the following steps: a) providing a composition of vitrifiable mixture of materials first techniques, containing 20 to 80% by weight of cullet; b) melting and refining of the batch mixture in a conventional unit, at temperatures below 1750 ° C; c) cooling the glass bath and shaping, to temperatures close to the processing temperature Va, and d) cooling in an annealing furnace, to room temperature. 19. A method of manufacturing a glass-ceramic according to one of claims 13 to 15, characterized in that the ceramization is carried out with the following program: a) increase in the temperature of the crystallizable glass, up to the temperature range d about 680 ° C, over 3 to 60 minutes; b) increasing the temperature of the crystallizable glass, within the nucleation temperature range of 680 to 800 ° C, for a time period of about 10 to 100 minutes; c) increasing the temperature of the glass containing seed crystals over a period of 5 to 80 minutes to the high speed crystal growth temperature range of 850 to 950 ° C; d) maintaining in the temperature range, at the maximum temperature of 850 to 950 ° C, up to 60 minutes, where crystals of the mixed crystal type of quartz-β grow on the crystallization seeds, and then; e) rapid cooling of the glass ceramic obtained to room temperature in less than 150 minutes, the ceramization of the glass having a total duration of less than 300 min. 20. Use of an object which comprises a glass-ceramic plate according to claim 17, as fire-resistant glass, fireplace glass, oven glass, hob, optionally with a coating on the underside, covering. in the field of lighting, as safety glass, optionally in a laminated composite structure, as a support plate or furnace lining in thermal processes.