DE102016211065B4 - Transparent, preferably colored glass ceramic article with low scattered light content and high strength, as well as processes for its preparation and its use - Google Patents

Abstract

Transparent, preferably with the aid of color oxides colored glass ceramic article with a scattered light content determined at a wavelength of 470 nm and normalized to a glass-ceramic material of 4 mm thickness of less than 70%, preferably less than 40%, more preferably less than 25%, comprising one first near-surface layer and an underlying core, wherein the core comprises keatite mixed crystals as the main crystal phase and wherein the core is present at a depth from 100 microns, as measured from the surface of the glass-ceramic material, the near-surface layer has a thickness of at least 10 microns, preferably has at least 20 microns, and the glass ceramic article contains at least 0.1- <4%, preferably 0.3 to 2% B ₂ O ₃ .

Description

The invention relates to a transparent, preferably colored glass ceramic with a near-surface layer and an underlying core, wherein the core comprises keatite mixed crystals as the main crystal phase, and a process for their preparation and their use.

Lithium aluminosilicate glass-ceramics (LAS glass ceramics) with low thermal expansion, good thermal shock resistance and high strength and their use, especially as cooking surfaces, have been known for many years.

A special role is played by the appealing design of the glass ceramic. Thus, the transparency of the glass ceramic can be adjusted over a wide range by means of adapted ceramization: Highly transparent glass ceramics with high quartz mixed crystals (HQMK) as the main crystal phase are just as possible as translucent or opaque glass ceramics with keatite mixed crystal (CMC) as the main crystal phase. In addition, coloring is through the use of color oxides such as V ₂ O ₅ , Fe ₂ O ₃ , Cr ₂ O ₃ , MnO ₂ , CoO, NiO, CuO, MoO ₃ , CeO ₂ , Nd ₂ O ₃ or other coloring components such as metals , rare earths and their oxides possible.

Due to the increasing use of LEDs and display components below the cooking surface, preference is given to using glass ceramics with little scattering, which have high quartz as their main crystal phase.

In contrast, glass ceramics with keatite mixed crystal as the main crystal phase often have a higher scattering or are even opaque, ie impermeable to visible light.

It is also known that keatite glass-ceramics, if they have a surface layer which differs from the core of the glass-ceramic microstructure, especially if this surface layer contains high-quartz mixed crystals, have particularly high strength values (as for example in the German Offenlegungsschrift DE 10 2010 006 232 A1 described), for example, allow the production of thinner glass-ceramics with sufficient strength for the application.

With the increasing use of glass ceramics, the importance of cost reduction in production is also increasing. In LAS glass-ceramics, two aspects are essential here: On the one hand, these are the raw material costs, with the content of lithium oxide making up a substantial proportion of the batch costs. On the other hand, the ceramization time and the ceramization temperatures are an important factor in terms of energy costs. This leads to an increase in production costs, in particular with keatite glass ceramics which require a higher maximum ceramization temperature than comparable high quartz glass ceramics.

A more cost-effective production of glass-ceramics could thus be achieved by reducing the lithium content, reducing the ceramification temperature and / or time, and saving material, e.g. a reduction in the thickness of the glass ceramic can be achieved. However, this is countered by the challenge that all the service properties of the glass-ceramic, e.g. Transparency, strength, thermal shock resistance must be maintained.

The British patent application GB 1095254 A discloses glass-ceramics in the system Li ₂ O-Al ₂ O ₃ -SiO ₂ with the nucleating agents TiO ₂ , ZrO ₂ and SnO ₂ as nucleators. Described is the production of translucent keatite glass ceramics. The material tends to devitrify and must be made in a slow uneconomical manufacturing process.

Also the British patent application GB 1108450 A describes keatite mixed crystal glass-ceramics. They are opaque and have no strength-enhancing surface layer.

The British property right GB 1 268 845 discloses a method for producing a crystallized glass which comprises an outer layer which builds a compressive stress and an inner layer. The outer layer comprises crystallites of high quartz mixed crystal, the inner layer crystallites of keatite. The crystallized glass has a minimum content of fluorine of 0.01 wt .-% and has a high flexural strength. However, no statements are made about the optical properties, such as transmission and / or scattering. Also, the fluorine content of these glass-ceramics, although useful against the background of a reduced temperature of keatite formation in terms of production technology, is to be regarded as critical from an environmental and safety point of view and should therefore be avoided as far as possible. Because the Moreover, the formation of keatite by fluorine is accelerated, making it difficult to produce a transparent material with low scattering.

This also shows, for example US 2004/110623 A , With a fluorine content between 1 and 3 wt .-%, although the temperature at which keatite is formed, only between 780 ° C and 835 ° C, but only opaque keatite glass-ceramics are obtained in this way.

Transparent LAS glass ceramics with a red coloration, which may also contain keatite as the main crystal phase, are the German patent DE 24 29 563 C2 disclosed. The coloring is produced with a mixture of different color oxides (MnO ₂ , NiO, CoO and Fe ₂ O ₃ ). The transformation into keatite glass-ceramics, however, is achieved by ceramization in numerous cycles ( 5 to 6 Cycles of more than 5 hours each), resulting in high process costs.

Further disclosed US 4,218,512 A glass-ceramic articles, which are prestressed by a compressive stress-exerting surface layer. This surface layer is produced by a control of the crystallization process and results in that on the surface of the glass-ceramic articles other crystal forms are present than in the interior. The thus obtained glass-ceramic articles are translucent. In particular, the ceramizing process disclosed in this patent includes rapid cooling of the article as soon as the desired structure has been established. Although transmissions of 70% are possible in this way, no statement is made about the scattering of the glass ceramic. Furthermore, very long ceramization times are required for this process, which include, for example, 21.5 hours, in which case a targeted cooling was not considered. Economic production is thus hardly possible with the proposed process. The use of boron to shorten the ceramization time or reduce the ceramization temperature is also not disclosed.

EP 0 853 071 A1 describes retractable LAS glass ceramics that may contain high quartz or keatite as the crystal phase. These glass-ceramics can contain up to 7% B ₂ O ₃ . The preparation of transparent keatite glass-ceramics and the ceramization conditions used for this purpose are not described.

The British patent application GB 1018384 A discloses the use of B ₂ O ₃ in combination with metal oxides such as MgO and ZnO to improve the melting properties. The nucleating agents used are TiO ₂ , Cr ₂ O ₃ and P ₂ O ₅ . Transparent Keatit glass ceramics with low Streuanteil can not be produced in this way.

The Japanese property rights application JP2001-052334 A describes glass-ceramics for use as a hard disk substrate containing high quartz and keatite as crystal phases. Here, the keatite content is at most 60% of the total crystal phase content. The glass ceramics produced in this way are milky, translucent and have a high degree of scattering.

Further disclosed US 4,285,728 A low-expansion glass-ceramics having as predominant crystal phases high quartz mixed crystals, keatite or both. The glass-ceramics can be both transparent, ie formed with a low dispersion, as well as opaque and continue to be colored or undyed. Furthermore, no statement is made as to which crystal phase is present in which glass ceramic. Also statements about a possible different distribution of crystal phases in the interior of the glass ceramic and at their interfaces is not made.

A glass ceramic with high quartz and / or keatite mixed crystals and high transmission in the near infrared (1050 nm) for use as a cold-light reflector is described in German industrial property law DE 101 10 225 A1 described. However, this glass-ceramic is visibly opaque or translucent as soon as keatite forms as crystal phase.

The American intellectual property right US 7,671,303 BB discloses transparent keatite mixed crystal or high quartz mixed crystal containing glass-ceramics having a white level of L ≥ 82 with Haze values> 50. The material is transparent with white-gray appearance. The production of glass ceramics with less scattering is not explained.

Furthermore, the German Offenlegungsschrift discloses DE 10 2007 025 893 A1 a glass-ceramic armored material. The main crystal phase of the glass ceramic comprises keatite mixed crystal. In this way, depending on the exact crystallization or Keramisierungsbedingungen and the resulting Transmittances up to 0.87 are realized, although it is not specified to which thicknesses such transmission values relate. Furthermore, the glass-ceramic armor material is characterized by a high strength, determined as bombardment. However, the glass ceramics described contain high levels of Li ₂ O of up to 10 wt .-%. Such glass ceramics therefore generally have high thermal expansion coefficients. Thus, such glass-ceramics, despite their high strengths, can not be used in applications with high temperature differences, for example as a chimney pane or as a hob. Furthermore, the formation of a high quartz-containing surface layer is not described.

The production of particularly high strength keatite mixed crystal glass-ceramics by a lesser-expansion surface layer, for example, a surface layer comprising high-quartz mixed crystals, discloses US patents US 3,764,444 A , In this case, the surface layer has a different composition than the core, which is achieved for example by an ion exchange. The simultaneous adjustment of transparency is not disclosed.

US 2005/0153142 A1 discloses glass-ceramics having a surface layer and an underlying core, wherein the surface layer and the core differ in that they have a different degree of crystallinity. Here, the crystallinity in the surface layer is more pronounced by having at least one crystallization-promoting element, for example, Cu, Zr, La, or Zn, having a higher content in the surface layer than in the core of the glass-ceramic. In this way, a pleasing visual impression of the glass ceramic is to be obtained. Although the glass ceramics disclosed therein may also comprise keatite mixed crystals, the preparation of transparent, keatite-comprising glass ceramics is not described.

The German patent application DE 10 2010 006 232 A1 discloses high-performance glass ceramics, which comprise three different microstructures, wherein the first external structure is predominantly glassy, in the second, adjoining microstructure has an average crystallite content and the third, internal microstructure is formed predominantly of a keatite mixed crystal. These high-performance glass-ceramics have high strength and are translucent to opaque. Particularly high strengths are achieved when Gahnit ZnAl ₂ O ₄ is present in the near-surface layers. This reveals the DE 10 2010 006 232 A1 Although glass ceramics with high strengths, but the described particularly high strength values are only achieved for glass ceramics, which include Gahnit. However, the presence of these crystalline phases leads to a very high scattering occurring in the glass-ceramics, which is exemplified in the US Pat DE 10 2010 006 232 A1 expressed as desirable white values of more than L * = 95. The also disclosed high heating rates are further detrimental to the formation of a transparent keatite glass-ceramic. The influence of B ₂ O ₃ on the ceramization and the structure is not disclosed.

Thus, both glass-ceramics containing keatite as the main crystal phase and glass ceramics having a small amount of grit and also glass ceramics having high breaking strength have already been known.

However, the production of such glass-ceramics generally requires high ceramification temperatures and / or long ceramization times, which lead to high energy costs in production, or a high lithium content and the resulting high raw material costs. The alternatively applied methods for subsequent increase in strength, such as thermal or chemical tempering, lead to an increase in the process costs.

The object of the invention is therefore to provide a transparent, preferably colored with the aid of color oxides glass ceramic, which is characterized by a low scattering and a high breaking strength and can be produced inexpensively in terms of energy and raw material use. Further aspects of the invention relate to a process for producing this glass ceramic and to the use thereof.

In the context of the present invention, the following definitions apply:

A glass ceramic is a material which is obtained in an at least two-stage process, wherein the first step is the complete melting of the material-forming components, so that a glass melt and from this by the solidification of the melt, a glassy body is obtained. In this case, the glass melt is composed so that the glass obtained is crystallizable, ie a controlled crystallization accessible. In a second step, the obtained vitreous body is subjected in a temperature treatment to a controlled crystallization, so that a glass ceramic, comprising a glassy phase and at least one crystalline phase. In the context of the present invention, a glass ceramic is also referred to as a glass-ceramic material.

The temperature treatment carried out in the second step of the glass-ceramic production is also referred to as ceramization and in particular comprises a crystallization. In addition to the crystallization of the desired main crystal phase, the ceramization also includes the steps of nucleation, crystal growth and / or the targeted conversion of an already existing crystal phase into a further crystalline phase. In summary, this process of nucleation, crystal growth, and / or crystal transformation can also be referred to as controlled crystallization. In the context of the present invention, the terms of ceramization and controlled crystallization are therefore also used interchangeably. The ceramization is preferably carried out as a one-step process, which consists of the above sections. The individual sections can be represented both by holding times at appropriately selected temperatures and by an adjustment of the heating ramps / cooling ramps. A glass having a composition which is amenable to controlled crystallization or ceramization is referred to in the sense of this invention as green glass or crystallizable glass.

In the context of the present invention, the terms high-quartz glass-ceramic and high-quartz mixed crystal glass-ceramic are used synonymously for glass ceramics having a main crystal phase whose crystalline structure, which can be determined by X-ray diffraction, is high quartz-like. Such a high quartz-like crystalline structure has, in particular, high-quartz and high-quartz mixed crystals as well as β-eukryptite. Therefore, the synonyms "β-quartz" or "β-eukryptite" can be found in the literature as crystal phase designation for this type of glass-ceramics.

Furthermore, in the context of the present invention, the terms keatite glass ceramic and keatite mixed crystal glass ceramic are used synonymously for glass ceramics which comprise as the main crystal phase a crystalline compound which has a keatite-like crystalline structure, determinable by X-ray diffraction. Such a keatite-like crystalline structure has in particular keatite and keatite mixed crystals and β-spodumene. Therefore, the synonym "β-spodumene" can also be found in the literature as crystal phase designation for this type of glass-ceramics.

For the purposes of the invention, a main crystal phase is present if its proportion exceeds more than 50% by volume of the sum of the crystal phase fractions.

A crystallite refers to a crystal or solid solution, ie a solid having a three-dimensionally ordered structure which has small dimensions, i. a maximum diameter of 1 μm. Usually, glass-ceramic materials have crystalline phases, ie, such crystallites, the maximum diameters often being significantly lower. In the context of the present invention, therefore, the terms "crystal", "mixed crystal" and "crystallite" are used largely synonymously.

A colored glass ceramic is a glass ceramic which has absorptions by the addition of one or more color oxides or other coloring components (such as colloid-forming metals) in the visible wavelength range, resulting in a homogeneous color impression for the viewer. In particular, the coloring may be so dark that non-luminous objects do not appear visible or unobtrusive through the glass ceramic, whereas luminous objects such as LEDs or display elements are visible.

As a decorative layer, also decoration or decor, a layer is referred to, which is applied to the glass-ceramic material and in addition to technical functions, such as the marking of hot areas or the cover of under a glass ceramic material technical components, also performs a decorative function, for example by causing certain color impressions or other visual effects, such as a metallic effect. Such decorative layers may be based on organic, semi-organic (comprising, for example, sol-gel inks, in particular pigmented sol-gel inks) or inorganically (including, for example, glass-flux-based inks). Decorative layers, which are applied at least in such a portion of the glass-ceramic material, which are exposed to high thermal stresses in use, are preferably formed semi-organic or inorganic.

The scattering of a material is determined in the context of the present invention with a Lamda950 UV / Vis spectrometer from Perkin Elmer with a 150 mm Ulbricht sphere. Separation of absorption, scattering and reflection is achieved by positioning the sample within the Ulbricht sphere and several measurements are carried out per sample, in which different portions of the radiation are coupled out of the sphere and thus removed from the measurement. This procedure allows an accurate measurement of the scattered fraction as a function of the wavelength. Particularly important for use as a cooking surface, the Streu share at 470 nm has been found, which corresponds to the wavelength of blue LEDs. The scattering at this wavelength is of particular importance, since only blue LEDs can be used to realize "white" displays with the aid of LEDs. Therefore, a large spread at this wavelength gives the impression of "blur" or "blurriness" in the LED displays, which negatively affects the visual appearance.

In the context of the present invention, the terms of the scattered light component and the haze are used interchangeably.

The object of the invention is achieved by a glass-ceramic article according to claim 1, a method according to claim 11 and a use according to claim 12. Preferred embodiments can be found in the dependent claims.

It has surprisingly been found that in these with at least 0.1 - <4 wt .-%, preferably 0.3 - 2 wt .-%, B ₂ O ₃ -containing glass-ceramics, an intrinsically strength-enhancing surface layer ("oberfächennahe Layer "), which has a thickness of at least 10 .mu.m, preferably at least 20 microns, forms. This near-surface layer consists of a layer in which are high-quartz mixed crystals whose lower thermal expansion in comparison to the keatite mixed-crystal-rich core of the glass-ceramic increases the strength over keto-containing glass-ceramics without such a surface layer. This increase in strength is caused by the construction of a stress profile, which is formed by the difference in thermal expansion in the surface and core of the glass ceramic during manufacture, analogous to the thermal or chemical bias of glasses or glass ceramics, but without that a further process step is necessary.

This results in the glass ceramic according to the invention a particularly low tendency to crack, expressed in "CIL" values of> 0.9 N.

In addition to high-quartz mixed crystals, the near-surface layer may already contain keatite mixed crystals. In contrast to the core of the glass ceramic but their proportion is less than 50% of the total crystal phase content. In addition, on the nearer side, a glassy surface zone with a thickness of between 50 and 1000 nm, preferably between 300 and 800 nm, which due to its smaller thickness alone would not lead to an increase in strength, can form. For thickness specifications for the near-surface layer, the sum of its thickness and the optionally existing glassy surface zone is given.

To determine the so-called "crack initiation load" (CIL), a specimen of the material to be tested, in this case a glass-ceramic material, fixed in a preferably nitrogen-purged receptacle is prepared by means of a material tester (micro combination tester from CSM) with a Vickers indenter VI-O3 punctually loaded. The default load is increased linearly to a selected maximum value within 30 seconds and reduced again in the same time without holding time. Due to the load, 0 to 4 cracks in the glass-ceramic can arise starting from the corners of the pyramidal impression of the Vickersindenter. The selected maximum value of the load is gradually increased until every impression 4 Cracks occur. For each force 5 to 20, preferably at least 10 measurements are carried out in order to be able to recognize the variation of the cracking, which is also dependent on the existing surface (previous damage). The mean value is calculated from the number of cracks with the same force.

The sample preferably remains during the measurement until the tears are counted in the receptacle. The assay is preferably conducted under a nitrogen atmosphere to avoid subcritical crack growth by ambient air moisture.

The number of cracks from the corners of the indentation of the Vicker indenter is determined in several experiments depending on the applied load. The determined number of cracks are plotted in relation to the indentation force in a diagram and a fit of a Boltzmann function on the load / crack curve is performed. Finally, the CIL value of the load at which an average of 2 cracks occur is read from this curve and output as the impact resistance parameter. Preferably, the load thus determined is at least 0.9N for the glass-ceramic materials of the present invention.

A description of this method can also be found in US 8,765,262 B2 ,

In addition to the high CIL, ie the low tendency to crack, the glass ceramics according to the invention also have a high impact resistance, as is necessary, for example, for use as a cooking surface. A recognized test method for determining the impact resistance is the so-called ball drop test, as in principle, for example in DE 10 2004 024 583 A1 is described. Shock resistance is currently measured here on square sections of the size 50 mm × 50 mm × 3 mm of a glass-ceramic pane to be tested. The test sample is placed in a test frame and a 200 g steel ball with a diameter of 36 mm is dropped onto the center of the sample. The measurement of the impact resistance is carried out in accordance with DIN 52306. The drop height is increased stepwise until the fracture occurs. Due to the statistical nature of the impact resistance, this test is performed on a series of at least 10 samples. The strength values are the mean value, the standard deviation and / or the 5% fractile of the measured value distribution. The latter value indicates at which fall height 5% of the tested samples were broken.

It is known that the impact resistance of a glass or glass ceramic plate is determined inter alia by more or less accidental surface damage. These because of their Zufallsbedingtheit difficult to control strength effects of surface damage usually lead to a high standard deviation of the measured value distribution and thus can seriously distort a comparative assessment of the impact resistance of different inspection lots. A possible way out is to increase the statistical scope of the audit, which may be a significant expense. Another possibility, which has established itself in the art, is to subject the surface of the plate made of glass or glass ceramic to a surface pretreatment of the same kind for all inspection lots in the form of a defined pre-injury. In the tests carried out here, this pre-injury consists of a single scratch, which is applied centrally on the underside of the test sample opposite the top contact point of the ball impact. The scratch is created with a diamond tip, in this case a Knoop indenter, by making this diamond point parallel to its longer axis with a constant contact force of 0.12 N and a constant speed of 20 mm / min over a length of at least 10 mm is guided in a straight line over the surface of the test sample.

The impact resistance of the LAS glass-ceramic which has undergone such contact-type contact can be determined in the manner described at the beginning by means of the ball-drop test. The standard deviation of typically less than 10% relative is only small, so that the measurement of a reliable statistical evaluation with an acceptable level of inspection lots is accessible. The ball drop test shows that the glass ceramics according to the invention have drop heights of more than 20 cm, preferably more than 25 cm and particularly preferably more than 30 cm (5% fractile of the Weibull distribution).

The formation of a high-quartz surface layer here seems to increase the impact resistance in a special way.

The high impact resistance of the glass ceramics according to the invention makes it possible to produce cooking surface substrates which have a smaller thickness compared to conventional cooking surfaces, without the serviceability decreasing due to a higher impact sensitivity. This allows the production of cooking surfaces with a thickness of less than 5 mm, preferably less than 4 mm, more preferably 3.0 mm and thus a material savings of up to 20%.

The thermal shock resistance and the resistance to temperature differences depend primarily on the CTE of the material, the operating temperature and the strength of the surface. As the CTE of the glass-ceramic material increases (eg, by reducing the Li ₂ O content), this must be compensated for by higher surface strength.

With the present invention, a combination of the lowest possible CTE and a surface strength maximized on account of the layer structure according to the invention succeeds. The thermal expansion, indicated by the thermal expansion coefficient α, also referred to as CTE, in the range from 20 ° C. to 700 ° C., the glass ceramic according to the invention is preferably less than 1.6 ppm / K, particularly preferably less than 1.5 ppm / K.

• • die Keramisierungstemperatur unter Beibehaltung der Keramisierungsdauer zu verringern oder die Keramisierungsdauer unter Beibehaltung der Keramisierungstemperatur zu verkürzen

und/oder

• • den Gehalt an Li₂O in der Glaskeramik zu verringern

Obviously, in the glass-ceramic according to the invention, the addition of B ₂ O ₃ promotes keatite formation in the glass-ceramic. This can be used to advantage:

• • to reduce the ceramization temperature while maintaining the ceramization time or to shorten the ceramization time while maintaining the ceramization temperature

and or

• • to reduce the content of Li ₂ O in the glass ceramic

It has proved to be advantageous if the sum of the Li ₂ O content and the B ₂ O ₃ content is> 2.8% by weight, preferably> 3.5% by weight, ie a low Li ₂ O content of only 2.5 Wt .-% can be used by the addition of at least 0.3 wt .-% B ₂ O ₃ , preferably at least 1 wt .-% B ₂ O ₃ , for the production of a transparent glass ceramic with low scattered light, without the Extend ceramization times so much that economical production is no longer possible.

Preferably, the total duration of the temperature treatment using the lowest maximum temperature, formed from the sum of all temperature steps, and possible heating times less than five hours.

• Al₂O₃ 18 - 23
• Li₂O 2,5 - 4,2 , bevorzugt 3,0 - < 4,0, besonders bevorzugt 3,3 - 3,9
• SiO₂ 60 - 69
• B₂O₃ 0,1- < 4, bevorzugt 0,3 - 2
• Fe₂O₃ 0,012 - 0,3
• ZnO 0 - 2
• Na₂O + K₂O 0,2 - 1,5
• MgO 0 - 2,0, bevorzugt 0 - 1,5
• CaO + SrO + BaO 0 - 4
• TiO₂ 2,3 - 4
• ZrO₂ 0,5 - 2
• P₂O₅ 0 - 3
• SnO₂ 0 - < 0,6
• Sb₂O₃ 0 - 1,5
• As₂O₃ 0 - 1,5
• TiO₂ + ZrO₂ + SnO₂3,8 - 6
• V₂O₅ 0 - 0,06 , bevorzugt 0,01 - 0,06

und gegebenenfalls weitere Farboxide oder färbende Komponenten, in Summe bis maximal 1,0 Gew.-%.According to a preferred embodiment of the invention, the glass ceramic contains the following components (in percent by weight based on oxide):

• Al ₂ O ₃ 18-23
• Li ₂ O 2.5 - 4.2, preferably 3.0 - <4.0, more preferably 3.3 - 3.9
• SiO ₂ 60-69
• B ₂ O ₃ 0.1- <4, preferably 0.3 - 2
• Fe ₂ O ₃ 0.012 - 0.3
• ZnO 0-2
• Na ₂ O + K ₂ O 0.2 - 1.5
• MgO 0-2.0, preferably 0-1.5
• CaO + SrO + BaO 0 - 4
• TiO ₂ 2,3-4
• ZrO ₂ 0.5 - 2
• P ₂ O ₅ 0-3
• SnO ₂ 0 - <0.6
• Sb ₂ O ₃ 0-1.5
• As ₂ O ₃ 0-1.5
• TiO ₂ + ZrO ₂ + SnO ₂ 3,8 - 6
• V ₂ O ₅ 0-0.06, preferably 0.01-0.06

and optionally further color oxides or coloring components, in total up to a maximum of 1.0 wt .-%.

In a further preferred embodiment, the glass ceramic essentially consists of the named components in the stated composition ranges. Here, "consisting essentially of" means that the listed components amount to at least 96% of the total composition.

In a further preferred embodiment, the glass ceramic consists of the named components in the stated composition ranges. The components Li ₂ O, Al ₂ O ₃ and SiO ₂ form the basic components of the crystal phase.

SiO ₂ is preferably present at least 60% by weight. SiO ₂ is preferably present at most 69% by weight. Higher SiO ₂ contents impair the meltability, ie lead to higher melting temperatures and thus to higher energy costs in the melt. At lower SiO ₂ contents, the keatite formation is inhibited.

Al ₂ O ₃ is preferably present at least 18% by weight. Al ₂ O ₃ is preferably present at most 23% by weight. A higher content of Al ₂ O ₃ may result in unwanted crystallization of secondary phases such as mullite, while too low a content of Al ₂ O ₃ inhibits the crystallization of the desired crystal phases high quartz and keatite.

Li ₂ O is preferably present at least 2.5 wt%, preferably at least 3.0%, more preferably at least 3.3 wt%. Li ₂ O is preferably present at most 4.2 wt .-%, preferably, the Li ₂ O content is less than 4.0 wt .-%, more preferably at most 3.9 wt .-%.
Lower Li ₂ O contents lead to an increase in the scattering in the glass ceramic, too high Li ₂ O contents are undesirable for reasons of cost, since Li ₂ O is a relatively expensive raw material.

The size of the crystals or crystallites and thus ultimately the resulting scattering of the material is essentially due to the nucleation. The material must be treated for a sufficiently long period in the range of nucleation temperature, so that a high germ density is achieved. Since the duration of ceramization should be kept as short as possible with regard to economical production of the glass ceramic, a sufficient amount of nucleating agents must be present in the composition for this purpose. This results in the condition that the sum of the nucleating agents TiO ₂ + ZrO ₂ + SnO _{2 is} preferably between 3.8 and 6 percent by weight. This allows the realization of ceramization times of less than 5 hours total duration using the lowest maximum temperature.

In addition to the total amount of nucleating agents, the amount of individual nucleating agents is also limited. In the case of TiO ₂ , too great an amount can lead to the formation of foreign phases, in particular magnesium titanate, which has a negative influence on the thermal expansion and thus the thermal shock resistance of the glass ceramic. The TiO ₂ content is preferably 2.3-4 wt .-%.

For ZrO ₂ and SnO ₂ , the preferred upper limits of 2% by weight for ZrO ₂ and <0.6% by weight for SnO ₂ result from the devitrification tendency, which increases sharply at relatively high levels, which makes shaping more difficult. SnO ₂ is optional. ZrO ₂ is preferably present at least 0.5% by weight.

The alkali oxides Na ₂ O and K ₂ O are used to adjust the viscosity and to improve the melting behavior, but their sum is preferably limited to 0.2-1.5% by weight in order to avoid the formation of undesired secondary phases or at least to restrict.

Although MgO promotes the formation of keatite mixed crystals, i. leads to a reduction in the maximum ceramification temperature, but at the same time also leads to the formation of undesired secondary phases, in particular to crystalline phases of complex mixed metal oxides, for example of spinel or magnesium titanate. On the one hand, this has a negative effect on the scattering of the glass-ceramic material and, on the other hand, also leads to an increase in the thermal expansion coefficient. MgO is optional. Its maximum level is preferably limited to 2.0% by weight, preferably 1.5% by weight.

Further alkaline earth oxides can contribute to the adjustment of the viscosity and possibly to the reduction of the scattered light component. Therefore, in particular CaO and / or SrO and / or BaO may also be present. The total maximum content of CaO and SrO and BaO is preferably limited to 4% by weight.

As already explained above, B ₂ O ₃ allows a lowering of the ceramization temperatures, in particular the maximum ceramization temperature and / or a shortening of the ceramization time. Alternatively, it allows a reduction in the lithium content of the glass-ceramic, which allows a reduction in raw material costs.
Furthermore, an advantageous effect of boron on the thickness of the strength-promoting surface layer was observed.

Fluorine also has a similar effect on the ceramization temperatures and times. Due to environmental and occupational safety aspects, however, preference is given to the use of fluorine.

P ₂ O ₅ can be used to improve the glass formation, but in amounts of more than 3 wt .-% to a deterioration of the transmission by increasing the scattering.

It has been observed that iron oxide Fe ₂ O ₃ can accelerate keatite formation, thus also acting as a nucleating agent. In addition, a reduction of the scattering by higher Fe ₂ O ₃ contents was observed, so that the iron content, expressed as Fe ₂ O ₃ , should preferably be at least 0.012%. The maximum Content should not exceed 0.3 wt .-%. Higher levels can lead to the formation of foreign phases and thus lead to a deterioration of performance properties. At the same time, iron oxide increases the inherent color of the glass ceramic. If an uncolored glass-ceramic is to be used, for example in connection with a colored underside coating as a cooking surface, the iron content must be limited, as a rule to less than 1000 ppm.

To assist the refining, the usual refining agents known from glass processing, such as SnO ₂ , MnO ₂ As ₂ O ₃ , Sb ₂ O ₃ , SO ₄ ^2- , Cl ^- , CeO _{2, can be used} in amounts of up to a total of not more than 2% by weight become. For reasons of environmental protection, the refining with SnO _{2 is} preferred.

According to a preferred embodiment, the content of arsenic, antimony, cadmium, lead and / or halide-containing compounds, already for environmental and health reasons, in each case in the sum of all compounds of an element below 0.1 wt .-%, based on the total mass of the glass-ceramic material.

The glass-ceramic material of the present invention is preferably selectively colored by the addition of color oxides or coloring components. As coloring matters, the color oxides known from glassmaking and processing such as V ₂ O ₅ , Fe ₂ O ₃ , Cr ₂ O ₃ , MnO ₂ , CoO, NiO, CuO, MoO ₃ , CeO ₂ , Nd ₂ O ₃ or others coloring components (metals, rare earths and their oxides, etc.). The total content of these coloring constituents, ie the color oxides and the other coloring components, is preferably at most 1% by weight.

• • die Größe von im Material enthaltenen Streupartikeln. Im Fall der vorliegenden Erfindung sind solche Streupartikel die Kristalle bzw. Kristallite, welche das glaskeramische Material umfasst.
• • der Brechzahlunterschied zwischen einer umhüllenden Matrix und darin eingebetteten Partikeln. Im Falle von glaskeramischem Material umfasst die Matrix Glas, und die darin eingebetteten Partikel umfassen Kristallite bzw. Kristalle.
• • Die Doppelbrechung des Materials. Im Falle eines glaskeramischen Materials ist hier insbesondere die Doppelbrechung der Kristallite zu berücksichtigen.

Transparency in the context of this invention refers to a property of a material which essentially results from three factors:

• • the size of scattering particles in the material. In the case of the present invention, such scattering particles are the crystals or crystallites comprising the glass-ceramic material.
• • the refractive index difference between an enveloping matrix and embedded particles. In the case of glass-ceramic material, the matrix comprises glass, and the particles embedded therein comprise crystallites or crystals.
• • The birefringence of the material. In the case of a glass-ceramic material, in particular the birefringence of the crystallites should be considered here.

In this sense, a glass-ceramic material is transparent, if the crystallite size is smaller than the wavelength of the light, the difference between refractive indices of the phases encompassed by the material, so for example, the main crystal phase and the surrounding, often vitreous matrix formed as low as possible and the Material comprised crystal phases have no or the lowest possible optical anisotropy. Such conditions can be set by a skillful, targeted selection of the composition and a precisely controlled controlled crystallization. The better these conditions are met for a given material, the higher the transmission and the lower the scatter.

In the context of the present invention, the transparency of a material, for example a glass-ceramic material, is therefore to be understood not only by having the lowest possible absorption coefficient for electromagnetic radiation in the wavelength range from 380 to 780 nm. Transparency of a material, for example a glass-ceramic material, is also determined in the present case by the smallest possible scattering of electromagnetic radiation in the wavelength range from 380 to 780 nm, in particular at the wavelengths 470 nm (blue LED), 530 nm (green LED) and 700 nm ( red LED).

According to yet another embodiment of the invention, the glass-ceramic material at least partially comprises a glassy surface zone having a thickness between 50 and 1000 nm, preferably between 300 and 800 nm.

Close to these possibly existing glassy surface zone, so preferably from 1 micron, the crystalline-containing areas.

Preferably, the near-surface layer of the glass-ceramic material at a depth of 1 .mu.m, preferably a depth of greater than 10 .mu.m, particularly preferably from 20 .mu.m to a maximum of 100 .mu.m, comprises high-quartz mixed crystal, preferably high-quartz mixed crystal as the main crystalline phase.

The near-surface layer thus consists of a layer with a thickness of 10 .mu.m to 100 .mu.m, in which high-quartz mixed crystal is present as a crystal phase, wherein the proportion of high-quartz mixed crystal preferably outweighs the proportion of other possibly present crystal phases, in particular keatite mixed crystals. In addition, on the nearer side, it may contain a glassy surface zone with a thickness between 50 and 1000 nm, preferably between 300 and 800 nm.

According to a further preferred embodiment of the invention, the glass-ceramic article is designed as a glass-ceramic disc with a thickness of less than 5 mm, preferably of less than 4 mm, particularly preferably of 3.0 mm to 3.2 mm.

According to a further preferred embodiment, at least one side of the glass-ceramic pane has at least one decorative layer.

The glass-ceramic according to the invention finds wide application possibilities, in particular in areas in which the material is subjected to operationally high thermal stresses and / or high mechanical loads. For example, these applications include the use of the glass ceramic article, preferably designed as glass ceramic, than, refractory glazing or window, as a door of a cooking chamber, for example in an electric range, especially in an electric range, which has a pyrolysis, or as a cover for heating elements, especially as a cook - or Bratfläche, as white goods, as a radiator cover, as a grill surface, fireplace fireplace, as a support plate or furnace lining in the ceramic, solar or pharmaceutical or medical technology, especially for production processes under high purity conditions, as a lining of furnaces in which chemical or physical Coating method can be carried out as a chemically resistant laboratory equipment, 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 Abde for reflectors, floodlights, projectors, projectors, photocopiers, for applications with thermo-mechanical stress, for example in night vision devices, as a wafer substrate, as an object with UV protection, as a material for housing components, for example, of electronic devices and / or covers for IT such as mobile phones , Laptops, scanner glasses or the like, as a facade panel, as fire-resistant glazing or as a component for ballistic protection. The use as a cooking surface or fireplace fireplace is particularly preferred.

For the preparation of the glass-ceramic according to the invention, a closely coordinated interaction between the composition of the glass-ceramic on the one hand and the ceramizing process on the other hand is necessary.

• • Läutern der Glasschmelze,
• • Formen des Grünglases unter Abkühlung der Schmelze,
• • Keramisieren des so hergestellten Grünglases in mehreren Abschnitten eines vorzugsweise einstufigen Prozesses, die vorzugsweise die folgenden Schritte umfassen:
  • ◯ einen ersten Keimbildungsschritt zur Ausbildung einer ausreichenden Anzahl an Kristallisationskeimen,
  • ◯ einen Kristallwachstumsschritt, bei dem Hochquarzmischkristalle auf den Kristallkeimen aufwachsen,
  • ◯ einem Kristallumwandlungsschritt, bei dem das vorkristallisierte glaskeramische Zwischenerzeugnis mit Hochquarzmischkristall als vorherrschender Kristallphase einem weiteren Keramisierungsschritt mit einer Maximaltemperatur Tmax und über eine Haltezeit t(Tmax) dieser Maximaltemperatur unterzogen wird, wobei die HQMK-Kristallphase zumindest teilweise in eine KMK-Kristallphase umgewandelt wird Die Maximaltemperatur ist bevorzugt ≤ 1050°C, besonders bevorzugt ≤ 980°C.

The inventive method for producing the glass-ceramic material according to the invention from a LAS glass preferably having the above composition provides the following steps starting from the glass melt:

• • refining the glass melt,
• Forming the green glass while cooling the melt,
• Ceramifying the green glass thus produced in several stages of a preferably single-stage process, which preferably comprise the following steps:
  • ◯ a first nucleation step to form a sufficient number of nuclei,
  • ◯ a crystal growth step in which high quartz mixed crystals grow on the crystal nuclei,
  • ◯ a crystal conversion step in which the pre-crystallized intermediate glass transition product with high quartz mixed crystal as the predominant crystal phase is subjected to a further ceramization step having a maximum temperature Tmax and a holding time t (Tmax) of this maximum temperature, at least partially converting the HQMK crystal phase into a KMK crystal phase Maximum temperature is preferably ≦ 1050 ° C., more preferably ≦ 980 ° C.

Starting from a precrystallized glass-ceramic intermediate product with HQMK as the predominant crystal phase, the method according to the invention commences correspondingly with the crystal conversion step. And starting from a green glass, the method according to the invention commences correspondingly with the nucleation step, followed by the crystal growth step and the crystal transformation step.

The glass-ceramic composition in connection with the production process makes it possible to produce the abovementioned layer structure and the crystal contents, as well as the inventive compositions Transmission characteristic and thus the advantageous material properties. The main crystal phase in the core of the glass ceramic and thus the main _{crystal phase of} the glass ceramic then consists of KMK, which in the composition range Li ( _1-2x-2y ) Mg _x Zn _y AlSi ₂ O ₆ - Li ( _1-2x-2y ) Mg ( _x ) Zn ( _y ) AlSi ₄ O ₁₀ with (0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.5 and 0 ≤ x + y ≤ 0.5).

1. a) Erhöhen der Temperatur des keramisierbaren Glases und/oder des glaskeramischen Materials auf den Temperaturbereich von etwa 620 °C bis 680 °C innerhalb von 3 Minuten bis 30 Minuten;
2. b) Erhöhen der Temperatur des keramisierbaren Glases und/oder des glaskeramischen Materials innerhalb des Temperaturbereichs der Keimbildung von 680 °C bis 800 °C über einen Zeitraum von etwa 10 Minuten bis 30 Minuten;
3. c) Erhöhen der Temperatur des Kristallisationskeime enthaltenden Glases und/oder glaskeramischen Materials innerhalb von 5 Minuten bis 30 Minuten Dauer in den Temperaturbereich hoher Kristallwachstumsgeschwindigkeit für Keatit-Mischkristalle von 850 °C bis 1050 °C;
4. d) Halten innerhalb des Temperaturbereiches bei der maximalen Temperatur von 850 °C bis 1050°C bis zu 20 Minuten, um das erfindungsgemäße Gefüge, also einen Kristallphasengehalt von mehr als 50 Vol.-% Keatitmischkristall im Kern der Glaskeramik und eine oberflächennahe Schicht, zu bilden;
5. e) rasches Abkühlen der erhaltenen Glaskeramik auf Raumtemperatur.

The ceramization program for crystal conversion according to the invention is described by the following ceramizing program with a total duration of less than 2 hours, preferably less than 80 minutes:

1. a) raising the temperature of the ceramizable glass and / or the glass-ceramic material to the temperature range of about 620 ° C to 680 ° C within 3 minutes to 30 minutes;
2. b) raising the temperature of the ceramizable glass and / or the glass-ceramic material within the temperature range of nucleation from 680 ° C to 800 ° C over a period of about 10 minutes to 30 minutes;
3. c) increasing the temperature of the nucleation-containing glass and / or glass-ceramic material within 5 minutes to 30 minutes in the temperature range high crystal growth rate for keatite mixed crystals of 850 ° C to 1050 ° C;
4. d) Keep within the temperature range at the maximum temperature of 850 ° C to 1050 ° C for up to 20 minutes to the structure of the invention, ie a crystal phase content of more than 50 vol .-% Keatitmischkristall in the core of the glass ceramic and a near-surface layer to form;
5. e) rapid cooling of the resulting glass-ceramic to room temperature.

The advantage of this ceramization process is that the ceramization conditions relevant for the product properties of the holding time t (Tmax) and the maximum ceramization temperature Tmax at which the ceramization processes and above all the phase transformation take place in a controlled manner are achieved quickly and the ceramization is not already achieved during the heating and so that it is less controlled.

If, instead of the ceramizable glass described above, a glass ceramic with high-quartz mixed crystal is used as the main crystal phase as the starting point, step b) of the ceramization program described above can be omitted or shortened.

The addition of B ₂ O ₃ significantly reduces the required maximum ceramization temperature and / or ceramization time compared to compositions containing no boron oxide. This will be further explained in the following by examples, wherein A1 to A9 represent exemplary embodiments and V1 to V3 represent comparative examples.

For both the embodiments and the comparative examples, Table 1 shows the compositions (in% by weight based on oxide), in each case three different maximum ceramization temperatures [° C] and the fractions of high quartz mixed crystal (HQMK) and keatite mixed crystal (KMK) present therein Core (in%, based on the sum of all crystalline fractions and the amorphous fraction in volume percent) and the thermal expansion coefficient α _20-700 [10 ^-6 / K] and the CIL [N] specified. Table 1 for some embodiments and comparative examples, depending on the respective ceramization temperature for the wavelengths 470 nm, 530 nm and 700 nm, the spectral transmission, ie τ (470 nm), τ (530 nm) and τ (700 nm), and the scatter proportion, in%. The maximum ceramization temperatures reported in the table were all achieved in a ceramization process that had a total duration of 90 minutes to 105 minutes. This total duration refers to the target temperatures and is calculated from the heating from room temperature to the said maximum temperature and the subsequent cooling to a temperature of 150 ° C. Heating rates and hold times in these temperature ranges were kept constant in all examples.

Table 1: Examples

Table 1: Examples A1 V1 A2 V2 A3 A4 A5 A6 A7 V3 A8 Li ₂ O 3.82 3.86 3.01 3.04 3.84 3.78 3.01 3.28 3.25 3.27 3.28 Na ₂ O 0.61 0.61 0.54 0.54 0.61 0.60 0.54 0.56 0.40 0.40 0.25 K20 0.27 0.27 0.24 0.24 0.27 0.27 0.24 0.25 0.32 0.32 0.21 MgO 0.31 0.31 1.22 1.24 0.31 0.30 1.22 0.94 1.19 1.20 0.45 CaO 0.43 0.43 0.68 0.68 0.43 0.42 0.68 0.60 0.25 0.25 0.55 BaO 2.24 2.26 2.02 2.04 2.25 2.22 2.02 2.10 1.99 2.00 1.46 ZnO 1.49 1.50 1.77 1.79 1.50 1.47 1.77 1.69 1.79 1.80 1.95 Al ₂ O ₃ 20.76 20.96 19,95 20.14 20.86 20.56 19.93 20.30 19.14 19.24 19,00 SiO ₂ 64.24 64.89 64.77 65.39 64.58 63.63 64.73 64,90 66.54 66.88 67.65 TiO ₂ 3.10 3.13 3.17 3.20 3.11 3.07 3.17 3.18 3.18 3.20 3.21 ZrO ₂ 1.36 1.37 1.27 1.28 1.37 1.35 1.26 1.27 1.02 1.03 1.08 SnO ₂ 0.25 0.25 0.25 0.26 0.25 0.25 0.25 0.25 0.25 0.25 0.25 Fe203 0,099 0,100 0,099 0,120 0,100 0.098 0,110 0.109 0.109 0,110 0.109 V ₂ O ₅ 0.026 0.026 0,040 0,038 0,038 0,038 0,035 0,035 MnO ₂ 0,025 0,025 0,025 0,025 0,025 0,025 0,025 0,025 0,025 0,025 B ₂ O ₃ 0.99 0.99 0.50 1.97 1.00 0.50 0.50 0.50 Max. Ceramization temp. 965 ° C 965 ° C 965 ° C 965 ° C 965 ° C 965 ° C 965 ° C 965 ° C 965 ° C 965 ° C 965 ° C HQMK share - 3 - 30 - - 0 0 0 3 6 KMK share 76 71 73 45 79 76 73 75 81 74 71 α (20,700 ° C) 1.32 nb 1.55 1.44 1.29 1.48 1.57 1.48 1.20 1.27 0.82 CIL 3.82 nb nb 1.24 nb nb nb nb nb nb nb τ (470nm)% 0.06 nb 21.41 0.06 46.48 3.65 0.08 0.23 0.03 0.08 0.63 τ (530nm)% 0.09 nb 38.94 0.09 59.01 11.32 0.29 0.42 0.47 0.10 1.03 τ (700nm)% 6.90 nb 67.97 6.90 78.04 49.16 13.07 13.53 6.08 6.61 18.55 Streu proportion (470 nm)% 0.32 nb 31.04 0.32 21,12 46.31 1.91 nb nb 0.43 0.43 Streu proportion (530 nm)% 0.43 nb 27.23 0.43 17.26 47.26 1.11 nb nb 0.29 0.35 Scattering fraction (700 nm)% 1.55 nb 17.42 1.55 10.20 32.81 1.54 nb nb 0.49 1.24 Max. Ceramization temp. 935 ° C 935 ° C 935 ° C 935 ° C 935 ° C 935 ° C 935 ° C 935 ° C 935 ° C 935 ° C 935 ° C HQMK share 10 69 44 64 56 - 20 48 64 70 72 KMK share 65 - 21 - 12 75 46 22 14 2 3 α (20,700 ° C) 1.33 nb 1.16 0.92 0.35 1.46 1.35 0.90 0.64 0.53 -0.15 CIL 2.25 nb nb 0.94 nb nb nb nb nb nb nb τ (470nm)% 0.11 nb 27.49 0.11 48.93 38.41 0.14 0.14 0.03 0.10 0.47 τ (530nm)% 0.15 nb 37.12 0.15 59.72 54.06 0.25 0.21 0.05 0.11 0.70 τ (700nm)% 9.61 nb 58.81 9.61 78.98 77.73 10.21 10.50 5.12 7.44 17,20 Streu proportion (470 nm)% 0.10 nb 26.69 0.10 8.85 24.18 0.51 nb ' nb nb nb Streu proportion (530 nm)% 0.08 nb 27.12 0.08 7.72 18.70 0.42 nb nb nb nb Scattering fraction (700 nm)% 0.11 nb 23.52 0.11 5.40 9.22 3.07 nb nb nb nb Max. Ceramization temp. 950 ° C 950 ° C 950 ° C 950 ° C 950 ° C 950 ° C 950 ° C 950 ° C 950 ° C 950 ° C 950 ° C HQMK share nb nb 1 nb 1 0 3 0 0 52 41 KMK share nb nb 71 nb 75 75 73 75 73 20 35 α (20,700 ° C) τ (470nm)% nbnb nbnb 1.56 nb nbnb 1.29 nb 1.47 nb 1.57 0.20 1.45 0.62 1.20 0.05 0.81 0.06 0.33 0.30 τ (530nm)% nb nb nb nb nb nb 0.47 1.13 0.08 0.07 0.52 τ (700nm)% nb nb nb nb nb nb 14.32 19,66 6.35 5.49 13.65 Streu proportion (470 nm)% nb nb nb nb nb nb 1.28 0.42 0.53 nb nb Streu proportion (530 nm)% nb nb nb nb nb nb 0.79 0.28 0.35 nb nb Scattering fraction (700 nm)% nb nb nb nb nb nb 1.43 0.85 0.47 nb nb nb = not determined

The materials listed in Table 1 were melted using conventional raw materials in the glass industry at temperatures of about 1600 to 1680 ° C and refined. The melting of the mixture was carried out initially in crucibles made of sintered silica glass, and the melted mixture was then recast in Pt / Rh crucible with inner crucibles made of silica glass and homogenized by stirring at temperatures of about 1550 ° C for 30 minutes. After leaving at 1640 ° C for 2 h castings of about 140 mm x 100 mm x 30mm in size were poured and relaxed in a refrigerator at about 620 to 680 ° C and cooled to room temperature. From the castings, the test samples for the measurement of the glassy state properties and the plates for the ceramization were prepared.

The ceramization was carried out using the ceramizing program described above with the three different maximum temperatures given in Table 1, namely 935 ° C, 950 ° C and 965 ° C. The particular crystal phase contents indicated in each case show that the examples according to the invention with B ₂ O ₃ ≥ 0.3 weight percent permit a lowering of the maximum temperature to 950 ° C. or even 935 ° C. and furthermore have keatite as main crystal phase. In comparison, the comparative examples without B ₂ O ₃ require ceramization temperatures of 965 ° C and higher to ensure sufficient conversion of the high quartz mixed crystals into keatite mixed crystals. This is particularly evident when comparing Example 1 and Comparative Example 1 or Example 2 and Comparative Example 2. Example 1 is almost completely converted into a keatite mixed crystal glass ceramic even at a maximum temperature of 935 ° C, whereas Comparative Example 1 at this temperature in the core only high quartz Having mixed crystals. The advantageous strength properties of the glass ceramics according to the invention are also achieved at this low temperature, as shown by the measured value of 2.25 for the CIL. An analogous behavior can be found in Comparison of Example 2 and Comparative Example 2 notice. The latter shows a significant high quartz content even at 965 ° C maximum temperature, while for the former already 950 ° C for complete conversion into keatite solid solution suffice.

The main difference between the mentioned compositions 1 and 2 and between Comparative Example 1 and 2 is in the content of Li ₂ O. The lowering of the Li ₂ O content of 3.85% to about 3% leads to a slowing in the conversion of the high quartz mixed crystal to keatite that an increase of Ceramization temperature (or alternatively Keramisierungdauer) makes necessary. As can be seen from the examples, this effect can be compensated for by the addition of B ₂ O ₃ without adversely affecting the properties of the glass ceramic. This allows a particularly economical production of glass-ceramics, wherein either the (expensive) raw material Li ₂ O can be reduced or the required process temperatures or times can be reduced.

The thermal expansion α _{20-700 of} the glass-ceramic _{according to} the invention is preferably less than 1.6 ppm / K, more preferably less than 1.5 ppm / K. The thermal expansion values generally increase with increasing keatite content, which must be taken into account when selecting the optimal ceramization. 1 shows the context for some selected examples, namely for A2, A3, A7, A8 and A9.

The bordered area represents the target area, which can be set by optimizing the ceramization. The lines connect readings of examples of equal composition and left to right increasing maximum temperature and are for illustration only. The crystal phase contents given in Table 1 were determined by X-ray diffraction measurements on a Panalytical X'Pert Pro diffractometer (Almelo, The Netherlands). As X-ray, a CuKα radiation (λ = 1.5060 Å) generated via a Ni filter was used. The standard X-ray diffraction measurements on powder and solid samples were carried out under a Bragg-Brentano geometry (θ-2θ). The X-ray diffraction patterns were measured between 10 ° and 100 ° (2θ angle). The quantification of the relative crystalline phase fractions was carried out by a Rietveld analysis. Absolute phase contributions were calculated using an internal standard. The measurement was carried out on ground sample material, whereby the volume fraction of the core region clearly dominates. The measured phase components therefore correspond to the phase distribution in the core of the glass ceramic.

X-ray diffraction thin-film measurements were carried out to determine the crystalline phase fractions in the near-surface layer. A parabolic X-ray mirror was used on the primary side and a parallel plate collimator on the secondary side. The X-ray diffraction patterns were measured between 10 ° and 60 ° (2θ angle). The angle of incidence of the primary beam was 0.5 ° Ω, which corresponds to an information depth (90%) of about 2 μm for these samples.

Table 2 shows by way of example the crystal phase content in the core of the glass ceramic and in the surface layer. Table 2: XRD results in the core and in the surface in comparison A5 A6 A7 ceramization 965 ° C 965 ° C 965 ° C XRD core HQMK share 0 0 0 KMK share 73 75 81 Thin film XRD [0.5 ° 2Theta] HQMK 14.00 9.00 25,00 KMK 26.00 24.00 19,00 ZrTiO ₄ 1.00 1.00 1.00 Amorphous 59,00 66,00 55,00 ceramization 935 ° C 935 ° C 935 ° C XRD core HQMK share 20 48 64 KMK share 46 22 14 Thin film XRD [0.5 ° 2Theta] HQMK 37,00 41,00 54,00 KMK 8.00 4.00 0.00 ZrTiO ₄ 1.00 1.00 1.00 Amorphous 54,00 54,00 45,00 Ceramization 950 ° C 950 ° C 950 ° C XRD core HQMK share 3 0 0 KMK share 73 75 73 Thin film XRD [0.5 ° 2Theta] HQMK 20.00 21,00 34,00 KMK 24.00 16.00 11.00 ZrTiO ₄ 1.00 1.00 1.00 Amorphous 55,00 61,00 54,00

All three examples show, after ceramization at a sufficiently high ceramification temperature, the structure of the glass ceramics according to the invention, in which keatite mixed crystal is present as the main crystal phase in the core of the glass ceramic, while a higher proportion of high quartz mixed crystals is present in the surface layer. As a result, the thermal expansion of the surface layer is reduced compared to the core of the glass ceramic, which leads to an increase in strength analogous to a chemical bias.

Table 3 shows the effect of the different ceramizations on the transmission of the glass ceramics.
The transmission is based on ISO 15368 (2001-08). For the determination of color values based on DIN 5033 the spectral data between 380 and 780 nm are used. These are evaluated in accordance with the CIE color system for the P-factor: 0.91, standard light C, observer angle 2 ° and 4 mm glass thickness and the Y value calculated therefrom. This light corresponds to white light with a color temperature of 6800 K and thus represents medium daylight. Table 3: Transmission Y of the glass-ceramics measured at 4 mm thickness A2 A3 A4 Ceramization 965 ° C 965 ° C 965 ° C transmission 4mm 4mm 4mm Transmission Y 44.2% 62.5% 17.0% Ceramization 935 ° C 935 ° C 935 ° C transmission 4mm 4mm 4mm Transmission Y 40.8% 63.4% 58.7%

As can be seen in Table 3, the choice of ceramization also allows an adjustment of the transmission. It is thus possible to choose the optimum transmission depending on the application. For use as a cooking surface, this means that the glass ceramic can be advantageously provided with a bottom coating, since at a high transmission, the color of the underside coating is not distorted. On the other hand, low-transmission glass-ceramics, often in combination with coloring by color oxides such as V ₂ O ₅ , Fe ₂ O ₃ , Cr ₂ O ₃ , MnO ₂ , CoO, NiO, CuO, MoO ₃ , CeO ₂ , Nd ₂ O ₃ , the use without further coating, which allows a more cost-effective production.

With regard to the use of the glass ceramics according to the invention as cooking surfaces and thus on the visual perception of mounted under a cooking surface LED, however, not only the averaged over the entire range of visible light transmission Y but also the spectral transmission and the scattering in the Wavelength of the selected LED, eg α (470 nm) and the scattered light component at 470 nm for blue LEDs. The values measured for the wavelengths 470 nm (blue), 530 nm (green) and 700 nm (red) in the exemplary embodiments are shown in Table 1.

Example 1 shows a particularly low scattering. In contrast, the lowering of the B ₂ O ₃ content in Example 3 compared to Example 1 leads to a significant increase in the scattering, which is still tolerable in the application. Example 4 shows that the scattering can be reduced by optimizing the ceramization. Keatite is already present as the main crystal phase at the lower temperature of 935 ° C, but the scattering is almost halved compared to the higher temperature of 965 ° C.

For use as a cooking surface, the combination of a relatively low transmission τ (470 nm) <10%, namely to avoid glare effects and undesired viewing of the under-cooktop technique, and a low scattered light content of <10% at 470 nm is particularly advantageous. This can be achieved, for example, by using 0.01% -0.06% of V ₂ O ₅ as the coloring component in the glass ceramics according to the invention.

2 shows the relationship of transmission and scattering at 470 nm for examples from Table 1.

Claims (12)

Transparent, preferably with the aid of color oxides colored glass ceramic article with a scattered light content determined at a wavelength of 470 nm and normalized to a glass-ceramic material of 4 mm thickness of less than 70%, preferably less than 40%, more preferably less than 25%, comprising one first near-surface layer and an underlying core, wherein the core comprises keatite mixed crystals as the main crystal phase and wherein the core is present at a depth from 100 microns, as measured from the surface of the glass-ceramic material, the near-surface layer has a thickness of at least 10 microns, preferably has at least 20 microns, and the glass ceramic article contains at least 0.1- <4%, preferably 0.3 to 2% B ₂ O ₃ . Glass ceramic article after Claim 1 containing the following components (in% by weight based on oxide): Al ₂ O ₃ 18-23 Li ₂ O 2.5-4.2, preferably 3.0- <4.0, more preferably 3.3-3 , 9 SiO ₂ 60-69 B ₂ O ₃ 0.1-4, preferably 0.3-2 Fe ₂ O ₃ 0.012-0.3 ZnO 0-2 Na ₂ O + K ₂ O 0.2-1, 5 MgO 0 - 2.0, preferably 0 - 1.5 CaO + SrO + BaO 0-4 TiO ₂ 2.3 - 4 ZrO ₂ 0.5 - 2 P ₂ O ₅ 0 - 3 SnO ₂ 0 - <0, 6 Sb ₂ O ₃ 0-1.5 As ₂ O ₃ 0-1.5 TiO ₂ + ZrO ₂ + SnO ₂ 3.8-6 V ₂ O ₅ 0-0.06, preferably 0.01-0.06 and optionally further color oxides or coloring components, in total up to a maximum of 1.0% by weight Glass ceramic article after Claim 1 or 2 characterized in that the content of Li ₂ O + B ₂ O ₃ > 2.8 weight percent, preferably> 3.5 weight percent. Glass ceramic article according to one of Claims 1 to 3 , characterized in that it has a crack initiation load (CIL) greater than 0.9N. Glass ceramic article according to one of Claims 1 to 4 , characterized in that it has a thermal expansion coefficient α of less than 1.6 ppm / K, preferably less than 1.5 ppm / K in the range of 20 ° C to 700 ° C. Glass ceramic article according to one of Claims 1 to 5 , characterized in that it has at 470 nm, a spectral transmission of less than 10% and a scattered light content of less than 10% and a V ₂ O ₅ content of 0.01 - 0.06 weight percent. Glass ceramic article according to one of Claims 1 to 6 , characterized in that the near-surface layer at least partially comprises a glassy surface zone having a thickness between 50 and 1000 nm, preferably between 300 and 800 nm. Glass ceramic article according to one of Claims 1 to 7 , characterized in that the near-surface layer comprises at a depth of greater than 1 micron to a maximum of 100 microns high quartz mixed crystal, preferably high quartz mixed crystal as the main crystal phase. Glass ceramic article according to one of Claims 1 to 8th , formed as a glass ceramic disc with a thickness of less than 5 mm, preferably less than 4 mm, particularly preferably from 3 mm to 3.2 mm. Glass ceramic article after Claim 9 , characterized in that at least one side of the glass-ceramic pane comprises at least one decorative layer Process for producing a transparent glass ceramic, preferably colored with the aid of color oxides, with a scattered light content determined at a wavelength of 470 nm and normalized to a glass-ceramic material of 4 mm thickness, of less than 70%, preferably less than 40%, more preferably less than 25 %, comprising a first, near-surface layer and an underlying core, wherein the core comprises keatite mixed crystals as the main crystal phase and a crystal phase is referred to as the main crystal phase, if their proportion exceeds more than 50 vol .-% of the sum of the crystal phase portions, and wherein the Core at a depth of 100 microns, measured from the surface of the glass-ceramic material, is present, the near-surface layer has a thickness of at least 10 microns and the at least 0.1- <4%, preferably 0.3 - 2% B ₂ O ₃ containing, with a total duration of less than 5 hours, preferably less than 2 hours, characterized in that the ceramization is carried out with the following program: a) providing a starting material comprising a ceramizable glass and / or a glass-ceramic material; b) raising the temperature of the ceramizable glass and / or the glass-ceramic material to the temperature range of 620-680 ° C within 3 to 30 minutes; c) raising the temperature of the ceramizable glass and / or the glass-ceramic material within the temperature range of 680 to 800 ° C over a period of about 10 to 30 minutes; d) increasing the temperature of the crystallization nuclei-containing glass and / or glass-ceramic material within 5 to 30 minutes duration in the temperature range from 850 to 1050 ° C, preferably 850-980 ° C; e) keeping within the temperature range at the maximum temperature of 850 to 1050 ° C, preferably 850 to 980 ° C; up to 20 minutes and then e) rapid cooling of the resulting glass-ceramic to room temperature. Use of a glass ceramic article according to one of Claims 1 to 9 as a fireproof glazing or window, as a door of a cooking chamber, for example in an electric range, in particular in an electric range, which has a pyrolysis function, or as a cover for heating elements, especially as a cooking or frying surface, as white goods, as a radiator cover, as a grill surface, as Kaminsichtscheibe , as a carrier plate or furnace lining in the ceramic, solar or pharmaceutical industry or medical technology, in particular 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, 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, as a wafer substrate, as an object with UV protection, as a material for housing components of, for example, electronic devices and / or covers for IT such as Mobile phones, laptops, scanner glasses or the like, as a facade panel, as fire-resistant glazing or as a component for ballistic protection.

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