PRODUCT COMPRISING A TRANSPARENT, VOLUME-COLOURED GLASS-CERAMIC

Abstract

A product having a transparent volume-coloured glass-ceramic is provided. The glass-ceramic includes, based on oxide, 58-72% by weight SiO.sub.2, 16-26% by weight Al.sub.2O.sub.3, 1.0-5.5% by weight Li.sub.2O, 2.0-<4.0% by weight TiO.sub.2, 0-<2.0 by weight ZnO, and 0.005-0.12% by weight MoO.sub.3, and where the glass-ceramic, based on a thickness of 4 mm, has a luminous transmittance τ.sub.vis of 0.5%-3.5%, and where the glass-ceramic has the property that after passage through the glass-ceramic, based on a thickness of 4 mm, light of the standard illuminant D65 has a colour locus in the white region A1 that in the CIExyY-2° chromaticity diagram is defined by the following coordinates:

TABLE-US-00001 A1 0.3 0.27 0.28 0.315 0.35 0.38 0.342 0.31 0.3 0.27.

Claims

1. A product comprising a transparent, volume-coloured glass-ceramic, where the glass-ceramic comprises, in % by weight based on oxide: SiO.sub.2 58-72, Al.sub.2O.sub.3 16-26, Li.sub.2O 1.0-5.5, TiO.sub.2 2.0-<4.0, ZnO 0-<2.0, MoO.sub.3 0.005-0.12, wherein the glass-ceramic, based on a thickness of 4 mm, has a luminous transmittance τ.sub.vis of 0.5%-3.5%, and wherein the glass-ceramic has a property that after passage through the glass-ceramic, based on a thickness of 4 mm, light of the standard illuminant D65 has a colour locus in the white region A1 that in the CIExyY-2° chromaticity diagram is defined by the following coordinates: TABLE-US-00007 A1 0.3 0.27 0.28 0.315 0.35 0.38 0.342 0.31 0.3 0.27.

2. The product of claim 1, wherein the luminous transmittance τ.sub.vis is 1.2%-2.8%.

3. The product of claim 1, wherein the glass-ceramic comprises MoO.sub.3, in % by weight based on oxide, of 0.030-0.070.

4. The product of claim 1, wherein the glass-ceramic further comprises a temperature T(pO.sub.232 1 bar) in a range 1550-1700° C.

5. The product of claim 1, wherein the glass-ceramic further comprises a temperature T(pO.sub.2=1 bar) in a range 1570-1680° C.

6. The product of claim 1, wherein the glass-ceramic comprises TiO.sub.2, in % by weight based on oxide, of 3.0-3.8.

7. The product of claim 1, wherein the glass-ceramic further comprises ZrO.sub.2, in % by weight based on oxide, of 0.1-2.5.

8. The product of claim 1, wherein the glass-ceramic further comprises ZrO.sub.2, in % by weight based on oxide, of 0.5-1.5.

9. The product of claim 1, wherein the glass-ceramic further comprises ZrO.sub.2 and has a ZrO.sub.2/TiO.sub.2 ratio in a range of 0.1-0.67.

10. The product of claim 1, wherein the ZrO.sub.2/TiO.sub.2 ratio is a range of 0.2-0.33.

11. The product of claim 1, where the glass-ceramic further comprises a constituent selected from a group consisting of: V.sub.2O.sub.5 in an amount of 0.0001-0.010 in % by weight based on oxide, V.sub.2O.sub.5 in an amount of 0.0005-0.0080 in % by weight based on oxide, V.sub.2O.sub.5 in an amount of 0.0010-0.0050 in % by weight based on oxide, Cr.sub.2O.sub.3 in an amount of 0-0.0100 in % by weight based on oxide, Cr.sub.2O.sub.3 in an amount of 0.0005-0.0090 in % by weight based on oxide, Cr.sub.2O.sub.3 in an amount of 0.0010-0.0060 ppm, Fe.sub.2O.sub.3 in an amount of 0.05-0.30 in % by weight based on oxide, Fe.sub.2O.sub.3 in an amount of 0.06-0.20 in % by weight based on oxide, Fe.sub.2O.sub.3 in an amount of 0.07-0.15 in % by weight based on oxide, and combinations thereof.

12. The product of claim 1, wherein the glass-ceramic has a thickness between 1 to 15 mm.

13. The product of claim 1, wherein the glass-ceramic further comprises V.sub.2O.sub.5 and Cr.sub.2O.sub.3 and has a (V.sub.2O.sub.5+Cr.sub.2O.sub.3)/MoO.sub.3 ratio in a range from at least 0.005 to 0.5.

14. The product of claim 1, wherein the glass-ceramic further comprises V.sub.2O.sub.5 and Cr.sub.2O.sub.3 and has a (V.sub.2O.sub.5+Cr.sub.2O.sub.3)/MoO.sub.3 ratio in a range from at least 0.03 to 0.15.

15. The product of claim 1, wherein the glass-ceramic has a MoO.sub.3/TiO.sub.2 ratio between 0.002 and 0.050.

16. The product of claim 1, wherein the glass-ceramic has a MoO.sub.3/TiO.sub.2 ratio between 0.008 and 0.030.

17. The product of claim 1, wherein the glass-ceramic further comprises SnO.sub.2 and wherein Li.sub.2O+SnO.sub.2<5.8.

18. The product of claim 1, wherein the glass-ceramic further comprises SnO.sub.2 and wherein Li.sub.2O+SnO.sub.2<4.5.

19. The product of claim 1, wherein the glass-ceramic has a property that after passage through the glass-ceramic, based on a thickness of 4 mm, light of the standard illuminant D65 has a colour locus in the white region A2 which in the CIExyY-2° chromaticity diagram is defined by the following coordinates: TABLE-US-00008 A2 X y 0.290 0.315 0.345 0.370 0.341 0.320 0.303 0.283 0.290 0.315.

20. The product of claim 1, wherein the product is configured for a use selected from a group consisting of a cooking appliance, a baking oven, a kitchen furnishing, a splash panel for a kitchen, an internal or external lining of stove, a viewing window of a stove, a grill, a refrigerator, a microwave appliance, a cover or facing of an extractor hood, a mobile phone, a tablet computer, a motor vehicle, a laboratory appliance, a laboratory furnishing, a fire protection glazing, a viewing window for a high-temperature process chamber, an IR emitter cover, a privacy screen, a cover of a user interface in a control panel, and a cover for an induction charging station.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0230] The invention is elucidated in more detail with reference to the figures.

[0231] FIG. 1 shows the black-body curve (“BBC”), the chromaticity coordinates of light of the standard illuminant D65 on direct viewing without glass-ceramic, “D65”), the position of the white regions A1 and A2, and the chromaticity coordinates of light of the standard illuminant D65 after the passage through a glass-ceramic of Examples 1-16 (“Ex. 1-16”) in a detail of the CIE chromaticity diagram in the CIExyY colour space.

[0232] FIG. 2 shows the same data as FIG. 1 for the overview of the position, but not in a detail, but instead with representation of the entire CIExyY colour space.

[0233] FIG. 3 shows the relationship between the resistivity and the frequency of the alternating current for different temperatures. The measurements were performed on a glass-ceramic according to Example 17.

[0234] FIG. 4 shows the relationship between the relative permittivity and the frequency of the alternating current for different temperatures. The measurements were performed on a glass-ceramic according to Example 17.

[0235] FIG. 5 shows the relationship between the loss factor and the frequency of the alternating current for different temperatures. The measurements were performed on a glass-ceramic according to Example 17.

[0236] FIG. 6 shows the relationship between the relative permittivity and the temperature of the glass-ceramic for an alternating current frequency of 1.9 GHz and 5.1 GHz. The measurements were performed on a glass-ceramic according to Example 17.

[0237] FIG. 7 shows the relationship between the loss factor and the temperature of the glass-ceramic for an alternating current frequency of 1.9 GHz and 5.1 GHz. The measurements were performed on a glass-ceramic according to Example 17.

[0238] FIG. 8 shows the relationship between the relative permittivity and the temperature of the glass-ceramic for an alternating current frequency of 1 MHz, 1.9 GHz and 5.1 GHz. The measurements were performed on a glass-ceramic according to Example 17.

[0239] FIG. 9 shows the relationship between the loss factor and the temperature of the glass-ceramic for an alternating current frequency of 10 KHz, 100 KHz, 1 MHz, 1.9 GHz and 5.1 GHz. The measurements were performed on a glass-ceramic according to Example 17.

[0240] FIG. 10a shows the spectral transmittance of a glass-ceramic of the invention (reference symbol 1) at a thickness of 4 mm in the wavelength range of 380 nm to 780 nm. Additionally FIG. 10a shows the spectral transmittance of the same glass-ceramic with an additional anti-scratch layer (reference symbol 2) at a thickness of 4 mm in the wavelength range 380 nm to 780 nm.

[0241] FIG. 10b shows the same data as FIG. 10a, but in the wavelength range 250-2500 nm.

DETAILED DESCRIPTION

[0242] The black-body curve BBC in FIGS. 1 and 2 shows the colour locus of the light emitted by a black-body radiator at different radiator temperatures on direct viewing (without glass-ceramic). Since the sun is a black-body radiator, light whose colour locos lies on the curve BBC is perceived to be particularly white, natural and pleasing.

[0243] Light of the standard illuminant D65 on direct viewing (without glass-ceramic) lies at the colour coordinates x=0.3127, y=0.3290. When D65 standard light passes through an absorbing glass-ceramic, the colour locus of the light may shift as a result of the absorption of the glass-ceramic. D65 standard light corresponds approximately to a black-body radiator having a temperature of approximately 6500 K.

[0244] For all of Examples 1 to 17, the colour locus of light of the standard illuminant D65 on passage through a corresponding sample is shifted only minimally along the BBC. Examples 1 to 4 lie almost exactly on the BBC. For all of the examples of the colour locus lies within the region A1. Examples 1 to 4 also lie within A2. Example 5 lies slightly below the BBC and just outside A2. Accordingly all of the examples have a very colour-neutral behaviour with respect to transmitted light. Colour shifts are only minimal.

[0245] Examples 1 to 17 are elucidated in more detail with Table 1. Examples 1 to 17 are glass-ceramic suitable for use in products of the invention. Comparative Example V1 is a glass-ceramic which does not conform to the invention.

[0246] The crystallisable starting glasses of the examples from Table 1 were produced in plants for the industrial production of LAS glass-ceramics. Sources of knowledge about melting operations for the industrial production of LAS glass-ceramics include EP 2 226 303 A2 and “Allgemeine Technologie des Glases, Grundlagen des Schmelzens and der Formgebung” [General Glass Technology, Principles of Melting and of Shaping], Prof. Dr H. A. Schaeffer, Erlangen, September 1985. The crystallisable starting glasses are also referred to below as green glass or green glasses.

[0247] Production took place using batch raw materials customary in the industry; for example:

TABLE-US-00005 Constituent Customary raw materials Al.sub.2O.sub.3 Spodumene, petalite, aluminium trihydroxide BaO Barium carbonate CaO Dolomite, calcium feldspar Fe.sub.2O.sub.3 Iron oxide, spodumene K.sub.2O Potassium feldspar, potassium nitrate Li.sub.2O Spodumene, petalite, lithium carbonate MgO Magnesite, dolomite Na.sub.2O Sodium feldspar, sodium nitrate, sodium chloride SiO.sub.2 Quartz sand, spodumene, petalite SnO.sub.2 Tin oxide TiO.sub.2 Rutile, synthetic titanium dioxide V.sub.2O.sub.5 Vanadium oxide ZnO Zinc oxide ZrO.sub.2 Zirconium silicate

[0248] The melting unit used was a continuously operated hybrid tank with oxyfuel burners and electrical additional heating. Likewise conceivable, however, is the use of a continuously operating all-electric tank or an exclusively fossil-heated tank as the melting unit. In the case of fossil-heated tanks, the fuel employed may alternatively be a mixture of fossil fuel and hydrogen, or hydrogen-based fuel without a fossil component. The use of synthetic fuels is also conceivable.

[0249] Designation “continuously operating” here means that batch is supplied continuously to the melting unit at one end and molten glass is withdrawn continuously at the opposite end.

[0250] Contrasting with these are discontinuously operating melting units. In the case of discontinuous operating units, the batch is introduced into a sufficiently cold crucible. The crucible is heated, and so the batch is melted and a glass melt is formed. The glass melt is subsequently poured from the crucible and subjected to hot shaping. When the crucible is sufficiently cooled again, it can be filled again with batch and a new melting operation performed. Therefore glass can be removed only discontinuously (batchwise) from a unit of this kind.

[0251] The specific melting performance of a unit indicates the number of metric tons of glass that can be produced on a daily basis, based on the base area of the unit. It is reported in units of metric tons per square metre per day (i.e. t/(m.sup.2 d).

[0252] The base area of the unit is a product of the distance between the inlet for the batch at the front end and the distributor at the rear end of the unit, in other words the total length of the glass melt in the unit, and also the width of the glass melt in the unit. A unit having a width of 8 m and a length of 15 m defined in this way has a base area of 120 m.sup.2, for example. Units having base areas of between around 50 and 250 m.sup.2 are commonplace for the production of glass-ceramic.

[0253] Another factor effecting the specific melting performance is the way in which the unit is operated. The hotter it is operated, the quicker the progress of thermal operations such as melting and refining in the glass melt. Consequently larger amounts of glass can be withdrawn from the unit at higher temperatures.

[0254] Where a unit is operated with a very low specific melting performance, the economics of the production operation are impaired. On operation with a very high specific melting performance, there is a risk of the high throughput having adverse effect on glass quality. This may be the case, for example, if the glass melt has such a short residence time in the unit that melting processes cannot take place completely or there is not sufficient time available for refining.

[0255] Glass-ceramics of the invention can be produced for example in units having a specific melting performance of 0.3 t/(m.sup.2 d) to 1.5 t/(m.sup.2 d), preferably 0.4 to 1.2 t/(m.sup.2 d), more preferably 0.5 to 1.1 t/(m.sup.2 d).

[0256] Surprisingly, glass-ceramics produced in a continuously operating melting unit having a specific performance of at least 0.3 t/(m.sup.2 d) and at most 1.5 t/(m.sup.2 d), especially in relation to glass-ceramics melted discontinuously in crucibles, exhibit improved properties such as a particularly good colour neutrality and a reduced requirement for colouring components in the composition of the glass-ceramic.

[0257] The produced observed for the examples from Table 1 was as follows:

[0258] Provision of the batch and introduction into the tank. To improve the melting behaviour, shards from the closed shard circuit of the tank or optionally from recycling circuits are typically added to the batch.

MELTING OF THE BATCH

[0259] Defining of the glass melt at glass melt temperatures of at least 1550° C., preferably 1600° C., more preferably 1650° C. The upper limit on the refining temperature is generally dependant on the temperature stability of the tank materials. The refining temperature is typically below 1850° C., preferably below 1800° C., more preferably below 1750° C. At temperatures of more than 1700° C. it is generally necessary for additional high-temperature refining units to be used. This means an additional energy expenditure for production, but in certain circumstances this can be competitive as a result of a greater specific melting performance and an improved glass quality.

[0260] Cooling of the glass melt to temperatures at which hot shaping, such as rolling, for example, is possible.

[0261] Implementation of hot shaping, for example rolling to a glass strip thickness of 4 mm, for example.

COOLING TO ROOM TEMPERATURE SEPARATION INTO INDIVIDUAL PLATES, AND STACKING

[0262] The examples from Table 1 were produced with a specific melting performance of 1 t/(m.sup.2 d). The examples were each produced in the form of rectangular plates having dimensions of 520×590×4 mm.

[0263] The water content of crystallisable glasses produced in this way for the production of the glass-ceramics is preferably between 0.01 and 0.09 mol/l, depending on the choice of the batch raw materials and the operating conditions at the melting stage. The method of determining water content is described for example in EP 1074520 A1. The water content can be adapted to the choice of the batch raw materials and the operating conditions.

[0264] The samples 1-17 are ceramised with a ceramisation process in a continuous furnace, with the following steps:

a) heating from room temperature to 740° C. at a heating rate of 30 K/min,
b) temperature increase from 740 to 825° C. at a heating rate of 6 K/min,
c) temperature increase from 825° C. to 930° C. at a heating rate of 18 K/min,
d) hold time of 6 min at 930° C. maximum temperature,
e) cooling to 800° C. at a cooling rate of 13 K/min, then rapid cooling to room temperature by discharge of the sample from the furnace.

[0265] The samples 18-24 are ceramised with a ceramisation process in a continuous furnace, with the following steps:

a) heating from room temperature to 740° C. at a heating rate of 30 K/min,
b) temperature increase from 740 to 825° C. at a heating rate of 6 K/min,
c) temperature increase from 825° C. to 930° C. at a heating rate of 15 K/min,
d) hold time of 6 min at 926° C. maximum temperature,
e) cooling to 800° C. at a cooling rate of 12 K/min, then rapid cooling to room temperature by discharge of the sample from the furnace.

[0266] It is known that the temperatures value, hold times and heating rates during ceramisation can be adapted, starting from the example given above, in order to optimise the properties of the resultant glass-ceramic. In particular it may be advantageous to select higher heating/cooling rates than those in the example indicated. The heating/cooling rates ought, however, not to be below the following values: 20 K/min in step a), 2 K/min in step b), 8 K/min in step c), and cooling at a cooling rate of at least 8 K/min in step d). Higher rates, as well as influencing the physical properties, are beneficial to the cycle times of the ceramisation process. The temperature in step d) may be varied for example in the range 890-940° C.

[0267] In practice the heating rates actually achievable are dependant both on the geometry/thermal mass of the plates for the ceramised, and on the heating units available.

[0268] Plates ceramised in this way may have a glassy, amorphous and lithium-depleted superficial zone at their surface. Such zones are known for example from WO 2012 019 833 A1. They improve the resistance of the glass-ceramic to chemical attacks. The glassy zone has a thickness of between 20 and 5000 nm, preferably of between 30 and 3000 nm, more preferably of between 50 and 1500 nm. By polishing it is possible to remove the glassy zone from a glass-ceramic surface after ceramisation.

[0269] The impurities caused by typical trace elements in the case of the technical raw materials used were—unless otherwise specified—up to 200 ppm B.sub.2O.sub.3, 30 ppm Cl, 1 ppm CoO, 3 ppm Cr.sub.2O.sub.3, 200 ppm Cs.sub.2O, 3 ppm CuO, 200 ppm F, 400 ppm HfO.sub.2, 3 ppm NiO, 500 ppm Rb.sub.2O, 5 ppm V.sub.2O.sub.5.

[0270] Table 1 shows the composition of Examples 1 to 17 and of Comparative Example V1 and also certain selected properties after the ceramisation. Baring customary impurities, the examples contain no Sb.sub.2O.sub.3 and As.sub.2O.sub.3. All examples contain MoO.sub.3 in the corresponding amounts as main colorant.

[0271] Comparative Example V1 was produced in a discontinuous process, rather than by the process described above in a continuous melting unit like Examples 1 to 17. The crystallisable starting glass was melted in each case from technical batch raw materials customary in the glass industry at temperatures of 1620° C., in 4 hours. After the melting of the batch in a crucible made of sintered silica glass, the melts were poured into Pt/Rh crucibles with an inner silica glass crucible and homogenized by stirring at temperatures of 1600° C., for 60 minutes. Following this homogenisation, the glass was refined at 1640° C. for 2 hours. Subsequently pieces with a size of around 120×140×30 mm.sup.3 were cast and were cooled to room temperature beginning from 640° C. in a cooling oven into dissipate stresses. The castings were divided into the sizes needed for the studies and for the ceramisation. The ceramisation took place by the process described above, which was also used to ceramise Examples 1 to 17.

[0272] In spite of the relatively high MoO.sub.3 content of 0.11% by weight, the comparison example has a luminous transmittance of more than 4% at a thickness of 4 mm. As a result of this high luminous transmittance, the poorer opacity of this glass-ceramic makes it unsuitable for use in products of the invention. The comparison example, furthermore, has an undesirably high ZnO fraction, of more than 2% by weight.

[0273] The comparative example has a temperature T(pO.sub.2=1 bar) of less than 1550° C. It therefore contains a relatively large amount of oxygen dissolved in the glass-ceramic. By comparison with this, Examples 1-5 have higher temperatures T(pO.sub.2=1 bar), in the range of 1592° C.-1633° C. They therefore contain less dissolved oxygen than Comparative Example V1.

[0274] Examples 1 to 17 contain 7 to 35 ppm V.sub.2O.sub.5 and 10 to 41 ppm Cr.sub.2O.sub.3. Accordingly, as is evident from FIG. 1, it is possible to establish the colour locus of transmitted light of the standard illuminant D65 at a desired value very precisely within the white region A1. The Comparative Example V1 contains neither V.sub.2O.sub.5 nor Cr.sub.2O.sub.3.

[0275] Table 1 lists the spectral transmittances at different wavelengths in the visible and near-infrared spectral range. All values reported are based on a glass-ceramic thickness of 4 mm. The values provide evidence of the advantageous transmission profile for the products of the invention, with low transmission in the visible spectral range from 380 to 780 nm and high transmission in the near infrared from 780 to 4000 nm. A profile of this kind makes the glass-ceramic suitable for producing cover plates which prevent viewing through the plate yet transmit thermal radiation or IR radiation for optical communication or sensor technology.

[0276] With these values as well it is apparent that the transmission of Comparative Example V1 at the specified wavelengths if significantly higher than that of Examples 1 to 17.

[0277] The colour coordinates in the CIExyY colour space (1931, 2°) of light of the standard illuminant D65 after passage through a sample 4 mm in thickness are likewise listed in Table 1. The Y coordinate is identical to the luminous transmittance τ.sub.vis according to DIN EN 410.

[0278] For the examples a determination was made of the thermal expansion in the 20° C. to 700° C. temperature range. All of the examples have values of significantly less than 1×10.sup.−6/K. On the basis of this low expansion, these glass-ceramics are particularly suitable for use in products such as hobs, for example, which are subject to high temperature change loads or high local temperature gradients.

[0279] Through x-ray diffraction it was possible to confirm all of the examples as comprising high-quartz solid solution (“HQSS”) as main crystal phase. This is also apparent from the fact that at a thickness of 4 mm the examples exhibited virtually no scattering acceptable to the naked eye for visible light.

[0280] The electrical properties of the glass-ceramics were studied representatively for all the examples on the basis of Example 17. The results are set out in FIGS. 3 to 9. The glass-ceramics exhibit electrical properties typical of LAS glass-ceramics. The resistivity for electrical current, R.sub.spec, decreases as the temperature of the glass-ceramic goes up. In addition the resistivity for alternating current goes down as the frequency of the alternating current goes up. In this regard, FIG. 3, illustratively, shows measurement data for the resistivity, recorded for temperatures from −15° C. to 350° C. and for frequencies from 10 KHz to 40 MHz. The values extend over more than 4 powers of ten, from 5.5*10.sup.4 Ω cm to 1.8*10.sup.9 Ω cm. This can presumably be explained by an altered mobility of charge carriers in the glass-ceramic under the corresponding conditions.

[0281] Accordingly, for alternated electrical voltages which may be present in particular between the upper side and the lower side of the glass-ceramic, there are also changes in the electrical properties of the glass-ceramic. These properties, quantified here with measurements of the parameters of the relative permittivity ϵ.sub.r and the loss factor tan (δ) of the glass-ceramic, are represented illustratively in FIGS. 4 to 9 as a function of the frequency of the applied alternating voltage and of the temperature of the glass-ceramic. FIGS. 4 and 5 show the frequency dependency of the relative permittivity and the loss factor for various temperatures occurring in particular in cooking applications of a glass-ceramic hob, over a frequency range from 10 KHz to 40 MHz. FIGS. 6 and 7 show the temperature dependency of the relative permittivity and the loss factor for the frequencies 1.9 GHz and 5.1 GHz. These frequencies are illustrative of a frequency range which is important in microwave heating applications or in wireless signal networks, such as WLAN/WiFi. FIGS. 8 and 9 likewise show the temperature dependencies as in FIGS. 6 and 7, extended to include illustrative frequencies from the 10 KHz to 40 MHz range and here to include a temperature range of up to 350° C. which is typical in specifically of cooking applications.

[0282] Alternating electrical voltages may find technical applications as a measurement signal or else for power transmission. In particular in the frequency range from 10 KHz to 40 MHz, in metrology, for example, capacitive touch sensors or else capacitive or incaptive temperature measurements, owing in particular to the temperature dependency of the electrical properties, may be provided. An important factor for touch sensors in particular is the relative permittivity at room temperature. The values for the relative permittivity here illustratively at 25° C., in the range 7.9 to 8.3, are stable over a relevant frequency range from 10 KHz to 40 MHz (FIG. 4). The glass-ceramics are especially suitable for capacitive touch sensors because the relative permittivity exhibits significantly higher values than for other materials with typically lower relative permittivities. Reference is for comparison include plastics (2-4), natural substances such as wood or paper (1-4), or glass (about 6-7).

[0283] The temperature dependency of the relative permittivity and of the loss factor (FIGS. 8 and 9), which is strongly pronounced particularly in the lower frequency range (10 KHz to 40 MHz), can be utilized for example for capacitive or inductive temperature measurements. A capacitive temperature measurement may be developed, for example, between a sensor electrode beneath the glass-ceramic and the base of a pan above the glass-ceramic. The relative permittivity which is relevant for capacitive measurements changes here, illustratively, between 25° C. and 350° C. from 8.1 to 25.8 by a factor of about 3, and this can be converted in a known way into an electrical signal change. In just the same way, the loss factor changing over the temperature can be utilized by an inductive temperature measurement. For example, the transmission loss between an emitting coil beneath the glass-ceramic and a sensor coil in a cooking vessel above the glass-ceramic changes at a signal frequency of 10 KHz between 25° C. and 350° C. from 0.03 to 8.9 by a factor of about 300, or between 50° C. and 150° C. from 0.05 to 0.25 by a factor of 5. A sensor coil beneath the glass-ceramic would also, via its self-induction, be sensitive to the temperature-dependent change in the electrical properties of the glass-ceramic material in its vicinity.

[0284] The temperature-dependent changes in the loss factor are also relevant for inductive heating, as is found, for example, in induction hobs. Typical operating frequencies here are between 10 KHz and 100 KHz. An increase in the loss factor with the temperature in this frequency range (FIG. 9) leads to a decrease in the power transmission into the cooking vessel and/or to a measurable increase in the power loss in the resident circuit of the inductive heating. This can also be utilized indirectly for a temperature measurement at the cooking vessel.

[0285] Further applications in which a part is played in particular by the loss factor in the context of power transmissions for different frequencies are inductive charging equipment for mobile terminal devices on hob plates or worktops made from glass-ceramic, or else applications in the field of electromobility. Also subject to the electrical properties in the manner described illustratively above are galvanically isolated signal transmissions through glass-ceramic which is used as a partition wall and protective wall in areas insulated and protected biological, chemically or physically (radiation).

[0286] As a further working example, a 4 mm glass ceramic according to Example 17 was coated with an anti-scratch layer. The layer was produced by means of reactive moderate-frequency sputtering as described in WO 2014/135490 A1. The coating is an x-ray-amorphous AlSiN layer with an Al:Si ratio of 50:50 at %. The layer thickness is about 1200 nm. FIGS. 10a and 10b are plots, for comparison, of the spectral transmittances of the uncoated glass-ceramic (reference symbol 1) and of the glass-ceramic with coating (reference symbol 2) against the wavelength of the light in nm. In comparison to the uncoated glass-ceramic, there is only an interference modulation of the transmission from the substantially transparent coating. As a result, the luminous transmittance is lowered slightly. The lowering amounts to around 8% in relative terms, corresponding to an absolute lowering of around 0.2%. The colour locus of light of the standard illuminant D65 is altered by the coating only to so small an extent that the change is imperceptible to the naked eye. The colour difference in the transmitted light can be calculated by the following formula:

ΔC=√{square root over ((x.sub.GK−x.sub.GK+S).sup.2+(y.sub.GK−y.sub.GK+S).sup.2)}.

[0287] In this formula, x.sub.GK/y.sub.CK are the colour coordinates of light of the standard illuminant D65 after passage through the uncoated glass-ceramic (here: x.sub.GK=0.3184, y.sub.GK=0.3270), and x.sub.GK+S/y.sub.GK+S are the colour coordinates after passage through the glass-ceramic with coating (here: x.sub.GK+S=0.3202, y.sub.GK+S=0.3284). For the coating shown in FIG. 3, accordingly, ΔC is only 0.002 and is therefore imperceptible to the human eye.

[0288] For comparison, again, reference may be made to the colour coordinates of light of the standard illuminant D65 when viewed directly, in other words without a glass-ceramic: x=0.3127, y=0.3290. In this case, therefore, the colour of light of the standard illuminant D65 is not markedly shifted either by the glass-ceramic on its own or by the combination of glass-ceramic with anti-scratch coating.

[0289] Products of the invention can be used in a multiplicity of applications.

[0290] In one embodiment of the product of the invention can be used in cooking appliance as a cooking plate.

[0291] In another embodiment a product of the invention may be used for covering a user interface in a control panel for controlling at least one household appliance, more particularly a cooking appliance, baking oven or refrigerator or an extractor hood.

[0292] In one development of this embodiment, the control panel may be configured for controlling multiple household appliances, such as for a cooking appliance, a baking oven and an extractor hood, for example. An extractor hood may also be integrated in the form of a downdraft extractor hood into a cooking appliance having a corresponding glass-ceramic cooking plate. For this purpose the product or the glass-ceramic may then have a cutout into which the downdraft extractor hood can be inserted. In a further embodiment the product may be designed as a cover for an extractor hood, more particularly a downdraft extractor hood. In modular cooking systems in particular, downdraft extractor hoods may be designed as a separate module without a cooking function. Since, however, such modules must be suitable for use in combination with modules having a cooking function, they are likewise required to meet the customary, very exactly requirements in terms of thermal and chemical stability. Furthermore, such modules may also have a user interface for the control of the downdraft remover. For user interfaces of this kind as well, the objective stated above in terms of colour-neutral displays must be fulfilled.

[0293] In a further embodiment, the product may be designed as the facing of an extractor group. In this embodiment it may be particularly aesthetic appealing if the hob of the cooking appliance and the facing of the extractor hood feature the same glass-ceramic. This embodiment is particularly advantageous if the extractor hood features a user interface arranged behind the glass-ceramic for controlling the extractor hood, or a user interface for the combined control of the extractor hood and the cooking appliance.

[0294] In baking ovens, especially pyrolysis ovens, the product of the invention will be used as part of door glazing.

[0295] In a further embodiment the product of the invention may be used in item of kitchen furniture, more particularly a kitchen cabinet, in a cooking table, as a worktop.

[0296] In another embodiment a product of the invention may be used as a splash protection plate for kitchens. Hence it may be used, for example, in the form of a plate as the back wall of a kitchen, in place of a tile mirror, for example. It may also be provided as a free-standing splash protection plate on a kitchen island. A splash protection plate of this kind may either be permanently installed, or have a recessible design. Recessible splash protection plates can be extended for the operation of a cooking appliance, to act as splash protection. After the end of cooking, they can then be recessed, for example, in the cooking appliance or in the worktop. In this case it may be particularly appealing aesthetically if both the cooking plate of the cooking appliance and the splash protection plate feature the same glass-ceramic of a product of the invention. The kitchen worktop additionally may likewise contain the same glass-ceramic.

[0297] Splash protection plates for kitchens are regularly splashed during cooking with hot liquids such as salt water or animal or vegetable fat. They are regularly cleaned with chemical cleaning products as well. Given the high thermal and chemical stability of products of the invention, they are especially suitable for use as a splash protection plate for kitchens.

[0298] In another embodiment a product of the invention may be used in a laboratory appliance, more particularly a hotplate, an oven, a balance or an item of laboratory furniture, more particularly an extractor, a cabinet or a bench, for covering a user interface or as a worktop.

[0299] In another embodiment a product of the invention may be used as a stove sightgIasses for combustion chambers and other high-temperature process chambers, as fire resistant glazing, as part of a housing for mobile electronic devices, especially mobile phones and tablet computers, as a cover for IR heating lamps or gas burners, especially in gas grills, as a privacy screen or as a cover for induction charging stations, for example for motor vehicles/automobiles, for example in the dashboard region or the centre console.

[0300] On the basis of its thermal chemical stability, the product in these context can be used in stoves both for interiors and for the exterior area. Such stoves may be fired for example with gas, wood or pellets.

[0301] High-temperature process chambers may for example be vacuum coating systems.

[0302] In grill appliances, there may be diverse possible uses for a product of the invention. It may be used to provide a protective cover for gas burners, concealing the gas burners in the switched-off state but revealing the flame in the switched-on state. As the colour of the flame is an important indicator for the proper operation of the grill, a true-colour representation of the kind possible with the present product is particularly advantageous for operational safety.

[0303] The product may be used, furthermore, as a grilling surface, particularly in gas, electric or charcoal grills, or as a viewing window in the hood of a grill.

TABLE-US-00006 TABLE 1 Examples of glass-ceramics for products of the invention Example 1 2 3 4 5 V1 Composition (wt %) Al.sub.2O.sub.3 wt % 21.29 21.29 21.25 21.24 21.16 21.24 BaO wt % 1.31 1.32 1.32 1.32 1.33 1.19 CaO wt % 0.44 0.44 0.441 0.438 0.436 0.55 Cr.sub.2O.sub.3 wt % 0.0041 0.0024 0.0024 0.0023 0.001 0 Fe.sub.2O.sub.3 wt % 0.091 0.092 0.091 0.090 0.098 0.070 K.sub.2O wt % 0.41 0.41 0.41 0.41 0.40 0.57 Li.sub.2O wt % 3.78 3.76 3.78 3.77 3.80 3.45 MgO wt % 0.31 0.30 0.31 0.30 0.30 0.35 MnO.sub.2 wt % 0.021 0.021 0.021 0.021 0.021 0.01 MoO.sub.3 wt % 0.052 0.055 0.055 0.054 0.068 0.11 Na.sub.2O wt % 0.56 0.57 0.56 0.56 0.56 0.52 P.sub.2O.sub.5 wt % 0.052 0.052 0.052 0.054 0.052 0 SiO.sub.2 wt % 65.21 65.19 65.22 65.25 65.26 65.55 SnO.sub.2 wt % 0.28 0.28 0.28 0.28 0.28 0.25 SrO wt % 0.0073 0.0073 0.0074 0.0075 0.0096 0 TiO.sub.2 wt % 3.64 3.64 3.65 3.65 3.66 3.41 V.sub.2O.sub.5 wt % 0.0023 0.0007 0.0008 0.0009 0.0035 0 ZnO wt % 1.58 1.59 1.58 1.58 1.58 2.12 ZrO.sub.2 wt % 0.90 0.91 0.91 0.91 0.92 1.05 Spectral transmittance at 4 mm thickness  470 nm % 1.7 2.5 2.4 2.4 1.2 5.4  600 nm % 1.5 1.7 1.6 1.6 0.8  700 nm % 4.1 4.0 3.9 4.0 2.9  950 nm % 35.0 33.7 33.5 33.7 33.1 46.6 1600 nm % 70.0 69.6 69.6 69.9 67.8 73.8 Transmittance at 4 mm thickness Y % 1.5 1.8 1.7 1.7 0.8 34.1 Colour coordinates in transmission at 4 mm thickness x 0.330 0.299 0.298 0.298 0.308 0.305 y 0.334 0.310 0.308 0.307 0.286 0.297 dissolved oxygen T (pO2 = 1 bar) ° C. 1633 1608 1592 1604 1610 1543 Density glassy g/cm.sup.3 2.46 2.46 2.46 2.46 2.46 Example 6 7 8 9 10 11 Composition (wt %) Al.sub.2O.sub.3 wt % 21.24 21.29 21.29 21.29 21.29 21.29 BaO wt % 1.31 1.31 1.31 1.31 1.31 1.31 CaO wt % 0.44 0.44 0.44 0.44 0.44 0.44 Cr.sub.2O.sub.3 wt % 0.0031 0.0025 0.0035 0.0035 0.0025 0.0035 Fe.sub.2O.sub.3 wt % 0.089 0.089 0.089 0.094 0.089 0.089 K.sub.2O wt % 0.43 0.41 0.41 0.41 0.41 0.41 Li.sub.2O wt % 3.74 3.74 3.74 3.74 3.74 3.74 MgO wt % 0.30 0.31 0.31 0.31 0.31 0.31 MnO.sub.2 wt % 0.020 0.021 0.021 0.021 0.021 0.021 MoO.sub.3 wt % 0.048 0.046 0.046 0.059 0.051 0.046 Na.sub.2O wt % 0.56 0.56 0.56 0.56 0.56 0.56 P.sub.2O.sub.5 wt % 0.021 0.052 0.052 0.052 0.052 0.052 SiO.sub.2 wt % 65.27 65.21 65.21 65.21 65.21 65.21 SnO.sub.2 wt % 0.28 0.28 0.28 0.28 0.28 0.28 SrO wt % 0.01 0.0073 0.0073 0.0073 0.0073 0.0073 TiO.sub.2 wt % 3.64 3.64 3.64 3.64 3.64 3.64 V.sub.2O.sub.5 wt % 0.0017 — — — — 0.0026 ZnO wt % 1.58 1.58 1.58 1.58 1.58 1.58 ZrO.sub.2 wt % 0.90 0.90 0.90 0.90 0.90 0.90 Transmittance at 4 mm thickness Y % 2.3 2.87 2.53 1.96 2.31 2.24 Colour coordinates in transmission at 4 mm thickness x 0.320 0.303 0.312 0.328 0.298 0.326 y 0.327 0.319 0.333 0.333 0.312 0.337 Example 12 13 14 15 16 17 Composition (wt %) Al.sub.2O.sub.3 wt % 21.29 21.29 21.29 21.29 21.29 21.34 BaO wt % 1.31 1.31 1.31 1.31 1.31 1.31 CaO wt % 0.44 0.44 0.44 0.44 0.44 0.44 Cr.sub.2O.sub.3 wt % 0.0025 0.0035 0.0035 0.0025 0.0035 0.0033 Fe.sub.2O.sub.3 wt % 0.094 0.094 0.089 0.094 0.094 0.092 K.sub.2O wt % 0.41 0.41 0.41 0.41 0.41 0.41 Li.sub.2O wt % 3.74 3.74 3.74 3.74 3.74 3.76 MgO wt % 0.31 0.31 0.31 0.31 0.31 0.30 MnO.sub.2 wt % 0.021 0.021 0.021 0.021 0.021 0.021 MoO.sub.3 wt % 0.046 0.046 0.051 0.051 0.051 0.054 Na.sub.2O wt % 0.56 0.56 0.56 0.56 0.56 0.57 P.sub.2O.sub.5 wt % 0.052 0.052 0.052 0.052 0.052 0.052 SiO.sub.2 wt % 65.21 65.21 65.21 65.21 65.21 65.16 SnO.sub.2 wt % 0.28 0.28 0.28 0.28 0.28 0.28 SrO wt % 0.0073 0.0073 0.0073 0.0073 0.0073 0.0073 TiO.sub.2 wt % 3.64 3.64 3.64 3.64 3.64 3.64 V.sub.2O.sub.5 wt % 0.0026 0.0026 0.0026 0.0026 0.0026 0.0015 ZnO wt % 1.58 1.58 1.58 1.58 1.58 1.59 ZrO.sub.2 wt % 0.90 0.90 0.90 0.90 0.90 0.91 Spectral transmittance at 4 mm thickness  470 nm % 2.6  600 nm % 2.1  700 nm % 5.2  950 nm % 37.4 1600 nm % 71.3 Transmittance at 4 mm thickness Y % 1.97 1.73 1.8 1.59 1.4 2.1 Colour coordinates in transmission at 4 mm thickness x 0.332 0.342 0.321 0.328 0.337 0.318 y 0.323 0.337 0.330 0.317 0.332 0.327 Density ceramized g/cm.sup.3 2.54 Example 18 19 20 21 22 23 Composition (wt %) Al.sub.2O.sub.3 wt % 21.27 21.27 21.35 21.28 21.57 21.52 BaO wt % 1.32 1.31 1.33 1.32 1.32 1.33 CaO wt % 0.44 0.44 0.44 0.44 0.44 0.44 Cr.sub.2O.sub.3 wt % 0.003 0.0032 0.0032 0.003 0.0031 0.0031 Fe.sub.2O.sub.3 wt % 0.092 0.093 0.093 0.091 0.092 0.092 K.sub.2O wt % 0.42 0.41 0.42 0.42 0.41 0.4 Li.sub.2O wt % 3.76 3.75 3.75 3.75 3.75 3.73 MgO wt % 0.3 0.31 0.3 0.31 0.3 0.3 MnO.sub.2 wt % 0.011 0.011 0.011 0.011 0.011 0.011 MoO.sub.3 wt % 0.063 0.061 0.059 0.059 0.057 0.058 Na.sub.2O wt % 0.57 0.57 0.57 0.58 0.56 0.55 P.sub.2O.sub.5 wt % 0.061 0.058 0.064 0.059 0.066 0.066 SiO.sub.2 wt % 65.3 65.27 65.21 65.25 64.98 65.02 SnO.sub.2 wt % 0.27 0.27 0.27 0.27 0.27 0.27 SrO wt % 0.013 0.014 0.009 0.009 0.015 0.015 TiO.sub.2 wt % 3.59 3.63 3.62 3.68 3.68 3.68 V.sub.2O.sub.5 wt % 0.0017 0.0016 0.0016 0.0015 0.0013 ZnO wt % 1.56 1.56 1.57 1.57 1.57 1.57 ZrO.sub.2 wt % 0.96 0.97 0.93 0.9 0.9 0.9 Spectral transmittance at 4 mm thickness  470 nm % 2.6 2.4 2.5 2.0 2.6 2.6  600 nm % 2.2 2.0 2.1 1.6 2.2 2.1  700 nm % 5.7 5.3 5.5 4.2 5.3 5.2  950 nm % 1600 nm % 69.8 69.7 69.9 69.5 70.1 69.9 Transmittance at 4 mm thickness Y % 2.2 2.0 2.1 1.6 2.2 2.2 Colour coordinates in transmission at 4 mm thickness x 0.325 0.324 0.323 0.318 0.319 0.316 y 0.328 0.327 0.327 0.322 0.325 0.325 Density glassy g/cm.sup.3 2.45 2.45 2.46 2.45 2.46 2.46 ceramized g/cm.sup.3 2.54 2.54 2.54 2.54 2.54 2.54 Example 24 Composition (wt %) Al.sub.2O.sub.3 wt % 21.23 BaO wt % 1.33 CaO wt % 0.44 Cr.sub.2O.sub.3 wt % 0.003 Fe.sub.2O.sub.3 wt % 0.092 K.sub.2O wt % 0.41 Li.sub.2O wt % 3.76 MgO wt % 0.3 MnO.sub.2 wt % 0.01 MoO.sub.3 wt % 0.059 Na.sub.2O wt % 0.54 P.sub.2O.sub.5 wt % 0.05 SiO.sub.2 wt % 65.37 SnO.sub.2 wt % 0.27 SrO wt % 0.014 TiO.sub.2 wt % 3.61 V.sub.2O.sub.5 wt % 0.0018 ZnO wt % 1.57 ZrO.sub.2 wt % 0.94 Spectral transmittance at 4 mm thickness  470 nm % 2.8  600 nm % 2.5  700 nm % 6.4  950 nm % 1600 nm % 70.1 Transmittance at 4 mm thickness Y % 2.5 Colour coordinates in transmission at 4 mm thickness x 0.328 y 0.331 Density glassy g/cm.sup.3 2.45 ceramized g/cm.sup.3 2.54