Glass ceramic and method for producing same

Abstract

A glass ceramic article is provided so that a reliable coloring with a defined transmittance is ensured. The reliable coloring of the glass ceramic article is based on a high content of iron oxide of more than 0.1 percent by weight which itself has a strongly coloring effect does not further reduce transmittance but rather interacts with vanadium oxide to attenuate the absorption caused by vanadium oxide.

Claims

1. A lithium aluminosilicate glass ceramic article comprising: titanium oxide; vanadium oxide as a color-imparting component in a proportion of at least 0.005; tin oxide in a proportion of 0.15 to 0.5 percent by weight; and a decoloring agent consisting of iron oxide in a proportion of more than 0.1 percent by weight, wherein the proportion by weight of the iron oxide is greater than the proportion by weight of vanadium oxide by a factor from 5 up to a factor of 20, wherein the article has a light transmittance in the visible spectral range when illuminated perpendicularly to a surface of the article that is greater than 2.5%, wherein the proportions of the tin oxide, the titanium oxide, and the iron oxide meet a relationship of (M(SnO.sub.2)+0.1*M(TiO.sub.2))/(M(Fe.sub.2O.sub.3))<4, and wherein M is the respective proportion in percent by weight of the component in the following brackets.

2. A lithium aluminosilicate glass ceramic article comprising: titanium oxide; vanadium oxide as a color-imparting component in a proportion of at least 0.005; tin oxide in a proportion of from 0.15 to 0.5 percent by weight; and a decoloring agent comprising iron oxide in a proportion of more than 0.1 percent by weight and/or cerium oxide in a proportion of at least 0.1 percent by weight, wherein the decoloring agent has a proportion by weight of a sum of the iron oxide and the cerium oxide that is greater than the proportion by weight of vanadium oxide by a factor from 7.7 up to a factor of 20, wherein the article has a light transmittance in the visible spectral range when illuminated perpendicularly to a surface of the article that is greater than 2.5%, wherein the proportions of the tin oxide, the titanium oxide, and the iron oxide and/or the cerium oxide meet a relationship of (M(SnO.sub.2)+0.1*M(TiO.sub.2))/(M(Fe.sub.2O.sub.3)+M(CeO.sub.2))<4, wherein M is the respective proportion in percent by weight of the component in the following brackets, and wherein the sum of the iron oxide and the cerium oxide is at most 0.6 percent by weight.

3. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the proportion of the vanadium oxide is at least 0.05 percent by weight.

4. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the proportion of vanadium content is at least 0.066/x percent by weight, wherein x is a thickness of the article in millimeters.

5. The lithium aluminosilicate glass ceramic article as claimed in claim 4, wherein the thickness is in a range from 2.5 to 7 millimeters.

6. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the proportion of the tin oxide is in a range from 0.2 to 0.45 percent by weight.

7. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the titanium oxide is in a proportion of less than 5 percent by weight.

8. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the titanium oxide is in a proportion of from 2.5 to 5 percent by weight.

9. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the relationship is (M(SnO.sub.2)+0.1*M(TiO.sub.2))/(M(Fe.sub.2O.sub.3)+M(CeO.sub.2))<3.

10. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the sum of the iron oxide and/or the cerium oxide includes iron oxide, and further comprising a ratio of Fe.sub.2O.sub.3 and V.sub.2O.sub.5 is such that in a range of wavelengths between 450 and 600 nanometers a coefficient of determination R.sup.2 resulting for a straight line fitted to a transmittance characteristic of the article using the method of least squares is more than 0.9.

11. The lithium aluminosilicate glass ceramic article as claimed in claim 10, wherein the coefficient of determination R.sup.2 is more than 0.95.

12. The lithium aluminosilicate glass ceramic article as claimed in claim 2, further comprising a proportion by weight of chromium or chromium oxide of less than 0.01%.

13. The lithium aluminosilicate glass ceramic article as claimed in claim 2, wherein the article is a glass ceramic cooktop and further comprising a self-luminous display element positioned below the glass ceramic cooktop and positioned to shine through the glass ceramic cooktop, wherein the self-luminous display element emits light in the visible spectral range with wavelengths of less than 570 nanometers.

14. The lithium aluminosilicate glass ceramic article as claimed in claim 13, further comprising an at least partially light-blocking coating on a lower surface of the glass ceramic cooktop.

15. The lithium aluminosilicate glass ceramic article as claimed in claim 14, wherein the light-blocking coating has a recess, wherein the self-luminous display element is positioned to shine through the recess.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described by way of exemplary embodiments and with reference to the accompanying drawings. In the drawings:

(2) FIG. 1 shows a glass ceramic cooktop including a glass ceramic plate according to the invention;

(3) FIG. 2 shows a variation of the glass ceramic cooktop illustrated in FIG. 1;

(4) FIG. 3 shows the spectral transmittance of two glass ceramics as a function of wavelength;

(5) FIG. 4 shows the spectral transmittance of the starting glasses of the two glass ceramics; and

(6) FIG. 5 shows the spectral transmittance of a glass ceramic according to the invention before and after a heat stress test.

DETAILED DESCRIPTION

(7) The invention is particularly suitable for glass ceramic cooktops. In this case the glass ceramic according to the invention exhibits a transmittance such that a very good visibility and color reproduction for self-luminous display elements is achieved. FIG. 1 shows a schematic side view of an exemplary embodiment of a glass ceramic cooktop 1 that comprises a glass ceramic article according to the invention in form of a glass ceramic plate 3. Glass ceramic plate 3 has an upper surface 31 and a lower surface 32. Heating elements 5 are arranged below lower surface 32 for heating cookware positioned oppositely on the upper surface 31 in a cooking zone 33, or optionally for directly heating food to be boiled or cooked. Glass ceramic plate 3 has a thickness d, typically in a range from 2 to 6 millimeters.

(8) Generally, without being limited to the illustrated example, at least one self-luminous display element 7 may additionally be arranged below the glass ceramic article or glass ceramic plate 3, which shines through the glass ceramic plate 3. By virtue of the improved transmittance of the glass ceramic plate 3 according to the invention, the latter now transmits in particular not only red light in a considerable intensity. Rather, it is possible to display yellow, green, and blue spectral ranges. Accordingly, in one embodiment of the invention the self-luminous display element 7 is adapted to emit light in the visible spectral range with wavelengths of less than 570 nanometers, preferably less than 510 nanometers. A suitable display element is a light emitting diode display, for example. In accordance with the transmittance in the yellow, green, and blue spectral ranges it is then possible to use display elements emitting in the yellow, green, or blue spectral ranges, for example correspondingly yellow, green, or blue light LEDs, as well as white light LEDs. It is also possible, that the display element consist of a color display that allows for a variety of indications and information for the user.

(9) Display element 7 may, for example, be arranged below a display and/or control area 35 of the glass ceramic plate 3, as illustrated. An arrangement in cooking zone 33 is also conceivable, for example for visually signalling which one of the cooking zones is currently enabled and heating. According to yet another embodiment, other than illustrated, the cooking zone may be curved to form a cooking vessel such as a wok. Also, the edges of the glass ceramic may be curved.

(10) Due to the properties of the glass ceramic which will be described in more detail below, even yellow, green, or blue spectral components of the light emitted by the display element 7 will be visible to an observer through the glass ceramic plate 3.

(11) FIG. 2 shows a variation of the embodiment illustrated in FIG. 1. Due to the comparatively high transmittance of the glass ceramic according to the invention, it may optionally be desirable in turn to reduce transmission in the visible spectral range. For this purpose, according to one embodiment of the invention exemplified in FIG. 2, an at least partially light-blocking coating 37 is provided on the lower surface 32 of glass ceramic plate 3.

(12) Light-blocking coating 37 is preferably heat resistant. This will be useful at least if the light-blocking coating 37 extends along cooking zone 33, as illustrated in FIG. 2.

(13) Both light absorbing and light reflecting coatings are contemplated as a light-blocking coating 37. The light-blocking coating 37 serves to ensure that the components of the cooktop arranged below ceramic glass plate 3 remain invisible for an observer. In order to change the design and aesthetics, the light-blocking coating 37 may be varied in color or may be patterned. Layers of organic or inorganic paints, such as lacquer or enamel layers are considered for the light-blocking coating 37. Also, metallic or optical interference reflective coatings may be used. Moreover, reflecting or absorbing coatings may be formed from metal compounds such as oxides, carbides, nitrides, or from mixed compounds of oxides, carbides, nitrides. Optionally, it is also possible to use a semiconductor coating such as a silicon layer as the light-blocking coating 37.

(14) In order not to affect the display capability, according to yet another embodiment of the invention it is contemplated that the light-blocking coating has at least one recess 38, with the self-luminous display element 7 arranged below glass ceramic plate 3 shining through the recess 38.

(15) For comparison, FIG. 3 shows spectral transmittance characteristics 17, 18 of two glass ceramics. The glass ceramic plates used for the measurement had a thickness of 3 mm and were irradiated perpendicularly to the surface.

(16) The transmittance characteristic designated by reference numeral 17 was measured on a glass ceramic having a low iron oxide content. By contrast, transmittance characteristic 18 was measured on a glass ceramic according to the invention which has a Fe.sub.2O.sub.3 content of more than 0.1 percent by weight which, moreover, is higher than the vanadium content. Here, the vanadium content is the same in both samples.

(17) Specifically, the two glass ceramics of transmittance characteristics 17, 18 have the same composition, in percent by weight, of:

(18) TABLE-US-00002 SiO.sub.2 65.14 Al.sub.2O.sub.3 20.9 Li.sub.2O 3.71 Na.sub.2O 0.59 K.sub.2O 0.22 MgO 0.37 ZnO 1.5 CaO 0.42 BaO 2.3 TiO.sub.2 3.1 ZrO.sub.2 1.34 SnO.sub.2 0.24 V.sub.2O.sub.5 0.026 MnO.sub.2 0.025

(19) The two glass ceramics only differ in the content of iron oxide. The glass ceramic with transmittance characteristic 17 has an Fe.sub.2O.sub.3 content of 0.093 percent by weight. By contrast, the glass ceramic plate according to the invention with transmittance characteristic 18 has an Fe.sub.2O.sub.3 content of 0.2 percent by weight. So, first, the content is greater than 0.1 percent by weight, as contemplated by the invention, and, second, it is greater than the content of vanadium oxide V.sub.2O.sub.5 by a factor of 7.7. And, the content of titanium oxide is lower than the preferred upper limit of 3.9 percent by weight or less.

(20) Preferably, the glass ceramics of the invention have a composition substantially comprising the following components, in wt %, on an oxide basis:

(21) TABLE-US-00003 Li.sub.2O 3.0-5.0 Na.sub.2O + K.sub.2O 0.2-1.5 MgO 0-2 CaO + SrO + BaO 0-4 ZnO 0-3 B.sub.2O.sub.3 0-2 Al.sub.2O.sub.3 18-25 SiO.sub.2 55-75 TiO.sub.2 1-5 ZrO.sub.2 0-2 P.sub.2O.sub.5 0-3 SnO.sub.2 0.15-0.5 TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 3.8-6 V.sub.2O.sub.5 0.005-0.05 Fe.sub.2O.sub.3 + CeO.sub.2 0.1-0.6

(22) Here, as in the other embodiments of the invention, either Fe.sub.2O.sub.3 or CeO.sub.2 or both components are present.

(23) Furthermore, the condition mentioned above is met: (M(SnO.sub.2)+0.1*M(TiO.sub.2))/(M(Fe.sub.2O.sub.3)+M(CeO.sub.2))<3. In this glass ceramic, the ratio of the weight fractions of these components has a value of 2.75.

(24) As can be seen from the graph of FIG. 3, the iron oxide reduces the absorption caused by the vanadium oxide in the visible spectral range, in particular between 750 and 450 nanometers, so that even with a high vanadium oxide content of more than 0.02 percent by weight, even with more than 0.025 percent by weight, a transmittance of more than 2.5%, even more than 5% is achieved in the visible spectral range between 450 and 750 nanometers. Specifically, the transmittance measured on a sample of 3 mm thickness with standard illuminant C in the visible range and corresponding to the color value Y was 28.5%. Furthermore, with standard illuminant A, a light transmittance of 31.5% was measured in the visible spectral range. Measurements of the visible transmission with standard illuminant D65 revealed a light transmittance of Y=28.4%.

(25) FIG. 3 illustrates another specific effect in conjunction with the decoloration of vanadium oxide due to a certain content of Fe.sub.2O.sub.3.

(26) Obviously, the effect of decoloration on absorption is stronger in the short wavelengths visible spectral range than in the longer wavelengths visible spectral range. A result thereof is that the transmittance characteristic becomes considerably more linear than that of the comparison sample with a lower content of Fe.sub.2O.sub.3.

(27) When fitting a straight line in the range of wavelengths from 450 to 700 nanometers using the method of least squares, the coefficient of determination R.sup.2 of transmittance characteristic 18 of the glass ceramic according to the invention has a value of 0.9857. By comparison, transmittance characteristic 17 of the comparison sample has a significantly lower value of 0.861. The coefficient of determination R.sup.2 is given by:

(28) $\begin{array}{cc}{R}^2=1-\frac{\underset{i=1}{\overset{n}{.Math.}}{\left(Y_i-{\hat{Y}}_i\right)}^2}{\underset{i=1}{\overset{n}{.Math.}}{\left(Y_i-\stackrel{\_}{Y}\right)}^2}.& \left(1\right)\end{array}$

(29) In this relationship, the Y.sub.i values denote the measured values of transmittance at different wavelengths, .sub.i the corresponding values of the straight line fitted to the measured values at the respective wavelength corresponding to Y.sub.i, and Y the average of the Y.sub.i values. The index i numbers the individual measured values of transmittance Y.sub.i up to the largest value n.

(30) The coefficient of determination takes a value between zero (no linear correlation) and one (perfect linear correlation of the measured values), depending on the linear correlation of the measured values. Therefore, the coefficient of determination of 0.9857 demonstrates that the transmittance characteristic is highly linear.

(31) This effect in particular also occurs in the yellow to blue spectral ranges. For an interval of wavelengths from 450 to 600 nanometers, a similarly high coefficient of determination R.sup.2 of 0.9829 is obtained for the glass ceramic according to the invention, while the coefficient of determination for the comparison sample is only 0.8589. Generally, as has been shown by way of this example, iron oxide may be metered to the mixture or the contents of Fe.sub.2O.sub.3 and V.sub.2O.sub.5 may be adjusted to such a ratio that with a given vanadium oxide content the spectral transmittance characteristic in a range of wavelengths between 450 and 600 nanometers becomes linear to such an extent that for a straight line fitted to the transmittance characteristic of the glass ceramic using the method of least squares a resulting coefficient of determination R.sup.2 is greater than 0.9, preferably greater than 0.95.

(32) This feature is particularly advantageous when using colored displays. If one or more self-luminous display elements emit light of different wavelengths, the approximately linear transmittance characteristic allows for an easier adjustment of the display elements in view of a true color reproduction.

(33) Both the coloring caused by the vanadium oxide and the decoloration caused by the iron oxide substantially occur only during ceramization of the starting glass. For comparison to FIG. 3, FIG. 4 shows two transmittance characteristics of the starting glasses of the two samples. In contrast to the data of FIG. 3, the measurements of FIG. 4 were carried out on samples of 4 mm thickness. Transmittance characteristic 19 was measured on the starting glass of the comparison sample, transmittance characteristic 20 on the starting glass of the glass ceramic according to the invention. Due to the higher content of Fe.sub.2O.sub.3 in the glass ceramic according to the invention, spectral transmittance is consistently lower here. Although the transmittance in the infrared range is also lower in the ceramized sample due to the iron oxide, as can be seen from FIG. 3, transmittance in the visible range is higher.

(34) Transmittance in the visible spectral range, or the Y value, further depends on the thickness of the glass ceramic plate. In the example of FIG. 3, the thickness was 3 mm, as mentioned above. When a thicker plate is used, for example one of 4 mm thickness, transmittance decreases for the same composition of the starting glass. Therefore, the vanadium oxide content may advantageously be adjusted as a function of the thickness of the plate. Specifically, according to a further embodiment of the invention it is contemplated that the vanadium oxide content is at least 0.066/x percent by weight, wherein x is the thickness of the glass ceramic in millimeters.

(35) Similarly it is possible to adjust the iron oxide content and/or the cerium oxide content as a function of the thickness of the plate in order to obtain specific transmittance values regardless of the thickness of the plate. According to yet another embodiment of the invention it is therefore intended that the iron oxide content or cerium oxide content is at least 0.4/x percent by weight, wherein x is the thickness of the glass ceramic in millimeters.

(36) The glass ceramic plates of the invention prove to be equally resistant under the extreme operating conditions of a glass ceramic cooktop when compared to conventional glass ceramics, both in terms of coloration and absorption. To illustrate this, FIG. 5 shows two transmittance characteristics 21, 22, both of which were measured on the glass ceramic of the invention on which the examples of FIGS. 3 and 4 were based. Accordingly, the glass ceramic has a Fe.sub.2O.sub.3 content of 0.2 percent by weight. From this glass ceramic, a sample of approximately 4 mm thickness was prepared, and transmittance characteristic 21 was measured on this sample.

(37) Then, a subsequent heat treatment was performed at 800 C. for a period of 10 hours, and then transmittance characteristic 22 was measured. According to that, transmittance in the visible spectral range after heat treatment is still 78% of the initial value with light transmission Y. Although transmittance decreases, the percentage reduction of transmittance is within the range of what is found for other volume-colored LAS glass ceramics. In absolute terms, transmittance in the visible spectral range in particular remains significantly higher than that of the comparison sample having a lower Fe.sub.2O.sub.3 content of less than 0.1 percent by weight.

(38) The effect of iron oxide and tin oxide on the transmittance of the glass ceramic is moreover well demonstrated by the exemplary embodiments listed in the table below:

(39) TABLE-US-00004 Sample 1 2 3 4 5 6 7 8 Component [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] CoO 0 0 0 0 0 0 0 0 NiO 0 0 0 0 0 0 0 0 Cr.sub.2O.sub.3 0 0 0 0 0 0 0 0 Nd.sub.2O.sub.3 0 0 0 0 0 0 0 0 Er.sub.2O.sub.3 0 0 0 0 0 0 0 0 MnO.sub.2 1 1 1 1 1 1 1 1 Fe.sub.2O.sub.3 100 500 750 1250 1500 2000 2500 3000 TiO.sub.2 31000 31000 31000 31000 31000 31000 31000 31000 SnO.sub.2 2500 2500 2500 2500 2500 2500 2500 2500 V.sub.2O.sub.5 220 220 220 220 220 220 220 220 ZrO.sub.2 13248 13248 13248 13248 13248 13248 13248 13248 (Sn + 0.1Ti)/Fe 56.00 11.20 7.47 4.48 3.73 2.80 2.24 1.87 Fe/V (5-30) 0.45 2.27 3.41 5.68 6.82 9.09 11.36 13.64 Y (4 mm) 1.89 2.1 2.25 2.58 2.77 3.17 3.64 4.17

(40) The samples have a thickness of 4 mm. The basic composition of samples 1 to 8 substantially corresponds to the composition specified in the description of FIG. 3, with 65.14 percent by weight of SiO.sub.2 and 20.9 percent by weight of Al.sub.2O.sub.3. The vanadium oxide content of 220 ppm (0.02 wt %) is somewhat lower than in the examples of FIG. 3 (260 ppm), the SnO.sub.2 content of 2500 ppm instead of 2400 ppm is slightly higher. As can be seen from the table, with the sample number increasing the iron oxide content was gradually increased from 100 ppm to 3000 ppm.

(41) In samples 1 to 3, the iron oxide content is still less than 1000 ppm, in sample 4 a Fe.sub.2O.sub.3 content of more than 1000 ppm is reached, with 1250 ppm. While in the comparison examples of samples 1 to 3 the transmittance (indicated as Y color value) is still less than 2.5%, this value is exceeded in sample 4. The transmittance clearly increases further as the Fe.sub.2O.sub.3 content increases, as can be seen from the transmittance values of samples 4 to 8, and with an Fe.sub.2O.sub.3 content of 3000 ppm and the given plate thickness of 4 millimeters, a transmittance of 4.17% is obtained in the visible spectral range.

(42) The preferred additional condition (M(SnO.sub.2)+0.1*M(TiO.sub.2))/(M(Fe.sub.2O.sub.3)+M(CeO.sub.2))<4, that the ratio of the components in percent by weight (designated (Sn+0.1Ti)/Fe in the table) is less than 4, is achieved in all samples 5 to 8 of the invention. In samples 6 to 8, this ratio is less than three.

(43) And, in all samples 4 to 8 of the invention, the ratio of weight fractions Fe.sub.2O.sub.3/V.sub.2O.sub.5 (abbreviated Fe/V in the table) is between 5 and 20 as preferably contemplated according to the invention, while in samples 1 to 3 this ratio has a value of less than 5.

(44) From these examples it is apparent that for a glass ceramic article of a given vanadium oxide containing composition a predetermined transmittance can be adjusted in a simple manner by adding a metered amount of iron oxide. The transmittance value of course also depends on the thickness of the glass ceramic article. If the thickness of the glass ceramic article is lower than the 4 mm thickness of the example, a smaller amount of iron oxide will be sufficient for a specific transmittance value. Therefore, in order to produce a glass ceramic article such as a glass ceramic cooktop with a predefined transmittance, first a transmittance value of 2.5% or more in the visible spectral range is predefined, wherein this transmittance value is higher than the transmittance value of a glass ceramic made from the same vanadium oxide containing mixture but with an iron oxide content of less than 0.1 percent by weight. Then, iron oxide is added to the melt or mixture to be melted in an amount which neutralizes the absorption caused by the vanadium oxide in the visible spectral range to such an extent that the predefined transmittance value is obtained in the glass ceramic with the intended thickness of the glass ceramic article. The method may likewise be performed using CeO.sub.2 instead of or in addition to Fe.sub.2O.sub.2.

(45) CeO.sub.2, likewise, is very effective as a decoloring agent for V.sub.2O.sub.5 containing glass ceramics, as will be shown by the following exemplary embodiment. Two lithium aluminosilicate glass ceramic samples of similar composition were prepared, the comparison sample with a V.sub.2O.sub.5 content of 0.2 percent by weight, and the sample according to the invention with a V.sub.2O.sub.5 content of 0.4 percent by weight, i.e. twice as high. If, additionally, 0.5 percent by weight of CeO.sub.2 is added to the latter mixture, transmittance remains almost the same, although, as mentioned before, V.sub.2O.sub.5 is a very strong color-imparting agent strongly absorbing in the visible spectral range. In other words, the addition of CeO.sub.2 compensates for a doubling of the V.sub.2O.sub.5 content in terms of transmittance in the visible spectral range.

(46) The mixtures of the sample according to the invention and the comparison sample have the following compositions:

(47) TABLE-US-00005 Component: Comparison Sample: Sample of the invention: Al.sub.2O.sub.3 22.47 22.21 K.sub.2O 0.20 0.20 Li.sub.2O 4.08 4.00 MgO 1.00 0.98 Na.sub.2O 0.64 0.64 P.sub.2O.sub.5 1.33 1.32 SiO.sub.2 65.84 65.35 SnO.sub.2 0.44 0.40 TiO.sub.2 1.80 1.80 V.sub.2O.sub.5 0.20 0.41 ZnO 0.20 0.20 ZrO.sub.2 2.00 2.00 ZnO 0.00 0.20 CeO.sub.2 0.00 0.50

(48) In a preferred embodiment, the CeO.sub.2 content should be at most 0.6 wt %. Higher contents are inefficient in view of the decreasing effect. Light transmittance of the ceramized samples in the visible spectral range is 1.2% in a comparison sample of 4 mm thickness, and is still 1.1% in the sample decolored with CeO.sub.2. At a wavelength of 600 nanometers, the comparison sample has a transmittance of 2.49%. The transmittance of the sample decolored using CeO.sub.2 is virtually the same, with 2.44%.