Method for modifying the transmission of glasses and glass ceramics and glass or glass ceramic articles that can be produced according to the method

Abstract

A product is provided that includes a volume-colored monolithic glass or glass ceramic element and to a method for producing same. The glass or glass ceramic element has a first region in which the coloration is modified so that light transmission of the first region differs from light transmission of a second, adjacent region. The light scattering in the region of modified coloration in the glass or glass ceramic remains the same as light scattering in the second, adjacent region with non-modified light transmission.

Claims

1. A product comprising: a volume-colored monolithic glass or glass ceramic element that has a first region and a second, adjacent region, the first region having been locally heated as compared to the second region by a laser beam so that the first region has a coloration that is different from that of the second, adjacent region and so that an absorption coefficient of the first region and thus light transmission through the first region is different from an absorption coefficient and thus light transmission of the second, adjacent region, wherein the first region has a spectral transmission that is greater than in the adjacent, second region within an entire spectral range between 420 nanometers and 780 nanometers, and wherein the first region has light scattering that differs from light scattering in the second region by not more than 20 percentage points.

2. The product as claimed in claim 1, wherein the glass or glass ceramic element comprises glass or glass ceramic comprising ions of a metal selected from the group consisting of vanadium, vanadium in combination with tin, vanadium in combination with titanium; rare earth elements, cerium, cerium in combination with chromium, cerium in combination with nickel, cerium in combination with cobalt, manganese, manganese in combination with tin, manganese in combination with titanium, iron, iron in combination with tin, iron in combination with titanium, and any combinations thereof.

3. The product as claimed in claim 2, wherein the glass or glass ceramic is volume-colored by vanadium oxide, wherein the first region has an integral light transmission in the visible spectral range that is increased relative to the second, adjacent region.

4. The product as claimed in claim 3, wherein the glass or glass ceramic comprises at least 0.005 percent by weight of vanadium oxide.

5. The product as claimed in claim 1, wherein the glass or glass ceramic element is a solarized glass element, wherein the solarized glass element comprises solarization sufficient to cause a volume-coloration due to light absorption in the visible spectral range, and wherein the first region has an integral light transmission that is increased compared to the second region.

6. The product as claimed in claim 1, wherein the glass or glass ceramic element is a diffusion-colored glass or glass ceramic element in which the first region exhibits increased light transmission in the visible spectral range compared to the adjacent region.

7. The product as claimed in claim 1, wherein the first region extends from a first surface to a second, opposite surface of the glass or glass ceramic element.

8. The product as claimed in claim 1, wherein the first region is a window that is surrounded along at least three edges thereof or along at least 50% of its periphery by adjacent non-brightened second regions.

9. The product as claimed in claim 1, wherein the glass or glass ceramic element has a face with a total surface area, wherein the first region occupies not more than one third of the total surface area.

10. The product as claimed in claim 1, wherein the glass or glass ceramic element comprises an aluminosilicate glass ceramic element in which the first region has a greater content of keatite mixed crystal.

11. The product as claimed in claim 1, wherein remission for visible light in the first region differs from remission of the second region by not more than 20 percentage points.

12. The product as claimed in claim 11, wherein in the first region transmission in the visible spectral range is greater by at least a factor of 2 compared to the second, adjacent region.

13. The product as claimed in claim 1, wherein the first region has a stress at the surface that is lower than a stress in a center of the volume of the first region.

14. The product as claimed in claim 1, wherein the glass or glass ceramic element has a thickness that varies along at least a portion of a surface, the portion having an absorption coefficient that locally varies as a function of the thickness.

15. The product as claimed in claim 14, wherein the first region is a window which is surrounded along at least three edges thereof or along at least 50% of its periphery by adjacent non-brightened second regions.

16. The product as claimed in claim 1, wherein the glass or glass ceramic element is a glass ceramic cooktop having a self-luminous display device disposed below the first region and emitting light that is visible through the first region.

17. The product as claimed in claim 1, wherein the first and second regions lack any coating thereon or joining therebetween.

18. A product, comprising: a volume-colored monolithic glass or glass ceramic element consisting of a first region and a second region adjacent to one another within the monolithic glass or glass ceramic element, the first and second regions having different colorations so that an absorption coefficient of the first region and a light transmission through the first region are different from an absorption coefficient of the second region and a light transmission through the second region, the first region having a spectral transmission that is greater than a spectral transmission in the second region within an entire spectral range between 420 nanometers and 780 nanometers, and the first region having a light scattering that differs from a light scattering in the second region by not more than 20 percentage points.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in more detail by way of exemplary embodiments and with reference to the accompanying figures. In the figures, the same reference numerals designate the same or equivalent elements.

(2) In the drawings:

(3) FIG. 1 and FIG. 2 show apparatus for performing the method of the invention;

(4) FIG. 3 shows X-ray diffraction spectra of a brightened region and a non-modified region of a volume-colored glass ceramic;

(5) FIG. 4 is a graph of spectral transmittance of a treated region and a non-treated region of a glass ceramic plate;

(6) FIG. 5 is a graph of the absorption coefficient of a treated region and a non-treated region of a glass ceramic cooking plate as a function of wavelength;

(7) FIG. 6 is a graph of spectral transmittance of a treated and a non-treated region of a solarized borosilicate glass;

(8) FIG. 7 is a graph of spectral transmittance of a treated and a non-treated region of a diffusion-colored borosilicate glass;

(9) FIG. 8 shows a glass ceramic cooktop comprising a glass ceramic plate according to the invention; and

(10) FIG. 9 shows a glass or glass ceramic article having a flat facet.

DETAILED DESCRIPTION

(11) The method of the invention for producing a glass or glass ceramic article with locally modified transmission will now be described in more detail with reference to FIG. 1. A ceramized glass ceramic plate 1 is provided, which has a first face 3 and a second face 5 and dimensions of 50 mm50 mm and a thickness of 4 mm. Glass ceramic plate 1 may have a knob pattern on one face, as usual. In particular, the glass ceramic plate is volume-colored by coloring metal ions. Such metal ions may for example be manganese ions, iron ions, rare earth ions such as in particular cerium ions, chromium, nickel, cobalt or vanadium ions. The coloring effect of these ions may depend on an interaction with other components of the glass or glass ceramic. That means, the coloration may be enhanced by interaction with other metal ions, or vice versa, may be attenuated. For example, manganese and iron ions exhibit interaction with tin and/or titanium, which is why preferably manganese or iron oxide is employed as a coloring agent preferably in combination with tin oxide and/or titanium oxide in the composition. Coloring ions of rare earth elements, in particular cerium ions, interact with ions of chromium, nickel and cobalt. Preferably, therefore, oxides of rare earth elements are employed as a coloring agent in combination with oxides of the above mentioned metals in the glass or glass ceramic composition. For vanadium, an interaction with tin, antimony, or titanium can be assumed.

(12) Generally, without being limited to the specific exemplary embodiments, the glass or glass ceramic therefore includes ions of at least one of the following metals or combinations of ions of the following metals:

(13) vanadium, in particular in combination with tin and/or titanium;

(14) rare earth elements, in particular cerium, in combination with chromium and/or nickel and/or cobalt;

(15) manganese in combination with tin and/or titanium;

(16) iron in combination with tin and/or titanium.

(17) Vanadium oxide is a very strong coloring agent. Generally in this case, coloration is only accomplished during ceramizing. It has been found that with the invention a volume-coloration caused by vanadium oxide may be offset, at least partially. To obtain a clearly visible effect in case of a glass ceramic colored by vanadium oxide, it is therefore contemplated according to one embodiment of the invention, without limitation to the exemplary embodiment, that the glass ceramic includes at least 0.005 percent by weight, preferably at least 0.01 percent by weight, more preferably at least 0.03 percent by weight of vanadium oxide. This causes a sufficiently strong coloration and accordingly a significant modification of transmission in locally brightened region 15.

(18) Glass ceramic plate 1 is placed on a slip-cast silicon oxide ceramic support 7 of 100 mm100 mm and of a thickness of 30 mm. The first face 3 bearing upon the silicon oxide ceramic support 7 is for example the smooth upper surface of glass ceramic plate 1. The upwardly facing second face 5 is the knobbed bottom face in this case.

(19) Generally, like in this example, it may be beneficial to irradiate the electromagnetic radiation to that surface which later faces away from the user. In a glass ceramic cooktop, one face of the glass ceramic plate typically has a knob pattern thereon and defines the surface facing away from the user. Irradiation is suitably effected on the surface facing away from the user because the surface that faces the radiation source tends to become warmer, which may lead to surface alterations. Such alterations will be less disturbing on the surface that faces away from the user.

(20) Silicon oxide ceramic support 7 and glass ceramic plate 1 are at room temperature. Above this arrangement, a laser scanner 13 with a focusing optical system with a focal length of 250 mm is installed in a manner so that laser beam 90 is incident perpendicular to the surface of glass ceramic plate 1. In the focus, laser beam 90 has a diameter of 1.5 mm. The arrangement of silicon oxide ceramic support 7 and glass ceramic plate 1 is placed at such a distance that the glass ceramic plate 1 is not in the focus of laser beam 90, so that the laser beam is defocused. In the exemplary embodiment, laser beam 90 has a diameter of 10 mm on glass ceramic plate 1. Laser radiation of a wavelength between 900 nm and 1100 nm is supplied from a laser 9 to laser scanner 13 via a transfer fiber 11. In this example, a diode laser is used as the laser 9, e.g. from company Laserline, which provides an adjustable output power between 0 W and 3000 W. Once the laser 9 has been enabled, glass ceramic plate 1 is locally irradiated with an output power of 1000 W and for a duration of 10 seconds. The glass ceramic is thereby heated at a rate of more than 250 K per minute, and within the period of irradiation the temperature exceeds a value at which an increase in integral light transmission of the glass ceramic material occurs. Typically, at this temperature the glass ceramic has a viscosity of less than 10.sup.14 dPa.Math.s. Then, the laser is turned off and the glass ceramic plate cools down in air. The cooling rate achieved in this way is more than 1 K per second, usually even more than 5 K per second, or more than 10 K per second, at least within a temperature range between the maximum temperature and 100 K below the maximum temperature, preferably down to the temperature at which the viscosity of the glass ceramic has a value of 10.sup.14 dPa.Math.s. In this manner, the color change, especially the brightening effect in this case, is frozen. In local region 15 which was heated by laser beam 90, transmission has locally become significantly higher across the entire thickness of the plate, which means that visible radiation can better pass through the glass ceramic plate 1. Adjacent regions 16 of the plate, or the rest of glass ceramic plate 1, remain dark, i.e. keep their low transmission in the visible range. Also, glass ceramic plate 1 is geometrically unchanged, in particular even in irradiated region 15. This applies to both, flatness and local thickness variations.

(21) According to another embodiment, the laser beam may be scanned over the surface of the glass or glass ceramic article by means of a laser scanner, so that a region 15 is heated, which has a larger surface area than the light spot of the laser beam on the surface of the glass or glass ceramic article.

(22) In a modification of the invention, the glass or glass ceramic article is optionally cooled superficially during irradiation, i.e. when being heated up. For this purpose, a cooling fluid 18 is brought into contact with the surface of the glass or glass ceramic article. Cooling fluid 18 may flow over the surface of the glass or glass ceramic article to enhance the cooling effect. Specifically, in the exemplary embodiment shown in FIG. 1, a film of cooling fluid is provided on the irradiated second face 5 of glass ceramic plate 1. The cooling fluid film could easily be caused to flow along the surface or along second face 5 by having the face 5 arranged obliquely, for example, and/or by continuously feeding cooling fluid 18. Otherwise than shown in FIG. 1, an arrangement may be provided in which both faces 3, 5 are in contact with a cooling fluid 18, preferably a flowing cooling fluid 18. Suitable for this purpose is an ethanol-water mixture. Generally, without being limited to the illustrated exemplary embodiment, it is preferred in this case that the ethanol content of the mixture does not exceed 50 percent by volume. Such a mixture is advantageous, because it absorbs less infrared radiation than pure water. The cooling fluid helps to avoid or at least reduce alterations of the surface such as warping or bulging. Additionally, the properties of the glass or glass ceramic article 1 may be positively influenced by the simultaneous cooling during irradiation. According to yet another embodiment of the invention, compressive stress may be produced at the surface. At least it is possible to prevent or reduce high tensile stresses at the surface after irradiation and cooling. Generally, therefore, a glass or glass ceramic article may be produced, in which in the first region 15 treated according to the invention stress at the surface is lower than in the center of the volume of the first region 15. Here, the term lower stress is not to be understood as an absolute value, but with sign. For example, the surface may almost be free of stress, while in the center of the volume there is a tensile stress which is a stress with positive sign. Even in this case, the stress is lower at the surface, since the interior stress is positive.

(23) Referring to FIG. 2, another exemplary embodiment will be explained, in which a coloration induced by solarization is modified, in particular at least partially offset, so that locally the product exhibits higher transmission in the visible spectral range. In this example, a glass tube 10 made of clear borosilicate glass (Fiolax glass), with a diameter of 15 mm and a length of 60 mm, which has been solarized by short-wave radiation is arranged horizontally at a distance of 10 mm above a silicon oxide ceramic support 7 sized 200 mm200 mm and having a thickness of 30 mm. Due to the solarization, the glass tube 10 is colored brown throughout its volume. At the beginning of the treatment, glass tube 10 is at room temperature.

(24) Above this assembly, a laser line generating optical system 14 is installed, which produces a line of 3 mm width and 56 mm length from a round laser beam, so that the laser beam 90 in the form of a laser line impinges onto the outer circumferential surface 100 of glass tube 10 perpendicularly to the longitudinal axis thereof. Via a fiber 11, laser line generating optical system 14 is supplied with laser radiation of a wavelength between 900 nm and 1100 nm. The laser source here, again, is a diode laser, such as available from Laserline company, which provides an adjustable power output between 0 W and 3000 W. Once the laser 9 has been enabled, glass tube 10 is locally irradiated with a power of 1000 W and a duration of 10 s. Then, laser 9 is switched off and glass tube 10 is allowed to freely cool in air. In the irradiated region, a clear transparent colorless ring has been produced along the circumference of glass tube 10. The rest of the glass tube 10 remains dark in color, i.e. it retains its low transmission in the visible range of wavelengths. Moreover, the glass tube is geometrically completely unchanged, even in the region 15 of irradiation. This applies both to roundness and to local evenness deviations.

(25) Thus, without being limited to the exemplary embodiment described above, the method permits to produce a product from or with a solarized and thus volume-colored monolithic glass element in which the solarization causes volume-coloration due to light absorption in the visible spectral range, and in which integral light transmission in the first region is increased compared to the second region.

(26) The two exemplary embodiments moreover have in common that the first region 15 with a transmission differing from an adjacent region, in particular with a higher transmission in the first region, extends from a first surface to a second, opposite surface of the glass or glass ceramic element. This is achieved by the electromagnetic radiation which penetrates through the article thus heating the entire glass or glass ceramic material between the two opposite surfaces. In the first exemplary embodiment, the first region extends from the first face 3 as the first surface to the second face 5 as the second surface. In the second exemplary embodiment, the opposite surfaces are defined by the outer circumferential surface 100 and the inner surface 101 of glass tube 10. However, it is also possible to achieve an increase in transmission to improve visibility of displays even when not the entire volume between the two surfaces is brightened, but only a layer, for example of a layer thickness that corresponds to half the thickness of the glass ceramic plate. Generally, of course, it is possible with the method of the invention to produce a plurality of regions 15 in the glass or glass ceramic element.

(27) According to one embodiment of the invention it is generally favorable for the method, without being limited to the particular exemplary embodiments of FIGS. 1 and 2, when a means is provided which reflects the electromagnetic radiation transmitted through the glass or glass ceramic article back into the glass or glass ceramic article. In particular, for this purpose, the glass or glass ceramic article may be placed on a support which reflects electromagnetic radiation back into the glass or glass ceramic material.

(28) By such reflection, the efficiency and speed of heating may be increased and so the process duration may be shortened. If an infrared laser is used, like in the example of FIG. 1, a support may be used which specifically reflects in the range of wavelengths of the laser radiation from 0.9 m to 1.1 m.

(29) If, as in the example shown in FIG. 1, a slip-cast silicon oxide ceramic support is used, an appropriately fine-grained silicon oxide ceramic can be used for this purpose. Generally, without limitation to silicon oxide ceramics, it is preferred according to a further embodiment of the invention that the average grain size of a ceramic, preferably of the slip-cast SiO.sub.2 ceramic that is used as a support for the glass or glass ceramic article, is smaller than the wavelength of the electromagnetic radiation. In this manner strong scattering of the radiation at the surface of the support is avoided. In case of broadband radiation sources, the average grain size of the ceramic should be less than the wavelength of the maximum spectral power density of the radiation transmitted through the glass or glass ceramic material, or, alternatively, the center wavelength of the spectrum of the radiation transmitted through the glass or glass ceramic material.

(30) According to another embodiment of the invention, instead of a ceramic surface such as that of the silicon oxide ceramic support 7 according to FIG. 1 and FIG. 2, a metallic reflecting support can be used. Suitable are aluminum or polished copper, for example. It is of course also possible to combine this embodiment with a ceramic support, by placing a metallic reflective layer or plate on the ceramic support. As an example, FIG. 2 shows a metallic plate 70 arranged on the silicon oxide ceramic support 7, which plate reflects the transmitted laser light having a wavelength preferably in the range from 0.9 m to 1.1 m.

(31) The heating and resulting color change of region 15 and subsequent cooling may optionally be followed by a thermal post treatment step to relieve tensile stresses. A thermal post treatment at a temperature of 800 C. and a holding time of 5 minutes already leads to a significant reduction of tensile stresses in a glass ceramic plate 1. The heating to a relaxation temperature in the thermal post treatment step may be accomplished using a laser, any other electromagnetic radiation source, or in a suitable furnace. When heating is effected using electromagnetic radiation, a radiation source may be used whose radiation is more strongly absorbed than the electromagnetic radiation used for heating in the first color modifying step. So in particular the surface of the glass or glass ceramic will be heated. Tensile stresses existing at the surface are particularly relevant with regard to the strength of the glass or glass ceramic element.

(32) In the example shown in FIG. 1, an optional cooling fluid is provided to prevent excessive heating of the surface. Another measure to produce a temperature gradient such that during irradiation the surface remains cooler than regions of the glass or glass ceramic below the surface would be an appropriate initial temperature profile of the glass or glass ceramic element to be treated. For example, initial starting temperature profiles with a suitable gradient across the thickness of the glass or glass ceramic may be created by freezing and/or pre-heating. For an appropriate starting profile, the volume may in particular be made hotter than the surface already prior to the actual exposure to the electromagnetic radiation. To give an example, the glass or glass ceramic article may be pre-heated with quenching of the surfaces prior to being exposed to the electromagnetic radiation.

(33) According to yet another embodiment of the invention, otherwise than shown in FIGS. 1 and 2, the laser beam 90 may be focused in the volume of the glass or glass ceramic. In this way, compressive stresses may possibly arise at the surfaces of the processed material.

(34) Generally, the glass or glass ceramic material may additionally be toughened prior to or after the color change. This may be accomplished by thermal or chemical tempering for selectively introducing near-surface zones of compressive stress, so that the material resists to or compensates for tensile stresses possibly induced by the process.

(35) FIG. 3 shows X-ray diffraction spectra of a monolithic glass ceramic element as obtained by the method explained with reference to FIG. 1. The tested glass ceramic is a lithium aluminosilicate glass ceramic volume-colored by vanadium oxide, as is used for cooking plates, for example. X-ray diffraction was used to compare the crystal phases, the content of crystal phases, and the crystallite size of a region 15 brightened by laser irradiation with those of adjacent, non-brightened regions 16.

(36) Additionally, the relative intensities of different crystal phases are marked with a diamond, a square, or a circle. Squares indicate X-ray diffraction peaks of high-quartz mixed crystal (HQMK), diamonds indicate X-ray diffraction peaks of lithium aluminosilicate or keatite mixed crystal (KMK, LiAlSi.sub.3O.sub.8), and circles indicate X-ray diffraction peaks of zirconium titanate (ZrTiO.sub.4) which was also detected in the glass ceramic. Curve 150 represents the X-ray diffraction spectrum of the brightened region, i.e. region 15 treated according to the invention, and curve 160 represents the X-ray diffraction spectrum of an adjacent, non-modified region 16. As can be seen, the curves are virtually identical, except for the different offset for purposes of illustration. The only result of a closer analysis of the intensities of the X-ray diffraction peaks is a very small increase in the content of the keatite mixed crystal phase. The results are summarized in the table below:

(37) TABLE-US-00001 HQMK phase KMK phase content content Crystallite size [nm] [+/10%] [+/10%] [+/5%] uncor- cor- uncor- cor- Sample HQMK KMK rected rected rected rected brightened 49 not 54 66 3 3 region deter- minable non- 48 not 55 67 1 1 modified deter- region minable

(38) For absorption correction in the columns designated corrected, the chemical composition of the glass ceramic and an assumed density of =2.5 g/cm.sup.3 were used.

(39) According to the table above and to FIG. 3, the content of the high-quartz mixed crystal phase does not change within the measurement error. Only the content of keatite mixed crystal shows a change which does not have any significant impact on the microstructure of the glass ceramic because of the low proportion of this crystal phase. That means, even if treated and non-treated regions of a glass ceramic element do not exhibit any significant structural differences, according to one embodiment a region of an aluminosilicate glass ceramic treated according to the invention, in particular of a lithium aluminosilicate glass ceramic, may be distinguished by an increased content of keatite mixed crystal.

(40) Changes in the crystal phases and/or their proportions may have an influence on light scattering. When light scattering in the material changes, this also leads to a change in remission when illuminating the treated region. As demonstrated in the above example, treated and non-treated regions are virtually identical in their morphology, in particular with respect to the existing crystal phases. Therefore, in an inventive product remission does not change either, or only marginally, when comparing a treated and a non-treated region. Therefore, according to a further embodiment, without limitation to the exemplary embodiment described above, remission for visible light in the first region differs from remission of the second region by not more than 20 percentage points, preferably by not more than 10 percentage points, more preferably by not more than 5 percentage points.

(41) FIG. 4 shows transmittances of a glass ceramic plate volume-colored by vanadium oxide and treated according to the invention, as a function of wavelength. Curve 151 in FIG. 4 represents spectral transmittance of a region 15 treated according to the invention, curve 161 represents spectral transmittance of an adjacent, non-treated region 16. From the two curves it can be seen that in the treated region 15 transmittance is significantly increased over the entire spectral range between 420 nanometers and 780 nanometers. This is advantageous when it is desired to improve transparency without significantly modifying the tint, in order to selectively make specific regions of the glass or glass ceramic article more transparent for luminous or non-luminous display elements, or, more generally, to provide viewing windows. Therefore, according to one embodiment of the invention and without limitation to the specific exemplary embodiment, spectral transmittance of the first region is higher than that of an adjacent, second region within the entire spectral range between 420 nanometers and 780 nanometers.

(42) What is also remarkable about the spectral transmittance of FIG. 4 is that transmittance in the blue and green spectral range increases more strongly than that in the red range. For example, at 500 nanometers transmittance increases from 0.0028 to 0.027, i.e. by a factor of more than nine. At 600 nanometers, the factor is lower, namely 4.7 in this case. It is just this what is particularly favorable to improve display capability for blue and/or green display elements or for color displays in volume-colored glass ceramics, especially glass ceramics colored by vanadium oxide. Therefore, according to yet another embodiment of the invention, the ratio of spectral transmittances of the first region to the second region is greater at a wavelength in a range from 400 to 500 nanometers than at a wavelength in a range from 600 to 800 nanometers.

(43) Below, the colors are listed as measured in the treated and non-treated regions 15, 16 in transillumination of the glass ceramic plate of 4 mm thickness, for different color models (xyY, Lab, Luv) and various standard light sources:

(44) TABLE-US-00002 region 16 region 15 Standard light type A x 0.6307 0.5782 y 0.3480 0.3805 Y 1.7 7.6 Standard light type D65 x 0.5550 0.4773 y 0.3540 0.3752 Y 1.2 6.2 Ra 25.6 22.0 Standard light type C x 0.5545 0.4763 y 0.3495 0.3685 Y 1.2 6.3 Yellowness I. 174.0 120.8 Standard light type A L* 13.6 33.2 a* 23.2 24.2 b* 19.1 27.7 C* 30.0 36.8 Standard light type D65 L* 10.6 30.0 a* 20.8 20.2 b* 13.8 22.9 C* 25.0 30.5 Standard light type C L* 10.8 30.2 a* 20.1 19.2 b* 14.1 23.2 C* 24.5 30.1 Standard light type A L* 13.6 33.2 u* 30.3 45.3 v* 0.9 4.3 Standard light type D65 L* 10.6 30.0 u* 22.6 36.6 v* 7.0 18.5 Standard light type C L* 10.8 30.2 u* 22.9 36.7 v* 7.8 20.3

(45) In the Lab, xyY, and Luv color models, parameters L and Y, respectively, denote the brightness. When using standard light type C or standard light type D65, the parameter Y in the xyY color model corresponds to transmission .sub.vis in the visible spectral range, and from a comparison of the Y values the increase in transmission can be determined. From the values given above it can be seen that transmission in the visible spectral range is increased by at least a factor of 2.5. Generally, it should be noted here that the transmission additionally depends on the refractive index and on the thickness of the transilluminated glass or glass ceramic element. However, it can be generally stated that according to one embodiment of the invention the transmittance in the visible spectral range between 380 and 780 nanometers is increased by at least a factor of 2.5, based on a thickness of 4 millimeters.

(46) The coloring by vanadium oxide, V.sub.2O.sub.5, as was the case in the exemplary embodiments of FIGS. 3 and 4 discussed above, has also been known from DE 10 2008 050 263 B4, according to which the coloring mechanism is a complex process. According to this document, a prerequisite for converting the vanadium oxide into the coloring state is a redox reaction. In the crystallizable initial glass, the V.sub.2O.sub.5 still colors relatively weakly and produces a slightly greenish tint. During ceramization the redox reaction occurs, the vanadium is reduced and the redox partner is oxidized.

(47) The refining agent functions as the primary redox partner, which was shown by Mossbauer investigations of Sb and Sn refined compositions. During ceramization, a part of the Sb.sup.3+ or Sn.sup.2+ in the initial glass is converted to the higher oxidation state Sb.sup.5+ and Sn.sup.4+, respectively. It was assumed that the vanadium is incorporated into the seed crystal in the reduced oxidation state as V.sup.4+ or V.sup.3+ and intensively colors therein due to electron charge transfer reactions. Also, as another redox partner, TiO.sub.2 may reinforce the coloring by vanadium oxide. Besides the type and quantity of the redox partners in the initial glass, the redox state that is adjusted in the glass for the melt also has an influence, according to DE 10 2008 050 263 B4. A low oxygen partial pressure, i.e. a melt adjusted as reducing, for example due to high melting temperatures, reinforces the coloring effect of the vanadium oxide.

(48) But it is also possible that the reduced V.sup.4+ or V.sup.3+ is not or not exclusively incorporated into the seed crystals, but possibly also into another structural environment, such as the high-quartz mixed crystal, or into clusters.

(49) With the invention, this coloration is locally modified by irradiation of high-energy radiation and heating of the glass ceramic.

(50) This may be associated with an impact on the coloring charge transfer process. Since the hypothetical electron transfer between donor and acceptor centers during charge transfer is significant for absorption, it can be assumed that the applied high-energy radiation and the heating cause a modification of the structure of these centers. This structural modification reduces the frequency/likelihood of electron transfers and thus absorption.

(51) Because of the sensitivity with which the coloring by vanadium reacts to partial pressure of oxygen and to redox processes during ceramizing, competing valency changes might be relevant for this. That is to say, the radiation in combination with the heating may possibly remove electrons from the donor or acceptor centers thereby passivating them for the charge transfer process.

(52) This hypothesis is supported by the observation that the reduced coloration can be reversed by thermal treatment. The thermodynamically more stable structural state of the centers can be restored. This re-increases the frequency of coloring charge transfers.

(53) FIG. 5 shows a diagram of absorption coefficient curves as a function of wavelength as measured in a region brightened according to the invention and in a non-treated region. As with the curve of light transmittance shown in FIG. 4, the glass ceramic in which the profile was measured has been colored by vanadium oxide. The higher light transmission of a brightened region 15 is now apparent here from the fact that in the visible spectral range the spectral absorption coefficient 152 of a brightened region is lower than the absorption coefficient 162 of an adjacent, non-brightened region 16. In particular, as in the example shown in FIG. 4, the absorption coefficient of the glass ceramic of a brightened region 15 may be lower than the absorption coefficient of the material of an adjacent, non-brightened region 16 over the entire visible spectral range. In the visible spectral range, the absorption coefficient decreases with increasing wavelength. Accordingly, spectral light transmittance increases, similarly to the example shown in FIG. 3.

(54) Also, it is obvious that the curves of spectral absorption coefficients 152, 162 cross each other in the infrared spectral range at a wavelength of about 1000 nanometers. Above this wavelength, the absorption coefficient of the first region 15 is higher than the absorption coefficient of an adjacent, second region 16.

(55) In the example shown, the absorption coefficient of first region 15 is higher than that of second region 16 in the infrared range up to a wavelength of 1650 nanometers.

(56) Generally, without being limited to the exemplary embodiment, according to one embodiment of the invention it is therefore contemplated, that in at least a spectral range having a wavelength of more than 900 nanometers the absorption coefficient of the first region 15 is greater than the absorption coefficient of a second, adjacent region 16, so that in the spectral range having a wavelength of more than 900 nanometers integral light transmission of first region 15 is lower than integral light transmission of the second, adjacent region 16 in this spectral range. Preferably, this spectral range extends at least between 1100 nanometers and 1400 nanometers, which also applies to the illustrated exemplary embodiment of FIG. 5.

(57) The spectral range mentioned above in particular applies to glass ceramics colored by vanadium oxide. This effect of a higher absorption coefficient of first region 15 in the infrared spectral range may also occur when coloration is effected by rare earth elements, in particular by cerium, preferably in combination with chromium and/or nickel and/or cobalt; by manganese, preferably in combination with tin and/or titanium; or by iron, preferably in combination with tin and/or titanium. However, the wavelength range may possibly differ from that of the example shown in FIG. 5.

(58) The higher absorption coefficient in the infrared spectral range of the first region 15 may be advantageous for example for a display device 23 arranged below region 15 of a glass ceramic plate. This reduces the risk that due to heat sources on the cooktop the display device is excessively heated through the glass ceramic and becomes damaged.

(59) FIG. 6 shows the spectral transmittance of a treated and a non-treated region of a solarized borosilicate glass. The borosilicate glass is marketed under the trade name Fiolax. The solarization was produced by irradiation with gamma rays. Then, as explained with reference to FIG. 2, the glass was heated by laser radiation of a wavelength of 1 m to a temperature between T.sub.g and the softening point. As in FIG. 4, curve 151 again shows the spectral transmittance of the so treated region 15 of the glass, curve 161 the spectral transmittance of an adjacent region 16 that has not been treated with the laser. The color change due to an increase in transmission caused by the laser treatment according to the invention is obvious. The treated region 15 exhibits a nearly constant transmission in the visible spectral range, which shows that the solarization has been substantially completely offset.

(60) Here, again, the increase in transmission is more pronounced in the blue and green spectral ranges than in the red spectral range. In this example, in the blue and also in the ultraviolet spectral range until a wavelength of about 300 nanometers not only the relative, but even the absolute increase in transmission is greater than in the red spectral range.

(61) Below, the measured color values for transillumination with standard light source are given for the treated region 15 and for an adjacent region 16:

(62) TABLE-US-00003 region 16 region 15 Standard light type A x 0.4602 0.4484 y 0.4079 0.4077 Y 67.8 79.1 Standard light type D65 x 0.3256 0.3136 y 0.3373 0.3299 Y 66.8 79.0 Ra 94.0 99.4 Standard light type C x 0.3231 0.3110 y 0.3248 0.3170 Y 66.8 79.0 Yellowness 12.4 1.2 Standard light type A L* 85.9 91.3 a* 3.9 0.2 b* 5.5 0.5 C* 6.8 0.5 Standard light type D65 L* 85.4 91.3 a* 2.3 0.0 b* 4.9 0.5 C* 5.4 0.5 Standard light type C L* 85.4 91.3 a* 2.0 0.0 b* 5.0 0.5 C* 5.4 0.5 Standard light type A L* 85.9 91.3 u* 8.8 0.5 v* 2.3 0.3 Standard light type D65 L* 85.4 91.3 u* 6.5 0.4 v* 7.0 0.7 Standard light type C L* 85.4 91.3 u* 6.4 0.4 v* 7.4 0.7

(63) Based on the Y values of the xyY color measurements, the resulting increase in transmission .sub.vis in the visible spectral range is at least 10%.

(64) According to yet another embodiment of the invention, a diffusion-colored glass or glass ceramic element is treated according to the invention and the coloration imparted by the diffusion ink is locally modified in this manner. Diffusion inks diffuse into the glass or glass ceramic and thereby also cause volume coloring. In this case, however, the material will usually not be colored throughout the volume, rather a volume-colored layer is resulting that extends from the surface to a certain depth into the material.

(65) In the example below, a borosilicate glass (again a Fiolax glass) was colored using a brown diffusion ink and was then locally treated with the laser as described with reference to FIG. 2. In this manner, a product is obtained from or with a diffusion-colored glass or glass ceramic element in which the first region 15 exhibits higher light transmission in the visible spectral range than an adjacent, non-treated region 16.

(66) FIG. 7 shows spectral transmittance of the diffusion-colored borosilicate glass for this embodiment of the invention, with curve 151 again representing the transmission of treated region 15 and curve 161 representing the transmission of non-treated region 16. Here, again, a significant increase in transmission is resulting in the treated region 15 and thus an alteration in color. As with the examples of FIG. 4 and FIG. 6, the relative increase in transmission in the blue and green spectral ranges is again greater than in the red spectral range. At a wavelength of 550 nanometers, the diffusion-colored glass exhibits a transmittance of less than 0.05. By the laser treatment, the transmittance at this wavelength is increased to more than 0.2, that is by more than a factor of 4.

(67) Below, the measured color values for transillumination with standard light source are given for the treated region 15 and for an adjacent region 16 of the diffusion-colored glass:

(68) TABLE-US-00004 region 16 region 15 Standard light type A x 0.6420 0.5704 y 0.3572 0.4217 Y 11.9 27.5 Standard light type D65 x 0.6186 0.5166 y 0.3798 0.4656 Y 8.4 23.3 Ra 24.4 17.8 Standard light type C x 0.6192 0.5197 y 0.3792 0.4626 Y 8.5 23.5 Yellowness 208.6 139.7 Standard light type A L* 41.1 59.5 a* 43.9 23.4 b* 69.7 81.5 C* 82.3 84.8 Standard light type D65 L* 34.7 55.4 a* 43.0 16.3 b* 59.4 82.8 C* 73.3 84.4 Standard light type C L* 35.1 55.6 a* 40.8 14.3 b* 60.0 84.0 C* 72.6 85.2 Standard light type A L* 41.1 59.5 u* 91.7 57.0 v* 6.0 18.7 Standard light type D65 L* 34.7 55.4 u* 87.5 54.6 v* 32.7 62.3 Standard light type C L* 35.1 55.6 u* 87.4 54.8 v* 36.4 67.5

(69) The increase in transmission in the visible spectral range as apparent from the Y values is by more than a factor of 2.

(70) FIG. 8 shows a glass ceramic cooktop 20 as one of the preferred applications of the invention. Glass ceramic cooktop 20 comprises a glass ceramic plate 1 that has a first face 3 which defines the upper surface in this example, and an opposite, second face which defines the lower surface. Underneath the lower surface or second face 5, a heating element 22 is arranged, for heating a cooking vessel placed upon the first face 3 above heating element 22. Glass ceramic plate 1 has a first region 15 which extends through the glass ceramic plate 1 from one surface to the opposite surface of the two faces 3, 5 and in which light transmission is increased relative to adjacent regions 16. Below first region 15, a preferably self-luminous display device 23 is disposed, and the light from the display device is visible through the first region 15. First region 15 was produced by a treatment according to the invention using a laser or another locally acting electromagnetic radiation source, by heating and subsequent cooling. In order to avoid that the parts arranged under glass ceramic plate 1, such as heating element 22, are visible when looking to upper surface 3, a volume-colored glass ceramic may be used, for instance with a vanadium oxide content of more than 0.02 percent by weight. Due to the local attenuation of absorption or the local brightening of the glass ceramic in region 15, the light from the display device 23 will nevertheless be transmitted through the glass ceramic plate 1 and will be clearly visible for an operator.

(71) Since very clear brightening effects can be achieved with the invention, the method is especially useful for dark glass ceramic plates to make them more transparent for display purposes. Therefore, without being limited to the exemplary embodiment, according to one embodiment of the invention a glass ceramic plate is used, in which integral light transmission in the visible spectral range of the second region 16 adjacent to first region 15 is not more than 5%, preferably not more than 2.5%. In other words, the glass ceramic plate which is the starting material for the cooking plate exhibits a correspondingly low transmission of not more than 5%, preferably not more than 2.5%.

(72) Display devices may also be arranged under a flat facet. An example of a glass ceramic plate 1 having such a flat facet 26 is shown FIG. 9. If now in an example as shown in FIG. 8 a display device is intended to be arranged below flat facet 26, a problem arising with volume-colored glass or glass ceramics is that due to the varying thickness of the material in the region of flat facet 26 light transmission also varies along the surface. Here, the invention now generally provides the possibility to compensate for variations in transmission caused by varying thicknesses of the glass or glass ceramic material. For this purpose, the treatment time and/or the power of incident electromagnetic radiation may be varied as a function of thickness. Above glass ceramic plate 1, FIG. 9 schematically shows a profile of absorption coefficient as a function of displacement coordinate x along the surface of glass ceramic plate 1. Here, flat facet 26 extends from the edge of the glass ceramic plate where, accordingly, the glass ceramic plate has the smallest thickness and therefore exhibits highest transmission without the treatment according to the invention. In order to keep transmission constant along flat facet 26, the absorption coefficient is gradually lowered starting from the edge until the inside border of flat facet 26. Thus, the area provided with flat facet 26 also forms the first region 15 in which the coloration differs from a second, adjacent region 16 next to flat facet 26 (i.e. the region with plane-parallel faces).

(73) In this way, light transmission keeps its constant value it has at the edge of the plate all along flat facet 26. At the inside border of flat facet 26, the absorption coefficient may then rapidly increase to the value of the non-treated glass ceramic. Therefore, there is a step in the profile of the absorption coefficient at this point. In this manner, flat facet 26 will appear as a uniformly brightened area. Therefore, without being limited to the specific application shown in FIG. 9, a glass or glass ceramic element may be provided, which has a thickness that varies along at least a portion of the surface, in which the absorption coefficient is locally varied by the treatment of the invention, i.e. in first region 15, as a function of the thickness, in particular in such a manner that light transmission which locally varies due to the varying thickness is evened out, at least partially. Specifically to this end, an absorption coefficient is adjusted that decreases with increasing thickness.

(74) In the exemplary embodiment of FIG. 8, the brightened first region 15 is a localized window which typically only extends over a small portion of the surface area of face 3. A flat facet 26 as shown in the example of FIG. 9 typically does not extend over a large portion of face 3 either. According to one embodiment of the invention, therefore, the total surface area of the one or more first regions 15 at a face of the glass or glass ceramic article occupies not more than one third of the surface area of this face 3.

(75) It will be apparent to those skilled in the art that the invention is not limited to the described exemplary embodiments but can be varied in many ways without departing from the scope of the subject matter of the claims. For example, besides a laser other radiation sources are likewise conceivable. For instance a high-performance short-arc lamp may be used. To achieve a localized exposure to the light, the glass or glass ceramic element may be masked appropriately. Another option is to use a microwave source with appropriate masking.

LIST OF REFERENCE NUMERALS

(76) 1 Glass ceramic plate 3 First face of 1 5 Second face of 1 7 Silicon oxide ceramic support 9 Laser 10 Glass tube 11 Transfer fiber 13 Laser scanner 14 Line generating optical system 15 Localized region with modified transmission 16 Region with non-modified transmission 18 Cooling fluid 20 Glass ceramic cooktop 22 Heating element 23 Display device 26 Flat facet 70 Metal plate 90 Laser beam 100 Outer circumferential surface of 10 101 Inner surface of 10 150 X-ray diffraction spectrum of 15 160 X-ray diffraction spectrum of 16 151 Spectral transmittance of 15 161 Spectral transmittance of 16 152 Spectral absorption coefficient of 15 162 Spectral absorption coefficient of 16