Glass ceramic substrate made of a transparent, colored LAS glass ceramic and method for producing it
10183888 ยท 2019-01-22
Assignee
Inventors
- Falk Gabel (Schlangenbad, DE)
- Oliver Hochrein (Mainz, DE)
- Evelin Weiss (Mainz, DE)
- Roland Dudek (Bad Kreuznach, DE)
- Uwe Martens (Mainz-Kastel, DE)
Cpc classification
C03C10/0027
CHEMISTRY; METALLURGY
International classification
C03B32/02
CHEMISTRY; METALLURGY
C03C10/00
CHEMISTRY; METALLURGY
Abstract
A glass ceramic substrate made of a transparent, colored LAS glass ceramic is provided. The glass ceramic has a gradient layer with keatite solid solution and an underlying core with high-quartz solid solution as predominant crystal phase. The keatite solid solution in a depth of 10 m or greater exceeds 50% of the sum of the high-quartz solid solution proportion and keatite solid solution proportion. The ceramization includes a crystal transformation step, in which the high-quartz solid solution is transformed at a maximum temperature in the range of 910 to 980 and a time period of between 1 and 25 minutes in part into the keatite solid solution.
Claims
1. A glass ceramic substrate made of a transparent, colored LAS glass ceramic comprising a composition (in wt %): Al.sub.2O.sub.3 18-23, Li.sub.2O 3.0-4.2, SiO.sub.2 60-69, ZnO 0-2, Na.sub.2O+K.sub.2O 0.2-1.5, MgO 0-1.5, CaO+SrO+BaO 0-4, B.sub.2O.sub.3 0-2, TiO.sub.2 2.3-4, ZrO.sub.2 0.5-2, P.sub.2O.sub.5 0-3, SnO.sub.2 0-<0.6, Sb.sub.2O.sub.3 0-1.5, As.sub.2O.sub.3 0-1.5, TiO.sub.2+ZrO.sub.2+SnO.sub.2 3.8-6, V.sub.2O.sub.5 0.01-0.06, and Fe.sub.2O.sub.3 0.03-0.2, wherein the LAS glass ceramic has a gradient layer and an underlying core, and wherein the LAS glass ceramic has keatite solid solution (KSS) as a predominant crystal phase in the underlying core and has high-quartz solid solution (HQSS) as a predominant crystal phase in the gradient layer, further comprising a depth profile of the HQSS and KSS crystal phase proportions, wherein the KSS crystal phase has a proportion in any depth 20 m that exceeds 50% of a sum of the HQSS and KSS crystal phase proportions.
2. The glass ceramic substrate according to claim 1, further comprising coloring oxides in sum total to a maximum of 1.0 wt %.
3. The glass ceramic substrate according to claim 1, wherein the LAS glass ceramic has a fracture strength, expressed in a CIL value, of at least 0.8 N in the case of an ambient humidity of 10% and/or a CIL value of at least 0.98 N in the case of an ambient humidity of 1%.
4. The glass ceramic substrate according to claim 1, wherein the depth profile of the HQSS and KSS crystal phase proportions is achieved by phase transformation of HQSS crystals to KSS crystals during ceramization with a maximum temperature T.sub.max and over a residence time t(T.sub.max) of this maximum temperature in a temperature-time region that is delimited by four straight lines, which connect the four corner points with the value pairs (T.sub.max=910 C.; t(T.sub.max)=25 minutes), (T.sub.max=960 C.; t(T.sub.max)=1 minute), (T.sub.max=980 C.; t(T.sub.max)=1 minute), and (T.sub.max=965 C.; t(T.sub.max)=25 minutes), wherein the ceramization method, including the time period for cooling to a temperature of 780 C., requires overall less than 60 min.
5. A glass ceramic substrate made of a transparent, colored LAS glass ceramic comprising a composition (in wt %): TABLE-US-00004 Al.sub.2O.sub.3 18-23, Li.sub.2O 3.0-4.2, SiO.sub.2 60-69, ZnO 0-2, Na.sub.2O + K.sub.2O 0.2-1.5, MgO .sup.0-1.5, CaO + SrO + BaO 0-4, B.sub.2O.sub.3 0-2, TiO.sub.2 2.3-4,.sup. ZrO.sub.2 0.5-2,.sup. P.sub.2O.sub.5 0-3, SnO.sub.2 .sup.0-<0.6, Sb.sub.2O.sub.3 .sup.0-1.5, As.sub.2O.sub.3 .sup.0-1.5, TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 3.8-6,.sup. V.sub.2O.sub.5 .sup.0.01-0.06, and Fe.sub.2O.sub.3 0.03-0.2, wherein the LAS glass ceramic has a gradient layer and an underlying core, wherein the LAS glass ceramic has keatite solid solution (KSS) as a predominant crystal phase in the underlying core and has high-quartz solid solution (HQSS) as a predominant crystal phase in the gradient layer, and wherein the LAS glass ceramic has a fracture strength, expressed in a CIL value, of at least 0.8 N at an ambient humidity of 10% and/or a CIL value of at least 0.98 N at an ambient humidity of 1%.
6. The glass ceramic substrate according to claim 5, further comprising coloring oxides in sum total to a maximum of 1.0 wt %.
7. The glass ceramic substrate according to claim 5, wherein the fracture strength is achieved by phase transformation of HQSS crystals to KSS crystals during ceramization with a maximum temperature T.sub.max and over a residence time t(T.sub.max) of this maximum temperature in a temperature-time region that is delimited by four straight lines, which connect the four corner points with the value pairs (T.sub.max=910 C.; t(T.sub.max)=25 minutes), (T.sub.max=960 C.; t(T.sub.max)=1 minute), (T.sub.max=980 C.; t(T.sub.max)=1 minute), and (T.sub.max=965 C.; t(T.sub.max)=25 minutes) wherein the ceramization method, including the time period for cooling to a temperature of 780 C., requires overall less than 60 in.
8. The glass ceramic substrate according to claim 5, wherein the LAS glass ceramic has a maximum scattered proportion, standardized to a glass ceramic with a thickness of 4 mm, is at most 15% at a wavelength of 470 nm.
9. The glass ceramic substrate according to claim 8, wherein the maximum scattered proportion does not exceed 20% in a wavelength range from 400 nm to 500 nm.
10. The glass ceramic substrate according to claim 5, wherein the LAS glass ceramic is free of arsenic and antimony, apart from unavoidable traces, and contains at least 0.1 wt % SnO.sub.2.
11. The glass ceramic substrate according to claim 5, wherein the composition meets a condition 1<Fe.sub.2O.sub.3/V.sub.2O.sub.5<8.
12. The glass ceramic substrate according to claim 6, wherein the coloring oxides comprise at least one substance selected from the group consisting of Cr, Mn, Co, Ni, Cu, Se, Mo, W, oxides thereof, and metal oxides of rare earths.
13. The glass ceramic substrate according to claim 5, wherein the LAS glass ceramic has an integral visual transmission in the visible range, standardized to a glass ceramic with a thickness of 4 mm, of .sub.vis, 4 mm<=5%.
14. The glass ceramic substrate according to claim 5, wherein the LAS glass ceramic has a spectral transmission, standardized to a glass ceramic with a thickness of 4 mm, of >0.1% at a wavelength of 470 nm and/or of >0.25% at a wavelength of 550 nm.
15. The glass ceramic substrate according to claim 5, further comprising a glassy surface zone on the gradient layer, the glassy surface zone has a thickness of 300-1000 nm.
16. The glass ceramic substrate according to claim 5, wherein the underlying core has a crystal proportion that is at most 82%.
17. The glass ceramic substrate according to claim 5, wherein the substrate is configured for a use selected from the group consisting of a covering for a heating element, whiteware, a grill top, a fireplace panel, a support plate, an oven lining, chemically resistant laboratory fixtures, a high-temperature article, an extremely low-temperature article, a furnace window for combustion furnaces, a heat shield for shielding of hot surroundings, a cover for reflectors, floodlights, projectors, beamers, photocopiers, an article exposed to thermomechanical loads, night vision device, wafer substrate, translucent articles with UV protection, material for housing components, electronic devices, glass cover screens for IT, cell phone, laptop, scanner glass plate, facade plate, fire-resistant glazing, and ballistic protection.
18. A glass ceramic substrate made of a transparent, colored LAS glass ceramic comprising a composition (in wt %): TABLE-US-00005 Al.sub.2O.sub.3 18-23, Li.sub.2O 3.0-4.2, SiO.sub.2 60-69, ZnO 0-2, Na.sub.2O + K.sub.2O 0.2-1.5, MgO .sup.0-1.5, CaO + SrO + BaO 0-4, B.sub.2O.sub.3 0-2, TiO.sub.2 2.5-4,.sup. ZrO.sub.2 0.5-2,.sup. P.sub.2O.sub.5 0-3, SnO.sub.2 .sup.0-<0.6, Sb.sub.2O.sub.3 .sup.0-1.5, As.sub.2O.sub.3 .sup.0-1.5, TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 3.8-6,.sup. V.sub.2O.sub.5 .sup.0.01-0.06, and Fe.sub.2O.sub.3 0.03-0.2, wherein the LAS glass ceramic has a gradient layer and an underlying core, and wherein the LAS glass ceramic has keatite solid solution (KSS) as a predominant crystal phase in the underlying core and has high-quartz solid solution (HQSS) as a predominant crystal phase in the gradient layer, further comprising a depth profile of the HQSS and KSS crystal phase proportions, wherein the KSS crystal phase has a proportion in any depth 20 m that exceeds 50% of a sum of the HQSS and KSS crystal phase proportions, wherein the depth profile of the HQSS and KSS crystal phase proportions is achieved by phase transformation of HQSS crystals to KSS crystals during ceramization with a maximum temperature T.sub.max and over a residence time t(T.sub.max) of this maximum temperature in a temperature-time region that is delimited by four straight lines, which connect the four corner points with the value pairs (T.sub.max=910 C.; t(T.sub.max)=25 minutes), (T.sub.max=960 C.; t(T.sub.max)=1 minute), (T.sub.max=980 C.; t(T.sub.max)=1 minute), and (T.sub.max=965 C.; t(T.sub.max)=25 minutes), and wherein the ceramization method, including the time period for cooling to a temperature of 780 C., requires overall less than 60 min.
19. A glass ceramic substrate made of a transparent, colored LAS glass ceramic comprising a composition (in wt %): TABLE-US-00006 Al.sub.2O.sub.3 18-23, Li.sub.2O 3.0-4.2, SiO.sub.2 60-69, ZnO 0-2, Na.sub.2O + K.sub.2O 0.2-1.5, MgO .sup.0-1.5, CaO + SrO + BaO 0-4, B.sub.2O.sub.3 0-2, TiO.sub.2 2.3-4,.sup. ZrO.sub.2 0.5-2,.sup. P.sub.2O.sub.5 0-3, SnO.sub.2 .sup.0-<0.6, Sb.sub.2O.sub.3 .sup.0-1.5, As.sub.2O.sub.3 .sup.0-1.5, TiO.sub.2 + ZrO.sub.2 + SnO.sub.2 3.8-6,.sup. V.sub.2O.sub.5 .sup.0.01-0.06, and Fe.sub.2O.sub.3 .sup.0.03-0.2; and wherein the LAS glass ceramic has a gradient layer and an underlying core, wherein the LAS glass ceramic has keatite solid solution (KSS) as a predominant crystal phase in the underlying core and has high-quartz solid solution (HQSS) as a predominant crystal phase in the gradient layer, wherein the LAS glass ceramic has a fracture strength, expressed in a CIL value, of at least 0.8 N at an ambient humidity of 10% and/or a CIL value of at least 0.98 N at an ambient humidity of 1%, wherein the fracture strength is achieved by phase transformation of HQSS crystals to KSS crystals during ceramization with a maximum temperature T.sub.max and over a residence time t(T.sub.max) of this maximum temperature in a temperature-time region that is delimited by four straight lines, which connect the four corner points with the value pairs (T.sub.max=910 C.; t(T.sub.max)=25 minutes), (T.sub.max=960 C.; t(T.sub.max)=1 minute), (T.sub.max=980 C.; t(T.sub.max)=1 minute), and (T.sub.max=965 C.; t(T.sub.max)=25 minutes), and wherein the ceramization method, including the time period for cooling to a temperature of 780 C., requires overall less than 60 min.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) These and further advantages and properties of the glass ceramic substrate according to the invention and of the method according to the invention will be explained below on the basis of figures. Shown are:
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DETAILED DESCRIPTION
(10) For measurement of the fracture strength, the crack initiation load (CIL) test method, which is known as such, is employed first and foremost; see, for example, U.S. Pat. No. 8,765,262 A. It provides that a sample of the glass ceramic fixed in placed in a holder, which has preferably been flushed with nitrogen, is subjected to a point load by means of a material testing device (Micro-Combi Tester of CSM) with a V-I-O3 Vickers indenter. The specified load is increased linearly within 30 seconds to a chosen maximum value and then reduced in the same time without any residence time. On account of the load, starting from the corners of the pyramidal indent of the Vicker indenter, 0 to 4 cracks can form in the glass ceramic. The chosen maximum value of the load is increased in steps until, with each indent, 4 cracks arise. At each force, at least 10 measurements are performed in order to be able to recognize variation of crack formation, which is also dependent on the existing surface (prior damage). The mean value is obtained from the number of cracks at the same force.
(11) The sample preferably remains in the holder during the measurement until counting of the cracks is complete. The investigation is preferably carried out under nitrogen atmosphere in order to prevent as much as possible any subcritical crack growth due to the moisture of the ambient air.
(12) What is thus determined each time in several tests is the number of cracks starting from the corners of the indent of the Vicker indenter as a function of the applied load. The determined numbers of cracks are plotted in a diagram in relation to the indent force and the load/crack curve is fitted to a Boltzmann function, as is illustrated in
(13) According to the invention, the load that is determined under an ambient humidity of 10% is at least 0.8 N, and the load that is determined under an ambient humidity of 1% is at least 0.98 N. The strength of the glass ceramic according to the invention and, in particular, the impact strength, which is important for the glass ceramic substrate of a cooktop (cf. EN 60335, UL 858, or CSA 22.2), however, can be determined in another way.
(14) An alternative certified test method for determining the impact strength is the so-called ball drop test (see, for example, DE 10 2004 024 583 A1). The test is performed on square cutout pieces of the size 100 mm100 mm of a glass ceramic panel to be tested. The measurement of the impact strength is carried out based on DIN 52306. In this case, the measured sample is placed in a test frame and a steel ball weighing 200 g and having a diameter of 36 mm is allowed to drop onto the center of the sample. The drop height is increased in steps until fracture occurs. On account of the statistical character of the impact strength, this test is conducted on a series of at least 10 samples. The mean value, the standard deviation, and/or the 5% fractile of the measured value distribution are or is determined as the characteristic values for strength. The last value gives the drop height at which 5% of the tested samples are fractured.
(15) It is known that the impact strength of a plate made of glass or glass ceramic is governed, among other things, by more or less incidental surface damage. These surface damage influences on the strength, which are difficult to control because of their incidental nature, usually lead to a high standard deviation of the measured value distribution and can thus severely falsify any comparative evaluation of the impact strength of different test lots. A possible recourse is to enlarge the statistical scope of the test, which, under these circumstances, may entail a substantial effort. Another possibility, which has become established among circles of experts consists in subjecting the surface of the plate made of glass or glass ceramic to surface pretreatment, which is identical for all test lots, in the form of defined prior damage. In the examples described below, this prior damage consists of a single scratch, which is made in the center on the bottom side of the measured sample opposite the contact point of the ball impact on the top side. The scratch is made with a diamond point, which, in this case, is a Knoop indenter, by passing this diamond point parallel to its longer axis with a constant applied force of 0.12 N and at a constant speed of 20 mm/min over a length of at least 10 mm in a straight line over the surface of the measured sample.
(16) The impact strength of the LAS glass ceramic that has been subjected to contact damage of this kind can be determined by means of the ball drop test in the way described above. The standard deviation of typically less than 10% is only relatively still small, so that the measurement is available for a reliable statistical evaluation, while, at the same time, allowing a reasonable scope of test lots.
(17) The measurement results determined using the described CIL test and the described ball drop test on samples subjected to prior damage are plotted in
(18) But not just for the comparative ceramic B1 with HQSS as predominant crystal phase in the core, but also for known translucent or opaque glass ceramics with KSS as main crystal phase, the impact strength lies in a markedly lower range after defined prior damage than for the glass ceramic according to the invention or the glass ceramic substrate. For example, a drop height of only 296 cm was determined in the described way in the ball drop test of the above-described kind for the examples B2 and B3 mentioned below in Table 2.
(19) The CIL limit value of 0.8 N for 10% ambient humidity or of 0.98 N for 1% ambient humidity corresponds to a mean height in the ball drop test of about 32 cm and a 5% fractile of about 26 cm. Preferably, the ball drop height determined in the ball drop test on a glass ceramic according to the invention, which has been subjected to defined prior damage as described above, is therefore at least 30 cm (mean value) and/or 25 cm (5% fractile) and most preferably at least 40 cm (mean value) and/or 35 cm (5% fractile).
(20) Whereas the determination of the impact strength has direct relevance as the standard specification for cooktopsreference is once again made to EN 60335, UL 858, or CSA 22.2there is no stipulation of a standard for the bending strength as another characteristic value for characterization of the mechanical strength. Nonetheless, the determination of the bending strength is another appropriate parameter measurement, on the basis of which it is possible to demonstrate an increase in strength due to the process according to the invention. The bending strength test, which is therefore carried out additionally in some cases, is conducted as a double ring test in accordance with EN 1288 Part 5 (R45). A glass ceramic cooktop with a thickness of 4 mm in the design according to the invention achieves a characteristic bending strength of 236 MPa for a Weibull modulus of 6.0 in the bending strength test with subsequent evaluation according to the Weibull model. In comparison to the comparative ceramic with HQSS as predominant crystal phase in the core of the same thickness with a characteristic bending strength of 171 MPa for a Weibull modulus of 7.3, this is a significant increase and confirms the overall strength-enhancing effect of the layer structure according to the invention and the crystal content. The use of the Weibull model for the statistical analysis of strength measurements is generally known among circles of experts from, for example: W. Weibull, A statistical theory of the strength of materials, Ingenirsvetenskapsakademiens Handlingar No. 151, 1-45 (1939).
(21) The bending strength was also determined in this way on B2 for comparison. The result is a confirmation: the Weibull analysis gave a value of 131 MPa, which in fact lies below the value for the HQSS glass ceramic.
(22) All analysis methods thus confirm that the specific strength and, in particular, the specific impact strength, which is very relevant for application as a cooktop, turns out to be extraordinarily high for the glass ceramic of the substrate according to the invention and enables a total load capability that is comparable to that for conventional glass ceramic panels with a thickness of 4 mm, even starting at a thickness of no more than 3.0 mm.
(23) Responsible for this is the specific crystal layer structure or the profile of the glass ceramic, the determination of which is explained below. The KSS crystal phase proportion and the HQSS crystal phase proportion are measured as a function of the depth. The crystal phase proportions are given here always in vol % and the mean crystallite sizes in nm. The crystal phase proportions are determined by means of thin-layer XRD (X-ray diffraction) on intact samples of the glass ceramics or by means of powder XRD on powders prepared from them. The reflections that are characteristic for the respective crystal phase (HQSS or KSS) were measured and the crystal phase proportion was determined from the integral areas of the reflections. These integral areas were related to those of standard samples with known phase content and the proportions of the crystal phases and of the other phases amorphous to X-rays were thereby determined. The crystallite sizes given herein were determined via the reflection broadening according to the so-called Scherrer formula in relation to a standard. According to experience, the relative errors in measurement lie at 10% with respect to the phase content and at 5% with respect to the crystallite size.
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(25) The diagrams show that, for the exemplary embodiments A1 and A2, the HQSS proportion initially increases slightly in a first segment. This increase may be ascribed to the known glassy zone of the surface of the glass ceramic that is several 100 nm to maximally 1 m thick, in which no crystallites are present. However, because the XRD measurement is integrated in each measurement step over a depth information of about 2 m, the content of HQSS within the first 2 m enters into the measured value at the surface, which, accordingly, is not determined with 0% in the scope of measurement error. The proportion of the HQSS phase then decreases each time successively in the direction of the core. In opposition to this, the proportion of the KSS in the direction of the core increases each time. In the case of A1, the proportion of the KSS at about 76 m corresponds to the proportion of the bulk value of approximately 75%, which is determined in a depth of 2000 m. The HQSS at 76 m has, at the same time, dropped to the bulk value of 0% at a depth of 2000 m. In the case of the exemplary embodiment A2, the proportion of HQSS drops only to 10%, and the bulk value in a depth of 2000 m is reached here at about 56 m. Correspondingly, at this depth, the maximum value for KSS is reached at 59%. Moreover, so-called X-ray-amorphous phases lie in the layers, that is, phases that cannot be detected by means of X-ray diffraction, such phases also including, in particular, the glass phase.
(26) Both exemplary embodiments show that the intersection point of the curves that represent the HQSS phase proportion and the KSS phase proportion lies between 0 and 10 m and, more specifically, between 2 and 8 m and thus, in any case, lies below 10 m. In other words, the KSS crystal phase proportion exceeds 50% of the sum of the HQSS and KSS crystal phase proportions at the latest in a depth of 10 m and beyond.
(27) In the comparative example B1, by contrast, it can clearly be seen that no KSS is present in the material, and the HQSS already reaches its maximum bulk value of 70% in a depth of 2000 m at 29 m.
(28) At the same time, in spite of the KSS formation in the core, the glass ceramic according to the invention is transparent and thus is also fundamentally well suited for multicolored displays, in particular, in the absence of interfering scattering centers. The transparency is determined by means of a scattered-light measurement in accordance with International Standard ISO 14782: 1999(E), standardized in each case to a glass ceramic with a thickness of 4 mm. The result of this scattered-light measurement in a wavelength range from 380 nm to 1000 nm is illustrated in a diagram in
(29) In order to arrive at the suitability of the glass ceramic for multicolored displays, the maximum scattered-light proportion (haze), determined in accordance with International Standard ISO 14782: 1999(E), standardized to a glass ceramic with a thickness of 4 mm and at a wavelength of 470 nm, is therefore preferably at most 15%, most preferably at most 12%.
(30) It is further preferred that the maximum scattered-light proportion (haze), determined in accordance with International Standard ISO 14782: 1999(E), standardized to a glass ceramic with a thickness of 4 mm, does not exceed 20% in a wavelength range from 400 nm to 500 nm and most preferably does not exceed 17%.
(31) Besides the transparency, the glass ceramic must have an adequate coloring as well, taking into consideration a good display capability; that is, it must bring about transmission losses through absorption in the visible wavelength range. The coloring should, in particular, be so dark that non-luminous objects are not perceptible with the naked eye through the LAS glass ceramic, but luminous objects are visible. A measurement parameter that represents this property is the integral transmission .sub.vis in the visual spectral range. .sub.vis, also referred to as Y, brightness, or luminance, is calculated from the transmission spectrum in the wavelength range 380 nm to 780 nm. To this end, the measured spectrum is convoluted with the emission spectrum of a standard light source (D65) and with the green proportion of the so-called tristimulus of the CIE color system.
(32) For the integral transmission of the glass ceramic according to the invention in the visible spectral range, standardized to a glass ceramic with a thickness of 4 mm, the following preferably holds: .sub.vis, 4 mm5%.
(33) This parameter adjustment ensures an adequate darkening of the non-luminous components located beneath the glass ceramic.
(34) Furthermore, the spectral transmission .sub.470 nm, 4 mm of the glass ceramic according to the invention, standardized to a glass ceramic with a thickness of 4 mm, is preferably greater than 0.1% at a wavelength of 470 nm.
(35) Finally, the spectral transmission .sub.550 nm, 4 mm of the glass ceramic according to the invention, standardized to a glass ceramic with a thickness of 4 mm, is preferably greater than 0.25% at a wavelength of 550 nm.
(36) The two last-mentioned parameter adjustments ensure separately an improved display capability and together an especially good color display capability. The invention thus combines for the first time properties that were until now not held to be reconcilable, such as a high strength, on the one hand, and a good display capability, owing to a low scatter and suitable transmission properties, on the other hand. It is therefore suitable to a special degree for applications with high aesthetic demand, such as cooktops or display and control panels. In the process, it makes possible an increase in the impact strength of the material as well as the manufacture of cooktops with lower material thicknesses of 3 mm, for example, which fulfill the requirements set forth in accordance with EN 60335 or UL 858 or CSA 22.2.
(37) These properties, which are apparently in part opposed to one another, are obtained by a coordinated interplay between the composition of the glass ceramic, on the one hand, and the ceramization method, on the other hand.
(38) The method according to the invention for the production of the glass ceramic substrate according to the invention from an LAS glass with the above composition, starting from the glass melt, provides the following steps: refinement of the glass melt, forming of the precursor glass with cooling of the melt, subjection of the precursor glass produced in this way to a nucleation step and subsequently a crystal growth step, in which the HQSS grows on the crystal nuclei, subjection of the glass ceramic intermediate product pre-crystallized to this form with high-quartz solid solution (HQSS) as predominant crystal phase to a crystal transformation step, in which the HQSS crystal phase is transformed in part into a KSS crystal phase, wherein the crystal transformation step is carried out with a maximum temperature T.sub.max and over a residence time t(T.sub.max) for this maximum temperature in a temperature-time region that is delimited by four straight lines, which connect the four corner points with the value pairs (T.sub.max=910 C.; t(T.sub.max)=25 minutes), (T.sub.max=960 C.; t(T.sub.max)=1 minute), (T.sub.max=980 C.; t(T.sub.max)=1 minute), and (T.sub.max=965 C.; t(T.sub.max)=25 minutes).
(39) Starting from a precrystallized glass ceramic intermediate product with high-quartz solid solution (HQSS) as predominant crystal phase, the method according to the invention correspondingly begins with the crystal growth step. And starting from a precursor glass, the method according to the invention correspondingly begins with the nucleation step, which is followed by the crystal growth step and the crystal transformation step.
(40) The glass ceramic composition in conjunction with the production method makes possible the creation of the above-mentioned layer structure and the crystal content as well as the transmission characteristic according to the invention and thus the advantageous material properties. The main crystal phase of the glass ceramic is then composed of KSS that is present in the composition range
(41) Li.sub.(1-2x-2y)Mg.sub.xZn.sub.yAlSi.sub.2O.sub.6-Li.sub.(1-2x-2y)Mg.sub.(x)Zn.sub.(y)AlSi.sub.4O.sub.10 with (0x0.5; 0y0.5 and 0x+y0.5).
(42) The ceramization program for crystal transformation according to the invention will be explained on the basis of
(43) TABLE-US-00002 TABLE 1 T.sub.max [ C.] t(T.sub.max) [min] Corner point 1 910, preferably 920 25, preferably 20 Corner point 2 960 1, preferably 2 Corner point 3 980 1, preferably 2 Corner point 4 965 25, preferably 20
(44) By way of example, in the diagram according to
(45) Preferably, the method according to the invention is enhanced in that the precursor glass or the glass ceramic intermediate product is heated over a time period of at most 60 minutes, preferably at most 45 minutes, and most preferably at most 30 minutes from room temperature to the maximum temperature T.sub.max.
(46) The advantage of this enhancement is that the ceramization conditions of the residence time t(T.sub.max) and the maximum ceramization temperature T.sub.max, which are relevant to the product properties and at which the ceramization processes and, above all, the phase transformation proceed in an especially controlled manner, are reached rapidly and the ceramization does not proceed already during the heating and thus in a less controlled manner.
(47) A ceramization method that is markedly more economic, because it is more rapid, was developed for the production of a transparent, high-strength glass ceramic containing KSS. This ensues, for example, from the comparison with the method described in US 20140238971 A. Whereas the method according to the invention, including the time period for cooling to a temperature of 780 C., requires overall less than 60 min and preferably less than 50 min, ceramization methods that have been described in the prior art take at least 80 minutes to reach a comparable temperature in the region of the so-called cooling curve, that is, in the phase after the maximum temperature.
(48) Owing to the short residence time at T.sub.max, it is then guaranteed that the KSS is formed in a size distribution, in a mean overall size, and with a phase proportion that make possible the high strength together with a low light scattering and thus a transmission according to the invention. This is all the more surprising in that these ceramization conditions according to the invention could also be combined with a more economical and more rapid method than is described in the prior art.
(49) It also appears that SnO.sub.2 engages crucially as nucleating agent in the crystal formation process and crystal growth process. Thus, it was observed that the transformation from an Sn-containing, uncolored, transparent glass body to an opaque glass ceramic is markedly retarded in comparison to an As-refined glass body. This also appears to be the case for the material and process according to the invention, for which reason it is also possible in the wavelength range of 380 nm-500 nm to produce transparent glass ceramics that, at the same time, have a high proportion of keatite phase. This is ascribed to the retarded crystal growth and the resulting small crystallite sizes.
(50) The production can be conducted with even more process reliability and the required product properties can be refined when the preferred parameters discussed below are maintained. The glass ceramic is preferably produced from an LAS glass that is free, apart from unavoidable traces, of arsenic and antimony and has at least 0.1 wt % SnO.sub.2.
(51) Fundamentally, the use of SnO.sub.2 as an environmentally compatible reductant (in contrast to Sb.sub.2O.sub.3 or As.sub.2O.sub.3) during the refinement and as a redox partner for a coloring oxide, such as, for example, V.sub.2O.sub.5 and/or Fe.sub.2O.sub.3, for coloring of the glass ceramic is known from DE 199 39 787 C2. In particular, outstanding color effects and bubble qualities can be obtained in combination with a high-temperature refinement above 1700 C.
(52) In regard to the coloring, it is especially preferred when, for the components Fe.sub.2O.sub.3 and V.sub.2O.sub.5 in the composition, the following condition is maintained: 1<Fe.sub.2O.sub.3/V.sub.2O.sub.5<8.
(53) Also in regard to the coloring, the ceramization conditions T.sub.max and t(T.sub.max) are chosen in an optimum way such that a post-darkening of the already colored glass ceramic does not occur. Together with a shorter separation time for the nucleating agent and a shorter volume crystallization, a .sub.vis of 0.5% is not undershot, even though passage through three regions that can contribute to the coloring and scattering occurs. Finally, any scattering is prevented, because the short ceramization period more or less freezes the low-scattering state.
(54) The further coloring oxides in the composition comprise at least one substance from the group composed of the elements Cr, Mn, Co, Ni, Cu, Se, Mo, W, the oxides thereof, and metal oxides of rare earths. In particular, these are Cr.sub.2O.sub.3, MnO.sub.2, MnO, CoO, Co.sub.2O.sub.3, NiO, Ni.sub.2O.sub.3, CuO, Cu.sub.2O, SeO, further metal oxides of the rare earths, and molybdenum compounds. These coloring substances enable the color locations and/or transmission values to be adjusted in a more targeted manner in case of need. Preferably, the content of Cr.sub.2O.sub.3 should be <100 ppm in order not to restrict too strongly the transmission particularly in the spectral range from 380 nm to 500 nm; otherwise, there would ensue a negative effect on the display capability for white and blue LEDs or color displays.
(55) Advantageously, the ZnO content is at least 0.2 wt %. ZnO is advantageous in regard to matching of the index of refraction between crystal phase and glass phase and thus has a positive effect on the transmission properties by minimization of the scatter. Moreover, the Zn-induced gahnite formation in non-colored glass ceramics serves to improve the lightness value L*.
(56) The MgO content is preferably at least 0.1 wt % and most preferably at least 0.25 wt %. The upper limit of the MgO content lies at preferably 1 wt %.
(57) Preferably, the Al.sub.2O.sub.3 content is 19-23 wt %. The Al.sub.2O.sub.3 content plays a crucial role in adjusting the Al/Si ratio in the KSS as well in the HQSS. In this way, it is possible, for example, to adjust the coefficient of thermal expansion of the glass ceramic. Al.sub.2O.sub.3 further has a positive effect on the chemical resistance of the glass ceramic.
(58) Furthermore, it has been found to be advantageous when the TiO.sub.2 content is 2.5-4 wt %.
(59) The ZrO.sub.2 content is preferably 0.5-1.9 wt %, more preferably 0.5-1.8 wt %, and most preferably 0.5-1.7 wt %.
(60) Within these limits for TiO.sub.2 and ZrO.sub.2, the nucleation behavior is especially favorable. On the one hand, it needs to be ensured that sufficient nucleating agent (zirconium titanate) is present in order to ensure a rapid and homogeneous ceramization. On the other hand, contents of TiO.sub.2 and, in particular, ZrO.sub.2 that are too high lead to a devitrification or spontaneous nucleation already during forming of the precursor glasses, which also opposes a homogeneous ceramization and good transparency.
(61) Preferably, the glass ceramic substrate has a glassy surface zone on the gradient layer with a thickness of 50-1000 nm, preferably 50-800 nm, most preferably 300-800 nm. This layer, which is formed by diffusion processes, particularly by diffusion of Li into the bulk or core, and thus brings about a Li enrichment in the interior and a depletion in the surface zone of the crystal, is to be evaluated as being positive in effect, particularly in terms of chemical attack processes.
(62) It has been found to be advantageous when the crystal proportion of all crystalline phases in the core is at most 90%, preferably at most 85%, most preferably at most 80%. An advantageous lower limit of at least 69% can be given. Further crystalline secondary phases are HQSS, rutile, gahnite, and zirconium titanate. It has also been found to be advantageous when the total keatite proportion is less than 80%.
(63) The crystallite proportion is important for the adjustment of the properties of the glass ceramic, in particular the thermal expansion. Because KSS exhibits a higher thermal expansion than HQSS, especially the KSS proportion needs to be limited in the way mentioned.
(64) In order that an adequate transparency of the glass ceramic substrate is ensured, the crystallites of the KSS phase in the core of the glass ceramic are preferably <130 nm, determined as above by XRD/X-ray diffraction measurement.
(65) The coefficient of thermal expansion .sub.20/700 of the resulting glass ceramic is preferably less than 1.310.sup.6/K. As a result, it lies in the range of known translucent LAS glass ceramics with KSS as main crystal phase.
(66) The temperature difference strength of the glass ceramic substrate according to the invention lies preferably at >800 C. The temperature difference strength of translucent KSS glass ceramics lies, by contrast, at typically 700 C.
(67) The temperature difference strength (TUF) describes the resistance of a plate-shaped object made of glass or glass ceramic to local temperature gradients. In conjunction with the application as a cooktop, the test of the temperature difference strength is defined as follows: As test sample, a square cutout piece of the size 250 mm250 mm of the glass ceramic panel to be tested is laid horizontally on a radiant heat element, which is typical of application and has an outer diameter of 1803 mm, so as to lie tightly against it and is positioned asymmetrically in such that the four midpoints of each side of the measured sample protrude by 252 mm, 352 mm, 352 mm, 452 mm over the outer edge of the heating element. The heating element type 200N8-D2830R of the company Ceramaspeed Ltd. with the characteristics 2300 W/220 V is suited as a radiant heating element, for example. If the heating element is operated, a temperature gradient is created between the heated region and the cold outer edge of the measured sample. The heating process of the heating element is controlled in such a way that, after 5.00.5 minutes, fracture occurs owing to the temperature gradient. The maximum temperature thereby reached on the surface of the measured sample lying opposite the heating element is recorded as the characteristic value of the temperature difference strength. Based on the statistical nature of the temperature difference strength, this test is conducted on a series of at least 10 samples. The mean value of the measured value distribution is taken as the temperature difference strength of the test lot.
(68) It is known that the strength of a plate made of glass or glass ceramic with respect to mechanically or thermally created tensile stresses is governed, among other things, by more or less incidental surface damage. In conjunction with the application as a cooktop, it may be assumed that the plate made of glass or glass ceramic experiences surface damage in the course of practical use, in particular by abrasive cleaning, cookware, etc. Any statement about the temperature difference strength, insofar as this is relevant to the intended use, therefore presumes of necessity some prior damage to the measured sample that corresponds to surface damage following conventional use in practice. According to experience, this can be achieved by sanding the surface of the measured sample with SiC sandpaper of 220 grain under an applied pressure of 1.2 N/cm.sup.2. The person skilled in the art is familiar with the fact that the sanding is only fully effective on the occurrence of fracture when the sanding occurs particularly in those regions in which tensile stresses are created during the test, namely, in a sanding direction that is perpendicular to the respective main stress direction. This includes, in particular, a sanding of the cold edge of the measured sample in the region of its midpoints on each side and perpendicular to the outer edges thereof.
(69) The glass ceramic substrate of the above-described type according to the invention finds application especially preferably as a covering for heating elements, in particular as a cooktop or roast top, as whiteware, as a heating element cover, as a grill top or fireplace panel, as a support plate or oven lining in the ceramic, solar, or pharmaceutical industry or in medical technology, in particular for production processes under cleanroom conditions, as a lining for ovens in which chemical or physical coating methods are carried out or as chemically resistant laboratory fixtures, as a glass ceramic article for high-temperature or extremely low-temperature applications, as a furnace window for combustion furnaces, as a heat shield for shielding of hot surroundings, as a cover for reflectors, floodlights, projectors, beamers, photocopiers, for applications involving thermomechanical loads, such as, for example, in night vision devices, or as wafer substrates, as translucent articles with UV protection, as material for housing components, for example, of electronic devices, and/or as glass cover screens for IT, such as cell phones, laptops, scanner glass plates, etc., or as facade plates, fire-resistant glazing, or as components for ballistic protection.
(70) According to the investigations of the inventors, it may even be sufficient that only one composition component, the maximum temperature during the ceramization or the ceramization time, departs from the range stipulated by the invention for the required properties of the glass ceramic according to the invention or the glass ceramic substrate composed of it to be absent. The influence that the interacting parameters have on the result is seen from the following Table 2.
(71) Table 2 compares 8 exemplary embodiments A0 to A3 to 4 comparative examples B1 to B4. Presented under the composition of the LAS glass ceramic are the relevant parameters of the ceramization, namely, the maximum ceramization temperature T.sub.max in C., the processing time (DLZ) in minutes, the residence time at the maximum ceramization temperature t@T.sub.max in minutes, and the heating rate to T.sub.max in Kelvin/minute. The parameters of processing time, residence time, and heating rate are each given initially as actual values; specifically, this means the way in which the ceramization of the respective example was carried out. The preferred ranges of these three parameters within which the ceramization has a successful outcome for the specific composition and the specific maximum ceramization temperature of the respective examples according to the invention follow below them. Listed underneath are the product parameters measured according to the above-described method in the following sequence: the phase content of the HQSS phase in %; the phase content of the KSS phase in %; the average crystallite size of the HQSS in nm, the average crystallite size of the KSS in nm (the phase contents as well as the crystallite sizes are each given in relation to the core (bulk) of the glass ceramic, measured on powdered samples); subjective optical transmission properties (colored, not colored); the measured transmission at a wavelength of 470 nm, standardized to a glass ceramic with a thickness of 4 mm, in %; the measured integral transmission .sub.vis in the visible spectral range, standardized to a glass ceramic with a thickness of 4 mm, in %; the maximum scattered proportion (haze), standardized to a glass ceramic with a thickness of 4 mm, at a wavelength of 470 nm; the coefficient of thermal expansion .sub.20/700 C. between 20 and 700 C. in 1/K; the temperature difference strength (TUF) in C.; and the impact strength, determined in the ball drop test and given as mean value and as 5% fractile, each in cm, as well as determined in the CIL method and given in N.
(72) All exemplary embodiments are KSS-forming in terms of the invention, which means that they have a dominant KSS phase proportion in the core. This also applies to the comparative examples B2 and B3. The other comparative examples do not form any KSS or only form it to a slight extent. By contrast, examples B2 and B3 show that the crystals become too large, this being ascribed to the maximum ceramization temperature in conjunction with the residence time. It results from this that these glass ceramics are not sufficiently transparent, but rather are instead translucent. Example B1 shows that, owing to the slight formation of KSS, the strength-increasing effect does not occur. The example is therefore deficient in display capability.
(73) TABLE-US-00003 TABLE 2 A (Exemplary embodiments) Example A0 A1 A2 A3 Composition Al2O3 % 20.780 20.780 20.780 20.500 As2O3 % BaO % 2.260 2.260 2.260 2.380 CaO % 0.430 0.430 0.430 0.360 CeO2 % Cr2O3 % 0.030 F % Fe2O3 % 0.094 0.094 0.094 0.088 K2O % 0.260 0.260 0.260 0.220 Li2O % 3.900 3.900 3.900 3.830 MgO % 0.310 0.310 0.310 0.200 MnO2 % Na2O % 0.620 0.620 0.620 0.500 Nd2O3 % NiO % P2O5 % 0.092 0.092 0.092 0.093 Sb2O3 % 0.018 0.018 0.018 SiO2 % 64.840 64.840 64.840 65.500 SnO2 % 0.260 0.260 0.260 0.290 SrO % 0.017 0.017 0.017 TiO2 % 3.120 3.120 3.120 3.030 V2O5 % 0.025 0.025 0.025 0.016 ZnO % 1.490 1.490 1.490 1.500 ZrO2 % 1.410 1.410 1.410 1.380 TOTAL 99.926 99.926 99.926 99.917 Parameters of the ceramization Tmax. C. 930 965 965 930 DLZ (actual) min 139.4 45.4 45.4 139.4 t@Tmax. min 18 6 4 18 (actual) Heating rate K/min 25 30 18 25 to Tmax. (actual) DLZ min <150 <50 <50 <150 (preferred range) t@Tmax. min <25 <10 <8 <25 (preferred range) Heating rate K/min >15 >3 >10 >15 to Tmax. (preferred range) Structure in the volume/bulk Phase % 15 12 21 content HQSS Phase % 57 78 57 53 content KSS Phase nm not not not content deter- deter- deter- HQSS mined mined mined Crystallite nm 128 128 128 size KSS Transmission (in relation to thickness 4 mm) @ 470 nm % 0.360% 0.450% 0.370% @ 400- % 500 nm .sub.vis 1.200% 1.500% 1.300% Haze @ 470 nm 9% 13% .sub.20-700 0.18 1.08 0.68 TUF C. >800 C. >800 C. Impact strength Mean value cm 48.3 3.8 46.2 5.6 36.0 3.8 5%-Fractile cm 42 37 30 CIL at 10% N >1.00 0.98 0.06 0.84 0.07 ambient humidity CIL at 1% N 1.75 1.20 1.03 1.26 ambient humidity B (Comparative examples) B2 B3 Example B1 (712-8) (712-6) B4 Composition Al2O3 % 20.780 20.780 20.780 22.000 As2O3 % BaO % 2.260 2.260 2.260 1.180 CaO % 0.430 0.430 0.430 0.035 CeO2 % Cr2O3 % F % Fe2O3 % 0.094 0.094 0.094 0.014 K2O % 0.260 0.260 0.260 0.300 Li2O % 3.900 3.900 3.900 3.780 MgO % 0.310 0.310 0.310 0.720 MnO2 % Na2O % 0.620 0.620 0.620 0.380 Nd2O3 % NiO % P2O5 % 0.092 0.092 0.092 1.470 Sb2O3 % 0.018 0.018 0.018 SiO2 % 64.840 64.840 64.840 65.500 SnO2 % 0.260 0.260 0.260 0.300 SrO % 0.017 0.017 0.017 TiO2 % 3.120 3.120 3.120 2.090 V2O5 % 0.025 0.025 0.025 ZnO % 1.490 1.490 1.490 ZrO2 % 1.410 1.410 1.410 2.230 TOTAL 99.926 99.926 99.926 99.999 Parameters of the ceramization Tmax. C. 930 1120 1000 930 DLZ (actual) min 45.4 120.0 120.0 139.4 t@Tmax. min 4 1 1 18 (actual) Heating rate K/min 15 40 40 25 to Tmax. (actual) DLZ min (preferred range) t@Tmax. min (preferred range) Heating rate K/min to Tmax. (preferred range) Structure in the volume/bulk Phase % 69 57 content HQSS Phase % 3 82 81 7 content KSS Phase nm 51 content HQSS Crystallite nm not 164 138 size KSS deter- mined Transmission (in relation to thickness 4 mm) @ 470 nm % 0.440% 0.000% 0.000% @ 400- % 500 nm .sub.vis 1.500% 0.000% Haze @ 470 nm 4% 27% .sub.20-700 0.11 1.29 1.29 TUF C. >800 C. 550 C. 500 C. Impact strength Mean value cm 19.0 2.7 29.0 6.0 29.0 6.0 21.2 7.0 5%-Fractile cm 15 18 18 10 CIL at 10% N 0.69 0.05 ambient humidity CIL at 1% N 0.84 ambient humidity