TRANSPARENT GLASS CERAMIC, ESPECIALLY AS A COVER PANE

Abstract

A transparent, chemically prestressable or chemically prestressed glass ceramic is provided. The glass ceramic has keatite as a main crystal phase, a transmittance greater than 80% at a thickness of 0.7 mm, a haze of less than or equal to 10, and a crystal phase content of at least 80% by weight of keatite solid solution based on all crystal phases in the glass ceramic.

Claims

1. A transparent, chemically prestressable or chemically prestressed glass ceramic, comprising: keatite as a main crystal phase; a transmittance greater than 80% at a thickness of 0.7 mm; a haze of less than or equal to 10; and a crystal phase content of at least 80% by weight of keatite solid solution based on all crystal phases in the glass ceramic.

2. The glass ceramic of claim 1, wherein the crystal phase content is at least 95% by weight.

3. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising SiO.sub.2, Al.sub.2O.sub.3, and Li.sub.2O as main components and SnO.sub.2 and ZrO.sub.2 as nucleating agents.

4. The glass ceramic of claim 3, wherein the SiO.sub.2 is present in a content of between 58 and 72 in percent by weight.

5. The glass ceramic of claim 3, wherein the Al.sub.2O.sub.3 is present in a content of between 18 to 23 in percent by weight.

6. The glass ceramic of claim 3, wherein the Li.sub.2O is present in a content of between 2 to 5.5 in percent by weight.

7. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising 1 to 2% by weight of SnO.sub.2 and/or 2 to 3% by weight of ZrO.sub.2.

8. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising less than 0.1% by weight of TiO.sub.2.

9. The glass ceramic of claim 3, comprising a ratio of Al.sub.2O.sub.3 to SiO.sub.2, both in percent by weight, of less than 0.33.

10. The glass ceramic of claim 1, wherein the glass ceramic has a composition that is substantially free of colouring components selected from a group consisting of V.sub.2O.sub.5, Nd.sub.2O.sub.3, CoO, and combinations thereof.

11. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising 0 to 2% by weight of B.sub.2O.sub.3.

12. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising less than 2% by weight of P.sub.2O.sub.5.

13. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising less than 1% by weight of Na.sub.2O+K.sub.2O.

14. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising a condition wherein Li.sub.2OZnOCaO is less than 3% by weight.

15. The glass ceramic of claim 1, wherein the glass ceramic has a composition that is substantially free of MgO and/or BaO.

16. The glass ceramic of claim 1, wherein the glass ceramic has a composition comprising less than 1.8% by weight of CaO+SrO, and/or 0 to 2% by weight of ZnO, and/or a sum of ZrO.sub.2+SnO.sub.2 that is greater than 3.6% by weight, and/or up to 2% of one or more fining agents selected from a group consisting of As.sub.2O.sub.3, Sb.sub.2O.sub.3, halides, and SO.sub.3.

17. The glass ceramic of claim 1, wherein the glass ceramic is configured as a cover pane having a thickness of 0.4 mm to 1 mm.

18. The glass ceramic of claim 17, wherein the cover pane is configured for a use selected from a group consisting of an electronic display device, a mobile electronic display device, a mobile touch panel, a mobile digital display device, a smartphone, and a smart watch.

19. A method for producing a prestressed glass ceramic, comprising: producing a silicate green glass by a melting process and a hot shaping process; exposing the silicate green glass to a temperature treatment having at least one nucleation step carried out in a temperature range of 690? ? C. to 850? C. for a period of 5 min to 72 hour and at least one ceramization step carried out in a temperature range of 780? C. to 1100? C. for a period of 3 min to 150 hours; and performing at least one ion exchange in an exchange bath having a composition of 100% by weight to 0% by weight KNO.sub.3, 0% by weight to 100% by weight NaNO.sub.3, 0% by weight to 5% by weight LiNO.sub.3 at a temperature of the exchange bath between 370? C. and 500? C. and for a period of between 2 hours and 50 hours such that the prestressed glass ceramic has keatite as a main crystal phase, a transmittance greater than 80% at a thickness of 0.7 mm, a haze of less than or equal to 10, and a crystal phase content of at least 80% by weight of keatite solid solution based on all crystal phases in the glass ceramic.

20. The method of claim 19, wherein the hot shaping process is selected from a group consisting of a floating process, a rolling process, a drawing process, and an ingot casting process.

Description

DETAILED DESCRIPTION

[0049] The disclosure is explained in greater detail below by means of examples.

[0050] The compositions of glass-ceramic materials according to the disclosure can be found in TABLE 1 (all data in percent by weight). The abbreviation MO stands for the sum of alkaline earth oxides and ZnO.

[0051] The materials listed in TABLE 1 were melted and refined at temperatures of approx. 1600? ? C. to 1680? ? C. using raw materials customary in the glass industry. The charge was first melted in crucibles made of sintered silica glass and then poured into Pt/Rh crucibles with inner crucibles made of silica glass and homogenized by stirring at temperatures of approx. 1550? C. for 30 minutes. After standing at 1640? ? C. for 2 hours, castings of approx. 140 mm?100 mm?30 mm size were cast and stress-relieved in an annealing lehr at approx. 620? C. to 680? C. and cooled to room temperature. The test samples for measuring the properties in the glassy state and for the ceramization processes were prepared from the castings.

[0052] As a rule, two-stage programs were used for the ceramization processes, and these are indicated in TABLE 1. In this case, the starting glasses are first heated from room temperature to a nucleation temperature above Tg and held there for a time sufficient for nucleation. The samples are then heated to the ceramization temperature and likewise held there. Three- or multi-level programs can also be used. Holding times for nucleation are from 5 min to 72 h, preferably 30 min to 2 h, and followed by a ceramization step of 3 min to 150 h, preferably 3 min to 8 h. Holding times can furthermore be replaced by slow heating rates.

[0053] On the ceramized samples, the crystal phases and their contents as well as the transmittance in the visible range t vis [%] (on samples with a thickness of 0.7 mm) and the colour values in the Lab system (standard illuminant D65) were determined by means of XRD.

[0054] Haze was measured with a Haze-Gard dual from BYK Additives & Instruments in accordance with the ASTM D 1003 and ISO 14782 standards (on samples with a thickness of 0.7 mm).

[0055] The crystal phase contents stated in TABLE 1 in % by weight, based on all crystal phases present in the glass ceramic, were determined by means of X-ray diffraction measurements on a Panalytical X'Pert Pro diffractometer (Almelo, Netherlands). CuK? radiation (?=1.5060 ?) generated via an Ni filter was used as the X-ray radiation. The standard X-ray diffraction measurements on powder and solid samples were carried out under a Bragg-Brentano geometry (?-2?). The X-ray diffraction patterns were measured between 10? and 100? (2? angles). The quantification of the relative crystalline phase components and the determination of the crystallite sizes were carried out via a Rietveld analysis. Measurement was carried out on ground sample material, resulting in a clear preponderance of the volume component of the core region. The measured phase components therefore correspond to the phase distribution in the core of the glass ceramic. KSS stands for keatite solid solution, HQSS for high-quartz solid solution. The samples indicated by a V correspond to comparative examples. The examples that are simply numbered are examples of embodiments. The abbreviation n.d. stands for not determined.

TABLE-US-00002 TABLE 1 1 2 V1 V2 3 4 V3 5 6 Al2O3 18.63 18.52 18.47 18.66 18.39 19.20 18.11 18.31 18.43 B2O3 0.40 CaO 0.20 0.20 0.20 1.01 0.80 0.20 0.20 0.20 0.21 Fe2O3 0.013 0.013 0.013 0.014 0.014 0.013 0.013 0.012 0.012 K2O 0.47 0.47 0.93 0.47 0.46 0.46 0.46 0.46 0.47 Li2O 4.51 4.40 4.42 4.47 4.36 4.52 4.93 4.38 4.42 Na2O 0.49 0.48 0.96 0.58 0.47 0.47 0.49 0.47 0.48 P2O5 0.02 2.12 SiO2 69.70 69.40 69.00 68.30 69.30 68.80 69.60 68.20 69.10 SnO2 1.41 1.46 1.45 1.45 1.46 1.47 1.47 1.42 1.43 SrO 0.49 0.50 0.49 0.98 0.77 0.49 0.49 0.49 1.43 TiO2 0 ZnO 1.44 1.53 1.43 1.44 1.43 1.94 1.64 1.41 1.43 ZrO2 2.50 2.52 2.49 2.50 2.49 2.33 2.50 2.46 2.47 Sum 99.85 99.90 99.85 99.89 99.95 99.90 99.91 99.94 99.88 Na2O + K2O 0.95 0.96 1.89 1.05 0.93 0.93 0.95 0.94 0.95 CaO + SrO 0.69 0.70 0.69 1.99 1.57 0.69 0.69 0.69 1.64 Li2O ? ZnO ? CaO 2.870 2.670 2.790 2.020 2.132 2.380 3.090 2.770 2.780 ZrO2 + SnO2 3.910 3.980 3.940 3.950 3.950 3.800 3.970 3.880 3.900 Al2O3/SiO2 0.27 0.27 0.27 0.27 0.27 0.28 0.26 0.27 10.27 Tg 686 696 705 698 n.d. 678 672 680 680 Nucleation 740? C. 735? C. 745? C. 740? C. 745? C. 700- 740? C. 740? C. 690- 760? C. 750? C. Ceramization 880? C. 875? C. 825? C. 850? C. 835? C. 840? C. 880? C. 880? C. 830? C. ? vis 90.30 90.4 65.30 opaque 83.00 89.80 transluscent- 84.90 86.6 opaque c* 1.2 2.5 12.40 n.d. 6.5 1.4 n.d. 6.4 4.2 L* 96.1 96.1 84.70 n.d. 93.0 95.9 n.d. 93.8 94.6 a* ?0.2 ?0.2 0.60 n.d. ?0.4 ?0.2 n.d ?0.7 ?0.4 b* 1.1 1.4 12.30 n.d. 6.4 1.4 n.d. 6.4 4.2 C n.d. 2.6 25.00 n.d. 11.9 2.6 n.d. n.d. n.d Haze 0.46 0.5 88.97 n.d. 5.2 0.73 n.d. n.d. n.d KSS phase content [% 98.4 98.50 98.60 97.9 98.10 98.3 97.7 98.1 97.9 by weight] Nucleating agent phase 1.6 1.50 1.40 1.3 1.50 1.7 1.7 1.5 1.7 content [% by weight] SnO2 phase content [% 0 0.00 0.00 0.8 0.40 0 0.6 0.4 0.4 by weight] KSS Crystallite size 65 69 70.00 82 75 69 88 72 73 [nm]

[0056] Examples 1 to 6 show transparent glass ceramics according to the disclosure with high transparency and low haze.

[0057] Comparative Example V1 shows the negative effect of a high alkali content. Transmittance decreased to 65% in the case of this composition.

[0058] In Comparative Example V2, 1% by weight of CaO was used, and with this composition the glass ceramic becomes opaque as a result, whereas at 0.8% in Example 3, transmittance is still at 83%. This concentration should thus be considered to be borderline.

[0059] Comparative Example V3 illustrates the significance that the condition Li.sub.2OZnOCaO<3 can have. With this example it is not possible to achieve good transmittance.

[0060] Example 5 contains 2% by weight of P.sub.2O.sub.5. This can lead to a reduction in the viscosity and therefore in the required processing temperatures; however, it does already have a negative effect on the transparency as can be seen in the example.

[0061] The haze was also determined in Examples 1, 2, 3 and 4. It can be seen that Examples 1, 2 and 4, which are in the particularly preferred composition range of the disclosure, have particularly low haze values of less than 1.