Glass ceramic with reduced lithium content

Abstract

A transparent colored glass ceramic, in particular an LAS glass ceramic, suitable for use as a cooking surface is provided. The transparent colored glass ceramic includes high-quartz solid solution (HQ s.s.) as a main crystal phase and exhibits thermal expansion of −1 to +1 ppm/K in the range from 20° C. to 700° C. The glass ceramic has from 3.0 to 3.6 percent by weight of lithium oxide (Li.sub.2O) as constituents and either is colored with 0.003 to 0.05 percent by weight of vanadium oxide (V.sub.2O.sub.5) or is colored with 0.003 to 0.25 percent by weight of molybdenum oxide (MoO.sub.3).

Claims

1. A transparent colored glass ceramic, comprising: high-quartz solid solution as a main crystal phase; a thermal expansion of −1 to +1 ppm/K in a range from 20° C. to 700° C.; 3.0 to 3.6 percent by weight of lithium oxide (Li.sub.2O); and a colorant that is 0.003 to 0.25 percent by weight of molybdenum oxide (MoO.sub.3).

2. The glass ceramic as claimed in claim 1, wherein the transparent colored glass ceramic is an LAS glass ceramic.

3. The glass ceramic as claimed in claim 1, further comprising: a coloring oxide in an amount from 0 to less than 0.01 percent by weight, the coloring oxide being selected from a group consisting of CoO, NiO, Cr.sub.2O.sub.3, CuO, Nd.sub.2O.sub.3, and combinations thereof.

4. The glass ceramic as claimed in claim 1, further comprising, in percent by weight: TABLE-US-00012 BaO 1.2 to 2.8; MgO 0 to less than 1; CaO 0 to less than 1; and ZnO 1.2 to 2.5.

5. The glass ceramic as claimed in claim 1, further comprising 0.1 to 0.6 percent by weight of tin oxide (SnO.sub.2).

6. The glass ceramic as claimed in claim 1, further comprising, in percent by weight: TABLE-US-00013 SiO.sub.2 62 to 68; Al.sub.2O.sub.3 18.5 to 23; Li.sub.2O 3.0 to 3.6; V.sub.2O.sub.5 0.03 to 0.05 SnO.sub.2 0.1 to 0.6; BaO 1.2 to 2.8; MgO 0 to less than 1; ZnO 1.2 to 2.5; TiO.sub.2 2.6 to 4.5; ZrO.sub.2 0.9 to 1.7; B.sub.2O.sub.3 0 to 2; P.sub.2O.sub.5 0 to 4; Na.sub.2O + K.sub.2O 0 to less than 1.5; CaO 0 to less than 1; SrO 0 to less than 1; Fe.sub.2O.sub.3 0 to 0.2.

7. The glass ceramic as claimed in claim 6, wherein the Li.sub.2O is 3.2 to 3.6 in percent by weight.

8. The glass ceramic as claimed in claim 6, wherein the MgO is 0.3 to less than 1 in percent by weight.

9. The glass ceramic as claimed in claim 6, wherein the Na.sub.2O+K.sub.2O is 0 to less than 1 in percent by weight.

10. The glass ceramic as claimed in claim 6, wherein the Fe.sub.2O.sub.3 is 0 to 0.15 in percent by weight.

11. The glass ceramic as claimed in claim 1, further comprising a ratio (BaO+ZnO) /MgO in percent by weight in a range from 2.2 to 35.

12. The glass ceramic as claimed in claim 1, further comprising a compound selected from a group consisting of arsenic (As), antimony (Sb), cadmium (Cd), lead (Pb), halides, sulfur (S) in an amount from 0 to less than 0.5 percent by weight.

13. The glass ceramic as claimed in claim 12, wherein the amount is less than 0.05 percent by weight.

14. The glass ceramic as claimed in claim 1, wherein the high-quartz solid solution accounts for more than 80% of existing crystal phases and wherein further crystal phases, only nucleation phases or keatite solid solution (K s.s.), account for an amount of not more than 20% of the crystal phase fraction.

15. The glass ceramic as claimed in claim 1, further comprising, at a thickness of 4 mm, a light transmittance from 0.5% to 70%.

16. The glass ceramic as claimed in claim 15, wherein the light transmittance is from 0.5% to 20%.

17. The glass ceramic as claimed in claim 1, further comprising a viscosity of 200 dPa•s at less than 1750° C.

18. The glass ceramic as claimed in claim 1, further comprising a thermal expansion, in a range from 20° C. to 700° C., of 0 to 0.41 ppm/K.

19. The glass ceramic as claimed in claim 1, wherein the glass ceramic is configured for a use selected from a group consisting of a cooking surface, a fireplace window, a grill surface, a roasting surface, a cover of a fuel element in a gas grill, an oven door, a pyrolysis oven door, a kitchen or laboratory surface, a cover for a lighting device, a fire-resistant glazing, a safety glass, a laminate composite, a support plate or furnace lining of a thermal processes, and a back cover for mobile electronic device.

20. The glass ceramic as claimed in claim 3, further comprising MnO.sub.2 in an amount from 0 to less than 0.3 percent by weight.

21. The glass ceramic as claimed in claim 1, wherein the colorant further comprises 0.003 to 0.05 percent by weight of vanadium oxide (V.sub.2O.sub.5).

22. The glass ceramic as claimed in claim 1, further comprising a transmission of equal to or more than 45% to equal to or less than 85% at a wavelength of 1600 nm.

23. A transparent colored glass ceramic, comprising: high-quartz solid solution as a main crystal phase; a thermal expansion of −1 to +1 ppm/K in a range from 20° C. to 700° C.; 3.0 to 3.6 percent by weight of lithium oxide (Li.sub.2O); a colorant comprising molybdenum oxide (MoO.sub.3); and a transmission of equal to or more than 45% and equal to or less than 85% at a wavelength of 1600 nm.

24. The glass ceramic of claim 23, wherein the colorant further comprises vanadium oxide (V.sub.2O.sub.5).

25. The glass ceramic of claim 23, wherein the colorant is 0.003 to 0.25 percent by weight of molybdenum oxide (MoO.sub.3).

26. The glass ceramic of claim 24, wherein the weight of vanadium oxide (V.sub.2O.sub.5) comprises 0.003 to 0.05 percent by weight of vanadium oxide (V.sub.2O.sub.5).

Description

DESCRIPTION OF THE DRAWINGS

(1) The sole FIGURE shows the spectral transmittance of a sample according to example 12.

DETAILED DESCRIPTION

(2) The invention will be explained in more detail below by way of exemplary embodiments and with reference to Tables 1 to 5 (each with a and b) and with reference to the sole FIGURE.

(3) Tables 1 to 5 show compositions according to analysis, in percent by weight, and properties, in particular of preferred glass ceramics and related green glasses as examples 1 to 27 in tables 1 to 4 (each with a and b), as well as non-inventive comparative examples C1 to C5 in tables 5a and 5b.

(4) TABLE-US-00002 TABLE 1A EXAMPLES 1 TO 8 Example 1 2 3 4 5 6 7 8 Li.sub.2O 3.35 3.20 3.32 3.22 3.27 3.25 3.3 3.31 Na.sub.2O 0.44 0.270 0.33 0.20 0.21 0.28 0.29 0.29 K.sub.2O 0.44 0.64 0.32 0.66 0.23 0.31 0.35 0.35 MgO 0.66 0.31 0.50 0.31 0.77 0.52 0.59 0.59 CaO 0.47 0.41 0.21 0.34 0.30 0.71 0.61 0.61 SrO 0.018 BaO 1.74 2.39 2.04 2.31 2.45 2.35 2.35 2.35 ZnO 1.39 1.96 1.94 1.92 1.82 1.97 2.06 2.15 Al.sub.2O.sub.3 19.80 18.50 19.20 18.50 18.40 18.60 18.70 18.60 SiO.sub.2 66.50 67.60 67.50 67.30 67.30 66.90 66.70 66.70 TiO.sub.2 3.25 3.15 3.15 3.16 3.38 3.32 3.33 3.15 ZrO.sub.2 1.24 1.16 1.01 1.24 1.43 1.42 1.36 1.45 P.sub.2O.sub.5 0.028 0.028 0.028 0.028 0.028 0.028 0.03 0.028 SnO.sub.2 0.250 0.240 0.240 0.250 0.250 0.250 0.250 0.250 Fe.sub.2O.sub.3 0.140 0.180 0.200 0.200 0.085 0.100 0.100 0.100 V.sub.2O.sub.5 0.030 0.029 0.028 0.030 0.029 0.029 0.029 0.028 B.sub.2O.sub.3 0.230 0.300 Properties, glassy V.sub.A (° C.) 1316 1341 1332 1334 1322 1322 1316 1320 T2,30 1666 1699 1685 1692 1678 1675 1669 1679 (200 dPa .Math. s) OEG (° C.) 1226 1245 1235 1235 1260 1275 1280 N/A V.sub.A − OEG (° C.) 90 96 97 99 62 47 36 N/A

(5) TABLE-US-00003 TABLE 1B EXAMPLES 1 TO 8 Properties, Example ceramized 1 2 3 4 5 6 7 8 HQ s.s. content 98.0 98.1 94.7 97.9 97.9 95.3 90.8 89.8 (%) HQ s.s. 42 38 40 N/A N/A 45 47 47 crystallite size (nm) K s.s. content 1.808 0.000 3.684 0.000 0.000 3.731 8.079 8.736 (%) K s.s. crystallite N/A N/A N/A N/A N/A N/A 33 N/A size (nm) ZrTiO.sub.4 content 0.181 0.780 0.526 0.922 0.912 0.560 0.718 1.115 (%) Rutile (TiO.sub.2) 0.0 1.2 1.1 1.2 1.2 0.4 0.4 0.4 content (%) τ.sub.vis (%) 2.2 2.4 3.3 3.0 0.7 1.1 0.7 1.0 T (470 nm) (%) 0.7 0.8 1.0 0.9 0.2 0.4 0.2 0.3 T (630 nm) (%) 6.4 6.8 9.0 8.1 2.5 3.5 2.5 3.1 T (950 nm) (%) 69.6 65.3 65.2 65.0 69.6 69.2 67.2 67.6 T (1600 nm) (%) 73.3 70.4 67.6 67.9 80.4 78.7 78.8 78.7 Density (g/ccm) N/A N/A N/A N/A N/A 2.57 2.57 2.57 α.sub.(20-700° C.) 0.4 0.16 0.07 0.1 0.17 0.21 0.28 N/A (ppm/K)

(6) TABLE-US-00004 TABLE 2A EXAMPLES 9 TO 16 Example 9 10 11 12 13 14 15 16 Li.sub.2O 3.22 3.28 3.27 3.36 3.30 3.46 3.31 3.30 Na.sub.2O 0.35 0.32 0.31 0.36 0.35 0.42 0.33 0.42 K.sub.2O 0.55 0.3 0.4 0.35 0.57 0.53 0.5 0.39 MgO 0.42 0.56 0.65 0.56 0.49 0.37 0.37 0.44 CaO 0.32 0.55 0.27 0.58 0.75 0.76 0.55 0.51 SrO 0.015 0.012 0.012 0.012 0.016 0.014 0.024 0.14 BaO 2.03 1.6 1.61 1.62 1.71 1.81 1.85 1.79 ZnO 1.71 1.83 2.01 1.99 2 2.01 1.92 1.95 Al.sub.2O.sub.3 20.9 20.9 20.9 21.4 21.2 21.4 21.6 21.6 SiO.sub.2 65.4 65.6 65.5 64.8 64.6 64.2 64.4 64.5 TiO.sub.2 3.29 3.35 3.35 3.21 3.29 3.3 3.27 3.23 ZrO.sub.2 1.43 1.33 1.33 1.34 1.38 1.31 1.41 1.4 P.sub.2O.sub.5 0.029 0.029 0.029 0.029 0.032 0.031 0.03 0.03 SnO.sub.2 0.25 0.25 0.25 0.27 0.25 0.25 0.25 0.25 Fe.sub.2O.sub.3 0.1 0.1 0.1 0.099 0.099 0.1 0.099 0.1 V.sub.2O.sub.5 0.03 0.028 0.03 0.028 0.028 0.027 0.026 0.027 B.sub.2O.sub.3 0 0 0 0 0 0 0 0 Properties, glassy V.sub.A (° C.) 1318 1308 1310 1290 1299 1281 1301 1298 T2,30 1665 1650 1651 1639 1638 1637 1650 1646 (200 dPa .Math. s) OEG (° C.) 1290 1275 1265 1270 1230 1260 1325 1270 V.sub.A − OEG (° C.) 28 33 45 20 69 21 −24 28

(7) TABLE-US-00005 TABLE 2B EXAMPLES 9 TO 16 Properties, Example ceramized 9 10 11 12 13 14 15 16 HQ s.s. content 99.2 99.3 99.3 98.9 99.0 99.1 98.9 98.9 (%) HQ s.s. 37 38 36 42 42 42 40 40 crystallite size (nm) K s.s. content 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 (%) K s.s. crystallite N/A N/A N/A N/A N/A N/A N/A N/A size (nm) ZrTiO.sub.4 content 0.787 0.717 0.714 1.116 0.987 0.865 1.141 1.101 (%) Rutile (TiO.sub.2) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 content (%) τ.sub.vis (%) N/A N/A N/A 3.7 2.7 2.4 N/A N/A T (470 nm) (%) N/A N/A N/A 1.0 0.7 0.6 N/A N/A T (630 nm) (%) N/A N/A N/A 10.5 7.8 7.2 N/A N/A T (950 nm) (%) N/A N/A N/A 72.7 71.1 70.8 N/A N/A T (1600 nm) (%) N/A N/A N/A 74.0 74.4 74.6 N/A N/A Density (g/ccm) 2.55 2.56 2.56 2.56 2.55 2.55 2.56 2.56 α.sub.(20-700° C.) 0.32 0.24 0.2 0.26 0.41 0.35 0.3 0.29 (ppm/K)

(8) TABLE-US-00006 TABLE 3A EXAMPLES 17 TO 21 Example 17 18 19 20 21 Li.sub.2O 3.29 3.27 3.29 3.31 3.30 Na.sub.2O 0.36 0.36 0.36 0.37 0.36 K.sub.2O 0.36 0.36 0.35 0.36 0.36 MgO 0.55 0.56 0.56 0.56 0.56 CaO 0.59 0.58 0.59 0.58 0.58 SrO 0.012 0.012 0.013 0.013 0.012 BaO 1.62 1.62 1.63 1.62 1.63 ZnO 2.01 2.00 1.99 1.92 1.90 Al.sub.2O.sub.3 21.4 21.4 21.5 21.4 21.4 SiO.sub.2 64.9 64.8 64.8 64.8 64.7 TiO.sub.2 3.23 3.22 3.16 3.2 4.02 ZrO.sub.2 1.34 1.33 1.34 1.35 0.68 P.sub.2O.sub.5 0.029 0.029 0.022 0.035 0.03 SnO.sub.2 0.12 0.4 0.25 0.24 0.22 Fe.sub.2O.sub.3 0.1 0.1 0.097 0.099 0.100 V.sub.2O.sub.5 0.026 0.025 0.033 0 0 MoO.sub.3 0 0 0 0.16 0.149 Properties, glassy V.sub.A (° C.) 1301 1299 1300 1300 N/A T2, 30 1595 1593 1591 1645 N/A (200 dPa .Math. s) OEG (° C.) 1230 1250 1265 1275 N/A V.sub.A − OEG (° C.) 71 49 35 25 N/A

(9) TABLE-US-00007 TABLE 3B EXAMPLES 17 TO 21 Properties, Example ceramized 17 18 19 20 21 HQ s.s. content (%) N/A N/A N/A N/A N/A HQ s.s. crystallite N/A N/A N/A N/A N/A size (nm) K s.s. content (%) N/A N/A N/A N/A N/A K s.s. crystallite size N/A N/A N/A N/A N/A (nm) ZrTiO.sub.4 content (%) N/A N/A N/A N/A N/A Rutile (TiO.sub.2) N/A N/A N/A N/A N/A content (%) τ.sub.vis (%) 19.0 1.8 1.8 5.0 0.6 T(470 nm) (%) 8.4 0.3 0.3 5.2 0.6 T(630 nm) (%) 34.6 6.1 6.1 7.0 0.96 T(950 nm) (%) 77.1 71.3 70.7 50.9 28.2 T(1600 nm) (%) 74.9 73.9 74.4 68.1 66.13 Density (g/ccm) 2.56 2.56 2.56 2.56 N/A α.sub.(20-700° C.) (ppm/K) 0.26 0.28 0.3 0.29 N/A

(10) TABLE-US-00008 TABLE 4A EXAMPLES 22 TO 27 Example 22 23 24 25 26 27 Li.sub.2O 3.22 3.25 3.38 3.56 3.27 3.58 Na.sub.2O 0.78 0.76 0.20 0.76 0.38 0.48 K.sub.2O 0.20 0.58 0.20 0.58 0.39 0.40 MgO 0.81 0.20 0.76 0.19 0.38 0.48 CaO 0.21 0.72 0.72 0.71 0.21 0.46 SrO 0 0 0 0 0 0 BaO 2.42 0.49 2.43 0.490 0.49 1.42 ZnO 1.16 0.90 1.25 1.26 1.96 1.40 Al.sub.2O.sub.3 19.8 22.9 22.4 22.6 22.4 21.1 SiO.sub.2 66.9 65.6 63.8 64.8 65.9 65.6 TiO.sub.2 2.68 3.45 2.69 4.08 3.15 3.57 ZrO.sub.2 1.44 0.65 1.55 0.43 1.14 1.05 P.sub.2O.sub.5 0 0 0 0 0 0 SnO.sub.2 0.20 0.19 0.38 0.39 0.19 0.29 Fe.sub.2O.sub.3 0.11 0.15 0.03 0.031 0.064 0.095 V.sub.2O.sub.5 0 0 0 0 0 0 MoO.sub.3 0.043 0.15 0.084 0.083 0.058 0.080 Properties, glassy V.sub.A (° C.) 1331 N/A 1295 1299 1313 1305 T2,30 N/A N/A N/A N/A N/A N/A (200 dPa .Math. s) OEG (° C.) 1260 N/A 1310 1305 N/A 1230 V.sub.A − OEG (° C.) 71 N/A −15 −6 N/A 75

(11) TABLE-US-00009 TABLE 4B EXAMPLES 21 TO 27 Example Properties, ceramized 22 23 24 25 26 27 HQ s.s. content (%) N/A N/A N/A N/A N/A N/A HQ s.s. crystallite size 43 73 53 60 70 42 (nm) K s.s. content (%) N/A N/A N/A N/A N/A N/A K s.s. crystallite size N/A N/A N/A N/A N/A N/A (nm) ZrTiO.sub.4 content (%) 1.1 N/A 0.9 N/A 0.7 N/A Rutile (TiO.sub.2) content N/A N/A N/A N/A N/A 0.6 (%) τ.sub.vis (%) 38.5 1.4 28.2 4.3 10.8 0.9 T (470 nm) (%) 33.2 0.6 23.7 2.4 5.2 1.6 T (630 nm) (%) 46.1 3.4 35.7 6.7 17.6 0.9 T (950 nm) (%) 71.6 17.2 75.4 42.5 61.8 26.0 T (1600 nm) (%) 72.3 25.4 84.2 78.3 76.7 65.9 Density (g/ccm) 2.54 2.49 2.56 2.50 2.52 2.54 α.sub.(20-700° C.) (ppm/K) 0.59 0.44 0.44 0.42 −0.23 0.26

(12) TABLE-US-00010 TABLE 5A COMPARATIVE EXAMPLES C1 TO C5 Comparative example C1 C2 C3 C4 C5 Li.sub.2O 3.63 2.92 3.80 2.93 3.50 Na.sub.2O 0.20 0.68 0.20 0.18 0.39 K.sub.2O 0.20 0.20 0.67 0.20 0.39 MgO 1.09 1.11 0.19 1.11 1.11 CaO 0.42 0.52 0.41 0.21 0.21 SrO 0 0 0 0 0 BaO 0.98 2.43 0.97 2.43 2.42 ZnO 1.84 1.82 1.61 1.83 1.25 Al.sub.2O.sub.3 18.30 18.30 21.40 21.00 20.80 SiO.sub.2 68.40 66.50 65.40 65.00 64.40 TiO.sub.2 3.56 2.97 3.35 3.06 3.56 ZrO.sub.2 0.93 1.53 1.03 1.28 0.94 P.sub.2O.sub.5 0.03 0.028 0.029 0.027 0.029 SnO.sub.2 0.240 0.250 0.250 0.250 0.250 Fe.sub.2O.sub.3 0.200 0.200 0.200 0.079 0.200 V.sub.2O.sub.5 0.029 0.030 0.028 0.028 0.030 B.sub.2O.sub.3 0.000 0.450 0.440 0.440 0.460 Properties, glassy V.sub.A (° C.) 1307 1308 1297 1300 1281 T2, 30 (200 dPa .Math. s) 1666 1660 1638 1638 1621 OEG (° C.) N/A N/A N/A N/A N/A V.sub.A − OEG (° C.) N/A N/A N/A N/A N/A

(13) TABLE-US-00011 TABLE 5B COMPARATIVE EXAMPLES V1 TO V5 Comparative example C1 C2 C3 C4 C5 Properties, ceramized HQ s.s. content (%) 0.0 9.6 98.5 99.1 97.1 HQ s.s. crystallite size N/A 34 93 48 40 (nm) K s.s. content (%) 97.709 88.554 1.303 0.000 2.004 K s.s. crystallite size 74 74 N/A N/A N/A (nm) ZrTiO.sub.4 content (%) 0.687 1.506 0.163 0.887 0.911 Rutile (TiO.sub.2) content 1.6 0.3 0.0 0.0 0.0 (%) τ.sub.vis (%) 0.1 1.1 8.4 4.6 1.6 T(470 nm) (%) 0.0 0.3 3.3 1.6 0.4 T(630 nm) (%) 0.3 3.5 18.3 11.9 4.8 T(950 nm) (%) 44.1 56.6 69.7 77.5 61.7 T(1600 nm) (%) 67.9 67.7 67.7 80.2 67.4 Density (g/ccm) N/A N/A N/A N/A N/A α.sub.(20-700° C.) (ppm/K) 0.93 1.51 −0.15 0.61 0.91

(14) The constituents of the starting glasses shown in Tables 1 to 5, which is the composition in percent by weight according to analysis, were melted at 1600 to 1650° C. for 4 hours in sintered silica glass crucibles. Subsequently, they were poured into silica glass crucibles, at 1550 to 1580° C., and homogenized for 30 minutes, by stirring. Refining with tin oxide was performed at 1620 to 1640° C. for 2 hours.

(15) The glasses were cast into castings which were slowly cooled in a lehr, from a temperature of about 20° C. below the transformation temperature (T.sub.g) to room temperature.

(16) Some of the castings were used to determine the glassy properties. The processing temperature (V.sub.a) at a viscosity of 10.sup.4 dPa.Math.s and the temperature corresponding to a viscosity of 200 dPa.Math.s were determined. In some cases, the upper devitrification temperature (OEG) was determined after 5 hours in Pt contact.

(17) The abbreviation N/A as used in the tables means that these data have not been determined.

(18) In addition, samples for the measurements of the glass ceramic properties were produced from the castings and ceramized.

(19) The following temperature-time profile was used as a ceramization program: a) rapid heating from room temperature to 740° C. within 26 minutes; b) temperature increase from 740 to 825° C. within 18 minutes (heating rate of 4.7° C./min); c) temperature increase from 825° C. to 930° C. within 4 minutes (heating rate of 26° C./min), holding time of 4 minutes at the maximum temperature of 930° C.; d) cooling to 800° C. within 16 minutes, then rapid cooling to room temperature.

(20) The ceramized samples were subjected to an XRD measurement to determine the crystal phases and crystallite sizes using Rietveld analysis.

(21) Since determination of the amorphous phase is subject to considerable uncertainty, the crystal phase fractions indicated in the tables are based on the total crystal phase content of the glass ceramic.

(22) Transmittance was determined on samples with a thickness of 3 to 4 mm and converted to a thickness of 4 mm for better comparability. Light transmittance (τ.sub.vis) as measured according to DIN EN 410 is listed in the tables.

(23) Thermal expansion (α) was measured in a range from 20° C. to 700° C.

(24) The density in the ceramized state was also determined on some samples.

(25) Examples 1 to 27 as shown in tables 1a and 1b, in tables 2a and 2b, and in tables 3a and 3b and 4a and 4b are examples of glass ceramics according to the invention, and the examples C1 to C5 shown in tables 5a and 5b are non-inventive comparative examples.

(26) The glass ceramics of the invention according to examples 1 through 27 have processing temperatures (V.sub.A) between 1281° C. and 1341° C. The upper devitrification temperature (OEG) is between 1226° C. and 1325° C.

(27) By a suitable selection of the composition it is possible to obtain particularly devitrification-stable green glass melts, that is to say melts exhibiting a particularly large margin between the processing temperature and the upper devitrification temperature (VA-OEG). This may be necessary depending on the requirements of the selected hot forming process.

(28) The temperatures at which the green glass melts have a viscosity of 200 dPa.Math.s range between 1591° C. and 1699° C. in the examples according to the invention.

(29) All green glass melts in the examples according to the invention include, after the ceramization described above, high quartz (HQ s.s.) as the main crystal phase, generally with more than 90% of the total crystal phase fraction. Other crystal phases are keatite solid solution (K s.s.) and the nucleation agent phases ZrTiO.sub.4 and rutile (TiO.sub.2). At the same time, crystallite sizes are so small, with 33 nm to 73 nm, that no disturbing scattering occurs when display elements are used below the glass ceramic.

(30) Light transmittance (τ.sub.vis), converted to 4 mm thickness, is between 0.6 and 38.5% in the exemplary embodiments.

(31) The sole FIGURE shows the spectral transmittance of the inventive sample according to example 12.

(32) The sample of example 12 exhibits a transmittance of 74% in the infrared (1600 nm). At 470 nm, transmittance of the glass ceramic is 1.0%, and at all wavelengths above it is greater than this value, so that good display capability for colored LEDs is given.

(33) Light transmittance (τ.sub.vis) of the glass ceramic of example 12 is 3.7%.

(34) Example 17 exhibits higher transmittance in the visible range of almost 20% (Table 3b). This can be advantageous if an underside coating is to be used or display elements are to be mounted below the glass ceramic.

(35) Example 19 was melted with more cost-efficient technical raw materials. Thereby, impurities of 0.0041 percent by weight of Cr.sub.2O.sub.3 were introduced, and 0.3232 percent by weight of Rh was introduced through crucible contact. As can be seen from the properties listed in Tables 3a and 3b, the desired utility properties for glass ceramic cooking surfaces are nevertheless achieved.

(36) Example 20 illustrates the coloring using MoO.sub.3. The composition contains 0.16% of MoO.sub.3 (Table 3a) and has a light transmittance (τ.sub.vis) of 5% when ceramized (Table 3b).

(37) Further examples for coloring with MoO.sub.3 are listed in Tables 4a and 4b. Light transmittances (τ.sub.vis) of the MoO.sub.3-colored examples are likewise in a range from 0.6% to 38.5%. Higher and lower light transmittances can also be adjusted by adjusting the composition accordingly.

(38) The glass ceramics according to the exemplary embodiments have a thermal expansion (α) of 0.07 to 0.59 ppm/K in the range from 20 to 700° C. (Tables 1b, 2b, and 3b), which meets the requirements for temperature-stable cooking surfaces.

(39) The non-inventive comparative examples C1 to C5 in table 5a illustrate the choice of composition ranges.

(40) Comparative Examples C1 and C2 contain excessively high contents of MgO (Table 5a). Both have keatite solid solution (K s.s.) as the main crystal phase and correspondingly high thermal expansions (α), as shown in Table 5b. This is also due to the excessively high SiO.sub.2 content in comparative example C1 and the insufficient Li.sub.2O content in C2 (Table 5a).

(41) Comparative Example C3 has a lithium content of 3.8% (Table 5a). Although the glass ceramic exhibits good properties, it implies higher raw material costs due to the high lithium content and therefore no advantage over the known prior art.

(42) Compared to the glass ceramics of the invention, comparative Example C4 also has an excessively high MgO content, with 1.11 wt %, and an insufficient Li.sub.2O content, as can be seen in Table 5a, so that thermal expansion between 20° C. and 700° C. increases to 0.6 ppm/K (Table 5a).

(43) Comparative Example C5 shows the adverse effect of an excessively high MgO content (Table 5a) on thermal expansion, which increases to 0.91 ppm/K in example C5 (Table 5b).