Lithium Silicate- Low Quartz Glass Ceramic

Abstract

Lithium silicate-low quartz glass ceramics are described which are characterized by a combination of very good mechanical and optical properties and can therefore be used in particular as restoration material in dentistry.

Claims

1. Lithium silicate-low quartz glass ceramic, which comprises lithium silicate as main crystal phase and low quartz as further crystal phase.

2. Glass ceramic according to claim 1, which comprises 59.0 to 79.0.

3. Glass ceramic according to claim 1, which comprises 8.0 to 15.0 wt.-% Li.sub.2O.

4. Glass ceramic according to claim 1, which comprises 0 to 9.0 wt.-% P.sub.2O.sub.5.

5. Glass ceramic according to claim 1, which comprises 1.0 to 8.0 wt.-% oxide of monovalent elements Me.sup.I.sub.2O selected from the group of K.sub.2O, Na.sub.2O, Rb.sub.2O, Cs.sub.2O and mixtures thereof.

6. Glass ceramic according to claim 1, which comprises 0 to 5.0 wt.-% K.sub.2O.

7. Glass ceramic according to claim 1, which comprises 1.0 to 9.0 wt.-% oxide of divalent elements Me.sup.IIO selected from the group of CaO, MgO, SrO, ZnO and mixtures thereof.

8. Glass ceramic according to claim 1, which comprises 1.0 to 6.0 wt.-% MgO.

9. Glass ceramic according to claim 1, which comprises 0 to 8.0 wt.-% oxide of trivalent elements Me.sup.III.sub.2O.sub.3 selected from the group of Al.sub.2O.sub.3, B.sub.2O.sub.3, Y.sub.2O.sub.3, La.sub.2O.sub.3, Ga.sub.2O.sub.3, In.sub.2O.sub.3 and mixtures thereof.

10. Glass ceramic according to claim 1, which comprises 1.0 to 6.0 wt.-% Al.sub.2O.sub.3.

11. Glass ceramic according to claim 1, which comprises SiO.sub.2 and Li.sub.2O in a molar ratio in the range of from 2.2 to 4.1.

12. Glass ceramic according to claim 1, which comprises lithium disilicate or lithium metasilicate as main crystal phase.

13. Glass ceramic according to claim 1, which has at least 20 wt.-% lithium disilicate crystals.

14. Glass ceramic according to claim 1, which has 0.2 to 28 wt.-% low quartz crystals.

15. Starting glass, which comprises the components of the glass ceramic according to claim 1.

16. Starting glass according to claim 15, which comprises nuclei for the crystallization of lithium metasilicate, lithium disilicate and/or low quartz.

17. Glass ceramic comprising lithium silicate as main crystal phase and low quartz as further crystal phase or starting glass comprising nuclei for the crystallization of lithium metasilicate, lithium disilicate and/or low quartz, wherein the glass ceramic and the starting glass are present in the form of a powder, a granulate, a blank or a dental restoration.

18. Process for the preparation of the glass ceramic according to claim 1, in which a starting glass comprising nuclei for the crystallization of lithium metasilicate, lithium disilicate and/or low quartz is subjected to at least one heat treatment in the range of 700? to 900? C.

19. Process according to claim 18, in which (a) the starting glass is subjected to a heat treatment at a temperature of 400 to 600? C. in order to form the starting glass with nuclei, and (b) the starting glass with nuclei is subjected to a heat treatment at a temperature of 700 to 900? C. in order to form the lithium silicate-low quartz glass ceramic.

20. Process of using the glass ceramic according to claim 1 as dental material, for coating dental restorations or for the preparation of dental restorations.

21. Process according to claim 20, wherein the glass ceramic is given a shape of the desired dental restoration comprising a bridge, inlay, onlay, veneer, abutment, partial crown, crown or facet, and is manufactured by pressing or machining.

22. Process for the preparation of a dental restoration comprising a bridge, inlay, onlay, veneer, abutment, partial crown, crown or facet, in which the glass ceramic according to claim 1 is given the shape of the desired dental restoration by pressing or machining.

Description

EXAMPLES

Examples 1 to 34Composition and Crystal Phases

[0082] A total of 34 glasses and glass ceramics according to the invention with the composition specified in Table I were prepared by melting corresponding starting glasses as well as subsequent heat treatments for controlled nucleation and crystallization.

[0083] The heat treatments used for controlled nucleation and controlled crystallization are also specified in Table I. The following meanings apply

TABLE-US-00009 T.sub.g Glass transition temperature, determined by means of DSC T.sub.g and t.sub.s Temperature and time used for melting the starting glass T.sub.Kb and t.sub.Kb Temperature and time used for nucleation of the starting glass T.sub.c and t.sub.c Temperature and time used for the crystallization T.sub.press and t.sub.press Temperature and time used for crystallization by hot pressing CR value Contrast value of the glass ceramic according to British Standard BS 5612 determined using: Apparatus: CM-3700d spectrometer (Konica- Minolta) Measurement parameters: Measurement area: 7 mm ? 5 mm Type of measurement: reflectance/ reflection Measurement range: 400 nm-700 nm Sample size: Diameter: 15-20 mm Thickness: 2 mm +/? 0.025 mm Plane parallelism: +/? 0.05 mm Surface roughness: about 18 ?m. CTE Coefficient of thermal expansion of the glass ceramic according to ISO 6872 (2008), measured in the range of 100 to 500? C. ?.sub.Biax Biaxial breaking strength, measured according to dental standard ISO 6872 (2008)

[0084] The amounts of the crystal phases were determined by means of the Rietveld method. For this, powders of the respective glass ceramic were used which were mixed with Al.sub.2O.sub.3 (product name: Taimicron TM-DAR, from: Taimei Chemicals, Co. Ltd., Japan) as internal standard in a ratio of 50 wt.-% glass ceramic to 50 wt.-% Al.sub.2O.sub.3. This mixture was slurried with acetone in order to achieve as good a thorough mixing as possible. The mixture was then dried at about 80? C. Then a diffractogram in the range 10 to 100? 2? was acquired by means of a D8 Advance diffractometer from Bruker using Cu.sub.K? radiation and a step size of 0.014? 2?. This diffractogram was then evaluated with the TOPAS software from Bruker, and the phase proportions were determined. For all diffractograms a lower limit of about 30 nm for the Li.sub.3PO.sub.4 crystallite size was used.

[0085] To produce the glasses and glass ceramics according to the invention the starting glasses in a range of 100 to 200 g were first melted from customary raw materials at 1500? C. or 1400? C. for a period of 1 to 3 hours, wherein the melting was very easily possible without formation of bubbles or streaks. By pouring the starting glasses into water, glass frits were prepared which were then melted a second time at 1500? C. or 1400? C. for 1 hour for homogenization.

[0086] A first heat treatment of the starting glasses at a temperature of 460 to 550? C. led to the formation of glasses with nuclei. As a result of a further heat treatment at 760 to 880? C., these nuclei-containing glasses crystallized to form glass ceramics with lithium silicate as main crystal phase and low quartz as further crystal phase, as was established by X-ray diffraction tests. Thus, lithium silicate-low quartz glass ceramics according to the invention were obtained.

[0087] A) Solid Glass Blocks

[0088] In Examples 1-26, 28 and 31-34 the glass ceramics were prepared from solid glass blocks. For this, the obtained glass granulates were melted again at the temperature T.sub.S for a period t.sub.S. The obtained melts of the starting glass were then poured into a graphite mould in order to produce solid glass blocks. These glass monoliths were then stress relieved at the temperature T.sub.Kb for a period t.sub.Kb, whereby nucleation was able to take place. The nuclei-containing starting glasses were then heated to a temperature T.sub.C for a period t.sub.C. Glass ceramics according to the invention with lithium disilicate as main crystal phase and low quartz as additional phase were thereby formed, as could be established by X-ray diffraction tests at room temperature.

[0089] It is assumed that in this process variant a volume crystallization of lithium disilicate and low quartz has taken place.

[0090] B) Powder Compacts

[0091] In Example 27 the glass ceramic was prepared from powder compacts. For this, the obtained glass granulate was ground in a zirconium oxide mill to a particle size of <90 ?m. About 4 g of this powder was then pressed to form cylindrical blanks and sintered in a sinter furnace (Programat? from Ivoclar Vivadent AG) at a temperature T.sub.C and a holding time of t.sub.C to form dense glass ceramic bodies. A glass ceramic according to the invention with lithium metasilicate as main crystal phase as well as lithium disilicate and low quartz as additional phases was formed by the sintering, as could be established by X-ray diffraction tests at room temperature.

[0092] C) Preparation of a Dental Restoration from Blocks According to A)

[0093] The glass ceramic blocks produced according to Examples 1-26, 28 and 31-34 were machined in a CAD/CAM unit to form desired dental restorations, such as crowns. For this, the crystallized blocks were provided with a suitable holder, and then given the desired shape in an inLab MC XL grinding unit from Sirona Dental GmbH, Germany. For the processing of the blanks according to the invention it was possible to use the same grinding parameters as for commercial e.max CAD blocks, Ivoclar Vivadent, Liechtenstein.

[0094] D) Hot Pressing of the Glass Ceramic

[0095] In Example 19, for which T.sub.press and t.sub.press are specified, the glass ceramic was prepared from solid glass blocks by hot pressing.

[0096] For this, the obtained glass granulate was melted again at the temperature T.sub.S for a period t.sub.S. The obtained melt of the starting glass was then poured into a pre-heated steel mould in order to produce rods. These monolithic glass rods were then stress relieved at a temperature T.sub.Kb for a period t.sub.Kb, whereby nucleation was able to take place. The rods were then sawn to form small cylinders with a mass of about 4 to 6 g. These small cylinders were then crystallized at a temperature T.sub.C for a period of t.sub.C. The nucleated and crystallized cylinders were then pressed to form a shaped body in a hot-pressing furnace at the temperature T.sub.press and for a holding time of t.sub.press. A glass ceramic according to the invention with lithium disilicate as main crystal phase and low quartz as further crystal phase had formed after the hot-pressing, as could be established by X-ray diffraction tests of the formed shaped body at room temperature.

[0097] E) Sintering of a Nucleated Glass

[0098] In Example 29 the starting glass was melted at 1500? C. for 2 h and then quenched in water. The obtained glass granulate was then nucleated at a temperature T.sub.Kb and for a time t.sub.Kb. The nucleated starting glass was comminuted to form a powder with an average particle size of 20 ?m. A test piece was prepared from this nucleated glass powder to determine the thermal expansion and to determine the optical properties, and crystallized and densely sintered at a temperature of T.sub.C and for a time t.sub.C. After the dense sintering a glass ceramic according to the invention with lithium disilicate as main crystal phase and low quartz as further additional phase had formed, as could be established by X-ray diffraction tests of the formed shaped body at room temperature.

TABLE-US-00010 TABLE 1 Example No. 1 2 3 4 5 Composition wt.-% wt.-% wt.-% wt.-% wt.-% SiO.sub.2 74.3 73.3 72.0 72.0 74.9 Li.sub.2O 11.2 12.6 13.3 12.3 10.7 K.sub.2O 3.4 3.2 3.5 3.4 3.4 Rb.sub.2O MgO 4.4 1.4 4.5 4.4 4.4 CaO 1.9 SrO Al.sub.2O.sub.3 2.8 3.5 2.8 2.8 2.8 Ga.sub.2O.sub.3 Er.sub.2O.sub.3 0.1 CeO.sub.2 0.8 V.sub.2O.sub.5 0.1 P.sub.2O.sub.5 3.9 4.1 3.9 3.9 3.8 F.sup.? Tb.sub.4O.sub.7 0.3 Tg/? C. 471 465 469 463 471 T.sub.s/? C., t.sub.s/min 1500, 120 1520, 120 1500, 120 1500, 120 1500, 120 T.sub.Kb/? C., t.sub.Kb/min 500, 30 480, 10 500, 30 500, 30 500, 30 T.sub.c/? C., t.sub.c/min 800, 30 800, 15 800, 30 810, 20 800, 30 Main crystal phase Li.sub.2Si.sub.2O.sub.5 (40.9) Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 (51.3) Li.sub.2Si.sub.2O.sub.5 (43.4) Li.sub.2Si.sub.2O.sub.5 (36.2) (wt.-%) Further crystal Low quartz Low quartz, Low quartz Low quartz Low quartz phases (17.5), Li.sub.3PO.sub.4 (0.2), (4.4), (20.7), (wt.-%) Li.sub.3PO.sub.4 (6.3) Li.sub.3PO.sub.4 (6.8) Li.sub.3PO.sub.4 (6.0), Li.sub.3PO.sub.4 (5.4) ?.sub.Biax/MPa 464 376 CR value 71.83 71.45 71.21 L* 94.15 89.46 93.90 a* ?0.45 0.48 ?0.40 b* 3.44 13.22 3.92 CTE/10.sup.?6K.sup.?1 (100-500? C.) Example No. 6 7 8 9 10 Composition wt.-% wt.-% wt.-% wt.-% wt.-% SiO.sub.2 72.3 72.6 70.1 73.0 75.6 Li.sub.2O 12.0 11.7 11.3 11.4 10.2 K.sub.2O 3.4 3.4 3.4 3.4 Rb.sub.2O 6.5 MgO 4.4 4.4 4.2 4.4 4.3 CaO SrO Al.sub.2O.sub.3 2.8 2.8 2.2 2.8 2.7 Ga.sub.2O.sub.3 Er.sub.2O.sub.3 0.1 0.1 0.1 0.1 CeO.sub.2 0.8 0.8 0.7 0.6 V.sub.2O.sub.5 0.1 0.1 0.1 0.1 P.sub.2O.sub.5 3.9 3.8 4.5 3.8 3.8 F.sup.? Tb.sub.4O.sub.7 0.3 0.3 0.3 0.4 T.sub.g/? C. 469 473 472 470 480 T.sub.s/? C., t.sub.s/min 1500, 120 1500, 120 1500, 120 1500, 120 1500, 120 T.sub.Kb/? C., t.sub.Kb/min 480, 60 520, 10 480, 120 470, 10 500, 30 T.sub.c/? C., t.sub.c/min 800, 30 820, 10 800, 10 780, 30 800, 30 Main crystal phase Li.sub.2Si.sub.2O.sub.5 (42.7) Li.sub.2Si.sub.2O.sub.5 (39.0) Li.sub.2Si.sub.2O.sub.5 (30.0) Li.sub.2Si.sub.2O.sub.5 (38.4) Li.sub.2Si.sub.2O.sub.5 (32.7) (wt.-%) Further crystal Low quartz Low quartz Low quartz Low quartz Low quartz phases (10.2), (12.1), (7.1), (14.8), (24.2), (wt.-%) Li.sub.3PO.sub.4 (6.0) Li.sub.3PO.sub.4 (6.0) Li.sub.3PO.sub.4 (7.1) Li.sub.3PO.sub.4 (5.6) Li.sub.3PO.sub.4 (6.3) ?.sub.Biax/MPa 371 395 456 326 347 CR value 69.27 68.94 77.14 68.63 71.28 L* 89.78 89.68 90.06 90.29 94.07 a* 0.34 0.18 ?0.13 0.85 ?0.46 b* 13.65 13.9 9.07 11.17 3.46 CTE/10.sup.?6K.sup.?1 (100-500? C.) 10.8 11.3 11.5 Example No. 11 12 13 14 15 Composition wt.-% wt.-% wt.-% wt.-% wt.-% SiO.sub.2 72.9 72.2 70.2 72.4 70.4 Li.sub.2O 11.3 11.6 12.5 10.9 12.1 K.sub.2O 2.1 3.4 3.3 3.4 3.1 Rb.sub.2O MgO 1.8 4.4 1.6 4.3 3.4 CaO 1.8 2.3 SrO 3.3 Al.sub.2O.sub.3 2.7 4.6 4.0 3.9 3.6 Ga.sub.2O.sub.3 2.5 Er.sub.2O.sub.3 0.2 0.2 0.1 CeO.sub.2 1.2 0.6 0.9 V.sub.2O.sub.5 0.1 0.1 0.1 P.sub.2O.sub.5 3.8 3.8 4.3 3.8 3.5 F.sup.? 0.3 Tb.sub.4O.sub.7 0.3 0.4 0.3 T.sub.g/? C. 453 477 464 472 462 T.sub.s/? C., t.sub.s/min 1500, 120 1500, 120 1500, 120 1500, 120 1500, 120 T.sub.Kb/? C., t.sub.Kb/min 460, 90 500, 30 500, 10 480, 40 540, 10 T.sub.c/? C., t.sub.c/min 800, 40 800, 30 800, 60 770, 60 790, 30 Main crystal phase Li.sub.2Si.sub.2O.sub.5 (45.0) Li.sub.2Si.sub.2O.sub.5 (38.7) Li.sub.2Si.sub.2O.sub.5 (38.1) Li.sub.2Si.sub.2O.sub.5 (34.4) Li.sub.2Si.sub.2O.sub.5 (39.0) (wt.-%) Further crystal Low quartz Low quartz Low quartz Low quartz Low quartz phases (19.3), (13.4), (9.4), (17.4), (9.9), (wt.-%) Li.sub.3PO.sub.4 (2.8), Li.sub.3PO.sub.4 (5.4) Li.sub.3PO.sub.4 (6.3) Li.sub.3PO.sub.4 (5.2) Li.sub.3PO.sub.4 (5.4) Ca.sub.2Sr.sub.3(PO.sub.4).sub.3F (5.5) ?.sub.Biax/MPa 397 350 377 285 CR value 70.06 64.56 69.01 70.84 L* 89.22 85.82 90.67 86.98 a* 0.50 2.6 1.95 1.80 b* 5.87 19.74 8.96 19.10 CTE/10.sup.?6K.sup.?1 (100-500? C.) 10.6 10.8 Example No. 16 17 18 19 20 Composition wt.-% wt.-% wt.-% wt.-% wt.-% SiO.sub.2 69.2 71.5 71.0 74.7 70.0 Li.sub.2O 11.5 10.5 10.0 9.8 10.5 K.sub.2O 3.2 3.3 3.3 3.3 3.2 Cs.sub.2O Rb.sub.2O MgO 3.1 3.5 3.8 4.3 3.8 CaO SrO ZnO Al.sub.2O.sub.3 3.1 2.8 3.0 2.9 3.8 Ga.sub.2O.sub.3 La.sub.2O.sub.3 3.4 Y.sub.2O.sub.3 2.9 In.sub.2O.sub.3 4.7 Er.sub.2O.sub.3 0.2 0.1 0.1 0.1 0.2 ZrO.sub.2 SnO.sub.2 CeO.sub.2 1.0 0.6 1.2 0.8 0.5 MnO.sub.2 V.sub.2O.sub.5 0.1 0.1 0.1 0.1 0.1 Ta.sub.2O.sub.5 3.8 P.sub.2O.sub.5 3.5 4.3 3.7 3.6 3.7 F.sup.? Tb.sub.4O.sub.7 0.4 0.4 0.4 0.4 0.4 T.sub.g/? C. 483 477 478 467 482 T.sub.s/? C., t.sub.s/min 1500, 120 1500, 120 1500, 120 1500, 120 1500, 120 T.sub.Kb/? C., t.sub.Kb/min 550, 30 480, 10 500, 40 470, 60 500, 20 T.sub.c/? C., t.sub.c/min 770, 20 760, 10 760, 20 750, 30 760, 30 T.sub.press/? C., t.sub.press/? C. 870, 25 Main crystal phase Li.sub.2Si.sub.2O.sub.5 (32.4) Li.sub.2Si.sub.2O.sub.5 (27.0) Li.sub.2Si.sub.2O.sub.5 (27.3) Li.sub.2Si.sub.2O.sub.5 (28.8) Li.sub.2Si.sub.2O.sub.5 (29.1) (wt.-%) Further crystal Low quartz Low quartz Low quartz Low quartz Low quartz phases (9.8), (19.9), (18.6), (24.3), (14.8), (wt.-%) Li.sub.3PO.sub.4 (4.9), Li.sub.3PO.sub.4 (6.0) Li.sub.3PO.sub.4 (5.1) Li.sub.3PO.sub.4 (3.8) Li.sub.3PO.sub.4 (4.4) ?.sub.Biax/MPa 299 290 320 367 CR value 57.60 56.69 46.84 64.29 63.10 L* 85.77 90.49 84.38 90.6 89.91 a* 0.16 ?0.50 0.05 0.38 1.70 b* 19.18 9.68 26.72 12.76 9.43 CTE/10.sup.?6K.sup.?1 (100-500? C.) 12.8 Example No. 21 22 23 24 25 Composition wt.-% wt.-% wt.-% wt.-% wt.-% SiO.sub.2 72.9 68.8 69.5 73.2 73.7 Li.sub.2O 12.5 11.4 11.5 11.7 11.5 K.sub.2O 3.5 3.3 3.3 0.8 3.3 Cs.sub.2O 1.3 Rb.sub.2O 1.3 MgO 4.4 3.2 1.1 2.9 CaO 1.5 SrO 2.4 3.6 ZnO 1.8 Al.sub.2O.sub.3 2.8 3.2 2.7 2.7 2.5 Ga.sub.2O.sub.3 La.sub.2O.sub.3 Y.sub.2O.sub.3 In.sub.2O.sub.3 Er.sub.2O.sub.3 0.2 0.2 0.1 0.1 ZrO.sub.2 2.1 SnO.sub.2 2.6 CeO.sub.2 1.1 1.8 1.5 1.5 MnO.sub.2 0.1 V.sub.2O.sub.5 0.1 0.2 0.2 0.1 Ta.sub.2O.sub.5 P.sub.2O.sub.5 3.9 3.6 3.7 3.8 3.3 F.sup.?1 Tb.sub.4O.sub.7 0.4 0.4 0.4 0.4 T.sub.g/? C. 473 483 461 467 472 T.sub.s/? C., t.sub.s/min 1500, 120 1500, 120 1500, 120 1500, 120 1500, 120 T.sub.Kb/? C., t.sub.Kb/min 500, 30 490, 30 480, 30 500, 20 500, 70 T.sub.c/? C., t.sub.c/min 800, 30 770, 40 800, 10 820, 30 830, 40 T.sub.press/? C., t.sub.press/? C. Main crystal phase Li.sub.2Si.sub.2O.sub.5 (48.0) Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 Li.sub.2Si.sub.2O.sub.5 (37.3) (wt.-%) Further crystal Low quartz Low quartz Low quartz Low quartz Low quartz phases (6.5), Li.sub.3PO.sub.4 Li.sub.3PO.sub.4 Li.sub.3PO.sub.4 (14.7), (wt.-%) Li.sub.3PO.sub.4 (6.6), Li.sub.3PO.sub.4 (25.1) ?.sub.Biax/MPa 487 CR value 74.98 53.15 L* 94.16 79.74 a* ?0.62 3.58 b* 3.40 34.15 CTE/10.sup.?6K.sup.?1 (100-500? C.) Example No. 26 27 28 29 Composition wt.-% wt.-% wt.-% wt.-% SiO.sub.2 73.0 74.8 68.9 75.8 Li.sub.2O 11.7 13.3 12.2 10.5 K.sub.2O 3.4 3.6 3.3 3.4 Cs.sub.2O Rb.sub.2O MgO 4.4 1.7 1.4 3.6 CaO 2.4 2.1 SrO ZnO Al.sub.2O.sub.3 3.7 4.2 3.9 2.9 Ga.sub.2O.sub.3 La.sub.2O.sub.3 Y.sub.2O.sub.3 In.sub.2O.sub.3 Er.sub.2O.sub.3 ZrO.sub.2 SnO.sub.2 CeO.sub.2 MnO.sub.2 V.sub.2O.sub.5 Ta.sub.2O.sub.5 P.sub.2O.sub.5 3.8 8.2 3.8 F.sup.? Tb.sub.4O.sub.7 T.sub.g/? C. 457 471 469 T.sub.s/? C., t.sub.s/min 1500, 120 1500, 180 1500, 150 1500, 120 T.sub.Kb/? C., t.sub.Kb/min 500, 30 490, 10 500, 30 T.sub.c/? C., t.sub.c/min 800, 30 780, 10 800, 30 880, 1 T.sub.press/? C., t.sub.press/? C. Main crystal phase Li.sub.2Si.sub.2O.sub.5 (45.0) Li.sub.2SiO.sub.3, Li.sub.2Si.sub.2O.sub.5, Li.sub.2Si.sub.2O.sub.5, (wt.-%) Further crystal Low quartz Low quartz Low quartz Low quartz phases (12.8), Li.sub.2Si.sub.2O.sub.5, Li.sub.3PO.sub.4 Li.sub.3PO.sub.4 (wt.-%) Li.sub.3PO.sub.4 (6.0), Li.sub.3PO.sub.4 ?.sub.Biax/MPa 320 CR value 67.66 76.6 L* 91.17 94 a* 0.08 ?0.21 b* 4.69 2.55 CTE/10.sup.?6K.sup.?1 (100-500? C.) 12.3 Example No. 30 31 32 33 34 Composition wt.-% wt.-% wt.-% wt.-% wt.-% SiO.sub.2 64.4 67.1 69.8 71.6 59.7 Li.sub.2O 14.6 13.3 11.9 11.1 12.9 Na.sub.2O 1.6 1.9 K.sub.2O 3.3 3.3 3.2 Cs.sub.2O 5.0 3.7 Rb.sub.2O MgO 3.9 3.2 0.2 CaO SrO ZnO 8.7 8.6 8.3 Al.sub.2O.sub.3 2.7 2.7 2.7 2.7 2.6 Ga.sub.2O.sub.3 La.sub.2O.sub.3 Er.sub.2O.sub.3 0.1 0.1 ZrO.sub.2 SnO.sub.2 CeO.sub.2 0.8 1.5 GeO.sub.2 8.9 MnO.sub.2 V.sub.2O.sub.5 0.1 0.1 Ta.sub.2O.sub.5 P.sub.2O.sub.5 6.3 5.0 3.8 3.7 4.2 F.sup.? Tb.sub.4O.sub.7 0.3 0.4 T.sub.g/? C. 455 459 458 463 T.sub.s/? C., t.sub.s/min 1500, 1500, 1500, 120 1500, 120 1400, 120 120 120 T.sub.Kb/? C., t.sub.Kb/min 500, 30 500, 30 500, 30 500, 30 500, 30 T.sub.c/? C., t.sub.c/min 840, 30 850, 30 800, 30 800, 30 820, 30 T.sub.press/? C., t.sub.press/? C. Main crystal phase Li.sub.2Si.sub.2O.sub.5 , Li.sub.2Si.sub.2O.sub.5, Li.sub.2Si.sub.2O.sub.5, Li.sub.2Si.sub.2O.sub.5 (33.8) Li.sub.2Si.sub.2O.sub.5 , (wt.-%) Further crystal Low Low Low quartz, Li.sub.3PO.sub.4, Low quartz, (15.6) Low quartz, phases quartz, quartz, Cs.sub.0.809AlSi.sub.5O.sub.12 Li.sub.3PO.sub.4, (5.8) Li.sub.3PO.sub.4 (wt.-%) Li.sub.3PO.sub.4 Li.sub.3PO.sub.4 Cs.sub.0.809AlSi.sub.5O.sub.12 (10.0) ?.sub.Biax/MPa 458 485 516 368 CR value 90.90 86.57 73.4 73.54 L* 95.87 95.52 90.36 82.05 a* ?0.24 ?0.24 0.32 4.63 b* 0.84 0.66 11.49 26.02 CTE/10.sup.?6K.sup.?1 (100-500? C.)