β-spodumene glass-ceramics that are white, opalescent, or opaque, with low titanium content, and tin-fined

Abstract

The present application relates to glass-ceramics that are white, opalescent or opaque, of the lithium aluminosilicate (LAS) type, containing a solid solution of β-spodumene as the main crystalline phase. The application also provides articles that are constituted, at least in part, of said glass-ceramics, precursor glasses for said glass-ceramics, and a method of preparing said article. Said glass-ceramics have a composition that is free from arsenic oxide and antimony oxide, with the exception of inevitable traces, and that contains the following, expressed as percentages by weight of oxides: 60% to 70% of SiO.sub.2, 18% to 23% of Al.sub.2O.sub.3, 3.0% to 4.3% of Li.sub.2O, 0 to 2% of MgO, 1 to 4% of ZnO, 0 to 4% of BaO, 0 to 4% of SrO, 0 to 2% of CaO, 1.3% to 1.75% of TiO.sub.2, 1% to 2% of ZrO.sub.2, 0.05% to 0.6% of SnO.sub.2, 0 to 2% of Na.sub.2O, 0 to 2% of K.sub.2O, 0 to 2% of P.sub.2O.sub.5, 0 to 2% of B.sub.2O.sub.3, with Na.sub.2O+K.sub.2O+BaO+SrO+CaO≤6% and Na.sub.2O+K.sub.2O≤2%, and a maximum of 500 ppm of Fe.sub.2O.sub.3.

Claims

1. A glass-ceramic that is white, opalescent or opaque, of the lithium aluminosilicate (LAS) type, containing a solid solution of β-spodumene as the main crystalline phase, the composition of which, exempt of arsenic oxide and antimony oxide, with the exception of inevitable traces, comprises, expressed as percentages by weight of oxides: 60% to 70% of SiO.sub.2, 18% to 23% of Al.sub.2O.sub.3, 3.0% to 4.3% of Li.sub.2O, 0 to 2% of MgO, 1 to 4% of ZnO, 0 to 4% of BaO, 0 to 4% of SrO, 0 to 2% of CaO, 1.3% to 1.75% of TiO.sub.2, 1% to 2% of ZrO.sub.2, 0.05% to 0.6% of SnO.sub.2, 0 to 2% of Na.sub.2O, 0 to 2% of K.sub.2O, 0 to 2% of P.sub.2O.sub.5, 0 to 2% of B.sub.2O.sub.3, with Na.sub.2O+K.sub.2O+BaO+SrO+CaO≤6% and Na.sub.2O+K.sub.2O≤2%, and a maximum of 500 ppm of Fe.sub.2O.sub.3, wherein the glass-ceramic comprises an optical transmission (Y) greater than or equal to 0.1% for a 4 mm thick panel.

2. The glass-ceramic according to claim 1, the composition of which comprises ZrO.sub.2 at a content greater than or equal to 1.5% and less than or equal to 1.9%.

3. The glass-ceramic according to claim 1, the composition of which comprises 0.1 to 4% of BaO.

4. The glass-ceramic according to claim 1, the composition of which comprises Na.sub.2O: 0-1%, K.sub.2O:0-1%, with Na.sub.2O+K.sub.2O+BaO+SrO+CaO≤5% and Na.sub.2O+K.sub.2O≤1.5%.

5. The glass-ceramic according to claim 1, the composition of which comprises Na.sub.2O: 0-1%, K.sub.2O: 0-1%, with Na.sub.2O+K.sub.2O+BaO+SrO+CaO≤5% and Na.sub.2O+K.sub.2O≤1%.

6. The glass-ceramic according to claim 1, the composition of which comprises an Al.sub.2O.sub.3 content greater than 20%.

7. The glass-ceramic according to claim 6, the composition of which comprises a MgO content less than or equal to 1.2%.

8. The glass-ceramic according to claim 1, the composition of which comprises a BaO+SrO content less than or equal to 1.5% and a CaO content greater than or equal to 0.5%; the composition of which is free of SrO, with the exception of inevitable traces, and comprises a BaO content less than or equal to 1.5%.

9. The glass-ceramic according to claim 6, the composition of which is exempt, with the exception of inevitable traces, of P.sub.2O.sub.5 or B.sub.2O.sub.3.

10. The glass-ceramic according to claim 1, the composition of which contains an Al.sub.2O.sub.3 content less than or equal to 20%.

11. The glass-ceramic according to claim 10, the composition of which contains an MgO content greater than 1.2%.

12. The glass-ceramic according to claim 10, the composition of which contains: 0 to 1.5% of BaO, 0 to 1.5% of SrO, 0 to 1% of Na.sub.2O, 0 to 1% of K.sub.2O, <1% of P.sub.2O.sub.5, <1% of B.sub.2O.sub.3, with Na.sub.2O+K.sub.2O+BaO+SrO+CaO≤5% and Na.sub.2O+K.sub.2O≤1%.

13. An article that is constituted, at least in part, by the glass-ceramic according to claim 1, comprising a cooktop, a worktop, a cooking utensil, a microwave oven plate, or a support for heat treatment.

14. A method of preparing an article according to claim 13, the method comprising in succession: melting a charge of vitrifiable raw materials, followed by fining the resulting molten glass; cooling the resulting fined molten glass and simultaneously shaping it into the shape desired for the intended article; and applying ceramming heat treatment to the shaped glass, which heat treatment comprises a first step of nucleation and a second step of crystal growth; wherein said charge of vitrifiable raw materials has a composition making it possible to obtain the glass-ceramic; and wherein said ceramming heat treatment comprises a crystal growth second step that is performed at least in part at a temperature higher than 1000° C.

Description

(1) The subject matter of the present application is illustrated below by the accompanying figures and the following examples. The figures and examples should be considered together.

(2) FIGS. 1A and 1B are photographs taken with a scanning electron microscope (at two different magnifications (see indicated scale)) showing the microstructure of a glass-ceramic (glass-ceramic C4a, presenting the composition specified in example C4), of composition containing too little Ti.sub.2O. The crystals observed are too big and the proportion of the amorphous residual phase is larger than the proportions observed in FIGS. 2, 3, 5A, and 5B. This leads to a material presenting a low flexural strength (a low value for its modulus of rupture (MOR)).

(3) FIGS. 2 and 3 are photographs taken with a scanning electronic microscope showing respectively the microstructure of a glass-ceramic of composition that does not contain CaO (glass-ceramic 3e, presenting the composition given in example 3) and of a glass-ceramic of composition that contains CaO, as a partial substitute for BaO (glass-ceramic 8b, presenting the composition given in Example 8). These figures show that partially substituting BaO with CaO reduces the size of the crystals.

(4) FIG. 4 shows the impact of such a partial substitution of BaO by CaO on the diffusion of light emitted by red LEDs. For a value of Y and a nucleation duration that are similar (i.e. Y≈21%-22% and a duration at *T.sub.n=240 min (glass-ceramic 3e and glass-ceramic 8b)), this substitution leads to better visibility of red displays (which is explained mainly by a reduction in the size of the crystals).

(5) FIGS. 5A and 5B are photographs taken with a scanning electron microscope showing the microstructure of two glass-ceramics (glass-ceramics 3a and 3b) presenting the same composition (the composition of example 3) and obtained under different ceramming conditions (more precisely with a different duration at *T.sub.n, 0 min for the glass-ceramic of example 3a at *T.sub.n=730° C., and 21 min for the glass-ceramic of example 3d at the same temperature *T.sub.n=730° C., with *T.sub.c (1105° C.) and the duration at *T.sub.c (21 min) being identical). The figures show that the nucleation duration at *T.sub.n has a large impact on the size of the crystals.

(6) FIG. 6 shows the impact of this duration at *T.sub.n (via the impact on the size of the crystals) on the diffusion of light emitted by red LEDs. The glass-ceramics that were tested (Y≈17.5%) were the glass-ceramics 2a (duration at *T.sub.n=0 min) and 2b (duration at *T.sub.n=120 min).

(7) In the same manner, FIG. 7 shows the impact of this duration at *T.sub.n (via the impact on the size of the crystals) on the diffusion of the light emitted by red LEDs. The glass-ceramics tested (Y≈20.6%) were the glass-ceramics 9b (duration at *T.sub.n=120 min) and 9c (duration at *T.sub.n=240 min).

(8) FIG. 8 shows the impact of lowering the ZnO content (ZnO being partly substituted by SiO.sub.2 (in the present case)) on the visibility of the light emitted by red LEDs. For similar lightness values (L*≈82.0-82.4), the visibility of the red LEDs is better for glass-ceramic 17a compared to glass-ceramic 16a (having a higher content in ZnO). *T.sub.n and T.sub.c respectively represent the nucleation temperature and the crystal growth temperature (see later).

EXAMPLES

(9) To produce 1 kilogram (kg) batches of precursor glass, the raw materials having the proportions specified in the first portions of Tables 1A, 1B, 1C, 1D, 1E, 2, and 3 below were mixed carefully (where the proportions are expressed in terms of oxides (as percentages by weight of oxides)).

(10) For melting, the mixtures were placed in platinum crucibles. The crucibles containing said mixtures were then inserted into a furnace preheated to 1550° C. They were subjected therein to a melting cycle of the following type: temperature maintained at 1550° C. for 30 min; temperature rise from 1550° C. to 1670° C. in 1 hour (h); and temperature maintained at 1670° C. for 330 min.

(11) The crucibles where then extracted from the furnace and the molten glass was poured onto a preheated steel plate. It was rolled to a thickness of 6 mm. Glass plates were thus obtained. They were annealed at 650° C. for 1 h and subsequently cooled slowly. The properties of the glass obtained are indicated in the second portions of Tables 1A, 1B, 1C, 1D, 1E, 2, and 3 below.

(12) The viscosities were measured with a rotational viscosity meter (Visco Tester VT550 HAAKE).

(13) T.sub.30 Pa.Math.s (° C.) corresponds to the temperature at which the viscosity of the glass is 30 Pa.Math.s.

(14) T.sub.liq (° C.) is the liquidus temperature. Specifically, the liquidus is given by a range of temperatures and associated viscosities: the range of temperatures given corresponds to the range of maximum temperatures for which there might be a risk of the glass devitrifying (unwanted formation of crystals). The higher temperature thus corresponds to the temperature above which no crystal is observed.

(15) The devitrification characteristics (low and high temperatures of the liquidus) were determined as follows. Samples of glass (0.5 cubic centimeters (cm.sup.3)) were subjected to the following heat treatment: placing into a furnace preheated to 1430° C.; maintaining this temperature for 30 min; lowering to the test temperature T at a rate of 10° C./min; maintaining the test temperature for 17 h; and cooling the samples rapidly by extracting them from the furnace.

(16) The crystals present, if any, can then be observed by an optical transmission microscope using polarized light. The ceramming cycle used is specified below: rapid temperature rise from 20° C. to 670° C., at a heating rate of 25° C./min; temperature rise from 670° C. to the nucleation temperature (T.sub.n) at a heating rate of 5° C./min; pause at T.sub.n for the specified duration (which duration may be zero so there is no pause; nucleation then taking place during the temperature rise from 670° C. to T.sub.n); temperature rise from T.sub.n to the crystal growth temperature (T.sub.c) at a heating rate of 7° C./min; pause at T.sub.c for the indicated duration; and rapid cooling to 20° C., at a cooling rate of 25° C./min.

(17) The parameters of the various ceramming cycles used are specified in Tables 1A, 1B, 1C, 1D, 1E, 2, and 3 below, where these parameters are maximum nucleation temperature (T.sub.n), duration of heating at said T.sub.n, crystal growth temperature (T.sub.c), and duration of heating at said T.sub.c.

(18) It can be understood that for most of the examples, the results given were obtained after a plurality of ceramming treatments. Thus, for example 1, three ceramming cycles are specified leading to three ceramics 1a, 1b, and 1c, all three of which present the specified composition. The properties of the glass-ceramics that were obtained are given in the last portions of Tables 1A, 1B, 1C, 1D, 1E, 2, and 3 below.

(19) a) The glass-ceramics (with the exception of that of comparative example C4a (see FIGS. 1A and 1B)) contained a solid solution of β-spodumene as the main crystalline phase (which was verified by X-ray diffraction). Thus, the glass-ceramics 1b, 3d, 6a, and 13d contained respectively 97%, 96.6%, 96.7% and 97.1% (by weight) of β-spodumene phase (relative to the total crystallized fraction). The percentages of the β-spodumene phase were determined by the Rietveld method. The sizes of the β-spodumene crystals were obtained by observation using a scanning electron microscope (SEM). Those crystals are in fact agglomerates of crystallites. Said sizes of the crystals are given in micrometers. The values given in Tables 1A, 1B, 1C, 1D, and 3 correspond to the minimum and maximum sizes observable in FIGS. 1A (1B), 2, 3, and 5A and 5B.

(20) L*, a*, and b* are the colorimetric coordinates of the CIELAB system (L*=lightness, and a* and b*=hue), measured with a colorimeter in reflection mode, under illuminant D65 with observation at 10°, on a sample having thickness of 4 mm.

(21) Transmission and diffusion measurements were performed at 4 mm using a Varian spectrophotometer (Cary 500 Scan model), fitted with an integrating sphere of a diameter of 150 mm. From those measurements, there were calculated the total integrated transmission Y (or TL) (%) in the visible spectrum (in the range 380 nm to 780 nm) of the XYZ color space and the “haze” or level of diffusion (%) using the ASTM D 1003-13 standard (with illuminant D65 and 2° observer). Transmission values are given at 625 nm (T.sub.625 nm) and at 1600 nm (T.sub.1600 nm)).

(22) The coefficients of thermal expansion between ambient (20° C.) and 700° C. (CTE.sub.20-700° C.) were measured using a high temperature dilatometer (DIL 402C, Netzsch) at a heating rate of 3° C./rain on glass-ceramic samples in the form of bars (50×4×4 mm).

(23) The modulus of rupture (MOR) is a measure of the flexural strength of the material and was performed in a “ring-on-ring” configuration. The sample support ring had a diameter of 15 mm and the piston ring, situated above the sample, had a diameter of 5 mm. The downward speed of the piston ring was 0.5 mm/min. The values expressed in the tables are averages of the breaking stress values measured on polished samples having a size of 32 mm in diameter and 2.1 mm in thickness.

(24) b) Examples 1 to 17 (Tables 1A, 1B, 1C, 1D and 1E) illustrate the present application.

(25) Concerning these examples, it is appropriate to make the following comments.

(26) The glass-ceramics exemplified are of a white color (see the specified values for L*, a*, and b*). Most are opalescent glass-ceramics; only glass-ceramics 12a and 13d are opaque glass-ceramics, as specified (see the specified values for Y).

(27) The consideration in parallel of the values of L* and Y of glass-ceramics 13a, 13b, 13c and 13d confirms that when the L* value increases, the Y value decreases (hence, as indicated above, the unobviousness in obtaining glass-ceramics according to the present application: L*≥80 with Y≥18).

(28) By considering the properties of glass-ceramics 13a, 14a, and 15a in parallel with the properties of glass-ceramics 13b and 14b it can be seen that, for similar ceramming, increasing the content of Fe.sub.2O.sub.3 from 190 ppm to 350 ppm and then to 430 ppm increases the value of a* and reduces the green component of the hue of the product.

(29) By considering in parallel the levels of transmission at 1600 nm (T.sub.1600 nm) of the glass-ceramics 2a-2b, 3a-3c, 6a-6b, and 9b-9c it can be seen that the increase in the duration at T.sub.n increases said levels of transmission.

(30) Lower contents of ZrO.sub.2 (1.65 wt % (exs. 16 and 17) <1.8 wt % (exs. 1 to 15)) allow to obtain glasses showing higher viscosities at the liquidus temperature (which is advantageous in reference to the problem of devitrification during the glass rolling).

(31) The consideration in parallel of the levels of transmission at 625 nm (T.sub.625 nm) of the glass-ceramics 16a and 17a shows that the reduction of the content of ZnO (here substituted by SiO.sub.2) allows to increase this transmission (31.4% for 17a compared to 29.4% for 16a), without degrading the lightness (L*: 82.45 for 17a compared to 82.05 to 16a) and, thus, to improve the visibility of the red LEDs (FIG. 8).

(32) c) Examples CA and CB (Table 2) are comparative examples. They confirm the coloring effect of SnO.sub.2.

(33) Adding tin, in a composition having a high titanium content, greatly degrades the lightness (L*) and develops a β-spodumene material that is very gray.

(34) d) Examples C1 to C5 (Table 3) are comparative examples.

(35) The compositions of the glass-ceramics (Cia, C2a, and C3a) of comparative examples C1 to C3 contain too much TiO.sub.2 (see below), the composition of the glass-ceramic (C4a) of the comparative example C4 does not contain enough (the glass-ceramic obtained has crystals that are too big, which prevents the creation of a material of the KW or KWTC type (see figures (photos) 1A and 1B), which explains why it has low flexural strength).

(36) The composition of the glass-ceramic C5a of comparative example C5 contains too much SnO.sub.2 (lightness L* too small for a low Y and a great deal of devitrification).

(37) On considering L* and Y value pairs, it can be understood that it is not possible to prepare glass-ceramics similar to a glass-ceramic of type KW (L*≈87-88, and Y≈10%-12%), or of KWTC type (L≈81-83 and Y≈18%-20%) with the compositions of examples C1, C2, C3, and C5 (specifically, increasing the value of L* of the glass-ceramics in question leads to a reduction in their Y value, and vice versa).

(38) e) Considering certain examples and comparative examples in parallel leads to the following additional comments.

(39) While using similar nucleation parameters (T.sub.n=730° C. and duration at T.sub.n=0 min), considering the following in parallel: the values of the L*, a*, and b* color points of glass-ceramics C1a, C2a, and C3a, (of comparative examples C1, C2, and C3); and the values of the L*, a*, and b* color points of glass-ceramics 1a, 1b, 2a, 3a, and 3b (of examples 1, 2, and 3);
reveals the positive impact of decreasing the TiO.sub.2 content. Glass-ceramics in accordance with the present application present greater lightness L* and values for a* that are less negative (hues that are less “green”) and they are therefore whiter.

(40) Considering the values of the L*, a*, and b* color points and the value of Y of the glass-ceramics 1b-3b and 1c-3d (of examples 1 and 3) reveals the positive impact of increasing the content of Al.sub.2O.sub.3. For similar Y values (about 11% or about 20.5%), increasing the alumina content from 18.26% to 20.5% by weight leads to materials having higher lightness L*, which are thus whiter.

(41) Considering the values of the L*, a*, and b* color points and the Y values of the glass-ceramics 1a, 4a, and C5a (of examples 1 and 4 and of comparative example C5) reveals the negative impact of increasing the content of SnO.sub.2. For similar Y values and nucleations (i.e. Y approximately 13%-15% and T.sub.n=730° C., duration at T.sub.n=0 min), increasing the tin content leads to a decrease of L* and an increase in the value of b* (hues that are “yellower”).

(42) TABLE-US-00001 TABLE 1A Examples 1 2 3 Composition (% by weight) SiO.sub.2 67.445 65.185 65.205 Al.sub.2O.sub.3 18.26 22.5 20.5 Li.sub.2O 3.33 4.1 3.33 MgO 1.17 0.4 1.17 ZnO 2.62 2.5 2.62 BaO 3.36 1 3.36 TiO.sub.2 1.5 1.6 1.5 ZrO.sub.2 1.8 1.8 1.8 SnO.sub.2 0.3 0.3 0.3 Na.sub.2O 0.2 0.2 0.2 P.sub.2O.sub.5 Fe.sub.2O.sub.3 0.015 0.015 0.015 CaO 0.4 BaO + Na.sub.2O + K.sub.2O + CaO + SrO 3.56 1.6 3.56 Na.sub.2O + K.sub.2O 0.2 0.2 0.2 Properties of glasses 1 2 3 T.sub.30 Pa .Math. s (° C.) 1653 1590 1611 T.sub.liq (° C.) Viscosity at T.sub.liq (Pa .Math. s) Ceramming T.sub.n (° C.) 730 730 730 730 730 730 730 730 730 730 Duration at T.sub.n (min) 0 0 120 0 120 0 0 120 120 240 T.sub.c (° C.) 1045 1060 1030 1070 1105 1105 1115 1125 1105 1100 Duration at T.sub.c (min) 21 21 21 21 21 21 21 21 21 21 Properties of glass-ceramics 1a 1b 1c 2a 2b 3a 3b 3c 3d 3e Reflection: L* 81.6 83.8 80.1 85.7 83.0 88 89.2 86.6 82.7 82.4 a* −0.47 −0.47 −0.18 −1.92 −2.6 −1.19 −1 −1.56 −2.31 −2.57 b* −0.61 −1.14 0.67 4.05 4.75 0.58 0.69 0.54 0.41 0.07 Transmission: Y (%) 14.8 11.4 20.5 17.8 17.3 13.3 11.1 14.6 20.6 22 Haze (%) 99 99 99 T.sub.625 nm (%) 22.0 18.4 27.4 23.7 24.7 20.1 17.5 22.0 28.1 30.0 T.sub.1600 nm (%) 60.0 59.3 69.4 66.7 75.2 63.9 61.9 66.3 71.1 72.5 CTE.sub.20-700° C. (×10.sup.−7/° C.) 8.8 8.8 9.6 9.4 Size of crystals (μm) 2.0-3.5 0.9-1.4 0.9-1.4 MOR (MPa) 118 Observations FIG.6 FIG. 5A FIG. 5B FIGS. 2 and 4

(43) TABLE-US-00002 TABLE 1B Examples 4 5 6 Composition (% by weight) SiO.sub.2 67.295 66.055 67.195 Al.sub.2O.sub.3 18.26 20.5 18.26 Li.sub.2O 3.33 3.73 3.33 MgO 1.17 1.17 1.17 ZnO 2.62 2.62 2.62 BaO 3.36 1.36 3.36 TiO.sub.2 1.5 1.5 1.75 ZrO.sub.2 1.8 1.8 1.8 SnO.sub.2 0.45 0.3 0.3 Na.sub.2O 0.2 0.2 0.2 P.sub.2O.sub.5 Fe.sub.2O.sub.3 0.015 0.015 0.015 CaO 0.75 BaO + Na.sub.2O + K.sub.2O + CaO + SrO 3.56 2.31 3.56 Na.sub.2O + K.sub.2O 0.2 0.2 0.2 Properties of glasses 4 5 6 T.sub.30 pa .Math. s (° C.) 1654 1614 1652 T.sub.liq (° C.) Viscosity at T.sub.liq (Pa .Math. s) Ceramming T.sub.n (° C.) 730 750 730 730 Duration at T.sub.n (min) 0 120 0 120 T.sub.c (° C.) 1045 1070 1060 1040 Duration at T.sub.c (min) 21 21 21 21 Properties of glass-ceramics 4a 5a 6a 6b Reflection: L* 81.2 85.5 83.5 82.9 a* −0.47 −0.36 −0.24 −0.31 b* −0.41 −0.41 −0.89 −1.33 Transmission: Y (%) 14.5 7.2 9.2 8.9 Haze (%) 99.5 99.5 T.sub.625 nm( %) 21.8 13.3 15.9 15.5 T.sub.1600 nm (%) 61.4 69.2 57.9 74.2 CTE.sub.20-700° C. (×10.sup.−7/° C.) Size of crystals (μm) 0.5-0.8 0.5-0.8 MOR (MPa) Observations

(44) TABLE-US-00003 TABLE 1C Examples 7 8 9 10 Composition (% by weight) SiO.sub.2 67.045 66.455 65.385 66.835 Al.sub.2O.sub.3 18.26 20.5 22.5 20 Li.sub.2O 3.33 3.33 4.1 3.45 MgO 1.17 1.17 0.4 1.17 ZnO 2.62 2.62 2.5 2.62 BaO 3.36 1.36 1 1.36 TiO.sub.2 1.75 1.5 1.6 1.5 ZrO.sub.2 1.8 1.8 1.8 1.8 SnO.sub.2 0.45 0.3 0.1 0.3 Na.sub.2O 0.2 0.2 0.2 0.2 P.sub.2O.sub.5 Fe.sub.2O.sub.3 0.015 0.015 0.015 0.015 CaO 0.75 0.4 0.75 BaO + Na.sub.2O + K.sub.2O + CaO + SrO 3.56 2.31 1.6 2.31 Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 Properties of glasses 7 8 9 10 T.sub.30 Pa .Math. s (° C.) 1653 1633 1589/1602 1637 T.sub.liq (° C.) 1350-1330 Viscosity at T.sub.liq (Pa .Math. s) 510-690 Ceramming T.sub.n (° C.) 730 750 750 750 730 750 750 750 Duration at T.sub.n (min) 120 60 240 240 0 120 240 240 T.sub.c (° C.) 1040 1070 1070 1090 1090 1100 1100 1065 Duration at T.sub.c (min) 21 21 21 21 21 21 21 21 Properties of glass-ceramics 7a 8a 8b 8c 9a 9b 9c 10a Reflection: L* 82.4 82.9 82.9 87.9 86.9 82.9 83.3 84.1 a* −0.52 −2.95 −2.8 −1.43 −0.46 −1.7 −1.82 −1.13 b* −0.79 0.89 0.85 1.09 0.24 0.8 0.73 0.28 Transmission: Y (%) 9.5 21.8 21.0 11.6 13.1 20.7 20.6 14 Haze (%) 97 98 99 T.sub.625 nm (%) 16.4 29.3 28.9 18.6 18.4 27.4 27.5 21.4 T.sub.1600 nm (%) 76.1 79.3 79.9 75.1 52.9 74.4 77.8 78.6 CTE.sub.20-700° C. (×10.sup.−7/° C.) 7.8 7.9 8.5 8.5 Size of crystals (μm) 0.5-1.0 05.-0.7 0.4-0.6 MOR (MPa) Observations FIGS 3 FIG. 7 and 4

(45) TABLE-US-00004 TABLE 1D Examples 11 12 13 14 15 Composition (% by weight) SiO.sub.2 64.105 66.815 66.761 66.745 66.737 Al.sub.2O.sub.3 20.5 18.26 20.5 20.5 20.5 Li.sub.2O 3.33 3.33 3.33 3.33 3.33 MgO 1.17 1.8 1.17 1.17 1.17 ZnO 2.62 2.62 2.62 2.62 2.62 BaO 3.36 3.36 1 1 1 TiO.sub.2 1.5 1.5 1.5 1.5 1.5 ZrO.sub.2 1.8 1.8 1.8 1.8 1.8 SnO.sub.2 0.3 0.3 0.2 0.2 0.2 Na.sub.2O 0.2 0.2 0.2 0.2 0.2 P.sub.2O.sub.5 1.1 Fe.sub.2O.sub.3 0.015 0.015 0.019 0.035 0.043 CaO 0.9 0.9 0.9 BaO + Na.sub.2O + K.sub.2O + CaO + SrO 3.56 3.56 2.1 2.1 2.1 Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 Properties of glasses 11 12 13 14 15 T.sub.30 Pa .Math. s (° C.) 1586 1630 1636/1609 1636 T.sub.liq (° C.) 1350-1330 Viscosity at T.sub.liq (Pa .Math. s) 560-750 Ceramming T.sub.n (° C.) 750 730 730 750 750 730 750 750 750 750 Duration at T.sub.n (min) 240 120 120 60 15 0 15 60 15 60 T.sub.c (° C.) 1100 1080 1025 1070 1100 1150 1160 1075 1110 1075 Duration at T.sub.c (min) 21 21 21 21 20 21 21 19 20 18 Properties of glass-ceramics 11a 11b 12a 13a 13b 13c 13d 14a 14b 15a Reflection: L* 88.4 82.9 87.8 83.3 87.9 93.3 94.65 82.2 87.7 80.0 a* −0.7 −1.4 −0.52 −2.43 −0.9 −0.28 0.1 −1.1 0 −0.7 b* 0.36 0.08 0.9 −0.45 −0.15 0.68 0.82 −0.4 −0.3 −1.0 Transmission: Y (%) 7 16.8 0.1 22.1 11.5 3.6 0.4 17.9 7.3 19.6 Haze (%) 99.5 opaque >99.5 opaque T.sub.625 nm (%) 12.8 24.3 0.5 30.1 18.2 6.7 1.2 26.4 13.3 28.8 T.sub.1600 nm (%) 64.0 68.5 33.1 79.4 69.1 35.2 19.1 76.7 60.7 76.0 CTE.sub.20-700° C. (×10.sup.−7/° C.) 8.0 10.3 Size of crystals (μm) 0.6-1.0 MOR (MPa) 109 Observations

(46) TABLE-US-00005 TABLE 1E Examples 16 17 Composition (% by weight)) SiO.sub.2 66.913 67.41 Al.sub.2O.sub.3 20.5 20.5 Li.sub.2O 3.33 3.33 MgO 1.17 1.17 ZnO 2.62 2.12 BaO 1 1 TiO.sub.2 1.5 1.5 ZrO.sub.2 1.65 1.65 SnO.sub.2 0.2 0.2 Na.sub.2O 0.2 0.2 P.sub.2O.sub.5 Fe.sub.2O.sub.3 0.017 0.020 CaO 0.9 0.9 BaO + Na.sub.2O + K.sub.2O + CaO + SrO 2.1 2.1 Na.sub.2O + K.sub.2O 0.2 0.2 Properties of glasses 16 17 T.sub.30 pa .Math. s (° C.) 1621 T.sub.liq (° C.) 1330-1310 Viscosity at T.sub.liq (Pa .Math. s)  770-1030 Ceramming Tn (° C.) 750 750 750 Duration at T.sub.n (min) 45 10 60 Tc (° C.) 1050 1075 1055 Duration at T.sub.c (min) 21 21 22 Properties of glass-ceramics 16a 16b 17a Reflection: L* 82.05 88.0 82.45 a* −2.25 −0.61 −2.67 b* −0.04 0.11 −0.38 Transmission: Y (%) 21.25 11.9 23.65 Haze (%) T.sub.625 nm (%) 29.45 18.25 31.4 T.sub.1600 nm (%) 77.4 64.9 79.4 CTE.sub.20-700° C. (×10.sup.−7/° C.) 9.5 Size of crystals (μm) MOR (MPa) Observations FIG. 8 FIG. 8

(47) TABLE-US-00006 TABLE 2 Comparative examples CA CB Composition (% by weight) SiO.sub.2 64.268 63.988 Al.sub.2O.sub.3 22.4 22.4 Li.sub.2O 4.13 4.13 MgO ZnO 0.8 0.8 BaO 3.6 3.6 TiO.sub.2 2.74 2.73 ZrO.sub.2 1.80 1.80 SnO.sub.2 0 0.29 Na.sub.2O 0.25 0.25 P.sub.2O.sub.5 Fe.sub.2O.sub.3 0.012 0.012 CaO Na.sub.2O + K.sub.2O + BaO + SrO+ CaO 3.85 3.85 Na.sub.2O + K.sub.2O 0.25 0.25 Ceramming T.sub.n (° C.) 730 730 Duration at T.sub.n (min) 0 0 T.sub.c (° C.) 1070 1070 Duration at T.sub.c (min) 21 21 Properties of glass-ceramics CAa CBa Reflection: L* 81.89 70.57 a* −1.55 0.02 b* −2.94 3.27

(48) TABLE-US-00007 TABLE 3 Comparative examples C1 C2 C3 C4 C5 Composition (% by weight) SiO.sub.2 65.016 66.585 64.485 68.346 67.046 Al.sub.2O.sub.3 22.72 20.6 22.5 18.26 18.26 Li.sub.2O 4.18 4.2 4.1 3.33 3.33 MgO 0.31 1.1 0.4 1.17 1.17 ZnO 0.19 1.4 2.5 2.62 2.62 BaO 1.22 0.8 1 3.36 3.36 TiO.sub.2 2.77 2.4 2.3 0.6 1.5 ZrO.sub.2 1.83 1.9 1.8 1.8 1.8 SnO.sub.2 0.3 0.3 0.3 0.3 0.7 Na.sub.2O 0.26 0.3 0.2 0.2 0.2 K.sub.2O 0.76 CaO 0.43 0.4 0.4 Fe.sub.2O.sub.3 0.014 0.015 0.015 0.014 0.014 BaO + Na.sub.2O + K.sub.2O + CaO + SrO 2.66 1.5 1.6 3.56 3.56 Na.sub.2O + K.sub.2O 1.02 0.3 0.2 0.2 0.2 Properties of glasses C1 C2 C3 C4 C5 T.sub.30 Pa .Math. s (° C.) 1624 T.sub.liq (° C.) 1350-1330 Viscosity at T.sub.liq (Pa .Math. s) 700-950 Ceramming T.sub.n (° C.) 730 730 730 780 730 Duration at T.sub.n (min) 0 0 0 480 0 Tc (° C.) 1085 1060 1070 1100 1045 Duration at T.sub.c (min) 21 21 21 21 21 Properties of glass-ceramics C1a C2a C3a C4a C5a Reflection: L* 78.3 76.5 78.45 88.9 79.8 a* −2.92 −3.25 −3.32 −0.89 −0.54 b* −1.47 2.11 3.32 0.79 1.18 Transmission: Y (%) 16.8 17.7 16.6 13.8 13.4 Haze (%) 94 87.5 98 T.sub.625 nm(%) 26.6 27.8 25.3 19.9 20.8 T.sub.1600 nm (%) 83.6 84.9 79.4 50.7 61.0 CTE.sub.20-700° C. (×10.sup.−7/° C.) 14.5 10 8.2 Size of crystals (μm) 7-17 MOR (MPa) 57 Observations FIGS. 1A and 1B

(49) f) Attention has also been given to resistance to aging at high temperature of glass-ceramic 9c of example 9. Its coefficient of thermal expansion between ambient (20° C.) and 700° C. (CTE.sub.20-700° C.) and its modulus of rupture (MOR) after aging at 725° C. for 500 h and at 980° C. for 50 h have been measured. The results are given in Table 4 below. Said results show little change in the values of CTE and MOR (at the end of said thermal aging).

(50) TABLE-US-00008 TABLE 4 Example 9 - Glass-ceramic 9c (750° C.-240 min + 1100° C.-21 min) 725° C. 980° C. t.sub.0 500 h 50 h CTE.sub.20-700° C. 8.5 8.4 9.4 MOR (MPa) 134 118