Transparent, colorless, tin-fined LAS glass-ceramics with improved microstructure and optical properties
Transparent, essentially colorless and non-diffusing β-quartz glass-ceramics and glass compositions for forming the same have a composition, free of arsenic oxide, antimony oxide and rare earth oxides except for inevitable trace amounts, which contains, expressed as percentages by weight of oxides 62-72% of SiO 2 , 20-23% of Al 2 O 3 , 2.8-5% of Li 2 O, 0.1-0.6% of SnO 2 , 1.9-4% of TiO 2 , 1.6-3% of ZrO 2 , less than 0.4% of MgO, 2.5-6% of ZnO and/or BaO and/or SrO, less than 250 ppm of Fe 2 O 3 ; and their crystallites present in the β-quartz solid solution have an average size of less than 35 nm. The glass-ceramics exhibit low thermal expansion and are easily obtained at an industrial scale.
1. A transparent, essentially colorless and non-diffusing glass-ceramic of the lithium aluminosilicate type containing a β-quartz solid solution as a main crystalline phase, wherein its composition, free of arsenic oxide, antimony oxide and rare earth oxides except for inevitable trace amounts, contains, expressed as percentages by weight of oxides:
62-72% of SiO 2 ,
20-23% of Al 2 O 3 ,
2.8-5% of Li 2 O,
0.1-0.6% of SnO 2 ,
1.9-4% of TiO 2 ,
1.6-3% of ZrO 2 ,
less than 0.4% of MgO,
2.5-6% of ZnO and/or BaO and/or SrO,
less than 250 ppm of Fe 2 O 3 ; and the crystallites present in said β-quartz solid solution have an average size of less than 35 nm.
2. The glass-ceramic according to claim 1 , the composition of which contains less than 0.1% of MgO.
3. The glass-ceramic according to claim 1 , the composition of which is free of MgO except for inevitable trace amounts.
4. The glass-ceramic according to claim 1 , the composition of which contains from 3.5 to 5% of at least one of ZnO, BaO and SrO.
5. The glass-ceramic according to claim 1 , the composition of which contains from 0.4 to 3% of ZnO, and from 0 to 5% of BaO.
6. The glass-ceramic according to claim 1 , the composition of which contains from 0.4 to 3% of ZnO, from 2.1 to 5% of BaO and from 0 to 5% of SrO.
7. The glass-ceramic according to claim 1 , the composition of which contains from 2.5 to 5% of BaO.
8. The glass-ceramic according to claim 1 , the composition of which, expressed as percentages by weight of oxides, contains, for at least 97% of its weight:
SiO 2 62-72
Al 2 O 3 20-23
Li 2 O 2.8-5
TiO 2 1.9-4
ZrO 2 1.6-3
SnO 2 0.1-0.6
MgO less than 0.4%
ZnO 0.4-3
BaO 0-5
SrO 0-5
CaO 0-1
Na 2 O 0-1
K 2 O 0-1
Fe 2 O 3 less than 250 ppm
CoO 0-30 ppm and
P 2 O 5 0-3.
9. The glass-ceramic according to claim 1 , the composition of which, expressed as percentages by weight of oxides, contains, for at least 97% of its weight:
SiO 2 63-69
Al 2 O 3 20-22
Li 2 O 3-4.5
TiO 2 2-3
ZrO 2 1.6-2
SnO 2 0.1-0.4
MgO less than 0.1%,
ZnO 0.4-2
BaO 2.1-5
CaO 0-1
Na 2 O 0-1
K 2 O 0-1
Fe 2 O 3 less than 180 ppm
CoO 0-10 ppm, and
P 2 O 5 0-2.
10. The glass-ceramic according to claim 1 , the composition of which, expressed as percentages by weight of oxides, contains, for at least 97% of its weight:
SiO 2 63-69
Al 2 O 3 20-22
Li 2 O 3-4
TiO 2 2-3
ZrO 2 1.6-2
SnO 2 0.1-0.4
MgO less than 0.1%,
ZnO 1.6-2
BaO 2.1-5
CaO 0-1
Na 2 O 0-1
K 2 O 0-1
Fe 2 O 3 less than 180 ppm
CoO 0-10 ppm and
P 2 O 5 0-2.
11. The glass-ceramic according to claim 1 , having an integrated transmission for a thickness of 5 mm, of more than 81%, a yellow index for a thickness of 5 mm, of less than 14, and a diffusion percentage for a thickness of 5 mm, of less than 2.5%.
12. An article selected from the group consisting of a fireproof door, a fireproof window, a cooking top plate, and a shielding comprising a glass-ceramic according to claim 1 .
13. A method for elaborating a glass-ceramic according to claim 1 , comprising a heat treatment of a batch mixture of raw materials able to vitrify, containing SnO 2 as a fining agent, under conditions which ensure melting, fining and ceramming by a first nucleation step and a second crystal growth step, wherein said batch mixture has a composition with which a glass-ceramic having a composition that is free of arsenic oxide, antimony oxide and rare earth oxides except for inevitable trace amounts, and which contains, expressed as percentages by weight of oxides:
62-72% of SiO 2 ,
20-23% of Al 2 O 3 ,
2.8-5% of Li 2 O,
0.1-0.6% of SnO 2 ,
1.9-4% of TiO 2 ,
1.6-3% of ZrO 2 ,
less than 0.4% of MgO,
2.5-6% of ZnO and/or BaO and/or SrO,
and less than 250 ppm of Fe 2 O 3 can be obtained, and wherein said ceramming is applied:
in a temperature interval between 650 and 850° C., for 15 minutes to 4 hours for the nucleation step, and
in a temperature interval between 860 and 935° C., for 10 minutes to 2 hours for the crystal growth step.