Glass-ceramic and a process for obtaining it

Details Glass-ceramic and a process for obtaining it Glass-ceramic and a process for obtaining it

2002-09-10

2004-03-16

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S007000, C065S033100, C065S033700

Reexamination Certificate

active

06706653

ABSTRACT:

The present invention relates to:
a glass-ceramic, which is opaque, and has a low, or zero thermal expansion coefficient, and which contains a solid solution of &bgr;-quartz as crystalline phase; and
a process of obtaining said glass-ceramic.
Glass-ceramics are materials which are known per se, and which are obtained by heat treatment of mineral glasses.
These materials are used in various contexts and notably as substrates of cooktops and as anti-fire windows (having a resistance to thermal breakage and thus preventing the spread of a fire).
It is important for the applications of this nature that said glass-ceramics resist not only to the high temperatures but also to the wide variations in temperature. In particular, in the case of a substrate for cooktops, such variations in temperature manifest themselves not only rapidly but also in a non-uniform manner within the mass, such that a part of said substrate can cool down or heat up very rapidly, while the temperature of the other parts can remain unchanged or can vary much more slowly. Thus, it is desirable to have glass-ceramics at one's disposal which have a very low thermal expansion coefficient, preferably with such a thermal expansion coefficient of zero or near-zero.
Furthermore, transparent and opalescent glass-ceramics, even opaque glass-ceramics are known.
More specifically:
the article by G. H. Beall entitled: “Crystallization and Chemical Strengthening of Stuffed &bgr;-Quartz Glass-Ceramics”, in the Journal of the American Ceramic Society, Vol. 50., No. 4., p. 181-190 (April 1967) gives, in general, &bgr;-quartz glass-ceramics (which contain a solid solution of &bgr;-quartz as crystalline phase). Said article indicates the ranges of composition of glass which enable said glass-ceramics to be obtained, the ranges of ceramming temperature, the ranges of expansion of said glass-ceramics, the possibility of high-temperature decomposition of the &bgr;-quartz phase. Said article describes no opaque glass-ceramic of zero expansion and all the time indicates ceramming times of at least 4 hours;
the patent application EP-A-0-853 071 describes &bgr;-quartz and &bgr;-spodumene glass-ceramics (which contain a solid solution of &bgr;-quartz or &bgr;-spodumene, respectively, as crystalline phase). The described &bgr;-quartz glass-ceramics (which are not opaque) are different from the ones of the invention;
the patent application EP-A-1 029 830 describes &bgr;-quartz glass-ceramics (which contain a solid solution of &bgr;-quartz as crystalline phase), which are opaque, and which are of zero expansion. However, said glass-ceramics are different from the glass-ceramics of the invention and the ceramming times described are of the order of at least 6 hours;
U.S. Pat. No. 5,070,045 describes &bgr;-quartz glass-ceramics (which contain a solid solution of &bgr;-quartz as crystalline phase), which have zero expansion, which are transparent, and obtainable by a ceramming treatment of less than 2 hours. The inventors wished to confer an opaque aspect to these glass-ceramics. Thus, they crystallised the glass precursors of said glass-ceramics at a higher temperature (around >950° C.). However, the opacification is then accompanied by a change of crystalline phase: the main phase becomes a solid solution of &bgr;-spodumene. This conversion has the drawback, for the applications sought after, of being accompanied with an increase in the thermal expansion coefficient. This latter value becomes close to 10×10
−7
K
−1
.
The glass compositions described in U.S. Pat. No. 5,070,045 thus have been modified, in accordance with the invention, in order to be able to generate opaque glass-ceramics of zero expansion. It was sought to obtain the opacification while at the same time preventing the conversion, of the &bgr;-quartz phase into the &bgr;-spodumene phase, which leads to the increase of the expansion. The conversion into other phases of high expansion (such as spinel) should also be avoided.
It is noted at this juncture that the presence of a solid solution of &bgr;-quartz, as main phase, is not in itself a guarantee to obtain a zero expansion. As indicated in the article cited above, numerous oxides can enter into the composition of this solid solution, in particular Li
2
O, MgO, ZnO and Al
2
O
3
. According to the content of each one of these oxides, the expansion of the crystalline phase and, thus, that of the glass-ceramic, can vary within a very large range (of around −20 to +50 10
−7
K
−1
for the glass-ceramic).
The technical problem tackled has thus been that of determining the levels of the various oxides not only in order to avoid the conversion into &bgr;-spodumene, but more specifically to obtain, advantageously in less than two hours, an opaque glass-ceramic with a thermal expansion coefficient of practically zero.
It was observed in a surprising way that this result could be obtained, with respect to the teaching of U.S. Pat. No. 5,070,045, by lowering the content of Li
2
O, while at the same time limiting the content of MgO and by increasing the content of ZnO; contents of Li
2
O, MgO and ZnO of the precursor mineral glasses and of the final glass-ceramics.
The glass-ceramics of the invention are thus opaque glass-ceramics which are particularly interesting with reference to their thermal expansion coefficient and which moreover have the advantage of being able to be obtained at the end of a short heat treatment (of less than two hours) of their precursor glasses.
According to a first object, the present invention relates to original glass-ceramics which:
are opaque;
have a low, or zero thermal expansion coefficient (of 0±5×10
−7
K
−1
, advantageously of 0±3×10
−7
K
−1
);
contain a solid solution of &bgr;-quartz as crystalline phase, and
which composition essentially consists, by weight percent, of:
65 to 72% of SiO
2
16 to 24% of Al
2
O
3
2 to less than 3% of Li
2
O,
0 to 1% of MgO
3 to 5% of ZnO
0 to 1.5% of BaO
2 to 4.5% of TiO
2
and/or ZrO
2
, with the Al
2
O
3
/(TiO
2
+ZrO
2
)ratio >5, 0 to an effective and non excessive amount of a least one fining agent, and less than 1% of K
2
O and/or Na
2
O.
From mineral glasses having the composition indicated above (for the glass-ceramics of the invention obtained from said mineral glasses) the inventors have surprisingly obtained opaque glass-ceramics which have low or zero thermal expansion coefficient and which contain a solid solution of &bgr;-quartz as crystalline phase.
In reference to the said composition (of the glass-ceramics of the invention as well as of the mineral glasses, precursors of the said glass-ceramics of the invention) the following may be precised.
SiO
2
is a classical component which forms mineral glasses. Its content must be less than 72% in order to limit the melting point and greater than 65% in order to avoid devitrification during cooling.
Al
2
O
3
is incorporated at a relatively significant content since it is one of the main constituents of the crystalline phase. Surprisingly, it was observed that it was easier to obtain an opaque material when said Al
2
O
3
content increased. With too high an Al
2
O
3
content, the tendency of devitrification during cooling becomes unacceptable.
Li
2
O is a key element of the crystallisation. When it is incorporated at a content of less than 2%, it is difficult to obtain the crystalline phase. When it is incorporated at a content of greater than 3%, the opacification tends to be accompanied by a conversion into &bgr;-spodumene.
MgO tends to increase the expansion: it is for this reason that its content is limited to 1%. Its presence is not indispensable.
K
2
O and Na
2
O also tends to increase the expansion: it is for this reason that their content (K
2
O alone, Na
2
O alone or K
2
O+Na
2
O in combination) is limited to 1%, advantageously 0.5% (the composition advantageously comprises less than 0.5% of K
2
O+Na
2
O). Their presence is in no way indispensable, is actually undesirable. Said eleme

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