Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
1999-09-16
2001-03-20
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S015000, C501S021000
Reexamination Certificate
active
06204212
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zirconium oxide and lithium oxide-containing borosilicate glass of increased resistance to attack by various chemical compounds and to the use of this improved borosilicate glass in several different applications.
2. Prior Art
Glasses, which have a very high resistance to chemical attack by both acidic and alkaline media, are required for glass-metal seals, which are exposed to a chemically corrosive environment, e.g. in chemical plants or reactor engineering. The thermal expansion properties of these sealing glasses must match those of high resistance metals or alloys that are employed. It is desirable that their linear expansion coefficients be close to or slightly under that of the metals to be sealed so that compressive strains build up in the glass on cooling of the seal. These compressive strains guarantee, on the one hand, a hermetic seal and, on the other hand, prevent the build up of tensile stresses in the glass, which would otherwise cause stress-corrosion. Glasses with thermal expansion coefficients &agr;
20/300
between 5.2 and 5.7×10
−6
/K are required as sealing glasses for glass-metal seals with Fe—Ni—Co alloys, e.g. Vacon® 11, which has a thermal expansion coefficient &agr;
20/300
=5.4×10
−6
/K, zirconium (&agr;
20/300
=5.9×10
−6
/K) or zirconium alloys.
The processing temperature V
A
at which the viscosity of the glass is 10
4
dPas is an essential parameter for characterizing the workability of a glass. It should be reduced since slight V
A
reductions have already led to a significant reduction in manufacturing costs, because the melting temperature is lowered. Furthermore a processing temperature V
A
which is as low as possible is of advantage in making glass-metal seals, since overheating of the parts to be sealed can be avoided, because they are already sealed either at a lower temperature or in a shorter time. Use of glasses with reduced or lowered processing temperatures V
A
can avoid volatilization and retrograde condensation of glass ingredients which can cause impairment of the sealing process and, in the worst case, unsealing. Furthermore the processing interval, i.e. the temperature difference of the processing temperature V
A
and the softening temperature E
w
, i.e. the temperature at which the viscosity of the glass amounts to 10
7.6
dPas, is important. The temperature range in which the glass can be processed is also designated as the “length” or “size” of the glass.
Glasses have already been described in the chemical literature which have a high resistance to chemical attack. However these glasses do not have the desired thermal expansion coefficients with simultaneously lower processing temperatures accompanying the high resistance to chemical attack.
German Patent DE 42 30 607 C1 discloses borosilicate glasses which have a high resistance to chemical attack and which can be sealed with tungsten. They have a thermal expansion coefficient &agr;
20/300
of at most=4.5×10
−6
/K.
The borosilicate glasses described in the published patent application DE 37 22 130 A1 and the zirconium-containing borosilicate glasses described in the German Patent DD 301 821 A7 have thermal expansion coefficients of at most 5.0 or 5.2×10
−6
/K which is too low for sealing with the above-mentioned high resistance metal.
The glasses of German Patent DE 44 30 710 C1 have a high proportion of SiO
2
, namely >75% by weight and >83% by weight SiO
2
and B
2
O
3
in combination with a weight ratio of SiO
2
/B
2
O
3
>8, and little Al
2
O
3
, which provides a high resistance to chemical attack but leads to a processing temperature which is disadvantageously too high.
The glasses of German Patent DE 195 35 708 C1 are highly resistant to chemical attack because of their high SiO
2
content, but have a disadvantageously high processing temperature and low thermal expansion. Also its resistance to attack by alkaline media drops with a decreasing SiO
2
/B
2
O
3
ratio, also with decreasing SiO
2
content.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a glass which fulfills the above-described requirements for both resistance to chemical attack, processing temperature and thermal expansion coefficient, and which makes a sufficiently tight seal with the above-mentioned metals and alloys possible.
This object is attained according to the invention by a zirconium-containing and lithium-containing borosilicate glass of a high resistance to chemical attack having the following composition(in % by weight on an oxide basis):
SiO
2
71 to <73%
B
2
O
3
7 to 10%
Al
2
O
3
5.5 to 9%
Li
2
O
0.5 to 2%
Na
2
O
0 to 10%
K
2
O
0 to 10%
MgO
0 to 2%
CaO
0 to 3%
SrO
0 to 3%
BaO
0 to 3%
ZnO
0 to 3%
ZrO
2
0.8 to 3%
CeO
2
0 to 1%,
with the proviso that a total amount of Li
2
O+Na
2
O+K
2
O is from 0.5 to 10.5%, and with the proviso that a total amount of MgO+CaO+SrO+BaO+ZnO is from 0 to 3%.
The glass according to the invention differs from the known glasses having a high resistance to chemical attack because of a lower SiO
2
content of from 71 to <73% by weight, preferably only up to 72.5% by weight. It was indeed completely surprising that glasses with so little SiO
2
could have both a high acid resistance and also a high alkali resistance so that they belong to the respective first resistance categories. Previously it was believed that good resistance could not be obtained when the SiO
2
content is reduced. The comparatively low SiO
2
content advantageously provides the desired properties, namely the lower processing temperature and comparatively higher thermal expansion coefficient.
The glass according to the invention contains at least 5.5% by weight, preferably at least 6% by weight, especially preferably at least 6.5% by weight, and at most 9% by weight, preferably at most 8.5% by weight of Al
2
O
3
. The glass is very crystallization stable, because of this comparatively high content, which facilitates large-scale manufacture. With higher content however the melting temperature increases without further improvements in the crystallization resistance.
The glass contains from 7 to 10% by weight, preferably at least 7.5% by weight, B
2
O
3
in order to lower the melting temperature with simultaneous improvement in the resistance to chemical attack. The melting temperature is not sufficiently lowered with smaller amounts of B
2
O
3
, while the acid resistance would decrease with larger amounts.
The simultaneous presence of Li
2
O (0.5 to 2 percent by weight, preferably up to 1.5% by weight, especially preferably 0.7 to 1.4% by weight) and ZrO
2
(at least 0.8% by weight, preferably at least 0.9% by weight, at most 3% by weight, preferably at most 2% by weight) contributes essentially to the outstanding resistance to chemical attack. Especially ZrO
2
improves the resistance to alkali or alkaline media. The Li
2
O content acts to oppose the increase in melting and processing temperatures called for by ZrO
2
. Indeed the melting temperature would be lowered with an Li
2
O content above 2% by weight, but the hydrolytic resistance would decrease and the glass is made more expensive. Moreover there is an increased danger of glass faults or imperfections at high ZrO
2
content, since particles of difficult-to-dissolve ZrO
2
raw material remain unmelted and are present in the product.
The other alkali oxides Na
2
O and K
2
O can be present in up to 10% by weight amounts and serve to lower the melting and processing temperature. Thus the entire alkali content (Li
2
O+Na
2
O+K
2
O) should not exceed 10.5% by weight, especially since the hydrolytic resistance would decrease. Preferably the content of the continuously facultative ingredient K
2
O is limited to 6% by weight, especially preferably to 4% by weight. Na
2
O is present, preferably from 3 to 7.5% by weight, especially preferably from 3.5 to 7.2% by weight. The sum total of the three alkali o
Brix Peter
Kunert Christian
Group Karl
Schott Glas
Striker Michael J.
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