Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
2001-01-19
2003-07-15
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C501S007000
Reexamination Certificate
active
06593258
ABSTRACT:
TECHNICAL FIELD
This invention relates to a transparent crystallized glass for use in a front window or an inspection window of a combustion apparatus for burning oil, coal, gas, wood, or the like, i.e., a space heater, a heating furnace, an annealing furnace, and so on.
BACKGROUND ART
The function of a front window in a space heater or the like is to enhance a space-heating effect by transmitting heat radiation emitted from a flame inside the heater to the outside as well as to increase visual warmth by allowing the flame to be visible. On the other hand, the function of an inspection window is to allow the burning condition of the flame to be observed from the outside. These windows must be resistant against a high temperature produced from the flame and heat shock at the ignition. Therefore, a window glass material used in the windows of such combustion apparatus is required to be transparent as well as to be low in thermal expansion and high in mechanical strength so that the heat resistance and the heat-shock resistance are excellent.
At present, as the material for the windows of the space heater, use is made of a borosilicate glass, a silica glass, and an Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass. However, the borosilicate glass is not sufficient in heat resistance and heat-shock resistance while the silica glass is excellent in thermal characteristics but is expensive. On the other hand, the Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass has a small coefficient of thermal expansion and a high mechanical strength so that the heat resistance and the heat-shock resistance are excellent, and it can be produced at a relatively low cost. Therefore, this glass is widely used.
However, the Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass has a problem that, when it is placed in a combustion atmosphere, the inner surface, that is, the burning-side surface of the window is subjected to chemical corrosion to produce microcracks which result in a considerable decrease in transparency and mechanical strength.
The reason why the above-mentioned problem occurs is as follows. In the combustion atmosphere of the combustion apparatus for burning oil, coal, gas, wood, or the like, SO
x
produced from sulfur contained in such fuel is present and reacts with H
2
O produced by combustion to produce H
2
SO
4
. Hydrogen ions produced from H
2
SO
4
cause an ion exchange reaction with lithium ions in crystals of the Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass to cause shrinkage in volume of the crystals, resulting in occurrence of cracks.
In order to prevent the above-mentioned problem, use is made of a technique of forming a coating film such as SiO
2
on the surface of the Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass. However, this technique is not preferable because the production cost becomes high. Alternatively, it is possible to suppress the occurrence of the ion exchange reaction with the hydrogen ions by reducing the content of the lithium ions in the Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass. However, there arises a problem that the transparency is deteriorated and the coefficient of thermal expansion is increased.
It is therefore an object of the present invention to provide an Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass which will not produce microcracks even if it is exposed to an atmosphere containing H
2
SO
4
produced by a fuel such as oil, coal, gas, wood, or the like for a long time, as well as a crystallizable glass for obtaining such crystallized glass.
DISCLOSURE OF THE INVENTION
As a result of various experiments, the present inventors have found out that &bgr;-OH contained in an Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass plays an important role in suppressing the occurrence of microcracks due to an ion exchange reaction with hydrogen ions, and thus have achieved the present invention.
Specifically, an Li
2
O—Al
2
O
3
—SiO
2
transparent crystallized glass according to the present invention is characterized in that &bgr;-quartz solid solution is produced as main crystals, an average coefficient of linear thermal expansion within a range between 30 and 380° C. is −10~10×10
−7
/°C., and &bgr;-OH in an amount of 0.28/mm or more is contained.
Further, an Li
2
O—Al
2
O
3
—SiO
2
crystallizable glass according to the present invention is characterized by having a property of being transformed into a crystallized glass in which &bgr;-quartz solid solution is produced as main crystals, an average coefficient of linear thermal expansion within a range between 30 and 380° C., is −10~10×10
−7
/°C., and &bgr;-OH in an amount of 0.28/mm or more is contained.
In the present invention, the amount of &bgr;-OH is obtained by the following formula (1) by the use of an infrared absorption spectrum of the crystallized glass.
Amount of &bgr;-OH={log(
T
3850
/T
3500
)}/
t
(1)
wherein
T
3850
: transmittance around 3850 cm
−1
T
3500
: minimum transmittance in an absorption band around 3500 cm
−1
t: thickness (mm) of crystallized glass at measurement of spectrum.
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patent: 4707458 (1987-11-01), Chyung et al.
patent: 5017519 (1991-05-01), Morimoto et al.
patent: 5070045 (1991-12-01), Comte et al.
patent: 5336643 (1994-08-01), Goto et al.
patent: 5591682 (1997-01-01), Goto et al.
patent: 5972816 (1999-10-01), Goto
patent: 6358869 (2002-02-01), Yamada et al.
patent: 6387509 (2002-05-01), Goto et al.
patent: 6413906 (2002-07-01), Shimatani et al.
patent: 59-116150 (1984-07-01), None
patent: 64-52631 (1989-02-01), None
patent: 8-104539 (1996-04-01), None
patent: 9-169542 (1997-06-01), None
patent: 9-188538 (1997-07-01), None
8-104539, translation, Apr. 1996.*
9-188538, translation, Jul. 1997.
Sakamoto Akihiko
Shimatani Narutoshi
Collard & Roe P.C.
Group Karl
Nippon Electric Glass Co. Ltd.
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