Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2000-08-02
2002-05-14
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S020000, C501S022000, C501S026000, C501S032000, C501S076000, C313S47700R, C220S00210A
Reexamination Certificate
active
06387833
ABSTRACT:
The present invention relates to a sealing composition useful for sealing a panel and a funnel of a color cathode ray tube.
A panel and a funnel of a color cathode ray tube used to be sealed by means of a sealing composition containing PbO—ZnO—B
2
O
3
—SiO
2
type low melting point glass. Namely, such a sealing composition is formed into a paste, which is then coated on the sealing portion and maintained at a temperature of from 440 to 4700° C. for from 30 to 40 minutes, whereby the panel and the funnel will be sealed. The interior of the sealed panel and funnel (i.e. a bulb of a color cathode ray tube) will then be evacuated to a high vacuum level of not higher than 10
−6
Torr at a high temperature of from 300 to 380° C.
As such a sealing composition, JP-A-8-225341 discloses a sealing composition comprising PbO—ZnO—B
2
O
3
—SiO
2
type low melting point glass and 4 wt % of an alumina powder.
In recent years, along with the progress in the trend for large size and flat panel of a color cathode ray tube, the weight of a bulb of a color cathode ray tube has increased (such a weight tends to be further increased, since it is necessary to further increase the glass thickness in order to prevent deterioration of the strength of the bulb of a color cathode ray tube due to the use of a flat panel). Under these circumstances, it is desired to reduce the weight of the bulb of a color cathode ray tube.
In order to reduce the weight of the bulb of a large sized color cathode ray tube, it is necessary to reduce the glass thickness of the panel and the funnel. However, there has been a problem that if the glass thickness is reduced, the strength at the sealing portion of the panel and the funnel decreases.
It is an object of the present invention to solve the above problems and to provide a sealing composition which is capable of improving the strength of the sealed product (such as a bulb of a color cathode ray tube).
The present invention provides a sealing composition having at least one member from &agr;-4PbO.B
2
O
3
crystal powder and Pb
3
O
4
powder incorporated in a total amount of from 0.0001 to 3 parts by weight to 100 parts by weight of a composition comprising at least 80 wt % and less than 98.99 wt % of a powder of PbO—ZnO—B
2
O
3
—SiO
2
type crystalline low melting point glass, from 0.01 to 5 wt % of a zircon powder, more than 1 wt % and not more than 19.99 wt % of an &agr;-alumina powder and from 0 to 10 wt % of a low expansion ceramic filler.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The sealing composition of the present invention is usually mixed with a vehicle to obtain a paste. Such a vehicle may, for example, be a solution having nitrocellulose dissolved in isoamyl acetate. The paste is coated, for example, at a portion where a panel and a funnel of a color cathode ray tube are to be sealed, and then baked. In such a case, the baking is carried out for example, by maintaining the coated paste at a temperature of from 400 to 450° C. for from 30 to 40 minutes.
In the present invention, crystalline glass is a glass which shows a heat generation peak in a differential thermal analysis wherein the temperature is raised at a rate of 10° C./min and maintained at from 350 to 500° C. for 2 hours. Namely, it is a glass which crystallizes under the above conditions. It is preferably a glass which shows a heat generation peak in a differential thermal analysis wherein the temperature is raised at a rate of 10° C./min and maintained at a temperature of from 400 to 450° C. for 2 hours.
In the present invention, low melting point glass is a glass having a softening point of not higher than 600° C.
In the present invention, PbO—ZnO—B
2
O
3
—SiO
2
type crystalline low melting point glass is a crystalline glass which contains PbO, ZnO, B
2
O
3
and SiO
2
and wherein, when it is maintained at a temperature of from 350 to 500° C., first crystals (2PbO.ZnO.B
2
O
3
) will appear as the time passes and then second crystals (&agr;-4PbO.B
2
O
3
) will precipitate. Preferably, it is a glass, wherein, when maintained at a temperature of from 400 to 450° C., the first crystals and the second crystals will precipitate as described above.
In the present invention, a low expansion ceramic filler is a ceramic powder having an average linear expansion coefficient within a range of from room temperature to 300° C. (hereinafter referred to simply as an expansion coefficient) being not higher than 70×10
7
/° C. However, zircon powder and &agr;-alumina powder are excluded.
In the sealing composition of the present invention, the low expansion ceramic filler is preferably at least one member selected from the group consisting of cordierite, mullite, lead titanate, silica, &bgr;-eucryptite, &bgr;-spodumene and &bgr;-quartz solid solution, from the viewpoint of handling efficiency.
The interior of the panel and the funnel of a color cathode ray tube, sealed by means of the sealing composition of the present invention, is evacuated usually to a high vacuum level of not higher than 10
−6
Torr at a high temperature of from 300 to 380° C. At that time, the baked sealing composition of the present invention is free from flowing or foaming.
The expansion coefficient of the baked sealing composition of the present invention (hereinafter referred to as the baked product) is preferably from 70×10
−7
to 110×10
−7
/° C., more preferably from 80×10
−7
to 110×10
−7
/° C., particularly preferably from 85×10
−7
to 105×10
−7
/° C., most preferably from 90×10
−7
to 100×10
−7
/° C., to adjust it to the expansion coefficients of the panel and the funnel of a color cathode ray tube.
Now, the composition of the sealing composition of the present invention will be described, wherein wt % will be represented simply by %.
The PbO—ZnO—B
2
O
3
—SiO
2
type crystalline low melting point glass powder is a component which provides fluidity and is essential. If it exceeds 98.99%, the expansion coefficient of the baked product tends to be too large, whereby the sealing portion is likely to break. It is preferably at most 98%. If it is less than 80%, the fluidity tends to be low, and the air tightness of the sealing portion is likely to be low. Preferably, it is at least 85%, more preferably at least 90%.
The zircon powder is a component which accelerates crystallization thereby to shorten the baking time or lowers the expansion coefficient of the baked product to a prescribed level, and thus it is essential. If it exceeds 5%, the fluidity tends to be too low. Preferably, it is at most 1%, more preferably at most 0.5%. If it is less than 0.01%, its effects tend to be too small. Preferably, it is at least 0.05%.
The &agr;-alumina powder is a component which increases the strength and lowers the expansion coefficient of the baked product to a prescribed level, and it is essential. If it exceeds 19.99%, the fluidity tends to be too low. Preferably, it is at most 15%, more preferably at most 10%. If it is less than 1%, its effects tend to be too small. Preferably, it is at least 1.5%, more preferably more than 2%, particularly preferably at least 2.5%, most preferably more than 4%.
The low expansion ceramic filler is a component which lowers the expansion coefficient of the baked product to a prescribed level, and it may be incorporated up to 10%. If it exceeds 10%, the fluidity is likely to be low. More preferably, it is at most 7%, particularly preferably at most 5%, most preferably at most 2%.
The &agr;-4PbO.B
2
O
3
crystalline powder and the Pb
3
O
4
powder are seed crystals for crystallization of the crystalline low melting point glass powder, and they accelerate precipitation of the second crystals (&agr;-4PbO.B
2
O
3
). It is essential that at least either component is incorporated. By the precipitation of the second crystals, precipitation of PbO crystals will be suppressed. Precipitation of PbO crystals deteriorates the electrical insulating property of
Ichikura Eiji
Kuroki Yuichi
Segawa Yutaka
Tanabe Ryuichi
Usui Hiroshi
Asahi Glass Company Limited
Sample David
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