Electric lamp and discharge devices – Cathode ray tube – Envelope
Reexamination Certificate
2002-06-05
2004-05-11
Wong, Don K. (Department: 2821)
Electric lamp and discharge devices
Cathode ray tube
Envelope
C501S017000, C501S021000, C501S022000
Reexamination Certificate
active
06734615
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a color cathode ray tube which is useful for a television receiver, a computer terminal display or the like, and a glass frit suitable for production of such a color cathode ray tube.
BACKGROUND ART
In a conventional color cathode ray tube, the envelope is constituted by a glass panel which supports a shadow mask for color selection and which serves as an image display portion, and a glass funnel which is located behind the glass panel and which supports a structure which provides a function of forming and scanning an electron beam.
In assembling a color cathode ray tube, in sealing of an electron gun and in a post process step such as an evacuation step, to be durable against the heat shock in a high temperature heat treatment and to maintain good airtightness, the glass panel and the glass funnel are sealed by carrying out a heat treatment at a high temperature of about 440° C. by means of a PbO—ZnO—B
2
O
3
type low melting glass which is crystalline and which has a high content of lead oxide.
JP-A-64-14128 discloses, as a preferred one of such a crystalline glass frit, a sealing composition comprising a PbO—B
2
O
3
type glass frit and from 0.1 to 15% of a low expansion filler and from 1 to 15% of an alumina filler, as represented by mass percentage, added to the glass frit, wherein the grain size of the alumina filler is suitably from 1 to 150 &mgr;m.
Further, JP-A-8-225341 discloses a sealing glass frit material comprising a PbO—ZnO—B
2
O
3
type low melting glass and an alumina powder added to the glass frit in an amount of not more than 5% as represented by mass percentage. It discloses an alumina powder having an average particle size smaller than 5 &mgr;m as a preferred one, and a sealing glass material having 3 wt % of alumina with an average particle size of 3.3 &mgr;m added thereto is exemplified.
In recent years, color cathode ray tubes become larger and the faceplate at the glass panel becomes flat, and resultingly a maximum value of a tensile deforming stress generated at the sealed portion between the glass panel and the glass funnel when the inside of a color cathode ray tube is vacuumized (hereinafter sometimes referred to as “tensile vacuum deforming stress”) tends to increase. Accordingly, to deal with this, the thickness of the glass at the sealed portion is increased. Therefore, in a conventional color cathode ray tube, the maximum tensile vacuum deforming stress generated at the sealed portion is substantially weak and is usually suppressed to be less than 7 MPa, and there has been no problem even though the strength at the sealed portion is not so high.
However, in view of weight saving of color cathode ray tubes, it is strongly desired to make the glass thin. However, when the glass becomes thin, generation of a tensile vacuum deforming stress of at least 7 MPa at the sealed portion between the glass panel and the glass funnel is inevitable. Accordingly, a sealed portion having a high strength, which tolerates a heavy tensile vacuum deforming stress generated at the sealed portion, and a glass frit which realizes such a high strength sealed portion, are desired. However, with a glass frit which is practically used at present or the above-described known glass frit, the strength is insufficient at the sealed portion especially at the sealing interface, and accordingly the above objects can not be achieved.
Specifically, with a glass frit which is used practically at present or a known glass frit, increase in strength of a sintered product of the glass frit and improvement in sealing properties of a color cathode ray tube may be attempted by addition of alumina. However, no good reactivity with the glass panel or the glass funnel can be obtained, and accordingly sealing force at the interface at the sealed portion with the glass is insufficient. Accordingly, in a case of color cathode ray tubes in which a heavy tensile vacuum deforming stress is generated at the sealed portion, since thermal stress is further applied thereto in a production step, they can not tolerate the stress and they are likely to break during their production.
For example, with respect to the sealing composition as disclosed in JP-A-64-14128, although improvement of the strength of the sealing composition itself is confirmed by addition of alumina, however, since the particle size and the addition amount of the alumina to be added are inappropriate, the area ratio of crystal at the sealing interface is only at a level of from 30 to 45%, and the sealing force at the interface at the sealed portion is low.
Further, with respect to the sealing glass frit material as disclosed in JP-A-8-225341, the strength of the sintered sealing glass frit material increases similarly by addition of alumina having an average particle size not larger than 5 &mgr;m, however, sealing strength at the sealing interface is inadequate. Namely, fluidity is improved by addition of alumina, whereby a seal shape which tolerates the stress can be obtained, however, no adequate reactivity with glass can be obtained with alumina having an average particle size of 3.3 &mgr;m, and the sealing strength at the sealing interface is inadequate.
Accordingly, in a case of color cathode ray tubes in which a maximum tensile vacuum deforming stress of at least 7 MPa is generated, since stress generated due to difference in temperature (hereinafter sometimes referred to as “thermal stress”) is further applied thereto in a production step, the sealed portion by means of the above glass frit can not tolerate such a stress, and color cathode ray tubes are likely to break during their production. Further, in order to suppress the maximum tensile vacuum deforming stress to be less than 7 MPa, the thickness of the glass has to be maintained, whereby weight saving of color cathode ray tubes tends to be difficult.
DISCLOSURE OF THE INVENTION
The present invention has been made to overcome the above problems.
The present invention provides a color cathode ray tube having a sealed portion at which a glass panel and a glass funnel are sealed by means of a sintered glass frit, wherein the maximum tensile stress generated at the above sealed portion is from 7 to 10 MPa when the inside of said color cathode ray tube is vacuumized, the above sintered glass frit contains at least a crystalline low melting glass and a low expansion filler containing &agr;-alumina, and the area ratio of crystal is at least 50% at the interface at the sealed portion at at least the region at which the above maximum tensile stress is generated.
In said cathode ray tube, the above frit sintered product glass preferably contains a low expansion filler containing &agr;-alumina and a crystalline low melting glass, and the contents of the respective components as represented by mass percentage based on the sintered glass frit are preferably such that the crystalline low melting glass: 90-99% and the low expansion filler: 1-10%, the content of the above and &agr;-alumina is preferably from 0.1 to 7% based on the sintered glass frit. Further, part of particles of the &agr;-alumina is preferably corpuscular &agr;-alumina having a particle size not larger than 3 &mgr;m, and the content of the corpuscular &agr;-alumina is preferably from 0.1 to 5% as represented by mass percentage based on the sintered glass frit.
Further, in the cathode ray tube of the present invention, the above sintered glass frit preferably contains a low expansion filler containing &agr;-alumina, a crystalline low melting glass and a crystallizing filler containing MgO and/or Fe
2
O
3
, and the contents of the respective components as represented by mass percentage based on the sintered glass frit are preferably such that the crystalline low melting glass: 90-98.9% and the low expansion filler: 1-9.9%, the content of the above &agr;-alumina is preferably from 0.1 to 7% based on the sintered glass frit. Further, the sum of the masses of MgO and Fe
2
O
3
as represented by mass percentage based on the sintered glass frit is preferably from 0.1 to 5%.
F
Ichikura Eiji
Segawa Yutaka
Sugawara Tsunehiko
Tanabe Ryuichi
Asahi Glass Company Limited
Tran Thuy Vinh
Wong Don K.
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