Electric lamp and discharge devices – Electrode and shield structures – Cathodes containing and/or coated with electron emissive...
Reexamination Certificate
1999-01-11
2001-06-05
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Electrode and shield structures
Cathodes containing and/or coated with electron emissive...
C313S3460DC, C313S350000, C313S353000, C313S354000, C313S355000, C313S356000, C313S351000
Reexamination Certificate
active
06242854
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an indirectly heated cathode for a cathode-ray tube used for a television receiver, a computer display, or the like and to a cathode-ray tube comprising the same. Particularly, the present invention relates to an alumina electrical insulating layer of a heater for an indirectly heated cathode used in an electron gun.
BACKGROUND OF THE INVENTION
FIG. 10
shows a heater
13
used for a conventional general indirectly heated cathode. In
FIG. 10
, an alumina electrical insulating layer
11
is formed by layering alumina particles on a surface of a metal-wire coil
14
by electrophoresis, spraying, or the like and then sintering it. The metal-wire coil
14
is made of tungsten or rhenium-tungsten alloy and is coiled. A metal cap
17
and a sleeve
10
for holding a cathode
9
are provided outside the heater
13
. The heater
13
supplies a sufficient amount of heat to the metal cap
17
and the sleeve
10
so that the cathode
9
emits thermoelectrons. The alumina electrical insulating layer
11
on the surface of the metal-wire coil
14
maintains the electric insulation between the sleeve
10
and the metal-wire coil
14
. Further, a dark layer
12
made of a mixture of tungsten-alumina particles and alumina particles is provided on the alumina electrical insulating layer
11
, thus increasing the heat transfer efficiency from the heater
13
to the sleeve
10
.
However, in an indirectly heated cathode provided with a heater having such an alumina electrical insulating layer, thermal stress is concentrated at uneven parts in the alumina electrical insulating layer during sintering and the practical operation. As a result, cracks
16
and deformation of the heater occur easily, thus causing decrease in volume of heat-transfer to a cathode, increase in heater temperature, bad electrical insulation between the heater and the cathode, heater breakdown, or the like. In addition, the operation temperature of the cathode decreases and therefore electron emission decreases, thus affecting the characteristics of a cathode-ray tube.
In order to solve such problems, various methods have been proposed. For instance, there are methods in which an alumina electrical insulating layer is strengthened by mixing a fibrous or whisker-like high-melting inorganic insulator with an inorganic insulator, thus preventing cracks (Japanese Patent Gazette Tokko Sho 44-1775) and on the contrary, porosity in an alumina electrical insulating layer is increased, thus preventing cracks from progressing (Publication of Unexamined Japanese Patent Application Tokkai Sho 60-221925).
However, in the conventional methods mentioned above, there were problems that materials were expensive and when increasing the porosity it was difficult to obtain a uniform alumina electrical insulating layer, thus significantly affecting the defective percentage in manufacturing a heater or damage on a heater after being incorporated into a cathode. Both above-mentioned methods were effective for a heater operated at relatively low temperature (about 1100° C. or less) but caused a short life of a heater operated at high temperature (at least about 1100° C.), for example, in an impregnated cathode.
SUMMARY OF THE INVENTION
In order to solve the conventional problems described above, the present invention aims to provide an indirectly heated cathode that can be produced stably and avoids the occurrence of cracks in an alumina electrical insulating layer, heater deformation, and the like even in the practical operation of a cathode-ray tube, thus lengthening the life of a heater. The present invention also aims to provide a cathode-ray tube comprising the indirectly heated cathode.
In order to attain the object described above, an indirectly heated cathode of the present invention comprises a heater and an electron-emitting part. The heater has an alumina electrical insulating layer formed by layering and sintering alumina particles on a surface of a metal wire. The electron-emitting part receives heat from the heater and emits thermoelectrons. The indirectly heated cathode is characterized in that alumina particles contained in the alumina electrical insulating layer have a purity of at least 99.7 wt % and alumina particles with a particle size of 2 &mgr;m or less included in the alumina particles used for forming the alumina electrical insulating layer have a Na content of 20 ppm or less or the alumina particles used for forming the alumina electrical insulating layer have a Si content of 100 ppm or less.
The cathode-ray tube of the present invention comprises a face plate having a phosphor screen on its inner surface, a funnel portion connected to the rear of the face plate, and a neck portion formed at the rear of the funnel portion. In the neck portion, an electron gun that emits electron beams is provided. In the cathode-ray tube, an indirectly heated cathode in the electron gun comprises a heater and an electron-emitting part. The heater has an alumina electrical insulating layer formed by layering and sintering alumina particles on a surface of a metal wire. The electron-emitting part receives heat from the heater and emits thermoelectrons. The indirectly heated cathode is characterized in that alumina particles contained in the alumina electrical insulating layer have a purity of at least 99.7 wt % and alumina particles with a particle size of 2 &mgr;m or less included in the alumina particles used for forming the alumina electrical insulating layer have a Na content of 20 ppm or less or the alumina particles used for forming the alumina electrical insulating layer have a Si content of 100 ppm or less.
In the indirectly heated cathode and the cathode-ray tube of the present invention, it is preferable that the alumina particles with a particle size of 2 &mgr;m or less are included in the alumina particles as a whole used for forming the electrical insulating layer in a ratio of 10-50 wt %. The life of the heater can be further lengthened by defining the particle size of the alumina particles and the Na content.
In the indirectly heated cathode and the cathode-ray tube of the present invention, it is preferable that the electron-emitting part is made of an oxide cathode material. When using the oxide cathode material, the electron-emitting part is suitable for an indirectly heated cathode operated at relatively low temperature. The oxide cathode material is effective especially when the alumina particles with a particle size of 2 &mgr;m or less are included in the alumina particles as a whole in a ratio of 10-50 wt %.
Further, in the indirectly heated cathode and the cathode-ray tube of the present invention, it is preferable that the alumina particles with a particle size of 2 &mgr;m or less, those with a particle size of 5-20 &mgr;m, and those with a particle size above 20 &mgr;m are included in the alumina particles as a whole in a ratio of 10-40 wt %, 40-70 wt %, and 10 wt % or less, respectively.
In the indirectly heated cathode and the cathode-ray tube of the present invention, it is preferable that the electron-emitting part is made of an impregnated cathode material. The impregnated cathode material is effective especially when the alumina particles with a particle size of 2 &mgr;m or less, those with a particle size of 5-20 &mgr;m, and those with a particle size above 20 &mgr;m are included in the alumina particles as a whole in a ratio of 10-40 wt %, 40-70 wt %, and 10 wt % or less, respectively.
In the indirectly heated cathode and the cathode-ray tube of the present invention, it is preferable that all the alumina particles used for forming the electrical insulating layer have a Na content of 20 ppm or less.
In the indirectly heated cathode and the cathode-ray tube of the present invention, it is also preferable that a dark layer made of a mixture of tungsten-alumina particles and alumina particles is further formed on the alumina electrical insulating layer.
In the indirectly heated cathode and the cathode-ray tube of the present invention, it is preferable th
Kawasaki Masaki
Koshino Hideo
Nakai Junya
Shimizu Tetsuya
Yamamoto Yoji
Haynes Mack
Matsushita Electronics Corporation
Merchant & Gould P.C.
Patel Ashok
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