Electron tube cathode

Details Electron tube cathode Electron tube cathode

2000-11-02

2002-12-17

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Electrode and shield structures

Cathodes containing and/or coated with electron emissive...

C313S337000, C313S3460DC, C313S355000, C313S307000

Reexamination Certificate

active

06495949

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electron tube cathode, and more particularly to an electron tube cathode enhancing life characteristics and electron emissivity by forming one or more reducing metal layer(s) between electron emissive material layers.
2. Description of the Related Art
In general, an electron tube cathode is a part emitting thermions as an electron source in an electron gun of a cathode ray tube for TV or a camera tube. The electron tube cathode is manufactured by forming an electron emissive material layer composed of electron emissive materials on a metal base. An example of a conventional electron tube cathode is disclosed in Japanese Patent Publication No. Sho 64-5417. As shown in
FIG. 1
, this electron tube cathode has a cathode sleeve
1
in which a heater
3
is placed. A base
5
is mounted on the upper opening of the cathode sleeve
1
, and an electron emissive material layer
7
is formed on the base
5
. The cathode sleeve
1
is composed of nichrome material, while the base contains a reducing material such as silicon (Si) or magnesium (Mg) by 0.01 to 0.09 wt % and is composed of high purity nickel (Ni) as a main ingredient. The electron emissive material layer
7
is composed as main ingredients of alkaline earth metal oxides including at least barium (Ba) and in addition strontium (Sr) or calcium (Ca). The electron emissive material layer
7
also contains rare earth metal oxide such as scandium oxide by 0.1 to 20 wt %. The heater
3
emits thermions from the electron emissive material layer
7
by means of electrical heating.
In order to form the electron emissive material layer
7
in an electron tube cathode having the structure described above, barium carbonate (BaCO
3
), strontium carbonate (SrCO
3
), calcium carbonate (CaCO
3
) and a predetermined amount of scandium oxide (Sc
2
O
3
) are first mixed together with a binder and a solvent to prepare a suspension. The suspension is sprayed onto the base
5
to a thickness of about 800 &mgr;m and thereafter heated by a heater during the cathode ray tube evacuating process. At this time, the carbonates of the alkaline earth metals are converted into alkaline earth metal oxides such as barium oxide (BaO), strontium oxide (SrO) and calcium oxide (CaO). Thereafter, a part of the alkaline earth metal oxides are reduced and activated so as to have semi-conductivity. Thus, the electron emissive material layer
7
composed of a mixture of the alkaline earth metal oxides and a rare earth metal oxide is formed on the base
5
. In the activating process, the reducing materials such as silicon and magnesium, which are contained in the base
5
, move to the interface between the alkaline earth metal oxides and the base
5
by diffusion, and react with the alkaline earth metal oxides. As a result of these reactions, a part of the alkaline earth metal oxides on the base
5
are reduced to be an oxygen deficient semiconductor, thereby facilitating electron emission.
In the electron tube cathode described above, a reducing material layer may be formed on the base by means of a sputtering method before forming a carbonate layer by means of a spraying method. In addition, in order to enhance life characteristics, a mixture layer of the carbonates and the reducing materials, which function to lower resistance of the non-conductive intermediate layer formed on the base, may be formed on the surface of the base, or a carbonate layer may be formed on the mixture layer.
However, the process of forming a reducing material layer on the surface of the base by means of the sputtering method poses a problem of failing to uniformly emit electrons, since the reaction between the reducing material and the base metal is not uniformly made over the entire base metal in the evacuating and activating processes. In addition, a difficulty is encountered in managing the manufacturing process, when forming a carbonate layer on the mixture layer of the reducing materials and the carbonates after forming the mixture layer. Moreover, forming the entire emissive material layer with a mixture of the carbonates and the reducing materials is likely to result in undesirable cathode condition patterns.
Japanese Patent Laid-Open No. Hei 2-75128 discloses a cathode in which an oxide layer of alkaline earth metal including barium is formed on a nickel base and contains scandium, and a metal layer containing at least one element selected from platinum (Pt), iridium (Ir) and rhodium (Rh) is formed between the nickel base and the oxide layer.
In the electron tube cathodes having the structure described above, although the rare earth metal oxides improves the supply of excess Ba, since the excess Ba supplying rate is controlled by the diffusion rate of the reducing material in the nickel base, the life characteristics of the cathode are greatly deteriorated in operating at a high current density over 2 A/cm
2
. Also, since the metal layer on the base is composed of a metal having a lower reducibility than tungsten (W) or molybdenum (Mo), it has almost no barium oxide reducing effect for enabling the operation at a high current density.
Japanese Patent Laid-Open Nos. Hei 3-230445 and Hei 2-267834 disclose a cathode composed of three emissive material layers. In Japanese Patent Laid-Open No. Hei 3-230445, a base is composed as a main ingredient of Ni containing reducing elements such as Si, Mg, etc., the first emissive material layer composed as main ingredients of alkaline earth metal oxides containing Sc
2
O
3
of 0.05 to 5 wt % and Ba is formed on the base, the second emissive material layer composed as main ingredients of alkaline earth metal oxides including barium and containing at least one metal element selected from the
1
B,
3
B and
5
B groups or oxide thereof by 0.01 to 5 wt % is formed on the first emissive material layer, and the third emissive material layer composed as main ingredients of alkaline earth metal oxides including barium is formed on the second emissive material layer.
In Japanese Patent Laid-Open No. Hei 2-267834, a base is composed as a main ingredient of Ni containing at least one reducing element, the first emissive material layer composed as main ingredients of alkaline earth metal including Ba is formed on the base, the second emissive material layer composed as main ingredients of at least one element selected from rare earth metal oxides, rare earth metals, heat resistant metal oxides and heat resistant metals is formed on the first emissive material layer, and the third emissive material layer composed as main ingredients of alkaline earth metal oxides including barium is formed on the second emissive material layer.
In case of a cathode having the structure consisting of three emissive material layers as described above, the reducing metal contained in the second layer exists in the form of a mixture with an emissive material. As mentioned above, however, this is likely to cause that the reaction between the reducing materials and the base metal may not uniformly occur over the entire base, and subsequently the electrons cannot be uniformly emitted. Also, the management of the manufacturing process is difficult, and undesirable cathode condition patterns are likely to be produced.
In Japanese Patent Laid-Open No. Hei 3-257735 and its counterpart U.S. Pat. No. 5,118,984, EP 445956, and Korean Patent Publication No. 93-11964, a base is composed as a main ingredient of Ni containing at least one reducing element selected from Si, Mg, W, zirconium (Zr) and aluminum (Al), a metal layer is formed on the base and contains at least one of W and Mo, and an emissive material layer composed as main ingredients of alkaline earth metal oxides including at least barium is formed on the metal layer and contains rare earth metal oxides of 0.01 to 25 wt %.
In this conventional cathode described above, as shown in
FIG. 2
, a heater
3
is arranged inside a cathode sleeve
1
, a base
5
is arranged on the upper opening of the sleeve, an emissive material layer
7
is formed

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