Electric lamp and discharge devices – Cathode ray tube – Ray generating or control
Reexamination Certificate
2000-06-26
2002-04-02
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Ray generating or control
C313S34600R, C313S447000
Reexamination Certificate
active
06366011
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a cathode-ray tube and, more particularly, to a cathode-ray tube having an electron gun with a cathode of a high current density in which stable electron emission characteristics are obtained for a long time.
Various cathode-ray tubes such as television picture tubes and image display electron tubes used as monitor tubes of information processing terminals have an electron gun for emitting one or a plurality of electron beams at one end of a vacuum envelope and a phosphor screen obtained by coating a fluorescent material film of one or a plurality of colors onto an inner surface at the other end of the vacuum envelope so that an electron beam emitted from the electron gun is two-dimensionally scanned by a magnetic field generated by a deflecting yoke attached to the outside of the vacuum envelope, thereby displaying a desired image.
For such a kind of recent cathode-ray tube, high definition of a display image is required in association with realization of a variety of display information, a high-density of display information. and the like. To realize high definition of a display image, it is necessary to remarkably improve focusing characteristics of the electron beam.
As means for improving the focusing characteristics to satisfy the requirement of the high definition mentioned above, it is considered to reduce an electron beam passage hole of a first grid electrode of the electron gun.
However, there arise a limitation on the reduction of the dimension of the diameter of the electron beam passage hole of the first grid electrode due to a limitation on a cut-off voltage, a limitation of an electrode manufacturing precision, and a limitation of a cathode loading. Particularly, the cathode loading relates to the life reliability of the cathode-ray tube and exerts a large influence in case of deciding the hole diameter of the first grid electrode.
Therefore, the reduction of the hole diameter of the first grid electrode is limited to no less than 0.40 mm except for a combined use of an impregnated cathode suitable for a high current density operation. The impregnated cathode, however, has problems such that the number of manufacturing steps is large and it is expensive.
A cathode-ray tube having a cathode with an electron emissive material layer to solve such problems has been disclosed in U.S. Pat. No. 5,216,320 issued Jun. 1, 1993, and assigned to the same assignee as that of the present invention.
FIG. 1
is a cross sectional view for explaining a structure similar to that of a cathode shown in the above U.S. patent. Reference numeral
13
denotes a cylindrical cathode sleeve;
14
a cap-shaped metal base;
15
an electron emissive material layer;
16
a heater;
19
a first layer made of an oxide of alkaline earth metal; and
20
a second layer made of an oxide of alkaline earth metal containing a rare earth metal (for example, barium scandate Ba
2
Sc
2
O
5
).
The cap-shaped metal base
14
, sealing one end of the cylindrical cathode sleeve
13
, is made of a material made of refractory metal, for example, nickel (Ni) as a main component and containing therein a reduction metal of a small quantity of silicon (Si) or magnesium (Mg). The heater
16
is placed in the cathode sleeve
13
, thereby constructing an indirectly heated cathode.
The electron emissive material layer
15
having a double layer structure is coated or formed on the upper surface of the metal base
14
. The electron emissive material layer
15
includes the first layer
19
made of an oxide of alkaline earth metal coated or formed so as to be in contact with the upper surface of the metal base
14
and the second layer
20
made of an oxide of alkaline earth metal which is coated or formed on the surface of the first layer
19
and contains an oxide of rare earth metal such as barium scandate or the like.
The electron emissive material layer
15
with the above structure may be manufactured as follows. A first layer of a carbonate of alkaline earth metal is first formed on the upper surface of the metal base
14
. A second layer of a carbonate of alkaline earth metal containing an oxide of rare earth metal such as barium scandate (Ba
2
SC
2
O
5
) or the like is formed on the first layer. After that, the carbonate of alkaline earth metal of each of the first and second layers is changed to an oxide of alkaline earth metal by a thermal decomposition at the time of a heat treatment in a manufacturing step of the cathode-ray tube, thereby obtaining the electron emissive material layer
15
comprising the first layer
19
and the second layer
20
laminated on the first layer
19
.
According to the structure of the electron emissive material layer
15
as mentioned above, the second layer
20
made of the oxide of alkaline earth metal formed on the electron emissive surface side of the electron emissive material layer
15
and containing the oxide of rare earth metal such as barium scandate or the like binds free barium (Ba) formed by a reduction element in the metal substance
14
to the second layer
20
and maintains free barium in the electron emissive material layer
15
so as to be in a high density state.
Thus, even when the cathode operates in a high current density state, an amount of Joule heat generated in the electron emissive material layer
15
is reduced and a degree of evaporation of barium also decreases.
As mentioned above, even when the cathode shown in
FIG. 1
is operated-in a high density current state, for example, in a large current state exceeding 2A/cm
2
, the reduction of current emission is small and the cathode of a long life can be realized.
SUMMARY OF THE INVENTION
In order to display an image on a display panel at high definition, it is necessary to reduce a diameter of an electron beam passage hole of an electrode, namely, a first grid electrode which is arranged adjacent to the cathode and has a hole to limit emanation of electrons emitted from the cathode.
According to the knowledge derived by experiments and study of the present inventors, an oxide of rare earth metal such as barium scandate or the like has functions of binding free barium and maintaining free barium in an electron emissive material layer in a high density state. However, since the rare earth metal oxide itself such as barium scandate or the like never contributes to the electron emission, when a content of the rare earth metal oxide such as barium scandate or the like in the electron emissive material layer is increased, an amount of emission of electrons from the electron emissive material layer decreases and the life of the cathode-ray tube is not always extended.
In addition, the rare earth metal oxide such as barium scandate or the like is an expensive material.
It is an object of the invention to provide a cathode-ray tube which is capable of displaying an image at high definition (high resolution) and has a sufficiently long operating life.
Another object of the invention is to provide an electron gun suitable for the above cathode-ray tube.
According to one aspect of the invention, there is provided a cathode-ray tube with an electron gun having a cathode, a control electrode, and another electrode, wherein the cathode has an electron emissive material layer for emanating electrons through a hole of the control electrode, the hole in the control electrode substantially having a diameter in a range from 0.3 mm to 0.4 mm, and the electron emissive material layer includes a first layer which is formed on a support and is made of an oxide of alkaline earth metal containing no oxide of rear earth metal and includes a second layer which is formed on the first layer and is mainly made of an oxide of alkaline earth metal substantially containing an oxide of rare earth metal of a quantity in a range from 0.8 wt % to 5.0 wt %.
As a diameter of electron beam passage hole formed on a first grid electrode (a control electrode arranged adjacent to the cathode) is smaller, a beam diameter at a crossover point in the electron gun more decreases and focusing char
Koizumi Sachio
Komiya Toshifumi
Antonelli Terry Stout & Kraus LLP
Day Michael H.
Hitachi , Ltd.
Williams Joseph
LandOfFree
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