Electric lamp and discharge devices – With support and/or spacing structure for electrode and/or... – For indirectly heated cathode
Reexamination Certificate
2001-01-10
2003-09-02
Oen, William (Department: 2855)
Electric lamp and discharge devices
With support and/or spacing structure for electrode and/or...
For indirectly heated cathode
Reexamination Certificate
active
06614147
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cathode ray tube having an electron gun employing an indirectly heated cathode, and in particular to a highly-reliable long-life cathode ray tube having prevented occurrence of leakage current by improving insulating characteristics between a cathode sleeve and a heater of the indirectly heated cathode.
Cathode ray tubes used for a color television receiver, a display monitor and the like are widely used in various fields as display means because of their capability of reproducing high-definition images.
Cathode ray tube of this kind includes a vacuum envelope formed of a panel portion, a neck portion and a funnel portion for connecting the panel portion and the neck portion, a phosphor screen formed of phosphors coated on an inner surface of the panel portion, an electron gun housed in the neck portion, and comprised of a plurality of electrodes such as an indirectly heated cathode, a control electrode and an accelerating electrode for projecting an electron beam toward the phosphor screen, and a deflection yoke mounted around the funnel portion for scanning the electron beam emitted from the electron gun over the phosphor screen. The electron gun usually employs an indirectly heated cathode.
FIG. 5
is a cross-sectional view of an essential part of an indirectly heated cathode and its vicinity of a prior art cathode ray tube. In
FIG. 5
, reference numeral
51
denotes an indirectly heated cathode structure, the indirectly heated cathode structure
51
comprises a tubular cathode sleeve
52
, a cap-shaped cathode cap
53
fixed at an end of the cathode sleeve
52
, an electron-emissive material layer
54
coated on a top surface of the cathode cap
53
, and a heater
55
a portion of which is disposed within the cathode sleeve
52
for heating the cathode cap
53
.
A portion of a spirally wound heating wire
55
a
of the heater
55
is covered with an insulating film
55
b
made chiefly of alumina and a coating film
55
c
containing alumina and tungsten powder. Of the insulating film
55
b
and the coating film
55
c
, the insulating film
55
b
covers all the heating wire
55
a
of the heater
55
extending to ends
55
e
except for end portions
55
d
for welding, and the coating film
55
c
covers the outer surface of approximately all the insulating film
55
b
except for the vicinity of the ends
55
e
of the insulating film
55
b
extending from a coil portion
55
f
on the side of the top of the cathode sleeve
52
to ends
55
g
beyond a flared bottom end
52
a
of the cathode sleeve
52
.
The coating film
55
c
contains a small amount of tungsten powder as described above and appears black, and the insulating film
55
b
is made chiefly of alumina and appears white, but the heater
55
appears black as a whole, and this type of heaters are generally called dark heaters.
The heater
55
is welded to heater supports
56
at its end portions
55
d
for welding. The cathode sleeve
52
is fixed to a small-diameter portion of a cathode cylinder
58
, a large-diameter portion of which is fixed to a tubular cathode support eyelet
57
. The cathode support eyelet
57
and the heater supports
56
are fixed to a pair of multiform glasses
61
via bead supports
59
and via heater lead straps
60
, respectively. Reference numeral
62
denotes a control electrode which is fixed to the multiform glasses
61
with a desired spacing between it and the electron-emissive material layer
54
.
The techniques for employing such dark heaters are described in the following references, for example.
Japanese Patent Publication No. Hei 8-3976 (published on Jan. 17, 1996) discloses a technique for improving withstand voltage characteristics by preventing deformation and cracking of an insulating alumina film of a heater using insulating alumina powder of specified average diameters.
Japanese Patent Application Laid-open No. Hei 7-161282 (laid-open on Jun. 23, 1995) discloses a technique for suppressing a leakage current between a heater and a cathode by combining a dark heater with a cathode sleeve having a silicon carbide film on its inner surface.
Japanese Patent Application Laid-open No. Hei 11-213859 (laid-open on Aug. 6, 1999) discloses a technique for suppressing a leakage current between a heater and a cathode by dispersing at least one of niobium and tantalum in a film made of a mixture of tungsten and alumina and coated on at least one of an inner surface of a cathode sleeve and a surface of the heater.
Japanese Patent Application Laid-open No. Hei 11-273549 (laid-open on Oct. 8, 1999) discloses a technique for suppressing a leakage current between a heater and a cathode by improving purity of alumina used for insulation of the heater and thereby increasing electrical resistance of the alumina itself.
Japanese Utility Model Publication No. Sho 60-3483 (Jan. 31, 1985) discloses a technique for preventing cracking of alumina by extending a dark-film region to cover a three-layer winding portion of each leg portion of a heater.
SUMMARY OF THE INVENTION
Cathode ray tubes employing such dark heaters have a feature in that heat can be efficiently radiated from a heater because the outer surface of the heater is darkened and thereby heat radiation efficiency of the surface of the heater is increased, and consequently, their reliability can be improved.
However, the prior art structure shown in
FIG. 5
, or the techniques disclosed in the above-cited references are not sufficient for preventing the leakage current between the heater and the cathode. In an automatic cutoff-voltage control circuit for controlling a cathode current to a predetermined value and used in a color television receiver or a display monitor, the leakage current between the heater and the cathode is superposed on the cathode current. Consequently, there is a problem in that, if the predetermined value of the cathode current in the color television receiver or the display monitor is not sufficiently large compared with a value of the leakage current between the heater and the cathode, the automatic cutoff-voltage control circuit cannot control the cutoff voltages of the electron beams for three colors of red, green and blue, a balance among the three colors is lost such that white balance is not obtained, the automatic cutoff-voltage control circuit is inoperable and the adjustment of the receiver or the monitor becomes difficult.
If the leakage current between the heater and the cathode begins to flow, the alumina film serving as a heater insulating film is heated by the leakage current, oxygen escapes from the alumina due to the heat, and electrical conductivity occurs in the oxygen-deficient alumina (Al
2
O
2.99
). As a result, there are various problems, and the heater is sometimes broken by a further increase in the leakage current, and therefore it is important in view of ensuring reliability of a cathode ray tube to prevent the leakage current between the heater and the cathode.
The following two causes are confirmed for occurrence of the leakage current between the heater and the cathode.
As for a first one of the two causes, it was found out that, in cathode ray tubes rejected for the leakage current between the heater and the cathode, many insulating films
55
b
which should otherwise be white have turned gray. The analysis confirmed that the cause of this coloration is tungsten.
Tungsten present within a cathode ray tube is used in the heating wire
55
a
of the heater
55
and the above-mentioned coating film
55
c
. If the two are compared with each other, tungsten contained in the coating film
55
c
is of a small powder size of about 1.0 &mgr;m in diameter, and is chemically active compared with the heating wire
55
a.
The degree of vacuum of the cathode ray tube is poorest immediately after flashing of getters in the manufacturing step, that is, about 10
−2
Pa. After flashing of the getters, decomposition of residual gases within the tube by an electron beam and adsorption of the residual gases by the getter film provide the ulti
Iwamura Norio
Komiya Toshifumi
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