U.S. patent number 3,928,783 [Application Number 05/423,107] was granted by the patent office on 1975-12-23 for thermionic cathode heated by electron bombardment.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Satoru Fukuhara, Shigeru Hosoki, Michio Ohtsuka.
United States Patent |
3,928,783 |
Hosoki , et al. |
December 23, 1975 |
Thermionic cathode heated by electron bombardment
Abstract
A structure wherein a cathode, which is made of a material, such
as lanthanum hexa-borides, prone to react with metals at high
temperatures and having a high electron emissivity, is held by a
supporter which is made of an electrically insulating material.
Concentric metal cylinders are arranged at the outer circumference
of the supporter to surround a part of the cathode and a heating
coil is arranged in the interstice between the metal cylinders. An
electron-emissive metal oxide layer is formed on the inside surface
of the inner metal cylinder. Thermions created from the oxide layer
strike the cathode, and the cathode is heated by the heat of the
electron bombardment.
Inventors: |
Hosoki; Shigeru (Hachioji,
JA), Ohtsuka; Michio (Hachioji, JA),
Fukuhara; Satoru (Kokubunji, JA) |
Assignee: |
Hitachi, Ltd.
(JA)
|
Family
ID: |
14837418 |
Appl.
No.: |
05/423,107 |
Filed: |
December 10, 1973 |
Foreign Application Priority Data
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|
|
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Dec 8, 1972 [JA] |
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47-122502 |
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Current U.S.
Class: |
313/270; 313/339;
313/305; 313/346R |
Current CPC
Class: |
H01J
1/20 (20130101) |
Current International
Class: |
H01J
1/20 (20060101); H01J 001/94 (); H01J 001/20 () |
Field of
Search: |
;313/337,340,346,339,46,305,338,347,270,90,464 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Lafferty, J. M., "Boride Cathodes," Jr. of Applied Physics, Vol.
27, 3-1951, pp. 299-309..
|
Primary Examiner: Smith; Alfred E.
Assistant Examiner: Punter; Wm. H.
Attorney, Agent or Firm: Craig & Antonelli
Claims
We claim:
1. A thermionic cathode structure comprising:
a cathode made of a material having a high electron emissivity;
a supporter made of an electrically-insulating and high
temperature-resistant material for supporting a part of said
cathode;
a first cylinder held in contact with said supporter and arranged
so as to surround a part of said cathode;
a second cylinder arranged concentrically with said first
cylinder;
a cathode heating coil arranged in an interstitial space between
said first and second cylinders;
an electron-emissive metal oxide layer formed on an inside surface
of said first cylinder;
flange means, extending from said first cylinder toward said
cathode past said oxide layer, for preventing electrons emitted
from said oxide layer from being mixed with electrons emitted from
a part of said cathode;
a power source connected to said coil for supplying heating power
to said coil; and
a power source connected between said cathode and said first
cylinder in order to cause electrons, emitted from said oxide
layer, to strike against said cathode.
2. The structure according to claim 1, wherein said cathode is made
of a boride.
3. The structure according to claim 1, wherein said oxide layer is
made of at least one member selected from the group consisting of
barium oxide, strontium oxide and calcium oxide.
4. In a thermionic cathode structure having a cathode;
an insulating body for insulatingly supporting said cathode;
and
a heating coil, disposed adjacent said cathode;
the improvement comprising:
means, disposed between said heating coil and said cathode and
surrounding at least part of said cathode, for emitting thermions
which impinge upon said cathode to heat said cathode, in response
to the heating of said heating coil, wherein said means comprises a
first cylinder supported by said insulating body and surrounding
said cathode and a layer of electron-emissive material coated on
the interior surface of said fist cylinder facing said cathode;
and
a flange extending from said first cylinder toward said cathode
past said layer of electron-emissive material, said
electron-emissive layer being confined between said flange and said
insulating body.
5. The improvement according to claim 4, wherein said means
comprises a wall made of an electron-emissive material which, when
heated by said coil, emits thermions which impinge upon said
cathode to heat said cathode.
6. The improvement according to claim 4, wherein said means further
includes a second cylinder surrounding said coil, with said coil
disposed between said second cylinder and said first cylinder.
7. The improvement according to claim 6, wherein said means further
includes a first power source for applying a potential between said
first cylinder and said cathode and a second power source for
applying a heating current to said heating coil.
8. The improvement according to claim 4, wherein said layer of
electron-emissive material is made of a material selected from the
group consisting of a metal oxide, a sintered body, and a porous
metal-impregnated layer.
9. The improvement according to claim 8, wherein said metal oxide
is made of at least one material selected from the group consisting
of barium oxide, strontium oxide and calcium oxide.
10. The improvement according to claim 4, wherein said cathode is
made of a boride.
11. The improvement according to claim 4, further including a layer
of graphite disposed between said insulating body and said
cathode.
12. The improvement according to claim 4, further including an
evaporation preventing plate disposed between said insulating body
and said electron-emissive wall.
13. The improvement according to claim 4, wherein said insulating
body has an extended portion defining a shade area between said
wall and said cathode to prevent the formation of an electrically
conductive film between said wall and said cathode.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a heating device for a cathode in
an electron tube and, more particularly, to a heating device for a
cathode which is made of a material, such as lanthanum hexa-borides
(LaB.sub.6) and yttrium hexa-borides (YB.sub.6), prone to react
with metals at high temperatures.
Description of the Prior Art
In general, borides such as lanthanum hexa-borides (LaB.sub.6) and
yttrium hexa-borides (YB.sub.6) have a small work function, and are
suitable as cathode materials. However, because they are liable to
react with metals at high temperatures, it has been difficult to
heat these borides to working temperatures of approximately
1,300.degree.-1,800.degree.C. During the heating of the cathode,
the direct heating type requires high power. Indirect heating type
is, therefore, effective in reducing power comsumption to as low a
value as possible.
FIG. 1 is a schematic diagram showing an example of a prior-art
cathode heating device, which is constructed such that a heating
coil 2, made of tungsten or the like, is held in a space
surrounding cathode 1, made of lanthanum hexa-boride (LaB.sub.6)
and heating power is supplied from a power source 3 to the coil 2.
Between the cathode 1 and the heating coil 2, an accelerating power
source 4 is connected for electron bombardment.
With such a construction, when the coil 2 is heated, the cathode 1
is heated by the radiant heat. Simultaneously therewith, thermions
emitted from the coil 2 are drawn to the cathode 1 by the voltage
of the accelerating power source 4, and the cathode 1 is heated by
heat which is generated by the electron bombardment. In order to
increase the temperature of the cathode 1 to working temperature
the heating coil 2 must be heated to a temperature of at least
about 2,500.degree.-2,800.degree.C when employing a tungsten wire.
However, when the coil 2 is heated to the above-mentioned
temperature, heat losses due to thermal conduction from the leads
at both ends of the coil 2 to the exterior and the heat loss due to
the thermal radiation from the coil 2 become so great as not to be
negligible.
FIG. 2 shows a sketch for roughly estimating the heat losses due to
thermal conduction and due to thermal radiation. Referring to the
figure, leads 5 of stainless steel are connected to both ends of
the heating coil 2 of tungsten in order to diminish the loss due to
the thermal conduction. Now, let T.sub.3 denote the temperature of
the central part of the heating coil 2 which is uniform, T.sub.2
denote the temperature of the point of contact between the coil 2
and the lead 5, and T.sub.1 denote the temperature of the end of
the lead 5 remote from the coil 2. It is assumed that the diameter
of the coil 2 is 0.02 cm, that the length of the coil extended in a
straight line is 2.0 cm, that the sectional area of the lead 5 is
4.45 .times. 10 .sup..sup.-2 cm.sup.2, that T.sub.3 =
2,800.degree.C, that T.sub.2 = 1,500.degree.C and that T.sub.1 =
1,000.degree.C. When, although not shown in the figure, a heat
shielding plate is provided around the coil 2, the temperature of
the heat shielding plate is assumed to be 1,000.degree.C. Then, the
radiation heat loss and the condition heat loss can be respectively
calculated to be approximately 40 W and 20 W.
In this manner, where the coil 2 is heated to a high temperature,
the heat loss due to thermal radiation becomes very large, since it
is proportional to the fourth power of the temperature in
accordance with Boltzmann's law. When mounting a plurality of heat
shielding plates in the space surrounding the coil 2 in the form of
concentric cylinders, the radiation heat loss can be reduced. In
this case, however, the heat capacity of the shielding plate
assembly itself becomes large, and the time required for increasing
the cathode temperature becomes large, resulting in the
disadvantage that the heating device is very difficult to use.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a device for
heating a cathode made of a material such as lanthanum hexa-borides
(LaB.sub.6), in which the heat loss by the thermal radiation is
small and the heating efficiency is high.
In order to accomplish this object, the present invention includes
a heating coil provided within an indirect heating case and an
electron-emissive metal oxide layer formed on the surface of the
indirect heating case facing to a cathode.
The other objects and features of the present invention will be
apparent from the following detailed description when read in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view depicting a prior-art heating
arrangement;
FIG. 2 is a schematic view for calculating losses due to thermal
radiation and thermal conduction from a heating coil;
FIG. 3 is a constructional view of an embodiment of the present
invention;
FIG. 4 is a schematic view for calculating thermal conduction;
FIG. 5 is a structural view of a cathode;
FIG. 6 is a constructional view showing another embodiment of the
present invention;
FIG. 7 is a sectional view of a portion of still another embodiment
of the present invention;
FIGS. 8a to 8d are sectional views of supporters for use in the
present invention; and
FIG. 9 is a structural view in the case where the heating device of
the present invention is applied to an actual cathode.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 3, a cathode 1 made of a boride material such as
lanthanum hexa-boride (LaB.sub.6) and yttrium hexa-boride
(YB.sub.6) is positioned in a hollow core portion of a supporter 6
made of a high temperature-resistant material, such as boron
nitride (BN), which is electrically insulating. Outside the
supporter 6, there are arranged concentric cylinder bodies 10a and
10b made of nickel or the like. The end parts of the cylinders on
one side are connected by a metal sheet 10c. Formed on the interior
surface of the cylinder 10b is an electron-emissive wall made of a
coating 9 which is made of an electron-emissive metal oxide such as
barium oxide (BaO), calcium oxide (CaO) and strontium oxide (SrO).
An electrode 7 of graphite or the like is mounted on one end of the
cathode 1, and is connected through a lead 8 to the positive
terminal of an electron accelerating power source 4 for electron
bombardment. The negative terminal of the power source 4 is
connected to the cylinder 10a. A heating coil 2 of tungsten or the
like insulated by an alumina coating layer is arranged between the
concentric cylinders 10a and 10b, and is supplied with heating
power from a power source 3.
With such a construction, when current flows through the coil 2 by
means of the heating power source 3 and the coil 2 is thus heated,
the cylinders 10a and 10b are also heated. As a consequence, the
oxide layer 9 is subjected to the indirect heating. When the oxide
layer 9 is heated to approximately 800.degree.C, thermions are
emitted. Since the cathode 1 is applied with a positive potential
through the electrode 7, which is made of graphite or the like
material and does not readily react with the cathode at high
temperatures the thermions emitted from the oxide layer 9 are
attracted toward the cathode 1 and impinge thereon. The cathode 1
is then heated to approximately 1,300.degree.-1,800.degree.C by the
heat of the electron bombardment. Since, in this case, the cylinder
10b has a temperature lower than the cathode 1, heat losses due to
thermal conduction and thermal radiation from the cathode 1 are
mostly fed-back to the cylinder 10b as is apparent from the
construction shown in the figure. Accordingly, once the cylinders
10a and 10b have been heated, the power required for maintaining
them at 800.degree.C may be very slight. Heat losses due to thermal
radiation from the cylinder 10a are extremely small, because the
temperature of this cylinder is as low as 800.degree.C.
As explained above, according to the cathode heating device of the
present invention, the cathode 1 is not heated directly by the
radiant heat of the heating coil 2, but is heated by the electron
bombardment heat in such way that the metal cylinders 10a and 10b
are heated and that thermions emitted from the oxide layer 9 formed
on the inside surface of the cylinder 10b are accelerated and
impinge upon the cathode 1, so that the cathode 1 can be heated to
a desired high temperature in the state in which the metal
cylinders 10a and 10b are at a temperature lower than that of the
cathode 1. From the heating by the electron bombardment heat, the
heat loss components are those due to thermal conduction from the
cathode 1 through the supporter 6 to the cylinder 10b and these
directly escaping due to thermal radiation from the cathode 1.
Since both these heat loss components are fed-back to the cylinder
10b surrounding the cathode, there is essentially no heat-loss, and
a highly efficient heating can be effected.
In the foregoing embodiment, the material of the cylinders 10a and
10b is not restricted to nickel, but may be any metal having
deoxidizing properties at a high temperature of about 800.degree.C.
The coating layer is not restricted to electron-emissive metal
oxide layer 9 but a layer of a sintered body or an impregnation
layer of a porous metal may be adopted, insofar as it has an
electron emissivity equivalent to that of the coating layer 9.
Although the disadvantages of the prior-art device have been
eliminated by the device of the embodiment shown in FIG. 3, even
the improved device has a few problems. One of them is that as the
loss heat from the cathode 1 is effectively fed-back to the
cylinder 10b, the temperature of the metal oxide layer 9 increases
more than is necessary, with the result that the deterioration of
the oxide layer 9 is hastened.
The excessive increase in the temperature of the oxide layer 9 in
FIG. 3 is attributable to the fact that the thermal feedback from
the cathode 1 is too great. As already explained, the causes for
the thermal feedback are 1 -- the thermal radiation from the
cathode 1 and 2 -- the thermal conduction through the supporter 6.
Rough estimates will be hereunder explained for both mechanisms of
thermal feedback.
The desired temperatures are approximately 1,500.degree.C for the
cathode 1 made of LaB or the like and approximately 800.degree.C
for the oxide layer 9. For the sake of simplicity, therefore, those
quantities of heat flow, for the respective heat transfer
mechanisms when the temperature difference between the specified
values is assumed may be determined.
FIG. 4 indicates the dimensions necessary for such calculations.
Let Qr be the quantity of heat which is imparted from the cathode 1
to the oxide layer 9 (the temperature of which is equal to that of
the cylinder 10b) by thermal radiation, and Qc be the quantity of
heat which is imparted through the supporter 6 by thermal
conduction. T.sub.10 and T.sub.20 are the temperatures of the
cathode 1 and the oxide layer 9 respectively. .epsilon. is the
emissivity, .sigma. is Stephen-Boltzmann's constant, and K the
coefficient of heat transfer of the supporter 6. Then, the
equations of heat transfer due to thermal conduction and radiation
are respectively given as follows: ##EQU1##
Now let T.sub.10 = 1,500.degree.C, T.sub.20 = 800.degree.C,
.epsilon.= 0.5 and .sigma.= 5.67 .times. 10.sup..sup.-12 Joule /sec
- cm.sup.2 - .degree.K.sup.4 . When employing boron nitride for the
supporter 6, the coefficient of heat transfer becomes K = 0.63
Joule/cm-sec-.degree.C. The dimensions in FIG. 4 are d.sub.1 = 0.1
cm, d.sub.2 = 0.4 cm, l.sub.1 = 0.2 cm and l.sub.2 = 0.3 cm. Then,
there are obtained:
It is understood that for the dimensions d.sub.1, d.sub.2, l.sub.1
and l.sub.2 given above, thermal conduction through the supporter 6
is very great when using boron nitride.
When the temperature difference between the cathode 1 and the oxide
layer 9 is to be kept at about 700.degree.C, a calculated value of
power corresponding to the heat loss component (about 400 Watts)
must be applied to the cathode 1. In actuality, thermal contact
resistances exist between the cathode 1 and the supporter 6 and
between the supporter 6 and the cylinder 10b, respectively, and the
coefficient of heat transfer K apparently becomes smaller by one
order or so. The power to be applied to the cathode 1 is, actually,
less than 10 Watts. Under this condition, the temperature
difference of 700.degree.C is not established, and the temperature
of the oxide layer 9 of approximately 1,300.degree.C has been
observed when the temperature of the cathode 1 is 1,500.degree.C.
Consequently, if a temperature difference of 700.degree.C is to be
maintained without changing the dimensions, an insulating material
having a smaller coefficient of heat transfer K by approximately
one order must be employed. (According to calculations, the heat
loss is about 400 W when employing boron nitride for the supporter
6, and hence, the coefficient of heat transfer must be decreased by
two orders for restraining the loss to less than 10 Watts. As
previously explained, however, thermal contact resistances exist
between the adjacent ones of the cathode 1 -- supporter 6--
cylinder 10b, and hence, the decrease of the coefficient of heat
transfer by approximately one order suffices. ) In this regard,
alumina has a coefficient of heat transfer of K = 0.07 Joule/ sec -
cm - .degree.C (at 800.degree.C), which is about one order smaller
than the coefficient of heat transfer of boron nitride. With a
cathode of the above dimensions, therefore, the use of alumina is
preferable to boron nitride. According to experiments, the value
700.degree.C has been observed as the temperature difference
between the cathode 1 and the oxide layer 9, and the life of the
oxide layer 9 has been extended.
Where the supporter 6 is specified beforehand, the desired
temperature difference can be established by appropriately
determining the dimensions d.sub.1, d.sub.2, l.sub.1 and l.sub.2 or
by making the areas and shapes of the contact surfaces between the
cathode 1 and the supporter 6 and between the supporter 6 and the
cylinder 10b different.
The power consumption of the cathode can thus be minimized by
selecting the insulating material of the supporter 6 in dependence
on the shape and dimensions of the cathode 1 with reference to
equations (1) and (2) for Qc and Qr respectively.
Where the cathode 1 is heated to a temperature (for example,
2000.degree.C) considerably higher than the usual working
temperature, the temperature of the contact part between the
supporter 6 and the cathode 1 increases, and both members
chemically react with each other in some cases. In that event, a
layer 11 made of graphite powder or a sintered body thereof may be
formed between the cathode 1 and the supporter 6, as shown in FIG.
5.
Another problem of the embodiment in FIG. 3 is that when the
cathode 1 is used at high temperatures for a long period of time, a
thin film which is electrically conductive is formed on the surface
of the supporter 6 of the high temperature-resistant insulating
material by the vaporization of the cathode material such as
LaB.sub.6, resulting in a degradation of the insulation between the
cathode 1 and the oxide layer 9.
In order to solve such a problem, an evaporation preventing plate
12 may be provided in proximity to the supporter 6 as illustrated
in FIG. 6. Since the evaporation preventing plate 12 functions as a
mask for the vaporization of the cathode material and prevents the
material from adhering to the surface of the supporter 6, there is
good electrical insulation between the cathode 1 and the oxide
layer 9 for a long period of time.
Even when a supporter 6' of a shape as shown in FIG. 7 is employed
instead of the provision of the preventing plate 12, the same
effect is achieved. An part 6a of the supporter 6' becomes the
so-called shade portion with respect to the cathode 1, and the
cathode material such as LaB.sub.6 cannot be readily deposited on
the portion. Even when supporters 6' of shapes illustrated in FIGS.
8a to 8d are used, the same effect is achieved. Any of the shapes
has an extended portion which defines a shade with respect to the
cathode 1 and on which LaB.sub.6, or the like cannot be readily
deposited, so that there is good electrical insulation between the
cathode 1 and the oxide layer 9 is even during the use of the
cathode for a long period of time.
FIG. 9 shows an overall concrete structure of the heating device of
the present invention. In the figure, the same constituent parts as
in FIG. 3 are assigned with the same symbols. The aperture of the
metal sheet 10c for connecting the end parts of the cylindrical
bodies 10a and 10b is made smaller than the inside diameter of the
cylindrical oxide layer 9. This serves to prevent thermions for the
electron bombardment, emitted from the oxide layer 9, from being
mixed into the thermions which are emitted from the cathode 1
towards the opening portion of a Wehnelt electrode 16 or a grid
electrode. The heating coil 2 has power supplied thereto through
lead wires from a power source (not shown) which is disposed
outside a cathode base 14 of glass or the like. The cylinder 10a is
connected to electrode terminals by lead wires 13.
While we have shown and described several embodiments in accordance
with the present invention, it is understood that the same is not
limited thereto but is susceptible of numerous changes and
modifications as known to those skilled in the art and we,
therefore, do not wish to be limited to the details shown and
described herein but intend to cover all such changes and
modifications as are encompassed by the scope of the appended
claims.
* * * * *