[0001] The present invention relates to a thermionic cathode.
[0002] A known thermionic cathode comprises a porous body of tungsten impregnated with Barium
Calcium Aluminate and coated with Osmium over its entire surface. The coating may
be 0.2 microns thick.
[0003] According to one aspect of the present invention there is provided a thermionic cathode
comprising:
a porous body of refractory metal selected from the group consisting of Tungsten and
Molybdenum and alloys thereof;
a material, dispersed within the body, constituted by a mixture of Barium Oxide, an
oxide of an alkaline earth metal other than Barium and at least one oxide selected
from the group consisting of Aluminium Oxide and Boron oxide; and a predetermined
pattern comprising a coating of a metal selected from the group consisting of Osmium,
Iridium, Ruthenium, Rhodium and Rhenium and alloys thereof on the surface of the said
body and exposed surface of the body, the predetermined pattern being such that no
portion of the surface of the coating is more than 30 micron from the exposed surface
of the body.
[0004] According to another aspect of the present invention, there is provided a method
of making a thermionic cathode comprising the step of forming, on a porous body of
refractory metal selected from the group consisting of Tungsten and Molybdenum and
alloys thereof, there being dispersed within the cathode a material constituted by
a mixture of Barium Oxide, an oxide of an alkaline earth metal other than Barium and
at least one oxide selected from the group consisting pf Aluminium Oxide and
Bor
o Oxide, a predetermined pattern comprising a coating of a metal selected from the
group consisting of Osmium, Iridium, Ruthenium, Rhodium and Rhenium and alloys thereof
on the surfa of the body and exposed surface of the body, the predetermined pattern
being such that no portion of the surface of the coatii is more than 30 micron from
the exposed surface of the body.
[0005] It will be appreciated that the cathode-of the invention t a long interface between
the coating and the exposed surface of the body. This is because the invention has
discovered that electron emission at an interface between the coating and the exposed
surface is enhanced.
[0006] The coating may have a thickness in the range 0.1 to 1.0 microns although 0.2 to
0.4 microns is preferred.
[0007] The coating may be in zones separated by exposed surfaces the body, adjacent zones
being'spaced apart at a distance in th range 1 to 20 microns. The zones may be regularly
or irregularly positioned on the body. The zones may be in the form of squares, or
strips having a width or widths in the rang 5 to 50 microns. Alternatively the coating
may be in the form of a convoluted strip, e.g. a spiral.
[0008] Embodiments of the invention are now described, by way of example, with reference
to the accompanying drawings, in which:-
Figures 1, 2 and 3 show portions of electron emitters according to the invention,
Figure 4 shows a graph useful in the description of the invention.
[0009] Figure 1 shows in schematic form a dispenser cathode form o electron emitter. A thin
walled tube 11 of refractory metal, such as molybdenum, has at one end a tablet of
refractory material, 12, such as tungsten sponge, impregnated with an alkaline earth
compound such as barium calcium aluminate. An osmium coating is applied to body 12
as a film 121. The film has hitherto been applied in a generally uniform manner with
the body 12 as a substrate. The film may be 1000 to 10,000 A thick A conductive filament,
13, heatable by the passage of electric current, is provided inside tube 11. The arrangement
of such cathodes is well-known and is not specifically described further.
[0010] It has now been found that electron emission occurs more readily at an interface
between the edge of the film and the body. Figure 4 is a graph of emission, in arbitrary
units X, against distance, plotted along a line crossing the interface. The interface
is at about the 60 micron point. The uncoated area A of refractory body, to the left
in Figure 4, has an emission of some 1 unit, while the coated area B to the right
has an emission of some 3 units. However the interface, C, has a much higher emission,
a peak of some 9 units, over a width of some 10 to 20 micron.
[0011] The reasons for the higher emission at the interface region are not known but it
is likely that in this region the rate of supply of Ba/Ba 0 actuator material to the
substrate, and the substrate composition there present, are more nearly optimum than
on either of the coated and uncoated portions.
[0012] A similar but less pronounced effect is produced on scribing a line through the osmium
film.
[0013] Thus, in accordance with the present invention, the coating is formed in a predetermined
pattern such that no portion of the surface of the coating is more than 30 micron
from an exposed surface of the body. This increases the length of interface for a
given area of substrate.
[0014] Figure 1 shows in the enlarged portion one form of osmium coating, 121, in accordance
with the invention, which increases the interface length for a given area of substrate.
The osmium film is present in elements 141, 142 some 40 microns square spaced by 10
microns from each other. These sizes and spacings are chosen with reference to measurements
such as are shown in Figure 4 which suggest that a 10 to 20 micron region at the edge
of, and probably extending into, the osmium film, is the more effective emitter. A
suitable method for the production of an osmium film in such a form is to sputter
coat or evaporate through a fine mesh. A suitable mesh is 750 threads per inch electrolytic
mesh, as used in camera tubes. Before coating the cathode surface this should be polished
to assist in good registration and the production of accurate, well-formed osmium
film elements.
[0015] Using the above technique, or similar techniques, a cathode can be produced with
a considerable proportion, in some cases over 50%, of the surface formed as the interface
region, having a film coating which is no more than 30 micron and preferably 10 to
20 micron from the uncoated surface. The film is preferably 0.2 to 0.4 microns thick.
[0016] Other suitable methods are to deposit a uniform film over the whole surface and etch
or cut away the film to leave the elements. Material may be cut away by ion-beam milling
or mechanical scribing. Masking techniques, for example photoresist masks, may also
be used.
[0017] Figures 2 and 3 show other patterns in which the film can be deposited, Figure 2
shows strips 241, 242 of osmium film 221 some 30 micron wide spaced 15 micron apart.
These could.be separate parallel strips or successive turns of a single spiral strip.
[0018] Figure 3 shows portions 341, 342 of continuous film 321 having a channel 35, which
produces the extended interface, to the substrate surface. The channel is 15 microns
wide and the portions are 40 micron wide.
[0019] The elements such as 141, 241, 341, 342 can have sizes of some 5 microns up to 50
microns across and be a strip, square, round or have irregular form. The elements
can be spaced by some 1 to 20 or more microns in a regular or irregular pattern. The
cathode surface may be plane or curved preferably into a concave form. The area of
the coating may be greater than the area of exposed surface of the tungsten body.
[0020] The coating has been described as a film but clearly particles or other forms of
material can be used to make the coating. For example the particles, some 40 to 50
micron across, could be pressed into the surface of a green tungsten compact and sintered.
The surface is then impregnated with an activator, e.g. tungsten/barium aluminate,
and may be polished if desired.
[0021] Materials other than osmium, e.g. iridium, ruthenium, rhodium, rhenium and alloys
of two or more of these five materials are also suitable using appropriate deposition
techniques. Instead of barium calcium aluminate, other materials may be used as impregnant
for the tungsten body. Such materials are disclosed in U.S. Patent 3,201,639 (Levi).
Furthermore, instead of tungsten, molybdenum may be used.
[0022] The technique described above permit the production of cathodes with emission increased
by factors of 2 or 3 for osmium. Typically "islands" of osmium should enhance output
by 30% to 90% and stripes by 60%-140% for a given area of cathode.
1. A thermionic cathode comprising:
a porous body of refractory metal selected from the group consisting of Tungsten and
Molybdenum and alloys thereof;
a material, dispersed within the body, constituted by a mixture of Barium Oxide, an
oxide of an alkaline earth metal other than Barium and at least one oxide selected
from the group consisting of Aluminium Oxide and Boron oxide; and a predetermined
pattern comprising a coating of a metal selected from the group consisting of Osmium,
Iridium, Ruthenium, Rhodium and Rhenium and alloys thereof on the surface of the said
body and exposed surface of the body, the predetermined pattern being such that no
portion of the surface of the costing is more than 30 micron from the exposed surface
of the body.
2. A cathode according to Claim 1, wherein the coating has a thickness in the range
of 0.1 to 1.0 microns.
3. A cathode according to Claim 1, wherein the coating has a thickness in the range
0.2 to 0.4 microns..
4. A cathode according to Claim 1, 2 or 3, wherein the said coating is in zones separated
by exposed surfaces of the body, adjacent zones being spaced apart at a distance in
the range 1 to 20 microns.
5. A cathode according to Claim 4, wherein the zones are in the form of strips each
having a width in the range 5 to 50 microns.
6. A cathode according to Claim 1, 2, 3 or 4, wherein the zones are in the form of
squares, the sides of each of which have a length in the range 5 to 50 microns.
7. A cathode according to Claim 4, 5 or 6, wherein the zones are arranged regularly
over the surface of the body.
8. A cathode according to Claim 4, 5 or 6, wherein the zones are arranged irregularly
over the surface of the body.
9. A cathode according to any one of claims 1 to 3, wherein the coating is in the
form of a single convoluted strip.
10. A cathode according to Claim 9, wherein the strip is in the form of a spiral.
11. A cathode according to any preceding claim wherein the coating is in the form
of a film.
12. A cathode according to any one of claims 1 to 10 wherein the coating is in particulate
form.
13. A cathode according to any preceding claim, wherein the oxide of an alkaline earth
metal other than Barium is Calcium Oxide.
14. A method of making a thermionic cathode comprising the step of forming, on a porous
body of refractory metal selected from the group consisting of Tungsten and Molybdenum
and alloys thereof, there being dispersed within the cathode a material constituted
by a mixture of Barium Oxide, an oxide of an alkali earth metal other than Barium
and at least one oxide selected from the group consisting of Aluminium Oxide and Boron
Oxide, a predetermi.ned pattern comprising
a coating of a metal selected from the group consisting of Osmium, Iridium, Ruthenium,
Rhodium and Rhenium and alloys thereof on the surface of the body, and exposed surface
of the body, the predetermined pattern being such that no portion of the surface of
the coating is more than 30 micron from an exposed surface of the body.
15. A method according to Claim 14, wherein the coating is formed in the predetermined
pattern by sputtering or evaporating through a mask defining the pattern.
16. A method according to Claim 14, wherein the coating is formed in the predetermined
pattern by coating the surface of the body and selectively removing the coating to
form the pattern.
17. A method according to Claim 14, wherein the said body is formed by compacting
and sintering powder of the said refractory metal, the said coating is formed by pressing
particles of the said metal into the surface of the body, and the said material is
dispersed within the cathode by impregnating the surface thereof with the said material.