U.S. patent number 4,675,570 [Application Number 06/851,896] was granted by the patent office on 1987-06-23 for tungsten-iridium impregnated cathode.
This patent grant is currently assigned to Varian Associates, Inc.. Invention is credited to Michael C. Green.
United States Patent |
4,675,570 |
Green |
June 23, 1987 |
Tungsten-iridium impregnated cathode
Abstract
Porous agglomerates are made from pure tungsten by sintering
fine particles together and mechanically breaking down the mass to
form some agglomerates considerably larger than the particles.
These agglomerates are mixed with fine iridium powder and sintered
to form a porous mass. The mass is machined to the cathode shapes
and impregnated with an alkaline earth aluminate. The large
agglomerates alloy with the iridium only on their outer surface.
Their pure tungsten interior provides the surfaces to reduce the
alkaline earth oxide to the metal which activates the cathode.
Inventors: |
Green; Michael C. (Palo Alto,
CA) |
Assignee: |
Varian Associates, Inc. (Palo
Alto, CA)
|
Family
ID: |
27082372 |
Appl.
No.: |
06/851,896 |
Filed: |
April 11, 1986 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
595789 |
Apr 2, 1984 |
|
|
|
|
Current U.S.
Class: |
313/346R;
252/514; 252/515; 313/346DC; 445/51 |
Current CPC
Class: |
H01J
9/04 (20130101); H01J 1/28 (20130101) |
Current International
Class: |
H01J
1/20 (20060101); H01J 1/28 (20060101); H01J
9/04 (20060101); H01J 001/14 (); H01J 019/06 () |
Field of
Search: |
;313/346R,346DC,341
;252/514,515 ;445/51 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Moore; David K.
Assistant Examiner: Wieder; K.
Attorney, Agent or Firm: Cole; Stanley Z. Fisher; Gerald M.
Warsh; Kenneth L.
Government Interests
This invention was made with Government support under Contact No.
DAAK20-81-C-0421 awarded by the Department of Defense. The
Government has certain rights in this invention.
Parent Case Text
This application is a continuation of application Ser. No. 595,789,
filed Apr. 2, 1984, now abandoned.
Claims
I claim:
1. A thermionic cathode comprising:
a porous matrix of an alloy of at least one noble metal of the
platinum group with a refractory metal of the group consisting of
tungsten, molybdenum and alloys of these,
porous agglomerates dispersed in said matrix having dimensions
large compared to the components of said matrix, said agglomerates
being composed of one of said refractory metals, and
the pores of said matrix and said agglomerates being filled with an
active material comprising at least one alkaline earth oxide.
2. The cathode of claim 1 wherein said active material further
comprises aluminum oxide.
3. The cathode of claim 1 wherein said platinum group metal is
iridium.
4. The cathode of claim 1 wherein said refractory metal is
tungsten.
5. The cathode of claim 1 further comprising a smooth surface
adapted for electron emission.
6. The cathode of claim 5 further comprising a uniform, homogeneous
layer on said smooth surface, said layer comprising said noble
metal and said refractory metal.
7. The cathode of claim 6 wherein the composition of said layer is
approximately the average composition of said matrix.
8. The cathode of claim 1 further comprising means for supporting
said cathode.
9. The cathode of claim 1 further comprising means for heating said
cathode to a temperature of about 1000.degree. C. to 1100.degree.
C.
10. A process for manufacturing a thermionic cathode comprising the
steps of:
forming a porous body by sintering together particles of a
refractory metal of the group consisting of tungsten, molybdenum
and alloys of these,
mechanically breaking down said porous body into fines and
agglomerates, said agglomerates being large compared to said
particles,
mixing said agglomerates with particles, of dimensions small
compared to said agglomerates, said particles containing at least
one noble metal of the group consisting of iridium, rhenium,
ruthenium and osmium,
compressing said mixture,
sintering said mixture to adhere said particles and said
agglomerates into a porous mass,
impregnating said mass with a molten oxide comprising an alkaline
earth oxide.
11. The process of claim 10 further comprising the steps, after
sintering said mixture, of:
impregnating said porous mass with a liquid,
converting said liquid to a solid to support said porous mass,
machining said mass to the shape of a desired cathode,
removing said solid.
12. The process of claim 11 further comprising the steps of:
machining on said cathode shape a smooth surface adapted for
electron emission,
depositing on said smooth surface a uniform, homogeneous layer
comprising said noble metal and said refractory metal.
13. The process of claim 12 wherein the composition of said layer
is approximately the average composition of said particles
containing said noble metal.
14. The process of claim 11 further comprising the step of affixing
said cathode to support means.
15. The process of claim 14 further comprising the step of
attaching to said support means a heater near said cathode capable
of heating said cathode to about 1000 to 1100 degrees Celsius.
Description
FIELD OF THE INVENTION
The invention pertains to thermionic cathodes in which a porous
body of refractory metal is impregnated with a molten oxide
containing an alkali-earth.
PRIOR ART
Early impregnated cathodes were made by sintering a body of
powdered tungsten to form a porous block. The porous material was
impregnated with molten copper or an organic polymer to make it
machinable to its desired shape. After machining this impregnant
was removed and the porous cathode was impregnated with a molten
barium aluminate. U.S. Pat. No. 2,700,000 issued Jan. 18, 1955 to
R. Levi such a cathode.
U.S. Pat. No. 3,373,307 issued Nov. 12, 1964 describes an improved
impregnated cathode in which the emissive surface is coated with
iridium or other metals of its group. The coating improves the
electron emission, but it has been found that the improvement is
often short-lived. A principal problem seems to be that, in an
electron tube where high current density is drawn from the cathode
at high voltage, ions are formed from the residual gas. They are
accelerated back to the cathode and sputter away a thin layer from
its surface, removing the iridium.
U.S. Pat. No. 4,165,473 issued Aug. 21, 1979 to Louis R. Falce and
assigned to the assignee of the present invention, discloses a
cathode in which particles of iridium or the like are dispersed
among the tungsten particles of the matrix. During sintering the
iridium partially alloys with the tungsten. This dispersed cathode
solved the problem of surface sputtering. It has been found,
however, that the sintering is a very delicate process. If the time
and temperature are enough to get a lot of alloying, the emission
is often poor. If the sintering is held to a minimum, the emission
is initially good, but interdiffusion of iridium and tungsten
occurs at operating temperature to form unreactive alloy. This in
turn causes the barium supply to the surface to fall off with a
resultant decay in emission. Also, shrinkage of the cathode button
can take place with the distortion of the emitting surface, which
impacts adversely on the electron optics of the gun.
PURPOSE OF THE INVENTION
An object of the invention is to provide a cathode with improved
emission over a long life span.
Another object is to provide a cathode which is tolerant of the
exact parameters of manufacture and operation.
These objects are realized by forming the porous matrix of a mix of
two kinds of grains: small particles of tungsten-iridium alloy, and
large grains composed of a porous matrix of pure tungsten.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic enlarged cross-section of a portion of a
cathode embodying the invention.
FIG. 2 is a cross-section of a concave cathode for a linear-beam
microwave tube.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As a basis for the physical form of my inventive cathode, following
is a brief description of my concept of the operation of a
dispenser cathode. Basically there are two requirements which in
prior-art cathodes were often at odds.
First, an emitting surface is required which has a low
work-function. This is provided by a thin (sometimes monatomic)
layer of an alkaline earth metal such as barium, strontium, calcium
or mixtures thereof and often containing oxygen as well.
The second requirement is a means for replenishing the active layer
as it is removed in operation by evaporation or sputtering.
In the following description I use the word "tungsten" to include a
number of refractory, moderately chemically active metals such as
molybdenum. I use the word "barium" as an example of an alkaline
earth metal or compound, which may additionally include calcium,
strontium, and alloys thereof as well.
In the original tungsten matrix cathodes the emitting layer is on
the surface of the porous tungsten. It has a moderately low work
function and thus gives emission capability of a few amperes per
square centimeter. The life is very good however, because the
surfaces of the tungsten matrix in contact with the barium
aluminate impregnant are chemically reducing at the operating
temperature of around 1000.degree. C., sufficient to react with the
oxide and produce free barium atoms. This barium can then be
transported to the active surface to re-activate it as fast as
surface material was removed.
The addition of a surface layer of iridium or other metals of the
platinum group produces a significant further lowering of the work
function and hence higher emission density. The work function is
believed to be affected by the extent to which the surface metal
can polarize the barium dipole layer. The iridium provides tighter
bonding of the barium atoms, reducing evaporation as well as work
function. Iridium, however, has low reducing power. When it is
added to the matrix as taught by U.S. Pat. No. 4,165,473 it alloys
with the tungsten, perhaps more as a surface coating than a bulk
alloy. This will decrease the reducing power of the tungsten and
slow the replenishment of lost barium.
In carrying out my invention as shown in FIG. 1, I provide
relatively large, porous "islands" of pure tungsten in a matrix 10
of tungsten-iridium alloy particles 12. The iridium provides a
surface 20 which can be activated to have a low work function. Even
when surface material is removed, the surface is regenerated. The
pure tungsten islands 14 are porous grains, each formed from many
fine tungsten particles 16 sintered together. Their large surface
area provides a reducing interface with the impregnant 18 to
produce an adequate supply of reactivating barium to the emitting
surface 20. The size and convolutions of the tungsten islands 14
are sufficient to prevent alloying with the iridium except on their
outer surfaces.
As an example of the process for producing the inventive cathode,
the following steps are performed:
1. Porous tungsten bar stock is made by compressing and sintering
fine tungsten powder particles of about 5 microns diameter, as is
well known in the art. The density of the resulting bar is about
72%, of that of solid tungsten.
2. The bar is impregnated with a liquid plastic monomer such as
methyl methacrylate which is then polymerized by heat as described
in U.S. Pat. No. 3,076,916 issued Feb. 5, 1963 to Otto G. Koppius.
The bar is broken down into a mixture of agglomerates by machining,
such as turning on a lathe.
3. The plastic infiltrant is removed from the agglomerates and any
carbon residues are cleaned up by firing in wet hydrogen at
750.degree. C.
4. Agglomerates larger than 150 microns are sieved out and
discarded.
5. The tungsten agglomerates and fines which pass through a 100
mesh sieve are tumble mixed with -325 mesh iridium powder in the
proportion of 60 parts by weight of tungsten to 40 parts of
iridium.
6. The powder mix is pressed at 50,000 psi and the resultant
compact sintered in hydrogen at 1720.degree. C. to give pure
tungsten agglomerates dispersed n a matrix of iridium-tungsten
alloy. Most of the agglomerates are large compared to the iridium
particles. The tungsten fines alloy with the iridium particles.
7. The sintered body is then manufactured into cathode elements by
conventional techniques:
7a. Molten copper is infiltrated into the pores to provide support
for machining. 7b. The cathode shapes are machined from the
copper-infiltrated bar. 7c. The copper is removed by chemical
etching and hydrogen firing. 7d. The cathode elements are
impregnated in hydrogen or vacuum with barium-calcium aluminate,
typically 6BaO:1CaO:2Al.sub.2 O.sub.3.
8. The emissive surface may be sputter coated with a codeposited
50:50 mixture of tungsten and iridium. This is nearly the same
composition as the iridium-tungsten alloy mixture, so its removal
by ion sputtering does not seriously affect the cathode.
This embodiment of the invention has been found to provide emission
current densities equal or superior to, and lives exceeding those
of, the best examples of the prior art. However, unlike the prior
art, this cathode can be reproducibly manufactured. Cathodes
capable of 8 amperes per square centimeter below 1050.degree. C.
brightness have been produced with greater than 90% yield. Running
temperatures for a given current density were within 10.degree. C.
of each other. The performance was very stable with operating time.
Cathodes have passed 4,000 hours at 8 A/cm.sup.2 with practically
no change.
The large agglomerates alloy with the iridium during sintering and
subsequent operation, but due to their size the alloying occurs
only at their outer surfaces. They are porous and are completely
infiltrated by the active oxide so that the reduction to produce
barium goes on freely in their interior.
FIG. 2 illustrates the incorporation of the emitting element (the
"cathode" proper) in a cathode structure as used in a linear-beam
microwave tube. Cathode 10' is machined to have a smooth concave
emitting surface 20' (usually spherical). Its base is fitted onto a
cylindrical support 22, as of molybdenum or tantalum and attached
thereto as by welding at junction 23. A radiant heater 24, as of
tungsten wire in a bifilar spiral is supported by its legs 25 by
the support means (not shown) of cylinder 22.
It will be obvious to those skilled in the art that many variations
of the above-described cathode and process of production can be
made within the true scope of the invention. The proportions of the
various components can cover a wide range. For example, I believe
the ratio of iridium to tungsten may vary from about 20% to about
80%. The "barium" may also include calcium, and/or strontium, or
mixtures thereof. The "tungsten" may be molybdenum, tungsten, or
their alloys. The "iridium" may be osmium, ruthenium, rhenium,
iridium or alloys thereof.
The particular embodiments described above are illustrative and not
intended to be limiting. The invention is to be limited only by the
following claims and their legal equivalents.
* * * * *