CA1159722A - Pellet of alkaline earth metal oxide impregnated with a solid, vaporizable organic protective material - Google Patents

Abstract

Abstract A dispenser cathode is fabricated by covering a reservoir of electron emitting material with a perforated metal foil having an appropriate pattern of pore-sized apertures thereon for providing uniform electron emission from the cathode surface. The electron emitting material is in the form of a pellet of barium oxide impregnated with a wax or resinous material to minimize chemical reduction of the barium oxide in air. The impregnated barium oxide pellet is sandwiched between the apertured foil and a support structure to which the foil is welded.
During tube bake-out or subsequently during cathode activation, the wax or resinous material evaporates and barium oxide migrates through the apertures to cover the surface of the foil in a uniform manner. The barium oxide pellet is obtained from the carbonate and is wax impregnated prior to assembly into the cathode. Thus no carbonate is present at the tube bake out or cathode activa-tion stages, thereby avoiding the carburization of electron emitting metal surfaces.

Description

- ~ lS9722 BACKGROUND OF THE INVENTION

2 This invention is a further developm~nt in the

3 fabrication of dispenser cathodes, which find application

4 qenerally in microwave tubes and linear beam devices.
Heretofore, the emitting surfaces of dispenser cathodes 6 have been made either from porous metal matrices whose pores 7 were filled with electron emitting material, or from porous 8 metal plugs covering reservoirs of electron emitting 9 material.
The porous metal bodies of prior art dispenser cathodes, 11 whether they were matrices filled with electron emitting 12 material or porous plugs covering reservoirs of electron 13 emitting material, did not have consistently uniform pore 14 size, pore length, or spacing between pores on the surface.
As a consequence, dispenser cathodes of the prior art 16 tended to exhibit non-uniform electron emission from their 17 surfaces.
18 U.S. Patent 4,101,800 ~issued July 18, 1978) described 19 a dispenser cathode comprising a reservoir of electron emitting material covered by a perforated metal foil. The 21 ~ pattern of perforations was such as to permit ~igration 22 of electron-emitting ~aterial from the reservoir to the 23 foil surface in such a way as to coat the surface uniformly, ~4 thereby providing a cathode surface of substantially uniform emissivity. The prior art has not, however, developed a 26 practicable method for fabricating a perforated metal foil 27 of the kind disclosed in U.S. Patent 4,101,800. Consequently, 28 the production of dispenser cathodes having uniform surface 29 porosity has not heretofore been commercially ~easible.
3~ A further problem with ~uch prior techn~ques h~s 31 arlsen from the conventional use of a bariu~ carbonate 32 ~tarting ~aterial during fabrication, and ~ts subsequent 33 conversion into the barium oxide electron emitting ~terial 34 duri-ng cathode activation by heating. The carbonate thereupon converts to the oxide, glving off carbon dioxide gas. ~his 36 results in the carborization or ox~datlon of the electron ~3~ ~

emittiny ~etal ~urfaces; further, ~uch carbori~ed or oxidized ~urfaces then reduce the active barium oxide into elemental barium, which evaporates at tube operating temperatures.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for fabricating a dispenser cathode having a ..... . .

, 1 uniform surface porosity, whereby uniform electron emission 2 from the surface can be achieved.
3 It is a concomitant object of this invention to provide 4 a method for controlling the porosity of the surface of a dispenser cathode in order to provide a surface of uniform 6 electron emission.
7 ~t is a further object of this invention to fabricate 8 a dispenser cathode having uniform surface porosity by a 9 method that minimizes carburization and oxidation of the cathode surface.
11 It is a particular object of the present invention 12 to provide a method for fabricating a dispenser cathode 13 having uniform pattern of electron emission from its surface 14 by sandwiching a reservoir of electron emitting material in oxide between an apertured foil and a supporting structure using 16 a bonding tQchniques, ~i~imizing carburizat~on of the 17 emitting surface.
18 In order to accomplish the aforementioned objects of 19 this invention, a quantity of material having a low work function (e.g., barium oxide) is placed on a supporting 21 structure, and a thin foil of refractory or platinum-group 22 metal having a desired pattern of uniformly sized and 23 evenly distributed apertures is placed on the support 24 structure so as to cover the barium oxide. The foil is bonded to the support structure by laser welding so as to 26 localize the heating effects due to the bonding process.
27 In order to prevent chemical reaction of the barium oxide ~8 with moisture in the air during fabrication of the cathode, 29 a specially treated pellet of barium oxide is used. This barium oxide pellet is formed by heating a solid pellet 31 of barium carbonate in a vacuum to liberate carbon dioxide, ~2 thereby leaving a porous pellet of barium oxide. m e porous 3jjmll2278 ' - 4 - 78-51 .. .. .. . .. . . .. ..... .. . .

pellet of barium oxide is then impregnated with a wax, 2 or ~ith a resinous ~aterial such as methyl methacrylate 3 or nitrocellulose, to provide a protective coating over 4 the barium oxide. Without such a protective coating, rapid S chemical reduction of the barium oxide to barium hydroxide 6 would occur. Barium hydroxide is not usable as an electron 7 emitting material.
8 In the fabrication of a dispenser ca~hode according to 9 the present invention, a pellet of barium oxide impregnated with a wax or a resinous material to prevent ~ny rapid 11 chemical reaction in air is placed on the surface of a metal 2 ~upporting member of the cathode structure. Ihe apertured 13 foil is placed over the barium oxide pellet, and is then 14 welded to the metal supporting member. A laser welding lS technique is preferred because laser welding can be 16 accomplished in areas of limited access, and effectively 17 localizes the heating effects of the welding process.
18 The heat generated during tube bake-out and processing, 19 or during cathode activation, causes the wax or resinous protective material to evaporate from the barium oxide 21 pellet.
22 The apertured metal foil of uniform pore size and 23 - distribution according to this invention may be obtained 24 by a photolithographic technique whereby a pattern of holes is chemically etched in a foil of a refractory 26 metal such as tungsten or molybdenum. After the holes 27 have been formed, the foil is then coated with iridium, 28 osmium or some other platinum-group metal. Typically, 29 the tungsten or molybdenum foil is O.OOl inch thic~, and the coating thereon is ab~ut one micron thic~.
31 Alternatively, a foil having a desired pattern of 32 uniformly sized and distributed pores according to the 3jjmll2278 ' - 5 - 78-Sl 1 present invention could be produced by deposition of a 2 layer of a platinum-group metal onto a substrate having 3 an array of appropriate dimensioned and spaced posts 4 projecting therefrom. ~fter the layer of metal has been deposited upon such a substrate, typically to a thickn~ss 6 in the range from 0.0005 inch to 0.0015, the substrate 7 with its projecting posts is removed either by chemical 8 etching or by evaporation~ Deposition of the platinum-9 group metal layer onto the substrate could be accomplished by chemical vapor deposition, sputter deposition, electro-11 plating or evaporation. Such techniques are well known 2 to those skilled in the art. Alternatively, the metal 13 layer could be formed by rolling fine particles of the 14 metal (i.e, particles less than one micron in diameter) onto the substrate and subsequently sintering the articles 16 to form a porous layer. The substrate could be made of 17 any material amenable to photoetching or subsequent 18 evaporation, whereby the posts could be formed by a photo-19 etching process, and whereby the entire substrate with its projecting posts could subsequently be removed from 21 the overlying foil by chemical etching and/or evaporation.
22 Suitable substrate materials are molybdenum, aluminum and 23 copper.
24 It is within the purview of the present invention to provide a shadow grid as an integral part of the foil 26 covering the reservoir of electron emitting material. To 27 accomplish this, the area of the foil destined to function 28 as the shadow grid is coated with a non-emitting material 29 such as zirconium or graphite. The non-emitting material coated onto specific non-perforated areas of the cathode 31 surface suppresses electron emission from these areas and 32 thereby functions in a manner analogous to a shadow grid 3jjmll2278 ' - 6 - 78-51 . ,~

l in a non-intercepting grided gun.
2 Other methods for accomplishing the objects of this 3 invention will become apparent to those skilled in the art 4 upon a perusal of the following description of the preferred e~mbodiment together with the accompanying drawing.

7 FIG. l is a cross-sectional view of a dispenser cathode 8 according to the present invention.
9 FIG. 2 is a pictorial flow diagram illustrating a step-by-step process for fabricating a metallic foil having ll uniformly sized and spaced apertures for use as the 2 emitting surface of a dispenser cathode.
13 FIG. 3 is a plan view of a dispenser cathode 14 emitting surface fabricated by the process illustrated in FIG. 2, with a shadow grid formed as an integral part 16 of the emitting surface.
17 FIG. 4 is a plan view of a dispenser cathode as in 18 FIG. 3, with an alternative design for the shadow grid.
19 FIG. 5 is a plan view of another dispenser cathode as in FIG. 3, with another alternative design for the 21 shadow grid.
22 FIG. 6 is a plan view of yet another dispenser 23 cathode as in FIG. 3, with a further alternative design 24 for the shadow grid.
FIG. 7 is a flow diagram sum~arizing the steps in 26 the fabrication of a reservoir of thermionically emitting 27 material according to the present invention.

2~ FIG. l shows a dispenser cathode 10 according to the present invention. The cathode structure comprises an 31 electron-emitting surface 11 covering a reservoir 12 of 32 thermionically emitting material such as barium oxide, 3jjmll2278 ' - 7 - 78-51 . - . .

i 159722 1 or a mixture of barium oxide in combination with calcium 2 oxide and/or strontium oxide.
3 The electron emitting surface 11 is an apertured metal 4 foil supported on a hollow elongate member 13, which is mo~lntable within an electron tube such as a klystron or a 6 travelling wave tube. The support member 13 is made of 7 a refractory material, and encloses a heater coil 14 that 8 is made of a material such as tungsten that can dissipate 9 electric power so as to achieve a temperature within the support structure 13 in the range from 800 to 1100C.
11 The support structure 13 may be made entirely of a refrac-12 tory metal such as tungsten or molybdenum; or it may be 13 a composite structure whose bottom portion is made of a 14 refractory insulating material such as alumina or beryllia, and whose upper portion is made of a refractory metal.
16 As shown in FIG. 1, the upper portion of the support 17 structure 13 is configured to retain a block 15 of refrac-18 tory metal such as tungsten, tantalum, or a porous tungsten-19 impregnated material. The block 15 need not be a separate member, but could be fabricated as an integral part of 21 a homogeneous support structure 13.
22 On the upper surface of the refractory metal block 15, 23 the reservoir 12 of material that emits electrons by ther-24 mionic emission at temperatures above 700C is provided.
The reservoir 12 would typically comprise a layer of barium 26 oxide. However, as discussed above, the reservoir layer 27 12 could also comprise a mixture of barium oxide in 28 combination with calcium oxide and/or strontium oxide, 29 depending upon the particular use intended for the tube in which the dispenser cathode 10 is to be mounted. On 31 top of the reservoir layer 12, the metal foil 11 is 32 disposed. The foil 11 is arranged as a cap structure 3jjmll2278 ' - 8 - 78-51 1 retaining the reservoir layer 12 in position. The foil 2 11 is bonded to the outside vertical wall of the support 3 structure 13 by an appropriate technique such as laser 4 welding, which localizes the heating effects of the bonding technique so as to minimize chemical decomposition of 6 the electron-emitting material constituting the reservoir 7 layer 12.
8 Barium oxide, when exposed to air, is quickly con-9 verted to barium hydroxide by the moisture in the air.
Barium hydroxide, which melts at 78C, is ineffective 11 as a thermionic electron-emitting material. Hence, the 12 application of a barium oxide layer to the surface of 13 a cathode has heretofore required rigid control of the 14 environment in which fabrication takes place.
According to the present invention, the barium oxide 16 reservoir layer 12 is applied to the top surface of the 17 refractory metal block 15 by the following technique.
18 First, a solid pellet of barium carbonate is heated in 19 a vacuum to liberate carbon dioxide, leaving barium oxide according to the equation BaCO3~~BaO ~ COz . The pellet 21 of barium oxide that remains after the carbon dioxiae 22 has been liberated is quite porous. Next, the porous 23~ bariu~ oxide pellet, while still under vacuum, is 24 impregnated with a wax such as eicosane, or a resinous material such as methyl methracrylate or nitrocellulose.
26 This wax or resinous coating, which permeates the barium 27 oxide pellet, protects the pellet from hydration in moist 28 air. Such coated pellets can easily be fabricated in 29 desired quantities by well-known techniques: e.g., in an inert atmosphere by back-filling a vacuum chamber with 31 argon.
32 A wax-impregnated or resin-impregnated barium oxide 3jjmll2278 , - - 9 - 78-51 . .

.

1 pellet is then placed on the surface of the refractory 2 met~ block 15. The apertured metal foil 11 is then 3 disposed to cover the barium oxide pellet; and the peri-4 meter of the foil 11 is then sealed to the outer wall of the support structure 18 by laser welding. Later, 6 during the tube bake-out or during the cathode activation 7 process, the heat thereby produced causes the wax or 8 resinous protective material to evaporate from the pellet 9 through the apertures in the foil 11. By this technique, not only can the barium oxide electron-emitting layer 11 12 be applied under ordinary atmospheric conditions, but 12 also the layer 12 can be heated to operating temperatures 13 without causing carburization or oxidation of the surface 14 of the foil 11.
Using prior techniques, carburization or oxidation 16 of the emitting surface could be caused by the release 17 of carbon dioxide gas during cathode ~ctivation. In the 18 prior art, the conventional method of applying a layer 19 of barium oxide to the surface of a cathode involved covering a quantity of barium carbonate with a porous ~1 foil and then welding the foil to a support structure.
22 Subsequent activation of the cathode by heating to 900C
23 would convert the barium carbonate to barium oxide, thereby 24 driving off carbon dioxide gas. This carbon dioxide gas would react with adjacent metal surfaces (including the 26 electron-emitting ~urface) to cause carburization or Z7 oxidation thereof. Furthermore, the carburized surface 28 would act as a reducing agent for the barium oxide, thereby 29 generating elemental barium that would evaporate at operating temperatures of the cathode. With tbe technique 31 of the present invention, on the other hand, there is 32 no carbon dioxide to be liberated from the wax- or 3jjmll2278 ' - 10 - 78-51 ,. ... . . ... .. .. ..... . . .

-\ 1 159722 1 resin-impregnated barium oxide pellet. Thus, the possibility 2 of carburization or oxidation of the surface of the foil 3 11 by the formation of the barium oxide reservoir layer 4 12 is eliminated.
In order to provide a uniform electron emission 6 density over the surface of the foil 11, a pattern o 7 apertures of uniformed size and of uniform distribution 8 with respect to each other are formed on the foil surface.
9 Such uniform porosity of the foil 11 is achieved according to the present invention by fabricating the foil 11 according 11 to one of the following techniques:
12 (1) Photolithography: A pattern of uniformly dimensioned 13 and spaced holes is chemically etched through a foil that is 14 made of a refractory metal such as tungsten or molybdenum, preferably about 0.001-inch thick. Thereafter, a coating 16 of iridium, osmium or other platinum-group metal is deposited 17 to a thickness of about one micron on one sur~ace (i.e., the 18 upper surface) of foil. This coating of iridium or other 19 platinum-group metal serves to enhance emissivity.
(2) Deposition on a Substrate: A layer of refractory 21 metal or platinum-group metal is deposited upon a substrate 22 by chemical vapor deposition, sputter deposition, electro-23- plating or evaporation. The material from which the substrate 24 is made depends upon the deposition technique used. The substrate is configured to have a flat surace with evenly 26 spaced posts protrudin~ therefrom, the posts having been 27 formed by a conventional photolithographic and chemical 28 milling technique. Preferably, the thickness of the substrate, 29 exclusive of the protruding posts, is about 0.01 inch. The posts are generally cylindrical, with a diameter in the 31 range from 0.0005 inch to 0.0010 inch, and extend about 32 0.0010 inch to 0.0015 inch above the flat surface of the 3jjmll2278 ~ 78-51 .~.~ .

, .. , . , .... ........ .. . -.

: , ': ' ' . . ' : . ~

1 15972~

1 substrate. In the usual embodiment, the posts are separated 2 by about 0.002 inch from center to center.
3 The second technique described above for 4 fabricating uniformly apertured metal foil for use in a dispenser cathode is illustrated in the pictorial 6 flow-diagram of FIG. 2. At step A, a substrate having 7 an array of posts protruding therefrom is pictured. The 8 posts are uniformly dimensioned and uniformly spaced with 9 respect to each other, and are produced on the substrate by a conventicnal lithographic technique and by chemical 11 milling. At step B, the substrate of step A is shown coated 2 with a layer of refractory metal or platinum-group metal.
13 The particular type of coating process used would depend 14 upon the nature of the substrate material. At step C, the coated substrate of step B is shown after having been 16 polished to achieve a metal layer of uniform thickness 17 with the substrate posts flush with the upper surface 18 of the metal layer. Polishing may be done by a conventional 19 method such as surface grinding. At step ~, the metal layer produced at step C is shown with the substrate 21 ~including its projecting posts) having been removed.
22 The substrate can be removed by a conventional method 23 such as chemical etching or evaporation, depending upon 24 the natuare of the substrate material.
The metal foil remaining after the substrate has 26 been removed, as pictured at step ~ in FIG. 2, has an 27 array of holes of predetermined size and spatial distribution.
28 This foil is used to cover the barium oxide reservoir `
29 12, as shown in FIG. 1, and provides the desired e}ectron-emitting pattern from the surface of the dispenser cathode.
31 ~uring cathode activation, barium oxide migrates up through 32 the apertures in the foil 11 and coats the entire surface 3jjmll2278 ' - 12 - 78-51 , .. -, .

.
- . .. ~ .. .

.

l thereof by a diffusion process known as Knudsen flow.
2 In this way, the non-perforated surface of the foil ll 3 becomes a thermionic electron source of substantially 4 uniform surface emissivity.
(3) Sintering: The formation of a metallic foil 6 on the surface of a substrate could also be accomplished 7 by the sintering of fine particles of a refractory or 8 platinum-group metal onto the surface of the substrate.
9 A porous metallic foil fabricated according to this invention can also be treated so as to provide a pattern ll of non-emitting portions on the upper surface of the 12 foil. Thus, after a foil of uniform porosity has been 13 fabricated according to the process described above in 14 connection with FIG. 2, a pattern of non-emitting surface areas can then be superimposed upon selected portions of 16 the upper surface of the foil to function in a manner 17 analogous to a shadow grid in a non-intercepting gridded 18 gun. These non-emitting areas comprise a coating of an l9 oxygen scavenging material such as zirconium or graphite, which can be deposited upon the surface of the foil by 21 sputter deposition or any other appropriate techni~ue 22 known to those skilled in the art.
23 ~ The configuration of the shadow grid portion of 24 the foil can be selected in accordance with the application for which the tube is intended. In FIG. 3, 26 the shadow grid is configured as a pattern of circles, 27 which is representative of shadow grid patterns used in 28 klystron tubes. In FIG. 4, the shadow grid is configured 29 as a pattern of hexagons, which represents another type of shadow grid pattern used in klystrons and also in 31 travelling wave tubes. In FIG. 5, a radial vane 32 configuration ~or the shadow grid pattern is shown, 3jjmll2278 , - 13 - 78-51 , .

~' ' ' ', , ~' '' , ~ 159722 1 which provides an advantage with respect to thermal 2 conductivity. In FIG. 6, the shadow grid comprises an 3 array of bars.
4 The steps in the fabrication of the layer 12 of thermionically emitting material according to the present 6 invention are summarized in the flow diagram of FIG. 7.
7 First, a quantity of alkaline earth carbonate material 8 is compacted to form a pellet. This pellet is then heated 9 in a vacuum at 900C to convert the carbonate to an oxide by driving off carbon dioxide. In the case of a barium 11 carbonate pellet, the heating converts the barium carbonate 12 pellet to a porous pellet of barium oxide. This resulting 13 oxide pellet is then impregnated in a vacuum with a wax 14 or with a resinous material. Finally, this impregnated lS oxide pellet is placed on the block 15 at the top of the 16 cathode support structure 13, and is covered with the 17 perforated foil 11.
18 The steps in the fabrication of a dispenser cathode 19 according to the present invention may be summarized as follows:
21 1) The heater 14 is assembled in the cathode structure 22 13.
23 2) The protected (i.e., wax- or resin-impregnated 24 oxide pellet is inserted in place on the support structure 13 to form the reservoir 12 of election-emitting material.
26 3) The perforated foil 11 is placed over the pro-27 tected oxide pellet.
28 4) The perimeter of the perforated foil 11 is laser 29 welded to the support structure 13 so as to sandwich the oxide pellet between the foil 11 and the support structure 31 13.
32 - This invention has been described above in terms 3jjmll2278 ' - 14 - 78-51 , . ..

1 of a particular embodiment. However, from a reading 2 of the above disclosure, other techniques for optimizing 3 the desired electron emitting pattern from the surface 4 of a dispenser cathode will suggest themselves to those skilled in the art. Consequently, the invention is 6 limited only by the following claims.

3jjmll2278 ' - 15 - 78-51 , .~,;~
. .i.. , .

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pellet of an alkaline oxide oxide material, said pellet being fabricated by the process of a) compacting a quantity of alkaline earth carbonate;
b) heating the compacted alkaline earth carbonate to convert said alkaline earth carbonate in an inert atmosphere to an alkaline earth oxide by driving off carbon dioxide; and c) impregnating the alkaline earth oxide thus formed with a vaporizable, organic protective material which is solid at room temperatures

2. The pellet of claim 1 wherein said step of impreg-nating said alkaline earth oxide with said protective material is performed in a vacuum.

3. The pellet of claim 1 wherein said protective material is wax.

4. The pellet of claim 3 wherein said wax is eico-sane.

5. The pellet of claim 1 wherein said protective material is a resinous material.

6. The pellet of claim 5 wherein said resinous material is methyl methacrylate.

7. The pellet of claim 5 wherein said resinous material is nitrocellulose.