JPH028413B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Description: An electron emissive material in a perforated metal foil having a suitable pattern of pore-sized pores thereon to produce uniform electron emission from the cathode surface. By covering the buildup of
A cathode is manufactured. The emissive material is in the form of barium oxide pellets impregnated with a wax or resin material to minimize chemical reactions of the barium oxide in the air. The impregnated barium oxide pellets are sandwiched between a perforated foil and a support structure, and the foil is welded to the support structure. During tube bakeout or subsequent cathode activation,
The wax or resin material evaporates and the barium oxide passes through the pores and evenly covers the surface of the foil. The desired pattern of holes in the foil can be formed by photolithography or by forming the foil on a substrate with an array of protruding columns (e.g. chemical vapor deposition, sputtering,
This is achieved by a vapor deposition method, a sintering method, etc.). When using photolithography, the desired pattern of holes is chemically etched directly onto the foil, and when using a technique that forms the foil on a substrate with an array of columns, the desired pattern of holes is etched directly onto the foil. The substrate with columns is removed by chemical etching after the foil is formed. Whichever technique is used to form the pores, the shadow grid can also be made into a cathode by depositing a layer of a reactive material such as zirconium or graphite on selected portions of the cathode surface. It can be formed as an integral part of the surface. This layer of reactive material prevents the formation of an emissive layer of barium oxide over the selected pattern on the cathode surface.

The present invention represents a further development in the art of manufacturing dispenser cathodes commonly applied to microwave tubes and linear beam devices.

Traditionally, the emissive surface of a dispenser cathode has been made of a porous metal matrix whose pores are filled with emissive material, or a porous metal plug that covers the accumulation of emissive material.

The porous metal body of a conventional dispenser cathode, whether it is a matrix filled with emissive material or a porous plug covering an accumulation of emissive material, has consistently uniform pores. size, pore length and spacing between pores on the surface. As a result, conventional dispenser cathodes tended to exhibit uneven electron emission from their surfaces.

U.S. Patent No. 4101800 (granted July 18, 1978)
describe a dispenser cathode consisting of an accumulation of electron emissive material covered by a perforated metal foil. The pattern of pores is such that it transports emissive material from the accumulation to the foil surface in such a way that it uniformly covers the surface, thereby creating a cathode surface with virtually uniform emissivity. provide. However, the prior art has not developed a viable method for manufacturing perforated metal foils of the type described in this patent. As a result, the production of dispenser cathodes with uniform surface porosity has not heretofore been commercially viable.

One object of the present invention is to provide a method for manufacturing a dispenser cathode having uniform surface porosity, thereby achieving uniform electron emission from the surface.

An additional object of the present invention is to provide a method of controlling the porosity of the surface of a dispenser cathode to provide a surface with uniform electron emission.

Another object of the present invention is to produce a dispenser cathode with uniform surface porosity by a method that minimizes carburization and oxidation of the cathode surface.

One particular object of the present invention is to sandwich the accumulation of emissive material between a perforated foil and a support structure using bonding techniques that minimize carburization of the emissive surface. It is an object of the present invention to provide a method of manufacturing a dispenser cathode having a uniform pattern of electron emission from its surface.

To achieve the above objects of the present invention, a quantity of a material with a low work function (e.g. barium oxide) is placed on the support structure to form pores of uniform size and uniform distribution. A thin foil of refractory metal or platinum group metal having the desired pattern is placed over the support structure to cover the barium oxide. In order to limit the heating effect for the bonding process, the foil is bonded to the support structure by laser welding. Specially treated pellets of barium oxide are used to prevent the barium oxide from chemically reacting with moisture in the air during cathode manufacture. The barium oxide pellets are made by heating solid pellets of barium carbonate in a vacuum to release carbon dioxide, thereby leaving behind porous pellets of barium oxide. The porous pellets of barium oxide are then impregnated with wax or with a resinous material such as methyl methacrylate or nitrocellulose to provide a protective coating over the barium oxide. Without such a protective coating, rapid chemical reduction of barium oxide to barium hydroxide occurs. Barium hydroxide cannot be used as an electron emitting material.

In manufacturing a dispenser cathode according to the invention, barium oxide pellets impregnated with a wax or resin material to prevent rapid chemical reactions in the air are placed on the surface of the metal support member of the cathode structure. . A perforated foil is placed over the barium oxide pellets,
It is then welded to the metal support member. Laser welding techniques are desirable because they can be performed in areas with limited access and are effective in localizing the heating effects of the welding process. The heat generated during the electron tube bakeout process or cathode activation causes the wax or resinous material to evaporate from the barium oxide pellets.

The perforated metal foil with uniform pore size and distribution according to the invention is obtained by photolithography technology,
In that technique, a pattern of holes is chemically etched into a foil of refractory metal, such as tungsten or molybdenum. After the holes are formed, the foil is coated with iridium, osmium or some other platinum group metal. Typical values for tungsten or molybdenum foil are 0.0254mm
(0.001 inch) thick and the overlying coating is about 1 micron thick.

Alternatively, a foil having a desired pattern of uniformly sized and uniformly distributed holes according to the present invention may be provided with an array of appropriately sized and appropriately spaced columns projecting therefrom. It may be made by depositing a platinum group metal onto a substrate having a platinum group metal. The metal layer on such a substrate typically ranges from 0.0127 mm (0.0005 inch) to
After being deposited to a thickness of 0.0015 inches, the substrate, along with the protruding columns, is removed by chemical etching or evaporation. The platinum group metal layer can be deposited onto the substrate by chemical vapor deposition, sputtering, electroplating or vapor deposition. Such techniques are well known to those skilled in the art. Another method is to roll press fine metal particles (particles with a diameter of 1 micron or less) onto the metal and then sinter the pressed body to form a porous layer. You can also make one. The substrate may be made of any material that is amenable to photoetching and subsequent evaporation, such that the columns are formed by the photoetching process and the entire substrate, including the protruding columns, is subsequently chemically etched. It is removed from the overlying foil by etching and/or evaporation. Suitable substrate materials are molybdenum, aluminum and copper.

Constructing the shadow grid as an integral part of the foil covering the emissive material deposits
It is within the scope of the invention. To accomplish this, the area of the foil intended to act as a shadow grid is coated with a non-emissive material such as zirconium or graphite. Non-emissive material coated over certain unperforated areas of the cathode surface suppresses electron emission from these areas, thereby acting similar to a shadow grid in a non-colliding gridded electron gun.

Other ways of achieving the objects of the invention will become apparent to those skilled in the art upon reading the following description of the embodiments thereof, with reference to the accompanying drawings.

FIG. 1 shows a dispenser cathode 10 according to the present invention. The cathode structure comprises an electron emissive surface 11 covering an accumulation 12 of thermionic emissive material, such as barium oxide or a mixture of barium oxide and calcium oxide or strontium oxide or a combination of both.

The electron emitting surface 11 is a perforated metal foil supported on a hollow elongated member 13 which can be mounted inside an electron tube such as a klystron or traveling wave tube. Support member 13
is made of a refractory material and has a heater coil 14 made of a material such as tungsten or the like capable of dissipating electrical power to achieve a temperature within the support structure 13 in the range of 800° to 1100°C. surround. The support structure 13 may be made entirely of a refractory metal, such as tungsten or molybdenum, or it may be made of a refractory insulating material, such as alumina or beryllia, with its lower part made of a refractory metal. It may also have a composite structure made of.

As shown in FIG. 1, the top of the support structure 13 is shaped to hold a block 15 of refractory metal, such as tungsten, tantalum, porous tungsten impregnated material, or the like. Block 15 need not be a separate part, but may be made as an integral part of homogeneous support structure 13.

The upper surface of the refractory metal block 15 is provided with an accumulation 12 of a substance that emits electrons by thermionic radiation at temperatures above 700°C. The reservoir 12 typically consists of a layer of barium oxide. However, as discussed above, the storage layer 12 may contain barium oxide and potassium oxide or strontium oxide, or both, depending on the particular application for which the electron tube to which the dispenser cathode 10 is to be installed is intended. It may also consist of a mixture of combinations of A metal foil 11 is placed on top of the storage layer 12 . The foil 11 is arranged in a cap structure that holds the storage layer 12 in place. The foil 11 is attached to the outside of the support structure 13 by a suitable technique, such as laser welding, which localizes the heating effect of the adhesive technique so as to minimize chemical decomposition of the emissive material that constitutes the storage layer 12. Glued to a vertical wall.

When barium oxide is exposed to air, it is rapidly converted to barium hydroxide by the moisture in the air. Barium hydroxide, which melts at 78°C, does not act as a thermionic emitter. Therefore, the addition of a barium oxide layer to the surface of the cathode has traditionally been
It required tight control of the ambient conditions in which its manufacture took place.

According to the invention, the barium oxide storage layer 12 is
It is attached to the top surface of the refractory metal block 15 by the following technique. A solid pellet of barium carbonate is first heated in a vacuum to release carbon dioxide and leave behind barium oxide according to the chemical reaction BaCO ₃ →BaO + CO ₂ . The barium oxide pellets that remain after the carbon dioxide is released are extremely porous. The porous barium oxide pellets, while still in vacuum, are then impregnated with a wax such as eicosane or a resinous material such as methyl methacrylate or nitrocellulose. This wax or resin coating, which permeates the barium oxide pellets, protects them from hydration in moist air.
Such coated pellets can be readily produced in desired quantities by well-known techniques, such as in an inert atmosphere by filling a vacuum chamber with argon.

The wax-impregnated or resin-impregnated barium oxide pellets are then formed into a refractory metal block 15.
placed on the surface of A perforated metal foil 11 is then placed over the barium oxide pellet, and the perimeter of the foil 11 is then sealed to the outer wall of the support structure 13 by laser welding. Later, during the bake-out of the electron tube or during the activation step of the cathode, the heat thereby generated evaporates the protective substance consisting of wax or resin from the pellet through the pores of the foil 11. This technique not only allows the barium oxide electron emissive layer 12 to be applied under normal atmospheric conditions;
2 can be heated to operating temperature without causing carburization or oxidation of the surface of the foil 11.

Using conventional techniques, the release of carbon dioxide gas during cathode activation can cause carburization or oxidation of the emissive surface.
In the prior art, the usual method of applying a layer of barium oxide to the surface of a cathode involved covering a quantity of barium carbonate with a porous foil and then welding the foil to a support structure. Subsequent activation of the cathode by heating to 900°C causes the barium carbonate to be converted to barium oxide, thereby releasing carbon dioxide gas. This carbon dioxide gas reacts with adjacent metal surfaces (including electron emitting surfaces) causing their carburization or oxidation. Furthermore, the carburized surface acts as a reducing agent for barium oxide, thereby generating elemental barium which evaporates at the operating temperature of the cathode. On the other hand, using the technique of the present invention, no carbon dioxide is released from wax-impregnated or resin-impregnated barium oxide pellets. In this way, there is no possibility that the surface of the foil 11 will be carburized or oxidized due to the formation of the barium oxide accumulation layer 12.

In order to produce a uniform electron emission density over the entire surface of the foil 11, a pattern of holes of uniform dimensions and uniformly distributed with respect to each other is formed on the surface of the foil. Such uniform porosity of the foil 11 is achieved, according to the invention, by making the foil 11 according to one of the following techniques.

(1) Deposition onto the substrate: The refractory metal is a layer of platinum group metal deposited onto the substrate by chemical vapor deposition, sputtering, electroplating, or vapor deposition. The materials that make up the substrate will depend on the deposition technique used. The substrate is shaped so that uniformly spaced columns have a flat surface projecting therefrom, and the columns are made by conventional photolithography or chemical milling techniques. Preferably, the thickness of the substrate, excluding the protruding columns, is about 0.254 mm (0.01 inch). The column is generally
0.0127mm (0.0005 inch) to 0.0254mm
(0.0010 inch) and approximately 0.0254 mm above the flat surface of the substrate.
mm (0.0010 inch) to 0.0381 mm (0.0015 inch) protruding. In a typical embodiment, the columns are spaced about 0.002 inches center-to-center.

The second technique described above for making a uniformly perforated metal foil for use in a dispenser cathode is illustrated in the schematic flow chart of FIG. In stage A, a substrate is defined having an array of columns projecting therefrom. The columns are uniformly sized and spaced apart and are fabricated on the substrate by conventional photolithography or chemical milling. In stage B, the substrate of stage A is shown coated with a layer of refractory metal or platinum group metal.
The particular type of coating process used will depend on the nature of the substrate material. In step C, the coated substrate of step B is polished to achieve a metal layer of uniform thickness with the pillars of the substrate flush with the top surface of the metal layer. Illustrated. Polishing can be performed by conventional methods such as surface grinding. In stage D, the metal layer produced in stage C is shown with its substrate (including the pillars protruding therefrom) removed. The substrate can be removed by conventional methods such as chemical etching, evaporation, etc., depending on the nature of the substrate material.

The metal foil remaining after the substrate is removed, as illustrated in step D of FIG. 2, has an array of holes with predetermined dimensions and spatial distribution. This foil is used to cover the barium oxide accumulation 12 as shown in FIG. 1, creating the desired electron emission pattern from the surface of the dispenser cathode. During activation of the cathode, the barium oxide migrates upward through the pores of the foil 11 and coats the entire surface of the foil by a diffusion process known as Knudsen flow.
In this way, the non-porous surface of foil 11 becomes a source of thermionic electrons with virtually uniform surface electron emission.

(3) Sintering method: Formation of the metal foil on the surface of the substrate is also accomplished by sintering fine particles of refractory metal or platinum group metal onto the surface of the substrate.

Porous metal foils made in accordance with the present invention can also be treated to provide a pattern of non-emissive portions on the top surface of the foil. In this way, after a foil with uniform porosity has been made according to the process described above in connection with FIG. For this purpose, a pattern of non-emissive surface areas can be superimposed on selected portions of the top surface of the foil. These non-emissive surface areas consist of a coating of oxygen scavenging material such as zirconium or graphite, which can be applied to the surface of the foil by sputtering or other suitable techniques well known to those skilled in the art. .

The shape of the shadow grid portion of the foil can be selected depending on the application for which the electron tube is intended. In Figure 3, the shadow grid is shaped as a circular pattern, which is similar to the shadow grid used in klystron tubes.
This is a typical pattern. In Figure 4, the shadow grid is shaped as a hexagonal pattern, which is similar to other types of shadow grids used in klystrons as well as in traveling wave tubes.
Represents a grid pattern. In FIG. 5, the radial vane configuration of the shadow grid pattern is shown, which has advantages with respect to thermal conductivity. In FIG. 6, the shadow grid consists of an array of rods.

The steps of manufacturing a layer of thermionic emissive material 12 according to the invention are summarized in the flow chart of FIG. First, a quantity of alkaline earth carbonate material is consolidated to form a pellet. The pellet is then heated to 900°C in vacuum to convert the carbonate to oxide by driving off the carbon dioxide. In the case of barium carbonate pellets, the heating converts the barium carbonate pellets into porous barium oxide pellets. The resulting oxide pellets are then impregnated with a wax or resin material under vacuum. Finally, this impregnated oxide pellet forms a block 15 on top of the cathode support structure 13.
and covered with a perforated foil 11.

The steps of manufacturing a dispenser cathode according to the invention are summarized as follows: 1) The heater 14 is incorporated into the cathode structure 13.

2) A protected (i.e., wax-impregnated or resin-impregnated) oxide pellet is inserted into position on the support structure 13 to create an accumulation 12 of electron emissive material.
form.

3) A perforated foil 11 is placed over the protected oxide pellet.

4) The periphery of the perforated foil 11 is laser welded to the support structure 13 such that the oxide pellet is sandwiched between the foil 11 and the support structure 13.

The invention has been described above with reference to specific embodiments.
However, other techniques for optimizing the desired electron emission pattern from the surface of the dispenser cathode will occur to those skilled in the art after reading the above description. The invention, therefore, is to be limited only by the scope of the following claims.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a dispenser cathode according to the present invention, and FIG. 2 is a cross-sectional view of a dispenser cathode according to the present invention; FIG. FIG. 3 is a diagrammatic flowchart illustrating the steps in the process of the dispenser manufactured by the process illustrated in FIG. 4 is a plan view of a dispenser cathode similar to FIG. 3 with a shadow grid of another design; FIG. 5 is a top view of a dispenser cathode with a shadow grid of another design. Plan view of another dispenser cathode similar to Figure 3, No. 6
FIG. 7 is a plan view of yet another dispenser cathode similar to FIG. 3 with a further shadow grid, and FIG. This is a flowchart. DESCRIPTION OF SYMBOLS 10... Dispenser cathode, 11... Electron emitting surface, 12... Accumulation of electron emitting material, 13... Hollow elongated member, 14... Heater coil, 15
…Fireproof metal block.

Claims (1)

Claims: 1. A method of manufacturing a dispenser cathode comprising an accumulation of electron emissive material sandwiched between a support structure and a perforated foil, the perforated foil comprising: a) depositing a metal coating on a substrate formed with a flat surface from which a plurality of columns protrude; b) the columns of the substrate being coplanar with the top surface of the metal layer; c) polishing the coated substrate to obtain a uniform thickness of the metal layer, and c) removing the substrate, including the protruding columns of the substrate. However, the method. 2. The method of claim 1, wherein the metal coating deposited on the substrate comprises a refractory metal. 3. The method of claim 1, wherein the metal deposited on the substrate comprises a platinum group metal. 4. The method of claim 1, wherein the metal is deposited on the substrate by chemical vapor deposition. 5. The method of claim 1, wherein the metal is deposited on the substrate by sputtering. 6. The method of claim 1, wherein the metal is deposited on the substrate by electroplating. 7. The method of claim 1, wherein the metal is deposited on the substrate by vapor deposition. 8. The method according to item 1, wherein the pillars are formed on the substrate by photolithography. 9. The method of claim 1, wherein the pillars are formed on the substrate by chemical milling. 10. The method of claim 1, wherein the coated substrate is polished by surface grinding. 11. The method of claim 1, wherein the substrate is removed by chemical etching. 12. The method of claim 1, wherein the substrate is removed by evaporation. 13. The method of claim 1, wherein said store of electron-emitting material consists of barium oxide. 14. The method of claim 1, wherein said accumulation of electron-emitting material consists of a mixture of barium oxide and calcium oxide. 15. The method of claim 1, wherein said accumulation of electron-emitting material consists of a mixture of barium oxide and strontium oxide. 16. The method of claim 1, wherein the accumulation of electron-emitting material consists of a mixture of barium oxide, calcium oxide, and strontium oxide. 17. said accumulation of electron emissive material comprises disposing a quantity of said electron emissive material on said support structure, said electron emissive material being sandwiched between said perforated foil and said support structure; 2. The method of claim 1, wherein the amount of electron emissive material is covered with the perforated foil to form a layer of . 18, wherein the quantity of electron emissive material disposed on the support structure is in the form of pellets;
The method according to paragraph 17. 19. The method of claim 18, wherein said pellets consist of alkaline earth oxide impregnated with wax. 20. The method according to item 19, wherein the wax is eicosane. 21. The method of claim 18, wherein the pellets consist of an alkaline earth oxide impregnated with a resinous material. 22. The method of paragraph 24, wherein the resin material comprises a methacrylate. 23. The method of paragraph 24, wherein the resin material comprises nitrocellulose.