GB1591789A

GB1591789A - Electron emitter

Info

Publication number: GB1591789A
Application number: GB2880076A
Authority: GB
Original assignee: EMI Varian Ltd
Current assignee: EMI Varian Ltd
Priority date: 1977-10-06
Filing date: 1977-10-06
Publication date: 1981-06-24

Description

(54) ELECTRON EMITTER (71) We, EMI-VARIAN LIMITED, a British company of Blyth Road, Hayes, Middlesex. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to electron emitters.

An electron emitter is a body arranged to release electrons efficiently and usable for example as a thermionic cathode in a microwave electron discharge tube. It is known to form such emitters from a matrix of a metallic material such as a high melting point metal or alloy, the matrix holding within its structure an activator material such as an alkaline earth oxide to bring about the emission of electrons from the emitter body when the matrix is heated. As the emitter is heated to temperatures in the order of 10000C and may be subject to considerable mechanical stress it is desirable that the matrix may be strong. The materials used for the matrix are often difficult to work and the formation of a sufficiently strong matrix with an open structure is not easy.

It is an object of the invention to provide an improved method of making an electron emitter.

According to one aspect of the invention there is provided a method of forming a thermionic electron emitter comprising a refractory metal, an activator comprising a compound of barium, and a reducing agent, in which method powders of the metal and of the activator are subjected to a milling action sufficiently energetic and for a sufficient time to reform the powders into grains, in which grains particles of the metal are welded together and particles of the activator are dispersed within the metal in such a manner as to allow the activator to emerge from within the metal to the surface of the grains, and the grains are formed into a matrix having an open structure, and in which the reducing agent is incorporated into the matrix.

According to another aspect, there is provided a method of forming a thermionic electron emitter comprising a refractory metal, an activator comprising a compound of barium, and a reducing agent in which method powders of the metal, the activator and the reducing agent are subjected to a milling action sufficiently energetic and for a sufficient time to reform the powders into grains in which grains particles of the metal are welded together and particles of the activator and of the reducing agent are dispersed within the metal in such manner as to allow reduced activator to emerge from within the metal to the surface of the grains, and in which method the grains are formed into a matrix having an open structure.

Embodiments of the invention will now be described with reference to the drawing accompanying the Provisional Specification representing one form of grain structure, shown m cross-section.

A known milling technique is called high energy agitation milling or mechanical alloying or high energy milling or energetic milling. This is a milling technique imposing much greater force on the material to be milled than in the conventional operation of a ball-mill or similar device. Usually highenergy milling is performed by placing the quantity of powdered material to be milled, and a larger quantity of milling elements, into a vessel in which an agitator is revolved to keep the material and elements in motion in a vacuum (or an inert gas) so that the elements collide with each other frequently and repeatedly, thereby subjecting the material to repeated impact compression.

Various machines and techniques are known. A summary of some is given in U.K.

Patent Specifications 1265343 and 1298944.

As described in these specifications the aim is to produce wrought composite particles that are hard and solid and are useful in producing hard materials, e.g. for cutting tools.

A thermionic emitter for use in a cathode of a microwave electron tube preferably has a body formed by a matrix of a support material and containing an activator as a source of electrons, the matrix permitting the migration of source substance to the emitter surface for electron emission.

Suitable substances for the matrix are high melting point relatively inert refractory metals such as occur in the transition series of elements, e.g. nickel, molybdenum, ruthenium, rhenium, tungsten, osmium, iridium and platinum or their alloys.

Suitable activators are compounds such as tungstates, aluminates, carbonates and oxides which include barium. A reducing agent, e.g. zirconium, added for convenience in the form of the hydride, may be included.

As mentioned above the usual aim of high energy milling techniques as used hitherto is to produce hard, solid grains. These would not be suitable for use in making a cathode.

However it has now been found that grains of a form suitable for electron emitters may be produced by an energetic milling technique. Accordingly powdered emitter materials i.e. a refractory metal, an activator and a reducing agent are subjected to milling forces energetic enough to only reform the powdered materials as grains of particles of material welded together, which grains may be of larger size than the powder. A matrix is then formed from the grains. The matrix may be formed by compressing the grains. The matrix may be formed to be self-supporting, e.g. as a cylinder, or supported on an inert substrate, e.g. as a surface coating, and applied by e.g.

sintering, pressing or rolling. The substrate may be nickel.

Typically the grains are some 40 micron in size and the starting powder is some 5 micron in size. However these sizes are only exemplary and there is also clearly a spread of sizes from the nominal value. Grains of up to 100 micron, which can be formed, are tolerable in many cases.

Techniques for producing a matrix, e.g.

by sintering, are known and appropriate ones will be readily apparent to those skilled in the art. Care is sometimes required in that, at the "activation" stage of the emitter, gas is released and this could disrupt the matrix. One alternative arrangement is to coat the materials for forming the matrix onto a passive nickel sheet before sintering, rolling or pressing them. The sheet is typically 0.005 inch thick with a coating 0.001 inch thick.

To make such a cathode, a starting material including at least a support component material, e.g. tungsten metal, and an activator component material, e.g.

barium strontium tungstate, Ba5Sr(WO5)2, is prepared. The component materials are in powder form fine enough to be subjected to energetic milling such as "high energy agitation milling" or "mechanical alloying" as mentioned above.

The drawing of part of a composite grain shows the grain form producible by such energetic milling of a support material with an activator material and a reducing agent.

It will be seen that filaments of activator material 11, 12 are in the grain surrounded by the metallic material 2. The filaments are in pores of the metallic material. The filaments are shown extending to the surface 31 of the grain. The grains can be formed into a porous cathode body in any suitable manner, e.g. compression and sintering. In use when such a body is heated the activator material in a filament such as 11 or 12 will be able to emerge easily from the grain as the reaction surface moves progressively along the filament path until substantially the whole filament is used up.

The reaction products of the activator component can accumulate around the mouth of each filament as shown in the drawing at 32. Regions of reducing agent may occur around the filament, e.g. at 41, 42. Forms other than filaments, e.g. laminar or fibrous elements may be produced in the grains.

A suitable mill for cathode material production is a vibratory mill using solid carbide balls in a molybdenum pot or preferably to be compatible with other cathode materials steel balls sprayed with molybdenum. The ball material should be chosen to reduce or avoid "pick-up" into the cathode material. Having regard to the volume of cathodes a suitable size of mill would have dimensions of some hundreds of millimetres and be operated for times of less than one hour.

Suitable materials for the support component are a high melting point metal or metal alloy. Such metals are iridium, molybdenum, nickel, osmium, platinum, rhodium, ruthenium and tungsten and alloys including such metals. Problems can arise with nickel-based matrices in that they are not dimensionally stable at high temperatures; a hardening material such as an inert material (e.g. Al203) when dispersed in the nickel would improve stability.

The activator component is a compound of barium or a compound of barium and one or more of calcium and strontium. A barium carbonate is suitable. In particular with tungsten as the metallic component barium strontiumtungstate BasSr(WO6)2 is a suitable source component. For nickel as the metallic component barium strontium calcium carbonate (BaSrCa)CO3 or barium strontium oxide (BaSr)O is suitable. In each case a reducing agent such as zirconium hydride ZrH2 is added.

Other reducing agents may be used with the specified and other cathode systems.

The class of suitable reducing agents is large. The criterion for the selection of a reducing agent is its effect on the equilibrium of the reaction by which the activator component brings about electron release. Typically the dissociation of barium oxide is involved and the reducing agent should act to combine with the oxygen released on the dissociation and permit the easy escape of the barium so that further dissociation can occur releasing more barium. Suitable reducing agents include the elements titanium, zirconium, hafnium, tungsten, thorium, magnesium, beryllium, scandium, yttrium, samarium, neodymium, preseodymium, lanthanum, uranium, aluminium, silicon, cerium, tantalum, manganese, boron, cadmium, chromium, molybdenum, gallium, zinc, vanadium, iron, tin.The reducing agent may be included in the grains formed by high-energy milling by providing it in powder form with the matrix material.

The technique described above permits the production of an electron emitter, such as a microwave tube cathode, with a structure improving the mechanical properties of the cathode by reforming the material for the matrix. It is also important that the high-energy milling method is much quicker than present techniques for cathode material production which involve hours or days of milling which save money or equipment investment. It is believed that the initial high rate of evaporation from the cathodes made by present techniques may be reduced. The electron emitters are also useful for other electron tubes such as cathode ray tubes and planar triodes.

WHAT WE CLAIM IS: 1. A method of forming a thermionic electron emitter comprising a refractory metal, an activator comprising a compound of barium, and a reducing agent, in which method powders of the metal and of the activator are subjected to a milling action sufficiently energetic and for a sufficient time to reform the powders into grains, in which grains particles of the metal are welded together and particles pf the activator are dispersed within the metal in such a manner as to allow the activator to emerge from within the metal to the surface of the grains, and the grains are formed into a matrix having an open structure, and in which method the reducing agent is incorporated into the matrix.

2. A method of forming a thermionic electron-emitter comprising a refractory metal, an activator comprising a compound of barium, and a reducing agent in which powders of the metal, the activator and the reducing agent are subjected to a milling action sufficiently energetic and for a sufficient time to reform the powders into grains of the material in which grains particles of the metal are welded together and particles of the activator and of the reducing agent are dispersed within the metal in such a manner as to allow reduced activator to emerge from within the metal to the surface of the grains, and in which method the grains are formed into a matrix having an open structure.

3. A method according to Claim 1 or 2 wherein the milling time is less than one hour.

4. A method according to any preceding claim wherein the grains are of greater size than the powders.

5. A method according to Claim 4 wherein the size of the powders is about 5 microns and the size of the grains is in the range of 40 to 100 microns.

6. A method according to any preceding claim comprising sintering the grains to form the matrix.

7. A method according to any one of claims 1 to 5 comprising supporting the grains as a coating on an inert substrate, and sintering, pressing, or rolling them on the substrate to form the matrix on the substrate.

8. A method according to claim 7, wherein the inert substrate is of nickel.

9. A method according to any preceding claim, wherein the refractory metal is selected from the group of iridium, molybdenum, nickel, osmium, platinum, rhenium, ruthenium, tungsten, and alloys thereof.

10. A method according to any preceding claim wherein the activator is selected from the group of tungstates, aluminates, carbonates and oxides which include barium or barium and one or more of calcium and strontium.

11. A method according to any preceding claim, wherein the reducing agent comprises titanium, zirconium, hafnium, tungsten, thorium, magnesium, beryllium, scandium, yttrium, samarium, neodymium, praseodymium, lanthanum, uranium, aluminium, silicon, carium, tantalum, manganese, boron, cadmium, chromium,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of barium or a compound of barium and one or more of calcium and strontium. A barium carbonate is suitable. In particular with tungsten as the metallic component barium strontiumtungstate BasSr(WO6)2 is a suitable source component. For nickel as the metallic component barium strontium calcium carbonate (BaSrCa)CO3 or barium strontium oxide (BaSr)O is suitable. In each case a reducing agent such as zirconium hydride ZrH2 is added. Other reducing agents may be used with the specified and other cathode systems. The class of suitable reducing agents is large. The criterion for the selection of a reducing agent is its effect on the equilibrium of the reaction by which the activator component brings about electron release. Typically the dissociation of barium oxide is involved and the reducing agent should act to combine with the oxygen released on the dissociation and permit the easy escape of the barium so that further dissociation can occur releasing more barium. Suitable reducing agents include the elements titanium, zirconium, hafnium, tungsten, thorium, magnesium, beryllium, scandium, yttrium, samarium, neodymium, preseodymium, lanthanum, uranium, aluminium, silicon, cerium, tantalum, manganese, boron, cadmium, chromium, molybdenum, gallium, zinc, vanadium, iron, tin.The reducing agent may be included in the grains formed by high-energy milling by providing it in powder form with the matrix material. The technique described above permits the production of an electron emitter, such as a microwave tube cathode, with a structure improving the mechanical properties of the cathode by reforming the material for the matrix. It is also important that the high-energy milling method is much quicker than present techniques for cathode material production which involve hours or days of milling which save money or equipment investment. It is believed that the initial high rate of evaporation from the cathodes made by present techniques may be reduced. The electron emitters are also useful for other electron tubes such as cathode ray tubes and planar triodes. WHAT WE CLAIM IS:

1. A method of forming a thermionic electron emitter comprising a refractory metal, an activator comprising a compound of barium, and a reducing agent, in which method powders of the metal and of the activator are subjected to a milling action sufficiently energetic and for a sufficient time to reform the powders into grains, in which grains particles of the metal are welded together and particles pf the activator are dispersed within the metal in such a manner as to allow the activator to emerge from within the metal to the surface of the grains, and the grains are formed into a matrix having an open structure, and in which method the reducing agent is incorporated into the matrix.

8. A method according to claim 7, wherein the inert substrate is of nickel.

molybdenum, gallium, zinc, vanadium, iron, or tin.

12. A method according to any one of claims I to 8 wherein the metal is tungsten, the activator is Ba5Sr(WO)2 and the reducing agent is ZrH2.

13. A method according to any one of claims 1 to 8 wherein the metal is nickel, the activator is (BaSrCa) CO3 or (BaSr)O and the reducing agent is ZrH2.

14. A method according to Claim 13, further comprising the step of dispersing Awl203 with the nickel to harden the nickel.

15. A method according to any preceding claim, wherein the milling is performed using a vibratory mill.

16. A method according to any one of claims 1 to 14, wherein the milling is performed using a mill comprising a vessel containing milling elements and an agitator for moving the elements.

17. A method of forming a thermionic electron emitter substantially as hereinbefore described with reference to the drawing accompanying the Provisional Specification.

18. A thermionic electron emitter made by the method of any preceding- claim.