US3697321A - Thermionic cathode and method of manufacturing the same - Google Patents

Abstract

A THORIUM FILM THERMIONIC CATHODE IN ELECTRON DISCHARGE DEVICES AND METHOD OF MANUFACTURING THE SAME WHEREIN A HIGH MELTING METAL CARRIER, SUCH AS TUNGSTEN OR TUNGSTEN ALLOY, CONTAMINATED WITH THORIUM OXIDE IS PROVIDED WITH A COATING CONTAINING AN INNER LAYER OF TUNGSTEN CARBIDE AND AN OUTER LAYER OF A HIGH MELTING METAL HAVING MIGRATINAL PROPERTIES AND FUNCTIONING AS AN EMISSION BASE FOR THORIUM, SUCH AS OSMIUM OR RHENIUM.

Description

US. Cl. 117-217 11 Claims ABSTRACT OF THE DISCLOSURE A thorium film thermionic cathode in electron discharge devices and method of manufacturing the same wherein a high melting metal carrier, such as tungsten or tungsten alloy, contaminated with thorium oxide is provided with a coating containing an inner layer of tungsten carbide and an outer layer of a high melting metal having migrational properties and functioning as an emission base for thorium, such as osmium or rhenium.

This invention relates to a thorium film thermionic cathode in electron discharge devices and to a method of manufacturing the same; more particularly the invention relates to thorium film thermionic cathodes having a high melting metal carrier contaminated with thorium oxide and provided with a layer of a material yielding maxi mum stabilization of emission properties and minimum effective work functions for the cathode and to the method of manufacturing the same.

R is generally well known that a reduction of the work function of pure tungsten thermionic cathodes can be achieved by the inclusion or addition of a relatively small amount of thorium oxide in the tungsten base metal. This improvement of emission is generally attributed to the reduction of thorium oxide to metallic thorium during high temperature annealing. The metallic thorium forms a surface coating (generally monatomic) on the tungsten carrier metal. Such thorium-film cathodes, or as they are generally referred to thoriatedtungsten cathodes, have a lower work function than pure tungsten cathodes and/or pure thorium cathodes. These type of thorium films must be produced through the reduction of thorium oxide added to a tungsten cathode and the film must be maintained on the tungsten cathode during the operation thereof.

Further, it is generally known that the above described thoriated tungsten cathode can be improved by converting the outer surface of the tungsten into tungsten carbide. A tungsten carbide coated thoriated tungsten cathode has a more favorable reducing effect on working temperatures than thorium oxide. Conversion of tungsten to tungsten carbide is generally achieved, for example, by carburization of the thoriated tungsten cathode base at elevated temperatures in a reducing gas atmosphere containing a vaporized hydrocarbon. This is generally known as the carburization process. General- 1y, to 40% of the outer cross-sectional area of the thoriated tungsten cathode base is converted to tungsten carbide.

Another method of reducing the work function of a thoriated high-emitting cathode is also known. This method consists of transferring normally existing hexagonal structured tungsten carbide crystals into modified cubic body-centered tungsten carbide crystals by subjecting the same to diffusion-annealing conditions at substantially elevated temperatures in the presence of foreign metals or metalloids of the Groups IB, VII-A "United States Patent 0 3,697,321 Patented Oct. 10, 1972 or VIII-A from the Periodic Chart of Atoms, such as silver, manganese or palladium.

A common disadvantageous feature of the aforesaid known thoriated tungsten cathodes is that the carbide layers, which function as the reduction zones, forms an emission base for the cathode on the surface of the cathode so that a relatively wide dispersion of the emission properties of the cathode is attained.

Accordingly, it is an object of the present invention to provide improved thoriated tungsten thermionic cathodes and a method of manufacturing the same eliminating the aforesaid problems.

It is another object of the present invention to provide a thoriated tungsten thermionic cathode having a minimal effective working function and a maximum stabilization of emission properties and a method of manufacturing the same.

It is yet a further object of the present invention to provide a thoriated tungsten thermionic cathode having an inner coating of tungsten carbide and an outer coating of a high melting metal having migrating properties capable of functioning as an emission base for thorium and a method of manufacturing the same.

Other features, objects and advantages of the present invention will become more apparent with the teachings of the principles of the said invention in connection with the disclosure of the preferred embodiments thereof in the specification and claims.

The present invention provides a thoriated-tungsten (or thorium film) thermionic cathode for utilization in electron discharge devices, such as transmitter tubes. The thoriated tungsten cathode structure is provided with an inner layer of tungsten carbide and an outer layer of a high melting metal having migrating properties for thorium and functioning as an emission base for thorium while having relatively higher work function than tungsten. The thoriated-tungsten cathode of the present invention provides effective separation of the reduction zone of the cathode from the emission zone thereof and thereby attains all desirable advantages of dispenser cathodes.

A preferred method of manufacturing the thoriatedtungsten cathode of the instant invention consists of providing a thoriated-tungsten base structure (i.e. the tungsten wire having incorporated therein a few parts per hundred of thorium oxide), and subjecting the same to the heretofore explained carburization process for a period of time suificient to convert an outer portion of the base structure to tungsten carbide thereby forming a substantially uniform coating surrounding the base structure. This carburized base structure is then provided with a further or outer substantially uniform coating of a high melting metal, which has migrational properties. The high melting metal coating on the outer layer of the carburized cathode structure provides a base for the migration of thorium thereon at working temperatures of the cathode so that deposition of thorium by diffusion through this outer layer is sufficiently and continuously assured. Further, this emission base (outer layer) is continuously retained throughout the entire life or working time of the particular cathode and thus provides an effective separation of the reduction zone (tungsten carbide) from the emission zone (such emission base or outer layer is formed of high melting metals having a relatively higher work function than tungsten and having migrational properties for thorium). In other words, the outer layer has a thickness sufficient to provide effective separation between the reduction zone and the emission zone and allow controlled subsequent diffusion of thorium therethrough. Preferably, this thickness is about The high melting metal utilized to form the outer coating-or. emission base of the thoriated-tungsten cathode of the present invention, in pure form, has a higher work function than tungsten. Preferably, this high melting metal is selected from this group and most preferably consisting of osmium, rhenium and mixtures thereof since these metals combine with thorium film to yield a particularly low etfective work function in cathode structures.

The method of applying the outer layer, which functions as the emission base of the thoriated-tungsten cathode structure, can be selected from electrolytical or non-electrolytical chemical-reduction deposition from a suitable solution containing the high melting metal. Once the outer or second layer is deposited or formed on the first or inner layer of tungsten carbide surrounding the base thoriated-tungsten cathode structure, the cathode is ready for installation and use in electron discharge devices.

Thus, comparing the preesnt thoriated-tungsten cathode having a tungsten carbide layer and methods of manufacturing the same ,to the aforesaid prior art thoriatedtungsten cathodes having a tungsten carbide layer composed of cubic body-centered crystals thereof (achieved by diffusion-annealing in the presence of foreign metals or metalloids from the Groups IB, VII-A or VIII-A) it will readily be appreciated that no structural conversion (i.e. crystal structure re-arrangement) of the carbide layer in the cathode of the present invention need be effected by high temperature annealing in order to activate the formed cathodes.

carburization of the metal carrier (i.e. tungsten) has an adverse effect on the mechanical strength, particularly the breaking strength of such metal. It has now been found thatwthe: mechanical strength of a carrier metal can be materially increased by utilizing a tungsten-rhenium alloy as the carrier metal. Preferably, the tungsten-rhenium alloy contains about to 25% by weight of rhenium. This increase in mechanical strength is further increased by utilizing an outer layer composed of rhenium. Rhenium counteracts embrittlement of cathode structures to such a degree that, among other things, practically no carbide is formed on the cathode structure. Thus, the carrier metal for the thermionic cathode of the present invention is composed of a relatively high melting metal selected from the group consisting of tungsten and tungsten-rhenium alloy. As will be appreciated, the carrier metal is contaminated with (or contains relatively small amounts of) thorium oxide.

Typical production methods that have been found eX- ceptionally suitable in the practice of the present invention are set forthin the following tables:

TABLE I (a) Mechanically producing a cathode structure from tungsten wire having a 1.5% thorium oxide content;

(1)) mounting the cathode structure on K-carriers;

(c) carburizing the cathode structure by pre-annealing under high vacuum conditions at 1700" C. for about minutes and then annealing in a benzol atmosphere at about 1620" C. for about 5 minutes to achieve a car- 'burized layer of W C in a thickness of about ,um.;

(d) galvanically coating the resultant structure with a layer of osmium in a thickness of about (LS/1111.;

(e) annealing the resultant structure under high vacuum conditions at 1 600, C. for about 10 minutes; and

(f) installing the finished cathode structure into the intended tube.

TABLE II (a) Mechanically producing a cathode structure from a tungsten-rhenium alloy containing about 24% (by weight) rhenium and having a 1.5 thorium-oxide content;

(b) mounting the cathode structure on K-carriers;

(c) carburizing the cathode structure by pre-annealing in nitrogen-hydrogen atmosphere containing 10% hydrogen gas at 1900 C. for about 1 minuteand then annealing in a nitrogen-hydrogen (:10 ratio) atmosphere having about 1.6% benzol vapor therein at about 1750 C. for about 1 minute to achieve a carburized layer of W C in a thickness of about 20 ,um.;

(d) applying a layer of vaporized rhenium by cathode atomizers in a thickness of about 0.7 ,um.;

(e) annealing the resultant structure under relatively high vacuum conditions at 1500 C. for about 10 minutes; and

(f) installing the finished cathode structure into the intended tube.

The manner in which the thorium film thermionic cathode of the instant invention can be produced will become more apparent to those versed in the art by reference to the following examples, which are intended only to be illustrative and not limiting the scope of the invention in any way.

EXAMPLE I A commercial wire of tungsten was obtained in blank etched form that had about 0.7% to 1.8% by weight of thorium oxide incorporated therein. This wire was mechanically shaped into a desired cathode form, subjected to inserted stress relieving annealing and mounted on an appropriate cathode carrier.

The cathode structure was then subjected to pre-annealing under relatively high vacuum conditions at temperatures ranging from about 1650 C. to 0 C. for a pe: riod of time ranging from about 5 to 15 minutes. Thereafter the cathode structure was carburized by annealing the same in a benzol vapor atmosphere (or some other hydrocarbon atmosphere) at temperatures in the range of about 1580 C. to 1670" C. for a period of time ranging from 3 to 8 minutes. The carburization results in a tungsten carbide layer thickness of about 25 ,um.

An osmium coating of about 0.5 urn. thickness was then applied by well known galvanic separation methods. The entire structure was then annealed under. relatively high vacuum conditions at temperatures in the range of about 1550 C. to 1650 C. for a period of time ranging from about 5 to 15 minutes. The cathode was then mounted in an appropriate electron discharge tube and produced excellent results as outlined hereinbefore.

EXA'MPLE II A commercial wire of tungsten-rhenium alloy (containing 24% of rhenium) having about 0.7 to 2.0% by weight of thorium oxide incorporated therein was obtained to produce a carburized thorium tungsten cathode with increased mechanical strength and increased shock resistance in accordance with the principles of the instant invention.

The thoriated tungsten-rhenium alloy wire was first deformed by mechanical manipulation into a suitable cathode shape and then mounted on a suitable cathode carrier.

The cathode structure was then subjected to a relatively short cleansing annealing in a 10% hydrogen-90% nitrogen atmosphere at about 1850 C. to 1950" C. temperature for a period of time ranging from 30 seconds to about 2 minutes. The cathode structure was then carburized in the same hydrogen-nitrogen atmosphere (10:90 ratio), to which was added about 1.6% by weight of benzol vapor at temperatures in the range of about 1700 C. to 1800 C. for a period of time ranging from 30 seconds to about 2 minutes. The carburization procedure produced a layer of tungsten carbide on the cathode structure having a thickness of about 20 um.

Thereafter a rhenium coating of about. 0.7 pm. thickness was applied by means of .cathode. atomization. The entire structure was then annealed under relatively high vacuum conditions at temperatures of about 1550 C. to 1650 C. for a period of time ranging from about 5 to 15 minutes.

The cathode was then mounted in a electron discharge device and exhibited the excellent properties described hereinbefore.

As will be appreciated, the various steps set forth in the above examples may be modified and interchanged between the various examples given.

It will be understood that modifications and variations of the above described preferred embodiments of the principles of the present invention may be effected without departing from the spirit or scope of the novel concepts set forth in the invention.

We claim:

1. A thoriated-tungsten thermionic cathode for electron discharge devices comprising a thoriated-tungsten cathode base structure, an inner layer on said base structure defining a reduction zone composed of a tungsten carbide substantially surrounding said cathode base structure and an outer layer on said inner layer defining a support or carrier layer for the emission zone composed of a relatively high melting metal having a relatively higher work function than tungsten and having migrational properties for thorium selected from the group consisting essentially of osmium and rhenium, said outer layer substantially uniformly surrounding said inner layer.

2. A thoriated-tungsten thermionic cathode for electron discharge devices comprising a thoriated-tungsten cathode base structure composed essentially of a high melting metal selected from the group consisting essentially of tungsten and a tungsten-rhenium alloy containing about to 25% rhenium, an inner layer on said base structure defining a reduction zone composed of a tungsten carbide substantially surrounding said cathode base structure and an outer layer on said inner layer defining a support or carrier layer for the emission zone composed of rhenium, said outer layer substantially uniformly sur rounding said inner layer.

3. A thoriated-tungsten thermionic cathode for electron discharge devices comprising a thoriated-tungsten cathode base structure, an inner layer on said base structure defining a reduction zone composed of a tungsten carbide substantially surrounding said cathode base structure and an outer layer on said inner layer defining a support or carrier layer for the emission zone composed of a relatively high melting metal having a relatively higher work function than tungsten and having migrational properties for thorium, said outer layer substantially uniformly surrounding said inner layer.

4. A thoriated-tungsten thermionic cathode as defined in 6 claim 3 wherein the outer layer is composed of osmium.

5. A thoriated-tungsten thermionic cathode as defined in claim 4 wherein the thickness of the outer layer is about 0.5 am.

6. A thoriated-tungsten thermionic cathode as defined in claim 3 wherein the outer layer is compose-d of rhenium.

7. A thoriated-tungsten thermionic cathode as defined in claim 6 wherein the thickness of the outer layer is about 0.7 pm.

8. A method of manufacturing a thermionic cathode for electronic discharge devices consisting essentially of the steps of: (1) providing a thoriated-tungsten cathode base structure, (2) subjecting said cathode base structure to a carburization process for a period of time, sufiicient to convert an outer portion of said cathode base structure into an inner layer which defines a reduction zone composed of a tungsten carbide substantially surrounding said cathode base structure, and (3) coating said inner layer with an outer layer, which defines a support or carrier layer for the emission zone, composed of a relatively high melting metal having a relatively higher work function than tungsten and having migrational properties for thorium, said outer layer substantially uniformly surrounding said inner layer.

9. A method of manufacturing a thermionic cathode as defined in claim 8 wherein the step (3) is effected through non-electrolytic chemical reduction deposition.

10. A method of manufacturing a thermionic cathode as defined in claim 8 wherein the step (3) is effected through electrolytic deposition.

11. A method of manufacturing a thermionic cathode as defined in claim 8 wherein the step (3) comprises coating rhenium by cathode atomization.

References Cited UNITED STATES PATENTS 2,204,391 6/1940 Allen 29-195 2,497,109 2/1950 Williams 117-217 3,373,307 3/1968 Zalm et a1. 313-311 3,488,549 1/1970 Amra 313-311 ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner US. Cl. X.R.