US3630770A

US3630770A - Method for fabricating lanthanum boride cathodes

Info

Publication number: US3630770A
Authority: US
Inventors: Louis J Favreau
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1969-04-30
Filing date: 1969-04-30
Publication date: 1971-12-28
Anticipated expiration: 1988-12-28
Also published as: DE2020709A1; FR2040505A1; GB1304216A; NL7006281A

Abstract

A method for fabricating cathodes with a lanthanum boride coating is disclosed in which the coating is applied to any base metal by spraying with a plasma flame spray. One plasma source is a spray gun utilizing an electric arc discharge through which a plasma gas is passed so that a plasma of ionized gas issues from the spray gun into which is injected a carrier gas having lanthanum boride powder suspended therein. The high-temperature plasma flame appearing at the nozzle of the spray gun melts the lanthanum boride (LaB6) almost instantly and the droplets formed thereby are carried to the surface of a prepared base material.

Description

2,659,685 11/1953 Lafferty States atent Louis J. Favreau Elnora, N.Y.

Apr. 30, 1969 Dec. 28, 1971 General Electric Company Inventor Appl. No. Filed Patented Assignee METHOD FOR FABRICATING LANTHANUM BORIDE CATHODES n aw/m OTHER REFERENCES Encyclopedia of Science & Technology Vol. 8, 1960 Mc- Graw-l-lill Primary Examiner-William L. Jarvis Attorneys-John F. Ahem, Paul A. Frank, Jerome C.

Squillaro, Frank L. Neuhauser, Oscar B. Waddell and Forman Joseph B.

ABSTRACT: A method for fabricating cathodes with a lanthanum boride coating is disclosed in which the coating is applied to any base metal by spraying with a plasma flame spray. One plasma source is a spray gun utilizing an electric arc discharge through which a plasma gas is passed so that a plasma of ionized gas issues from the spray gun into which is injected a carrier gas having lanthanum boride powder suspended therein. The high-temperature plasma flame appearing at the nozzle of the spray gun melts the lanthanum boride (LaB almost instantly and the droplets formed thereby are carried to the surface of a prepared base material.

METHOD FOR FABRICATING LANTI'IANIJM BORIDE CATI'IODES BACKGROUND OF THE INVENTION This invention relates generally to improved cathodes for electron discharge devices and more particularly pertains to a method for making an improved cathode by the application of lanthanum boride.

The borides of the alkaline earth and rare earth metals and thorium have been found to improve the thermionic emission characteristics of cathode materials such as nickel, tungsten, tantalum, molybdenum, and rhenium when such metals are coated with the aforementioned borides. The desirable properties of the rare earth metals are well known in the art and are fully discussed, for example, in US. Pat. No. 2,639,399 to J. M. Lafferty. As disclosed therein, lanthanum boride, LaB, is particularly desirable for electron emitters. One of the difiiculties previously encountered in utilizing lanthanum boride however is that it did not adhere readily to cathode base metals. Accordingly, there was a serious need for the development of a process for applying lanthanum boride (LaB to cathode base metals in such a manner that the LaB would adhere to the base metal. The present invention describes such a process.

SUMMARY OF THE INVENTION Briefly, the present invention pertains to a method for fabricating lanthanum boride cathodes including the steps of cleaning the base cathode metal by sandblasting techniques and spraying the base metal with molten lanthanum boride which may conveniently be obtained from a plasma flame spray gun operating with a SOO-ampere-arc current and with plasma gas passed through the arc to create a plasma flame into which is injected lanthanum boride powder suspended in a gaseous carrier. The resulting spray issuing from the nozzle of the spray gun is pennitted to strike the base metal to form a coating thereon. This coating is not only harder and more tenacious than any prior art coatings, but also produces a cathode with unusually higher electron emission characteristics than lanthanum boride cathodes made by conventional methods such as painting, vacuum sintering and cataphoretic deposition.

It is therefore an object of this invention to provide a method for applying lanthanum boride to cathode base metals so that a harder and more tenacious bond exists therebetween.

It is another object of this invention to provide a lanthanum boride coating to base metals which does not mar or chip when scraped with a sharp instrument and is not damaged when the base metal with the lanthanum boride coating is bent, twisted, or cut into strips.

It is a further object of this invention to provide a lanthanum boride cathode which exhibits higher thermionic emission characteristics than those made by conventional methods.

Other attendant advantages and features of this invention will be better understood from the following description taken in connection with the accompanying drawing. The scope of the invention will be more particularly pointed out in connec tion with the appended claims.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. l of the drawing, there is illustrated a plasma flame spray gun 1] for producing lanthanum boride coatings in accordance with the invention. Spray guns found suitable for this purpose are marketed by Metco of Westbury, Long Island, NY. and are designated by type Nos. 2M and 3MB.

The plasma flame spray gun It provides a plasma flame 12 which can be controlled over a wide temperature range so as to ensure the proper melt temperature for the lanthanum boride. The heat for spraying is generated by an electric arc of extremely high intensity and is transferred to the material to be sprayed by a plasma gas which is brought to the spray gun 11 through a gas line 13 which terminates at a point behind the electric arc. The plasma gas, being applied under pressure, drives the electric are through the nozzle of the spray gun Ill and directs the flame 12 toward the material to be sprayed.

The plasma gas consists essentially of a primary gas selected from the group consisting of nitrogen, argon, and helium and a secondary gas of hydrogen. The primary gas is mixed with from S to 15 percent by volume of the secondary gas to form the desired plasma gas.

The electric are utilized in the spray gun I 1 is provided by a direct current which is supplied from a rectifier type power supply unit through electrical conductors l4 and 15. As a result of the high temperatures created by the plasma flame, it is necessary to provide a circulating coolant through the nozzle of the spray gun. This coolant, which may be water, for example, is conducted to and away from the nozzle by a coolant line to appropriately connected to a heat exchanger.

The lanthanum boride is brought to the spray gun 11 in the form of a powder suspended in a carrier gas stream which is taken from the primary gas nitrogen, argon, or helium. The lanthanum boride suspended in the carrier gas stream is brought to the nozzle of the spray gun I] by a line 17. The lanthanum boride thus introduced into the nozzle of the spray gun 11 is melted by the flame 12 and the droplets formed thereby are propelled to the surface being coated by the blast of plasma gas as it rushes through the nozzle orifice.

As illustrated in FIG. I, the plasma flame 12 is directed toward strips of cathode base material such as, for example, tantalum, tungsten, nickel, rhenium, or molybdenum. The strips 18 are supported by a support member 19 which is held in a fixed position by a vise 20. The strips 18, the support member 19, and vise are all enclosed in a hood 21 which collects the hot gases and rents them to the outside air.

Having thus described apparatus useful in practicing the present invention, the method for depositing lanthanum boride on base metal cathodes will now be described. Assuming that it is desired to make a cathode of lanthanum boride coated upon a rhenium base having a configuration such as illustrated in FIG. 2, a piece of rhenium metal is selected so that cathode base strips can be subsequently cut therefrom. Before cutting and spraying however, the surface of the rhenium must be prepared properly. This is accomplished by washing the metal thoroughly in a solvent and then blasting the metal with an abrasive such as alumina with a particle size of 325-mesh at an air pressure of 7080 psi. The degree of roughening produced by the abrasive blasting naturally depends upon the type and mesh size of the abrasive, the air pressure, and the hardness of the surface. The foregoing air pressure and the particle size were found acceptable for the making of lanthanum boride coated cathodes, but are indicated solely for the purpose of illustration and are not meant by way of limitation.

After preparing the surface of the rhenium, the rhenium metal is then cut into strips of the size desired for the finished cathode. Considerable care must be exercised so as not to contaminate the surface of the rhenium after it is cleaned. The strips 18 thus out are placed or formed on the support member R9 in preparation for the plasma spraying.

The best lanthanum boride coatings have been achieved when the plasma gas has been formulated in approximate proportions of percent to 15 percent by volume of argon gas to hydrogen gas, respectively. Argon has been found to be preferable to nitrogen since nitrogen reacts with the lanthanum to cause nitriding which adversely affects the emission characteristics of the resultant cathode if exposed to moisture in the air. Helium, however, produces no such affects and could be used interchangeably with argon. The means for achieving this proportion or ratio of gases can readily be achieved by measuring the individual flow rates of the two gases into a unitary gas line. Such apparatus is a part of the prior art since it is found in the Metco type 2M and 3Mb plasma spray guns with which the present invention may be practiced, and accordingly need not be discussed further.

The purpose of the hydrogen in the plasma gas is to cause burning with the atmospheric oxygen in the area surrounding the plasma spray so as to provide more protection for the lanthanum boride droplets from undesirable chemical action than that afforded by the presence of the inert gas alone.

As described previously, the primary gas is also utilized as a carrier gas for the lanthanum boride powder. This is accomplished by introducing a portion of the primary gas into a powder feed unit (not shown) in which lanthanum boride powder is contained. The size of the lanthanum boride powder may range between 5 to 150 microns with very good results in the final coating; however, it is preferable to use lanthanum boride powder in the 5- to lO-micron size for the coatings described herein. The powder contained in the powder feed unit is agitated so that as the primary gas passes therethrough, the lanthanum boride powder is picked up and conveyed to the spray gun 11 through the gas line 17.

While various arc currents have been employed in the coating process, the most desirable results have been achieved when the are current for the spray gun is approximately 500 amperes. With this are current and the plasma gas supplied to the spray gun in the aforementioned proportions, the lanthanum boride powder introduced into the gas stream is melted by the plasma flame 12 and the droplets formed thereby are propelled to the surface of the rhenium strips. As illustrated in FlG. l, the plasma flame spray gun 22 is positioned or spaced from the rhenium strips by a distance such that the tip or point of the plasma flame 12 just strikes the surface of the rhenium strips. in this way, minimum heating of the base metal occurs and the best coating results are obtained.

FIG. 3 illustrates a cross-sectional view of a cathode strip 18 with a coating 22 of lanthanum boride applied in accordance with the instant invention. As described previously, the lanthanum boride coating applied by the foregoing process is much harder and forms a more tenacious bond with the base material than by any of the prior art processes. An additional advantage of the lanthanum boride coating applied in accordance with the instant invention resides in the higher thermionic emission characteristics of the resultant cathode which exceed those of lanthanum boride cathodes produced by conventional techniques. The exact reason for this improvement is not clearly understood; however, it may be a result of the existence of free lanthanum or possibly a result of an increase in surface area of emission.

Additionally, it has been found that cathodes made in accordance with the present invention are almost completely activated, i.e., capable of emission, apparently because some slight decomposition of the lanthanum boride takes place which releases free lanthanum metal. This has the additional advantage of eliminating the requirement of high-temperature activation and therefore may permit the use of tantalum and other metals as base metals for cathodes where lower emission characteristics can be tolerated.

While the invention has been described in connection with a specific type of plasma device, namely, a plasma flame spray gun, obviously other plasma spray means may likewise be utilized. Similarly, whereas the invention has been described with regard to cathode strips, obviously other configurations such as wires, cylinders, rods, discs, or any other shaped cathode may be used. Therefore, it is intended that the following claims include all such variations as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A method for making a lanthanum-bonde-coated cathode having improved thermionic emission characteristics and a tenacious bond between the lanthanum boride and a cathode base metal, said method comprising spraying molten lanthanum boride on the surface of said cathode base metal.

2. The method for making a lanthanum-boride-coated cathode as recited in claim 1 wherein the step of spraying comprises:

formulating a plasma gas with hydrogen and a gas selected from the group consisting of argon, nitrogen, and helium,

introducing the plasma gas into an electric arc to create a plasma of ionized gas, and

injecting lanthanum boride powder into the plasma of ionized gas whereby the heat created by the electric arc melts the lanthanum boride.

3. The method for making a lanthanum-boride-coated cathode as recited in claim 2 further comprising the step of:

cleaning the surface of the metal to be sprayed by washing with a solvent and blasting with an abrasive.

4. The method for making a lanthanum-boride-coated cathode as recited in claim 3 wherein the step of injecting lanthanum boride powder into the plasma of ionized gas further comprises suspending the lanthanum boride powder in a carrier gas selected from the group consisting of argon, nitrogen, and helium.

5. The method for making a lanthanum-boride-coated cathode as recited in claim 2 wherein the step of formulating a plasma gas comprises:

mixing 5 to 15 percent of hydrogen with 95 to percent by volume respectively, of a gas selected from the group consisting of argon, nitrogen, and helium.

6. The method of claim 1 wherein the cathode base metal is selected from the group consisting of tungsten, rhenium, tantalum, nickel, and molybdenum.

k i t

Claims

1. A method for making a lanthanum-boride-coated cathode having improved thermionic emission characteristics and a tenacious bond between the lanthanum boride and a cathode base metal, said method comprising spraying molten lanthanum boride on the surface of said cathode base metal.

2. The method for making a lanthanum-boride-coated cathode as recited in claim 1 wherein the step of spraying comprises: formulating a plasma gas with hydrogen and a gas selected from the group consisting of argon, nitrogen, and helium, introducing the plasma gas into an electric arc to create a plasma of ionized gas, and injecting lanthanum boride powder into the plasma of ionized gas whereby the heat created by the electric arc melts the lanthanum boride.

3. The method for making a lanthanum-boride-coated cathode as recited in claim 2 further comprising the step of: cleaning the surface of the metal to be sprayed by washing with a solvent and blasting with an abrasive.

5. The method for making a lanthanum-boride-coated cathode as recited in claim 2 wherein the step of formulating a plasma gas comprises: mixing 5 to 15 percent of hydrogen with 95 to 85 percent by volume respectively, of a gas selected from the group consisting of argon, nitrogen, and helium.