US2491284A

US2491284A - Electrode for electron discharge devices and method of making the same

Info

Publication number: US2491284A
Application number: US715902A
Authority: US
Inventors: Raymond W Sears
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1946-12-13
Filing date: 1946-12-13
Publication date: 1949-12-13
Anticipated expiration: 1966-12-13

Description

Dec. 13, 1949 R. W. SEARS ELECTRODE FOR ELECTRON DISCHARGE DEVICES AND METHOD OF MAKING THE SAME Filed Dec.

5r METAL rALLOV BASE ME TAL /N I/E N TOR R544 SEARS BV 04mm 6.74mi,

A TTORNEV R. w. SEARS 2,493,28 ELECTRODE FOR ELECTRON DISCHARGE DEVICES AND METHOD OF MAKING THE SAME Filed Dec. 13, 1946 r METAL BASE METAL INVENTOR Mm (2v Mia/L ,4 7TORNE r Patent actress ELEC'IRQDE F lBl-R. EL EQTRQN DKSCHARGE @BJWEEES asp IVEE'EEQD @11 THE Application December it, 119%, Serial No. 715,992

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This invention relates to electronic discharge devices and more particularly to such devices capable of supplying a high power output.

This is a continuation in part of my application, Serial No. 497,330, filed August l, 194$;novz abandoned.

In the usual power rectifier or amplifier device, the output electrode or anode is formed of a. highly refractory metal, such as tungsten, molybdenum or tantalum which can sustain the high eficiency output or the device without appreciable buckling or distortion clue to the intense heat energy dissipated in the anode. In order to facilitate the dissipation of heat energy. it is common practice to apply a black body material the anode surface in the form of a heat radiating coating. Carbon, a desirable coating substance at low energy levels. is unsatisfactory for operating temperatures above 10%" C. In the high energy range it is more practical to employ high melting point materials as the coating substance, such as zirconium, titanium and tantalum. However, the problem of adherence of the coat ing material to the highly refractory base metal. heretofore, has greatly handicapped the manuiacture of such devices and the longevity of operation of power rectifiers and similar devices, since peeling and chipping of the coating rentiered the device unsuitable after a short period due to localizeci burning of the anode at the un protected spots. Furthermore, the binder ma torial. usually of a low melting point substance. exposed by the chipping, readily vaporized and; retarded the efficiency of the device by diffusing deleterious gases or vapors not absorbed by the getter properties of the and which reduced the high vacuum in the crevice.

Another disadvantage of employing these materials as coatings on a highly refractory base metal is the readiness of the material, perticu-= iarly zirconium, who heated in the presence of oxygen to change the character of the black body heat radiating material to a white insulating oxide which is not a satisfactory heat radiator. a further characteristic of the coating material employed as a high temperature heat radiation layer on the electrode develops during manufacture whenthe electrode is outgassecl in pumping procedure. If the coating attains a temperature of 1501? under high vacuum conditions. zirconium sisters to a hard brittle and compact matrix which destroys the good heat radiating and gas absorbing properties of the coating on the electrode. Similar efiects occur in the cases of coatings of tantalum or titanium.

Alli

An object of this invention is to overcome these difiicultles in high power discharge devices.

Another object of the invention is to materially enhance the operating life of discharge devices by eliminating low melting point metals or binders from the coating of the electrode.

A further object of the invention is to facilitate the eflicient application of the coating sub stance to the base metal electrode to insure constant adherence.

Another object oi the invention is to provide a method of the application whereby an alloy interface is formed to produce a more stable bond be tween the coating and the base metal.

A further object is to provide a heat treatment for the electrode and coating which insures positive adherence of the coating without endangering the properties of the coating by high sintemperatures.

These objects are obtained in accordance with this invention by introducing an agent in the nature of a. catalyst in the processing of the coating on the base metal, the agent being a metal of the iron group, such as nickel, cobalt and iron, which is vaporized during the heat treatment oi the coating and. promotes the formation of an alloy interface of the coating and base metals which firmly bonds the porous coating to the base metal.

in a preferred aspect of the invention, the coating material is applied to the base metal and the electrode is heated in nickel vapor in a low pressure environment to facilitate the alloying of the coating metal to the base metal at the iuterface and provide a permanent bond between the coating and the base metal. The temperature of treatment is sufficient to completely eliminate all. binder material from the coating and produce alloying on the surface of the base metal through the catalyst-like action of the nickel vapor. The nickel does not combine with the base and coat ing metals since it is continually in the vapor phase so that be low melting point metal entersinto the constituents of the composite electrode. The iron group metal agent may be introduced as a part of the coating substance and during the heat treatment is reduced to the vapor state to perform its function in partially alloying the coating to the base metal. Any desired degree of interfacial alloy thickness may be obtained by varying the temperature and time of treatment in the vapor or the amount of metal agent incorporated in the coating material.

These and other aspects of the invention will be more clearly understood from the following molybdenum cylindrical container or boat surrounds the electrode as a heating chamber or oven and a small piece of sheet metal. for example nickel, approximately 2 milligrams weight, is welded to the inner surface of the boat. fhe bottle is evacuated by the pumps to a low pressure, of the order of 1 x ill" millimeters of mer cury, to secure a high vacuum in the system. The molybdenum container is heated by high frequency induction by surrounding the bottle in the vicinity of the boat with an external high frequency coil supplied with voltage cur rent to inductively heat the container to a term perature sufficient to radiate heat to the coated electrode so that the temperature oi the electrode is raised to 110G C. to 130W these tempera tures being below the sintering temperature 1509 C., 01 the zirconium coating. The temperature of the electrode may be observed by the use of an optical pyrorneter.

During the heating cycle the nickel, attached to the inner surface of the container, will be completely converted to the vapor phase under the low pressures within the bottle and necessarily increases the pressure within the bottle slightly to l x 10 to 2 x i0" millimeters of mercury de pending on the temperature. In the nickel vapor atmosphere at a temperature of 1300 l1. and a pressure of l K 16 the interfacial film of ziz conium at the surface or the molybdenum cleatrocie is alloyed therewi h to form a bonding linlrage between the case metal and the outer porous coating. The nic l is continually in the vapor phase so that it does not combine with the interfacial alloy and, therefore, does not introduce low melting point metal in the alloy. The nickel vapor acts in the nature of a catalyst in promoting alloys "5? between molybdenum and zirconium at temperatures below the sintering temperature of zirconium and molybdenum. After a period of five to seven minu es at the above temperature and pressure the desired intcriacial alloy is formed and the nickel vapor condenses on the cooler po tion of the glass settle. The nickel may be vaporized at a lower tern ature of 1100 at which point the vapor of the nickel. vapor is 2 13"" millimeters oi mercury, and heating for the same of five minutes. Of course. any desired degree of alloyage of the in.- terface material may be accomplished by varying the pressure, amount of nickel or the time ol treatment during the heating process. Further more, the iron group vapor treating method achieves con bonding of the porous coating to the i r. electrode at temperatures 280 to 406 C. below the sintering temperature of the coating material so that the porous suriece of the matrix is unimpaired during this heat treating method.

An alternative method may be practiced, in accordance with invention, which secures the same results wherein the iron group metal agent is incorporated in the coating metal or compound as a powdered material mixed with the coating metal and binder. The amount of the powdered metal agent incorporated in the coating material is substantially the same as used in the example heretofore described, being about per cent by volume of the coating material, i. e., 2 milligra. for the case of the specific tube heretofore noted, and the degree of allcyage at the interface being secured by determination of the amount of the iron group metal incorporated in the process, the time of treatment, temperature of the electrode and the pressure extant within the system. lllurlag the heating step of the process at the temperatures heretofore mentioned, the binder is vaporized and completely removed from the coating and the iron group metal agent is reduced to the vapor state by diffusion from the coating whereupon alloying of the coating and base is secured at the interface after a short period of heating, approximately five to ten minutes.

During the above-mentioned treatment the iron group metal agent in the vapor state is com pleteiy evaporated from the coating and condenses 0n the cool wall of the treating chamber.

Either of the methods above described may be used in producing coatings of tantalum or titanium. The quantity of iron group metal, the term perature, pressure and time of treatment are of the same order of magnitude as in the specific illustrations given for the forming of the zirconium coating.

While the invention been disclosed in corn nection with an output electrode or anode it is, of course, understood that the invention may be applied to other electrodes, such as a grid or com trol electrode, in which the porous coating is applied thereto for the purpose of inhibiting primary and secondary emission of electrons and to render the electrode relativel cool by heat racliation. Therefore, the invention should be lirnited only Within the scope of the appended claims.

W hat is claimed is:

l. The method of heat treating an electrode for an electron discharge device which comprises heating an electrode of metal of the group consisting of molybdenum and tungsten having a coating thereon of a metal oi the group consisting of titanium and zirconium in vacuum in the presence of a metal of the iron group in the vapor phase, and alloying an interfacial layer of said coating metal with the surface of said electrode.

The method oi heat treating a refractory metal electrode for an electron discharge device. said electrode being of the group consisting of tungsten and molybdenum and having a porous refractory metal coating thereon of the group consisting of Zirconium and titanium, which comprises heating said electrode and coating in a low pressure atmosphere at high temperature, introducing nickel vapor during the heating, and forming an alloy between said coating metal and electrode at the interfacial surface of said electrode at a temperature below the sintering temperature of said coating.

3. method of heat treating a refractory metal electrode for electron discharge devices having a porous refractory metal coating thereon. electrode being of the group consisting of tungsten. and molybdenum and said coating being of the group consisting of zirconium and titanium which comprises, heating said electrode and coating in a highly evacuated vessel at a temperature between 1100" C. to 13-00" 0., introduclug vaporized metal of the iron group during the heating period, and alloying said coating metal to the surface of said electrode to form an intermediate bonding film between said electrode and said coatin metal.

l. The method of coating an electron discharge device electrode having a highly refractory metal core of the group consisting of molybdenum and tungsten with a heat radiating highly porous metal of the group consisting of zirconium and titanium, which comprises, mixing a viscous binder carrier with said low vapor pressure metal, adding a powdered metal of the iron group, ap-

- plying the mixture to said metal core, and heat-