US2874077A

US2874077A - Thermionic cathodes

Info

Publication number: US2874077A
Application number: US691999A
Authority: US
Inventors: Perry R Joseph; Constantin S Szegho
Original assignee: Rauland Borg Corp
Current assignee: Rauland Borg Corp
Priority date: 1957-10-23
Filing date: 1957-10-23
Publication date: 1959-02-17
Anticipated expiration: 1976-02-17

Description

Feb. 17, 1959 P. R. JOSEPH ETAL THERMIONIC CATHODES Filed Oct. 2:5, 1957 FIG. 4

4 :ment Vo|1oge(VRMS) SZEGHO INVENTORS.

PERRY R. JOSEPH SERGE PAKSWER CONSTANTIN S.

THEIR ATTORNEY.

United States Patent O THERMIONIC CATHODES Perry R. Joseph, Zion, Serge Pakswer, Elmhurst, and Constantin S. Szegho, Chicago, Ill., assignors to The Rauland Corporation, a corporation of Illinois Application October 23, 1957, Serial No. 691,999

12 Claims. (Cl. 117-201) This invention relates generally to vacuum tubes and more particularly. to thermionic cathodes and to a new and improved method and apparatus for processing cathodes which are adaptable for mounting in an envelope after the processing operations are completed. This application is a continuation-in-part of the co-pending application of Perry R. Joseph, et al., Serial No. 604,961, filed August 29, 1956, now abandoned, for Thermiouic Cathodes, and assigned to the same assignee as the present application.

In the process of making cathodes for conventional vacuum tubes, as presently used in television and radio circuitry, it is customary to coat a suitable metallic substrate or base such as nickel, or the like, with a coating of barium carbonate or a mixture of barium carbonate with carbonates of other alkaline-earth metals.

After the carbonate coating has been sprayed or otherwise applied to the substrate, itis necessary to employ an activation procedure in order to render the coating thermionically emissive. This procedure usually consists of first mounting the tube elements in an envelope which is then exhausted in a conventional manner; then the coated cathode is heated to a predetermined temperature for a short time. This does two things: first the carbonates are changed to oxides and second a further rise in temperature makes the oxides thermionically electronemissive. After the change is completed and the cathode coating is rendered thermionically emissive, the temperature of the cathode is then reduced for a short period of time to age the coating for normal operation, generally drawing some emission current from a positively charged electrode. The exact details of this activation processing, of course, are conventional and vary greatly in actual commercial production.

Oxide-coated cathodes produced in this manner possess many undesirable characteristics. Among these characteristics is the inability of the carbonate or the oxide coating to adhere to the metallic substrate during physical abuse of the cathode. Due to the relatively weak adherence of the carbonate coating, the cathodes must be handled with great care during mounting of the tube elements in the envelope. Another disadvantage of oxide-coated cathodes is that the oxide coating is easily contaminated or poisoned by residual gases which may still be present in the tube envelope after evacuation and gettering, thus greatly impairing the emission characteristics of the cathode. Furthermore, the cathodes are sputtered or otherwise destroyed by positive ion bombardment.

In the process of making cathodes for magnetrons and gas discharge tubes, it is customary to mix approximately an equal weight of nickel powder with the carbonate coating and to coat the cathodes with this mixture. After processing, such cathodes are reputed to have less tendency to sparking, improved durability when exposed to gases and physical abuse, and more resistance to ion bombardment. Recently, new types of cathodes have been proposed, one of which is the so-called Barium L- Cathode which has a plug or cap of porous tungsten or 2,874,077 Patented Feb. 17, 1959 molybdenum positioned to enclose the oxide mixture. When the cathode is heated, the active barium filters through the pores of the plug and covers the surface thereof to produce an emission surface of low work function.. The barium is then replenished by active material from the reservoir below the plug during operation of the tube.

Another type of cathode is the so-called sintered cathode. This type of cathode comprises a mixture of powdered nickel and carbonate pressed into pills or cylinders and sintered, preferably in an inert gas at a predetermined temperature. Such cathodes are reputed to be particularly robust against bad vacuum and let-down to air. However, the manufacture of sintered cathodes, to date, has necessitated several steps of complicated, expensive and time consuming operation and is not readily adaptable to the use of commercial mass production techniques.

Therefore, one of the principal objects of this invention is to devise a new and improved thermionic cathode.

Another object of this invention is to devise a new and improved sintered cathode which has the highly desirable characteristics of durability against contamination by gases, durability against physical abuse, high resistance to ion bombardment and low work function.

A further object of this invention is to devise a new and improved sintered cathode that can be processed externally and then may afterwards be mounted in an envelope which is subsequently evacuated and gettered in a conventional manner, and which may still later be removed from the exhausted envelope without damage to the cathode, thus making it possible to salvage the tube elements contained in a poorly exhausted tube during production.

Still another object of this invention is to provide a new and improved method for producing such sintered cathodes which is relatively simple, inexpensive, and readily adaptable to automatic assembly and mass production techniques.

In accordance with one feature of the present invention, a new and improved method for processing a carbonate coated cathode by utilizing a metallic elementhaving a melting temperature higher than the normal operating temperature of the cathode comprises the following steps in the presence of an inert atmosphere: The cathode is first heated to a predetermined temperature of the same order of magnitude as its normal operating temperature to reduce the carbonate coating to a thermionically emissive oxide. Particles of metal from the metallic element are then evaporated onto the oxide coating of the cathode for a predetermined time. Thereafter, the temperature of the cathode is maintained at least equal to its normal operating temperature to sinter the metal thereon. I

In accordance with still another feature of the present invention, a new and improved method and means have been provided for processing cathodes by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: The cathode is first heated to a predetermined temperature in the same order of magnitude as its normal operating temperature to reduce the carbonate coating to a thermionically emissive oxide. The metallic anode is then bombarded with electrons, or the like, and heated thereby to substantially its melting temperature to evaporate particles of metal therefrom and to deposit a layer of metallic particles onto the oxide coating of the cathode for a predetermined time. The cathode is then maintained at a predetermined temperature at least equal to its normal operating temperature to sinter the metallic particles thereon.

In accordance with still another feature of the present invention, a new and improved sintered cathode has been provided and consists essentially of a metallic substrate having a layer of thermionically emissive material coated thereon and a pervious layer of metal,'having a melting temperature higher than the normal operating temperature of said cathode, deposited on the first layer and permeated by thermionically emissive material from the cathode coating.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

Figure 1 is a schematic view, partly in section, depicting one form of apparatus for use in carrying out the process of the present invention;

Figures 2A and 2B are cross-sectional views of the cathode during processing;

Figure 3 is a view similar to that of Figure 1 showing an alternative form of apparatus for performing the process of the invention; and

Figure 4 is a graph showing the relationship between filament voltage and cathode temperature in carrying out an illustrative embodiment of the inventive process.

7 It is to be noted at the outset that throughout the specification and appended claims the term element is intended in its physical sense rather than in its chemical sense.

In the schematic view of Figure 1 there is illustrated one form of apparatus which is useful in processing and forming sintered cathodes in accordance with the present invention. A metallic substrate or cathode sleeve 10, composed of a suitable metal such as nickel, is supported within a glass envelope 11 in any conventional manner, such as by means of a ceramic washer 12. A coating 13, for instance a mixture of barium and strontium carbonates, is sprayed or otherwise formed on the depending face of cathode sleeve 10. A heater element 14 is inserted in cathode sleeve and is energized by a suitable source of electrical power, not shown, through connecting

lead wires

15 and 16. A metallic element or sleeve 17, of a material having a melting temperature higher than the normal operating temperature of cathode sleeve- 10, such as nickel, tungsten, tantalum, stainless steel, or the like, is mounted in axial alignment with cathode sleeve 10 by means of a ceramic ring 18 and is properly spaced in close proximity from carbonate coating 13 by

spacer rods

19 and 20. A second heater element or filament 21 is inserted in sleeve 17 and is also energized by a suitable source of electrical power, not shown, through connecting

lead wires

22 and 23. Base element 24 is secured to envelope 11 in any conventional manner to form an air tight .chamber and, in addition, supports spacers 19 and which, in turn, support

sleeves

10 and 17 in a predetermined spaced relation.

In the process of forming sintered cathodes in accordance with the present invention, the tube elements are mounted in envelope 11, as heretofore shown, which is then exhausted to a desired vacuum. If desired, the envelope may be filled with an inert gas such as helium although there is no particular advantage to be obtained in this manner since evacuation to a pressure of less than one millimeter of mercury provides a sufficiently good inert atmosphere. Cathode sleeve 16 is then heated by filament 14 to a predetermined temperature substantially equal to its normal operating temperature to reduce carbonate coating 13 to an oxide and to render the oxide coating thermionically emissive. The process to this point -may be entirely conventional. After the coating has been rendered thermionically emissive, metallic element 17 is then heated by filament 21 to effect evaporation of a metallic vapor, therefrom and to deposit a pervious layer of metallic particles 25 onto oxide coating 13, as shown in Figure 2. Thereafter, the temperature of cathode sleeve 10 is maintained at a predetermined value at least equal to and preferably greater than its normal operating temperature to sinter the metallic particles onto oxide coating 13' and in addition to permeate the metallic layer with oxide material from coating 13 as illustrated in Figure 2B.

After the processing is completed, the cathode may be removed from the envelope for storage and may later be mounted in another envelope, based, evacuated, and gettered in a conventional manner for commercial use. Such a processing technique is readily adaptable to the use of automation techniques for mass production of such sintered cathodes to maintain their cost at a minimum. In addition, it has been found that cathodes processed in this manner are particularly robust against poor vacuum and let-down to air so that it is now possible to salvage the cathode structure of a poorly exhausted tube without damage to the oxide coating, an operation which has not heretofore been possible. Such cathodes have excellent abrasive resistance even when brushed with a glass fiber eraser; they also possess the highly desirable characteristics of durability against contamination by gases remaining in the envelope after exhausting and gettering as well as a high resistance against destruction due to ion bombardment. The emitting surface of the cathode, in addition, possesses the highly desirable characteristic of a low work function.

In the alternative apparatus of Figure 3, metallic element 17 is heated directly instead of being indirectly heated as shown in Figure 1. In Figure 3, the elements are first mounted within the envelope as shown in Figure l, and then envelope 11 is evacuated to provide the required inert atmosphere. Cathode sleeve 10 is then heated to a predetermined temperature substantially equal to its normal operating temperature by filament 14 to reduce carbonate coating 13 to a thermionically emissive oxide and to create an electron space charge field about the oxide coating. After carbonate coating 13 has been reduced and rendered thermionically emissive, a positive electrical potential with respect to that of cathode sleeve 16 is applied to metallic element 17 from a source of unidirectional electrical potential, not shown, through connecting lead wires 26 and 27. Therefore, metallic element 17 now operates in the same manner as the anode of a conventional vacuum tube; application of positive potential to anode 17 directs a stream of electrons from the oxide coating toward anode 17 to effect bombardment thereof, which, in turn, increases the temperature of anode 17. As the bombardment by electrons is increased, a metallic vapor is evaporated or sputtered from anode 17 and deposited on cathode coating 13 in the form of a pervious layer of metallic particles 25 as shown in Figure 2A. Thereafter, the process is continued in the same manner described in connection with Figure l; the temperature of cathode sleeve 19 is maintained at a predetermined temperature at least equal to or greater than its normal operating temperature to sinter metallic particles 25 onto oxide coating 13' and to permeate the metallic layer with the oxide material to form, again, as illustrated in Figure 213, a thermionically emissive surface on the outermost surface of the metallic coating.

It is recognized that anode 17 may effectively be bombarded from a source of ions in any well known manner instead of being heated by electron bombardment. However, electron bombardment is preferred due to the inherent simplicity of the apparatus required to so utilize the electrons emitted from the cathode being processed.

As an actual working example of the process, cathode sleeve 10, composed of nickel and sprayed with a suspension of a suitable carbonate or carbonate mixture in a liquid binder, such as a nitrocellulose solution, amyl acetate for instance, is mounted in the manner heretofore shown at a spacing of approximately /2 centimeter from anode 17 which is also constructed of nickel.

Ceramic washers

12 and 18 are shielded both on top and bottom to prevent any deposit of evaporated metal thereon. Envelope 11 is evacuated to a pressure of the order of 2x10 millimeters of mercury, and an alternating voltage of approximately 4 volts (R. M. S.) is then applied to filament 14 for approximately 15 seconds which heats the cathode to a temperature of approximately 680 C. as shown by the graph of Figure 4. At this time the binder is driven off as evidenced by a rise in pressure which can be measured by an ionization gauge. The pressure then decreases to some intermediate value and stabilizes at that point. The voltage applied to filament 14 is then increased to approximately 8 volts which raises the temperature of the cathode to approximately 875 C. After a time lapse of approximately 15 seconds, reduction of the carbonate coating to an oxide begins as indicated by a slight rise in pressure in the envelope. After the reduction of the carbonate coating has started, the voltage applied to filament 14 is raised to from 9 to 10 volts, increasing the temperature of the cathode to approximately 915 C.960' C., depending on the pressure rise in the envelope, which is maintained preferably under one micron or 10X 10* millimeters of mercury at all times.

After about 30 seconds the reduction is completed and with continued heating, coating 13 is then rendered thermionically emissive as indicated by a rapid pressure drop down to the order of l l0- millimeters of mercury.

At this point, the filament voltage is increased to 12 volts, corresponding to a cathode temperature of approximately 1040 C., and a positive continuous or pulse potential of 50 volts is applied to nickel anode 17. The anode voltage is slowly increased to 850 volts, and the cathode current rises rapidly to from 120 to 150'milliamperes, at which time anode 17 becomes molten and a vapor of metallic particles is evaporated therefrom and is deposited on the oxide coating of the cathode in the form of a pervious layer of metallic particles. When the anode becomes molten ionization of the anode vapor occurs, at which time the anode potential is reduced to zero to prevent excessive anode material from being deposited on the cathode.

The metallic layer deposited on the oxide coating causes its electron emission to decrease. To compensate for this decrease in electron emission, the anode voltage is again increased to 850 volts D. C. T his-causes the oxide from the cathode coating to filter through the pores and crevices of the metallic layer to form a thermionically emissive layer on the outermost surface of the cathode. The emission increases rapidly to from 300 to 400 milliamperes at this time, and the anode voltage is then reduced to about 50 volts. The cathode is subsequently formed or aged by slowly raising the anode voltage back to 800 volts, until the cathode current stabilizes at from 90 to 120 milliamperes. At this point the cathode processing is completed and the cathode may either be operated at its normal operating voltage of approximately 6.3 V. A. C. corresponding to a temperature of approximately 800 C. or may be removed fro-m the apparatus for storage or assembly in a tube as the case may be.

It is to be pointed out that a very important aspect of the present invention resides in the fact that the various values of cathode temperatures utilized in each stage of the process are not critical. Because of the non-critical relationship of the various temperatures, each stage of the process may be successfully completed on a timetemperature basis within any reasonable range of temperatures. In other words, if in any stage of the process a temperature lower or higher than that shown in the specific example were utilized, it would be accompanied only by a respectively corresponding longer or shorter period of time in order to complete that particular stage of the process. In actual production, it is of course de- 6 sired to complete the process in a minimum of time, and therefore it is apparent that higher cathode temperatures are then utilized.

Thus the present invention provides a new and improved sintered cathode possessing the highly desirable characteristics of excellent abrasive resistance, immunity to poor vacuum or let-down to air, durability against contamination by gases, a high resistance against destruction due to ion bombardment, a thermionically-emissive surface of low work function, and adaptability to commercial mass production techniques.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A method of processing a carbonate coated cathode by utilizing a metallic element having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: heating said cathode to a predetermined temperature of the same order of magnitude as said normal operating temperature to reduce said carbonate coating to an oxide coating; evaporating particles of metal from said metallic element onto the oxide coating of said cathode for a predetermined time While maintaining said cathode at a temperature at least equal to said normal operating temperature to sinter said particles thereon; and thereafter maintaining said cathode at a temperature at least equal to said normal operating temperature for a predetermined time to stabilize said metalsintered oxide coating.

2. A method of processing a carbonate coated cathode by utilizing a metallic element having a melting temperature higher than the normal operating temperature of said cathode, comprising the following steps in the presence of an inert atmosphere: heating said cathode to a predetermined temperature of the same order of magnitude as said normal operating temperature to reduce said carbonate coating to an oxide coating; evaporating particles of metal from said metallic element onto the oxide coating of said cathode for a predetermined time while maintaining said cathode at a temperature substantially higher than said normal operating temperature to sinter said particles thereon; and thereafter maintaining said cathode at a temperature substantially higher than said normal operating temperature for a predetermined time to stabilize said metal-sintered oxide coating.

3. A method of improving the operating characteristics and the life of a thermionically emissive cathode by utilizing a metallic element having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: heating said cathode to a predetermined temperature at least equal to and of the same order of magnitude as said normal operating temperature; evaporating particles of metal from said metallic element and depositing a pervious layer of said metal particles onto said cathode while maintaining said cathode at a temperature at least equal to said normal operating temperature to sinter said metallic layer thereon; and thereafter maintaining said cathode at a temperature at least equal to said normal operating temperature for a predetermined time to permeate said pervious layer with thermionically emissive material from said cathode.

4. A method of improving the operating characteristics and the life of a thermionically emissive cathode by utilizing a metallic element having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: heating said cathode to a pre determined temperature at least equal to and of the same order of magnitude as said normal operating temperature; evaporating particles of metal from said metallic element and depositing a pervious layer of said metal particles onto said cathode while maintaining said cathode at a temperature substantially higher than said normal operating temperature to sinter said metallic layer thereon; and thereafter maintaining said cathode at a temperature substantially higher than said normal operating temperature for a predetermined time to permeate said pervious layer with thermionically emissive material from said cathode.

5. The method of processing carbonate coated cathodes by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: heating said cathode to a predetermined temperature in the same order of magnitude as said normal operating temperature to reduce said carbonate coating to an oxide coating and to render said oxide coating thermionically emissive; bombarding said anode with electrons emitted by said oxide coating to heat said anode to substantially said melting temperature and evaporate particles of metal therefrom onto said cathode for a predetermined time while maintaining said cathode ata temperature at least equal to said normal operating temperature to sinter said metal thereon; and thereafter maintaining said cathode at a predetermined temperature at least equal to said normal operating temperature for a predetermined time to process said cathode.

6. The method of processing carbonate coated cathodes by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: heating said cathode to a predetermined temperature in the same order of magnitude as said normal operating temperature to reduce said carbonate coating to an oxide coating and to render said oxide coating thermionically emissive; bombarding said anode with electrons emitted by said oxide coating to heat said anode to substantially said melting tem perature and evaporate particles of metal therefrom onto said cathode for a predetermined time while maintaining said cathode at a temperature substantially higher than said normal operating temperature to sinter said metal thereon; and thereafter maintaining said cathode at a predetermined temperature substantially higher than said normal operating temperature for a predetermined time to process said cathode.

7. The method of improving the operating characteristics and the life of a thermionically emissive cathode by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: heating said cathode to a predetermined temperature in the same order of magnitude as said normal operating temperature; bombarding said anode with electrons emitted by said cathode to heat said anode to substantially said melting temperature and evaporate particles of metal therefrom and to deposit a pervious layer of said metal onto said cathode while maintaining said cathode at a temperature at least equal to said normal operating temperature to sinter said metallic layer thereon; and thereafter maintaining said cathode at a predetermined temperature at least equal to said normal operating temperature and for a predetermined time to permeate said metallic layer with thermionically emissive material from said cathode.

8. The method of improving the operating characteristics and the life of a thermionically emissive cathode by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: heating said cathode to a predetermined temperature in the same order of magnitude as said normal operating temperature; bombarding said anode with electrons emitted by said cathode to heat said anode to substantially said melting temperature and evaporate particles of metal therefrom and to deposit a pervious layer of said metal onto said cathode while maintaining said cathode at a temperature substantially higher than said normal operating temperature to sinter said metallic layer thereon; and thereafter maintaining said cathode at a predetermined temperature substantially higher than said normal operating temperature and for a predetermined time to permeate said metallic layer with thermionically emissive material from said cathode.

9. The method of improving the operating characteristics and the life of a cathode by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: coating said cathode with a material which is rendered thermionically emissive when reduced; heating said cathode to a predetermined temperature at least equal to and of the same order of magnitude as said normal operating temperature for a predetermined time to reduce said coating; applying a positive potential to said anode to direct an electron stream from said cathode to said anode to evaporate particles of metal therefrom and deposit a pervious layer of said metal onto said cathode while maintaining said cathode at a temperature at least equal to said normal operating temperature to sinter said metallic layer thereon; and thereafter maintaining said cathode at a predetermined temperature at least equal to said normal operating temperature and for a predetermined time to permeate said metallic layer with thermionically emissive material from said cathode.

10. The method of improving the operating characteristics and the life of a cathode by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: coating said cathode with a material which is rendered thermionically emissive when reduced; heating said cathode to a predetermined temperature at least equal to and of the same order of magnitude as said normal operating temperature for a predetermined time to reduce said coating; applying a positive potential to said anode to direct an electron stream from said cathode to said anode to evaporate particles of metal therefrom and deposit a pervious layer of said metal onto said cathode while maintaining said cathode at a temperature substantially higher than said normal operating temperature to sinter said metallic layer thereon; and thereafter maintaining said cathode at a predetermined temperature substantially higher than said normal operating temperature and for a predetermined time to permeate said metallic layer with thermionically emissive material from said cathode.

11. The method of improving the operating characteristics and the life of a cathode by utilizing a metallic anode having a melting temperature higher than the normal operating temperature of said cathode comprising the following steps in the presence of an inert atmosphere: coating said cathode with a material which is rendered thermionically emissive when reduced; heating said cathode to a predetermined temperature at least equal to and of the same order of magnitude as said normal operating temperature for a predetermined time to reduce said coating; applying a positive potential to said anode to direct an electron stream from said cathode to said anode to evaporate particles of metal therefrom and deposit a pervious layer of said metal onto said cathode while maintaining said cathode at a temperature at least equal to said normal operating temperature to sinter said metallic layer thereon; and thereafter maintaining said cathode at a predetermined temperature substantially higher than said normal operating temperature and for a predetermined time sufficient to stabilize the electron stream from said cathode to permeate said metallic layer with thermionically emissive material from said cathode.

12. A cathode comprising: a metallic substrate; a layer of thermionically emissive material on said substrate; and a disjunct layer of metal particles having a melting temperature higher than the normal operating temperature of said cathode sintered to said first-mentioned layer and permeated by said thermionically emissive material.

References Cited in the file of this patent -UNITED STATES PATENTS 1,760,454 Ulrey May 27, 1930 10 Ramsay et a1. June 29, 1937 Kott Dec. 28, 1937 Kolligs Feb. 14, 1939 Germeshausen Dec. 27, 1949 Stanier May 30, 1950 Hughes et al. Ian. 18, 1955 Tversen Sept. 25, 1956 FOREIGN PATENTS 'Italy Aug. 9, 1938 Great Britain Apr. 29, 1940

Claims

1. A METHOD OF PROCESSING A CARBONATE COATED CATHODE BY UTILIZING A METALLIC ELEMENT HAVING A METLTING TEMPERATURE HIGHER THAN THE NORMAL OPERATING TEMPERATURE OF SAID CATHODE COMPRISING THE FOLLOWING STEPS IN THE PRESENCE OF AN INERT ATMOSPHERE: HEATING SAID CATHODE TO A PREDETERMINED TEMPERATURE OF THE SAME ORDER OF MAGNITUDE AS SAID NORMAL OPERATING TEMPERATURE TO REDUCE SAID CARBONATE COATING TO AN OXIDE COATNG; EVAPORATING PARTICLES OF METAL FROM SAID METALIC ELEMENT ONTO THE OXIDE COATING OF SAID CATHODE FOR A PREDETERMINED TIME WHILE MAINTAINING SAID CATHODE AT A TEMPERATURE AT LEAST EQUAL TO SAID NORMAL OPERATING TEMPERATURE TO SINTER SAID PARTICLES THEREON; AND THEREAFTER MAINTAINING SAID CATHODE AT A TEMPERATURE AT LEAST EQUAL TO SAID NORMAL OPERATING TEMPERATURE FOR A PREDETERMINED TIME TO STABLIZE SAID METALSINTERD OXIDE COATING.