US2476590A - Cathode coating - Google Patents

Description

Jly 19, 1949. G. s. Il-:vANs

CATHODE COATING original Filed March 14, 1941 fan/P0555 56,55 Mfr/u 5,955 MEI-su v ATTORNEY m @mim atented July 19, M1949 cA'rHoDE vcorrrINGf George S. Evans, Fairmont, W. Va., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporationof Pennsylvania Continuation of application Serial No. 383,359, l March 14, 1941. This application July 3, 1943,

Serial No. 493,338

My invention relates to cathodes for discharge devices and especially to coated cathodes, and the present application is a continuation of my copending application, Ser. No. 383,359, filed March 14, 1941.

An object of my invention is to increase the rated peak current per unit area of cathode surface.

Another object is to provide a coated cathode able to withstand overloads withoutdestructive sparking. y

Other objects and advantages of the invention will be apparent from the following description and drawing in which:

Fig. 1 is a cross section through a base metal and coating formed thereon according to my invention.

Fig. 2 is a cross section through a base metal and standard coating thereon.

Coated cathodes utilizing alkaline earth oxides are very widely used as cathodes of discharge devices. The peak current demanded of a cathode usually determines its size. The present practice is to rate an oxide coated cathode at around five amperes peak currentl per square inch of coated area, depending to some extent upon the shape of thecathode. An increasein the capacity of the cathode would naturally be of great advantage.

Unfortunately, alkaline earth oxide cathodes exhibit a phenomenon known as. sparking. Sparking is a formation of an arc-like spot on the coated surface when the cathode is` overloaded. The arc-like spot moves over the surface, knocking Yoil the coating and soon rendering the cathode inoperable. The bond between the oxide coating and the base metal has a great influence on the ability of the cathode to withstand overloads without sparking, and if -a spark spot does form, a good bond will minimize the ydestruction of the coating resulting from the sparking.

In addition, the cathodervoltage drop has great influence on the life of an alkaline earth oxide coating, especially in vapors such as mercury and in the heavy. gases. A cathode coating with a lower cathode drop than the present type of coating minimizes the loss of life due to ion bombardment. Furthermore, the manner in which the present alkalinek earth oxide coating is applied to the cathode has a great influence on its emission, over-load capacity, and life. A new type of coating which need not be so carefully and exactly appliedwould be Vhighly desirable.

My invention comprises a new. cathode vcoating 14 Claims. (Cl. Z50-277.5)

v which has ahigher emission capacity, greater overload capacity, less destruction due to sparking, a lower cathode drop, and requires less skill 1n Yapplying toits base. I utilize a carbonate of one or more of the alkaline earth metals, and preferably utilize only barium carbonate. The base metal preferably is nickel but may be nickelcobalt-ferrotitanium sold under the trade name K OnaI, or any other suitable metal or alloy. I 'he barium carbonatev is preferably ball milled toa small particle size. The barium carbonate is mixed with a binder and then sprayed, painted, or applied by dippingto the base metal as in the prior art. While the cathode may be heat treated according to my invention before mounting it in the tube, I preferably mount the coated cathode in the tube and apply my invention after the tube has been processed during exhaust to the point of cathode activation.

The invention Ais to heat the cathode very quickly to atemperature at which the coating fuses. Specifically, the cathode should be heated to around 1100 C. at such a rate that the barium carbonate has small chance to totally or even appreciably decompose due to the heating before 1100 is reached. If the heating of the barium carbonate proceeded slowly as in the present practice of activating the cathodes, then the barium Vcarbonate decomposes to barium oxide and carbon dioxide. The barium oxide will not fuse except at extremely high temperatures.

The decomposition of the barium carbonate starts at about 500 C. and my invention is to heat the barium carbonate from 500 to a temperature of 1100 C. within thirty seconds, and preferably within twenty seconds or less. 'I'his rapid heating provides a phase consisting of barium carbonate with a certain percentage of barium oxide and carbon dioxide in the coating. The barium carbonate-barium oxide phasev will fuse to the base metal at around 1100 C. and the iluiy granular coating will be transformed into a smooth coating with the evolution of some of the carbon dioxide which has been dissociated from the original coating.

The fusion of the barium carbonate-barium oxide phase does not seem to be dependent on the alloying of the barium oxide with another metal oxide such as is required in the cathodes known to the art. The rapidly heated barium carbonate will fuse on base metals whose oxides the prior Iart teaches us willnot alloy with barium oxide to produce a satisfactory fused coating. Furthermore. barium carbonate can be fused by quick a rating of ve amperes per square inch for the standard coating on similar cathodes. 'lh bond;

between the base metal and the fused coating is so excellent that when a spark spot isA formed: on the coating due to an overload., only a small point on the coating is affected, as compared to the extensive stripping oi of; the coating; on astandard cathode. Y

Tests made on standard coating and my fused coating show that the voltage drop for the fused coating, at a certain current up to the current at which sparking occurs, is half the voltage tube drop for the standard coating at the samer current. Since the coating fuses, the loose parts of the coating, caused lay-*improper or careless method. of application, are melted down to good contactwith the base metal or underlying coating.

On the evidence of magnified photographs, I have illustratedE in Fig., 1 a cross section through aV base metal coated in accordance'with my invention, and in Fig.` 2 a cross section through a base metal and standard alkaline earth cathode. The standard oxide coated cathode in` Fig,Y 2 shows that. the surface hasla large specic area and that the coating. is continue ous. The standard coating is of a semi-ceramic crust in nature, being porous, and havingan appreciable thickness. In contrast, a; magnification of' an oxide surface fused by my method shows in Fig. 1 that 'the oxide4 exists in discontinuous patches on the. base metal, the base metal being exposed through the discontinuities of the coating. I believe that the discontinuous patches, which contain emission material comparable in quantity tothe ordinary coating becauseythe density isi greater, produces a more permanent and better bond because the material has fused onto the base metal in sucha manner that the coating can better followV the. expansion of the base metal due to heating. Thev ordinary coating, of a crust nature, does not follow the base metal expansion so well, causing- Vthe bond to breaky with the result that the coating flakes.v

A second advantage I believeA is obtained with the fused, coating as shown by the magnication, is that thehigh resistance nature. of a semiceramic crust` is obvated by the dense patches. This renders the cathode morel resistant to spark'- ing, as tests show. AV third advantage Ilbelieve is obtained is ythatV the: uncovered base metal,` which during operation is commonly thought to become active by virtue. of barium contaminaE tion from adjacent sources of barium, is. directly exposed to the discharge. path. One theory in the art devised to explain. the. mechanism of emission from oxide coated; c-iailflodes` has asits premise emission from the contaminated biase metal.v In accordance with this theory, the fused coating with exposed emitting` base metal arcas is believed to give better performance,y and testsv with my coating conrm this.

In Fig, 1 are disclosd the patches ofV coating fused to a density of material much higher than that of the standard coating. These discontinuous patches adhere tol the i935@ :metal and', withstand expansion of the metal. vThe 4 appearance of the patches of coating is somewhat globular.

My fused coating has better emission, more indestructibility, lower cathode drop characteristcs, and an adherence that lends itself admirably to.` applications involving high peak currents, such as ignitron firing, spotwelding, etc.

I claim:

1. A method of preparing a coated cathode which comprises applying a carbonate of an alkaline earth metal to a base metal and raisingv thev temperature vof the coating to approximately:A 11009 C. before the alkaline earth carbonate is appreciably decomposed due to the heatg. Y 2L The, method of preparing a coated cathode which comprises applying a carbonate of barium to a base metal and raising the temperature of the coating to approximately 1100 C. before the barium carbonate is appreciably decomposed due to theheatingf. f

3. The method or, ereperns a @atedY Gained@ whichl comprises applying; en; alkaline e. ih carbonatetof a base metal and raisins thetig-L perature of the coating from 500 C to 11.00 Cf. within thirty seconds f' 4f., Themethodf Q2 preparing, al seated @these which comprises 'applying an alkaline earth carbonate t0l a base metaland lvaifiirig,Y the temperature of .the Goaine'frcm 500 Q@ to 11.1199 C. within twenty seconds.

5f The: method. 0fl pre1@arms` a coated cathode which comprises applying barium: carbonate t9 a base metal and raising the; temperature ofthe coating from 5.0.0.o C: to. 1lQ0 Cl Within thirty seconds. Y

6. The method of preparing a coated cathode which commises applying pentiumA Carberlate t0 a base metal and raisingthetemperature,of the coating from 5.00, Cj. .to 1100or Qjwithin twenty seconds;

7. A thermionitype cathode .Comprising e substantialj base of stable metal thereby capable of withstanding heatv and current of the, order employedY in thermionic. cathodes, and` a barium oxide barium carbonate compound electrdne emitting material' on'Y said base in the formi of. promiscuous discrete' particles 0f irregular size. and shape, said material being stablej when heated andl emissive due to heating the cathode;

8. A cathode comprising a base "portion and a fused: barium carbonate coating' thereon having adhesionV to the base portionA by fusionthere? withand thereon;

9.' A cathode comprising a base-portion and a fused' barium" carbona'lie-bariumY oxide coating thereonY having adhesion to; theA baseY portion by fusion therewitnfandlthereon. Y

l0". A cathode comprising a base portionA of, nickel and a fused -barium'carbonate Vcoatingv thereon having adhesion tothe base-portion'by fusion therewithandithereon.

11. A4 cathode comprising ai base portionof, nickel and a fusedk barium carbonate-barium ox*- ide coatingV thereonV having adhesion tothe base portion by fusion therewith andithereon.

12. Al cathode comprising a` baseV portion or an alloy of nickel-cobalt and ferrotitanium andl a fused coating` thereon having adhesion to the base.V portion by*V fusion therewitlrand1 thereon.

13. A.k cathode comprising a.` base portion of an 'alloyof: nickel-cobaltv and ferrotitanium andv a fused bariumv .carbonate coating' thereon hav-A ing adhesionto the'fbase portion'byffusonthere with andthereon.i i

5 14. A cathode comprising a base portion of an alloy of nickel-cobalt and ferrotitanium land a fused barium carbonate-barium oxide coating thereon having adhesion to the base portion by fusion therewith and thereon.

GEORGE S. EVANS.

REFERENCES CITED UNITED STATES PATENTS Number Name Date Schaefer et a1. July 7, 1931 Number Name Date Marden Apr. 25, Duiend'ack et a1. May 16, Lowry May 29, Pearcy Nov. 20, Forst Sept, 24, Essig Dec. 29, Varian Mar. 30, Edwards et a1. May 25, Kolligs et al. Sept. 5, Bandringa et a1. Apr. 23, I-Iickok Sept. 16,

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