US2447973A - Coated anode for electron discharge devices - Google Patents

Description

8- P. D. WILLIAMS 2,447,973

COATED ANODE FOR ELECTRON DISCHARGE DEVICES Original Filed April 6, 1946 INVENTOR Paul D. Williams ATTORNEY Patented Aug. 24, 1948 1 UNITED STATES PATENT OFFICE Paul D. Williams, Palo Alto, C alif., assignor to Eitel-McCullough, Inc., San Bruno, Calif; a corporation of California originali application' April 6, 1946, Serial No. 660,263. Divided and this application April 21, 1947, Serial N0. 742,773 j This is a division of my copending application Serial No. 660,263, filed April 6, 1946.

My invention relates to electrodes for electron tubes.

It is among the objects of my invention to improve the heat dissipation and gettering properties of electrodes such as anodes. v

Another object is to provide an electrode having a surface layer which is stable at high temperatures.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of my invention. It is to be understood that I do not limit myself to this disclosure of species of my invention as I may adopt variant embodiments thereof within the scope of the claim.

Referring to the drawing:

The figure of the drawing is an elevational view of a tube embodying my improvements.

In terms of broad inclusion, my electrode for an electron tube comprises a surface layer of metallic particles bonded together and to the electrode. The particles making up the surface layer include carbon and a metal such as zirconium, tantalum or molybdenum, zirconium being preferred, the carbon being combined with the metal to form a metallic carbide. In the case of an anode the improved electrode preferably comprises a hollow body of sheet metal, such as molybdenum, upon the outer surface of which the carbide layer is applied. The surface layer is preferably formed by coating the electrode with a mixture of finely divided metal and carbon, the major proportion of which is metal, and then heating the electrode to convert the carbon to the carbide and sinter the particles together and to the electrode.

In greater detail, and referring to the drawing, a tube illustrating my improvements comprises an envelope 2 having a reentrant stem 3 carrying an exhaust tubulation 4. A base 6 with prongs 1 is provided at the lower end of the tube. The envelope encloses a cathode 8, grid II and tubular anode '9, the anode having a cap t2 supported by bracket [3 on a lead [4 sealed to the upper portion of the envelope.

Grid II is of the cage type having a base ring 2| supported by brackets 22 on rods 23, one of which serves as a grid lead connected to a base prong 1 by a conductor 24. Cathode 8 is of the filamentary type comprising a spiral of thori- 1 Claim. (01. 250-285) 1 ated tungsten wire welded top and bottom to a The above described tube structure is merely for purposes of illustration and may be varied Within Wide limits.

My improvements, as embodied in anode 9, oomprisea surface, layer I5 of carburized metallic particles bonded or sintered together and to the anode, body. Body 9 is of a'suitable refractory metal'having a high melting point and low vapor pressure, molybdenum being preferred because it can be fabricated in sheet form and is relatively cheap compared to other metals in this class.

The particles in surface layer I5 comprise carbon combined as a carbide with a refractory metal or a combination of refractory metals, such as zirconium, tantalum, molybdenum or tungsten, zirconium being the preferred metal for several reasons hereinafter recited. The surface layer is preferably formed by preparing a fluid mixture of the finely divided metal and carbon, and applying this mixture to the electrode by brushing or spraying. In the case of a coating containing molybdenum, for example, the proportions are preferably parts by Weight of molybdenum powder to 10 parts of carbon powder, it being noted that the major proportion of the mixture is of the metal. In the case of a coating containing zirconium the ratio is preferably 91.2 parts by weight of zirconium powder to 6 parts carbon. These proportions provide about half the theoretical amount of carbon required to convert all of the metal in the coating to the carbide. In either case the powders are mixed with a suitable vehicle such as amyl acetate to a consistency for brushing or spraying.

After coating, the electrode is heated to a temperature sufficient to convert the carbon to a carbide and to sinter the particles together and to the electrode body. This heating is carried out in an oxygen free atmosphere such as in vacuum or in an inert gas such as argon or helium. Heating in vacuum is preferred because the coating is outgassed simultaneously with the carburizing step. With a coating containing molybdenum powder I have secured the desired result by firing the electrode in a vacuum furnace to about 1800 C. for about 5 minutes. With the preferred coating containing zirconium powder the firing is preferably done at a lower temperature for a longer period, say at 1400 C. for about 30 minutes. After firing, the anode structure is ready for sealing into the envelope, the tube from this point on being processed and evacuated in the usual manner.

My coated electrode has several advantages.

fact that an excess of metal is provided in the 'j coating, over and above that required to satisfy the carbon, means that the finaLcoating con- Y sirableinan anode.

tains some pure metal along with the. metallic carbide, so that the surface layer I5 comprises an intimate admixture of metalcarbideand uncombined metal. Some of thepuremetal is probably present as uncombined metal in'the core parts of the metallic carbide particles and some probably as metal particles between the carbide particles. In any event, the presence of this uncombined metal aids in binding the metallic carbide together in the coating, resulting in a very tightly cemented layer after the-sintering operation. Such a firmly bonded layer is important to prevent the particles from being dislodged under the electrical and thermal stresses set up in the tube during operation.

While any of the metals mentioned in connection with the coating composition yield the above mentioned desirable characteristics in the surface layer, I prefer to use zirconium powder because it has additional advantages. For example a layer composed of .zirconium and carbonv hasgoo'd gettering (ga absorption) properties for cleaning up residual gases in the tube.

-;.Also, a surface layer containing zirconium carbide tends to suppress electron emission from the electrode. Both of these characteristics ar de- ,I-claim:

Anianqderfor ;an; electron tube, comprising a ;:meta;llic1body,-;and:.a surface layer comprising an .intimateladmixtureoi zirconium carbide and uncombineldflzirconium bonded together and to the landdebody.

PAUL D. WILLIAMS.

REFERENCES CITED The following references-are of recordin the file of this patent:

UNITED STATES PATENTS Number Name Date 2,232,083 Strohfeldt Feb. 18,-1941 2,368,060 Wooten Jan. 23, 1945

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