Cathode

Jan. 8, 1957

2,777,086

E. A. LEDERER

CATHODE

Filed July 26, 1952 r .

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United States Patent Office

2,777,086

Patented Jan. 8, 1957

2,777,086 CATHODE

Ernest A. Lederer, Essex Fells, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania

Application July 26, 1952, Serial No. 301,026

Claims. (Cl. 313—193)

My invention relates to thermionically-emissive cathodes and in particular relates to additions to the base material of thermionic cathodes that are activated by diffusion of the active material from within and are particularly adapted for operation in a hydrogen atmosphere.

In the U. S. Patent No. 2,121,589 to W. Espe, issued June 21, 1938, a cathode is described consisting of a porous tungsten base enclosing a cavity containing the oxides of the alkaline earth metals. The entire assembly is adapted to be heated. At the operating temperature which is near 1200° K., barium, the other alkaline earth metals and alkali metals diffuse through the porous tungsten base producing an emissive layer thereon which is capable of emitting electrons.

Cathodes of this type are found to operate satisfactorily in many respects in high-vacuum tubes and tubes having atmospheres of one of the inert gases such as argon, or even in mercury atmospheres, but their operation is poor in atmospheres containing hydrogen, since the electronemissivity falls off badly and barium is lost rapidly by evaporation. While the operativeness of my invention is in no sense dependent upon the correctness of this theory, it is my present opinion that this poor performance where hydrogen is present is due to reduction by the latter of a thin film of oxides or oxygen which is present at the tungsten surface in the case of such cathodes operating in a non-reducing atmosphere.

One object of my invention is accordingly to provide a novel type of thermionically-emissive cathode capable of operating satisfactorily in a reducing atmosphere.

Another object is to provide a tungsten-base cathode having a coating containing barium which will operate with high efficiency in an atmosphere containing a reducing gas.

Still another object of my invention is to provide a cathode having many of the characteristics of the type of cathode as described but which will operate much more satisfactorily than the latter in an electrical discharge tube having a hydrogen atmosphere.

Still another object to provide a new and improved thermionically-emissive cathode for electrical discharge tubes.

Other objects of my invention will become apparent upon reading the following description taken in connection with the drawings in which the single figure shows an electrical discharge tube having a cathode embodying the principles of my invention.

Referring in detail to the drawings which show the application of my cathode to a thyratron, a vacuum-tight container 1, which may be of glass, is provided with an anode 2 which may be of molybdenum supported on a lead 3 which is sealed through the top of container 1. A grid 4 supported in a grid-cage 5 is positioned just below the anode 2 and attached at its lower end to a grid-lead 6 and a support-pin 7 sealed to a stem 8 of conventional form. A pair of baffles 9 and 11 are supported from the grid-cage across the direct path between anode 2 and the emissive face of cathode disc 12. The cathode structure consists of a molybdenum cylinder 13 having an upper chamber covered by a sintered cathode disc 12 comprising a mixture of tungsten having admixed with it from 5 percent to 15 percent by weight of mag5 nesium oxide or other oxide which is isomorphic with oxides of the alkaline-earth metals and does not react with metallic barium or evaporate substantially in the temperature range encountered by the cathode during preparation and operation.

The porous tungsten cathode disc 12 may be attached to the end of the walls of chamber in cylinder 13 by welding, and this is facilitated by interposing a ring 14 of platinum or ruthenium or ruthenium powder painted between the tungsten and the molybdenum walls. The 15 cathode disc 12 thus forms the top of a chamber in which is placed a pellet 15 containing one or more alkaline-earth metals, such for example as a pellet made up of barium carbonate and strontium carbonate. The carbonates probably change to the oxides in the course of 20 tube manufacture.

The lower chamber of cylinder 13 contains a cathode heater 16 of usual type supported on in-leads 17 which are sealed through press 8. The cylinder 13 is enclosed by a cathode-shield 18 which may be of molyb25 denum and which is supported on a cathode-lead 19 sealed through press 8. The container 1 is exhausted and provided with an atmosphere of hydrogen at a pressure of around 0.5 mm. of mercury by methods conventional in the electric discharge tube art.

The sintered tungsten base of cathode disc 12 may be formed by mixing tungsten powder with one to twenty percent by weight of magnesium oxide, molding the mixture under pressure to produce a rod, and sintering the latter in a hydrogen-atmosphere furnace at a temper35 ature of about 1700 degrees centigrade for a time sufficient to yield an ingot having a density of about 11 to 12.