US1921066A - Cathode for electron discharge devices and method of making the same - Google Patents

Description

Patented Aug. 8, 1933 CATHODE FOR 1 92 0 6 f ,7 ELECTRON DISCHARGE DE- VICES AND METHOD THE 7 :SAME r Leslie HerbertBedford, Aldwych, London,.Eng

" land, assignor to. Western Electric Company,

Incorporated, New York, N. Y., a Corporation of New York No Drawing. Application August 13, 1929, Serial 7 N 0. 385,697, and in Great Britain November 16,

, 2 Claims. (01. 250-275) This invention relates to thermionicfilaments or cathodes for use in electron discharge devices, valves or, the like. i a

The object of the invention is to providesuch a cathode or filament which has good emission and long life.

According to one feature of thejinvention a filament or cathode is manufactured so that it has an active surface which comprises one or more alkaline earth metals, and one or'more'me tallic oxides.

According to another feature the alkaline cathode and are alloyed together. Preferably said surface comprises metallic barium andba- 1 rium oxide.

, ide or strontium oxide or a'mixture.

Other features of the invention will be disclosed in the following description with reference' to a preferred method or process of pro-' ducingthe cathode or filament of the present inventioni In thermionic filaments of the oxide type the thermionic activity is attributed by some to the alkaline earth oxide and by others to a supposed film of alkaline earth metal. Ihave conducted experiments which lead to the hypothesis that there exists a complex of. barium metal and barium oxide which has a much higher activity than either constituent alone and the same is probably true of the other alkaline earth metals and of the rare earth metals. The present in-- vention is believed to provide a cathode having 'an active surface comprising a high percentage 7 of this complex. Thermionic filaments made in In order to obtain a cathode according to the procedure may be iol-- invention the following lowe d- A core or filament is provided with a coating.

of oxide, which may for instance be barium ox- The en-. velope containing the filament and other-electrodes is then evacuated and during the evacua tion a carefully determined amount 'of alkaline earth or rare earth metal is applied or added to the filament. This is more easily performed and controlled bypassing an ionizing current' through low pressure vapour of alkaline earth or ried to the cathodeby a potential gradient. The

following method is preferably employed to effect the introduction of the barium source into the envelope. A mixture of barium oxide, barium. peroxide, and metallic aluminum is heated, whereupon at a' certain temperature (depending principally upon proportions) a reaction rium and aluminum oxide. readilydistilled from the mixture. The application to the thermionic device may take place at any stage, that is to say either before or after the performance of the thermit reaction, or after earth or rare earth metals, and oneor more metallic oxides, form the active surface of the,

thedistillation of the barium; Measures are desirable to prevent oxidation or hydrating of the source while it is being applied, for preference,

to the plate of the valve. Saturated hydrocarbons carefully dried overv sodium have proved suitable as protection, but anyother form may be used, such as another metal. It is convenient to provide a small pocket in the plate (anode) of the valve which may contain the barium source and in orderto control the egress of the barium vapour therefrom a covering plate may be used to provide only a restricted outlet from said pocket. Any other means of reducing the vapour pressure of the barium may-be used instead of the above. r

. During the operation of the device, the space lytic effect, to establish a concentration of oxide at the cathode surface, with a corresponding concentration of alkaline earth or rare earth metal at thecore; counteracting this is the normal difiusion effect tending to annul all concentration gradients. The relative concentration of oxide at the surface is also increased by the evaporation of the metals which generally have a higher vapour pressure than the oxides.

current from the cathode tends, by an electro: 1

For these reasons the addition of alkaline'earth.

or, rare earth metals is carried further than is required by the proportion of the complex by a certain amount which is determined by the con- 'ditions under which the cathode is to run.. In

certain cases it is necessary to add a further exelectron discharge devices which comprises pro- -viding the surface of the cathode core with a rare earth metals,- the resulting ions being carthermionically active earth metal oxide, evacu- 2. The process of manufacturing'a cathode for electron discharge devices which comprises producing a coating of alkaline earth oxideson the cathode core, preparing a freaction mixture of said metal-metal oxide comlthrough the vapor of barium oxide, barium peroxide and aluminum, heating said mixture to reaction temperature to produce a product of metallic barium and aluminum' oxide, heating said' product in the presence of said cathode to distill the metallic barium therefrom,v passing an ionizing current said distilled, barium whereby the barium is drawn to the oxide surfaceof said cathode, and thereafter heating said cathode to intermix said barium with the oxides at the surface of said cathode. V

. "LESLIE HERBERT BEDFORD.

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