GB325492A - Thermionic cathodes for electric discharge devices - Google Patents

Abstract

325,492. Standard Telephones & Cables, Ltd., and Bedford, L. H. Nov. 16, 1928. Vacuum tubes. -The core of a thermionic cathode is provided with an active surface consisting of one or more alkaline earth metals by forming on a coating of the oxide or oxides, a deposit of one or more alkali metals from the vapour of the metal or metals; preferably barium and barium oxide are employed. The core may be a platinum nickel alloy containing 5 per cent of nickel and a coating of barium oxide is pro. vided in any known manner, or a coating of nickel oxide is produced by heating in air to a temperature of 900‹ to 1000‹ C. The filament is then mounted in the envelope within which is placed a source of metallic barium, situated preferably on the anode. Preferably as the source of the barium, a mixture of barium oxide. and peroxide is heated with aluminium to form metallic barium and aluminium oxide, the reaction vessel being filled up with an excess of aluminium oxide as soon as the reaction has taken place to prevent oxidation of the metallic barium. A portion of this mixture is ground, when cold, under molten paraffin wax and placed in a recess in the anode. A cover plate is secured over the recess, apertures being left to the edges to allow the escape of the barium vapour and to limit its effective vapour pressure. After exhaustion and removal of the paraffin vapour, the temperature is raised to distil off part of the barium vapour, the nickel oxide, when this is employed, being reduced. The envelope is sealed off and the distillation continued by heating the anode by electron bombardment from the cathode, the barium ions being directed to the cathode by a voltage gradient.