The base metal of the oxide-coated cathode

Since the discovery of its excellent thermionic emission performances, 100 years ago by A. Wehnelt, the oxide-coated cathode has been the most widely used electron emitter in vacuum electronic devices: in vacuum valves up to the late 60\´s, and afterwards in CPT\´s and CDT\´s. Schematically, this cathode is composed of a 60 to 150 thick nickel alloy substrate called the base metal, covered with a 50 to 100 thick porous layer of alkaline-earth oxide. The exact definition of its "emitting system", i.e. base metal and emissive oxide, varies from one company to another. It is noteworthy that the emissive oxide is either a mixture of BaO, SrO or BaO, SrO and CaO, respectively called double or triple oxide having always sensitively the same respective composition. The base metal composition is of primary importance for the cathode emitting properties and is very often proprietary of the cathode manufacturing companies. Actually, the definition of base metal is more an "art" than a science because of the large number of parameters to take into account and of their potential interactions.