Chemistry and surface physics phenomena involved in the activation of impregnated tungsten dispenser cathodes

摘要

Summary form only given. In most fast wave and slow wave vacuum electron devices dispenser cathodes, which are made of porous tungsten impregnated with barium calcium aluminate, are used as the electron source. A very necessary process called activation must occur that results in barium uniformly covering the surface and remaining there for a sufficient time to provide a lowered work function. The key to this is the formation of a dipole of barium over oxygen over the metal substrate from which the electrons are pulled. A fully activated surface must have every available site occupied by the barium-oxygen dipole. This not only provides the lowest work function but also assures a constant and uniform source of electrons during the life of the device. The chemical reactions that take place and the processes required to remove the unwanted species from the surface are described.