Structural engineering of impregnated scandate ScBa cathodes of new generation

Abstract

The results of research focusing on the thermionic emission and structure of impregnated WВа and ScВа scandate cathodes are summarized through the concept of a structural scale hierarchy in impregnated cathodes. Generalized systematization of data on the relationship between the emission characteristics of impregnated WВа and ScВа scandate cathodes based on W and W–Re skeletons with the structure and composition of the emitter layer of adsorbed substances determined by Auger spectrometry is proposed. The principles behind structural engineering of high-emission and longeval impregnated scandate cathodes are presented; they consists in improving the phase composition and structure at structural levels of different scales. The stability of the macro- and microscopic structural level is ensured by refractory skeletons made of tungsten and rhenium powders of uniform size, having an average particle size of at least 4 µm. The phase composition of the emissive substance  must include a required amount of the anion-deficient b-Ba₂ScAlO₅ perovskite phase. Compliance with these requirements allows impregnated ScBa cathodes to acquire reproducible and stable thermionic characteristics such as work function j_ef = 1.65–1.71 eV at 1070 K and service life longer than 10,000 h at a current density of 15–20 А/см² and elevated test temperatures of 1230 and 1340 K. They are provided by microscopic-level parameters. Barium scandate cathodes with an additional mesostructural level—films with a nanoscale, heterophase, and homogeneous structure—including the anion-deficient b-Ba₂ScAlO₅ perovskite phase, scandium tungstate, and tungsten show a higher level of work function j_ef (950 K) = 1.29 eV and j_ef (1000 K) = 1.34 eV and a saturation current density of 100–140 A/cm² at 1200–1400 K and over 20 A/cm² at 950 K. The service life of such a ScBa cathode is over 20,000 h at a test temperature of 1150 K and a current density of 10 A/cm² and at least 2,000 h at a current density of 80 A/cm² and a test temperature of 1250 K. Barium alumoscandate and barium tungstate compounds are proposed to be regarded as independent emissive materials with a lower work function.