Activation-induced surface chemistry and interface evolution in osmium-coated M-type thermionic dispenser cathodes

We report the structural, emission, and surface characteristics of an M-type thermionic cathode fabricated by BCA (Ba, Ca, Al) impregnation into a porous tungsten matrix. Mercury intrusion porosimetry and SEM confirmed ∼17 percent porosity, and EDS mapping verified uniform BCA distribution within the pore structure. The cathode exhibited strong thermionic emission, with zero-field current densities of 6–10 A/cm 2 as surface temperature increased from 1020 to 1090 °C. The work function, extracted using the Richardson-Dushman equation, was 1.48 eV, in agreement with reported M-type values. Lifetime testing under pulsed conditions showed stable initial emission and a decay time of 7.6 years. Photoelectron spectroscopy revealed surface transformation after activation, including BaO 2 decomposition and formation of Ba–O–W and Ba–O–Os bonding states. These surface modifications enhanced emission stability and reduced the work function. The results highlight the effectiveness of controlled porosity, uniform impregnation, and surface chemistry tuning in realizing high-performance and long-lifetime thermionic cathodes.