Enhanced thermionic energy conversion and thermionic emission from doped diamond films through methane exposure

Thermionic electron emitters are a crucial component in applications ranging from high power telecommunication, electron guns, space thrusters and direct thermal to electrical energy converters. One key characteristic of diamond based electron sources is the negative electron affinity (NEA) properties of hydrogen terminated surfaces which can significantly reduce the emission barrier. Nitrogen and phosphorus doped diamond films have been prepared by plasma assisted chemical vapor deposition on metallic substrates for thermionic emitter application. Electron emission current versus temperature was measured and analyzed with respect to the Richardson-Dushman relation, with work function and Richardson constant deduced from the results. Initial emission measurements up to 500 °C in vacuum were followed by emitter characterization while the sample was exposed to methane. Vacuum measurements indicated a work function of 1.18 eV and 1.44 eV for phosphorus and nitrogen doped diamond films, respectively. Introduction of methane resulted in a significant increase of the emission current which was ascribed to contribution from ionization processes which increase charge transfer from the emitter surface. This phenomenon was utilized in a thermionic energy conversion structure by introduction of methane in the inter electrode gap where a two-fold increase in output power was observed upon introduction of the gaseous species.