High current density diamond based electron emitters for vacuum thermionic energy conversion*

Vacuum thermionic energy conversion utilizes thermionic emission to release electrons from an emitter into vacuum and collection at a counter-electrode. In our approach for an efficient thermionic emitter a multi-layer diamond thin film structure was synthesized by plasma-assisted CVD on a metallic substrate with controlled surface roughness including a nanodiamond pretreatment step. Introduction of nitrogen during ultra-nanocrystalline diamond (UNCD) film growth resulted in a low resistivity interstitial layer significantly enhancing emission current density which can be related to the Richardson constant. The top layer of polycrystalline nitrogen doped diamond was exposed to a hydrogen plasma inducing negative electron affinity characteristic presenting a low effective emitter work function $<$ 1.3 eV. Thermionic emission from this material commences at temperatures as low as 260$^{o}$C and observes the law of Richardson -- Dushman. From a data fit a significant Richardsons constant $>$ 2 A/cm$^{2}$ K$^{2}$ was extracted and at a temperature of 500$^{o}$C a thermionic emission current $>$ 5 mA was measured. This may well be the highest current density reported from a thermionic emitter operating at the moderate temperature of 500$^{o}$C.