Sub 500 °C Vacuum Thermionic Energy Conversion based on Doped Diamond Thin Film Structures.

Franz Alexander Koeck and Robert J Nemanich

This research presents a demonstration of vacuum thermionic energy conversion at temperatures less than 500 °C. Vacuum thermionic energy conversion is an efficient means of transforming heat directly into electricity where a thermionic emitter is separated from a cooled collector by a vacuum gap. The negative electron affinity of diamond is a crucial aspect of obtaining a low temperature thermionic electron emitter. In this research we have prepared nitrogen - doped and ultra - nanocrystalline (UNCD) diamond multilayered thin film emitter structures on metallic substrates by microwave plasma assisted CVD. Electron emission is described in terms of the Richardson - Dushman equation which is based on the work function φ and Richardson constant A. Our new film structure demonstrate significant improvements in the emission which is attributed to the low resistivity UNCD interface layer as well as selective metal substrates. Films are assembled into a thermionic converter configuration where the emitter was separated from a collector with similar properties. At increased emitter temperatures, while keeping a cooler collector, a self - generated voltage appears at the electrodes. Operating the converter at 500 °C resulted in an open source voltage of V₀ ≈ 0.42 V. With an electrical load of variable resistance the output voltage of the converter was recorded. Maximum output power was achieved at a load voltage of ~ 0.1 V indicative of vacuum thermionic energy conversion at 500 °C. This research is supported through the Office and Naval Research TEC MURI.

MRS Fall 2008, SESSION T4: 1:45 PM T4.2