XXXII. Thermionic Diodes as Energy Converters†

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Abstract
Power is dissipated in a resistor connected directly between anode and cathode of a thermionic diode when the cathode is raised to operating temperature. A diode is thus a direct heat-to-electricity converter. This paper analyses the conversion phenomena in a planar system and relates power output to the various diode parameters. In particular, it is shown that the limitation on power conversion efficiency brought about by the space charge barrier can be made arbitrarily small by sufficiently reducing the anode to cathode spacing. However, spacings of the order of 0·01 mm are needed to achieve reasonable efficiencies The paper is of interest in a pedagogical sense, since it throws light on various phenomena in the planar diode, including the effects of cathode and anode work functions. Numerical results are presented which are of general interest in diode theory, and there is a comparison of the exact diode characteristic solution (using η-ξ tables) and the well known Langmuir approximate expansion formula.