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Home  >  Volume 22 (2012)

Modification of Richardson-Dushmaan Equation for the Figure of Merit of a Thermionic Energy Converter Using Pure Tungsten and Tantalum as the Emitters by Alkasim Abubakar. Volume 22 (November, 2012), pp 119 – 128
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In this paper, a comparative analysis of maximum conversion efficiencies of thermionic converters of heat to electricity made of pure Tungsten, W and that made of  Tantalum, Ta metals as cathodes were done in terms of the potential difference between the top potential barrier in the inter electrode space and the Fermi level of the emitter, VE , the potential drop across a load impedance connected in series to the converter, VL and the potential drop to the necessary electrical connection to the collector, VC. An expression for the maximum conversion efficiency has been developed, which yields optimum values of load impedance, collector lead geometry and emitter work function in terms of collector voltage, emitter temperature, effective emitter emmissivity using the theoretical Dushmann constant, A. The constant depends on the work functions of the emitting metal surfaces, which experimentally varies from one metal to another. The modification of the Richardson equation was done based on this variation. The results show that low value of collector voltage is required for a high efficiency. Low radiation heat loss is required for a high conversion efficiency and relatively low values of  emitter work function are required for maximum conversion efficiency at ordinary emitter temperature. A constant deviation of 4% from the theoretical efficiency was observed. Also, the maximum conversion efficiency of Tantalum, Ta metal is higher than that of Pure Tungsten, W.

Keywords: Thermionic converters, emitter, potential drop, Richardson-Dushmann constant.