Re/W/Mo and Ir/W/Ta Graded-Structure Emitters of Thermionic Energy Converter

Abstract

Rhenium and iridium exhibit excellent thermionic electron emission characteristics in a cesium plasma converter, owing to their low work function in cesium vapor. The maximum power density and output characteristics of an emitter surface with iridium or thenium layers were simulated, and the surface was expected to be improved compared with other refractory metals such as polycrystalline rhenium, tungsten and molybdenum. The rhenium/tungsten/molybdenum graded structure was fabricated by chemical vapor deposition. In order to improve the thermal stability of the rhenium layer at elevated temperatures, tungsten layer was inserted between the rhenium layer and the molybdenum substrate to form graded-composition interfaces. The iridium/tungsten/ tantalum graded-structure emitter has been successfully produced by annealing at high temperature following hot isostatic pressing. When the tantalum component reacted with the iridium emitter, a 2226 K low-melting-point phase appeared. A three-layer structure with tungsten as the intermediate layer was selected to enhance the thermal stability of the emitter. The procedure of an elastic thermal stress analysis of functionally graded emitters by the axisymmetric finite-element method is presented. Thermal stresses are reduced to form graded-composition interfaces of rhenium/tungsten/molybdenum and iridium/tungsten/tantalum systems in the simulated model. The maximum power of a thennionic energy converter of a rhenium/tungsten/molybdenum emitter is 7.9 W/cm 2 and that of an iridium/tungsten/tantalum emitter is 2.6 W/cm 2 .