Thermionic Properties and Thermal Stability of Emitter with a (0001) Oriented Rhenium Layer and Graded Structure

Abstract

Thermionic energy conversion is a power generation method which can be used to convert thermal energy into electric energy directly. A thermionic energy converter is a nonmechanical device that has high reliability. Research has been carried out on thermionic energy converters for use in space and ocean environments. Rhenium exhibits excellent thermionic electron emission characteristics in a cesium plasma converter. Chemical-vapor-deposited rhenium with a preferred (0001) orientation should have advantages over polycrystalline rhenium with respect to thermionic emission. Rhenium layers with (0001)-oriented three-dimensional surfaces have been successfully deposited on molybdenum substrates. The work function of rhenium emitters was estimated to be 4.9 eV from the results of power generation tests. The maximum power of a thermionic energy converter incorporating a rhenium emitter is 2.0 W/cm2 when the temperature of the emitter is 1800 K, that of the molybdenum collector is 990 K and that of the cesium reservoir is 561 K. The composition and surface morphology of the rhenium layer may change as molybdenum from the substrate diffuses into the rhenium layer at high operating temperatures. In order to improve the thermal stability of the rhenium layers at elevated temperatures, tungsten layers were inserted between the rhenium layers and the molybdenum substrates to form graded composition interfaces. The tungsten layers prevented excessive diffusion of molybdenum into the rhenium layers.