Output Characteristics and Work Function Measurement of LaB₆ Thermionic Converter

Abstract

This paper reports on design and test fabrication of a poly-crystalline lanthanum hexa-boride (LaB₆) thermionic converter, and experimental results of the output power generation characteristics both in unignited and ignited mode operation, and the measurement of the LaB₆ emitter and the LaB₆ collector work function in cesium (Cs) vapor based on the data in unignited mode V-I characteristics.
Poly-crystalline LaB₆ emitter work function ø_E was obtained from the forward saturation current J_for in unignited mode V-I curves. The bare work function of the LaB₆ emitter ø_Ebare is 3.4 eV, and ø_E in Cs vapor is, for example, 2.68 eV at T_E/T_R=3.0, here T_E is the emitter temperature, T_R is the cesium reservoir temperature.
Poly-crystalline LaB₆ collector work function ø_c in Cs vapor was measured from the reverse saturation current J_rev in unignited mode V-I curves. Although ø_c depends on the emitter and/or the collector temperature, the minimum ø_c is 1.35 eV when T_c=720K, T_c/T_R=1.53, here T_c is the collector temperature.
The value of ø_cmin=1.35 eV is far less than the value of ø_cmin=1.6_??_1.7 eV of refractory metal collectors. Therefore the LaB₆ thermionic converter has possibility to greatly improve power generation efficiency of a thermionic converter.
The maximum power density of the LaB₆ thermionic converter in ignited mode operation was 0.39W/cm², when output voltage V=0.5V, output current density J=0.78A/cm², T_E=1, 746K, T_c=1, 018 K, T_R=454 K, and the interelectrode spacing d=1.1mm. The value of the barrier index was V_B=2.5eV.