Fabrication and surface characterization of composite refractory compounds suitable for thermionic converters

Procedures for fabricating high purity single crystals of LaB6 and SmB6 have been developed. Auger spectroscopy shows that a clean surface can be obtained by thermal heating at 1700 K; such a clean surface gives a value of B/La of 2.3 to 2.6 for the (100) face. The measured thermionic and FERP work functions of the (100) face of LaB6 are 2.47 and 2.28 eV, respectively. The electron reflection coefficient for LaB6(100) increases near zero primary energy to 0.5. Flash thermal desorption of an oxygen dosed surface showed that B2O3 is the desorption product at 1600 to 1700 K. A work function increase of 1.6 eV was observed on oxygen adsorption on LaB6(100). Thermal desorption of Co occurs at approximately 1300 K. A reduction of the work function to 1.39 eV was observed on adsorption of a partial monolayer of Cs on LaB6(100). The evaporation of LaB6 occurs as atomic La and B with a value of B/La of 6 to 3 in the temperature range 1700 to 2000 K. The activation energies of desorption for La and B are 6.3 + or - 0.3 and 6.8 + or - 0.3 eV, respectively.