Structure of lanthanum-hexaboride-coated rhenium filaments

Abstract

Rhenium filaments electrophoretically coated with lanthanum hexaboride have been studied using a scanning electron microscope. Activation of these filaments appears to produce a coalesced layer of lanthanum hexaboride, nearest to the rhenium surface. This coalescence does not, however, extend to the surface of the filament, owing presumably to the relatively low thermal conductivity of the material. A definite cavity exists between the coalesced inner layer and the primary crystals in the surface layer which may partially explain the thermal ion desorption data from these surfaces. The results confirm that the preferential diffusion of lanthanum to the surface of the boride is not the activation mechanism for the low work function of these materials, and that the rhenium is acting as a truly inert support.