Effective work function measurements of advanced cathode materials via a thermionic projection microscope system

An experimental method for simultaneously determining the effective thermionic work functions of major planes of single-crystal surfaces is reported. This method has been used to examine transition metal carbides and the emitting surface of a prototype, thermionic fuel element (TFE) cathode. In addition to generating work function values, this method maps variations in the distribution of work functions. Applications for transition metal carbides could include cathodes for advanced thermionic energy conversion, radiation immune microcircuitry, β-SiC substrates, or high current density field emission cathodes. Transition metal carbide single-crystal specimens were prepared by arc floating zone refinement from sintered stock. Hemispherical cathodes, 0.75 mm in diameter, were made from these carbides and from a machined sample of a high creep strength, emitter prototype. Work function ordering for various crystal planes is reported through the thermionic projection microscope image-processing method. To understand the work function values obtained, scanning electron microscopy and optical microscopy were utilized. An optical image of a cross section of the fuel element which was electrochemically etched to reveal the crystalline structure and orientation of the CVD-W layer is presented