HIGH-CURRENT DENSITY FIELD-EMISSION FROM TRANSITION-METAL CARBIDES

We report on fabrication, performance and modeling of field emission cathodes made from refractory transition metal carbides. The primary carbides under study include HfC, TaC, and ZrC. Single crystal carbide specimens were prepared by arc floating zone refinement from sintered stock. Emission patterns are shown along with emission stability data. Work function ordering of various crystal planes determined by means of field emission microscopy is reported and comparisons are made with results from thermionic projection microscopy. Typically, the carbides show that phi210,310 < phi100 almost-equal-to phi110 >> phi111. Results of effective thermionic work function measurements are presented for macroscopic clean crystal surfaces. An emission increase mode is described wherein applied tip voltages can be decreased by half, while simultaneous emission current increases of a thousand-fold are observed. Stable currents of 0.5 mA DC and lifetimes > 2400 h are reported. Stable pulsed field emission of > 10 mA for a single emitter at pulse lengths of 10 mus and duty factors of 0.01-0.001 are reported. Single-pulse currents as high as 48 mA were observed from a single emitter tip. Current densities have been calculated to be in the high 10(8) A/cm2 range.