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

We report on fabrication, performance and modeling of field emission c athodes made from refractory transition metal carbides. The primary ca rbides under study include HfC, TaC, and ZrC. Single crystal carbide s pecimens were prepared by arc floating zone refinement from sintered s tock. Emission patterns are shown along with emission stability data. Work function ordering of various crystal planes determined by means o f 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. Resul ts of effective thermionic work function measurements are presented fo r macroscopic clean crystal surfaces. An emission increase mode is des cribed wherein applied tip voltages can be decreased by half, while si multaneous emission current increases of a thousand-fold are observed. Stable currents of 0.5 mA DC and lifetimes > 2400 h are reported. Sta ble pulsed field emission of > 10 mA for a single emitter at pulse len gths of 10 mus and duty factors of 0.01-0.001 are reported. Single-pul se 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 r ange.