Operation and oxidation of thermionic dispenser cathodes studied by high resolution photoemission

Thermionic dispenser cathodes are practical devices whose action is dominated by their surface properties. At their operating temperature of 1450 K a near-monolayer barium oxide coating in dynamic equilibrium maintains a low work function. We have used high resolution synchrotron radiation x-ray photoelectron spectroscopy (SRXPS) to help elucidate the mechanism of operation and poisoning (by oxidation) of two types of cathode, one made from a pure tungsten matrix (B type) and the other coated with an osmium/tungsten alloy (CD type). SRXPS measurements have been made on cathodes both at room temperature and at their operating temperature. Comparison between these confirms that room temperature experiments are representative of operational cathodes, with no significant change in the chemical state of the surface species. For the B type cathode, the core level binding energies are consistent with the presence of a single layer of barium oxide at the surface, while for the CD type, the Ba is shifted to a lower binding energy and the O is shifted to higher binding energy suggesting additional electron transfer to the O. Each of the W 4f peaks for the CD type cathode is split, with the lower binding energy peak being W metal and the higher binding energy peak WO. The W has a lower binding energy than for the B cathode, ascribed to the lower coordination number of W in the W/Os lattice. Valence band spectra of reactivated cathodes showed none of the known contamination features and thereby confirm that the study is representative of operational cathodes. The valence bands of both types of cathodes are dominated by a peak at about 6.0 eV, attributed to O 2p with limited hybridization with the substrate valence orbitals. Poisoning of the cathodes by progressive exposure at room temperature to small amounts of oxygen produced a substantial increase in the work function and dramatic changes in the core level spectra. For the tungsten cathode, oxygen appears to react preferentially with the barium surface component, whereas for the alloy cathode the oxygen reacts almost exclusively with the tungsten atoms, to form WO2 and WO3. We conclude that, although the two types of cathode are superficially similar, in fact they have completely different surface and electronic structure. (C) 1998 American Vacuum Society. [S0734-2101(98)01904-6].