| Popis: |
The structure and composition of the emissive layers of activated and stabilize B-type and M-type cathode surfaces at operating temperature were investigated using X-ray photoelectron spectroscopy and high spatial resolution Auger Spectroscopy. The results indicate that the activating layer consists of a monolayer or submonolayer film of barium and oxygen of stoichiometry close to Ba101. The emissive layer was found to be vertically ordered, with barium concentrated in the first monolayer above oxygen. The primary difference observed between the B-type and M-type surfaces was increased coverage of the underlying metal by the barium/oxygen layer in the M-type case. The spectra of the emissive layer, although quite different from those of 'bulk' barium oxide, were consistent with a barium-oxygen bonding interaction. A theoretical model of the bonding of barium and oxygen to each other and to the underlying metal matrix surface was proposed, consistent with the above experimental data and with the published results of other workers in the field. A calculation of the lattice stabilization energy in bulk ionic BaO indicated that the bonding in the emissive layer must be predominantly covalent. In Appendix A, a new user-oriented method of plotting cathode emission test data in the form of Miram-plots of normalized current density against temperature is described. Examples are given of the manner in which such plots clearly delineate changes in cathode performance capability with time and specific cathode failure modes. The cathode physics of a novel failure mode of a high current density cathode involving saturation of pore center emission is described. |
