Work function variation versus alloy concentration for dispenser cathodes

The dependence of the work function (∅) of dispenser cathodes on the concentration of Ir and Re alloyed with the tungsten emitting surface has been determined for controlled porosity dispenser (CPD) cathodes and matrix type cathodes. ∅ versus concentration curves show a minimum with a distinct cusp near 50/50 concentration for CPD cathodes and near 40% tungsten for matrix cathodes. Our matrix cathode data (using Ir) is consistent (relative to the concentration at which a minimum occurs) with Os/W data derived from other work 6,15]. For all alloys the concentration at which the minimum occurs is independent of which noble metal is involved, but does depend on the nature of the tungsten substrate, and perhaps the deposition technique used to deposit the noble metal film. The data is interpreted in terms of a previously proposed model 7] involving enhanced bonding between Ba-Ir and O-W compared to Ba-W and O-Ir bonds. In addition we suggest a model which explains the cusp as well as other characteristics of the work function concentration curves in terms of a postulated surface phase transition.