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Two types of oxygenated collectors were tested against the same emitter, to evaluate their ability to provide stable, enhanced thermionic performance. These two types were: sublimed Mo oxide layers, and a Nb-O single crystal alloy. Initially, the Mo oxide layers provided excellent thermionic performance, but degradation occurred, latter associated with emitter material transport and condensation on the collectors. By contrast, when used in conjunction with an internal getter, the Nb-O alloy resulted in somewhat lower, but stable performance, after an activation process. This activation process was associated with changes in the surface chemistry.

The Conductively Coupled Multi-Cell Thermionic Fuel Element combines the best aspects of the single cell and multi-cell TFE designs. The key element in this TFE design is the emitter tri-layer. The emitter tri-layer places strict requirements on material properties especially the insulator material. Emitter tri-layers composites consisting of scandium oxide and tungsten were fabricated and tested. Scandium oxide has met the requirements of thermal stability, mechanical strength and electrical resistance lifetime tests. Initial results on cesium plasma compatibility test indicate that scandium oxide will meet the chemical compatibility requirement.