Component development for externally heated cylindrical thermionic converter

The development, fabrication, and evaluation of the components fof' an externally heated cylindrical converter having an interelectroLte spacing of 0. 010-hich are described.The objective of the current program is the development of converter x'eliability and long term performance capability.Previous 2 converters of the same basic geometry and size produced 5. 9 v/att/cm 2 (10 am/cm ) at an emitter temperature of 1600°C.The converter contains a 2 70 cm tungsten emitter vapor deposited by hydrogen reduction of tungsten hexafluoride and an insulated niobium collector prepared by electrodeposition of niobiimi over an alumiiia coated niobium-1 zirconium coolant tube.The more novel developments include the tri-layer collector, electron beam welded nested type tantalum bellows and gas pressure bonded tantalum-alumina cylindrical seals.The fabrication of an alternative planar graded cermet metal-ceramic seal based on gas pressure bonding niobium-alumina cermet layers is described.Gas pressure bonding techniques were also employed in fabricating the "fuel"emitter assembly by consolidating segmented tungsten "fuel" cores and bonding these in a tantalum cladding to the tungsten emitter.All converter components are joined by electron beam welding to provide maximimi reliability of the envelope.Component evaluation and process control are summarized.