Review of Refractory Materials for Alkali Metal Thermal-to-Electric Conversion Cells

Refractory alloys are being considered as structural materials in multitube, vapor-anode alkali metal thermalto-electric conversion (AMTEC) cells for future use in radioisotope space electric power systems. In these power systems, the AMTEC cells would operate at a heat source temperature of » 1150 K and a radiator temperature of » 550 K, for a 7‐15 year mission lifetime. In addition to high strength, low density, and low brittle-to-ductile transition temperature, suitablematerialsmustbecompatiblewith thesodium working e uid and havelowthermal expansion and low vapor pressure ( 900 K). C-103 (niobium‐10% hafnium‐1% titanium‐0.5% zirconium ) is also suitable, particularly for thecell’ s colderstructurebecause of its higher strength andlowerthermalconductivity.However,thecompatibilityoftheseniobiumalloyswithsodiumattypicaloperating temperatures and in the presence of minute amounts of oxygen (>5‐10 ppm) for up to 15 years needs further evaluation. Despite the limited availability of rhenium, Mo ‐Re alloys, with a rhenium content of 14 ‐45%, are also good choices as structural materials in vapor anode AMTEC cells. However, their relatively higher density and thermal conductivity could lower the cell’ s performance and increase its specie c mass.