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 thermal-to-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 similar to 1150 K and a radiator temperature of similar to 550 K, for a 7-15 year mission lifetime. In addition to high strength, low density, and low brittle-to-ductile transition temperature, suitable materials must be compatible with the sodium working fluid and have low thermal expansion and low vapor pressure (< 133 nPa), Refractory metals and alloys are reviewed for their potential use as structural materials in vapor-anode AMTEC cells. Results indicate that Nb-1Zr (niobium-1% zirconium) is a suitable choice, particularly for the hot-end structures of the cell (> 900 K), C-103 (niobium-10% hafnium-1 % titanium-0.5 % zirconium) is also suitable, particularly for the cell's colder structure because of its higher strength and lower thermal conductivity. However, the compatibility of these niobium alloys with sodium at typical operating 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 specific mass.