Investigation of molybdenum-44.5%rhenium as cell wall material in an AMTEC based space power system

A new generation of radioisotope space power systems based on AMTEC (Alkali Metal Thermal to Electrical Conversion) technology is presently being developed. The future application of this technology, as the electrical power system for outer planet deep space missions, is ultimately dependent on it being robust enough to withstand the mission’s operational environments (high temperatures, dynamic loadings, long mission durations etc). One of the critical material selections centers on the cell wall whose physical and chemical properties must provide it with sufficient strength and material compatibility to successfully complete the mission. Niobium-1%zirconium has been selected as the baseline cell wall material with a molybdenum/rhenium alloy being the cell wall backup material. While these refractory materials have been commercially available for a number of years, several of their physical and mechanical properties have not been completely characterized especially within the expected operating parameters of an AMTEC based space power system. Additional characterization of the selected refractory alloys was initiated by fabricating mechanical test specimens out of ∼0.5 mm $(0.020″)$ thick sheets of material. Test specimens were heat treated at 1073K and 1198K for up to 150 hours under an argon cover gas containing small concentrations of oxygen. Room temperature and high temperature (1073K/1198K) mechanical tests were performed to determine the effect of time, temperature, and atmosphere on the mechanical properties of the refractory alloys. In addition, since the fabrication of AMTEC cell walls requires the welding of sheet material into a cylindrical shape, preliminary electron beam welding studies were performed. Comparison of the various test results obtained on Mo-44.5%Re and Nb-1%Zr samples are discussed.