Compatibility of CoO, Cosub(0.5)Mgsub(0.5)O, and CoAl2O4 with containment metals

Long-term interactions of refractory metals and noble metals with CoO, Cosub(0.5)Mgsub(0.5)O, and CoAl2O4 were investigated between 10000C and 18500C to select encapsulation materials for high-temperature radioisotopic power applications of 60Co. Rhenium was the only refractory metal that exhibited no solid-state reaction with the oxides of cobalt. However, vapor transport of rhenium probably by ReO3(g) was found to be extensive at temperatures above 15000C because of intrinsic high oxygen pressures over the cobalt oxides and small (<10C) temperature gradients. Transport kinetics analyses suggest rhenium, then, may not be suitable container for cobalt oxides for thousands of hours at temperatures over 12000C. Short-term exposures (1000 h at 15000C) and thermodynamics analyses indicate that iridium will be compatible with cobalt oxides and that vapor transport will be negligible. Superalloys were shown to react with cobalt oxides at 10000C, primarily by reduction of cobalt oxides by chromium. (Auth.)