Rate of Sublimation of Yb14MnSb11, a Thermoelectric Material for Space Power Applications

Abstract

The compound Yb14MnSb11 is a p-type thermoelectric material of interest for space power applications. However, average rates of sublimation previously measured at 1000°C were unacceptably high. In at least one study, Yb2O3 was observed on the surface after testing. In this study, the rate of sublimation of Yb14MnSb11 was measured at 1000°C by use of a vacuum thermogravimetric analyzer (TGA) which continuously measures weight loss as a result of sublimation. This experiment revealed that the rate of sublimation decreased with time, but also resulted in formation of Yb2O3 on the surface, even though the base pressure at the start of the test was 1.9 × 10−4 Pa (1.4 × 10−6 torr). Subsequently, the effect of the Yb2O3 on the rate of sublimation was evaluated by performing interrupted vacuum furnace tests in which the sample was weighed after exposure at 1000°C for different times. During the weighing periods, the accumulated oxide scale was either completely removed or left to accumulate further on the surface. The interrupted furnace tests showed that formation of Yb2O3 on the surface was the likely cause of the reduction in the rate of sublimation of the Yb14MnSb11 when measured by use of the vacuum TGA, at least for the measured test duration. Therefore, uncoated material in the vacuum of space, where oxygen is absent, would be likely to sublime at a continuous rate in excess of 5 × 10−3 g/cm2/h.