Influence of Stoichiometry and Aging at Operating Temperature on Thermoelectric Higher Manganese Silicides

Thermoelectric higher manganese silicides, MnSix, were synthesized by magnesioreduction followed by spark plasma sintering with different nominal compositions (x = 1.65, 1.74, and 1.80) and various postsynthesis annealing durations (0, 48, 96, and 336 h). The composite Nowotny chimney-ladder crystal structures of the resulting samples were investigated by synchrotron X-ray powder diffraction. The modulation vector component gamma, generally considered corresponding to the stoichiometry (x) of the material, was accurately determined by Rietveld refinement using a (3 + 1)D superspace approach. Regardless of the initial nominal composition, all the samples have a similar gamma similar to 1.736 after 48 h of annealing at 900 K. This result suggests that MnSix, at a temperature of 900 K, is better described as a defined compound with x close to 1.736, rather than intermediate solid-solution phases with 1.725 < x < 1.75 as predicted by the commonly accepted phase diagram. At the fixed nominal composition MnSi1.74, gamma increases significantly from 1.7313(2) to 1.7411(1) after 336 h of annealing, indicating that the thermal history influences the Si stoichiometry. The evolution of gamma with time is concomitant with a power factor drop (-19%), attributed to a decrease in charge carrier concentration. The drop of the power factor, partially compensated by a decrease in thermal conductivity, results in a -12% reduction of the maximum figure-of-merit ZT, after prolonged annealing under realistic application conditions.