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

Thermoelectric higher manganese silicides, MnSi x , 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 γ, 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 γ ∼ 1.736 after 48 h of annealing at 900 K. This result suggests that MnSi x , 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, γ 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 γ 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.