Microstructure and Thermoelectric Properties of Higher Manganese Silicides

Higher manganese silicides (HMS) bulk materials with the nominal compositions of MnSix (x = 1.60, 1.65, 1.68, 1.73, 1.81, 1.85) were prepared by arc-melting, annealing and spark plasma sintering. The phase structures and morphologies of the samples were investigated, and their thermoelectric properties were measured from 300°C to 600°C. The analyses of phase compositions revealed that the intensity of X-ray diffraction peaks was firstly enhanced and then weakened along with x value increasing. The phase composition were of HMS and MnSi for x<1.73 samples, and were of the HMS and Si for x≥1.73 samples. It was shown that the thermoelectric properties of the samples were affected strongly by the metallic MnSi phase and the semiconducting Si phase. The electrical conductivity decreased continuously, while the Seebeck coefficients increased with x value increasing. In addition, the thermal conductivity decreased firstly and then increased with x value increasing. Furthermore, it was showed that the thermal conductivity remained constant at lower temperature and then increased sharply because of the contribution of electron thermal conductivity at temperature higher than 450°C. The MnSi1.68 sample had the highest dimensionless figure of merit ZT 0.36 at 400°C, as a result of its highest electrical performance (power factor) and moderately low thermal conductivity.