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 struc-tures and morphologies of the samples were investigated,and their thermoelectric properties were measured from 300℃ to 600℃.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 x1.73 samples,and were of the HMS and Si for x≥1.73 samples.It was shown that the thermoelectric proper-ties of the samples were affected strongly by the metallic MnSi phase and the semiconducting Si phase.The elec-trical conductivity decreased continuously,while the Seebeck coefficients increased with x value increasing.In ad-dition,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℃.The MnSi1.68 sample had the highest dimensionless figure of merit ZT 0.36 at 400℃,as a result of its highest electrical performance(power fac-tor) and moderately low thermal conductivity.