Enhancing Thermoelectric Properties of Higher Manganese Silicide (HMS) by Partial Ta Substitution

In this work, the thermoelectric properties of the p-type higher manganese silicide (HMS) were enhanced by partially substituting Mn with Ta. The ingots of the compound Mn36.4-xTaxSi63.6, where x=0.00, 0.25, 0.50. 0.75 and 1.00, were synthesized via arc melting which were then cast into a ribbon shape by a melt spinning process. After that, the crushed ribbons were consolidated to form a bulk sample by spark plasma sintering. The x-ray diffraction analysis showed that the single phase of HMS existed for x up to 0.50. Above that, the evidence of secondary phases was found, confirmed by scanning electron microscope imaging with elemental mapping. For thermoelectric properties measurement, the Seebeck coefficient and electrical conductivity were insignificantly different in the pure-phase samples. On the other hand, the samples with secondary phases showed increased electrical conductivities but slightly decreased Seebeck coefficients. The thermal conductivity was suppressed in all Ta-substituted samples. The lowest lattice thermal conductivity was found in the sample with the impurity phase (TaSi2) due to the enhanced phonon scattering. Consequently, the ZT of the Ta-substituted HMS was enhanced with the peak ZT of 0.37 at 813 K, which is about 28% higher than that of the pristine HMS.