ICT2014 Abstracts

Higher manganese silicides (HMS) are regarded as promising p-type thermoelectric materials because their complex crystal structure results in low lattice thermal conductivity and their constituent elements are earth-abundant, inexpensive, and non-toxic. It is shown here that the already low thermal conductivity of HMS can be reduced further to approach the minimum thermal conductivity by substitution of Mn with heavier Rhenium (Re) to obtain Re_xMn_1-xSi_1.8. The solubility limit of Re is determined to be about x = 0.18. Elemental inhomogeneityand the formation of ReSi_1.75 inclusions with 50-200 nm size are found within the HMS matrix. With increasing Re content in the range of 0 ≤ x ≤ 0.12, the hole concentration increases, while the hole effective mass and mobility decrease. Consequently, the power factor does not markedly change at low Re content of x ≤ 0.04 before it drops considerably at higher Re contents. The lattice thermal conductivity is primarily suppressed by Re substitution, consistent with calculations showing that the reduction results from point defect scattering. The sample with x = 0.12 exhibits the lowest lattice thermal conductivity of 1.6 W m^-1 K^-1 at 723 K, which approaches the calculated minimum lattice thermal conductivity value of 1.4 W m^-1 K^-1 at 723 K. Compared to pure HMS, the reduced lattice thermal conductivity in Re_xMn_1-xSi_1.8 results in an 25% increase of the peak figure of merit ZT to reach 0.57±0.08 at 800 K for x = 0.04.