Approaching the minimum thermal conductivity in rhenium-substituted higher manganese silicides

Higher manganese silicides (HMS) made of earth-abundant and non-toxic elements are regarded as promising p-type thermoelectric materials because their complex crystal structure results in low lattice thermal conductivity. It is shown here that the already low thermal conductivity of HMS can be reduced further to approach the minimum thermal conductivity via partial substitution of Mn with heavier rhenium (Re) to increase point defect scattering. The solubility limit of Re in the obtained Re_xMn_1-xSi_1.8is determined to be about x = 0.18. Elemental inhomogeneity and the formation of ReSi_1.75inclusions with 50-200 nm size are found within the HMS matrix. It is found that the power factor does not change markedly at low Re content of x ≤ 0.04 before it drops considerably at higher Re contents. Compared to pure HMS, the reduced lattice thermal conductivity in Re_xMn_1-xSi_1.8results in a 25% increase of the peak figure of merit ZT to reach 0.57 ± 0.08 at 800 K for x = 0.04. The suppressed thermal conductivity in the pure Re_xMn_1-xSi_1.8can enable further investigations of the ZT limit of this system by exploring different impurity doping strategies to optimize the carrier concentration and power factor.