Enhanced thermoelectric power factor of Re-substituted higher manganese silicides with small islands of MnSi secondary phase

Abstract

Higher manganese silicides (HMS) are promising thermoelectric materials owing to the abundance of the constituent elements in the earth crust, environmental friendliness and good chemical stability at high temperatures. However, the metallic MnSi layers with a lateral size as large as ∼50 μm are formed in the melt-grown HMS samples. These large MnSi layers are characterized with relatively high electrical and thermal conductivities and low Seebeck coefficient, which can degrade the thermoelectric performance of the melt-grown samples. Here, we report the synthesis and thermoelectric properties of Re-substituted HMS with relatively small-size MnSi platelets via melt-quenching, followed by ball-milling, and consolidated by spark plasma sintering. As compared to the samples prepared by either solid-state reaction or mechanical alloying, the reduced lateral size of MnSi in the quenched sample leads to an increased carrier concentration without a reduction in the carrier mobility according to our electrical transport measurements. As a result, the thermoelectric power factor is increased to 1.9 ± 0.2 × 10⁻³ W m⁻¹ K⁻² at 860 K, which is about 20% higher than that of the sample prepared by solid-state reaction. In addition, the lattice thermal conductivity of the quenched sample remains nearly the same as the samples prepared by two other synthesis methods. Therefore, a figure-of-merit ZT of 0.64 ± 0.08 at 823 K is obtained for the quenched sample, compared to 0.57 ± 0.07 and 0.26 ± 0.03 obtained from the two other samples prepared by different methods.