Structure relations with transport properties in p-type thermoelectric materials: Iron silicides

Abstract

Understanding the structure and its relations with the transport properties is important for designing a good thermoelectric (TE) material. In this work, we investigate the relationship between those properties of the low-cost and eco-friendly materials, β-Fe1-xMnxSi2 (0≤x≤0.10), prepared by direct arc melting and heat treatment process. The metallic phases (tetragonal α-Fe2Si5 and cubic ε-FeSi) are transformed into the semiconductor phase (orthorhombic β-FeSi2) through heat treatment. The amount of β-FeSi2 significantly decreases at x ≥ 0.09. Mn atoms act as an acceptor and improve the carrier density (nH of holes) but decrease the mobility (μH). By substituting Mn to β-FeSi2, the Seebeck coefficient (S) is more uniform at high temperatures and the electrical resistivity (ρ) effectively decreases, leading to an improvement in the power factor. The thermal conductivity (κ) slightly increases with Mn doping. However, with increasing Mn content, the formation of secondary phases increases, resulting in the reduction of solid solution of Mn in β-phase; therefore, the electrical transport deteriorates. As a result, the optimum doping level for improving TE performance is obtained at x = 0.05 sample, verifying with crystal structure analysis that the optimum doping level is in the range of 0 ≤ x ≤ 0.08 because the amount of semiconducting β-FeSi2 drastically drops at x ≥ 0.09. Our study reveals that by judging the variation of the crystal structures, we could achieve the optimum doping level for improving TE transport properties.