Phase Formation and Thermoelectric Properties of Doped Higher Manganese Silicides (Mn₁₅Si₂₆)

We herein report substitutional doping effects on the electronic and thermal transport properties of higher manganese silicides (HMS) Mn₁₅Si₂₆. Polycrystalline bulks of Mn_0.972A_0.028Si_1.80 and MnSi_1.75B_0.028 (A = V, Cr, Mo/B = Al, Ge) were fabricated by a solid-state reaction combined with the spark plasma sintering technique, and their thermoelectric properties were evaluated. We found that thermoelectric performance of Mn₁₅Si₂₆ was significantly enhanced due to the simultaneous improvement in electronic transport and phonon scattering via partial substitution of foreign atoms at Mn- and/or Si-sites. Through the small amount of Cr doping at the Mn-site and Al and Ge doping at the Si-site, the power factor was improved due to enhancement in density of the state's effective mass. Thermal transport properties could be also manipulated due to the point defect phonon scattering effect, and reduced lattice thermal conductivity was obtained with Ge-doped HMS. As a consequence, the maximum dimensionless figure␣of merit ZT of 0.64 at 773 K (increased 50% compared to undoped Mn₁₅Si₂₆) was obtained in Ge-doped Mn₁₅Si₂₆.