Nano- and Microstructure Engineering: An Effective Method for Creating High Efficiency Magnesium Silicide Based Thermoelectrics

Considering the effect of CO<sub>2</sub> emission together with the depletion of fossil fuel resources on future generations, industries in particular the transportation sector are in deep need of a viable solution to follow the environmental regulation to limit the CO<sub>2</sub> emission. Thermoelectrics may be a practical choice for recovering the waste heat, provided their conversion energy can be improved. Here, the high temperature thermoelectric properties of high purity Bi doped Mg<sub>2</sub>(Si,Sn) are presented. The samples Mg<sub>2</sub>Si<sub>1–x–y</sub>Sn<sub><i>x</i></sub>Bi<sub><i>y</i></sub> with x­(Sn) ≥ 0.6 and y­(Bi) ≥ 0.03 exhibited electrical conductivities and Seebeck coefficients of approximately 1000 Ω<sup>–1</sup> cm<sup>–1</sup> and −200 μV K<sup>–1</sup> at 773 K, respectively, attributable to a combination of band convergence and microstructure engineering through ball mill processing. In addition to the high electrical conductivity and Seebeck coefficient, the thermal conductivity of the solid solutions reached values below 2.5 W m<sup>–1</sup> K<sup>–1</sup> due to highly efficient phonon scattering from mass fluctuation and grain boundary effects. These properties combined for <i>zT</i> values of 1.4 at 773 K with an average <i>zT</i> of 0.9 between 400 and 773 K. The transport properties were both highly reproducible across several measurement systems and were stable with thermal cycling.