Ru Alloying Induced Enhanced Thermoelectric Performance in FeSi₂-Based Compounds

β-FeSi2 has been considered a promising material for thermoelectric applications, but its thermoelectric performance is greatly limited by the overhigh lattice thermal conductivity. In thermoelectrics, alloying an effective method to reduce the lattice thermal conductivity. In this study, we chose the Co-doped β-FeSi2 (Fe0.94Co0.06Si2) as the matrix and then prepared a series of Ru alloyed Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). X-ray characterizations show that all samples crystallize in the β-FeSi2 structure. The elemental mappings detect an inhomogeneous Ru distribution in Fe0.89Ru0.05Co0.06Si2, which is attributed to the different Ru solution contents in ε-FeSi and α-FeSi2+δ before the formation of β-FeSi2 and the slow diffusion behavior of Ru during the annealing process. The Ru-alloying obviously reduces the lattice thermal conductivity via introducing the mass and strain field fluctuations to interrupt the phonon transports, while it has a weak effect on electrical transport properties. Finally, a maximum zT value of 0.33 at 900 K has been obtained for Fe0.89Ru0.05Co0.06Si2, which is about 27% higher than that for Fe0.94Co0.06Si2.