The effects of various transition-metal dopants on the electrical and thermal transport properties of Fe${}_{1-x}$$M$${}_x$Si alloys ($M$ $=$ Co, Ir, Os) are reported. The maximum thermoelectric figure of merit $Z$$T_{\max }$ is improved from 0.007 at 60 K for pure FeSi to $\mathit{ZT}$ $=$ 0.08 at 100 K for 4% Ir doping. A comparison of the thermal conductivity data among Os-, Ir-, and Co-doped alloys indicates strong electron-phonon coupling in this compound. Because of this interaction, the common approximation of dividing the total thermal conductivity into independent electronic and lattice components (κ${}_{\mathrm{total}}$ $=$ κ${}_{\mathrm{electronic}}$ $+$ κ${}_{\mathrm{lattice}}$) fails for these alloys. The effects of grain size on thermoelectric properties of Fe${}_{0.96}$Ir${}_{0.04}$Si alloys are also reported. The thermal conductivity can be lowered by ∼50% with little or no effect on the electrical resistivity or Seebeck coefficient. This results in $Z$$T_{\max }$ $=$ 0.125 at 100 K, still approximately a factor of 5 too low for solid-state refrigeration applications.

• Received 7 December 2010

DOI:https://doi.org/10.1103/PhysRevB.83.125209