Thermoelectric properties of Co-, Ir-, and Os-doped FeSi alloys: Evidence for strong electron-phonon coupling

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