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-xM_xSi alloys (M = Co, Ir, Os) are reported. The maximum thermoelectric figure of merit ZT _max is improved from 0.007 at 60 K for pure FeSi to 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_total = κ_electronic + κ _lattice) fails for these alloys. The effects of grain size on thermoelectric properties of Fe_0.96Ir_0.04Si 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 ZT_max = 0.125 at 100 K, still approximately a factor of 5 too low for solid-state refrigeration applications.