Thermoelectric properties of FeSi and Related Alloys: Evidence for Strong Electron-Phonon Coupling

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 thermoelectric figure of merit ZT 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. The common approximation of dividing the total thermal conductivity into electronic and lattice components ($\kappa _{Total}=\kappa _{electronic}+\kappa _{lattice})$ fails spectacularly for these alloys. The effects of nanostructuring on thermoelectric properties of Fe$_{0.96}$Ir$_{0.04}$Si alloys are also reported. The thermal conductivity can be lowered by about 50{\%} with little or no effect on the electrical resistivity or Seebeck coefficient. This results in ZT$_{max}$ = 0.125 at 100 K, still about a factor of five too low for solid-state refrigeration applications. Research sponsored by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. DOE.