Thermoelectric properties of p-type Fe-doped TiCoSb half-Heusler compounds

TiFexCo1-xSb half-Heusler materials with randomly distributed TiO2 particles have been synthesized by arc melting and annealing. Thermoelectric properties were measured in the temperature range of similar to 300-850 K. TiCoSb shows n-type conduction, while TiFexCo1-xSb transfers to p-type conduction when x >= 0.01. The electrical conductivity of p-type TiFexCo1-xSb increased with increasing Fe content. The maximum Seebeck coefficient reached about 300 mu V/K at 850 K for x=0.15. Since Fe powder contained a trace of Fe2O3, a small amount of TiO2 particles formed during the synthesis process. The lattice thermal conductivity dramatically decreased with increasing Fe content, which was mostly caused by the introduction of in situ formed TiO2 particles, as well as the effects of mass fluctuation and strain field fluctuation due to the substitution of Fe to the Co site. The dimensionless figure of merit (ZT=S-2 sigma T/kappa) was significantly improved over the whole temperature region, and a maximum ZT value of 0.45 has been obtained for the composition of TiFe0.15Co0.85Sb at 850 K.