Origin of low thermal conductivity in Nb_1-xTi_xFe_1.02Sb half-Heusler thermoelectric materials

Nb_1-xTi_xFe_1.02Sb half-Heusler thermoelectric materials were synthesized trough arc melting and subsequent spark plasma sintering (SPS). Doping effect of Ti at Nb-site results in high power factor due to the optimization of hole carrier concentration. Further, the lattice thermal conductivity reduced as the Ti content increased, which mainly resulted from point defects and mass fluctuation by Ti doping as well as coherent nano-scale second phase, Ti-rich and Fe-deficient Ti_1.18Fe_0.57Sb. For the sample of Nb_0.6Ti_0.4Fe_1.02Sb, a minimum lattice thermal conductivity of ∼2.08 W/mK was obtained. This is attributed to the enhanced phonon scattering at the alternating white and dark lamellar interphase boundaries of (Nb_0.6,Ti_0.4)FeSb half-Heusler and Nb-doped Ti_1.18Fe_0.57Sb. A maximum thermoelectric figure of merit, ZT, of ∼0.81 was obtained at 973 K, which is comparable with values previously reported for this system.