Effects of various grain structure and sizes on the thermal conductivity of Ti-based half-Heusler alloys

Half-Heusler alloys with the general formula TiNiSn_1-XSb_X are currently being investigated for their potential as thermoelectric (TE) materials. These materials exhibit high thermopower (40-250 μV/K) and low electrical resistivity values (0.1-8 mΩ-cm) which yields a relatively large power factor (α²σT) of 0.2-1.0 W/m^.K at room temperature. For these materials to be used in thermoelectric applications, the relatively high thermal conductivity (λ≈10 W/m^.K) that is evident in these materials must be reduced. We have investigated the effect of Sb-doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of TiNiSn. As expected, it is observed that large concentrations of Zr-doping on Ti-site reduce the lattice thermal conductivity as compared to the parent compounds. However an unusual result is observed with Sb-doping. The lattice thermal conductivity increases somewhat randomly with small amounts (< 5%) of Sb-doping at the Sn-site. This doping should have little effect on the thermal conductivity. A systematic investigation of grain structure in these Sb-doped materials has been performed as an attempt to explain the anomalous behavior of thermal conductivity due to Sb-doping. In addition, effects of grain size reduction on the thermal conductivity in ball milled and shock compressed samples have been investigated in hopes of reducing the lattice thermal conductivity