Effect of boundary scattering on the thermal conductivity of TiNiSn-based half-Heusler alloys

TiNiSn-based half-Heusler alloys have been of significant interest for their potential as thermoelectric materials. They exhibit promising electronic transport properties as revealed through high Seebeck coefficient and moderate electrical resistivity values. The chief disadvantage of these materials is a comparatively high lattice thermal conductivity. Attempts to tune the lattice thermal conductivity (K-L) in these materials have led to the comparison and analysis of the thermal conductivity of two series of Ti- and Zr-based half-Heusler alloys. In the first series, Ti1-yZryNiSn0.95Sb0.05, a significant reduction in K-L is observed, with the substitution of large concentrations of Zr (y >= 25%) at Ti site, which is most likely due to mass fluctuation scattering. In the second series, TiNiSn1-xSbx, a nonsystematic increase in K-L is observed, with minute amounts of Sb doping (x <= 5%) at the Sn site. Extensive microstructural analysis in a TiNiSn1-xSbx series reveals a correlation between KL and the average grain diameter in these materials, which is in good agreement with theoretical predictions related to phonon boundary scattering. In addition, a comparison of the calculated phonon mean free path in each of the series of compounds shows some insight into the two different phonon scattering mechanisms.