Effects of phase separation on the thermoelectric properties of (Ti, Zr, Hf)NiSn half-Heusler alloys

Ti substitution is a common strategy to further reduce the lattice thermal conductivity of (Zr,Hf)NiSn half-Heusler thermoelectric alloys, but phase separations are also observed in the resulting substituted samples. The n-type (Ti_x(Zr_0.5Hf_0.5)_1-x) _0.99Ta_0.01NiSn alloys (x = 0.34, 0.4, 0.5, and 0.6), which are produced by arc-melting followed by annealing, show a phase separation into a Ti-poor primary dendritic half-Heusler phase and a Ti-rich interdendritic half-Heusler phase. For example, the x = 0.4 alloy consists of two phases: (Ti_0.36Zr_0.32Hf_0.32)_0.99Ta _0.01NiSn and (Ti_0.44Zr_0.28Hf _0.28)_0.99Ta_0.01NiSn. Using a detailed study of the phonon-scattering parameters of the single phase (Ti, Zr, Hf)NiSn compound as a function of mass and strain field fluctuations, a quantitative model of the lattice thermal conductivity has been successfully established. Based on this, the effective lattice thermal conductivity of the phase-separated samples can be well explained by the classical theory of composited medium. The theoretical calculation and the measured results indicate that both phases in the x = 0.4 sample possess the lowest lattice thermal conductivity. As a result, the dimensionless figure of merit ZT of this alloy is greater than that of the other compositions in the whole temperature range measured. A peak ZT of 1.0 at 750-800 K is obtained for this sample.