Effects of microstructural evolution on the thermoelectric properties of spark-plasma-sintered Ti_0.3Zr_0.35Hf_0.35NiSn half-heusler compound

The MNiSn (M = Ti, Zr, Hf) half-Heusler semiconducting compounds are widely investigated due to their good potential for thermoelectric (TE) power generation applications. In the current work, the evolution of the transport and structural properties of the Ti_0.3Zr_0.35Hf _0.35NiSn compound upon various thermal treatments was studied. The nominal composition was arc melted, ball milled and spark plasma sintered (SPS). Following SPS, large Hf-rich domains were found by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). Subsequently, the samples were subjected to homogenization treatments at 1163 K for 480 h and 610 h under argon atmosphere. Following these thermal treatments, the relative amount of the Hf-rich domains was reduced and they became smaller in size, with increasing thermal treatment duration. Nevertheless, no uniphased structure was reached. The dissolution of the Hf-rich domains in the half-Heusler matrix resulted in increase of both the Seebeck coefficient and electrical resistivity values and a decrease of the carrier concentration, attributed to elimination of these metallic domains. Resulting from the high atomic disorder caused by substitution at the M site, low thermal conductivity values of ∼3.8 W m^-1 K^-1 were obtained leading to high ZT values of up to 0.82 following SPS.