Enhanced thermoelectric performance of a new half-Heusler derivative Zr9Ni7Sn8 bulk nanocomposite: enhanced electrical conductivity and low thermal conductivity

Varying the valence electron concentration per unit cell (VEC) in a half-Heusler (HH) material gives a large number of structures and substructures that can be exploited to improve the thermoelectric performance. Herein, we studied Zr9Ni7Sn8 with VEC = 17.25, which is smaller than 18 for normal ZrNiSn half-Heusler, to explore the structural modifications for improvement of thermoelectric performance. The structural analysis employing XRD, SEM and TEM confirms the resulting material to be a composite of HH and Ni3Sn4-type phases. Rietveld analysis estimates the volume fraction of HH to be 75.6 ± 1.2% and 24.6 ± 0.8% for Ni3Sn4 phase. Interestingly, the present composite results in a substantial increase in electrical conductivity (σ) by ∼75% and a drastic reduction in thermal conductivity (κ) by ∼56%, leading to a thermoelectric figure of merit (ZT) of 0.38 at 773 K, which is ∼85% higher than in normal HH ZrNiSn. Further, the nanostructuring of the composite, achieved by mechanical milling, derives a significantly reduced κ (i.e. from 4.56 W m−1 K−1 to 3.36 W m−1 K−1, at 323 K), yielding a ZT of 0.90 at 773 K, which is >300% enhancement over the normal HH. The experimental results have been compared with the Bergman and Fel model for calculating effective thermoelectric parameters in composites.