Demonstration of a phonon-glass electron-crystal strategy in (Hf,Zr)NiSn half-Heusler thermoelectric materials by alloying
The general phonon-glass electron-crystal strategy is to disrupt phonon transport without affecting electron transport. Disruption of phonon thermal conductivity by alloying and nanostructuring is well known but a direct comparison of the scattering strength of electrons to that of phonons from disorder has not been made. Here we show that the point defect disorder of Zr/Hf atoms in MNiSn (M = Hf, Zr, and Ti) half-Heusler alloys effectively reduces lattice thermal conductivity as predicted from point defect scattering. However the introduced local atomic disorder produces a negligible effect on the electron scattering process and the conduction band structure. The electron scattering potential observed on the conduction band electrons is less than 0.1 eV, ten times less than that typically observed. This phenomenon can be understood from the existence of intrinsic disorder in the MNiSn system causing distinct mass and strain difference that effectively screens the effect of the induced disorder of Hf/Zr. The highest zT = 1.1 was obtained for Zr0.2Hf0.8NiSn0.985Sb0.015 at 1000 K. This substantial improvement in zT is due to alloying on the Zr/Hf site and demonstrates a dramatic improvement in TE performance that does not require nanoscale microstructures to avoid scattering of charge carriers.