Fine tuning of thermoelectric performance in phase-separated half-Heusler compounds

Abstract

Two successful recipes to enhance the thermoelectric performance, namely carrier concentration optimization and reduction of thermal conductivity, have been combined and applied to the p-type (Ti/Zr/Hf)CoSb_1−xSn_x system. An intrinsic micrometer-scale phase separation increases the phonon scattering and reduces the lattice thermal conductivity. A substitution of 15% Sb by Sn optimizes the electronic properties. Starting from this, further improvement of the thermoelectric properties has been achieved by a fine tuning of the Ti to Hf ratio. The microstructuring of the samples was studied in detail with high-resolution synchrotron powder X-ray diffraction and element mapping electron microscopy. Linking the structural with the thermoelectric properties, a record thermoelectric figure of merit for p-type half-Heusler compounds of ZT ≈ 1.2 at 710 °C in Ti_0.25Hf_0.75CoSb_0.85Sn_0.15 was achieved. The phase separation approach can form a significant alternative to nanostructuring processing, saving time, energy consumption and increasing the thermoelectric efficiency.