Thermoelectric properties in phase-separated half-Heusler materials

The conversion of dissipated heat into electricity is the basic principle of thermoelectricity. In this context, half-Heusler (HH) compounds are promising thermoelectric (TE) materials for waste heat recovery. They meet all the requirements for commercial TE applications, ranging from good efficiencies via environmentally friendliness to being low cost materials. This work focused on the TE properties of Ti0.3Zr0.35Hf0.35NiSn-based HH materials. This compound undergoes an intrinsic phase separation into a Ti-poor and Ti-rich HH phase during a rapid solidification process. The resulting dendritic microstructure causes a drastic reduction of the thermal conductivity, leading to higher TE efficiencies in these materials. The TE properties and temperature dependence of the phase-separated Ti0.3Zr0.35Hf0.35NiSn compound were investigated. The TE properties can be adjusted depending on the annealing treatment. The extension of annealing time for 21 days at 1000 °C revealed a reduction of the thermal conductivity and thus an enhancement of the TE performance in this sample. An increase of annealing temperature caused a change of the phase fraction ratio in favor of the Ti-rich phase, leading to an improvement of the electronic properties. rnInspired by the TE properties of the Ti0.3Zr0.35Hf0.35NiSn HH compound, the performance of different n- and p-type materials, realized via site substitution with donor and acceptor elements was examined. The fabrication of a TE n- and p-type material pair based on one starting compound can guarantee similar TE and mechanical properties and is enormous beneficial for device engineering. As donor dopants V, Nb and Sb were tested. Depending on the lattice position small doping levels were sufficient to attain distinct improvement in their TE efficiency. Acceptor-induced doping with Sc, Y and Co caused a change in the transport behavior from n- to p- type conduction, revealing the highest Seebeck coefficients obtained in the MNiSn system. rnThen, the long-term stability of an exemplary n- and p-type HH compound was proven. Surprisingly, the dendritic microstructure can be maintained even after 500 cycles (1700 h) from 373 to 873 K. The TE performance of both n- and p-type materials showed no significant change under the long-term treatment, indicating the extraordinary temperature stability of these compounds. Furthermore both HH materials revealed similar temperature-dependence of their mechanical properties. This work demonstrates the excellent suitability of phase-separated HH materials for future TE applications in the moderate temperature range.rn