Peter Rogl¹, Andrij Grytsiv¹, Matthias Gürth¹, Philipp Sauerschnig¹, Jan Vrestal², Vitalij Romaka³, Gerda Rogl¹, Kunio Yubuta⁴, Ernst Bauer¹

¹Christian Doppler Laboratory for Thermoelectricity at the Institute of Materials Chemistry and Research, University of Vienna, Vienna, Austria and at the Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria
²Department of Chemistry, Masaryk University, Kotlarska 2, Brno 61137, CR
³Department of Materials Science and Engineering, Lviv Polytechnic National University, 79013 Lviv, Ukraine
⁴Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan

Hitherto non-centrosymmetric multi-component half-Heusler phases have proven to be excellent thermoelectrics which are able to efficiently convert waste exhaust heat of internal combustion engines into electricity. Large-scale production and particularly nanostructuring of Half Heusler-based thermoelectric materials by preferably system-inherent phases need a profound knowledge not only of isothermal phase relations, temperature dependent solubilities but also of the solidification behavior.

The current presentation will cover a detailed description of the state of art of research on Half-Heusler thermoelectrics leading to optimised >98% dense bulk TE-material with ZT-values (ZT = figure of merit) reaching ZT = 1.4 for p-type grades and ZT = 1.5 for n-type grades. The presentation will focus on a detailed experimental investigation (backed by SEM/TEM analyses, DFT) of the constitution of the {Ti,Zr}-Ni-Sn systems including a liquidus projection and a Scheil diagram, as well as a CALPHAD calculation. For the binary systems TiNiSn-ZrNiSn and TiNiSn-HfNiSn, thermodynamic spinodal/binodal curves as well as coherent spinodal demixing (nanostructuring of thermoelectrics) have been determined from solubility data and DFT calculations.

Although among Ti_1-x-yZr_xHf_yNiSn_1-zSb_z compounds the thermoelectric behavior reached ZT ~ 1.5 at 850 K, we reached already a ZT~1.2 for Hf-free n-type Ti_0.5Zr_0.5NiSn_0.98Sb_0.02 and {Ti,Zr,Nb}Ni Sn_0.98Sb_0.02 yielding a thermoelectric efficiency of ~ 11%. These excellent TE data prove that Hf-free and therefore cheap half-Heusler alloys can be promising materials for a large scale production route.