Topic：High ZT Spinodal Half-Heusler Alloys for Efficient Thermoelectric Energy Conversion

Speaker：Peter Franz Rogl

Time：9:00am., (Fri.) May 11,2018

Venue：Room 468, Lee Hsun Building

High ZT Spinodal Half-Heusler Alloys for Efficient Thermoelectric Energy Conversion

Peter Franz Rogl
Christian 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 
Email: peter.franz.rogl@univie.ac.at

Multi-component half-Heusler phases (noncentrosymmetric MgAgAs-type) have proven to be excellent thermoelectrics. The Large-scale production and particularly nanostructuring of (Ti,Zr)NiSn-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 presentation covers a detailed experimental investigation of the constitution of the systems Ti-Ni-Sn and Zr-Ni-Sn including liquidus projections and Scheil diagrams, as well as CALPHAD calculations of the entire ternaries. For the quasi-binary systems TiNiSn - ZrNiSn and TiNiSn - HfNiSn, thermodynamic spinodal/binodal curves have been determined from solubility data and DFT calculations. With the measured relevant elastic moduli also the corresponding depression of the critical points of coherent spinodal demixing have been derived. From a series of compounds Ti_1-x-yZr_xHf_yNiSn and respective Sn/Sb substituted compounds the thermoelectric behavior was characterized and found to reach ZT=1.6 at 870 K but for Hf-free, n-type Ti_0.5Zr_0.5NiSn_0.98Sb_0.02 a ZT ~ 1.2 at 870 K yielding an thermoelectric efficiency ŋ ~ 11%. For the p-type counterparts alloys Nb_1-xM_xFe(Sb,Sn) (M=Ti or Y, Zr) reaching ZT=1.2 at 870 K are used. These excellent TE data on spinodally demixed alloys prove that Hf-free and therefore cheap half-Heusler alloys can be promising materials for a large scale production route. The experimental data (backed by SEM/TEM analyses) are compared with DFT results and discussed.