2018-Sustainable Industrial Processing Summit
SIPS2018 Volume 5. Zehetbauer Intl. Symp. / SISAM
| Editors: | F. Kongoli, S. Kobe, M. Calin, J.-M. Dubois, T. Turna |
| Publisher: | Flogen Star OUTREACH |
| Publication Year: | 2018 |
| Pages: | 154 pages |
| ISBN: | 978-1-987820-90-4 |
| ISSN: | 2291-1227 (Metals and Materials Processing in a Clean Environment Series) |
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Spinodal Decomposition in High-Entropy Half-Heusler Thermoelectrics with High ZT~1.5
Peter Rogl1;1UNIVERSITY OF VIENNA, FACULTY OF CHEMISTRY, Wien, Austria;
Type of Paper: Keynote
Id Paper: 189
Topic: 42
Abstract:
Besides skutterudites and Zintl phases, Half Heusler (HH) alloys are currently the most promising candidates for thermoelectric (TE) devices at elevated temperatures; they can be used in a wide range of temperatures, and their starting materials are abundant and cheap [1]. In particular, the nanostructuring of TiNiSn-based thermoelectric materials - not only by ball-milling but also by preferably system-inherent phase separation - has accomplished multicomponent HH alloys with attractive ZTs for n-type TE materials based on (Ti,Zr)-Ni-Sn. These values could be achieved on the basis of a profound knowledge not only on isothermal phase relations, temperature dependent solubilities, but also on solidification behavior.
The detailed experimental investigation of the constitution of the (Ti,Zr)-Ni-Sn systems, including liquidus projections, Scheil solidification diagrams, as well as CALPHAD modelling, provided the necessary basis for an elaborate synthesis (annealing/hot-pressing) route in order to get a suitable and reproducible microstructure. In addition, exploiting inherent but coherent binodal/spinodal demixing at subsolidus temperatures within the sections TiNiSn-ZrNiSn and TiNiSn-HfNiSn, we were able to achieve for the n-type half Heusler alloy Ti0.5Zr0.25Hf0.25NiSn a ZTmax = 1.5 at 825 K. The demixing is a balanced effect of destabilisation of the solid solution by a positive enthaphy of mixing, compensated by elastic strain energy (coherent binodal) but also by the entropy of mixing. In this respect, the five component thermoelectric material can be considered as a so-called pseudoternary high-entropy alloy system. The experimental data are backed by SEM/TEM analyses as well as by DFT results.