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Issue 36, 2016
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Origins of phase separation in thermoelectric (Ti, Zr, Hf)NiSn half-Heusler alloys from first principles

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Abstract

A high thermoelectric performance has been achieved in half-Heusler alloys of composition MNiSn (M = Ti, Zr, Hf). The enhanced properties are attributed to the formation of microscale Ti-rich and Ti-poor grains. The mechanism of phase separation remains unclear, as the composition and microstructure of the grains are highly dependent upon synthesis conditions. This work uses first principles density functional theory, combined with the cluster expansion method, to calculate a thermodynamic phase diagram of the full pseudo-ternary (Hf1−xyZrxTiy)NiSn system. The results show that ZrNiSn and HfNiSn are fully miscible at all temperatures, and that a miscibility gap between (Zr, Hf)NiSn and TiNiSn exists at temperatures below 850 K. For temperatures above 850 K, a solid solution is found to minimize the free energy. The low critical temperature of 850 K challenges the prevailing interpretation that the microstructure is created by an intrinsic phase separation mechanism, such as spinodal decomposition. Calculated migration barriers for Ti, Zr, and Hf atoms show that Ti-rich and Ti-poor grains may be non-equilibrium states that are kinetically trapped after solidification.

Graphical abstract: Origins of phase separation in thermoelectric (Ti, Zr, Hf)NiSn half-Heusler alloys from first principles

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Supplementary files

Article information


Submitted
13 Jun 2016
Accepted
16 Aug 2016
First published
16 Aug 2016

J. Mater. Chem. A, 2016,4, 13949-13956
Article type
Paper

Origins of phase separation in thermoelectric (Ti, Zr, Hf)NiSn half-Heusler alloys from first principles

A. Page, A. Van der Ven, P. F. P. Poudeu and C. Uher, J. Mater. Chem. A, 2016, 4, 13949
DOI: 10.1039/C6TA04957E

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