Correlation between processing conditions, microstructure and charge transport in half-Heusler alloys

Pierre Ferdinand P. Poudeu


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Julien PA Makongo, Xiaoyuan Zhou, Dinesh K Misra, Ctirad Uher, and Pierre FP Poudeu (2013)

Journal of Solid State Chemistry, 201:280-287.

Five bulk samples of n-type Zr0.25Hf0.75NiSn0.975Sb0.025 half-Heusler (HH) alloy were fabricated by reacting elemental powders via (1) high temperature solid state (SS) reaction and (2) mechanical alloying (MA), followed by densification using spark plasma sintering (SPS) and/or hot pressing (HP). A portion of the sample obtained by SS reaction was mechanically alloyed before consolidation by hot pressing (SS-MA-HP). X-ray powder diffraction and transmission electron microscopy studies revealed that all SS specimen (SS-SPS, SS-HP, SS-MA-HP) are single phase HI-I alloys, whereas the MA sample (MA-SPS) contains metallic nanoprecipitates. Electronic and thermal transport measurements showed that the embedded nanoprecipitates induce a drastic increase in the carrier concentration (n), a large decrease in the Seebeck coefficient (S) and a marginal decrease in the lattice thermal conductivity (kappa(l)) of the MA-SPS sample leading to lower ZT values when compared to the SS-HP samples. Constant values of S are observed for the SS series regardless of the processing method. However, a strong dependence of the carrier mobility (mu), electrical conductivity (sigma) and kappa(l) on the processing and consolidation method is observed. For instance, mechanical alloying introduces additional structural defects which enhance electron and phonon scattering leading to moderately low values of mu and large reduction in kappa(l). This results in a net 20% enhancement in the figure of merit (ZT=0.6 at 775 K). HH specimen of the same nominal composition with higher ZT is anticipated from a combination of SS reaction, MA and SPS (SS-MA-SPS). (C) 2013 Elsevier Inc. All rights reserved.

Times Cited: 1

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