The effect of an isoelectronic Ti-Zr substitution on Heusler nanoprecipitation and the thermoelectric properties of a (Ti0.2,Zr0.8) Ni1.1Sn half-Heusler alloy

The microstructure and thermoelectric properties of an off-stoichiometric quaternary (Ti-0.2,Zr-0.8) Ni1.1Sn half-Heusler (HH) alloy was investigated in three heating cycles. A high density of coherent nanoprecipitates with an average diameter of similar to 13 nm and an interprecipitate spacing of similar to 6 nm was observed in the alloy. Formation of the extremely fine nanoprecipitates, most likely to be the 'full' Heusler (FH) phase, was not only strongly related to the degree of excess Ni concentration ratio in the alloy, but also appeared to be affected by the Ti-Zr substitutions. We noticed the behaviour of both electrical resistivity (r) and Seebeck coefficient (S) of the alloy was closely associated with the microstructure evolution of the FH-nanoprecipitates, which depended on their phase instability at elevated temperature and the cyclic heating process. The r and S reduced after the 1st heating cycle and stabilised thereafter in the subsequent heating cycles. Despite of the presence of the metallic FH-nanoprecipitates, the stabilised S maintained similar magnitudes to S of the ZrNiSn (without FH-nanoprecipitates) and did not show degradation of S as previously seen in the ZrNi1.1Sn containing relatively much larger FH-nanoprecipitates. The high density of FH-nanoprecipitates and the presence of TieZr point defects were responsible for the significant reduction of thermal conductivity (k) of the alloy, about 30% and 20% less than k of the ZrNiSn and ZrNi1.1Sn alloys, respectively. Moreover, further reduction of k was noticed due to formation of the diffuse HH/FH interfaced FH-nanoprecipitates from the cyclic heating process. Consequently, the alloy has shown a maximum dimensionless figure of merit (ZT) up to 0.81 at 870 K. (C) 2015 Elsevier B.V. All rights reserved.