2, University of Glasgow, Glasgow, , United Kingdom
Intermetallic half-Heusler based materials have long been considered as promising thermoelectric materials for high-temperature waste heat recovery.These narrow band gap semiconductors with 18-valence electrons tend to have high Seebeck coefficients (S) and low electrical resistivity (ρ), which results in large thermoelectric powerfactors (S2/ρ).Their thermoelectric figures of merit, ZT = S2T/ρκ, are limited by relatively large thermal conductivity values (κ), which is a sum of lattice and electronic components.
Reductions in κ while retaining S2/ρ were originally reported for Ni-rich TiNiSn and was attributed to the occupancy of vacant tetrahedral sites in the half-Heusler structure, acting as phonon scattering centres .This led to great interest in half-Heuslers with formula XNi1+ySn (X = Ti, Zr, Hf) [3,4]. Among half-Heusler alloys, pristine ZrNiSn itself exhibits alloy scattering dominated charge transport due to the presence of intrinsic interstitial Ni . This raises the prospect of decreasing κ by excess Ni atoms in ZrNiSn. Here, we systematically explore the impact of excess Ni on S2/ρ and κ, along with Sb substitution to adjust the carrier concentration to optimize the thermoelectric properties of the ZrNi1+ySn1-zSbz half-Heuslers. All compositions were prepared via solid-state reactions and hot pressing. The unit cell composition was determined from neutron powder diffraction. The microstructure and elemental distribution were analysed using scanning and transmission electron microscopy. In agreement with the literature, Sn/Sb substitution resulted in a semiconducting to metal-like transition with r is reduced by 2-orders of magnitude, while S is only reduced by a factor of two. This resulted in S2/ρ =5 mWm-1K-2 at 700 K, which is comparable to state-of-the-art ZrNiSn materials prepared using arc melting. The introduction of excess Ni resulted in reduced S2/ρ =3 mWm-1K-2 at 700 K in compositions co-doped with Sb. This appears caused by a reduction in carrier mobility. In terms of thermal transport, κlat is decreased by 20% with excess Ni and therefore does not compensate for the lower S2/ρ. These results in a ca.50% reduction in zT compared to optimized ZrNiSn without excess Ni. The reduced ZT values contrast with TiNiSn, were improvements are generally observed. A comparison of the different impact of excess Ni in TiNiSn and ZrNiSn will be presented.
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