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Heusler materials for thermoelectric applications: phase separation, atomic site disorder, and interstitials


By converting heat into electricity and vice versa, thermoelectric devices have a range of applications, including solid state refrigeration, powering space probes, and converting waste heat into energy. The performance of thermoelectric materials is characterized by the thermoelectric figure of merit (zT) which requires a large-magnitude Seebeck coefficient (S), reduced electrical resistivity (ρ), and reduced thermal conductivity (κ). Heusler (XY2Z) and half-Heusler (XYZ) materials with 18 and 24 valence electron counts are of particular interest for waste-heat thermoelectric applications due to their derivation from affordable elements, mechanical reliability, and thermal stability at operating conditions. In this work, we discuss four unique studies in which we engineer, characterize, and explore three different Heusler variants: NbCoSn, TiFe2Sn, and TiNiSn. Through these vignettes, we find a connection between atomic structural disorder and thermoelectric performance.

We begin by exploring the effect of phase separation in the NbCoSn–NbCo2Sn system in the form of NbCo1+xSn. The introduction of Heusler particulates to a half-Heusler matrix is an established technique for zT enhancement by reducing thermal conductivity without impacting electrical performance. In addition to the successful application of this technique to NbCoSn, we find that an annealing treatment regulates antisite disorder in this system. This plays a critical role in improving thermoelectric performance.

We explored variations of Heusler compound TiFe2Sn, revealing the role of antisite disorder. First, we considered variations on the Ti–Fe ratio as Ti1+xFe2−xSn, concluding that all compounds contain Ti–Fe antisite disorder and this disorder can be reduced by preparing a Ti–rich variation of this compound.

Next, we attempted to modify the charge carrier concentration as TiFe2Sn1−ySby. This technique modifies the Seebeck coefficient, but does not enhance the Seebeck coefficient as expected by electrical band structure calculations. This discrepancy arises from calculations completed for a perfect crystal that do not include the observed antisite defects.

Next, we examined a novel 18 valence electron solid solution formed between half-Heusler compounds TiNiSn and NbCoSn: (TiNiSn)1−x(NbCoSn)x. We first confirm the successful preparation of the solid solution and then examined the thermoelectric properties. Characterization of thermoelectric performance shows that the introduction of substitutional atoms degrades electrical performance while improving thermal conductivity. Intermediate compound (TiNiSn)0.5(NbCoSn)0.5 has the most promising thermoelectric performance of the alloys, owing to a high-temperature Seebeck coefficient that is larger than that of any other composition.

Finally, we consider a heat treatment study that evaluates the presence of Ni-interstitials in Ni-rich TiNiSn in relation to their impact on thermoelectric performance. Heat treatments are designed so that samples are thermodynamically encouraged to either phase separate into TiNiSn and TiNi2Sn or to accommodate excess Ni as Ni-interstitials. This work shows that the presence of Ni-interstitials enhances the thermoelectric performance of TiNiSn by simultaneously increasing the Seebeck coefficient and reducing thermal conductivity.

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