The half-Heuslers NbCoSn and NbCoSb have promising thermoelectric properties. Here, an investigation of the NbCoSnSb (y = 0, 0.05; 0 ≤ z ≤ 1) solid-solution is presented. In addition, the p-type doping of NbCoSn using Ti and Zr substitution is investigated. Rietveld analysis reveals the gradual creation of Nb vacancies to compensate for the n-type doping caused by the substitution of Sb in NbCoSn. This leads to a similar valence electron count (∼18.25) for the NbCoSnSb samples (z > 0). Mass fluctuation disorder due to the Nb vacancies strongly decreases the lattice thermal conductivity from 10 W m K (z = 0) to 4.5 W m K (z = 0.5, 1). This is accompanied by a transition to degenerate semiconducting behaviour leading to large power factors, S/ρ = 2.5-3 mW m K and figures of merit, ZT = 0.25-0.33 at 773 K. Ti and Zr can be used to achieve positive Seebeck values, e.g. S = +150 μV K for 20% Zr at 773 K. However, the electrical resistivity, ρ = 27-35 mΩ cm, remains too large for these materials to be considered useful p-type materials.
Half-Heusler (HH) alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron x-ray diffraction study of several TiNiSn- and VFeSb-based HH alloys. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with α
av = 10.1 × 10−6 K−1 between 400 and 848 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced α
av = 8.9 × 10−6 K−1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are linked to low thermal conductivities, but also reduce the bandgap and lower the onset of thermal bipolar transport.
TiNiCu 0.05 Sn was sintered using Spark Plasma Sintering (SPS) and a new derivative processing method, hybrid Flash-SPS (hFSPS). The high heating rate achieved (7700 °C/min) produced almost single-phase samples with high density. The sample sintered at 1040 °C showed a higher power factor and a lower thermal conductivity than the SPS sample, resulting in a higher ZT at 350 °C
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