Half-Heusler alloys based on TiNiSn are promising thermoelectric materials characterized by large power factors and good mechanical and thermal stabilities, but they are limited by large thermal conductivities. A variety of strategies have been used to disrupt their thermal transport, including alloying with heavy, generally expensive, elements and nanostructuring, enabling figures of merit, ZT ≥ 1 at elevated temperatures (>773 K). Here, we demonstrate an alternative strategy that is based around the partial segregation of excess Cu leading to grain-by-grain compositional variations, the formation of extruded Cu "wetting layers" between grains, and-most importantly-the presence of statistically distributed interstitials that reduce the thermal conductivity effectively through point-defect scattering. Our best TiNiCuSn (y ≤ 0.1) compositions have a temperature-averaged ZT = 0.3-0.4 and estimated leg power outputs of 6-7 W cm in the 323-773 K temperature range. This is a significant development as these materials were prepared using a straightforward processing method, do not contain any toxic, expensive, or scarce elements, and are therefore promising candidates for large-scale production.

TiNiSn is an intensively studied half-Heusler alloy that shows great potential for waste heat recovery. Here, we report on the structures and thermoelectric properties of a series of metal-rich TiNi1+ySn compositions prepared via solid-state reactions and hot pressing. A general relation between the amount of interstitial Ni and lattice parameter is determined from neutron powder diffraction. High-resolution synchrotron X-ray powder diffraction reveals the occurrence of strain broadening upon hot pressing, which is attributed to the metastable arrangement of interstitial Ni. Hall measurements confirm that interstitial Ni causes weak n-type doping and a reduction in carrier mobility, which limits the power factor to 2.5–3 mW m−1 K−2 for these samples. The thermal conductivity was modelled within the Callaway approximation and is quantitively linked to the amount of interstitial Ni, resulting in a predicted value of 12.7 W m−1 K−1 at 323 K for stoichiometric TiNiSn. Interstitial Ni leads to a reduction of the thermal band gap and moves the peak ZT = 0.4 to lower temperatures, thus offering the possibility to engineer a broad ZT plateau. This work adds further insight into the impact of small amounts of interstitial Ni on the thermal and electrical transport of TiNiSn.

Rietveld analysis of neutron powder diffraction data has been used to investigate the compositions of XNiSn (X = Ti, Zr, Hf) half-Heusler alloys prepared by solid state reactions. All samples containing Ti have 2-3% excess Ni, whereas the samples with X = Zr, Hf are almost stoichiometric.Samples with mixed X-metals are characterised by the presence of 3-4 distinct X 1-x X' x Ni 1+y Sn half-Heusler phases. Variable temperature and time dependent neutron powder diffraction for X = Ti and X = Ti 0.5 Hf 0.5 demonstrates that both the amount of excess Ni and the phase distribution are stable up to at least 600 °C. Debye temperatures of 367(2) K and 317 (2) K were obtained from the thermal displacement parameters. The samples containing Ti are characterised by a ~0.15 eV bandgap and a monotonously decreasing Seebeck coefficient. The compositions with Zr and Hf have similar bandgap values but show ambipolar transitions. Analysis of the thermoelectric transport data of degenerately doped Ti 0.5 Zr 0.5 NiSn 1-z Sb z samples using a single parabolic band model demonstrates that the transport is limited by alloy scattering and yielded an effective carrier mass of 2.5(1) m e .

Addition of Cu to XNiSn half-Heuslers improves homogeneity and reduces thermal conductivity without affecting electron mobility.

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.

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Half-Heusler alloys are leading contenders for application in thermoelectric generators. However, reproducible synthesis of these materials remains challenging. Here, we have used in situ neutron powder diffraction to monitor the synthesis of TiNiSn from elemental powders, including the impact of intentional excess Ni. This reveals a complex sequence of reactions with an important role for molten phases. The first reaction occurs upon melting of Sn (232 °C), when Ni 3 Sn 4 , Ni 3 Sn 2 , and Ni 3 Sn phases form upon heating. Ti remains inert with formation of Ti 2 Ni and small amounts of half-Heusler TiNi 1+y Sn only occurring near 600 °C, followed by the emergence of TiNi and full-Heusler TiNi 2 y ’ Sn phases. Heusler phase formation is greatly accelerated by a second melting event near 750–800 °C. During annealing at 900 °C, full-Heusler TiNi 2 y ’ Sn reacts with TiNi and molten Ti 2 Sn 3 and Sn to form half-Heusler TiNi 1+ y Sn on a timescale of 3–5 h. Increasing the nominal Ni excess leads to increased concentrations of Ni interstitials in the half-Heusler phase and an increased fraction of full-Heusler. The final amount of interstitial Ni is controlled by defect chemistry thermodynamics. In contrast to melt processing, no crystalline Ti–Sn binaries are observed, confirming that the powder route proceeds via a different pathway. This work provides important new fundamental insights in the complex formation mechanism of TiNiSn that can be used for future targeted synthetic design. Analysis of the impact of interstitial Ni on the thermoelectric transport data is also presented.