Impact of Interstitial Ni on the Thermoelectric Properties of the Half-Heusler TiNiSn

Abstract

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.

Substitution Versus Full-Heusler Segregation in TiCoSb

“…There are therefore significant differences in microstructure for samples prepared via SSR and AM. Both SSR and AM samples surprisingly also contain some unreacted elemental Ti, as previously observed for TiNiSn [36]. It seems unlikely that this is due to slow reaction kinetics in the case of the AM samples and is more likely to be a consequence of the thermodynamics of this system.…”

Suppression of thermal conductivity without impeding electron mobility in n-type XNiSn half-Heusler thermoelectrics

“…60 For our samples, the alloy scattering most likely originates from the X-site, as there is no evidence for the presence of substantial amounts of interstitial Ni from the NPD analysis (Table S1 †). Recent literature has conrmed the suppression of the electron mobility by interstitial Ni, 46,62,63 and our results on interstitial Cu are therefore highly relevant as they afford the possibility of increased phonon scattering without detriment to the electrical transport. This very different impact is supported by our DFT calculations that show the presence of in-gap states for interstitial Ni, which provide a possible route for carrier scattering, while these are absent in the Cu case.…”

“…The peak shape was tted using multiple HH phases, with a lattice parameter difference of Da ¼ 0.0025Å and xed prole parameters so that the parent material TiNiSn is described as a single HH phase. 46 The increasing homogenisation is quantied by the reduced number of phases required to describe the peak which, in turn, conrms that Cu causes a mineralising effect. Mineralising effects are commonly observed for reactions that exploit a molten ux (e.g.…”

“…k lat $ 4.5 W m À1 K À1 for $5% interstitial site occupancy, with a much stronger impact than expected from alloy phonon scattering only. 43,46 In addition, Cu interstitials are effective ntype dopants and lead to rapid S 2 /r improvements, enabling ZT ¼ 0.6 at 773 K and commercially viable power outputs. 43 However, our previous study did not address the impact of interstitial Cu on the electron mobility.…”

Recent Advances in Thermoelectric Performance of Half-Heusler Compounds

“…between Hf and Sn sites, are also possible. The study of the effect of antisite disorder on TE properties has recently gained much attention [23][24][25][26][27][28][29]. The electronic structure that determines effective mass and mobility, as well as phonon scattering that gives rise to lattice thermal conductivity, are sensitive to the type of structural disorder in half-Heusler compounds [29].…”