Improving the thermoelectric properties of half-Heusler TiNiSn through inclusion of a second full-Heusler phase: microwave preparation and spark plasma sintering of TiNi1+xSn

Abstract

Half-Heusler thermoelectrics offer the possibility to choose from a variety of non-toxic and earth-abundant elements. TiNiSn is of particular interest and - with its relatively high electrical conductivity and Seebeck coefficient - allows for optimization of its thermoelectric figure of merit, reaching values of up to 1 in heavily-doped and/or phase-segregated systems. In this contribution, we used an energy- and time-efficient process involving solid-state preparation in a commercial microwave oven and a fast consolidation technique, Spark Plasma Sintering, to prepare a series of Ni-rich TiNi1+xSn with small deviations from the half-Heusler composition. Spark Plasma Sintering plays an important role in the process by being a part of the synthesis of the material rather than solely a densification technique. Synchrotron powder X-ray diffraction and microprobe data confirm the presence of a secondary TiNi2Sn full-Heusler phase within the half-Heusler matrix. We observe a clear correlation between the amount of full-Heusler phase and the lattice thermal conductivity of the samples, resulting in decreasing total thermal conductivity with increasing TiNi2Sn fraction. This trend shows that phonons are scattered effectively as a result of the microstructure of the materials with full-Heusler inclusions in the size range of microns to tens of microns. The best performing samples with around 5% of TiNi2Sn phase exhibit maximum figures of merit of almost 0.6 between 750 K and 800 K which is an increase of ca. 35% compared to the zT of the parent compound TiNiSn.

Incommensurate crystal supercell and polarization flop observed in the magnetoelectric ilmeniteMnTiO3

“…It is possible that compositional surface impurities are undetectable in low quantities, similar to other compounds such as the half-heusler TiNiSn. [40] However, electron microprobe measurements used to examine the surface of a portion of the Fe-containing crystal presented no detectable compositional defects away from the nominal stoichiometry along the bulk of the crystal, except at regions where the crystal was cut along the edges, where the stoichiometry could not be accurately determined. [23] We note that while the Fe-containing crystal was polished before the electron microprobe measurement, the single crystal portion selected to be used for Mössbauer spectroscopy was also polished before powdering.…”

Tuning the magnetocaloric response in half-Heusler/Heusler MnNi1+xSb solid solutions

“…29,30 Stoichiometric amounts of elemental powders were ground together (10 min. ), pressed into a pellet, and sealed in an evacuated fused silica ampoule.…”

Improving the thermoelectric performance of TiNiSn half-Heusler via incorporating submicron lamellae eutectic phase of Ti70.5Fe29.5: a new strategy for enhancing the power factor and reducing the thermal conductivity

“…5(b). Here, we use the temperature dependent Lorenz number calculated specically for TiNiSn by Birkel et al, 58 and the bipolar contribution was taken into account by assuming k lattice $ 1/T. 60 Thus, we notice that the lattice thermal conductivity signicantly contributes to the total thermal conductivity, as envisaged from Fig 5 (b) and (c).…”