Phase stability and property evolution of biphasic Ti–Ni–Sn alloys for use in thermoelectric applications

Abstract

Thermoelectric properties and phase evolution have been studied in biphasic Ti-Ni-Sn materials containing full-Heusler TiNi 2 Sn embedded within half-Heusler thermoelectric TiNiSn. Materials prepared by levitation induction melting followed by annealing were of the nominal starting composition of TiNi 1+x Sn, with x between 0.00 and 0.25. Phases and microstructure were determined using synchrotron X-ray diffraction and optical and electron microscopy. The full-Heusler phase is observed to be semi-coherent with the half-Heusler majority phase. Differential thermal analysis was performed to determine melting temperatures of the ternary compounds and extrapolate a liquidus contour surface for the Ti-Ni-Sn system. The thermal conductivity is reduced in full-Heusler material. This leads to an increased thermoelectric figure of merit, ZT , from 0.35 for the stoichiometric compound to 0.44 for TiNi 1.15 Sn. Density functional theory calculations using hybrid functionals (HSE06) were carried out to determine band alignments between the half-and full-Heusler compounds, as well as comparative energies of formation. The HSE06 band structure of TiNiSn is presented as well.

Microstructure Evolution of Biphasic TiNi1+x Sn Thermoelectric Materials

“…The distinct dendritic shape of the fH regions clearly implies TiNi 2 Sn is the primary phase formed during solidification, even though the overall composition is much closer to TiNiSn. The behavior is consistent with the incongruent melting of the hH phase, as discussed elsewhere [21,22].…”

“…It is further noted in the latter figure that both the vestigial fH dendritic regions and the surrounding hH matrix contain submicrometer scale precipitates. These observations were reported in an earlier study [21] and are further analyzed below using TEM, to gain insight on the nature, morphology, size distribution and orientation of the precipitates within each phase.…”

“…The as-cast material was given the initial heat treatment depicted in Fig. 1 (1173K/24h + 1123K/192K) for comparison with previous studies [21]. While it is recognized that the composition investigated should be two-phase at all temperatures below the solidus, this lower temperature (LT) treatment is designated as "homogenization" because it should remove much of the non-equilibrium segregation produced during solidification.…”

“…The Ti-Ni-Sn alloys investigated were synthesized by induction levitation melting followed by annealing according to a procedure established in an earlier publication [21]. Briefly, the heat treatments involve two cycles, depicted in Figure 1.…”

Natural off-stoichiometry causes carrier doping in half-Heusler filled tetrahedral structures

“…This leads to a sublinear relation between carrier density vs. T as shown in the solid red curve, which is consistent with the negative d /dT observed from experiments (e.g. [43,66] [60,70,71]. All HSE fundamental gaps are of the Γ-X indirect character, except HfIrSb which has a direct gap (see band structures in Fig.…”