Among the possible avenues for increasing the efficiency of global energy usage, thermoelectrics are an exciting, solid-state option. Thermoelectric materials, which convert an internal temperature gradient into a voltage and vice versa, have found applications in refrigeration as well as power generation from waste heat. TiNiSn, a semiconductor of the half-Heusler (hH) crystal structure, is of particular interest due to its very favorable electronic transport properties, conductivity (sigma) and Seebeck coefficient ( S), at relevant temperature regimes (between 600 K and 900 K). Unfortunately, its overall efficiency is hampered by a comparatively high thermal conductivity (kappa). In the design of thermoelectric materials, a number of approaches have been taken to increase the thermoelectric figure of merit, ZT = ( S2sigma/kappa)T, where T is temperature. In this work we examine how microstructure can be used to alter these thermoelectric propertiesin a biphasic Ti--Ni--Sn materials containing full-Heusler (fH) TiNi2Sn embedded within hH thermoelectric TiNiSn.

We explored a wide range of Ni compositions in TiNi1+xSn---from stoichiometric TiNiSn to high Heusler volume fraction, TiNi1.25Sn---materials prepared by levitation induction melting followed by annealing. Phase distributions and microstructure were characterized using synchrotron x-ray diffraction and optical and electron microscopy. In a sample of the nominal composition TiNi1.15Sn, a significant decrease in thermal conductivity (about 30%) is observed for the biphasic material despite the metallic second-phase particles existing at the micrometer scale; a 50% increase in the electrical conductivity is also measured. These result in a maximum figure of merit, ZT, of 0.44 at 800 K, which is 25% greater than is observed for the x = 0 sample. Density functional theory calculations using hybrid functionals were performed to determine band alignments between the half- and full-Heusler compounds, as well as comparative energies of formation.

We discuss the effects of thermal treatment on the microstructure of biphasic materials comprising hH and fH phases, as well as on their associated thermal transport properties. The fH phase is observed to be semi-coherent with the hH majority phase. Microstructural analysis, both in 2-D and 3-D, shows that the fH solidifies first and the hH then nucleates on these fH dendrites as the melt becomes depleted of Ni. The various heat treatments generated microstructures containing second phase precipitates ranging from ~10 nm to a few micrometers, reducing the connectivity of the percolated network observed previously. The materials were characterized with regard to morphology, size, shape and orientation relationship of the fH precipitates with the hH matrix. The precipitate size and morphology are very sensitive to Ni concentration in fH/hH phase. The thermal conductivity is found to drop by 50% as the precipitate size increases; however, this enhancement was not retained after high temperature exposure.

In addition to the microstructural implications of TiNi 1+xSn alloying, the (crystal) structural implications of excess Ni in the TiNiSn hH compound were investigated next through a combination of synchrotron x-ray and neutron scattering studies, in conjunction with first principles density functional theory calculations on supercells. Despite the phase diagram suggesting that TiNiSn is a line compound with no solid solution, for small x in TiNi 1+xSn there is indeed an appearance---from careful analysis of the scattering---of some solubility, with the excess Ni occupying the interstitial tetrahedral site in the hH structure. The analysis performed here would point to the excess Ni not being statistically distributed, but rather occurring as coherent nanoclusters. First principles calculations of energetics, carried out using supercells, support a scenario of Ni interstitials clustering, rather than a statistical distribution.

Finally, we also used a unique advanced tomographic technique with multimodal characterization capabilities, necessary to understand TiNiSn/TiNi2Sn composites. The three-dimensional microstructure of levitation melted TiNi 1.20Sn was characterized using the TriBeam system, a scanning electron microscope equipped with a femtosecond laser for rapid serial sectioning, to map the character of interfaces, in a 155 microm x 178 microm x 210 microm volume. We found the phase boundaries and grain boundaries to be decoupled and that the predominant phases present in the material, TiNiSn and TiNi2Sn, have a percolated structure. The distribution of coherent interfaces and high-angle interfaces has been measured quantitatively.