TiNiSn-based half-Heusler alloys have been of significant interest for their potential as thermoelectric materials, which arises from their promising electronic transport properties. They exhibit high Seebeck coefficients and moderate electrical resistivity values but have comparatively high lattice thermal conductivities (κ

Thermal conductivity of two series of Ti-based half-Heusler alloys have been investigated and compared. A significant reduction in the lattice thermal conductivity (/spl kappa//sub L/) in Ti/sub 1-y/Zr/sub y/NiSn/sub 0.95/Sb/sub 0.05/ via mass fluctuation scattering (y>20%) is observed. However, small amounts of Sb (x/spl les/5%) in TiNiSn/sub 1-x/Sb/sub x/ do not yield similar results. A non-systematic increase in /spl kappa//sub L/ in the TiNiSn/sub 1-x/Sb/sub x/ series was observed, even with the amounts (x/spl les/5%) of Sb-doping. Extensive investigations of the grain structure in these materials show that there is a direct correlation between /spl kappa//sub L/ and the average grain size in these materials. These results are in good agreement with the theoretical predictions of Goldsmid et al. Theoretical calculations relating to the phonon mean free path in both the series of compounds would also be presented and discussed.

It is well known that most of the half-Heuslers with potential thermoelectric properties are n-type due to electron domination in the transport. However, p-type half-Heusler alloys can be prepared by appropriate combination of elements. These alloys are multi-component and exhibit resistivity values 1000-3000 mu Omega cm and thermopower values hundreds of mu V/K. Such values are typical for state-of-the-art thermoelectric materials and the power factor S-2 sigma can be in the range of 10-15 mu W cm(-1) K-2 at room temperature. In addition to the high power factors, low thermal conductivities are expected due to the presence of heavy elements. We have studied a series of such alloys and the details are presented in this paper.

Electrical transport measurements have been performed on doped and undoped TiCoSb half-Heusler phases. The semiconducting properties are found to be more robust than those reported for MNiSn (M = Ti, Zr, Hf ). Undoped TiCoSb phases exhibit large n-type Seebeck coefficients and high resistivities that reach -500 µV K-1 at 300 K and ~1500 Ω cm at 4.2 K, respectively. A tendency towards carrier localization is seen in several disordered phases. The effects due to n-type and p-type dopants are readily manifested in the thermopower, from which moderately heavy electron and hole band masses are inferred. The unusual properties measured are consistent with the prediction of a wide bandgap for the TiCoSb phase. A resistivity minimum is observed at 500-600 K for undoped and V-doped TiCoSb. Consequently, the semiconducting gap has not been determined.

Half-Heusler alloys are currently being investigated for their potential as thermoelectric materials [1], [2]. They exhibit high negative thermopower (40-250μV/K) and favorable electrical resistivity (0.1-8mW•cm) at room temperature. Attractive power factors (α 2 σT) of about (0.2-1.0W/m•K) at room temperature and about 4W/m•K at 600K [3] have been reported in these materials. But in order to achieve a high figure-of-merit in the half-Heusler alloys, the relatively high thermal conductivity in these materials (∼ 10 W/m•K) must be reduced. The thermal conductivity in these materials is composed of mainly a lattice contribution, compared to a very small electronic component. The challenge is to reduce the relatively high lattice thermal conductivity in these materials. Reported in this paper is a significant reduction of lattice thermal conductivity (∼1.5 - 3.5W/m•K) in some Ti-based half-Heusler alloys. Samples have been prepared by ball milling and followed by shock-compaction that has resulted into reduced grain sizes in these materials. The effects of the microstructure on the thermal transport properties of the Half-Heusler alloys have been investigated and are presented and discussed herein.

Half-Heusler alloys with the general formula TiNiSn 1-X Sb X are currently being investigated for their potential as thermoelectric (TE) materials. 1,2,3,4 These materials exhibit high thermopower (40-250μV/K) and low electrical resistivity values (0.1 - 8mΩ-cm) which yields a relatively large power factor (α 2 σT) of (0.2 - 1.0) W/m♦K at room temperature. The challenge is to reduce the relatively high thermal conductivity (≈ 10 W/m♦K) that is evident in these materials. The focus of this research is to investigate the effect of Sb-doping on the Sn site and Zr doping on the Ti site on the thermal conductivity of TiNiSn. Highly doped half-Heusler alloys have shown marked reduction in thermal conductivity to values on the order of 3.5 - 4.5 W/m♦K. Systematic determination of thermal conductivity in a variety of these doped materials as well as Sb and Zr doped TiNiSn are presented and discussed.

Substituted semiconductor alloys exhibit effects due to doping and disorder. To focus on the effect of disorder on transport properties, we have investigated isoelectronic Zr/sub 1-x/Hf/sub x/NiSn (x=0.0-0.3) and heavily substituted Zr/sub 0.5/Hf/sub 0.5/Ni/sub 0.7/Pd/sub 0.3/Sn/sub 0.95/, half Heusler alloys. Upon substitution, Zr/sub 1-x/Hf/sub x/NiSn alloys exhibit resistivity ratios R=/spl rho//sub 4.2K///spl rho//sub 295K/ as large as /spl sim/300 and thermally activated conduction in the temperature range 100-295K. At 4.2K, the (Zr,Hf)NiSn alloys possess carrier densities of /spl sim/10/sup 16/ to 10/sup 18/ cm/sup -3/ and carrier mobilities in the range /spl sim/100-350 cm/sup 2//V-s. The opposite signs exhibited by the Hall coefficient and thermopower, along with the saturation of resistivity at low temperature in ZrNiSn and Zr/sub 0.5/Hf/sub 0.5/Ni/sub 0.7/Pd/sub 0.3/Sn/sub 0.95/ alloys indicate band overlapping. Various scenarios of bandgap structures are discussed.