Investigation of thermoelectric properties of titanium-based half-Heusler alloys

Half-Heusler alloys with the general formula MNiSn (M = Ti, Zr, Hf) have recently been of significant interest due their potential as thermoelectric materials. They not only exhibit interesting electronic and lattice transport, but also reveal unusual optic and magnetic properties. The electronic and thermal transport properties of the Ti-based half-Heusler alloys are investigated at Clemson University in collaboration with Dr. Joe Poon and his group at the University of Virginia, where these samples were synthesized. The Ti-based half-Heusler alloys (measured at Clemson University) exhibit very high Seebeck coefficients (-40 to -250 muV/K) and favorable electrical resistivities (0.1 to 8 mO-cm), resulting into promising power factors (alpha 2sigmaT) (≈1.0 Wm-1K-1 at 300 K and ≈4.2 Wm-1K-1 at 650 K). However, the thermal conductivity in these materials is also very high, compared to our goal of an ideal thermoelectric material. Hence, the thermal conductivity, (composed mainly of the lattice contributions), here needs to be reduced or 'tuned'. The lattice thermal conductivity is approximately two to three times greater in these materials than their electronic components.; In an attempt to 'tune' the lattice thermal conductivity in these materials, large quantities of Zr substituted at the Ti site in Ti 1-yZryNiSn0.95Sb0.05 reduce the lattice thermal conductivity via mass fluctuation scattering. In a complimentary study, we found the lattice thermal conductivity increases randomly with small amounts of Sb-doping at Sn site in TiNiSn1-xSbx. Further investigation of the microstructural analysis in the TiNiSn1-x Sbx reveals grain boundary scattering effects prominent in the TiNiSn1-xSbx alloys.; A correlation between the grain size (≤10mum) and the lattice thermal conductivity in these materials have also been observed, which is in good agreement with the theoretical predictions by Sharp, Poon and Goldsmid. Reductions in grain size (∼sub-micron or nano-sized grains) in these materials have been achieved by ball milling followed by the shock compaction (performed by Dr. Thadhani and his group at the GA Institute of Technology) on the original ingots. Significant reductions in the room temperature lattice thermal conductivity (≈3 Wm-1K-1) in these nano-grained samples proved to be highly encouraging.; The microstructure of the Zr substituted compounds has been investigated is also in good agreement with the theoretical predictions. Two different phonon scattering phenomena are thus observed in the lattice thermal conductivity behavior in these materials, namely, mass fluctuation scattering effect (due to Zr substitution) and grain boundary scattering effect (due to Sb doping). Mean free path calculations in these materials also confirm these results.