Transport and thermoelectric properties in semiconducting and semimetallic half-Heusler compounds

A study of the thermoelectric properties in several half-Heusler alloy systems is presented. Half Heusler alloys are intermetallic compounds that crystallize in the MgAgAs-type structure. Several of the half-Heusler compounds HfNiSn, TiNiSn, and ZrNiSn were found to exhibit semiconducting behavior with Seebeck coefficients as large as −200 to −400 μV/K at room temperature. In view of their large thermopower and only moderately high resistivity, their potential as TE materials was readily pointed out.; We tried to maximize the power factor of TiNiSn alloys from small amount doping. This method lowers the Seebeck coefficient a little bit and lowers the electrical conductivity dramatically, where the thermal conductivity stays about the same. The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi₂Te₃ alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T ∼ 650K where it is ∼4.5 W/m K making these materials very attractive for potential power generation considerations.; We tried to decrease the thermal conductivity from multi-component substitution. This method will increase the phonon scattering and lower the thermal conductivity. (ZrHf)(CoPt)(SbSn) is the system we chose. The large mass difference between Zr, Hf and between Co, Pt helps increase the phonon scattering. It is also easy to keep VEC to be 18 and achieve the electron or hole doping effect by varying the content of different components slightly. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300K, which is among the lowest reported for this class of structural phases.; We used ball milling to decrease the grain size of the half-Heusler alloys. The boundary scattering decreases the thermal conductivity. The lattice thermal conductivity decreases from ∼10 W/m K to 3.7 W/m K in ball milled and shock compacted alloys. This method opens possibilities for optimizing figure of merit by adjusting the grain sizes in these half-Heusler alloys.