Screening and Fabrication of Half-Heusler phases for thermoelectric applications

Half-Heusler phases have gained recently much interest as thermoelectric materials. Screening of possible systems was performed by ab-initio simulation using VASP-software. The energy-versus-Volume (E(V)) curves were calculated and calibrated. For TiCoSb, NbNiSn, FeMoSb the stability of Half-Heusler phase against concurrent crystal structures like TiNiSi, ZrCoAl, ZrBeSi, FeSiV, ZrNiP and Full Heusler was confirmed. However, the thermo-dynamical driving force as calculated from the difference in lattice energies is less than 0.1eV/atom. Hence, the fabrication of Half Heusler phases is a challenge and requires three steps, surface activation of the raw material by ball milling, arc-melting of pressed pellets and finally long-term annealing treatment in a vacuum furnace. On doped TiCoSb specimens, Seebeck coefficients up to 0.1 mV/K, on NiNbSn 0.16 mV/K were measured, although the microstructure was not yet optimized.