The Role of Ti in the Improvement of Thermoelectric Performance in Nb 0.6− x Ti x Ta 0.4 FeSb

High Resolution Image Download MS PowerPoint Slide Half-Heusler (HH) materials with the stoichiometry Nb 0.6− x Ti x Ta 0.4 FeSb (0 ≤ x ≤ 0.175) were synthesized by mechanical alloying. The role of Ti doping on electrical and thermal transport properties is studied by both experimental and theoretical approaches. Powder X-ray diffraction, Rietveld refinements, and transmission electron microscopy analyses confirm the HH phases and the incorporation of Ti atoms in the crystal structure. Electrical conductivity and Seebeck coefficient measurements indicate that Ti substitution induces an effective hole doping, significantly increasing σ. However, the increase in hole concentration is accompanied by a moderate decrease in the Seebeck coefficient, keeping the S values of heavily doped phases at relatively high levels. Interestingly, density functional theory (DFT) calculations, using the Korringa−Kohn−Rostoker method with the coherent potential approximation (KKR-CPA) method to account for such a complex chemical disorder, reveal aspects related to the role and unique contribution of Ti in the formation of the electronic structure of the (Nb,Ta)FeSb system in the vicinity of the Fermi level ( E F ). Ti substitution seems to contribute to the sharpness enhancement of the total electronic density of states (DOS) close to E F, since Ti presents a higher and sharper DOS than those of Nb and Ta. According to the Mott relation, this probably affects positively the thermopower of heavily doped phases. Theoretical estimations of the Seebeck coefficient based on the electronic DOS, calculated by the KKR-CPA method, tend to support this evidence. In addition, Ti substitution in combination with Nb/Ta alloying and nanostructuring induces an effective phonon scattering, strongly reducing the lattice thermal conductivity. Theoretical calculations of lattice thermal conductivity are in very good agreement with the experimental measurements, revealing the strong effect of Ti on the enhancement of mass fluctuation scattering and the reduction of the phonon mean free path. As a result, a powerful enhancement of ZT is achieved for Nb 0.45 Ti 0.15 Ta 0.4 FeSb, reaching a maximum value of 1 at 600 °C, and highlighting the multiple roles of Ti in the improvement of thermoelectric (TE) performance of M V FeSb-based HH phases.