Phase formation, stability, and oxidation in (Ti, Zr, Hf)NiSn half-Heusler compounds (Phys. Status Solidi A 6∕2014)
Krzysztof Gałązka, Sascha Populoh, Leyre Sagarna, Lassi Karvonen, Wenjie Xie, Alessandra Beni, Patrik Schmutz, Jürg Hulliger and Anke Weidenkaff
Article first published online: 11 JUN 2014 | DOI: 10.1002/pssa.201470235
Thermoelectricity offers the unique possibility of a direct conversion of heat to electricity. Half-Heusler compounds based on TiNiSn are of great interest for thermoelectric research due to their promising electrical properties. However, for an efficient energy conversion their thermal conductivity has to be lowered. One strategy is to reduce the phonon mean free path by isoelectric substitution of Ti by Zr and Hf, as in the (Ti0.33Zr0.33Hf0.33)NiSn in this work (see pp. 1259–1266). The complex phase composition of these compounds is revealed in the SEM image, where brighter (Zr,Hf)-rich half-Heusler grains are separated by Ti-rich intergranular phases. Gałązka et al. analyze the stability of the compound by in-situ Atomic Force Microscopy and Scanning Kelvin Probe Microscopy, which allow real-time tracking of topographic and chemical changes of the sample surface at temperatures up to 423 K. The sequence of experimental images reveals topography and Volta Potential Difference changes resulting from irreversible phase segregation on the grain boundaries within the Ti-rich intergranular phase.