Abstract
In spite of their relatively high lattice thermal conductivity , the (, Zr, or Hf) half-Heusler compounds are good thermoelectric materials. Previous studies have shown that can be reduced by sublattice alloying on the site. To cast light on how the alloy composition affects , we study this system using the phonon Boltzmann-transport equation within the relaxation time approximation in conjunction with density functional theory. The effect of alloying through mass-disorder scattering is explored using the virtual crystal approximation to screen the entire ternary phase diagram. The lowest lattice thermal conductivity is found for the compositions; in particular, there is a shallow minimum centered at with taking values between 3.2 and 4.1 W/mK when the Ti content varies between 20% and 80%. Interestingly, the overall behavior of mass-disorder scattering in this system can only be understood from a combination of the nature of the phonon modes and the magnitude of the mass variance. Mass-disorder scattering is not effective at scattering acoustic phonons of low energy. By using a simple model of grain boundary scattering, we find that nanostructuring these compounds can scatter such phonons effectively and thus further reduce the lattice thermal conductivity; for instance, with a grain size of nm experiences a 42% reduction of compared to that of the single crystal.
- Received 6 November 2016
DOI:https://doi.org/10.1103/PhysRevB.95.045202
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