In spite of their relatively high lattice thermal conductivity κ , the XNiSn (X = Ti, Zr, or Hf) half-Heusler compounds are good thermoelectric materials. Previous studies have shown that κ can be reduced by sublattice alloying on the X site. To cast light on how the alloy composition affects κ , 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 Ti x Zr y Hf 1−x−y NiSn phase diagram. The lowest lattice thermal conductivity is found for the Ti x Hf 1−x NiSn compositions; in particular, there is a shallow minimum centered at Ti 0.5 Hf 0.5 NiSn with κ 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, Ti 0.5 Hf 0.5 NiSn with a grain size of L = 100 nm experiences a 42% reduction of κ compared to that of the single crystal.

Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κ Lat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κ Lat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κ Lat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κ Lat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials.

The energy conversion efficiency of solar cells based on multicrystalline silicon is greatly deteriorated by dislocations. However, an in-depth understanding on the dislocation motion dynamics down to atomic scale is still lacking. In this paper, we propose a novel atomistic approach to simulate the kink migration and kink-pair formation which govern dislocation motion in silicon, namely the kinetic Activation-Relax Technique (k-ART). With this method, long timescale events can be simulated and complex energy landscapes can be explored. Four mechanisms for kink migration are observed, with total activation energy of 0.16, 0.25, 0.32 and 0.25 eV. New non-trivial kink structures that participate in kink migration are identified due to the open-ended search algorithm for saddle points in k-ART. In addition, a new pathway for kink-pair formation, with a minimum activation energy of 1.11 eV is discovered. The effect of shear stress on kink migration is also investigated. It shows that shear stress shifts the energy barriers of available events to lower energies, resulting in a change of the preferred kink-migration mechanism and a reduction of kink-pair formation energy. 65 teratomic potentials. NEB simulation require knowledge 66 of the initial and final states, and an initial guess for 67 the connecting pathway. This means that only the path-68 way closest to the initial guess is explored, leaving other 69 possible pathways unexplored. This can be problematic 70 with complex energy landscapes, where non-trivial but 71 relevant pathways may be present. 72 Core structure of kinks on dislocations and their role in 73 dislocation motion in silicon have been considered to be 74 of high complexity 26. To thoroughly sample the energy 75 landscape around such complex structures and reveal 76 new possible non-trivial structures, an open-ended search 77 algorithm is a necessity. In principle, such a method can 78 795