| dc.description.abstract | Over the past few years, the half-Heusler materials have attracted attention for their potential within thermoelectric applications. This is mainly due to the flexibility of half-Heusler crystal structure. The half-Heuslers comprise three elements, XYZ, that crystallize in the face-centered cubic structure F$\overline 4$3m. Thus, materials that comprise nontoxic, environmentally friendly, inexpensive and abundant elements are possible.
%, as opposed to many of the commercialized thermoelectric materials such as the lead and bismuth tellurides.
Moreover, many of these half-Heuslers are low band gap semiconductors with good electrical properties, making them attractive for thermoelectric applications. On the other hand, the half-Heuslers are associated with too high lattice thermal conductivity to be applicable as thermoelectric materials. However, previous studies suggest that the lattice thermal conductivity may be reduced through materials' engineering, where two of the most common methods proposed in literature are alloying on one of the atomic sites, and nanostructuring.
Consequently, the present work investigates the effect of alloying on the X-position relative to the Bi-position of XNiBi, X=(Sc, Y or La), based half-Heuslers using density functional theory together with the temperature dependent effective potential method. The different half-Heusler alloys are described within the virtual crystal approximation, and the lattice thermal conductivity is calculated with the Boltzmann transport equation within the relaxation time approximation. This enables independent investigations of different contributions to the lattice thermal conductivity. In the Sc$_x$Y$_y$La$_{1-x-y}$NiBi alloys, the minimum lattice thermal conductivity was calculated to 4.3 W/mK for Sc$_{0.24}$La$_{0.76}$NiBi even though the maximum mass-disorder occurs for higher concentrations of Sc. This behaviour may be explained by the dominance of the anharmonic contributions to the lattice thermal conductivity. Consistent with the results from the specialization project, increasing the concentration of La in Sc$_x$Y$_y$La$_{1-x-y}$NiBi was shown to increase the anharmonic scattering of the acoustic phonon modes. Since the mass-disorder in Sc$_x$Y$_y$La$_{1-x-y}$NiBi primarily targets the optic phonon modes, which carry less heat than the acoustic, the alloying on the X-position does not reduce the lattice thermal conductivity efficiently. Alloying on the Bi-position of YNiBi with As or Sb, on the other hand, targets the acoustic phonon modes. Thus, the lattice thermal conductivity is reduced to 2.4 W/mK in YNiBi$_{0.36}$As$_{0.64}$. The impact of the scattering of acoustic phonon modes on the lattice thermal conductivity is further emphasized in the nanostructured materials. Through a simple model for grain boundary scattering, nanoscaled grains were introduced to the bulk materials such that the acoustic phonon modes were targeted. Consequently, in the Sc$_x$La$_{1-x}$NiBi binary alloy with grains of 50 nm in diameter, the minimum lattice thermal conductivity is reduced with more than 45\% compared to the minimum value of the bulk alloys; from 4.3 W/mK for Sc$_{0.24}$La$_{0.76}$NiBi to 2.4 W/mK for Sc$_{0.5}$La$_{0.5}$NiBi.
The present work indicates that the lattice thermal conductivity of XNiBi, X=(Sc, Y or La), based half-Heuslers may be reduced sufficiently through nanostructuring and alloying to be competitive with state-of-the-art thermoelectric materials. Moreover, the understanding of how different mechanisms influence the lattice thermal conductivity achieved in this work may aid the continued progress of the performance of thermoelectric materials. | |
