Lattice thermal conductivity of bulk and polycrystalline PbTe from computer simulations

Our results show that the presence of both vacancies and grain boundaries can separately reduce the value of the thermal conductivity to $\sim 0.5$ W/mK, regardless of the temperature, i.e. a factor of 4 at room temperature. A similar decrease is observed in the presence of interstitials and voids, but in this case concentrations are more limited. However, small grains are not stable at high and intermediate temperatures, so in the second part of the thesis we study if grain boundaries survive or not against thermal coarsening under operational conditions.

We attacked this problem by means of a multiscale dynamical modelling approach applied to porous, polycrystalline PbTe through a phase-field model informed by molecular dynamics simulations. The main hypothesis considered was that voids act as Zener pinning particles to stop grain growth and stabilise grain boundaries. We analyzed the stability of voids and determined metastable void sizes at different temperatures and observed that the presence of voids can stabilise grains with radii of the order of hundreds of nm.

A simple serial model allowed us to calculate the thermal conductivity of these porous polycrystalline materials and showed that it can be reduced by a 35 %, with lower reductions at higher temperatures.