Combined effect of nanoscale grain size and porosity on lattice thermal conductivity of bismuth-telluride-based bulk alloys

Abstract

Here, we investigate the combined effect of the nanoscale crystal grains and porosity on the lattice thermal conductivity of bismuth-telluride-based bulk alloys using both experimental studies and modeling. The fabricated bulk alloys exhibit average grain sizes of 30 < d < 60 nm and porosities of 12% < Φ < 18%. The total thermal conductivities were measured using a laser flash method at room temperature, and they were in the range 0.24 to 0.74 W/m/K. To gain insight into the phonon transport in the nanocrystalline and nanoporous bulk alloys, we estimate the lattice thermal conductivities and compare them with those obtained from a simplified phonon transport model that accounts for the grain size effect in combination with the Maxwell-Garnett model for the porosity effect. The results of this combined model are consistent with the experimental results, and it shows that the grain size effect in the nanoscale regime accounts for a significant portion of the reduction in lattice thermal conductivity.