Microstructures and nanostructures in long-term annealed AgPb ₁₈SbTe₂₀ (LAST-18) compounds and their influence on the thermoelectric properties

This article reports on the role of annealing on the development of microstructure and its concomitant effects on the thermoelectric properties of polycrystalline AgPb_mSbTe_2+m (m = 18, lead-antimony- silver-tellurium, LAST-18) compounds. The annealing temperature was varied by applying a gradient annealing method, where a 40-mm-long sample rod was heat treated in an axial temperature gradient spanning between 200 and 600 °C for 7 days. Transmission electron microscopy investigations revealed Ag ₂Te nanoparticles at a size of 20-250 nm in the matrix. A remarkable reduction in the thermal conductivity to as low as 0.8 W/mK was also recorded. The low thermal conductivity coupled with a large Seebeck coefficient of ∼320 μV/K led to high ZT of about 1.05 at 425 °C for the sample annealed at 505 °C. These results also demonstrate that samples annealed above 450 °C for long term are more thermally stable than those treated at lower temperatures.