Microstructures and nanostructures in long-term annealed AgPb18SbTe20 (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 AgPbmSbTe2+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 degrees C for 7 days. Transmission electron microscopy investigations revealed Ag2Te nanoparticles at a size of 20-250 nm in the matrix. A remarkable reduction in the thermal conductivity to as low as 0.8W/mK was also recorded. The low thermal conductivity coupled with a large Seebeck coefficient of similar to 320 mu V/K led to high ZT of about 1.05 at 425 degrees C for the sample annealed at 505 degrees C. These results also demonstrate that samples annealed above 450 degrees C for long term are more thermally stable than those treated at lower temperatures.