Tin Telluride-Based Nanocomposites of the Type AgSnmBiTe2+m (BTST-m) as Effective Lead-Free Thermoelectric Materials

In the medium temperature range tin telluride has emerged as a compensational thermoelectric material for the well established compound lead telluride. We present tin telluride-based nanocomposites of the composition AgSn m BiTe 2+ m (Bismuth–Tin–Silver–Tellurium, BTST- m ) showing superior thermoelectric performance. Nanopowders with varying concentration ratios between silver bismuth telluride and tin telluride were synthesized by mechanical alloying. Three different compacting routes were applied: cold pressing/annealing, hot pressing, and short-term sintering. A strong interdependence between the compacting method and thermoelectric properties was found with hot pressed and short-term sintered samples exhibiting the best results. Alike for the already known antimony analogue, ZT could be increased to around 1. Scanning electron microscopy indicates that the observed tremendous reduction of thermal conductivity originates from precipitation of nanoparticles and a high number of grain boundaries. Powder X-ray diffraction and transmission electron microscopy revealed the formation of the rock salt-type structure with linearly varying lattice parameters depending on m according to a solid solution, in agreement with a microscopically homogeneous spatial distribution of the constituting elements. However, imaging on the nanostructure shows the formation of coherently ingrown nanoinclusions and diffuse scattering as an indication of short-range ordering.