Tin Telluride-Based Nanocomposites of the Type AgSn_<i>m</i>BiTe_2+<i>m</i> (BTST‑<i>m</i>) 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_mBiTe_2+<i>m</i> (Bismuth–Tin–Silver–Tellurium, BTST-<i>m</i>) 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, <i>ZT</i> 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 <i>m</i> 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