Effect of microstructure on thermoelectric conversion efficiency in metastable δ-phase AgSbTe2

Herein, the effect of AgSbTe2 microstructure on the thermoelectric conversion efficiency of an AgSbTe2-based alloy was studied. The as-sintered sample exhibited material separation into a multiphase mixture of cubic Sb-rich AgSbTe2 (Ag21Sb28Te51), monoclinic Ag2Te with an inhomogeneous distribution, and an Sb-rich zone. The samples were subjected to heat treatment for 24 h at 523, 573, and 673 K. The results indicated that AgSbTe2 was metastable against Ag2Te and Sb2Te3 at 573 K. The accelerated kinetics induced the separation of AgSbTe2 into less-Sb-rich δ’-AgSbTe2 and Sb2Te3 with sub-µm sizes. The observations also revealed that AgSbTe2 was stable against Ag2Te and Sb2Te3 at 673 K. Micrometer-scale Ag2Te precipitates and rhombohedral Sb2Te3 phases were dissolved in the main matrix. Furthermore, single-phase δ-AgSbTe2 was stabilized with nanoscale precipitates of Ag2Te and Sb-rich nanodots. The thermoelectric transport properties of the heat-treated samples were investigated. The results indicated that the thermoelectric performance of the single-phase metastable structure with nanoscale precipitates was superior to that of the multiphase structure. Multiphase AgSbTe2 exhibited a low thermal conductivity; however, the effective Seebeck coefficient was balanced by the presence of multiple phases. This resulted in a low thermoelectric power factor. Metastable single-phase AgSbTe2, with Ag2Te and Sb-rich nanodots, exhibited a high Seebeck coefficient with a slightly low p-type conductivity. This resulted in a high thermoelectric power factor, and the presence of nanoscale precipitates lowered the thermal conductivity. It was concluded that the thermoelectric performance of metastable single-phase δ-AgSbTe2 was superior to that of multiphase AgSbTe2.