Formation of Te-rich phase and its effect on microstructure and thermoelectric properties of hot-extruded Bi-Te-Se bulk materials

A mechanical alloying (MA) and hot-extrusion technique was used to prepare n-type Bi-Te-Se bulk materials. The formation and distribution of Te-rich phase as well as its effect on microstructure and thermoelectric properties of the extruded samples have been investigated. The formation of Te-rich phase is mainly associated with sublimation of Te and eutectic reaction. The former occurs at any extrusion temperature and leads to formation of small-sized Te-rich phase, while the latter occurs at ≥400 °C and the resultant Te-rich phase possesses large sizes. The Te-rich phase was distributed in different morphologies depending on the extrusion temperature. Small-sized Te-rich phase was distributed around the grain boundaries in extruded samples. When excessive amount of Te was incorporated, the average grain size and orientation degree were found to be reduced due to formation of more Te-rich phase. Furthermore, the addition of excessive Te led to decrease of the Seebeck coefficient and increase of thermal conductivity, thus resulting in a significant reduction in ZT value.