High temperature thermoelectric properties evolution of Pb_1-xSn_xTe based alloys

In thermodynamic engines, such as thermoelectric (TE) converters, maximizing the Carnot efficiency, by applying large temperature differences, is required for maximizing the heat to electricity energy conversion. This requirement is a key factor for the desire to maximize the hot side temperature of TE converters as much as possible. Yet, practical stability considerations, such as sublimation of volatile species, composing many of the currently available TE materials, limit the hot side temperature to a certain level, depending on the specific composition. Pb_1-xSn_xTe based TE compositions are known as highly efficient and stable up to a temperature of around 500 °C. For a better understanding of the degradation mechanisms in such compositions, the current research is focused on accelerated degradation studies at a slightly higher temperature of 520 °C for periods of up to 900 h, following hot pressing. It was found that in this class of TE materials, besides of the already reported perferial surface sublimation, sublimation of volatile species from grain boundaries is also apparent, degrading the TE figure of merit, ZT, by up to 44%. The mechanisms for this performance degradation are discussed in details. Furthermore, ZT enhancement of up to 40% is currently being reported by using hot-pressing synthesis rather than the previously reported cold pressing and sintering.