Suppressing the dynamic precipitation and lowering the thermal conductivity for stable and high thermoelectric performance in BaCu2Te2 based materials†
Dynamic precipitation phenomena are usually observed in Cu- and Ag-based thermoelectric compounds undergoing a temperature rise, affecting the stability and thermoelectric figure of merit zT. The present work demonstrates that rational controlling of the Cu content would eliminate unstable factors in BaCu2Te2, beneficial for achieving stable and reliable thermoelectric materials. Over-stoichiometric Cu in BaCu2Te2 can not only optimize the hole concentration approaching the optimal one, but also lead to the stable degenerate semiconducting behaviors. More importantly, despite the intrinsically low lattice thermal conductivity originating from the small phonon group velocities and mean free path, the excess Cu content generates widely distributed Cu-rich nano-precipitates with a size of 20–50 nm. These nano-precipitates significantly contribute to low-frequency phonon scattering for further reducing the thermal conductivity, and thus more than 25% of the κ reduction has been achieved at room temperature (0.95 W m−1 K−1 for BaCu2Te2 and 0.70 W m−1 K−1 for BaCu2.08Te2). A high figure of merit zT = 1.31 at 833 K has been realized for the sample BaCu2+xTe2 (x = 0.04), which is superior to the state of the art p-type Zintl phase thermoelectric materials.