Evolvement of microstructure and lattice thermal conductivity in Na doped PbTe–PbS pseudo−binary system

PbTe–PbS pseudo−binary is a renowned thermoelectric system largely due to its phase separation driven low thermal conductivity. To understand the thermal conductivity evolvement as well as its correlation with microstructural features (especially the nano-scale precipitates), systematic transmission electron microscope studies were performed in this work on a series of PbTe–PbS samples with gradually varying PbS or Na content. We confirmed the co-existence of two groups of precipitates in these samples, i.e. INPs (incipient nano-precipitates) ∼5 nm and PbS-rich ones ranging from 10 to a few hundreds of nano-meters. We also studied their individual size and concentration evolvement with varying PbS and Na. The effect of evolving microstructures on phonon transport was discussed and understood in a modified Callaway model, which conclusively led to the conclusion that significant impedance on heat conduction could only be realized in the case of nano-precipitates with a proper size and number density distribution.