Nanoscale Phase Immiscibility in High-ZT Bulk Lead Telluride Thermoelectric Materials

Renewable energy initiatives have increased interest in thermoelectric materials as an option for inexpensive and environmentally friendly waste heat-to-power generation. Unfortunately, low efficiencies have limited their wide-scale utilization. This work describes the synthesis and characterization of bulk nanostructured thermoelectric materials wherein natural phase immiscibility is manipulated to selectively generate nanoscale inclusions of a second phase that improve their efficiency through reductions in lattice thermal conductivity.;The PbTe-PbS system exhibits natural phase separation by nucleation and growth or spinodal decomposition phase transformations depending on composition and temperature treatment. Through rapid quenching, nearly ideal solid solution alloys of PbTe-PbS are observed by powder X-ray diffraction. However, characterization by solid-state NMR and IR reflectivity show that solid solutions are obtained for rapidly quenched samples within the nucleation and growth region of the phase diagram, but samples within the spinodal decomposition region exhibit very slight phase immiscibility. We report the temperatures of phase separation using high temperature powder X-ray diffraction. Microscopy reveals that phase separation in PbTe-PbS naturally produces nanoinclusions. A decrease in lattice thermal conductivity is observed as a result of the solid solution-to-nanostructured phase transformation in this materials system, increasing thermoelectric figure of merit.;Sn addition to PbTe-PbS produces a pseudobinary system of PbTe-PbSnS 2. This materials system produces microscale lamellae that effectively reduce lattice thermal conductivity. Unfortunately, the PbSnS2 inclusions also scatter electrons, reducing electrical conductivity and producing only a minimal increase in thermoelectric figure of merit. We additionally investigate PbSnS2 as prepared through Bridgman crystal growth.;PbTe-PbS doped with Na appears to increase the kinetic rate of phase separation, so that rapid quenching does not produce solid solutions. Na segregation in and at the interfaces of PbS nanocrystals results in the formation of cuboctahedral nanostructures that reduce lattice thermal conductivity. Additionally, at high temperatures Na incorporation in PbTe-PbS appears to promote carriers into a different electronic energy band of PbTe, significantly enhancing the electronic transport. The enhancement in thermoelectric figure of merit by concurrent reductions in lattice thermal conductivity and enhancement in electronic properties make this material particularly attractive for future device fabrication.