Nanostructuring and Band Alignment as Routes to High ZT in PbTe Thermoelectric Materials

Global climate change has necessitated rapid advances in renewable energy technologies and has led to increased interest in thermoelectric materials as an environmentally friendly source of energy by converting waste heat into electrical power. This work describes the synthesis and exploration of the nanostructured nature of a system previously considered a solid solution and the synthesis and characterization of two bulk thermoelectric material systems designed to incorporate nanostructures and concurrently manipulate the band structure.;The PbTe-PbSe thermoelectric system has long been considered a solid solution as no second phase precipitation is observed with bulk powder X-ray diffraction techniques. However, characterization with transmission electron microscopy reveals extensive nanostructuring found in pristine PbTe-PbSe and in highly doped PbTe-PbSe doped with 2% Na, 2% Sb or 0.15% PbI2. The thermoelectric properties are consistent with previous literature reports and indicate that nanostructuring in these types of systems may be difficult to differentiate from point defect scattering. We report ZT values from ∼0.8-1.6 for all the samples studied within, and all samples exhibited nanostructuring. We conclude the nanostructuring within the PbTe-PbSe system is extensive and thermodynamically stable, and no true solid solution behavior was found despite the solid-solution-like behavior observed with bulk analysis techniques.;A pseudo-ternary lead chalcogenide system of (PbTe)1-2x(PbSe) x(PbS)x was designed to incorporate the modified band structure and high power factor of the PbTe-PbSe system with the low thermal conductivity of the PbTe-PbS system. The thermoelectric properties of the 2% Na doped PbTe-PbSe-PbS system are found to be superior to those of 2% Na doped PbTe-PbSe and PbTe-PbS. PbTe-PbSe-PbS exhibits an increased the power factor by virtue of valence band modification combined with a very reduced lattice thermal conductivity deriving from alloy scattering and point defects. The presence of sulfide ions in the rock-salt structure alters the band structure and creates a plateau in the electrical conductivity and thermopower. The very low total thermal conductivity values are due to phonon scattering in solid solution defects rather than from the assistance of nanostructures.;Band alignment in n-type systems should not be neglected, as the thermoelectric efficiencies of n-type PbTe systems are inherently lower than p-type PbTe systems. PbTe was alloyed with different second phases, including CdSe, CdS, GaSb, InSb, InP, and Pb2Sb2Se5, which were selected because their conduction band minima was close to the position of PbTe. The purpose of the second phase is to scatter phonons to increase the material's ZT without scattering electrons, which also contributes to a high ZT value. Only Pb2Sb2Se5 had a significant impact on the lattice thermal conductivity at fractions above 2%, and the ZT values were found to be a function of the individual carrier concentration level and not increased over pristine PbTe.