PbQ–NaSbQ2 (Q = Te, Se, S) and SnTe–NaPnTe2 (Pn = Sb, Bi) Thermoelectric Alloys

Slade, Tyler John

doi:https://doi.org/10.21985/n2-xdzh-q242

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PbQ–NaSbQ2 (Q = Te, Se, S) and SnTe–NaPnTe2 (Pn = Sb, Bi) Thermoelectric Alloys

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Thermoelectric modules that convert heat into electrical energy are attractive for improving global energy management. This thesis reports the synthesis and characterization of two new families of lead and tin chalcogenide alloys and focuses on the impact of the grain boundaries, phase segregation, and atomic vacancies on the electronic and thermal transport properties. Alloying NaSbQ2 (Q = Te, Se, S) into PbQ forms new quaternary semiconductors NaPbmSbQm+2 where the three cations randomly occupy the same crystallographic position. NaSbQ2 enhances phonon scattering and favorably changes the electronic structure, resulting in high thermoelectric performance in the telluride and selenide families. The synthetic procedure has a profound impact on the microstructure and physical properties of NaPbmSbTem+2. As–cast ingots form two phase composites, with nano to micron–scale phase segregation depending on the NaSbTe2 fraction and exhibit degenerate p–type conduction. On the contrary, sintered pellets form single phase solid solutions with weakly n–type transport. The selenide and sulfide families exhibit irregular charge transport behavior, with thermally activated electrical conductivity below 500 K and a change to metallic transport above. This work demonstrates the thermally activated conduction stems from scattering of charge carriers by the grain boundaries and furthermore proposes a chemical framework that explains the magnitude of the scattering in PbQ–NaSbQ2 alloys and other thermoelectric compounds. The tin–analogues, SnTe–NaPnTe2 (Pn = Sb, Bi), are found to have distinct thermoelectric properties that are determined by the intrinsic defects. Addition of NaSbTe2, but not NaBiTe2, to SnTe enhances the concentration of native Sn vacancies. The vacancies both strengthen phonon scattering and raise the charge carrier concentration, which suppresses detrimental intrinsic conduction. Therefore, the Sn vacancies allow superior thermoelectric performance in SnTe–NaSbTe2 than SnTe–NaBiTe2. The final chapter explores the Fermi–level dependence of the sound velocities (acoustic phonon velocities) in eight thermoelectric semiconductors. Raising the charge carrier concentration, in both p– and n–type directions, to highly degenerate values over 1020 cm-3 suppresses the sound velocity by up to 16 percent, demonstrating charge carriers play an important and previously unrecognized role in determining the phonon transport properties of heavily doped semiconductors.

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