THERMOELECTRIC STUDIES OF THE TIN TELLURIDE

Abstract

The lead-free tin telluride (SnTe) is considered as a potential candidate to substitute lead telluride (PbTe) for thermoelectric power generation based on their similar crystal and electronic structures. However, the relatively high lattice thermal conductivity and low Seebeck coefficient of pristine SnTe are detrimental for real-life applications. This dissertation explored elements-doping/substituting of SnTe to overcome those shortcomings and improve SnTe thermoelectric performance. A series of the Sn1-xGexTe phases were synthesized and studied. When the Ge amount reaches 50% or higher, Sn1-xGexTe undergoes a phase transition from the rock-salt structure (Fm3̅m) to the rhombohedral one (R3m). The Sn0.5Ge0.5Te phase was explored in more details because it delivers the best thermoelectric performance with the Sn1-xGexTe series. The electron-richer Sb and Bi were substituted on the Sn/Ge site to optimize the charge transport properties, and Cu2Te was added into the matrix to improve the thermoelectric performance further. The In/Sb and In/Bi co-doping on the Sn/Ge sites was employed for Seebeck coefficient optimization. A comparative study of the electronic structure of the Sn0.5Ge0.5Te-based samples was performed. The calculations indicated a band convergence and changes in the valence band, thus providing insight into the co-doping effects. Suppression of the lattice thermal conductivity of SnTe was performed via alloying with AgSnSe2 and PbTe, which introduced strong atomic disorder. Additionally, AgSnSe2 showed a hole donor behavior in SnTe, and the increased carrier concentration compensated for the reduction in the carrier mobility, thus rendering a decent electrical conductivity in alloyed samples. As a result, the alloying effectively improved the samples' thermoelectric performance.