Date of Award

8-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Advisor

Tritt, Terry M

Committee Member

Tritt , Terry M

Committee Member

Manson , Joseph R

Committee Member

Daw , Murray S

Committee Member

Drymiotis , Fivos R

Abstract

The field of thermoelectric research has attracted a lot of interest in hope of helping address the energy crisis. In recent years, low-dimensional thermoelectric materials have been found promising and thus become a popular school of thought. However, the high complexity and cost for fabricating low-dimensional materials give rise to the attempt to further improve conventional bulk polycrystalline materials. Polycrystals are featured by numerous grain boundaries that can scatter heat-carrying phonons to significantly reduce the thermal conductivity κ whereas at the same time can unfortunately deteriorate the electrical resistivity ρ. Aiming at the dualism of the grain boundaries in determining the transport properties of polycrystalline materials, a novel concept of 'grain boundary engineering' has been proposed in order to have a thermoelectrically favorable grain boundary. In this dissertation, a polycrystalline p-type Bi2Te3 system has been intensively investigated in light of such a concept that was realized through a hydrothermal nano-coating treatment technique.
P-type Bi0.4Sb1.6Te3 powder was hydrothermally treated with alkali metal salt XBH4 (X = Na, K or Rb) solution. After the treatment, there formed an alkali-metal-containing surface layer of nanometers thick on the p-Bi2Te3 grains. The Na-treatment, leaving the Seebeck coefficient α almost untouched, lowered κ the most while the Rb-treatment at the same time increased α slightly and decreased ρ the most. Compared to the untreated sample, Na- and Rb-treatments improved the dimensionless figure of merit ZT by ~ 30% due to the reduced κ and ~ 38% owing to the improved the power factor PF, respectively. The grain boundary phase provides a new avenue by which one can potentially decouple the otherwise inter-related α, ρ and κ within one thermoelectric material. The morphologic investigation showed this surface layer lacked crystallinity, if any, and was possibly an amorphous phase.
Once Na- and Rb-treatments with various molar ratios were applied to the same sample, a similar grain boundary layer formed with a compositional gradient along the depth direction. The Hall effect measurements showed that the grain boundary phase introduced new carriers into the system and thereby compensated the loss in mobility. With α almost untouched, the &rho to κ ratio has been optimized by varying the Na:Rb ratio in the starting solution. As a result, the Na:Rb = 1:2 ratio yielded the best ZTvalue of ~ 0.92 at 350K, comparable with that of the state-of-the-art p-Bi2Te3 commercial ingot.
Besides ZT, the hydrothermal treatment lessened the temperature dependence of compatibility factor S of as-treated polycrystalline samples, helping a thermoelectric device have overall better performance even if it did not work under its optimal condition.

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