Semester

Fall

Date of Graduation

2019

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xueyan Song

Committee Member

Yun Chen

Committee Member

Hailin Li

Committee Member

Jacky Prucz

Committee Member

David Tucker

Abstract

Worldwide energy consumption losses more than 60% of the energy after its conversion, most of these losses are in the form of waste heat. Therefore, it is a matter of utmost importance to harness waste heat and reutilize this energy source to improve the overall conversion efficiency. Thermoelectric (TE) technology is one of the rising alternatives to harvest the excessive amount of energy lost as waste heat for a better and sustainable energy future. TE materials can utilize waste heat to improve efficiency in the ever-increasing demand for energy by converting a temperature difference directly into electricity due to the Seebeck effect. The energy conversion efficiency of TE materials is described by the figure of merit ZT, which is defined as ZT= S2ρ‑1κ‑1T, where S, ρ, S2ρ-1, and κ are the Seebeck coefficient, electrical resistivity, electrical power factor(PF) and thermal conductivity, respectively. P-type calcium cobaltite Ca3Co4O9-δ is a promising candidate for TE applications over conventional TE materials due to its high thermal stability in the air at high temperatures, low cost, lightweight, and non-toxicity. Single crystal Ca3Co4O9-δ already possess an excellent TE performance, approaching that of the well-developed conventional TE materials. However, TE energy conversion efficiency of polycrystalline Ca3Co4O9-δ remains low and accounts to ~30-60% of the single crystals. To overcome the low performance of the ZT in Ca3Co4O9-δ ceramics, dopants were introduced through both the cation stoichiometric substitution and novel non-stoichiometric addition. This thesis work shows that dopants intragranular non-stoichiometric addition and concurrent intergranular segregation at the grain boundaries dramatically decrease the electrical resistivity, simultaneously increase the Seebeck coefficient, and ultimately result in the polycrystalline ceramics outperforming the single crystals over a wide range of temperatures.

A record high thermoelectric figure of merit ZT~0.9 at 1073 K was achieved for Ca3Co4O9‑δ ceramics that is surpassing the Ca3Co4O9-δ single crystals. Furthermore, the engineered Ca3Co4O9-δ polycrystalline ceramics also outperformed the best reported p-type SiGe from 373 K to 973K, while Ca3Co4O9-δ ceramics are having just 5-10% of the cost of SiGe and will be performing directly in air. Most of all, the present work presents a novel approach to engineer the lattice and grain boundary in ceramics to decouple the strongly coupled thermoelectric parameters of S, ρ, and κ to largely increase the electrical power factor of thermometric oxide ceramics.

Embargo Reason

Patent Pending

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