Semester

Spring

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xueyan Song

Committee Co-Chair

Victor Mucino

Committee Member

Yun Chen

Committee Member

Hailin Li

Committee Member

Dustin McIntyre

Abstract

Most of the energy lost during the conversion process comes in the form of waste heat. The world's energy consumption loses over 60% of its energy after conversion. In order to increase the efficiency of the conversion process, it is essential to harness waste heat and reutilize this energy resource. Heat recovery technology as thermoelectric (TE) technology is one of the growing alternatives to harvest excessive amounts of energy lost as waste heat for a more efficient, safe, and sustainable future. Due to the Seebeck effect, TE materials have the potential to convert waste heat directly into electricity, improving efficiency to meet an ever-increasing energy demand. In TE materials, the energy conversion efficiency is measured 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. N-type calcium manganite CaMnO3 is of significant interest in various energy applications over conventional TE materials because it is an earth-abundant material, non-toxic, environmentally friendly, and has a high thermal stability in air at high temperatures. Unfortunately, polycrystalline CaMnO3 has been regarded as poor TE materials since its energy conversion efficiency remains low. To improve the performance of the ZT in CaMnO3 ceramics, both cation stoichiometric substitution and novel non-stoichiometric addition of dopants were employed. This thesis work shows that a synergistic approach of dopant substitution or addition, and secondary phase at the grain boundaries leads to the dramatic decrease of the electrical resistivity and simultaneous increase the Seebeck coefficient, producing an overall increase in power factor and outperforming the current state of the art at a large temperature range.

A high thermoelectric figure of merit ZT~0.67 at 773 K was achieved for CaMnO3 ceramics that is by far the highest ZT reported for various perovskites oxide ceramics. Moreover, the present work presents a novel approach of dramatically increasing the energy conversion efficiency of thermoelectric CaMnO3 ceramics through the combination of lattice dopants substitution and secondary phase segregation at the grain boundaries.

Embargo Reason

Publication Pending

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