Date of Award

Spring 2026

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Dongkyu Lee

Abstract

The development of highly efficient and cost-effective thermoelectric (TE) materials is critical for converting waste heat into electricity in advanced power generators. For high-temperature TE applications, transition metal oxides (TMOs) have attracted significant interest because they are comparatively safe, earth-abundant, and stable under oxidizing conditions where many conventional heavy-metal and chalcogenide TE materials degrade. However, the TE performance of TMOs is limited by the intrinsic trade-off between the Seebeck coefficient (S) and electrical conductivity (σ), both of which depend inversely on charge carrier concentration. This constraint limits the achievable power factor (PF = S²∙σ) and remains a central obstacle for high-performance oxide-based TE technologies.

In this work, epitaxial strain engineering is employed as a route to mitigate the S–σ trade-off in CaMnO3 (CMO) TE thin films by coupling lattice distortion with defect chemistry. CMO films are synthesized by pulsed laser deposition on single-crystal substrates: SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7, SrLaAlO4, and LaAlO3 to impose distinct strain states and enable a systematic study of how epitaxial strain influences oxygen-vacancy formation and electronic transport.

Increasing tensile strain induces a monotonic expansion of the CMO unit cell and promotes oxygen-vacancy formation. While oxygen vacancies increase electron carrier concentration, they also enhance defect scattering that reduces carrier mobility. As tensile strain is reduced toward the relaxed state, vacancy-related scattering is suppressed, enabling simultaneous improvements in σ and S, and thereby alleviating the conventional S–σ trade-off. Fully relaxed CMO films exhibit room-temperature electrical conductivity approximately four orders of magnitude higher than highly tensile-strained films and achieve the highest thermoelectric power factor, up to six orders of magnitude greater than that of strained films. These results demonstrate that strain-mediated control of oxygen-vacancy concentration provides a viable pathway to mitigate the S–σ trade-off and maximize TE performance in TMO thin films.

Rights

© 2026, Ebenezer Seesi

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