Date of Award

Summer 2025

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Dongkyu Lee

Abstract

The discovery of highly efficient and cost-effective thermoelectric (TE) materials is essential to envision high-performance TE power generators that enable the direct conversion of waste heat into electricity. Transition metal oxides (TMOs) are considered promising candidates for high-temperature TE applications due to their excellent thermal stability and low cost. However, their maximum power output (power factor, PF = σ•S2) is constrained by the intrinsic trade-off between thermopower (S) and electrical conductivity (σ), both of which are inversely related to carrier concentration. This trade-off poses a significant barrier to improving the performance of TE devices. Among various strategies to overcome this limitation, this study focuses on two emerging strategies: 1. strain engineering and 2. redox engineering, utilizing epitaxial thin films synthesized pulsed laser deposition (PLD) as a model system.

Epitaxial strain is a powerful approach for controlling the physical properties of materials. While it has demonstrated potential for tuning the TE properties of oxides, the underlying mechanisms are not yet fully understood. To investigate the impact of strain on TE properties, epitaxial SrRuO3 (SRO) thin films were grown on three different single crystal substrates: LaAlO3, (LaAlO3)0.29(SrAl0.5Ta0.5O3)0.71, and SrTiO3, which induce complete relaxation, -0.53% compressive strain, and -1.47 % compressive strain, respectively. The PF of SRO increased with increasing compressive strain. In-situ X-ray diffraction and TE property analysis revealed that compressive strain reduces the concentration of oxygen vacancies while maintaining carrier concentration. As a result, electrical conductivity increases due to enhanced carrier mobility, while thermopower remains unchanged, leading to an improvement in PF.

Nanoinclusion strategies, such as embedding metallic particles in a TE material, are also known to effectively tune TE properties. However, maintaining the stability and integrity of metal nanoparticles within an oxide matrix is inherently challenging. To overcome these limitations, this work exploits metal exsolution—a process that arises from the unique redox flexibility and defect chemistry of TMOs, wherein nanoparticles emerge from the oxide matrix under reducing conditions. Two different material systems: La0.7Ca0.2Ni0.25Ti0.75O3 (LCNTO) and Sr0.95Ti0.76Nb0.19Ni0.05O3 (STNNO) were used to investigate the influence of exsolved metal particles on the TE properties of TMOs. In LCNTO, the exsolved Ni particles after reduction significantly enhanced the PF by increasing both S and σ. The enhancement in S was attributed to energy-dependent electron scattering at the interfaces, while the conducting network formed by metal particles accounted for the increased σ. Similarly, STNNO thin films exhibited a significant increase in PF due to exsolved Ni particles; however, in this case, the improvement was attributed solely to the enhanced σ, without an energy filtering effect.

Building on these findings, a simplified, one-step exsolution process was developed by tuning the PLD growth conditions. By reducing the oxygen partial pressure (< 0.3 mTorr), spontaneous exsolution of Ni nanoparticles was achieved during film growth, yielding a PF improvement of approximately seven orders of magnitude.

These results demonstrate that the combination of strain and redox engineering provides a robust and scalable framework for enhancing the thermoelectric performance of TMOs. Together, these strategies address long-standing limitations in oxide TE materials and open new pathways for the development of efficient, high-temperature TE applications.

Rights

© 2025, Mohammad Jamal El Loubani

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