Relating charge transport properties to the multi-scale chemo-mechanical evolution of crystallizing mixed conducting oxide thin films

Buckner, Haley B

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https://hdl.handle.net/2142/122124 Copy

Description

Title

Relating charge transport properties to the multi-scale chemo-mechanical evolution of crystallizing mixed conducting oxide thin films

Author(s)

Buckner, Haley B

Issue Date

2023-11-22

Director of Research (if dissertation) or Advisor (if thesis)

Perry, Nicola H

Doctoral Committee Chair(s)

Perry, Nicola H

Committee Member(s)

• Krogstad, Jessica
• Zuo, Jian Min
• Bellon, Pascal

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• crystallization
• mixed ionic/electronic conductor
• conductivity
• microstructure

Abstract

Mixed ionic and electronic conductors (MIECs) exhibit inherent hierarchical functionality, meaning the surface catalyzes electrochemical reactions while the bulk transports ionic and electronic charge carriers. In non-stoichiometric oxide MIECs, this facilitates reversible changes in bulk oxygen concentration through oxygen exchange with the surrounding environment, motivating their use as solid oxide fuel/electrolysis cell cathodes, electrocatalysts for aqueous water splitting, and gas separation membranes. In Chapter 1 of this work, I introduce the central themes motivating my investigation into the electro-chemo-mechanical evolution of crystallizing mixed conducting perovskite oxide thin films. My primary research questions revolve around understanding the point defect chemistry differences between amorphous and crystallized phases, the multi-scale structural changes during crystallization, and the implications of these changes for ionic and electronic charge transport. I also explore the tunability of the resultant microstructure through crystallization conditions. Ultimately, my objective is to gain insights that will enable the deliberate formation of hierarchical structures through the crystallization of amorphous-grown thin films promote to the rational design of high-performance mixed conducting perovskite oxide materials. In Chapter 2, I delve into the chemo-mechanical evolution of crystallizing SrTi0.65Fe0.35O3-d (STF35) thin films and its impact on electrical conductivity. By utilizing X-ray absorption near-edge spectroscopy (XAS), (scanning) transmission electron microscopy (S/TEM) imaging, and in-situ impedance spectroscopy, I provide a framework for understanding how crystallization coupled with oxidation affects the properties of STF35 films. X-ray absorption near edge spectroscopy (XANES) measurements reveal a shift of the Fe K-edge position indicating an increase in average Fe oxidation state. The increase in Fe oxidation state occurs with the of crystallization. The close coupling of oxidation and crystallization largely influences changes in local cation environment, resulting in an increase in symmetry and alignment of B-site cation coordination units. Lattice contractions arising from oxidation and crystallization drive the microstructural evolution, particularly the formation of extensive porosity. These changes with crystallization result in a significant increase in electrical conductivity, mostly due to an increase in hole concentration and mobility. Comparison of crystallinity and conductivity points to a potential percolation-type behavior for electronic conductivity. Chapter 3 extends my exploration to La0.7Sr0.3Ga0.6Fe0.4O3-d (LSGF3040) thin films, allowing us to directly link crystal structure development to ionic transference numbers, ionic conductivity, and total conductivity. Using in-situ grazing incidence X-ray diffraction, I observe that the LSGF film crystallizes directly into a perovskite structure while suppressing secondary phase formation. My impedance findings demonstrate that both total and ionic conductivity increase with crystallinity, although electronic conductivity experiences a more rapid rise. This insight enables the tuning of ionic transference numbers with crystallinity and was corroborated with DC polarization measurements. Changes in microstructure, such as blocking grain boundaries, were observed to increase the activation energy for ionic transport in crystallized LSGF compared to amorphous. In Chapter 4, I investigate the influence of oxygen availability on the microstructure of crystallized STF35 thin films. Utilizing electrochemical pumping, oxygen is driven into our out of a crystallizing STF35 thin film on a YSZ substrate. The crystallization kinetics and resulting microstructure are characterized with spatially resolved optical transmission, and advanced electron imaging and diffraction techniques, revealing potential for directing crystallite nucleation behavior through oxygen availability during crystallization. However, the effects of oxygen availability on the microstructure evolution are coupled with electric field effects from the applied bias. Most significantly, a sufficiently high applied bias creates an inhomogeneous cation distribution through the bulk of the crystallized film, giving evidence of enhanced ion (and defect) migration. At lower biases, the cation distribution remains homogeneous, but the electric field effects are suspected to be linked to enhanced grain growth. Differences in pore formation (open vs closed porosity) and cracking in film crystallized on different substrates or using more complex geometries are presented in the Appendix, highlighting the need for more work on understanding and directing pore network formation. In the concluding chapter, we summarize the key findings from our study on the chemo-mechanical evolution of mixed conducting perovskite oxide thin films. We reflect on the changes in point defect chemistry and microstructure during crystallization and their impact on ionic conductivity, motivating further exploration of methods to manipulate microstructure. We also propose future work to investigate the formed pore network in crystallized films and elucidate the factors influencing porosity through strain engineering. Overall, in-situ crystallization of amorphous-grown perovskite oxide thin films provides a promising route for the fabrication of high performance MIECs for low to intermediate temperature solid state ionic applications. This dissertation demonstrated multiple approaches to effectively monitor changes in point defect chemistry, multi-scale structure, and charge transport properties and explore strategies for directing the chemo-mechanical evolution via crystallization of amorphous thin films.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Haley Buckner

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