Experimental Evidence for the Influence of Electromagnetic Fields on Atomic Structure
Creators: name: Nakamura, Nathan
Publisher: name: Carnegie Mellon University
Year: 2020
Subjects: subject: Mechanical Engineering
subject: FOS: Mechanical engineering
subject: FOS: Mechanical engineering

description: Electromagnetic (EM) fields can significantly alter ceramic synthesis, inducing rapid, low-temperature crystallization and phase transitions not observed via conventional routes. Due to these benefits, EM field-assisted methods hold tremendous promise and may be used to access atomic structures or phase compositions not attainable without EM field exposure. However, a lack of understanding surrounding the mechanisms underlying EM field effects in materials has limited the widespread application of these processes. The mechanisms of EM field-induced structural changes are generally attributed to one of two sources: (1) thermal effects such as rapid heating rates or more efficient heating, or (2) non-thermal EM field effects such as field-induced mass transport or defect generation. Distinguishing between these two potential mechanisms has proven difficult, and a key component missing from prior studies is information regarding how applied EM fields influence the local atomic structure and resultant phase stability. Open questions also exist regarding the magnitude of local electric field intensities near nucleation sites and the interplay between local electric field intensity, local atomic structure, and phase stability.This dissertation sheds light on the mechanisms underlying EM field-assisted synthesis, demonstrating that EM fields induce local electric field intensities and oxygen defects which influence atomic structure in ways not replicable via purely thermal processes. This is accomplished via synchrotron x-ray characterization and the development of in-situ techniques monitoring electric fields and atomic structure during EM field exposure. A microwave radiation (MWR)-assisted thin film growth technique is utilized which localizes electric field absorption to the thin film substrate, enabling the study of atomic structure at a known EM field interaction site. Synchrotron x-ray pair distribution function (PDF) analysis and x-ray absorption spectroscopy (XAS) are used to analyze both the local and long-range atomic structure of MWR-grown ceramic oxide thin films and demonstrate that MWR exposure results in large oxygen defect concentrations and local structural distortions. Coupling these results with first principles calculations indicates that the local distortions modify the free energy landscape and explain the phase stability observed during EM field-assisted synthesis.Two novel in-situ characterization techniques which provide information regarding dynamic changes as MWR is applied are also developed in this dissertation. The first capability developed provides the ability to monitor atomic structure during MWR exposure via in-situ synchrotron x-ray characterization. A custom-built microwave reactor is designed which enables simultaneous MWR input and x-ray input/output while allowing for custom sample environments inside the microwave waveguide. MWR-assisted growth of SnO2 nanoparticles is monitored throughout phase formation and reveals that the synthetic pathway during MWR-assisted synthesis differs significantly compared to synthesis without EM field exposure. The source of these differences is found to be distortions on the oxygen sublattice which predate crystalline phase formation, further confirming the role of oxygen defects in mediating EM field-assisted reactions. The second technique, microwave cyclic voltammetry (MW-CV), measures local electric field intensities at the substrate-thin film interface via changes in surface charge buildup during MWR exposure. An orders-of-magnitude increase in the electric field intensity at the interface is found to occur due to MWR absorption. Linking these results with x-ray PDF data indicates that the high local electric fields can serve as a driving force for defect formation.These findings, along with the instrumentation developed enabling in-situ characterization, open the door for future investigations to explore tuning EM field parameters to achieve desired atomic structures and material properties.