Synchrotron-based X-ray Imaging to Characterize Structural and Chemical Changes in Energy Materials

The development of increasingly complex, multiphase, and micro-scale composite materials has led to a simultaneous increase in the demand for characterization techniques capable of examining and evaluating the quality of these material systems. These types of composites, which can include such devices as fuel cell and battery electrodes and dense gas separation membranes, display an intimate link between the microstructural arrangement of the various constituent phases and the device performance. This connection is often manifested through coupled transport, reaction, and degradation processes occurring at the micro-scale and conformal to the detailed phase distribution. Therefore, direct examination of the microstructure is critical to understanding how a composite material will perform within the device of its intended purpose. This work is aimed at developing new characterization capabilities, particularly through the use of transmission x-ray microscopy and x-ray nanotomography, to examine these types of dense composite materials. The new tools developed in this dissertation add to existing characterization methods, but provide valuable new opportunities for the direct visual as well as quantitative evaluation of real structures in three dimensions at the nanoscale. Full-field absorption contrast and x-ray absorption near edge structure spectroscopy (XANES) methods are applied using a synchrotron-based microscope with a tunable incident x-ray beam. The elemental and chemical specificity permitted by these methods is exploited to obtain accurate identification and mapping of multiple solid phases in 3-D, including unintentional/poisoning phases introduced during material fabrication or operation. The developed imaging capabilities are then applied to several different materials relevant to real applications of heterogeneous functional materials, and provide a means for evaluating the likely implications of the unintentional phases on the performance of the microstructure. In addition, a statistical approach is introduced to help guide the determination of a representative volume element (RVE) sample size for the accurate measurement of microstructural properties from experimental data.