Grain Boundary Segregation and Energy in Multiphase Oxide Ceramics
The properties of polycrystalline ceramics are dependent on the properties of their constituents: grains, surfaces, and grain boundaries. Grain boundary behavior has been widely studied for different ceramics, particularly single-phase systems with added dopants. However, there is a wide research gap to understand how various processing techniques affect multiphase ceramics and their grain boundaries. Also, while the measurements of grain boundary energies in single-phase ceramics have been carried out for decades, there is limited understanding of how grain boundary energies vary between single phase and multiphase ceramics due to changes in local grain boundary chemistry. In this thesis, we thus seek to expand the understanding of grain boundary segregation and energy anisotropy in multiphase oxide ceramics.The effects of using different sintering techniques (conventional, flash and spark plasma sintering) on grain boundary segregation are investigated in a 3-phase polycrystalline ceramic (8 mol% Y2O3 stabilized ZrO2 (YSZ), α-Al2O3 and MgAl2O4). Using aberration-corrected STEM-EDS, we show Al segregation at YSZ-YSZ boundaries, and Y/Zr segregation at Al2O3-Al2O3, MgAl2O4-MgAl2O4 and MgAl2O4-Al2O3 boundaries. YSZ-MgAl2O4 and YSZ-Al2O3 heterointerfaces, in contrast, do not show elemental segregation. Our results indicate that while the type of segregation at any particular interface does not change with different sintering processes, the quantity of segregants is directly affected by processing type. Next, we determine the grain boundary and interfacial energies in conventionally-sintered single and multiphase oxides (combination of YSZ, Al2O3 and/or MgAl2O4 phases) using AFM. We find that grain boundary energies play a critical role in microstructural evolution in multiphase ceramics. Furthermore, we report that energies of heterointerfaces are intermediate between the grain boundary energies of the constituent phases. Lastly, we expand on the role of interfaces in ceramics by focusing on a thin-film oxide system (La0.6Sr0.4FeO3 or LSF). The phase decomposition (single-to-multiphase) via exsolution is characterized using STEM-EDS, and we report a distinct variety of Fe-based nanostructures formed in the host LSF matrix. Importantly, the grain boundaries and interfaces are found to be the primary sites for Fe-nucleation in LSF. This thesis, and any resulting publications from it, will serve as a beneficial guide to understand the role of grain boundaries and interfaces in multiphase engineering systems in relation to their thermal, electrical, mechanical, and chemical properties.