|Ph.D Student||Sternlicht Hadasa|
|Subject||Mechanisms of Grain Boundary Motion in SrTiO3|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Wayne D. Kaplan|
|Full Thesis text|
Grain boundary motion is a well-known phenomenon and one of the fundamental processes which define microstructural evolution, and thus many of the properties of polycrystalline materials. Although methodologies to study grain boundary migration kinetics have been established, and grain boundary mobility (velocity normalized by the driving force) can be experimentally measured, the atomistic mechanism by which a grain boundary moves has yet to be fully evaluated. Understanding the mechanism of grain boundary motion is important for both fundamental and applied issues related to microstructural evolution of materials, such as the role of grain boundary atomistic structure on growth, and controlling the grain size of polycrystalline material systems in order to optimize their engineering properties.
Following the terrace ledge kink (TLK) model, grain boundaries were described as stepped planes which move by step-motion along the boundary plane during grain growth. The concept of steps at grain boundaries includes line defects; such that steps can have both a step and a dislocation character (so called disconnections). In the past, high symmetry grain boundaries (in bicrystals) were (almost) exclusively studied, since their atomistic structure can be determined. However, these make up a small portion of available boundaries and do not necessarily exist in nature or represent the general case.
The present work focuses on the atomistic mechanism by which general grain boundaries migrate. To do this, aberration corrected electron microscopy was employed to characterize disconnections at grain boundaries in a model system (SrTiO3). The main goal of the research was to correlate between experimentally determined grain boundary mobility and an atomistic mechanism, while identifying specific grain boundary disconnections. General grain boundaries in SrTiO3 were studied ex-situ, and compared to thin films of SrTiO3 which were studied in-situ. The step and dislocation components of the identified disconnections were found to be anisotropic and of the same nature at boundaries annealed at a variety of conditions. In order to address changes in the measured grain boundary mobility, the chemistry of the boundaries was quantified using energy dispersive spectroscopy and electron energy loss spectroscopy. It was found that these changes in mobility can be explained by the formation of second phases. The existence of steps in both in-situ and ex-situ experiments indicates that they are active during grain boundary motion. As such, step motion is very likely associated with the mechanism of grain growth in general grain boundaries.