Evolution and Impact of Microstructure in Functional Materials

Abstract

A series of investigations are conducted to determine, first, how microstructure evolves in the synthesis of optical materials via template-directed eutectic solidification and, second, how microstructure affects the chemical transport properties of function materials with diffusional hindrance at grain boundaries. As the first and primary part of this thesis, self-organization techniques, such as eutectic solidification, are being explored to enable fabrication of large-area optical metamaterials and to overcome the limitations of traditional metamaterial synthesis methods. Directing the solidification of a eutectic material through a non-reactive periodic template yields more complex morphologies with different length scales than those of either the native eutectic material or the template alone. Phase-field simulations utilizing the smoothed boundary method (SBM) to include template-eutectic interactions are conducted, and the predictions are used to guide the design of template geometries and the selection of eutectic material systems. A combination of phase-field and heat transfer simulations elucidate the morphological evolution of four cases of geometric confinement. The first case is a printed AgCl-KCl eutectic filament. The simulated temperature profile reveals an inward solidification direction at the filament edges, whereas the filament center solidifies along the printing direction. In another case, eutectic solidification is guided by the surface of a three-dimensional cage structure. The cage’s nonuniform thermal conductivity results in a curved solidification front. Additionally, simulations indicate in an investigation of a solidification velocity-dependent rod-to-lamellar transition in the eutectic microstructure of AgCl-CsAgCl2, that the structure which forms initially on the cool surface will persist through the bulk. Finally, through phase-field simulations with asymmetric eutectic-template interfacial energies confined within a cylindrical channel, core-shell nanowire morphologies are realized. Solidifying binary eutectic materials confined to a template consisting of an array of pillar obstacles produces periodic structures with a high degree of order. These pillar templates can also be used to exert control over lamellar orientation. The relationship between undercooling and lamellar orientation is explored via phase-field simulations and the cause of lamellar reorientation within a template is discovered. Highly ordered mesostructures develop when solidifying along the pillar axis. A parametric study is conducted to investigate the effects of minority-phase volume fraction, template volume fraction, and solidification velocity on these mesostructures. The second part of this thesis considers interfaces and grain boundaries that can enhance or hinder transport, which alter the properties of polycrystalline solids from their intrinsic bulk properties. Diffusion in polycrystalline materials can be hindered at grain boundaries in several material systems, including those of solid oxide fuel cells and batteries. A hindered grain boundary diffusion model employing SBM is developed, analyzed, and utilized to study a nanocrystalline solid oxide fuel cell material (yttria-stabilized zirconia) and a battery cathode material (nickel manganese cobalt oxide). Further, effective diffusivities are extracted from the concentration profiles produced by the model for a range of grain morphologies. The anisotropy of grain morphologies plays a critical role in the overall transport behavior, which cannot be quantified with preexisting mean-field approximations. Steady-state concentration profiles are used to guide the development of a universal expression for predicting effective diffusivities of complex polycrystalline solids without computationally intensive simulations. The universal expression predicts effective diffusivity more accurately than the Maxwell Garnett equation does by up to 57%, depending on anisotropy. This approach enables efficient simulation of transport in larger-scale systems while accurately capturing the effects of grain morphologies.