Modeling microstructural evolution during crystallization: from organic thin films to electrodeposited metals

Abstract

Many materials with diverse technological applications, such as flexible electronics, have complex microstructures that form when crystallization occurs under non-equilibrium conditions. In this thesis, we develop theoretical models and perform numerical simulations to study microstructural evolution during non-equilibrium phase transformations in two physical systems: crystallization in organic thin films and electrodeposition into porous materials.

In our studies of organic thin film crystallization, we develop phase-field models to simulate dendritic and spherulitic crystal growth in organic thin films. In particular, we find that a combination of thermodynamic and kinetic effects are important for the formation of a wide variety of morphologies with regions of different out-of-plane molecular orientations or polymorphs. We also develop a model for twisted crystal growth, which we use to simulate curved dendrites and banded spherulites. Additionally, for crystallization guided by a substrate-pattern-defined channel, we derive an analytical expression for the steady-state growth velocity as a function of channel width, and using experimental data we extract a critical channel width that gives the minimum feature size achievable with this patterning technique.

In our efforts to better understand electrodeposition in porous materials, we develop a phase-field model for template-assisted electrodeposition of nanowires. We simulate nanowire growth in single straight pores as well as templates composed of many pores, and we find that variations in pore shapes as well as permeability of the template material to ionic diffusion can significantly increase nanowire length inhomogeneity. We also develop a framework for statistically analyzing nanowire length distributions to determine the variations in growth rates or nucleation times necessary to cause a given spread in nanowire lengths. Finally, we perform simulations of metal plating through a porous battery separator and develop a cell-level lithium-ion battery model to simulate the effects of various shapes of separator pore closure.