Extending classical nucleation theory: Understanding the effects of trace additives and inhomogeneous concentration distributions
Nucleation is the first step in the conversion of a metastable phase to a more stable one. It involves the formation of a post-critical nucleus that is thermodynamically fa- vorable to grow and is important in most phase transitions. Our understanding of the thermodynamics and kinetics of nucleation is largely based on classical nucleation theory (CNT). It was formulated on the basic principle developed by Gibbs, who reasoned that the free energy required to form a nucleus involves two competing contributions: (1) a favorable bulk driving force and (2) an unfavorable surface penalty, and the subsequent work by Volmer and Weber, Farkas, Becker and Doring, and Zeldovich that developed rate laws. Almost 100 years later, most studies of nucleation still borrow at least some el- ements of CNT. Although CNT provides a simple and intuitive framework to understand nucleation, it is not applicable in many important situations.
In this thesis, we develop corollary theories that adapt CNT to understand solute precipitate nucleation when different factors affect the driving force or surface energy. We introduce a spatio-temporal survival probability model to provide the first stochastic model of nucleation due to solute enrichment ahead of a crystallizing front. The model predicts the distribution of nucleation times as a function of CNT rate parameters and growth conditions. Finally, we investigate how adsorbing additives can promote nucle- ation. We created a theoretical framework for modeling the thermodynamics and kinetics of solute precipitate nucleation when molecular surfactants can adsorb onto nuclei and reduce the surface energy. We also used lattice simulations to study how properties of adsorbing molecular and oligomeric additives affect nucleation barriers.