This dissertation focuses on identifying and analyzing the mechanism of solid nucleation in liquid thin films. In doing so, we identify and describe a previously unrecognized mechanism of nucleation in condensed systems referred to as transmorphic nucleation. This cluster-shape-change-based mechanism is revealed as a general heterogeneous nucleation mechanism applicable to discontinuous phase transformations occurring in continuous or pre-patterned thin films, as well as in numerous materials systems that possess morphologically and chemically non-trivial heterogeneous-nucleation-catalyzing interfaces (e.g., polycrystalline materials, embedded nano-crystals, and materials with structured interfaces).
Identifying, deciphering, and modeling the nature and details associated with how a new phase can nucleate in thin-film materials can be both scientifically meaningful for understanding discontinuous phase transformations in general, and technologically important for engineering various thin-film-based and nano-material-based applications and devices in particular. Classical nucleation theory (CNT) has long been established and regarded as the most practicable treatment that captures the thermodynamic and kinetic essence of the nucleation phenomenon in condensed systems in the simplest and most effective manner. Through a close examination of the theory, we identify and propose morphological equilibrium hypothesis (MEH) as an essential element of CNT. Our shape-transition-based model for transmorphic nucleation in thin films presented in this thesis illustrates that this hypothesis can be violated. As such, the CNT formulation is lacking in capturing the occurrence of the MEH-deviating shape evolution of the clusters, as for instance encountered during the process of transmorphic nucleation.
In this dissertation, we conceptually, theoretically, and numerically examine and analyze the kinetic pathway through which nucleation of solids takes place in encapsulated liquid thin films. This example was selected for investigation because it is a particularly simple system, which in turn permits one to make clear, definitive, and general conclusions. A new nucleation mechanism of transmorphic nucleation is discovered in the process. This mechanism is defined generally as the nucleation mechanism through which supercritical clusters are generated from subcritical clusters during an irreversible and morphological-equilibrium-deviating shape evolution initiated when the fluctuating embryos encounter a local growth-inducing element in the catalyzing interface. Both thermodynamic and kinetic analyses in accordance with our transmorphic nucleation mechanism are carried out using a novel adaptation of established theoretical formulations and numerical modeling methods. The kinetic pathway of transmorphic nucleation is described, and transmorphic nucleation temperature window is thermodynamically identified. The kinetic aspect of transmorphic nucleation in thin films is uniquely captured by keeping track of two coupled population distribution profiles of equilibrium-morphology-adhering cluster shapes.
Overall, the thesis starts with critical and deconstructive examination of CNT. It builds on our theory of phase initiation and evolution in condensed systems, i.e., Gibbs-Thomson variation (GTV) and Gibbs-Thomson function (GTF), and our interpretation of CNT to investigate steady-state and transient transmorphic nucleation in thin films. The thesis also examines and analyzes all other modes of shape-transition-affected nucleation in thin films outside the transmorphic nucleation domain to provide the comprehensive description of the entire map of nucleation mechanisms in thin-film systems. As far as the implications of the current work on the classical theory of nucleation is concerned, we illustrate how the phenomenon of transmorphic nucleation which violates MEH that forms the basis of CNT, reveals this previously unrecognized limitation of the current formulation of the classical theory of nucleation.
The results presented in this dissertation further show that the GTV-based approach, which we identify as the foundation upon which CNT is formulated, can address the MEH-violating shape evolution of subcritical to supercritical clusters. Moreover, the aforementioned reformulation of cluster evolution in this dissertation can be of value for understanding and manipulating phase initiation and evolution involving all of the three Gibbs-Thomson phenomena (i.e., nucleation, coexistence, and free growth) in small, controlled materials systems for optimizing various confined and interface-rich materials that are increasingly becoming technologically important.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/xxbs-pt21 |
| Date | January 2024 |
| Creators | Shen, Bonan |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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