Browsing by Author "Neilson, Jamie, committee member"

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Open Access
Determination of reliable minimum, disproof-based particle formation mechanisms: investigation of a second-generation Ir(0)n nanoparticle system
(Colorado State University. Libraries, 2021) Whitehead, Christopher Breck, author; Finke, Richard, advisor; Neilson, Jamie, committee member; Van Orden, Alan, committee member; Shipman, Patrick, committee member
A long-sought goal in particle formation is an understanding of the chemical reaction mechanism. The complete understanding of the associated processes (nucleation, growth, and agglomeration) will yield particle size and distribution control. Mechanistic control and knowledge will yield improvements in the development of renewable energy and catalytic materials. The current state of chemical reaction mechanisms and the direct methods to study them are presented in an in-depth literature review in Chapter II. The best, state-of-the-art case studies are examined and the minimum criteria for a reliable, disproof-based chemical mechanism are presented. The experimental work presented in this dissertation centers on a second-generation {[(1,5-COD)IrI•HPO4]2}2– precursor to Ir(0)~150(HPO42–)x nanoparticle system. The exhaustive investigation of the reaction speciation and the dependence of IrI and HPO42– concentrations on the reaction kinetics are presented in Chapter III. Based on the reaction kinetics and there experimentally determined nucleation step, the molecular mechanism of Ir(0)~150(HPO42–)x nanoparticle formation is elucidated. Next, in Chapter IV, the second-generation {[(1,5-COD)IrI•HPO4]2}2– precursor to Ir(0)~150(HPO42–)x nanoparticle system is monitored directly by X-ray absorption spectroscopy and small-angle X-ray scattering and indirectly by in-house cyclohexene reporter reaction, gas-liquid chromatography, proton nuclear magnetic resonance, and transmission electron microscopy. A total of 6 physical methods are used to follow the particle formation kinetics. Finally, mechanism-enabled population balance modeling is applied as a final test of the proposed mechanism.
Open Access
Normalizing Parseval frames by gradient descent
(Colorado State University. Libraries, 2024) Caine, Anthony, author; Peterson, Chris, advisor; Shonkwiler, Clayton, advisor; Adams, Henry, committee member; Neilson, Jamie, committee member
Equinorm Parseval Frames (ENPFs) are collections of equal-length vectors that form Parseval frames, meaning they are spanning sets that satisfy a version of the Parseval identity. As such, they have many of the desirable features of orthonormal bases for signal processing and data representation, but provide advantages over orthonormal bases in settings where redundancy is important to provide robustness to data loss. We give three methods for normalizing Parseval frames: that is, flowing a generic Parseval frame to an ENPF. This complements prior work showing that equal-norm frames could be "Parsevalized" and potentially provides new avenues for attacking the Paulsen problem, which seeks sharp upper bounds on the distance to the space of ENPFs in terms of norm and spectral data. This work is based on ideas from symplectic geometry and geometric invariant theory.
Open Access
Selective halogenation of pyridines and diazines via unconventional intermediates
(Colorado State University. Libraries, 2022) Levy, Jeffrey N., author; McNally, Andrew, advisor; Bandar, Jeffrey, committee member; Neilson, Jamie, committee member; Hansen, Jeffrey, committee member
Pyridines and diazines are prevalent in pharmaceuticals, agrochemicals, ligands, and other organic materials, and it's vital that synthetic chemists can selectively functionalize these heterocycles. We have shown that heterocyclic phosphonium salts and Zincke imine intermediates can be used to regioselectively functionalize pyridine rings. This dissertation describes the development of these strategies with an emphasis on new approaches to selectively halogenate pyridines, which we view as a long-standing challenge in organic chemistry. Chapter One introduces the importance of pyridines and diazines, as well as established methods and limitations in halogenating these azines. Chapter Two provides an overview of the synthesis and reactivity of heterocyclic phosphonium salts, and then describes a new strategy to access 4-halogenated pyridines via these reagents. Chapter Three examines further developments of heterocyclic phosphonium salts, including as how they can be used to selectively add amines and fluoroalkyl substituents to pyridines. Chapter Four provides an overview of pyridine ring-opening reactions and then shows how this approach can be applied to selectively halogenate the 3-position of pyridines. Chapter Five describes how modifications to the ring-opening strategy can be used to change halogenation site-selectivity. This chapter also shows that the ring-opened intermediates can be used to form isotopically labeled pyridines and aniline derivatives.
Open Access
Understanding and utilization of thermal gradients in spark plasma sintering for graded microstructure and mechanical properties
(Colorado State University. Libraries, 2022) Preston, Alexander David, author; Ma, Kaka, advisor; Weinberger, Chris, committee member; Neilson, Jamie, committee member; Heyliger, Paul, committee member
Spark plasma sintering (SPS), also commonly known as electric field assisted sintering, utilizes high density electric currents and pressure to achieve rapid heating and significantly shorter sintering times for consolidating metal and ceramic powders, which could otherwise be difficult, time consuming, and energy intensive. SPS has attracted extensive research interests since the early 1990's, with the promise of efficient manufacturing of refractory materials, ultrahigh temperature ceramics, nanostructured materials, functionally graded materials, and non-equilibrium materials. Thermal gradients occur in SPS tooling and the samples during sintering, which can be a drawback if homogeneous properties are desirable, as the temperature inhomogeneity can lead to large gradients in microstructure such as porosity, grain size, and phase distribution. Many researchers have looked to mitigate or control these gradients by design and use of specialized tooling. However, the effect of the starting powder is relatively less investigated or overlooked. Feedstock powders can come in various shapes, particle size distributions, and surface chemistry. Effects of these powder characteristics on the SPS process and the consequent microstructure of the sintered parts remain as a gap in the fundamental knowledge of SPS. To fill in this gap, my research investigated the role of thermal gradients during SPS, and how the thermal gradients subsequently affect the location-specific pore distribution, and the consequent mechanical properties of the materials. From a practical point of view, design and fabrication of a bulk sample with a fully dense surface and an engineered pore architecture in the sample interior via one-step SPS will enable mechanical properties unattainable via conventional processing of fully dense bulk materials, such as alike combination of lightweight, high surface hardness, and wear resistance, and high toughness. Therefore, the overarching goal of my research was to provide fundamental insights into the material processing - microstructure - properties correlation so that the field assisted sintering technology can be advanced to control location-specific microstructure. To fulfill this goal, two metallic materials were selected in my study, austenitic stainless steel and commercially pure titanium, representing inherently heavy but widely used alloys, and a pure metal that is inherently lightweight, these materials were used to investigate the effects of powder morphology on the sintering behavior. The pure Ti was selected specifically to gain fundamental insight into the effect of powder shape on sintering, while mitigating the concern of alloying/precipitation events and integrating FEM with my experimental work. This work identified a relationship between decreasing pore size and increasing yield strength in stainless steel, which was attributed to fine precipitate formation surrounding submicron pores inducing local stiffening. Whereas larger pores where precipitates were not found are concluded to not have the necessary driving force for the precipitation event to occur. Ball milled stainless steel powders with higher aspect ratios were also shown to have smaller porosity gradients in comparison to their spherical gas atomized counterparts. A thermal electric finite element model is also proposed which incorporates the master sintering curve to simulate densification as an alternative to the more computationally costly and difficult to parametrize fully coupled thermal-electric-mechanical finite element model. Results from the combined model indicate strong agreement with experimental results within 2% accuracy of measured densification. Additionally, the model predicts higher porosity gradients for gas atomized powders in comparison to ball milled powders which is experimentally verified.