The mechanisms of thermal transport in defect-free silicon nanostructures are examined using a combination of lattice dynamics (LD) calculations and the Boltzmann transport equation (BTE). To begin, the thermal conductivity reduction in thin films is examined using a hierarchical method that first predicts phonon transport properties using LD calculations, and then solves the phonon BTE using the lattice Boltzmann method. This approach, which considers all of the phonons in the first Brillouin-zone, is used to assess the suitability of common assumptions used to reduce the computational effort. Specifically, we assess the validity of: (i) neglecting the contributions of optical modes, (ii) the isotropic approximation, (iii) assuming an averaged bulk mean-free path (i.e., the Gray approximation), and (iv) using the Matthiessen rule to combine the effect of different scattering mechanisms. Because the frequency-dependent contributions to thermal conductivity change as the film thickness is reduced, assumptions that are valid for bulk are not necessarily valid for thin films. Using knowledge gained from this study, an analytical model for the length-dependence of thin film thermal conductivity is presented and compared to the predictions of the LD-based calculations. The model contains no fitting parameters and only requires the bulk lattice constant, bulk thermal conductivity, and an acoustic phonon speed as inputs. By including the mode-dependence of the phonon lifetimes resulting from phonon-phonon and phonon-boundary scattering, the model predictions capture the approach to the bulk thermal conductivity better than predictions made using Gray models based on a single lifetime. Both the model and the LD-based method are used to assess a procedure commonly used to extract bulk thermal conductivities from length-dependent molecular dynamics simulation data. Because the mode-dependence of thermal conductivity is not included in the derivation of this extrapolation procedure, using it can result in significant error. Finally, phonon transport across a silicon/vacuum-gap/silicon structure is modelled using lattice dynamics and Landauer theory. The phonons transmit thermal energy across the vacuum gap via atomic interactions between the leads. Because the incident phonons do not encounter a classically impenetrable potential barrier, this mechanism is not a tunneling phenomenon. The heat flux due to phonon transport can be 4 orders of magnitude larger than that due to photon transport predicted from near-field radiation theory.

This thesis describe methods to improve the interfacial stability of an interpenetrating phase composite (IPC) polyvinylurethanecarbonate), and to increase the hydrophobicity of the polymer phase. The current IPCs introduce covalent bonding between the phases via silanizing agents to enhance the interfacial stability. Incorporation of the silanizing agents was also intended to reduce the IPC’s sensitivity to interfacial hydration, thereby enhancing the IPC’s resistance to degradation during aging. Lysine diisocyanate was used to increase the hydrophobic character in the polyvinylurethanecarbonate resin. The polymer resins were infiltrated into porous CPP blocks with 25 volume% interconnected porosity and polymerized to produce the IPCs. After mechanical testing following a aging study it was found that the silanizing agents contributed to stability of the mechanical properties under aqueous conditions. It was concluded that the mechanical properties and stability were comparable to available biodegradable composites, as well as being biocompatible to a preosteoblast model cell line.

Doctor of Philosophy / Mechanical and Nuclear Engineering / Douglas S. McGregor / There is a need for compact, rugged neutron detectors for a variety of applications including national security and oil well logging. A solid form neutron detector would have a higher efficiency than present day gas filled ³He and ¹⁰BF ₃ detectors, which are standards currently used in the industry today. A sub-branch of the III-V semiconductors is the filled tetrahedral compounds, known as Nowotny-Juza compounds (A[superscript I]B[superscript II]C[superscript V]). These materials are desirable for their cubic crystal structure and semiconducting electrical properties. Originally studied for photonic applications, Nowotny-Juza compounds have not been fully developed and characterized. Nowotny-Juza compounds are being studied as neutron detection materials here, and the following work is a study of LiZnP and LiZnAs material development and device characterization. Precursor binaries and ternary materials of LiZnAs and LiZnP were synthesized in-house in vacuum sealed quartz ampoules with a crucible lining. Synthesized powders were characterized by x-ray diffraction, where lattice constants of 5.751 ± .001 Å and 5.939 ± .002 Å for LiZnP and LiZnAs, respectively, were determined. A static vacuum sublimation in quartz was performed to help purify the synthesized ternary material. The resulting material from the sublimation process showed characteristics of a higher purity ternary compound. Bulk crystalline samples were grown from the purified material. Ingots up to 9.0 mm in diameter and 13.0 mm in length were harvested. Individual samples were characterized for crystallinity on a Bruker AXS Inc. D2 CRYSO, energy dispersive x-ray diffractometer, and a Bruker AXS D8 DISCOVER, high-resolution x-ray diffractometer with a 0.004° beam divergence. High-resolution XRD measurements indicated reasonable out-of-plane and in-plane ordering of LiZnP and LiZnAs crystals. Devices were fabricated from the LiZnP and LiZnAs crystals. Resistivity of devices were determined within the range of 10⁶ – 10¹¹ Ω cm. Charge carrier mobility and mean free drift time products were characterized for electrons at 8.0 x 10⁻⁴ cm² V⁻¹ ± 4.8% and 9.1 x 10⁻⁴ cm² V⁻¹ ± 4.4% for LiZnP and LiZnAs respectively. Sensitivity to 337 nm laser light (3.68 eV photons) was observed, where an absorption coefficient of 0.147 mm⁻¹ was determined for LiZnAs devices. Thermal neutron sensitivity was evaluated with unpurified and purified LiZnP and LiZnAs devices. Sensitivity was observed, however material quality and crystalline quality significantly hindered device performance.

Doctor of Philosophy / Chemistry / Christer B. Aakeroy / A series of co-crystallizations between four biimidazole based compounds with nine symmetric aliphatic di-acids and fifteen perfluorinated halogen-bond donors were carried out to determine if a MEPS based ranking can be used to effectively assign selectivity in hydrogen- and halogen-bond interactions. The results suggested that a simple electrostatic view provides a reliable tool for successfully implementing the practical co-crystal synthesis with desired connectivity. MEPS based selectivity guidelines for halogen-bond interactions were explored in co-crystallizations between twelve asymmetric ditopic acceptors and nine halogen-bond donors. If the difference between the two acceptor sites is below 35 kJ/mol, no selectivity was observed; above 65 kJ/mol halogen bond selectivity dominates and mid ΔE range was recognized as the grey area where predictions cannot be made. To examine competition between hydrogen and halogen bonds, five heteroaryl-2-imidazoles were co-crystallized with fifteen halogen-bond donors. It was found that halogen bonds prefer best the acceptor site, demonstrating that a suitably activated halogen-bond donor can compete with a strong hydrogen-bond donor. The benefits of ‘double activation’ for promoting halogen bond effectiveness was explored with nine haloethynylnitrobenzenes. The positive potential on halogen atoms was enhanced through a combination of an sp-hybridized carbon and electron-withdrawing nitro group(s). Iodoethynylnitrobenzenes were identified as the most effective halogen-bond donors reported to date and the compounds were exploited for the interaction preferences of nitro group and nitro⋯X-Csp interactions were identified as synthetic tools for energetic co-crystal assembly. A synthetic strategy for the deliberate assembly of molecular polygons was developed utilizing bifurcated halogen bonds constructed from N-oxides and complementary halogen-bond donors via co-crystallization. A convenient, effective, and scalable protocol for stabilizing volatile liquid chemicals with co-crystallization was achieved. Through the use of halogen-bonding, liquid iodoperfluoroalkanes were transformed into crystalline materials with low-vapor pressure, considerable thermal stability and moisture resistance. To stabilize the energetic compound ethylenedinitramine, a co-crystallization approach targeting the acidic protons was employed. Eight co-crystals were obtained and the acceptors were identified as supramolecular protecting groups leading to diminished reactivity and enhanced stability while retaining the desirable energetic properties.

Doctor of Philosophy / Department of Chemistry / Jun Li / Nanostructured electrode materials for electrochemical energy storage systems have been shown to improve both rate performance and capacity retention, while allowing considerably longer cycling lifetime. The nano-architectures provide enhanced kinetics by means of larger surface area, higher porosity, better material interconnectivity, shorter diffusion lengths, and overall mechanical stability. Meanwhile, active materials that once were excluded from use due to bulk property issues are now being examined in new nanoarchitecture. Silicon was such a material, desired for its large lithium-ion storage capacity of 4,200 mAh g[superscript]-1 and low redox potential of 0.4 V vs. Li/Li[superscript]+; however, a ~300% volume expansion and increased resistivity upon lithiation limited its broader applications. In the first study, the silicon-coated vertically aligned carbon nanofiber (VACNF) array presents a unique core-shell nanowire (NW) architecture that demonstrates both good capacity and high rate performance. In follow-up, the Si-VACNFs NW electrode demonstrates enhanced power rate capabilities as it shows excellent storage capacity at high rates, attributed to the unique nanoneedle structure that high vacuum sputtering produces on the three-dimensional array. Following silicon’s success, titanium dioxide has been explored as an alternative highrate electrode material by utilizing the dual storage mechanisms of Li+ insertion and pseudocapacitance. The TiO[subscript]2-coated VACNFs shows improved electrochemical activity that delivers near theoretical capacity at larger currents due to shorter Li[superscript]+ diffusion lengths and highly effective electron transport. A unique cell is formed with the Si-coated and TiO[subscript]2-coated electrodes place counter to one another, creating the hybrid of lithium ion battery-pseudocapacitor that demonstrated both high power and high energy densities. The hybrid cell operates like a battery at lower current rates, achieving larger discharge capacity, while retaining one-third of that capacity as the current is raised by 100-fold. This showcases the VACNF arrays as a solid platform capable of assisting lithium active compounds to achieve high capacity at very high rates, comparable to modern supercapacitors. Lastly, manganese oxide is explored to demonstrate the high power rate performance that the VACNF array can provide by creating a supercapacitor that is highly effective in cycling at various high current rates, maintaining high-capacity and good cycling performance for thousands of cycles.

Master of Science / Mechanical and Nuclear Engineering / Kevin B. Lease / Gurpreet Singh / Energy has always been one of the most important factors in any type of human or industrial endeavor. Clean energy and alternative energy sources are slowly but steadily replacing fossil fuels, the over-dependence on which have led to many environmental and economic troubles over the past century. The main challenge that needs to be addressed in switching to clean energy is storing it for use in the electrical grid and transportation systems. Lithium ion batteries are currently one of the most promising energy storage devices and tremendous amount of research is being done in high capacity anode and cathode materials, and better electrolytes and battery packs as well, leading to overall high efficiency and capacity energy storage systems. Polymer derived ceramics (PDCs) are a special class of ceramics, usually used in high temperature applications, but some silicon based PDCs have demonstrated good electrochemical properties in lithium ion batteries. The goal of this research is to explore a special hybrid ceramic of titanium dioxide (TiO₂) and silicon oxy carbide (SiOC) ceramic derived from 1,3,5,7 -- tetravinyl -- 1,3,5,7 -- tetramethylcyclotetrasiloxane (TTCS) polymer for use in lithium ion batteries and investigate the source of its properties which might make the ceramic particularly useful in some highly specialized energy storage applications.

Doctor of Philosophy / Chemical Engineering / Mary E. Rezac / Membrane contact reactors (MCRs) have been evaluated for the selective hydro-treating of model reactions; the partial hydrogenation of soybean oil (PHSO), and the conversion of lactic acid into commodity chemicals. Membranes were rendered catalytically active by depositing metal catalyst onto the polymer "skin" of an asymmetric membrane. Hydrogen was supplied to the support side of the membrane and permeated from the support side to the skin side, where it adsorbed directly onto the metal surface. Liquid reactant was circulated over the membrane, allowing the liquid to come into direct contact with the metal coated surface of the membrane, where the reaction occurred. Our membrane contact reactor approach replaces traditional three-phase batch slurry reactors. These traditional reactors possess inherent mass transfer limitations due to low hydrogen solubility in liquid and slow diffusion to the catalyst surface. This causes hydrogen starvation at the catalyst surface, resulting in undesirable side reactions and/or extreme operating pressures of 100 atmospheres or more. By using membrane reactors, we were able to rapidly supply hydrogen to the catalyst surface. When the PHSO is performed in a traditional slurry reactor, the aforementioned hydrogen starvation leads to a high amounts of trans-fats. Using a MCR, we were able to reduce trans-fats by over 50% for equal levels of hydrogenation. It was further demonstrated that an increase in temperature had minimal effects on trans-fat formation, while significantly increasing hydrogenation rates; allowing the system to capture higher reaction rates without adversely affecting product quality. Additionally, high temperatures favors the hydrogenation of polyenes over monoenes, leading to low amounts of saturated fats. MCRs were shown to operator at high temperatures and: (1) capture high reaction rates, (2) minimize saturated fats, and (3) minimize trans-fats. We also demonstrated lactic acid conversion into commodity chemicals using MCRs. Our results show that all MCR experiments had faster reaction rate than all of our controls, indicating that MCRs have high levels of hydrogen coverage at the catalyst. It was also demonstrated that changing reaction conditions (pressure and temperature) changed the product selectivities; giving the potential for MCRs to manipulate product selectivity.

Master of Science / Department of Mechanical and Nuclear Engineering / Steven J. Eckels / Christopher M. Sorensen / Large specific surface areas and high concentrations of reactive edge and defect sites make NanoActive™ metal oxide powders ideal chemical adsorbents. These powders are dispersed in aerosol form to remediate toxic wastes and neutralize chemical and biological warfare agents. In the destructive adsorption of toxic chemicals, effective application requires particles be as small as possible, thus, maximizing surface area and number of edge and defect sites. Other applications, e.g. smoke clearing, require particles be large so they will settle in a timely manner. Ideally, particle size control could be engineered into powder dispersion devices. The purpose of this study was to explore particle cohesion and aerodynamic deaggregation mechanisms to enhance the design of powder dispersion devices. An aerosol generator and four experimental nozzles were designed to explore the most commonly referenced deaggregation mechanisms: particle acceleration, particles in shear and turbulent flows, and particle impaction. The powders were then dispersed through the nozzles with increasing flow rates. A small angle light scattering device was used to make in situ particle size measurements. The nozzle designed for impaction deaggregated the NanoActive™ MgO particles to a lesser degree than the other three nozzles, which deaggregated the particles to a similar degree. Flows in three of the four nozzles were simulated in a commercial computational fluid dynamics package. Theoretical particle and aggregate stresses from the literature were calculated using simulated data. These calculations suggest particle acceleration causes internal stresses roughly three orders of magnitude larger than shear and turbulent flows. These calculations, coupled with experimental data, lead to the conclusion that acceleration was the most significant cause of particle deaggregation in these experiments. Experimental data also identified the dependence of deaggregation on primary particle size and agglomerate structure. NanoActive™ powders with smaller primary particles exhibited higher resistance to deaggregation. Small primary particle size was thought to increase the magnitude of van der Waals interactions. These interactions were modeled and compared to theoretical deaggregation stresses previously mentioned. In conclusion, deaggregation is possible. However, the ideas of particle size control and a universal dispersion device seem elusive considering the material dependent nature of deaggregation.

La matière condensée moderne porte un intérêt particulier pour la classe de matériaux formée par les isolants topologiques. Ils sont différents des isolants typiques par leurs intéressantes propriétés quantiques; ils se comportent comme des isolants dans leur intérieur, mais contiennent des états conducteurs sur leur surface. On peut mieux comprendre le comportement de certains systèmes en matière condensée, tel que les chaînes de polyacétylène, en étudiant un système unidimensionnel simple : le modèle de Su-Schrieffer-Heeger (SSH). Le modèle SSH décrit des fermions sans spin sautant sur un réseau unidimensionnel où les amplitudes de saut alternent d’un site à l’autre. Ce modèle, bien que simpliste, expose les propriétés clés des isolants topologiques tel que les états délocalisés dans tout le réseau ainsi que les états exponentiellement localisés aux frontières du réseau. Dans ce projet, nous étudions le modèle SSH, mais en ajoutant un défaut central dans le réseau qu’on appelle un soliton. Dans notre cas, le soliton consiste en un site central donc les amplitudes de saut sont les mêmes d’un côté et de l’autre. Nous trouvons un ensemble de solutions complet incluant des états de basse énergie localisés aux frontières ainsi que des états de haute énergie localisés au soliton. / Topological insulators are a class of materials that have attracted much attention in modern condensed matter. They are different from typical insulators as they exhibit interesting quantum properties; they behave as insulators in their interior but have conducting states on their surface. We can better understand the properties of low dimensional condensed matter systems (like poly-acetylene chains) by studying a toy model known as the Su-Schrieffer-Heeger (SSH) Model. The SSH model describes spinless fermions hopping on a one-dimensional lattice with staggered hopping amplitudes. Such a toy model exhibits key properties of topological insulators, such as bulk states (delocalized states across the lattice) and edge states (exponentially localized states at the boundaries of the lattice). In this project, we study the SSH model with an added central defect to the chain, which we call a soliton. In our case, the soliton consists of a central site with the same hopping amplitude on either side. We study the impact of such a defect on the properties of the system; we find a complete set of solutions including near-zero-energy edge states as well as high-energy states localized at the soliton.

Le fer et les alliages à base de fer présentent un intérêt considérable pour la communauté de la modélisation des matériaux en raison de l’immense importance technologique de l’acier. Les alliages ferritiques à base de fer sont largement utilisés dans les industries aéronautique et nucléaire en raison de leur résistance mécanique élevée, de leur faible dilatation à haute température et de leur résistance à la corrosion. Ces propriétés sont cependant affectées par des défauts ponctuels intrinsèques et extrinsèques. Dans cette thèse, nous décrivons en détail la cinétique des défauts ponctuels dans le fer α en utilisant la technique d’activation-relaxation cinétique (ARTc), une méthode de Monte Carlo cinétique hors réseau avec construction de catalogue à la volée. Plus précisément, nous nous intéressons aux mécanismes de diffusion du carbone (C) et des amas de lacunes dans le fer α. Dans un premier temps, nous étudions l’effet de la pression sur la diffusion du carbone dans le joint de grains de fer α. Nous constatons que l’effet de la pression peut fortement modifier la stabilité et la diffusivité du carbon dans le joint de grains d’une manière qui dépend étroitement de l’environnement local et de la nature de la déformation. Ceci peut avoir un impact majeur sur l’évolution des matériaux hétérogènes, avec des variations de pression locale qui altéreraient fortement la diffusion à travers le matériau. Nous étudions également l’évolution structurale des amas de lacunes contenant de deux à huit lacunes dans le fer α. Nous décrivons en détail le paysage énergétique, la cinétique globale et les mécanismes de diffusion associés à ces défauts. Nos résultats montrent des mécanismes de diffusion complexes même pour des défauts aussi simples que de petits amas de lacunes. Enfin, dans le dernier chapitre, nous discutons une approche de gestion de petites barrières par bassin local dans ARTc. Les simulations de Monte Carlo cinétiques deviennent inefficaces dans les systèmes où le paysage énergétique est constitué de bassins avec de nombreux états reliés par des barrières énergétiques très faibles par rapport à celles nécessaires pour quitter ces bassins. Au fur et à mesure que le système évolue état par état, il est beaucoup plus susceptible d’effectuer des événements répétés (appelés oscillateurs) à l’intérieur du bassin d’énergie de piégeage que de s’échapper du bassin. De tels osccilateurs ne font pas progresser la simulation et ne fournissent que peu d’informations au-delà d’uen première évaluation de ces états. Notre algorithme de bassin local détecte, à la volée, des groupes d’états oscillants et les consolide en bassins locaux, que nous traitons avec la méthode de taux moyen d’auto-construction de bassin (bac-MRM), une approche de type équation maîtresse selon la méthode du taux moyen. / Iron and iron-based alloys are of considerable interest to the materials modelling community because of the immense technological importance of steel. Iron-based ferritic alloys are widely used in aeronautic and nuclear industries due to their high mechanical strength, low expansion at high temperatures, and corrosion resistance. These properties are affected by intrinsic and extrinsic point defects, however. In this thesis, we describe in detail the kinetics of point defects in α−iron using the kinetic activation-relaxation technique (kART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building. More specifically, we focus on the diffusion mechanisms of carbon and vacancy clusters in α−iron. First, we study the pressure effect on carbon diffusion in the grain boundary (GB) of α−iron. We find that the effect of pressure can strongly modify the C stability and diffusivity in the GB in ways that depend closely on the local environment and the nature of the deformation. This can have a major impact on the evolution of heterogeneous materials, with variations of local pressure that would strongly alter diffusion across the material. We also study the structural evolution of vacancy clusters containing two to eight vacancies in α−iron. We describe in detail the energy landscape, overall kinetics, and diffusion mechanisms associated with these defects. Our results show complex scattering mechanisms even for defects as simple as small vacancy clusters. Finally, in the last chapter, we discuss a local basin approach to managing low barrier events in the kART. Kinetic Monte Carlo simulations become inefficient in systems where the energy landscape consists of basins with numerous states connected by very low energy barriers compared to those needed to leave these basins. As the system evolves state by state, it is much more likely to perform repeated events (so-called flickers) inside the trapping energy basin than to escape the basin. Such flickers do not progress the simulation and provide little insight beyond the first identification of those states. Our local basin algorithm detects, on the fly, groups of flickering states and consolidates them into local basins, which we treat with the basin-auto-constructing Mean Rate Method (bac-MRM), a master equation-like approach based on the mean-rate method.