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Characterizing The Mechanical Behavior Of Single And Polycrystalline Silicon Carbide Using Nanoindentation., Amit Datye

Doctoral Dissertations

This research aims at enhancing the fundamental understanding of mechanisms controlling the deformation and fracture of silicon carbide based ceramics (single- and poly-crystal). The role of microstructure and material properties on the energy absorption capability of SiC is studied. This research helps to improve the ability to quantitatively predict the initiation and propagation of fracture and the interaction between fracture and plasticity, which provides a step towards a mechanistic understanding of deformation and failure properties of ceramic single crystals and polycrystals. The validity of the indentation-cracking method for toughness measurement is examined using nanoindentation tests with different indenters (spherical, pyramidal). …

Thermodynamic Modeling Of Uranium And Oxygen Containing Ternary Systems With Gadolinium, Lanthanum, And Thorium, Jacob Wesley Mcmurray

Doctoral Dissertations

The CALPHAD method is used to assess the thermodynamic properties and phase relations in the U-M-O system where M = Gd, La, and Th. A compound energy formalism (CEF) model for fluorite UO_2±x [urania] is extended to represent the complex U_1-yM_yO_2±x [urania solid solution] phases. The lattice stabilities for fictive GdO₂ [gadolinia] and LaO₂ [lanthana] fluorite structure compounds are calculated from density functional theory (DFT) for use in the CEF for U_1-yM_yO_2±x [urania solid solution phase] while U⁶⁺ [uranium 6 plus cation] is introduced into the …

Elucidating Fundamental Mechanisms In Focused Electron- And Ion-Beam Induced Synthesis, Carlos M. Gonzalez

Doctoral Dissertations

A focused electron beam deposition process (FEBID) coupled with in-situ infrared pulsed laser assist (LA-EBID) has been implemented for higher purity tungsten nanowires using W(CO)₆ [tungsten hexacarbonyl] as parent precursor gas. Nanowires made of Co from Co₂(CO)₈ [dicobalt octacarbonyl] and Pt from MeCpPtIVMe₃ [trimethyl methylcyclopentadienyl platinum] have also been realized by using inert focused ion beams of helium and helium and neon, respectively. In all cases, higher electrical conductivities, higher purities and larger grain sizes have been obtained when compared with preceding traditional additive edit techniques. These new approaches will make possible successful nanoscale direct-write …

Influence Of Texture And Grain Size On The Plastic Anisotropy In A Wrought Mg Alloy: Synchrotron X-Ray Diffraction And Visco-Plastic Self-Consistent Modeling, Yi Wang

Doctoral Dissertations

The combined effects of texture and grain size on the yielding and hardening behavior in a hot-rolled AZ31B Mg alloy were studied by using synchrotron x-ray diffraction and visco-plastic self-consistent (VPSC) simulation methods.

First, the influence of texture on Hall-Petch relationships (namely, critical resolved shear stresses and hardening parameters) in a Mg alloy was investigated to establish a constitutive basis for the VPSC simulation.

Then, the changes in dominant deformation mechanisms (basal, prismatic, and pyramidal slip as well as extension twin) and their relative interactions were studied systematically as a function of the initial texture using a VPSC scheme. The …

A Study Of Energy Resolution And Non-Proportionality Of Yalo3:Ce And Gd3ga3al2o12:Ce, Samuel Bradley Donnald

Doctoral Dissertations

For many radiation detection applications, energy resolution is one of the most important detector characteristics. In well designed scintillation detectors, the energy resolution is primarily driven by two main factors - the light output and light yield non-proportionality. A great deal of research has already focused on understanding and improving the light yield of scintillation detectors; however, light yield non-proportionality is less well understood. Until recently, light yield non-proportionality was thought to be an intrinsic scintillator property with very little sample-to-sample variation. In this work, two materials have been investigated that demonstrate a variable light yield non-proportionality. The first material …

Investigation Of Deformation And Failure Mechanisms In Nanoindentation Mechanics, Yuzhi Xia

Doctoral Dissertations

This dissertation further extends nanoindentation to study the initiation of plasticity in single crystals in nanoscale stressed volumes, to the instrumented tests for damage evaluation and monitoring, and to the fundamental issues in deformation and failure mechanisms in relationship to the defect evolutions.

In the first project, model Fe-Cr, Fe-Ni and Fe-Cr-Ni alloys that are the basis of many structural steels were synthesized as single crystals and characterized. The compositions investigated were Fe-15Cr, Fe-30Cr, Fe-30Ni and Fe-15Cr-15Ni (atomic percent). Several key mechanical properties were determined which will be useful in further studies of irradiation/deformation-induced defects. Incipient plasticity and slip characteristics …

Plastic Anisotropy Of Complex Crystals And Hierarchically Structured Alloys Using Micro-Mechanical Computational Analysis, Lin Li

Doctoral Dissertations

The material anisotropy is one of the most important material properties that cannot be disregarded in today’s world of materials designing and manufacturing. As new materials being developed and new material demands are introduced the inevitable focus on anisotropic materials has been brought under the spotlight. In this dissertation, several experimental and simulation project regarding material anisotropic effects on hexagonal close packed crystals such as Silicon Carbide as well and hierarchically structured solid solution ferritic based alloys. The general purpose was to demonstrate the improvement on various intended material properties using finite element method. Since indentation is a widely used …

Single Fiber Mechanical Properties Using Nano-Tensile Testing And Carbon Fiber Structure-Property Relationship, Matthew Erich Kant

Doctoral Dissertations

Single carbon fibers are studied using a nano-tensile testing system. This system has unprecedented load and displacement resolution, nN and nm respectively, and the ability to perform dynamic testing for storage and loss modulus during quasi-static tensile extension. Furthermore, improved fiber mounting and alignment procedures coupled with the precision of the nano-tensile testing system assist in unprecedented resolution in single fiber mechanical testing for axial modulus and strength. Hence, using these unique capabilities, the moduli and their statistical distribution of many high performance carbon fibers are reported here. From this, a simplified single parameter model describing the strain dependent modulus …

Fatigue, Fracture, And Environmentally-Assisted Behavior Of Advanced Engineering Materials, Zhi Tang

Doctoral Dissertations

The objective of the present study is to provide a fundamental understanding of fatigue, fracture, and environmentally-assisted behavior of high-entropy alloys (HEAs). The work involves fatigue, fracture, and environmentally-assisted behavior of a new kind of advanced engineering materials, called HEAs. Three tasks are studied: (1) microstructures and fracture mechanisms of HEAs, (2) fatigue failure and life prediction of HEAs, and (3) corrosion and environmentally-assisted behavior of HEAs.

In the first task, microstructural stability and fracture mechanism of the AlCoCrFeNi alloy are studied and compared with thermodynamic calculations. In the second task, high-cycle fatigue-failure mechanisms of the cold-rolled Al0.5CoCrCuFeNi alloy are …

Characterization Of The Fine-Scale Weld Deposit Microstructure And Its Influence On The Elevated Temperature Properties Of 2.25cr - 1mo - 0.25v Weldments In Heavy Wall Pressure Vessels, Dewey Joshua Burgess

Doctoral Dissertations

The research herein was conducted to characterize the fine-scale microstructure of 2.25Cr‑1Mo‑0.25V (22V) submerged arc weld deposits and to study the influence of the microstructure on creep behavior and reheat cracking susceptibility. Scanning electron microscopy and transmission electron microscopy examinations concentrated on carbide morphology and evolution as a function of time and temperature, since the majority of properties that are associated with 22V weld deposits are attributable to the carbide type and location throughout the microstructure.

Five distinct carbides were observed in the range of heat treatments studied: MC, M2C, M23C6, M7C3, and M6C. It was shown that each carbide …

Zone Refining Of Raw Materials For Metal Halide Scintillator Crystal Growth, Thomas Williams Mcalexander

Masters Theses

In recent years, scintillators have been researched for homeland security applications where properties such as high light output and low energy resolution are required for efficient gamma ray spectroscopy. Several promising scintillators, including SrI₂ [Strontium Iodide], CsSrI₃ [Cesium Strontium Iodide]_, and KSr₂I₅ [Potassium Strontium Iodide] are activated with EuI₂ [Europium Iodide]. The quality of EuI₂ raw material is generally poor and inconsistent as-received from the manufacturer. Material purity is known to affect its scintillation performance, and so purification processes have been investigated to improve the performance of Eu-doped [Europium-doped] scintillators. In this …

Reverse Engineering Of Reciprocating Saw, Nicholas Alexander Cavopol, Candice Kinsler, Carly Jania, Joseph Richard Creekmore

Chancellor’s Honors Program Projects

No abstract provided.

Monte Carlo Modeling Of Ion Beam Induced Secondary Electrons, Uk Huh

Doctoral Dissertations

Modeling ion beam induced secondary electron (iSE) production within matter for simulating ion beam induced images has been studied. When the complex nature of ion beam interactions with matter is account for, a detailed quantitative model of the ion interactions with matter, Monte Carlo simulation will be the best choice to be able to compute and predict iSE yields faster and more accurately. In order to build Monte Carlo simulation software incorporated with a reliable database of stopping power tables, for wide variety of range of materials, there have been numerous attempts to experimentally measure ion stopping power tables and …

Neutron Diffraction Study Of Engineering Materials Subjected To Complex Loadings, Jeffrey R. Bunn

Doctoral Dissertations

No studies using diffraction have specifically aimed to study the (hkl) specific shear strain of lattice planes to the application of pure shear due to applied torsion. Also no measures of strain resultant from combined loads of tension and shear stress applied simultaneously have been performed. Current techniques simply measure the planar response to axial stress and cannot provide shear strains directly. Shear moduli are not necessarily related through traditional continuum mechanics to planar Youngs’ moduli (E_hkl) from these axial experiments for all materials. This is of significance as most engineering components fail under the …

Deformation Of A Face-Centered Cubic Fecocrmnni High Entropy Alloy, Chao Zhu

Doctoral Dissertations

A polycrystalline, face-centered cubic high entropy alloy (HEA) FeCoCrMnNi was investigated by nanoindentation and stress relaxation. A literature review on HEAs was presented in Chapter 1 and a description of methodology was followed in Chapter 2.

In Chapter 3, the nature of incipient plasticity was characterized with instrumented indentations over various loading rates and temperatures. The maximum shear stress for initiating the incipient plasticity attained the theoretical strength and was relatively insensitive to grain orientation. However, it strongly depended upon the temperature, indicating a thermally activated nucleation process. The estimated activation energy and volume signify that the nucleation is mediated …

Phenotypic Modulation Of Smooth Muscle Cells On Biodegradable Elastomeric Substrates, Xifeng Liu

Doctoral Dissertations

Cardiovascular disease is the number one killer in the U.S. Cardiovascular tissue engineering holds enormous potential by providing synthetic materials as vessel replacements. This dissertation focused on the development of novel biodegradable and photo-crosslinkable polymers with controlled surface chemistry, stiffness, and topographical features in regulating smooth muscle cell (SMC) adhesion, proliferation and phenotypic conversion for cardiovascular tissue engineering applications. Chapter II presents a facile synthesis route to obtain a series of photocrosslinkable poly(epsilon-caprolactone) triacrylates (PCLTA) with varied mechanical properties and further demonstrated tunable cell responses using these polymer system. Chapter III demonstrates a model polymer network from PCLTA that can …

Temperature And Alloying Effects On The Mechanical Properties Of Equiatomic Fcc Solid Solution Alloys, Zhenggang Wu

Doctoral Dissertations

Compared to decades-old theories of strengthening in dilute solid solutions, the mechanical behavior of concentrated solid solutions is relatively poorly understood. A special subset of these materials includes alloys in which the constituent elements are present in equal atomic proportions, including the high-entropy alloys of recent interest. A unique characteristic of equiatomic alloys is the absence of “solvent” and “solute” atoms, resulting in a breakdown of the textbook picture of dislocations moving through a solvent lattice and encountering discrete solute obstacles. To clarify the mechanical behavior of this interesting new class of materials, we investigate here a family of equiatomic …

Theoretical Studies Of The Growth And Functionality Of Layered Materials, Wei Chen

Doctoral Dissertations

In this thesis, we present several projects on the growth and functionality of layered materials, using density functional theory (DFT) method and phenomenological modeling approach. Beyond the understanding of growth mechanisms and exploration of properties, we propose novel avenues to realize controllable growth processes and layered materials with desirable properties. The contents have three major parts:

(1) Graphene growth on Cu(111) and Ni(111) substrates. We first demonstrate that the inherent multi-orientational degeneracy of the graphene islands on Cu(111) in the early stages of nucleation could result in the prevalence of grain boundaries (GBs). Next, we propose a possible solution to …

Fabrication And Characterization Of Cu-Co Alloy Nanoparticles Via Pulsed Laser Dewetting, Shaofang Fu

Masters Theses

The fabrication and characterization of Cu[Copper]-Co[Cobalt] alloy nanoparticles are addressed in this thesis. First, Cu-Co alloy thin films with total thickness of ~10 nm [nanometer] were deposited on both 100 nm SiO₂ [silicon dioxide] coated silicon wafers and quartz substrates using RF magnetron sputtering system. Three structures were studied in this work: (1) ~10 nm co-sputtered Cu-Co alloy thin film. (2) ~5 nm (at center) Cu thin film covered by ~5 nm (at center) Co thin film. (3) ~5 nm (at center) Co thin film covered by ~5 nm (at center) Cu thin film. In addition, ~10 nm pure …

Low Temperature Thermally Activated Cvd Of Silicon On Molybdenum For Oxidation Protection, Brian C. Jolly

Masters Theses

The United States Global Threat Reeducation Initiative (GTRI) is promoting the production of molybdenum-99 without the use of uranium-235. One possible route, called neutron capture technology, is through the activation of molybdenum-98 via irradiation. GE-Hitachi has identified a location within their boiling water reactors for molybdenum-98 irradiation, but the elevated temperatures present risk oxidizing the molybdenum. This work explored using a silicon coating to protect the molybdenum from oxidation. With silane gas as the precursor, chemical vapor deposition was employed to produce the silicon coating. Characterization was performed and showed a discrete silicon layer was deposited with little to no …

Radiation-Induced Radicals In Polyurea-Crosslinked Silica Aerogel, Benjamin Michael Walters

Masters Theses

Free radicals are atoms or molecules with an odd number of electrons in an outer shell. Since electrons typically occur in pairs, this leaves one electron that is unpaired. In seek of another electron to pair with, free radicals react with and steal electrons from neighboring molecules, which then become free radicals themselves. This can start a chain reaction, cascading into large scale damage.

Ionizing radiation can tear through molecules, just as bullets can tear through things that we see. If free radicals can be detected, and seen to increase in a material upon radiation exposure, this can indicate molecular …

An Evaluation Of Void Formation In Ex-Service And Creep Tested Hp Alloy Tubing Used For Hydrocarbon Reforming, Zane William Fox Palmer

Masters Theses

HP alloy tubing is commonly used in petroleum reforming facilities for its high temperature strength and resistance to corrosion. Unfortunately, the tubes can experience failure during service caused by the formation of voids in the tube wall brought on by creep stress generated by the pressure differential and thermal gradient between the inside and outside diameters of the tube. The purpose of this project was to analyze the formation of voids in eight ring sections, each from a different ex-service tube, from a total of three different manufacturers (identified as M1, M2, and M3). Further analysis of the ex-service tubes …

Structural Dynamics And Charge Transport In Room Temperature Ionic Liquids, Philip James Griffin

Doctoral Dissertations

Room temperature ionic liquids are an important class of materials due to their chemical tunability and numerous advantageous physicochemical properties. As a result, ionic liquids are currently being investigated for use in a wide array of chemical and electrochemical applications. Despite their great potential, however, the relationship between the chemical structure and physicochemical properties of ionic liquids is not well understood.

To this end, this dissertation presents experimental studies of the reorientational structural dynamics and charge transport properties of a variety of room temperature ionic liquids using quasielastic light scattering spectroscopy and broadband dielectric spectroscopy.

Studies of a series of …

Ab Initio Studies Of Proton Transport In Proton Exchange Membranes, Jeffrey Keith Clark

Doctoral Dissertations

A molecular-level understanding of the factors that contribute to transport properties of proton exchange membranes (PEMs) for fuel cell applications is needed to aid in the development of superior membrane materials. Ab initio electronic structure calculations were undertaken on various PEM ionomer fragments to explore the effects of local hydration, side chain connectivity, protogenic group separation, and specific side chain chemistry on proton dissociation and transfer at low hydration. Cooperative interactions between both intra- and inter-molecular acidic groups and hydrogen bond connectivity were found to enhance proton dissociation at very low degrees of hydration. The energetics associated with proton transfer …

Synthesis And Characterization Of Magnetic Nanowires Prepared By Chemical Vapor Deposition, Siwei Tang

Doctoral Dissertations

Various metal silicide and germanide magnetic nanowires were synthesized using a home-built CVD [chemical vapor deposition] system. The morphology, composition, and magnetic properties of the nanowires were studied and correlated with growth parameters such as temperature, pressure, time, and source-substrate distance.

One of the compositions targeted for synthesis was MnSi [manganese silicide]. In bulk, this material orders helimagnetically at T_c [curie temperature] = 30K, with a helical pitch of about 20 nm. After extensive study, we learned that the thickness of the silicon dioxide layer on the substrate is a critical parameter for the growth of MnSi nanowires. An …

Nanolayer Polymeric Coatings To Enhance The Performance And Service Life Of Inorganic Membranes For High Temperature-High Pressure Biomass Pretreatment And Other Applications, Vincent C. Kandagor

Doctoral Dissertations

Membrane technology has become increasingly attractive in several applications including water filtration, food industry, oil and gas, and biomedical applications. Most recently the quest for renewable, bioenergy has called for use of membranes in biomass pretreatment and other stages of producing biofuel. The success and advancement of the membrane technology for these various applications has, however, been impeded by the fouling of membranes, which causes the pores in the microporous structure to block, resulting in reduced efficiency, and in some cases, total failure of the membranes system. This challenge leads to a tremendous increase in the cost of using membranes …

Investigation Of The Phase Transformation Kinetics And Texture Evolution In A Trip Steel Under Complex Loads, Ercan Cakmak

Doctoral Dissertations

The martensitic phase transformation kinetics and its relation with texture evolution and deformation/transformation microstructures under complex loading conditions were investigated in a 304L stainless steel that exhibits the transformation induced plasticity (TRIP) effect when strained at ambient temperature. The applied load paths included torsional and biaxial deformation scenarios including simultaneous biaxial torsion/tension and torsion/compression as well as stepwise deformation of tension followed by torsion. Synchrotron x-ray and electron back-scatter diffraction techniques were used to investigate the phase transformation-microstructure-texture evolutions relations.

Under torsional deformation, the inhomogeneous distribution of martensite phase fractions were recorded through the radius of solid cylindrical specimens consistent …

A Study Of Indentation Cracking In Brittle Materials Using Cohesive Zone Finite Elements, Kurt E. Johanns

Doctoral Dissertations

Cohesive zone finite element simulations of pyramidal indentation cracking in brittle materials have been carried out in order to: (1) critically examine indentation cracking models that relate fracture toughness to indentation data; (2) determine the underlying physical mechanisms of indentation crack growth from a continuum view and their relationship to material properties; (3) explore the influence of indenter geometry on crack extension; and (4) provide a platform from which future simulations can add more complex material behavior as well as guidance for experimental measurements of fracture toughness. Standard fracture toughness geometries in addition to simplified indentation geometries were simulated in …

Atomic Structure Of The Vicinal Interface Between Silicon Carbide And Silicon Dioxide, Peizhi Liu

Doctoral Dissertations

The interface between silicon carbide (SiC) and silicon dioxide (SiO­₂) is generally considered to be the cause for the reduced electron mobility of SiC power devices. Previous studies showed an inverse relationship between the mobility and the transition layer width at SiC/SiO₂ interface. In this research the transition region at the interface was investigated with atomic resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS).

From a tilting series of high resolution TEM imaging and a through focal series of Z-contrast imaging, the 3D atomic structure of the SiC/SiO₂ vicinal interface was constructed. The vicinal …

Low Pressure Chemical Vapor Deposition Of Semiconducting Boron Carbide Thin Films On Silicon, Thomas Gregory Wulz

Masters Theses

Boron carbide thin films were grown on the (100) plane of n-type silicon in a low pressure chemical vapor deposition (CVD) system from the thermal decomposition of boron trichloride and methane reactant gases with hydrogen as a carrier gas. Boron trichloride to methane molar ratio was 5, while the boron trichloride to hydrogen molar ratio was 3.5. Thin film deposition was carried out at 900 degrees Celsius at 25 Torr. The thin films were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Energy Dispersive X-Ray Spectroscopy (EDS), Laser Induced Breakdown Spectroscopy (LIBS), and current-voltage characteristics. The crystallography of …