Latest content added for UNT Digital Library Collection: UNT Theses and Dissertationshttps://digital.library.unt.edu/explore/collections/UNTETD/browse/?fq=untl_institution:UNT&fq=untl_decade:2010-2019&fq=dc_rights_access:public&fq=str_degree_department:Department+of+Materials+Science+and+Engineering2019-08-29T10:25:12-05:00UNT LibrariesThis is a custom feed for browsing UNT Digital Library Collection: UNT Theses and DissertationsDesign of a Polymeric Coating for Protecting Thermoelectric Materials from Sublimation and Oxidation2019-08-29T10:25:12-05:00https://digital.library.unt.edu/ark:/67531/metadc1538763/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1538763/"><img alt="Design of a Polymeric Coating for Protecting Thermoelectric Materials from Sublimation and Oxidation" title="Design of a Polymeric Coating for Protecting Thermoelectric Materials from Sublimation and Oxidation" src="https://digital.library.unt.edu/ark:/67531/metadc1538763/small/"/></a></p><p>Thermoelectric (TE) devices can undergo degradation from reactions in corrosive environments and at higher operating temperatures by sublimation and oxidation. To prevent the degradation, we have applied two high temperature polymers (HTPs) as coatings for TE materials. Sintering temperatures were from 250°C to 400°C. We explain why dip coating is better technique in our study and had two potential HTPs for tests. By applying TGA (thermogravimetric analysis), we were able to figure out which HTPs have better thermal resistivity. Besides, TGA also help us to find proper curing cycles for HTPs. EDS and SEM results show that the coatings prevent oxidation and sublimation of TE materials. We also shorten HTP curing cycle time and lower the energy costs.</p>Fabrication of Large-Scale and Thickness-Modulated Two-Dimensional Transition Metal Dichalcogenides [2D TMDs] Nanolayers2019-06-09T21:09:49-05:00https://digital.library.unt.edu/ark:/67531/metadc1505271/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1505271/"><img alt="Fabrication of Large-Scale and Thickness-Modulated Two-Dimensional Transition Metal Dichalcogenides [2D TMDs] Nanolayers" title="Fabrication of Large-Scale and Thickness-Modulated Two-Dimensional Transition Metal Dichalcogenides [2D TMDs] Nanolayers" src="https://digital.library.unt.edu/ark:/67531/metadc1505271/small/"/></a></p><p>This thesis describes the fabrication and characterization of two-dimensional transition dichalcogenides (2D TMDs) nanolayers for various applications in electronic and opto-electronic devices applications. In Chapter 1, crystal and optical structure of TMDs materials are introduced. Many TMDs materials reveal three structure polytypes (1T, 2H, and 3R). The important electronic properties are determined by the crystal structure of TMDs; thus, the information of crystal structure is explained. In addition, the detailed information of photon vibration and optical band gap structure from single-layer to bulk TMDs materials are introduced in this chapter. In Chapter 2, detailed information of physical properties and synthesis techniques for molybdenum disulfide (MoS2), tungsten disulfide (WS2), and molybdenum ditelluride (MoTe2) nanolayers are explained. The three representative crystal structures are trigonal prismatic (hexagonal, H), octahedral (tetragonal, T), and distorted structure (Tʹ). At room temperature, the stable structure of MoS2 and WS2 is semiconducting 2H phase, and MoTe2 can reveal both 2H (semiconducting phase) and 1Tʹ (semi-metallic phase) phases determined by the existence of strains. In addition, the pros and cons of the synthesis techniques for nanolayers are discussed. In Chapter 3, the topic of synthesized large-scale MoS2, WS2, and MoTe2 films is considered. For MoS2 and WS2 films, the layer thickness is modulated from single-layer to multi-layers. The few-layer MoTe2 film is synthesized with two different phases (2H or 1Tʹ). The all TMDs films are fabricated using two-step chemical vapor deposition (CVD) method. The analyses of atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy confirm that the synthesis of high crystalline MoS2, WS2, and MoTe2 films are successful. The electronic properties of both MoS2 and WS2 exhibit a p-type conduction with relatively high field effect mobility and current on/off ratio. In Chapter 4, vertically-stacked few-layer MoS2/WS2 heterostructures on SiO2/Si and flexible polyethylene terephthalate (PET) substrates is presented. Detailed structural characterizations by Raman spectroscopy and high-resolution/scanning transmission electron microscopy (HRTEM/STEM) show the structural integrity of two distinct 2D TMD layers with atomically sharp van der Waals (vdW) heterointerfaces. Electrical transport measurements of the MoS2/WS2 heterostructure reveal diode-like behavior with current on/off ratio of ~ 104. In Chapter 5, optically uniform and scalable single-layer Mo1-xWxS2 alloys are synthesized by a two-step CVD method followed by a laser thinning. Post laser treatment is presented for etching of few-layer Mo1-xWxS2 alloys down to single-layer alloys. The optical band gap is controlled from 1.871 to 1.971 eV with the variation in the tungsten (W) content, x = 0 to 1. PL and Raman mapping analyses confirm that the laser-thinning of the Mo1-xWxS2 alloys is a self-limiting process caused via heat dissipation to SiO2/Si substrate, resulting in fabrication of spatially uniform single-layer Mo1-xWxS2 alloy films.</p>Design Principles for Metal-Coordinated Frameworks as Electrocatalysts for Energy Storage and Conversion2019-01-19T21:34:31-06:00https://digital.library.unt.edu/ark:/67531/metadc1404610/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1404610/"><img alt="Design Principles for Metal-Coordinated Frameworks as Electrocatalysts for Energy Storage and Conversion" title="Design Principles for Metal-Coordinated Frameworks as Electrocatalysts for Energy Storage and Conversion" src="https://digital.library.unt.edu/ark:/67531/metadc1404610/small/"/></a></p><p>In this dissertation, density functional theory calculations are performed to calculate the thermodynamic and electrochemical properties of metal coordinated frameworks for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Gibb's free energy, overpotential, charge transfer and ligands effect are evaluated. The charge transfer analysis shows the positive charges on the metal coordinated frameworks play an essential role in improving the electrochemical properties of the metal coordinated frameworks. Based on the calculations, design principles are introduced to rationally design and predict the electrochemical properties of metal coordinated frameworks as efficient catalysts for ORR and OER. An intrinsic descriptor is discovered for the first time, which can be used as a materials parameter for rational design of the metal coordinated frameworks for energy storage and conversion. The success of the design principles provides a better understanding of the mechanism behind ORR and OER and a screening approach for the best catalyst for energy storage and conversion.</p>High Temperature Water as an Etch and Clean for SiO2 and Si3N42019-01-19T21:34:31-06:00https://digital.library.unt.edu/ark:/67531/metadc1404614/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1404614/"><img alt="High Temperature Water as an Etch and Clean for SiO2 and Si3N4" title="High Temperature Water as an Etch and Clean for SiO2 and Si3N4" src="https://digital.library.unt.edu/ark:/67531/metadc1404614/small/"/></a></p><p>An environmentally friendly, and contamination free process for etching and cleaning semiconductors is critical to future of the IC industry. Under the right conditions, water has the ability to meet these requirements. Water becomes more reactive as a function of temperature in part because the number of hydronium and hydroxyl ions increase. As water approaches its boiling point, the concentration of these species increases over seven times their concentrations at room temperature. At 150 °C, when the liquid state is maintained, these concentrations increase 15 times over room temperature. Due to its enhanced reactivity, high temperature water (HTW) has been studied as an etch and clean of thermally grown SiO2, Si3N4, and low-k films. High temperature deuterium oxide (HT-D2O) behaves similarly to HTW; however, it dissociates an order of magnitude less than HTW resulting in an equivalent reduction in reactive species. This allowed for the effects of reactive specie concentration on etch rate to be studied, providing valuable insight into how HTW compares to other high temperature wet etching processes such as hot phosphoric acid (HPA). Characterization was conducted using Fourier transform infrared spectroscopy (FTIR) to determine chemical changes due to etching, spectroscopic ellipsometry to determine film thickness, profilometry to measure thickness change across the samples, scanning electron microscopy (SEM), contact angle to measure changes in wetting behavior, and UV-Vis spectroscopy to measure dissolved silica in post etch water. HTW has demonstrated the ability to effective etch both SiO2 and Si3N4, HT-D2O also showed similar etch rates of Si3N4 indicating that a threshold reactive specie concentration is needed to maximize etch rate at a given temperature and additional reactive species do not further increase the etch rate. Because HTW has no hazardous byproducts, high temperature water could become a more environmentally friendly etchant of SiO2 and Si3N4 thin films.</p>Mechanisms of Formation and Effects of Transition Metal Oxides in Silicon Nitride on Steel Dry Sliding Contacts2019-01-19T21:34:31-06:00https://digital.library.unt.edu/ark:/67531/metadc1404592/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1404592/"><img alt="Mechanisms of Formation and Effects of Transition Metal Oxides in Silicon Nitride on Steel Dry Sliding Contacts" title="Mechanisms of Formation and Effects of Transition Metal Oxides in Silicon Nitride on Steel Dry Sliding Contacts" src="https://digital.library.unt.edu/ark:/67531/metadc1404592/small/"/></a></p><p>Silicon nitride on steel sliding contacts may provide advantageous tribological properties over traditional self-mated pairs, however the friction and wear behavior at high sliding speeds (>1 m/s) is not well understood. Previous studies at low sliding speeds (< 1 m/s) have found that the wear mechanisms change as a function of the operating parameters, e.g. atmosphere, sliding speed, load, and temperature, due to the formation of transition metal oxides such as Fe2O3 and Fe3O4. This study detected transient effects of the dry silicon nitride on steel contact over a range of sliding speeds to understand their relation to tribochemical reactions and the resulting tribological behavior. Two sets of dry silicon nitride on steel experiments were conducted at 1.45 GPa maximum Hertzian pressure. The first set were low sliding speed reciprocating experiments, conducted at an average of 0.06 m/s, conducted at variable operating temperature, ranging from 23 °C to 1000 °C. In the low sliding speed experiments, transitions of the wear mechanism from adhesive wear, to abrasive wear, then to oxidative wear was observed when the operating temperature increased. The second set were high sliding speed experiments, conducted at variable sliding speeds, ranging from 1 m/s to 16 m/s. In the high sliding speed experiments, a transition from adhesive wear to oxidative wear was observed when the sliding speed surpassed 4.5 m/s. The high sliding speed experiments were accompanied by in-situ instrumentation which detected the presence of a tribofilm which correlated to a reduction in friction, and its formation was linked to tribochemical reactions induced by high flash temperatures. Both sets of experiments had a maximum estimated contact temperature of 1000 °C where oxidative wear was prevalent. Although, the low sliding speed experiments underwent severe bulk oxidation and thermal softening effects, while the high sliding speed experiments experienced localized flash heating events with temperatures sufficient to form a semi-coherent tribofilm that was lubricious and significantly improved wear resistance. Therefore, the effects of transition metal oxides in sliding contacts are determined to be significantly influenced on their mechanisms of formation and interrelated to the operating parameters as found for dry sliding silicon nitride on steel contacts.</p>Molecular Dynamics Simulations of the Structures and Properties of Aluminosilicate and Borosilicate Glasses2019-01-19T21:34:31-06:00https://digital.library.unt.edu/ark:/67531/metadc1404517/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1404517/"><img alt="Molecular Dynamics Simulations of the Structures and Properties of Aluminosilicate and Borosilicate Glasses" title="Molecular Dynamics Simulations of the Structures and Properties of Aluminosilicate and Borosilicate Glasses" src="https://digital.library.unt.edu/ark:/67531/metadc1404517/small/"/></a></p><p>Silicate glasses are the most common glass types and have impact on almost every aspect in our lives: from window, containers, to glass fibers for telecommunications. Unlike their crystalline counterparts, glass materials lack long-range order in their atomic arrangement but their structures do possess short and medium range characteristics that play critical roles in their physical and chemical properties. Despite active development of characterization techniques that have contributed to the understanding of glass structures, there remain key challenges in obtaining essential structural features of glasses. Atomistic computer simulations have become an increasingly important method in elucidating the atomic structures and in interpretation and/or prediction of composition-structure-property relationships of complex materials. In this dissertation, classical molecular dynamic (MD) simulations were used to investigate the atomic structures, dynamic and other properties of two important glass systems—aluminosilicate glasses and borosilicate glasses, which are the basis of most industrial and technologically important glasses. Firstly, a comprehensive study of peralkaline Na2O-Al2O3-SiO2 glass with varying Al2O3/SiO2, Na2O/Al2O3, Na2O/SiO2 ratios has been performed to obtain better understanding of the composition–structure–property relationships in this glass system. More than 99% of Al were 4-coordinated in these glasses, validating that Na+ tend to charge balance [AlO4]- network forming units first and then, excess Na+ was used to create non-bridging oxygen (NBO) on Si. As the drop of Na/Al ratio, the percentage of NBO decreases, indicating an increase of the glass network connectivity. In addition, polyhedral connection probability results show that Al tend to be randomly distributed in the glass structure, suggesting a violation of Lowenstein's rule. These structural properties were further used to explain macroscopic properties of glass, such as change of glass transition temperature (Tg) and hardness (Hv) with glass composition. Secondly, molecular dynamics simulations were used to understand the structural, thermal mechanical and diffusion behaviors of spodumene (LiAlSi2O6) crystalline phases and glasses. It was found that β-LiAlSi2O6 has a structure much closer to the glass phase. The α-LiAlSi2O6 phase, however, has a more closed-packed structure and higher density. The diffusion behaviors were also found to be closely related to the atomic structures. Thirdly, the surface atomic structures of a series of sodium borosilicate glasses were studied using recently developed compositional dependent partial charge potentials. This provides insight into: a) the structural difference between glass surface and bulk glass; b) the evolution of bulk and surface structures as the change of glass composition. Lastly, pressure and temperature effects on the structure and properties of borosilicate glass were investigated in detail. A serial data derived from different compression temperatures and pressures enable us to explore the link between the microstructure and macroscopic physical properties. The results show that compression temperature and pressure play important roles in glass densification process and may result various glass densification mechanism. This dissertation demonstrates that atomistic simulations coupled with effective potentials and careful validations have become an effective method in research and design of complex glass materials.</p>Friction Stir Welding of Dissimilar Metals2019-01-19T21:34:31-06:00https://digital.library.unt.edu/ark:/67531/metadc1404577/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1404577/"><img alt="Friction Stir Welding of Dissimilar Metals" title="Friction Stir Welding of Dissimilar Metals" src="https://digital.library.unt.edu/ark:/67531/metadc1404577/small/"/></a></p><p>Dissimilar metals joining have been used in many industry fields for various applications due to their technique and beneficial advantages, such as aluminum-steel and magnesium-steel joints for reducing automobile weight, aluminum-copper joint for reducing material cost in electrical components, steel-copper joints for usage in nuclear power plant, etc. The challenges in achieving dissimilar joints are as below. (1) Big difference in physical properties such as melting point and coefficient of thermal expansion led to residual stress and defects. (2) The miscibility issues resulted in either brittle intermetallic compound layer at the welded interface for miscible combinations (such as, aluminum-steel, aluminum-copper, aluminum-titanium, etc.) or no metallurgical bonding for immiscible combinations (such as magnesium-copper, steel-copper, etc.). For metallurgical miscible combinations, brittle intermetallic compounds formed at the welded interface created the crack initiation and propagation path during deformational tests. (3) Stress concentration appeared at the welded interface region during tensile testing due to mismatch in elastic properties of dissimilar materials. In this study, different combinations of dissimilar metals were joined with friction stir welding. Lap welding of 6022-T4 aluminum alloy/galvanized mild steel sheets and 6022-T4 aluminum alloy/DP600 steel sheets were achieved via friction stir scribe technology. The interlocking feature determining the fracture mode and join strength was optimized. Reaction layer (intermetallic compounds layer) between the dissimilar metals were investigated. Butt welding of 5083-H116 aluminum alloy/HSLA-65 steel, 2024-T4 aluminum alloy/316 stainless steel, AZ31/316 stainless steel, WE43/316 stainless steel and 110 copper/316 stainless steel were obtained by friction stir welding. The critical issues in dissimilar metals butt joining were summarized and analyzed in this study including IMC and stress concentration.</p>Effects of Transition Metal Oxide and Mixed-Network Formers on Structure and Properties of Borosilicate Glasses2019-01-19T21:34:31-06:00https://digital.library.unt.edu/ark:/67531/metadc1404587/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1404587/"><img alt="Effects of Transition Metal Oxide and Mixed-Network Formers on Structure and Properties of Borosilicate Glasses" title="Effects of Transition Metal Oxide and Mixed-Network Formers on Structure and Properties of Borosilicate Glasses" src="https://digital.library.unt.edu/ark:/67531/metadc1404587/small/"/></a></p><p>First, the effect of transition metal oxide (e.g., V2O5, Co2O3, etc.) on the physical properties (e.g., density, glass transition temperature (Tg), optical properties and mechanical properties) and chemical durability of a simplified borosilicate nuclear waste glass was investigated. Adding V2O5 in borosilicate nuclear waste glasses decreases the Tg, while increasing the fracture toughness and chemical durability, which benefit the future formulation of nuclear waste glasses. Second, structural study of ZrO2/SiO2 substitution in silicate/borosilicate glasses was systematically conducted by molecular dynamics (MD) simulation and the quantitative structure-property relationships (QSPR) analysis to correlate structural features with measured properties. Third, for bioactive glass formulation, mixed-network former effect of B2O3 and SiO2 on the structure, as well as the physical properties and bioactivity were studied by both experiments and MD simulation. B2O3/SiO2 substitution of 45S5 and 55S5 bioactive glasses increases the glass network connectivity, correlating well with the reduction of bioactivity tested in vitro. Lastly, the effect of optical dopants on the optimum analytical performance on atom probe tomography (APT) analysis of borosilicate glasses was explored. It was found that optical doping could be an effective way to improve data quality for APT analysis with a green laser assisted system, while laser spot size is found to be critical for optimum performance. The combined experimental and simulation approach adopted in this dissertation led to a deeper understanding of complex borosilicate glass structures and structural origins of various properties.</p>Process Improvement of Surface Preparation of Structuraly Bonded Helicopter Detail Parts2019-01-19T21:34:31-06:00https://digital.library.unt.edu/ark:/67531/metadc1404599/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1404599/"><img alt="Process Improvement of Surface Preparation of Structuraly Bonded Helicopter Detail Parts" title="Process Improvement of Surface Preparation of Structuraly Bonded Helicopter Detail Parts" src="https://digital.library.unt.edu/ark:/67531/metadc1404599/small/"/></a></p><p>The objective of this study was to increase the bond strength at the surface interface of a thin stainless-steel panel for structural bonding on a helicopter. To achieve this objective, six activation methods for applying the coating to the panel in the surface preparation process are presented and explored. Adhesion and roughness tests were conducted to determine which method consistently initiates the etch and improves the bond at the surface. Based on the test results, three methods proved to be effective in initiating the etch. Of the three effective methods, only one method exhibited significantly improved bond strength at the surface interface as well as consistently initiated the etch in solution. The applicability of this method is discussed, and recommendations are presented for further study.</p>Laser Surface Modification of AZ31B Mg Alloy Bio-Implant Material2018-09-26T18:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc1248459/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1248459/"><img alt="Laser Surface Modification of AZ31B Mg Alloy Bio-Implant Material" title="Laser Surface Modification of AZ31B Mg Alloy Bio-Implant Material" src="https://digital.library.unt.edu/ark:/67531/metadc1248459/small/"/></a></p><p>Magnesium and its alloys are considered as the potential biomaterials due to their biocompatibility and biodegradable characteristics but suffer from poor corrosion performance. Various surface modification techniques are employed to improve their corrosion resistance. In present case, laser surface melting was carried out on AZ31B Mg alloy with various laser energy densities using a continuous wave ytterbium laser. Effect of laser treatment on phase and microstructure evolution was evaluated by X ray diffraction and scanning electron microscopy. Multi-physics thermal model predicted time temperature evolution along the depth of the laser treatment zone. Additionally, electrochemical method and bio-immersion test were employed to evaluate the corrosion behavior in simulated body fluid medium. Microstructure revealed grain refinement and even distribution of Mg17Al12 phase along the grain boundary for laser treated samples leading to substantial enhancement in the corrosion resistance of the laser treated samples compared to the untreated alloy. The laser processed samples also possessed a superior wettability in SBF solution than the untreated sample. This was further reflected in enhanced bio-integration behavior of laser processed samples. By changing the parameters of laser processing such as power, scanning speed, and fill spacing, a controllable corrosion resistance and bioactivity/biocompatibility of the implant material was achieved.</p>Room and Elevated Temperature Sliding Wear Behavior of Cold Sprayed Ni-WC Composite Coatings2018-09-26T18:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc1248400/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1248400/"><img alt="Room and Elevated Temperature Sliding Wear Behavior of Cold Sprayed Ni-WC Composite Coatings" title="Room and Elevated Temperature Sliding Wear Behavior of Cold Sprayed Ni-WC Composite Coatings" src="https://digital.library.unt.edu/ark:/67531/metadc1248400/small/"/></a></p><p>The tribological properties of cold sprayed Ni-WC metal matrix composite (MMC) coatings were investigated under dry sliding conditions from room temperature (RT) up to 400°C, and during thermal cycling to explore their temperature adaptive friction and wear behavior. Characterization of worn surfaces was conducted using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy to determine the chemical and microstructural evolution during friction testing. Data provided insights into tribo-oxide formation mechanisms controlling friction and wear. It was determined that the steady-state coefficient of friction (CoF) decreased from 0.41 at RT to 0.32 at 400˚C, while the wear rate increased from 0.5×10-4 mm3/N·m at RT to 3.7×10-4 mm3/N·m at 400˚C. The friction reduction is attributed primarily to the tribochemical formation of lubricious NiO on both the wear track and transfer film adhered to the counterface. The increase in wear is attributed to a combination of thermal softening of the coating and a change in the wear mechanism from adhesive to more abrasive. In addition, the coating exhibited low friction behavior during thermal cycling by restoring the lubricious NiO phase inside the wear track at high temperature intervals. Therefore, cold sprayed Ni-WC coatings are potential candidates for elevated temperature and thermally self-adaptive sliding wear applications.</p>Ultrasonic Processing of Aluminum 2139 and 70502018-09-26T18:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc1248496/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1248496/"><img alt="Ultrasonic Processing of Aluminum 2139 and 7050" title="Ultrasonic Processing of Aluminum 2139 and 7050" src="https://digital.library.unt.edu/ark:/67531/metadc1248496/small/"/></a></p><p>Acoustics is the study of all sound waves, with ultrasound classified as those frequencies above 20,000 Hz. Currently, ultrasound is being used in many industries for a variety of purposes such as ultrasonic imaging, ultrasonic assisted friction stir welding, and ultrasonic spot welding. Despite these uses, the effects of ultrasound on phase stability and resultant mechanical properties has been minimally analyzed. Here we study the impact waves play in ultrasonic welding and design an apparatus to maximize waves entering aluminum alloy samples. Aluminum 2139 and 7050 are used because they are precipitation strengthened by metastable phases so temperature change, and the corresponding phase stability, can greatly impact their strength. Results suggest that the ultrasonic welder primarily imposes a localized temperature spike due to friction, averaging over 200°C in a few seconds, which generally lowers the Vickers hardness due to coarsening or even dissolution of strengthening precipitates. Conversely, the new design increases the Vickers hardness by up to 30% over the initial hardness of approximately 63HV for aluminum 2139 and 83HV for aluminum 7050, respectively, while only increasing the temperature by an average of approximately 10°C. This new design was unable to achieve peak hardness, but the strengthening it achieved in two minutes was equivalent to one month of natural aging. If this system was able to be fine-tuned, it could serve as a quick strengthening process for recently weakened aluminum alloys, such as after friction stir welding.</p>Processing of NiTi Shape Memory Alloys through Low Pressure and Low Temperature Hydrogen Charging2018-06-06T23:02:11-05:00https://digital.library.unt.edu/ark:/67531/metadc1157656/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157656/"><img alt="Processing of NiTi Shape Memory Alloys through Low Pressure and Low Temperature Hydrogen Charging" title="Processing of NiTi Shape Memory Alloys through Low Pressure and Low Temperature Hydrogen Charging" src="https://digital.library.unt.edu/ark:/67531/metadc1157656/small/"/></a></p><p>Many industries including the medical, aerospace, and automobile industries have increasingly adopted the use of shape memory alloys (SMAs) for a plethora of applications due to their unique thermomechanical properties. From the commercially available SMAs in the market, binary NiTi SMAs have shown the most desirable properties. However, SMA properties can be significantly affected by the fabrication process. One of the most familiar applications of NiTi SMAs is in the design of actuating devices where the shape memory effect properties are highly advantageous. Spring NiTi SMA actuators are among the most commonly used and are generally made by torsion loading a straight wire. Consequently, stress concentrations are formed causing a reduction in recovery force. Other methods for producing springs and other NiTi SMA components is the fast emerging manufacturing method of additive manufacturing (AM). AM often uses metal powders to produce the near-net shape components. A major challenge for SMAs, in particular, is their well-known composition sensitivity. Therefore, it is critical to control composition in NiTi SMAs. In this thesis, a novel method for processing NiTi SMAs for pre-alloyed NiTi SMA powders and springs is presented. A low pressure and low temperature hydriding-pulverization-dehydriding method is used for preparing the pre-alloyed NiTi SMA powders with well-controlled compositions, size, and size distributions from wires. By hydrogen charging as-drawn martensitic NiTi SMA wires in a heated H3PO4 solution, pulverizing, and dehydriding, pre-alloyed NiTi powders of various well-controlled sizes are produced. In addition, a low pressure and low temperature hydriding-dehydriding method is used for producing NiTi SMA helixes from wires. The helix pattern in the pre-alloyed NiTi SMA wires was obtained by hydrogen charging NiTi SMA 500 μm diameter wires at different time intervals, followed by dehydriding to remove the hydrogen. The wires, powders, and resulting helixes were characterized using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD). The relationship between the wire diameter, powder particle size, and helix geometry as a function of hydrogen charging time is investigated. Lastly, the recovery behavior due to the shape memory effect is also investigated after dehydriding.</p>Alloy Development and High-Energy X-Ray Diffraction Studies of NiTiZr and NiTiHf High Temperature Shape Memory Alloys2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157525/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157525/"><img alt="Alloy Development and High-Energy X-Ray Diffraction Studies of NiTiZr and NiTiHf High Temperature Shape Memory Alloys" title="Alloy Development and High-Energy X-Ray Diffraction Studies of NiTiZr and NiTiHf High Temperature Shape Memory Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc1157525/small/"/></a></p><p>NiTi-based shape memory alloys (SMAs) offer a good combination of high-strength, ductility, corrosion resistance, and biocompatibility that has served them well and attracted the attention of many researchers and industries. The alloys unique thermo-mechanical ability to recover their initial shape after relatively large deformations by heating or upon unloading due to a characteristic reversible phase transformation makes them useful as damping devices, solid state actuators, couplings, etc. However, there is a need to increase the temperature of the characteristic phase transformation above 150 °C, especially in the aerospace industry where high temperatures are often seen. Prior researchers have shown that adding ternary elements (Pt, Pd, Au, Hf and Zr) to NiTi can increase transformation temperatures but most of these additions are extremely expensive, creating a need to produce cost-effective high temperature shape memory alloys (HTSMAs). Thus, the main objective of this research is to examine the relatively unstudied NiTiZr system for the ability to produce a cost effective and formable HTSMA. Transformation temperatures, precipitation paths, processability, and high-temperature oxidation are examined, specifically using high energy X-ray Diffraction (XRD) measurements, in NiTi-20 at.% Zr. This is followed by an in situ XRD study of the phase growth kinetics of the favorable H-phase nano precipitates, formed in NiTiHf and NiTiZr HTSMAs, based on prior thermo-mechanical processing in a commercial NiTi-15 at.% Hf HTSMA to examine the final processing methods and aging characteristics. Through this research, knowledge of the precipitation paths in NiTiZr and NiTiHf HTSMAs is extended and methods for characterization of phases and strains using high energy XRD are elucidated for future work in the field.</p>Surface Chemistry and Work Function of Irradiated and Nanoscale Thin Films Covered Indium Tin Oxides2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157651/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157651/"><img alt="Surface Chemistry and Work Function of Irradiated and Nanoscale Thin Films Covered Indium Tin Oxides" title="Surface Chemistry and Work Function of Irradiated and Nanoscale Thin Films Covered Indium Tin Oxides" src="https://digital.library.unt.edu/ark:/67531/metadc1157651/small/"/></a></p><p>In this study, we used UV-ozone Ar sputtering, X-ray photoelectron and ultra-violet photoelectron spectroscopies and sputtering based depositions of RuO2 and Se nano-layers on indium tin oxides (ITOs). We elucidated the effect of Ar sputtering on the composition and chemistry of Sn rich ITO surface. We demonstrated that while a combination of UV-ozone radiation and Ar sputtering removes most of the hydrocarbons responsible for degrading the work function of ITO, it also removes significant amount of the segregated SN at the ITO surface that's responsible for its reasonable work function of 4.7eV. We also demonstrated for the first time that sputtering cleaning ITO surface leads to the reduction of the charge state of Sn from Sn4+ to Sn2+ that adds to the degradation of the work function. For the nano-layers coverage of ITO studies, we evaluated both RuO2 and Se. For RuO2 coated ITO, XPS showed the formation of a Ru-Sn-O ternary oxide. The RuO2 nano-layer reduced the oxidation state of Sn in the Sn-rich surface of ITO from +4 to +2. The best work function obtained for this system is 4.98eV, raising the effective work function of ITO by more than 0.5 eV. For the Se coated ITO studies, a systematic study of the dependence of the effective work function on the thickness of Se overage and its chemistry at the Se/ITO interface was undertaken. XPS showed that Se reacts with Sn at the Sn-rich surface of ITO determined the presence of both negative and positive oxidation state of Se at the Se/ITO interface. The Se also reduced the oxidation state of Sn from Sn4+ to Sn2+ in the Sn-rich ITO surface. The highest effective work function obtained for this system is 5.06eV. A combination of RuO2/Se nanoscale coating of optimally cleaned ITO would be a good alternative for device applications that would provide work function tuning in addition to their potential ability to act as interface stabilizers and a barrier to reaction and inter-diffusion at ITO/active layers interfaces responsible for long term stability of devices and especially organic solar cells and organic light emitting diodes.</p>A Study on NiTiSn Low-Temperature Shape Memory Alloys and the Processing of NiTiHf High-Temperature Shape Memory Alloys2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157642/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157642/"><img alt="A Study on NiTiSn Low-Temperature Shape Memory Alloys and the Processing of NiTiHf High-Temperature Shape Memory Alloys" title="A Study on NiTiSn Low-Temperature Shape Memory Alloys and the Processing of NiTiHf High-Temperature Shape Memory Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc1157642/small/"/></a></p><p>Shape memory alloys (SMAs) operating as solid-state actuators pose economic and environmental benefits to the aerospace industry due to their lightweight, compact design, which provides potential for reducing fuel emissions and overall operating cost in aeronautical equipment. Despite wide applicability, the implementation of SMA technology into aerospace-related actuator applications is hindered by harsh environmental conditions, which necessitate extremely high or low transformation temperatures. The versatility of the NiTi-based SMA system shows potential for meeting these demanding material constraints, since transformation temperatures in NiTi can be significantly raised or lowered with ternary alloying elements and/or Ni:Ti ratio adjustments. In this thesis, the expansive transformation capabilities of the NiTi-based SMA system are demonstrated with a low and high-temperature NiTi-based SMA; each encompassing different stages of the SMA development process. First, exploratory work on the NiTiSn SMA system is presented. The viability of NiTiSn alloys as low-temperature SMAs (LTSMAs) was investigated over the course of five alloy heats. The site preference of Sn in near-equiatomic NiTi was examined along with the effects of solution annealing, Ni:Ti ratio adjustments, and precipitation strengthening on the thermomechanical properties of NiTiSn LTSMAs. Second, the thermomechanical processability of NiTiHf high-temperature SMA (HTSMA) wires is presented. The evolution of various microstructural features (grain size reduction, oxide growth, and nano-precipitation) were observed at incremental stages of the hot rolling process and linked to the thermal and mechanical responses of respective HTSMA rods/wires. This work was carried out in an effort to optimize the rolling/drawing process for NiTiHf HTSMAs.</p>Processing-Structure-Property Relationships of Spark Plasma Sintered Boron Carbide and Titanium Diboride Ceramic Composites2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157631/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157631/"><img alt="Processing-Structure-Property Relationships of Spark Plasma Sintered Boron Carbide and Titanium Diboride Ceramic Composites" title="Processing-Structure-Property Relationships of Spark Plasma Sintered Boron Carbide and Titanium Diboride Ceramic Composites" src="https://digital.library.unt.edu/ark:/67531/metadc1157631/small/"/></a></p><p>The aim of this study was to understand the processing – structure – property relationships in spark plasma sintered (SPS) boron carbide (B4C) and B4C-titanium diboride (TiB2) ceramic composites. SPS allowed for consolidation of both B4C and B4C-TiB2 composites without sintering additives, residual phases, e.g., graphite, and excessive grain growth due to long sintering times. A selection of composite compositions in 20% TiB2 feedstock powder increments from 0% to 100%, was sintered at 1900°C for 25 minutes hold time. A homogeneous B4C-TiB2 composite microstructure was determined with excellent distribution of TiB2 phase, while achieving ~99.5% theoretical density. An optimum B4C-23 vol.% TiB2 composite composition with low density of ~3.0 g/cm3 was determined that exhibited ~30-35% increase in hardness, fracture toughness, and flexural bend strength compared to commercial armor-grade B4C. This is a result of a) no residual graphitic carbon in the composites, b) interfacial microcrack toughening due to thermal expansion coefficient differences placing the B4C matrix in compression and TiB2 phase in tension, and c) TiB2 phase aids in crack deflection thereby increasing the amount of intergranular fracture. Collectively, the addition of TiB2 serves as a strengthening and toughening agent, and SPS shows promise for the manufacture of hybrid ceramic composites.</p>Preventing Thermal Degradation of Pvc Insulation by Mixtures of Cross-Linking Agents and Antioxidants2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157547/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157547/"><img alt="Preventing Thermal Degradation of Pvc Insulation by Mixtures of Cross-Linking Agents and Antioxidants" title="Preventing Thermal Degradation of Pvc Insulation by Mixtures of Cross-Linking Agents and Antioxidants" src="https://digital.library.unt.edu/ark:/67531/metadc1157547/small/"/></a></p><p>Poly(vinyl chloride)(PVC) wire and cable insulation has poor thermal stability, causing the plasticizer to separate from the PVC chain and produce an oily residue, lowering the tensile elongation at break and thus increasing brittleness. We have added 4 wt.% of three different types of cross-linking agents and antioxidants, as well as mixtures of both, to improve the thermal stability of the plasticizer and tensile properties of PVC after thermal exposure. We performed tensile tests, tribological tests, profilometry, scanning electron microscopy(SEM) and water absorption determination before and after thermal exposure at 136 ℃ for 1 week. After adding the agents, elongation at break increased by 10 to 20 % while the wear rate and water absorption were lower than for the control sample. Less voids are seen in the SEM images after adding these two kinds of agents. The thermal resistance of the PVC cable insulation is best enhanced by combinations of cross-linking agents and antioxidants.</p>Investigation into the Semiconducting and Device Properties of MoTe2 and MoS2 Ultra-Thin 2D Materials2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157626/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157626/"><img alt="Investigation into the Semiconducting and Device Properties of MoTe2 and MoS2 Ultra-Thin 2D Materials" title="Investigation into the Semiconducting and Device Properties of MoTe2 and MoS2 Ultra-Thin 2D Materials" src="https://digital.library.unt.edu/ark:/67531/metadc1157626/small/"/></a></p><p>The push for electronic devices on smaller and smaller scales has driven research in the direction of transition metal dichalcogenides (TMD) as new ultra-thin semiconducting materials. These ‘two-dimensional' (2D) materials are typically on the order of a few nanometers in thickness with a minimum all the way down to monolayer. These materials have several layer-dependent properties such as a transition to direct band gap at single-layer. In addition, their lack of dangling bonding and remarkable response to electric fields makes them promising candidates for future electronic devices. For the purposes of this work, two 2D TMDs were studied, MoS2 and MoTe2. This dissertation comprises of three sections, which report on exploration of charge lifetimes, investigation environmental stability at elevated temperatures in air, and establishing feasibility of UV laser annealing for large area processing of 2D TMDs, providing a necessary knowledge needed for practical use of these 2D TMDs in optoelectronic and electronic devices.
(1) A study investigating the layer-dependence on the lifetime of photo-generated electrons in exfoliated 2D MoTe2 was performed. The photo-generated lifetimes of excited electrons were found to be strongly surface dependent, implying recombination events are dominated by Shockley-Read-Hall effects (SRH). Given this, the measured lifetime was shown to increase with the thickness of exfoliated MoTe¬2; in agreement with SRH recombination. Lifetimes were also measured with an applied potential bias and demonstrated to exhibit a unique voltage dependence. Shockley-Read-Hall recombination effects, driven by surface states were attributed to this result. The applied electric field was also shown to control the surface recombination velocity, which lead to an unexpected rise and fall of measured lifetimes as the potential bias was increased from 0 to 0.5 volts.
(2) An investigation into the environmental stability of exfoliated 2D MoTe2 was conducted using a passivation layer of amorphous boron nitride as a capping layer for back-gated MoTe2 field effect transistor (FET) devices. A systematic approach was taken to understand the effects of heat treatment in air on the performance of FET devices. Atmospheric oxygen was shown to negatively affect uncoated MoTe2 devices while BN-covered FETs showed remarkable chemical and electronic characteristic stability. Uncapped MoTe2 FET devices, which were heated in air for one minute, showed a polarity switch from n- to p-type at 150 °C, while BN-MoTe2 devices switched only after 200 °C of heat treatment. Time-dependent experiments at 100 °C showed that uncapped MoTe2 samples exhibited the polarity switch after 15 min of heat treatment while the BN-capped device maintained its n-type conductivity. X-ray photoelectron spectroscopy (XPS) analysis suggests that oxygen incorporation into MoTe2 was the primary doping mechanism for the polarity switch.
(3) The feasibility of UV laser annealing as a post-process technique to sinter 2D crystal structures from sputtered amorphous MoS2 was explored. Highly crystalline materials are sought after for their use in electron and opto-electronic devices. Sputtered MoS2 has the advantage of potential for large area deposition and high scalability, however, it requires high temperatures (>350 °C) for their crystalline growth. Which creates difficulty for devices grown on polymer substrates. Low-temperature and room temperature deposition results in amorphous films which is detrimental for electric devices. A one-step lase annealing procedure was developed to provide amorphous to crystalline conversion of nanometer thin MoS2 films. Samples were annealed using an unfocused laser beam from a KrF (248 nm) excimer source. The power density was found to be 1.04 mJ/mm2. Raman analysis of laser annealed MoS2 was shown to exhibit a significant improvement of the 2D MoS2 crystallinity compared to as-deposited films on both SiO2/Si, as well as polydimethylsiloxane (PDMS) substrates. Annealed samples showed improvement of their conductivity on an order of magnitude. A top-gated FET device was fabricated on flexible PDMS substrates using Al2O3 as a gate oxide. Measured field effect mobility of annealed samples showed significant improvement over as-deposited devices.</p>Recycling of PVC and XLPE for High Impact Resistance in Spool Development2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157640/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157640/"><img alt="Recycling of PVC and XLPE for High Impact Resistance in Spool Development" title="Recycling of PVC and XLPE for High Impact Resistance in Spool Development" src="https://digital.library.unt.edu/ark:/67531/metadc1157640/small/"/></a></p><p>My work focuses on taking waste wire-grade PVC = poly(vinyl chloride) and waste XLPE = cross-linked polyethylene and recycle them into small wire/cable spool technology in order to reduce waste cost and reduce cost of spool production. The PVC and XLPE were provided by Encore Wire Corp. of McKinney, TX; they have also defined the standard to which I am comparing my results. The end goal is to incorporate as much PVC and XLPE into the spools while maintaining material toughness, impact resistance, as well as cost-effectiveness in the implementation of the waste materials. The work has been divided into two primary sections, the first is focused on improving material strength through the addition of ceramic fillers. The second section is focused on adding PVC and XLPE into a stronger and highly cohesive polymer matrix and optimizing the concentration of the waste products. Since XLPE is non-polar while PVC is strongly polar, compatibilizers such as CPE (chlorinated polyethylene) and MA-DCP (maleic anhydride with dicumyl peroxide) were used to improve interactions between polar and non-polar constituents. Testing involved the tensile mechanical properties, tribology and thermal properties, namely dynamic mechanical analysis (DMA) and evaluation of thermal degradation by thermogravimetric analysis (TGA). Combining PVC and XLPE together is not economically feasible with current compatiblizers. At the same time, introduction of PVC waste or XLPE waste with sufficient properties of the resulting composites is doable.</p>Defining a Relationship between the Flexibility of Materials and Other Properties2018-06-06T13:19:50-05:00https://digital.library.unt.edu/ark:/67531/metadc1157637/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1157637/"><img alt="Defining a Relationship between the Flexibility of Materials and Other Properties" title="Defining a Relationship between the Flexibility of Materials and Other Properties" src="https://digital.library.unt.edu/ark:/67531/metadc1157637/small/"/></a></p><p>Brittleness of a polymeric material has a direct relationship with the material's performance and furthermore shares an inverse relationship with that material's flexibility. The concept of flexibility of materials has been understood but merely explained with a hand-waving manner. Thus, it has never been defined by a calculation, thereby lacking the ability to determine a definite quantitative value for this characteristic. Herein, an equation is presented and proven which makes determining the value of flexibility possible. Such an equation could be used to predict a material's flexibility prior to testing it, thus saving money and valuable time for those in research and in industry. Substantiating evidence showing the relationship between flexibility of polymers and their respective mechanical properties is presented. Further relating the known tensile properties of a given polymer to its flexibility is expanded upon by proving its relationship to the linear coefficient of thermal expansion for each polymer. Additionally, determining flexibility for polymers whose chemical structures have been compromised by respective solvents has also been investigated to predict a solvent's impact on a polymer after exposure. Polymers examined through literature include polycarbonate (PC), polystyrene (PS), teflon (PTFE), styrene acrylonitrile (SAN), acrylonitrile butadiene styrene (ABS), poly(ethersulfone) (PES), low density polyethylene (LDPE), polypropylene (PP), poly(methyl methacrylate) (PMMA), and poly(vinylidene fluoride) (PVDF). Further testing and confirmation was made using PC, PS, ABS, LDPE, PP, and PMMA.</p>Non-Isothermal Laser Treatment of Fe-Si-B Metallic Glass2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062821/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062821/"><img alt="Non-Isothermal Laser Treatment of Fe-Si-B Metallic Glass" title="Non-Isothermal Laser Treatment of Fe-Si-B Metallic Glass" src="https://digital.library.unt.edu/ark:/67531/metadc1062821/small/"/></a></p><p>Metallic glasses possess attractive properties, such as high strength, good corrosion resistance, and superior soft magnetic performance. They also serve as precursors for synthesizing nanocrystalline materials. In addition, a new class of composites having crystalline phases embedded in amorphous matrix is evolving based on selective crystallization of metallic glasses. Therefore, crystallization of metallic glasses and its effects on properties has been a subject of interest. Previous investigations from our research group related to laser assisted crystallization of Fe-Si-B metallic glass (an excellent soft magnetic material by itself) showed a further improvement in soft magnetic performance. However, a fundamental understanding of crystallization and mechanical performance of laser treated metallic glass was essential from application point of view. In light of this, the current work employed an integrated experimental and computational approach to understand crystallization and its effects on tensile behavior of laser treated Fe-Si-B metallic glass. The time temperature cycles during laser treatments were predicted using a finite element thermal model. Structural changes in laser treated Fe-Si-B metallic glass including crystallization and phase evolution were investigated with the aid of X-ray diffraction, differential scanning calorimetry, resistivity measurements, and transmission electron microscopy. The mechanical behavior was evaluated by uniaxial tensile tests with an InstronTM universal testing machine. Fracture surfaces of the metallic glass were observed using scanning electron microscopy and site specific transmission electron microscopy.
Fe-Si-B metallic glass samples treated with lower laser fluence (<0.49 J/mm2) underwent structural relaxation while higher laser flounces led to partial crystallization. The crystallization temperature experienced an upward shift due to rapid heating rates of the order of 104 K/s during laser treatments. The heating cycle was followed by termination of laser upon treatment attainment of peak temperature and rapid cooling of the similar order. Such dynamic effects resulted in premature arrest of the crystallite growth leading to formation of fine crystallites/grain (~32 nm) of α-(Fe,Si) as the major component and Fe2B as the minor component. The structural relaxation, crystallization fractions of 5.6–8.6 Vol% with α-(Fe,Si) as the main component, and crystallite/grain size of the order of 12 nm obtained in laser fluence range of 0.39-0.49 J/mm2 had minimal/no influence on tensile behavior of the laser treated Fe-Si-B metallic glass foils. An increase in laser fluence led to progressive increase in crystallization fractions with considerable amounts of Fe2B (2-6 Vol%) and increase in grain size to ~30 nm. Such a microstructural evolution severely reduced the strength of Fe-Si-B metallic glass. Moreover, there was a transition in fracture surface morphology of laser treated Fe-Si-B metallic glass from vein pattern to chevron pattern. Tensile loading lacked any marked influence on the crystallization behavior of as-cast and structurally relaxed laser-treated metallic glass foils. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallized metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.</p>Modeling of High Strain Rate Compression of Austenitic Shape Memory Alloys2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062835/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062835/"><img alt="Modeling of High Strain Rate Compression of Austenitic Shape Memory Alloys" title="Modeling of High Strain Rate Compression of Austenitic Shape Memory Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc1062835/small/"/></a></p><p>Shape memory alloys (SMAs) exhibit the ability to absorb large dynamic loads and, therefore, are excellent candidates for structural components where impact loading is expected. Compared to the large amount of research on the shape memory effect and/or pseudoelasticity of polycrystalline SMAs under quasi-static loading conditions, studies on dynamic loading are limited. Experimental research shows an apparent difference between the quasi-static and high strain rate deformation of SMAs. Research reveals that the martensitic phase transformation is strain rate sensitive. The mechanism for the martensitic phase transformation in SMAs during high strain rate deformation is still unclear. Many of the existing high strain rate models assume that the latent heat generated during deformation contributes to the change in the stress-strain behavior during dynamic loading, which is insufficient to explain the large stress observed during phase transformation under high strain rate deformation. Meanwhile, the relationship between the phase front velocity and strain rate has been studied. In this dissertation, a new resistance to phase transformation during high strain rate deformation is discussed and the relationship between the driving force for phase transformation and phase front velocity is established. With consideration of the newly defined resistance to phase transformation, a new model for phase transformation of SMAs during high strain rate deformation is presented and validated based on experimental results from an austenitic NiTi SMA. Stress, strain, and martensitic volume fraction distribution during high strain rate deformation are simulated using finite element analysis software ABAQUS/standard. For the first time, this dissertation presents a theoretical study of the microscopic band structure during high strain rate compressive deformation. The microscopic transformation band is generated by the phase front and leads to minor fluctuations in sample deformation. The strain rate effect on phase transformation is studied using the model. Both the starting stress for transformation and the slope of the stress-strain curve during phase transformation increase with increasing strain rate.</p>Effects of HALSs and Nano-ZnO Worked as UV Stabilizers of Polypropylene2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062903/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062903/"><img alt="Effects of HALSs and Nano-ZnO Worked as UV Stabilizers of Polypropylene" title="Effects of HALSs and Nano-ZnO Worked as UV Stabilizers of Polypropylene" src="https://digital.library.unt.edu/ark:/67531/metadc1062903/small/"/></a></p><p>This work reports the outdoor weathering performance of ultraviolet (UV)-stabilized polypropylene (PP) products (using PP resins from Encore Wire). Different hindered amine light stabilizers (HALSs) and nano-ZnO were used to stabilize PP-film-based formulations that were exposed under UV light for 6 weeks simulating for in harsh outdoor weather of Dallas, Texas, USA in 2016. Characterization of the exposed PP film products was done in terms of mechanical and friction spectroscopic properties. The PP film formulations were divided into 15 categories based on the type of HALS and nano-ZnO incorporated. This was done to derive meaningful comparison of the various film formulations. Following exposure under UV light, the lifetimes of certain formulations were determined. On the basis of the mechanical and friction properties, it was determined that generally, the HALS or nano-ZnO stabilized PP film give better properties and if those two kinds of UV stabilizers can work together.</p>Molecular Dynamics Simulations of the Structure and Properties of Boron Containing Oxide Glasses: Empirical Potential Development and Applications2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062909/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062909/"><img alt="Molecular Dynamics Simulations of the Structure and Properties of Boron Containing Oxide Glasses: Empirical Potential Development and Applications" title="Molecular Dynamics Simulations of the Structure and Properties of Boron Containing Oxide Glasses: Empirical Potential Development and Applications" src="https://digital.library.unt.edu/ark:/67531/metadc1062909/small/"/></a></p><p>Potential parameters that can handle multi-component oxide glass systems especially boron oxide are very limited in literature. One of the main goals of my dissertation is to develop empirical potentials to simulate multi-component oxide glass systems with boron oxide. Two approaches, both by introducing the composition dependent parameter feature, were taken and both led to successful potentials for boron containing glass systems after extensive testing and fitting. Both potential sets can produce reasonable glass structures of the multi-component oxide glass systems, with structure and properties in good agreement with experimental data. Furthermore, we have tested the simulation settings such as system size and cooling rate effects on the results of structures and properties of MD simulated borosilicate glasses. It was found that increase four-coordinated boron with decreasing cooling rate and system size above 1000 atoms is necessary to produce converged structure. Another application of the potentials is to simulate a six-component nuclear waste glass, international simple glass (ISG), which was for first time simulated using the newly developed parameters. Structural features obtained from simulations agree well with the experimental results. In addition, two series of sodium borosilicate and boroaluminosilicate glasses were simulated with the two sets of potentials to compare and evaluate their applicability and deficiency. Various analyses on the structures and properties such as pair distribution function, total correlation function, coordination number analysis, Qn distribution function, ring size distribution function, vibrational density of states and mechanical properties were performed. This work highlights the challenge of MD simulations of boron containing glasses and the capability of the new potential parameters that enable simulations of wide range of mixed former glasses to investigate new structure features and design of new glass compositions for various applications.</p>Three-Dimensional Carbon Nanostructure and Molybdenum Disulfide (MoS2) for High Performance Electrochemical Energy Storage Devices2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062842/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062842/"><img alt="Three-Dimensional Carbon Nanostructure and Molybdenum Disulfide (MoS2) for High Performance Electrochemical Energy Storage Devices" title="Three-Dimensional Carbon Nanostructure and Molybdenum Disulfide (MoS2) for High Performance Electrochemical Energy Storage Devices" src="https://digital.library.unt.edu/ark:/67531/metadc1062842/small/"/></a></p><p>My work presents a novel approach to fabricate binder free three-dimensional carbon nanotubes/sulfur (3DCNTs/S) hybrid composite by a facile and scalable method increasing the loading amount from 1.86 to 8.33 mg/cm2 highest reported to date with excellent electrochemical performance exhibiting maximum specific energy of ~1233Wh/kg and specific power of ~476W/kg, with respect to the mass of the cathode. Such an excellent performance is attributed to the fact that 3DCNTs offers higher loading amount of sulfur, and confine polysulfide within the structure. In second part of the thesis, molybdenum disulfide (MoS2) is typically studied for three electrochemical energy storage devices including supercapacitors, Li-ion batteries, and hybrid Li-ion capacitors. The intrinsic sheet like morphology of MoS2 provides high surface area for double layer charge storage and a layered structure for efficient intercalation of H+/ Li+ ions. My work demonstrates the electrochemical analysis of MoS2 grown on different substrates including copper (conducting), and carbon nanotubes. MoS2 film on copper was investigated as a supercapacitor electrode in three electrode system exhibiting excellent volumetric capacitance of ~330F/cm3 along with high volumetric power and energy density in the range of 40-80 W/cm3 and 1.6-2.4 mWh/cm3, respectively. Furthermore, we have developed novel binder-free 3DCNTs/ MoS2 as an anode materials in half cell Li-ion batteries. The vertically oriented morphology of MoS2 offers high surface area and active electrochemical sites for efficient intercalation of Li+ ions and demonstrating excellent electrochemical performance with high specific capacity and cycling stability. This 3DCNTs/ MoS2 anode was coupled with high surface area southern yellow pine derived activated carbon (SYAC) cathode to obtain hybrid 3DCNTs/ MoS2 || SYAC Li-ion capacitor (LIC), which delivers large operating voltage window of 1-4.0V with excellent cycling stability exhibiting capacitance retention of ~80% after 5000 cycles.</p>Surface Degradation Behavior of Bulk Metallic Glasses and High Entropy Alloys2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062863/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062863/"><img alt="Surface Degradation Behavior of Bulk Metallic Glasses and High Entropy Alloys" title="Surface Degradation Behavior of Bulk Metallic Glasses and High Entropy Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc1062863/small/"/></a></p><p>In this study, the surface degradation behavior was studied for typical examples from bulk metallic glasses (BMGs), metallic glass composites (MGCs) and high entropy alloys (HEAs) alloy systems that are of scientific and commercial interest. The corrosion and wear behavior of two Zr-based bulk metallic glasses, Zr41.2Cu12.5Ni10Ti13.8Be22.5 and Zr57Cu15.4Ni12.6Al10Nb5, were evaluated in as-cast and thermally relaxed states. Significant improvement in corrosion rate, wear behavior, and friction coefficient was seen for both the alloys after thermal relaxation. Fully amorphous structure was retained with thermal relaxation below the glass transition temperature. This improvement in surface properties was explained by annihilation of free volume, the atomic scale defects in amorphous metals resulting from kinetic freezing. Recently developed MGCs, with in situ crystalline ductile phase, demonstrate a combination of mechanical properties and fracture behavior unseen in known structural metals. The composites showed higher wear rates but lower coefficient of friction compared to monolithic amorphous glasses. No tribolayer formation was seen for the composites in sharp contrast to that of the monolithic metallic glasses. Corrosion was evaluated by open circuit potential (OCP) analysis and potentiodynamic polarization. Site-specific corrosion behavior was studied by scanning vibration electrode technique (SVET) to identify formation of galvanic couples. Scanning kelvin probe microscope was used to map elecropositivity difference between the phases and linked to wear/corrosion behavior. Phases with higher elecropositivity were more susceptible to surface degradation. Wear and corrosion synergy in marine environment was evaluated for two high entropy alloys (HEAs), CoCrFeMnNi and Al0.1CoCrFeNi. Between the two alloys, Al0.1CoCrFeNi showed better wear resistance compared to CoCrFeMnNi in dry and marine conditions due to quicker passivation, a higher magnitude of polarization resistance and significantly larger pitting resistance.</p>P-type Doping of Pulsed Laser Deposited WS2 with Nb2018-01-27T07:36:46-06:00https://digital.library.unt.edu/ark:/67531/metadc1062806/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1062806/"><img alt="P-type Doping of Pulsed Laser Deposited WS2 with Nb" title="P-type Doping of Pulsed Laser Deposited WS2 with Nb" src="https://digital.library.unt.edu/ark:/67531/metadc1062806/small/"/></a></p><p>Layered transition metal dichalcogenides (TMDs) are potentially ideal semiconducting materials due to their in-plane carrier transport and tunable bandgaps, which are favorable properties for electrical and optoelectronic applications. However, the ability to make p-n junctions is the foundation of semiconductor devices, and therefore the ability to achieve reproducible p- and n-type doping in TMD semiconducting materials is critical. In this work, p-type substitutional doping of pulsed laser deposited WS2 films with niobium is reported. The synthesis technique of the PLD target with dopant incorporation which also ensures host material stoichiometry is presented. Hall electrical measurements confirmed stable p-type conductivity of the grown films. Structural characterization revealed that there was no segregation phase of niobium in the fabricated films and x-ray phtoelectron spectroscopy (xps) characterization suggest that the p-type doping is due to Nb4+ which results in p-type behavior. Stable hole concentrations as high as 10E21(cm-3) were achieved. The target fabrication and thin film deposition technique reported here can be used for substitutional doping of other 2D materials to obtain stable doping for device applications.</p>Additive Manufacturing of Metastable Beta Titanium Alloys2017-10-09T11:44:47-05:00https://digital.library.unt.edu/ark:/67531/metadc1011883/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1011883/"><img alt="Additive Manufacturing of Metastable Beta Titanium Alloys" title="Additive Manufacturing of Metastable Beta Titanium Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc1011883/small/"/></a></p><p>Additive manufacturing processes of many alloys are known to develop texture during the deposition process due to the rapid reheating and the directionality of the dissipation of heat. Titanium alloys and with respect to this study beta titanium alloys are especially susceptible to these effects. This work examines Ti-20wt%V and Ti-12wt%Mo deposited under normal additive manufacturing process parameters to examine the texture of these beta-stabilized alloys. Both microstructures contained columnar prior beta grains 1-2 mm in length beginning at the substrate with no visible equiaxed grains. This microstructure remained constant in the vanadium system throughout the build. The microstructure of the alloy containing molybdenum changed from a columnar to an equiaxed structure as the build height increased. Eighteen additional samples of the Ti-Mo system were created under different processing parameters to identify what role laser power and travel speed have on the microstructure. There appears to be a correlation in alpha lath size and power density. The two binary alloys were again deposited under the same conditions with the addition of 0.5wt% boron to investigate the effects an insoluble interstitial alloying element would have on the microstructure. The size of the prior beta grains in these two alloys were reduced with the addition of boron by approximately 50 (V) and 100 (Mo) times.</p>Developing Precipitation Hardenable High Entropy Alloys2017-10-09T11:44:47-05:00https://digital.library.unt.edu/ark:/67531/metadc1011755/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1011755/"><img alt="Developing Precipitation Hardenable High Entropy Alloys" title="Developing Precipitation Hardenable High Entropy Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc1011755/small/"/></a></p><p>High entropy alloys (HEAs) is a concept wherein alloys are constructed with five or more elements mixed in equal proportions; these are also known as multi-principle elements (MPEs) or complex concentrated alloys (CCAs). This PhD thesis dissertation presents research conducted to develop precipitation-hardenable high entropy alloys using a much-studied fcc-based equi-atomic quaternary alloy (CoCrFeNi). Minor additions of aluminium make the alloy amenable for precipitating ordered intermetallic phases in an fcc matrix. Aluminum also affects grain growth kinetics and Hall-Petch hardenability.
The use of a combinatorial approach for assessing composition-microstructure-property relationships in high entropy alloys, or more broadly in complex concentrated alloys; using laser deposited compositionally graded AlxCrCuFeNi2 (0 < x < 1.5) complex concentrated alloys as a candidate system. The composition gradient has been achieved from CrCuFeNi2 to Al1.5CrCuFeNi2 over a length of ~25 mm, deposited using the laser engineered net shaping process from a blend of elemental powders. With increasing Al content, there was a gradual change from an fcc-based microstructure (including the ordered L12 phase) to a bcc-based microstructure (including the ordered B2 phase), accompanied with a progressive increase in microhardness. Based on this combinatorial assessment, two promising fcc-based precipitation strengthened systems have been identified; Al0.3CuCrFeNi2 and Al0.3CoCrFeNi, and both compositions were subsequently thermo-mechanically processed via conventional techniques. The phase stability and mechanical properties of these alloys have been investigated and will be presented. Additionally, the activation energy for grain growth as a function of Al content in these complex alloys has also been investigated.
Change in fcc grain growth kinetic was studied as a function of aluminum; the apparent activation energy for grain growth increases by about three times going from Al0.1CoCrFeNi (3% Al (at%)) to Al0.3CoCrFeNi. (7% Al (at%)). Furthermore, Al addition leads to the precipitation of highly refined ordered L12 (γ′) and B2 precipitates in Al0.3CoCrFeNi. A detailed investigation of precipitation of the ordered phases in Al0.3CoCrFeNi and their thermal stability is done using atom probe tomography (APT), transmission electron microscopy (TEM) and Synchrotron X-ray in situ and ex situ analyses.
The alloy strengthened via grain boundary strengthening following the Hall-Petch relationship offers a large increment of strength with small variation in grain size. Tensile strength of the Al0.3CoFeNi is increased by 50% on precipitation fine-scale γ′ precipitates. Furthermore, precipitation of bcc based ordered phase B2 in Al0.3CoCrFeNi can further strengthen the alloy. Fine-tuning the microstructure by thermo-mechanical treatments achieved a wide range of mechanical properties in the same alloy. The Al0.3CoCrFeNi HEA exhibited ultimate tensile strength (UTS) of ~250 MPa and ductility of ~65%; a UTS of ~1100 MPa and ductility of ~30%; and a UTS of 1850 MPa and a ductility of 5% after various thermo-mechanical treatments. Grain sizes, precipitates type and size scales manipulated in the alloy result in different strength ductility combinations. Henceforth, the alloy presents a fertile ground for development by grain boundary strengthening and precipitation strengthening, and offers very high activation energy of grain growth aptly suitable for high-temperature applications.</p>Evolution of Precipitates and Their Influence on the Mechanical Properties of β-Titanium Alloys2017-10-09T11:44:47-05:00https://digital.library.unt.edu/ark:/67531/metadc1011759/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1011759/"><img alt="Evolution of Precipitates and Their Influence on the Mechanical Properties of β-Titanium Alloys" title="Evolution of Precipitates and Their Influence on the Mechanical Properties of β-Titanium Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc1011759/small/"/></a></p><p>Over the last few decades, body-centered-cubic (bcc) beta (β) titanium alloys have largely been exploited as structural alloys owing to the richness in their microstructural features. These features, which lead to a unique combination of high specific strength and ductility, excellent hardenability, good fatigue performance, and corrosion resistance, make these alloys viable candidates for many applications, including aerospace, automobile, and orthopedic implants. The mechanical properties of these alloys strongly depend on the various phases present; which can be controlled by thermomechanical treatments and/or alloy design. The two most important and studied phases are the metastable ω phase and the stable α phase. The present study focuses on the microstructural evolution and the mechanical behavior of these two phases in a model β-Ti alloy, binary Ti-12wt. %Mo alloy, and a commercial β-Ti alloy, β-21S.
Microstructures containing athermal and isothermal ω phases in the binary Ti-12wt. %Mo alloy are obtained under specific accurate temperature controlled heat treatments. The formation and the evolution of the ω-phase based microstructures are investigated in detail via various characterization techniques such as SEM, TEM, and 3D atom probe tomography. The mechanical behavior was investigated via quasi-static tensile loading; at room and elevated temperatures. The effect of β phase stability on the deformation behavior is then discussed.
Similar to the Ti-12wt. %Mo, the formation and the evolution of the athermal and isothermal ω phases in the commercial β-21S alloy was studied under controlled heat treatments. The structural and compositional changes were tracked using SEM, TEM, HR-STEM, and 3D atom probe tomography (3D-APT). The presence of additional elements in the commercial alloy were noted to make a considerable difference in the evolution and morphology of the ω phase and also the mechanical behavior of the alloys. The Portevin-Le Chatelier (PLC) like effect was observed in
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this alloy at elevated temperature and this has been attributed to the shearing of the ω precipitates and the dynamic precipitation of the α phase within these channels.
The formation of the stable α phase in the commercial β-21S alloy due to the influence of precursor phases, like the metastable ω phase, is investigated. It is evident from the microstructural characterization, using SEM, TEM, HR-STEM, and 3D-APT, that the ω phase does play a role on the fine scale α precipitation. The mechanical behavior of the β+α microstructure, investigated via tensile testing, shows that these alloys are ideal candidate for precipitation hardening. The exceptional strength values obtained in this alloy have been attributed to a combination of several factors.</p>Laser Additive Manufacturing of Magnetic Materials2017-10-09T11:44:47-05:00https://digital.library.unt.edu/ark:/67531/metadc1011873/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc1011873/"><img alt="Laser Additive Manufacturing of Magnetic Materials" title="Laser Additive Manufacturing of Magnetic Materials" src="https://digital.library.unt.edu/ark:/67531/metadc1011873/small/"/></a></p><p>A matrix of variably processed Fe-30at%Ni was deposited with variations in laser travel speeds as well and laser powers. A complete shift in phase stability occurred as a function of varying laser travel speed. At slow travel speeds, the microstructure was dominated by a columnar fcc phase. Intermediate travel speeds yielded a mixed microstructure comprised of both the columnar fcc and a martensite-like bcc phase. At the fastest travel speed, the microstructure was dominated by the bcc phase. This shift in phase stability subsequently affected the magnetic properties, specifically saturation magnetization. Ni-Fe-Mo and Ni-Fe-V permalloys were deposited from an elemental blend of powders as well. Both systems exhibited featureless microstructures dominated by an fcc phase. Magnetic measurements yielded saturation magnetizations on par with conventionally processed permalloys, however coercivities were significantly larger; this difference is attributed to microstructural defects that occur during the additive manufacturing process.</p>Design Principle on Carbon Nanomaterials Electrocatalysts for Energy Storage and Conversion2017-07-12T03:17:08-05:00https://digital.library.unt.edu/ark:/67531/metadc984279/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc984279/"><img alt="Design Principle on Carbon Nanomaterials Electrocatalysts for Energy Storage and Conversion" title="Design Principle on Carbon Nanomaterials Electrocatalysts for Energy Storage and Conversion" src="https://digital.library.unt.edu/ark:/67531/metadc984279/small/"/></a></p><p>We are facing an energy crisis because of the limitation of the fossil fuel and the pollution caused by burning it. Clean energy technologies, such as fuel cells and metal-air batteries, are studied extensively because of this high efficiency and less pollution. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential in the process of energy storage and conversion, and noble metals (e.g. Pt) are needed to catalyze the critical chemical reactions in these devices. Functionalized carbon nanomaterials such as heteroatom-doped and molecule-adsorbed graphene can be used as metal-free catalysts to replace the expensive and scarce platinum-based catalysts for the energy storage and conversion. Traditionally, experimental studies on the catalytic performance of carbon nanomaterials have been conducted extensively, however, there is a lack of computational studies to guide the experiments for rapid search for the best catalysts. In addition, theoretical mechanism and the rational design principle towards ORR and OER also need to be fully understood.
In this dissertation, density functional theory calculations are performed to calculate the thermodynamic and electrochemical properties of heteroatom-doped graphene and molecule-adsorbed graphene for ORR and OER. Gibb's free energy, overpotential, charge transfer and edge effect are evaluated. The charge transfer analysis show the positive charges on the graphene surface caused by the heteroatom, hetero-edges and the adsorbed organic molecules play an essential role in improving the electrochemical properties of the carbon nanomaterials. Based on the calculations, design principles are introduced to rationally design and predict the electrochemical properties of doped graphene and molecule-adsorbed graphene as metal-free catalysts for ORR and OER. An intrinsic descriptor is discovered for the first time, which can be used as a materials parameter for rational design of the metal-free catalysts with carbon nanomaterials for energy storage and conversion. The success of the design principle provides a better understanding of the mechanism behind ORR and OER and a screening approach for the best catalyst for energy storage and conversion.</p>Exceptional Properties in Friction Stir Processed Beta Titanium Alloys and an Ultra High Strength Steel2017-07-12T03:17:08-05:00https://digital.library.unt.edu/ark:/67531/metadc984167/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc984167/"><img alt="Exceptional Properties in Friction Stir Processed Beta Titanium Alloys and an Ultra High Strength Steel" title="Exceptional Properties in Friction Stir Processed Beta Titanium Alloys and an Ultra High Strength Steel" src="https://digital.library.unt.edu/ark:/67531/metadc984167/small/"/></a></p><p>The penchant towards development of high performance materials for light weighting engineering systems through various thermomechanical processing routes has been soaring vigorously. Friction stir processing (FSP) - a relatively new thermomechanical processing route had shown an excellent promise towards microstructural modification in many Al and Mg alloy systems. Nevertheless, the expansion of this process to high temperature materials like titanium alloys and steels is restricted by the limited availability of tool materials. Despite it challenges, the current thesis sets a tone for the usage of FSP to tailor the mechanical properties in titanium alloys and steels. FSP was carried out on three near beta titanium alloys, namely Ti6246, Ti185 and Tiβc with increasing β stability index, using various tool rotation rates and at a constant tool traverse speed. Microstructure and mechanical property relationship was studied using experimental techniques such as SEM, TEM, mini tensile testing and synchrotron x-ray diffraction. Two step aging on Ti6246 had resulted in an UTS of 2.2GPa and a specific strength around 500 MPa m3/mg, which is about 40% greater than any commercially available metallic material. Similarly, FSP on an ultra-high strength steel―Eglin steel had resulted in a strength greater than 2GPa with a ductility close to 10% at around 4mm from the top surface of stir zone (SZ). Experimental techniques such as microhardness, mini-tensile testing and SEM were used to correlate the microstructure and properties observed inside SZ and HAZ's of the processed region. A 3D temperature modeling was used to predict the peak temperature and cooling rates during FSP. The exceptional strength ductility combinations inside the SZ is believed to be because of mixed microstructure comprised of various volume fractions of phases such as martensite, bainite and retained austenite.</p>Structure Evolution and Nano-Mechanical Behavior of Bulk Metallic Glasses and Multi-Principal Element Alloys2017-07-12T03:17:08-05:00https://digital.library.unt.edu/ark:/67531/metadc984260/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc984260/"><img alt="Structure Evolution and Nano-Mechanical Behavior of Bulk Metallic Glasses and Multi-Principal Element Alloys" title="Structure Evolution and Nano-Mechanical Behavior of Bulk Metallic Glasses and Multi-Principal Element Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc984260/small/"/></a></p><p>Bulk metallic glasses and multi-principal element alloys represent relatively new classes of multi-component engineering materials designed for satisfying multiple functionalities simultaneously. Correlating the microstructure with mechanical behavior (at the microstructural length-scales) in these materials is key to understanding their performance. In this study, the structure evolution and nano-mechanical behavior of these two classes of materials was investigated with the objective of fundamental scientific understanding of their properties. The structure evolution, high temperature nano-mechanical behavior, and creep of two Zr-based alloys was studied: Zr41.2Ti13.8Cu12.5Ni10.0Be22 (Vitreloy1) and Zr52.5Ti5Cu17.9Ni14.6All0 (Vitreloy105). Devitrification was found to proceed via the formation of a metastable icosahedral phase with five-fold symmetry. The deformation mechanism changes from inhomogeneous or serrated flow to homogenous flow near 0.9Tg, where Tg is the glass transition temperature. The creep activation energy for Vitreloy1 and Vitreloy105 were 144 kJ/mol and 125 kJ/mol, respectively in the range of room temperature to 0.75Tg. The apparent activation energy increased drastically to 192 kJ/mol for Vitreloy1 and 215 kJ/mol for Vitreloy105 in the range of 0.9Tg to Tg, indicating a change in creep mechanism. Structure evolution in catalytic amorphous alloys, Pt57.5Cu14.7Ni5.3P22.5 and Pd43Cu27Ni10P20, was studied using 3D atom probe tomography and elemental segregation between different phases and the interface characteristics were identified.
The structure evolution of three multi-principal element alloys were investigated namely CoCrNi, CoCrFeMnNi, and Al0.1CoCrFeNi. All three alloys formed a single-phase FCC structure in as-cast, cold worked and recrystallized state. No secondary phases precipitated after prolonged heat treatment or mechanical working. The multi-principal element alloys showed less strain gradient plasticity compared to pure metals like Ni during nano-indentation. This was attributed to the highly distorted lattice which resulted in lesser density of geometrically necessary dislocations (GNDs). Dislocation nucleation was studied by low load indentation along with the evaluation of activation volume and activation energy. This was done using a statistical approach of analyzing the "pop-in" load marking incipient plasticity. The strain rate sensitivity of nanocrystalline Al0.1CoCrFeNi alloy was determined by in situ compression of nano-pillars in a Pico-indenter. The nanocrystalline alloy demonstrated a yield strength of ~ 2.4 GPa, ten times greater than its coarse grained counterpart. The nanocrystalline alloy exhibited high strain rate sensitivity index of 0.043 and activation volume of 5b3 suggesting grain boundary dislocation nucleation.</p>Microstructural Evolution and Mechanical Response of Materials by Design and Modeling2017-07-12T03:17:08-05:00https://digital.library.unt.edu/ark:/67531/metadc984205/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc984205/"><img alt="Microstructural Evolution and Mechanical Response of Materials by Design and Modeling" title="Microstructural Evolution and Mechanical Response of Materials by Design and Modeling" src="https://digital.library.unt.edu/ark:/67531/metadc984205/small/"/></a></p><p>Mechanical properties of structural materials are highly correlated to their microstructure. The relationship between microstructure and mechanical properties can be established experimentally. The growing need for structural materials in industry promotes the study of microstructural evolution of materials by design using computational approaches. This thesis presents the microstructural evolution of two different structural materials. The first uses a genetic algorithm approach to study the microstructural evolution of a high-temperature nickel-based oxide-dispersion-strengthened (ODS) alloy. The chosen Ni-20Cr ODS system has nano Y2O3 particles for dispersion strengthening and submicron Al2O3 for composite strengthening. Synergistic effects through the interaction of small dispersoids and large reinforcements improved high-temperature strength. Optimization considered different weight factors on low temperature strength, ductility, and high temperature strength. Simulation revealed optimal size and volume fraction of dispersoids and reinforced particles. Ni-20Cr-based alloys were developed via mechanical alloying for computational optimization and validation. The Ni-20Cr-1.2Y2O3-5Al2O3 alloy exhibited significant reduction in the minimum creep rate (on the order of 10-9 s-1) at 800oC and 100 MPa. The second considers the microstructural evolution of AA 7050 alloy during friction stir welding (FSW). Modeling the FSW process includes thermal, material flow, microstructural and strength modeling. Three-dimensional material flow and heat transfer model was developed for friction stir welding process of AA 7050 alloy to predict thermal histories and extent of deformation. Peak temperature decreases with the decrease in traverse speed at constant advance per revolution, while the increase in tool rotation rate enhances peak temperature. Shear strain is higher than the longitudinal and transverse strain for lower traverse speed and tool rotation rate; whereas for higher traverse speed and tool rotation rate, shear and normal strain acquire similar values. Precipitation distribution simulation using TC-PRISMA predicts the presence of η' and η in the as-received AA 7050-T7451 alloy and mostly η in the friction stir welded AA7050 alloy, which results in the lower predicted strength of friction stir welded alloy. Further, development of modeling assists in process optimization and innovation, and enhances the progression rate. Accelerating the development process requires coupling experimental methods with predictive modeling. The overall purpose of this work was to develop an integrated computational model with predictive capabilities. In the present work, an application tool to predict thermal histories during FSW of AA7050 was developed using COMSOL software.</p>Catalytic Properties and Mechanical Behavior of Metallic Glass Powders2017-07-12T03:17:08-05:00https://digital.library.unt.edu/ark:/67531/metadc984273/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc984273/"><img alt="Catalytic Properties and Mechanical Behavior of Metallic Glass Powders" title="Catalytic Properties and Mechanical Behavior of Metallic Glass Powders" src="https://digital.library.unt.edu/ark:/67531/metadc984273/small/"/></a></p><p>Lack of crystalline order and microstructural features such as grain/grain-boundary in metallic glasses results in a suite of remarkable attributes including very high strength, close to theoretical elasticity, high corrosion and wear resistance, and soft magnetic properties. By altering the morphology and tuning of composition, MGs may be transformed into high-performance catalytic materials. In this study, the catalytic properties of metallic glass powders were demonstrated in dissociating toxic organic chemicals such as AZO dye. BMG powders showed superior performance compared to state of the art crystalline iron because of their high catalytic activity, durability, and reusability. To enhance the catalytic properties, high energy mechanical milling was performed to increase the surface area and defect density. Iron-based bulk metallic glass (BMG) of composition Fe48Cr15Mo14Y2C15B6 was used because of its low cost and ability to make large surface area by high energy ball milling. AZO dye was degraded in less than 20 minutes for the 9 hours milled Fe-BMG. However, subsequent increase in ball milling time resulted in devitrification and loss of catalytic activity as measured using UV-Visible spectroscopy. Aluminum-based bulk metallic glass (Al-BMG) powder of composition Al82Fe3Ni8Y7 was synthesized by arc-melting the constituent elements followed by gas-atomization. The particle size and morphology were similar to Fe-BMG with a fully amorphous structure. A small percentage of transition metal constituents (Fe and Ni) in a mostly aluminum alloy showed high catalytic activity, with no toxic by-products and no change in surface characteristics. Al-alloy particles, being light-weight, were easily dispersed in aqueous medium and accelerated the redox reactions. The mechanism of dye dissociation was studied using Raman and Infrared (IR) spectroscopy. Breaking of -C-H- and - C-N- bonds of AZO dye was found to be the primary mechanism. Mechanical behavior of individual BMG particles was evaluated by in situ pico-indentation in a scanning electron microscope (SEM) to understand the fracture mechanisms. Catastrophic shear banding was found to be the primary fracture mode, which supported the observation of flake formation during high energy ball milling.</p>Improving the Long-term Performance of PVC Compositions2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955074/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955074/"><img alt="Improving the Long-term Performance of PVC Compositions" title="Improving the Long-term Performance of PVC Compositions" src="https://digital.library.unt.edu/ark:/67531/metadc955074/small/"/></a></p><p>PVC are extensively applied in many fields, such as cables, pipes, vehicles, shoes, toys and infusion bags. Generally, plasticizers are blended with PVC to improve the ability of process in industrial production; however, the toxic plasticizers will gradually migrate to the surface of products and such a leakage results in brittleness of plasticized PVC and environmental pollution. In other words, humans are frequently exposed to the potential risks. According to previous researches, cross-linked PVC was proved that it was able to hinder the migration of plasticizer. Thus, in this research, we selected some commercially used cross-linking agents and employed six different tests based on mechanical, tribological and microscopy analysis in order to seek the best solution against plasticizer migration. Thus, we expected to develop a cross-linked flexible PVC which performed improved long-term performance and extended lifetime.</p>Defect Behaviors in Zinc Oxide and Zinc Titanates Ceramics from First Principles Computer Simulations2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955093/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955093/"><img alt="Defect Behaviors in Zinc Oxide and Zinc Titanates Ceramics from First Principles Computer Simulations" title="Defect Behaviors in Zinc Oxide and Zinc Titanates Ceramics from First Principles Computer Simulations" src="https://digital.library.unt.edu/ark:/67531/metadc955093/small/"/></a></p><p>ZnO and ZnO-TiO2 ceramics have intriguing electronic and mechanical properties and find applications in many fields. Many of these properties and applications rely on the understanding of defects and defect processes in these oxides as these defects control the electronic, catalytic and mechanical behaviors. The goal of this dissertation is to systematically study the defects and defects behaviors in Wurtzite ZnO and Ilmenite ZnTiO3 by using first principles calculations and classical simulations employing empirical potentials. Firstly, the behavior of intrinsic and extrinsic point defects in ZnO and ZnTiO3 ceramics were investigated. Secondly, the effect of different surface absorbents and surface defects on the workfunction of ZnO were studied using DFT calculations. The results show that increasing the surface coverage of hydrocarbons decreased the workfunction. Lastly, the stacking fault behaviors on ilmenite ZnTiO3 were investigated by calculating the Generalized Stacking Fault (GSF) energies using density functional theory based first principles calculations and classical calculations employing effective partial charge inter-atomic potentials. The gamma-surfaces of two low energy surfaces, (110) and (104), of ZnTiO3 were fully mapped and, together with other analysis such as ideal shear stress calculations.</p>Sliding Friction and Wear Behavior of High Entropy Alloys at Room and Elevated Temperatures2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955078/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955078/"><img alt="Sliding Friction and Wear Behavior of High Entropy Alloys at Room and Elevated Temperatures" title="Sliding Friction and Wear Behavior of High Entropy Alloys at Room and Elevated Temperatures" src="https://digital.library.unt.edu/ark:/67531/metadc955078/small/"/></a></p><p>Structure-tribological property relations have been studied for five high entropy alloys (HEAs). Microhardness, room and elevated (100°C and 300°C) temperature sliding friction coefficients and wear rates were determined for five HEAs: Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4; Co Cr Fe Ni Al0.25 Ti0.75; Ti V Nb Cr Al; Al0.3CoCrFeNi; and Al0.3CuCrFeNi2. Wear surfaces were characterized with scanning electron microscopy and micro-Raman spectroscopy to determine the wear mechanisms and tribochemical phases, respectively. It was determined that the two HEAs Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4 and Ti V Nb Cr Al exhibit an excellent balance of high hardness, low friction coefficients and wear rates compared to 440C stainless steel, a currently used bearing steel. This was attributed to their more ductile body centered cubic (BCC) solid solution phase along with the formation of tribochemical Cr oxide and Nb oxide phases, respectively, in the wear surfaces. This study provides guidelines for fabricating novel, low-friction, and wear-resistant HEAs for potential use at room and elevated temperatures, which will help reduce energy and material losses in friction and wear applications.</p>The Role of Misfit Strain and Oxygen Content on Formation and Evolution of Omega Precipitate in Metastable Beta-titanium Alloys2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955080/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955080/"><img alt="The Role of Misfit Strain and Oxygen Content on Formation and Evolution of Omega Precipitate in Metastable Beta-titanium Alloys" title="The Role of Misfit Strain and Oxygen Content on Formation and Evolution of Omega Precipitate in Metastable Beta-titanium Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc955080/small/"/></a></p><p>β-Ti alloys are widely used in airframe and biomedical applications due to their high ductility, high hardenability, and low elastic modulus. The phase transformations in β-Ti alloys are rather complex due to formation of metastable phases during various thermo-mechanical treatments. One such critical metastable phase, the hexagonal omega (ω) phase, can form in β-Ti alloys under quenching from the high temperature β phase and/or isothermal aging at intermediate temperature. Despite a substantial amount of reported works on the ω phase, there are several critical issues related to the ω formation need to be resolved, e.g. role of alloying elements and oxygen content. Therefore, this dissertation has attempted to provide insights into ω transformation in low misfit (Ti-Mo) and high misfit (Ti-V) binary systems as well as multicomponent (Ti-Nb-Zr-Ta) alloys.
The evolution of ω structure, morphology and composition from the early stage (β-solution+quenched) to later stages after prolonged aging are systematically investigated by coupling transmission electron microscopy (TEM), atom probe tomography (APT) and high-energy synchrotron X-ray diffraction techniques. The influence of aging temperature and duration on characteristic of ω phase in Ti-Mo, and Ti-V alloys is addressed in details. It is found that compositional changes during aging can alter the structure, size and morphology of ω precipitates. In low misfit alloys, the ellipsoidal morphology of ω phase was retained during isothermal aging, while in high misfit alloys it changed from ellipsoidal to cuboidal morphology after prolonged aging. Secondly, ω transformation in biomedical Ti-Nb-Zr-Ta alloy is probed in which the micro-hardness was sensitive to microstructural changes. Furthermore, the evolution of oxygen concentration in ω precipitates during various aging conditions in binary Ti-Mo and Ti-V alloys are reported. It has been accepted that interstitial elements such as oxygen can largely alter mechanical behavior and the microstructure of Ti-alloys. Recently, oxygen is intentionally added to some biomedical alloys to improve their performances. However, a careful understanding of the effect of oxygen on ω phase transformation is still lacking in the literature. In this work, the role of oxygen on ω phase formation in biomedical TNTZ alloys is investigated. Although it is traditionally accepted that oxygen suppresses ω transformation, our observations revealed contradictory results during isothermal aging of TNZT alloys. The results of our investigations provide a novel insight into understanding the effect of interstitial elements on metastable phase transformation in β-Ti alloys. It is concluded that depending upon the nature of alloying elements and/or the applied thermo-mechanical treatments, oxygen may play a different role in ω transformations.</p>Design and Manufacture of Molding Compounds for High Reliability Microelectronics in Extreme Conditions2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955110/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955110/"><img alt="Design and Manufacture of Molding Compounds for High Reliability Microelectronics in Extreme Conditions" title="Design and Manufacture of Molding Compounds for High Reliability Microelectronics in Extreme Conditions" src="https://digital.library.unt.edu/ark:/67531/metadc955110/small/"/></a></p><p>The widespread use of electronics in more avenues of consumer use is increasing. Applications range from medical instrumentation that directly can affect someone's life, down hole sensors for oil and gas, aerospace, aeronautics, and automotive electronics. The increased power density and harsh environment makes the reliability of the packaging a vital part of the reliability of the device. The increased importance of analog devices in these applications, their high voltage and high temperature resilience is resulting in challenges that have not been dealt with before. In particular packaging where insulative properties are vital use polymer resins modified by ceramic fillers. The distinct dielectric properties of the resin and the filler result in charge storage and release of the polarization currents in the composite that have had unpredictable consequences on reliability. The objective of this effort is therefore to investigate a technique that can be used to measure the polarization in filled polymer resins and evaluate reliable molding compounds. A valuable approach to measure polarization in polymers where charge release is tied to the glass transition in the polymer is referred to as thermally stimulated depolarization current (TSDC) technique. In this dissertation a new TSDC measurement system was designed and fabricated. The instrument is an assembly of several components that are automated via a LabVIEW program that gives the user flexibility to test different dielectric compounds at high temperatures and high voltage. The temperature control is enabled through the use of dry air convection heating at a very slow rate enabling controlled heating and cooling. Charge trapping and de-trapping processes were investigated in order to obtain information on insulating polymeric composites and how to optimize it. A number of material properties were investigated. First, polarization due to charges on the filer were investigated using composites containing charged and uncharged particles using quartz and ion exchange montmorillonite silicates in an epoxy matrix. The thermally-activated charge release shows a difference in the composite characteristics and preparation. This difference indicates that the trap levels depend on the de-trapping process and on the chemical nature of the trap site. Using a numerical approach to the release spectra, a model was developed to examine through short time testing, important parameters such as glass transition temperature, residual polarization, depolarization peak, window polarization modeling and activation energy of relaxations. Second the design of mold compounds that could combine manufacturing (temperature of molding), geometric (thickness of packaging material), composition (amount and size of filler) effects was developed using a novel design of experiments approach. The statistical DOE enabled the determination of which causes should be considered when designing a mold compound that has minimal polarization both as singular variables as well as combined variables. Finally, the DOE approach was used to develop a high temperature reliable molding compound through use of combined fillers of thermally conductive and nonconductive fillers having different shapes. Through the systematic approach to developing a technique and designing a mold compound addressing the multiple impacts on reliability of packaging, the dissertation provides an approach to the design, selection, performance and durability of molding compounds.</p>Angular Analysis of a Wide-Band Energy Harvester based on Mutually Perpendicular Vibrating Piezoelectric Beams2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955086/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955086/"><img alt="Angular Analysis of a Wide-Band Energy Harvester based on Mutually Perpendicular Vibrating Piezoelectric Beams" title="Angular Analysis of a Wide-Band Energy Harvester based on Mutually Perpendicular Vibrating Piezoelectric Beams" src="https://digital.library.unt.edu/ark:/67531/metadc955086/small/"/></a></p><p>The recent advancements in electronics and the advents of small scaled instruments has increased the attachment of life and functionality of devices to electrical power sources but at the same time granted the engineers and companies the ability to use smaller sources of power and batteries. Therefore, many scientists have tried to come up with new solutions for a power alternatives. Piezoelectric is a promising material which can readily produce continuous electric power from mechanical inputs. However, their power output is dependent upon several factors such as, system natural frequency, their position in the system, the direction of vibration and many other internal and external factors. In this research the working bandwidth of the system is increased through utilizing of two different piezoelectric beam in different directions. The dependency of output power with respect to rotation angle and also the frequency shift due to the rotation angle is studied.</p>In Vitro Behavior of AZ31B Mg-Hydroxyapatite Metallic Matrix Composite Surface Fabricated via Friction Stir Processing2016-08-31T22:41:47-05:00https://digital.library.unt.edu/ark:/67531/metadc862762/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc862762/"><img alt="In Vitro Behavior of AZ31B Mg-Hydroxyapatite Metallic Matrix Composite Surface Fabricated via Friction Stir Processing" title="In Vitro Behavior of AZ31B Mg-Hydroxyapatite Metallic Matrix Composite Surface Fabricated via Friction Stir Processing" src="https://digital.library.unt.edu/ark:/67531/metadc862762/small/"/></a></p><p>Magnesium and its alloys have been considered for load-bearing implant materials due to their similar mechanical properties to the natural bone, excellent biocompatibility, good bioactivity, and biodegradation. Nevertheless, the uncontrollable corrosion rate in biological environment restrains their application. Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is a widely used bio-ceramic which has bone-like mineral structure for bone fixation. Poor fracture toughness of HA makes it not suitable for load-bearing application as a bulk. Thus, HA is introduced into metallic surface in various forms for improving biocompatibility. Recently friction stir processing (FSP) has emerged as a surface modification tool for surface/substrate grain refinement and homogenization of microstructure in biomaterial. In the pressent efforts, Mg-nHA composite surface on with 5-20 wt% HA on Mg substrate were fabricated by FSP for biodegradation and bioactivity study. The results of electrochemical measurement indicated that lower amount (~5% wt%) of Ca in Mg matrix can enhance surface localized corrosion resistance. The effects of microstructure,the presence of HA particle and Mg-Ca intermetallic phase precipitates on in vitro behavior of Mg alloy were investigated by TEM, SEM, EDX,XRD ,and XPS. The detailed observations will be discussed during presentation.</p>Microstructure for Enhanced Plasticity and Toughness2016-08-31T22:41:47-05:00https://digital.library.unt.edu/ark:/67531/metadc862825/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc862825/"><img alt="Microstructure for Enhanced Plasticity and Toughness" title="Microstructure for Enhanced Plasticity and Toughness" src="https://digital.library.unt.edu/ark:/67531/metadc862825/small/"/></a></p><p>Magnesium is the lightest metal with a very high specific strength. However, its practical applicability is limited by its toughness and reliability. Mg, being HCP has low ductility. This makes the improvement of toughness a grand challenge in Mg alloys. Friction stir processing (FSP) is a thermomechanical technique used to effect microstructural modification. Here, FSP was utilized to affect the toughness of WE43 sheets through microstructural modification. Room temperature Kahn-type tests were conducted to measure the toughness of WE43 sheets. Microscopic techniques (SEM, TEM) was utilized to study the effect of various microstructural factors like grain size, texture, constituent particles, precipitates on crack initiation and propagation. Tensile properties were evaluated by mini-tensile tests. Crack growth in WE43 sheets was also affected by mechanics and digital image correlation (DIC) was utilized to study the plastic zone size. The underlying mechanisms affecting toughness of these sheets were understood which will help in formulating ways in improving it.
WE43 nanocomposites were fabricated via FSP. Uniform distribution of reinforcements was obtained in the composites. Improved mechanical properties like that of enhanced strength, increased hardness and stiffness were obtained. But contrary to other metal matrix composites which show reduction in ductility with incorporation of ceramic reinforcements, the nanocomposites showed good strength-ductility combination. The composites were precisely characterized and mechanisms governing this property were studied. The nano-length of the reinforcements was observed to be the main criteria and the dislocation-particle interaction, the main reason behind the strength-ductility property.</p>Friction Stir Welding of High Strength Precipitation Strengthened Aluminum Alloys2016-08-31T22:41:47-05:00https://digital.library.unt.edu/ark:/67531/metadc862787/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc862787/"><img alt="Friction Stir Welding of High Strength Precipitation Strengthened Aluminum Alloys" title="Friction Stir Welding of High Strength Precipitation Strengthened Aluminum Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc862787/small/"/></a></p><p>Rising demand for improved fuel economy and structural efficiency are the key factors for use of aluminum alloys for light weighting in aerospace industries. Precipitation strengthened 2XXX and 7XXX aluminum alloys are the key aluminum alloys used extensively in aerospace industry. Welding and joining is the critical step in manufacturing of integrated structures. Joining of precipitation strengthened aluminum alloys using conventional fusion welding techniques is difficult and rather undesirable in as it produces dendritic microstructure and porosities which can undermine the structural integrity of weldments. Friction stir welding, invented in 1991, is a solid state joining technique inherently benefitted to reduces the possibility of common defects associated with fusion based welding techniques.
Weldability of various 2XXX and 7XXX aluminum alloys via friction stir welding was investigated. Microstructural and mechanical property evolution during welding and after post weld heat treatment was studied using experimental techniques such as transmission electron microscopy, differential scanning calorimetry, hardness testing, and tensile testing. Various factors such as peak welding temperature, cooling rate, external cooling methods (thermal management) which affects the strength of the weldment were studied. Post weld heat treatment of AL-Mg-Li alloy produced joint as strong as the parent material. Modified post weld heat treatment in case of welding of Al-Zn-Mg alloy also resulted in near 100% joint efficiency whereas the maximum weld strength achieved in case of welds of Al-Cu-Li alloys was around 80-85% of parent material strength. Low dislocation density and high nucleation barrier for the precipitates was observed to be responsible for relatively low strength recovery in Al-Cu-Li alloys as compared to Al-Mg-Li and Al-Zn-Mg alloys.</p>Friction Stir Welding of Precipitation Strengthened Aluminum 7449 Alloys2016-08-31T22:41:47-05:00https://digital.library.unt.edu/ark:/67531/metadc862775/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc862775/"><img alt="Friction Stir Welding of Precipitation Strengthened Aluminum 7449 Alloys" title="Friction Stir Welding of Precipitation Strengthened Aluminum 7449 Alloys" src="https://digital.library.unt.edu/ark:/67531/metadc862775/small/"/></a></p><p>The Al-Zn-Mg-Cu (7XXX series) alloys are amongst the strongest aluminum available. However, they are considered unweldable with conventional fusion techniques due to the negative effects that arise with conventional welding, including hydrogen porosity, hot cracking, and stress corrosion cracking. For this reason, friction stir welding has emerged as the preferred technique to weld 7XXX series alloys. Aluminum 7449 is one of the highest strength 7XXX series aluminum alloy. This is due to its higher zinc content, which leads to a higher volume fraction of eta' precipitates. It is typically used in a slight overaged condition since it exhibits better corrosion resistance. In this work, the welds of friction stir welded aluminum 7449 were studied extensively. Specific focus was placed in the heat affected zone (HAZ) and nugget. Thermocouples were used in the heat affected zone for three different depths to obtain thermal profiles as well as cooling/heating profiles. Vicker microhardness testing, transmission electron microscope (TEM), and differential scanning calorimeter (DSC) were used to characterize the welds. Two different tempers of the alloy were used, a low overaged temper and a high overaged temper. A thorough comparison of the two different tempers was done. It was found that highly overaged aluminum 7449 tempers show better properties for friction stir welding. A heat gradient along with a high conducting plate (Cu) used at the bottom of the run, resulted in welds with two separate microstructures in the nugget. Due to the microstructure at the bottom of the nugget, higher strength than the base metal is observed. Furthermore, the effects of natural aging and artificial aging were studied to understand re-precipitation. Large improvements in strength are observed after natural aging throughout the welds, including improvements in the HAZ.</p>Workfunction tuning of AZO Films Through Surface Modification for Anode Application in OLEDs.2016-08-31T22:41:47-05:00https://digital.library.unt.edu/ark:/67531/metadc862818/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc862818/"><img alt="Workfunction tuning of AZO Films Through Surface Modification for Anode Application in OLEDs." title="Workfunction tuning of AZO Films Through Surface Modification for Anode Application in OLEDs." src="https://digital.library.unt.edu/ark:/67531/metadc862818/small/"/></a></p><p>Widespread use of organic light emitting diodes (OLEDs) in solid state lighting and display technologies require efficiency and lifetime improvements, as well as cost reductions, inclusive of the transparent conducting oxide (TCO). Indium tin oxide (ITO) is the standard TCO anode in OLEDs, but indium is expensive and the Earth's reserve of this element is limited. Zinc oxide (ZnO) and its variants such as aluminum-doped ZnO (AZO) exhibit comparable electrical conductivity and transmissivity to ITO, and are of interest for TCO applications. However, the workfunction of ZnO and AZO is smaller compared to ITO. The smaller workfunction of AZO results in a higher hole injection barrier at the anode/organic interface, and methods of tuning its workfunction are required.
This dissertation tested the hypothesis that workfunction tuning of AZO films could be achieved by surface modification with electronegative oxygen and fluorine plasmas, or, via use of nanoscale transition metal oxide layers (MoOx, VOx and WOx). Extensive UPS, XPS and optical spectroscopy studies indicate that O2 and CFx plasma treatment results in an electronegative surface, surface charge redistribution, and a surface dipole moment which reinforces the original surface dipole leading to workfunction increases. Donor-like gap states associated with partially occupied d-bands due to non-stoichiometry determine the effective increased workfunction of the AZO/transition-metal oxide stacks. Reduced hole injection barriers were engineered by ensuring that the surface ad-layers were sufficiently thin to facilitate Fowler-Nordheim tunneling. Improved band alignments resulted in improved hole injection from the surface modified AZO anodes, as demonstrated by I-V characterization of hole only structures. Energy band alignments are proposed based on the aforementioned spectroscopies.
Simple bilayer OLEDs employing the surface modified AZO anodes were fabricated and characterized to compare their performance with standard ITO. Anodes consisting of AZO with MoOx or VOx interfacial layers exhibited 50% and 71% improvement in power efficiency (PE) and external quantum efficiency (EQE), respectively, compared to ITO at a working voltage of 9 V. The efficiencies of dipole reinforced AZO (O2/CFx plasma treated) anodes were comparable to ITO. The improved performance of the surface modified AZO anodes compared to as-deposited AZO is ascribed to improved hole injection, improved charge balance, and improved radiative recombination kinetics. The results suggest that surface modified AZO anodes are a promising alternative to ITO, given the lower cost and Earth abundance of Al and Zn.</p>Compostable Soy-Based Polyurethane Foam with Kenaf Core Modifiers2016-08-31T22:41:47-05:00https://digital.library.unt.edu/ark:/67531/metadc862866/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc862866/"><img alt="Compostable Soy-Based Polyurethane Foam with Kenaf Core Modifiers" title="Compostable Soy-Based Polyurethane Foam with Kenaf Core Modifiers" src="https://digital.library.unt.edu/ark:/67531/metadc862866/small/"/></a></p><p>Building waste and disposable packaging are a major component in today's landfills. Most of these are structural or thermally insulative polymer foams that do not degrade over a long period of time. Currently, there is a push to replace these foams with thermoplastic or biodegradable foams that can either be recycled or composted. We propose the use of compostable soy-based polyurethane foams (PU) with kenaf core modifiers that will offer the desired properties with the ability to choose responsible end-of-life decisions. The effect of fillers is a critical parameter in investigating the thermal and mechanical properties along with its effect on biodegradability. In this work, foams with 5%, 10%, and 15% kenaf core content were created. Two manufacturing approaches were used: the free foaming used by spray techniques and the constrained expansion complementary to a mold cavity. Structure-property relations were examined using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermal conductivity, compression values, scanning electron microscopy (SEM), x-ray micro-computed tomography (micro-CT), and automated multiunit composting system (AMCS). The results show that mechanical properties are reduced with the introduction of kenaf core reinforcement while thermal conductivity and biodegradability display a noticeable improvement. This shows that in application properties can be improved while establishing a responsible end-of-life choice.</p>Stable Nanocrystalline Au Film Structures for Sliding Electrical Contacts2016-06-28T16:28:55-05:00https://digital.library.unt.edu/ark:/67531/metadc849672/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc849672/"><img alt="Stable Nanocrystalline Au Film Structures for Sliding Electrical Contacts" title="Stable Nanocrystalline Au Film Structures for Sliding Electrical Contacts" src="https://digital.library.unt.edu/ark:/67531/metadc849672/small/"/></a></p><p>Hard gold thin films and coatings are widely used in electronics as an effective material to reduce the friction and wear of relatively less expensive electrically conductive materials while simultaneously seeking to provide oxidation resistance and stable sliding electrical contact resistance (ECR). The main focus of this dissertation was to synthesize nanocrystalline Au films with grain structures capable of remaining stable during thermal exposure and under sliding electrical contact stress and the passing of electrical current. Here we have utilized a physical vapor deposition (PVD) technique, electron beam evaporation, to synthesize Au films modified by ion implantation and codeposited ZnO hardened Au nanocomposites. Simultaneous friction and ECR experiments of low fluence (< 1x10^17 cm^-2) He and Ar ion implanted Au films showed reduction in friction coefficients from ~1.5 to ~0.5 and specific wear rates from ~4x10^-3 to ~6x10^-5 mm^3/N·m versus as-deposited Au films without significant change in sliding ECR (~16 mΩ). Subsurface microstructural changes of He implanted films due to tribological stress were analyzed via site-specific cross-sectional transmission electron microscopy (TEM) and revealed the formation of nanocrystalline grains for low energy (22.5 keV) implantation conditions as well as the growth and redistribution of cavities. Nanoindentation hardness results revealed an increase from 0.84 GPa for as-deposited Au to ~1.77 GPa for Au uniformly implanted with 1 at% He. These strength increases are correlated with an Orowan hardening mechanism that increases proportionally to (He concentration)1/3. Au-ZnO nanocomposite films in the oxide dilute regime (< 5 vol% ZnO) were investigated for low temperature aging stability in friction and ECR. Annealing at 250 °C for 24 hours Au-(2 vol%)ZnO retained a friction coefficient comparable to commercial Ni hardened Au of ~ 0.3 and sliding ECR values of ~35 mΩ. Nanoindentation hardness increases of these films (~2.6 GPa for 5 vol% ZnO) are correlated to microstructure via high resolution TEM and scanning electron microscope cross-sections to both Hall-Petch and Orowan strengthening mechanisms. Also presented is a correlation between electrical resistivity and grain size in the oxide dilute range based on the Mayadas-Shatzkes (M-S) electron scattering model. Using the M-S model in combination with a model describing solute drag stabilized grain growth kinetics we present a new technique to probe grain boundary mobility and thermal stability from in-situ electrical resistivity measurements during annealing experiments.</p>