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The Metabolic Physiology of Planarian Flatworms

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Using a high throughput closed respirometry method to measure oxygen consumption, I determined metabolic rates in asexual and sexual Schmidtea mediterranea and Girardia dorotocephala, as a function of temperature, taxon, stressors, reproductive mode, age, regeneration, and specific dynamic action. This study has shown that oxygen consumption can reliably be measured in planaria using optode closed respirometry, and also provided a reliable method for measuring wet mass in planaria, which has been a challenge to researchers in the past. This research revealed that oxygen consumption in S. mediterranea is 1.5-2.1X greater in the sexual strain over the asexual strain at 13-18°C. Within the sexual strain, oxygen consumption is 1.5 -2.2X greater in sexually mature adults over the sexually immature groups (hatchlings, juveniles, and regenerating sexuals). Furthermore, I was able to quantify differences in sexual morphology between these groups exhibiting significant differences in oxygen consumption. The results of this research supports a theory of higher metabolic costs with sexual maturity in S. mediterranea. Therefore, this study has established sexual and asexual S. mediterranea as simple, yet attractive models for investigating energetic costs between sexual and asexual phenotypes. This research also provided quantitative values for specific dynamic action in planaria, with a maximum …
Date: August 2019
Creator: Lewallen, Melissa A
Object Type: Thesis or Dissertation
System: The UNT Digital Library

The Geography of Retail Clinics Post Implementation of the Affordable Care Act

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Retail clinics are walk-in clinics designed for convenience and for servicing minor health issues and certain acute conditions. The model began as a way of bringing both convenience and care to areas that have lower levels of access to primary care resources. With the implementation of Affordable Care Act (ACA) in March 2010, populations that were previously uninsured were now required to have access to some level of health insurance. These populations presented a potential new market for retail clinics. This research shows that post implementation of the ACA, retail clinics tend to locate in areas with higher incomes and, generally, greater access to primary care.
Date: August 2019
Creator: Portillo, Ethan
Object Type: Thesis or Dissertation
System: The UNT Digital Library

489 Days

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489 Days is an animated documentary about the harrowing experiences of Egyptian-American Mohamed Soltan, who survived 16 months of hunger strike in an Egyptian prison. Caught up in the political turmoil which followed the Arab Spring uprisings, Soltan was unjustly incarcerated between August 2013 and May 2015, when the United States government intervened to release him weeks after an Egyptian court sentenced him to life in prison. The film is also the larger story of an estimated 60,000 political detainees currently held in Egypt without due process, and in violation of local and international human rights conventions.
Date: August 2019
Creator: Elmalky, Rania
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Prevalence and Proportionality of Dyslexia in Texas Public and Charter School Districts

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Passed in 1985, the Texas Education Code (TEC) §38.003, Screening and Treatment of Dyslexia and Related Disorders, required public school districts and charters to identify and provide remediation services for students with dyslexia. While Texas was the first state to pass such a requirement, the question remains: What is the prevalence and proportionality of dyslexia in Texas public school districts and charters? In Phase 1 of this study, a secondary analysis using point prevalence and disproportionality calculations was conducted to analyze the impact of this more than 30-year-old law. In order to better help understand these findings, semi-structured interviews with district leaders were conducted in Phase 2 to gain insight on how students were identified with dyslexia across the state. The results of Phase 1 showed the estimated prevalence of dyslexia in Texas public school districts and charters to be low in comparison to the literature. Additionally, the findings suggested a discrepancy in identifications between gender and district type (public school versus charter) and across racial and ethnic groups. Meanwhile, the results of Phase 2 revealed that leadership, support, funding, and accountability impact dyslexia identifications. This study emphasizes the need to further explore and analyze how to best identify and …
Date: August 2019
Creator: Sneed, Samantha
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Weapons of Mass Deception: Opacity and the Israeli Nuclear Program

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Access to nuclear technology and growing concern over the spread of nuclear weapons triggered an international debate in the 1960s that led to the creation of the Nonproliferation Treaty. Ratified in 1970, NPT was designed to prevent the horizontal spread of nuclear weapons and limit destructive uses of nuclear energy. At the same time, it also normalized the arsenals of existing nuclear states and encouraged exchanges of nuclear information, technology, and materials for peaceful purposes. Nonproliferation policy relies on a theory of the development process that identifies a nuclear frontier to locate evidence of nuclear capabilities. Absent from the proliferation model, however, are cases of covert nuclear weapons programs. For almost 50 years, it has been generally accepted that Israel is a nuclear weapon state, yet Israeli officials have never confirmed nor denied the possession of nuclear weapons. Israel has not signed NPT and has not appeared to conduct a nuclear test, in effect absolving the nuclear program's main reactor from international inspections. Uncertainty surrounding the Israeli program stems from a tradition of deliberate secrecy and deception that constitutes a national policy of opacity. This study argues that opacity has armed Israel with the privilege of nuclear immunity, exempting its …
Date: August 2019
Creator: Beattie, Kathleen E
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Universal Suicide Risk Screening in the Parkland Health and Hospital System: Evaluation of the Parkland Algorithm for Suicide Screening

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Suicide is a significant public health issue in the US. Despite national and international prioritization since 1996, little definitive progress has been made in terms of identification and intervention in cases of elevated suicide risk. Forty percent of those who died by suicide attended an emergency department within a year of death. Therefore, universal suicide risk screening in emergency departments could prove a vital component to a national suicide prevention strategy. The present study empirically evaluated the universal suicide risk screening program recently implemented at Parkland Health and Hospital System. The sample consisted of patients over 18 years of age (N=333,855; Mage=42.7, 32% male) screened as part of routine clinical care from May 4th, 2015, through November 3rd, 2015. The Parkland Algorithm for Suicide Screening (PASS) is part of a clinical decision support system for responses to Columbia - Suicide Severity Rating Scale Clinical Practice Screener (C-SSRS) items, leading to an automated clinical response via three suicide risk stratification levels: no action for no risk identified, psychiatric social worker assessment for moderate risk identified, and psychiatrist/psychologist interview for high risk identified. The present study used receiver operating characteristic (ROC) curve analysis, which found the PASS predicted disposition (z=30.46, p<.001, AUC=.78, …
Date: August 2019
Creator: Goans, Christian
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Who Does Online Dating Benefit? Association of Adult Attachment with Relationship and Sexual Satisfaction for Online Daters

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Research on associations between online dating and later relationship and sexual satisfaction is limited. The purpose of this study is to examine the association between meeting a romantic partner online or in person and later relationship and sexual satisfaction for heterosexual males and females. Main analysis results suggest that men report higher relationship satisfaction when they met their partner online and women with a preoccupied and dismissing attachment style reported higher sexual satisfaction when they met their partner online. Overall, there were few differences in relationship and sexual satisfaction for heterosexual men and women who met their partner online or in person.
Date: August 2019
Creator: Woolford, Brittany
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Disasters, Smart Growth and Economic Resilience: An Empirical Analysis of Florida Cities

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This dissertation examines the relationship between economic resilience, disaster experience, and smart growth policies at the local government level. The study is based upon three research questions that examine spatial distribution of economic resilience in Florida cities, and examines the impact of disaster experience, and smart growth policies adopted by local governments on economic resilience. Based upon the bounce-forward approach (Cowell, 2013; Klein et al. 2003), economic resilience is defined using three dimensions—economic stability, economic equity, and economic diversity. The spatial analysis is conducted by mapping economic resilience scores across 780 Census Designated Places in Florida through standard deviation method of classification, and conducting cluster-outlier analysis. Results suggest difference in economic resilience within coastal and inland communities—with higher scores mostly situated inland. East Central Florida, Tampa Bay, and South Florida were identified as high economic resilience clusters, and Northwest Florida was identified as low resilience cluster. Impact of disaster experience, and smart growth policies on economic resilience was examined based upon logic of focusing events by Birkland (1997, 2010). Data was collected from the U.S. Census, the National Climatic Data Center, and the Energy Sustainable Florida Communities Survey conducted by Florida State University in 2009. Results suggest significant association between …
Date: August 2019
Creator: Chatterjee, Vaswati
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Beyond Nothingness: A Broader Nihilism in Cinema Paradiso by Stephen Goss

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Stephen Goss composed Cinema Paradiso, a six-movement suite for solo guitar, as an homage to films and film directors. Goss cites nihilism as a theme in Dogville, the film that inspires the fourth movement, "Mandalay," but I assert that all the films and many musical devices throughout the piece can be read through the lens of nihilism. The first movement, "Paris, Texas," depicts the stark landscape of the opening scene of the 1984 Wim Wenders film of the same name. "Modern Times" chronicles Charlie Chaplin's slapstick-laden descent from the factory to the insane asylum in the opening sequence of his 1936 Modern Times. "Noir" is a tribute to the procedures of film noir: violent storylines that depict the harshness of life, dim lighting, and anti-hero characters, all accompanied by jazz. Lars von Trier's Dogville provides the movement "Mandalay" with its nihilistic meaning, but Goss writes that he invokes the musical style of Kurt Weill's opera The Rise and Fall of the City of Mahagonny. Just as the book people of François Truffaut's Fahrenheit 451 had to pass on books orally, Goss has burned the score for his "451," forcing guitarists to learn it by watching a video and listening to …
Date: August 2019
Creator: Kyzer, Dan
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Lookouts

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Lookouts follows Matt and Joann, two fire tower lookouts for the United States Forest Service, as they discuss their happiness living in seclusion as well as the decline of fire towers due to technological advances.
Date: August 2019
Creator: Dejarnett, Claire
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Extrinsic Doping of Few Layered Tungsten Disulfide Films by Pulsed Laser Deposition

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This dissertation tested the hypothesis that pulsed laser deposition (PLD) could be used to create targeted dopant profiles in few layered WS2 films based on congruent evaporation of the target. At the growth temperatures used, 3D Volmer-Weber growth was observed. Increased energy transfer from the PLD plume to the growing films degraded stoichiometry (desorption of sulfur) and mobility. Sulfur vacancies act as donors and produce intrinsic n-type conductivity. Post deposition annealing significantly improved the crystallinity, which was accompanied by a mobility increase from 6.5 to 19.5 cm2/Vs. Preparation conditions that resulted in excess sulfur, possibly in the form of interstitials, resulted in p-type conductivity. Current-voltage studies indicated that Ohmic contacts were governed by surface properties and tunneling. Extrinsic p-type doping of few layered WS2 films with Nb via pulsed laser deposition using ablation targets fabricated from WS2, S and Nb powders is demonstrated. The undoped controls were n-type, and exhibited a Hall mobility of 0.4 cm2/Vs. Films doped at 0.5 and 1.1 atomic percentages niobium were p-type, and characterized by Fermi levels at 0.31 eV and 0.18 eV from the valence band edge. That is, the Fermi level moved closer to the valence band edge with increased doping. With increased …
Date: August 2019
Creator: Rathod, Urmilaben Pradipsinh P
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Phase Transformation and Elastic Constants in Binary Titanium Alloys: An Atomistic Study

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The current understanding of the mechanical properties and deformation behavior of some individual phases in titanium alloys is limited due to the fine scale at which these phases precipitate within the β-phase matrix. The α and ω phases represent the most widely observed phases in titanium alloys depending on the alloy composition and also the heat treatment procedure adopted during processing. The possibility of precipitating ω-phase depends on the content of the β-stabilizers within the system. Although a significant compositional partitioning occurs within ω-phase upon aging treatment, the knowledge of ω-phase mechanical properties as a function of composition is very limited. The initial part of the current work focuses on the effect of common β-stabilizers elements on the phase stability and mechanical properties of the ω-phase using first-principles calculations. A relation between the bonding nature, the phase stability, and elastic properties was proposed. Thereafter αʺ martensitic phase was investigated in Ti-Nb and Ti-Nb-O alloys. The phase stability and martensitic start temperature of αʺ-phase was studied as a function of Nb and oxygen content. Also, the effect of the lattice shear distortion induced by oxygen atom on stabilizing β-phase was investigated. Subsequently the effect of the β-stabilizers' elements on stacking faults …
Date: August 2019
Creator: Salloom, Riyadh Farooq
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Application-Focused Investigation of Monovalent Metal Complexes for Nanoparticle Synthesis

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Over the last 20 years, there has occurred an increase in the number, scope, and impact of nanomaterials projects. By leveraging the Surface Plasmon Resonance of metallic nanoparticles for labelling, sensing, and treatment, researchers have demonstrated the versatile utility of these nanomaterials in medicine. The literature provides evidence of use of simple, well-known chemistry for nanomaterials synthesis when the focus is new applications of nanomaterials. A case in point, is the synthesis of metallic nanoparticles, whereby HAuCl4, CuCl2, Cu(acac)2, and AgNO3 are typically employed as nanoparticle precursors. Unfortunately, the use of these precursors limits the number of applications available to these materials - particularly for AuNPs in medicine, where the byproducts of nanoparticle synthesis (most often surface-adsorbed reductants, toxic stabilizers, and growth directors) cause nanoparticles to fail clinical trials. Despite the several thousand publications detailing the advancements in nanoparticle therapeutics, as of 2017, there were only 50 FDA-approved nanoparticle formulations. Less than 10 were based on metallic nanoparticles. This is a problem because many of these nanoparticle therapeutics demonstrate potent cell killing ability and labeling of cells. A solution to this problem may be the use of weakly coordinated, monovalent metal complexes, which require only one electron to reduce them …
Date: August 2019
Creator: Kamras, Brian Leon
Object Type: Thesis or Dissertation
System: The UNT Digital Library

How the Conflict of Autonomous and Controlled Motivation Influences Sales Controls to Inside Sales Agents' Work Outcomes

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Through the use of multiple methodologies and analytical approaches, this dissertation combines (1) sales control; (2) call center service; and (3) motivational theory to extend sales control literature beyond its current state, to consider the conflicting motivational perspectives an inside sales agent has to experience. To achieve this unification, this dissertation consists of three essays intended to: (1) identify the influence of autonomous and controlled motivation on operational sales outcome controls and performance; (2) explore the influence these motivators have on sales controls and sales performance; and, (3) understand the impact of autonomous and controlled motivation on sales agent tenure.
Date: August 2019
Creator: Conde, Gonzalo R
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Turkish Gazis' (Injured Veterans) Transition into Civilian Life

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The aim of this study was to describe the dimensions of Turkish Gazis' transition to civilian life, to explore the main factors that make this process easier or more difficult, and their psychological integration, that is, specifically, satisfaction with their life. To that end, this study examined the impact of combat-related traumatic stressor (e.g., functional limitations), personal resources (social support, sense of mastery), perceived mental health on Turkish veterans' adjustment into civilian life and their psychological integration. The data was collected in Turkey in 2015 by the researcher with the help of Türkiye Harp Malulü Gaziler Şehit Dul ve Yetimler Derneği, a non-profit veteran organization. The final sample included 240 Turkish Gazis. The level of perceived transition into civilian life and veterans' life satisfaction were the dependent variables in this study. A series of ordinary least squares (OLS) regression was conducted. Hayes' PROCESS macro 3.0 was utilized to measure the direct, indirect and moderation effects of variables on transition and life satisfaction among Gazis. The results demonstrated that perceived available social support, perceived sense of mastery and mental health partially mediated the association between Gazis' functional limitations and transition into civilian life and they fully mediated the association between functional …
Date: August 2019
Creator: Celebi, Mehmet
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Method Development for Corrosion Testing of Carbon Steel and Ni-based Alloy Coatings Exposed to Gas Hydrate Formation Environments

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Gas hydrate formation and corrosion can cause serious safety and flow assurance problems in subsea environments. One aspect that has been given less attention is the corrosion behavior of materials in gas hydrate formation environment (GHFE). This work introduces a new technique/method for corrosion testing of materials exposed to low temperatures GHFEs. This technique allows pH monitoring, and control of test conditions like temperature. In this work, GHFE is defined as an environment that includes water, methanol and its degraded products in the presence of corrosive agents like CO2 and chloride salt at gas hydrate formation temperatures (GHFT). After 20 hrs immersion in CO2-saturated salinity environment at GHFT, as-deposited Ni-Mo alloy coating has the highest corrosion resistance of 33.28 kΩ cm2. The corrosion resistance dropped to 14.36 kΩ cm2 and 11.11 kΩ cm2 in the sweet low-salinity and sweet high-salinity test solutions respectively. The combined results of SEM/EDX showed that the Ni-Mo coating oxide layer broke down quicker in sweet high-salinity environment than sweet low-salinity environment. When carbon steel was immersed in a CO2-saturated high salinity environment at GHFT, there was slight overall change in corrosion rate (CR) as salt concentration increase from 3 wt% to 25 wt%. In degraded …
Date: August 2019
Creator: Ozigagu, Christopher E.
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Electrochemical Deposition of Metal Organic-Modified-Ceramic Nanoparticles to Improve Corrosion and Mechanical Properties

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Corrosion is an unstoppable process that occurs spontaneously in many areas of industry, specially, oil and gas industries. Therefore, the need of developing protective coating to lower the cost of corrosion is very consistent. Among different methods, electrodeposition has been a popular method since it offer many advantages such as low cost, ability to control the surface and thickness of the coating, ability to perform at low temperature and pressure, and very convenience. Ceramic nanoparticles have been widely incorporated into metal coating and used as a protective layer to improve both corrosion and hardness properties. Diazonium synthesis was used to modify cerium oxide nanoparticles by grafting with ferrocene for use in nickel nanocomposite coating. Citric acid and citrate salt were used as stabilizing ligands for yttrium oxide and praseodymium oxide nanoparticles in nickel plating solution to prevent the formation of hydroxide, thus, higher amount of nanoparticles was able to incorporate into nanocomposite coatings. These fabricated coatings were evaluate for the corrosion and mechanical properties using many different instruments and electrochemical techniques. As modified cerium oxide, stabilized yttrium oxide or praseodymium oxide added into nickel coatings. The results showed an increase in hardness and corrosion resistance leading to the overall improvement …
Date: August 2019
Creator: Ngo, Ngan Kim
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Optimizing Production System Maintenance Policies when Cyber Threats are Considered

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In a production environment, physical and cyber-related failures become unavoidable because of the complexity of highly connected manufacturing systems and a finite equipment life cycle. The main purpose of this thesis is to investigate optimal maintenance outsourcing and replacement policies in the presence of cyber-threats, as well as policies to achieve channel coordination via cost subsidization. Although well-developed maintenance outsourcing literature has addressed many advanced and trending issues such as the costs and benefits of new technology adoptions, learning effects, forgetting effects, and systems with back-up machines, no study has looked at the effect of cyber threats on connected production systems. Besides filling this gap, this thesis addresses the most common replacement policies including preventive maintenance with minimal repairs and age replacement. The advent of the Internet of Things (IoT) has enabled the creation of "smart" manufacturing systems, However the resulting connected format makes these systems potential targets for cyber-attacks. Manufacturers have to face a difficult decision as to whether they should adopt costly security technologies or let the manufacturing systems be vulnerable to cyber-threats. This study develops a model addressing this dilemma by providing some insights into the effects of cost subsidization and installation of security systems on the …
Date: August 2019
Creator: Ta, Anh V
Object Type: Thesis or Dissertation
System: The UNT Digital Library

A Novel Two-Stage Adaptive Method for Estimating Large Covariance and Precision Matrices

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Estimating large covariance and precision (inverse covariance) matrices has become increasingly important in high dimensional statistics because of its wide applications. The estimation problem is challenging not only theoretically due to the constraint of its positive definiteness, but also computationally because of the curse of dimensionality. Many types of estimators have been proposed such as thresholding under the sparsity assumption of the target matrix, banding and tapering the sample covariance matrix. However, these estimators are not always guaranteed to be positive-definite, especially, for finite samples, and the sparsity assumption is rather restrictive. We propose a novel two-stage adaptive method based on the Cholesky decomposition of a general covariance matrix. By banding the precision matrix in the first stage and adapting the estimates to the second stage estimation, we develop a computationally efficient and statistically accurate method for estimating high dimensional precision matrices. We demonstrate the finite-sample performance of the proposed method by simulations from autoregressive, moving average, and long-range dependent processes. We illustrate its wide applicability by analyzing financial data such S&P 500 index and IBM stock returns, and electric power consumption of individual households. The theoretical properties of the proposed method are also investigated within a large class of …
Date: August 2019
Creator: Rajendran, Rajanikanth
Object Type: Thesis or Dissertation
System: The UNT Digital Library

A Global Spatial Model for Loop Pattern Fingerprints and Its Spectral Analysis

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The use of fingerprints for personal identification has been around for thousands of years (first established in ancient China and India). Fingerprint identification is based on two basic premises that the fingerprint is unique to an individual and the basic characteristics such as ridge pattern do not change over time. Despite extensive research, there are still mathematical challenges in characterization of fingerprints, matching and compression. We develop a new mathematical model in the spatial domain for globally modeling loop pattern fingerprints. Although it is based on the well-known AM-FM (amplitude modulation and frequency modulation) image representation, the model is constructed by a global mathematical function for the continuous phase and it provides a flexible parametric model for loop pattern fingerprints. In sharp contrast to the existing methods, we estimate spatial parameters from the spectral domain by combining the exact values of frequencies with their orientations perpendicular to the fingerprint ridge flow. In addition, to compress fingerprint images and test background Gaussian white noise, we propose a new method based on periodogram spacings. We obtain the joint pdf of these m-dependent random variables at Fourier frequencies and derive the asymptotic distribution of the test statistic.
Date: August 2019
Creator: Wu, Di
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Understanding the Micromechanism of Cyclic Loading Behavior of Ultrafine Grained Alloys

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In the current study, we have investigated the cyclic loading behavior of conventional as well as novel alloy system exhibiting fine and ultrafine-grained structure. While in case of conventional alloy systems (here aluminum alloy AA5024), the effect of three different grain sizes was investigated. Improvement in fatigue properties was observed with decreasing grain size. The unique microstructure produced via Friction stir processing was responsible for the improved fatigue response. Additionally, microstructures consisting of a high fraction of special boundaries within the fine and ultrafine-grained regime were also subjected to cyclic loading. The hierarchical features introduced in the eutectic high entropy alloy deflected the persistent slip bands, responsible for fatigue cracking, thus resulted in delayed crack initiation and improved fatigue life. The selective nature of fatigue was learnt in the fine grain Al0.5CoCrFeNi, where the introduction of hierarchical features did not result in improved fatigue properties. The weak links in the microstructure, while not affecting the tensile properties, got exposed during cyclic loading. Further study on the medium entropy alloy revealed the inherent reason for the improved fatigue properties. The medium entropy alloys utilized the benefit of UFG single-phase FCC matrix. The UFG matrix showed signs of transformation of FCC phase …
Date: August 2019
Creator: Shukla, Shivakant
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Ceramide Biosynthesis and NEET Proteins Impact Development, Function, and Maintenance of the Caenorhabditis elegans Germline

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I used the C. elegans genetic model to examine the role of ceramide biosynthesis (sphingolipid pathway) and iron regulation and found that each process impacts germline development and function. Using a sphingolipid specific antibody mAb15B4, I found that sphingolipids are associated with germ granules (P granules) within C. elegans and zebrafish; thus, suggesting conservation of macromolecules associated with germ granules. Phenotype analysis of ceramide biosynthesis mutants in C. elegans revealed that this pathway is essential for normal germline function in the aging adult hermaphrodite; specifically, precocious germline senescence was observed. Furthermore, I found that disruption of ceramide biosynthesis, via the hyl-2 deletion mutation, negatively impacts mAb15B4 localization at the P granules. Through genetic suppression analysis, I determined that insulin signaling and lipid biosynthesis can modulate the mAb15B4 localization to P granules. Additional, phenotype analysis showed that ceramide biosynthesis dysfunction decreased fecundity, and led to germline structure defects and uterine tumors. Through suppression analysis, I determined that modulation of the insulin signaling pathway suppressed the precocious germline senescence due to ceramide biosynthesis dysfunction. Since the presence of uterine tumors is associated with reproductive senescence I concluded that ceramide biosynthesis has a role in germline maintenance in the aging of the germline …
Date: August 2019
Creator: King, Skylar Dawn
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Phase Transformations in Refractory High Entropy Alloys

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High entropy alloys (HEAs) based on refractory elements have shown a great potential for high temperature structural applications. In particular, the ones containing Al, exhibits a microstructure similar to the γ-γ' in Ni-based superalloys. While these alloys exhibit impressive strengths at room temperature (RT) and at elevated temperatures, the continuous B2 matrix in these alloys is likely to be responsible for their brittle behavior at RT. Phase stability of five such alloys are studied by thermo-mechanical treatments and characterization techniques using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two of these alloys showed an inverted microstructure, where the disordered BCC phase becomes continuous, and therefore, they were characterized in detail using SEM, TEM, atom probe tomography (APT) and synchrotron x-ray diffraction experiments. The phenomenon of phase inversion lead to a better combination of strength and ductility as compared to the non-inverted microstructure.To enhance the stability of B2 intermetallic phase which provides the strength when present in a BCC matrix, multicomponent B2 phase compositions stable at 1000°C in some of the above studied alloys, were melted separately. The aim was to establish a single phase B2 at 1000°C and understand the mechanical behavior of these single-phase multicomponent B2 intermetallic …
Date: August 2019
Creator: Soni, Vishal
Object Type: Thesis or Dissertation
System: The UNT Digital Library

Dynamic Deformation and Shear Localization in Friction-Stir Processed Al0.3CoCrFeNi and Fe50Mn30Co10Cr10 High-Entropy Alloys

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High entropy alloys (HEAs) are a relatively new class of solid solution alloys that contain multiple principal elements to take advantage of their high configurational entropy, sluggish diffusion, lattice distortion, and the cocktail effect. In recent development, work hardening mechanisms known as twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) have been found active in Al0.3CoCrFeNi (molar fraction) and Fe50Mn30Co10Cr10 (at %) HEA compositions. Friction-stir processing was done to increase the mechanical properties and improve the microstructure of the alloys for the purpose of high strain rate performance. Quasi-static tensile tests as well as top-hat geometry Split-Hopkinson pressure bar tests were conducted to view the mechanical properties as well as view the microstructural evolution at dynamic strain rates. Overall, the Al0.3CoCrFeNi condition after friction-stir processing and heat treatment has proved to have the best mechanical properties, and selecting from the conditions in this study, Al0.3CoCrFeNi has better shear localization resistance.
Date: August 2019
Creator: Macdonald, Neil
Object Type: Thesis or Dissertation
System: The UNT Digital Library