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Evaluation of Salt Tolerance at the Seedling Stage and Genetic Diversity in USDA Tomato (\u3ci\u3eSolanum lycopersicum\u3c/i\u3e) Germplasm, and Genomic Insights into Leafminer Resistance and Tallness in Spinach (\u3ci\u3eSpinacia oleracea\u3c/i\u3e) through Genome-Wide Association Studies (GWAS) and Genomic Prediction Approaches
No full textGenetic diversity and stress tolerance mechanisms in tomato (Solanum lycopersicum) and spinach (Spinacia oleracea) were explored through advanced genomic tools, focusing on salt tolerance in tomato and leafminer resistance and plant height in spinach. Using genome-wide association studies (GWAS) and genomic prediction (GP), this research uncovers key insights into the genetic architecture of these traits, paving the way for improving crop resilience. In tomato, the evaluation of 71 and 280 USDA accessions under saline stress identified genotypes with significant salt tolerance. Population structure analysis revealed three genetic groups, emphasizing the influence of domestication on diversity. These findings provide a strong foundation for breeding salt-tolerant tomato varieties tailored to regions affected by salinity. For spinach, GWAS identified significant single nucleotide polymorphism (SNP) markers associated with leafminer resistance and tallness. Candidate genes related to pest resistance and height were identified, offering valuable targets for breeding programs. Genomic prediction models showed high accuracy in selecting resistant and optimized genotypes, supporting the acceleration of spinach breeding efforts. The integration of genomic technologies with traditional breeding approaches enhances the ability to develop more resilient and productive crop varieties. These findings offer critical contributions to sustainable agriculture, particularly in addressing challenges posed by salinity stress and pest infestations
Interface, Thermal, and Mechanical Properties of Multidimensional Carbon-Based Nanomaterials
No full textThere is a broad range of carbon-based nanomaterials with different dimensions and mechanical properties. Among these are 3D carbon nanotubes (CNTs), 2D cyclo[18]carbon (C18) rings, and 1D carbyne chains. Carbyne has the highest stiffness, but it is highly reactive when not encapsulated. This dissertation uses molecular dynamics (MD) and density functional theory (DFT) to investigate the interface, thermal, and mechanical properties of these low dimensional carbon materials. In this dissertation, MD is used to predict the mechanical and thermal properties of carbon-based materials including carbyne, CNTs, C18 rings, and hybrid C18-carbyne structures. DFT is utilized to investigate interface properties of carbyne on Ni and Cu(111) surfaces and validate properties of the carbyne on these surfaces predicted by MD. Based on the predicted interface, thermal, and mechanical properties of the materials, this dissertation also focuses on the low dimensional materials’ possible use as components of nanocomposites and their high temperature mechanical properties. Cu matrix nanocomposites with the 1D carbyne and 1D-2D C18-carbyne are developed and the mechanical properties of the nanocomposite under tensile loading are predicted. Then, cumulenic and polyynic carbyne are pyrolyzed to determine how heating the chains affects their mechanical properties. During investigation of the carbyne’s interface properties, it is observed that carbyne does not maintain cumulenic or polyynic bonding on Ni(111) without dielectric screening via graphene or CNTs, while the chain maintains cumulenic bonding on the Cu(111) surface. When compared to the other low dimensional materials, carbyne requires the largest force to fracture during tensile testing and has the highest thermal conductivity. The thermal conductivity of these low dimensional materials on a Cu surface is also studied, and carbyne again has the highest thermal conductivity. Metallic interface effects on the low dimensional materials’ mechanical properties are also investigated. During comparison of the mechanical properties of the low dimensional materials on Ni and Cu(111) surfaces during three-point bending, carbyne encapsulated in a CNT is the stiffest structure on both metals and shields the carbyne from the Ni surface. We then predict the mechanical properties when the low dimensional materials are embedded in a Cu matrix. Cu matrix nanocomposites with carbyne and C18-carbyne are generated and studied at room and elevated temperatures. The Cu-C18-carbyne nanocomposite has the highest elastic modulus at 300K due to its 2D ring interface with the Cu and thus high adsorption energy. However, at 600K and 900K, the 1D carbyne placed in a line defect of the Cu has the highest elastic modulus due to its smaller displacement of Cu atoms in the matrix. It is also determined that pyrolysis can increase carbyne’s resistance to compression due to the formation of graphitic nanostructures at high temperatures. The thermal and mechanical properties of carbon-based materials are highly dependent on their dimensions, interfaces, and bonding. This study demonstrates that carbyne and its hybrid structures are impressive materials with a promising outlook for future studies and applications
Wind Tunnel Investigation of the Along-wind Dispersion in Finite-Duration Neutrally Buoyant Gas Releases
Get PDFThis thesis investigates the along-wind dispersion of hazardous gas releases within a turbulent boundary layer using controlled experiments in an ultra-low-speed wind tunnel, recognizing the limitations of numerical modeling and field tests. This study utilizes a neutrally buoyant gas mixture and dual Flame Ionization Detectors (FIDs) to capture representative, repeatable data on gas cloud behavior over finite durations. Ensemble averaging of 65 trials across varying release durations and downwind distances provided detailed insights into gas dispersion and the distinct time phases within finite-duration releases. Analysis revealed that along-wind dispersion coefficients depend on turbulence and vertical wind shear, with normalized coefficients demonstrating consistent trends. The results underscore the challenges in capturing reliable time distribution coefficients and highlight the effectiveness of wind tunnel experiments in assessing hazardous gas behavior. This work contributes to improved dispersion modeling approaches, enabling better prediction of hazardous gas release impacts and enhancing industrial safety protocols
Guided Pathways: How Early Major Declaration Impacts Student Graduation Rates
Get PDFIncreasing graduation rates continues to be a high priority for administrators at community colleges nationwide. While several factors affect graduation rates, many researchers focus on how a student’s status as declared or undecided may impact their likelihood of graduating. This study aims to examine the potential relationship between matriculating with or without a major declared and degree completion. There exists a perception that an early decision about a major is a critical step in ensuring students graduate. Many institutions require or strongly encourage students to declare a major before enrolling in their first semester. Previous research about undeclared students and degree completion is lacking and dated. Conceptually framed within Astin’s (1993) input–environment–output model, logistic regression analyses will be conducted using institutional records and National Student Clearinghouse data for the Fall 2016 cohort at a community college in Oklahoma
High Performance Silicon Carbide based Converters for Powertrain Electrification
Get PDFWith the increasing concern towards environment protection, electric vehicles (EVs) and hybrid electric vehicles (HEVs) have become popular in recent years. They not only help reduce carbon emissions, but also come with other benefits such higher efficiency and better user experience. Many countries and vehicle manufactures have made their plans to keep up with this trend in vehicle electrification. Powertrain is an assembly of components in a vehicle that pushes it forward. As for EVs and HEVs, their powertrains are composed of one or more power electronics converters, which are significant to the system. Silicon carbide MOSFETs have shown superior characteristics and thus are being widely used in different applications including EVs and HEVs. This dissertation focuses on key technologies to improve the performance of high-power SiC converters in EV and HEVs’ powertrain systems. The architecture of this dissertation can be divided into two major parts according to the targets investigated. The first part of the dissertation centers around the power converter topology in powertrain system, including DC-DC boost converter and inverter. The second part investigates two different current balancing methods to help realize balanced current sharing between parallel connected SiC MOSFETs, which is often used to increase power rating of converters. A DC-DC converter is applied in some EV/HEV powertrains to boost the battery voltage for the DC bus of inverter. Light load scenario happens frequently in EV/HEVs’ driving profile, but the efficiency of conventional boost converter is low in that range. Therefore, a composite DC-DC converter topology is proposed in chapter 2 to enhance the light load efficiency. A multi-variable optimization method during the converter operation is also studied for the same purpose. A 30-kW composite converter prototype was built and the light load efficiency improvement brought by the proposed optimization method has been validated in experiments. Inverter is another key power electronics converter in the electrified powertrain system. Soft switching technology may bring multiple benefits such as higher efficiency and reduced electromagnetic interference issue. The auxiliary resonant commutated pole (ARCP) soft switching inverter topology has been selected for high power motor drive applications. Simulations study and an optimization design method are presented in chapter 3. A 10-kW ARCP soft switching inverter was assembled. Double pulse test and R-L load continuous have been performed to validate the soft switching functionality. The ARCP inverter has been tested to have higher efficiency while lower voltage slew rate compared to the conventional hard switching inverter. A hybrid closed-loop current balancing method is proposed in chapter 4. Causes of steady-state and transient imbalance current issues are studied via simulation. The steady steady-state imbalance current is first suppressed by inserting inductors between AC output of paralleled half-bridge modules and the load. As for heavy-duty electrified vehicles which have large profiles, the inserted inductors can be implemented by the self-inductance of power cables. The remaining transient imbalanced current can be thereby sensed easily during the steady state with conventional low-cost sensors. Based on this, a closed-loop control by regulating modulation reference signals of different paralleled phase legs is introduced to further balance the remaining current differences. The proposed method has been applied to high power inverter with 2 and 4 SiC modules paralleled in each phase. A good current sharing has been observed in both cases. Active gate drivers have been widely investigated to regulate switching performance of SiC MOSFETs, which could also be adopted for current balancing purposes. An active gate drive control method is proposed in chapter 5 for steady-state current balancing, which is realized by controlling gate voltages in PWM scheme to regulate the equivalent on-state drain-to-source resistance of paralleled MOSFETs. A time delay is applied to different paralleled MOSFETs suppress the transient unbalanced current together with the proposed method. An active gate driver board was assembled and tested. The proposed method shows good robustness when tested with different number of SiC modules paralleled and at different temperatures. The currents and losses are balanced among all the parallel connected SiC modules on all the testing cases. The total losses in tests using proposed active gate driver method are not increased when compared to losses in tests using conventional gate driver method
Exploring the Effects of Mindful Nature Walks on College Student Anxiety
Get PDFIndividuals pursuing a degree in higher education face challenges that lead to greater levels of anxiety than that of the general U.S. adult population. This disparity, further heightened by the COVID-19 pandemic, presents a need for higher education institutions to search for ways to address this spike in student anxiety and other mental health concerns. This mixed-methods study explored mindful nature walks as a low-cost and easily accessible way for college students to reduce levels of experienced anxiety. Using validated quantitative measures, this study examined the impact of three weekly, consecutive 30-minute mindful nature walks on state and general anxiety levels experienced by a sample of 11 students at the University of Arkansas. Follow-up interviews with five of these participants qualitatively investigated participants’ experiences with the intervention and their feedback for future integration. Quantitative results reveal a statistically significant reduction in state anxiety and a non-significant reduction in general anxiety. Qualitative results further strengthen these findings with participants’ positive experiences with this intervention and present their desire for future integration of mindful nature walks into university programs, educational efforts, and mental health services
Comparing Children’s Engagement in Storybook Reading with and without Access to AAC with a Literacy Supportive Feature
Get PDFPurpose: This thesis evaluates the effects of comparing children’s engagement in storybook reading with and without access to high-tech augmentative and alternative communication (AAC) with a literacy supportive feature. Literacy skills are a fundamental cornerstone for engagement in the lives of young children. Among these skills, the ability to decode words, or phonetically sound them out, plays a pivotal role in the acquisition of literacy. Proficiency in literacy also broadens the communication capabilities of individuals with developmental disabilities who rely on augmentative and alternative communication (AAC) methods. However, existing AAC technologies currently fall short in adequately strengthening literacy development, particularly in terms of decoding skills. Specially, for the very individuals who depend on them. Hence, the present study\u27s primary aim was to conduct an initial assessment of the overall impact of a new AAC feature designed to bolster decoding abilities on engagement within a common literacy activity for children – shared book reading. Method: The study involved two young children who presented with challenges of both functional speech and literacy skills. One participant had a diagnosis Costello syndrome, while the other had autism spectrum disorder (ASD). The research utilized a single-subject approach, utilizing an ABAB design. In Condition A, participants did not have access to AAC. This was both of the participants current standard of care. In Condition B, participants had access to AAC designed to support foundational literacy. Results: Each participant demonstrated higher engagement with access to AAC. Specifically, they participated in the linguistic routines of the activity more often in the condition where participants had access to AAC. Conclusion: These findings provide initial indications that AAC with a technology feature offering decoding models when AAC picture symbols are selected can be supportive of engagement in literacy activities for young children with developmental disabilities. Specifically, those who have limited speech and language. It is important to note that this feature is not meant to substitute formal instruction but is a tool to enhance literacy for individuals with developmental disabilities who rely on AAC. It is also important to note that this feature should not be used in any instance where it could reduce linguistic participation. In the current study, AAC with the literacy feature supported rather than reduced participation. Future larger scale research is needed