Dissertation Defense Announcements

Physical activity offers a range of health benefits but can be difficult to initiate and maintain. Self-regulatory processes are one route to understanding health behavior change. While affective mechanisms like positive affect and reward processing provide a valuable neurobiological pathway to elucidate individual motivations for physical activity. Theories and models that emphasize the role of affect and intrinsic and extrinsic motivations are applied in the current study to deepen our understanding of physical activity behaviors. Psychosocial sources of individual motivations, such as improving mood or changing one’s physical appearance, can provide insight into ways to alter affective-cognitive mechanisms to encourage sustainable physical activity behaviors. The present study applied a mixed-method approach to elucidate themes of affect and motivation in social media content. A sample of 2,585 Twitter posts were collected in mid-July 2022. A motivational and health behavior change qualitative codebook was first developed to guide thematic coding analyses. Thematic coding results revealed a high frequency of extrinsic motivation and goal and change-oriented facilitators of physical activity. Intrinsic motivation included the highest percentage of positive attitudes compared to other motivation types. Health-oriented themes, satisfaction, dissatisfaction with physical appearance, and weight loss were also relevant. LIWC-22 analyses supported the role of positive affect and informed health themes. BERT topic modeling analyses provided overarching physical activity topic themes for motivation and physical activity. Interpretations of the current results were presented, and future directions were suggested.

Nanotechnology has the potential to revolutionize various fields, addressing complex issues such as cancer treatment, waste remediation, and energy storage. To achieve this, precise engineering of nanocrystals at the atomic level is essential, going beyond mere control of size and shape. The unique properties of nanomaterials, which differ from their bulk counterparts, are influenced by surface chemistry, defects, and local structure. These characteristics are determined by the synthesis methods used, making a deep mechanistic understanding of these processes crucial for engineering nanoscale structures and properties.

To contribute to the rapidly evolving field of nanoscience, this dissertation focuses on the solution-based synthesis of vanadium oxide nanocrystals. Vanadium oxides are promising candidates for applications in catalysis, sensing, cathode materials for high-density lithium batteries, smart windows, neuromorphic computing, and optical switching. However, vanadium oxides exhibit multiple oxidation states (+2 to +5) and polymorphs. Consequently, the colloidal synthesis of high-quality vanadium oxide nanocrystals in a specific oxidation state and stoichiometry remains challenging.

This dissertation advances the synthesis of vanadium oxide nanocrystals, emphasizing the effects of synthetic parameters on their oxidation state and crystal structure. Key findings include the successful synthesis of anosovite V₃O₅ nanocrystals via a hot-injection method, marking the first colloidal synthesis of this rare phase from a readily available precursor. By adjusting vanadium precursor-to-alcohol-to-amine ratio, controlled reduction of vanadium was achieved to selectively synthesize V₃O₅ and V₂O₃ nanocrystals. The dissertation also presents an alcohol-mediated valence-state controlled synthesis method for selective preparation of pure corundum-structured V₂O₃ and anosovite V₃O₅ nanocrystals. Comprehensive characterization, including spectroscopic ellipsometry and diffuse reflectance spectroscopy, reveals unique optical properties deviating from bulk behavior, attributed to the nanoscale size effects. In addition, a heat-up method was developed for synthesizing VOx nanocrystals by thermal decomposition of vanadyl acetylacetonate, demonstrating the formation of vanadium monoxide nanocrystals. The reaction pathways for the formation of these nanocrystals via hot-injection method and heat-up methods were analyzed with ATR-FTIR spectroscopy. The findings will advance the fundamental understanding of vanadium oxide nanocrystal synthesis and pave the way for their application as advanced functional nanomaterials.

The invention of advanced Ni-containing concentrated solid solution alloys has significantly broadened the compositional space of alloy design. Unlike conventional alloys that typically consist of a principal solvent element with minor additions of various solute atoms, concentrated solid solution alloys involve multiple elements in equal or near-equal compositions. These concentrated solid solution alloys have exhibited remarkable properties such as exceptional toughness and superior radiation resistance. The exceptional performance of these concentrated solid solution alloys is generally attributed to specific intrinsic properties of concentrated solid solution alloys such as high entropy effect, severe lattice distortion and short ordering effect. However, being relatively new emerging materials, the theoretical understanding and experimental exploration of these alloys are still ongoing and not comprehensively understood.

This dissertation work will provide a systematic investigation on surface properties including mechanical properties and radiation damage of Ni-based concentrated solid solution alloys by using ex-situ and in-situ indentation techniques. The study explores the surface properties of a batch of Ni-containing concentrated solid solution alloys with addition of different 3d transition elements including binary NiCo, NiFe, Ni80Cr20, Ni80Mn20 and quaternary NiCoFeCr. First, initial investigations use ex-situ nanoindentation to obtain mechanical properties of five alloys including hardness, elastic modulus and strain rate sensitivity. A complete methodology is developed to acquire accurate property information directly from nanoindentation in a high throughput manner, which considers the indentation size effect and pile-up effect. Furthermore, the strengthening in Ni-concentrated solid solution alloys is attributed to being driven by solid solution strengthening induced by mismatch in atomic size. Notably, the intrinsic properties of alloying elements play a more critical role in strengthening than the number of alloying elements. Second, based on previous work, nanoindentation is further employed to evaluate the early-stage irradiation induced hardening in NiCo, NiFe and NiCoFeCr. It proposes using nanoindentation to detect early-stage irradiation-induced defects and hypothesizes that interactions between these defects and dislocations carried by deformation during indentation can quantify irradiation-induced defects. This approach successfully quantifies irradiation-induced hardening in NiCo, NiFe, and NiCoFeCr. Quantitative analysis reveals that irradiation-induced defects harden NiFe and NiCoFeCr, but no significant hardening is observed in NiCo. Additionally, irradiation-induced hardening is associated with the evolution of geometrically necessary dislocations and is interpreted by changes in the plastic zone during indentation. Finally, in-situ flat-punch indentation provides real-time observations of deformation behaviors, aiming to derive accurate stress-strain curves. A new protocol addresses thermal drift effects and contact issues, often neglected in measurements.

This comprehensive study on Ni-based concentrated solid solution alloys enhances understanding of their mechanical properties and irradiation resistance by using ex-situ and in-situ nano-mechanical techniques. Methodologies are developed for nanoindentations to acquire meaningful property information directly from surface in a high throughput manner. This systematic work offers valuable insights into the strengthening mechanisms and irradiation hardening mechanisms of Ni-based concentrated solid solution alloys. The robust experimental evidence supports that the exceptional properties of concentrated solid solution alloys are not solely determined by the number of elements but also determined by the intrinsic performance of alloying elements. These results provide new insights for alloying design strategy of concentrated solid solution alloys.

Approximately 30% of individuals with autism have complex communication needs (CCN). These individuals are unable to use vocal speech as their primary form of language and typically require support across several areas of communication such as comprehension, pragmatics, phonology, semantics, and syntax (Ganz et al., 2022; Reichle, 2019). Researchers have found that communication skills can greatly impact academic, behavioral, social, and postsecondary outcomes (Carter et al., 2012; Chiang, 2008; Matson et al., 2013; Park et al., 2012; Pillay & Bronlow, 2017). Fortunately, augmentative and alternative communication (AAC) has been effectively used to increase communication for individuals with intellectual and developmental disabilities (IDD; Crowe et al., 2022). Most often, individuals with autism are only taught to request using single words or short phrases using AAC devices (Ganz et al., 2017; Muharib et al., 2018; Tincani et al., 2020). Another way to expand communication through AAC is to teach sentence structure. Researchers have used an intervention package consisting of response prompting, sentence frames, and technology like AAC to teach students with autism and CCN to construct sentences (Pennington et al., 2021; Pennington, Flick, et al. 2018; Pennington, Foreman, et al. 2018; Pennington & Rockhold, 2018). Additionally, matrix training has been used as a generative framework to increase language for individuals with autism who use vocal speech (Frampton et al., 2016, 2019; Jimenez-Gomez et al., 2019; Kohler & Malott, 2014) and AAC (Marya et al., 2021; Naoi et al., 2006; Nigam et al., 2006; Tönsing et al., 2014). This study examined the effects of matrix training, response prompting, and sentence frames on sentence writing for four students, ages 10–18 with ASD and CCN in a specialized private school located in the southeastern United States. Three teachers, ages 23–46 served as the interventionists in the study. A series of A-B designs with modifications was used to examine the effects of the intervention package on the percentage of trained and untrained correct sentences, percentage of subject-verb combinations, and the percentage of correct word selections. Teachers presented photos of subject-verb combinations for students to write about using pre-programmed arrays with words and symbol supports on speech-generating devices. Overall, results indicated that across all interventions, there were no effects on the percentage of trained and untrained correct sentences and subject-verb combinations for all participants. Two students, however, increased their percentage of correct word selections. Overall, teachers found the intervention acceptable and beneficial for students in the classroom. Furthermore, three of four students preferred this writing intervention over their typical writing instruction in the classroom. Implications of this study provide several considerations for practitioners who would like to use matrix training to teach subject-verb combinations and/or sentence writing with students who have autism and CCN.