Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Illuminating dynamic phenomena within organic microstructures with time resolved broadband microscopies(Montana State University - Bozeman, College of Letters & Science, 2024) Hollinbeck, Skyler Robert; Chairperson, Graduate Committee: Erik Grumstrup; This is a manuscript style paper that includes co-authored chapters.Materials derived from organic chromophore subunits are currently at the forefront of academic and industrial interest. This strong interest is driven in part by the tunability of their extant properties through engineering of both the intra-molecular and inter-molecular structure. The structure of organic materials affects optoelectronic properties because organic chromophores are sensitive to dipole-dipole and charge-transfer coupling interactions. This sensitivity presents both opportunities for tuning functional properties through designing specific packing geometries, and liabilities arising from the disruptive effects of structural disorder. Many organic materials are built from weak noncovalent interactions between chromophores, leading solid-state deposition, and crystallization to be susceptible to microscopic variations in environmental conditions. Structural heterogeneity is regularly intrinsic to organic materials, and even self-assembled systems of covalently linked chromophores exhibit defects. Ergo, in order to disentangle the effects of structural heterogeneity from the inherent properties of the material, the study of organic materials must be anchored with techniques that are capable of correlating optoelectronic properties and excited state evolution with microscale morphological characteristics. We have employed broadband pump-probe microscopies, in conjunction with steady-state and time resolved fluorescence techniques, to examine the effects of structure and coupling on excited state dynamics in solid-state organic microstructures. The study of perylene diimide (PDI) materials revealed that kinetically trapping PDI (KT-PDI) enhanced long-range ordering and led to distinct excited state evolution, delocalized charge-transfer states and long-lived charge separated species. In the MOF PCN-222, excitation energy dependent excited state behavior was observed. Pumping the first excited state (Q-band) led to immobile excited states that were relatively unaffected by local defect densities, whereas pumping the second excited state (Soret-band) led to mobile subdiffusive excited state species whose lifetimes are significantly impacted by morphologically correlated defect sites. Finally, we present progress made toward the construction and utilization of a frequency modulated-femtosecond stimulated Raman microscope, yielding spectra that resolve the location of photoinduced anion formation in KT-PDI. The work presented herein highlights broadband time-resolved microscopy as a potent tool for studying the structure-function relationship and photophysical behavior in molecular solids, deepening our understanding of how structural characteristics influence excited state evolution.Item Numerical modelling of nanoparticle diffusion and microstructure formation during selective laser melting process(Montana State University - Bozeman, College of Engineering, 2022) Alam, Taosif; Chairperson, Graduate Committee: M. Ruhul AminSelective laser melting (SLM) is a popular metal additive manufacturing technique that has a wide range of industrial applications lately. This additive process allows the development of new metal matrix nanocomposites by fusing metallic powders with nanoparticles. However, the molten pool flow generated by a moving laser heat source has complex fluid dynamics which redistribute the nanoparticles. Consequently, the microstructures of the solidified molten pool are affected by the local distribution of nanoparticles, which is reflected in their mechanical properties. Smaller grains can increase the strength and isotropic behavior of the solid layers. Therefore, the current research aims to numerically investigate the relationships among the SLM process parameters, nanoparticle transport, and microstructure evolution to explore the formation of nanocomposites. The current study formulated a three-dimensional computational fluid dynamics (CFD) model of the SLM process in a commercial software package, ANSYS FLUENT. A volumetric laser heat source model melted the aluminum alloy powders and the underlying solid substrate. The difference between the powder and the solid or liquid state of the metal alloy was defined using an effective thermal conductivity model. Lagrangian particle transport calculation was performed to track TiB 2 nanoparticles in the molten pool. This model was coupled with a 2D Cellular Automata (CA) model to simulate the solidified microstructure using MATLAB. Finally, a detailed parametric analysis was conducted to study the effects of varying laser power, scanning speed, and preheating temperature. The numerical results showed that the maximum temperature and Marangoni convection in the molten pool increased at higher laser powers, higher preheating temperatures, and lower scanning speeds. The particle-voided region was significantly large with high Marangoni convection but decreased with weaker Marangoni convection. The simulated microstructure was dominated by large columnar grains when nanoparticles were not considered. The introduction of nanoparticles disrupted the columnar grain growth by promoting small, randomly oriented, equiaxed grains. A decrease of 30%-40% in average grain diameter was measured at the cross-section of the solidified layer when nanoparticles were present. The qualitative comparison of the microstructures showed that the grains were smaller in the uniformly distributed particle region compared to the particle-voided region.Item Fiber shape effects on the compressive strength of unidirectional carbon fiber composites: a computational study(Montana State University - Bozeman, College of Engineering, 2020) Clarke, Ryan; Chairperson, Graduate Committee: David A. MillerThe tensile strength tends to be much higher than the compressive strength for carbon fiber reinforced polymer composites because of a change in failure modes. Current research activities are looking at novel precursors for reducing overall costs of carbon fiber production. The potential cost savings in new precursor carbon fiber make it economically feasible to use in large structural components. Some fiber precursors and manufacturing methods produce carbon fibers that have a kidney-shaped cross-section whereas traditional carbon fiber is circular. The aim of this study is to investigate the differences in compressive failure responses between fiber shapes in carbon fiber composites. A finite element micromechanical model was developed in ABAQUS of a single carbon fiber embedded in a square matrix with periodic boundary conditions. Two fiber cross-sectional geometries were examined: circular and kidney shaped. Three factors that affect the compressive failure response of carbon fiber reinforced polymers were investigated. These include fiber misalignment, volume fraction, and multiaxial loading. The results showed negligible differences between the compressive failure response of fibers with different cross-sectional shapes. Compressive strength was shown to have a decaying sensitivity to increasing fiber misalignment. Decreasing the volume fraction did decrease the compressive strength but also increased the compressive failure strain. In addition, adding in-plane shear loads proved detrimental to the compressive load-carrying capacity of a composite structure. This research showed minimizing fiber misalignment in manufacturing processes is only beneficial for high tolerance processes. In addition, decreasing volume fraction could be beneficial for highly flexible structures. Finally, the results demonstrated the need to minimize multiaxial loading for optimal composite compressive response.