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2060 results
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1 - 10 of 2060
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ItemDesign of Materials Tolerant to Dynamic Tensile Spall Failure(Georgia Institute of Technology, 2024-12-16) Frawley, Keara G.Materials tolerant to dynamic tensile or “spall” failure are of interest for applications involving high-velocity impact and blast loading. Metals and polymers generally have favorable responses to such extreme conditions and are therefore useful materials in shock- absorbing applications, such as the automotive industry, body armor, or other protection and shielding devices. The complex stress states and high strain rates involved in events leading to spall failure are typically different from the conditions under which most material testing is conducted to determine mechanical properties. Hence, it has been difficult to predict how spall strength, i.e., resistance to dynamic tensile failure, relates with typical mechanical properties such as hardness, toughness, strength, and moduli. This work focuses on utilizing machine learning (ML) to determine the relationship between these key properties and spall strength, with the goal of developing predictive models and a better understanding of the spall response behavior of metals, alloys, and polymers. Various methods were utilized to generate databases of spall strengths and key properties of metals and polymers through literature surveys and experiments. Sources included peer-reviewed journal articles and gas gun plate-on-plate impact experiments. Data analytic methods, such as the Pearson correlation and the coefficient of determination, were used to correlate the properties to spall strength. The first main result of this work is a model that predicts the spall strength of metals and alloys. The study provides design guidelines for efficiently screening metals based on commonly available mechanical properties that most influence the spall strength values, minimizing reliance on intensive experimental procedures. Furthermore, the model has been extended to predict the spall strength values of a class of complex alloys for which there is limited data available: high entropy alloys (HEAs). The second main result is a database of the spall strengths of 23 unique polymers, either experimentally determined in this work or available in the literature. This database also includes the available mechanical and physical properties of the various polymers, and correlates those to predict the spall strength based on a physically-based energy balance model available in the literature. Additionally, an initial exploration using Molecular Dynamics (MD) calculations was conducted on a simple polymer, polyethylene, to evaluate how this computational approach might perform across a broader range of polymers. By learning more about the influence of material properties on the spall strengths of different classes of materials, we can better understand and predict the spall response of untested materials.
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ItemAluminumnano-Layersuperconducting Thin Films(Georgia Institute of Technology, 2024-12-16) Dadkhah, ShohrehThe fascinating properties of superconductors have led to widespread use in applications such as quantum computing, MRI machines, and particle accelerators. However, understanding how factors like material thickness influence these properties remains a key challenge in the field. We focus on investigating the relationship between the thickness of aluminum nanolayers in superconductors and their corresponding resistivity and critical temperature. Aluminum, being a well-characterized low-temperature superconductor, provides an ideal model system for exploring these effects. By systematically varying the thickness of the aluminum films and conducting comprehensive microstructural and electrical characterization, this study aims to elucidate the fundamental mechanisms that govern superconductivity in thin films. The research presented in this thesis not only contributes to the fundamental understanding of superconductivity but also has practical implications for the design and optimization of superconducting materials for technological applications. By advancing our knowledge of how thickness and microstructure affect the superconducting properties of aluminum-based films, this work lays the groundwork for future innovations in both low and high-temperature superconducting technologies.
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ItemPrecise Synthesis, Characterization, and Applications of Nonlinear Polymers with Varied Architectures(Georgia Institute of Technology, 2024-12-05) Choi, WoosungThe precise synthesis of non-linear polymers with desired architectures is important in polymer science due to their unique properties compared to the linear counterparts, including superior rheological properties, abundant terminal groups, and potential for novel applications. Controlled radical polymerization (CRP) offers significant advantages in the synthesis of non-linear polymers owing to the versatility in monomer selection, end-group functionality, and precise structural control. However, there are still challenges and complexity in precisely synthesizing non-linear polymers via CRP techniques, including inter-/intramolecular radical coupling, non-uniform chain growth, and difficulty in accurate characterization. This thesis aims to carefully design and craft non-linear polymers with varied architectures, thoroughly characterize and understand their properties, and explore their unique functionalities for their potential use in small molecule detection, sustainable materials development, and stimuli-responsive recyclable emulsifiers. First, we demonstrated how carefully designed non-linear polymers can be advantageous towards their linear counterparts by crafting nanometer-scale Janus star polymers without the need for self-assembly. The importance and difficulties in characterization were discussed. Then, we explored in what manner non-linear polymers with precisely controlled architectures can serve as unimolecular templates to grow stable plasmonic nanocrystals with intriguing morphology by utilizing carefully designed bottlebrush-like diblock copolymers to render polymer-ligated gold nanostars. We showed that our polymer-ligated gold nanostars exhibited improved colloidal and morphological stability. Finally, a novel strategy to render degradable yet mechanically robust polymer films was developed by leveraging the multifunctionality of star polymers. We showed that such degradable and crosslinkable star polymers could find their potential applications in advanced packaging materials with further optimization. Therefore, the goal of this thesis is to address the importance and challenges in the precise synthesis of non-linear polymers by demonstrating their unique functionalities and applications originating from their distinct and well-controlled architectures. This work advances polymer engineering through careful design and understanding of various non-linear polymers while exploring their novel applications to address contemporary challenges in materials science and technology.
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ItemMolecular Beam Epitaxy Synthesis and Characterization of Indium Selenide Films(Georgia Institute of Technology, 2024-12-03) Voigt, Cooper AugustusIn this thesis, synthesis and physical characterization of In2Se3 films, characterization of the effect of cooling rate on the β→α phase transition, and electrical characterization β-, γ-, and κ-In2Se3 thin-film transistors has been demonstrated. The effects of substrate temperature, substrate choice, and Se/In flux ratio during molecular beam epitaxy synthesis on the phase composition, surface morphology and stoichiometry of indium selenide films was investigated. Higher substrate temperature combined with higher Se/In ratio promoted formation of β-In2Se3 over γ and/or κ-In2Se3. In2Se3 flake lateral dimensions were observed to increase as substrate temperature increased on all substrates, and the largest lateral dimensions were observed in films synthesized on HOPG at 973 K. No evidence of α-In2Se3 was observed in the Raman spectra of all the films. In2Se3 powders and films were cooled at controlled rates through the β→α phase transition temperature, ranging from 1200 K/hr to 12 K/hr. Some evidence of β to α-In2Se3 was observed in in-situ XRD, but it was independent of cooling rate. A method of quantifying phase fraction and stoichiometry of indium selenide films via Raman and X-ray photoelectron spectroscopies was developed. Se 3d XPS data of single-phase β-In2Se3¬, γ-In2Se3, InSe, κ-In2Se3 and selenium films were obtained and the distinct Se 3d binding energies of each of the films enabled their differentiation via XPS. Raman spectra of each film was also measured on each film and the results correlated with those from XPS. STEM and XRD characterization of the κ-In2Se3 film support the existence of this phase, distinct from γ-In2Se3. The accuracy of Wagner, Scofield, Thermo-modified Scofield, and Crist libraries of relative sensitivity factors for XPS were compared and the most accurate quantification of the selenium-indium stoichiometry of the films was achieved by utilizing an indium sensitivity factor that is 13-times (Crist) larger than the selenium sensitivity factor. Finally, thin-film transistors with β-, γ-, and κ-In2Se3¬ channels were fabricated and tested. Raman spectroscopy through optically transparent gate stacks revealed no β→α phase transition upon applying a vertical field of 4 x 10-2 V/nm. Transfer characteristics (VGS sweep -3 V to 3 V to -3V, VDS = 2 V) at 300K and 78 K revealed hysteresis in all devices at 300 K, but disappearance of the hysteresis in γ-In2Se3 at 78 K , and persistence of the hysteresis in β-In2Se3 and κ-In2Se3 device at 78 K. The disappearance of hysteresis at 78 K in γ-In2Se3¬ suggests that the room temperature hysteresis on this device arose from charge trapping rather than ferroelectricity. Hysteresis in all devices occurred in a clockwise direction, which could be expected for ferroelectricity in an FeSFET with the device dimensions used in this study. Stepwise increase in the VGS sweep range was performed on all devices, from ±0.5 V to ±3V in 0.5 V increments, and the resultant hysteresis width plotted as a function of sweep range. The increase in the hysteresis width was gradual, similar to what is expected of either charge trapping, or thermally activated ferroelectric domain motion.
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ItemExploration of the Additive Manufacturing Process Development Space using High-throughput Mechanical Property Assays(Georgia Institute of Technology, 2024-12-02) Courtright, Zachary StephenAdditive Manufacturing (AM) of metals is a breakthrough technology that may fabricate parts more quickly and allow for the development of novel materials. Recent research has uncovered a variety of discrepancies in the development of AM processes which must be addressed. One such discrepancy is in the mechanical testing of AM samples so they may be optimized, qualified, and certified. This is mainly due to the highly anisotropic nature of AM builds and the costs associated with fabricating and testing mechanical test coupons. To mitigate this issue and find processing parameters that produce a sample with optimal mechanical properties, lower cost, High-Throughput (HT) mechanical testing protocols must be employed at multiple steps along the AM process development cycle. In addition to the primary goal of developing and applying HT experimental protocols to better inform AM process development, it is critical to derive the greatest value from inherently limited experimental data. To address this challenge in part, collaborative research applied Machine Learning (ML) protocols informed by experimental data collected for this dissertation to both improve the throughput of mechanical test data analysis and generate Process-Structure-Property (PSP) linkages. This created a pathway toward the application of rapid experimental mechanical test data collection to inform ML models so Edisonian trial-and-error development methodologies may become less pertinent to the industrial infusion of new manufacturing technologies. Indentation-type methods are ideal for mitigating the mechanical testing discrepancy in AM process or alloy development. Two primary benefits of indentation-type mechanical testing present themselves. 1). A significantly reduced sample volume compared to ASTM-E8 standard samples, and 2). A configuration that is highly conducive to HT automation. These two benefits lead to cost reductions concerning generating the relevant mechanical property data necessary to down-select AM process parameters, build orientations, alloys, and post-processing procedures. This cost reduction has the potential to convince AM developers to apply mechanical testing earlier in their development cycle, so they can reduce the number of builds or heat treatment cycles necessary to determine optimal processing windows. The research described in this dissertation focuses on adding to the already growing amount of indentation-type test data from spherical microindentation and Small Punch Test (SPT). It centers specifically on additive manufactured alloys relevant to aeronautical and astronautical applications. Known issues with AM of aerospace relevant materials, such as microstructural and property heterogeneity, were addressed by performing tests in multiple planes with respect to the build orientation of samples. SPT was a significant focus in this dissertation due to its ability to predict plastic mechanical properties such as yield strength, ultimate strength, and uniform elongation. The data produced with indentation-type tests was validated, in part, by standard ASTM-E8 tensile tests. The two primary end goals of the research within this dissertation were to use HT experimental data to enable the utilization of proven ML protocols to produce reliable correlations between indentation-type mechanical test results, processing parameters, and microstructural features in AM samples and to develop HT experimental and data analysis protocols for SPT.
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ItemThe Effects of Controlled Porosity on the Dynamic Compression and Tensile Failure Strength of Additively Manufactured 316L Stainless Steel(Georgia Institute of Technology, 2024-11-06) Sloop-Cabral, Taylor A.Process-inherent microstructural heterogeneities in materials can have marked effects on the shock wave propagation and the resulting tension-induced spall failure. Incorporating intentional heterogeneities in the form of micro-scale pores of controlled size and distribution can allow for a deeper investigation of their effects on shock wave motion and interactions. The research was conducted on 316L SS printed using Powder Bed Fusion (PBF) to generate a better understanding of the effects of size, fraction, and location of pre-existing pores on dynamic mechanical properties. A high-throughput experimental method involving multiple samples simultaneously impacted in each experiment employing the 80-mm diameter single-stage gas gun and multiplexed PDV diagnostics was utilized. The resulting analysis revealed shock-wave mitigation caused by the collapse of pre-existing pores that varied dependent on the pore size and location within the sample leading to a decrease in damage in the spall plane. When many pores are present at a lower peak stress, a higher number of pores and a larger pore size led to an increase in the dissipation of the shock wave as well as a dispersion of the shock wave. An investigation into isolated pores at higher peak stresses revealed that a single pore most effectively disrupts the shock wave and limits the spall damage experienced by the material, with larger pores having a more exaggerated effect. However, the presence of multiple pores at a higher peak stress, both in the plane of the impact direction as well as perpendicular to it, does not dissipate the shock wave as effectively, and more damage is observed. This work provides insights into the shock mitigation mechanisms of alloys containing small volume fractions of pores, and furthers our understanding of the resulting microstructural deformation processes dependent on the pore size and location
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ItemThe impact of sintering temperatures, humidity levels, and un-heated applied pressures on the dielectric properties and density of alumina(Georgia Institute of Technology, 2024-09-06) Wu, Maddy BiqianAlumina is one of the most common ceramic materials in our daily lives, offering many beneficial properties. It can function as a pure capacitor or an effective insulator, making the study of its electrical properties essential. This thesis investigates the dielectric properties, specifically impedance and permittivity, of both sintered and non-sintered alpha-alumina samples with different particle sizes ranging from nano to micron size range. For non-sintered specimens, the effects of varying compaction pressures from 100 MPa to 450 MPa were examined using the Carver Unheated Press to compress the powder into pellets. All five alumina specimens investigated exhibited clear RC behaviors with impedance decreasing with increasing applied pressure. However, some samples showed a noticeable rebound in the Nyquist impedance plot at certain pressure ranges. The Spark Plasma Sintering technique was employed to sinter the alumina powders into pellets. To better understand how sintering temperatures affect the dielectric properties of alumina, seven different temperatures ranging from 1000°C to 1300°C, with 50°C increments, were utilized. It was found that alumina specimens sintered below 1200°C are highly susceptible to ambient humidity even when their density was over 90%, whereas those sintered at or above 1200°C are resistant to humid environments. These trends were analyzed through comprehensive impedance and dielectric spectroscopy measurements as a function of frequency, humidity, and sintering temperature. This analysis revealed that specimens with lower bulk density exhibit lower dielectric permittivity at high frequencies than those with higher densities, but significant increases occur at frequencies below 105 Hz when exposed to humidity. A notable transition at 1150°C sintering temperature was also identified to be related to the difference in response for the low-density and high-density samples. Detailed structural and compositional analyses of these samples were conducted using X-ray diffraction (XRD), X-ray fluorescence (XRF), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and particle size analysis.
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ItemInvestigation of Distribution and Transport of CO2 in Polyimine/MCM-41 Hybrid System: Modeling and Simulation Approach(Georgia Institute of Technology, 2024-07-27) Chen, JunheThis study focuses on advancing Direct Air Capture (DAC) technology for atmospheric carbon dioxide reduction by employing novel polyimine materials. From a computational perspective, The study investigates traditional poly(ethylenimine) (PEI) sorbents and novel poly(propylenimine) (PPI) and their derivatives, addressing the pressing environmental challenge of CO2 accumulation. The research encompasses the following objectives: exploring CO2 transport in the hyperbranched PEI/MCM-41 hybrid system and conducting an in-depth investigation of CO2 transport in hyperbranched PPI membranes and corresponding MCM-41 systems. Central to the dissertation is the use of multiscale modeling, including First Principles Calculations and Molecular Dynamics (MD) simulations, to understand molecular behaviors in the CO2 capture processes. This involves the development of new force field parameters to accurately describe interactions between CO2, water, amine groups, and silica surfaces, alongside characterizing CO2 distribution and movement through pair correlation and mean-square displacement analyses. The study is set to provide significant insights into the enhancement of CO2 capture within these systems and contribute to the development of new materials with improved capture performance. The research outcomes are expected to substantially elevate the efficacy and practicality of DAC technology, potentially leading to reduced costs and energy demands and aligning with global efforts towards carbon neutrality and sustainable industrial processes.
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ItemElucidating exciton-polaritons in organic and two-dimensional metal halide semiconductors(Georgia Institute of Technology, 2024-07-27) Quiros Cordero, VictoriaExciton-polaritons combine properties of light (i.e., low effective mass, delocalization) and electron-hole pairs bound by Coulombic forces or excitons (i.e., many-body interactions), opening doors for light-based computing, long-range energy harvesting, and tunable chemical reactivity, to name a few of their applications. Polaritons emerge in semiconductors placed within optical microcavities when non-dissipative, coherent energy exchange between excitons and the microcavity optical modes (i.e., standing electromagnetic waves) dominates over the system's energy losses. This regime is also known as strong light-matter coupling. This Ph.D. thesis overcomes challenges of exciton-polaritons in organics and two-dimensional metal halide semiconductors at two distinct fronts: the fabrication of photonic structures with strong light-matter coupling and the characterization of exciton-polariton photophysics. We enable polaritons in a wider variety of materials by developing non-destructive fully solution-processed microcavities, which offer the required electromagnetic field enhancements for strong light-matter coupling and are highly compatible with temperature-sensitive semiconductors. These microcavity structures are produced from solution at ambient conditions using common processing methods (i.e., dip coating) and comprise a high-refractive-index titanium oxide hydrate/poly(vinyl alcohol) hybrid material and low-refractive-index commodity polymers. On the other front, we identify nonlinear processes building up and hindering the population of exciton-polaritons in these material classes. We resolve nonlinear processes that are faster than the polariton lifetime (<< 1 ps) and comparable to the exciton lifetime (>> 1 ps) by utilizing excitation correlation photoluminescence spectroscopy and two-dimensional coherent spectroscopy, respectively. This is a key milestone towards developing guidelines for populating large ensembles of exciton-polaritons, as required for polariton-based technologies. The work presented in this thesis moves us forward toward practical applications of room-temperature exciton-polaritons.
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ItemSimulation of Polymer Imprinting and Embossing using Smoothed Dissipative Particle Dynamics(Georgia Institute of Technology, 2024-05-15) St Julien, James W.A simulation of an imprinting process using Smoothed Dissipative Particle Dynamics is presented. Finite Element Methods (FEM) are commonplace in industry for the simulation of materials processing; however, FEM has a limited capacity to simulate more complex material behaviors. Finite Difference Methods (FDM) are alternative methods that can overcome many of the limitations that FEM faces, though they face their own unique challenges. Smoothed Dissipative Particle Dynamics (SDPD) is a FDM method incorporating a discretized Navier-Stokes Equations to describe the motions of fluids, which allows for the utilization of known fluid properties to simulate flow behavior. Couette Startup flow was simulated first using SDPD to establish the validity of the simulation method, showing good agreement with analytical results. Embossing simulations under different settings were then conducted using SDPD. Cavity filling modes and their dependence on die parameters is demonstrated for single and multi-cavity die. Within a single cavity die, we see a transition of flow peak deformation from single peak to dual peak flow at a cavity half width to film thickness ratio (w/h_i =1), showing results consistent with FEM simulations. As a particle-based simulation method, SDPD can allow for modeling of more complex fluid behaviors; this is demonstrated by simulations exhibiting elastic-viscoplastic relaxation and droplet formation/material separation. These results demonstrate the potential of SDPD in simulating materials processing scenarios which would prove difficult using FEM methods such as materials separation, common in spraying or jetting processes.