Browsing by Author "Li, Lin"
Now showing 1 - 20 of 37
Results Per Page
Sort Options
Item Atomistic modeling and structure-property relationship of topologically accurate complex nanotube junction architectures(University of Alabama Libraries, 2020-08) Nakarmi, Sushan; Barkey, Mark; Unnikrishnan, Vinu; University of Alabama TuscaloosaCarbon nanotubes have remarkable material properties and are ideal for different space applications including thermal management devices, light-weight mechanical shock absorbers, and fiber-reinforced composites. Nanotube junctions, which are the interconnections of carbon nanotubes, have properties different from the pristine structures and are promising materials for constructing unit blocks with excellent material properties. However, widespread application of the junctions and nanostructures is limited due to the lack of understanding of their mechanical, thermal, and electronic properties. The overall objective of the current research is to provide a computational methodology to construct atomistic models of nanostructures and study their thermal and mechanical properties under different operating conditions. In the first part of the research, the topologically accurate atomistic models of the junctions are created using a novel CAD-based remeshing and optimization strategies. The most energetically stable configurations are chosen to build 3D architectures, thus, providing an economical way to construct complex and larger dimension nanostructures. The created macro-structures can be used directly in the atomistic simulations to study their structure-property relationships. In this dissertation, the thermal and mechanical characterization of pristine nanotubes and complex nanotube multiterminal junctions have been studied using molecular dynamics (MD) simulation. At the nanoscale, the thermal conductivity of nanotube is found to be dependent on size, strain, temperature, and defects. The effects of each of these parameters on the thermal transport of nanostructures have been determined using MD. This is followed by the comparative study of the phonon density of states and phonon dispersion relations of different configurations. The study provides guidelines for creating nanotube heat transfer devices with desired thermal specifications. In addition to being highly conductive, nanotubes and junctions have very high strength and modulus. Although an extensive amount of research is available with the characterization of the pristine nanotubes, there lacks a proper understanding of the mechanical characteristics of the complex structures (multi-terminal junctions and micro-blocks). With the atomistic models of these structures created, the tensile and compressive strengths of such complex architectures have been presented. These computational models will provide the much needed next step for the realization of nanotube junctions for the industrial applications.Item Chemical variation induced nanoscale spatial heterogeneity in metallic glasses(Taylor & Francis, 2018) Wang, Neng; Ding, Jun; Luo, Peng; Liu, Yanhui; Li, Lin; Yan, Feng; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; Chinese Academy of Sciences; Institute of Physics, CASMetallic glasses possess amorphous structures with inherent heterogeneity at the nanoscale. A combined experimental and modeling investigation to elucidate the chemical effect on such nanoscale heterogeneity in a Cu-Zr-Al metallic glass system is conducted. By using the dynamic atomic force microscopy, we reveal a reduction of the nanoscale spatial heterogeneity in the local viscoelastic response after introducing Al into the Cu50Zr50 metallic glass. The change of such nanoscale heterogeneity can be contributed to the variation of local atomic structures. The addition of Al increases the population of the icosahedral short-range ordered clusters, thus reducing the structural heterogeneity at the nanoscale. IMPACT STATEMENTThis paper provides a combination between the nanoscale experimental and theoretical understanding of the chemical variation induced spatial heterogeneity in CuZrAl metallic glass and their impacts on the mechanical properties.Item Correlation between heat input and residual stresses from friction stir welding of AA5052 plate(University of Alabama Libraries, 2018) Zhu, Ning; Brewer, Luke N.; University of Alabama TuscaloosaThis thesis investigates the connections between friction stir welding (FSW) parameters, simple energy/heat input metrics, and the resultant residual stresses on AA5052-H32 plates. A range of weldments were produced with different tool rotational and traverse speeds to produce the same values of the pseudo heat index (PHI). Average residual stresses inside the stir zone and peak residual stress in the thermo-mechanically affected zone were systematically recorded using laboratory x-ray diffraction. In addition, thermal cycles on the advancing side of the welds were collected and analyzed for comparison with the predictions of heat input based upon FSW parameters. Based upon these results, the PHI is not a good predictor of the peak residual stress for welding conditions which produced sound welds. Increasing traverse speed, V, with fixed rotational speed does increase the residual stresses inside the stir zone. The data in this thesis suggests that there is a complex relationship between frictional heating and mechanical stirring of the material. As a result, there is a rotational speed, which requires minimum torque during welding.Item Crystallization Characteristics in Co-Based Magnetic Amorphous Nanocomposite Alloys(University of Alabama Libraries, 2022) Koenig, Alicia Grace; Thompson, Gregory B.; University of Alabama TuscaloosaThis document will describe analytical procedures for the microstructural characterization of Co-based soft magnetic amorphous nanocomposite materials and the evolution of that microstructure after heat treatment. Atom probe tomography analysis of these alloys reveals increased chemical diffusion with additional solute content, as well as a reduction in defects in the crystalline phase. It was confirmed that Co and Fe partition preferentially to the crystalline phase, and that other elements (B, Si, Nb, Mn) segregate to the amorphous matrix. It was found that a combination of FCC/HCP structures were the basis for the crystalline phase.Differential scanning calorimetry was used to evaluate the characteristics of phase transition as a function of solute content, revealing an increase in the necessary energy for the formation of the primary crystallization phase with higher solute concentrations. From this data, the primary crystallization temperature was estimated and anneals were performed at a temperature just below that to slow the kinetics of crystallite nucleation and growth. Postmortem atom probe tomography and transmission electron microscopy data revealed that the alloy undergoes a constant nucleation condition, and that the added solute content suppresses the nucleation and growth behavior of both the primary and secondary crystalline phases.Finally, as strain annealing has been shown to improve the desired magnetic properties in these alloys, but the mechanisms are not yet understood, a method for applying digital image correlation techniques to tensile testing in an in situ tensile testing environment is described. This will establish precedent for applying these analyses to in situ tensile testing of sputter-deposited magnetic alloys, and eventually to in situ thermomechanical testing.Item The Density and Molecular Phase Field Methods(University of Alabama Libraries, 2022) Jacobson, David Wallace; Thompson, Gregory B.; University of Alabama TuscaloosaDensity phase field (DPF) methods have arisen as a means of more closely linking free energy functionals with grain boundary physics as opposed to using functionals that are purely phenomenological. In their current form, DPF methods exhibit a number of theoretical and computational problems that limit their applicability. These issues include the following:(1) being unable to simulate moving grain boundaries, (2) low computational performance due to high order gradient energy terms, and (3) failing to predict stable bulk equilibriums. We solve the mobility and performance issues mentioned above by coupling the density field of DPF simulations with traditional order parameters. The stable equilibrium problem is solved through the development of a criteria list that can be used to determine the set of DPF free energy functionals that correctly predict bulk equilibrium states. A subset of the free energy functionals that meet aforementioned criteria were identified and studied further because of their direct connection with atomistic physics. Termed the Molecular Phase Field (MoPF) method, these free energy functionals are constructed from interatomic potentials. Grain boundaries simulated using the MoPF method are material specific and their calculated excess energies are a natural consequence of the interatomic potential parameters used as model inputs. Lennard Jones, M-N, Mie, and Morse potential parameters have been calculated for over 30 different transition metals such that MoPF simulations of these metals may be carried out. Finally, MoPF models allow for the thermodynamic description of specific grain boundaries to be incorporated within phase field models such that the large variation in properties over the grain boundary phase space might be more accurately represented in phase field simulations.Item Electrospun single-phase spinel magnetic high entropy oxide nanoparticles via low-temperature ambient annealing(Royal Society of Chemistry, 2023) Han, Xiao; Li, Dian; Zhou, Jingyi; Zheng, Yufeng; Kong, Lingyan; Li, Lin; Yan, Feng; University of Alabama Tuscaloosa; University of Nevada Reno; Arizona State University; Arizona State University-TempeHigh entropy oxide nanoparticles (HEO NPs) with multiple component elements possess improved stability and multiple uses for functional applications, including catalysis, data memory, and energy storage. However, the synthesis of homogenous HEO NPs containing five or more immiscible elements with a single-phase structure is still a great challenge due to the strict synthetic conditions. In particular, several synthesis methods of HEO NPs require extremely high temperatures. In this study, we demonstrate a low cost, facile, and effective method to synthesize three- to eight-element HEO nanoparticles by a combination of electrospinning and low-temperature ambient annealing. HEO NPs were generated by annealing nanofibers at 330 degrees C for 30 minutes under air conditions. The average size of the HEO nanoparticles was similar to 30 nm and homogenous element distribution was obtained from post-electrospinning thermal decomposition. The synthesized HEO NPs exhibited magnetic properties with the highest saturation magnetization at 9.588 emu g(-1) and the highest coercivity at 147.175 Oe for HEO NPs with four magnetic elements while integrating more nonmagnetic elements will suppress the magnetic response. This electrospun and low-temperature annealing method provides an easy and flexible design for nanoparticle composition and economic processing pathway, which offers a cost- and energy-effective, and high throughput entropy nanoparticle synthesis on a large scale.Item Enhancing Mechanical Properties and Design of Metallic Glasses Through Nanoscale Heterogeneities and Machine Learning Optimization(University of Alabama Libraries, 2023) Gu, Yucong; Daniewicz, Steven R.; Li, LinMetallic glasses (MGs) are a type of metal alloy that has a disordered atomic structure similar to glass. They possess unique properties such as high strength, elasticity, and corrosion resistance. However, their susceptibility to catastrophic failure through shear banding has limited their widespread use. Recently, the local ordering in MGs has been identified within the amorphous structure, which can influence the physical and mechanical properties of these materials. This dissertation investigates the effect of local ordering and nanoscale heterogeneities on shear band behaviors and optimizes MG designs accordingly. Firstly, a mesoscale shear transformation zone (STZ) dynamic model is employed to investigate the deformation behaviors of MGs. The presence of nanoscale heterogeneity results in a Hall-Petch-like relationship between yield stress and spatial correlation length of heterogeneity. Secondly, dynamic mechanical responses of MGs are studied via experiments and simulations on thin film MGs with various nanoscale heterogeneities. The strain rate sensitivity transition is attributed to a shift in deformation mechanisms from structure-dictated strain localization to stress-dictated strain percolation into a shear band. Finally, a data-driven design framework is developed using artificial neural networks (ANN) and a genetic algorithm (GA) to optimize dual-phase MG designs. The ANN models are trained using simulation data from the STZ dynamic model, and the GA is used to guide the development of new dual-phase MGs with improved mechanical properties. Additionally, the data-driven design framework is extended to optimize material performance under both mechanical and electrochemical processes, minimizing material degradation under the simultaneous action of wear and corrosion based on hierarchical ANN models trained on multiphysics simulations. This dissertation integrates machine learning with multiscale and multiphysics models to uncover the mechanisms that explain the emergence of mechanical behaviors of MGs and explores a massive design space for property optimization. These findings provide valuable insights into the material structure-property relationships, enabling informed decision-making for material design.Item Flexible nano-memory device by zinc oxide nanorod arrays(University of Alabama Libraries, 2016) Tang, Chaolong; Song, Jinhui; University of Alabama TuscaloosaMemory is one of the key techniques for information technology. It is highly desired to have memory device with flexible characteristic for special applications. The challenges for archiving flexible memory are not only restrained by current materials but also existing memory mechanism, while nanomaterials exhibit size dependent properties that are different from their bulk form, discovering in new materials and architectures design as well as novel working principle provides an alternative approach to meet the objectives. In this dissertation, a comprehensive study on developing flexible nano-memory device from fundamental semiconducting nanomaterial to device’s architecture design and performance is presented. With the strong basis support, the as-fabricated flexible nano-memory device exhibits extraordinary memory characteristics and excellent flexibility. First, controllable synthesis of zinc oxide (ZnO) nanorod/nanowire (NR/NW) is the prerequisite to provide desired nanomaterials and nanostructures, the effect from substrate roughness is the key to grow well-aligned three-dimensional (3D) ZnO nanostructures for fabricating patterned nano-memory. Second, because the performance of device is closely related to the material’s property, electrical property of one-dimensional (1D) ZnO nanomaterial by experimental and theoretical approaches have been characterized. The results reveal that the electrical resistance has nonlinear length dependence in the single crystal ZnO microbelt/nanobelt (MB/NB), which is significantly different with the bulk counterpart. Finite element simulation can identify the crystallography of the anisotropic semiconducting nanomaterial. Third, when nanomaterials with a height-diameter ratio in-between zero-dimensional (0D) and 1D, its photoelectric properties will not follow the traditional Ohm’s law because of the additional nanoconfinement from the third dimension. Photoelectric property change in half-dimensional (0.5D) ZnO nanomaterials as a function of illumination light intensity and materials geometry has been systematically studied. A new proposed model could more accurately predict the photoelectric characteristics of 0.5D semiconducting nanomaterials. Last, based on fundamental synthesis and properties characterization above, a new flexible nano-memory device based on ZnO NW arrays is fabricated. Benefited from 3D nanostructures and the unique polar charges screening effect induced electric hysteresis loop memory mechanism, the nano-memory device has outstanding performances in unit down to nanoscale, operation speed up to gigahertz, as well as excellent flexibility.Item Going Beyond Shockley-Queisser Limit Perovskite Chalcogenides Tandem Solar Cell(University of Alabama Libraries, 2023) Gokul Menon, Harigovind; Yan, Feng; Daniewicz, StevenThe efficiencies of single junction solar cell (SJ SC) technologies like silicon and cadmium telluride have been rapidly increasing and are nearing their fundamental Shockley Queisser (SQ) efficiency limit. New novel methods must be developed to go beyond the limitations of SJ SCs. A promising approach is the combination of an efficient narrow bandgap (NBG) absorber with a wide bandgap (WBG) absorber to form a tandem solar cell that can effectively utilize the solar spectrum. The aim of this work is the realization of mechanically stacked perovskite-antimony selenide (Sb2Se3) and perovskite-cadmium telluride (CdTe) 4T tandem solar cells. Perovskite materials with their excellent performance and tunable bandgap make it an ideal candidate for tandem solar cells. Sb2Se3 with a bandgap of 1.2eV and a theoretical efficiency limit of over 30% and CdTe with a bandgap of 1.5eV and an efficiency over 22% make them potential NBG absorbers. In this work, we first simulated 4T tandem cells pairing a 1.6eV WBG Perovskite solar cell (PSC) with a 1.2 eV NBG Sb2Se3 cell and a 1.6eV WBG PSC with a 1.5 eV NBG CdTe cell using Solar Cell Capacitance Simulator (SCAPS). We obtained a simulated tandem PCE of 23.14 % for the perovskite Sb2Se3 tandem and a PCE of 23.37% for the perovskite CdTe tandem. We then developed two WBG perovskite top cells with a bandgap of 1.6eV and 1.77eV to study the impact of bandgap on the tandem architecture. We also developed a CdTe solar cell with Cu doping which had a PCE of 17.94% and a Sb2Se3 solar cell with a PCE of 5.76%. As a substitute for the opaque metal back electrode for the top cell, we developed an indium tin oxide (ITO) transparent electrode using DC sputtering to fabricate a semitransparent top cell. By mechanically stacking the sub-cells, we obtained an excellent 4T tandem PCE of 16.13% for the perovskite-Sb2Se3 tandem and a tandem PCE of 19.41% for the perovskite-CdTe tandem. The experimental results show promising tandem cell performance and pave the way to go beyond the SQ limit.Item Grain Boundary Engineering of Stainless Steel Via Additive Manufacturing(University of Alabama Libraries, 2023) McAllister, Sarah; Daniewicz, Steven RAdditive manufacturing followed by thermomechanical processing was investigated as a means to achieve grain boundary engineering. Different combinations of a 930°C recovery treatment, 0-30% cold-rolling, and a 1150°C solution annealing treatment were systematically applied to 316L stainless steel samples produced by laser powder bed fusion. The resulting microstructures were analyzed, including calculations of the special boundary and triple junction statistics, in order to assess the effects of each processing route. It was determined that a temperature above the recovery temperature was necessary to recrystallize the as-built microstructure and to relax internal strain created by the additive process. The lowest amount of cold-rolling resulted in the highest twin boundary length percentage in each sample set, so applying additional cold work did not help toward achieving grain boundary engineering. Longer time at the solution annealing temperature, however, did improve the grain boundary character distribution. While one sample was able to reach a twin boundary length percentage over 60%, none of the samples showed a breakup of the random-angle grain boundary network. Further research on breaking up the random-angle grain boundary network is recommended.Item High-Efficiency and Stable Carbon-Based Planar Perovskite Solar Cells(University of Alabama Libraries, 2023) Sankaranarayanan Nair, Vijayaraghavan; Daniewicz, StevePerovskite solar cells (PSCs) have attracted much attention both in research and industrial domains. The power conversion efficiencies (PCE) of PSCs have been improved from 3.8% to 25.8% in just over a decade, rivaling that of silicon solar cells. Hybrid perovskite materials have exceptional optoelectrical properties and can be processed using cost-effective solution-based methods. In contrast, the fabrication of silicon solar cells requires high-vacuum, high-temperature, and energy-intensive processes. The combination of excellent optoelectrical properties and cost-effective manufacturing makes hybrid perovskite a winning candidate for solar cells.As the PCE of PSCs improves and their long-term stability increases, one of the crucial hurdles to be addressed is the cost-effective scalable fabrication and its long-term stability. Most high-efficiency solar cells require an energy-consuming method of depositing the cathode to complete the cell. These high-efficiency devices also require highly expensive noble metal electrodes. Herein, following recommendations from the literature, carbon-based nanomaterials are utilized, and their effects on the efficiency, fill factor (FF), open-circuit voltage (Voc), and short circuit density (Jsc) are analyzed. The PSCs and carbon counter electrodes are fabricated at low temperatures (~100 degrees Celsius). As per the literature, carbon-based devices exhibited an efficiency of >19%. This endeavor further buttresses the fact that carbon nanomaterials are promising candidates for the future of low-cost, high-performance, and scalable production of PSCs. Thus, complex vacuum deposition of expensive cathodes to complete the cells might be eliminated.In this thesis, I focus on several novel interface engineering techniques, that can reduce the surface roughness of the carbon electrode, and improve the interface it forms with the underlying layer. These interfacial engineering techniques improved the charge collection efficiency of the carbon, and thereby reduced the recombination happening at the interface. As a result, the PCE could be enhanced from ~13% to >18%.In addition to these techniques, a high-quality narrow band gap perovskite layer was developed with low dimensional 2D perovskite passivation to further improve the device performance. Together with the improved perovskite film quality and optimized film composition, along with interface engineered carbon electrode, an impressive PCE of >21% was attained.Item Influence of Processing Variables on the Morphology and Microstructure of a Non-Stoichiometric Complex Concentrated Alloy(University of Alabama Libraries, 2022) Mitchell, Aidan Matthew; Weaver, Mark L.; University of Alabama TuscaloosaThe development of high-entropy alloys has been a major area of research in recent years.The compositionally complex alloy Al_1_0Co_2_5Cr_8Fe_1_5Ni_3_6Ti_6 has been promoted as an optimizedchemistry exhibiting medium hardness with good ductility and high tensile strength. This thesisaims to provide insight into the growth of Al_1_0Co_2_5Cr_8Fe_1_5Ni_3_6Ti_6 as a surface coating and into theinfluence of coating morphology on its interdiffusion behavior. Coatings were deposited via highrate unbalanced direct current magnetron sputtering. Deposition conditions were selected tosurvey processing-property relations relative to the structure zone model for thick film growthproposed by Thornton. Thick micro-scale coatings were deposited onto CMSX-8 Ni-based superalloy substrates at various temperatures and Ar gas pressures and later annealed to observe diffusion behavior. Coating morphology, chemical analysis, and crystallographic identification were performed using scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction. Coatings produced at 5 mTorr Ar pressure displayed dense Zone T or mixed Zone T + Zone 2 morphologies and the least amount of diffusion into the substrate upon annealing. Some of the samples at high pressures and temperatures possessed porous, fibrous Zone 2 morphologies caused by a high incident angle between the substrate and sputter gun.Item Mechanical Properties of Nanoparticles and Nanotubes with Continuous Ceramic Coatings and Cross-Links(University of Alabama Libraries, 2023) Kayang, Kevin Walier; Volkov, Alexey NThe deposition of continuous ceramic coatings on nanoparticles (NPs) or introducing covalent cross-links between carbon nanotubes (CNTs) can strongly improve the mechanical properties of the porous nanocomposite. The continuous coating specifically acts as a binding agent that promotes the mechanical integrity of a nano-powder and transforms it into a porous nanocomposite. This dissertation quantifies the effect of continuous coating on the mechanical properties of individual NPs, and cross-links on the mechanical properties of CNTs as well as their nanocomposite structures using atomistic simulations. The simulations of quasi-static compression of single-material SiC NPs showed that 6H-SiC NPs demonstrate stronger resistance to compression compared to 3C-SiC NPs with the dominant mechanism of deformation dependent on the type of the SiC polymorph, NP size, temperature, and lattice orientation. For impact interaction between SiC NPs, coefficients of restitution as functions of the NP size, impact parameters, and impact velocity are determined based on results of atomistic simulations. Simulations of linear arrays composed of core-shell Si/SiC NPs reveal dramatic differences in the dominant mechanisms of inelastic deformation, mechanical properties, and phase changes between the arrays with and without continuous coating (overlap of ceramic shells). Overall, the continuous SiC coating on Si NPs can increase the material modulus and strength in up to 1.5 orders of magnitude, turning a nano-powder into a porous nanomaterial with excellent mechanical properties. The simulations of Si NPs arranged in a face centered cubic structure with continuous 6H-SiC coating showed that a 1 nm thick coating induces a dramatic order-of-magnitude increase in the elastic modulus of a pure Si nano-powder while a further increase in thickness results in a significant increase in 12-fold over a Si nano-powder. A combined atomistic-mesoscopic study performed to reveal the effect of CNT radius on the deformation mechanisms and mechanical properties of low-density CNT films with covalent cross-links showed that the mechanical properties of CNT films are found to be strongly dependent on the CNT radius while the stretching rigidity of individual nanotubes and volumetric CL density are identified as the key factors that dominate the effect of CNT chirality on mechanical properties of CNT films.Item A model of thermal aging of hyper-elastic materials with an application to natural rubber(University of Alabama Libraries, 2017) Korba, Ahmed G.; Barkey, Mark E.; University of Alabama TuscaloosaUnderstanding the degradation of material properties and stress-strain behavior of rubber-like materials that has been exposed to elevated temperature is essential for rubber among components design and lifetime prediction. The complexity of the relationship between hyper-elastic materials, crosslinking density, and chemical composition present a difficult problem for the accurate prediction of mechanical properties under thermal aging. In the first part of the current research, a new and relatively simple mathematical formulation is presented to expresses the change in material properties of natural rubber subjected to various elevated temperatures and aging times. The aging temperatures ranged from 76.7 °C to 115.0 °C, and the aging times ranged from 0 to 600 hours. Based on the experimental data, the natural rubber mechanical properties under thermal aging showed a similar behavior to the rate of change of the crosslinking density (CLD) with aging time and temperature as determined as of the research. Three mechanical properties have been chosen to be studied: the ultimate tensile strength, the fracture stretch value, and the secant modulus at 11.0% strain. The proposed phenomenological model relates the mechanical properties with the rate of change of the CLD based on a form of Arrhenius equation. The proposed equations showed promising results compared to the experimental data with an acceptable error margin of less than 10% in most of the cases studied. In the second part of the current research, a closed form set of equations that was based on basic continuum mechanics assumptions has been proposed to define the material stress-strain behavior of natural rubber as an application of hyper-elastic materials. The proposed formulas include the influence of aging time and temperature. The newly proposed “Wight Function Based” (WFB) method has been verified against the historic Treloar’s test data for uni-axial, bi-axial and pure shear loadings of Treloar’s vulcanized rubber material, showing a promising level of confidence compared to the Ogden and the Yeoh methods. Tensile testing was performed on strip specimens that were thermally aged then subjected uni-axial tension and hardness tests. A non-linear least square optimization tool in Matlab (Lscurvefitt) was used for all fitting purposes.Item Modeling and process-structure-property-performance study of perovskite solar cells(University of Alabama Libraries, 2017) Shaik, Shoieb; Li, Dawen; University of Alabama TuscaloosaThis dissertation study mainly falls into two parts: simulation study and experimental investigation of the process-structure-property-performance relationship in perovskite solar cells. Herein, a controllable fabrication of annealing-free perovskite films with tunable crystal grain size and morphology via a seeded approach has been developed. Specifically, a solution of lead iodide (PbI_2) was spin-coated on a substrate, and a low concentration solution of Methylammonium iodide (MAI) was dropped onto the PbI_2 film to form perovskite seed before introducing high concentration solution of MAI. The fast, annealing-free seeded nucleation and growth leads to dense and uniform perovskite thin films exhibited controllable crystal grains. In another project, a polymer additive assisted approach to facilitate the growth of uniform, dense, and ultra-smooth perovskite thin films has also been demonstrated. In specific, a polymer, Polyamidoamine (PAMAM) dendrimers, was incorporated into the blend solution of lead iodide (PbI_2) and Methylammonium iodide (MAI) to regulate the nucleation and growth thereby tuning the morphology and crystallinity. The PAMAM addition not only realized compact perovskite thin films without pinholes in it, but also increased the stability. In the simulation study, both the organic bulk heterojunction solar cells and pervoskite solar cells have been systematically investigated to help understand the device operation and guide the experiments.Different electron transport layers (ETL) and hole transport layers (HTL) were used to study the effect of band gap alignment with adjacent layers and improve the transport of charges. The change in band gap not only facilitated in collection of charges but also improved the overall power conversion efficiency (PCE) of the device in study. Recombination of charges in the bulk active region and its effect on overall PCE was also studied.Item Multi-scale modeling of spatial heterogeneity effect on the shear banding behaviors in metallic glasses(University of Alabama Libraries, 2018) Wang, Neng; Li, Lin; University of Alabama TuscaloosaStronger than steels but able to be shaped and molded like plastics, the mechanical properties of metallic glasses (MGs) are proven to be scientific interest and potential applications in industry. However, MGs suffer from negligible plasticity prior to catastrophic failure in the form of a single shear band at room temperature, which precludes their immediate application as structural components. The structural and property heterogeneity have been found recently in MG. These nanoscale heterogeneities may influence the initiation and propagation of the shear band, which would thus improve the plasticity. To understand the structural and property heterogeneity effect on the shear band behaviors, this thesis is divided into three sections. First, Activation-relaxation technique (ART) and dynamic atomic force microscopy (DAFM) are employed to prove the existence of the nanoscale heterogeneity. ART discovers that the activation energy possesses a normal distribution, reflecting the non-uniform local structure. This non-uniformity should correspond to the different motifs found in the molecular dynamics simulation. The energy dissipation resulted from the DAFM also exhibits a normal distribution, thus the inelastic spatial heterogeneity is confirmed. Furthermore, the correlation length of the inelasticity is identified based on the 2D scanning figures from DAFM. Second, a mesoscale modeling technique, shear transformation zone dynamics (STZD) will be employed. A series of configurations with the spatial elastic heterogeneity will be built up. We find that the organization of such nanometer-scale shear transformation events into shear-band patterns is dependent on the spatial heterogeneity of the local shear moduli. A critical spatial correlation length of elastic heterogeneity is identified for the simulated MGs to achieve the best tensile ductility, which is associated with a transition of shear-band formation mechanisms, from stress-dictated nucleation and growth to structure-dictated strain percolation, as well as a saturation of elastically soft sites participating in the plastic flow. Third, a state variable, excess free volume, is incorporated into STZD model in order to introduce the strain softening which is typical during MG deformation. The stress ‘overshoot’ and cyclic hardening of MGs have been successfully captured by the model. We found that it is the dynamic competition between free volume creation and annihilation that give rise to the signature stress overshoot in the homogeneous deformation regime at elevated temperatures during tension and cause the removal of large free volume sites in the confined deformation region during nanoindentation.Item On the Mechanical Behavior of Nanocrystalline Ni-Based Alloys(University of Alabama Libraries, 2022) Koenig, Thomas Robert; Thompson, Gregory B.; University of Alabama TuscaloosaThe fabrication and characterization of a ternary nanocrystalline (NC) stabilized Ni-Cu-P alloy was reported. The complexities of fabrication a low contamination, equiaxed NC material was discussed. For comparison, elemental Ni was prepared via sputter deposition, highlighting the difficulty in fabricating an idealized structure within the confines of the parametric space. An exchange between the ideal grain size distribution and film coalescence was discussed with the ramifications of substrate heat, deposition pressure, and deposition rate considered. These results were then compared to a kinetic model to assess the validity of the model and its ability to capture the complex growth behavior noted between the Ni films.A series of ternary NC Ni-Cu-P thin films were fabricated, characterized, and loaded via in situ annealing and nanoindentation. Examination of the thermal stability behavior revealed differences in precipitation behavior as a function of Cu and P solute content. Furthermore the Ni-40Cu-0.6P (at.%) alloy was noted as the only NC stabilized composition at 550 °C. The Ni-40Cu-0.3P deposit developed nanoscale precipitates upon annealing to temperatures of 550 °C which ultimately resided in the matrix. It was found that the ternary Ni-Cu-P films are harder than previously studied binary Ni-1P and Ni-4P (at.%) systems. Adding Cu to the Ni-P system promoted solid solution strengthening which mitigated softening in those binary alloys previously reported.A MEMS device was then employed for in situ thermomechanical testing of the stabilized Ni-40Cu-0.6P film for comparison with a binary Ni-40Cu counterpart. Digital image correlation (DIC) was employed to data mine microstructural evolutions that occurred while the films were loaded. Increased fracture strength was reported with the P solute addition, irrespective of the loading temperature. The ramifications of adding this stabilizing solute were discussed with respect to the fracture profile and microstructural stability.Item Phase stability and deformation behavior of nanostructured copper based multilayers(University of Alabama Libraries, 2018) Guo, Qianying; Thompson, Gregory B.; University of Alabama TuscaloosaAs metallic multilayers exhibit large surface area-to-volume ratios, their physical and chemical properties can be size dependent. These size dependent changes facilitate both crystallographic phase transformations as well as alternations in deformation mechanisms. These topics were studied using a series of Cu based thin film architectures, where Cu was deposited between either crystalline phases, i.e. Nb or V for phase stability studies, or glassy phase, i.e. Cu45Zr55, for deformation studies. The phase stability of all the layers were monitored through real time stress measurements during the growth of the films coupled with post growth characterization methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). Face centered cubic (fcc) phase transformation was detected in both multilayered systems, explained by the reduction of interfacial energy. An additional vitrification phase transformation was observed in the crystalline and Cu/Nb system owing to clusters formations at the interfaces. For the in situ deformation studies, the crystalline Cu/ amorphous Cu45Zr55 multilayers were studied by indentation and high cycle fatigue in the TEM. Precession electron diffraction (PED) quantified the grain size, grain misorientation and texture. Grain rotation, facilitated by the free surface of the TEM foil, and grain growth, were observed to be primary deformation mechanisms in the crystalline layers of the multilayers.Item Phase stability in ti/bcc multilayered thin films(University of Alabama Libraries, 2016) Wan, Li; Thompson, Gregory B.; University of Alabama TuscaloosaMaterials structures with large surface area-to-volume ratios can exhibit size dependent physical and chemical properties that are different than their bulk form. These changes are often related to the material adopting a different crystallographic phase. Often these phase transformations are serendipitously observed with the criteria for their stability difficult to ascertain. This work elucidates the underpinnings of phase stability behavior in the nanoscale regime by providing a systematic study using Ti/bcc multilayered thin film architectures. The influences of lattice misfit, layer thickness, composition and chemical intermixing on the phase stability are determined. In situ thin film growth stresses of these materials are measured and correlated to the interfacial stress evolution to help rationalize the stability behavior. X-ray and electron diffraction have been employed to determine the phase with atom probe tomography used to characterize the chemical compositions within the materials and across the interfaces. This work will delineate how intrinsic film stress drives compositional intermixing across such interfaces which can thermodynamically promote phase transformations.Item Precursor gas comparison for the growth of silicon carbide fibers via laser chemical vapor deposition(University of Alabama Libraries, 2020) Matt, Mia Catherine; Thompson, Gregory B.; University of Alabama TuscaloosaLaser chemical vapor deposition (LCVD) is a processing technique that can be used to grow fibers. In this study, the relationships between deposited SiC fiber composition, fiber microstructure, and fiber mechanical properties (ambient temperature failure stress in tension) were compared using two different precursor gases - Tetramethylsilane (TMS) and Dimethylsilane (DMS) – using laser chemical vapor deposition (LCVD). Each set of fibers was grown at 2, 4 and 6 bar at a growth rate of 50 um/s. Furthermore, each set of fibers contained a nanocrystalline core. In some cases, the presence of nodular structures were noted, but these features were comprised of nanocrystalline grains. The crystalline structure for both fibers were indexed by X-ray diffraction as the 3C-SiC cubic phase. The fibers were carbon-rich. In general, the TMS fibers had generally higher average stress at failures that were 2280 MPa as compared to the average DMS fibers being 1150 MPa. Considerable spread in the tensile strength at failure was noted for the DMS fibers and is contributed to residual stresses, as they fibers were qualitatively more delicate to handle. A Weibull analysis revealed that the fibers had a low Weibull modulus, which is an indication of a large and unpredictable variation of flaws within the fibers.