<p>Applications of Aluminum (Al) alloys in harsh environments involving high stress and high temperatures are often hindered because of their inherently low strength and poor performance at high temperatures. The strongest commercial Al alloys reported up to date have a maximum strength less than 700 MPa. Although ultrafine grained Al alloys prepared by severe plastic deformation have higher strength, they encounter grain growth at moderate temperatures. </p> <p>This thesis focuses on adopting transition metal solutes and non-equilibrium approach to fabricate high-strength, thermally stable nanotwinned Al alloys. To understand the underlying deformation mechanisms of nanotwinned Al alloys, <i>in-situ</i> micromechanical tests, high resolution and analytical transmission microscopy and atomistic simulations were used. Our studies show that nanotwinned supersaturated Al-Fe alloys have a maximum hardness and flow stress of ~ 5.5 GPa and 1.6 GPa, respectively. The apparent directionality of the vertical incoherent twin boundaries renders plastic anisotropy and compression-tension asymmetry in the nanotwinned Al-Fe alloys, revealed by systematic <i>in-situ</i> tensile and compressive micromechanical experiments conducted from both in-plane and out-of-plane directions. Moreover, the nanotwinned Al-Fe alloys experience no apparent softening when tested at 200 °C. When selectively incorporating with one additional solute as stabilizer, the ternary nanotwinned Al alloys can preserve an exceptionally high flow stress, exceeding 2 GPa, prior to precipitous softening at an annealing temperature of > 400 °C. The thesis offers a new perspective to the design of future strong, deformable and thermally stable nanostructured Al alloys. </p>

This thesis deals with an introduction to the theory of switching power supplies and with properties of magnetic materials. Special attention is paid to nanocrystalline materials. It also discusses the issues of simulating the behavior of ferromagnets. The properties of commonly used ferrite material and nanocrystalline materials was measured. Using the created program for the design of forward converter is designed a switching power supply to verify its proposal. Design optimization is based on using a nanocrystalline core, which allows to reduce the operating frequency.

Nanocrystalline diamond (NCD) films possess great mechanical properties (low friction coefficient, high hardness etc.), chemical properties (e.g. low corrosivity or chemical inertness) and good biocompatibility. This makes them perspective materials for protective coatings of medical implants and devices. As bacteria biofilms are often very resistant to antibacterial treatment, materials with anti-bacterial or at least anti-adhesive properties are needed. The interaction of NCD films with bacteria has not been properly examined yet. The aim of this thesis was to introduce and optimize the methods for routine bacterial biofilm cultivation and analysis, use them to investigate the ability of NCD films to inhibit the attachment and biofilm formation of Escherichia coli and correlate it with the NCD surface hydrophobicity. The materials used for the study were hydrogenated NCD (hydrophobic), oxidized NCD (hydrophilic) and uncoated glass. For bacterial biofilm growth, cultivation in six-well plates and continuous cultivation in CDC Bioreactor was used. Several methods were tested for quantitative biofilm detachment and analysis. The putative anti-bacterial properties of NCD material were not confirmed in this work. Higher bacterial attachment to NCD films in comparison to the uncoated glass was...

Im quaternären System Erdalkalimetall-Silicium-Chlor-Wasserstoff bildet sich bei der Umsetzung des Metalls mit einer SiCl4-H2-Atmosphäre eine quaternäre Phase. Diese metastabile Phase zerfällt beim Abkühlen in das Metallchlorid und Silicium in nanokristalliner Form. Die vorliegende Arbeit hat sich mit der tiefergehenden Charakterisierung der quaternären Phase beschäftigt. Dazu wurden die Eigenschaften des quaternären Systems aus den Eigenschaften der sechs binären und vier ternären Systemen abgeleitet. Die Oberfläche wurde erstmals mit Photoelektronenspektroskopie charakterisiert. Zusätzlich gelang erstmalig die Verfolgung der Bildungsreaktion durch Messung des Spannungsabfalls über das Reaktionssystem. Erste Ansätze zur Aufklärung des Bildungsmechanismus ausgehend von den Ergebnissen der Charakterisierung wurden zusätzlich aufgezeigt.

Damage to the central nervous system (CNS) leads to the generation of an immune response which culminates with the encapsulation of the damaged area. The encapsulation, known as a glial scar, essentially breaks neural signal pathways and blocks signal transmissions to and from the CNS. The effect is the loss of motor and sensory control for the damaged individual. One method that has been used successfully to treat this problem is the use of a brain-machine interface (BMI) which can intercept signals from the brain and use these signals to control a machine. Although there are many types of BMI devices, implantable devices show the greatest promise with the ability to target specific areas of the CNS, with reduced noise levels and faster signal interception, and the fact that they can also be used to send signals to neurons. The largest problem that has plagued this type of BMI device is that the materials that have been used for their construction are not chemically resilient, elicit a negative biological response, or have difficulty functioning for extended periods of time in the harsh body environment. Many of these implantable devices experience catastrophic failure within weeks to months because of these negative factors. New materials must be examined to advance the future utilization of BMI devices to assist people with CNS damage or disease. We have proposed that two semiconductor materials, cubic silicon carbide (3C-SiC) and nanocrystalline diamond (NCD), which should provide solutions to the material biocompatibility problems experienced by implantable BMI devices. We have shown in this study that these two materials show chemical resilience to neuronal cellular processes, and we show evidence which indicates that these materials possess good biocompatibility with neural cell lines that, in the worst case, is comparable to celltreated polystyrene and, in most cases, even surpasses polystyrene. We have utilized 3C-SiC within an electrode device and activated the action potential of differentiated PC12 cells. This work details our initial efforts to modify the surfaces of these materials in order to improve cellular interaction and biocompatibility, and we examine our current and future work on improving our implantable BMI devices.

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<p>This research study has been devoted to the study of magnetic properties and magnetic transport of nanocrystalline Ni and Ni-15% Fe alloys consisting of randomly oriented grains with an average size of 23 and 12 (nm), respectively. The structures of the deposits were confirmed by the XRD analysis using Rietveld refinement technique. The as-deposited Ni and Ni-15%Fe sample was comprised exclusively of the γ phase with lattice parameter of 3.5270 (nm) and 3.5424 (nm), respectively. The small increase in lattice parameter was attributed to the replacement of iron solutes in the Ni sites in lattice. Texture analysis of nanocrystalline Ni and Ni-15%Fe revealed that textures components of both materials is qualitatively the same and vary in terms of volume fraction. Both material showed strong <100> fibre texture with some contribution of the <111> component. The calculated volume fraction of the <100> and <111> components were respectively 17.157% and 3.201% for Ni and, 22.032% and 6.160% for Ni-15%Fe and the rest being confined to the random texture.</p> <p>Magnetic measurements show that all samples exhibit low loss hysteresis loops with high permeabilities. The presence of 15%Fe in Ni leads to enhancement of the saturation magnetization (M_s) regardless of the direction of the applied field. M_s shows an increase from 60.169 (emu/gr) in nanocrystalline Ni to 93.67 (emu/gr) in Ni-15%Fe sample at T=2K. No strong temperature–dependence of the magnetization was observed for samples, but the magnetization of the Ni-15%Fe samples at T=2K were slightly higher than that of T=298K. The coercivity values of nanocrystalline Ni-15%Fe were in all cases smaller than that of nanocrystalline Ni samples. Good agreement between random anisotropy model (RAM) theory and experiment for nanocrystalline Ni and Ni-15%Fe samples was observed. The ferromagnetic exchange length (L_ex) was larger than the average grain size (D) for samples at all times. The effective magnetic anisotropy constants (K_eff) of the nanocrystalline Ni and Ni-15%Fe alloys were measured using the law of approach to saturation. At T=2K, the K_eff of Ni-15%Fe samples were measured to be 1.7037´10⁵ (erg/cm³) and 2.71996 ´10⁵ (erg/cm³) at field parallel and perpendicular, respectively. These values were almost half of the values obtained for nanocrystalline Ni samples 4.66091´10⁵ (erg/cm³) and 4.19703´10⁵ (erg/cm³). Temperature dependence measurements showed that K_eff constants decrease with increasing temperature. The angular dependence MR studies on nanocrystalline Ni and Ni-15%Fe resulted in a twofold, and a fourfold symmetric behaviour, respectively. The field dependence MR measured at various sample tilt with respect to the applied field, showed various trends from pure positive MR to pure negative MR, which partially could be explained by magnetocrystalline anisotropy of the samples.</p> / Master of Applied Science (MASc)

The research reported in this dissertation focuses on the response of grain boundaries in polycrystalline metallic nanostructures to applied strain using molecular dynamics simulations and empirical interatomic force laws. The specific goals of the work include establishing how local grain boundary structure affects deformation behavior through the quantitative estimation of various plasticity mechanisms, such as dislocation emission and grain boundary sliding. The effects of strain rate and temperature on the plastic deformation process were also investigated. To achieve this, molecular dynamics simulations were performed on both thin-film and quasi-2D virtual samples constructed using a Voronoi tessellation technique. The samples were subjected to virtual mechanical testing using uniaxial strain at strain rates ranging from 105s-1 to 109s-1. Seven different interatomic embedded atom method potentials were used in this work. The model potentials describe different metals with fcc or bcc crystal structures. The model was validated against experimental results from studying the tensile deformation of irradiated austenitic stainless steels performed by collaborators at the University of Michigan. The results from the model validation include a novel technique for detecting strain localization through adherence of gold nanoparticles to the surface of an experimental sample prior to deformation. Similar trends with respect to intergranular crack initiation were observed between the model and the experiments. Simulations of deformation in the virtual samples revealed for the first time that equilibrium grain boundary structures can be non-planar for model potentials representing fcc materials with low stacking fault energy. Non-planar grain boundary features promote dislocation as deformation mechanisms, and hinder grain boundary sliding. This dissertation also reports the effects of temperature and strain rate on deformation behavior and correlates specific deformation mechanisms that originate from grain boundaries with controlling material properties, deformation temperature and strain rate. / Ph. D.