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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
391

The fast calculation of magnetic field using the local refinement method /

Wang, Tongyu, 1973- January 2004 (has links)
The speed of the Finite Element Method (FEM) is an obstacle to the fast calculation of magnetic fields. A fast Local Refinement Method (LRM) using the first-order FEM is presented for quickly tracking the magnetic field changes while electromagnetic models have small changes made to their shape. This method resolves the potentials in the local mesh or submesh extracted from the whole mesh, with a boundary condition that is calculated by the initial solution based on the whole mesh. Instead of being re-meshed in the local area, the extracted submesh is coarsened and reshaped by the LRM to speed up the calculation time by sharply decreasing the time used for building the S matrix and solving the matrix equation Ax = b. The new potentials in the submesh are, with an acceptable error, embedded back into the whole problem to update the magnetic fields which provide designers or users with a fast visual feedback to their adjustment.
392

Studies of compounds related to Cu(In-xGax)Se solar cells

Wang, Haiping, 1969- January 2001 (has links)
Crystals of Cu(In1-xGax)3Se 5 were grown by the horizontal and vertical Bridgman methods. A non-contact carbon coating was used to avoid the adhesion between Cu(In1-x Gax)3Se5 ingots and the inner ampoule walls. The composition along and across the as grown ingots with different starting Ga contents was analyzed and the results were interpreted by the established pseudobinary phase diagrams. Results of XRD confirmed that the lattice constants of the Cu(In1-xGax)3Se 5 crystals varied linearly with the Ga content. Results of X-ray Laue back-reflection showed that the Cu(In1-xGax) 3Se5 ingots contained large single crystal regions. Hall effect measurements carried out on the grown samples revealed that the Cu(In 1-xGax)3Se5 crystals were highly resistive with rather low carrier concentrations. The morphology of as-grown or cleaved sample surfaces of the Cu(In1-xGax) 3Se5 ingots were also studied under optical microscope and SEM. / Metallic Na was, for the first time, introduced into Cu(In1-x Gax)3Se5 compounds to observe the doping effects. The introduction of Na increased the electron concentration significantly for CuIn3Se5 samples (x = 0) but did not show a significant effect on Cu(In1-xGax) 3Se5 samples with x > 0. The increase in electron concentration in the CuIn3Se5 samples after the Na diffusion could be explained by defect generation related to Se and In sites. / Crystals of CuInSe2 were also grown by the horizontal Bridgman method for the first time with the addition of metallic sodium. Degradation in crystalline quality and a change of conductivity type from p- to n-type were observed in ingots grown from melts containing more than 0.25 at. % Na. Experiments of Na diffusion were also carried out on CuInSe 2 crystals in a sealed glass ampoule to observe the doping effect. Hot probe measurements indicated that the sodium-treated CuInSe2 samples changed from p-type to n-type. / MIS devices were fabricated on the Na-treated CuIn3Se 5 material for electrical characterization. Dark current density-voltage characteristics and differential capacitance-voltage characteristics of the MIS devices were measured at room temperature. An energy band diagram of the MIS devices has been constructed based on the band lineup data reported in the literature. The current transport mechanism was examined and a dominant multi-step tunneling process was proposed. / Samples of Cu(In1-xGax)3Se 5 with x ≤ 0.5 were found to be strongly photoconductive over the wavelength range from 700 to 1100 nm even at room temperature. It was observed that the sensitivity of photoconductivity was greatly influenced by surface preparation conditions. Chemically etched samples showed the highest photoconductivity, believed to be due to the reduced surface recombination velocity. / Capacitance measurements were carried out to investigate the interface and bulk properties of ZnO/CdS/Cu(In1-xGax)Se 2 solar cells. Results from the steady state C-V measurements showed evidence of interface or surface states, especially for the samples without annealing. DLTS technique was used to determine the deep levels in Cu(In 1-xGax)Se2 crystals with Ga content varying from 0 to 1. Different deep levels for holes with different DLTS spectra were found in the Cu(In1-xGax)Se2 crystals, with different Ga contents. The present results showed that the Ga content has an important effect on the formation of deep levels in Cu(In1-x Gax)Se2 crystals.
393

Analog and mixed-signal test methods using on-chip embedded test cores

Hafed, Mohamed M. January 2002 (has links)
A robust method has been developed for the test and characterization of analog and mixed-signal integrated circuits. The method relies on a compact, robust, and easily synthesized integrated test core capable of emulating the function of external automatic test equipment. The core consists of a 2 x N memory whose contents are periodically circulated, a coarse analog filter, and a voltage comparator. One half of the circular memory is used to generate analog signals without the need for multi-bit digital-to-analog converters. The second half is used to generate extremely accurate DC levels, the latter being programmed using a clever software encoding technique that relies on some form of sigma-delta modulation. The DC levels, in combination with the comparator, enable multi-bit digitization using a progressive multiple conversion pass procedure. In order to accommodate broadband circuit phenomena, a delayed-clock sub-sampling mechanism is also employed, in which the digitizer sample clock is consistently delayed over multiple runs of the periodic test signal. One method of delaying the clock is to use a voltage-controlled delay line tuned by a delay-locked loop. The timing resolution of this approach is determined by the value of the consistent clock delay and not its period. / A divide-and-conquer approach to the test of deeply embedded analog integrated circuits using the proposed test core is described. Multiple test configurations are presented that can span a wide range of phenomena to be tested both internally to the integrated circuit and externally through I/O interfaces. The applicability of these configurations to increasing test parallelism both at the core and die levels is investigated. Performance limits of the proposed test core are also derived by drawing a comparison to conventional circuits used for data-conversion applications. The same fundamental limitations on integrated circuit performance are shown to affect the test core electronics, although test-specific requirements, such as forcing periodicity and the reliance on software signal processing, help further enhance on-chip measurement accuracy and repeatability. Finally, several successful experimental prototypes that demonstrate the viability of the proposed approach are presented. The prototypes range from concept proving test core integrated circuits to ones containing multiple simultaneously operated test cores and completely embedded circuits under test. In total, several hundred different test cores have been demonstrated, which is further testimony to the practicality of the proposed techniques.
394

Electromagnetic fields generated by ocean currents and the potential for using geomagnetic data in ocean and climate studies

Tyler, Robert H. January 1995 (has links)
The ocean currents flowing through the earth's main magnetic field are known to induce secondary magnetic fields. Hence, variations in the ocean circulation induce variations in the net magnetic field. This research is aimed at exploring the potential for using geomagnetic data to study variability in ocean circulation and climate. / First, general relativity theory is used to formally establish the proper set of electromagnetic equations to be used for observers in a rotating (accelerating) frame of reference observing a medium (the ocean, in this case) with relative velocity. Extra terms due to rotation are derived and described and a generalized Schiff's charge density is shown to be potentially significant for the application to ocean circulation. / We extend the theory of electromagnetic fields generated by ocean currents. Many analytical solutions are found for idealized ocean features including sheared flow, jets, and a Stommel gyre. Results indicate that the ocean-induced magnetic fields will typically have magnitudes of 10's-100's of nT within the ocean. Outside of the ocean, the magnitudes are smaller (typically 1-10 nT) but decay on scales set by the horizontal scale of the ocean feature. / We investigate the time-scales associated with the adjustment of electromagnetic fields generated by low-frequency ocean currents. We find that the time scales can be quite long, prohibiting a quasistatic assumption in the treatment of the electromagnetic fields generated by the important tidal, inertial, and diurnal-frequency ocean currents. / Three-dimensional explicit time-dependent and steady-state finite-difference numerical models are constructed to study the electromagnetic fields generated by more realistic ocean current and conductivity features. / The ocean currents generate electromagnetic forces on the fluid at the surface of the earth's core. If these forces lead to significant core motion, the effect of the oceans on the generation and variability of the earth's magnetic field may be nontrivial. We estimate the form and magnitude of these forces and make comparisons with observations. Despite many uncertainties, we find evidence to suggest the ocean forcing mechanism may be significant and conclude that this process should be further investigated in the context of a larger study. / This work indicates that it is likely that the geomagnetic record has captured oceanic signals. From a preliminary data analyses we find that aside from the oceanic tidal signals, the magnetic record shows other signals of possible oceanic origin including an apparent correlation between magnetic records from the equatorial Pacific and the Southern Oscillation Index. We discuss the prerequisites that are necessary to extract information about ocean circulation variability from the geomagnetic record.
395

Structural and magnetotransport properties of nickelcobalt multilayers

Freitag, James M. (James Mac) January 1996 (has links)
Ferromagnetic/ferromagnetic Ni/Co multilayers with component layer thicknesses ranging from 40 A down to 5 A were prepared by DC-magnetron sputtering. Due to the fact that Ni and Co alloys share a common d band, it is expected that the total resistance of the multilayers, including the elemental resistance of the layers and the resistance of the interdiffused alloyed region at the interfaces, will be lower than for other 3d transition metal combinations. Consequently, the magnetoresistance ratio $ Delta rho/ rho$ is expected to be enhanced. / Structural characterization by grazing-angle X-ray reflectivity reveals high-quality layered structures with a well-defined composition modulation along the film growth direction. Wide-angle X-ray diffraction scans display the polycrystalline nature of the Ni/Co multilayers which grow in an FCC phase with a preferred (111) orientation and a fraction of (200) structural domains. / Measurements of the magnetotransport properties of these multilayers indicate that the magnetoresistance (MR) effect, $ Delta rho sim0.35 mu Omega cdot$cm, is roughly constant over the entire compositional range. The MR ratio $ Delta rho/ rho,$ which is as high as 3.0% in a Si/(Ni40A/Co5A) $ times$ 6 multilayer, is therefore more strongly dependent on the zero-field resistivity $ rho.$ By fitting a semi-classical model to the resistivity compositional variation, we determined the interface contribution to the resistivity. The MR measurements as well as the magnetic anisotropy of the films, studied by vibrating sample magnetometry (VSM) and magneto-optical Kerr effect (MOKE) magnetometry, are consistent with the origin of the observed MR effect being anisotropic magnetoresistance (AMR). The highest magnetic sensitivity measured at zero-field and constant in the range from ${ sim}{-}10$ Oe to +10 Oe was 0.1%/Oe. This value compares well with other alloys being developed as magnetic sensors. (Abstract shortened by UMI.)
396

Radio-frequency energy quantification in magnetic resonance imaging

Alon, Leeor 23 October 2014 (has links)
<p> Mapping of radio frequency (RF) energy deposition has been challenging for 50+ years, especially, when scanning patients in the magnetic resonance imaging (MRI) environment. As result, electromagnetic simulation software is often used for estimating the specific absorption rate (SAR), the rate of RF energy deposition in tissue. The thesis work presents challenges associated with aligning information provided by electromagnetic simulation and MRI experiments. As result of the limitations of simulations, experimental methods for the quantification of SAR were established. A system for quantification of the total RF energy deposition was developed for parallel transmit MRI (a system that uses multiple antennas to excite and image the body). The system is capable of monitoring and predicting channel-by-channel RF energy deposition, whole body SAR and capable of tracking potential hardware failures that occur in the transmit chain and may cause the deposition of excessive energy into patients. Similarly, we demonstrated that local RF power deposition can be mapped and predicted for parallel transmit systems based on a series of MRI temperature mapping acquisitions. Resulting from the work, we developed tools for optimal reconstruction temperature maps from MRI acquisitions. The tools developed for temperature mapping paved the way for utilizing MRI as a diagnostic tool for evaluation of RF/microwave emitting device safety. Quantification of the RF energy was demonstrated for both MRI compatible and non-MRI-compatible devices (such as cell phones), while having the advantage of being noninvasive, of providing millimeter resolution and high accuracy.</p>
397

A fourth-order adaptive mesh refinement solver for Maxwell's Equations

Chilton, Sven 28 May 2014 (has links)
<p> We present a fourth-order accurate, multilevel Maxwell solver, discretized in space with a finite volume approach and advanced in time with the classical fourth-order Runge Kutta method (RK4). Electric fields are decomposed into divergence-free and curl-free parts; we solve for the divergence-free parts of Faraday's Law and the Amp&egrave;re-Maxwell Law while imposing Gauss' Laws as initial conditions. We employ a damping scheme inspired by the Advanced Weather Research and Forecasting Model to eliminate non-physical waves reflected off of coarse-fine grid boundaries, and Kreiss-Oliger artificial dissipation to remove standing wave instabilities. Surprisingly, artificial dissipation appears to damp the spuriously reflected waves at least as effectively as the atmospheric community's damping scheme.</p>
398

Analytical and computational investigations of a magnetohydrodynamics (MHD) energy-bypass system for supersonic gas turbine engines to enable hypersonic flight

Benyo, Theresa Louise 13 June 2014 (has links)
<p> Historically, the National Aeronautics and Space Administration (NASA) has used rocket-powered vehicles as launch vehicles for access to space. A familiar example is the Space Shuttle launch system. These vehicles carry both fuel and oxidizer onboard. If an external oxidizer (such as the Earth's atmosphere) is utilized, the need to carry an onboard oxidizer is eliminated, and future launch vehicles could carry a larger payload into orbit at a fraction of the total fuel expenditure. For this reason, NASA is currently researching the use of air-breathing engines to power the first stage of two-stage-to-orbit hypersonic launch systems. Removing the need to carry an onboard oxidizer leads also to reductions in total vehicle weight at liftoff. This in turn reduces the total mass of propellant required, and thus decreases the cost of carrying a specific payload into orbit or beyond. However, achieving hypersonic flight with air-breathing jet engines has several technical challenges. These challenges, such as the mode transition from supersonic to hypersonic engine operation, are under study in NASA's Fundamental Aeronautics Program. </p><p> One propulsion concept that is being explored is a magnetohydrodynamic (MHD) energy- bypass generator coupled with an off-the-shelf turbojet/turbofan. It is anticipated that this engine will be capable of operation from takeoff to Mach 7 in a single flowpath without mode transition. The MHD energy bypass consists of an MHD generator placed directly upstream of the engine, and converts a portion of the enthalpy of the inlet flow through the engine into electrical current. This reduction in flow enthalpy corresponds to a reduced Mach number at the turbojet inlet so that the engine stays within its design constraints. Furthermore, the generated electrical current may then be used to power aircraft systems or an MHD accelerator positioned downstream of the turbojet. The MHD accelerator operates in reverse of the MHD generator, re-accelerating the exhaust flow from the engine by converting electrical current back into flow enthalpy to increase thrust. Though there has been considerable research into the use of MHD generators to produce electricity for industrial power plants, interest in the technology for flight-weight aerospace applications has developed only recently. </p><p> In this research, electromagnetic fields coupled with weakly ionzed gases to slow hypersonic airflow were investigated within the confines of an MHD energy-bypass system with the goal of showing that it is possible for an air-breathing engine to transition from takeoff to Mach 7 without carrying a rocket propulsion system along with it. The MHD energy-bypass system was modeled for use on a supersonic turbojet engine. The model included all components envisioned for an MHD energy-bypass system; two preionizers, an MHD generator, and an MHD accelerator. A thermodynamic cycle analysis of the hypothesized MHD energy-bypass system on an existing supersonic turbojet engine was completed. In addition, a detailed thermodynamic, plasmadynamic, and electromagnetic analysis was combined to offer a single, comprehensive model to describe more fully the proper plasma flows and magnetic fields required for successful operation of the MHD energy bypass system. </p><p> The unique contribution of this research involved modeling the current density, temperature, velocity, pressure, electric field, Hall parameter, and electrical power throughout an annular MHD generator and an annular MHD accelerator taking into account an external magnetic field within a moving flow field, collisions of electrons with neutral particles in an ionized flow field, and collisions of ions with neutral particles in an ionized flow field (ion slip). In previous research, the ion slip term has not been considered. </p><p> The MHD energy-bypass system model showed that it is possible to expand the operating range of a supersonic jet engine from a maximum of Mach 3.5 to a maximum of Mach 7. The inclusion of ion slip within the analysis further showed that it is possible to 'drive' this system with maximum magnetic fields of 3 T and with maximum conductivity levels of 11 mhos/m. These operating parameters better the previous findings of 5 T and 10 mhos/m, and reveal that taking into account collisions between ions and neutral particles within a weakly ionized flow provides a more realistic model with added benefits of lower magnetic fields and conductivity levels especially at the higher Mach numbers. (Abstract shortened by UMI.)</p>
399

Mineral Magnetism of Environmental Reference Materials: Iron Oxyhydroxide Nanoparticles

Gonzalez Lucena, Fedora 30 September 2010 (has links)
Iron oxyhydroxides are ubiquitous in surface environments, playing a key role in many biogeochemical processes. Their characterization is made challenging by their nanophase nature. Magnetometry serves as a sensitive non-destructive characterization technique that can elucidate intrinsic physical properties, taking advantage of the superparamagnetic behaviour that nanoparticles may exhibit. In this work, synthetic analogues of common iron oxyhydroxide minerals (ferrihydrite, goethite, lepidocrocite, schwertmannite and akaganéite) are characterized using DC and AC magnetometry (cryogenic, room temperature), along with complementary analyses from Mössbauer spectroscopy (cryogenic, room temperature), powder X-ray diffraction and scanning electron microscopy. It was found that all of the iron oxyhydroxide mineral nanoparticles, including lepidocrocite, schwertmannite and akaganéite were superparamagnetic and therefore magnetically ordered at room temperature. Previous estimates of Néel temperatures for these three minerals are relatively low and are understood as misinterpreted magnetic blocking temperatures. This has important implications in environmental geoscience due to this mineral group’s potential as magnetic remanence carriers. Analysis of the data enabled the extraction of the intrinsic physical parameters of the nanoparticles, including magnetic sizes. The study also showed the possible effect on these parameters of crystal-chemical variations, due to elemental structural incorporation, providing a nanoscale mineralogical characterization of these iron oxyhydroxides. The analysis of the intrinsic parameters showed that all of the iron oxyhydroxide mineral nanoparticles considered here have a common magnetic moment formation mechanism associated with a random spatial distribution of iv uncompensated magnetic spins, and with different degrees of structural disorder and compositional stoichiometry variability, which give rise to relatively large intrinsic magnetization values. The elucidation of the magnetic nanostructure also contributes to the study of the surface region of the nanoparticles, which affects the particles’ reactivity in the environment.
400

Numerical studies of conductance fluctuations in disordered metals

Houari, Ahmed January 1990 (has links)
We compute conductance fluctuations in a variety of disordered mesoscopic systems through direct numerical evaluation of the Kubo-Greenwood formula for the conductivity. / Our model Hamiltonian is quite different from the Anderson tight-binding Hamiltonian which has been most commonly used in studies of electronic structure and properties of disordered systems. It is reminiscent of the Kronig-Penney model in that $ delta$-like atomic potentials are specified by a single parameter. / Our model structures range from substitutional binary alloys to topologically disordered "glasses", and include systems where the disorder is caused by random small displacements of atoms from their crystalline lattice positions. / We test the universal nature of conductance fluctuations for the model with substitutional disorder. We study systems which are always larger than the elastic mean free path, but not always smaller than the localization length of the electron wavefunctions. In those systems where universality is expected, we confirm the ergodic theorem of Lee and Stone and observe the universal amplitudes of the fluctuations both in two-dimensional systems with and without a magnetic field, and in quasi-one-dimensional geometry. / We have also performed the first studies of conductance fluctuations in strongly disordered systems and observed a universal relationship between the amplitude of the fluctuations and the value of the conductance itself: this relationship does not depend upon the nature of the disorder.

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