Thesis (M.S.) University of Missouri-Columbia, 2005. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (July 10, 2006) Includes bibliographical references.

This dissertation tested the hypothesis that pulsed laser deposition (PLD) could be used to create targeted dopant profiles in few layered WS2 films based on congruent evaporation of the target. At the growth temperatures used, 3D Volmer-Weber growth was observed. Increased energy transfer from the PLD plume to the growing films degraded stoichiometry (desorption of sulfur) and mobility. Sulfur vacancies act as donors and produce intrinsic n-type conductivity. Post deposition annealing significantly improved the crystallinity, which was accompanied by a mobility increase from 6.5 to 19.5 cm2/Vs. Preparation conditions that resulted in excess sulfur, possibly in the form of interstitials, resulted in p-type conductivity. Current-voltage studies indicated that Ohmic contacts were governed by surface properties and tunneling. Extrinsic p-type doping of few layered WS2 films with Nb via pulsed laser deposition using ablation targets fabricated from WS2, S and Nb powders is demonstrated. The undoped controls were n-type, and exhibited a Hall mobility of 0.4 cm2/Vs. Films doped at 0.5 and 1.1 atomic percentages niobium were p-type, and characterized by Fermi levels at 0.31 eV and 0.18 eV from the valence band edge. That is, the Fermi level moved closer to the valence band edge with increased doping. With increased Nb doping, the hole concentrations increased from 3.9 x1012 to 8.6 x1013 cm-2, while the mobility decreased from 7.2 to 2.6 cm2/Vs, presumably due to increased ionized impurity scattering. X-ray photoelectron spectroscopy indicates that Nb substitutes on W lattice sites, and the measured peak shifts toward lower binding energy observed corresponded well with the UPS data. Throughout, a clear correlation between degraded stoichiometry and decreased mobility was observed, which indicates that point defect and ionized impurity scattering is a dominant influence on carrier transport in PLD few-layered WS2 films. The approach demonstrates the potential of PLD for targeted doping of transition metal dichalcogenides.

No description available.

A laser-based ultrasonic technique using a pulsed laser for stimulating ultrasonic waves in fiber-reinforced composite materials is the subject of investigation. For convenience, the material is chosen to be homogeneous transversely isotropic. The study is strictly limited to the laser power regimes that are suitable for nondestructive evaluation. An elastodynamic methodology is presented based on integral formulation in order to develop a representation for the dynamic responses in terms of the characteristics of the source that originated the motion. This requires a computation of elastodynamic Green function which represents the displacement field from the idealized synthetic sources localized precisely in both space and time. A two-dimensional numerical analysis utilizing a finite difference method for computation of the Green function in a finite plate is developed which provides the basis for quantitative nondestructive evaluation of fiber reinforced composite materials. Numerical results are presented for the surface displacement at the epicenter. Prediction based on numerical simulations are compared with experimental results. / Ph. D.

The multi-order response of the tuned secondary circuit of a Tesla transformer, following impulse excitation from its tuned primary circuit, is presented and analysed at the fundamental resonant frequency and at higher-order mode frequencies. A novel way of modifying the frequency response of the secondary coil is then investigated by utilising a technique normally applied to the design of a certain type of filter known as a helical filter. In general, these are used in radio and microwave frequency circuits in order to pass certain frequencies with little attenuation whilst significantly attenuating other frequencies. Design techniques, developed over several decades, modify and optimise the performance of such filters. The frequency response of the helical filter is modified by altering the geometry of the helical resonator component therein, which is typically in the form of an air-cored single-layer solenoid. A Tesla transformer whose secondary is constructed to be some form of single-layer solenoidal winding resonates at its designed frequency - its fundamental mode - but also at non-integer harmonics (higher-order anharmonic frequencies, also known as overtones). Those multi-order oscillatory voltages and currents energised in the secondary circuit have been identified and measured and research has determined the fundamental and higher-order mode frequencies and amplitudes for various experimental secondary winding configurations derived from helical filter design techniques. Applied to the Tesla transformer secondary winding, such techniques lead to a new design with a performance that is improved by the suppression of higher- order anharmonic frequencies whilst imparting little change to the fundamental response. It is anticipated that this feature will lead to Tesla transformers which exhibit enhanced spectral purity and which will be better suited to use in certain pulsed power applications than conventionally wound designs.

No description available.

The plasma opening switch (POS) is used in pulsed power systems where a very fast opening and high current switch is required. Plasma is injected into the switch, which carries a large conduction current, before it opens in a process that lasts for a few nanosecond and transfers the current to a parallel-connected load at a much increased voltage and with a much shorter rise time. The conduction and opening times of the switch are dependent on plasma parameters such as the distribution, speed and species, all of which are determined by the plasma source. Most of the earlier reported work involves large dimension POSs and a correspondingly high input current (more than 100 kA) and uses carbon plasma. One main objective of the present research was to achieve a low input current (20 kA) and miniaturised POS by using hydrogen plasma rather than carbon plasma on account of its lower mass. A cable gun was selected for producing the plasma, since although this produces both hydrogen and carbon plasma these arise different times during its operation. For the present application a Tesla transformer was used in preference to a Marx generator to produce an initial high voltage pulse for the system, on the basis of its simpler design and cost effectiveness. This transformer together with an associated water PFL (pulse forming line) and pressurised switch was capable of producing a load current in excess of 20 kA with a rise time of 53 ns, which was fed through the POS to the final load. Special diagnostics arrangements were necessary to measure the fast high current and voltage pulse a in nonintrusive way. Faraday cups and a high speed camera were used to measure the plasma parameters. The overall system built (i.e. including the POS) is capable of producing a 22 kA current with a rise time of 5 ns, and of generating a power of more than 10 GW. Much of the work detailed in the thesis has already been presented in peer reviewed journals and at prestigious international conferences.

No description available.

The deposition of WOx and FeOx thin films by reactive PLD and post-deposition annealing in an oxygen environment have been investigated. The influence of the deposition parameters on the growth and structure of WOx as well as the electronic and structural properties of FeOx thin films are presented. WOx thin films have been deposited onto native oxide Si (100) and SrTiO3 (100) substrates, whilst FeOx films were deposited onto glass and MgO (100). The films have been analysed using X-ray diffraction (XRD), texture pole figure analysis, X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and resistance vs temperature measurements. The WOx films deposited on both Si and SrTiO3 substrates were found to exhibit a dependence on the primary texture with fluence in the range of 5.3 J cm-2 to 14.7 J cm-2. The WOx films deposited on SrTiO3 were observed to exhibit a biaxial texture whilst the films on Si displayed a (002) WO3 out of plane fibre texture. The primary texture of the WOx films on SrTiO3 was observed to evolve from (200)/(020) WO3 to a single (002) WO3 texture with increasing fluence. The FeOx films deposited via reactive PLD onto glass substrates were found to exist in the Fe2O3 oxidation state for all parameters used. The production of Fe3O4 by post-deposition annealing of Fe films on glass and MgO (100) substrates in an oxygen environment is also presented. On both substrates it was necessary to use a two-stage anneal process to produce Fe3O4; an initial 175 °C oxygen anneal followed by a 500 °C vacuum anneal. The presence of Fe3O4 was confirmed by the existence of the Verwey transition at close to 120 K, during resistance vs temperature scans. The anneal parameters required to produce Fe3O4 are shown to be different for MgO substrates in comparison to glass.

This study presents the overall design of a 3U CubeSat equipped with commercial-off-the shelf hardware, Teflon-fueled micro-Pulsed Plasma Thrusters (µPPT) and an attitude determination and control system. The µPPT is sized by the impulse bit and pulse frequency required for continuous compensation of expected maximum disturbance torques at altitudes between 400 and 1000 km, and to perform stabilization of up to 20 deg/s and slew maneuvers of up to 180 degrees. The study involves realistic power constraints anticipated on the 3U CubeSat. Attitude estimation is implemented using the q-method for static attitude determination of the quaternion using pairs of the spacecraft-sun and magnetic field vectors. The quaternion estimate and the gyroscope measurements are used with an extended Kalman filter to obtain the attitude estimates. Proportional and derivative control algorithms use the static attitude estimation in order to calculate the angular momentum required to compensate for the disturbance torques and to achieve specified stabilization and slewing maneuvers or combinations. Two control methods are developed: paired firing method, and separate control algorithm and thruster allocation methods which determines the optimal utilization of the available thrusters and introduces redundancy. Simulations results are presented for a 3U CubeSat under stabilization, pointing, and pointing and spinning scenarios.