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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.
101

Quantification of Laser-Induced Breakdown Spectroscopy at Low Energies

Taschuk, Michael Thomas Unknown Date (has links)
Laser-induced breakdown spectroscopy (LIBS) is an elemental characterisation technique using spectrally resolved emission froma laser-induced plasma to determine the composition of a sample. Due to limited quantitifcation of emission levels, in the literature, it is difficult to compare results between different lab groups, and only qualitative comparisons can be made with theory. As a result, understanding of the underlying physical processes which govern LIBS has lagged the growth of applications. Most applications of the LIBS technique have employed laser pulse energies in the range of 10 - 100mJ, focal spot sizes of ��� 100 ��m, and an accumulation of 10 - 100 spectra for a single measurement. The high energies, large focal spots and number of shots acquired improves the sensitivity of LIBS. The primary focus of this thesis is the quantification of the LIBS technique, LIBS equipment and the extension of LIBS to much lower pulse energies. This new regime, referred to as ��LIBS, utilises pulse energies below 100 ��J. In this thesis a theory of LIBS detector systems is developed, and used to define responsivity, noise-equivalent integrated spectral brightness and noise-equivalent spectral brightness in terms useful for the LIBS experimentalist. Four LIBS detection systems have been characterised. Laser ablation plume dynamics and absolute emission levels from a millijoule energy LIBS system were studied and compared with simple physical models for shock wave expansion and stagnation. A simple model for the emission is compared with the absolute emission levels of the LIBS plasma. The scaling of LIBS emission below 100 ��J pulse energies is investigated. The number of photons emitted is found to be a small fraction of the number of atoms ablated for the energy range between 100 nJ and 100 ��J. Using a thin film target, it is found that the ablated region which contributes to the LIBS emission is restricted to a layer much shallower than the ablation crater. Finally, two applications of the ��LIBS technique are presented. Surface mapping of Al alloys with sub-microjoule laser pulses is demonstrated. Latent fingerprint detection and imaging is demonstrated using the ��LIBS technique
102

Laser Induced Hierarchical Coatings on Titanium Alloy

Kurella, Anil Kumar 01 August 2009 (has links)
Biomaterials research is an exciting and challenging area. It is exciting because of its potential applications and need for improving the quality of life. It is challenging because of the complexity with which natural biomaterials function in their environments. The gap that exists in terms of maturity and sophistication of the currently used synthetic materials from natural biomaterials is huge. It is only in the last few decades with the evolution of advanced material analytical techniques that researchers are starting to understand the complexity of nature. One such particular feature that has attracted our interest is the hierarchical nature of the bioimplant surfaces. The present work is one small step in that direction where we tried to engineer a surface that is multi-scale in nature and biocompatible at these length scales. During a discovery phase a multi-scale textured zirconia coating was done on titanium alloy using a pulsed laser. Following proof of concept a bioactive calcium phosphate based coating was deposited on titanium alloy surface using a continuous wave laser. Based on detailed morphological and chemical analysis it was evident that the multi-phase coating had a multi-scale arrangement. Owing to the complexity of the coating a fractal based approach was used to interpret the morphology of the coatings. It appeared that at higher laser processing speeds star shaped calcium titanate features exist inside calcium phosphate and titania ring like structures. By tailoring a thermal model with current material system temperature calculations were made for various laser processing speeds. Using temperature predictions and knowledge of the phase constituents the series of self assembling steps that led to the formation of star and ring shaped arrangement are discussed. The biocompatibility of the coatings was evaluated by immersing in simulated body fluids. The morphological and chemical evolution of hydroxyapatite precipitation along the calcium phosphate rich ring like structures coupled with the porous structure supports the possibility of enhanced osteointegration. The presence of calcium titanate ensured an interaction between the substrate and the precursor coating material. Wear measurements indicated that the laser processed samples possessed better mechanical properties than unprocessed surfaces.
103

Electron Beam Induced Etching

Lassiter, Matthew Gordon 01 August 2009 (has links)
The mechanisms of electron beam induced etching have been studied both experimentally and theoretically. Specifically, a steady-state and a time-dependent continuum model of the process have been developed which uniquely includes the effect of the etch product desorption and diffusion. Both analytical and numerical methods were employed for the modeling, and various experimental designs were used for validation. Initially, a steady-state model was developed to understand an observed so-called “moat” profile which could adequately be described by a finite etch product surface residence time. Subsequently a thorough time-dependent model was written to investigate scanning parameter effects on EBIE. A design of experiments was performed to validate the model and to extract the fundamental parameters for the etching of silica by xenon difluoride. Finally, two technical applications were explored: spontaneous etching passivation on Ta-based extreme ultraviolet lithography masks and carbon nanotube etching.
104

Supply Chain Management with Demand Substitution

Song, Laigang 01 August 2009 (has links)
Demand substitution is a very common practice, but due to its inherent difficulty of mathematical modeling, little has been done on the impact of the demand substitution to the supply chain network.This dissertation studied the impact of demand substitution to a supply chain network. One of the most important measurements of supply chain network, Bullwhip effect, is studied under the demand substitution case. To help understand the influence, a new qualitative measurement of the bullwhip effect is proposed to better capture the essence of the uncertainty associated with supply chain networks. Then a mathematical model is formulated to investigate the bullwhip effect of two products substitution case. Due to the difficulty of the mathematical modeling of the demand substitution process, “Metamodel” methodology is applied to study the relationship among different aspects of the supply chain network. Finally, a network based algorithm is proposed to represented the demand substitution process. Graphical Evaluation And Review Technique (GERT) is used to solve the network model. The results demonstrated the effectiveness of the network model to approximate the demand substitution problem. In the end, the dissertation concludes with a summary of the contributions to the state of the art.
105

Differentiated Intrusion Detection and SVDD-based Feature Selection for Anomaly Detection

Kang, Inho 01 August 2007 (has links)
Most of existing intrusion detection techniques treat all types of attacks equally without any differentiation of the risk they pose to the information system. However, certain types of attacks are more harmful than others and their detection is critical to protection of the system. This study proposes a novel differentiated anomaly detection method that can more precisely detect intrusions of specific types of attacks. Although many researchers have been developed many efficient intrusion detection methods, fewer efforts have been made to extract effective features for host-based intrusion detection. In this study, we propose a new framework based on new viewpoints about system activities to extract host-based features, which can guide further exploration for new features. There are few feature selection methods for anomaly detections although lots of studies have been done for the feature selection both in classification and regression problems. This study proposes new support vector data description (SVDD)-based feature selection methods such as SVDD-R2-recursive feature elimination (RFE), SVDD-RFE and SVDDGradient method. Concrete experiments with both simulated and the Defense advanced research projects agency (DARPA) datasets shows promising performance of the proposed methods. These achievements in this dissertation could significantly contribute to anomaly detection field. In addition, the proposed differentiated detection and SVDD-based feature selection methods would benefit even other application areas beyond intrusion detection
106

Investigation of Magnetic Field Dependent Electroluminescence and Charge Injection in Organic Light Emitting Diodes

Wu, Yue 01 August 2007 (has links)
After 20 years of development, conjugated polymers have been extensively applied in organic light emitting diodes (OLED), solar cells, transistors, and chemical or bio-sensors. Recently it is discovered that magnetic field can tune the electroluminescence intensity and conductivity in OLEDs, leading to the development of organic magneto-optoelectronics. However, the underlying mechanisms are still unclear. In this dissertation, we investigated a wide range of conjugated polymers and low molecular weight molecules and proposed that the magnetic field effect on electroluminescence and magnetoresistance arise from the magnetic field enhanced polaron pair dissociation and reduced triplet-charge reaction. The final magnetic field effects are determined by the sum of the two contributions. The magnetic field effect on polaron pair dissociation can be tuned by varying the spin-orbital coupling of the organic semiconductor. Stronger spin-orbital coupling leads to the reduction of magnetic field effect on both electroluminescence and magnetoresistance. Phosphorescent dye doping can also tune the magnetic field effects through energy transfer process and intermolecular interaction. Triplet-charge reaction can be largely controllable by manipulating the bipolar injection. It has found that unbalanced bipolar injection enhance the triplet-charge injection, leading to more positive magnetoresistance and more negative magnetic field effect on electroluminescence. Balanced bipolar injection reduces triplet charge reaction, resulting in more negative magnetoresistance and more positive magnetic filed effect on electroluminescence. The triplet-charge reaction can also be morphologically tuned. In poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) based OLEDs, low energy crystalline domains can be induced in PFO amorphous matrix by either high boiling point solvent or annealing treatments. The low energy domains can both spatially confine both excitons and charges to enhance the triplet-charge reaction. Consequently the enhanced triplet-charge reaction reduces the magnitude of magnetic field effects Our study successfully built a bridge between the magnetic field effects and the spin dependent excitonic processes in OLEDs. Scientifically, the excitonic processes, e.g. intersystem crossing, triplet-charge reaction, can be investigated by simply measuring the magnetic responses. Technically, this tunable magnetic field effects have the potential to be used to in new generation smart screens, magnetic sensors.
107

A Digital-to-Analog Converter Architecture for Multi-Channel Applications

Hale, Mark D. 01 May 2008 (has links)
Systems-on-chip with the capability of driving multiple analog voltages are useful for a variety of applications, including multiple actuator control for robotics applications, automated test equipment systems, industrial automation, programmable logic controllers, and satellite ywheel motor control. Such applications require a DAC for each analog output. A multi-channel architecture that saves power and area by sharing hardware is needed. This work introduces a new single-ramp multi-channel 12-bit DAC architecture. The architecture includes a low power Gray code counter, ramp generator, digital comparator, analog memory units, and control logic. The new multi-channel DAC architecture allows hardware sharing between multiple channels, and enables Systems-on-Chip to have multiple analog outputs for stimulating transducers or motors. The DAC architecture is to be used in a variety of space and defense applications as part of the BAE Systems RAD6000 microcontroller project.
108

Moisture Degradation of CTD-403: Testing the Application of Cyanate Ester Insulations to Fusion Devices

Morgan, Bradley Ian 01 December 2008 (has links)
The fusion devices currently being developed present several challenges for magnet designers. One challenge lies within the electrical insulation, which must be able to withstand extreme temperatures (both cryogenic and elevated temperatures), large shear and compressive stresses, high operating voltages, and high levels of incident radiation. To address the need for better performing insulation systems, Composite Technology Development, Inc. (CTD) has developed CTD-403, a cyanate ester resin with increased radiation resistance, ease of processing and fabrication, low moisture absorption characteristics, and high mechanical and electrical strength at cryogenic and elevated temperatures. In this thesis, CTD-403 resin based insulation systems were tested under the operation guidelines of the Quasi-Poloidal Stellarator (QPS). Because the coils of QPS are water cooled, it is important to understand the effects of long term humidity exposure on the insulation. The effects of humidity on moisture absorption characteristics, dimensional stability, mechanical and electrical properties were characterized. Increasing the humidity level caused a corresponding increase in the saturation level, while increasing the temperature of exposure magnified these effects causing more pronounced non-Fickian behavior. The mechanical properties degraded with an increase in humidity level. Elevated temperature effects are more pronounced on the compressive properties than the tensile properties. The glass transition temperature was more greatly affected by elevated temperature exposure as opposed to increased humidity exposure, decreasing in both instances. The dielectric strength decreases with increased humidity level and exposure time. Based on these findings, the performance of cyanate ester resin based insulation systems is shown superior to that of traditional epoxy based resin systems in all performance measures. The degradation of the mechanical and electrical properties of the insulation from long term effects of humidity and elevated temperature exposure are shown to be within the performance criteria bounds, and the insulation is therefore recommended for use in current and future fusion programs.
109

Removing Acetic Acid from the UREX+ Process

Mitchell, Jessica Anne 01 December 2008 (has links)
Acetic acid needs to be removed from a waste stream in the UREX+ process so that nitric acid can be recycled and possible interference with downstream steps can be avoided. Acetic acid arises from acetohydroxamic acid (AHA) used to suppress plutonium in the first step of the UREX+ process. Later, it is hydrolyzed into hydroxyl amine nitrate and acetic acid. Many common separation technologies were examined, and solvent extraction was determined to be the best choice under process conditions. Solvents already used in the UREX+ process were then tested to determine if they would be sufficient for the removal of acetic acid. The tributyl phosphate (TBP)-dodecane diluent, used in both UREX and NPEX, was determined to be a solvent system that gave sufficient distribution coefficients for acetic acid in addition to a high separation factor from nitric acid. This solvent system was tested under various TBP concentrations in the dodecane to create a concentration plot that can be used for further flow sheet development. Each step in the UREX+ process was examined to determine if there was any acetic acid interference in the performance of any step of the UREX+ flow sheet that would make it necessary to remove the acetic acid prior to that step. It was found that no interference with acetic acid was present. Therefore, the acetic acid removal step can be placed essentially anywhere in the process. For simplicity, it has been proposed to place the removal step at the end of the process after TALSPEAK where all desirable metals have already been extracted and the nitric acid waste stream is prepared to be recycled.
110

A High Voltage CCD Sensor Control Chip for the Large Synoptic Survey Telescope (LSST)

Ning, Zuoliang 01 December 2008 (has links)
This thesis presents the design of a Sensor Control Chip (SCC) developed to provide the required clock and bias signals for the Large Synoptic Survey Telescope’s CCD imagers. The circuit consists of current-summing DACs followed by trans-impedance operational amplifiers to control the rail voltages of the clock signals and bias voltages. The clocks are input to the SCC through LVDS receivers and converted internally to the required amplitude for driving the CCDs. The ASIC is designed to drive clock signals with 20-V adjustable output voltage swing and a maximum output current of 150 mA. The prototype chip has been fabricated in a 0.8-um BCD-SOI process, and is designed to operate down to 175K. Design techniques used in the ASIC will be presented, along with room temperature and operational temperature test results obtained from prototype chips. Test results have shown that the prototype chip is fully functional and agrees well with simulation results.

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