Advisor: M. W. Tretheway
Committee: C. Lissenden, J. Cusumano, J. Todd
Evaluation of a Variable Inertia System in Force-Frequency Shifting Technique for Large Structure Vibration Applications
Advisor: M. C. Demirel
Committee: A. Lakhtakia, J. Sofo, J. Todd
A Molecular Dynamics Study of Engineered Green Fluorescent Protein
Proteins are the molecular machines of nature. A special protein, called the green fluorescent protein (GFP), emits green fluorescent light under certain conditions. GFP is a commonly used protein in biotechnology applications because of its intrinsic fluorescence property. A set of experiments was performed to develop engineered GFPs that could be useful for bio-sensing, cancer diagnosis and bio-nanotechnology, but some engineered GFPs turned out to be dysfunctional. A new computational model was developed to understand the causes of the experimentally observed dysfunction, and to investigate the GFP chemistry happening at the molecular level. Results from the model revealed the reason for dysfunctional engineered GFPs, the penetration of protein structures with water molecules.
Advisor: M. W. Horn
Committee: R. Messier, J. Todd
Advisor: L. H. Friedman
Committee: M. Horn, C.Muhlstein, J. Todd
Analytic Treatment of Metallic Multilayer Strength at All Length Scales
Multilayers are interesting engineering composites composed of alternating layers of different materials. They are of great interest for both engineers and material scientists. Because of the special multilayered structure, novel properties can be obtained from multilayers. The novel properties achieved make them attractive for a number of engineering applications, such as X-ray optics, thin film magnetic recording medium, microelectromechanical system (MEMS) and hard coatings. In addition, they provide good systems to investigate relation between structures and material properties, which is a very fundamental aspect of material science.
The application of hard coatings, our focus here, is due to the superior mechanical strength observed in multilayers. Under certain circumstances, one may observe strength that is several times larger than those observed in constituent materials. Since proposed in early 70's, a lot of efforts have been spent on understanding the hardening mechanisms and predicting the superior strength of multilayers, from both experimental and theoretical aspects. Much interest has been directed to a strong size effect observed in multilayers, where mechanical strength is dependent on layer thickness. The most successful explanation for this size effect comes from dislocation pileup theory, which concerns with a linear array of dislocations and is a powerful tool to investigate material's properties. The most important result of pileup theory is the Hall-Petch relation, which is also the relation traditionally used to account for the pronounced size effect. However, more rigorous application of dislocation pileup theory predicts significant deviation due to elastic inhomogeneity, discreteness of dislocation and dislocation source operation.
This thesis is concerned with two steps towards better understanding and predicting of multilayer strength based on dislocation pileup theory. In particular, the effects from dislocation discreteness and dislocation source operation on multilayer strength are investigated, combined with the effect of elastic inhomogeneity reported by others. First, effect of dislocation discreteness has important impact on multilayer strength when layer thickness is small (< 100 nm). Thus, a finite size correction is needed. An analytic expression for the finite size correction is firstly proposed, thus, analytic formulas for multilayer strength that are applicable at all length scales are developed from a piecewise approach. Second, the dislocation source effect is taken into consideration, thus all the three effects (elastic inhomogeneity, discreteness of dislocation and dislocation source operation) are accounted for. Under the framework of the piecewise approach developed, analytic formulas linking macroscopically measurable strength to some microscopic material parameters and other easily obtainable material constants are provided for multilayer strength. In this way, all length scales are linked, and all the three effects are incorporated. This model is then applied to Cu/Ni multilayers, and the theory prediction is compared with experimental data.
The analytic formulas proposed in this thesis provide a more accurate prediction of multilayer strength than the traditional Hall-Petch relation, especially at small length scales. The analytic theory for material's strength is developed from microscopic modeling. An analytic formula provides more transparency than a computer model, which tends to operate as a black box. Thereby, it enables greater intuitive understanding of how strength depends on various multilayer characteristics and provides a quick quantitative estimate of how varying these characteristics will affect the final strength. This aids in characterization, design and behavior prediction of new materials.
Advisor: A. E. Segall
Committee: C. Lissenden, T. Eden, J. Todd
A Study on the Sliding Wear of Cryogenically Treated Aluminum Alloys and an Inverse Solution for Determining a wear Interface Temperature
The research conducted in this study is aimed at ascertaining a replacement sheave wheel material used in an aircraft recovery system which is used on aircraft carriers in the United States Navy. A replacement material is needed because of the costly sheave wheel maintenance and replacement. The primary cause of sheave wheel failure is excessive wear, which is caused by rubbing and friction. The amount of rubbing can be reduced by using a light weight material. Therefore, five different aluminum alloys were tested. The aluminum alloys that were tested were 2024-T351, 6061-T651, 7055-T7751, 7075-T651, and B390. A Cryogenic treatment which consisted of lowering the temperature of the material to -160 °C, was also applied to all of the test materials. All five materials were tested with and without a cryogenic treatment, which resulted in ten unique materials. To determine wear properties, a aluminum pin was forced into a rotating ring. The ring contained 45° slots to simulate the wire rope sheave wheel interaction. After the wear tests were conducted, the specimens were studied under a microscope to determine what was happening during the wear test. The study determined that the B390 outperformed the other candidate materials in terms of wear. The cryogenic treatment increased the wear resistance of 4 of the 5 alloys. 7075-T651 aluminum was the only alloy to lose wear resistance.
Internal temperature measurements were also taken in the wear specimen and near the wear interface. Mathematical methods were then used to predict what the wear interface temperature was during the wear test. Computer simulations were then used to verify the prediction. The prediction was found to be extremely accurate.
Advisor: C. J. Lissenden
Committee: M. Trethewey, A. Segall, J. Todd
Life Prediction Methodology for Cracked Nuclear Reactor Coolant Pump Shafts
Cracks have occurred in reactor coolant pump shafts found in commercial nuclear power plants. Several of these pump shafts have cracked entirely while the pump was running. Unexpected pump shaft failure can lead to significant financial losses and decreased safety for people who work at the plant. Ongoing research addresses ways to detect and monitor cracks. Once a crack has been detected and its length is known, it would be most useful if the remaining time until failure could be estimated. This study developed the basis for a set of steps that could be followed to predict the time until failure. Both semi-elliptical and circumferential cracks were considered since both types have been observed in failed pump shafts.
The first step of the life prediction process was to define the critical point at which failure would occur. Typically, the crack grows due the action of force applied at the end of the shaft by the pump impeller. Three different ways that the shaft might fail were considered. The first way was brittle fracture. Brittle fracture occurs when a crack grows steadily to a critical depth and then fails catastrophically. The depth at which fast fracture occurs would be the critical crack depth. The second way to define failure was by plasticity. The remaining uncracked section of the shaft becomes fully plastic when all of it has permanently deformed. This condition occurs when a critical crack depth is reached. The third failure definition was maximum allowable shaft deflection. As the pump shaft rotates, the deflection of the shaft from its position at rest is monitored. The deflection increases as the crack grows. This causes instability in the pump, and operation should be terminated when a certain allowable deflection has been reached. The crack depth corresponding to this maximum allowable deflection could then be defined as the critical crack depth. The shortest of these three critical crack depths would then be the type of failure that would actually occur.
Once the critical crack depth was determined by the three failure definitions, an equation called the Paris Law was applied. The Paris Law estimates the remaining number of shaft revolutions until failure by using the initial and critical crack depths, forces applied to the shaft, the shape of the crack, and shaft material information. The operating speed of the shaft was then used to convert the number of revolutions to a length of time until failure would occur.
Computer programs were written to apply the equations associated with the failure definitions and the Paris Law. All of the calculations involved in the life prediction process were heavily dependent on the quality of shaft load and material property data. Since some of this information was not readily available, the life prediction method cannot yet be applied in commercial nuclear power plants. Continued research is necessary in order to provide better input information for an accurate and reliable life prediction method.
Advisor: C. E. Bakis
Committee: H. Hofmann, C. Lissenden, J. Todd
Electrical and Mechanical Characterization of High-Purity Aluminum Composite Conductors at 20 Kelvin
Energy conversion provides the means by which we travel, transport goods, and manufacture products. Often the energy stored in fossil fuels is converted into mechanical energy by an internal combustion engine, such as the kind typically found in automobiles. However, to reduce pollution and limit our dependence on fossil fuels, highly efficient, light weight, low volume electric motors are advantageous. High efficiency electric motors can offer improved performance when operating at temperatures well below room temperature. This is because the materials used to carry the electricity have extremely low electrical resistance and therefore lose little energy in the form of heat. Such super-cooled motors could be used for space travel or even in electric powered airplanes. In this investigation, the performance of materials used to fabricate coil windings for super-cooled electric motors is evaluated. The conducting material of particular interest is high-purity aluminum wire. Despite the ability of super- cooled, high-purity aluminum to carry energy efficiently, it is very weak mechanically. Therefore, for the aluminum wire to be able to operate in light-weight, small-volume motors, it must be stabilized or structurally reinforced. A polymer-based material is used to stabilize the aluminum wire in this investigation. It is found that the remarkably low electrical resistance of pure aluminum is preserved in the non-metallic matrix, which is a notable improvement over previous attempts to use metallic support materials.
Advisor: B. R. Tittmann
Committee: J. Rose, A. Segall, J. Todd
The Design of a Unique Two Dimensional Phased Array with Low Channel Applications for Imaging Defects on a Metal Surface
This research was focused on the design and development of a device that uses ultrasound waves called a ''transducer''. For this project, this transducer will evaluate a target, which is a piece of metal with ultrasonic sound waves. The target metal is placed in a water environment, and the sound waves will travel from the transducer, through the water, and reflect off the metal surface. The transducer will be able to detect changes to the surface of the metal that can include, corrosion, cracking, and warping by receiving back that reflected waves. The overall objectives of this research were to investigate different transducer choices, to learn computer simulation programs to aid in the design process, and to bring into fruition the theoretical transducer design.
The transducer type that was designed and built is called a phased array transducer. Phased arrays are being built for a growing number of applications in the field of ultrasonics. This specific transducer will allow for testing to be done when the electronics are limited in size and number. When confidence is found that the smaller arrays can detect changes in a metal surface, more complicated phased arrays can be explored.
Phased arrays are widely used in the medical field. However with advances in electronic capabilities, these transducers can be implemented also in industry. The boarder impart of this research work is that it will help to continue to introduce phased arrays into industry applications. Eventually this research work will lead to high temperature applications and the development of a high temperature phased array transducer. High temperature arrays allow for a more accurate assessment of materials in high temperatures and high pressures. These transducers simplify inspections, decrease in labor and inspection time.
Inspection detection used to check for the safety of the material if there are any cracks or twisting or expansions of the metal. Being able to characterize the metal is useful because it will make the metal's behavior more predictable and reliable especially when used in power plants, industry and in special applications.
Advisor: C. E. Bakis
Committee: I. Smid, A. Segall, J. Todd
Processing and Thermomechanical Characterization of Epoxy-Based Carbon Nanocomposites
The objective of this investigation is to evaluate the potential of using carbon nanotubes and nanofibers as nano-reinforcements to improve the mechanical and thermal properties of an epoxy resin. ''Nano'' refers to the dimensions of the reinforcement to be on an order of magnitude of 10-9 meters. Low time-dependent deformation (creep), low change in dimension with temperature changes, and an ability to retain stiffness at elevated temperatures are the material attributes of interest in this investigation. The processing methods explored for making the nano-reinforced composites employed an air release agent, chemicals to separate the reinforcement in the resin, mechanical agitation, and in certain cases, a solvent. As-processed nanotubes, nanofibers, and purified nanotubes were investigated in concentrations ranging from 0% to 2.5%. The equipment utilized to gage the degree of success of each processing method and material formulation included an electron microscope, a dilatometer, a dead-weight load frame, strain gauges, and strain gauge circuitry. The results indicate that (a) nanotubes and nanofibers can benefit all of the properties of interest, (b) that nanotubes are more effective than nanofibers at equal mass fractions in meeting the objectives, and (c) that nanotubes are more difficult to process than nanofibers at equal mass fractions on the account of higher viscosity of the former. The improvements in thermomechanical properties are not dramatic, however, and suggestions for utilizing better nanocomposites are provided.
Advisors: M. Urquidi-Macdonald, D. Macdonald
Committee: J. Mahaffy, J. Todd
An Electrochemical Model of Activity Transport in Pressurized Water Reactors
World energy demand is always calling for more efficient and reliable forms of electricity generation. Many solutions have been proposed to meet this call, one of which is to continue and expand our use of nuclear power as a clean, emissions free method of electrical generation. In order for nuclear power plants to remain economically viable, we must ensure they remain safe for both nearby residents and those who work at the plant, whether it be operating or repairing the reactors.
One problem facing nuclear plant owners and operators is the accumulation of radiation fields in various parts of the plant. This includes locations that must be serviced or inspected on a regular basis, leading to the potential for human exposure to radiation. While human exposure to radiation is always considered a very important safety factor, this question can become an economic one for plant owners. Currently, the US government sets a limit on the annual dose of radiation energy a person may absorb while on the job. If an employee exceeds this limit, the employer may be forced to hire another employee to take the formers job, while paying both of them.
This thesis seeks to develop a model that will predict the accumulation of radiation in various areas of nuclear power plants. The model will take into account the basic scientific properties of some components of the plant, the parameters under which the plant is being operated, and the length of plant operation when calculating the accumulated radiation. Furthermore, this model will emphasize the use of basic scientific principles, and refrain from using empirically fitted solutions whenever possible.
The goal of developing a model of this nature to predict where, and quantify how much, radiation will accumulate is to gain understanding of the physical processes behind the phenomenon. Once the processes are better understood, a better chance will exist for being able to control radiation accumulation, and hence increase the efficiency and safety of nuclear power plants.
Advisor: A. E. Segall
Committee: C. Lissenden, B. Shaw, J. Todd
CO2 Laser Healing of Laser-Induced Damage of Alumina Ceramics
Laser machining of alumina ceramics provides an attractive alternative to traditional grinding. A well-known drawback of this method is that laser machining causes very intense localized heating of the ceramic, which leads to the formation of a thin resolidified layer on the surface containing a network of microscopic cracks. This recast layer makes laser-machined ceramics vulnerable to liquid penetration, and degrades the ceramics' structural properties.
The experiments conducted and outlined in this study develop a method of healing the laser-machining flaws with the same industrial CO2 laser used to make the initial cut.
Though the laser must be focused to a tiny diameter for cutting, this treatment requires it to be defocused so that the beam power is spread over a larger area. This laser, when defocused and aligned such that the beam was directed parallel to the surface, provided a less intense heating of the ceramic workpiece yet still provided enough power to modify the surface.
Preliminary trials indicated that this method could successfully alter the laser-machined surface and increase the surface quality and smoothness. Further research is required to optimize the beam parameters for industrial use of this healing method.
Advisor: O. O. Awadelkarim
Committee:
Ti and Ti/W on 4H-SiC Schottky Diodes: Measurements and Simulations of Current-Voltage Characteristics
Power electronic circuits are often employed to perform a variety of tasks in automobiles, consumer electronics, aircrafts and space vehicles. Tasks such as triggering the engine, power management, lighting, sensing and a host of others are efficiently performed by today's power electronic circuits. Typically, these circuits are designed to handle hundreds of volts and carry thousands of Amperes. Today's power devices are primarily based on silicon. The material properties of silicon make it difficult to operate at temperatures above 150 °C.
Silicon carbide has shown excellent promise for high temperature electronics. On paper, material properties of silicon carbide excel compared to silicon. However, a great deal of processing challenges has deferred realizing the full potential silicon carbide has to offer. Titanium deposited on silicon carbide makes up a Schottky structure. The schottky structure is like an electric switch. The properties of silicon schottky have been studied extensively over the past few decades. Schottky structures are often used as a test vehicle to probe the material quality.
In this study, Schottky structures made out of two different metal (Titanium and titanium-tungsten) systems have been analyzed. The behavior of these Schottky diodes is anomalous due to the presence of defects in the material. Often simplifying assumptions have to be made to be able to analyze and explain such anomalous behavior. However, such assumptions can lead to faulty interpretations. Herein, a method has been developed which doesn't rely on such simplifying assumptions. The method provides a simple, powerful method to extract meaningful physical parameters. These parameters are a better way to gauge the material quality and can help improve the performance. Numerical simulations provide alternate means to process these diodes to get optimal performance, even in the presence of defects.
Advisor: B. A. Shaw
Committee: A. Segall, E. Sikora, J. Todd
Accelerated Corrosion Testing of Sacrificial Coating Systems
Advisor: P. M. Lenahan
Committee: S. Ashok, J. Ruzyllo, J. Todd
Spin Dependent Recombination Observation of Deep Level Defects in 4H Silicon Carbide PiN Diodes
Nearly all microelectronic circuits today are fabricated with silicon. Silicon-based integrated circuits are utilized in everything from televisions and cellular phones to alarm clocks and coffee makers. However, there is the need for microelectronic devices to perform in certain areas where silicon-based devices would fail. For instance, the high temperatures encountered inside jet engines or on the exterior of spacecrafts would drastically change the electrical properties of silicon to the point where the devices would not function properly. To account for this, significant research has gone into developing silicon carbide; a material whose properties will allow it to be suitable for high temperature and high power applications.
To this point, silicon carbide based devices have shown unreliable electrical characteristics. These reliability issues have been attributed to a relatively large amount of atomic defects which are generated during fabrication of the devices. The purpose of this study was to use a magnetic resonance technique, known as spin dependant recombination, to investigate the nature of some of the defects present in silicon carbide diodes. We were successful in determining one defect which is likely related to the unreliable electrical characteristics of the diodes. The resulting knowledge of this defect can be used to modify processing parameters to improve the reliability of silicon carbide diodes.
This page was last updated on March 16, 2006.