Mechanical Engineering Commons^™

Micromechanical Studies Of Intergranular Strain And Lattice Misorientation Fields And Comparisons To Advanced Diffraction Measurements, Lili Zheng

Doctoral Dissertations

Inhomogeneous deformation fields arising from the grain-grain interactions in polycrystalline materials have been evaluated using a crystal plasticity finite element method and extensively compared to neutron diffraction measurements under fatigue crack growth conditions. The roles of intergranular deformation anisotropy, grain boundary damage, and non-common deformation mechanisms (such as twinning for hexagonal close packed crystals) are systematically evaluated. The lattice misorientation field can be used to determine the intragranular deformation behavior in polycrystals or to describe the deformation inhomogeneity due to dislocation plasticity in single crystals. The study of indentation-induced lattice misorientation fields in single crystals sheds lights on the understanding …

Surface Wetting And Friction Studies Of Nano-Engineered Surfaces On Copper Substrate, Julius Sheldon Morehead

Graduate Theses and Dissertations

Nano-engineered-textures on a material surface can dramatically improve the wetting and non-wetting properties of a surface, and they also show promise to address friction issues that affect surfaces in contact. In this work, aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) was used to produce nano-textures on copper (Cu) substrates. A study was performed to examine the effects of changing the annealing conditions and a-Si thickness on nano-texture formation. The creation of various nano-topographies and chemically modifying them using octafluorocyclobutane (C4F8) was performed to control hydrophilicity, hydrophobicity, and oil affinity of nano-textured surfaces. A video-based contact angle measurement system was used …

Modeling And Control Of A Flexible Ionic Polymer Metal Composite(Ipmc) Actuator For Underwater Propulsion, Shivakanth Gutta

UNLV Theses, Dissertations, Professional Papers, and Capstones

The goal of this research is to model and control the underwater vehicle propelled by IPMC actuator. IPMC consists of an ionic membrane sandwiched between two metallic electrodes. When an external voltage is applied, IPMC undergoes large deformation due to transport of ions. Due to its ability to work in aqueous environments, it can be used for developing small scale underwater vehicles.

First, Finite element approach is used to describe the dynamics of the both single and segmented IPMC actuator. In the approach presented, each element is attached with a local coordinate system that undergoes rigid body motion along with …

Modeling Of A Novel Solar Down Beam Test Facility Utilizing Newtonian Optics, Ryan J. Hoffmann

UNLV Theses, Dissertations, Professional Papers, and Capstones

As advances in concentrated solar energy progress there will inevitably be an increase in the demand of resources for testing new conceptions. Currently, there are limited facilities available for taking concentrated solar energy concepts from the laboratory bench scale to the engineering test scale. A proposed solution is a scientific and developmental facility that provides highly concentrated solar energy at ground level. The design presented is a solar down beam test facility utilizing a Newtonian optics approach with a flat rectangular down beam mirror to reflect and concentrate the sun's rays at ground level.

Literature review suggests a hyperbolic reflector …

Evaluation Of Cathode Materials For Low Temperature (500-700c) Solid Oxide Fuel Cells, Alexander M. Lassman

Master's Theses

Solid oxide fuel cells (SOFC) have gained a great deal of interest, due to their potential for high efficiency power generation and ability to utilize hydrogen fuel, as well as various hydrocarbon-based fuels. A recent trend in SOFC development has been towards lower operating temperatures (500-700°C), which can substantially reduce the cost and complexity of the system. This thesis presents an investigation into state of the art Ba- and La- based cathode materials for use in low temperature (500-700°C) solid oxide fuel cells.

Synthesis of A-site deficient [A=0.97] Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ (BSCF) was …

Air Plasma Sprayed Thermal Barrier Coatings: Experiments And Finite Element Analysis, Charles H. Thistle

Master's Theses

The purpose of this research was to examine the primary in-service failure mechanism of an air plasma sprayed thermal barrier coating commonly used in combustor applications, and to use that failure mechanism as a basis in developing a life prediction strategy. The research consisted of an experimental phase, in which the failure mechanism was identified and key features of the coating system measured, and a modeling phase, in which the findings of the experimental phase were used to build a system specific finite element model of the coating in order to extract quantitative data relevant to coating life.

Observations were …

Time-Dependent Crack Growth Behavior Of Alloy 617 And Alloy 230 At Elevated Temperatures, Shawoon Kumar Roy

UNLV Theses, Dissertations, Professional Papers, and Capstones

Two Ni-base solid-solution-strengthened superalloys: INCONEL 617 and HAYNES 230 were studied to check sustained loading crack growth (SLCG) behavior at elevated temperatures appropriate for Next Generation Nuclear Plant (NGNP) applictaions with constant stress intensity factor (K _max = 27.75 MPa[checkmark]m) in air. The results indicate a time-dependent rate controlling process which can be characterized by a linear elastic fracture mechanics (LEFM) parameter - stress intensity factor (K). At elevated temperatures, the crack growth mechanism was best described using a damage zone concept. Based on results and study, SAGBOE (stress accelerated grain boundary oxidation embrittlement) is considered the primary reason for …

Immersion Cooling Of Photovoltaic Cells In Highly Concentrated Solar Beams, Ahmed Darwish

UNLV Theses, Dissertations, Professional Papers, and Capstones

Concentrated solar radiation can be utilized to generate electrical power from photovoltaic cells, but concentrated solar radiation increases the photovoltaic cell’s temperature. This increase in temperature can lead to degradation of the cell efficiency, and too high of a temperature can damage the cell’s integrity. This is particularly important in dish and tower systems where a maximum uniform flux may be difficult to achieve. While a variety of approaches have been used to the keep the cells cool, most are based upon removal of heat from the back (opposite surface of the incident flux exposed surface) of the cell. This …

Peripheral Soldering Of Flip Chip Joints On Passive Rfid Tags, Md Syful Islam

UNLV Theses, Dissertations, Professional Papers, and Capstones

Flip chip is the main component of a RFID tag. It is used in billions each year in electronic packaging industries because of its small size, high performance and reliability as well as low cost. They are used in microprocessors, cell phones, watches and automobiles. RFID tags are applied to or incorporated into a product, animal, or person for identification and tracking using radio waves. Some tags can be read from several meters away or even beyond the line of sight of the reader. Passive RFID tags are the most common type in use that employ external power source to …

High Frequency Thermally Actuated Single Crystalline Silicon Micromechanical Resonators With Piezoresistive Readout, Amir Rahafrooz

Electronic Theses and Dissertations

Over the past decades there has been a great deal of research on developing high frequency micromechanical resonators. As the two most common and conventional MEMS resonators, piezoelectric and electrostatic resonators have been at the center of attention despite having some drawbacks. Piezoelectric resonators provide low impedances that make them compatible with other low impedance electronic components, however they have low quality factors and complicated fabrication processes. In case of electrostatic resonators, they have higher quality factors but the need for smaller transductions gaps complicates their fabrication process and causes squeezed film damping in Air. In addition, the operation of …

A Feasibility Study Of Model-Based Natural Ventilation Control In A Midrise Student Dormitory Building, Steven James Gross

Dissertations and Theses

Past research has shown that natural ventilation can be used to satisfy upwards of 98% of the yearly cooling demand when utilized in the appropriate climate zone. Yet widespread implementation of natural ventilation has been limited in practice. This delay in market adoption is mainly due to lack of effective and reliable control. Historically, control of natural ventilation was left to the occupant (i.e. they are responsible for opening and closing their windows) because occupants are more readily satisfied when given control of the indoor environment. This strategy has been shown to be effective during summer months, but can lead …

Interface Engineered Diamond Coatings For Dry Machining Applications, Humberto Arturo Gomez Vega

USF Tampa Graduate Theses and Dissertations

Several studies have been propose to improve the adhesion of diamond films on cemented carbide tool materials, however a systematic study in identifying the role of the factors that affect the final diamond adhesion and the resulting machining performance of the tool under real manufacturing conditions is still unexplored. CVD diamond film's extraordinary qualities bring little benefit if the film fails to adhere sufficiently to the substrate. Inadequate adhesion undermines tool performance and longevity, causing unpredictable behavior under load and possibly leading to unexpected failure of the tool in the production line. This dissertation investigates the effects of different surface …

Diamond Based-Materials: Synthesis, Characterization And Applications, Qiang Hu

USF Tampa Graduate Theses and Dissertations

The studies covered in this dissertation concentrate on the various forms of diamond films synthesized by chemical vapor deposition (CVD) method, including microwave CVD and hot filament CVD. According to crystallinity and grain size, a variety of diamond forms primarily including microcrystalline (most commonly referred to as polycrystalline) and nanocrystalline diamond films, diamond-like carbon (DLC) films were successfully synthesized. The as-grown diamond films were optimized by changing deposition pressure, volume of reactant gas hydrogen (H2) and carrier gas argon (Ar) in order to get high-quality diamond films with a smooth surface, low roughness, preferred growth orientation and high sp3 bond …

Phase Change Materials As A Thermal Storage Device For Passive Houses, Kevin Ryan Campbell

Dissertations and Theses

This study describes a simulation-based approach for informing the incorporation of Phase Change Materials (PCMs) in buildings designed to the "Passive House" standard. PCMs provide a minimally invasive method of adding thermal mass to a building, thus mitigating overheating events. Phase change transition temperature, quantity, and location of PCM were all considered while incrementally adding PCM to Passive House simulation models in multiple climate zones across the United States. Whole building energy simulations were performed using EnergyPlus from the US Department of Energy. A prototypical Passive House with a 1500 Watt electric heater and no mechanical cooling was modeled. The …

Polydimethylsiloxane Mechanical Properties Measured By Macroscopic Compression And Nanoindentation Techniques, Zhixin Wang

USF Tampa Graduate Theses and Dissertations

In this thesis, the relationship between the elastic modulus of PDMS and the base/agent ratio (the amount of crosslinking) is studied. Reliable macroscopic compression test instrument was developed. Preload method was applied for the nanoindentation flat punch test to develop full contact.

In chapter 2, an easy instrument setup for macroscopic compression test is described. A series of PDMS samples with different base/agent ratios were tested using the macroscopic compression method. The relationship between PDMS elastic modulus and its base/agent ratio was established.

In chapter 3, PDMS nanoindentation DMA tests provide stable data with different test control models. The storage …