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Articles 271 - 300 of 335

Full-Text Articles in Materials Science and Engineering

Residual Strength And Fatigue Characterization Of Scs-6/Ti-6-4, Sang-Myung Lee Dec 1996

Residual Strength And Fatigue Characterization Of Scs-6/Ti-6-4, Sang-Myung Lee

Theses and Dissertations

Metal matrix composite have potential in a variety of aerospace applications, but they should be better understood before they can be employed. Despite the many studies that have contributed to characterizing the fatigue response of titanium alloy MMCs, few have researched the residual strength. The purpose of this study was to investigate the residual strength and the frequency effects on the residual strength of an unidirectional laminate of SCS-6/Ti-6-4, a titanium alloy matrix based composite under tension-tension, load-controlled conditions at elevated temperatures and different frequencies. Specimens were fatigued at frequencies of 1 and 0.01Hz, and at maximum stress of 900MPa …

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Optical Detection Properties Of Silicon-Germanium Quantum Well Structures, Michael R. Gregg Oct 1996

Optical Detection Properties Of Silicon-Germanium Quantum Well Structures, Michael R. Gregg

Theses and Dissertations

A study has been carried out on Si/SiGe multi quantum well structures to determine their applicability as normal incidence infrared detectors in the spectral range of 2-12 micrometers. The research effort was primarily experimental; however, extensive calculations were performed to initially explain the experimental data and then used to design subsequent structures. Multiple quantum well structures grown on both Si[001] and Si[110] substrates via molecular beam epitaxy were studied by photoluminescence, absorption, and photoresponse measurements over a wide parameter space. Variables included quantum well depth and width, well doping, number of wells and growth temperature. Well widths were varied from …

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Low-Velocity Impact On Composite Sandwich Plates, Eric J. Herup Jul 1996

Low-Velocity Impact On Composite Sandwich Plates, Eric J. Herup

Theses and Dissertations

The objective of the research is to analytically model the response of composite sandwich plates to low velocity impact. A displacement based, plane stress, finite element code is modified for this purpose. Major new algorithms include 5th order Hermitian interpolation, three dimensional equilibrium integration for transverse stress calculations, sandwich core modeling as an elastic plastic foundation, loading by simulated contact with a spherical indentor, adaptive mesh, damage prediction, damage progression via stiffness reduction, and local-global analysis for displacement. An experimental effort is also included in which composite sandwich plates with graphite epoxy facesheets and Nomex honeycomb core are subjected to …

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Design Of Gradient Index Optical Thin Films, Jeffrey J. Druessel Jun 1996

Design Of Gradient Index Optical Thin Films, Jeffrey J. Druessel

Theses and Dissertations

Gradient index thin films provide greater flexibility for the design of optical coatings than the more conventional 'layer' films. In addition, gradient index films have higher damage thresholds and better adhesion properties. This dissertation presents an enhancement to the existing inverse Fourier transform gradient index design method, and develops a new optimal design method for gradient index films using a generalized Fourier series approach. The inverse Fourier transform method is modified to include use of the phase of the index profile as a variable in rugate filter design. Use of an optimal phase function in Fourier-based filter designs reduces the …

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Structural Damage Identification From Limited Measurement Data, Richard G. Cobb Mar 1996

Structural Damage Identification From Limited Measurement Data, Richard G. Cobb

Theses and Dissertations

The research focused on the development of a new method to identify damaged structural elements from a large flexible space structure on-orbit, using limited measured modal data. Limited measured modal data is loosely defined as measured data containing only a few modal frequencies and less than 10% of the total structural degrees-of-freedom. This effort was decomposed into four specific tasks. The first is the identification of partial modal properties from measured data of the nominal space structure. Second, the finite element model must be adjusted to match the measured nominal partial data. The third task is an analysis of the …

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Electrical Characterization Of 4h-Silicon Carbide P-N Junction Diodes, Michael E. Dunn Dec 1995

Electrical Characterization Of 4h-Silicon Carbide P-N Junction Diodes, Michael E. Dunn

Theses and Dissertations

The current conduction mechanisms of 4H-SiC p+n mesa diodes were studied using current-voltage-temperature (I-V-T), capacitance-voltage-temperature (C-V-T), deep level transient spectroscopy (DLTS), optical observations, and reverse breakdown measurements. Temperature and voltage dependencies of diffusion, recombination, and tunneling current processes are shown to be consistent with Sah-Noyce-Shockley theory. Recombination currents having an ideality factor of A=1.85-2.1 yielded an activation energy of EA=1.56 eV, whereas for ideal recombination, A=2 and EA=1.6 eV. Forward I-V curves of poor diodes dominated by tunneling and recombination processes, showing low reverse breakdown voltages of approx. 100 V, can be correlated to DLTS results …

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Photoluminescence And Electroluminescence Of Erbium And Neodymium Implanted Semiconductors, James R. Hunter Dec 1995

Photoluminescence And Electroluminescence Of Erbium And Neodymium Implanted Semiconductors, James R. Hunter

Theses and Dissertations

Low temperature photoluminescence (PL) and electroluminescence (EL) measurements were used to study the excitation of erbium- and neodymium-implanted GaAs and AlxGa1-xAs (x=0. 1, 0.3) pn-junctions. The rare-earth (RE) emissions were investigated as a function of ion dose, aluminum mole fraction, laser excitation power, and applied forward bias voltage for the implanted samples. Low temperature PL was also measured from Er doped silicon grown by the metalorganic chemical vapor-phase deposition (MOCVD) method using various growth parameters.. The MOCVD-grown Si samples were studied as a function of metalorganic source temperature, silane (SiH4) flow, growth time, and …

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Electrical Characterization Of 4h-And 6h-Silicon Carbide Schottky Diodes, Jeffrey C. Wiemeri Dec 1995

Electrical Characterization Of 4h-And 6h-Silicon Carbide Schottky Diodes, Jeffrey C. Wiemeri

Theses and Dissertations

The electrical properties of silicon carbide (SiC) make it an excellent candidate for use in high temperature/high power devices due to its wide bandgap, high breakdown field, high electron mobility, etc. In this work two Schottky diodes of platinum (Pt) on n-type carbon faced 4H-SiC and three Schottky diodes of nickel (Ni) on n-type silicon faced 6H-SiC were electrically characterized. The diodes exhibited good performance up to 698 K for both the forward and reverse (up to -100v) bias voltages. The barrier heights measured by I-V-T tests were found to be 0.32 and 0.61 eV for the Pt/4H-SiC samples, but …

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Evaluation Of The Sensitivity Of Radar Cross Section Predictions To Uncertainties In Material Characteristics, Greg A. Barnhart Dec 1995

Evaluation Of The Sensitivity Of Radar Cross Section Predictions To Uncertainties In Material Characteristics, Greg A. Barnhart

Theses and Dissertations

The purpose of this study was to evaluate the effects of material characteristics uncertainties on Radar Cross Section (RCS) predictions. Many methods have been developed to predict the RCS of metal objects, but for material coated objects, these methods depend on the accuracy of measured material characteristics. Material characteristics of three dielectrics were measured by two separate X-band waveguide set-ups. RCS measurements were then made to evaluate the accuracy of RCS predictions using these measured material characteristics. A six inch square slab of each material was measured with and without a metal plate backing. A six inch square flat metal …

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Thermal Stresses In End-Heated Layered Media, Jerry R. Couick Dec 1995

Thermal Stresses In End-Heated Layered Media, Jerry R. Couick

Theses and Dissertations

Thermal stresses in semi-infinite layered beams heated on the end are calculated using an extension to simple bimetallic thermostat theory. Recently, researchers have used the concept of interfacial compliance to determine interlaminar stresses in a simple thermostat of finite length subjected to a uniform temperature increase. In the present work, the thermostat theory is extended to apply to the beams of interest. A closed-form solution to the problem is obtained. It is not applicable within about one beam thickness (St Venant boundary region) of the end. Various classes of layered materials are analyzed to determine if significant stresses exist outside …

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Optical Characterization Of Indium Arsenide Antimonide Semiconductors Grown By Molecular Beam Epitaxy, Michael A. Marciniak Sep 1995

Optical Characterization Of Indium Arsenide Antimonide Semiconductors Grown By Molecular Beam Epitaxy, Michael A. Marciniak

Theses and Dissertations

The material parameters and crystalline quality of undoped, MBE-grown InAs1-xSbx nearly lattice-matched to (100) GaSb (-0.617% ≤ Δ a-a ≤ +0.708%) similar to material used for mid-infrared semiconductor lasers were determined by optical characterization. Absorption measurements at temperatures between 6-295 K determined the energy gap and wavelength-dependent absorption coefficient for each sample. The compositional dependence of the energy gap was anomalous when compared to previously reported data, suggesting phase separation existed in the material. The samples were also studied by temperature- and excitation-dependent photoluminescence (PL), which, for the majority of cases, showed only a single band-edge peak, …

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Satellite Surface Material Composition From Synthetic Spectra, Eugene L. Caudill Dec 1994

Satellite Surface Material Composition From Synthetic Spectra, Eugene L. Caudill

Theses and Dissertations

The objective of this research was to determine if measurements from a Sagnac interferometer could provide reliable estimates of satellite material composition. The Sagnac interferometer yields a spatial interferogram that can sampled by a linear detector array. The interferogram is related to the spectrum of the source through a Fourier transform. Here, spectral reflectivities of nine common satellite materials were used to simulate the spectrum on obtains from an ideal Sagnac interferometer in the beam-train of a ground-based telescope whose mission is to view satellites. The signal-to-noise ratio of the spectrum was varied to simulate the effect of range variation …

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Investigation Into The Behavior Of Geometrically Nonlinear Composite Arches, John C. Bailey Dec 1994

Investigation Into The Behavior Of Geometrically Nonlinear Composite Arches, John C. Bailey

Theses and Dissertations

This research modifies the existing finite element formulation of a potential energy based large deformation and moderate rotation theory. Hermitian shape functions replace the existing linear bending angle interpolations. Negligible differences between the two formulations indicate the underlying kinematics limit the accuracy, not the finite element interpolations. Using the new program, numerous nonlinear arch geometries are modeled to investigate the effects of arc length and thickness variations. Local and global snapping phenomena are captured as well as through the thickness shear driven nonlinearities.

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Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite Subjected To Tension-Tension Cycling With Hold Time, Scott A. Grant Dec 1994

Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite Subjected To Tension-Tension Cycling With Hold Time, Scott A. Grant

Theses and Dissertations

This study was carried out to investigate the elevated temperature behavior of the SiC-MAS5 cross- ply [0-90]4S ceramic matrix composite manufactured by Corning Inc. to fatigue with loading waveforms that combine the characteristics of stress rupture and high cycle fatigue. The test results were compiled in the form of S-N (cycles to failure), S-T (exposure time versus cycles to failure), S-S (energy exposure versus cycles to failure), normalized modulus degradation, strain progression, and hysteresis loop progression. From the mechanical behavior demonstrated by these curves, relationships between the effect of the environment and loading waveform were developed. In addition, a …

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Characterization Of Fatigue Behavior Of 2-D Woven Fabric Reinforced Ceramic Matrix Composite At Elevated Temperature, Daniel J. Groner Dec 1994

Characterization Of Fatigue Behavior Of 2-D Woven Fabric Reinforced Ceramic Matrix Composite At Elevated Temperature, Daniel J. Groner

Theses and Dissertations

This study investigated the fatigue behavior and associated damage mechanisms in notched and unnotched enhanced Sic-SiC ceramic matrix composite specimens at 1100°C. Stiffness degradation, strain variation, and hysteresis were evaluated to characterize material behavior. Microscopic examination was performed to characterize damage mechanisms. During high cycle-low stress fatigue tests, far less fiber-matrix interface debond was evident than in low cycle-high stress fatigue tests. Notched specimens exhibited minimal stress concentration during monotonic tensile testing and minimal notch sensitivity during fatigue testing. Damage mechanisms were also similar to unnotched.

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Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite At Elevated Temperature Under Tension-Tension Loading, Craig D. Steiner Dec 1994

Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite At Elevated Temperature Under Tension-Tension Loading, Craig D. Steiner

Theses and Dissertations

This study investigated the fatigue behavior and damage mechanisms of a [0-90]4s SiC-MAS ceramic matrix composite under tension-tension loading at two elevated temperatures and two frequencies. Stress and strain hystereses, maximum and minimum strain, and modulus of elasticity were evaluated to characterize the material behavior. Microscopy and fractography were used to evaluate damage progression and mechanisms. Fatigue life was independent of frequency at both temperatures.

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Total Ionizing Dose Effects In Mosfet Devices At 77 K, Kevin J. Daul Dec 1994

Total Ionizing Dose Effects In Mosfet Devices At 77 K, Kevin J. Daul

Theses and Dissertations

Total ionizing dose effects on thermal oxide and reoxidized nitrided oxide (RNO) MOSFET devices at 77 K were studied. The MOSFETs were immersed in liquid nitrogen and irradiated, using a 60Co source, up to 1 Mrad(Si) at a dose rate of 107 rads(Si)-sec. Drain current-gate voltage characteristics were obtained and used to determine threshold voltage and transconductance. At 77 K the subthreshold slopes indicated no observed buildup of interface states in any of the transistors. Furthermore, all transistors experienced very little change in the transconductance. Typical negative shifts in threshold voltage as dose increased were observed in all of …

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Photoluminescence Spectroscopy Of 4h- And 6h-Sic, William A. Davis Dec 1994

Photoluminescence Spectroscopy Of 4h- And 6h-Sic, William A. Davis

Theses and Dissertations

Typical undoped bulk grown SiC shows n- or p-type conductivity due to residual impurities such as nitrogen, boron, or aluminum. In order to produce high resistivity material, vanadium can be used as a compensating dopant. Since vanadium is an amphoteric dopant in SiC, it produces either a donor state, VSi4+(3d1) → VSi5+(3d0), or an acceptor state, VSi4+(3d1) → VSi3+(3d2). Thus, vanadium doping can compensate both n- and p-type conductivity. In this work, vanadium doped and undoped 4H- and 6H-SiC grown …

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Model Of A Single Impurity In A Wide Bandgap Semiconductor Describing Electric Field Screening, Anthony N. Dills Dec 1994

Model Of A Single Impurity In A Wide Bandgap Semiconductor Describing Electric Field Screening, Anthony N. Dills

Theses and Dissertations

A mathematical model of the influence on electric field screening arising from a single impurity in a wide bandgap semiconductor has been numerically investigated and compared with analytically derived solutions. The parameter set chosen to perform the comparison of analytical solution and numerical solution is based upon a bismuth silicate crystal. Both the analytical calculations and the numerical calculations are an attempt to mathematically model the internal electric field within a semiconductor. Two types of impurities were looked at: a single donor level and a single trap impurity level. In general, after an abrupt application of a voltage across the …

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Design, Fabrication, Modeling, And Optical Characterization Of Organic Polymer Nonlinear Directional Couplers, John N. Berry Dec 1994

Design, Fabrication, Modeling, And Optical Characterization Of Organic Polymer Nonlinear Directional Couplers, John N. Berry

Theses and Dissertations

This research was an investigation into the suitability of a recently developed polymer, polyphenylene, as a material for integrated optical circuits (IOCs). Polymers show great promise in the area of IOCs because of material processing advantages, compatibility with most existing integrated circuit technology, and relatively strong nonlinear optical characteristics. This thesis contains an overview of: dielectric waveguides, linear and nonlinear directional coupler theory; various models useful in the design and analysis of optical waveguides; the fabrication of three different waveguide designs; the experimental apparatus and procedure used to optically characterize the waveguides; and the experimental results of the characterization. Waveguiding …

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The Junction Characteristics And Current Conduction Mechanisms Of Gainp2 N+P Diodes And Solar Cells, Kitt C. Reinhardt Dec 1994

The Junction Characteristics And Current Conduction Mechanisms Of Gainp2 N+P Diodes And Solar Cells, Kitt C. Reinhardt

Theses and Dissertations

This work involves an investigation of GaInP2 n+p diode and solar cell dark current mechanisms, the defect centers that affect these mechanisms, and the response of dark current and solar cell photovoltaic parameters to I MeV electron irradiation and thermal annealing. Dark current due to carrier diffusion, recombination, and tunneling were identified, and recombination current was found to dominate at the maximum power-point voltage of the GaInP2 solar cells. Carrier recombination was found to occur via defect centers at the perimeter and within the bulk of the junction. Two deep majority-hole trap centers were found at E …

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The Excitation Mechanism Of Praseodymium-Doped Semiconductors, Paul L. Thee Jun 1994

The Excitation Mechanism Of Praseodymium-Doped Semiconductors, Paul L. Thee

Theses and Dissertations

This study on praseodymium Pr luminescence in AlxGa1-xAs was conducted to enhance the understanding of the excitation mechanism. Pr was implanted at 390 keV with doses from 5 x 1012 to 5 x 1013 cm2 into AlxGa1-xAs x0.0 to 0.50 wafers which were annealed using the rapid thermal annealing RTA method. Low temperature photoluminescence PL was conducted using an Ar-ion laser and Ge detector. PL emissions of Pr from all hosts include peaks near 1.3 and 1.6 µm which are assigned to the intra-4f transitions of 1G4 yielding 3H5 …

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Rapid And Accurate Timing Simulation Of Radiation-Hardened Digital Microelectronics Using Vhdl, Charles P. Brothers Jr. Mar 1994

Rapid And Accurate Timing Simulation Of Radiation-Hardened Digital Microelectronics Using Vhdl, Charles P. Brothers Jr.

Theses and Dissertations

This dissertation presents the development of a fast, accurate, timing simulation capability based on VHSIC Hardware Description Language VHDL without the use of back annotation of timing delay information. This VHDL-based timing simulator is intended for use with radiation-hardened microelectronics in simulating timing of circuit operation in pre-radiation, post-radiation 1 MradSi total dose, and ionizing dose radiation environments. Development of the timing models are presented. The implementation of the timing models are incorporated into a VHDL library composed of basic logic gates and flip-flops. Simulations of complex circuits were run in SPICE and VHDL to assess the timing accuracy and …

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Static Fracture Behavior Of A Ceramic Matrix Composite At Elevated Temperatures, David M. Agins Dec 1993

Static Fracture Behavior Of A Ceramic Matrix Composite At Elevated Temperatures, David M. Agins

Theses and Dissertations

This study investigated systematically the damage initiation damage progression, and failure modes of a ceramic matrix composite, Nicalon/CAS, when loaded in tension at elevated temperatures in the presence of air. A cross-ply lay-up, [0/90]2s, was studied. The primary means of analysis were stress-strain data taken during monotonic and incremental load tests and crack density assessments. Testing was conducted at room temperature 700°C, and 850°C. The stress-strain response was consistent for monotonic and incremental loading, except for a decrease in failure load at the elevated temperatures. Initial damage progression was also consistent, but the damage leading to failure and …

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Investigation Of Tension-Compression Fatigue Behavior Of A Cross-Ply Metal Matrix Composite At Room And Elevated Temperatures, Elizabeth A. Boyum Dec 1993

Investigation Of Tension-Compression Fatigue Behavior Of A Cross-Ply Metal Matrix Composite At Room And Elevated Temperatures, Elizabeth A. Boyum

Theses and Dissertations

This research, the first load-controlled tension-compression fatigue testing to be performed on a MMC, extends the existing knowledge of MMC fatigue damage mechanisms to include the tension compression loading condition. To accomplish this, a (0/90)2, SCS-6/Ti-15-3 laminate was subjected to tension- tension fatigue at room temperature, and tension-compression fatigue at both room temperature and 427°C. Stress and strain data was taken to evaluate the macro-mechanic behavior of the material. Microscopy and fractography were performed to characterize the damage on a micro-mechanic level. On a maximum applied stress basis, the room temperature tension-tension specimens had longer fatigue lives than the room …

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Influence Of Embedded Optical Fibers On Compressive Strength Of Advanced Composites, Stefan B. Dosedel Dec 1993

Influence Of Embedded Optical Fibers On Compressive Strength Of Advanced Composites, Stefan B. Dosedel

Theses and Dissertations

This study investigated the effects of embedding optical fibers into advanced composite materials. This combination was meant to simulate 'smart structures' that have been shown to sense several different variables in the composite including strain, temperature, and damage. A laminate orientation taken from an existing aircraft structure was used to fabricate sixteen groups of specimens which were subjected to compression testing in an IITRI fixture to determine the ultimate compressive strength and modulus of elasticity. Ten of these groups were fabricated with optical fibers while the other six were control groups and contained no optical fibers. This study showed that …

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Investigation Of Compressive Behavior For Metal Matrix Composites With A Circular Hole, Janet L. Gooder Dec 1993

Investigation Of Compressive Behavior For Metal Matrix Composites With A Circular Hole, Janet L. Gooder

Theses and Dissertations

This study investigated the notched and unnotched macromechanical response due to micromechanical behavior of [0/±45/90]S SCS-6/β 21 S and [0/90] 2S Sigma/β 21S subjected to static compressive loads at room temperature. The stress-strain behavior and mechanical response in the presence of a circular hole was examined. A clear description of the progressive nature of damage, which ultimately resulted in failure, was provided. The effect of elevated temperature was shown for SCS-6/β 21S. The ultimate compressive strength and elastic modulus were reported for both MMCs. Sigma/β 21S and SCS-6/β 21S were mildly notch sensitive at room temperature. Notch sensitivity of …

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Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite Under Strain Controlled Tension-Tension And Tension-Compression Loading, James J. Gudaitis Dec 1993

Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite Under Strain Controlled Tension-Tension And Tension-Compression Loading, James J. Gudaitis

Theses and Dissertations

This stuidy investigates the fatigue response of a cross-ply [0/90]2s Nicalon/Calcium Aluminosillicate (Nicalcc/CAS) ceramic matrix composite at room temperature under strain controlled tension-tension and tension-compression fatigue loading. The primary objectives were to determine strain fatigue limits for both loading cases and to complete a fatigue life diagram (Δe-N) for all tests. Failure mechanisms were also studied. The average initial modulus value for this lay-up was 120 GPa. The ultimate tensile strain was 0.00988 mn/mm (285 MPa). Eight tests were conducted with maximum strains that ranged from 0.00300 mm/mm to 0.00722 mm/mm. The tension-tension fatigue limit was found to be …

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Nonlinear Large Deformation Theory Of Composite Arches Using Truncated Rotations, Daniel A. Miller Ii Dec 1993

Nonlinear Large Deformation Theory Of Composite Arches Using Truncated Rotations, Daniel A. Miller Ii

Theses and Dissertations

This research has been directed toward capturing large cross sectional rotation during bending of composite arches. A potential energy based nonlinear finite element model that incorporates transverse shear strain was modified to include large bending rotations. Large rotation kinematics were derived in a vector format leading to nonlinear strain that was decomposed into convenient forms for inclusion in the potential energy function. Problem discretization resulted in a finite element model capable of large deformations. Riks method and displacement control solution techniques were used. Numerous problems and examples were compared and analyzed. Code vectorization and parallelization were briefly examined to improve …

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Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite At Elevated Temperatures Under Tension-Tension Loading, Richard J. Tuznik Dec 1993

Fatigue Behavior Of A Cross-Ply Ceramic Matrix Composite At Elevated Temperatures Under Tension-Tension Loading, Richard J. Tuznik

Theses and Dissertations

A study was conducted which investigated the behavior of a Nicalon/ Calcium-Aluminosilicate (Nicalon/CAS) cross-ply [0/90]2S ceramic matrix composite at elevated temperatures under tension-tension fatigue loading. Tension-tension fatigue tests were performed with a load ratio of R = 0.10 and a frequency of 10 Hertz at 700°C and 850°C. These results were compared to results from a previous study conducted at room temperature. Material behavior and damage was recorded by fatigue life curves, elastic modulus' maximum and minimum strain, acetate replication techniques, and post-mortem fractography. Analysis of these results showed identical matrix dominated damage behavior at RT and 700°C. Ultimate …

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