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Articles 361 - 390 of 436

Full-Text Articles in Materials Science and Engineering

Numerical Study Of Optical Delay In Semiconductor Multilayer Distributed Bragg Reflector And Tunable Microcavity Structures, Michael I. K. Etan Mar 2001

Numerical Study Of Optical Delay In Semiconductor Multilayer Distributed Bragg Reflector And Tunable Microcavity Structures, Michael I. K. Etan

Theses and Dissertations

The Air Force has a growing need for the greater bandwidth, speed, and flexibility offered by optical communication links. Future space systems and airborne platforms will most likely use optical signals for efficient power transmission and to minimize the possibility of spoofing and eavesdropping. Tunable optical delays play an important role in the implementation of free space optical communication links. The primary challenge in implementing these systems is the active maintenance of coherent wave fronts across the system's optical aperture. For space applications, this aperture may he hundreds of meters in diameter. Spatial segmentation of a large aperture into smaller …

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Optical Investigation Of Molecular Beam Epitaxy AlXGa1-XN To Determine Material Quality, Judith L. Mcfall Mar 2000

Optical Investigation Of Molecular Beam Epitaxy AlXGa1-XN To Determine Material Quality, Judith L. Mcfall

Theses and Dissertations

The purpose of this research was to determine the quality of AIGaN samples with various mole fractions of aluminum doped with silicon. The samples utilized for this study were composed of an AIN buffer layer sandwiched between the sapphire substrate and AIGaN epilayer grown by molecular beam epitaxy (MBE). Cathodoluminescence (CL) and photoluminescence (PL) were employed to determine the mole fraction of aluminum in each sample. These techniques also gave insight into the material's nonuniformity, defects, and impurities. CL was run at 4 different beam energies (2,5,10, & 15 keV) with four different currents (1,10,50, & 90 µA) for the …

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Mechanics Of A Functionally-Graded Titanium Matrix Composite, G. Brandt Miller Mar 2000

Mechanics Of A Functionally-Graded Titanium Matrix Composite, G. Brandt Miller

Theses and Dissertations

Functionally-graded Titanium Matrix Composites, (F/G TMCs) combine the ideal properties of titanium matrix composites with the more practical machining qualities of monolithic (unreinforced) alloy. This material shows great promise in application to aerospace structural components - even in parts whose design requirements have defied the use of composite materials in the past. Successful implementation of such a material would lead to enhanced aircraft performance. However, the basic properties of a functionally-graded titanium matrix composite need to be investigated. The composite/alloy transition region, or joint area, may be less strong than its constituents and therefore determine the overall performance of the …

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Chemical Mechanical Polishing Optimization For 4h-Sic, Craig L. Neslen Mar 2000

Chemical Mechanical Polishing Optimization For 4h-Sic, Craig L. Neslen

Theses and Dissertations

Scratch free surfaces are required for substrates used in epitaxial growth. Silicon carbide (SiC) is a substrate material that is used in the epitaxial growth of SiC, GaN, and InGaN electronic devices. Preliminary chemical mechanical polishing (CMP) studies of 1 3/8" 4H-SiC wafers were performed in an attempt to identify the polishing parameter values that result in a maximum material removal rate and thus reduce substrate polishing time. Previous studies reported increased material removal rates associated with increasing polishing temperature, slurry pH, pressure, and polishing pad speed. In the current study, the effects of temperature, slurry pH, polishing pressure, and …

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Effects Of Foreign Object Damage From Small Hard Particles On The High-Cycle Fatigue Life Of Ti-6a1-4v, Joseph L. Hamrick Ii Dec 1999

Effects Of Foreign Object Damage From Small Hard Particles On The High-Cycle Fatigue Life Of Ti-6a1-4v, Joseph L. Hamrick Ii

Theses and Dissertations

Thin rectangular samples of Ti-6Al-4V were damaged by four methods to represent foreign object damage found in turbine engine blades: 1) impact with 2 mm and 5 mm diameter glass spheres at 305 m/s, 2) impact with 2 mm and 4 mm diameter steel spheres at 305 m/s, 3) quasi-static displacement controlled indentation using steel chisels with 1 mm, 2 mm and 5 mm diameter tips and 4) shearing notches with a 2 mm diameter chisel point under a quasi-static loading condition. Finite element analysis was used to study the relationship between the stress state created by the plastic damage …

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Electrical And Optical Characterization Of Intrinsic And Ion-Implantation Induced Defects In 6h- And 4h-Sic, Michael B. Scott Dec 1999

Electrical And Optical Characterization Of Intrinsic And Ion-Implantation Induced Defects In 6h- And 4h-Sic, Michael B. Scott

Theses and Dissertations

Deep level transient spectroscopy (DLTS), Hall effect, and cathodolummescence (CL) measurements are used to characterize the intrinsic and ion-implantation induced defects in high-temperature (475 and 500 °C) ion-implanted epitaxial n-type 6H- and 4H-SiC, ion-implanted with Cr, Mg, Ar, N, and P atoms. Comparison of room-temperature and high-temperature ion-implanted 6H-SiC:Mg and :Cr indicate the significance of high-temperature ion implantation on the activation of the ion-implanted atoms and damage-recovery of the crystalline lattice. The effects of high-temperature annealing on both damage-recovery and implanted ion activation are detected and analyzed, from 1200 to 1800 °C. Trap parameters of both damage-related and species-related defects …

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Electrical Characterization Of Ion-Implanted 4h-Silicon Carbide, Christian P. Morath Mar 1999

Electrical Characterization Of Ion-Implanted 4h-Silicon Carbide, Christian P. Morath

Theses and Dissertations

Electrical characterization has been performed on ion-implanted p-type 4H-SiC to assess the activation efficiency and implantation-related damage recrystallization with the intention of developing an implantation/annealing scheme. Low doped (Na - Nd = 5x1015cm3 epitaxial p-type layers grown by MOCVD were implanted with Al or B at doses ranging from 1x1013 to 1x1014/cm2 at room temperature or 500°C. The electrical technique of Temperature Dependent Hall Effect (TDHE) indicated that Al and B act as shallow acceptors 4H-SiC with ionization energies of ~252 and ~285 meV, respectively. The highest activation efficiency for Al and B …

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Measurement Of Ultrafast Carrier Recombination Dynamics In Mid-Infrared Semiconductor Laser Material, William T. Cooley Dec 1997

Measurement Of Ultrafast Carrier Recombination Dynamics In Mid-Infrared Semiconductor Laser Material, William T. Cooley

Theses and Dissertations

Shockley-Read-Hall, radiative, and Auger recombination rates in mid-infrared laser structures are measured and reported using time resolved photoluminescence (TRPL) frequency upconversion. The mid-IR lasers studied were actual InAsSb/InAlAsSb multiple-quantum-well (MQW) diode lasers emitting near 3.3 micrometers which were previously characterized for laser performance. This effort extends the initial studies and reports on the carrier recombination dynamics. Shockley-Read-Hall, radiative and Auger recombination rates at low temperature (77 K) were measured and found to be ASRH ≈ 10 x 107sec-1, Brad ≈ 2 x 10-10 cm3sec-1 and CAuger < 10-29 cm6 …

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Experimental Investigation Of Nonlinear Dynamics In Single Mode Semiconductor Laser Diodes With Phase Conjugate Feedback, Gordon T. Hengst Aug 1997

Experimental Investigation Of Nonlinear Dynamics In Single Mode Semiconductor Laser Diodes With Phase Conjugate Feedback, Gordon T. Hengst

Theses and Dissertations

The semiconductor laser diode offers a unique system to investigate nonlinear dynamics when optical feedback is applied. Although there is extensive research of laser diodes with optical feedback from normal dielectric mirrors, very little has been done experimentally to analyze the effects of degenerate phase conjugate feedback from a BaTiO3 crystal. This research experimentally investigated the dynamics of a single-mode laser diode with weak phase conjugate feedback using both the self-pumped and double phase conjugate geometries. The experimental results validated a mathematical model which numerically evaluates the Lang-Kobayahsi coupled differential equations. The model simulated the nonlinear behavior of a …

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Thickness Effects On A Cracked Aluminum Plate With Composite Patch Repair, Joel J. Schubbe Jun 1997

Thickness Effects On A Cracked Aluminum Plate With Composite Patch Repair, Joel J. Schubbe

Theses and Dissertations

Post-repair fatigue crack growth was investigated in 3.175, 4.826, and 6.35 mm thick aluminum panels asymmetrically repaired with boron/epoxy composite patches bonded to the plates with FM73 sheet adhesive. Patches were uniaxial with patch to panel stiffness ratios ranging from 0.46 to 1.3. Experimental fatigue tests were carried out at 120 MPa, R=0.1, and 10 Hz (sinusoidal) to measure patched and unpatched face crack lengths, center crack opening displacements, and selected strains. Crack growth data was acquired using optical, eddy current, and post-test analysis methods. Crack growth rates were calculated using the incremental polynomial method. A three-layer Mindlin plate finite …

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Investigation Of The Optical Properties Of Ordered Semiconductor Materials, Jack E. Mccrae Jr. Jan 1997

Investigation Of The Optical Properties Of Ordered Semiconductor Materials, Jack E. Mccrae Jr.

Theses and Dissertations

Optical studies have been conducted upon CdGeAs2 and ZnGeP2, two of the most promising semiconductors being developed for mid-infrared non-linear optics applications. These experiments included photoluminescence (PL) studies of both compounds as well as photoreflectance (PR) measurements upon CdGeAs2. In addition, Hall effect measurements were carried out upon CdGeAs2, to aid in interpretation of the optical data. PL was measured as a function of laser power, sample temperature, and crystal orientation for CdGeAs2. One broad weak peak near 0.38 eV, and another somewhat narrower and often far brighter peak near 0.57 …

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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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