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Articles 271 - 300 of 436
Full-Text Articles in Materials Science and Engineering
A Framework For Studying The Physical Degradation Characteristics Of Dvds And Their Relationship To Digital Errors, Brian K. Saville
A Framework For Studying The Physical Degradation Characteristics Of Dvds And Their Relationship To Digital Errors, Brian K. Saville
Theses and Dissertations
The methods used to store data on DVD-R discs have been proven to work over the last 15 years. However, there has been a growing concern that these discs will be outlasted by the paper records they were meant to replace. The data on a DVD-R is stored as optical contrasts which have the potential to be misread and even damaged. This damage may occur either on the surface or internally to the disc, especially on the recording layer itself. The literature is saturated with studies attempting to determine the time period in which discs may fail and what the …
Distributed Spacing Stochastic Feature Selection And Its Application To Textile Classification, Jeffrey D. Clark
Distributed Spacing Stochastic Feature Selection And Its Application To Textile Classification, Jeffrey D. Clark
Theses and Dissertations
Many situations require the need to quickly and accurately locate dismounted individuals in a variety of environments. In conjunction with other dismount detection techniques, being able to detect and classify clothing (textiles) provides a more comprehensive and complete dismount characterization capability. Because textile classification depends on distinguishing between different material types, hyperspectral data, which consists of several hundred spectral channels sampled from a continuous electromagnetic spectrum, is used as a data source. However, a hyperspectral image generates vast amounts of information and can be computationally intractable to analyze. A primary means to reduce the computational complexity is to use feature …
Study Of Novel Materials For Organic Photovoltaics And Organic/Inorganic Hybrid Solar Cells, Viney Saini
Study Of Novel Materials For Organic Photovoltaics And Organic/Inorganic Hybrid Solar Cells, Viney Saini
Theses and Dissertations
As the human race evolves and continues to progress in the future, we will face some obstacles during the advancement. Challenges like water, food, and poverty crisis, pollution, population explosion, and diseases are impediments in the development of mankind. Above all, with the development arises the need to fulfill the energy requirements. There are number of technologies available to answer the present energy needs, e.g. coal, nuclear, oil/natural gas, hydropower, etc. All these technologies have their advantages and disadvantages, with disadvantages being greenhouse gas emissions and other environmental impacts. One technology which has lagged behind due to high cost is …
Experimental Investigation Of Natural Fiber Reinforced Composite, Irene Samy Fahim
Experimental Investigation Of Natural Fiber Reinforced Composite, Irene Samy Fahim
Theses and Dissertations
There is a growing use of polymers in different applications worldwide. This phenomenon resulted in an enormous increase in the amount of plastic waste. These wastes have significant impacts on the environment including pollution and depletion of resources. Recycling plastic waste and reusing it helps in alleviating environmental degradation. The amount of recycled plastics all over the world not utilized is huge. U.S.A recycled 5.7% of the total plastics generated and Western Europe recycled 39% of total amount of plastics consumed [1]. This research studies the potential usage of virgin and recycled polymer reinforced with natural fiber such as rice …
Electromagnetic Modeling And Measurement Of Adaptive Metamaterial Structural Elements, Matthew E. Jussaume
Electromagnetic Modeling And Measurement Of Adaptive Metamaterial Structural Elements, Matthew E. Jussaume
Theses and Dissertations
This document addresses two major obstacles facing metamaterial development: uncertainty in the characterization of electromagnetic field behavior in metamaterial structures and the relatively small operational bandwidth of metamaterial structures. To address the first obstacle, a method of prediction aided measurement is developed and exploited to examine the field interactions within metamaterial devices. The fusion of simulation and measurement techniques enhances the understanding of the physical interactions of fields in the presence of metamaterials. To address the second obstacle, this document characterizes the effectiveness of an adaptive metamaterial design that incorporates a microelectromechanical systems (MEMS) variable capacitor. Applying voltages to the …
Memristive Properties Of Thin Film Cuprous Oxide, Brett C. Castle
Memristive Properties Of Thin Film Cuprous Oxide, Brett C. Castle
Theses and Dissertations
Memristive properties of thin film copper oxides with different grain sizes were characterized using tunneling atomic force microscopy (TUNA) and optical reflection measurements. The thin films containing copper ions of different chemical states were prepared by thermal oxidation of metallic copper thin films, deposited via magnetron sputtering onto silicon wafer substrates at an elevated temperature for various lengths of time. The TUNA measurements showed a memristive hysteresis in the I-V curves under an applied bias profile with an initial bias of -3.5V, a ramp up to 3.5V, and subsequent return to -3.5V. Histogram analysis of the barrier height distribution for …
An Analytical Model Of Nanometer Scale Viscoelastic Properties Of Polymer Surfaces Measured Using An Atomic Force Microscope, Jacob B. Goldberg
An Analytical Model Of Nanometer Scale Viscoelastic Properties Of Polymer Surfaces Measured Using An Atomic Force Microscope, Jacob B. Goldberg
Theses and Dissertations
The United States Air Force and the Department of Defense is increasingly interested in nanomaterials. To study these materials, one needs to measure the mechanics of materials on the nanoscale. Over the past few decades the atomic force microscope (AFM) has been used in various methods to establish local surface properties at the nanoscale. In particular, surface elasticity measurements are crucial to understanding nanoscale surface properties. Problems arise, however, when measuring soft surfaces such as polymers and biological specimens, because these materials have a more complex viscoelastic response. This research focuses on modeling an AFM dynamic nanoindentation experiment intended to …
All Solid-State Mid-Ir Laser Development, Nonlinear Absorption Investigation And Laser-Induced Damage Study, Torrey J. Wagner
All Solid-State Mid-Ir Laser Development, Nonlinear Absorption Investigation And Laser-Induced Damage Study, Torrey J. Wagner
Theses and Dissertations
In this research, nonlinear optical absorption coefficients and laser-induced damage thresholds are measured in Ge and GaSb, which are materials that are used in IR detectors. Using a simultaneous fitting technique to extract nonlinear absorption coefficients from data at two pulse widths, two-photon and free-carrier absorption coefficients are measured in Ge and GaSb at 2.05 and 2.5 μm for the first time. At these wavelengths, nonlinear absorption is the primary damage mechanism, and damage thresholds at picosecond and nanosecond pulse widths were measured and agreed well with modeled thresholds using experimentally measured parameters. The damage threshold for a single-layer Al …
Characterization And Measurement Of Passive And Active Metamaterial Devices, Christopher A. Lundell
Characterization And Measurement Of Passive And Active Metamaterial Devices, Christopher A. Lundell
Theses and Dissertations
This document addresses two major obstacles facing metamaterial development: uncertainty in the characterization of electromagnetic field behavior in metamaterial structures and the relatively small operational bandwidth of metamaterial structures. To address the first obstacle, a new method to characterize electromagnetic field behavior in a metamaterial is presented. This new method is a bistatic radar cross section (RCS) measurement technique. RCS measurements are well-suited to measuring bulk metamaterial samples because they show frequency dependence of scattering angles and offer common postprocessing techniques that can be useful for visualizing results. To address the second obstacle, this document characterizes the effectiveness of an …
Optical And Electrical Characterization Of Bulk Grown Indium-Gallium-Arsenide Alloys, Austin C. Bergstrom
Optical And Electrical Characterization Of Bulk Grown Indium-Gallium-Arsenide Alloys, Austin C. Bergstrom
Theses and Dissertations
Advances in crystal growth techniques have allowed increased quality in growth of bulk ternary InxGa1-xAs. Here, the optical and electrical properties of samples grown through the vertical Bridgman (or multi-component zone melting growth) method have been investigated through photoluminescence spectroscopy and Hall effect measurements. Indium mole fractions varied from 0.75 for 1. Hall effect measurements at temperatures ranging from 10 to 300 K revealed moderate n-type doping with carrier concentrations ranging from 1.5 to 9.6×1016 cm-3 at 10 to 15 K. Carriers from deep donor levels became appreciable between 50 and 100 K. Hall …
Rubidium Recycling In A High Intensity Short Duration Pulsed Alkali Laser, Wooddy S. Miller
Rubidium Recycling In A High Intensity Short Duration Pulsed Alkali Laser, Wooddy S. Miller
Theses and Dissertations
Laser induced fluorescence was used to study how pump pulse duration and alkali recycle time effects maximum power output in a Diode Pumped Alkali Laser (DPAL) system. A high intensity short pulsed pump source was used to excited rubidium atoms inside a DPAL-type laser. The maximum output power of the laser showed a strong dependence upon the temporal width of the pump pulse in addition to the input pump intensity. A linear relationship was observed between the maximum output power and the pulse width due to the effective lifetime of the excited state, defined as the time it takes for …
Afm-Patterned 2-D Thin-Film Photonic Crystal Analyzed By Complete Angle Scatter, Nicholas C. Herr
Afm-Patterned 2-D Thin-Film Photonic Crystal Analyzed By Complete Angle Scatter, Nicholas C. Herr
Theses and Dissertations
The purpose of this research was to use an atomic force microscope (AFM) to generate a 2-D square array of sub-wavelength surface features from a single material over a region large enough to permit optical characterization. This work is an extension of previous AFIT nano-patterning work and is in response to the small subunit sizes demanded for the production of optical metamaterials and photonic crystals. A diamond nano-indentation AFM probe was used to produce a 325-μm by 200-μm array of indentations in a 120-nm thick polystyrene film deposited on silicon. Indentation spacing of 400 nm produced well-defined surface features with …
Hard Collisions In Rubidium Using Sub-Doppler Spectroscopy, Douglas E. Thornton
Hard Collisions In Rubidium Using Sub-Doppler Spectroscopy, Douglas E. Thornton
Theses and Dissertations
To better understand the laser kinetics of an alkali gain medium, hard collisions, or velocity-changing collisions, has been studied and a velocity-changing collisional rate has been calculated. Previous works have studied these collisions, but no rate has been calculated. Using the precise tool of sub-Doppler spectroscopy, atomic hard collisions can be observed. The collected spectra are fitted with two different line shapes to demonstrate the accuracy of this method. From the fits, the number of hard collisions can be extracted. The time scale of the hard collisions in rubidium is interpolated by varying the chopping frequency of the pump beam, …
Thermo-Mechanical Characterization Of Silicon Carbide-Silicon Carbide Composites At Elevated Temperatures Using A Unique Combustion Facility, Ted T. Kim
Theses and Dissertations
This research investigated four different types of CMCs in a simulated gas turbine engine hot section condition that involved the simultaneous application of a combustion environment and mechanical fatigue loading using a unique burner rig facility developed for this study at AFIT. Three of the materials were woven Melt-Infiltrated (MI) BN/SiC reinforced by Hi-Nicalon Type S (Hi-Nic-S), Sylramic (Syl) and Sylramic with insitu BN coating (Syl-iBN) fibers, respectively. They were made by the slurry cast method. The other CMC was MI Hi-Nic-S/BN/SiC made using the prepreg MI processing. Prepreg MI CMC and Sylramic-iBN fiber reinforced CMC showed better resistance to …
Ceramic Matrix Composite Characterization Under A Combustion And Loading Environment, Andrew R. Nye
Ceramic Matrix Composite Characterization Under A Combustion And Loading Environment, Andrew R. Nye
Theses and Dissertations
Lightweight materials that can withstand high temperatures and corrosive environments are constantly sought after in the aerospace industry, typically for Gas Turbine Engine (GTE) application. These materials need to retain their strength throughout the long service period they would see in the combustor and turbine components of a GTE. One material that is ideal for these types of applications is an oxide/oxide Ceramic Matrix Composite (CMC). The fatigue behavior of the oxide/oxide CMC NextelTM 720/Alumina (N720/A) was investigated in a unique high temperature environment. N720/A consisted of an 8-harness satin weave of NextelTM aluminum oxide/silicon oxide fibers bound …
Creep And Fatigue Interaction Characteristics Of Pwa1484, Christian J. Pierce
Creep And Fatigue Interaction Characteristics Of Pwa1484, Christian J. Pierce
Theses and Dissertations
One of the main methods for predicting the service life of jet engine turbine blades is through creep analysis. A sample of the turbine blade material will be mechanically tested to characterize its creep performance. This characterization will be used to determine safe operating conditions for the material. While methods for modeling creep behavior are generally well developed, this constant load creep testing does not fully represent the loading conditions present in a jet engine due to cyclic loading caused by the mission profile and throttle movements. As the industry seeks to become more accurate in physics based modeling of …
Effects Of Contact Load On The Fretting Fatigue Behavior Of In-100 At Elevated Temperature, Alfred G. Traylor Ii
Effects Of Contact Load On The Fretting Fatigue Behavior Of In-100 At Elevated Temperature, Alfred G. Traylor Ii
Theses and Dissertations
This thesis studied the effects of contact load of 8006 N on the fretting fatigue behavior of IN-100 at an elevated temperature of 600°C. First, S-N curves were created for a range of applied axial stresses at an identical stress ratio of 0.03 at 8006 N. A condition of partial slip condition necessary to fretting fatigue was verified by generating hysteresis loops, plotting shear force against axial stress. The half-contact width and the crack initiation location were determined for all of the fretting specimens using optical and scanning electron microscopes. The contact widths were consistent with expected analytical values and …
Effects Of Environment On Creep Behavior Of Nextel720/Alumina-Mullite Ceramic Composite With ±45º Fiber Orientation At 1200ºc, Muzaffer Ozer
Effects Of Environment On Creep Behavior Of Nextel720/Alumina-Mullite Ceramic Composite With ±45º Fiber Orientation At 1200ºc, Muzaffer Ozer
Theses and Dissertations
The present effort will investigate the off-axis creep behavior of the Nexte720/Alumina-Mullite CMC, a material system which relies on a porous alumina/mullite matrix for damage tolerance as compared to the pure alumina matrix in earlier studies. This research evaluated the creep behavior of N720/AM with a ± 45° fiber orientation at 1200°C in: laboratory air, steam, and argon environments. Creep-rupture tests at the creep stress levels of: 32, 30, 26, and 20 MPa were conducted in each environment. Additionally 35 MPa stress level examined in air and 13 MPa stress level examined both in air and steam. The N720/AM with …
Production And Characterization Of High Repetition Rate Terahertz Radiation In Femtosecond-Laser-Induced Air Plasma, Michael L. Dexter
Production And Characterization Of High Repetition Rate Terahertz Radiation In Femtosecond-Laser-Induced Air Plasma, Michael L. Dexter
Theses and Dissertations
The purpose of this research was to produce and characterize high repetition rate terahertz radiation in ionized air plasma. An 800 nanometer, 50 femtosecond, 0.35 Watt, 40 KHz, pulsed Ti:Sapphire laser system was used as the source infrared beam. This beam was focused onto a second harmonic generation crystal to produce a collinear, perpendicularly polarized secondary beam at 400 nm. After realigning the polarization of the fundamental to the second harmonic and compensating for group velocity dispersion introduced by the optics, both beams were recombined and focused by a 3.75 cm focal length mirror to form an air plasma. An …
A Comparative Study Of The Bidirectional Reflectance Distribution Function Of Several Surfaces As A Mid-Wave Infrared Diffuse Reflectance Standard, Bradley Balling
A Comparative Study Of The Bidirectional Reflectance Distribution Function Of Several Surfaces As A Mid-Wave Infrared Diffuse Reflectance Standard, Bradley Balling
Theses and Dissertations
The Bi-Directional Reflectance Distribution Function (BRDF) has a well defined diffuse measurement standard in the ultraviolet, visible, and near infrared (NIR), Spectralon(trade name). It is predictable, stable, repeatable, and has low surface variation because it is a bulk scatterer. In the mid-wave IR (MWIR) and long-wave IR (LWIR), there is not such a well-defined standard. There are well-defined directional hemispherical reflectance (DHR) standards, but the process of integrating BRDF measurements into DHR for the purpose of calibration is problematic, at best. Direct BRDF measurement standards are needed. This study use current calibration techniques to ensure valid measurements and then systematically …
Analysis And Application Of The Bi-Directional Scatter Distribution Function Of Photonic Crystals, Robert B. Lamott
Analysis And Application Of The Bi-Directional Scatter Distribution Function Of Photonic Crystals, Robert B. Lamott
Theses and Dissertations
Photonic crystals (PCs) are periodic structures built from materials with different refractive indices repeated at sub-wavelength intervals, which results in unusual optical characteristics, including narrowband laser protection, and zero reflectance and high absorption anomalies. Most of the research into the optical properties of PCs has concentrated only on the small range of wavelengths and angles where these effects occur. To better understand where all light leaving a PC is scattered, a Complete Angle Scatter Instrument was used to analyze the scatter from three Guided Mode Resonance Filters designed for laser protection. In the plane of incidence, measurements of the scatter …
Wave Optics Simulation Of Optically Augmented Retroreflections For Monostatic/Bistatic Detection, John J. Tatar Iii
Wave Optics Simulation Of Optically Augmented Retroreflections For Monostatic/Bistatic Detection, John J. Tatar Iii
Theses and Dissertations
Optical devices interrogated with a laser in the appropriate band can exhibit strong, deterministic reflections of the incident beam. This characteristic could be exploited for optical target detection and identification. The distribution of reflected power is strongly dependent on the geometry of the interrogation scenario, atmospheric conditions, and the cross section of the target optical device. Previous work on laser interrogation systems in this area has focused on analytic models or testing. To the best of my knowledge, I am presenting for the first time an approach to predict reflected power for a variety of interrogation configurations, targets, and propagation …
Investigation Of Electrical And Optical Properties Of Bulk Iii-V Ternary Semiconductors, Travis C. Gomez
Investigation Of Electrical And Optical Properties Of Bulk Iii-V Ternary Semiconductors, Travis C. Gomez
Theses and Dissertations
Bulk grown III-V ternary semiconductors of In0.08Ga0.92Sb and In0.15Ga0.85As were investigated through Hall-effect and photoluminescence measurements to determine carrier concentration, mobility, sheet resistivity, and luminescence spectrum. In the past, epitaxial layers of ternary compounds have been grown on binary compound substrates, and thus very limited lattice matched ternary alloys were available. Recently, bulk grown ternary substrates have been developed, and it has presented a renewed interest in using these substrates to grow high quality ternary compounds for use in many next generation optoelectronic devices. The results of photoluminescence (PL) study for the …
Non-Destructive Evaluation Of Aerospace Composites, Jeremy D. Johnson
Non-Destructive Evaluation Of Aerospace Composites, Jeremy D. Johnson
Theses and Dissertations
Five methods of non-destructive material evaluation (NDE) were used to inspect various forms of damage commonly found in aerospace fiberglass composites: voids, edge and sub-surface delaminations, surface burning, and cracking. The images produced by X-ray, X-ray Computed Tomography, terahertz (THz) imaging, ultrasound, and flash IR thermography were analyzed for the detection of defects. Test results and analysis of each NDE method's capabilities provide a comparison study of conventional techniques versus the emerging technology of THz imaging for the non-destructive evaluation of aerospace composite materials. A comparison guide to the five methods' damage detection effectiveness for fiberglass composites is provided by …
Nondestructive Evaluation Of Aircraft Composites Using Terahertz Time Domain Spectroscopy, Christopher D. Stoik
Nondestructive Evaluation Of Aircraft Composites Using Terahertz Time Domain Spectroscopy, Christopher D. Stoik
Theses and Dissertations
Terahertz (THz) time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Material properties of glass fiber composite were measured using both transmission and reflection configuration. The interaction of THz with a glass fiber composite was then analyzed, including the effects of scattering, absorption, and the index of refraction, as well as effective medium approximations. THz TDS, in both transmission and reflection configuration, was used to study composite damage, including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive …
The Effect Of Radiation On The Electrical Properties Of Aluminum Gallium Nitride/Gallium Nitride Heterostructures, John W. Mcclory
The Effect Of Radiation On The Electrical Properties Of Aluminum Gallium Nitride/Gallium Nitride Heterostructures, John W. Mcclory
Theses and Dissertations
AlGaN/GaN Heterojunction Field Effect Transistors (HFETs) were irradiated at low temperature and the temperature dependent changes to drain current, gate current, capacitance, and transconductance were measured. The results were compared to the charge control model of the drain current and trap-assisted tunneling model of the gate current to determine the source of the radiation-induced changes. AlGaN/GaN HFETs demonstrated threshold voltage shifts and drain current changes after irradiation. After electron and neutron irradiation applied at ~80 K, measurement of the drain current at this temperature showed an increase that saturated after 1013 electrons/cm2 or 1010 neutrons/cm2 due …
Analysis Of Multi-Layered Materials Under High Velocity Impact Using Cth, Jason K. Lee
Analysis Of Multi-Layered Materials Under High Velocity Impact Using Cth, Jason K. Lee
Theses and Dissertations
Multi-layer armor containing ceramic and metallic layers has become more common in the past two decades. Typically, ceramics have high compressive strength which combined with their low density make them highly desirable for armor applications. This research effort numerically simulates high velocity impact of cylindrical projectiles on multi-layer metallic and ceramic targets of finite thickness. The impact of the projectile occurs normal to the surface of the target. The projectiles used are made of either S7 tool steel or tungsten. The targets consist of either rolled homogeneous armor, 4340 steel and boron carbide ceramic or rolled homogeneous armor and boron …
Effects Of Environment On Creep Behavior Of Nextel 720/Alumina-Mullite Ceramic Composite At 1200°C, Christopher L. Genelin
Effects Of Environment On Creep Behavior Of Nextel 720/Alumina-Mullite Ceramic Composite At 1200°C, Christopher L. Genelin
Theses and Dissertations
The creep behavior of an oxide-oxide ceramic matrix composite (CMC) was investigated at 1200°C in laboratory air, in steam and in argon. The composite consisted of a porous alumina-mullite matrix reinforced with laminated, woven mullite/alumina (Nextel/720) fibers. The composite had no fiber coating and relied on its porous alumina/mullite matrix for flaw tolerance. Tensile stress-strain behavior was investigated and the tensile properties were measured at 1200°C in laboratory air. Tensile creep behavior of the CMC was examined for creep stress levels of 73, 91, 114 and 136 MPa. Creep run-out, set to 100 h, was achieved for stress levels ≤ …
Electrical Activation Studies Of Silicon Implanted Aluminum Gallium Nitride With High Aluminum Mole Fraction, Elizabeth A. Moore
Electrical Activation Studies Of Silicon Implanted Aluminum Gallium Nitride With High Aluminum Mole Fraction, Elizabeth A. Moore
Theses and Dissertations
This research demonstrates a method for producing highly conductive Si-implanted n-type aluminum gallium nitride (AlxGa1-xN) alloys, and represents a comprehensive analysis of the resulting material's electrical and optical properties as a function of Al mole fraction, anneal temperature, anneal time, and implantation dose. Highly conductive alloys are critical to the fabrication of devices operating in deep UV, high-temperature, high-power, and high-frequency environments, and thus this research is significant in regard to the application of such devices. The AlxGa1-xN wafers of this study, with Al concentrations of 10 to 50%, were implanted at …
Effect Of Prior Aging On Fatigue Behavior Of Im7/Bmi 5250-4 Composite At 191°C, Christine G. Ladrido
Effect Of Prior Aging On Fatigue Behavior Of Im7/Bmi 5250-4 Composite At 191°C, Christine G. Ladrido
Theses and Dissertations
IM7/BMI 5250-4 with carbon fiber orientations [± 45] and [0/90] were aged in air at 191 °C for up to 1000 hours. The weight loss was analyzed during the aging process. Tension to failure tests were performed on both the unaged and aged specimens to establish a baseline for the Ultimate Tensile Strengths and Young's Modulus. Tension-tension cyclic load fatigue testing was conducted on the specimen to determine the effect of prior aging on the fatigue response of the composite. Cyclic strain accumulation was not influenced by prior aging, but it was influenced by the stress levels applied. [± 45] …