Showing papers by "Wright-Patterson Air Force Base published in 1996"

Preparation of amorphous diamond-like carbon by pulsed laser deposition: a critical review

Abstract: A critical review of the pulsed laser deposition (PLD) of amorphous diamond-like carbon (DLC) films is presented. A short review of the PLD process is followed by a review of various experimental configurations for DLC deposition and a discussion of the influence of process parameters on the composition and energy of ablated carbon plumes. Particular emphasis is given to the relationship between plume properties and film structure and mechanical characteristics. For the first time, a cumulative influence of the laser power density (fluence) and wavelength on the formation and properties of DLC films is shown. The influence of bias, additional auxiliary energy, substrate temperature, and the presence of hydrogen is also discussed. A fluence-wavelength region for DLC formation is proposed and correlated with the kinetic energy of ablated carbon species. It is shown that lower fluences are required to produce DLC films when shorter-wavelength lasers are used. The latest available results on applications of PLD DLC films as protective coatings for reducing friction and wear are also discussed. Methods are proposed to improve film adhesion to steel substrates, so that DLC films can be used in highly loaded friction contacts. Finally, process improvements that are necessary to permit scaling up PLD for growing DLC films are outlined.

Aircraft Anti-Icing and De-Icing Techniques and Modeling

Abstract: Nomenclature Ac = accumulation parameter Cd drag coefficient Ci = lift coefficient Cm = moment coefficient Cp = pressure coefficient c = specific heat at constant pressure, J/(kg-K); airfoil chord, m D = propeller diameter, m; flexural stiffness, N-m D drag force, N d = droplet diameter, m E = total collection efficiency / = freezing fraction g = acceleration caused from gravity, m/s Hi = ice thickness, m Hp = plate thickness, m h = airfoil projected height, m hc = convective heat transfer coefficient, W/(m-K) hfg = heat of vaporization, J/kg hsi = heat of fusion, J/kg / = airfoil drag constant K = thermal conductivity, W/(m-K); inertia parameter K0 = modified inertia parameter k = roughness diameter, m LWC = liquid water content, kg/m M = local Mach number MVD = median volume droplet diameter, m m = mass, kg ra = mass flow rate, kg/s m' = mass flow rate per unit width, kg/(m-s) m" = mass flux, kg/(m-s) n = normal direction P = pressure, Pa p spatial pressure distribution, N/m <2 = heat rate, W q = normal pressure distribution, N/m

Characterization of wear protective AlSiO coatings formed on Al-based alloys by micro-arc discharge treatment

Abstract: The wear life of components manufactured from Al-based alloys can be drastically increased by the application of ceramic coatings. However, coatings deposited by conventional methods such as vacuum deposition or plasma spray have either insufficient adhesion to Al-based materials or the deposition process causes the component to overheat. A recently developed micro-arc discharge oxidizing (MDO) technique allows for the formation 100–200 μm thick AlSiO coating on the surface of Al alloys. A composite Al2O3SiO2 coating is formed at room temperature as a result of a reactive process between Al in the alloy itself and O and Si supplied by an electrolyte. AlSiO coatings were investigated with XPS, Vickers and nanoindentation hardness tests, ball-on-disk, and block-on-ring friction and wear tests. Coatings were found to consist of at least two phases: a hard Al2O3 phase and a softer aluminasilicate phase. A maximum hardness of 17 GPa was found for coatings with highest content of Al2O3 phase. The tribological properties of AlSiO coatings with different composition are discussed. The lowest friction coefficient was found for the Al0.26Si0.08O0.66 coating and was measured around 0.15–0.25 depending on the test environment. The application of this coating decreased the wear rate of components fabricated from an Al-based alloy by several orders of magnitude and permitted operation of coated friction pairs at 1 GPa contact load.

Electro-optical switching characteristics of volume holograms in polymer dispersed liquid crystals

Abstract: Electrically switchable volume holograms lead to the possiblity of real-time electro-optical control of diffractive optic components. We report here on the development of a novel photopolymer-liquid crystal composite material system for writing in a fast single step, high diffraction efficiency volume holograms, capable of switching in applied electric fields of low voltage. Switching of a first-order Bragg diffracted beam into the zero-order with an applied field of ~10 V/µm was observed. With the addition of a surfactant to our pre-polymer syrup, we observed lowering of the switching fields to ~5 V/µm. We report response times for switching and relaxation in the order of microseconds. Low voltage, high resolution scanning electron microscopy studies show that the Bragg gratings formed consist of periodic polymer dispersed liquid crystal planes. The addition of surfactant leads to formation of very uniform small (20–40 nm) nematic droplets. A simple model based on the shape of the liquid crystal droplets...

Textured magnetoplumbite fiber-matrix interphase derived from sol-gel fiber coatings

Abstract: A calcium hexaluminate (CaAl{sub 12}O{sub 19}, magnetoplumbite structure) sol was used to coat alumina and yttrium-aluminum-garnet (YAG) single-crystal fibers and single-crystal alumina plates. When the coated substrates were either annealed or hot-pressed in polycrystalline alumina and YAG matrices, the calcium hexaluminate basal cleavage planes were aligned parallel with the fiber-matrix interface. A complex series of reactions and phase transformations contributed to texture formation on alumina substrates. The alumina fibers and plates seeded the phase transformation of sol-derived transition aluminas to {alpha}-Al{sub 2}O{sub 3}, CaAl{sub 12}O{sub 19} and CaAl{sub 4}O{sub 7} formed between the seeded {alpha}-Al{sub 2}O{sub 3}, and CaAl{sub 4}O{sub 7} later reacted with the seeded {alpha}-Al{sub 2}O{sub 3} to form CaAl{sub 12}O{sub 19}, resulting in a single-phase coating. Several different mechanisms may be responsible for the texture. The microstructure, phase evolution, and possible mechanisms for texture formation of CaAl{sub 12}O{sub 19} powders, sol-derived thin films, and coated plates and fibers, with and without hot-pressed matrices, were studied and are discussed. Deflection and propagation of cracks within the fiber-matrix interphase in thin foils suggests that such an interphase may protect fibers from matrix cracks.

Wet Chemical Digital Etching of GaAs at Room Temperature

Abstract: A new room temperature wet chemical digital etching technique for GaAs is presented which uses hydrogen peroxide and an acid in a two‐step etching process to remove GaAs in approximately 15 A increments. In the first step, GaAs is oxidized by 30% hydrogen peroxide to form an oxide layer that is diffusion limited to a thickness of 14 to 17 A for time periods from 15 to 120 s. The second step removes this oxide layer with an acid that does not attack unoxidized GaAs. These steps are repeated in succession until the desired etch depth is obtained. Experimental results are presented for this digital etching technique demonstrating the etch rate and process invariability with respect to hydrogen peroxide and acid exposure times.

Aurally Aided Visual Search under Virtual and Free-Field Listening Conditions

Abstract: We examined the minimum latency required to locate and identify a visual target (visual search) in a two-alternative forced-choice paradigm in which the visual target could appear from any azimuth (0° to 360°) and from a broad range of elevations (from 90° above to 70° below the horizon) relative to a person's initial line of gaze. Seven people were tested in six conditions: unaided search, three aurally aided search conditions, and two visually aided search conditions. Aurally aided search with both actual and virtual sound localization cues proved to be superior to unaided and visually guided search. Application of synthesized three dimensional and two-dimensional sound cues in the workstations are discussed.

Design and implementation of a direct digitization GPS receiver front end

Abstract: A direct digitization approach greatly reduces the hardware requirements in traditional front end design. Further, the hardware that has been eliminated is typically the source of a number of potential difficulties including age-based, temperature-based, and/or nonlinear performance. This paper presents a case study on the design and implementation of direct digitization Global Positioning System (GPS) receiver front end. First, sensitivity and dynamic range issues for a generic front end are discussed with particular attention given to the unique requirements in the direct digitization approach. Second, two GPS front end implementations are compared. The first is the direct digitization of the input signal at radio frequency (RF) as is the case in the true digital receiver or software radio. The second uses a more standard approach of downconverting the input signal to an intermediate frequency (IF) for further processing of digitization. Experimental data is presented which characterizes the relative signal-to-noise ratio for both implementations as well as the results of initial acquisition processing of true GPS data.

Colonoscopy without sedation

Abstract: PURPOSE AND METHODS: Two hundred fifty-eight consecutive nonsedated colonoscopies were prospectively evaluated over an 18-month period The cecum was intubated in 250 cases (97 percent) Polypectomies or biopsies were performed in 77 cases (31 percent) Indications for colonoscopy included history of polyps (32 percent), hematochezia (28 percent), family history of colon cancer (16 percent), prior history of colon cancer (9 percent), change in bowel habits (12 percent), abnormal flexible sigmoidoscopy (6 percent), and inflammatory bowel disease (3 percent) RESULTS: Sedation was withheld for the following reasons: patient preference (57 percent), medical problems (2 percent), prior resection (16 percent), surgeon preference (65 percent), lack of intravenous access (3 percent), and age (1 percent) Three patients (15 percent) required sedation to complete the procedure Procedure-related pain was based on a scale from zero to three One hundred thirtynine patients (61 percent) rated their pain as either zero (no pain) or one (mild pain), and 217 percent (8 percent) indicated that they would prefer their next colonoscopy to be performed without sedation One hundred twelve patients had previously undergone sedated colonoscopy Ninety-two (92 percent) preferred nonsedated to sedated procedures Average procedure time was 137 (range, 05 to 85) minutes CONCLUSIONS: Several series have studied nonsedated endoscopic upper and lower gastrointestinal evaluations None, however, have evaluated patient comfort and satisfaction In our prospective series of 258 consecutive nonsedated colonoscopies, we found the procedure to be safe, effective, and well accepted

Bow shock/jet interaction in compressible transverse injection flowfields

Abstract: In flowfields created by transverse injection into supersonic cross-flows, the bow and separation shocks formed upstream of the injectant plume are dominant features. In the present investigation, the interaction between these features and the large-scale eddies that develop at the jet/freestream interface has been examined.

Body-axis rolling motion critical states of a 65-degree delta wing

Abstract: Dynamic wind-tunnel test results of a 65-deg swept delta wing are reviewed. These tests involved bodyaxis rolling motions at moderate (15- to 35-deg) angles of attack in both the Institute for Aerospace Research 2 X 3 m low-speed wind tunnel and the 7 X 10 ft Subsonic Aerodynamic Research Laboratory facility at Wright-Patter son Air Force Base. They included static, forced oscillation, and free-to-roll experiments with flow visualization. Multiple trim points (attractors) for body-axis rolling motions and other unusual dynamic behavior were observed. These data are examined in light of the nonlinear indicial response theory. The analysis confirms the existence of critical states with respect to roll angle. When these singularities are encountered in a dynamic situation, large and persistent transients are induced. Conventional means of representing the nonlinear forces and moments hi the aircraft equations of motion, notably the locally linear model, are shown to be inadequate for these cases. Finally, the impact of these findings on dynamic testing techniques is discussed.

Breakup processes of liquid jets in subsonic crossflows

Abstract: The breakup processes of liquid jets injected into subsonic air crosse ows were experimentally studied. Test liquids, injector diameters, and air Mach numbers were varied to provide a wide range of jet operation conditions. Results indicate that for larger injection velocity conditions liquid jets penetrate relatively far into the crosse ows and exhibit surface breakup processes before the column breaks. Liquid column trajectories were correlated by liquid/air momentum e ux ratios based on a force analysis of a cylindrical liquid element subjected to an aerodynamic drag force. Drag coefe cients were inferred from the column trajectories and were found to exhibit a weak dependence on liquid viscosity. The heights of the column fracture points were correlated using the time required for an analogous droplet to complete an aerodynamic secondary breakup process. The success of the resulting correlation justie es the assumption that the aerodynamic forces acting on a droplet and those acting on a liquid column have similar effects. This result, combined with the trajectory correlation, leads to the conclusion that the liquid column always breaks at the same streamwise location, in agreement with the present experimental observation.

Developmental toxicity of JP-8 jet fuel in the rat.

Abstract: JP-8 aviation fuel is being phased in as a replacement for both JP-4 and JP-5 jet fuels presently in use by the Air Force and Navy, respectively. At the present time, 11% of active-duty Air Force personnel are women of child-bearing age. This study was undertaken to determine the threat posed to the unborn fetus should female active-duty personnel come in contact with JP-8 while pregnant. Time-mated Sprague-Dawley rats were dosed orally with JP-8 at 0, 500, 1000, 1500 and 2000 mg kg-1 day-1 on days 6-15 of pregnancy. The number and type of fetal malformations and variations observed did not differ significantly between dose groups. Dams in the 1000, 1500 and 2000 mg kg-1 day-1 groups gained significantly less body weight during pregnancy than did control dams. Embryo toxicity was indicated by a significant reduction in fetal body weight in the 1500 and 2000 mg kg-1 day-1 dose groups.

Comparison of Ni/Au, Pd/Au, and Cr/Au Metallizations for Ohmic Contacts to p-GaN

Abstract: Reactions between electron beam evaporated thin films of Ni/Au, Pd/Au, and Cr/Au on p-GaN with a carrier concentration of 9.8 × 1016 cm−3 were investigated in terms of their structural and electronic properties both as-deposited and following heat treatments up to 600°C (furnace anneals) and 900°C (RTA) in a flowing N2 ambient. Auger electron spectroscopy (AES) depth profiles were used to study the interfacial reactions between the contact metals and the p-GaN. The electrical properties were studied using room temperature current-voltage (1-V) measurements and the predominant conduction mechanisms in each contact scheme were determined from temperature dependent I-V measurements. The metallization schemes consisted of a 500 A interfacial layer of Ni, Pd, or Cr followed by a 1000 A capping layer of Au. All schemes were shown to be rectifying as-deposited with increased ohmic character upon heat treatment. The Cr/Au contacts became ohmic upon heating to 900°C for 15 seconds while the other schemes remained rectifying with lower breakdown voltages following heat treatment. The specific contact resistance of the Cr/Au contact was measured to be 4.3×10−1 Ωcm2. Both Ni and Cr have been shown to react with the underlying GaN above 400 °C while no evidence of a Pd:GaN reaction was seen. Pd forms a solid solution with the Au capping layer while both Ni and Cr tend to diffuse through the capping layer to the surface. All contacts were shown to have a combination of thermionic emission and thermionic field emission as their dominant conduction mechanism, depending on the magnitude of the applied reverse bias.

White-light imaging by use of a multiple passband acousto-optic tunable filter.

Abstract: White-light imaging was accomplished by operation of a TeO(2) acousto-optic tunable filter (AOTF) with 40 simultaneous overlapping passbands from 400 to 700 nm. The AOTF was chromatically compensated by a wedge applied to the output surface of the AOTF, and the measured spatial resolution correlated well with predictions. Switching off specific rf's applied to the AOTF produced optical rejection corresponding to the inactive passbands. A rejection ratio of 30 dB was demonstrated, and the rejection level was found to be controlled by leakage through the sidelobes of adjacent passbands.

Gliomatosis cerebri: Results with radiation therapy

Abstract: BACKGROUND This review was undertaken to determine the outcome for patients diagnosed in the modern era and treated with radiation therapy. METHODS Using the tumor registries of six institutions in a large metropolitan area, cases of gliomatosis were identified and retrospectively reviewed. RESULTS The clinical course for each patient was unique. Deterioration during treatment, brief stabilization, and reversal of the clinical signs and symptoms with stability and high quality of life at 16 months from diagnosis characterized Patients 1, 2, and 3, respectively. CONCLUSIONS Radiotherapy for gliomatosis appears to stabilize or improve neurologic function in some patients. Its impact on survival will await additional reports and longer follow-up. Cancer 1996;78:1789-93.

Preparation of thin sections of coated fibers for characterization by transmission electron microscopy

Abstract: A method of preparing transmission electron microscopy (TEM) specimens of coated ceramic fibers has been developed, which produces large electron transparent areas due to the minimal preferential milling of the fiber, coating, or epoxy matrix. Multiple individual fibers or tows are impregnated with a high-temperature epoxy and contained to assure a high fiber-to-epoxy volume ratio. The samples are then sectioned and mechanically thinned either parallel or normal to the fiber axes using a wedge polisher on diamond lapping films to achieve a thickness of less than 5 {micro}m. The thinned sample is then ion-milled to electron transparency in less than 30 min, giving representative specimens of the coating, fiber, and coating-fiber interface. This technique is also well suited to preparing extremely flat specimens for scanning electron microscopy analysis of thin coatings. Examples of TEM sections of coated fibers prepared using this technique are presented.

Effect of the spin split-off band on optical absorption in p-type Ga1-xInxAsyP1-y quantum-well infrared detectors.

Abstract: Experimental investigations of p-type ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ quantum-well intersubband photodetectors (QWIP's) led to the discovery of unique features in photoresponse spectra of these devices. In particular, the strong 2--5-\ensuremath{\mu}m photoresponse of these QWIP's was not anticipated based on previous experimental and theoretical results for p-type GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As QWIP's. Our theoretical modeling of p-type QWIP's based on the ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ system revealed that the intense short-wavelength photoresponse was due to a much stronger coupling to the spin-orbit split-off components in the continuum than occurs for GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As QWIP's. Due to the strong influence of the spin split-off band, an eight-band Kane Hamiltonian was required to accurately model the measured photoresponse spectra. This theoretical model is first applied to a standard p-type GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As QWIP, and then to a series of GaAs/${\mathrm{Ga}}_{0.51}$${\mathrm{In}}_{0.49}$P, GaAs/${\mathrm{Ga}}_{0.62}$${\mathrm{In}}_{0.38}$${\mathrm{As}}_{0.22}$${\mathrm{P}}_{0.78}$, ${\mathrm{Ga}}_{0.79}$${\mathrm{In}}_{0.21}$${\mathrm{As}}_{0.59}$${\mathrm{P}}_{0.41}$/${\mathrm{Ga}}_{0.51}$${\mathrm{In}}_{0.49}$P, and ${\mathrm{Ga}}_{0.79}$${\mathrm{In}}_{0.21}$${\mathrm{As}}_{0.59}$${\mathrm{P}}_{0.41}$/${\mathrm{Ga}}_{0.62}$${\mathrm{In}}_{0.38}$${\mathrm{As}}_{0.22}$ ${\mathrm{P}}_{0.78}$ QWIP's. Through this analysis, the insignificance of spin split-off absorption in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As QWIP's is verified, as is the dual role of light-hole extended-state and spin split-off hole-extended-state absorption on the spectral shape of ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ QWIP's. \textcopyright{} 1996 The American Physical Society.

Critical states and flow structure on a 65-deg delta wing

Abstract: Swept and delta wings maneuvering at moderate and high angles of attack produce highly nonlinear and often discontinuous aerodynamic forces and moments that are difficult to model. The nonlinear indicial response (NIR) methodology and the concept of critical states accompanied by changes in the flow structure and topology could provide a rational framework for the analyses and modeling of these flows. The analysis of surface oil-flow photographs and laser light sheet high-speed video images of smoke flow has been performed. The correlation of the structural and topological changes in the flow with force and moment data follows. Critical states are often accompanied by changes in the flow topology and not all topological changes produce measurable changes hi the forces and moments, however, a useful relationship may exist.

EEG-based control for human-computer interaction

Abstract: An interface whereby brain responses can control machines has been developed by the Armstrong Laboratory. This EEG-based control uses the magnitude of the steady-state visual evoked response (SSVER) as a control signal. The SSVER is identified and monitored using non-invasive scalp electrodes and advanced signal processing technology. With biofeedback, users learn to increase or decrease the magnitude of the VSYER to an evoking stimulus. These responses are translated into commands that control the operation of a physical device or computer program. After further development this innovative interface could revolutionize human interaction with complex systems.