SEARCH · Engineering Papers

Results for “REACTIVITY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 793 records · Page 44

All sources NASA NTRS DOE OSTI

NASA NTRS1991

A method of producing high quality oxide and related films on surfaces

Aluminum oxide or aluminum nitride films were deposited on molecular beam epitaxy (MBE) grown GaAS(100) using a novel cryogenic-based reactive thin film deposition technique. The process involves the condensation of molecular oxygen, ammonia, or other gases normally used for reactive thin film deposition on the substrate before the metal is deposited. The metal vapor is deposited into this layer and reacts with the molecular solid to form the desired compound or a precursor that can be thermally decomposed to generate the desired compound. The films produced by this method are free of impurities, and the low temperatures can be used to control the film and interfacial structure. The process can be easily integrated with existing MBE systems. Ongoing research using the same apparatus suggests that photon or electron irradiation could be used to promote the reactions needed to produce the intended material.

Ruckman, Mark W.↗

NASA NTRS1991

X-ray photoelectron spectroscopy study of chemically-etched Nd-Ce-Cu-O surfaces

Acetic acid, Br2, and HCl solutions are investigated for removing insulating species from Nd(1.85)Ce(0.15)CuO(4-delta) (NCCO) thin film surfaces. X-ray photoelectron spectroscopy (XPS) shows that the HCl etch is most effective, yielding O 1s spectra comparable to those obtained from samples cleaned in vacuum and a clear Fermi edge in the valence band region. Reduction and oxidation reversibly induces and eliminates, respectively, Fermi level states for undoped samples, but has no clearly observable effect on the XPS spectra for doped samples. Reactivity to air is much less for NCCO compared to hole superconductors, which is attributed to the lack of reactive alkaline earth elements in NCCO.

Vasquez, R. P.↗

NASA NTRS1992

Immune responsiveness and risk of illness in U.S. Air Force Academy cadets during basic cadet training

It has been proposed, but not confirmed, that environmental stressors alter immune function and increase the risk of viral infection among healthy individuals. This hypothesis was evaluated by examining the relationship among stress, immune function, and illness in 96 first-year U.S. Air Force Academy cadets during orientation and four weeks later during the stressful environment of basic cadet training (BCT). Perceived stress and well-being levels of cadets were assessed via questionnaire. Immune responsiveness was analyzed by PHA-stimulated thymidine uptake in mononuclear leucocytes and by serologic evidence of reactivation of the Epstein-Barr virus (EBV). Results showed significant declines in in vitro PHA-induced lymphocyte transformation (-35 percent)and subjective well-being (-19 percent) from orientation to BCT with corresponding, significant increases in perceived stress (+32 percent). Despite significantly altered in vitro immune responsiveness, there was no serologic evidence of EBV reactivation nor was there an association between these measures and risk of illness as determined by medical chart review and self-reported symptoms.

Lee, David J.↗

NASA NTRS1992

Microrover Operates With Minimal Computation

Small, light, highly mobile robotic vehicles called "microrovers" use sensors and artificial intelligence to perform complicated tasks autonomously. Vehicle navigates, avoids obstacles, and picks up objects using reactive control scheme selected from among few preprogrammed behaviors to respond to environment while executing assigned task. Under development for exploration and mining of other planets. Also useful in firefighting, cleaning up chemical spills, and delivering materials in factories. Reactive control scheme and principle of behavior-description language useful in reducing computational loads in prosthetic limbs and automotive collision-avoidance systems.

Miller, David P.↗

NASA NTRS1992

Multi-layer light-weight protective coating and method for application

A thin, light-weight, multi-layer coating is provided for protecting metals and their alloys from environmental attack at high temperatures. A reaction barrier is applied to the metal substrate and a diffusion barrier is then applied to the reaction barrier. A sealant layer may also be applied to the diffusion barrier if desired. The reaction barrier is either non-reactive or passivating with respect to the metal substrate and the diffusion barrier. The diffusion barrier is either non-reactive or passivating with respect to the reaction barrier and the sealant layer. The sealant layer is immiscible with the diffusion barrier and has a softening point below the expected use temperature of the metal.

Wiedemann, Karl E.↗

NASA NTRS1993

Time-dependent Computational Studies of Premixed Flames in Microgravity

This report describes the research performed at the Center for Reactive Flow and Dynamical Systems in the Laboratory for Computational Physics and Fluid Dynamics, at the Naval Research Laboratory, in support of NASA Microgravity Science and Applications Program. The primary focus of this research is on investigating fundamental questions concerning the propagation and extinction of premixed flames in earth gravity and in microgravity environments. Our approach is to use detailed time-dependent, multispecies, numerical models as tools to simulate flames in different gravity environments. The models include a detailed chemical kinetics mechanism consisting of elementary reactions among the eight reactive species involved in hydrogen combustion, coupled to algorithms for convection, thermal conduction, viscosity, molecular and thermal diffusion, and external forces. The external force, gravity, can be put in any direction relative to flame propagation and can have a range of values. Recently more advanced wall boundary conditions such as isothermal and no-slip have been added to the model. This enables the simulation of flames propagating in more practical systems than before. We have used the numerical simulations to investigate the effects of heat losses and buoyancy forces on the structure and stability of flames, to help resolve fundamental questions on the existence of flammability limits when there are no external losses or buoyancy forces in the system, to understand the interaction between the various processes leading to flame instabilities and extinguishment, and to study the dynamics of cell formation and splitting. Our studies have been able to bring out the differences between upward- and downward-propagating flames and predict the zero-gravity behavior of these flames. The simulations have also highlighted the dominant role of wall heat losses in the case of downward-propagating flames. The simulations have been able to qualitatively predict the formation of multiple cells and the cessation of cell-splitting. Our studies have also shown that some flames in a microgravity environment can be extinguished due to a chemical instability and without any external losses. However, further simulations are needed to more completely understand upward-propagating and zero-gravity flames as well as to understand the potential effect of radiative heat losses.

Kailasanath, K.↗

NASA NTRS1992

Materials science research in microgravity

There are several important attributes of an extended duration microgravity environment that offer a new dimension in the control of the microstructure, processing, and properties of materials. First, when gravitational effects are minimized, buoyancy driven convection flows are also minimized. The flows due to density differences, brought about either by composition or temperature gradients will then be reduced or eliminated to permit a more precise control of the temperature and the composition of a melt which is critical in achieving high quality crystal growth of electronic materials or alloy structures. Secondly, body force effects such as sedimentation, hydrostatic pressure, and deformation are similarly reduced. These effects may interfere with attempts to produce uniformly dispersed or aligned second phases during melt solidification. Thirdly, operating in a microgravity environment will facilitate the containerless processing of melts to eliminate the limitations of containment for reactive melts. The noncontacting forces such as those developed from electromagnet, electrostatic, or acoustic fields can be used to position samples. With this mode of operation, contamination can be minimized to enable the study of reactive melts and to eliminate extraneous crystal nucleation so that novel crystalline structures and new glass compositions may be produced. In order to take advantage of the microgravity environment for materials research, it has become clear that reliable processing models based on a sound ground based experimental experience and an established thermophysical property data base are essential.

Perepezko, John H.↗

NASA NTRS1993

An architecture for object-oriented intelligent control of power systems in space

A control system for autonomous distribution and control of electrical power during space missions is being developed. This system should free the astronauts from localizing faults and reconfiguring loads if problems with the power distribution and generation components occur. The control system uses an object-oriented simulation model of the power system and first principle knowledge to detect, identify, and isolate faults. Each power system component is represented as a separate object with knowledge of its normal behavior. The reasoning process takes place at three different levels of abstraction: the Physical Component Model (PCM) level, the Electrical Equivalent Model (EEM) level, and the Functional System Model (FSM) level, with the PCM the lowest level of abstraction and the FSM the highest. At the EEM level the power system components are reasoned about as their electrical equivalents, e.g, a resistive load is thought of as a resistor. However, at the PCM level detailed knowledge about the component's specific characteristics is taken into account. The FSM level models the system at the subsystem level, a level appropriate for reconfiguration and scheduling. The control system operates in two modes, a reactive and a proactive mode, simultaneously. In the reactive mode the control system receives measurement data from the power system and compares these values with values determined through simulation to detect the existence of a fault. The nature of the fault is then identified through a model-based reasoning process using mainly the EEM. Compound component models are constructed at the EEM level and used in the fault identification process. In the proactive mode the reasoning takes place at the PCM level. Individual components determine their future health status using a physical model and measured historical data. In case changes in the health status seem imminent the component warns the control system about its impending failure. The fault isolation process uses the FSM level for its reasoning base.

Holmquist, Sven G.↗

NASA NTRS1992

Calculations of ion-molecule deuterium fractionation reactions involving HD

Gas-phase chemical models of deuterium fractionation in dense interstellar clouds utilize a small number of exothermic reactions to achieve fractionation. Although HD is a major repository of deuterium, it appears not to exchange deuterium with many molecular ions. Useful semiquantitative reasons have been given for the unusual lack of reactivity of exothermic ion-HD deuterium exchange systems, but quantum chemical studies are needed to understand these ideas in more detail and to determine if the lack of reactivity pertains at very low temperatures not studied in the laboratory, or whether tunneling can drive the reactions. Accordingly, the potential energy surfaces of three representative ion-molecule exchange reactions involving protonated ions and HD have been investigated with ab initio quantum chemical techniques. Our results generally confirm the semiquantitative picture as to which reactions are likely to occur and show that tunneling at low temperatures is unlikely to alter this picture.

Maluendes, Sergio A.↗

NASA NTRS1992

Atmospheric measurements of peroxyacetyl nitrate and other organic nitrates at high latitudes - Possible sources and sinks

Measurements of PAN and other reactive nitrogen species during the NASA Arctic Boundary Layer Expedition (ABLE 3A) are described, their north-south and east-west gradients in the free troposphere are characterized, and the sources and sinks of PAN and NO(y) are assessed. Large concentrations of PAN and NO(y) are present in the Arctic/sub-Arctic troposphere of the Northern Hemisphere during the summer. Mixing ratios of PAN and a variety of other molecules are more abundant in the free troposphere compared to the boundary layer. Coincident PAN and O3 atmospheric structures suggest that phenomena that define PAN also define the corresponding O3 behavior. Model calculations, correlations between NO(y) and anthropogenic tracers, and the compositions of NO(y) itself suggest that the Arctic/sub-Arctic reactive nitrogen measured during ABLE 3A is predominantly of anthropogenic origin with a minor component from the stratosphere.

Singh, H. B.↗

NASA NTRS1993

The nature and role of quantized transition states in the accurate quantum dynamics of the reaction O + H2 yields OH + H

Accurate quantum mechanical dynamics calculations are reported for the reaction probabilities of O(3P) + H2 yields OH + H with zero total angular momentum on a single potential energy surface. The results show that the reactive flux is gated by quantized transition states up to the highest energy studied, which corresponds to a total energy of 1.90 eV. The quantized transition states are assigned and compared to vibrationally adiabatic barrier maxima; their widths and transmission coefficients are determined; and they are classified as variational, supernumerary of the first kind, and supernumerary of the second kind. Their effects on state-selected and state-to-state reactivity are discussed in detail.

Chatfield, David C.↗

NASA NTRS1993

Models for direct Monte Carlo simulation of coupled vibration-dissociation

A new model for reactive collisions is developed within the framework of a particle method, which simulates coupled vibration-dissociation (CVD) behavior in high-temperature gases. The fundamental principles of particle simulation methods are introduced with particular attention given to the probability functions employed to select thermal and reactive collisions. Reaction probability functions are derived which favor vibrationally excited molecules as reaction candidates. The new models derived here are used to simulate CVD behavior during thermochemical relaxation of constant-volume O2 reservoirs, as well as the dissociation incubation behavior of postshock N2 flows for comparisons with previous models and experimental data.

Haas, Brian L.↗

NASA NTRS1992

A walking prescription for statically-stable walkers based on walker/terrain interaction

The walker/terrain interaction phenomena for the control of a statically stable walking machine are described. The algorithms, measures, and knowledge of walker/terrain interaction phenomena are then combined to form a prescription for how to walk on general terrain. This prescription consists of two parts: nominal control and reactive control. The function of nominal control is the evaluation and execution of planned motions, based on predicted foot force redistributions, to achieve reliable locomotion. The function of reactive control is the monitoring of walker/terrain interaction in real-time to detect anomalous conditions and then respond with the appropriate reflexive actions. Simulations and experiments have been used to test and verify various aspects of the walking prescription.

Nagy, Peter V.↗

NASA NTRS1993

Stratospheric ClO and ozone from the Microwave Limb Sounder on the Upper Atmosphere Research Satellite

Concentrations of atmospheric ozone and of ClO (the predominant form of reactive chlorine responsible for stratospheric ozone depletion) are reported for both the Arctic and Antarctic winters of the past 18 months. Chlorine in the lower stratosphere was almost completely converted to chemically reactive forms in both the northern and southern polar winter vortices. This occurred in the south long before the development of the Antarctic ozone hole, suggesting that ozone loss can be masked by influx of ozone-rich air.

Waters, J. W.↗

NASA NTRS1992

Flux-split algorithms for hypersonic flows

This study reviews numerical techniques for the simulation of fluid flows spanning the range of reactive regimes, from local chemical equilibrium to full thermochemical non-equilibrium. In particular, characteristic-based algorithms are considered, of the flux-vector and the flux-difference type, which are becoming ever more popular due to their accurate rendition of physically complicated, shock-wave dominated flows. Consideration is given to the problems associated with modeling the thermo-chemical behavior of reactive mixtures of thermally perfect gases, and a few topics of current research are outlined.

Cinnella, P.↗

NASA NTRS1957

Nonmetallic Material Compatibility with Liquid Fluorine

Static tests were made on the compatibility of liquid fluorine with several nonmetallic materials at -3200 F and at pressures of 0 and 1500 pounds per square inch gage. The results are compared with those from previous work with gaseous fluorine at the same pressures, but at atmospheric temperature. In general, although environmental effects were not always consistent, reactivity was least with the low-temperature, low-pressure liquid fluorine. Reactivity was greatest with the warm, high-pressure gaseous fluorine. None of the liquids and greases tested was found to be entirely suitable for use in fluorine systems. Polytrifluorochloroethylene and N-43, the formula for which is (C4F9)3N, did not react with liquid fluorine at atmospheric pressure or 1500 pounds per square inch gage under static conditions, but they did react when injected into liquid fluorine at 1500 pounds per square inch gage; they also reacted with gaseous fluorine at 1500 pounds per square inch gage. While water did not react with liquid fluorine at 1500 pounds per square inch gage, it is known to react violently with fluorine under other conditions. The pipe-thread lubricant Q-Seal did not react with liquid fluorine, but did react with gaseous fluorine at 1500 pounds per square inch gage. Of the solids, ruby (Al2O3) and Teflon did not react under the test conditions. The results show that the compatibility of fluorine with nonmetals depends on the state of the fluorine and the system design.

Price, Harold G , Jr↗

NASA NTRS1994

Melt-Infiltration Process For SiC Ceramics And Composites

Reactive melt infiltration produces silicon carbide-based ceramics and composites faster and more economically than do such processes as chemical vapor infiltration (CVI), reaction sintering, pressureless sintering, hot pressing, and hot isostatic pressing. Process yields dense, strong materials at relatively low cost. Silicon carbide ceramics and composites made by reactive melt infiltration used in combustor liners of jet engines and in nose cones and leading edges of high-speed aircraft and returning spacecraft. In energy industry, materials used in radiant-heater tubes, heat exchangers, heat recuperators, and turbine parts. Materials also well suited to demands of advanced automobile engines.

Behrendt, Donald R.↗

NASA NTRS1993

Analytical and experimental investigation of the feasibility of accelerated lifetime testing of materials exposed to an atomic oxygen beam

The interaction of atomic particles with surfaces is of both scientific and technological interest. Past work emphasizes the measurement of high-energy sputtering yields. Very little work utilized low-energy beams for which chemical and electronic effects can be important. Even less work has been carried out using well-defined low-energy projectiles. The use of low-energy, reactive projectiles permits one to investigate surface processes that have not been well characterized. As the energy of the projectile decreases, the collisional cascades and spikes, that are common for high-energy projectiles, become less important, and chemical and electronic effects can play a significant role. Aspects of particle-surface interactions are of concern in several areas of technology. For example, the erosion, desorption, and glow of surfaces of spacecraft in orbit are important in the arena of space technology. The materials studied under this contract are of possible use on the exterior portions of the power generation system of Space Station Freedom. Under the original designs, Space Station Freedom's power generation system would generate potential differences on the surface as high as 200 volts. Ions in the plasma that often surround orbiting vehicles would be accelerated by these potentials leading to bombardment and erosion of the exposed surfaces. The major constituent of the atmosphere, approximately 90 percent, in the low earth orbit region is atomic oxygen. Since atomic oxygen is extremely reactive with most materials, chemical effects can arise in addition to the physical sputtering caused by the acceleration of the oxygen ions. Furthermore, the incident oxygen ions can remain embedded in the exposed surfaces, altering the chemical composition of the surfaces. Since the effective binding energy of a chemically altered surface can be quite different from that of the pure substrate, the sputtering yield of a chemically altered surface is usually different also. The low-energy O+ sputtering yield measurements, reported here, will help quantify the erosion rates for materials exposed to the low-earth orbit environment. These measurements are of technological importance in another respect. In most surface analysis techniques, a surface is bombarded with ions, electrons or photons. Information concerning the structure of the surface and near-surface bulk, abundance of impurities and defects, as well as other surface properties are obtained either from the desorbed species or from the scattered projectiles. Because of their low penetration depth, low-energy ions provide an advantage over other techniques because they provide information that is more indicative of conditions on the surface rather than integrated effects arising from deeper in the bulk. A better understanding of the microscopic processes involved in these interactions is not only of basic scientific interest, but will also aid the scientific community by increasing the accuracy and usefulness of these surface analysis techniques.

Albridge, Royal↗