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Results for “Reaction Kinetics”

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.

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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

The platinum microelectrode/Nafion interface - An electrochemical impedance spectroscopic analysis of oxygen reduction kinetics and Nafion characteristics

The objectives of this study were to use electrochemical impedance spectroscopy (EIS) to study the oxygen-reduction reaction under lower humidification conditions than previously studied. The EIS technique permits the discrimination of electrode kinetics of oxygen reduction, mass transport of O2 in the membrane, and the electrical characteristics of the membrane. Electrode-kinetic parameters for the oxygen-reduction reaction, corrosion current densities for Pt, and double-layer capacitances were calculated. The production of water due to electrochemical reduction of oxygen greatly influenced the EIS response and the electrode kinetics at the Pt/Nafion interface. From the finite-length Warburg behavior, a measure of the diffusion coefficient of oxygen in Nafion and diffusion-layer thickness was obtained. An analysis of the EIS data in the high-frequency domain yielded membrane and interfacial characteristics such as ionic conductivity of the membrane, membrane grain-boundary capacitance and resistance, and uncompensated resistance.

Parthasarathy, Arvind↗
NASA NTRS1995

An overview of the Evaluation of Oxygen Interactions with Materials 3 experiment: Space Shuttle Mission 46, July-August 1992

The Evaluation of Oxygen Interactions with Materials 3 (EOIM-3) flight experiment was developed to obtain benchmark atomic oxygen reactivity data and was conducted during Space Transportation System Mission 46 (STS-46), July 31 to August 7, 1992. In this paper, we present an overview of EOIM-3 and the results of the Lyndon B. Johnson Space Center (JSC) materials reactivity and mass spectrometer/carousel experiments. Mass spectrometer calibration methods are discussed briefly, as a prelude to a detailed discussion of the mass spectrometric results produced during STS-46. Mass spectrometric measurements of ambient O-atom flux and fluence are in good agreement with the values calculated using the MSIS-86 model of the thermosphere as well as estimates based on the extent of O-atom reaction with Kapton polyimide. Mass spectrometric measurements of gaseous products formed by O-atom reaction with C(13) labeled Kapton revealed CO, CO2, H2O, NO, and NO2. Finally, by operating the mass spectrometer so as to detect naturally occurring ionospheric species, we characterized the ambient ionosphere at various times during EOIM-3 and detected the gaseous reaction products formed when ambient ions interacted with the C(13) Kapton carousel sector. By direct comparison of the results of on-orbit O-atom exposures with those conducted in ground-based laboratory systems, which provide known O-atom fluences and translational energies, we have demonstrated the strong translational energy dependence of O-atom reactions with a variety of polymers. A 'line-of-centers' reactive scattering model was shown to provide a reasonably accurate description of the translational energy dependence of polymer reactions with O atoms at high atom kinetic energies while a Beckerle-Ceyer model provided an accurate description of O-atom reactivity over a three order-of-magnitude range in translational energy and a four order-of-magnitude range in reaction efficiency. Postflight studies of the polymer samples by x-ray photoelectron spectroscopy and infrared spectroscopy demonstrate that O-atom attack is confined to the near-surface region of the sample, i.e. within 50 to 100 A of the surface.

Koontz, Steven L.↗
NASA NTRS1972

Hydrogen atom scrambling in ion-molecule reactions of methane and ethylene.

The extent of hydrogen atom exchange in the reaction, CH3(+) + CH4 yields C2H5(+) + H2, is determined by examining the product distribution for the reactions CH3(+) + CD4 and CD3(+) + CH4 as a function of relative kinetic energy from thermal energies to 10 eV. It is found that the reaction of CH4(+) with the parent neutral proceeds both via proton transfer and hydrogen abstraction accompanied by approximately 10% hydrogen atom exchange during the reaction.

Huntress, W. T., Jr.↗
NASA NTRS1992

Effect of non-equilibrium flow chemistry on the heating distribution over the MESUR forebody during a Martian entry

Effect of flow field properties on the heating distribution over a 140 deg blunt cone was determined for a Martian atmosphere using Euler, Navier-Stokes (NS), viscous shock layer (VSL), and reacting boundary layer (BLIMPK) equations. The effect of gas kinetics on the flow field and the surface heating distribution were investigated. Gas models with nine species and nine reactions were implemented into the codes. Effects of surface catalysis on the heating distribution were studied using a surface kinetics model having five reactions.

Chen, Yih-Kang↗
NASA NTRS1971

Kinetic limitations on the diffusional control theory of the ablation rate of carbon.

It is shown that the theoretical maximum oxidation rate is limited in many cases even at temperatures much higher than 1650 deg K, not by oxygen transport, but by the kinetics of the carbon-oxygen reaction itself. Mass-loss rates have been calculated at air pressures of 0.01 atm, 1 atm, and 100 atm. It is found that at high temperatures the rate of the oxidation reaction is much slower than has generally been assumed on the basis of a simple linear extrapolation of Scala's 'fast' and 'slow' rate expressions. Accordingly it cannot be assumed that a transport limitation inevitably must be reached at high temperatures.

Maahs, H. G.↗
NASA NTRS1976

Dissolution kinetics of small amounts of oxygen in tantalum alloy T-111 and internal oxide displacement reactions during annealing

Oxygen was added to T-111 (Ta-8W-2Hf, wt. %) at 820 and 990 C at an oxygen pressure of about 0.0003 torr. The technique employed permitted predetermined and reproducible doping of T-111 up to 3.0 at. % oxygen. Based on the temperature dependence of the doping reaction, it is concluded that the initial rates of oxygen pickup are probably controlled by solution of oxygen into the T-111 lattice. Although hafnium oxides are more stable than those of tantalum or tungsten, analyses of extracted residues indicate that the latter oxides predominate in the as-doped specimens, presumably because of the higher concentrations of tantalum and tungsten in the alloy. However, high-temperature annealing promotes gettering of dissolved oxygen and of other oxides to form hafnium oxides. Small amounts of tantalum and tungsten oxides were still present after high-temperature annealing. Tungsten oxide (WO3) volatilizes slightly from the surface of T-111 at 990 C. The vaporization of WO3 has no apparent affect on the doping reaction.

Stecura, S.↗
NASA NTRS1978

Experimental Studies of the Formation/Deposition of Sodium Sulfate in/from Combustion Gases

Processes related to the hot corrosion of gas turbine components were examined in two separate investigations. Monochromatic laser light was used to probe condensation onset and condensate film growth (via interference of reflected light) on electrically heated ribbons immersed in seeded, flat flame combustion product gases. Boron trichloride is used as the seed gas in these preliminary experiments conducted to obtain precise measurements of the dew point/deposition rates. Because of the importance of gaseous Na(g) as a precursor to NaSO4 formation, the kinetics and mechanisms of the heterogeneous reaction H(g) + NaCl(s) yields Na(g) + HCl(g) was studied using atomic absorption spectroscopy combined with microwave discharge-vacuum flow reactor techniques at moderate temperatures. Preliminary results indicate the H-atom attack of solid NaCl vaporization is negligible; hence the corresponding gas phase (homogeneous) reaction no role in the observed Na(g) production.

Rosner, D. E.↗
NASA NTRS1983

NASA redox storage system development project

The operating temperature was raised from 25 C to 65 C, which enhanced the kinetics of the chromium electrode charging reactions. The design of the auxiliary electrochemical cell, which is used to keep both reactants at the same state of charge, was modified, leading to better and more stable performance. Preliminary testing has shown that the four tank mode of operation improves energy efficiency as much as 5 percentage points over the conventional two tank mode. Another variation in operating mode, the use of mixed reactants, potentially offers several very attractive advantages. Preliminary reactant cost studies lend further weight to the feasibility of the mixed reactant concept. Electrode studies show that reproducibility of performance is very dependent on the pyrolysis temperature at which the carbon/graphite felt substrate is formed. Membrane development work continued to concentrate on cost reduction and the enhancement of resistivity and selectivity.

Source record↗
NASA NTRS1983

Kinetics and mechanism of the oxidation of S(IV) by ozone in aqueous solution with particular reference to SO2 conversion in nonurban tropospheric clouds

Results are presented from a laboratory study of the kinetics of the S(IV)-O3 reaction in aqueous solution, including measurements of the effects of UV radiation, dissolved transition metals, and an antioxidant (hydroquinone) on the rate. On the basis of the results, relative rates of S(IV) conversion by O3 in tropospheric cloud water are compared with those predicted for H2O2 and for O2. The reaction mechanism is discussed, with an outline given of the elements of a possible reaction scheme. Application of the rate constants obtained to SO2 conversion in cloud water predicts conversion rates by ozone to be competitive with those by H2O2 at pH above about 4.5 and to dominate at pH above about 5.5. It is pointed out that since these pH's are typical for nonurban tropospheric cloud water, ozone is a potentially important contributor to the overall oxidative conversion of SO2 to sulfate in the nonurban troposphere.

Maahs, H. G.↗
NASA NTRS1987

Hydroxide decomposition of dimethylsulfoniopropionate to form dimethylsulfide

The kinetics of DMS production resulting from reaction of OH(-) with DMSP were investigated as a function of hydroxide concentration and temperature. The reaction was first-order with respect to DMSP and OH(-). The second order rate constant at 20+/-1 C is 0.0044/M/sec. The activation energy for this reaction is 14.4 kcal/mode. The investigation indicates that the rate of reaction of DMSP with OH(-) is very slow at the pH of seawater, suggesting that DMSP, which may be a major precursor of DMS in seawater, decomposes in the ocean by other mechanisms. A bacterium which produces DMS from DMSP quantitatively at rates many orders of magnitude higher than indicated by OH(-1) decomposition has been cultured, suggesting that enzymatic processes accelerate the production of DMS from DMSP in seawater.

Dacey, John W. H.↗
NASA NTRS1988

Stability of bulk Ba2YCu3O(7-x) in a variety of environments

Small bars of ceramic Ba2YCu3O(7-x) were fabricated and subjected to environments similar to those that might be encountered during some NASA missions. These conditions include ambient conditions, high humidity, vacuum, and high fluences of electrons and protrons. The normal state resistivity or critical current density (J sub c) were monitored during these tests to assess the stability of the material. When normal state resistivity is used as a criterion, the ambient stability of these samples was relatively good, exhibiting only a 2 percent degradation over a 3 month period. The humidity stability was shown to be very poor, and to be a steep function of temperature. Samples stored at 50 C for 40 min increased in normal state resistivity by four orders of magnitude. Kinetic analysis indicates that the degradation reaction is second order with water vapor concentration. It is suspected that humidity degradation also accounts for the ambient instability. The samples were stable to vacuum over a period of at least 3 months. Degradation of J sub c in a 1 MeV electron fluence of 9.7 x 10 to the 14th e(-)/sq cm was determined to be no more than about 2 percent. Degradation of J sub c in a 8.7 x 10 to the 14th p(+)/sq cm of 42 MeV protons was found to be grain size dependent. Samples with smaller grain size and initial J sub c of about 240 A/sq cm showed no degradation. while that with larger grain size and an initial J sub c of about 30 A/sq cm degraded to 37 percent of its original value.

Gaier, James R.↗
NASA NTRS1989

Two-dimensional analysis of two-phase reacting flow in a firing direct-injection diesel engine

The flow field, spray penetration, and combustion in two-stroke diesel engines are described. Fuel injection begins at 345 degrees after top dead center (ATDC) and n-dodecane is used as the liquid fuel. Arrhenius kinetics is used to calculate the reaction rate term in the quasi-global combustion model. When the temperature, fuel, and oxygen mass fraction are within suitable flammability limits, combustion begins spontaneously. No spark is necessary to ignite a localized high temperature region. Compression is sufficient to increase the gaseous phase temperature to a point where spontaneous chemical reactions occur. Results are described for a swirl angle of 22.5 degrees.

Nguyen, H. Lee↗
NASA NTRS1990

The impedance of a tubular electrode - A model for a porous electrode

A cylindrical tube is used as the basis for a two-dimensional mathematical model to calculate the impedance of a flooded porous electrode. The model incorporates charge-transfer, mass-transfer, and ohmic resistances to obtain the axial and radial dependencies of the concentration and potential profiles. A linearized Butler-Volmer kinetic expression for a simple redox reaction O + e yielding R is used, in conjuction with analytical expressions for the surface concentration and overpotential, to compute the open-circuit impedance. The results of the two-dimensional model, which omits double-layer charging, are compared with the results of a more standardly applied one-dimensional model in which radial variations are neglected, with and without double-layer charging. The simpler-to-apply one-dimensional model is found to be satisfactory when mass-transfer and ohmic resistances are small with respect to charge-transfer resistance. The omission of double-layer charging does not introduce error into the two-dimensional model in the frequency range in which capacitive-like effects are caused by mass-transfer limitations.

Viner, Andrew S.↗
NASA NTRS1989

Gas-phase formation of silicon carbides, oxides, and sulphides from atomic silicon ions

A systematic experimental study of the kinetics and mechanisms of the chemical reactions in the gas phase between ground-state Si(+)2p and a variety of astrophysical molecules. The aim of this study is to identify the reactions which trigger the formation of chemical bonds between silicon and carbon, oxygen and sulphur, and the chemical pathways which lead to further molecular growth. Such knowledge is valuable in the identification of new extraterrestrial silicon-bearing molecules and for an assessment of the gas-phase transition from atomic silicon to silicon carbide and silicate grain particles in carbon-rich and oxygen-rich astrophysical environments.

Bohme, Diethard K.↗
NASA NTRS1991

A numerical study of shock wave/boundary layer interaction in nonequilibrium chemically reacting air - The effects of catalytic walls

This paper presents a numerical study that investigate the effects of nonequilibrium chemistry, and in particular, wall catalycity on the separated flow region generated by an oblique shock wave impinging upon a flat plate boundary layer. To obtain a solution to this problem, the full two dimensional Navier-Stokes equations were solved using MacCormack's predictor-corrector time dependent technique on a rectangular grid. Nonequilibrium chemistry was included by utilizing the 5 species, 17 reaction modified Dunn-Kang chemical kinetics model. Separate results were obtained for: calorically perfect, chemically reacting - noncatalytic wall, and chemically reacting - fully catalytic wall cases, for a given set of flow conditions. A direct comparison of all three cases revealed a slight decrease in the peak heat transfer for the noncatalytic wall case, as compared to the calorically perfect case. On the other hand, the fully catalytic wall case had a tremendous increase in the peak surface heat transfer. It is concluded that, for the particular conditons treated here (nearly frozen flow in the free stream), the effects of the nonequilibrium chemically reacting flow on the shock-wave/boundary-layer interaction depend critically on the catalycity of the wall, having virtually no effect for the case of a noncatalytic wall, and exerting a tremendous effect for a fully catalytic wall.

Grumet, Adam A.↗
NASA NTRS1994

Venus climate stability and volcanic resurfacing rates

The climate of Venus is to a large degree controlled by the radiative properties of its massive atmosphere. In addition, outgassing due to volcanic activity, exospheric escape processes, and surface/atmosphere interactions may all be important in moderating the abundances of atmospheric CO2 and other volatiles. We have developed an evolutionary climate model for Venus using a systems approach that emphasizes feedbacks between elements in the climate system. Modules for atmospheric radiative transfer, surface/atmosphere interactions, tropospheric chemistry, and exospheric escape processes have so far been developed. Climate feedback loops result from interconnections between modules, in the form of the environmental parameters pressure, temperature, and atmospheric mixing ratios. The radiative transfer module has been implemented by using Rosseland mean opacities in a one dimensional grey radiative-convective model. The model has been solved for the static (time independent) case to determine climate equilibrium points. The dynamics of the model have also been explored by employing reaction/diffusion kinetics for possible surface atmosphere heterogeneous reactions over geologic timescales. It was found that under current conditions, the model predicts that the climate of Venus is at or near an unstable equilibrium point. The effects of constant rate volcanism and corresponding exsolution of volatiles on the stability of the climate model were also explored.

Bullock, M. A.↗
NASA NTRS1994

Proton exchange membrane fuel cells for space and electric vehicle applications: From basic research to technology development

The proton exchange membrane fuel cell (PEMFC) is one of the most promising electrochemical power sources for space and electric vehicle applications. The wide spectrum of R&D activities on PEMFC's, carried out in our Center from 1988 to date, is as follows (1) Electrode Kinetic and Electrocatalysis of Oxygen Reduction; (2) Optimization of Structures of Electrodes and of Membrane and Electrode Assemblies; (3) Selection and Evaluation of Advanced Proton Conducting Membranes and of Operating Conditions to Attain High Energy Efficiency; (4) Modeling Analysis of Fuel Cell Performance and of Thermal and Water Management; and (5) Engineering Design and Development of Multicell Stacks. The accomplishments on these tasks may be summarized as follows: (1) A microelectrode technique was developed to determine the electrode kinetic parameters for the fuel cell reactions and mass transport parameters for the H2 and O2 reactants in the proton conducting membrane. (2) High energy efficiencies and high power densities were demonstrated in PEMFCs with low platinum loading electrodes (0.4 mg/cm(exp 2) or less), advanced membranes and optimized structures of membrane and electrode assemblies, as well as operating conditions. (3) The modeling analyses revealed methods to minimize mass transport limitations, particularly with air as the cathodic reactant; and for efficient thermal and water management. (4) Work is in progress to develop multi-kilowatt stacks with the electrodes containing low platinum loadings.

Srinivasan, Supramaniam↗
NASA NTRS1997

Laboratory Investigations of Stratospheric Halogen Chemistry

A final report for the NASA-supported project on laboratory investigations of stratospheric halogen chemistry is presented. In recent years, this project has focused on three areas of research: (1) kinetic, mechanistic, and thermochemical studies of reactions which produce weakly bound chemical species of atmospheric interest; (2) development of flash photolysis schemes for studying radical-radical reactions of stratospheric interest; and (3) photochemistry studies of interest for understanding stratospheric chemistry. The first section of this paper contains a discussion of work which has not yet been published. All subsequent chapters contain reprints of published papers that acknowledge support from this grant.

Wine, Paul H.↗