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At least 901 records · Page 50

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NASA NTRS1990

In situ analysis of CO during chemisorption and oxidation on graphite: Supported Pt by FTIR-microspectrometry

For chemisorption and oxidation on Pt/HOPG (highly-orientated pyrolytic) graphite, reflectance Fourier Transform Infrared (FTIR)-microspectrometry reveals a variable state and reactivity for CO. Even for model surface science systems, where surface heterogeneity is minimal, surface diffusion may be too slow relative to the reaction rate to avoid segregation of reactants into surface islands under steady-state conditions. Thus in CO oxidation on Pt (where the relevant surface diffusion coefficients are such that D sub O less than D sub CO) then reactant CO islands exists at the perimeters of which the surface reaction is thought to occur. Furthermore CO can chemisorb on metals in linear and bridge forms to extents which vary with precise faces predominantly exposed coverage, etc. Infrared has long been used to probe the nature of adsorbed CO on model film and heterogeneous surfaces, but it may now be that FTIR-microspectrometry will allow the state of this adsorbate and reactant to be investigated with a spatial resolution of 4.4 microns on model (and real) catalytic surfaces.

Self, Valerie A.↗

NASA NTRS2014

Diffusivity in Alumina Scales Grown on Al-MAX Phases

Ti3AlC2, Ti2AlC, and Cr2AlC are oxidation resistant MAX phase compounds distinguished by the formation of protective Al2O3 scales with well controlled kinetics. A modified Wagner treatment was used to obtain interfacial grain boundary diffusivity, deltaD(sub gb,O,int.), from scale growth rates and corresponding grain size. It is based on the p(O2)(exp -1/6) dependency of the double charged oxygen vacancy and oxygen diffusivity, coupled with the effective diffusion constant for short circuit grain boundary paths. Data from the literature for MAX phases was analyzed accordingly, and deltaD(sub gb,O,int.) was found to nearly coincide with the Arrhenius line developed for Zr-doped FeCrAl, where: deltaD(sub gb,O,int.) = 1.8x10(exp -10) exp(-375 kJ/RT) cubic meters/s. Furthermore, this oxidation relation suggests the more general format applicable to bulk samples under ambient conditions: deltaD(sub gb,O) = 7.567x10(exp -8) exp(-544 kJ/RT) p(O2)(exp -1/6) cubic meters/[s x Pa(exp -1/6)]. Data from many other FeCrAl(X) studies were similarly assessed to show general agreement with the relation for deltaD(sub gb,O,int.). This analysis reinforces the view that protective alumina scales grow by similar mechanisms for these Al-MAX phases and oxidation resistant FeCrAl alloys.

aluminum oxide↗

DOE OSTI2026

Buried Interfaces in Organic Photocathodes for H 2 Evolution: Fermi-Level Pinning and Recombination

Herein, we demonstrate how Fermi-level pinning at buried contacts impacts solar fuel generation in all-polymer photocathodes by systematically comparing the effects of work function, hydroxyl coverage, and hydrogen evolution using chemically modified indium tin oxide (ITO) supports. Photovoltages and net photocathode performance are improved when the ITO is passivated using phosphonic acids, independent of work function, suggesting that the passivation reduces Fermi-level pinning at the buried interface arising from blended heterojunction interactions with surface metal hydroxyls. Transient photovoltage decay reveals differences in recombination mechanisms, supported by light intensity-dependent measurements. Briefly, nonpassivated, hydrophilic contacts exhibit trap-assisted recombination, while passivated, hydrophobic contacts follow bimolecular recombination. We then investigate changes in electroactivity of hole-transfer processes as a function of scan rate and repetitive cycling using a diffusion-controlled molecular redox probe, analogous to a hole-only device achieved via the electrolyte. The nonpassivated buried contacts exhibit higher overpotentials for oxidation, indicative of hole injection/extraction barriers. We observe irreversible electron transfer via the hole-transport level of the blended heterojunction and a strong cycle dependence, consistent with changes in the hole trap state density. Passivation results in more reversible redox behaviors, consistent with more Ohmic-like contacts. Collectively, these results provide context toward the realization of durable organic photoelectrodes with optimized photovoltages and net solar-to-hydrogen conversion efficiencies via fundamental understanding of the rates of carrier generation, recombination, and transport in high-dielectric aqueous environments and opportunities to characterize buried interfaces under device-relevant electric fields.

buried interfaces↗

DOE OSTI2020

Lithium Manganese Spinel Cathodes for Lithium-Ion Batteries

Spinel LiMn 2 O 4 , whose electrochemical activity was first reported by Professor John B. Goodenough’s group at Oxford in 1983, is an important cathode material for lithium-ion batteries, which attracts continuous academic and industrial interests. It is cheap and environmentally friendly, and has excellent rate performance with 3-dimensional (3D) Li + diffusion channels. However, it suffers from severe degradations, especially under extreme voltages and during high-temperature operations. In this review, the current understanding and future trends of the spinel cathode and its derivatives with cubic lattice symmetry (LiNi 0.5 Mn 1.5 O 4 that shows high-voltage stability, and Li-rich spinels that show reversible hybrid anion- and cation-redox activities) shall be discussed. Special attention is given to the degradation mechanisms, further development of spinel cathodes, and concepts of utilizing the cubic spinel structure to stabilize high-capacity layered cathodes and as robust framework for high-rate electrodes. Lastly, “Good spinel” surface phases like LiNi 0.5 Mn 1.5 O 4 are distinguished from “bad spinel” surface phases like Mn 3 O 4 .

25 ENERGY STORAGE↗

DOE OSTI2021

Faster Form of Electron Magnetic Reconnection with a Finite Length X-Line

Observations in Earth’s turbulent magnetosheath downstream of a quasiparallel bow shock reveal a prevalence of electron-scale current sheets favorable for electron-only reconnection where ions are not coupled to the reconnecting magnetic fields. In small-scale turbulence, magnetic structures associated with intense current sheets are limited in all dimensions. And since the coupling of ions are constrained by a minimum length scale, the dynamics of electron reconnection is likely to be 3D. Here, both 2D and 3D kinetic particle-in-cell simulations are used to investigate electron-only reconnection, focusing on the reconnection rate and associated electron flows. A new form of 3D electron-only reconnection spontaneously develops where the magnetic X-line is localized in the out-of-plane (z) direction. The consequence is an enhancement of the reconnection rate compared with two dimensions, which results from differential mass flux out of the diffusion region along z, enabling a faster inflow velocity and thus a larger reconnection rate. This outflow along z is due to the magnetic tension force in z just as the conventional exhaust tension force, allowing particles to leave the diffusion region efficiently along z unlike the 2D configuration.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1992

Constraints on the acceleration of anomalous cosmic rays

The recent determination of an upper limit of 0.2 pc on the distance to the source of the anomalous cosmic rays, and the implied upper limit on their age imposes a significant observational constraint on possible acceleration mechanisms. Adiabatic cooling in the expanding wind makes the constraints significantly more stringent. In the light of these constraints, the rate of second-order Fermi acceleration is much too low to play a significant role. Diffusive shock acceleration, even at the strong termination shock of the solar wind, is sufficient only if one discards the standard expression for the maximum acceleration rate based on the Bohm limit. Only diffusive acceleration at the quasi-perpendicular solar wind termination shock has a high enough acceleration rate to be the source of the anomalous component.

Jokipii, J. R.↗

NASA NTRS1997

Leading Edge Flame Detachment: Effect on Burning Rate of Ammonium Perchlorate Propellants

Inner details of the transition from premixed to diffusion controlled burning are examined by considering the transition at the individual particle flamelet level. These considerations lead to prediction of observable singular burning rate behavior of bimodal AP formulations. Burning rate measurements verify the predictions, supporting the view that particle flamelets become detached and retreat to more remote premixed locations at definite (particles size dependent) pressures, revealed by the burning rate tests.

Price, E. W.↗

DOE OSTI2022

Reactive Transport Modeling of Mineral Precipitation and Carbon Trapping in Discrete Fracture Networks

In this study we use numerical experiments to analyze reactive flow and transport behavior in discrete intersecting fracture networks, focusing on (a) how reaction-induced changes in physical and chemical properties affect flow connectivity and (b) how fracture networks developed in the Earth's critical zone contribute to carbon sequestration via mineral weathering reactions. In the first part of the study, we used two-dimensional reactive flow and transport simulations to analyze the impacts of mixing in a natural discrete fracture network. We concluded that reaction-induced changes can substantially alter the flow connectivity, especially at fracture intersections. The second set of simulations considered the problem of natural weathering of fractured mafic and ultramafic rocks in the partially saturated Earth's critical zone as a function of infiltration rates, fracture permeability, and partially saturated flow parameters. As a model system, we considered an incongruent reaction network with dissolution of forsterite and precipitation of magnesite. The behavior is complex in terms of the rate-controlling processes because of the multicomponent nature of the system as shown by the grid Peclet number: the CO 2 behavior is gas diffusion-controlled in the partially saturated zone, while the rate of water flow via the Damkӧhler number controls Mg 2+ transport through the fracture network. The amounts of carbon that can be trapped are modest, but the naturally fractured domain considered here provides a useful “base case” against which various engineered solutions can be compared.

58 GEOSCIENCES↗

NASA NTRS1979

Experiments on tandem diffusers with boundary-layer suction applied in between

Experiments were performed on conical diffusers of various configurations with the same, but rather unusually large, 16:1 area ratio. Because available performance data on diffusers fall short of very large area ratio configurations, an unconventional design, consisting of two diffusers following each other in tandem, was proposed. Both diffusers had the same area ratio of 4:1, but had different taper angles. While for the first diffuser (called leading) the angle remained constant, for the second (called follower), the taper angle was stepped up to higher values. Boundary layer control, by way of suction, was applied between the diffusers, and a single slot suction ring was inserted between them. The leading diffuser had an enclosed nominal divergence angle 2 theta = 5 degrees, while the follower diffusers had either 10, 20, 30, or 40 degrees, respectively, giving 4 combinations. The experiments were performed at four different Reynolds numbers with various suction rates. The rates indicate a general improvement in the performance of all diffusers with boundary layer suction. It appears that the improvement of the pressure recovery depends on both the Reynolds number and the suction rate, and the largest increase, 0.075, was found at the lowest R sub e when the follower divergence was 2 theta = 40 degrees.

Barna, P. S.↗

NASA NTRS1980

Long-time creep behavior of the niobium alloy C-103

The creep behavior of C-103 was studied as a function of stress, temperature, and grain size for test times to 19000 hr. Over the temperature range 827 to 1204 C and the stress range 6.89 to 138 MPa, only tertiary (accelerating) creep was observed. The creep strain epsilon can be related to time t by an exponential relation epsilon = epsilon(0) + K e raised to power (st) - 1), where epsilon (0) is initial creep strain, K is the tertiary creep strain parameter, and s is the tertiary creep rate parameter. The observed stress exponent 2.87 is similar to the three power law generally observed for secondary (linear) creep of Class I solid solutions. The apparent activation energy 374 kj/g mol is close to that observed for self diffusion of pure niobium. The initial tertiary creep rate was slightly faster for fine grained than for coarse-grained material. The strain parameter K can be expressed as a combination of power functions of stress and grain size and an exponential function of temperature. Strain time curves generated by using calculated values for K and s showed reasonable agreement with observed curves to strains of at least 4 percent. The time to 1 percent strain was related to stress, temperature, and grain size in a similar manner as the initial tertiary creep rate.

Titran, R. H.↗

DOE OSTI2021

Methodologies for Design, Characterization and Testing of Electrolytes that Enable Extreme Fast Charging of Lithium-ion Cells

Selection, testing and validation of electrolyte candidates for Li-ion cells are discussed, based on a 10-minute target for extreme fast charge (XFC). A combination of modeling and laboratory measurements create a timely and synergistic approach to identifying candidate electrolyte formulations. Multi-solvent systems provide a balanced set of properties, wherein lower molecular-weight solvents offer reduced viscosity, increased species diffusivity, and mitigation of concentration polarization at high charge rates. Carefully selected formulations can exhibit peak conductivity and usable conductivity range of two to three times that of the baseline EC-EMC (3:7, wt.) + LiPF 6 . Candidates are also chosen based on stability and longevity within the cell environment. Lab testing coincides with property predictions from the Advanced Electrolyte Model (AEM) and a macro-scale cell model. Furthermore, cell testing utilized coin and pouch cells having NMC532 or NMC811 cathodes with graphite electrodes. Results indicate combinations of low-molecular weight solvents are key for fast-charge electrolytes as they extend the useful conductivity range to both low and higher salt concentrations, and possess higher self-diffusivities compared to conventional solvents. This reduces impacts from concentration polarization. The choice of electrolyte also influences the tendency for lithium metal deposition at the anode, as showcased by experimental and modeling results herein.

25 ENERGY STORAGE↗

NASA NTRS1990

Coarsening in binary solid-liquid mixtures

A theory of Ostwald ripening has been developed for a solid-liquid mixture cosisting of a low volume fraction array of spherical solid particles in a liquid wherein the coarsening process proceeds via the transport of both heat and mass. It is found that the simultaneous transport of heat and mass during ripening does not alter the exponents of the temporal power laws governing the ripening process from their classical values but does alter the amplitudes of these power laws. The growth rate of the cube of the average particle radius, the rate constant, is found to depend both on the alloy solute concentration and the ratio of the thermal to solutal diffusivities. In most metallic systems, a large decrease in the rate constant can be expected with small additions of solute to a pure metal. Possible extensions of this theory to the analogous problem of ripening in isothermal ternary alloys are also discussed.

Voorhees, P. W.↗

NASA NTRS1990

Three-dimensional modelling of horizontal chemical vapor deposition. I - MOCVD at atmospheric pressure

A systematic numerical study of the MOCVD of GaAs from trimethylgallium and arsine in hydrogen or nitrogen carrier gas at atmospheric pressure is reported. Three-dimensional effects are explored for CVD reactors with large and small cross-sectional aspect ratios, and the effects on growth rate uniformity of tilting the susceptor are investigated for various input flow rates. It is found that, for light carrier gases, thermal diffusion must be included in the model. Buoyancy-driven three-dimensional flow effects can greatly influence the growth rate distribution through the reactor. The importance of the proper design of the lateral thermal boundary conditions for obtaining layers of uniform thickness is emphasized.

Ouazzani, Jalil↗

DOE OSTI2024

Dynamic Behavior of Pt Multimetallic Alloys for Active and Stable Propane Dehydrogenation Catalysts

Improving the use of platinum in propane dehydrogenation catalysts is a crucial aspect to increasing the efficiency and sustainability of propylene production. A known and practiced strategy involves incorporating more abundant metals in supported platinum catalysts, increasing its activity and stability while decreasing the overall loading. Here, using colloidal techniques to control the size and composition of the active phase, we show that Pt/Cu alloy nanoparticles supported on alumina (Pt/Cu/Al 2 O 3 ) displayed elevated rates for propane dehydrogenation at low temperature compared to a monometallic Pt/Al 2 O 3 catalyst. We demonstrate that the enhanced catalytic activity is correlated with a higher surface Cu content and formation of a Pt-rich core and Cu-rich shell that isolates Pt sites and increases their intrinsic activity. However, rates declined on stream because of dynamic metal diffusion processes that led to a more uniform alloy structure. This transformation was only partially inhibited by adding excess hydrogen to the feed stream. Instead, cobalt was introduced to provide trimetallic Pt/Cu/Co catalysts with stabilized surface structure and stable activity and higher rates than the original Pt/Cu system. In conclusion, the structure–activity relationship insights in this work offer improved knowledge of propane dehydrogenation catalyst development featuring reduced Pt loadings and notable thermal stability for propylene production.

Alloys↗

NASA NTRS2002

High Temperature Corrosion of Silicon Carbide and Silicon Nitride in Water Vapor

Silicon carbide (SiC) and silicon nitride (Si3N4) are proposed for applications in high temperature combustion environments containing water vapor. Both SiC and Si3N4 react with water vapor to form a silica (SiO2) scale. It is therefore important to understand the durability of SiC, Si3N4 and SiO2 in water vapor. Thermogravimetric analyses, furnace exposures and burner rig results were obtained for these materials in water vapor at temperatures between 1100 and 1450 C and water vapor partial pressures ranging from 0.1 to 3.1 atm. First, the oxidation of SiC and Si3N4 in water vapor is considered. The parabolic kinetic rate law, rate dependence on water vapor partial pressure, and oxidation mechanism are discussed. Second, the volatilization of silica to form Si(OH)4(g) is examined. Mass spectrometric results, the linear kinetic rate law and a volatilization model based on diffusion through a gas boundary layer are discussed. Finally, the combined oxidation and volatilization reactions, which occur when SiC or Si3N4 are exposed in a water vapor-containing environment, are presented. Both experimental evidence and a model for the paralinear kinetic rate law are shown for these simultaneous oxidation and volatilization reactions.

Opila, E. J.↗

NASA NTRS1996

NOx Emissions from a Lobed Fuel Injector/Burner

The present experimental study examines the performance of a novel fuel injector/burner configuration with respect to reduction in nitrogen oxide NOx emissions. The lobed injector/burner is a device in which very rapid initial mixing of reactants can occur through strong streamwise vorticity generation, producing high fluid mechanical strain rates which can delay ignition and thus prevent the formation of stoichiometric diffusion flames. Further downstream of the rapid mixing region. this flowfield produces a reduced effective strain rate, thus allowing ignition to occur in a premixed mode, where it is possible for combustion to take place under locally lean conditions. potentially reducing NOx emissions from the burner. The present experiments compare NO/NO2/NOx emissions from a lobed fuel injector configuration with emissions from a straight fuel injector to determine the net effect of streamwise vorticity generation. Preliminary results show that the lobed injector geometry can produce lean premixed flame structures. while for comparable flow conditions, a straight fuel injector geometry produces much longer. sooting diffusion flames or slightly rich pre-mixed flames. NO measurements show that emissions from a lobed fuel injector/burner can be made significantly lower than from a straight fuel injector under comparable flow conditions.

Mitchell, M. G.↗

DOE OSTI2021

Low diffusive methane emissions from the main channel of a large Amazonian run-of-the-river reservoir attributed to high methane oxidation

The global development of hydropower dams has rapidly expanded over the last several decades and has spread to historically non-impounded systems such as the Amazon River's main low land tributaries in Brazil. Despite the recognized significance of reservoirs to the global methane (CH4) emission, the processes controlling this emission remain poorly understood, especially in Tropical reservoirs. Here we evaluate CH4 dynamics in the main channel and downstream of the Santo Antônio hydroelectric reservoir, a large tropical run-of-the-river (ROR) reservoir in Amazonia. This study is intended to give a snapshot of the CH4 dynamics during the falling water season at the initial stage after the start of operations. Our results show substantial and higher CH4 production in reservoirs' littoral sediment than in the naturally flooded areas downstream of the dam. Despite the large production in the reservoir or naturally flooded areas, high CH4 oxidation in the main channel keep the concentration and fluxes of CH4 in the main channel low. Similar CH4 concentrations in the reservoir and downstream close to the dam suggest negligible degassing at the dam, but stable isotopic evidence indicates the presence of a less oxidized pool of CH4 after the dam. ROR reservoirs are designed to disturb the natural river flow dynamics less than traditional reservoirs. If enough mixing and oxygenation remain throughout the reservoir's water column, naturally high CH4 oxidation rates can also remain and limit the diffusive CH4 emissions from the main channel. Nevertheless, it is important to highlight that our results focused on emissions in the deep and oxygenated main channel. High emissions, mainly through ebullition, may occur in the vast and shallow areas represented by bays and tributaries. However, detailed assessments are still required to understand the impacts of this reservoir on the annual emissions of CH4.

Amazon, aquatic, biogeochemistry, carbon cycling, ↗

NASA NTRS1993

Obliquity variation in a Mars climate evolution model

The existence of layered terrain in both polar regions of Mars is strong evidence supporting a cyclic variation in climate. It has been suggested that periods of net deposition have alternated with periods of net erosion in creating the layered structure that is seen today. The cause for this cyclic climatic behavior is variation in the annually averaged latitudinal distribution of solar insolation in response to obliquity cycles. For Mars, obliquity variation leads to major climatological excursion due to the condensation and sublimation of the major atmospheric constituent, CO2. The atmosphere will collapse into the polar caps, or existing caps will rapidly sublimate into the atmosphere, dependent upon the polar surface heat balance and the direction of the change in obliquity. It has been argued that variations in the obliquity of Mars cause substantial departures from the current climatological values of the surface pressure and the amount of CO2 stored in both the planetary regolith and polar caps. In this new work we have modified the Haberle et al. model to incorporate variable obliquity by allowing the polar and equatorial insolation to become functions of obliquity, which we assume to vary sinusoidally in time. As obliquity varies in the model, there can be discontinuities in the time evolution of the model equilibrium values for surface pressure, regolith, and polar cap storage. The time constant, tau r, for the regolith to find equilibrium with the climate is estimated--depending on the depth, thermal conductivity, and porosity of the regolith--between 10(exp 4) and 10(exp 6) yr. Thus, using 2000-yr timesteps to move smoothly through the 0.1250 m.y. obliquity cycles, we have an atmosphere/regolith system that cannot be assumed in equilibrium. We have dealt with this problem by limiting the rate at which CO2, can move between the atmosphere and regolith, mimicking the diffusive nature and effects of the temperature and pressure waves, by setting the time rate of change of regolith storage proportional to the difference between equilibrium storage and current storage.

Tyler, D.↗