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At least 235 records · Page 13

Terrestrial nitrous oxide cycles and atmospheric effects

The basic processes that cause N2O emission from soils are briefly discussed, and the rate of the processes is shown to vary widely in space and time, depending on soil, climate, and agrotechnical conditions. Although significant amounts of N2O are indeed emitted from the land, the complexity of the soil processes involved and the wide variation of measured rates still prevents the quantitative evaluation, global budgeting, and reliable prediction of atmospheric N2O. Increased atmospheric N2O abundance increases the levels of odd-nitrogen in the stratosphere, which, in turn, decreases the stratospheric ozone density via a catalytic cycle. Using appropriate atmospheric models and current chemical kinetic data, it has been found that the dependence of ozone reduction on N2O increase is nearly linear; a simulated doubling of N2O leads to a predicted decrease of about 14 percent in total ozone column density. A 10 percent increase in N2O yields a predicted increase in nonmelanoma skin cancer of several percent, and a possible raising of surface temperature of 0.03 K.

Whitten, R. C.↗

Silicon-Based Optical Modulator with Ferroelectric Layer

According to a proposal, a silicon dioxide layer in a high-speed, low-power, silicon- based electro-optical modulator would be replaced by a layer of lead zirconate titanate or other ferroelectric oxide material. The purpose of this modification is to enhance the power performance and functionality of the modulator. In its unmodified form, the particular silicon- based electro-optical modulator is of an advanced design that overcomes the speed limitation of prior silicon-based electro- optical modulators. Whereas modulation frequencies of such devices had been limited to about 20 MHz, this modulator can operate at modulation frequencies as high as 1 GHz. This modulator can be characterized as a silicon-waveguide-based metal oxide/semiconductor (MOS) capacitor phase shifter in which modulation of the index of refraction in silicon is obtained by exploiting the free-charge-carrier-plasma dispersion effect. As shown in the figure, the modulator includes an n-doped crystalline silicon slab (the silicon layer of a silicon- on-insulator wafer) and a p-doped polycrystalline silicon rib with a gate oxide layer (the aforementioned silicon dioxide layer) sandwiched between them. Under accumulation conditions, the majority charge carriers in the silicon waveguide modify the index of refraction so that a phase shift is induced in the optical mode propagating in the waveguide. The advantage of using an MOS capacitor phase shifter is that it is possible to achieve high modulation speed because there are no slow carrier-generation or -recombination processes involved in the accumulation operation. The main advantage of the proposed substitution of a ferroelectric oxide layer for the silicon dioxide layer would arise from the spontaneous polarization effect of the ferroelectric layer: This spontaneous polarization would maintain accumulation conditions in the absence of applied voltage. Consequently, once the device had been switched to a given optical state, it would remain in that state, even in the absence of applied voltage (in other words, even with power turned off). A secondary advantage is that because the ferroelectric layer would have an index of refraction larger than that of silicon dioxide, there could be some reduction of optical losses attributable to fabrication of the modulator

Sheldon, Douglas↗

The Galim LL/EH Polymict Breccia: Evidence for Impact-Induced Exchange Between Reduced and Oxidized Meteoritic Material

Galim is a polymict breccia consisting of a heavily shocked (shock stage S6) LL6 chondrite, Galim (a), and an impact-melted EH chondrite, Galim (b). Relict chondrules in Galim (b) served as nucleation sites for euhedral enstatite grains crystallizing from the impact melt. Many of the reduced phases typical of EH chondrites (e.g., Si-bearing metallic Fe-Ni; Ti-bearing troilite) are absent. Galim (b) was probably shock-melted while in contact with a more oxidized source, namely, Galim (a); during this event, Si was oxidized from the metal and Ti was oxidized from troilite. Galim (a) contains shock veins and recrystallized, unzoned olivine. The absence of evidence for reduction in Galim (a) may indicate that the amount of LL material greatly exceeded that of EH material; shock metamorphism may have taken place on the LL parent body. Shock-induced redox reactions such as those inferred for the Galim breccia appear to be restricted mainly to asteroids because the low-end tail of their relative-velocity distribution permits mixing of intact disparate materials (including accretion of projectiles of different oxidation states), whereas the peak of the distribution leads to high equilibration shock pressures (allowing impact-induced exchange between previously accreted, disequilibrated materials). Galim probably formed by a two-stage process: (I) accretion to the LL parent body of an intact EH projectile at low relative velocities, and (2) shock metamorphism of the assemblage by the subsequent impact of another projectile at significantly higher relative velocities.

Rubin, Alan E.↗

Mineral-Catalysed Formation of Marine NO and N 2 O on the Anoxic Early Earth

Microbial denitrification converts fixed nitrogen species into gases in extant oceans. However, it is unclear how such transformations occurred within the early nitrogen cycle of the Archean. Here 5we present experimental anoxic surface-catalyzed reduction of nitrite and nitrate via green rust and magnetite combined with diffusion and photochemical modeling. We find that in a Fe 2+ -rich marine environment, Fe minerals could have catalyzed abiotic denitrification reactions leading to the formation of nitric oxide (NO) and nitrous oxide (N 2 O). Nitrate did not exhibit reactivity in the presence of either mineral or aqueous Fe2+, however, both minerals induced rapid nitrite reduction 10to NO and N 2 O. While N 2 O escaped into the atmosphere (63% of nitrite-nitrogen, with green rust as catalyst), NO remained associated with precipitates (7%) serving as a potential shuttle to the benthic ocean.The modeling suggests that marine N 2 O emissions would have sustained 0.8-6 ppb of atmospheric N 2 O without a protective ozone layer. Our findings add detail to the as yet incompletely documented Archean nitrogen cycle, implying a globally distributed process driven 15by chemical kinetics similar to those of modern enzymatically mediated conversions.

Steffen Buessecker↗

Apollo 14 - Oxide, metal, and olivine mineral chemistries in 14072 with a bearing on the temporal relationships of subsolidus reduction

One of three primary objectives of the reported study is related to the definition of the distribution of troilite and Fe and the evaluation of the role of FeS in subsolidus reduction assemblages. Another objective was concerned with the definition of the lower limit of stabilizing elements such as Mg, Al, and Cr in ulvospinel to determine the terminal compositions at which reduction is inhibited. The third objective involves a comparison of the mineral chemistries of decomposed olivine with unaltered olivine in an attempt to determine the temperature limits. The sample was selected because it is one of the most intensely reduced samples returned from the moon. The model invoked for reduction is a magmatic-deuteric process which requires an environment approximating that of a closed system in which the volatile constituents are contained and in which reduction proceeds continuously with crystallization.

Haggerty, S. E.↗

Redox chemistry at liquid/liquid interfaces

The interface between two immiscible liquids with immobilized photosynthetic pigments can serve as the simplest model of a biological membrane convenient for the investigation of photoprocesses accompanied by spatial separation of charges. As it follows from thermodynamics, if the resolvation energies of substrates and products are very different, the interface between two immiscible liquids may act as a catalyst. Theoretical aspects of charge transfer reactions at oil/water interfaces are discussed. Conditions under which the free energy of activation of the interfacial reaction of electron transfer decreases are established. The activation energy of electron transfer depends on the charges of the reactants and dielectric permittivity of the non-aqueous phase. This can be useful when choosing a pair of immiscible solvents to decrease the activation energy of the reaction in question or to inhibit an undesired process. Experimental interfacial catalytic systems are discussed. Amphiphilic molecules such as chlorophyll or porphyrins were studied as catalysts of electron transfer reactions at the oil/water interface.

Non-NASA Center↗

Effects of Potassium Permanganate Oxidation on Subsurface Microbial Activity

In situ chemical oxidation has the potential for degrading large quantities of organic contaminants and can be more effective and timely than traditional ex situ treatment methods. However, there is a need to better characterize the potential effects of this treatment on natural processes. This study focuses on potential inhibition to anaerobic dechlorination of trichloroethene (TCE) in soils from a large manufacturing facility as a result of in situ oxidation using potassium permanganate (KMn04)Previous microcosm studies established that natural attenuation occurs on-site and that it is enhanced by the addition of ethanol to the system. A potential remediation scheme for the site involves the use of potassium permanganate to reduce levels of TCE in heavily contaminated areas, then to inject ethanol into the system to "neutralize" excess oxidant and enhance microbial degradation. However, it is currently unknown whether the exposure of indigenous microbial populations to potassium permanganate may adversely affect biological reductive dechlorination by these microorganisms. Consequently, additional microcosm studies were conducted to evaluate this remediation scheme and assess the effect of potassium permanganate addition on biological reductive dechlorination of TCE. Samples of subsurface soil and groundwater were collected from a TCE-impacted area of the site. A portion of the soil was pretreated with nutrients and ethanol to stimulate microbial activity, while the remainder of the soil was left unamended. Soil/groundwater microcosms were prepared in sealed vials using the nutrient-amended and unamended soils, and the effects of potassium permanganate addition were evaluated using two permanganate concentrations (0.8 and 2.4 percent) and two contact times (1 and 3 weeks). TCE was then re-added to each microcosm and TCE and dichloroethene (DCE) concentrations were monitored to determine the degree to which microbial dechlorination occurred following chemical oxidation. Evidence of microbial degradation was generally detected within four weeks after TCE addition. Increases in DCE concentrations were consistent with decreases in TCE. The concentration of TCE in the nutrient-amended samples exposed to 2.4% KMnO4 for one week degraded somewhat more slowly than the samples exposed to the 0.8% KMnO4. The rates of degradation did not correlate with the length of KMn04 exposure for the nutrient-amended microcosms. Microbial degradation of TCE in the unamended microcosms was generally similar to that observed in the nutrient-amended microcosms. One treatment condition (unamended, one week exposure, 2.4% KMnO4) was exposed to elevated levels of ethanol and showed little evidence of degradation. It is suspected that the high levels of ethanol were toxic to the microorganisms. The results of the study indicate that exposure of indigenous soil and groundwater microbial populations to KMnO4 at concentrations of 0.8 to 2.4% do not impair the ability of the microbial populations to dechlorinate TCE. Consequently, the combination of chemical oxidation followed by enhanced biological reductive dechlorination appears to be a viable remedial strategy for highly-impacted subsurface areas of the site.

Rowland, Martin A.↗

Transparent and Electrically Conductive Carbon Nanotube-Polymer Nanocomposite Materials for Electrostatic Charge Dissipation

In recent years, nanocomposite materials have been extensively studied because of their superior electrical, magnetic, and optical properties and large number of possible applications that range from nano-electronics, specialty coatings, electromagnetic shielding, and drug delivery. The aim of the present work is to study the electrical and optical properties of carbon nanotube(CNT)-polymer nanocomposite materials for electrostatic charge dissipation. Single and multi-wall carbon nanotubes were grown by catalytic chemical vapor deposition (CCVD) on metal/metal oxide catalytic systems using acetylene or other hydrocarbon feedstocks. After the purification process, in which amorphous carbon and non-carbon impurities were removed, the nanotubes were functionalized with carboxylic acid groups in order to achieve a good dispersion in water and various other solvents. The carbon nanostructures were analyzed, both before and after functionalization by several analytical techniques, including microscopy, Raman spectroscopy, and X-Ray photoelectron spectroscopy. Solvent dispersed nanotubes were mixed (1 to 7 wt %) into acrylic polymers by sonication and allowed to dry into 25 micron thick films. The electrical and optical properties of the films were analyzed as a function of the nanotubes' concentration. A reduction in electrical resistivity, up to six orders of magnitude, was measured as the nanotubes' concentration in the polymeric films increased, while optical transparency remained 85 % or higher relative to acrylic films without nanotubes.

Dervishi, E.↗

Iron-Tolerant Cyanobacteria for Human Habitation beyond Earth

In light of the President's Moon/Mars initiative, lunar exploration has once again become a priority for NASA. In order to establish permanent bases on the Moon and proceed with human exploration of Mars, two key problems will be addressed: first, the production of O2 and second, the production of methane (CH4). While O2 is required for life support systems (LSS), both liquid O2 and CH4 are needed as an oxidizer and a propellant, respectively for the Lunar Surface Access Module (LSAM) and the Crew Exploration Vehicle (CEV). Unlike previous propulsion systems, the new CEV will use liquid oxygen (LO2) as an oxidizer and liquid methane (LCH4) as a propellant. Existing technology (e.g. hydrogen reduction) for the production of liquid oxygen from lunar regolith is very energy intensive and requires high temperature reactors. We propose an alternative approach using iron-tolerant cyanobacteria. We have found that iron-tolerant cyanobacteria (IT CB) are capable of etching iron-bearing minerals, which may lead to bonds breaking between Fe and O of common lunar mare basalt Feoxides including ilmenite, pseudobrookite, ferropseudobrookite, and armalcolite with the subsequent release of both Fe, Ti and oxygen as by-products. We also propose to use CB biomass for CH4 production as carbon stock and a propellant. Both processes can be accomplished in an energy and cost effective manner because sunlight will be used as an energy source and allows the reactions at ambient temperatures between 10-60 C. Current evaluations include assessing the thermodynamics of such biogenic reactions using a variety of nutrients and atmospheric parameters, as well as assessing the rates and species variation effects of the driving reactions.

Brown, Igor↗

A Model of Reduced Kinetics for Alkane Oxidation Using Constituents and Species for N-Heptane

The reduction of elementary or skeletal oxidation kinetics to a subgroup of tractable reactions for inclusion in turbulent combustion codes has been the subject of numerous studies. The skeletal mechanism is obtained from the elementary mechanism by removing from it reactions that are considered negligible for the intent of the specific study considered. As of now, there are many chemical reduction methodologies. A methodology for deriving a reduced kinetic mechanism for alkane oxidation is described and applied to n-heptane. The model is based on partitioning the species of the skeletal kinetic mechanism into lights, defined as those having a carbon number smaller than 3, and heavies, which are the complement of the species ensemble. For modeling purposes, the heavy species are mathematically decomposed into constituents, which are similar but not identical to groups in the group additivity theory. From analysis of the LLNL (Lawrence Livermore National Laboratory) skeletal mechanism in conjunction with CHEMKIN II, it is shown that a similarity variable can be formed such that the appropriately non-dimensionalized global constituent molar density exhibits a self-similar behavior over a very wide range of equivalence ratios, initial pressures and initial temperatures that is of interest for predicting n-heptane oxidation. Furthermore, the oxygen and water molar densities are shown to display a quasi-linear behavior with respect to the similarity variable. The light species ensemble is partitioned into quasi-steady and unsteady species. The reduced model is based on concepts consistent with those of Large Eddy Simulation (LES) in which functional forms are used to replace the small scales eliminated through filtering of the governing equations; in LES, these small scales are unimportant as far as the overwhelming part of dynamic energy is concerned. Here, the scales thought unimportant for recovering the thermodynamic energy are removed. The concept is tested by using tabular information from the LLNL skeletal mechanism in conjunction with CHEMKIN II utilized as surrogate ideal functions replacing the necessary functional forms. The test reveals that the similarity concept is indeed justified and that the combustion temperature is well predicted, but that the ignition time is over-predicted, a fact traced to neglecting a detailed description of the processes leading to the heavies chemical decomposition. To palliate this deficiency, functional modeling is incorporated into this conceptual reduction in addition to the modeling the evolution of the global constituent molar density, the enthalpy evolution of the heavies, the contribution to the reaction rate of the unsteady lights from other light species and from the heavies, the molar density evolution of oxygen and water, and the mole fractions of the quasisteady light species. The model is compact in that there are only nine species-related progress variables. Results are presented showing the performance of the model for predicting the temperature and species evolution. The model reproduces the ignition time over a wide range of equivalence ratios, initial pressure, and initial temperature.

Harstad, Kenneth G.↗

Reactive Distillation and Air Stripping Processes for Water Recycling and Trace Contaminant Control

Reactive distillation designs are considered to reduce the presence of volatile organic compounds in the purified water. Reactive distillation integrates a reactor with a distillation column. A review of the literature in this field has revealed a variety of functional reactive columns in industry. Wastewater may be purified by a combination of a reactor and a distiller (e.g., the EWRS or VPCAR concepts) or, in principle, through a design which integrates the reactor with the distiller. A review of the literature in reactive distillation has identified some different designs in such combinations of reactor and distiller. An evaluation of reactive distillation and reactive air stripping is presented with regards to the reduction of volatile organic compounds in the contaminated water and air. Among the methods presented, an architecture is presented for the evaluation of the simultaneous oxidation of organics in air and water. These and other designs are presented in light of potential improvements in power consumptions and air and water purities for architectures which include catalytic activity integrated into the water processor. In particular, catalytic oxidation of organics may be useful as a tool to remove contaminants that more traditional distillation and/or air stripping columns may not remove. A review of the current leading edge at the commercial level and at the research frontier in catalytically active materials is presented. Themes and directions from the engineering developments in catalyst design are presented conceptually in light of developments in the nanoscale chemistry of a variety of catalyst materials.

Boul, Peter J.↗

Stress-Activated Positive Holes (O− in a Matrix of O2–) Cause DNA Damage in Surface-Dwelling Organisms: Unveiling Mutation-Induced Secrets of Nature

Peroxy defects consist of pairs of tightly bonded oxygen anions in the –1 valence state such as in O3X/OO\YO3 with X, Y = Si4+, Al3+ etc. They commonly occur in igneous, metamorphic and many sedimentary rocks. When such rocks are stressed by tectonic forces, peroxy defects break up, releasing highly mobile electronic charge carriers: defect electrons in the O2– sublattice, i.e. unbound O–, known as “positive holes”, h•. The h• can flow out of stressed rock volumes, spreading far and wide, causing electric currents and electrochemical reactions. This study explores how the h• impact the electron flow in the electron transport chain (ETC) of organisms on the surface of rocks such as gabbro and granite. We found that, by forming hydroxyl radicals and superoxide anions, the h• disrupt the in vivo coordination of reduction-oxidation reactions that are essential for the timing of the redox cycle. Our observations show that stress activation of h• delays the sprouting of certain plant seeds and impedes the growth of yeast cultures, Saccharomyces cerevisiae. The h• induce mutations and affect plant development as evidenced by reduced stem length and leaf size. At the same time, the h• serve as a source of abiotic oxidation, capable of forming various organic compounds in situ. Through the generation of radical species that create new carbon-carbon bonds the h• facilitate the abiotic synthesis of hydrocarbons and other organic molecules essential to life, including porphyrins. Our findings highlight the intricate interplay between positive holes, redox timing, and biological processes, revealing their significant role in influencing the growth and development of organisms in tectonically stressed crustal environments. Understanding these effects enhances our broader comprehension of redox biology and the influence of environmental factors on cellular development in natural settings.

astrobiology↗

Thermal barrier coating life prediction model development

Thermal barrier coatings (TBCs) for turbine airfoils in high-performance engines represent an advanced materials technology with both performance and durability benefits. The foremost TBC benefit is the reduction of heat transferred into air-cooled components, which yields performance and durability benefits. This program focuses on predicting the lives of two types of strain-tolerant and oxidation-resistant TBC systems that are produced by commercial coating suppliers to the gas turbine industry. The plasma-sprayed TBC system, composed of a low-pressure plasma-spray (LPPS) or an argon shrouded plasma-spray (ASPS) applied oxidation resistant NiCrAlY (or CoNiCrAlY) bond coating and an air-plasma-sprayed yttria (8 percent) partially stabilized zirconia insulative layer, is applied by Chromalloy, Klock, and Union Carbide. The second type of TBC is applied by the electron beam-physical vapor deposition (EB-PVD) process by Temescal.

T E Strangman↗

Surface activation of air oxidation of hydrazine on kaolinite. 2. Consideration of oxidizing/reducing entities in relationship to other compositional, structural, and energetic factors

The rates (previously reported) for the air oxidation of hydrazine on kaolinite and substituent oxides of kaolinite showed a complex dependence on the relative amounts of several structural oxidizing/reducing entities within the reaction-promoting solids. The rates indicated an important role of the clay but no dominant role of any one of the oxidizing/reducing entities. In this paper we review (a) the reaction-promoting activity of these centers as studied in other systems, (b) various spectroscopic results showing interaction between these entities in clays, and (c) reported spectroscopic studies of the complexation between hydrazine and aluminosilicate surfaces as a whole, in an effort to propose a mechanism for the reaction. Whereas some uncertainties remain, the present synthesis concludes that a mechanism operating through single electron/hole transfers and hydrogen atom transfers by discrete centers is adequate to explain the observed rate behaviors including the observed second order dependence of the oxidation rate on catalyst amount. The effects of these operations on the catalyst can result in no alteration of, or complete or partial electronic relaxation of its contingent of trapped separated charge pairs. The degree to which surface complexation as a whole, intercalation, or luminescent processes may also be associated with the reaction cannot be adequately assessed with the information in hand.

NASA Program Exobiology↗

Experimental investigation of jet impingement on surfaces of fine particles in a vacuum environment

The erosion characteristics of a bed of fine particles under vacuum conditions (10 -4 torr) resulting from an impinging jet exhaust were investigated for various particle sizes and jet heights above the dust surface. The investigation included measurements of the craters during erosion, measurements of the visibility reduction, and photographs of the erosion process. The results of this investigation indicate that there exists a finite jet height above which particles will not be eroded. This height, for given nozzle conditions, varies with surface-particle size. In addition, the results indicate that the magnitude of surface erosion also depends on-surface-particle size; erosion is most rapid with the coarsest particles. These and other associated test results are presented and compared with available theory.

Jet exhaust↗

Systematic Sensitivity Analysis of Metabolic Controllers During Reductions in Skeletal Muscle Blood Flow

An acute reduction in oxygen delivery to skeletal muscle is generally associated with profound derangements in substrate metabolism. Given the complexity of the human bioenergetic system and its components, it is difficult to quantify the interaction of cellular metabolic processes to maintain ATP homeostasis during stress (e.g., hypoxia, ischemia, and exercise). Of special interest is the determination of mechanisms relating tissue oxygenation to observed metabolic responses at the tissue, organ, and whole body levels and the quantification of how changes in oxygen availability affect the pathways of ATP synthesis and their regulation. In this study, we apply a previously developed mathematical model of human bioenergetics to study effects of ischemia during periods of increased ATP turnover (e.g., exercise). By using systematic sensitivity analysis the oxidative phosphorylation rate was found to be the most important rate parameter affecting lactate production during ischemia under resting conditions. Here we examine whether mild exercise under ischemic conditions alters the relative importance of pathways and parameters previously obtained.

Radhakrishnan, Krishnan↗

A Two-Stage Regenerable Filter for Collection and Disposal of Carbon Fines

NASA is investigating the use of the Plasma Pyrolysis Assembly (PPA), which is a methane post-processing technology with the goal to recover hydrogen from the Carbon dioxide Reduction Assembly (CRA) currently onboard the International Space Station (ISS). The PPA can theoretically recover 75% of hydrogen from methane produced by the CRA. During methane processing the PPA creates roughly 40 mg/hr of carbon dust when operating at 4 crew member levels. The unwanted fines need to be removed from the stream to prevent any clogging of downstream components. The Regenerable Carbon Filter (RCF) was designed by Umpqua Research Company to address the carbon particulate created in the PPA. The RCF includes two stages. The first is an electrostatic precipitator designed to collect ultrafine particles. The second is a low pressure drop physical filter. Both filters are made of heat tolerant materials to allow regeneration of the filtration capacity by O2 oxidation of captured carbon. This paper will discuss the RCF hardware and test plans.

Filtration, Environmental Control, Life Support↗

Vapor Phase Catalytic Ammonia Reduction

This paper discusses the development of a Vapor Phase Catalytic Ammonia Reduction (VPCAR) teststand and the results of an experimental program designed to evaluate the potential of the technology as a water purification process. In the experimental program the technology is evaluated based upon product water purity, water recovery rate, and power consumption. The experimental work demonstrates that the technology produces high purity product water and attains high water recovery rates at a relatively high specific power consumption. The experimental program was conducted in 3 phases. In phase I an Igepon(TM) soap and water mixture was used to evaluate the performance of an innovative Wiped-Film Rotating-Disk evaporator and associated demister. In phase II a phenol-water solution was used to evaluate the performance of the high temperature catalytic oxidation reactor. In phase III a urine analog was used to evaluate the performance of the combined distillation/oxidation functions of the processor.

Flynn, Michael T.↗