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At least 523 records · Page 29

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NASA NTRS1985

Isotope effects associated with the anaerobic oxidation of sulfite and thiosulfate by the photosynthetic bacterium, Chromatium vinosum

The purple photosynthetic bacterium Chromatium vinosum, strain D, catalyzes several oxidations of reduced sulfur compounds under anaerobic conditions in the light: e.g., sulfide --> sulfur --> sulfate, sulfite --> sulfate, and thiosulfate --> sulfur + sulfate. Here it is shown that no sulfur isotope effect is associated with the last of these processes; isotopic compositions of the sulfur and sulfate produced can differ, however, if the sulfane and sulfonate positions within the thiosulfate have different isotopic compositions. In the second process, an observed change from an inverse to a normal isotope effect during oxidation of sulfite may indicate the operation of 2 enzymatic pathways. In contrast to heterotrophic anaerobic reduction of oxidized sulfur compounds, anaerobic oxidations of inorganic sulfur compounds by photosynthetic bacteria are characterized by relatively small isotope effects.

Non-NASA Center↗

NASA NTRS1999

Nitric-oxide synthase is a mechanical signal transducer that modulates talin and vinculin expression

Mechanical stimuli can cause changes in muscle mass and structure which indicate that mechanisms exist for transducing mechanical stimuli into signals that influence gene expression. Myotendinous junctions show adaptations to modified muscle loading which suggest that these are transcriptionally distinct domains in muscle fibers that may experience local regulation of expression of structural proteins that are concentrated at these sites. Vinculin and talin are cytoskeletal proteins that are highly enriched at myotendinous junctions that we hypothesize to be subject to local transcriptional regulation. Our findings show that mechanical stimulation of muscle cells in vivo and in vitro causes an increase in the expression of vinculin and talin that is mediated by nitric oxide. Furthermore, nitric oxide-stimulated increases in vinculin and talin expression occur through a protein kinase G-dependent pathway and therefore differ from other mechanisms through which nitric oxide has been shown previously to modulate transcription. Analysis of vinculin mRNA distribution in mechanically stimulated muscle fibers shows that the mRNA is highly concentrated at myotendinous junctions, which supports the hypothesis that myotendinous junctions are distinct domains in which the expression of cytoskeletal proteins is modulated by mechanical stimuli through a nitric oxide and protein kinase G-dependent pathway.

NASA Discipline Musculoskeletal↗

NASA NTRS1997

Oxidative acylation using thioacids

Several important prebiotic reactions, including the coupling of amino acids into polypeptides by the formation of amide linkages, involve acylation. Theae reactions present a challenge to the understanding of prebiotic synthesis. Condensation reactions relying on dehydrating agents are either inefficient in aqueous solution or require strongly acidic conditions and high temperatures. Activated amino acids such as thioester derivatives have therefore been suggested as likely substrates for prebiotic peptide synthesis. Here we propose a closely related route to amide bond formation involving oxidative acylation by thioacids. We find that phenylalanine, leucine and phenylphosphate are acylated efficiently in aqueous solution by thioacetic acid and an oxidizing agent. From a prebiotic point of view, oxidative acylation has the advantage of proceeding efficiently in solution and under mild conditions. We anticipate that oxidative acylation should prove to be a general method for activating carboxylic acids, including amino acids.

Non-NASA Center↗

NASA NTRS1995

The regulation of methane oxidation in soil

The atmospheric concentration of methane, a greenhouse gas, has more than doubled during the past 200 years. Consequently, identifying the factors influencing the flux of methane into the atmosphere is becoming increasingly important. Methanotrophs, microaerophilic organisms widespread in aerobic soils and sediments, oxidize methane to derive energy and carbon for biomass. In so doing, they play an important role in mitigating the flux of methane into the atmosphere. Several physico-chemical factors influence rates of methane oxidation in soil, including soil diffusivity; water potential; and levels of oxygen, methane, ammonium, nitrate, nitrite, and copper. Most of these factors exert their influence through interactions with methane monooxygenase (MMO), the enzyme that catalyzes the reaction converting methane to methanol, the first step in methane oxidation. Although biological factors such as competition and predation undoubtedly play a role in regulating the methanotroph population in soils, and thereby limit the amount of methane consumed by methanotrophs, the significance of these factors is unknown. Obtaining a better understanding of the ecology of methanotrophs will help elucidate the mechanisms that regulate soil methane oxidation.

Review, Academic↗

NASA NTRS2015

Uncertainties in Isoprene Photochemistry and Emissions: Implications for the Oxidative Capacity of Past and Present Atmospheres and for Climate Forcing Agents

Current understanding of the factors controlling biogenic isoprene emissions and of the fate of isoprene oxidation products in the atmosphere has been evolving rapidly. We use a climate-biosphere-chemistry modeling framework to evaluate the sensitivity of estimates of the tropospheric oxidative capacity to uncertainties in isoprene emissions and photochemistry. Our work focuses on trends across two time horizons: from the Last Glacial Maximum (LGM, 21 000 years BP) to the preindustrial (1770s); and from the preindustrial to the present day (1990s). We find that different oxidants have different sensitivities to the uncertainties tested in this study, with OH being the most sensitive: changes in the global mean OH levels for the LGM-to-preindustrial transition range between -29 and +7, and those for the preindustrial-to-present day transition range between -8 and +17, across our simulations. Our results suggest that the observed glacial-interglacial variability in atmospheric methane concentrations is predominantly driven by changes in methane sources as opposed to changes in OH, the primary methane sink. However, the magnitudes of change are subject to uncertainties in the past isoprene global burdens, as are estimates of the change in the global burden of secondary organic aerosol (SOA) relative to the preindustrial. We show that the linear relationship between tropospheric mean OH and tropospheric mean ozone photolysis rates, water vapor, and total emissions of NOx and reactive carbon first reported in Murray et al. (2014) does not hold across all periods with the new isoprene photochemistry mechanism. Our results demonstrate that inadequacies in our understanding of present-day OH and its controlling factors must be addressed in order to improve model estimates of the oxidative capacity of past and present atmospheres.

methane↗

NASA NTRS2014

Role of Oxidative Damage in Radiation-Induced Bone Loss

During prolonged spaceflight, astronauts are exposed to both microgravity and space radiation, and are at risk for increased skeletal fragility due to bone loss. Evidence from rodent experiments demonstrates that both microgravity and ionizing radiation can cause bone loss due to increased bone-resorbing osteoclasts and decreased bone-forming osteoblasts, although the underlying molecular mechanisms for these changes are not fully understood. We hypothesized that excess reactive oxidative species (ROS), produced by conditions that simulate spaceflight, alter the tight balance between osteoclast and osteoblast activities, leading to accelerated skeletal remodeling and culminating in bone loss. To test this, we used the MCAT mouse model; these transgenic mice over-express the human catalase gene targeted to mitochondria, the major organelle contributing free radicals. Catalase is an anti-oxidant that converts reactive species, hydrogen peroxide into water and oxygen. This animal model was selected as it displays extended lifespan, reduced cardiovascular disease and reduced central nervous system radio-sensitivity, consistent with elevated anti-oxidant activity conferred by the transgene. We reasoned that mice overexpressing catalase in mitochondria of osteoblast and osteoclast lineage cells would be protected from the bone loss caused by simulated spaceflight. Over-expression of human catalase localized to mitochondria caused various skeletal phenotypic changes compared to WT mice; this includes greater bone length, decreased cortical bone area and moment of inertia, and indications of altered microarchitecture. These findings indicate mitochondrial ROS are important for normal bone-remodeling and skeletal integrity. Catalase over-expression did not fully protect skeletal tissue from structural decrements caused by simulated spaceflight; however there was significant protection in terms of cellular oxidative damage (MDA levels) to the skeletal tissue. Furthermore, we used an array of countermeasures (Antioxidant diets and injections) to prevent the radiation-induced bone loss, although these did not prevent bone loss, analysis is ongoing to determine if these countermeasure protected radiation-induced damage to other tissues.

oxidative damage↗

NASA NTRS2017

Coatings for Oxidation and Hot Corrosion Protection of Disk Alloys

Increasing temperatures in aero gas turbines is resulting in oxidation and hot corrosion attack of turbine disks. Since disks are sensitive to low cycle fatigue (LCF), any environmental attack, and especially hot corrosion pitting, can potentially seriously degrade the life of the disk. Application of metallic coatings are one means of protecting disk alloys from this environmental attack. However, simply the presence of a metallic coating, even without environmental exposure, can degrade the LCF life of a disk alloy. Therefore, coatings must be designed which are not only resistant to oxidation and corrosion attack, but must not significantly degrade the LCF life of the alloy. Three different Ni-Cr coating compositions (29, 35.5, 45wt. Cr) were applied at two thicknesses by Plasma Enhanced Magnetron Sputtering (PEMS) to two similar Ni-based disk alloys. One coating also received a thin ZrO2 overcoat. The coated samples were also given a short oxidation exposure in a low PO2 environment to encourage chromia scale formation. Without further environmental exposure, the LCF life of the coated samples, evaluated at 760C, was less than that of uncoated samples. Hence, application of the coating alone degraded the LCF life of the disk alloy. Since shot peening is commonly employed to improve LCF life, the effect of shot peening the coated and uncoated surface was also evaluated. For all cases, shot peening improved the LCF life of the coated samples. Coated and uncoated samples were shot peened and given environmental exposures consisting of 500 hrs of oxidation followed by 50 hrs of hot corrosion, both at 760C). The high-Cr coating showed the best LCF life after the environmental exposures. Results of the LCF testing and post-test characterization of the various coatings will be presented and future research directions discussed.

LCF life↗

NASA NTRS2018

Transition Metal Oxides as Cathodes in Li-O2 battery: A First Principles Investigation

Li-O2 batteries have traditionally used carbon based electrodes (graphite, buckypaper) as the cathode of choice due to its good electrical conductivity, stability against non-aqueous electrolytes like Dimethyl ether (DME) and ease of handling. But, the carbon cathode also leads to formation of carbonate by-products that increase overpotentials during charging leading to degradation of cathode and reduction of cyclability. In this work, we investigate some of the well-known oxides as cathodes with focus on the interface between the oxide surfaces and the discharge product: Li2O2, in the Li-O2 battery using first principles computations. Our results show that attention must be paid on choosing the appropriate surface of the oxides. We extend the analysis to suggest other possible oxide chemistries that should be investigated as cathodes in Li-O2 batteries.

Li-O2 battery↗

NASA NTRSDate not supplied

Development of a Microlith® Catalytic Oxidizer for Exploration Trace Contaminant Control

The state-of-the-art in long mission duration spacecraft trace contaminant control processes rely on high-temperature catalytic oxidation for light contaminant removal. Target compounds for oxidation include metabolic methane, carbon monoxide, and low molecular weight volatile organics such as formaldehyde and methanol. Precision Combustion, Inc. and NASA Marshall Space Flight Center have been developing and testing prototype high temperature catalytic oxidizers based on PCI’s patented Microlith® technology to meet the requirements of future spaceflight exploration missions. To this end, our latest generation Microlith oxidizer, featuring an integrated heat recuperator, was subjected to endurance testing to simulate catalyst ageing over a Mars transit mission duration. Periodic reactor health testing indicates our approach results in a robust contaminant control solution for exploration missions beyond low earth orbit. Based on the demonstrated performance of this unit, a next-generation prototype was designed to meet exploration contaminant load control demands while upgrading the prototype form and fit to match flight-compatible interfaces. Prototype design considerations intended to reduce power consumption, impacting process thermal and hydraulic performance, are discussed herein.

Trace Contaminant Control↗

NASA NTRS2022

Energetics of Reactions between Ceramic Coating Materials and their Binary Oxide Components with Silicate Melts

This paper summarizes our previous and current studies of using high-temperature calorimetry to investigate the energetics of reactions of ceramic coating materials (e.g., yttrium disilicate and 7-wt% yttria-stabilized zirconia) and their binary oxide components with silicate melts in the CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) system. Such interactions are found to become stronger (more exothermic) with increasing difference in acid–base character between these materials and the melt. Our results suggest that the reactivity between the coating materials and the melt increases with decreasing thermodynamic stability (less exothermic enthalpy of formation from oxide components) of the coating material. They also suggest that ceramic coating materials made from binary oxides that have less exothermic enthalpies of solution and mixing are less susceptible to CMAS melt corrosion when in contact with an acidic, relatively polymerized, melt rich in SiO 2 . Thus, we propose that new coating material formulation and CMAS melt corrosion mitigation strategies should be optimized based on the energetic contributions of their binary oxide components.

CMAS↗

NASA NTRS2023

The Effect of Oxidation in Environmental Barrier Coatings Subject to Foreign Object Damage

Oxidation and foreign object damage (FOD) are two key failure modes for environmental barrier coatings (EBCs). For EBCs with a silicon (Si) bond coat, a thermally grown oxide (TGO) layer forms at the EBC –bond coat interface. The TGO layer is considered the weak interface, and directly influences the life of the EBC. Moreover, FOD has been shown to cause significant subsurface damage resulting in delamination or spallation of the EBC which can occur at the EBC -bond coat interface or the bond coat -substrate interface. The purpose of this work is to investigate the synergistic effects of oxidation and FOD in a ytterbium disilicate (𝑌𝑏2𝑆𝑖2𝑂7) EBC. FOD testing is conducted on as-deposited and steam oxidized samples at room temperature using a 1.59-mm hardened steel ball projectile. The resulting damage is characterized by optical profilometry and scanning electron microscopy (SEM). Additionally, a quasi-static, mechanical assessment of the EBC bond strength in the presence of a growing TGO is presented.

environmental barrier coatings (EBCs)↗

NASA NTRSDate not supplied

A Parallelized Oxidation-Driven Surface Recession Framework in DSMC Code, SPARTA

Spacecrafts rely on ablative thermal protection systems (TPS) made of composites consisting of a carbon-based reinforcement and a polymeric matrix. These materials are designed to withstand high-temperature oxidation and surface recession during re-entry into the Earth's atmosphere. However, ablation occurs due to a complex interplay of thermal, mechanical, and chemical factors, making it challenging to determine the individual impact of each on the TPS's overall degradation. In this study, we have developed an ablation model that can leverage a finite rate carbon oxidation model to predict material recession and surface states more accurately. Stochastic PArallel Rarified-gas Time-accurate Analyzer (SPARTA), a direct-simulation Monte Carlo (DSMC) code, is modified to allow oxidation-driven ablation of implicitly defined carbon surfaces. In SPARTA, implicit surfaces are generated from the grid corner point values via a marching cubes algorithm, therefore creating a new set of surface elements every time ablation is performed. The finite-rate oxidation model developed by Gopalan et. al can perform both gas-surface and pure-surface reactions and is now adapted to tally surface data on a per grid cell basis. The ablation functionality was also adjusted so once the reactions have occurred, the number of reactions leading to CO formation can be converted to corner point reduction values; therefore, carbon removal is directly proportional to surface recession. We also briefly discuss some unique challenges associated with parallelizing this dynamic surface state and geometry. Finally, we analyze the performance of this parallelized implicit chemistry model with simple 2D and 3D benchmark cases by producing surface state statistics, area changes over time, and visualization across a range of surface temperatures and processors with and without load-balancing.

DSMC↗

NASA NTRS1971

Soot oxidation rates in gas turbine engines

A basis is proposed for extrapolating soot oxidation rate measurements obtained in laboratory flames to the more extreme operating conditions of gas turbine combustion chambers. The proposal is based on the observation that, within probable experimental uncertainty, the limited soot oxidation measurements correlate with the more extensive measurements of the surface oxidation rates of macroscopic samples of pyrographite. The soot oxidation rates thus determined for the conditions of a typical gas turbine combustion chamber are considerably lower than estimates which were based on simple extrapolations of the flame data.

Radcliffe, S. W.↗

NASA NTRS1972

Improved oxidation-resistant carbon and graphite materials

A variety of silicon-carbide treated carbon and graphite products were prepared, and oxidation and strength properties determined. Carbon and graphite materials used in the preliminary study include fiber, tape, felt, foam, bulk, and laminar configurations. The silicon-carbide treatment used in preparing the oxidation resistant specimens employs either vacuum or ultrasonic impregnation of various silicone-silicon mixtures into the matrix material. The impregnation phase is followed by controlled thermal reaction at which time the silicon carbide is synthesized and takes the form of an integral protective boundary layer. Thickness of the layer ranges from a fraction of a mil to 10 mils, depending on the matrix material and processing parameters. Oxidation resistance of all the silicon-carbide-treated specimens investigated was found to be significantly improved. Mechanical strengths of the laminar composites were also found to be somewhat improved. At room temperature the specific strength of the silicon-carbide treated fibers and tapes was degraded to some extent; however, their relative strengths were significantly increased at elevated temperatures in oxidizing atmosphere.

Muir, H. M.↗

NASA NTRS1972

Oxidation resistant porous material for transpiration cooled vanes

Porous metal sheet with controlled permeability was made by space winding and diffusion bonding fine wire. Two iron-chromium-aluminum alloys and three-chromium alloys were used: GE 1541 (Fe-Cr-Al-Y), H 875 (Fe-Cr-Al-Si), TD Ni Cr, DH 245 (Ni-Cr-Al-Si) and DH 242 (Ni-Cr-Si-Cb). GE 1541 and H 875 were shown in initial tests to have greater oxidation resistance than the other candidate alloys and were therefore tested more extensively. These two materials were cyclic furnace oxidation tested in air at 1800 and 2000 F for accumulated exposure times of 4, 16, 64, 100, 200, 300, 400, 500, and and 600 hours. Oxidation weight gain, permeability change and mechanical properties were determined after exposure. Metallographic examination was performed to determine effects of exposure on the porous metal and electron beam weld joints of porous sheet to IN 100 strut material. Hundred hour stress rupture life and tensile tests were performed at 1800 F. Both alloys had excellent oxidation resistance and retention of mechanical properties and appear suitable for use as transpiration cooling materials in high temperature gas turbine engines.

Madsen, P.↗

NASA NTRS1972

Quantitative X-ray diffraction analysis of oxides formed on superalloys

Methods were developed for quantitative analysis by X-ray diffraction of the oxides Al2O3, NiO, Cr2O3, CoO, and CoCr2O4 within a standard deviation of about 10 percent of the weight fraction reported or within 1 percent absolute. These error limits assume that the sample oxides are well characterized and that the physiochemical structure of the oxides in the samples are identical with those in the synthesized standards. Results are given for the use of one of the techniques in the analysis of spalls from a series of oxidation tests of the cobalt base alloy WI-52.

Garlick, R. G.↗

NASA NTRS1972

The oxidation behavior of Ni-Cr-Al-2ThO2 alloys at 1093 and 1204 C.

A pack diffusion process has been developed which permits the introduction of nearly 6 wt % Al into solid solution in the near surface region of TDNiCr (Ni-20 wt % Cr-2 vol % ThO2) and Ni-20Cr. Alumina scales, adherent under cyclic heating and cooling conditions, were produced on TDNiCr-5.86Al upon exposure to an environment of 1330 N/sq m (10 torr) or 101,000 N/sq m (760 torr) air at temperatures of 1093 and 1204 C. While the same oxidation kinetics were observed in isothermal tests for Ni-14.6Cr-5.86Al as were obtained for the TDNiCr-5.86Al, the dispersion-strengthened alloy exhibited superior oxide scale adhesion during cyclic testing. At 1204 C, continuous weight gains were observed under all test conditions for TDNiCr-5.86Al, in contrast to the weight loss with time which occurred several hours after exposure of TDNiCr to an oxidizing environment. TDNiCr with an initial aluminum surface concentration of 4.95 wt % has nearly comparable oxidation resistance to the TDNiCr-5.86Al alloy.

Seltzer, M. S.↗

NASA NTRS1973

Thermogravimetric study of the reduction of oxides of nickel and chromium

The controlled hydrogen, carbon, and hydrogen-carbon reductions of the oxides of nickel and chromium were evaluated by thermogravimetric means. The materials studied were nickel (nickelous) oxide (NiO) and chromic sesquioxide (Cr2O3), mixed NiO-Cr2O3, and oxidized nickel - 20-percent chromium (Ni-20Cr). NiO was effectively reduced by all three atmospheres, Cr2O3 only by hydrogen-carbon, NiO-Cr2O3 by hydrogen and hydrogen-carbon, and oxidized Ni-20Cr by hydrogen and hydrogen-carbon and to a considerable extent by carbon alone. The results suggest that the presence of nickel promotes the reduction of Cr2O3. However, no definite explanation could be reached for the effectiveness of the hydrogen-carbon reduction of Cr2O3.

Herbell, T. P.↗