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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

Decadal Changes in Summertime Reactive Oxidized Nitrogen and Surface Ozone over the Southeast United States

Widespread efforts to abate ozone (O3) smog have significantly reduced emissions of nitrogen oxides (NOx) over the past 2 decades in the Southeast US, a place heavily influenced by both anthropogenic and biogenic emissions. How reactive nitrogen speciation responds to the reduction in NOx emissions in this region remains to be elucidated. Here we exploit aircraft measurements from ICARTT (International Consortium for Atmospheric Research on Transport and Transformation - July-August 2004), SENEX (Southeast Nexus - June-July 2013), and SEAC4RS (Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys - August-September 2013) and long-term ground measurement networks alongside a global chemistry-climate model to examine decadal changes in summertime reactive oxidized nitrogen (RON) and ozone over the Southeast US. We show that our model can reproduce the mean vertical profiles of major RON species and the total (NO (sub y)) in both 2004 and 2013. Among the major RON species, nitric acid (HNO3) is dominant (approximately 42-45 percent), followed by NOx (31 percent), total peroxy nitrates (Sigma PNs; 14 percent), and total alkyl nitrates (Sigma ANs; 9-12 percent) on a regional scale. We find that most RON species, including NOx, Sigma PNs, and HNO3, decline proportionally with decreasing NOx emissions in this region, leading to a similar decline in NO (sub y). This linear response might be in part due to the nearly constant summertime supply of biogenic VOC (Volatile Organic Compounds) emissions in this region. Our model captures the observed relative change in RON and surface ozone from 2004 to 2013. Model sensitivity tests indicate that further reductions of NOx emissions will lead to a continued decline in surface ozone and less frequent high-ozone events.

Li, Jingyi↗
NASA NTRS1998

Surface Area, and Oxidation Effects on Nitridation Kinetics of Silicon Powder Compacts

Commercially available silicon powders were wet-attrition-milled from 2 to 48 hr to achieve surface areas (SA's) ranging from 1.3 to 70 sq m/g. The surface area effects on the nitridation kinetics of silicon powder compacts were determined at 1250 or 1350 C for 4 hr. In addition, the influence of nitridation environment, and preoxidation on nitridation kinetics of a silicon powder of high surface area (approximately equals 63 sq m/g) was investigated. As the surface area increased, so did the percentage nitridation after 4 hr in N2 at 1250 or 1350 C. Silicon powders of high surface area (greater than 40 sq m/g) can be nitrided to greater than 70% at 1250 C in 4 hr. The nitridation kinetics of the high-surface-area powder compacts were significantly delayed by preoxidation treatment. Conversely, the nitridation environment had no significant influence on the nitridation kinetics of the same powder. Impurities present in the starting powder, and those accumulated during attrition milling, appeared to react with the silica layer on the surface of silicon particles to form a molten silicate layer, which provided a path for rapid diffusion of nitrogen and enhanced the nitridation kinetics of high surface area silicon powder.

Bhatt, R. T.↗
NASA NTRS1994

Prebiotic Oxidative Polymerization of 2,3 Dimercaptopropanol on the Surface of Iron(III) Hydroxide Oxide

The oxidation of 2,3-Dimercapto-1-propanol by ferric ions on the surface of iron (III) hydroxide oxide yielded polydisulfide polymers. This polymerization occured readily at low dithiol concentration under mild aqueous conditions. Polydisulfide polymers up to the 15-mer were synthesized from 1 mM dithiol in 5 ml water reacted with iron (III) hydroxide oxide (20 mg, 160 micro mole Fe) for 3 days under anaerobic conditions at 40 C and pH 4. About 91% of the dithiol was converted to short soluble oligomers and 9% to insoluble larger oligomers that were isolated with the mineral phase. Reactions at higher dithiol concentrations with the same ratio of dithiol to mineral gave a higher yield of the larger insoluble oligomers. The relationship of these results to prebiotic polymer synthesis will be discussed.

Weber, Arthur L.↗
NASA NTRS1973

Friction-induced surface activity of some hydrocarbons with clean and oxide-covered iron

Sliding friction studies were conducted on a clean and oxide-covered iron surface with exposure of that surface to various hydrocarbons. The hydrocarbons included ethane, ethylene ethyl chloride, methyl chloride, and vinyl chloride. Auger cylindrical mirror analysis was used to follow interactions of the hydrocarbon with the iron surface. Results with vinyl chloride indicate friction induced surface reactivity, adsorption to surface oxides, friction sensitivity to concentration and polymerization. Variation in the loads employed influence adsorption and accordingly friction. In contrast with ethyl and vinyl chloride, friction induced surface reactivity was not observed with ethane and ethylene.

Buckley, D. H.↗
NASA NTRS1986

Sputtering and native oxide formation on (110) surfaces of Cd(1-x)Mn(x)Te

Native oxides on the surface of Cd(1-x)Mn(x)Te (X between 0 and 0.7) have been analyzed on the basis of X-ray photoemission spectroscopy measurements. Depth profile analysis revealed a significant increase in the thickness at higher Mn concentrations and a strong Mn segregation to the surface, respectively. Sputter-induced damage on cleaved (110)-oriented surfaces was analyzed by photoreflectance and photoluminescence measurements. The damage was found to be larger on CdTe than on the alloy. Thermal annealing showed nearly complete restoration for the surface of the alloy, while CdTe revealed irreversible modifications in the near-surface regime upon sputtering and post annealing.

Neff, H.↗
NASA NTRS1977

Process of forming catalytic surfaces for wet oxidation reactions

A wet oxidation process was developed for oxidizing waste materials, comprising dissolved ruthenium salt in a reactant feed stream containing the waste materials. The feed stream is introduced into a reactor, and the reactor contents are then raised to an elevated temperature to effect deposition of a catalytic surface of ruthenium black on the interior walls of the reactor. The feed stream is then maintained in the reactor for a period of time sufficient to effect at least partial oxidation of the waste materials.

Jagow, R. B.↗
NASA NTRS1990

Characterization of the surfaces of platinum/tin oxide based catalysts by Fourier Transform Infrared Spectroscopy (FTIR)

A Pt/SnO2 catalyst has been developed at NASA Langley that is effective for the oxidation of CO at room temperature (1). A mechanism has been proposed to explain the effectiveness of this catalyst (2), but most of the species involved in this mechanism have not been observed under actual catalytic conditions. A number of these species are potentially detectable by Fourier Transform Infrared Spectroscopy (FTIR), e.g., HOSnO sub x, HO sub y PtO sub z, Pt-CO, and SnHCO3. Therefore a preliminary investigation was conducted to determine what might be learned about this particular catalyst by transmission FTIR. The main advantage of FTIR for this work is that the catalyst can be examined under conditions similar to the actual catalytic conditions. This can be of critical importance since some surface species may exist only when the reaction gases are present. Another advantage of the infrared approach is that since vibrations are probed, subtle chemical details may be obtained. The main disadvantage of this approach is that FTIR is not nearly as sensitive as the Ultra High Vacuum (UHV) surface analytical techniques such as Auger, Electron Spectroscopy for Chemical Analysis (ESCA), Electron Energy Loss Spectroscopy (EELS), etc. Another problem is that the assignment of the observed infrared bands may be difficult.

Keiser, Joseph T.↗
NASA NTRS1974

Effect of surface pre-oxidation on hydrogen permeation in alpha titanium

The effect of a specific oxidation treatment, to which a titanium specimen was subjected, on the permeation of hydrogen through the titanium specimen was studied. The oxidation treatment involved the maintenance of the specimen in an oxygen atmosphere at a temperature of 270 C for a time of four hours. It was found that the oxidation treatment has the effect of decreasing the permeation rate. However, the first power pressure dependency is not affected.

Shah, K. K.↗
NASA NTRS1978

Early stages of the oxidation of metal surfaces

Photoemission cross sections were calculated for the ZnO4(-6) cluster using the self consistent-chi alpha- scattered wave theory to display the main features of the ultraviolet and X-ray photoemission data from ZnO. A solid model is suggested for an absolute photoemission intensity comparison resulting in chi alpha intensities which are roughly 70% of the experimental values. Together with the experimental data, the calculations allow a complete determination of the electronic structure of a ZnO surface.

Gatos, H. C.↗
NASA NTRS1985

Environmental effects on the tensile strength of chemically vapor deposited silicon carbide fibers

The room temperature and elevated temperature tensile strengths of commercially available chemically vapor-deposited (CVD) silicon carbide fibers were measured after 15 min heat treatment to 1600 C in various environments. These environments included oxygen, air, argon and nitrogen at one atmosphere and vacuum at 10/9 atmosphere. Two types of fibers were examined which differed in the SiC content of their carbon-rich coatings. Threshold temperature for fiber strength degradation was observed to be dependent on the as-received fiber-flaw structure, on the environment and on the coating. Fractographic analyses and flexural strength measurements indicate that tensile strength losses were caused by surface degradation. Oxidation of the surface coating is suggested as one possible degradation mechanism. The SiC fibers containing the higher percentage of SiC near the surface of the carbon-rich coating show better strength retention and higher elevated temperature strength.

Bhatt, R. T.↗
NASA NTRS1986

Oxidation-resistant reflective surfaces for solar dynamic power generation in near Earth orbit

Reflective surfaces for space station power generation systems are required to withstand the atomic oxygen-dominated environment of near Earth orbit. Thin films of platinum and rhodium, which are corrosion resistant reflective metals, have been deposited by ion beam sputter deposition onto various substrate materials. Solar reflectances were then measured as a function of time of exposure to a RF-generated air plasma. Similarly, various protective coating materials, including MgF2, SiO2, Al2O3, and Si3N4, were deposited onto silver-coated substrates and then exposed to the plasma. Analysis of the films both before and after exposure by both ESCA and Auger spectroscopy was also performed. The results indicate that Pt and Rh do not suffer any loss in reflectance over the duration of the tests. Also, each of the coating materials survived the plasma environment. The ESCA and Auger analyses are discussed as well.

Gulino, D. A.↗
NASA NTRS1987

Oxidation-resistant reflective surfaces for solar dynamic power generation in near earth orbit

Reflective surfaces for Space Station power generation systems are required to withstand the atomic oxygen-dominated environment of near earth orbit. Thin films of platinum and rhodium, which are corrosion resistant reflective metals, have been deposited by ion beam sputter deposition onto various substrate materials. Solar reflectances were then measured as a function of time of exposure to a RF-generated air plasma.

Gulino, Daniel A.↗
NASA NTRS1989

Characterization of the surfaces of platinum/tin oxide based catalysts by Fourier transform spectroscopy (FTIR)

The Laser Atmospheric Wind Sounder (LAWS) Program has as one of its goals the development of a satellite based carbon dioxide laser for making wind velocity measurements. The specifications for this laser include the requirement that the laser operate at a repetition rate of 10 Hertz continuously for three years. Earth-based carbon dioxide lasers can operate for only a short time on a single charge of gas because the lasing action causes the CO2 to break down into CO and O2. Therefore, earth-based CO2 lasers are generally operated in a flow through mode in which the spent gas is continually exhausted and fresh gas is continually added. For a satellite based system, however, a recirculation system is desired because it is not practical to send up extra tanks of CO2. A catalyst which could enable a recirculating CO2 laser to function continuously for three years needs to be developed. In the development of a catalyst system there are many variables. Obviously, not all possible formulations can be tested for three years, therefore, an accurate model which is based on the reaction mechanism is needed. The construction of a multistep reaction mechanism is similar to the construction of a jigsaw puzzle. Different techniques each supply a piece of the puzzle and the researcher must put the pieces together. Transmission infrared spectroscopy was shown to be very useful in supplying some of the information needed to elucidate reaction mechanisms. The purpose was to see what kind of information might be obtained about the NASA catalyst using infrared absorption spectroscopy. Approximately 200 infrared spectra of the prototype Pt/tin oxide catalyst and its precursor components are observed under a variety of different conditions. The most significant observations are summarized.

Keiser, Joseph T.↗
NASA NTRSDate not supplied

Scalable Variable Charge Molecular Dynamics Simulations of Metal-Oxide Systems

Interfaces between metals and oxides are an important feature in many technologically relevant materials, e.g., oxidation of metal surfaces, oxide-dispersion strengthened (ODS) alloys, dielectric components, and thermal barrier coatings among others. Experimental studies of such interfaces are challenging since the majority are buried within the bulk, making computational modeling an attractive alternative. Molecular dynamics (MD) simulations operate at the length scales relevant to many interface-mediated mechanisms, but the requisite interatomic potentials for metal-oxide systems require computationally expensive variable charge schemes to account for the disparate bonding types, thus often limiting their effectiveness. Here we introduce several improvements to the charge transfer interatomic potential (CTIP) model which enable greater computational efficiency for large scale MD simulations. Then, using a new CTIP parametrization for the Ni-O system, we demonstrate its capabilities to capture critical atomic scale mechanisms associated with metal-oxide interfaces. Long time scale simulations (>10 ns) are used to investigate high temperature oxidation and oxide precipitation from the melt, and large length scale simulations (> 1 million atoms) are used to study the interaction of dislocations with oxide particles. We have implemented the new CTIP model in the widely used, open-source MD code LAMMPS.

Gabriel Plummer↗