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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,495 records · Page 83
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Theoretical approach for optical response in electrochemical systems: Application to electrode potential dependence of surface-enhanced Raman scattering

We propose a theoretical approach for optical response in electrochemical systems. The fundamental equation to be solved is based on a time-dependent density functional theory in real-time and real-space in combination with its finite temperature formula treating an electrode potential. Solvation effects are evaluated by a dielectric continuum theory. The approach allows us to treat optical response in electrochemical systems at the atomistic level of theory. We have applied the method to surface-enhanced Raman scattering (SERS) of 4-mercaptopyridine on an Ag electrode surface. It is shown that the SERS intensity has a peak as a function of the electrode potential. Furthermore, the real-space computational approach facilitates visualization of variation of the SERS intensity depending on an electrode potential.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

When small is big: the role of impurities in electrocatalysis.

Improvements in the fundamental understanding of electrocatalysis have started to revolutionize the development of electrochemical interfaces for the efficient conversion of chemical energy into electricity, as well as for the utilization of electrons to produce new chemicals that then can be re-used in energy conversion systems. Here, some facets of the role of trace level of impurities (from 10-7 to 10-6 M) in electrocatalysis of the oxygen reduction reaction, hydrogen oxidation and evolution reactions, and CO oxidation reactions are explored on well-characterized platinum single crystal surfaces and high surface area materials in alkaline and acidic environments. Of particular interest is the effect of anions (e.g., Cl-, NO3-) and cations (i.e., Cu2+) present in the supporting electrolytes as well as surface defects (i.e., ad-islands) that are present on metal surfaces. The examples presented are chosen to demonstrate that a small level of impurities may play a crucial role in governing the reactivity of electrochemical interfaces.

Strmcnik, Dusan↗
DOE OSTI2017

Elucidating the Copper–Hägg Iron Carbide Synergistic Interactions for Selective CO Hydrogenation to Higher Alcohols

CO hydrogenation to higher alcohols (C 2 +OH) provides a promising route to convert coal, natural gas, shale gas, and biomass feedstocks into value-added chemicals and transportation fuels. However, the development of nonprecious metal catalysts with satisfactory activity and well-defined selectivity toward C 2 +OH remains challenging and impedes the commercialization of this process. Here, we show that the synergistic geometric and electronic interactions dictate the activity of Cu-0-chi-Fe 5 C 2 binary catalysts for selective CO hydrogenation to C 2 +OH, outperforming silica-supported precious Rh-based catalysts, by using a combination of experimental evidence from bulk, surface-sensitive, and imaging techniques collected on real and high-performance Cu-Fe binary catalytic systems coupled with density functional theory calculations. The closer is the d-band center to the Fermi level of Cu-0-chi-Fe 5 C 2 (510) surface than those of chi-Fe 5 C 2 (510) and Rh(111) surface, and the electron-rich interface of Cu-0-chi-Fe 5 C 2 (510) due to the delocalized electron transfer from Cu-0 atoms, facilitates CO activation and CO insertion into alkyl species to C-2-oxygenates at the interface of Cu-0-chi-Fe 5 C 2 (510) and thus enhances C 2 H 5 OH selectivity. Starting from the CHCO intermediate, the proposed reaction pathway for CO hydrogenation to C 2 H 5 OH on Cu-0-chi-Fe 5 C 2 (510) is CHCO + (H) -> CH 2 CO + (H) -> CH 3 CO + (H) -> CH 3 CHO + (H) -> CH 3 CH 2 O + (H) -> C 2 H 5 OH. This study may guide the rational design of high-performance binary catalysts made from earth-abundant metals with synergistic interactions for tuning selectivity.

CO hydrogenation↗
DOE OSTI2014

Mixed quantum classical calculation of proton transfer reaction rates: From deep tunneling to over the barrier regimes

We present mixed quantum classical calculations of the proton transfer (PT) reaction rates represented by a double well system coupled to a dissipative bath. The rate constants are calculated within the so called nontraditional view of the PT reaction, where the proton motion is quantized and the solvent polarization is used as the reaction coordinate. Quantization of the proton degree of freedom results in a problem of non-adiabatic dynamics. By employing the reactive flux formulation of the rate constant, the initial sampling starts from the transition state defined using the collective reaction coordinate. Dynamics of the collective reaction coordinate is treated classically as over damped diffusive motion, for which the equation of motion can be derived using the path integral, or the mixed quantum classical Liouville equation methods. The calculated mixed quantum classical rate constants agree well with the results from the numerically exact hierarchical equation of motion approach for a broad range of model parameters. Moreover, we are able to obtain contributions from each vibrational state to the total reaction rate, which helps to understand the reaction mechanism from the deep tunneling to over the barrier regimes. The numerical results are also compared with those from existing approximate theories based on calculations of the non-adiabatic transmission coefficients. It is found that the two-surface Landau-Zener formula works well in calculating the transmission coefficients in the deep tunneling regime, where the crossing point between the two lowest vibrational states dominates the total reaction rate. When multiple vibrational levels are involved, including additional crossing points on the free energy surfaces is important to obtain the correct reaction rate using the Landau-Zener formula.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Advanced nanoscale characterization of aluminum nanoparticles with modified surface morphology via atmospheric helium and carbon monoxide plasmas

Aluminum nanoparticles (nAl) have the potential as energetic additives in explosive/propellant formulations. Scalable methodologies must be pursued to mitigate the inactive amorphous alumina shell surrounding the active aluminum (Al) core with modified surface morphology and chemistry for increased combustion effects. This work explores the feasibility of making reactive core/shell nAl with thinned oxide shells and modified surface coatings via a two-step atmospheric plasma surface treatment process in a custom dielectric barrier discharge plasma reactor. The commercial nAl of nominal average size similar to 40-60nm was first treated with helium (He) followed by He/carbon monoxide (CO) plasmas for different durations. The resultant samples were characterized via high-resolution transmission electron microscopy (HRTEM) and Fourier transform IR (FTIR) spectra. HRTEM images revealed sporadic patchy gamma -alumina deposits on particle surfaces and in gaps among particles for all samples, suggesting the non-uniform plasma effects of the He/CO glow. Nanoscale chemical analyses via scanning transmission electron microscopy elemental mapping and x-ray energy dispersive spectroscopy were further performed. Although no carbon-associated structure appeared in electron energy loss spectroscopy (EELS) spectra, the presence of carbonaceous materials was confirmed as a thin dispersive layer evenly distributed on the nAl surface suggesting either its amorphous nature or is present at a level insufficient to generate satisfactory EELS spectra. The trend of intensity profiles for key elements acquired by drawing lines across a single particle on the elemental maps confirmed that carbonaceous materials only existed on the surface and they were most likely carboxylates that increased with increased He/CO treatment duration, as evident by FTIR results. Finally, this work demonstrated the success of atmospheric plasma-treated reactive nAl with comprehensively characterized surface features via advanced microscopy and spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Urban Land Surface Effects on Summertime Clouds and Moist Convection in Houston Under Different Synoptic Conditions

Urban landscapes modify cloud formation and convection through complex thermodynamic and aerodynamic processes; however, their influence under different synoptic regimes remains poorly understood. This study investigates the impact of the Houston metropolitan area on summertime cloud cover and convective cell characteristics using a combination of satellite observations, radar data, and high-resolution process-based modeling. We isolate urban effects by comparing model simulations with realistic urban land cover against hypothetical scenarios where all urban areas are replaced by rural vegetation. Results reveal that Houston's land cover consistently enhances cloud fraction and convective activity relative to surrounding rural areas, altered by large-scale meteorological forcing. Under weakly forced conditions, enhanced surface heat flux primarily contributes to driving low-level convergence and vertical ascent, leading to over 8% cloud fraction increase between 2 and 6 km over the city. Under strongly forced conditions, urban influences manifest differently depending on cloud type. For non-convective clouds, the city acts as a barrier that decelerates and lifts moist southerly inflow, increasing low cloud cover over the urban core, while decreasing it downwind the city. For convective clouds, both synoptic ascent and urbanization modulate moisture redistribution and cloud structure, producing modest cloud enhancement over the city and slight suppression over the downwind area. Urbanization exerts small changes in the intensity of the convective cells; however, it significantly decreases their duration and traveling distance. This work highlights the importance of accounting for land surface heterogeneity in modeling clouds and precipitation and demonstrates that urban impacts on clouds are highly regime-dependent.

Liu, Ye [Pacific Northwest National Laboratory (PN↗
DOE OSTI2021

The role of defects and solid/liquid interfacial interactions on controlling anisotropic growth of novel two-dimensional materials

Atomic-scale understanding about how interfacial properties are modified in the presence of defects and how they are correlated to the properties of nucleation and growth is critical for controlled materials growth on solid/liquid interfaces. The goal of this proposal is to establish preliminary data on the critical role of surface defects and chemical compositions of solvent on the structure, dynamics, reactivity, and free energetics at graphene/ionic liquid (IL) interfaces and their impact on two-dimensional (2D) growth of transition metal oxides on the surface. This study aims to develop generalized physical/chemical principles for directional growth at solid/liquid interfaces to achieve atomic-level control of two-dimensional materials synthesis. The proposed study of the effects of surface defects on the interfacial structure and reactivity will improve our understanding of the critical roles of heterogeneity, disorder, and solvents on 2D materials synthesis and lead to better control of directional growth of materials at solid/liquid interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Influence of surface potential on the adhesive force of radioactive gold surfaces

Radioactive particles may acquire surface potential through self-charging, and thus can behave differently from natural aerosols in atmospheric systems with respect to aggregation, deposition, resuspension, and transport to areas surrounding a radioactive source. Here, this work focuses on the adhesive force between radioactive particles and metallic surfaces, which relates to the deposition and resuspension of particles on surrounding surfaces. Scanning surface potential microscopy was employed to measure the surface potential of radioactive gold foil. Atomic force microscopy was used to investigate the adhesive force for gold that acquired surface charge either by irradiation or by application of an equivalent electrical bias. Overall, the adhesive force increases with increasing surface potential or relative humidity. However, a behavior that does not follow the general trend was observed for the irradiated gold at a high decay rate. A comparison between experimental measurements and calculated values revealed that the surface potential promotes adhesion. The contribution of the electrostatic force at high levels of relative humidity was lower than the one found using theoretical calculations due to the effects caused by enhanced adsorption rate of water molecules under a high surface charge density. Lastly, the results of this study can be used to provide a better understanding of the behavior of radioactive particles in atmospheric systems.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2018

Field Effect in PbSnTe:In Films with Low Conductivity in the Mode of Injection from Contacts and Space-Charge Limitation of the Current

At T = 4.2 K, a strong change (up to 4 times) in the space-charge-limited current (SCLC) is observed for the first time at samples based on semi-insulating PbSnTe:In films grown by molecular-beam epitaxy on BaF{sub 2}(111) substrates. The obtained results agree with experiments on the effect of treatment of the surface of PbSnTe:In films on the space-charge-limited current with a variation in the current to 10{sup 3} times or more. At a qualitative level, the model is considered which assumes the substantial contribution of localized surface states to space charge formed in the mode of space-charge-limited current due to the injection of charge carriers from contacts.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Near surface inversion layer recombination in Al{sub 2}O{sub 3} passivated n-type silicon

On n-type silicon, negatively charged surface passivation layers create a near surface recombination channel, which could significantly reduce the effective carrier lifetime at low injection levels (Δn < 10{sup 14 }cm{sup −3}). This effect is described by Shockley Read Hall recombination at homogeneously distributed defects in the silicon wafer. In the near surface region, fixed charges in the dielectric layer significantly change the carrier concentrations and the recombination rate of defects. Sentaurus device simulations show that the contribution of the near surface recombination to the effective carrier lifetime depends on the properties of the involved defects. The lifetime reduction is strongest when the involved defects have an energy level in the lower half of the band gap and a very high electron to hole capture cross section ratio. For the simulation, a very low defect density in the order of 10{sup 8 }cm{sup −3} is assumed, which is a realistic value in highly pure float zone silicon. Quasi-steady state photoconductance measurements on n-type silicon with Al{sub 2}O{sub 3} passivation are done and fitted with the recombination model. Very good correlation between simulation and experiment is achieved when the involved recombination centers have an electron to hole capture cross section ratio of 10{sup 7} and an energy level of −0.2 eV w.r.t. the intrinsic level. The simulated defect properties are discussed in respect of transition metal and doping related defects reported in literature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interaction of light with the ZnO surface: Photon induced oxygen “breathing,” oxygen vacancies, persistent photoconductivity, and persistent photovoltage

ZnO surfaces adsorb oxygen in the dark and emit CO{sub 2} when exposed to white light, reminiscent of the lungs of living creatures. We find that this exchange of oxygen with the ambient affects the integrity of the ZnO surface. Thus, it forms a basis for several interesting surface phenomena in ZnO, such as photoconductivity, photovoltage, and gas sensing, and has a role in ZnO electrical conduction. Using x-ray photoelectron spectroscopy on ZnO nanowires, we observed a decomposition of ZnO under white light and formation of oxygen-depleted surface, which explains photoconductivity by the electron donation of oxygen vacancies. Our findings suggest that the observed decomposition of the ZnO lattice may only take place due to photon-induced reduction of ZnO by carbon containing molecules (or carbo-photonic reduction), possibly from the ambient gas, accounting in a consistent way for both the reduced demands on the energy required for decomposition and for the observed emission of lattice oxygen in the form of CO{sub 2}. The formation of oxygen-vacancy rich surface is suggested to induce surface delta doping, causing accumulation of electrons at the surface, which accounts for both the increase in conductivity and the flattening of the energy bands. Using surface photovoltage spectroscopy in ultra high vacuum, we monitored changes in the deep level spectrum. We observe a wide optical transition from a deep acceptor to the conduction band, which energy position coincides with the position of the so called “green luminescence” in ZnO. This green transition disappears with the formation of surface oxygen vacancies. Since the oxygen vacancies are donors, while the green transition involves surface acceptors, the results suggest that the initial emission of oxygen originates at the defect sites of the latter, thereby eliminating each other. This suggests that the green transition originates at surface Zn vacancy acceptors. Removing an oxygen atom from a Zn vacancy completes the vacancy to become a full ZnO molecule vacancy, which does not produce deep levels. Our results explain why ZnO finds use as an electrical detector for oxygen and for carbon containing gas molecules. They may also shed new light on photocatalytic uses of ZnO. It is suggested that similar surface phenomena may affect other semiconducting oxides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Relationships among climatological vertical moisture structure, column water vapor, and precipitation over the central Amazon in observations and CMIP5 models

Bias and spread in Coupled Model Intercomparison Project Phase 5 simulated vertical specific humidity (q) structure are examined and related to both precipitation and column water vapor (cwv) near Manaus, Brazil, site of the recent Green Ocean Amazon campaign. Simulated seasonal mean q profiles are typically too dry, especially at low levels and during the local dry season, consistent with previously identified surface hydroclimate biases in the Amazon. Multimodel empirical orthogonal function analysis of the models' monthly climatological q profiles indicates two significant modes of ensemble spread in moisture vertical structure, with the leading mode peaked at low levels and the second mode in the lower free troposphere (LFT). While both modes project onto simulated cwv spread, only the first projects on precipitation, suggesting inconsistent sensitivity of simulated rainfall to LFT moisture. Finally, relative to observations, models with high cwv and low-level moisture errors tend to exhibit high precipitation error.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

Influence of carbon content and nitrogen vacancies on the bonding structure and mechanical performance of graphite-like BC{sub x}N thin films

X-ray absorption near edge structure (XANES) was used to investigate two sets of graphite-like BC{sub x}N thin films with similar B:C:N ratios but different amounts of nitrogen vacancies, which become oxygen-filled in the surface-most region. The two sets of samples were grown on Si (100) at room temperature by ion beam assisted deposition using two different ion/atom ratios. Nitrogen vacancy defects were detected in the B1s XANES spectra due to an oxygen decoration mechanism taking place at the film surface, which is correctly described by a core-level shift model. Analysis of the O1s XANES spectra showed two different types of oxygen incorporation in the samples under study. The tribomechanical properties of the BC{sub x}N samples were tested by the pin-on-disk technique, revealing the substantial role played by both the carbon intake in the hexagonal BCN planes and the concentration of nitrogen vacancies generated during the growth.

36 MATERIALS SCIENCE↗
DOE OSTI2021

The influence of surface heat fluxes on the growth of idealized monsoon depressions

This study explores the effect of surface sensible and latent heat fluxes on monsoon depressions using a series of idealized convection-permitting simulations. Each experiment is initialized with a small-amplitude wave that is allowed to grow within an environment representative of the South Asian monsoon. Comparing experiments with and without interactive surface heat fluxes, it is found that these fluxes enhance the growth of the simulated vortices. Without interactive surface fluxes, the strengthening period is short and the vortices fail to reach intensities characteristic of stronger monsoon depressions. Using a large set of experiments in which the vertical and meridional shear are systematically varied, it is found that surface heat fluxes enhance intensity the most when the upper-level shear is weak, the lower-level shear and associated moist static energy (MSE) gradient are sufficiently steep, and the lower-level meridional shear is strong. Here, these experiments reveal two different regimes of convection-coupled monsoon depression growth: one in which convection is driven by MSE advection and one in which it is driven by surface heat fluxes and quasigeostrophic forcing for ascent. Both regimes require sufficiently strong meridional shear to achieve initial growth by barotropic instability.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

River Deltas and Sea-Level Rise

Future sea-level rise poses an existential threat for many river deltas, yet quantifying the effect of sea-level changes on these coastal landforms remains a challenge. Sea-level changes have been slow compared to other coastal processes during the instrumental record, such that our knowledge comes primarily from models, experiments, and the geologic record. Here we review the current state of science on river delta response to sea-level change, including models and observations from the Holocene until 2300 CE. We report on improvements in the detection and modeling of past and future regional sea-level change, including a better understanding of the underlying processes and sources of uncertainty. We also see significant improvements in morphodynamic delta models. Still, substantial uncertainties remain, notably on present and future subsidence rates in and near deltas. Observations of delta submergence and land loss due to modern sea-level rise also remain elusive, posing major challenges to model validation. There are large differences in the initiation time and subsequent delta progradation during the Holocene, likely from different sea-level and sediment supply histories. Modern deltas are larger and will face faster sea-level rise than during their Holocene growth, making them susceptible to forced transgression. Regional sea-level projections have been much improved in the past decade and now also isolate dominant sources of uncertainty, such as the Antarctic ice sheet. Vertical land motion in deltas can be the dominant source of relative sea-level change and the dominant source of uncertainty; limited observations complicate projections. River deltas globally might lose 5% (~35,000 km 2 ) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario.

58 GEOSCIENCES↗
DOE OSTI2025

Subtropical Jet Regulates Arabian Winter Precipitation: A Viable Mechanism

The present study explores the mechanism governing wintertime (November–April) precipitation over the Arabian Peninsula (AP) using a 17-yr-long (2002–18) high-resolution WRF simulation. The composite analysis of strong precipitation events suggests that the equatorward extension of the upper-level jet together with the embedded upper-level trough creates a positive (cyclonic) midlevel vorticity and subsequently generates an anomalous lower-level convergence through Ekman pumping. This leads to the development of an anomalous surface low, which is further enhanced in the presence of the existing Red Sea trough over the AP. This surface low weakens the persistent anticyclone over the AP, shifting it further eastward to the Arabian Sea. The eastward shift in the lower-level anticyclone contributes to the transport of warm, moist air from the Arabian Sea and the Red Sea toward the AP. This warm, moist air converges with the cold and dry air advected by the midlatitude jet and creates a moisture convergence zone, leading to the initiation of convection. We test the proposed mechanism through numerical experiments with modified upper-level wind and demonstrate that a strong, southward intrusion of the jet can indeed lead to precipitation over the AP. The above mechanism also explains the interannual variability of precipitation over the AP. During wet years, we notice approximately 3 m s -1 stronger jet core magnitude and about a 2° equatorward shift of the jet compared to dry years. While the equatorward extension of the jet explains about 21% of the interannual variability, the jet magnitude explains around 7% of the variability during wet years.

54 ENVIRONMENTAL SCIENCES↗