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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Does vapor pressure deficit drive the seasonality of δ 13 C of the net land-atmosphere CO 2 exchange across the United States?: The Influence of VPD on δ 13 C of NEE

The seasonal pattern of the carbon isotope content (δ 13 C) of atmospheric CO 2 depends on local and nonlocal land-atmosphere exchange and atmospheric transport. Previous studies suggested that the δ13C of the net land-atmosphere CO 2 flux (δsource) varies seasonally as stomatal conductance of plants responds to vapor pressure deficit of air (VPD). We studied the variation of δ source at seven sites across the United States representing forests, grasslands, and an urban center. Using a two-part mixing model, we calculated the seasonal δsource for each site after removing background influence and, when possible, removing δ 13 C variation of nonlocal sources. Compared to previous analyses, we found a reduced seasonal (March–September) variation in δ source at the forest sites (0.5‰variation). We did not find a consistent seasonal relationship between VPD and δ source across forest (or other) sites, providing evidence that stomatal response to VPD was not the cause of the global, coherent seasonal pattern in δsource. In contrast to the forest sites, grassland and urban sites had a larger seasonal variation in δ sourc e (5‰) dominated by seasonal transitions in C 3 /C 4 grass productivity and in fossil fuel emissions, respectively. Our findings were sensitive to the location used to account for atmospheric background variation within the mixing model method that determined δsource. Special consideration should be given to background location depending on whether the intent is to understand site level dynamics or regional scale impacts of land-atmosphere exchange. The seasonal amplitude in δ 13 C of land-atmosphere CO 2 exchange (δ source ) varied across land cover types and was not driven by seasonal changes in vapor pressure deficit. The largest seasonal amplitudes of δsource were at grassland and urban sites, driven by changes in C 3 /C 4 grass productivity and fossil fuel emissions, respectively. Mixing model approaches may incorrectly calculate δs ource when background atmospheric observations are remote and/or prone to anthropogenic influence.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Steering CO 2 hydrogenation toward C–C coupling to hydrocarbons using porous organic polymer/metal interfaces

The conversion of CO 2 into fuels and chemicals is an attractive option for mitigating CO 2 emissions. Controlling the selectivity of this process is beneficial to produce desirable liquid fuels, but C–C coupling is a limiting step in the reaction that requires high pressures. Here, we propose a strategy to favor C–C coupling on a supported Ru/TiO 2 catalyst by encapsulating it within the polymer layers of an imine-based porous organic polymer that controls its selectivity. Such polymer confinement modifies the CO 2 hydrogenation behavior of the Ru surface, significantly enhancing the C 2+ production turnover frequency by 10-fold. We demonstrate that the polymer layers affect the adsorption of reactants and intermediates while being stable under the demanding reaction conditions. Our findings highlight the promising opportunity of using polymer/metal interfaces for the rational engineering of active sites and as a general tool for controlling selective transformations in supported catalyst systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Measurement of the mass-changing, charge-changing, and production cross sections of 11 C, 11 B, and 10 B nuclei in 12 C + p interactions at 13.5 GeV/𝑐 per nucleon

A good knowledge of nuclear fragmentation cross sections is important to interpret the fluxes of secondary cosmic rays from the Galaxy. Here we report new measurements of nuclear fragmentation with the NA61/SHINE experiment at the CERN SPS. The specific focus is on cross sections important for the production of boron in the Galaxy from the interactions of 12 C nuclei with hydrogen in the interstellar medium, including the contribution from the decay of the short-lived 11 C fragments. The data were taken with the secondary 12 C beam at beam momentum of 13.5 GeV/𝑐per nucleon and two fixed targets, polyethylene (CH 2 ) and graphite (C), from which we derive the cross sections of carbon on hydrogen. We present the measurement of the fragmentation cross sections of 11 C, 11 B, and 10 B as well as the mass- and charge-changing cross sections.

nuclear fragmentation↗
DOE OSTI2021

Local Resistance Measurement for Degradation of c-Si Heterojunction with Intrinsic Thin Layer (HIT) Solar Modules

Silicon heterojunction with intrinsic thin layer (or HIT) modules typically degrade at a rate of less than 1% annually in solar fields with dominant degradation in open-circuit voltage and some degradation in series resistance. However, detailed mechanisms can differ from module to module. Here, we study increases in local series resistance that occur over long-term field deployment, indicated by cell areas where the photoluminescence intensity does not degrade but the electroluminescence degrades significantly. To directly measure the local series resistance, we have cored out the local electroluminescence-degraded area, and we measured the sheet resistance by 4-point-probe and local nm-scale resistance using scanning spreading resistance microscopy (SSRM). The results by 4-point-probe show scattered sheet resistance that can be caused, for example, by nonuniform current paths through the transparent conductive oxide layer, the a-Si:H emitter, or the near-junction c-Si inversion layer. In contrast, the SSRM results indicate a relatively uniform and non-degraded resistivity on smaller nanometer spatial scales. SSRM is an atomic force microscopy-based two-terminal resistance mapping technique that measures the local resistance in nm-volume beneath the probe. The consistent resistances measured on the control and degraded samples can exclude the degradation of transparent conductive oxide resistance.

41 EE - Solar Energy Technologies Office (EE-4S)↗
NASA NTRS1991

The optical depth of the 158 micron forbidden C-12 II line - Detection of the F = 1 - 0 forbidden C-13 II hyperfine-structure component

The detection of the F = 1 - 0 hyperfine component of the 158-micron forbidden C-13 II fine-structure line in the interstellar medium is reported. A 12-point intensity map was obtained of the forbidden C-13 distribution over the inner 190-arcsec (R.A.) X 190-arcsec (decl.) regions of the Orion Nebula using an imaging Fabry-Perot interferometer. The forbidden C-12 II/C-13 II line intensity ratio varies significantly over the region mapped. It is highest (86 +/-0) in the core of the Orion H II region, and significantly lower (62 +/-7) in the outer regions of the map, reflecting higher optical depth in the forbidden C-12 II line here. It is suggested that this enhanced optical depth is the result of limb brightening of the optically thin forbidden C-13 II line at the edges of the bowl-shaped H II region blister.

Stacey, G. J.↗
DOE OSTI2018

C–H Bond Addition across a Transient Uranium–Nitrido Moiety and Formation of a Parent Uranium Imido Complex

Uranium complexes in the + 3 and + 4 oxidation states were prepared using the anionic PN – (PN – = (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) ligand framework. New complexes include the halide starting materials, (PN) 2 U III I (1) and (PN) 2 U IV Cl 2 (2), which both yield (PN) 2 U IV (N 3 ) 2 (3) by reaction with NaN 3 . Compound 3 was reduced with potassium graphite to produce a putative, transient uranium–nitrido moiety that underwent an intramolecular C–H activation to form a rare example of a parent imido complex, [K(THF) 3 ][(PN)U IV (=NH)[ i Pr 2 P(C 6 H 3 Me)N(C 6 H 2 Me 2 CH 2 )]] (4). Here, calculated reaction energy profiles strongly suggest that a C–H insertion becomes unfavorable when a reductant is present, offering a distinctively different reaction pathway than previously observed for other uranium nitride complexes.

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

Validation of full-wave simulations for mode conversion of waves in the ion cyclotron range of frequencies with phase contrast imaging in Alcator C-Mod

Mode conversion of fast waves in the ion cyclotron range of frequencies (ICRF) is known to result in current drive and flow drive under optimised conditions, which may be utilized to control plasma profiles and improve fusion plasma performance. To describe these processes accurately in a realistic toroidal geometry, numerical simulations are essential. Quantitative comparison of these simulations and the actual experimental measurements is important to validate their predictions and to evaluate their limitations. The phase contrast imaging (PCI) diagnostic has been used to directly detect the ICRF waves in the Alcator C-Mod tokamak. The measurements have been compared with full-wave simulations through a synthetic diagnostic technique. Recently, the frequency response of the PCI detector array on Alcator C-Mod was recalibrated, which greatly improved the comparison between the measurements and the simulations. In this study, mode converted waves for D-{sup 3}He and D-H plasmas with various ion species compositions were re-analyzed with the new calibration. For the minority heating cases, self-consistent electric fields and a minority ion distribution function were simulated by iterating a full-wave code and a Fokker-Planck code. The simulated mode converted wave intensity was in quite reasonable agreement with the measurements close to the antenna, but discrepancies remain for comparison at larger distances.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Comparison of the physical, chemical and electrical properties of ALD Al 2 O 3 on c- and m- plane GaN: Comparison of the physical, chemical and electrical properties of ALD Al 2 O 3 on c- and m- plane GaN

Our study compares the physical, chemical and electrical properties of Al 2 O 3 thin films deposited on gallium polar c- and nonpolar m -plane GaN substrates by atomic layer deposition (ALD). Correlations were sought between the film's structure, composition, and electrical properties. The thickness of the Al 2 O 3 films was 19.2 nm as determined from a Si witness sample by spectroscopic ellipsometry. We measured the gate dielectric was slightly aluminum-rich (Al:O=1:1.3) from X-ray photoelectron spectroscopy (XPS) depth profile, and the oxide-semiconductor interface carbon concentration was lower on c -plane GaN. The oxide's surface morphology was similar on both substrates, but was smoothest on c -plane GaN as determined by atomic force microscopy (AFM). Circular capacitors (50-300 μm diameter) with Ni/Au (20/100 nm) metal contacts on top of the oxide were created by standard photolithography and e-beam evaporation methods to form metal-oxide-semiconductor capacitors (MOSCAPs). Moreover, the alumina deposited on c -plane GaN showed less hysteresis (0.15 V) than on m -plane GaN (0.24 V) in capacitance-voltage (CV) characteristics, consistent with its better quality of this dielectric as evidenced by negligible carbon contamination and smooth oxide surface. These results demonstrate the promising potential of ALD Al 2 O 3 on c -plane GaN, but further optimization of ALD is required to realize the best properties of Al 2 O 3 on m -plane GaN.

36 MATERIALS SCIENCE↗
DOE OSTI2018

An atomistic building block description of C-S-H - Towards a realistic C-S-H model

Understanding the atomistic structure of Calcium silicate hydrate (C-S-H), the main product of cement hydration, is of paramount importance to better formulate sustainable cement. The existing atomistic models are not in total agreement with experimental results and fail to explain the nanosized nature of C-S-H. Here, we present a new approach for describing the complexity of these structures at the molecular level, enabling a detailed comparison of C-S-H models. The new methodology encodes a full, large scale atomistic C-S-H structure by a simple, readable string of characters, similar to the way the base sequence in DNA encodes a vast range of different proteins. We then use the methodology to assess 14 Å tobermorite-defect structures and their stabilities using DFT and classical molecular dynamics. Finally we highlight how the model may be extended to develop reliable atomistic C-S-H models for a range of Ca/Si ratios and conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electrocatalytic CO 2 reduction on earth abundant 2D Mo 2 C and Ti 3 C 2 MXenes

Mo 2 C and Ti 3 C 2 MXenes were investigated as earth-abundant electrocatalyts for the CO 2 reduction reaction (CO 2 RR). Mo 2 C and Ti 3 C 2 exhibited faradaic efficiencies of 90% (250 mV overpotential) and 65% (650 mV overpotential), respectively, for the reduction of CO 2 to CO in acetonitrile using an ionic liquid electrolyte. The use of ionic liquid 1-ethyl-2-methylimidazolium tetrafluoroborate as an electrolyte in organic solvent suppressed the competing hydrogen evolution reaction. Density functional theory (DFT) calculations suggested that the catalytic active sites are oxygen vacancy sites on both MXene surfaces. Also, a spontaneous dissociation of adsorbed COOH species to a water molecule and adsorbed CO on Mo 2 C promote the CO 2 RR.

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

A LABORATORY STUDY OF C{sub 3}H{sup +} AND THE C{sub 3}H RADICAL IN THREE NEW VIBRATIONALLY EXCITED {sup 2}Σ STATES USING A PIN-HOLE NOZZLE DISCHARGE SOURCE

Rotational lines of the positive molecular ion C{sub 3}H{sup +} and of the neutral C{sub 3}H radical in three new vibrationally excited states with {sup 2}Σ symmetry have been detected in a supersonic molecular beam in the centimeter-wave band. The fundamental rotational line of the ion is quite weak, but is observed with similar intensity in a dc discharge through several different hydrocarbon gases when helium is the buffer gas. Under these conditions, the fractional abundance of C{sub 3}H{sup +} relative to C{sub 3}H is estimated to be of order 10{sup −4}, i.e., toward the lower end of the ratio (10{sup −3}–10{sup −4}) found for protonated ions using the same discharge nozzle. For each new {sup 2}Σ state of the C{sub 3}H radical, spectroscopic constants, including those describing hydrogen hyperfine structure, have been determined to high precision. Lines of one {sup 2}Σ state (B = 11271 MHz) are particularly intense in our molecular beam; for this state and a second one (B = 11306 MHz), millimeter-wave transitions have also been observed between 180 and 340 GHz using a long path dc glow absorption spectrometer. On the basis of intensity measurements with this spectrometer, the inferred rotation–vibration constant α, and theoretical calculations, the state with B = 11271 MHz is tentatively assigned to the ν{sub 5} bending mode, predicted to lie ∼300 cm{sup −1} above ground.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

The Streptomyces master regulator BldD binds c-di-GMP sequentially to create a functional BldD2-(c-di-GMP)4 complex

Streptomyces are ubiquitous soil bacteria that undergo a complex developmental transition coinciding with their production of antibiotics. This transition is controlled by binding of a novel tetrameric form of the second messenger, 3 -5 cyclic diguanylic acid (c-diGMP) to the master repressor, BldD. In all domains of life, nucleotide-based second messengers allow a rapid integration of external and internal signals into regulatory pathways that control cellular responses to changing conditions. c-di-GMP can assume alternative oligomeric states to effect different functions, binding to effector proteins as monomers, intercalated dimers or, uniquely in the case of BldD, as a tetramer. However, at physiological concentrations c-di-GMP is a monomer and little is known about how higher oligomeric complexes assemble on effector proteins and if intermediates in assembly pathways have regulatory significance. Here, we show that c-di-GMP binds BldD using an ordered, sequential mechanism and that BldD function necessitates the assembly of the BldD2-(c-di-GMP)4 complex.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Anisotropic H c 2 , thermodynamic and transport measurements, and pressure dependence of T c in K 2 Cr 3 As 3 single crystals

We present a detailed study of single crystalline K 2 Cr 3 As 3 and analyze its thermodynamic and transport properties, anisotropic H c2 (T), and initial pressure dependence of T c . In zero field, the temperature-dependent resistivity is metallic. Deviation from a linear temperature dependence is evident below 100 K and a T 3 dependence is roughly followed from just above T c (~10K) to ~40K. Anisotropic H c2 (T) data were measured up to 140 kOe with field applied along and perpendicular to the rodlike crystals. For the applied field perpendicular to the rod, H c2 (T) is linear with a slope ~–70 kOe/K. For field applied along the rod, the slope is about –120 kOe/K below 70 kOe. Above 70 kOe, the magnitude of the slope decreases to ~–70 kOe/K. The electronic specific heat coefficient γ, just above T c , is 73 mJ/mol K 2 ; the Debye temperature Θ D is 220 K. As a result, the specific heat jump at the superconducting transition ΔC~2.2γT c . Finally, for hydrostatic pressures up to ~7 kbar, T c decreases under pressure linearly at a rate of –0.034K/kbar.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Measurement of branching fractions and C P violation parameters in B → ω K decays with first evidence of C P violation in B 0 → ω K s 0

In this article, we present a measurement of the branching fractions and charge-parity-( C P -)violating parameters in B → ω K decays. The results are obtained from the final data sample containing 772 × 10 6 B B ¯ pairs collected at the Υ ( 4 S ) resonance with the Belle detector at the KEKB asymmetric-energy e + e - collider. We obtain the branching fractions B ( B 0 → ω K 0 ) = ( 4.5 ± 0.4 ( stat ) ± 0.3 ( syst ) ) × 10 - 6 , B ( B + → ω K + ) = ( 6.8 ± 0.4 ( stat ) ± 0.4 ( syst ) ) × 10 - 6 which are in agreement with their respective current world averages. For the C P -violating parameters, we obtain A ω K S 0 = - 0.36 ± 0.19 ( stat ) ± 0.05 ( syst ) , S ω K S 0 = + 0.91 ± 0.32 ( stat ) ± 0.05 ( syst ) , A ω K + = - 0.03 ± 0.04 ( stat ) ± 0.01 ( syst ) , where A and S represent the direct and mixing-induced C P asymmetry, respectively. We find no evidence of C P violation in the decay channel B + → ω K + ; however, we obtain the first evidence of C P violation in the B 0 → ω K S 0 decay channel at the level of 3.1 standard deviations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Calibration of Distributed Temperature Sensors Using Commercially Available SMF-28 Optical Fiber From 22 °C to 1000 °C

Optical frequency domain reflectometry (OFDR) is a family of optical techniques which can be used to produce distributed temperature measurements from the spectral shift of an interference pattern based on the Rayleigh backscatter signature of an optical fiber. Adaptive signal processing techniques have recently been used with OFDR to record meaningful spectral shift data from commercially available SMF-28 optical fibers heated beyond 950 °C. Furthermore, a correlation between the measured spectral shift and temperature has not yet been developed at these high temperatures. To extend the measurable temperature range of OFDR in SMF-28, this work describes the development of such a correlation from room temperature (22 °C) to 1000 °C. The relationship between spectral shift and temperature change over this range was found to be best characterized by the fourth-order polynomial ΔT=(-4.241 * 10 -11 )S 4 + (-2.017 * 10 -7 )S 3 + (-3.677 * 10 -4 )S 2 + (-0.8057)S, where ΔT represents the temperature difference compared to the reference temperature, and S represents the spectral shift measured by the fibers. The calibration developed in this work assumes that the fiber has been fully annealed by heating the fiber to 1000 °C for a few hours. This paper is the first to demonstrate the calibration and use of SMF-28 distributed optical fiber sensors up to 1000 °C, enabled using adaptive OFDR-based signal processing.

47 OTHER INSTRUMENTATION↗
NASA NTRS1985

The 157-micron forbidden C II luminosity of the Galaxy. II - The presence of knotlike features in the forbidden C II emission

The measurement by Stacey et al. (1983) of the diffuse 157-micron emission of singly ionized carbon from the Galactic plane is augmented by measurements at two additional galactic longitudes. The results indicate that the total forbidden C II flux from the Galaxy is about 6 x 10 to the 7th solar luminosity - a factor of 8 lower than the previous estimate. It is likely that the measurement at l(II) = 8.0 deg was due to a knot in the forbidden C II emission. The results indicate that the forbidden C II flux has a half width of roughly 0.34 deg, in agreement with the (C-12)O (J = 1-0) half widths. The forbidden C II emission probably arises in at the edges of molecular clouds.

Stacey, G. J.↗
NASA NTRS1986

Mapping of carbon concentrations in the Allende meteorite with the C-12(d,p)C-13 method

Carbon concentrations in a 4.2 x 4.2 mm area of the Allende meteorite were mapped with the C-12(d,p)C-13 nuclear method. The x,y resolution perpendicular to the deuteron beam was 65 microns; the depth resolution was 0.6 micron. The matrix of the meteorite contains about 1 wt pct carbon, which is uniformly distributed at the 65-micron level of resolution. The matrix contains most, perhaps close to all, C of this meteorite. The coarse-grained constituents, such as chondrules, show a very strong enhancement of C in the depth range 2 to about 0 micron; the matrix does not show this enhancement. The enhancement may be due to contamination of the coarse-grained areas or else to migration of C in solid solution from the interiors of comparatively large crystals to their freshly polished surfaces.

Van Der Stap, C. A. H.↗