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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Cloud-Scale Droplet Number Sensitivity to Liquid Water Path in Marine Stratocumulus

Cloud droplet number concentration (N c ) is retrieved from remotely observed marine stratocumulus cloud liquid water path (LWP), cloud optical depth (τ c ), and cloud thickness, using an optical model that assumes liquid water content (LWC) increases linearly from cloud base. Assuming that LWC is vertically uniform would underestimate τ c by 5% and N c by 14%. Individual retrievals of N c from 10-min averages vary by orders of magnitude from long-term averages. Surface cloud condensation nuclei (CCN) number concentration N CCN is weakly but significantly correlated with N c (R = 0.3) for the day leading and 6 hr following N c . Consistent with coalescence and drizzle scavenging cloud droplets, lag correlations show that N c decreases for 1 hr after the peak area-average rain rate. Greater observed LWP for lower N c [d(log N c )/d(log LWP) = 2.3] is consistent with enhanced entrainment drying of clouds with greater N c and consistent with removal of N c by thicker clouds with more coalescence and drizzle. Stronger precipitation in clouds with greater N c is the opposite sensitivity as expected were LWP to be controlled by the “cloud lifetime” indirect aerosol effect. The strong sensitivity of N c to LWP suggests that cloud dynamic and thermodynamic forcings drive macrophysical variability that controls N c in southeastern tropical Pacific stratocumulus clouds. Regressions are relatively insensitive to assumptions about the covariance of errors among the sensors.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Light absorption enhancement of black carbon in a pyrocumulonimbus cloud

Abstract Pyrocumulonimbus (pyroCb) firestorm systems have been shown to inject significant amounts of black carbon (BC) to the stratosphere with a residence time of several months. Injected BC warms the local stratospheric air, consequently perturbing transport and hence spatial distributions of ozone and water vapor. A distinguishing feature of BC-containing particles residing within pyroCb smoke is their thick surface coatings made of condensed organic matter. When coated with non-refractory materials, BC’s absorption is enhanced, yet the absorption enhancement factor ( E abs ) for pyroCb BC is not well constrained. Here, we perform particle-scale measurements of BC mass, morphology, and coating thickness from inside a pyroCb cloud and quantify E abs using an established particle-resolved BC optics model. We find that the population-averaged E abs for BC asymptotes to 2.0 with increasing coating thickness. This value denotes the upper limit of E abs for thickly coated BC in the atmosphere. Our results provide observationally constrained parameterizations of BC absorption for improved radiative transfer calculations of pyroCb events.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Textured Nanoporous Mo:BiVO4 Photoanodes with High Charge Transport and Charge Transfer Quantum Efficiencies for Oxygen Evolution

We have developed a simple spin coating method to make high-quality nanoporous photoelectrodes of monoclinic BiVO4 and studied the ability of these electrodes to transport photogenerated carriers to oxidize sulfite and water. Samples containing molybdenum and featuring [001] out-of-plane crystallographic texture show a photocurrent and external quantum efficiency (EQE) for sulfite oxidation as high as 3.1 mA cm-2 and 60%, respectively, at 1.23 V versus reversible hydrogen electrode. By using an optical model of the electrode stack to accurately determine the fraction of electrode absorptance due to the BiVO4 active layer, we estimate that on average 70 +/- 5% of all photogenerated carriers escape recombination. A comparison of internal quantum efficiency as a function of film processing, illumination direction, and film thickness shows that electron transport is efficient and hole transport limits the photocurrent (hole diffusion length <40 nm). We find that Mo addition primarily improves electron transport and texturing mostly improves hole transport. Mo enhances electron transport by thinning the surface depletion layer or passivating traps and recombination centers at grain boundaries and interfaces, while improved hole transport in textured films may result from more efficient lateral hole extraction due to the texturing itself or the reduced density of deep gap states observed in photoemission measurements. Photoemission data also reveal that the films have bismuth-rich, vanadium- and oxygen-deficient surface layers, while ion scattering spectroscopy indicates a Bi-V-O surface termination. Without added catalysts, the plain BiVO4 electrodes oxidized water with an initial photocurrent and peak EQE of 1.7 mA cm-2 and 30%, respectively, which equates to a hole transfer efficiency to water of >64% at 1.23 V. The electrodes quickly photocorrode during water oxidation but show good stability during sulfite oxidation and indefinite stability in the dark. By improving the hole transport efficiency and coating these nanoporous BiVO4 films with an appropriate protective layer and oxygen evolution catalyst, it should be possible to achieve highly efficient and stable water oxidation at a practical pH.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Towards high efficiency thin-film crystalline silicon solar cells: The roles of light trapping and non-radiative recombinations

Thin-film solar cells based on silicon have emerged as an alternative to standard thick wafers technology, but they are less efficient, because of incomplete absorption of sunlight, and non-radiative recombinations. In this paper, we focus on the case of crystalline silicon (c-Si) devices, and we present a full analytic electro-optical model for p-n junction solar cells with Lambertian light trapping. This model is validated against numerical solutions of the drift-diffusion equations. We use this model to investigate the interplay between light trapping, and bulk and surface recombination. Special attention is paid to surface recombination processes, which become more important in thinner devices. These effects are further amplified due to the textures required for light trapping, which lead to increased surface area. We show that c-Si solar cells with thickness of a few microns can overcome 20% efficiency and outperform bulk ones when light trapping is implemented. The optimal device thickness in presence of light trapping, bulk and surface recombination, is quantified to be in the range of 10–80 μm, depending on the bulk quality. These results hold, provided the effective surface recombination is kept below a critical level of the order of 100 cm/s. We discuss the possibility of meeting this requirement, in the context of state-of-the-art techniques for light trapping and surface passivation. We show that our predictions are within the capability of present day silicon technologies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Mueller matrix ellipsometric detection of profile asymmetry in nanoimprinted grating structures

Mueller matrix ellipsometry (MME) is applied to detect foot-like asymmetry encountered in nanoimprint lithography (NIL) processes. We present both theoretical and experimental results which show that MME has good sensitivity to both the magnitude and direction of asymmetric profiles. The physics behind the use of MME for asymmetry detection is the breaking of electromagnetic reciprocity theorem for the zeroth-order diffraction of asymmetric gratings. We demonstrate that accurate characterization of asymmetric nanoimprinted gratings can be achieved by performing MME measurements in a conical mounting with the plane of incidence parallel to grating lines and meanwhile incorporating depolarization effects into the optical model. The comparison of MME-extracted asymmetric profile with the measurement by cross-sectional scanning electron microscopy also reveals the strong potential of this technique for in-line monitoring NIL processes, where symmetric structures are desired.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Interface effects in nanometer-thick yttrium iron garnet films studied by magneto-optical spectroscopy

The properties of nanometer-thick yttrium iron garnet (YIG) films are strongly influenced by interfaces. This work employs spectral ellipsometry (SE) and magneto-optic polar Kerr rotation (PKR) to characterize YIG films with thickness, t, from 6 nm to 30 nm grown on Gd 3 Ga 5 O 12 (GGG) substrates oriented parallel to (111) plane. The films display a surface roughness of 0.35 nm or lower. The analysis of the SE data at the photon energies of 1 eV < E < 6.5 eV provided the t and permittivity values. The PKR at 1.3 eV < E < 4.5 eV is reasonably explained with the optical model for the YIG film/GGG substrate system. Even better agreement is achieved by assuming a 1.07-nm-thick layer sandwiched between YIG and GGG that has Fe 3+ sublattice magnetization opposite to that in the YIG volume. This suggests the existence of antiferromagnetic coupling between the Gd 3+ and tetrahedral Fe 3+ .

36 MATERIALS SCIENCE↗
DOE OSTI2016

Melt front propagation in dielectrics upon femtosecond laser irradiation: Formation dynamics of a heat-affected layer

Several studies in dielectrics have reported the presence of a thin heat-affected layer underneath the ablation crater produced by femtosecond laser irradiation. In this work, we present a time-resolved microscopy technique that is capable of monitoring the formation dynamics of this layer and apply it to the study of a phosphate glass exposed to single pulses below the ablation threshold. A few nanoseconds after laser excitation, a melt front interface can be detected, which propagates into the bulk, gradually slowing down its speed. By means of image analysis combined with optical modeling, we are able to determine the temporal evolution of the layer thickness and its refractive index. Initially, a strong transient decrease in the refractive index is observed, which partially recovers afterwards. The layer resolidifies after approximately 1 μs after excitation, featuring a maximum thickness of several hundreds of nanometers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Optimal Heliostat Assignment Strategy for Multiple-Receiver Systems

Next-generation concentrating solar power concepts currently being developed under the U.S. Department of Energy's Gen3 program include several novel heliostat field and receiver arrangements. Among these concepts are solar fields utilizing multiple receivers or several distinct receiver flux control zones, and each of which has particular power or peak flux constraints. In this paper, we present a linear program that identifies optimal heliostat-to-receiver assignment strategies for multi-receiver or multi-zone solar fields based on detailed calculations made by an optics modeling package. Two distinct problems are addressed: namely, (1) a design problem that chooses the optimal set of heliostats to include in a final layout given receiver constraints and performance, and (2) an operations problem that assigns existing heliostats to a receiver, maximizing field power output while maintaining receiver power constraints. The optimization methodology is integrated into the concentrating solar power tower modeling package SolarPILOTTM, and results are presented showing that heliostat assignment strategies can impact both initial heliostat field layout characteristics and the cost of thermal energy produced by the field. The methodology succeeds in maintaining power requirements but at the expense of producing different flux profiles on each receiver surface. Consequently, the paper also discusses additional measures that are applied to ensure that desirable flux distribution properties are maintained, reducing local flux mean-absolute-deviation values from a baseline of 17% or greater to less than 5% compared to a desired reference flux profile.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2021

Optimization of four terminal rear heterojunction GaAs on Si interdigitated back contact tandem solar cells

High-efficiency, four-terminal tandem solar cells composed of thin GaAs films mechanically stacked onto interdigitated back contact silicon solar cells with a glass interlayer are demonstrated. The optimal thickness of the absorber layer of a rear heterojunction GaAs subcell for use in four terminal tandem solar cells was studied. GaAs top cells with absorber layer thicknesses of 1.5, 1.9, 2.3, 2.8, and 3.5 µm were fabricated on glass and mechanically stacked onto interdigitated back-contact Si bottom cells. All tandem cells were found to have efficiencies above 30% under the AM1.5 G spectrum demonstrating a relatively weak sensitivity to thickness in the four-terminal configuration. We found the 2.8 µm absorber layer cell to have the highest top cell and tandem cell efficiency at 26.38% and 32.57%, respectively. Optical modeling with transfer matrix method for the planar top cell and Lambertian light trapping in the textured Si subcell, along with drift-diffusion Hovel equations, were used to show photon recycling enhancement to the effective diffusion length and VOC of the top cell as a result of the low-index glass interlayer.

14 SOLAR ENERGY↗
DOE OSTI2017

Absolute calibration of Doppler coherence imaging velocity images

Here, a new technique has been developed for absolutely calibrating a Doppler Coherence Imaging Spectroscopy interferometer for measuring plasma ion and neutral velocities. An optical model of the interferometer is used to generate zero-velocity reference images for the plasma spectral line of interest from a calibration source some spectral distance away. Validation of this technique using a tunable diode laser demonstrated an accuracy better than 0.2 km/s over an extrapolation range of 3.5 nm; a two order of magnitude improvement over linear approaches. While a well-characterized and very stable interferometer is required, this technique opens up the possibility of calibrated velocity measurements in difficult viewing geometries and for complex spectral line-shapes.

spectroscopy and imaging↗
DOE OSTI2026

Nonmonotonic-potential description of polarization effects, fusion, and nuclear rainbows in elastic scattering of 6 Li + 12 C at 4.5–600 MeV

The experimental differential cross-section (CS) and analyzing power (AP) data of the 6 Li + 12 C elastic scattering over a wide laboratory energy scale (4.5MeV≤𝐸 lab ≤600MeV) are analyzed within the framework of the optical model (OPM) using nonmonotonic (NM) nucleus–nucleus potentials. The real part of the NM potentials is derived from the Pauli-embodied energy density-functional (EDF) formalism with the sudden approximation. The real part of the noncentral spin-orbit and tensor terms, as well as the imaginary parts, are treated phenomenologically. The effect of the radius of sensitivity on the CS and AP data is found to be more important at lower energies. The diffractive and refractive scattering with Airy structures in the whole angular region of the elastic scattering across the studied energy range is successfully described within the OPM using the NM 6 Li + 12 C potential. The near- and far-side (N and F) decomposition of the total elastic-scattering amplitudes has also been studied using our NM potentials. The evolution of the Airy minima in the angular distributions, coupled with the fitting of the AP data, provides an accurate description of Airy minima of different orders. The OPM calculations with the NM potentials describe exceptionally well the CS, vector analyzing power (VAP), and tensor analyzing power data at 𝐸 lab =9.0,19.24,20,30, and 50MeV. In agreement with our past successful descriptions of CS and the opposite signs of the VAP data for the 6 Li and 7 Li elastic scattering using NM potentials in OPM, the present results appear to provide a better fit, so far, than those obtained from the coupled-channels method. The fusion cross sections of 6 Li + 12 C have been predicted in the energy range (4.5MeV≤𝐸 lab ≤20MeV), fitting the experimental data well in the range 𝐸 lab =2.97–11.77MeV. The EDF potential without any energy dependence and renormalization is also found to describe satisfactorily the experimental CS and AP data at energies up to several hundreds of MeV.

6 ≤ A ≤ 19↗
DOE OSTI2020

Using Fourier-Plane Imaging Microscopy for Determining Transition-Dipole-Moment Orientations in Organic Light-Emitting Devices

Here, we use Fourier-plane imaging microscopy (FIM) to determine the transition-dipole-moment orientation in doped organic emissive thin films. The use of FIM enables precise, sensitive, and rapid measurement of dipole orientation in the emission layer of an organic light-emitting device (OLED). An optical model of a stratified birefringent multilayer is introduced for interpreting results obtained by FIM. Using the model, we determine the average orientation of transition-dipole-moment vectors of three phosphorescent dopant emitters. The dipole alignment measured by FIM quantitatively explains the difference in OLED efficiencies using these archetype dopant molecules. FIM provides a nondestructive tool to measure and ultimately improve the outcoupling efficiency of OLEDs and other light-emitting devices.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Reply to “Comment on ‘Reexamining the relation between the binding energy of finite nuclei and the equation of state of infinite nuclear matter' ”

In their comment to our paper [1], Bertsch and Stroberg [2] provide three criticisms. Two of these concern the interpretation of our dispersive optical model (DOM) results and their relation to the liquid drop model (LDM) parameters. The third criticism focuses on the potential systematic uncertainties on our DOM results associated with missing three-body contributions. Before addressing these critiques, we want to state that the key message of our paper remains whether or not the DOM results agree with the LDM predictions in the nuclear interior. Here, the key point is that the standard determination of the saturation energy from the LDM is not ideal since the total binding energy has a minimal contribution from the core of the nucleus as pointed out in Figs. 1–3 of our paper.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Direct cross-section measurement of the weak r -process Sr 88 ( α , n ) Zr 91 reaction in ν -driven winds of core-collapse supernovae

About half of the heavy elements beyond iron are known to be produced by the rapid neutron capture process, known as the r process. However, the astrophysical site producing the r process is still uncertain. Chemical abundances observed in several cosmic sites indicate that different mechanisms should be at play. For instance, the abundances around silver measured in a subset of metal-poor stars indicate the presence of a weak r process. This process may be active in neutrino-driven winds of core collapse supernovae where (α,n) reactions dominate the synthesis of Ζ ≈ 40 elements in the expelled materials. Scarcely measured, the rates of (α,n) reactions are determined from statistical Hauser-Feshbach calculations with α-optical-model potentials, which are still poorly constrained. Further, the uncertainties of the (α,n) reaction rates therefore make a significant contribution to the uncertainties of the abundances determined from stellar modeling. In this work, the 88 Sr (α,n)⁢ 91 Zr reaction which impacts the weak r-process abundances has been probed at astrophysics energy for the first time; directly measuring the total cross sections at astrophysical energies of 8.37–13.09 MeV in the center of mass (3.8–7.5 GK). Two measurements were performed at ATLAS with the electrically segmented ionization chamber MUSIC, in inverse kinematics, while following the active target technique. The cross sections of this α-induced reaction on 88 Sr, located at the shell closure N = 50, have been found to be lower than expected, by a factor of 3, despite recent statistical calculations validated by measurements on neighboring nuclei. This result encourages more experimental investigations of (α,n) reactions, at Ν = 50 and towards the neutron-rich side, to further test the predictive power and reliability of such calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Level density and mechanism of deuteron-induced reactions on Fe 54 , 56 , 58

Here, deuteron elastic cross sections, as well as neutron, proton, and α-particle emission spectra, from d+ 54,56,58 Fe reactions have been measured with deuteron beam energies of 5, 7, and 9 MeV. Optical model parameters have been tested against our experimental data. The fraction of total reaction cross section responsible for the formation of compound nuclei has been deduced from the angular distributions. The degree of discrepancy between calculated and experimental compound cross sections was found to increase with increasing neutron number. The nuclear level densities of the residual nuclei 55 Co, 57 Co, 55 Fe, 57 Fe, 52 Mn, and 54 Mn have been deduced from the compound double differential cross sections. The Gilbert-Cameron model with Iljinov parameter systematics [A. S. Iljinov and M. V. Mebel, Nucl. Phys. A 543, 517 (1992)] was found to have a good agreement with our results.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Neutron scattering cross section measurements for Fe 56

Elastic and inelastic differential cross sections for neutron scattering from 56 Fe have been measured for several incident energies from 1.30 to 7.96 MeV at the University of Kentucky Accelerator Laboratory. Scattered neutrons were detected using a C 6 D 6 liquid scintillation detector using pulse-shape discrimination and time-of-flight techniques. The deduced cross sections have been compared with previously reported data, predictions from evaluation databases ENDF, JENDL, and JEFF, and theoretical calculations performed using different optical model potentials using the TALYS and EMPIRE nuclear reaction codes. The coupled-channel calculations based on the vibrational and soft-rotor models are found to describe the experimental (n,n 0 ) and (n,n 1 ) cross sections well.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

( d , n ) proton-transfer reactions on Be 9 , B 11 , C 13 , N 14 , 15 , and F 19 and spectroscopic factors at E d = 16 MeV

The (d,n) reaction has been studied with targets of 9 Be, 11 B, 13 C, 14,15 N, and 19F at E d = 16MeV using a deuterated liquid-scintillator array. Advanced spectral unfolding techniques with accurately measured scintillator response functions were employed to extract neutron energy spectra without the need for long-path neutron time-of-flight. An analysis of the proton-transfer data at forward angles to the ground states of the final nuclei, using finite-range distorted-wave Born approximation analysis with common bound-state, global, and local optical-model parameter sets, yields a set of self-consistent spectroscopic factors. These are compared with the results of several previous time-of-flight measurements, most done many years ago for individual nuclei at lower energy and often analyzed using zero-range transfer codes. In contrast to some of the earlier published data, our data generally compare well with simple shell-model predictions, with little evidence for uniform quenching (reduction from shell-model values) that has previously been reported from analysis of nucleon knock-out reactions. Data for low-lying excited states in 14 N from 13 C(d,n) also is analyzed and spectroscopic information relevant to nuclear astrophysics obtained. Here, a preliminary study of the radioactive ion beam induced reaction 7 Be(d,n), E( 7 Be) = 30MeV was carried out and indicates further improvements are needed for such measurements, which require detection of neutrons with E n < 2MeV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗