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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Net carbon uptake has increased through warming-induced changes in temperate forest phenology

The timing of phenological events exerts a strong control over ecosystem function and leads to multiple feedbacks to the climate system1. Phenology is inherently sensitive to temperature (though the exact sensitivity is disputed2) and recent warming is reported to have led to earlier spring, later autumn3,4 and increased vegetation activity5,6. Such greening could be expected to enhance ecosystem carbon uptake7,8, though reports also suggest decreased uptake for boreal forests4,9. Here we assess changes in phenology of temperate forests over the eastern US during the past two decades, and quantify the resulting changes in forest carbon storage. We combine long-term ground observations of phenology, satellite indices, and ecosystem-scale carbon dioxide flux measurements, along with 18 terrestrial biosphere models. We observe a strong trend of earlier spring and later autumn. In contrast to previous suggestions4,9 we show that carbon uptake through photosynthesis increased considerably more than carbon release through respiration for both an earlier spring and later autumn. The terrestrial biosphere models tested misrepresent the temperature sensitivity of phenology, and thus the effect on carbon uptake. Our analysis of the temperature-phenology-carbon coupling suggests a current and possible future enhancement of forest carbon uptake due to changes in phenology. This constitutes a negative feedback to climate change, and is serving to slow the rate of warming.

Keenan, Trevor↗
DOE OSTI2023

Mitigating Green's function Monte Carlo signal-to-noise problems using contour deformations

The Green's function Monte Carlo (GFMC) method provides accurate solutions to the nuclear many-body problem and predicts properties of light nuclei starting from realistic two- and three-body interactions. Controlling the GFMC fermion-sign problem is crucial, as the signal-to-noise ratio decreases exponentially with Euclidean time, requiring significant computing resources. Inspired by similar scenarios in lattice quantum field theory and spin systems, in this work, we employ integration contour deformations to improve the GFMC signal-to-noise ratio. Machine learning techniques are used to select optimal contours with minimal variance from parameterized families of deformations. As a proof of principle, we consider the deuteron binding energies and Euclidean density response functions. We only observe mild signal-to-noise improvement for the binding energy case. On the other hand, we achieve an order of magnitude reduction of the variance for Euclidean density response functions, paving the way for computing electron- and neutrino-nucleus cross-sections of larger nuclei.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Simple eigenvalue-self-consistent $\overline{Δ}GW_0$

We show that a rigid scissors-like GW self-consistency approach, labeled here $\overline{Δ}GW_0$, can be trivially implemented at zero additional cost for large scale one-shot $G_0W_0$ calculations. The method significantly improves one-shot $G_0W_0$ and for large systems is very accurate. $\overline{Δ}GW_0$ is similar in spirit to ev$GW_0$ where the self-consistency is only applied on the eigenvalues entering Green's function, while both W and the eigenvectors of Green's function are held fixed. $\overline{Δ}GW_0$ further assumes that the shift of the eigenvalues is rigid scissors-like so that all occupied states are shifted by the same amount and analogously for all the unoccupied states. We show that this results in a trivial modification of the time-dependent $G_0W_0$ self-energy, enabling an a posteriori self-consistency cycle. The method is applicable for our recent stochastic-$GW$ approach, thereby enabling self-consistent calculations for giant systems with thousands of electrons. The accuracy of $\overline{Δ}GW_0$ increases with the system size. For molecules, it is up to 0.4-0.5 eV away from coupled-cluster single double triple (CCSD(T)), but for tetracene and hexacene, it matches the ionization energies from both CCSD(T) and ev$GW_0$ to better than 0.05 eV. For solids, as exemplified here by periodic supercells of semiconductors and insulators with 6192 valence electrons, the method matches ev$GW_0$ quite well and both methods are in good agreement with the experiment.

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

A class of exact solutions for population balances with arbitrary internal coordinates

We develop a novel transformation that maps the linear, nonhomogeneous, multidimensional population balance equation (PBE) into an advection equation that is readily solved using the method of characteristics. The PBEs targeted by this transformation exclude aggregation, breakage, and time dependent birth and death rates. In addition, internal coordinates are assumed to grow independently of each other. The ensuing general formulation is then used to recover closed‐form analytical solutions for problems with monosurface and bulk‐diffusion growth‐rates as well as Gaussian‐type nucleation. For completeness, we derive the multidimensional Green's functions for our approach. This is followed by a brief discussion on how the proposed framework may be used for code verification of moment methods such as the quadrature method of moments and the direct quadrature method of moments. Finally, a sample Mathematica code is provided to derive analytical solutions for the single‐internal‐coordinate case given user‐specified growth, birth, and death rates. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1691–1698, 2015

Saad, Tony↗
DOE OSTI2021

Accelerated impurity solver for DMFT and its diagrammatic extensions

Here, we present ComCTQMC, a GPU accelerated quantum impurity solver. It uses the continuous-time quantum Monte Carlo (CTQMC) algorithm wherein the partition function is expanded in terms of the hybridisation function (CT-HYB). ComCTQMC supports both partition and worm-space measurements, and it uses improved estimators and the reduced density matrix to improve observable measurements whenever possible. ComCTQMC efficiently measures all one and two-particle Green's functions, all static observables which commute with the local Hamiltonian, and the occupation of each impurity orbital. ComCTQMC can solve complex-valued impurities with crystal fields that are hybridized to both fermionic and bosonic baths. Most importantly, ComCTQMC utilizes graphical processing units (GPUs), if available, to dramatically accelerate the CTQMC algorithm when the Hilbert space is sufficiently large. We demonstrate acceleration by a factor of over 600 (100) in a simulation of δ-Pu at 600 K with (without) crystal fields. In easier problems, the GPU offers less impressive acceleration or even decelerates the CTQMC. Here we describe the theory, algorithms, and structure used by ComCTQMC in order to achieve this set of features and level of acceleration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Conducting polymer-based electrochemical sensors: Progress, challenges, and future perspectives

Conducting polymers are promising due to their unique properties, such as excellent electrical and optical properties, physical and chemical stability, high conductivity, and effective redox properties with high-temperature stability and biocompatibility. Due to these properties, conducting polymers are useful in diverse applications like sensors, batteries, oil industries, biosensors, biomedicines, catalysis, cancer treatment, etc. This review article aims to discuss the recent trends and analysis of conducting polymer-based electrochemical sensors in diverse areas with all required sensor characteristics, such as the derived limit of detection, utilized techniques for the sensing analysis and derived linear dynamic range with the stability of the sensors. Conducting polymers and their nanocomposites-based electrochemical sensors have demonstrated exceptional capabilities towards detecting various biomolecules, heavy metals, pesticides, and viruses like SARS-COV-2. Incorporation of redox mediators, use of conducting hydrogels, and molecular imprinting are promising strategies for better performance of the derived sensor. The article has demonstrated the existing challenges and limitations and provided solutions in the field. In the future, conducting polymers-based electrochemical sensors can be utilized in wearable sensors and integrated with IoT devices for better reach in real-time applications. They can also be made more accessible with precise control and data output by following specific methodologies. Utilizing green and sustainable conducting polymers can be crucial in advancing eco-friendly practices in the future. Conducting polymer-based electrochemical sensors has affectivity in neurochemical and pathogen sensing, which is essential for brain function and mental health.

42 ENGINEERING↗
DOE OSTI2019

Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States

We present here a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal–organic chromophores of the generic molecular formula [Re(phen)(CO) 3 (CNAr)] + [CNAr = 2,6-diisopropylphenyl isocyanide ( 1 ), 4-phenyl-2,6-diisopropylphenyl isocyanide ( 2 ), 4-phenylethynyl-2,6-diisopropylphenyl isocyanide ( 3 ), and 4-biphenyl-2,6-diisopropylphenyl isocyanide ( 4 )]. This particular series features varied degrees of π-conjugation length in the CNAr moiety, resulting in significant modulation in the resultant photophysical properties. All molecules possess long-lived [8–700 μs at room temperature (RT)], strongly blue-green photoluminescent and highly energetic excited states (λ max,em = 500–518 nm; Φ = 14–64%). Each of these chromophores has been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to supra-nanosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in manipulating the excited-state decay in three of these molecules ( 2 – 4 ), wherein the triplet metal-to-ligand charge-transfer ( 3 MLCT) state along with two distinct triplet ligand-centered ( 3 LC) excited-state configurations (phen and CNAr) conspire to produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1 , the phenanthroline 3 LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT character. With more extended π conjugation in the CNAr subunit ( 2 – 4 ), the initially generated 3 LC(phen)/ 3 MLCT excited state ultimately migrates to the CNAr 3 LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature unique examples of inorganic excimer formation, as demonstrated by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.

14 SOLAR ENERGY↗
DOE OSTI2014

Insights into the Structural Changes Occurring upon Photoconversion in the Orange Carotenoid Protein from Broadband Two-Dimensional Electronic Spectroscopy

Carotenoids play an essential role in photoprotection, interacting with other pigments to safely dissipate excess absorbed energy as heat. In cyanobacteria, the short time scale photoprotective mechanisms involve the photoactive orange carotenoid protein (OCP), which binds a single carbonyl carotenoid. Blue-green light induces the photoswitching of OCP from its ground state form (OCPO) to a metastable photoproduct (OCPR). OCPR can bind to the phycobilisome antenna and induce fluorescence quenching. The photoswitching is accompanied by structural and functional changes at the level of the protein and of the bound carotenoid. In this study, we use broadband two-dimensional electronic spectroscopy to look at the differences in excited state dynamics of the carotenoid in the two forms of OCP. Our results provide insight into the origin of the pronounced vibrational lineshape and oscillatory dynamics observed in linear absorption and 2D electronic spectroscopy of OCPO and the large inhomogeneous broadening in OCPR, with consequences for the chemical function of the two forms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

EMHOLE v. 2.1

Program EMHOLE is an electromagnetic (EM) forward modeling algorithm that calculates time-domain electric vector e (with SI unit V/m) and magnetic vector h (in A/m) responses generated by a variety of point body sources situated within a homogeneous and isotropic wholespace. A mathematically exact (i.e., without low-frequency or far-field approximation) numerical evaluation procedure is utilized to simulate EM responses (alternately, Green functions) observed at a set of multicomponent receivers distributed throughout the wholespace.

Aldridge, David F.↗
DOE OSTI2016

Description of Hydration Water in Protein (Green Fluorescent Protein) Solution

The structurally and dynamically perturbed hydration shells that surround proteins and biomolecules have a substantial influence upon their function and stability. This makes the extent and degree of water perturbation of practical interest for general biological study and industrial formulation. Here, we present an experimental description of the dynamical perturbation of hydration water around green fluorescent protein in solution. Less than two shells (~5.5 Å) were perturbed, with dynamics a factor of 2–10 times slower than bulk water, depending on their distance from the protein surface and the probe length of the measurement. Furthermore, this dependence on probe length demonstrates that hydration water undergoes subdiffusive motions (τ ∝ q –2.5 for the first hydration shell, τ ∝ q –2.3 for perturbed water in the second shell), an important difference with neat water, which demonstrates diffusive behavior (τ ∝ q –2 ). Our results help clarify the seemingly conflicting range of values reported for hydration water retardation as a logical consequence of the different length scales probed by the analytical techniques used.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Disintegration in Compost Conditions and Water Uptake of Green Composites from Poly(Lactic Acid) and Hazelnut Shell Flour

Green composites of poly(lactic acid)-PLA and hazelnut shell flour (HSF) with and without epoxidized linseed oil (ELO) as plasticizer/compatibilizer were subjected to different aging conditions such as water uptake by immersion and disintegration in compost soil. The effect of the hydrolytic degradation was analyzed by measuring the weight gain as a function of the immersion time in water and calculating the corresponding diffusion coefficients. As expected, the water diffusion coefficient increases with HSF content while no remarkable change is obtained for plasticized compositions with ELO. Differential scanning calorimetry reveals a noticeable increase in crystallinity after the degradation process by water immersion. Degradation in controlled compost soil was followed thorough measurements of weight changes. In general, the weight change for a particular degradation time is lower as the HSF content increases. In addition, presence of ELO as plasticizer/compatibilizer delays the degradation process in compost soil. Scanning electron microscopy highlighted a noticeable deterioration of aged samples after 2 weeks with multiple crack formation and high surface abrasion due to microbial activity after 4 weeks.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A high-finesse Fabry-Perot cavity with a frequency-doubled green laser for precision Compton polarimetry at Jefferson Lab

Here, a high-finesse Fabry-Perot cavity with a frequency-doubled continuous wave green laser (532 nm) has been built and installed in Hall A of Jefferson Lab for high precision Compton polarimetry. The infrared (1064 nm) beam from a ytterbium-doped fiber amplifier seeded by a Nd:YAG nonplanar ring oscillator laser is frequency doubled in a single-pass periodically poled MgO:LiNbO 3 crystal. The maximum achieved green power at 5 W infrared pump power is 1.74 W with a total conversion efficiency of 34.8%. The green beam is injected into the optical resonant cavity and enhanced up to 3.7 kW with a corresponding enhancement of 3800. The polarization transfer function has been measured in order to determine the intra-cavity circular laser polarization within a measurement uncertainty of 0.7%. The PREx experiment at Jefferson Lab used this system for the first time and achieved 1.0% precision in polarization measurements of an electron beam with energy and current of 1.0 GeV and 50 μA.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2023

TeraChem protocol buffers ( TCPB ): Accelerating QM and QM/MM simulations with a client–server model

The routine use of electronic structures in many chemical simulation applications calls for efficient and easy ways to access electronic structure programs. Here, we describe how the graphics processing unit (GPU) accelerated electronic structure program TeraChem can be set up as an electronic structure server, to be easily accessed by third-party client programs. We exploit Google’s protocol buffer framework for data serialization and communication. The client interface, called TeraChem protocol buffers (TCPB), has been designed for ease of use and compatibility with multiple programming languages, such as C++, Fortran, and Python. To demonstrate the ease of coupling third-party programs with electronic structures using TCPB, we have incorporated the TCPB client into Amber for quantum mechanics/molecular mechanics (QM/MM) simulations. The TCPB interface saves time with GPU initialization and I/O operations, achieving a speedup of more than 2× compared to a prior file-based implementation for a QM region with ~250 basis functions. We demonstrate the practical application of TCPB by computing the free energy profile of p-hydroxybenzylidene-2,3-dimethylimidazolinone (p-HBDI - )—a model chromophore in green fluorescent proteins—on the first excited singlet state using Hamiltonian replica exchange for enhanced sampling. All calculations in this work have been performed with the non-commercial freely-available version of TeraChem, which is sufficient for many QM region sizes in common use.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Apparent Explosion Moments from Rg Waves Recorded on SPE

Seismic moments for the first four chemical tests making up phase I of the Source Physics Experiments (SPE) are estimated from 6-Hz Rg waves recorded along a single radial line of geophones under the assumption that the tests are pure explosions. These apparent explosion moments are compared with moments determined from the reduced displacement potential method applied to free-field data. Light detection and ranging (lidar) observations, strong ground motions on the free surface in the vicinity of ground zero, and moment tensor inversion results are evidence that the fourth test SPE-4P is a pure explosion, and the moments show good agreement, 8×10 10 N·m for free-field data versus 9×10 10 N·m for Rg waves. In stark contrast, apparent moments for the first three tests are smaller than near-field moments by factors of 3–4. Relative amplitudes for the three tests determined from Rg interferometry using SPE-4P as an empirical Green’s function indicate that radiation patterns are cylindrically symmetric within a factor of 1.25 (25%). This fact assures that the apparent moments are reliable even though they were measured on just one azimuth. Spallation occurred on the first three tests, and ground-based lidar detected permanent deformations. As such, the source medium suffered late-time damage. In conclusion, destructive interference between Rg waves radiated by explosion and damage sources will reduce amplitudes and explain why apparent moments are smaller than near-field moments based on compressional energy emitted directly from the source.

47 OTHER INSTRUMENTATION↗
DOE OSTI2011

DIFFUSIVE PARTICLE ACCELERATION IN SHOCKED, VISCOUS ACCRETION DISKS: GREEN'S FUNCTION ENERGY DISTRIBUTION

The acceleration of relativistic particles in a viscous accretion disk containing a standing shock is investigated as a possible explanation for the energetic outflows observed around radio-loud black holes. The energy/space distribution of the accelerated particles is computed by solving a transport equation that includes the effects of first-order Fermi acceleration, bulk advection, spatial diffusion, and particle escape. The velocity profile of the accreting gas is described using a model for shocked viscous disks recently developed by the authors, and the corresponding Green's function distribution for the accelerated particles in the disk and the outflow is obtained using a classical method based on eigenfunction analysis. The accretion-driven, diffusive shock acceleration scenario explored here is conceptually similar to the standard model for the acceleration of cosmic rays at supernova-driven shocks. However, in the disk application, the distribution of the accelerated particles is much harder than would be expected for a plane-parallel shock with the same compression ratio. Hence the disk environment plays a key role in enhancing the efficiency of the shock acceleration process. The presence of the shock helps to stabilize the disk by reducing the Bernoulli parameter, while channeling the excess binding energy into the escaping relativistic particles. In applications to M87 and Sgr A*, we find that the kinetic power in the jet is {approx}0.01 M-dot c{sup 2}, and the outflowing relativistic particles have a mean energy {approx}300 times larger than that of the thermal gas in the disk at the shock radius. Our results suggest that a standing shock may be an essential ingredient in accretion onto underfed black holes, helping to resolve the long-standing problem of the stability of advection-dominated accretion disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Nonequilibrium electron dynamics in pump-probe spectroscopy: Role of excited phonon populations

We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green's function formalism. The transient modi cations in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green's functions. The pump induced changes manifest in both the electronic and phononic spectral functions. We nd that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibrium decay rates and the equilibrium scattering rates apart. The decay rates are found to be time-dependent, and this is illustrated in the experimentally observed population decay of photoexcited Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Robust half-metallicity of hexagonal SrNiO{sub 3}

In the rich panorama of the electronic and magnetic properties of 3d transition metal oxides SrMO{sub 3} (M=Ti, V, Cr, Mn, Fe, Co, Ni, Cu), one member (SrNiO{sub 3}) is missing. In this paper we use GGA+U method based on density functional theory to examine its properties. It is found that SrNiO{sub 3} is a ferromagnetic half-metal. The charge density map shows a high degree of ionic bonding between Sr and other atoms. Meanwhile, a covalent-bonding Ni–O–Ni–O–Ni chain is observed. The spin density contour of SrNiO{sub 3} further indicates that the magnetic interaction between Ni atoms mediated by O is semicovalent exchange. The density of states are examined to explore the unusual indirect magnetic-exchange mechanism. Corresponding to the total energies results, a robust half-metallic character is observed, suggesting a promising giant magneto-optical Kerr property of the material. The partial density of states are further examined to explore the origin of ferromagnetic half-metallicity. The O atoms are observed to have larger contribution at fermi level than Ni atoms to the spin-polarized states, demonstrating that O atoms play a critical role in ferromagnetic half-metallicity of SrNiO{sub 3}. Hydrostatic pressure effect is examined to evaluate how robust the half-metallic ferromagnetism is. - Graphical abstract: (a) The total energy as a function of the lattice constant a for hexagonal SrNiO3 with various magnetic phases. (b) The total electronic density of states for hexagonal SrNiO{sub 3} with FM configuration from GGA+U calculations. (c) Total electron-density distribution in the (110) plane. The colors gradually change from cyan (through pink) to yellow corresponding to charge density value from 0 to 4.0. (d) The magnetization density map in the (110) plane. The colors range from blue (through green) to red corresponding to magnetization density value from −0.15 to 0.45. Black and white contours stand for positive and negative values, respectively. - Highlights: • Hexagonal SrNiO{sub 3} is studied using first-principles method for the first time. • It is predicted that SrNiO{sub 3} is a ferromagnetic half metal. • The half-metallic ferromagnetism survives upon a pressure up to 20 GPa.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗