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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 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Oxidation kinetics of SPS-densified U 3 Si 2 fuels—Microstructure impact

Here, U 3 Si 2 is a potential candidate for accident tolerant fuels because of its high uranium density and excellent thermal conductivity in comparison to UO 2 . However, U 3 Si 2 suffers from oxidation, steam corrosion, and subsequent disintegration/pulverization. The detailed investigation of kinetics that incorporates fundamental treatment of oxidation of U 3 Si 2 is scarcely reported, and the oxidation mechanisms have not been fully elucidated. In this paper, the oxidation behavior of microcrystalline (mc - ) and nanocrystalline (nc - ) U 3 Si 2 have been systematically investigated using a thermogravimetric analysis (TGA) apparatus through a series of isothermal and non-isothermal kinetic studies. The isothermal kinetic study with a model-fitting approach indicates oxidation activation energy of 85 kJ/mol for dense mc - U 3 Si 2 and 96.4 kJ/mol for nc - U 3 Si 2 pellets, while the isoconversional approach leads to an activation energy in the range of 70–85 kJ/mol for mc - U 3 Si 2 and 75–86 kJ/mol for nc - U 3 Si 2 with three most common model-free methods, including Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, and Friedman methods. The derivation of oxidation activation energies using both isothermal and isoconversional methods highlights the approach to evaluate the oxidation resistance of nuclear materials using TGA quantitatively and makes it possible to compare among various nuclear fuels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Kinetics of atoms in a bichromatic field formed by elliptically polarised waves

The kinetics of atoms in a bichromatic field formed by waves with an arbitrary elliptical polarisation is considered. Analytical expressions are derived for the force and the friction and diffusion coefficients within the model of an atom with the j {sub g} = 1/2 → j {sub e} = 3/2 optical transition. These expressions are presented in the general form as an expansion in gradients of light field parameters. Some specific features of the kinetics of atoms are found, which are due to the interference contributions to the friction and diffusion coefficients. The kinetics of atoms in the field of an optical lattice formed as a result of the interference of a wave with a uniform elliptical polarisation and a nonuniformly polarised dissipative field is analysed. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

The Harrison Diffusion Kinetics Regimes in Solute Grain Boundary Diffusion

Knowledge of the limits of the principal Harrison kinetics regimes (Type-A, B and C) for grain boundary diffusion is very important for the correct analysis of the depth profiles in a tracer diffusion experiment. These regimes for self-diffusion have been extensively studied in the past by making use of the phenomenological Lattice Monte Carlo (LMC) method with the result that the limits are now well established. The relationship of those self-diffusion limits to the corresponding ones for solute diffusion in the presence of solute segregation to the grain boundaries remains unclear. In the present study, the influence of solute segregation on the limits is investigated with the LMC method for the well-known parallel grain boundary slab model by showing the equivalence of two diffusion models. It is shown which diffusion parameters are useful for identifying the limits of the Harrison kinetics regimes for solute grain boundary diffusion. It is also shown how the measured segregation factor from the diffusion experiment in the Harrison Type-B kinetics regime may differ from the global segregation factor.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2014

When and how does a prominence-like jet gain kinetic energy?

A jet is a considerable amount of plasma being ejected from the chromosphere or lower corona into the higher corona and is a common phenomenon. Usually, a jet is triggered by a brightening or a flare, which provides the first driving force to push plasma upward. In this process, magnetic reconnection is thought to be the mechanism to convert magnetic energy into thermal, nonthermal, and kinetic energies. However, most jets could reach an unusual high altitude and end much later than the end of its associated flare. This fact implies that there is another way to continuously transfer magnetic energy into kinetic energy even after the reconnection. The picture described above is well known in the community, but how and how much magnetic energy is released through a way other than reconnection is still unclear. By studying a prominence-like jet observed by SDO/AIA and STEREO-A/EUVI, we find that the continuous relaxation of the post-reconnection magnetic field structure is an important process for a jet to climb up higher than it could through only reconnection. The kinetic energy of the jet gained through the relaxation is 1.6 times that gained from the reconnection. The resultant energy flux is hundreds of times larger than the flux required for the local coronal heating, suggesting that such jets are a possible source to keep the corona hot. Furthermore, rotational motions appear all the time during the jet. Our analysis suggests that torsional Alfvén waves induced during reconnection could not be the only mechanism to release magnetic energy and drive jets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Boron nitride nanotube precursor formation during high-temperature synthesis: kinetic and thermodynamic modelling

In this work, we performed integrated modelling of the chemical pathways of formation for boron nitride nanotube (BNNT) precursors during high-temperature synthesis in a B/N 2 mixture. Integrated modelling includes quantum chemistry, Quantum–classical molecular dynamics, thermodynamic modelling, and kinetic approaches. We demonstrate that BN compounds are formed via the interaction of molecular nitrogen with small boron clusters, rather than through interactions with less reactive liquid boron. (This process can also be described as N 2 molecule fixation.) Liquid boron evaporates to produce these boron clusters (B m with m ≤ 5), which are subsequently converted into B m N n chains. The production of such chains is crucial to the growth of BNNTs because these chains form the building blocks of bigger and longer BN chains and rings, which are in turn the building blocks of fullborenes and BNNTs. Additionally, kinetic modelling revealed that B 4 N 4 and B 5 N 4 species in particular play a major role in the N 2 molecule fixation process. Furthermore, the formation of these species via reactions with B 4 and B 5 clusters is not adequately described under the assumption of thermodynamic equilibrium, as is demonstrated in our kinetic modelling. Thus, the accumulation of both B 4 N 4 and B 5 N 4 depends on the background gas pressure and the gas cooling rate. Long BN chains and rings, which are precursors of the fullborene and BNNT growth, form via self-assembly of components B 4 N 4 and B 5 N 4 . Our modelling results—particularly the increased densities of B 4 N 4 and B 5 N 4 species at higher gas pressures—explain the experimentally observed effect of gas pressure on the yield of high-quality BNNTs. The catalytic role of hydrogen was also studied; it is shown that HBNH molecules can be the main precursor of BNNT synthesis in the presence of hydrogen.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Temperature response of Rubisco kinetics in Arabidopsis thaliana : thermal breakpoints and implications for reaction mechanisms

Enhancement of Rubisco kinetics could improve photosynthetic efficiency, ultimately resulting in increased crop yield. However, imprecise knowledge of the reaction mechanism and the individual rate constants limits our ability to optimize the enzyme. Membrane inlet mass spectrometry (MIMS) may offer benefits over traditional methods for determining individual rate constants of the Rubisco reaction mechanism, as it can directly monitor concentration changes in CO 2 , O 2 , and their isotopologs during assays. However, a direct comparison of MIMS with the traditional radiolabel method of determining Rubisco kinetic parameters has not been made. Here, the temperature responses of Rubisco kinetic parameters from Arabidopsis thaliana were measured using radiolabel and MIMS methods. The two methods provided comparable parameters above 25 °C, but temperature responses deviated at low temperature as MIMS-derived catalytic rates of carboxylation, oxygenation, and CO 2 /O 2 specificity showed thermal breakpoints. Here, we discuss the variability and uncertainty surrounding breakpoints in the Rubisco temperature response and the relevance of individual rate constants of the reaction mechanisms to potential breakpoints.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Bhatnagar-Gross-Krook subgrid model for neutrino quantum kinetics

Here, we present a new subgrid model for neutrino quantum kinetics, which is primarily designed to incorporate effects of collective neutrino oscillations into neutrino-radiation-hydrodynamic simulations for core-collapse supernovae and mergers of compact objects. We approximate the neutrino oscillation term in a quantum kinetic equation using the Bhatnagar-Gross-Krook (BGK) relaxation-time prescription, and the transport equation is directly applicable for classical neutrino transport schemes. The BGK model is motivated by recent theoretical indications that nonlinear phases of collective neutrino oscillations settle into quasisteady structures. We explicitly provide basic equations of the BGK subgrid model for both multiangle and moment-based neutrino transport to facilitate the implementation of the subgrid model in the existing neutrino transport schemes. We also show the capability of our BGK subgrid model by comparing it to fully quantum kinetic simulations for fast neutrino-flavor conversion. We find that the overall properties can be well reproduced in the subgrid model; the error of angular-averaged survival probability of neutrinos is within ~20%. By identifying the source of error, we also discuss perspectives to improve the accuracy of the subgrid model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Kinetic theory for classical and quantum many-body chaos

For perturbative scalar field theories, the late-time-limit of the out-of-time-ordered correlation function that measures (quantum) chaos is shown to be equal to a Boltzmann-type kinetic equation that measures the total gross (instead of net) particle exchange between phase-space cells, weighted by a function of energy. This derivation gives a concrete form to numerous attempts to derive chaotic many-body dynamics from ad hoc kinetic equations. A period of exponential growth in the total gross exchange determines the Lyapunov exponent of the chaotic system. Physically, the exponential growth is a front propagating into an unstable state in phase space. As in conventional Boltzmann transport, which follows from the dynamics of the net particle number density exchange, the kernel of this kinetic integral equation for chaos is also set by the 2-to-2 scattering rate. This provides a mathematically precise statement of the known fact that in dilute weakly coupled gases, transport and scrambling (or ergodicity) are controlled by the same physics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Sustained virological response with intravenous silibinin: individualized IFN-free therapy via real-time modelling of HCV kinetics

Providing here our aims and project background, we note that intravenous silibinin (SIL) is a potent antiviral agent against hepatitis C virus (HCV) genotype-1. In this proof of concept case-study we tested: (i) whether interferon-alfa (IFN)-free treatment with SIL plus ribavirin (RBV) can achieve sustained virological response (SVR); (ii) whether SIL is safe and feasible for prolonged duration of treatment and (iii) whether mathematical modelling of early on-treatment HCV kinetics can guide duration of therapy to achieve SVR. With our method, a 44 year-old female HCV-(genotype-1)-infected patient who developed severe psychiatric adverse events to a previous course of pegIFN+RBV, initiated combination treatment with 1200 mg/day of SIL, 1200 mg/day of RBV and 6000 u/day vitamin D. Blood samples were collected frequently till week 4, thereafter every 1-12 weeks until the end of therapy. The standard biphasic mathematical model with time-varying SIL effectiveness was used to predict the duration of therapy to achieve SVR. Our results show that, based on modelling the observed viral kinetics during the first 3 weeks of treatment, SVR was predicted to be achieved within 34 weeks of therapy. Provided with this information, the patient agreed to complete 34 weeks of treatment. IFN-free treatment with SIL+RBV was feasible, safe and achieved SVR (week-33). In conclusion, we report, for the first time, the use of real-time mathematical modelling of HCV kinetics to individualize duration of IFN-free therapy and to empower a patient to participate in shared decision making regarding length of treatment. SIL-based individualized therapy provides a treatment option for patients who do not respond to or cannot receive other HCV agents and should be further validated.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

Nucleation of diindenoperylene and pentacene at thermal and hyperthermal incident kinetic energies

The authors have examined the nucleation of diindenoperylene (DIP) on SiO{sub 2} employing primarily atomic force microscopy and focusing on the effect of incident kinetic energy employing both thermal and supersonic sources. For all incident kinetic energies examined (E{sub i} = 0.09–11.3 eV), the nucleation of DIP is homogeneous and the dependence of the maximum island density on the growth rate is described by a power law. A critical nucleus of approximately two molecules is implicated by our data. A re-examination of the nucleation of pentacene on SiO{sub 2} gives the same major result that the maximum island density is determined by the growth rate, and it is independent of the incident kinetic energy. These observations are readily understood by factoring in the size of the critical nucleus in each case, and the island density, which indicates that diffusive transport of molecules to the growing islands dominate the dynamics of growth in the submonolayer regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Kinetics of Enzymatic Mercury Methylation at Nanomolar Concentrations Catalyzed by HgcAB

Methylmercury (MeHg) is a potent bioaccumulative neurotoxin that is produced by certain anaerobic bacteria and archaea. Mercury (Hg) methylation has been linked to the gene pair hgcAB , which encodes a membrane-associated corrinoid protein and a ferredoxin. Although microbial Hg methylation has been characterized in vivo , the cellular biochemistry and the specific roles of the gene products HgcA and HgcB in Hg methylation are not well understood. Here, we report the kinetics of Hg methylation in cell lysates of Desulfovibrio desulfuricans ND132 at nanomolar Hg concentrations. The enzymatic Hg methylation mediated by HgcAB is highly oxygen sensitive, irreversible, and follows Michaelis-Menten kinetics, with an apparent K m of 3.2 nM and V max of 19.7 mol · min -1 · mg -1 total protein for the substrate Hg(II). Although the abundance of HgcAB in the cell lysates is extremely low, Hg(II) was quantitatively converted to MeHg at subnanomolar substrate concentrations. Interestingly, increasing thiol/Hg(II) ratios did not impact Hg methylation rates, which suggests that HgcAB-mediated Hg methylation effectively competes with cellular thiols for Hg(II), consistent with the low apparent K m . Supplementation of 5-methyltetrahydrofolate or pyruvate did not enhance MeHg production, while both ATP and a nonhydrolyzable ATP analog decreased Hg methylation rates in cell lysates under the experimental conditions. These studies provide insights into the biomolecular processes associated with Hg methylation in anaerobic bacteria. The concentration of Hg in the biosphere has increased dramatically over the last century as a result of industrial activities. The microbial conversion of inorganic Hg to MeHg is a global public health concern due to bioaccumulation and biomagnification of MeHg in food webs. Exposure to neurotoxic MeHg through the consumption of fish represents a significant risk to human health and can result in neuropathies and developmental disorders. Anaerobic microbial communities in sediments and periphyton biofilms have been identified as sources of MeHg in aquatic systems, but the associated biomolecular mechanisms are not fully understood. In the present study, we investigate the biochemical mechanisms and kinetics of MeHg formation by HgcAB in sulfate-reducing bacteria. These findings advance our understanding of microbial MeHg production and may help inform strategies to limit the formation of MeHg in the environment.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2012

Growth kinetics and micromorphology of NH{sub 4}Cl:Mn{sup 2+} crystals formed in the NH{sub 4}Cl-MnCl{sub 2}-H{sub 2}O-CONH{sub 3} system

The growth kinetics and elementary growth processes on the surface of NH{sub 4}Cl:Mn{sup 2+} heterogeneous crystals formed in the NH{sub 4}Cl-MnCl{sub 2}-H{sub 2}O-CONH{sub 3} system are experimentally studied. It is found that a change in the composition of complexes in an NH{sub 4}Cl crystal from Mn(NH{sub 4}){sub 2}Cl{sub 4} {center_dot} 2H{sub 2}O to MnCl{sub 2} {center_dot} 2CONH{sub 3} leads to the occurrence of a local maximum in the kinetic curve and a change in the shape of dislocation growth centers from flat to conical. The growth kinetics of {l_brace}100{r_brace} faces of heterogeneous NH{sub 4}Cl:Mn{sup 2+} crystals is described within the Bliznakov model using the Fowler-Guggenheim adsorption isotherm, which takes into account the lateral interaction of adsorbed particles.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Anomalous kinetic energy of a system of dust particles in a gas discharge plasma

The system of equations of motion of dust particles in a near-electrode layer of a gas discharge has been formulated taking into account fluctuations of the charge of a dust particle and the features of the nearelectrode layer of the discharge. The molecular dynamics simulation of the system of dust particles has been carried out. Performing a theoretical analysis of the simulation results, a mechanism of increasing the average kinetic energy of dust particles in the gas discharge plasma has been proposed. According to this mechanism, the heating of the vertical oscillations of dust particles is initiated by induced oscillations generated by fluctuations of the charge of dust particles, and the energy transfer from vertical to horizontal oscillations can be based on the parametric resonance phenomenon. The combination of the parametric and induced resonances makes it possible to explain an anomalously high kinetic energy of dust particles. The estimate of the frequency, amplitude, and kinetic energy of dust particles are close to the respective experimental values.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Kinetics of generation, relaxation, and accumulation of electronic excitations under two-photon interband picosecond absorption in tungstate and molibdate crystals

Under two-photon 523.5 nm interband picosecond laser excitation, we measured the kinetics of induced absorption in PbWO{sub 4}, ZnWO{sub 4}, and PbMoO{sub 4} crystals with 532 to 633 nm continuous probe radiation. We obtained real-time information about the dynamics of the generation, relaxation, and accumulations of electronic excitations over a wide time range (from picoseconds to hundreds of seconds) and the 77-300 K temperature range. For the studied crystals, exponential temperature-independent growth of the induced absorption (IA) with 60 ns rise time reflects the dynamics of the generation of electronic excitation. The kinetics of the IA exponential growth with temperature-dependent 3.5-11 {mu}s time constants reflect the dynamics of energy migration between neighboring tungstate (molibdate) ions to traps for the studied crystals. The multiexponential relaxation absorption kinetics strongly depend on temperature, and the relaxation decay time of induced absorption increased from tens to hundreds of milliseconds to seconds under crystal cooling from 300 to 77 K. We found that the increase in the laser pump repetition rate (0-10 Hz) leads to the accumulation of electronic excitations. Control of the repetition rate and the number of excitations allowed us to change the relaxation time of the induced absorption by more than two orders of magnitude. Due to accumulation of excitations at 77 K, the absorption relaxation time can exceed 100 s for PbWO{sub 4} and PbMoO{sub 4} crystals. In the initially transparent crystals, two-photon interband absorption (2PA) leads to crystals opacity at the 523 and 633 nm wavelengths. (An inverse optical transmission of the crystals exceeds 50-55 at a 50-100 GW/cm{sup 2} pump intensity.) Measured at {approx}1 mW probe radiation of 532 and 633 nm wavelengths, the induced absorption values are comparable with those obtained under two-photon absorption at {approx}5 kW pump power. An optical 2PA shutter for the visible spectral range is proposed with a variable shutting time from hundreds of microseconds to tens of seconds.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

On the accuracy of a one-dimensional approach to the solution of kinetic equations with velocity-dependent collision frequencies

The solution of many problems in light-induced gas kinetics can be simplified significantly using quantum kinetic equations in the context of the so-called one-dimensional approximation, in which the initial equations are averaged over transverse (relative to the direction of radiation) velocities. The errors introduced in such an approach are usually assumed to be small; however, this has been confirmed quantitatively only on the basis of the simplest (two- and three-level) particle models. We analyze the accuracy of the one-dimensional approximation for multilevel particles quantitatively for the light-induced drift (LID) effect in cesium atoms in the atmosphere of inert buffer gases. It is shown that in the case of the so-called “normal” LID, one-dimensional kinetic equations can always be used instead of three-dimensional equations without a risk of losing some important fine details in the dependence of the drift velocity on the radiation frequency. In the case of anomalous LID, the error of the one-dimensional approximation is also insignificant, but it can be disregarded only in the case of light buffer particles. For comparable masses of resonant and buffer particles, the one-dimensional approximation may give a noticeable error in determination of drift velocity amplitudes; however, the positions of drift velocity zeros and extrema depending on radiation-frequency detuning can be described successfully. Results show that the error introduced by using the one-dimensional approximation for multilevel particles turns out to be more significant than for the simplest particle models.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Kinetic equations in polaron theory for the case of spatial inhomogeneity

Construction of a kinetic equation for a dynamical system interacting with a boson field in the case of spatial inhomogeneity is based on a method expounded in [1–3]. In the present article, it is shown that approaches considered in [1, 2] can be generalized for the case of spatial inhomogeneity. An arbitrary operator function that depends on the momentum and spatial variable is applied for derivation of a kinetic equation. We consider a method of studying an electron-phonon system by means of exclusion of boson operators from corresponding operator equations. In particular, interaction of an electron with a boson field is described by a kinetic equation for a polaron in the case of spatial inhomogeneity. In the relevant limit, a Boltzmann equation for the polaron is obtained.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Chemical Kinetics in Microdroplets

Micrometer-sized compartments play significant roles in driving heterogeneous transformations within atmospheric and biochemical systems as well as providing vehicles for drug delivery and novel reaction environments for the synthesis of industrial chemicals. Many reports now indicate that reaction kinetics are accelerated under microconfinement, for example, in sprays, thin films, droplets, aerosols, and emulsions. These observations are dramatic, posing a challenge to our understanding of chemical reaction mechanisms with potentially significant practical consequences for predicting the complex chemistry in natural systems. Here we introduce the idea of kinetic confinement, which is intended to provide a conceptual backdrop for understanding when and why microdroplet reaction kinetics differ from their macroscale analogs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Hydrogen-bond driven loop-closure kinetics in unfolded polypeptide chains

Characterization of the length dependence of end-to-end loop-closure kinetics in unfolded polypeptide chains provides an understanding of early steps in protein folding. Here, loop-closure in poly-glycine-serine peptides is investigated by combining single-molecule fluorescence spectroscopy with molecular dynamics simulation. For chains containing more than 10 peptide bonds loop-closing rate constants on the 20-100 nanosecond time range exhibit a power-law length dependence. However, this scaling breaks down for shorter peptides, which exhibit slower kinetics arising from a perturbation induced by the dye reporter system used in the experimental setup. The loop-closure kinetics in the longer peptides is found to be determined by the formation of intra-peptide hydrogen bonds and transient beta-sheet structure, that accelerate the search for contacts among residues distant in sequence relative to the case of a polypeptide chain in which hydrogen bonds cannot form. Hydrogen-bond-driven polypeptide-chain collapse in unfolded peptides under physiological conditions found here is not only consistent with hierarchical models of protein folding, that highlights the importance of secondary structure formation early in the folding process, but is also shown to speed up the search for productive folding events.

08 HYDROGEN↗