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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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Equation of state and transport properties of warm dense helium via quantum molecular dynamics simulations

The equation of state, self-diffusion, and viscosity coefficients of helium have been investigated by quantum molecular dynamics (QMD) simulations in the warm dense matter regime. Our simulations are validated through the comparison with the reliable experimental data. The calculated principal and reshock Hugoniots of liquid helium are in good agreement with the gas-gun data. On this basis, we revisit the issue for helium, i.e., the possibility of the instabilities predicted by chemical models at around 2000 GPa and 10 g/cm{sup 3} along the pressure isotherms of 6309, 15 849, and 31 623 K. Our calculations show no indications of instability in this pressure-temperature region, which reconfirm the predictions of previous QMD simulations. The self-diffusion and viscosity coefficients of warm dense helium have been systematically investigated by the QMD simulations. We carefully test the finite-size effects and convergences of statistics, and obtain numerically converged self-diffusion and viscosity coefficients by using the Kubo-Green formulas. The present results have been used to evaluate the existing one component plasma models. Finally, the validation of the Stokes-Einstein relationship for helium in the warm dense regime is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Replica-exchange molecular dynamics simulations of cellulose solvated in water and in the ionic liquid 1-butyl-3-methylimidazolium chloride

Ionic liquids have become a popular solvent for cellulose pretreatment in biorefineries due to their efficiency in dissolution and their reusability. Understanding the interactions between cations, anions, and cellulose is key to the development of better solvents and the improvement of pretreatment conditions. While previous studies described the interactions between ionic liquids and cellulose fibers, shedding light on the initial stages of the cellulose dissolution process, we study the end state of that process by exploring the structure and dynamics of a single cellulose decamer solvated in 1-butyl-3-methyl-imidazolium chloride (BmimCl) and in water using replica-exchange molecular dynamics. In both solvents, global structural features of the cellulose chain are similar. However, analyses of local structural properties show that cellulose explores greater conformational variability in the ionic liquid than in water. For instance, in BmimCl the cellulose intramolecular hydrogen bond O3H'••• O5 is disrupted more often resulting in greater flexibility of the solute. Our results indicate that the cellulose chain is more dynamic in BmimCl than in water, which may play a role in the favorable dissolution of cellulose in the ionic liquid. Here, the calculation of the configurational entropy of the cellulose decamer confirms its higher conformational flexibility in BmimCl than in water at elevated temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Fragile to strong crossover at the Widom line in supercooled aqueous solutions of NaCl

We study by molecular dynamics simulations the dynamical properties of an aqueous solution of NaCl at a concentration of 0.67 mol/kg upon supercooling. In a previous study of the same ionic solution, we have located the liquid-liquid critical point (LLCP) and determined the Widom line connected to the liquid-liquid transition. We present here the results obtained from the study of the self-intermediate scattering function in a large range of temperatures and densities approaching the LLCP. The structural relaxation is in agreement with the mode coupling theory (MCT) in the region of mild supercooling. In the deeper supercooled region the α-relaxation time as function of temperature deviates from the MCT power law prediction showing a crossover from a fragile to a strong behavior. This crossover is found upon crossing the Widom line. The same trend was found in bulk water upon supercooling and it appears almost unchanged by the interaction with ions apart from a shift in the thermodynamic plane toward lower pressures and higher temperatures. These results show that the phenomenology of supercooled water transfers from bulk to solution where the study of the supercooled region is experimentally less difficult.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1978

Effects of kinetics on the crystallization of quartz normative basalt 15597 - An experimental study

Equilibrium experiments on a residual liquid composition from the dynamic crystallization study on quartz normative basalt (QNB) 15597 are used to evaluate the effects of kinetics on the crystallization of pyroxene-porphyritic basalts. The results indicate that the liquids coexisting with pyroxene phenocryst rims before the precipitation of groundmass plagioclase + pyroxene have left the equilibrium liquid line of descent and metastably penetrated the plagioclase primary phase volume. The precipitation of groundmass plagioclase + pyroxene brings the basalt system closer to equilibrium with respect to the residual liquid and crystals. At rapid cooling rates pyroxene phenocryst growth produces substantial compositional gradients in the residual silicate liquid at the pyroxene/liquid interface. The compositional gradients enhance the nucleation and growth of groundmass phases at the phenocryst interface, because effects related to constitutional supercooling for these phases (e.g., plagioclase, pyroxene, etc.) favor nucleation at the interface.

Grove, T. L.↗
NASA NTRS1982

Strain rate characterization of unidirectional graphite/epoxy composite

For an investigation of problems of composites at high strain rates with dominant wave propagation effects, such as dynamically loaded composite jet engine blades, the longitudinal, transverse, and in-plane shear properties of unidirectional SP 288/AS graphite/epoxy were measured at various strain rates ranging from quasi-static to over 500/s. The test specimens were rings 10.16 cm in diameter, 2.54 cm long, 6 to 8 plies thick, and with fibers at 0, 90, and 10 deg off-axis. Quasi-static testing was conducted in a fixture which applies hydraulic pressure to the ring specimens, and dynamic testing was conducted by applying an internal pressure pulse through a liquid in a special fixture. Dynamic stress-strain curves are presented. For the 0-deg specimens, the modulus increases with strain up to 20 percent of the static value. The modulus and strength increase sharply in the 90-deg rings with strain rate reaching values two to three times the corresponding static rate.

Daniel, I. M.↗
DOE OSTI2020

Thermodynamics–Structure–Dynamics Correlations and Nonuniversal Effects in the Elastically Collective Activated Hopping Theory of Glass-Forming Liquids

We employ the microscopic Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of activated dynamics in combination with crystal-avoiding simulations to study four inter-related questions for metastable monodisperse hard sphere fluids. The first is how significantly improved integral equation theory structural input (Modified-Verlet (MV) closure) changes the dynamical predictions of ECNLE theory. The main consequence is a modest enhancement of the importance of the collective elastic barrier relative to its local cage contribution, which increases the alpha relaxation time and fragility relative to prior results based on the Percus–Yevick closure. Second, ECNLE-MV theory predictions for the alpha time and self-diffusion constant in the metastable regime are quantitatively compared to our new simulations. The small adjustment of a numerical prefactor that enters the collective elastic barrier leads to quantitative agreement over three decades. Third, using the more accurate MV structural input, ECNLE theory is shown to predict thermodynamics–structure–dynamics “correlations” based on various long and short wavelength scalar properties all related to static two-point collective density fluctuations. The logarithm of the alpha relaxation time scales as a power law with these scalar metrics with an exponent that is significantly lower in the less dense noncooperative activated regime compared to the very dense highly cooperative regime. However, the discovered correlation of activated relaxation with a thermodynamic property (dimensionless compressibility) is not causal in ECNLE theory, but rather reflects a strong connection between the local structural quantities that quantify kinetic constraints in the theory with the amplitude of long wavelength density fluctuations. Fourth, the consequences of chemically specific nonuniversalities associated with the onset condition and relative importance of collective elasticity are studied. In conclusion, the predicted thermodynamics–structure–dynamics correlations are found to be robust, albeit with nontrivial shifts of the onset condition.

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

Hydrogen bond dynamics in bulk alcohols

Hydrogen-bonded liquids play a significant role in numerous chemical and biological phenomena. In the past decade, impressive developments in multidimensional vibrational spectroscopy and combined molecular dynamics–quantum mechanical simulation have established many intriguing features of hydrogen bond dynamics in one of the fundamental solvents in nature, water. The next class of a hydrogen-bonded liquid—alcohols—has attracted much less attention. This is surprising given such important differences between water and alcohols as the imbalance between the number of hydrogen bonds, each molecule can accept (two) and donate (one) and the very presence of the hydrophobic group in alcohols. Here, we use polarization-resolved pump-probe and 2D infrared spectroscopy supported by extensive theoretical modeling to investigate hydrogen bond dynamics in methanol, ethanol, and isopropanol employing the OH stretching mode as a reporter. The sub-ps dynamics in alcohols are similar to those in water as they are determined by similar librational and hydrogen-bond stretch motions. However, lower density of hydrogen bond acceptors and donors in alcohols leads to the appearance of slow diffusion-controlled hydrogen bond exchange dynamics, which are essentially absent in water. We anticipate that the findings herein would have a potential impact on fundamental chemistry and biology as many processes in nature involve the interplay of hydrophobic and hydrophilic groups.

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

Structure, dynamics, and electrochemistry of choline chloride/ethylene glycol eutectic solvents at an electrode surface explored by molecular dynamics simulations

Choline chloride and ethylene glycol mixtures with 1:2, 1:4, and 1:6 molar ratios on the surfaces of graphite and gold electrodes were studied using classical molecular dynamics simulations. Both neutral and charged electrodes were considered. The liquid composition, solvation structure, molecular orientation, and dynamics at the electrode surface are significantly different from those of the bulk liquid. These properties strongly depend on the electrode material and charge density, whereas they are less sensitive to the overall solvent composition. The effect of the electrode on the composition, structure, and orientation of the liquid fades beyond ∼10 Å from the surface of the electrode. This distance corresponds to about two layers of the structured solvent, despite the fact that the layered structure extends to at least five layers or about 25 Å. However, the electrode influences solvent dynamics over a longer distance. The electrochemical properties of the eutectic solvent at both electrode surfaces were also studied. The simulations captured the experimental differential capacitance shapes for both electrode systems, although the magnitudes and exact shapes differ. The simulations further revealed that the solvent in the first solvation layer can both overscreen and underscreen the electrode charges depending on the electrode material and electrode potential.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

Superfluid helium sloshing dynamics induced oscillations and fluctuations of angular momentum, force and moment actuated on spacecraft driven by gravity gradient or jitter acceleration associated with slew motion

The generalized mathematical formulation of sloshing dynamics for partially filled liquid of cryogenic superfluid helium II in dewar containers driven by the gravity gradient and jitter accelerations associated with slew motion for the purpose to perform scientific observation during the normal spacecraft operation are investigated. An example is given with the Advanced X-Ray Astrophysics Facility-Spectroscopy (AXAF-S) for slew motion which is responsible for the sloshing dynamics. The jitter accelerations include slew motion, spinning motion, atmospheric drag on the spacecraft, spacecraft attitude motions arising from machinery vibrations, thruster firing, pointing control of spacecraft, crew motion, etc. Explicit mathematical expressions to cover these forces acting on the spacecraft fluid systems are derived. The numerical computation of sloshing dynamics is based on the non-inertia frame spacecraft bound coordinate, and solve time-dependent, three-dimensional formulations of partial differential equations subject to initial and boundary conditions. The explicit mathematical expressions of boundary conditions to cover capillary force effect on the liquid-vapor interface in microgravity environments are also derived. The formulations of fluid moment and angular moment fluctuations in fluid profiles induced by the sloshing dynamics, together with fluid stress and moment fluctuations exerted on the spacecraft dewar containers have also been derived. Examples are also given for cases applicable to the AXAF-S spacecraft sloshing dynamics associated with slew motion.

Hung, R. J.↗
DOE OSTI2014

In-silico investigation of Rayleigh instability in ultra-thin copper nanowire in premelting regime

Motivated by the recent experimental reports, we explore the formation of Rayleigh-like instability in metallic nanowires during the solid state annealing, a concept originally introduced for liquid columns. Our molecular dynamics study using realistic interatomic potential reveals instability induced pattern formation at temperatures even below the melting temperature of the wire, in accordance with the experimental observations. We find that this is driven by the surface diffusion, which causes plastic slips in the system initiating necking in the nanowire. We further find the surface dominated mass-transport is of subdiffusive nature with time exponent less than unity. Our study provides an atomistic perspective of the instability formation in nanostructured solid phase.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1993

Sloshing dynamics on rotating helium dewar tank

The generalized mathematical formulation of sloshing dynamics for partially filled liquid of cryogenic superfluid helium II in dewar containers driven by both the gravity gradient and jitter accelerations applicable to scientific spacecraft which is eligible to carry out spinning motion and/or slew motion for the purpose to perform scientific observation during the normal spacecraft operation are investigated. An example is given with Gravity Probe-B (GP-B) spacecraft which is responsible for the sloshing dynamics. The jitter accelerations include slew motion, spinning motion, atmospheric drag on the spacecraft, spacecraft attitude motions arising from machinery vibrations, thruster firing, pointing control of spacecraft, crew motion, etc. Explicit mathematical expressions to cover these forces acting on the spacecraft fluid systems are derived. The numerical computation of sloshing dynamics were based on the non-inertia frame spacecraft bound coordinate, and solve time dependent, three-dimensional formulations of partial differential equations subject to initial and boundary conditions. The explicit mathematical expressions of boundary conditions to cover capillary force effect on the liquid vapor interface in microgravity environments are also derived. The formulations of fluid moment and angular moment fluctuations in fluid profiles induced by the sloshing dynamics, together with fluid stress and moment fluctuations exerted on the spacecraft dewar containers were derived. Results were widely published in the open journals.

Hung, R. J.↗
DOE OSTI2018

Formation of the Excess Free Volume in Triple Junctions during Nickel Crystallization

The formation of an excess free volume in triple junctions during crystallization has been studied by the molecular dynamics model using nickel as an example. It is shown that an excess free volume that forms during nickel crystallization in triple junctions predominantly forms as a result of the fixation of the liquid phase volume when contacting three crystallization fronts that contains, after crystallization, a high fraction of the free volume. In some cases, as the free volume is concentrated in triple junctions, a comparatively small crystalline subgrain (from one to several nanometers in diameter) forms, and the subgrain has the orientation different from those of contacting grains and exists in the extended state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

V-T theory for the Self-Intermediate Scattering Function in a Monatomic Liquid

In V-T theory the atomic motion is harmonic vibrations in a liquid-specific potential energy valley, plus transits, which move the system rapidly among the multitude of such valleys. Here, in its first application to the self intermediate scattering function (SISF), V-T theory produced an accurate account of molecular dynamics (MD) data at all wave numbers q and time t. Recently, analysis of the mean square displacement (MSD) resolved a crossover behavior that was not observed in the SISF study. Our purpose here is to apply the more accurate MSD calibration to the SISF, and assess the results. We derive and discuss the theoretical equations for vibrational and transit contributions to the SISF. The time evolution is divided into three successive intervals: the vibrational interval when the vibrational contribution alone accurately accounts for the MD data; the crossover when the vibrational contribution saturates and the transit contribution becomes resolved; and the diffusive interval when the transit contribution alone accurately accounts for the MD data. Finally, the resulting theoretical error is extremely small at all q and t. V-T theory is compared to mode-coupling theories for the MSD and SISF, and to recent developments in Brownian motion experiments and theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Numerical experiment of thermal conductivity in two-dimensional Yukawa liquids

A newly improved homogenous nonequilibrium molecular dynamics simulation (HNEMDS) method, proposed by the Evans, has been used to compute the thermal conductivity of two-dimensional (2D) strongly coupled complex (dusty) plasma liquids (SCCDPLs), for the first time. The effects of equilibrium external field strength along with different system sizes and plasma states (Γ, κ) on the thermal conductivity of SCCDPLs have been calculated using an enhanced HNEMDS method. A simple analytical temperature representation of Yukawa 2D thermal conductivity with appropriate normalized frequencies (plasma and Einstein) has also been calculated. The new HNEMDS algorithm shows that the present method provides more accurate results with fast convergence and small size effects over a wide range of plasma states. The presented thermal conductivity obtained from HNEMDS method is found to be in very good agreement with that obtained through the previously known numerical simulations and experimental results for 2D Yukawa liquids (SCCDPLs) and with the three-dimensional nonequilibrium molecular dynamics simulation (MDS) and equilibrium MDS calculations. It is shown that the HNEMDS algorithm is a powerful tool, making the calculations very efficient and can be used to predict the thermal conductivity in 2D Yukawa liquid systems.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1992

Effect of asymmetric gravity jitter excited slosh waves at liquid-vapor interface under microgravity

The dynamical behavior of fluids affected by the asymmetric gravity jitter oscillations, in particular the effect of surface tension on partially-filled rotating fluids (cryogenic liquid helium and helium vapor) in a sub-scale Gravity Probe-B Spacecraft propellant dewar tank imposed by time-dependent various directions of background gravity environment have been investigated. Results show that lower frequency gravity jitter imposed on the time-dependent variations of the direction of background gravity induced a greater amplitude of oscillations and a stronger degree of asymmetry in liquid-vapor interface geometry than that made by the higher frequency gravity jitter. Furthermore, the greater the components of background gravity in radial and circumferential directions will provide a greater contribution in driving more to the increasing amplitude and degrees of symmetry of liquid-vapor interface profiles which, in turn, modify the disturbance of moment of inertia and angular momentum of spacecraft.

Hung, R. J.↗
DOE OSTI2016

Structural and Rotational Dynamics of Carbon Dioxide in 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids: The Effect of Chain Length

Ionic liquids (ILs) have been proposed as possible carbon dioxide (CO 2 ) carbon capture media, and so it is useful to understand the dynamics of both the dissolved gas and its IL environment as well as how altering the IL affects these dynamics. With increasing alkyl chain length, it is well established that ILs obtain a mesoscopic structural feature assigned to polarapolar segregation, and the change in structure with chain length affects dynamics. In this work, the dynamics of CO 2 in a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs), where alkyl is ethyl, butyl, hexyl, or decyl, were investigated using ultrafast infrared spectroscopy by measuring the reorientation and spectral diffusion of carbon dioxide in the ILs. It was found that reorientation of the carbon dioxide occurs on 3 timescales, which correspond to 2 different timescales of restricted wobbling-in-a-cone motions and a long time complete diffusive reorientation. The complete reorientation slows with increasing chain length but less than the increases in viscosity of the bulk liquids. Spectral diffusion, measured with 2D IR spectroscopy, is caused by a combination of the liquids’ structural fluctuations and reorientation of the CO 2 . The data were analyzed using recent theory that takes into account both contributions to spectral diffusion and extracts the structural spectral diffusion. Different components of the structural fluctuations have distinct dependences on the alkyl chain length. All of the dynamics are fast compared to the complete orientational randomization of the bulk ILs, as measured with optical heterodyne detected optical Kerr effect measurements. The results indicate a hierarchy of constraint releases in the liquids that give rise to increasingly slower dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Radiolysis and Radiation-Driven Dynamics of Boehmite Dissolution Observed by In Situ Liquid-Phase TEM

We report over the last several decades, there have been several studies examining the radiation stability of boehmite and other aluminum oxyhydroxides, yet less is known about the impact of radiation on boehmite dissolution. Here, we investigate radiation effects on the dissolution behavior of boehmite by employing liquid-phase transmission electron microscopy (LPTEM) and varying the electron flux on the samples consisting of either single nanoplatelets or aggregated stacks. We show that boehmite nanoplatelets projected along the [010] direction exhibit uniform dissolution with a strong dependence on the electron dose rate. For nanoplatelets that have undergone oriented aggregation, we show that the dissolution occurs preferentially at the particles at the ends of the stacks that are more accessible to bulk solution than at the others inside the aggregate. In addition, at higher dose rates, electrostatic repulsion and knock-on damage from the electron beam causes delamination of the stacks and dissolution at the interfaces between particles in the aggregate, indicating that there is a threshold dose rate for electron-beam enhancement of dissolution of boehmite aggregates.

54 ENVIRONMENTAL SCIENCES↗