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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 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Baryogenesis and gravitational waves from runaway bubble collisions

We propose a novel mechanism for production of baryonic asymmetry in the early Universe. The mechanism takes advantage of the strong first order phase transition that produces runaway bubbles in the hidden sector that propagate almost without friction with ultra-relativistic velocities. Collisions of such bubbles can non-thermally produce heavy particles that further decay out-of-equilibrium into the SM and produce the observed baryonic asymmetry. This process can proceed at the very low temperatures, providing a new mechanism of post-sphaleron baryogenesis. In this paper we present a fully calculable model which produces the baryonic asymmetry along these lines as well as evades all the existing cosmological constraints. We emphasize that the Gravitational Waves signal from the first order phase transition is completely generic and can potentially be detected by the future eLISA interferometer. We also discuss other potential signals, which are more model dependent, and point out the unresolved theoretical questions related to our proposal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Influence of Copper Composition on Cu 2 BaSn(S,Se) 4 Solution-Deposited Films and Photovoltaic Devices with Over 5% Efficiency

Cu 2 BaSn(S,Se) 4 is currently in the spotlight for prospective environmentally friendly, stable, thin-film solar cell application, with demonstrated device power conversion efficiency (PCE) exceeding 5% for vacuum-deposited absorbers. As suggested by first-principles calculations, experimental studies involving related Cu 2 ZnSn(S,Se) 4 and Cu(In,Ga)(S,Se) 2 absorbers prove that the detailed chemical composition typically plays a sensitive role in altering defects and electronic properties of these complicated compound semiconductors. Herein, the copper composition of Cu 2 BaSn(S,Se) 4 has been systematically modified, employing a solution-based deposition approach, to provide a more complete picture of the phase stability and optoelectronic property sensitivity for this material. X-ray diffraction and scanning electron microscopy show that phase purity is preserved over a film Cu content range of nominally 0.94 ≤ [Cu]/[Ba + Sn] ≤ 1.01. Terahertz spectroscopy and Hall effect measurements reveal that the majority carrier hole density of ~10 13 cm –3 and mobility (~5 cm 2 /V s), as well as the minority carrier lifetime (a bulk lifetime of 180 ps and a surface recombination velocity >106 cm/s), are nominally independent of Cu content. The champion PCEs exceed 4.7% for all copper compositions in the phase-pure region, with a record value of 5.1%, similar to the reported values for record vacuum-deposited devices. Furthermore, these results suggest that Cu 2 BaSn(S,Se) 4 films and solar cells (at the current performance level) may be less sensitive to Cu stoichiometry compared to kesterite materials and therefore may provide a more stable material platform to prepare thin-film solar cells.

(CBTSSe)↗
DOE OSTI2022

A Phase-Field Model for In-Space Manufacturing of Binary Alloys

The integrity of the final printed components is mostly dictated by the adhesion between the particles and phases that form upon solidification, which is a major problem in printing metallic parts using available In-Space Manufacturing (ISM) technologies based on the Fused Deposition Modeling (FDM) methodology. Understanding the melting/solidification process helps increase particle adherence and allows to produce components with greater mechanical integrity. We developed a phase-field model of solidification for binary alloys. The phase-field approach is unique in capturing the microstructure with computationally tractable costs. The developed phase-field model of solidification of binary alloys satisfies the stability conditions at all temperatures. The suggested model is tuned for Ni-Cu alloy feedstocks. We derived the Ginzburg-Landau equations governing the phase transformation kinetics and solved them analytically for the dilute solution. We calculated the concentration profile as a function of interface velocity for a one-dimensional steady-state diffuse interface neglecting elasticity and obtained the partition coefficient, k, as a function of interface velocity. Numerical simulations for the diluted solution are used to study the interface velocity as a function of undercooling for the classic sharp interface model, partitionless solidification, and thin interface.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structural transitions and hysteresis in clump- and stripe-forming systems under dynamic compression

In using numerical simulations, we study the dynamical evolution of particles interacting via competing long-range repulsion and short-range attraction in two dimensions. The particles are compressed using a time-dependent quasi-one dimensional trough potential that controls the local density, causing the system to undergo a series of structural phase transitions from a low density clump lattice to stripes, voids, and a high density uniform state. The compression proceeds via slow elastic motion that is interrupted with avalanche-like bursts of activity as the system collapses to progressively higher densities via plastic rearrangements. The plastic events vary in magnitude from small rearrangements of particles, including the formation of quadrupole-like defects, to large-scale vorticity and structural phase transitions. In the dense uniform phase, the system compresses through row reduction transitions mediated by a disorder-order process. We also characterize the rearrangement events by measuring changes in the potential energy, the fraction of sixfold coordinated particles, the local density, and the velocity distribution. At high confinements, we find power law scaling of the velocity distribution during row reduction transitions. We observe hysteresis under a reversal of the compression when relatively few plastic rearrangements occur. The decompressing system exhibits distinct phase morphologies, and the phase transitions occur at lower compression forces as the system expands compared to when it is compressed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Acoustic anomalies in SrTiO{sub 3}−BiFeO{sub 3} solid solutions

The results of acoustic investigations of solid solutions SrTiO{sub 3}−BiFeO{sub 3} in the temperature range from 100 to 650 K have been presented. The measurements of the velocity and attenuation of the longitudinal ultrasonic mode at a frequency of 10 MHz were carried out by the pulse-echo method. The observed anomalies in velocity and attenuation correlate with the maxima of the dielectric constant in the temperature range of the relaxor state. In addition, the attenuation peaks in the temperature range 400–600 K, which define Burns and T* temperatures, which are characteristic of relaxors, have been identified. The obtained results have allowed the clarifying of the phase diagram of the solid solution system SrTiO{sub 3}−BiFeO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Graphite Dust Deposition on HTGR Steam Generator: Effect of Particle-wall Impact - 18520

Generated by abrasion of fuel spheres, graphite dust is an important issue for the operation and maintenance of high-temperature gas-cooled reactor (HTGR), since the transport of fission product is closely coupled with graphite dusts. AVR experiments on dust specific radioactivity indicate that graphite dust deposition on steam generator (SG) should be of special interest. Most previous simulations on graphite dust deposition assume a hit-and-stick nature for particles, which may overestimate the deposition rate. In this work we uses a critical velocity model to account for particle-wall interaction. Combined with computational fluid dynamics simulation and discrete phase method, the deposition of graphite dust in the upper 22 layers of SG tubes was studied with particle-wall interaction considered. The result shows that the deposition rate is highly overestimated if hit-and-stick is assumed. For particles larger than 5 {mu}m, the difference can be as large as 3 times. Particle deposition rate reaches maximum at about 3 {mu}m. As for the spatial distribution, few particles deposit on the first layer of tubes due to relative low Stokes number while more depositions occur after entering bundle zone, which indicates that the unsteady wake among tubes could be a major mechanism for the deposition. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

Controlling the stability of both the structure and velocity of domain walls in magnetic nanowires

For magnetic nanowire devices, the precise control of both domain wall (DW) motion and pinning behaviour is essential for reliable functional performance. The domain wall velocity and wall structure are typically sensitive to the driving field or spin-polarized current, and the pinning behaviour depends on the walls' structure and chirality, leading to variability in behaviour. Here, a systematic study combining experimental measurements and micromagnetic simulations of planar nanowires with small fixed-angle structural modulations on both edges was undertaken to study the domain wall reversal regime. A phase diagram for the reversal field as a function of modulation amplitude was obtained that shows that three DW reversal regime. A range of field and modulation amplitudes were identified in which stable DW reversal occurs, where the wall velocity is constant as a function of field and the wall structure is stable, which is well suited to applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Evaluation of interfacial area transport models for horizontal bubbly flow

This paper presents evaluation results of interfacial area transport equation (IATE) for horizontal air-water bubbly flow. The steady-state one-dimensional, one-group IATE for adiabatic air-water bubbly flow developed by Talley [1] is employed in the current evaluation study. The models evaluated include: (a) drift-flux-based closure relations for void-weighted bubble velocity and void fraction; (b) frictional pressure drop model based on the Lockhart-Martinelli approach; (c) bubble interaction mechanisms; and (d) covariance associated with asymmetric bubble distribution in horizontal two-phase flow. In order to evaluate the models, a local two-phase flow database obtained by the four-sensor conductivity probe in fourteen different test conditions is employed, which covers a wide range of flow conditions in horizontal bubbly flow regime, with superficial liquid and gas velocities ranging from 3.50-6.00 m/s and 0.09- 1.00 m/s, respectively. It is demonstrated that both the void weighted bubble velocity and void fraction are predicted well with an average percent difference of ±2.7%. The fictional pressure loss can be predicted with an average percent difference of ±1.12%. The covariance parameters calculated from the modeling approach generally compare well with those calculated from experimental data within ±20% difference. It is found that the IATE generally predicts the measured interfacial area concentration with an average percent difference of ±5.7%. Furthermore, the effect of liquid velocity on individual bubble interaction mechanisms is investigated. It is found that both the contributions of random collision and turbulent impact decrease as liquid velocity decreases. (authors)

42 ENGINEERING↗
DOE OSTI2015

Theory of dark matter superfluidity

We introduce a novel theory of dark matter (DM) superfluidity that matches the successes of the Λ cold dark matter ( Λ CDM ) model on cosmological scales while simultaneously reproducing the modified Newtonian dynamics (MOND) phenomenology on galactic scales. The DM and MOND components have a common origin, representing different phases of a single underlying substance. DM consists of axionlike particles with mass of order eV and strong self-interactions. The condensate has a polytropic equation of state P ~ ρ 3 giving rise to a superfluid core within galaxies. Instead of behaving as individual collisionless particles, the DM superfluid is more aptly described as collective excitations. Superfluid phonons, specifically, are assumed to be governed by a MOND-like effective action and mediate a MONDian acceleration between baryonic matter particles. Our framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not); due to the higher velocity dispersion in clusters, and correspondingly higher temperature, the DM in clusters is either in a mixture of superfluid and the normal phase or fully in the normal phase. The rich and well-studied physics of superfluidity leads to a number of observational signatures: an array of low-density vortices in galaxies; merger dynamics that depend on the infall velocity vs phonon sound speed; distinct mass peaks in bulletlike cluster mergers, corresponding to superfluid and normal components; and interference patters in supercritical mergers. Remarkably, the superfluid phonon effective theory is strikingly similar to that of the unitary Fermi gas, which has attracted much excitement in the cold atom community in recent years. The critical temperature for DM superfluidity is of order mK, comparable to known cold atom Bose–Einstein condensates. Identifying a precise cold atom analog would give important insights on the microphysical interactions underlying DM superfluidity. Tantalizingly, it might open the possibility of simulating the properties and dynamics of galaxies in laboratory experiments.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The dependence of Ammonium-Nitrate Fuel-Oil (ANFO) detonation on confinement

As detonation is a coupled fluid-chemical process, flow divergence inside the detonation reaction zone can strongly influence detonation velocity and energy release. Such divergence is responsible for the diameter-effect and failure-diameter phenomena in condensed-phase explosives and particularly dominant in detonation of nonideal explosives such as Ammonium Nitrate and Fuel Oil (ANFO). In this study, the effect of reaction zone flow divergence on ANFO detonation was explored through variation of the inert confinement and explosive diameter in the rate-stick geometry with cylinder expansion experiments. New tests are discussed and compared to prior experiments. Presented results include the detonation velocity as a function of diameter and confinement, reaction zone times, detonation product isentropes and energies, as well as sonic surface pressures and velocities. Product energy densities and isentropes were found to increase with detonation velocity, indicating more complete chemical reaction with increased detonation velocity. In addition, detonation reaction zone times were found to scale with the acoustic transit time of the confiner wall and used to show that the ANFO diameter effect scaled with the reaction zone time for a particle along the flow centerline, regardless of the confinement. Such a result indicates that the ANFO reaction mechanisms are sufficiently slow that the centerline fluid expansion timescale is a limiting factor controlling detonation velocity and energy release.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

INVESTIGATION OF THE FORMATION AND SEPARATION OF AN EXTREME-ULTRAVIOLET WAVE FROM THE EXPANSION OF A CORONAL MASS EJECTION

We address the nature of EUV waves through direct observations of the formation of a diffuse wave driven by the expansion of a coronal mass ejection (CME) and its subsequent separation from the CME front. The wave and the CME on 2011 June 7 were well observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Following the solar eruption onset, marked by the beginning of the rapid increasing of the CME velocity and the X-ray flux of accompanying flare, the CME exhibits a strong lateral expansion. During this impulsive expansion phase, the expansion speed of the CME bubble increases from 100 km s{sup -1} to 450 km s{sup -1} in only six minutes. An important finding is that a diffuse wave front starts to separate from the front of the expanding bubble shortly after the lateral expansion slows down. Also a type II burst is formed near the time of the separation. After the separation, two distinct fronts propagate with different kinematic properties. The diffuse front travels across the entire solar disk, while the sharp front rises up, forming the CME ejecta with the diffuse front ahead of it. These observations suggest that the previously termed EUV wave is a composite phenomenon and driven by the CME expansion. While the CME expansion is accelerating, the wave front is cospatial with the CME front, thus the two fronts are indiscernible. Following the end of the acceleration phase, the wave moves away from the CME front with a gradually increasing distance between them.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

EVIDENCE FOR A GAS-RICH MAJOR MERGER IN A PROTO-CLUSTER AT z = 2.5

Gas-rich major mergers in high-redshift proto-clusters are important events, perhaps leading to the creation of the slowly rotating remnants seen in the cores of clusters in the present day. Here, we present a deep Jansky Very Large Array observation of CO J = 1-0 emission line in a proto-cluster at z = 2.5, USS1558-003. The target field is an extremely dense region, where 20 Hα emitters (HAEs) are clustering. We have successfully detected the CO emission line from three HAEs and discovered a close pair of red and blue CO-emitting HAEs. Given their close proximity (∼30 kpc), small velocity offset (∼300 km s{sup –1}), and similar stellar masses, they could be in the early phase of a gas-rich major merger. For the red HAE, we derive a total infrared luminosity of L {sub IR} = 5.1 × 10{sup 12} L {sub ☉} using MIPS 24 μm and radio continuum images. The L{sub IR}/L{sub CO}{sup ′} ratio is significantly enhanced compared to local spirals and high-redshift disks with a similar CO luminosity, which is indicative of a starburst mode. We find the gas depletion timescale is shorter than that of normal star-forming galaxies regardless of adopted CO-H{sub 2} conversion factors. The identification of such a rare event suggests that gas-rich major mergers frequently take place in proto-clusters at z > 2 and may involve the formation processes of slow rotators seen in local massive clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Predictive process simulation of cryogenic implants for leading edge transistor design

Two cryogenic implant TCAD-modules have been developed: (i) A continuum-based compact model targeted towards a TCAD production environment calibrated against an extensive data-set for all common dopants. Ion-specific calibration parameters related to damage generation and dynamic annealing were used and resulted in excellent fits to the calibration data-set. (ii) A Kinetic Monte Carlo (kMC) model including the full time dependence of ion-exposure that a particular spot on the wafer experiences, as well as the resulting temperature vs. time profile of this spot. It was calibrated by adjusting damage generation and dynamic annealing parameters. The kMC simulations clearly demonstrate the importance of the time-structure of the beam for the amorphization process: Assuming an average dose-rate does not capture all of the physics and may lead to incorrect conclusions. The model enables optimization of the amorphization process through tool parameters such as scan speed or beam height.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Spherical ion oscillations in a positive polarity gridded inertial-electrostatic confinement device

A pulsed, positive polarity gridded inertial electrostatic confinement device has been investigated experimentally, using a differential emissive probe and potential traces as primary diagnostics. Large amplitude oscillations in the plasma current and plasma potential were observed within a microsecond of the discharge onset, which are indicative of coherent ion oscillations about a temporarily confined excess of recirculating electron space charge. The magnitude of the depth of the potential well in the established virtual cathode was determined using a differential emissive Langmuir probe, which correlated well to the potential well inferred from the ion oscillation frequency for both hydrogen and argon experiments. It was found that the timescale for ion oscillation dispersion is strongly dependent on the neutral gas density, and weakly dependent on the peak anode voltage. The cessation of the oscillations was found to be due to charge exchange processes converting ions to high velocity neutrals, causing the abrupt de-coherence of the oscillations through an avalanche dispersion in phase space.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Simultaneous use of camera and probe diagnostics to unambiguously identify and study the dynamics of multiple underlying instabilities during the route to plasma turbulence

We use multiple-tip Langmuir probes and fast imaging to unambiguously identify and study the dynamics of underlying instabilities during the controlled route to fully-developed plasma turbulence in a linear magnetized helicon plasma device. Langmuir probes measure radial profiles of electron temperature, plasma density and potential; from which we compute linear growth rates of instabilities, cross-phase between density and potential fluctuations, Reynold's stress, particle flux, vorticity, time-delay estimated velocity, etc. Fast imaging complements the 1D probe measurements by providing temporally and spatially resolved 2D details of plasma structures associated with the instabilities. We find that three radially separated plasma instabilities exist simultaneously. Density gradient driven resistive drift waves propagating in the electron diamagnetic drift direction separate the plasma into an edge region dominated by strong, velocity shear driven Kelvin-Helmholtz instabilities and a central core region which shows coherent Rayleigh-Taylor modes propagating in the ion diamagnetic drift direction. The simultaneous, complementary use of both probes and camera was crucial to identify the instabilities and understand the details of the very rich plasma dynamics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Inviscid evolution of large amplitude filaments in a uniform gravity field

The inviscid evolution of localized density stratifications under the influence of a uniform gravity field in a homogeneous, ambient background is studied. The fluid is assumed to be incompressible, and the stratification, or filament, is assumed to be initially isotropic and at rest. It is shown that the center of mass energy can be related to the center of mass position in a form analogous to that of a solid object in a gravity field g by introducing an effective gravity field g{sub eff}, which is less than g due to energy that goes into the background and into non-center of mass motion of the filament. During the early stages of the evolution, g{sub eff} is constant in time and can be determined from the solution of a 1D differential equation that depends on the initial, radially varying density profile of the filament. For small amplitude filaments such that ρ{sub 0} ≪ 1, where ρ{sub 0} is the relative amplitude of the filament to the background, the early stage g{sub eff} scales linearly with ρ{sub 0}, but as ρ{sub 0}→∞, g{sub eff}→g and is thus independent of ρ{sub 0}. Fully nonlinear simulations are performed for the evolution of Gaussian filaments, and it is found that the time t{sub max}, which is defined as the time for the center of mass velocity to reach its maximum value U{sub max}, occurs very soon after the constant acceleration phase and so U{sub max}≈g{sub eff}(t=0)t{sub max}. The simulation results show that U{sub max}∼1/t{sub max}∼√(ρ{sub 0}) for ρ{sub 0} ≪ 1, in agreement with theory and results from previous authors, but that U{sub max} and t{sub max} both scale approximately with √(ρ{sub 0}) for ρ{sub 0} ≫ 1. The fact that U{sub max} and t{sub max} have the same scaling with ρ{sub 0} for large amplitude filaments is in agreement with the theory presented in this paper.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Optimal driving of Bose-Einstein condensates in optical cavities

We apply quantum optimal control to enhance the performance of the experimental setup of Ref. [1], speeding up the system dynamics at time scales of the order of the quantum speed limit. We perform a fast crossing of the quantum phase transition the system undergoes under realistic experimental conditions, and we present a new scaling-based strategy to compute optimal pulses for systems at the thermodynamical limit.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Cold melting of beryllium: Atomistic view on Z-machine experiments

Analysis of phase diagram of beryllium at high pressures and temperatures obtained as a result of ab initio calculations and large scale classical molecular dynamics simulations of beryllium shock loading have shown that the so called cold melting takes place when shock wave propagates through polycrystalline samples. Comparison of ab initio calculation results on sound speed along the Hugoniot with experimental data obtained on Z-machine also evidences for possible manifestation of the cold melting. The last may explain the discrepancy between atomistic simulations and experimental data on the onset of the melting on the Hugoniot.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗