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At least 271 records · Page 15

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NASA NTRS1974

Nuclei of planetary nebulae

The nuclei of planetary nebulae are examined both observationally and theoretically. It is seen that the region occupied by these stars in the logT-logL diagram is quite wide but consistent with a general progression of stars from high to low luminosity, with a noticeable but not large increase in luminosity during the early phase. The 'evolutionary path' is intrinsically quite wide and may indicate the evolution of stars under different conditions or non-monotonic passage along the mean path. Among the several theoretical approaches to this subject, only the double shell burning models seem to offer enough luminosity and short enough timescales to match the observations.

Odell, C. R.↗

DOE OSTI2021

Bulk nanocrystalline Al alloys with hierarchical reinforcement structures via grain boundary segregation and complexion formation

Grain size engineering, particularly reducing grain size into the nanocrystalline regime, offers a promising pathway to further improve the strength-to-weight ratio of Al alloys. Unfortunately, the fabrication of nanocrystalline metals often requires non-equilibrium processing routes, which typically limit the specimen size and require large energy budgets. In this study, multiple dopant elements in ternary Al alloys are deliberately selected to enable segregation to the grain boundary region and promote the formation of amorphous complexions. Three different fully dense bulk nanocrystalline Al alloys (Al-Mg-Y, Al-Fe-Y, and Al-Ni-Y) with small grain sizes were successfully fabricated using a simple powder metallurgy approach, with full densification connected directly to the onset of amorphous complexion formation. All the compositions demonstrate densities above 99% with grain sizes <60 nm following consolidation via hot pressing at 585 °C. The very fine grain structure results in excellent mechanical properties, as evidenced by nanoindentation hardness values in the range of 2.2-2.8 GPa. Detailed microstructural characterization verifies the segregation of all dopant species to grain boundaries as well as the formation of amorphous complexions, which suggests their influential role in aiding effective consolidation and endowing thermal stability in the alloys. Moreover, nanorods with a core-shell structure are also observed at the grain boundaries, which likely contribute to the stabilization of the grain structure while also strengthening the materials. Lastly, intermetallic particles with sizes of hundreds of nanometers form in all systems. As a whole, the results presented here demonstrate a general alloy design strategy of segregation and boundary evolution pathway that enables the fabrication of multiple nanocrystalline Al alloys with hierarchical microstructures and improved performance.

36 MATERIALS SCIENCE↗

DOE OSTI2025

In-Situ Atomic-Scale Revelation of Amorphous Metallic Iron Formation during Hydrogen-Driven Reduction of Iron Oxides

The transition to hydrogen as a green reductant in metal production is critical for decarbonizing the metallurgical industry, yet atomic-scale mechanisms governing reduction pathways and phase evolution remain unresolved. Using in-situ environmental transmission electron microscopy, we identify a hidden pathway that reveals dynamic formation of amorphous metallic iron (Fe) during the hydrogen-driven reduction of ferrous oxides of Fe 3 O 4 and FeO. Real-time imaging uncovers three coexisting transformation routes: (i) Fe 3 O 4 → FeO, (ii) Fe 3 O 4 → amorphous Fe, and (iii) FeO → amorphous Fe. The resulting amorphous Fe exhibits fluid-like mobility, enabling its rapid aggregation and crystallization into core-shell nanostructures, with a crystalline core enveloped by an amorphous shell. Complementary ab initio molecular dynamics simulations trace the amorphous Fe formation to interfacial strain at the metal/oxide interfaces, where large lattice mismatches destabilize the metal lattice during initial metallization. This interplay between thermodynamics and kinetics governs phase evolution: thermodynamics favors a self-limiting amorphous Fe overlayer, while rapid oxide reduction kinetics drives amorphous overgrowth. Our findings demonstrate that amorphous intermediates bypass rate-limiting crystalline steps, providing mechanistic insights to optimize H 2 -based processes for sustainable steelmaking. In conclusion, these insights bridge the gap between macroscopic process engineering and atomic-scale dynamics, with broader implications for catalysis and nanostructured material synthesis, where oxide reduction pathways critically shape functional phases and microstructures.

36 MATERIALS SCIENCE↗

NASA NTRS1989

Influence of heating mode on three-dimensional mantle convection

Numerical models of three-dimensional thermal convection in highly viscous spherical shells with different combinations of internal and basal heating consistently have upwelling concentrations in the form of cylindrical plumes and downwelling in planar sheets. As the proportion of internal heating increases, the number of upwelling plumes increases, and downwelling sheets become more vigorous and time-dependent. With any amount of basal heating, the entire convective pattern, during its evolution, is anchored to the upwelling plumes. As the proportion of internal heating increases, the heat flow carried by the upwelling plumes remains a large fraction of the basal heat flow. Downwelling sheets carry only a minor fraction (approximately 30 percent) of the basal heat flow (even when the shell is entirely heated from below), but they advect almost all of the internally generated heat. The relatively large number of plumes in the earth's mantle (inferred from hotspots), the possibility that downwelling slabs are vigorous enough to penetrate the lower mantle, and the small fraction of terrestrial surface heat flow carried by plumes all suggest that the mantle is predominantly heated from within.

Bercovici, D.↗

DOE OSTI2023

Type-I CdS/ZnS Core/Shell Quantum Dot-Gold Heterostructural Nanocrystals for Enhanced Photocatalytic Hydrogen Generation

Developing Type-I core/shell quantum dots is of great importance toward fabricating stable and sustainable photocatalysts. However, the application of Type-I systems has been limited due to the strongly confined photogenerated charges by the energy barrier originating from the wide-bandgap shell material. In this project, we found that through the decoration of Au satellite-type domains on the surface of Type-I CdS/ZnS core/shell quantum dots, such an energy barrier can be effectively overcome and an over 400-fold enhancement of photocatalytic H(2) evolution rate was achieved compared to bare CdS/ZnS quantum dots. Transient absorption spectroscopic studies indicated that the charges can be effectively extracted and subsequently transferred to surrounding molecular substrates in a subpicosecond time scale in such hybrid nanocrystals. Based on density functional theory calculations, the ultrafast charge separation rates were ascribed to the formation of intermediate Au2S layer at the semiconductor-metal interface, which can successfully offset the energy confinement introduced by the ZnS shell. In conclusion, our findings not only provide insightful understandings on charge carrier dynamics in semiconductor-metal heterostructural materials but also pave the way for the future design of quantum dot-based hybrid photocatalytic systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1976

The evolution of rotating stars. 1: Method and exploratory calculations for a 7 solar mass star

A method was developed which allows us to study the evolution of rotating stars beyond the main sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7 solar mass star, rotating according to these different cases, were computed from the ZAMS to the double shell source stage. Each sequence was begun with a (typical) equatorial velocity of 210 km/sec. On the main sequence, the effects of rotation are of minor importance. As the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution.

Endal, A. S.↗

NASA NTRS1983

The evolution of the infrared emission from the Type II supernova 1980k in NGC 6946 - The dust formation model

The paper presents 1-4 micron photometry of supernova 1980 k in NGC 6946 obtained over a period of 1 year following the outburst. During the period between 1980 November 1 and December 19, the infrared emission probably originated from the extended atmosphere of the expanding star. The JHKL colors and a 1.3-2.6-micron spectrum observed during this period correspond to those of a blackbody with an average temperature of about 5000 K. Observations around 1981 May 31 showed that the supernova developed an infrared excess after 1980 December. This infrared excess persisted through 1981 October and is consistent with the appearance of thermal emission from about 700 to 900 K dust in addition to a hotter photosphere. The similarity of this behavior to that of the infrared evolution of some novae suggests that dust formation may be occurring in the supernova ejecta. The hypothesis, that the emission arises from preexisting grains in a circumstellar shell which are heated by the supernova outburst, is also consistent with the data.

Dwek, E.↗

NASA NTRS1988

Stellar nucleosynthesis and chemical evolution of the solar neighborhood

Current theoretical models of nucleosynthesis (N) in stars are reviewed, with an emphasis on their implications for Galactic chemical evolution. Topics addressed include the Galactic population II red giants and early N; N in the big bang; star formation, stellar evolution, and the ejection of thermonuclearly evolved debris; the chemical evolution of an idealized disk galaxy; analytical solutions for a closed-box model with continuous infall; and nuclear burning processes and yields. Consideration is given to shell N in massive stars, N related to degenerate cores, and the types of observational data used to constrain N models. Extensive diagrams, graphs, and tables of numerical data are provided.

Clayton, Donald D.↗

NASA NTRS1993

Hydrodynamics of Relativistic Fireballs

Many models of gamma-ray bursts involve a fireball, which is an optically thick concentration of radiation energy with a high ratio of energy density to rest mass. We examine analytically and numerically the evolution of a relativistic fireball. We show that, after an early rearrangement phase, most of the matter and energy in the fireball is concentrated within a narrow shell. The shell propagates at nearly the speed of light, with a frozen radial profile, and according to a simple set of scaling laws. The spectrum of the escaping radiation is harder at early times and softer later on. Depending on the initial energy-to-mass ratio, the final outcome of a fireball is either photons with roughly the initial temperature or ultrarelativistic baryons. In the latter case, the energy could be converted back to gamma-rays via interaction with surrounding material.

Piran, Tsvi↗

NASA NTRS1977

Planetary magnetism and the interiors of the moon and Mercury

Various theories regarding lunar and Mercurian magnetic properties are discussed in terms of the thermal evolutions of these bodies. In particular, the extinct dynamo, the primordial field of external origin, local mechanisms, and the active dynamo hypotheses are reviewed. The theory involving magnetization by an internal dynamo is applied to Mercury, noting that it implies the existence of a molten metallic core, or shell. Possible sources of the energy required for core differentiation are discussed, including accretional heating, long-lived radioactive isotopes, and other radioactive heat sources. Thermal processes which might keep the core molten are suggested along with processes permitting the flow of heat through the mantle. Conclusions suggested by the dynamo hypothesis are reviewed in terms of current models of the thermal evolution of Mercury and the moon.

Cassen, P.↗

NASA NTRS1981

Convective overshooting in the evolution of very massive stars

Possible convective overshooting in stars of 30-120 solar masses are considered, including a merger between the convective core and the intermediate zone, and penetration by the outer convection zone into the hydrogen-shell region when the star is a supergiant. Convective mixing between the core and inner envelopes is found to lead to a brief renewal of hydrogen burning in the core, and a moderate widening of the main sequence bond in the H-R diagram. Deep penetration by the outer convection zone is found to force the star out of the red supergiant configuration and into a configuration near the main sequence. This would account for the apparent spread of the uppermost part of the main sequence and the concentration of luminous supergiants towards earlier spectral types. In addition, heavy mass loss need not be assumed to achieve the points of agreement, and are tentatively considered unimportant from an evolutionary point of view.

Stothers, R.↗

DOE OSTI2024

Hydrodynamical Simulations of Proton Ingestion Flashes in Type I X-Ray Bursts

We perform the first multidimensional fluid simulations of thermonuclear helium ignition underneath a hydrogen-rich shell. This situation is relevant to Type I X-ray bursts on neutron stars that accrete from a hydrogen-rich companion. Using the low-Mach number fluid code MAESTROeX, we investigate the growth of the convection zone due to nuclear burning, and the evolution of the chemical abundances in the atmosphere of the star. We also examine the convective boundary mixing processes that cause the evolution to differ significantly from previous one-dimensional simulations that rely on mixing-length theory. We find that the convection zone grows outward as penetrating fluid elements cool the overlying radiative layer, rather than directly from the increasing entropy of the convection zone itself. Simultaneously, these flows efficiently mix composition, carrying carbon out of and protons into the convection zone even before contact with the hydrogen shell. We discuss the implications of these effects for future modeling of these events and observations.

astrophysical fluid dynamics↗

DOE OSTI2020

Role of shell composition and morphology in achieving single-emitter photostability for green-emitting “giant” quantum dots

The use of the varied chemical reactivity of precursors to drive the production of a desired nanocrystal architecture has become a common method to grow thick-shell graded alloy quantum dots (QDs) with robust optical properties. Conclusions on their behavior assume the ideal chemical gradation and uniform particle composition. In this work, advanced analytical electron microscopy (high-resolution scanning transmission electron microscopy coupled with energy dispersive spectroscopy) is used to confirm the nature and extent of compositional gradation and these data are compared with performance behavior obtained from single-nanocrystal spectroscopy to elucidate structure, chemical-composition, and optical-property correlations. In particular, the evolution of the chemical structure and single-nanocrystal luminescence was determined for a time-series of graded-alloy “CdZnSSe/ZnS” core/shell QDs prepared in a single-pot reaction. In a separate step, thick (~6 monolayers) to giant (>14 monolayers) shells of ZnS were added to the alloyed QDs via a successive ionic layer adsorption and reaction (SILAR) process, and the impact of this shell on the optical performance was also assessed. By determining the degree of alloying for each component element on a per-particle basis, we observe that the actual product from the single-pot reaction is less “graded” in Cd and more so in Se than anticipated, with Se extending throughout the structure. The latter suggests much slower Se reaction kinetics than expected or an ability of Se to diffuse away from the initially nucleated core. It was also found that the subsequent growth of thick phase-pure ZnS shells by the SILAR method was required to significantly reduce blinking and photobleaching. However, correlated single-nanocrystal optical characterization and electron microscopy further revealed that these beneficial properties are only achieved if the thick ZnS shell is complete and without large lattice discontinuities. Thus, we identify the necessary structural design features that are required for ideal light emission properties in these green-visible emitting QDs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1977

Star dust

Recent infrared techniques have revealed that the dust which is a major constituent of the universe, is composed of refractory grains produced by certain classes of stars, condensed in their atmospheres and blown into interstellar space by the radiation pressure of these stars. In some cases stars are surrounded by dust shells which consist of carbon refractories in the case of a carbon-rich environment, and metallic silicates of the kind that produced terrestrial planets in the case of oxygen-rich environments. A few of these infrared stars (called cygnids) exhibit a unique morphology that suggests the formation of a planetary stage in the evolution of a planetary nebula. Comets which are bright in the infrared and believed to be the remnants of the most primitive material in the solar nebula, are found to inject the astrophysical dust into our solar system together with asteroidal debris. Certain novae are also found to condense grains which are blown out in their shells after the explosion.

Ney, E. P.↗

DOE OSTI2025

An Unconventional Dark Radical Chemistry in Dense Molecular Clouds: Directed Gas-Phase Formation of Naphthyl Radicals

The synthetic pathways to aromatic molecules inside photon-shielded dense molecular clouds remain a fundamental, unsolved enigma in astrochemistry and astrophysics, with low-temperature molecular growth routes involving aromatic radicals, such as prototype bicyclic naphthyl (C 10 H 7 • ), implicated as key sources. Here, exploiting crossed molecular beam experiments augmented by electronic structure calculations, unexpected pathways are exposed leading to the gas-phase formation of 1- and 2-naphthyl via barrierless bimolecular reactions of atomic carbon (C) with indene (C 9 H 8 ) and of dicarbon (C 2 ) with styrene (C 8 H 8 ) accompanied by ring expansion and cyclization together with aromatization. These facile routes challenge conventional wisdom that aromatic radicals are formed in deep space solely via “bright” gas-phase photochemistry of their closed-shell polycyclic aromatic hydrocarbon (PAH) precursors. A hitherto disregarded “dark” aromatic radical chemistry with aromatic radicals synthesized via gas-phase reactions offers new concepts on the chemical evolution of the chemistry of dark molecular clouds eventually culminating in the rapid formation of aromatics, fullerenes, and carbonaceous nanostructures.

Aromatic compounds↗

DOE OSTI2022

The Impact of Resolution on Double-detonation Models for Type Ia Supernovae

Abstract Thermonuclear supernovae are the result of the violent unbinding of a white dwarf (WD), but the precise nature of the explosion mechanism(s) is a matter of active debate. To this end, several specific scenarios have been proposed to explain the observable traits of Type Ia supernovae. A promising pathway is the double-detonation scenario, where a WD accretes a shell of helium-rich material from a companion and a detonation in the resulting helium shell is the primary cause of the explosion. Through a set of two-dimensional grid-based simulations of this scenario we clearly distinguish three phases of evolution: external helium-rich detonation, core compressive heating, and a final core carbon burn. Though final disruption of the whole system is achieved at all resolutions, only models with minimum resolutions of 4 km and better exhibit all three phases. Particularly, core compression detonation is only observed for higher resolutions, producing qualitatively different nucleosynthetic outcomes. We identify the effect of finer spatial resolution on the mixing of hot silicon at the interface between the detonating helium layer and the underlying C/O WD as a primary driver of these dynamic differences.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2026

On the Importance of the Convective Urca Process in 3D Simulations of a Simmering White Dwarf

Type Ia supernovae are bright thermonuclear explosions that are important to numerous areas of astronomy. However, the origins of these events are poorly understood. One proposed setting is that of a near Chandrasekhar mass white dwarf that undergoes runaway carbon burning in the core. During the thousand years leading up to the explosion, the white dwarf undergoes a simmering phase where slow carbon burning heats the core and drives convection. A poorly understood aspect of this phase is the convective Urca process, which links convection with weak nuclear reactions. We use the low Mach number code MAESTROeX to perform full 3D simulations as is required to accurately capture the turbulent convection. We present simulations with and without the A=23 convective Urca process, which have relaxed to a steady state. We characterize the effects of the convective Urca process on the neutrino losses, the nuclear energy generation, and the convective boundary. We find that the size of the convection zone is substantially reduced by the convective Urca process, though convection still extends past the Urca shell. Our findings on the structure of the convective zone and the compositional changes can be used to inform 1D stellar models that track the longer-timescale evolution.

FOS: Physical sciences↗

NASA NTRS2004

The Origin and Evolution of Deep Plasmaspheric Notches

Deep plasmaspheric notches can extend over more than 2 RE and 3 hours MLT in the magnetic equatorial plane. They can last for days and exhibit varying structure. In this presentation, the low L-shell portion of the recovery-time plasmaspheric convection plume will be explored as the source for these deep evacuations in density. Interactions with the ring current will be explored as the source of features observed in these density depletions.

Gallagher, D.↗