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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Adjoint DSMC for nonlinear spatially-homogeneous Boltzmann equation with a general collision model

We derive an adjoint method for the Direct Simulation Monte Carlo (DSMC) method for the spatially homogeneous Boltzmann equation with a general collision law. This generalizes our previous results in Caflisch et al., which was restricted to the case of Maxwell molecules, for which the collision rate is constant. The main difficulty in generalizing the previous results is that a rejection sampling step is required in the DSMC algorithm in order to handle the variable collision rate. We find a new term corresponding to the so-called score function in the adjoint equation and a new adjoint Jacobian matrix capturing the dependence of the collision parameter on the velocities. The new formula works for a much more general class of collision models.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Fabrication of homogeneous titania/MWNT composite materials

Highlights: {yields} Homogenous titania coverage on MWNT surface in a controllable way. {yields} Various titanium alkoxy precursors are suitable for layer formation. {yields} Acetone and ethanol are the best to promote interaction between MWNT and titania. -- Abstract: MWNT/titania nanocomposites were prepared by an impregnation method and subsequent heat treatment at 400 {sup o}C. Precursor compounds such as titanium (IV) propoxide and titanium (IV) ethoxide were used to cover the surface of CNTs under solution conditions. Electron microscopy and X-ray diffraction techniques were carried out to characterize the as-prepared titania layers.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Phase-separation induced homogeneous nucleation and growth of Cs{sub 3}LaCl{sub 6} nanoparticles in chalcohalide glass

Graphical abstract: The phase-separation induced nucleation and growth of Cs{sub 3}LaCl{sub 6} nanocrystals has been studied for the first time. It is experimentally evidenced that the doped active rare earth ions are incorporated into the Cs{sub 3}LaCl{sub 6} nanocrystals, resulting in the intensified down- and up-conversion emissions. - Highlights: • A novel transparent glass ceramic containing Cs{sub 3}LaCl{sub 6} nanocrystals was fabricated. • Crystallization behaviors of Cs{sub 3}LaCl{sub 6} nanophase are systematically investigated. • The up-/down-conversion emissions are greatly intensified after crystallization. - Abstract: The phase-separation induced nucleation and growth of Cs{sub 3}LaCl{sub 6} nanocrystals has been studied in the GeS{sub 2}-Ga{sub 2}S{sub 3}-La{sub 2}S{sub 3}-LaCl{sub 3}-CsCl system for the first time. Remarkably, the precipitated chloride nanocrystals are spherical and distributed homogeneously in the glass matrix. Benefiting from the uniform structure, high transparency of the glass ceramic is maintained after heat treatment by reducing adverse optical scattering. As revealed by the absorption spectra and Judd–Ofelt calculations, the doped active rare earth ions are partially partitioned into the low-phonon-energy Cs{sub 3}LaCl{sub 6} nanocrystals, resulting in the decrease of the non-radiative transition probabilities and therefore intensified photoluminescence emission of Nd{sup 3+} and up-conversion one of Er{sup 3+}.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Comparison of pencil beam–based homogeneous vs inhomogeneous target dose planning for stereotactic body radiotherapy of peripheral lung tumors through Monte Carlo–based recalculation

This study was conducted to ascertain whether homogeneous target dose planning is suitable for stereotactic body radiotherapy (SBRT) of peripheral lung cancer under appropriate breath-holding. For 20 peripheral lung tumors, paired dynamic conformal arc plans were generated by only adjusting the leaf margin to the planning target volume (PTV) edge for fulfilling the conditions such that the prescription isodose surface (IDS) encompassing exactly 95% of the PTV (PTV D{sub 95}) corresponds to 95% and 80% IDS, normalized to 100% at the PTV isocenter under a pencil beam (PB) algorithm with radiologic path length correction. These plans were recalculated using the x-ray voxel Monte Carlo (XVMC) algorithm under otherwise identical conditions, and then compared. Lesions abutting the parietal pleura or not were defined as edge or island tumors, respectively, and the influences of the target volume and its location relative to the chest wall on the target dose were examined. The median (range) leaf margin required for the 95% and 80% plans was 3.9 mm (1.3 to 5.0) and −1.2 mm (−1.8 to 0.1), respectively. Notably, the latter was significantly correlated negatively with PTV. In the 80% plans, the PTV D{sub 95} was slightly higher under XVMC, whereas the PTV D{sub 98} was significantly lower, irrespective of the dose calculation algorithm used. Other PTV and all gross tumor volume doses were significantly higher, while the lung doses outside the PTV were slightly lower. The target doses increased as a function of PTV and were significantly lower for island tumors than for edge tumors. In conclusion, inhomogeneous target dose planning using smaller leaf margin for a larger tumor volume was deemed suitable in ensuring more sufficient target dose while slightly reducing lung dose. In addition, more inhomogeneous target dose planning using <80% IDS (e.g., 70%) for PTV covering would be preferable for island tumors.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2025

Geant4 simulations of sampling and homogeneous hadronic calorimeters with dual readout for future colliders

Calorimeters with dual readout measure both scintillation and Cherenkov light produced in their active media. They offer improvements in energy resolution and, therefore, have become increasingly interesting due to the need for precision jet measurements at Higgs factories. Furthermore, this paper presents GEANT4 simulations of single-particle responses in sampling and homogeneous calorimeters, and demonstrates the effect of inclusion of Cherenkov light in the reconstruction of energies.

Detector modeling and simulations↗
DOE OSTI2021

Microstructure homogenization of concrete used in nuclear power plants

Almost all nuclear power plants in the United States are operating past their intended lifetimes or are requesting lifetime extensions. Therefore, understanding changes to the concrete containment structure over time is crucial to evaluate the structure’s continued viability. Concrete materials are heterogeneous particulate composites that exhibit viscoelastic material properties, which can lead to slow deformation over time, causing stress redistribution and the potential for creep cracking. A code to generate random, three dimensional (3D) concrete microstructures has been developed and paired with finite element analysis to predict the long-term viscoelastic properties of concrete. Data from these simulations are used to develop constitutive equations for the viscoelastic behavior of the homogenized concrete. The codes in this work are used to virtualize laboratory experiments, to obtain long-term creep data in a faster, cheaper manner. To validate this work, the simulated creep behavior of concrete is compared to 800 d of experimental data that has been extended to 27 y of data using the Time-Temperature superposition (TTS) principal. Excellent agreement between the simulation results and experimental data is seen.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Thermal fluctuations in the dissipation range of homogeneous isotropic turbulence

In this work, using fluctuating hydrodynamics we investigate the effect of thermal fluctuations in the dissipation range of homogeneous isotropic turbulence. Simulations confirm theoretical predictions that the energy spectrum is dominated by these fluctuations at length scales comparable to the Kolmogorov length. We also find that the extreme intermittency in the far-dissipation range predicted by Kraichnan is replaced by Gaussian thermal equipartition.

42 ENGINEERING↗
DOE OSTI2026

Is there an exact magnetic moment for charged particle motion in a time-dependent, homogeneous magnetic field?

The non-perturbative guiding-centre model provides an exact alternative to full-orbit simulations of charged particle dynamics in situations where traditional guiding-centre theory may fail. We demonstrate that the charged particle motion in a homogeneous, time-varying magnetic field is a solvable example of the non-perturbative guiding-centre model. This entails showing that the exact magnetic moment of Qin and Davidson can be constructed to be asymptotic to the adiabatic invariant series of Kruskal. In contrast to the perturbative invariant, the exact invariant contains information about parametric resonances. These resonances destroy the conservation of the usual magnetic moment over very long times. This refutes some previous claims about the all-time invariance of the magnetic moment.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Bioconversion of Homogeneous Linear C-Lignin to Polyhydroxyalkanoates

Here, the bioconversion of homogeneous linear catechyl lignin (C-lignin) to polyhydroxyalkanoates (PHA) was examined for the first time in this study. C-lignins from vanilla, euphorbia, and candlenut seed coats (denoted as C1, C2, and C3, respectively) varied in their molecular structures, which showed different molecular weight distributions, etherification degrees, and contents of hydroxyl groups. A notable amount of nonetherified catechol units existed within C1 and C2 lignins, and these catechol units were consumed during fermentation. These results suggested that the nonetherified catechol structure was readily converted by Pseudomonas putida KT2440. Since the weight-average molecular weight of C2 raw lignin was 26.7% lower than that of C1, the bioconversion performance of C2 lignin was more outstanding. The P. putida KT2440 cell amount reached the maximum of 9.3 × 10 7 CFU/mL in the C2 medium, which was 37.9 and 82.4% higher than that in the C1 and C3 medium, respectively. Accordingly, PHA concentration reached 137 mg/L within the C2 medium, which was 41.2 and 149.1% higher than the C1 and C3 medium, respectively. Overall, C-lignin, with a nonetherified catechol structure and low molecular weight, benefits its microbial conversion significantly.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Crystal Growth and Elemental Homogeneity of the Multicomponent Rare-Earth Garnet (Lu 1/6 Y 1/6 Ho 1/6 Dy 1/6 Tb 1/6 Gd 1/6 ) 3 Al 5 O 12

We report high-entropy aluminum garnets were grown as bulk single crystals using the micro-pulling-down method, taking the synthesis of complex ceramics a step further from the conventional preparation of polycrystalline materials. We studied the effects of growth parameters on the elemental distribution in high optical quality crystals of (Lu 1/6 Y 1/6 Ho 1/6 Dy 1/6 Tb 1/6 Gd 1/6 ) 3 Al 5 O 12 containing six cations (yttrium and rare-earths) taken in equimolar amounts. A single garnet structure was confirmed by powder X-ray diffraction. Electron microprobe measurements were obtained to correlate the radial distribution of rare-earth elements with pulling rates and molten zone height. The nature of the elemental distribution in the radial direction was associated with ionic radius: smaller rare-earths concentrated in the center of the crystal, while larger rare-earths segregated toward the outer edge of the cylindrical crystal. Faster pulling rates led to a flattening of the concentration profiles toward the nominal concentration, promoting a more homogeneous radial elemental distribution, while varying the molten zone height did not have a significant effect. The demonstrated success with crystal growth enables the practical availability of single crystals of multicomponent aluminum garnets for further discovery of new phenomena and applications.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. Finally, the details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Predicting the Rate of Homogeneous Intermetallic Nucleation within Steep Composition Gradients

Simulations of isothermal homogeneous nucleation from deeply undercooled amorphous melts exhibit systematic variations in nucleation behavior depending upon the strength of an imposed composition gradient. Data from molecular dynamics (MD) simulations in a model Ni/Al system permit quantification of the nucleation rate of the NiAl-B2 intermetallic phase and indicate that nucleation proceeds in a polymorphous mode. The nucleation rate decreases with increasing gradient, and nucleation is completely suppressed above a critical gradient. Based on an argument of crystal nucleus stability, a simple estimate provides good prediction of the critical gradient. Here, a modified classical nucleation model parameterized with thermodynamic and kinetic quantities calculated independently predicts the nucleation rates and their variation with the imposed gradient, matching the MD results very well.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Evaluation of Chemical and Structural Homogeneity in Single Particles of Li 1-x Ni 0.33 Mn 0.33 Co 0.33 0 2

Identifying irreversible phenomena at single particles of battery cathode materials is an important step to understanding degradation pathways upon cycling. LiNi 0.33 Mn 0.33 Co 0.33 O 2 (LiNMC(111)) single particles of size 200 x 300 nm were studied before and after cycling using Scanning X-ray Diffraction Microscopy (SXDM) and ptychographic microscopy. Here, with resolutions of 30 and 5 nm, respectively, it was possible to map the (003) reflection and nickel oxidation state changes in single particles at different states of charge. While the compositions measured by the c lattice parameter from SXDM were found to be homogeneous, mapping at higher resolution using ptychography revealed that Ni oxidation states decrease sharply 0-25 nm from the edge, then trail into a steady oxidation state toward the interior of the particle. Within the 30 nm length scale, we conclude that LiNi 0.33 Mn 0.33 Co 0.33 O 2 single particles are highly structurally reversible.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Light-Driven Iodine Loss and Photoluminescence Homogenization in Mixed-Halide Perovskite Semiconductors

Carrier-induced instabilities in lead halide perovskites are often investigated as either transient phenomena, e.g., photoinduced halide segregation or permanent performance changes, e.g., photodegradation, while the mechanistic links between them remain unclear. Here, we aim to connect these observations by studying a model mixed-halide system, MAPb(Br x I 1–x ) 3 . By combining grazing-incidence X-ray diffraction, photothermal deflection spectroscopy, and photoluminescence measurements with hyperspectral microscopy, we investigate the role of mobile halide defects and local chemistry on reversible and long-term instabilities in these materials. Our results show that mixed-halide perovskites are uniquely susceptible to photoinduced changes with illumination driving initial iodide redistribution (i.e., halide segregation), eventual selective iodine expulsion, and subsequent changes in photoinduced halide segregation behavior. By quantifying structural and compositional changes, we estimate an approximately 3–5% iodine loss in our mixed-halide samples after only 24 h of illumination. Further, using microscale measurements, we identify pre-existing iodide-rich domains as key contributors to both the observed transient photostability and permanent iodine loss in MAPbBrI 2 , and see evidence that extended light soaking results in iodide redistribution that improves optoelectronic homogeneity. Overall, our results emphasize the importance of carrier-induced halide oxidation in creating a dynamic defect landscape in mixed-halide perovskites and provide a framework for interpreting apparent light-driven changes in optoelectronic behavior through the lens of permanent compositional changes.

electrical conductivity↗
DOE OSTI2017

β-SiH-containing tris(silazido) rare-earth complexes as homogeneous and grafted single-site catalyst precursors for hydroamination

A series of homoleptic rare-earth silazido compounds and their silica-grafted derivatives were prepared to compare spectroscopic and catalytic features under homogeneous and interfacial conditions. Trivalent tris(silazido) compounds Ln{N(SiHMe 2 ) t Bu} 3 (Ln = Sc (1), Y (2), Lu (3)) are prepared in high yield by salt metathesis reactions. Solution-phase and solid-state characterization of 1–3 by NMR and IR spectroscopy and X-ray diffraction reveals Ln←H–Si interactions. These features are retained in solvent-coordinated 2·Et 2 O, 2·THF, and 3·THF. The change in spectroscopic features characterizing the secondary interactions (ν SiH , 1 J SiH ) from the unactivated SiH in the silazane HN(SiHMe 2 ) t Bu follows the trend 3 > 2 > 1 ≈ 2·Et 2 O > 2·THF ≈ 3·THF. Ligand lability follows the same pattern, with Et 2 O readily dissociating from 2·Et 2 O while THF is displaced only during surface grafting reactions. 1 and 2·THF graft onto mesoporous silica nanoparticles (MSN) to give Ln{N(SiHMe 2 ) t Bu} n @MSN (Ln = Sc (1@MSN), Y (2@MSN)) along with THF and protonated silazido as HN(SiHMe 2 ) t Bu and H 2 N t Bu. The surface species are characterized by multinuclear and multidimensional solid-state (SS) NMR spectroscopic techniques, as well as diffuse reflectance FTIR, elemental analysis, and reaction stoichiometry. A key 1 J SiH SSNMR measurement reveals that the grafted sites most closely resemble Ln·THF adducts, suggesting that siloxane coordination occurs in grafted compounds. These species catalyze the hydroamination/bicyclization of aminodialkenes, and both solution-phase and interfacial conditions provide the bicyclized product with equivalent cis:trans ratios. As a result, similar diastereoselectivities mediated by catalytic sites under the two conditions suggest similar effective environments.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Scaling-Relation-Based Analysis of Bifunctional Catalysis: The Case for Homogeneous Bimetallic Alloys

Here, we present a generic analysis of the implications of energetic scaling relations on the possibilities for bifunctional gains at homogeneous bimetallic alloy catalysts. Such catalysts exhibit a large number of interface sites, where second-order reaction steps can involve intermediates adsorbed at different active sites. Using different types of model reaction schemes, we show that such site-coupling reaction steps can provide bifunctional gains that allow for a bimetallic catalyst composed of two individually poor catalyst materials to approach the activity of the optimal monomaterial catalyst. However, bifunctional gains cannot result in activities higher than the activity peak of the monomaterial volcano curve as long as both sites obey similar scaling relations, as is generally the case for bimetallic catalysts. These scaling-relation-imposed limitations could be overcome by combining different classes of materials such as metals and oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Uniform, Homogenous Coatings of Carbon Nanohorns on Arbitrary Substrates from Common Solvents

ABSTRACT: We demonstrate a facile technique to electro- phoretically deposit homogenous assemblies of single-walled carbon nanohorns (CNHs) from common solvents such as acetone and water onto nearly any substrate including insulators, dielectrics, and three-dimensional metal foams, in many cases without the aid of surfactants. This enables the generation of pristine film-coatings formed on time scales as short as a few seconds and on three-dimensional templates that enable the formation of freestanding polymer-CNH supported materials. As electrophoretic deposition is usually only practical on conductive electrodes, we emphasize our observation of efficient deposition on nearly any material, including nonconductive substrates. The one-step versatility of deposition on these materials provides the capability to directly assemble CNH materials onto functional surfaces for a broad range of applications. In this manner, we utilized as-deposited CNH films as conductometric gas sensors exhibiting better sensitivity in comparison to equivalent single-walled carbon nanotube sensors. This gives a route toward scalable and inexpensive solution-based processing routes to manufacture functional nanocarbon materials for catalysis, energy, and sensing applications, among others.

Oakes, Landon↗