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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Nonorthogonal Configuration Interaction for Singlet Fission: Beyond the Dimer

Non-orthogonal configuration interaction with fragment calculations are presented for a number of compounds that show singlet fission properties: (i) four perylene-diimide derivatives, (ii) crystalline pentacene and its (B,N)-substituted variant, and (iii) a regular and a distorted stack of three indolonaphthyridine molecules. The electronic couplings between the singlet excitonic states (S 1 ) and the singlet-coupled double triplet (T 1 T 1 ), the so-called singlet fission coupling, were computed from ensembles with two and three molecules, and except for some small deviations when charge transfer states were included, results are virtually the same. Ensembles of three molecules were used to study the mechanisms of triplet separation, double triplet diffusion, and singlet and triplet exciton diffusion. The calculations show that apart from the standard mechanism for the generation of two uncoupled triplet states (S 1 → T 1 T 1 → T 1 ...T 1 ), there are two other possible pathways: the direct generation from the singlet excitonic state (S 1 → T 1 ...T 1 ) and the process in which the excitonic state evolves in a superposition of T 1 T 1 and T 1 ...T 1 states. Furthermore, the electronic coupling for triplet diffusion is in general much smaller than for singlet diffusion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2004

Determining the Thermal Properties of Space Lubricants

Many mechanisms used in spacecrafts, such as satellites or the space shuttle, employ ball bearings or gears that need to be lubricated. Normally this is not a problem, but in outer space the regular lubricants that are used on Earth will not function properly. Regular lubricants will quickly vaporize in the near vacuum of space. A unique liquid called a perfluoropolyalkylether (PFPE) has an extremely low vapor pressure, around l0(exp -10) torr at 20 C, and has been used in numerous satellites and is currently used in the space shuttle. Many people refer to the PFPEs as "liquid Teflon". PFPE lubricants however, have a number of problems with them. Lubricants need many soluble additives, especially boundary and anti-wear additives, in them to function properly. All the regular known boundary additives are insoluble in PFPEs and so PFPEs lubricate poorly under highly loaded conditions leading to many malfunctioning ball bearings and gears. JAXA, the Japanese Space Agency, is designing and building a centrifuge rotor to be installed in the International Space Station. The centrifuge rotor is part of a biology lab module. They have selected a PFPE lubricant to lubricate the rotor s ball bearings and NASA bearing experts feel this is not a wise choice. An assessment of the centrifuge rotor design is being conducted by NASA and part of the assessment entails knowing the physical and thermal properties of the PFPE lubricant. One important property, the thermal diffusivity, is not known. An experimental apparatus was set up in order to measure the thermal diffusivity of the PFPE. The apparatus consists of a constant temperature heat source, cylindrical Pyrex glassware, a thermal couple and digital thermometer. The apparatus was tested and calibrated using water since the thermal diffusivity of water is known.

Maldonado, Christina M.↗
DOE OSTI2026

The Cascade Effectiveness of 3-Terminal Tandem Photocathode Architectures as Applied to CO 2 Reduction

Cascade catalysis for photoelectrochemical CO 2 reduction (CO 2 R) decouples the overall reaction into sequential steps occurring on separately optimized catalysts (for example, Ag and Cu) between which an intermediate species such as CO is transferred. A 3-terminal tandem (3TT) photovoltaic architecture advantageously holds two different catalytic regions at different potentials under a single illumination source, but its overall efficiency is low. Using a stochastic reaction-diffusion model, we have examined 3TT photocathode design principles, focusing on the coupling of surface chemistry and transport of CO both inside the boundary layer and outward, into the bulk electrolyte. We find that ensuring that the lateral diffusion distance within the boundary layer is short compared to the boundary layer thickness and controlling bulk flow are key, with interdigitated designs showing an overall conversion efficiency improvement by 2 orders of magnitude compared to the side-by-side case without flow. These findings can be adapted for other cascaded architectures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

The Role of Oxygen Concentration in Enabling High Gradients in Niobium SRF Cavities

We studied the role of O concentration with depth in the performance of Nb SRF cavities. An ensemble of electropolished 1.3 GHz cavities, which initially showed high field Q-slope (HFQS), was subjected to sequential testing and treatment with in-situ low temperature baking at various temperatures. We find that increasing the bake duration causes (i) an increase in the onset of HFQS until it is absent up to quench (ii) a non-monotonic relationship with the quench field (iii) an evolution of the RBCS toward a non-equilibrium behavior that drives anti-Q slope. Our data is qualitatively explained by assuming an O diffusion model and suggests that the mitigation of HFQS that arises from 120 C in-situ LTB is mediated by the diffusion of O from the native oxide which prevents the precipitation of proximity-coupled Nb nano-hydrides, in turn enabling higher quench fields. The decrease in quench field for cavities in which O has been diffused >90 nm from the RF surface may be due to a reduction of the field limit in the SS bilayer structure. We also suggest that the evolution of the RBCS occurs due to the absence of proximity coupled inclusions, bringing about non-equilibrium effects.

Bafia, D.↗
DOE OSTI2022

Robust estimation of charge carrier diffusivity using transient photoluminescence microscopy

Transient microscopy has emerged as a powerful tool for imaging the diffusion of excitons and free charge carriers in optoelectronic materials. In many excitonic materials, extraction of diffusion coefficients can be simplified because of the linear relationship between signal intensity and local excited state population. However, in materials where transport is dominated by free charge carriers, extracting diffusivities accurately from multidimensional data is complicated by the nonlinear dependence of the measured signal on the local charge carrier density. To obtain accurate estimates of charge carrier diffusivity from transient microscopy data, statistically robust fitting algorithms coupled to efficient 3D numerical solvers that faithfully relate local carrier dynamics to raw experimental measurables are sometimes needed. Here, we provide a detailed numerical framework for modeling the spatiotemporal dynamics of free charge carriers in bulk semiconductors with significant solving speed reduction and for simulating the corresponding transient photoluminescence microscopy data. To demonstrate the utility of this approach, we apply a fitting algorithm using a Markov chain Monte Carlo sampler to experimental data on bulk CdS and methylammonium lead bromide (MAPbBr 3 ) crystals. Parameter analyses reveal that transient photoluminescence microscopy can be used to obtain robust estimates of charge carrier diffusivities in optoelectronic materials of interest, but that other experimental approaches should be used for obtaining carrier recombination constants. Additionally, simplifications can be made to the fitting model depending on the experimental conditions and material systems studied. Our open-source simulation code and fitting algorithm are made freely available to the scientific community.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Machine-learning-assisted deciphering of microstructural effects on ionic transport in composite materials: A case study of Li 7 La 3 Zr 2 O 12 -LiCoO 2

The effective diffusivity of ionic species in multiphase materials is critical for the design and function of composite materials for electrochemical energy storage. In practice, effective diffusivity depends sensitively not only on the intrinsic diffusivities of constituting materials but also on their topological arrangement; nevertheless, these coupled contributions are oversimplified in most analytical models. Here, we combine atomistically informed mesoscale modeling and machine learning (ML) analysis to unravel how such features affect effective diffusivity in two-phase composites. Using the Li 7 La 3 Zr 2 O 12 -LiCoO 2 composite solid-state battery cathode as a model system, we compute effective diffusivity for 600 distinct dense polycrystalline microstructures with different topological configurations of grains, grain boundaries, and heterointerfaces. We verify that in addition to atomic-scale variabilities, microstructural feature diversity can significantly impact effective transport properties. Across the ensemble of test microstructures, this often results in bimodal distributions of effective diffusivity that encompass two qualitatively distinct operating mechanisms, which we identify via flux analysis. An ML approach reveals that the most critical determining factors for effective diffusivity are the connectivity of bulk phases and their heterointerfaces. The role of ionic mobility at the heterointerfaces is also discussed. These insights highlight the combined importance of microstructure and interface engineering in tuning the transport properties of ionic species in composite materials. In conclusion, our framework can also be extended for understanding generic microstructure-property relationships in other complex multiphase materials.

25 ENERGY STORAGE↗
DOE OSTI2021

Incorporating point defect generation due to jog formation into the vector density-based continuum dislocation dynamics approach

During plastic deformation of crystalline materials, point defects such as vacancies and interstitials are generated by jogs on moving dislocations. A detailed model for jog formation and transport during plastic deformation was developed within the vector density-based continuum dislocation dynamics framework. Here as a part of this model, point defect generation associated with jog transport was formulated in terms of the volume change due to the non-conservative motion of jogs. Balance equations for the vacancies and interstitials including their rate of generation due to jog transport were also formulated. A two-way coupling between point defects and dislocation dynamics was then completed by including the stress contributed by the eigen-strain of point defects. A jog drag stress was further introduced into the mobility law of dislocations to account for the energy dissipation during point defects generation. A number of test problems and a fully coupled simulation of dislocation dynamics and point defects generation and diffusion were performed. The results show that there is an asymmetry of vacancy and interstitial generation due to the different formation energies of the two types of defects. The results also show that a higher hardening rate and a higher dislocation density are obtained when the point defect generation mechanism is coupled to dislocation dynamics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Recent Progress on Numerical Modeling for Microgravity Electric Field Flames: Preprint

This paper presents the recent progress on the NASA Physical Science Informatics (PSI) project PeleLM CFD of Ion Driven Winds from Diffusion Flames in simulating the E-FIELD Flames microgravity results. The focus of this project is to comprehensively simulate the behavior of a small diffusion flame under the influence of an externally applied electric field in zero-gravity. To date, the capability of accurately simulating electric field flames has eluded researchers because the system exhibits dramatic ranges of coupled temporal and spatial scales. Moreover, in earth gravity the hot combustion products are subject to buoyancy effects that are difficult to isolate from those generated by the electric field. This work is an implementation in an existing powerful simulation framework (PeleLM) for this problem using PeleLMeX in order to validate the model and investigate the complex coupled system. The current effort includes establishing the domain of a diffusion coflow burner, examining the boundary conditions, flame geometry and ignition with gravitational forces and also with an electric field applied. A detailed model is used that includes the chemistry of charged ions and chemiluminescent flame intermediates to capture any feedback between ion-driven convection and combustion behavior, and to allow quantitative comparisons with experimental measurements.

coflow flames↗
NASA NTRS1972

A study of the unidentified interstellar diffuse features.

The interstellar diffuse lines at 5780 and 5797 A have been observed in 66 stars using the Washburn Observatory echelle spectrograph coupled with an image tube. At a resolution of 0.17 A, the profiles of these diffuse lines are asymmetric and steeper to the blue. There is also a broad, shallow feature shortward of 5780 A. The FWHM of 5780 and 5797 are respectively 2.7 A and 1.1 A. Equivalent widths of 5780, 5797 have been measured at a resolution of 0.26 A, and they present a very tight correlation. It is very likely that these two diffuse lines are carried by the same agent. This investigation proposes that the two lines are produced by the pure electronic transitions of the impurity centers in solid grains, with the broad, shallow absorption feature shortward of 5780 A as the absorption sideband associated with the parent pure electronic line.

Wu, C.-C.↗
NASA NTRS1993

An Eulerian/Lagrangian coupling procedure for three-dimensional vortical flows

A coupled Eulerian/Lagrangian method is presented for the reduction of numerical diffusion observed in solutions of 3D vortical flows using standard Eulerian finite-volume time-marching procedures. A Lagrangian particle tracking method, added to the Eulerian time-marching procedure, provides a correction of the Eulerian solution. In turn, the Eulerian solution is used to integrate the Lagrangian state-vector along the particles trajectories. While the Eulerian solution ensures the conservation of mass and sets the pressure field, the particle markers describe accurately the convection properties and enhance the vorticity and entropy capturing capabilities of the Eulerian solver. The Eulerian/Lagrangian coupling strategies are discussed and the combined scheme is tested on a constant stagnation pressure flow in a 90 deg bend and on a swirling pipe flow. As the numerical diffusion is reduced when using the Lagrangian correction, a vorticity gradient augmentation is identified as a basic problem of this inviscid calculation.

Felici, Helene M.↗
NASA NTRS2009

A Study of Ablation-Flowfield Coupling Relevant to the Orion Heatshield

The coupled interaction between an ablating surface and the surrounding aerothermal environment is studied. An equilibrium ablation model is coupled to the LAURA flowfield solver, which allows the char ablation rate (m-dot(sub c)) to be computed as part of the flowfield solution. The wall temperature (T(sub w)) and pyrolysis ablation rate (m-dot(sub g)) may be specified by the user, obtained from the steady-state ablation approximation, or computed from a a material response code. A 32 species thermochemical nonequilibrium flowfield model is applied, which permits the treatment of C, H, O, N, and Si containing species. Coupled ablation cases relevant to the Orion heatshield are studied. These consist of diffusion-limited oxidation cases with Avcoat as the ablation material. The m-dot(sub c) values predicted from the developed coupled ablation analysis were compared with those obtained from a typical uncoupled ablation analysis. The coupled results were found to be as much as 50% greater than the uncoupled values. This is shown to be a result of the cumulative effect of the two fundamental approximations inherent in the uncoupled analysis.

Johnston, Christopher O.↗
NASA NTRS1994

A Coupled Soil-Atmosphere Model of H2O2 on Mars

The Viking Gas Chromatograph Mass Spectrometer failed to detect organic compounds on Mars, and both the Viking Labeled Release and the Viking Gas Exchange experiments indicated a reactive soil surface. These results have led to the widespread belief that there are oxidants in the martian soil. Since H2O2 is produced by photochemical processes in the atmosphere of Mars, and has been shown in the laboratory to reproduce closely the Viking LR results, it is a likely candidate for a martian soil oxidant. Here, we report on the results of a coupled soil/atmosphere transport model for H202 on Mars. Upon diffusing into the soil, its concentration is determined by the extent to which it is adsorbed and by the rate at which it is catalytically destroyed. An analytical model for calculating the distribution of H202 in the martian atmosphere and soil is developed. The concentration of H202 in the soil is shown to go to zero at a finite depth, a consequence of the nonlinear soil diffusion equation. The model is parameterized in terms of an unknown quantity, the lifetime of H202 against heterogeneous catalytic destruction in the soil. Calculated concentrations are compared with a H202 concentration of 30 nmoles/cu cm, inferred from the Viking Labeled Release experiment. A significant result of this model is that for a wide range of H202 lifetimes (up to 105 years), the extinction depth was found to be less than 3 m. The maximum possible concentration in the top 4 cm is calculated to be approx. 240 nmoles/cu cm, achieved with lifetimes of greater than 1000 years. Concentrations higher than 30 nmoles/cu cm require lifetimes of greater than 4.3 terrestrial years. For a wide range of H202 lifetimes, it was found that the atmospheric concentration is only weakly coupled with soil loss processes. Losses to the soil become significant only when lifetimes are less than a few hours. If there are depths below which H202 is not transported, it is plausible that organic compounds, protected from an oxidizing environment, may still exist. They would have been deposited by meteors, or be the organic remains of past life.

Bullock, Mark A.↗
NASA NTRS1978

Diffuse auroral precipitation

The paper summarizes the development of a plausibly coherent view of the self-consistent coupling of convection in the plasma sheet to auroral particle precipitation and the ionosphere. Attention is given to an understanding of the plasma instabilities responsible for diffuse auroral precipitation. The electrostatic ion and electron cyclotron harmonic loss-cone instabilities seem to be the best candidates, although they depend sensitively upon the cold electron density and temperature deep in space on auroral field lines. However, there is little or no experimental or theoretical information about these parameters. For this reason, further theoretical progress on cyclotron harmonic instabilities will be limited until the cold electron temperature is known. Also necessary are a complete phenomenological understanding of ion harmonic modes and a modeling of the cold electron distribution on auroral field lines.

Ashour-Abdalla, M.↗
DOE OSTI2020

Thermochemically-Informed Mass Transport Model for Zr in U-Zr Fuel

Recent improvements to the coupled Thermochimica-MOOSE/BISON code system have enabled efficient calculations of species transport based on direct evaluation of composition and temperature dependent chemical potentials of the species. This presents an alternative to the traditional approach to species transport in nuclear fuels, which has been to employ a diffusion formulation that combines concentration-gradient driven Fickian diffusion with a Soret term based on a heat of transport fit to experimental data. Here we describe the application of the coupled code system to the diffusion of Zr in U-Zr metallic fuel. New classes implemented in BISON to solve this problem are documented. The Zr concentration profile after 50 years of diffusion is found to be strongly dependent on the assumptions made pertaining how to mobility is calculated in multi-phase regions of the fuel element. Two assumptions are compared (simple averaging of mobilities and using the majority phase mobility), and good qualitative agreement with experimental measurements is obtained using the majority phase assumption.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS1985

Comparative magnetospheric/ionospheric studies using Dynamics Explorer spacecraft and ground-based radars

Coincident magnetosphere-ionosphere observations between the Dynamics Explorer 1 and 2 spacecraft with ground-based radar are reviewed. Ionospheric-magnetospheric coupling processes in the plasmasphere, main trough, auroral zone, and polar cap are inferred from the density, temperature, composition, and angular distributions of the low-energy plasma observed from the E-region ionosphere out into the magnetosphere to an altitude of 2.5 Re. From the polar cap to the inner plasmasphere, parallel and perpendicular electric fields, polar wind flow, and equilibrium diffusion appear to be involved in the ionospheric-magnetospheric coupling of the low energy plasma.

Green, J. L.↗
NASA NTRS1985

Comparative magnetospheric/ionospheric studies using Dynamics Explorer spacecraft and ground-based radars

Coincident observations between the Dynamics Explorer 1 and 2 spacecraft with Chatanika and Arecibo ground-based radar are briefly reviewed. Ionospheric-magnetospheric coupling processes in the plasmasphere, main trough, auroral zone, and polar cap are inferred from the density, temperature, composition, and angular distributions of the low-energy plasma observed from the E-region ionosphere out into the magnetosphere to an altitude of 2.5 earth radii (Re). From the polar cap to the inner plasmasphere, parallel and perpendicular electric fields, polar wind flow, and equilibrium diffusion appear to be involved in the ionospheric-magnetospheric coupling of the low energy plasma.

Green, J. L.↗
NASA NTRS2003

Oscillatory Extinction Of Spherical Diffusion Flames

Since extinction has been observed in an oscillatory manner in Le greater than 1 premixed flames, it is not unreasonable to expect that extinction could occur in an unsteady manner for diffusion flames. Indeed, near-limit oscillations have been observed experimentally under microgravity conditions for both candle flames and droplet flames. Furthermore, the analysis of Cheatham and Matalon on the unsteady behavior of diffusion flames with heat loss, identified an oscillatory regime which could be triggered by either a sufficiently large Lewis number (even without heat loss) or an appreciable heat loss (even for Le=1). In light of these recent understanding, the present investigation aims to provide a well-controlled experiment that can unambiguously demonstrate the oscillation of diffusion flames near both the transport- and radiation-induced limits. That is, since candle and jet flames are stabilized through flame segments that are fundamentally premixed in nature, and since premixed flames are prone to oscillate, there is the possibility that the observed oscillation of these bulk diffusion flames could be triggered and sustained by the oscillation of the premixed flame segments. Concerning the observed oscillatory droplet extinction, it is well-known that gas-phase oscillation in heterogeneous burning can be induced by and is thereby coupled with condensed-phase unsteadiness. Consequently, a convincing experiment on diffusion flame oscillation must exclude any ingredients of premixed flames and other sources that may either oscillate themselves or promote the oscillation of the diffusion flame. The present experiment on burner-generated spherical flames with a constant reactant supply endeavored to accomplish this goal. The results are further compared with those from computational simulation for further understanding and quantification of the flame dynamics and extinction.

Law, C. K.↗
NASA NTRS1998

Theoretical Evaluation of the Radiation Hazards from Cosmic Rays Within Space Vehicles

We may summarize our efforts as follows: a. Improvement of our calculations of the radial dose distribution from delta rays ejected in the passage of heavy ions through matter through the application of new data to a previous calculation by Kobetich and Katz (1968). Supplementing this calculation, we have found the radial distribution of electron energy spectra and the radial distribution of microdosimetric quantities (Cucinotta et al, 1996, 1997). b. Extension of the Katz model of cellular survival to bacteria, to lethal mutations in C. Elegans in vivo, to mutation induction in vitro, to thindown in radiobiology (observed experimentally at GSI, Darmstadt, and there called "Darmstadt hooks", predicted by Katz theory years before GSI was constructed). c. Coupling the Katz theory of RBE to the NASA theory of the diffusion of heavy ion beams in matter to yield predictions of the effects for monoenergetic heavy ion beams as well as range modulated beams used for cancer therapy. Here we have directed attention to the role of "ion-kill" (the effects produced by heavy ions passing through the nucleus of a cell), responsible for increased RBE, decreased OER, and reduced repair. We predict that the use of beams of heavy ions in cancer therapy will create late effect problems for fractionated therapy. We highlight also the damage by "ion-kill", from single heavy ions in the cosmic rays, to the central nervous system in space flight. d. The coupling of Katz theory and the NASA theory of heavy ion diffusion and penetration through matter, and knowledge of the space radiation environment, has been applied to design of shielding, to the cell damage in space flight.

Katz, Robert↗