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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Predicting metal–metal interactions. I. The influence of strain on nanoparticle and metal adlayer stabilities

Strain-engineering of bimetallic nanomaterials is an important design strategy for developing new catalysts. In this paper, we introduce an approach for including strain effects into a recently introduced, density functional theory (DFT)-based alloy stability model. The model predicts adsorption site stabilities in nanoparticles and connects these site stabilities with catalytic reactivity and selectivity. Strain-based dependencies will increase the model’s accuracy for nanoparticles affected by finite-size effects. In addition to the stability of small nanoparticles, strain also influences the heat of adsorption of epitaxially grown metal-on-metal adlayers. In this respect, we successfully benchmark the strain-including alloy stability model with previous experimentally determined trends in the heats of adsorption of Au and Cu adlayers on Pt (111). For these systems, our model predicts stronger bimetallic interactions in the first monolayer than monometallic interactions in the second monolayer. We explicitly quantify the interplay between destabilizing strain effects and the energy gained by forming new metal–metal bonds. While tensile strain in the first Cu monolayer significantly destabilizes the adsorption strength, compressive strain in the first Au monolayer has a minimal impact on the heat of adsorption. Hence, this study introduces and, by comparison with previous experiments, validates an efficient DFT-based approach for strain-engineering the stability, and, in turn, the catalytic performance, of active sites in bimetallic alloys with atomic level resolution.

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

General microscopic model of magnetoelastic coupling from first principles

Magnetoelastic coupling, i.e., the change of crystal lattice induced by a spin order, is not only scientifically interesting, but also technically important. In this work, we propose a general microscopic model from firstprinciples calculations to describe the magnetoelastic coupling and provide a way to construct the microscopic model from density functional theory calculations. Based on this model, we reveal that there exists a previously unexpected contribution to the electric polarization induced by the spin order in multiferroics due to the combined effects of magnetoelastic coupling and piezoelectric effect. Interestingly and surprisingly, we find that this latticedeformation contribution to the polarization is even larger than that from the pure electronic and ion-displacement contributions in BiFeO 3 . We conclude this model of magnetoelastic coupling can be generally applied to investigate the other magnetoelastic phenomena.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

A modeling framework for optimizing current density in four-terminal tandem solar cells: A case study on GaAs/Si tandem

Here we present a modeling framework to quantify current density losses and determine optimization steps for four-terminal tandem solar cells, which consist of a high-bandgap thin-film top cell and an industrial-type Si wafer bottom cell. We demonstrate the framework on the example of a 21.3% efficient stacked four-terminal GaAs/Si tandem solar cell. In a first step, we develop an optical model for stacked tandem configuration and carry out the current loss analysis for this tandem device. Photon absorption in the tandem is broken down into different channels. All parasitic absorptions that do not contribute to current generation are considered part of a loss channel. In a second step, we quantify the potential current gains from single-parameter optimizations, the redistribution of photon losses into other channels, and the synergistic gains that can be achieved by co-optimization. Finally, using the simulation model, we are able to determine the best sequence of optimizing the short-circuit current density in the GaAs/Si tandem solar cell to possibly achieve 37.8 mA/cm 2 with experimentally feasible parameters.

14 SOLAR ENERGY↗
DOE OSTI2022

Microstructure in matrix elements

We investigate the simple model of Pennington, Shenker, Stanford and Yang for modeling the density matrix of Hawking radiation, but further include dynamics for EOW branes behind the horizon. This allows interactions that scatter one interior state to another, and also allows EOW loops. At strong coupling, we find that EOW states are no longer random; the ensemble has collapsed, and coupling constants encode the microscopic matrix elements of Hawking radiation. This suggests strong interior dynamics are important for understanding evaporating black holes, without any ensemble average. In this concrete model the density matrix of the radiation deviates from the thermal state, small off-diagonal fluctuations encode equivalences between naively orthogonal states, and bound the entropy from above. For almost evaporated black holes the off-diagonal terms become as large as the diagonal ones, eventually giving a pure state. We also find the unique analytic formula for all Renyi entropies.

2D gravity↗
DOE OSTI2015

One-dimensional time-dependent fluid model of a very high density low-pressure inductively coupled plasma

A time-dependent two-fluid model has been developed to understand axial variations in the plasma parameters in a very high density (peak n e~ > 5x10 19 m –3 ) argon inductively coupled discharge in a long 1.1 cm radius tube. The model equations are written in 1D, with radial losses to the tube walls accounted for by the inclusion of effective particle and energy sink terms. The ambipolar diffusion equation and electron energy equation are solved to find the electron density n e (z,t) and temperature T e (z,t), and the populations of the neutral argon 4s metastable, 4s resonant, and 4p excited state manifolds are calculated in order to determine the stepwise ionization rate and calculate radiative energy losses. The model has been validated through comparisons with Langmuir probe ion saturation current measurements; close agreement between the simulated and measured axial plasma density profiles and the initial density rise rate at each location was obtained at p Ar = 30-60 mTorr. Lastly, we present detailed results from calculations at 60 mTorr, including the time-dependent electron temperature, excited state populations, and energy budget within and downstream of the radiofrequency (RF) antenna.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1981

The abundance and excitation of the carbon chains in interstellar molecular clouds

Emission lines from the carbon chains HC3N, HC5N, HC7N and HC9N were observed at 3 mm, 7 mm, and 1.4 cm in a number of dark clouds, Orion A and IRC(plus)10216. Non-LTE models were constructed to describe excitation and column densities. Component models for the Taurus dark cloud TMC-1 suggested that relative molecular abundances do not vary substantially along the cloud ridge, whereas the H2 density does by a factor of three. Data available for other dark clouds showed that the decrease in abundance with length from one carbon chain to the next is nearly constant, being close to 2.3. The decline in carbon chain abundance with length is steeper in Orion KL than in dark clouds by a factor of approximately four. Abundance ratios derived for the carbon star IRC(plus)10216 are uncertain, due to difficulties in modeling excitation rates in this environment.

Bujarrabal, V.↗
DOE OSTI2025

Predicting the evolution of biomass bulk density through feedstock preprocessing: Discrete element modeling, regression analysis, and pilot-scale validation

Bulk density is an important material property of biomass feedstocks, influencing handling, storage, transport costs, and conversion efficiency. In this study, predictive regression models for loose and tapped bulk densities of Alamo and Cave-in-Rock switchgrass are developed using a comprehensive dataset generated via calibrated bonded-sphere discrete element method (DEM) simulations. Here, a key contribution of this study is the use of a DEM-based approach, which correlates density with moisture content and particle size distribution parameters and enables analysis across a continuous particle size range, overcoming limitations of purely experimental data. For comparison, regression models are also developed using only experimental data from pilot-scale runs at the Biomass Feedstock National User Facility at Idaho National Laboratory. Validation against pilot-scale data showed reasonable prediction accuracy for both model types, particularly for smaller particle sizes (post-secondary grinding). While the experimental model showed slightly better performance matching the validation data in some cases, the DEM-based model benefits from a much larger dataset, reduced predictor multicollinearity, and continuous parameter coverage, highlighting the utility of validated simulation models for developing robust predictive tools for biomass preprocessing applications.

09 - BIOMASS FUELS↗
NASA NTRS2002

Subcutaneous administration of insulin-like growth factor (IGF)-II/IGF binding protein-2 complex stimulates bone formation and prevents loss of bone mineral density in a rat model of disuse osteoporosis

Elevated serum levels of insulin-like growth factor binding protein-2 (IGFBP-2) and a precursor form of IGF-II are associated with marked increases in bone formation and skeletal mass in patients with hepatitis C-associated osteosclerosis. In vitro studies indicate that IGF-II in complex with IGFBP-2 has high affinity for bone matrix and is able to stimulate osteoblast proliferation. The purpose of this study was to determine the ability of the IGF-II/IGFBP-2 complex to increase bone mass in vivo. Osteopenia of the femur was induced by unilateral sciatic neurectomy in rats. At the time of surgery, 14-day osmotic minipumps containing vehicle or 2 microg IGF-II+9 microg IGFBP-2/100g body weight/day were implanted subcutaneously in the neck. Bone mineral density (BMD) measurements were taken the day of surgery and 14 days later using a PIXImus small animal densitometer. Neurectomy of the right hindlimb resulted in a 9% decrease in right femur BMD (P<0.05 vs. baseline). This loss in BMD was completely prevented by treatment with IGF-II/IGFBP-2. On the control limb, there was no loss of BMD over the 14 days and IGF-II/IGFBP-2 treatment resulted in a 9% increase in left femur BMD (P<0.05). Bone histomorphometry indicated increases in endocortical and cancellous bone formation rates and in trabecular thickness. These results demonstrate that short-term administration of the IGF-II/IGFBP-2 complex can prevent loss of BMD associated with disuse osteoporosis and stimulate bone formation in adult rats. Furthermore, they provide proof of concept for a novel anabolic approach to increasing bone mass in humans with osteoporosis.

NASA Discipline Regulatory Physiology↗
DOE OSTI2024

Model Quality and Measurement Density Impact on Volt/Volt Ampere Reactive Optimization Performance

The operation of the utility grid is being reshaped by the continuous addition of distributed energy resources and advanced metering infrastructure, which challenge existing grid control strategies. Some utilities deploy advanced distribution management systems (ADMS) to assist with the consolidation of various applications and to augment situational awareness in response to the new power delivery dynamics. An ADMS is an integrated software platform that provides utilities with a way to enhance their reliability, control, and optimization with advanced applications, such as volt/VAR optimization (VVO). A VVO application could serve as a vehicle to deliver cost savings by providing the utility with a method to reduce rates by controlling the voltage and decreasing the energy usage in their service territory. Some utilities are reluctant to integrate an ADMS, because it is a significant investment that requires approval from the public regulatory commission and/or their customers. This paper evaluates the impact on VVO performance when using a lower-quality network model supplemented with additional measurements, which could provide an implementation for cost savings. The results show that a better model quality would provide the highest energy savings; however, some level of telemetry is necessary in all scenarios to prevent voltage exceedances.

24 POWER TRANSMISSION AND DISTRIBUTION↗
NASA NTRS1984

New empirical models of the electron temperature and density in the Venus ionosphere with application to transterminator flow

Pioneer Venus Orbiter (PVO) electron temperature probe measurements from the Venus years between December 1978 and December 1982 have been used to construct new empirical models of electron temperature and density. The models are used to obtain a two-dimensional solution of the momentum equation for the nightward ion flow velocities believed to be largely responsible for the maintenance of the nightside ionosphere. The velocities at the terminator rise from the neutral atmospheric wind velocity of about 300 m/s at 150 km to a peak velocity exceeding 2000 m/s above 500 km, in general agreement with PVO measurements of ion drift in that region.

Theis, R. F.↗
NASA NTRS2020

Mapping tall shrub biomass in Alaska at landscape scale using structure-from-motion photogrammetry and lidar

Warming in arctic and boreal regions is increasing shrub cover and biomass. In southcentral Alaska, willow (Salix spp.) and alder (Alnus spp.) shrubs grow taller than many tree species and account for a substantial proportion of aboveground biomass, yet they are not individually measured as part of the operational Forest Inventory and Analysis (FIA) Program. The goal of this research was to test methods for landscape-scale mapping of tall shrub biomass in upper montane and subalpine environments using FIA-type plot measurements (n = 51) and predictor variables from imagery-based structure-from-motion (SfM) and airborne lidar. Specifically, we compared biomass models constructed from imagery acquired by unmanned aerial vehicle (UAV; ~1.7 cm pixels), imagery from the NASA Goddard's Lidar, Hyperspectral, and Thermal Airborne Imager (G-LiHT; ~3.1 cm pixels), and concomitant G-LiHT small-footprint lidar. Tall shrub biomass was most accurately predicted at 5 m resolution (R^2 = 0.81, RMSE = 1.09 kg/sq. m) using G-LiHT SfM color and structure variables. Lidar-only models had lower precision (R^2 = 0.74, RMSE = 1.26 kg/sq. m), possibly due to reduced model information content from variable multicollinearity or lower data density. Separate models for upper montane zones with trees and shrubs and subalpine zones with only shrubs were always chosen over single models based on minimization of Akaike's Information Criterion, indicating the need for variable sets robust to overhanging tree canopy. Decreasing point density from UAV (5000–8000 pts./sq. m) to the G-LiHT SfM point cloud (500–2000 pts./sq. m) had little impact on model fit, suggesting that high-resolution airborne imagery can extend SfM approaches well beyond line-of-sight restrictions for UAV platforms. Overall, our results confirmed that SfM from high-resolution imagery is a viable approach to estimate shrub biomass in the boreal region, especially when an existing lidar terrain model and local field calibration data are available to quantify uncertainty in the SfM point cloud and landscape-scale estimates of shrub biomass.

Michael Alonzo↗
DOE OSTI2014

Research Update: Spatially resolved mapping of electronic structure on atomic level by multivariate statistical analysis

Atomic level spatial variability of electronic structure in Fe-based superconductor FeTe0.55Se0.45 (Tc = 15 K) is explored using current-imaging tunneling-spectroscopy. Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Similar statistical analysis of modeled calculated density of states of chemically inhomogeneous FeTe1 x Sex structures further confirms that the two types of chalcogens, i.e., Te and Se, can be identified by their electronic signature and differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the scanning tunneling microscopy data including separation of atomic identities, proximity, and local configuration effects and can be universally applicable to chemically and electronically inhomogeneous surfaces.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Density profiles in stellarators: an overview of particle transport, fuelling and profile shaping studies at TJ-II

We provide an overview of activities carried out at the TJ-II stellarator for improving our understanding of- and developing plasma physics models for particle density profiles in stellarators. Namely, we report on recent progress in turbulent particle transport simulation, validation of pellet deposition models, density profile shaping for performance control and new experimental techniques for edge turbulence and plasma-neutral interaction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Heterolysis of H 2 Across a Classical Lewis Pair, 2,6-Lutidine∙BCl3: Synthesis, Characterization, and Mechanism

In this study, we report that 2,6-lutidine∙trichloroborane (Lut∙BCl 3 ) reacts with H 2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut∙BHCl 2 . The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut∙BCl 3 was calculated to react with H 2 and form the ion pair, [LutH + ][HBCl 3 – ], with a barrier of ΔH ≠ =24.7 kcal mol –1 (ΔG ≠ =29.8 kcal mol –1 ). Metathesis with a second molecule of Lut∙BCl 3 produced Lut∙BHCl 2 and [LutH + ][BCl 4 – ]. The overall reaction is exothermic by 6.0 kcal mol –1 (Δ r G°=–1.1). Alternate pathways were explored involving the borenium cation (LutBCl 2 + ) and the four-membered boracycle [(CH 2 {NC 5 H 3 Me})BCl 2 ]. Barriers for addition of H 2 across the Lut/LutBCl 2 + pair and the boracycle B—C bond are substantially higher (ΔG ≠ =42.1 and 49.4 kcal mol –1 , respectively), such that these pathways are excluded. The barrier for addition of H 2 to the boracycle B—N bond is comparable (ΔH ≠ =28.5 and ΔG ≠ =32 kcal mol –1 ). Conversion of the intermediate 2-(BHCl 2 CH 2 )-6-Me(C 5 H 3 NH) to Lut∙BHCl 2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl 3 . Intramolecular protodeboronation, which could form Lut∙BHCl 2 directly, is prohibited by a high barrier (ΔH ≠ =52, ΔG ≠ =51 kcal mol –1 ).

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

Pairing tendencies in the doped Kitaev-Heisenberg model

Here, we study the impact of hole doping on the Kitaev-Heisenberg model on the honeycomb lattice. We investigate the pairing tendencies and correlation functions in the framework of a t - J - K model using density matrix renormalization group calculations on three-leg cylinders. In the case of the pure Kitaev model, which realizes a quantum spin-liquid phase at half-filling, we find that binding of two holes only occurs at low values of the hopping, where the holes are slow. We have theoretically verified that pair formation occurs in the limit of immobile holes, where the pure Kitaev model remains exactly solvable. When we instead fix the hopping at an intermediate, more realistic, value, and vary the Heisenberg and Kitaev interaction strengths, we find pairing tendencies only in the Néel phase. This is in contrast to prior mean-field calculations, highlighting the importance of accounting for the kinetic energy of dopants in generalized Kitaev models. Interestingly, we also find signatures of pair-density wave formation over the studied range of model parameters, namely, a periodic modulation of the charge density as well as the spin-spin and pair-pair correlations in real space. Moreover, we present a comparative study of the different correlations as a function of doping. We finally discuss the potential for experimentally observing the studied physics in quantum materials and heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Probing Galaxy assembly bias in BOSS galaxies using void probabilities

ABSTRACT We measure the void probability function (VPF) of galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS). The VPF provides complementary information to standard two-point statistics in that it is sensitive to galaxy bias in the most extreme underdensities in the cosmic web. Thus, the VPF is ideal for testing whether halo occupation of galaxies depends on large-scale density, an effect known as galaxy assembly bias. We find that standard HOD model – one parameterized by halo mass only – fit only to the two-point function, accurately predicts the VPF. Additionally, for HOD models where density dependence is explicitly incorporated, the best-fitting models fit to the combination of the correlation function and the VPF have zero density dependence. Thus, galaxy assembly bias is not a strong source of systematic uncertainty when modelling the clustering of massive galaxies.

Walsh, Kilian↗