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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Recent developments in mathematical aspects of relativistic fluids

Abstract We review some recent developments in mathematical aspects of relativistic fluids. The goal is to provide a quick entry point to some research topics of current interest that is accessible to graduate students and researchers from adjacent fields, as well as to researches working on broader aspects of relativistic fluid dynamics interested in its mathematical formalism. Instead of complete proofs, which can be found in the published literature, here we focus on the proofs’ main ideas and key concepts. After an introduction to the relativistic Euler equations, we cover the following topics: a new wave-transport formulation of the relativistic Euler equations tailored to applications; the problem of shock formation for relativistic Euler; rough (i.e., low-regularity) solutions to the relativistic Euler equations; the relativistic Euler equations with a physical vacuum boundary; relativistic fluids with viscosity. We finish with a discussion of open problems and future directions of research.

Disconzi, Marcelo (ORCID:0000000234497778)↗
DOE OSTI2021

Some aspects of the five-dimensional Lovelock black hole spacetime: Strong homotopy retract, perihelion precession and quasistationary levels

Highlights: • Some topological aspects are explored. • The perihelion precession is calculated. • The general Heun functions are applied to obtain analytical solutions. • The quasibound states and the radial wave eigenfunctions are obtained. In this work we explore some mathematical physics aspects of the spherically symmetric Lovelock black hole in high dimensions. Intended for this aim, we thoroughly consider the metric corresponding to the five-dimensional Lovelock black hole spacetime. We construct the strong retractions by the geodesic equations on the background under consideration. As a result, from the topological point of view, we construct the theory of strong homotopy retract, which will allow us, in principle, to better understand some of its suitable applications on astrophysics and cosmology, in particular, in the analysis of the spacetime singularities. We find the solutions of the equation of motion for both radial and angular coordinates, and then we describe the outer (“exterior”) and lower (“interior”) apparent horizons. Indeed, the outer apparent horizon is the last surface from which the light waves could still escape from the black hole. Thus, it is meaningful to analyze some physical phenomena related to quantum particles propagating outside the exterior apparent horizon, in particular, we discuss the quasistationary levels of scalar fields and their radial wave functions, which are given in terms of the general Heun functions. We also calculate the perihelion precession in this background.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

An international review of occupant-related aspects of building energy codes and standards

In light of recent research, it is evident that occupants are playing an increasingly important role in building energy performance. Furthermore, despite the important role of building energy codes and standards in design, the occupant-related aspects are typically simple and have not kept up with the leading research. This paper reviews 23 regions’ building energy codes and standards by first comparing their quantitative aspects and then analyzing their mandated rules and approaches. While the present paper focuses on offices, general recommendations are applicable to other building types as well. The review revealed a wide range of occupant-related values, approaches, and attitudes. For example, code-specified occupant density varies by nearly a factor of three between different codes. This underlines the need for development of advancement in occupant behavior modeling approaches for future occupant-centric building performance codes and standards. Moreover, occupants are often referred to only implicitly; underlying expectations about energy-saving occupant behavior from building occupants varies greatly; and, only a few codes address occupant feedback and system usability. Based on the findings of the review, a set of initial recommendations for future building energy codes is proposed.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2020

Temporal dynamics of large-scale structures for turbulent Rayleigh–Bénard convection in a moderate aspect-ratio cylinder

We investigate the spatial organization and temporal dynamics of large-scale, coherent structures in turbulent Rayleigh–Bénard convection via direct numerical simulation of a 6.3 aspect-ratio cylinder with Rayleigh and Prandtl numbers of 9.6×10 7 and 6.7, respectively. Fourier modal decomposition is performed to investigate the structural organization of the coherent turbulent motions by analysing the length scales, time scales and the underlying dynamical processes that are ultimately responsible for the large-scale structure formation and evolution. We observe a high level of rotational symmetry in the large-scale structure in this study and that the structure is well described by the first four azimuthal Fourier modes. Two different large-scale organizations are observed during the duration of the simulation and these patterns are dominated spatially and energetically by azimuthal Fourier modes with frequencies of 2 and 3. Studies of the transition between these two large-scale patterns, radial and vertical variations in the azimuthal energy spectra, as well as the spatial and modal variations in the system's correlation time are conducted. Rotational dynamics are observed for individual Fourier modes and the global structure with strong similarities to the dynamics that have been reported for unit aspect-ratio domains in prior works. It is shown that the large-scale structures have very long correlation time scales, on the order of hundreds to thousands of free-fall time units, and that they are the primary source for a horizontal inhomogeneity within the system that can be observed during a finite, but a very long-time simulation or experiment.

42 ENGINEERING↗
DOE OSTI2023

Neoclassical transport in strong gradient regions of large aspect ratio tokamaks

We present a new neoclassical transport model for large aspect ratio tokamaks where the gradient scale lengths are of the size of the ion poloidal gyroradius. Previous work on neoclassical transport across transport barriers assumed large density and potential gradients but a small temperature gradient, or neglected the gradient of the mean parallel flow. Using large aspect ratio and low collisionality expansions, we relax these restrictive assumptions. We define a new set of variables based on conserved quantities, which simplifies the drift kinetic equation whilst keeping strong gradients, and derive equations describing the transport of particles, parallel momentum and energy by ions in the banana regime. The poloidally varying parts of density and electric potential are included. Studying contributions from both passing and trapped particles, we show that the resulting transport is dominated by trapped particles. We find that a non-zero neoclassical particle flux requires parallel momentum input which could be provided through interaction with turbulence or impurities. We derive upper and lower bounds for the energy flux across a transport barrier in both temperature and density and present example profiles and fluxes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Chemically Tunable Aspect Ratio Control and Laser Refrigeration of Hexagonal Sodium Yttrium Fluoride Upconverting Materials

Hexagonal sodium yttrium fluoride with Na 3x Y 2-x F 6 stoichiometry (β-NaYF) is a promising material for luminescence upconversion applications due to the narrow crystal field splitting of the Yb(III) ion’s lower 2 F 7/2 manifold. However, growing single crystals of β-NaYF remains an outstanding challenge due to thermal expansion stresses that cause cracking during melt growth. Here, we demonstrate a novel hydrothermal synthesis of β-NaYF with the ability to tune the aspect ratio from microplatelets to microrods with aspect ratios that match computationally predicted cavity (Mie) resonances. These crystals have a root-mean-square roughness below 1 nm after calcination, which makes them ideal for optical cavities. The β-NaYF microcrystals are doped with 10% Yb(III) cations and are used to build optomechanical laser-refrigeration devices consisting of a hexagonal β-NaYF crystal located at the end of a cantilever. Laser refrigeration of these devices by >12.5 °C is observed using calibrated measurements of both the cantilever’s fundamental eigenfrequency and a Boltzmann fit to crystal field luminescence from the Yb(III) ions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High aspect ratio organic light-emitting diodes

Reliability is particularly challenging for organic light-emitting diodes (OLEDs) used in solid-state lighting applications, because OLED lifetime is inversely proportional to luminance, and most lighting applications demand high luminance. Here we introduce a strategy to overcome this tradeoff by constructing OLEDs on a substrate with sub-mm, high aspect ratio surface texture. By creating more active OLED area per unit lighting panel area, the device current density required to generate a given panel luminance decreases. We validate this approach for fluorescent and phosphorescent OLEDs, demonstrating good thickness uniformity on corrugated substrates with area enhancement factors up to 1.4x using a standard thermal evaporator. Relative to planar controls at the same panel current density, the high aspect ratio devices achieve a 2.7-fold increase in operating lifetime and up to a 40% increase in external light extraction efficiency, indicating that this approach offers a powerful pathway to improve the efficiency and lifetime of OLED lighting.

OLED↗
DOE OSTI2024

Development of a high performance MagLIF target platform using high aspect ratio coated liners and low-mix laser preheat

We report on a series of Magnetized Liner Inertial Fusion (MagLIF) experiments conducted on the Z pulsed power facility that utilized high aspect ratio (ratio of outer radius to wall thickness) liners with dielectric coatings and low-mix laser preheat configurations. The liners consisted of an aspect ratio of 10.6 beryllium tube coated with 75 μm of epoxy on the outside that have been shown to maintain a better implosion stability than uncoated beryllium and have demonstrated consistent stagnation performances in previous experiments [Ampleford et al., Phys. Plasmas 31, 022703 (2024)]. Two-dimensional HYDRA simulations were used to design three different “co-injection” laser configurations, whereby a second laser is used to provide an early prepulse before the main pulse, to reduce LEH foil mix while increasing the fuel density and coupled energy. The laser preheat energy for each configuration was constrained using dedicated laser experiments before being applied to the integrated MagLIF experiments on Z. The DD neutron yield for experiments using co-injection preheat configurations is found to increase with the specific preheat energy in line with simulations. The highest neutron yield achieved in this study of 1.1 × 10 13 matches the highest reported in a MagLIF experiment to date and is a factor 3.5 times higher than similar experiments using preheat with no phase plate smoothing. We attempt to assess the effects of mix and morphology to explain the improved performance; however, neither factor is found to be conclusive within the uncertainty of the measurements.

Harvey-Thompson, A. J.↗
DOE OSTI2024

Inverse aspect-ratio expanded tokamak equilibria

Following Greene et al. [Phys. Fluids 14, 671 (1971)] and Connor et al. [Phys. Plasmas 31, 577 (1988); Plasma Phys. Control. Fusion 34, 161 (1992); and Nucl. Fusion 33, 1533 (1993)], the Grad-Shafranov equation for an axisymmetric tokamak plasma equilibrium is solved via an expansion in the, supposedly small, inverse aspect-ratio of the plasma, ϵ. The displacements of equilibrium magnetic flux-surfaces due to plasma shaping are assumed to be $\mathcal{O}$(ϵ) smaller than the minor radii of the surfaces, but no other restriction is placed on the nature of the shaping. The solution of the Grad-Shafranov equation is matched to a vacuum solution that extends to infinity, and consists of an expansion in toroidal functions. The external poloidal magnetic field generated by a finite set of discrete external poloidal magnetic field-coils is calculated, and incorporated into the toroidal function expansion. In this manner, the shape of a large aspect-ratio tokamak plasma is directly related to the currents flowing in the external poloidal field-coils. Finally, a pedestal in the plasma pressure, and the associated spike in the bootstrap current, are incorporated into the model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Molecular aspect ratio effect on axial thermal transport in solution-spun carbon nanotube fibers

Neat, densely packed, and highly aligned carbon nanotube fibers (CNTFs) have appealing room-temperature axial thermal conductivity (⁠k⁠) and thermal diffusivity (⁠α⁠) for applications in lightweight heat spreading, flexible thermal connections, and thermoelectric active cooling. Although CNTFs are regularly produced from different input carbon nanotubes (CNTs), prior work has not quantified how the CNT molecular aspect ratio r (i.e., molecular length-to-diameter ratio) influences k and in well-aligned, packed CNTFs. Here, we perform self-heated steady-state and three-omega thermal measurements at room temperature on CNTF suspended in vacuum. Our results show that k increases from 150 to 380 W / mK for viscosity-averaged molecular aspect ratios increasing from r = 960 to 5600 and nanotube diameters of ~2 nm, which we attribute to the effects of thermal resistances between CNT bundles. CNTFs made with varying volume fraction Φ of constituent high-r and low-r CNT have properties that fall within or below the typical macroscopic rule-of-mixtures bounds. The thermal diffusivity α scales with k, leading to a sample-averaged volumetric heat capacity of 1.5 ± 0.3 MJ / m 3 K⁠. This work's findings that fibers made from longer CNT have larger k and α at room temperature motivate further investigation into thermal transport in solution-spun CNTF.

42 ENGINEERING↗
DOE OSTI2025

Surrogate models to optimize plasma-assisted atomic layer deposition in high aspect ratio features

In this work, we explore surrogate models to optimize plasma enhanced atomic layer deposition (PEALD) in high aspect ratio features. In plasma-based processes such as PEALD and atomic layer etching (ALE), surface recombination can dominate the reactivity of plasma species with the surface, which can lead to unfeasibly long exposure times to achieve full conformality inside nanostructures like high aspect ratio vias. Using a synthetic dataset based on simulations of PEALD, we train artificial neural networks to predict saturation times based on cross section thickness data obtained for partially coated conditions. The results obtained show that just two experiments in undersaturated conditions contain enough information to predict saturation times within 10% of the ground truth. A surrogate model trained to determine whether surface recombination dominates the plasma–surface interactions in a PEALD process achieves 99% accuracy. This demonstrates that machine learning can provide a new pathway to accelerate the optimization of PEALD processes in areas such as microelectronics. Our approach can be easily extended to ALE and more complex structures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Freestanding high-aspect-ratio gold masks for low-energy, phase-based x-ray microscopy

High-resolution, x-ray phase contrast microscopy, a key technique with promising potential in biomedical imaging and diagnostics, is based on narrow-slit high-aspect-ratio gold gratings. Here we present the development, fabrication details, and experimental testing of the freestanding 10 μm thick gold membrane masks with an array of 0.9–1.5 μm void slit apertures for a novel low-energy x-ray microscope. The overall mask size is 4 mm × 4 mm, with a grating pitch of 7.5 μm, 6.0–6.6 μm wide gold bars are supported by 3 μm wide crosslinks at 400 μm intervals. The fabrication process is based on gold electroplating into a silicon mold coated with various thin films to form a voltage barrier, plating base, and sacrificial layer, followed by the mold removal to obtain the freestanding gold membrane with void slit apertures. We discuss key aspects for the materials and processes, including gold structures homogeneity, residual stresses, and prevention of collapsing of the grid elements. We further demonstrate the possibility to obtain high-resolution, high contrast 2D images of biological samples using an incoherent, rotating anode x-ray tube.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Reanalysis of the 𝛽− $\overline{v}$ 𝑒 Angular Correlation Measurement from the aSPECT Experiment with New Constraints on Fierz Interference

On the basis of revisions of some of the systematic errors, we reanalyzed the electron-antineutrino angular correlation (𝑎 coefficient) in free neutron decay inferred from the recoil energy spectrum of the protons which are detected in 4⁢𝜋 by the aSPECT spectrometer. With 𝑎=−0.104 02⁢(82), the new value differs only marginally from the one published in 2020. Here, the experiment also has sensitivity to 𝑏, the Fierz interference term. From a correlated (𝑏,𝑎) fit to the proton recoil spectrum, we derive a limit of 𝑏 =−0.0098⁢(193) which translates into a somewhat improved 90% confidence interval region of −0.041 ≤ 𝑏 ≤ 0.022 on this hypothetical term. Tighter constraints on 𝑏 can be set from a combined analysis of the PERKEO III (𝛽 asymmetry) and aSPECT measurement which suggests a finite value of 𝑏 with 𝑏 (𝑐) =−0.0181 ± 0.0065 deviating by 2.82⁢𝜎 from the standard model.

Beta decay↗
DOE OSTI2024

Foreword: Special Section on Multiphysics Aspects of Power Electronics Packaging - Power Die, Power Module, and Converter Level - Part 1

Power electronics are increasingly being used to condition electricity for a wide array of applications, such as transportation (on land, air, and water), data centers, radio frequency, directed energy, wind, solar, and grid-tied applications. To increase power density, performance, efficiency, and reliability and reduce cost, innovations and developments are needed in the multiphysics packaging of power electronics at a die, module, and converter level. This includes fundamental research and development related to emerging high-voltage, high-temperature, and high-switching-frequency power electronics; packaging materials; thermal materials and interfaces; fluid-based thermal management technologies; reliability; condition monitoring; and prognostics. To address these important aspects, this Special Section on Multiphysics Aspects of Power Electronics Packaging includes several articles to be published in two parts. More details are given below on the articles included in the first part.

condition monitoring↗
DOE OSTI2024

GraphCH: A Deep Framework for Assessing Cyber-Human Aspects in Insider Threat Detection

Insider threat is one of the most damaging cyber attacks that could cause the loss of intellectual property and enterprise data security breaches. Action sequence data such as host logs are used to investigate such threats and develop anomaly-based AI detectors. However, insider threat actions are similar to legitimate user activities, causing AI detectors to fail and suffer from high false alarm rates. Therefore, user cyber activity logs are inadequate to fully unfold insider threats. In this study, we adopt human psychological principles of risk-taking and impulsiveness along with host data to assess the influence and usefulness of human behavioral aspects in insider threat detection. Here, we hypothesize that individuals' impulsive and risk-taking behavior correlates with cyberspace activities. To validate our hypothesis, we conducted an IRB-approved study recruiting 35 participants who work in a large U.S. university and collected their cyber and psychological data for 90 days. Host and human-behavioral data analysis and mapping indicate that impulsive and risk-taking users trigger more system errors causing (un)intentional insider threats and are susceptible to attackers' social engineering and cognitive hacking. Utilizing cyber-human aspects, we introduce a Cyber-Human Graph Neural Network (GNN) based framework GraphCH to identify abnormal user behaviors and detect insider threats.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2023

Integrating Qualitative and Quantitative Aspects of Risk Management

This topical report summarizes the integration of qualitative and quantitative aspects of the risk management workflow for the Southwest Regional Partnership on Carbon Sequestration (SWP) Phase III Demonstration Project, located near the community of Farnsworth in northernmost Texas. Detail on some aspects of this subject has been previously provided in internal project reports on qualitative risk assessment processes, in a published paper summarizing quantitative risk modeling, and in many other published papers on specific risk modeling efforts. These prior documents are cited but the details are not repeated here. The report documents work undertaken for Task 7.4.1 of U.S. Department of Energy award number DE- FC26-05NT42591.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2025

High Aspect Ratio OLEDs

Long-term reliability is especially challenging for organic light emitting diodes (OLEDs) used in solid-state lighting applications because OLED lifetime is inversely proportional to luminance, and most lighting applications demand high luminance. This project explored a new strategy to overcome this tradeoff by constructing OLEDs on a substrate with sub-mm, high aspect ratio surface texture. By creating more active OLED area per unit lighting panel area, the device current density required to generate a given panel luminance decreases. We validated this approach for both fluorescent and phosphorescent OLEDs, demonstrating good thickness uniformity on corrugated substrates with area enhancement factors up to 1.4x using a standard thermal evaporator. Relative to planar controls at the same panel current density, the high aspect ratio devices achieve a 2.7-fold increase in operating lifetime and up to a 40% increase in external light extraction efficiency. Experiments carried out on a pilot-line OLED manufacturing system demonstrated good coating uniformity, indicating that this approach has the potential to be implemented at scale and therefore offers a powerful pathway to improve the efficiency and lifetime of OLED lighting.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

Branched High Aspect Ratio Nanostructures Fabricated by Focused Helium Ion Beam Induced Deposition of an Insulator

Helium ion beam induced deposition using the gaseous precursor pentamethylcyclopentasiloxane is employed to fabricate high aspect ratio insulator nanostructures (nanopillars and nanocylinders) that exhibit charge induced branching. The branched nanostructures are analyzed by transmission electron microscopy. It is found that the side branches form above a certain threshold height and that by increasing the flow rate of the precursor, the vertical growth rate and branching phenomenon can be significantly enhanced, with fractalesque branching patterns observed. The direct-write ion beam nanofabrication technique described herein offers a fast single-step method for the growth of high aspect ratio branched nanostructures with site-selective placement on the nanometer scale.

36 MATERIALS SCIENCE↗