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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Imaging Light‐Induced Migration of Dislocations in Halide Perovskites with 3D Nanoscale Strain Mapping

Abstract In recent years, halide perovskite materials have been used to make high‐performance solar cells and light‐emitting devices. However, material defects still limit device performance and stability. Here, synchrotron‐based Bragg coherent diffraction imaging is used to visualize nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. Significant strain heterogeneity within MAPbBr 3 (MA = CH 3 NH 3 + ) crystals is found in spite of their high optoelectronic quality, and both 〈100〉 and 〈110〉 edge dislocations are identified through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, dramatic light‐induced dislocation migration across hundreds of nanometers is uncovered. Further, by selectively studying crystals that are damaged by the X‐ray beam, large dislocation densities and increased nanoscale strains are correlated with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. These results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability.

36 MATERIALS SCIENCE↗
NASA NTRS1994

Numerical Simulation of Cylindrical, Self-field MPD Thrusters with Multiple Propellants

A two-dimensional, two-temperature, single fluid MHD code was used to predict the performance of cylindrical, self-field magnetoplasmadynamic (MPD) thrusters operated with argon, lithium, and hydrogen propellants. A thruster stability equation was determined relating maximum stable J(sup 2)/m values to cylindrical thruster geometry and propellant species. The maximum value of J(sup 2)/m was found to scale as the inverse of the propellant molecular weight to the 0.57 power, in rough agreement with limited experimental data which scales as the inverse square root of the propellant molecular weight. A general equation which relates total thrust to electromagnetic thrust, propellant molecular weight, and J(sup 2)/m was determined using reported thrust values for argon and hydrogen and calculated thrust values for lithium. In addition to argon, lithium, and hydrogen, the equation accurately predicted thrust for ammonia at sufficiently high J(sup 2)/m values. A simple algorithm is suggested to aid in the preliminary design of cylindrical, self-field MPD thrusters. A brief example is presented to illustrate the use of the algorithm in the design of a low power MPD thruster.

Lapointe, Michael R.↗
DOE OSTI2020

The role of edge resonant magnetic perturbations in edge-localized-mode suppression and density pump-out in low-collisionality DIII-D plasmas

Two-fluid nonlinear MHD simulations using the TM1 code demonstrate that the formation of magnetic islands at the top and bottom of the H-mode pedestal, together with the strong screening of resonant fields in the gradient region of the pedestal, can account for ELM suppression and density pump-out by n = 2 resonant magnetic perturbations (RMPs) in low-collisionality DIII-D ITER similar shape (ISS) plasmas. Using experimentally relevant transport coefficients, neoclassical resistivity, electron collisionality, and RMP amplitudes, nonlinear MHD simulations reproduce the observed level of density reduction (density pump-out) in DIII-D due to the formation of narrow magnetic islands in the resistive foot of pedestal. For large amplitude RMPs ( B r / B t > 1 × 10 -4 ) simulations predict resonant field penetration and resulting in pressure reduction at the top of the pedestal, consistent with experimental observations at the onset of ELM suppression. The predicted reduction in the height and width of the pedestal by magnetic island enhanced collisional transport provides a quantitative mechanism for the stabilization of the peeling-ballooning mode (PBM). Importantly, these simulations predict strong screening of resonant fields in the steep gradient region of the pedestal due to strong E × B rotation and diamagnetic flows. Yet, if the plasma resistivity is made artificially larger (~ 10X) than neoclassical, the simulations predict magnetic stochasticity throughout the plasma edge and the collapse of the pedestal due to the reduction in the penetration threshold with increasing resistivity. A scaling relation for resonant field penetration at the pedestal top, using several hundred nonlinear simulations, reproduces the density and E × B rotation dependence of the ELM suppression threshold observed in DIII-D. Simulations using ITER model equilibria indicate that the penetration threshold at the top of the ITER pedestal will be lower than in DIII-D due to the anticipated lower perpendicular flow velocities in ITER, resulting in the weaker screening of resonant fields.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Max Data Report Jet Stability versus Inlet Geometry

This document describes experiments investigating the effect of inlet geometry on the flow field within a glass tank where two jets mix and impinge upon the lid. The setup mimics the outlet plenum of a fast reactor where core exit flows of different temperatures can mix in ways that induce thermal cycling in neighboring structures.

Lomperski, S.↗
DOE OSTI2016

SPRUCE Stability of Peatland Carbon to Rising Temperatures: Supporting Data

This data set reports the results of extensive field measurements and laboratory analyses of samples from the SPRUCE experiment. Results indicate that ecosystem-scale warming of over 2 m of peat exponentially increased CH4 emissions —but not ecosystem respiration of CO2. Multiple lines of evidence, including incubations and in situ analyses of 14C, dissolved gases, and microbial community metabolic potential, indicate that CH4 emissions increased due to surface processes and not degradation of deeper buried carbon. The supporting data provided include the summarized and aggregated data used to generate the six figures in the main text of the paper and the 12 supplementary figures.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2015

Perpendicular magnetic tunnel junctions with double barrier and single or synthetic antiferromagnetic storage layer

The magnetic properties of double tunnel junctions with perpendicular anisotropy were investigated. Two synthetic antiferromagnetic references are used, while the middle storage magnetic layer can be either a single ferromagnetic or a synthetic antiferromagnetic FeCoB-based layer, with a critical thickness as large as 3.0 nm. Among the different achievable magnetic configurations in zero field, those with either antiparallel references, and single ferromagnetic storage layer, or parallel references, and synthetic antiferromagnetic storage layer, are of particular interest since they allow increasing the efficiency of spin transfer torque writing and the thermal stability of the stored information as compared to single tunnel junctions. The latter configuration can be preferred when stray fields would favour a parallel orientation of the reference layers. In this case, the synthetic antiferromagnetic storage layer is also less sensitive to residual stray fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1992

Dynamic interactions between hypersonic vehicle aerodynamics and propulsion system performance

Described here is the development of a flexible simulation model for scramjet hypersonic propulsion systems. The primary goal is determination of sensitivity of the thrust vector and other system parameters to angle of attack changes of the vehicle. Such information is crucial in design and analysis of control system performance for hypersonic vehicles. The code is also intended to be a key element in carrying out dynamic interaction studies involving the influence of vehicle vibrations on propulsion system/control system coupling and flight stability. Simple models are employed to represent the various processes comprising the propulsion system. A method of characteristics (MOC) approach is used to solve the forebody and external nozzle flow fields. This results in a very fast computational algorithm capable of carrying out the vast number of simulation computations needed in guidance, stability, and control studies. The three-dimensional fore- and aft body (nozzle) geometry is characterized by the centerline profiles as represented by a series of coordinate points and body cross-section curvature. The engine module geometry is represented by an adjustable vertical grid to accommodate variations of the field parameters throughout the inlet and combustor. The scramjet inlet is modeled as a two-dimensional supersonic flow containing adjustable sidewall wedges and multiple fuel injection struts. The inlet geometry including the sidewall wedge angles, the number of injection struts, their sweepback relative to the vehicle reference line, and strut cross-section are user selectable. Combustion is currently represented by a Rayleigh line calculation including corrections for variable gas properties; improved models are being developed for this important element of the propulsion flow field. The program generates (1) variation of thrust magnitude and direction with angle of attack, (2) pitching moment and line of action of the thrust vector, (3) pressure and temperature distributions throughout the system, and (4) performance parameters such as thrust coefficient, specific impulse, mass flow rates, and equivalence ratio. Preliminary results are in good agreement with available performance data for systems resembling the NASP vehicle configuration.

Flandro, G. A.↗
DOE OSTI2016

Retention of intermediate polarization states in ferroelectric materials enabling memories for multi-bit data storage

A homogeneous ferroelectric single crystal exhibits only two remanent polarization states that are stable over time, whereas intermediate, or unsaturated, polarization states are thermodynamically instable. Commonly used ferroelectric materials however, are inhomogeneous polycrystalline thin films or ceramics. To investigate the stability of intermediate polarization states, formed upon incomplete, or partial, switching, we have systematically studied their retention in capacitors comprising two classic ferroelectric materials, viz. random copolymer of vinylidene fluoride with trifluoroethylene, P(VDF-TrFE), and Pb(Zr,Ti)O{sub 3}. Each experiment started from a discharged and electrically depolarized ferroelectric capacitor. Voltage pulses were applied to set the given polarization states. The retention was measured as a function of time at various temperatures. The intermediate polarization states are stable over time, up to the Curie temperature. We argue that the remarkable stability originates from the coexistence of effectively independent domains, with different values of polarization and coercive field. A domain growth model is derived quantitatively describing deterministic switching between the intermediate polarization states. We show that by using well-defined voltage pulses, the polarization can be set to any arbitrary value, allowing arithmetic programming. The feasibility of arithmetic programming along with the inherent stability of intermediate polarization states makes ferroelectric materials ideal candidates for multibit data storage.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

THE IBEX RIBBON AND THE PICKUP ION RING STABILITY IN THE OUTER HELIOSHEATH. I. THEORY AND HYBRID SIMULATIONS

The nearly circular band of energetic neutral atom emission dominating the field of view of the Interplanetary Boundary Explorer ( IBEX ) satellite, is most commonly attributed to the effect of charge exchange of secondary pickup ions (PUIs) gyrating about the magnetic field in the outer heliosheath and the interstellar space beyond. Several models for the PUI dynamics of this mechanism have been proposed, each requiring either strong or weak scattering of the initial pitch angle. Conventional wisdom states that ring distributions tend to generate waves and scatter onto a shell on timescales too short for charge exchange to occur. We performed a careful study of ring and thin shell proton distribution stability using theoretical tools and hybrid plasma simulations. We show that the kinetic behavior of a freshly injected proton ring is a far more complicated process than previously thought. In the presence of a warm Maxwellian core, narrower rings could be more stable than broader toroidal distributions. The scattered rings possess a fine structure that can only be revealed using very large numbers of macroparticles in a simulation. It is demonstrated that a “stability gap” in ring temperature exists where the protons could retain large gyrating anisotropies for years, and the wave activity could remain below the level of the ambient magnetic fluctuations in interstellar space. In the directions away from the ribbon, however, a partial shell distribution is more likely to be unstable, leading to significant scattering into one hemisphere in velocity space. The process is accompanied by turbulence production, which is puzzling given the very low level of magnetic fluctuations measured in the outer heliosheath by Voyager 1 .

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2009

NanoSail-D: A Solar Sail Demonstration Mission

During the past decade, within the United States, NASA Marshall Space Flight Center (MSFC) was heavily engaged in the development of revolutionary new technologies for in-space propulsion. One of the major in-space propulsion technologies developed was a solar sail propulsion system. Solar sail propulsion uses the solar radiation pressure exerted by the momentum transfer of reflected photons to generate a net force on a spacecraft. To date, solar sail propulsion systems have been designed for large spacecraft in the tens to hundreds of kilograms mass range. Recently, however, MSFC has been investigating the application of solar sails for small satellite propulsion. Likewise, NASA Ames Research Center (ARC) has been developing small spacecraft missions that have a need for amass-efficient means of satisfying deorbit requirements. Hence, a synergistic collaboration was established between these two NASA field Centers with the objective of conducting a flight demonstration of solar sail technologies for small satellites. The NanoSail-D mission flew onboard the ill-fated Falcon Rocket launched August 2, 2008, and, due to the failure of that rocket, never achieved orbit. The NanoSail-D flight spare is ready for flight and a suitable launch arrangement is being actively pursued. Both the original sailcraft and the flight spare are hereafter referred to as NanoSail-D. The sailcraft consists of a sail subsystem stowed in a three-element CubeSat. Shortly after deployment of the NanoSail-D, the solar sail will deploy and mission operations will commence. This demonstration flight has two primary technical objectives: (1) to successfully stow and deploy the sail and (2) to demonstrate deorbit functionality. Given a near-term opportunity for launch on Falcon, the project was given the challenge of delivering the flight hardware in 6 mo, which required a significant constraint on flight system functionality. As a consequence, passive attitude stabilization of the spacecraft will be achieved using permanent magnets to detumble and orient the body with the magnetic field lines and then rely on atmospheric drag to passively stabilize the sailcraft in an essentially maximum drag attitude. This paper will present an introduction to solar sail propulsion systems and an overview of the NanoSail-D spacecraft.

Johnson, Les↗
DOE OSTI2017

Dynamics of Equilibrium Points in a Uniformly Rotating Second-Order and Degree Gravitational Field

Using tools such as periodic orbits and invariant manifolds, the global dynamics around equilibrium points (EPs) in a rotating second-order and degree gravitational field are studied. For EPs on the long axis, planar and vertical periodic families are computed, and their stability properties are investigated. Invariant manifolds are also computed, and their relation to the first-order resonances is briefly discussed. For EPs on the short axis, planar and vertical periodic families are studied, with special emphasis on the genealogy of the planar periodic families. Our studies show that the global dynamics around EPs are highly similar to those around libration points in the circular restricted three-body problem, such as spatial halo orbits, invariant manifolds, and the genealogy of planar periodic families.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Amphiphilic Polyphosphazene for Fluorocarbon Emulsion Stabilization

High internal phase emulsions (HIPEs) have been of great interest for fabricating fluorinated porous polymers having controlled pore structures and excellent physicochemical properties. However, it remains a challenge to prepare stable fluorocarbon HIPEs, due to the lack of suitable surfactants. By randomly grating hydrophilic and fluorophilic side chains to polyphosphazene (PPZ), a comb-like amphiphilic PPZ surfactant with biodegradability is designed and synthesized for stabilizing water/fluorocarbon oil-based emulsions. The hydrophilic–lipophilic balance of PPZs can be controlled by tuning the grating ratio of the two side chains, leading to the preparation of stable water-in-oil HIPEs and oil-in-water emulsions, and the production of fluorinated porous polymers and particles by polymerizing the oil phase. Finally, these fluorinated porous polymers show excellent thermal stability and, due to the hydrophobicity and porous structure, applications in the field of oil/water separation can be achieved.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Linearly first- and second-order, unconditionally energy stable schemes for the phase field crystal model

In this paper, we develop a series of linear, unconditionally energy stable numerical schemes for solving the classical phase field crystal model. The temporal discretizations are based on the first order Euler method, the second order backward differentiation formulas (BDF2) and the second order Crank–Nicolson method, respectively. The schemes lead to linear elliptic equations to be solved at each time step, and the induced linear systems are symmetric positive definite. We prove that all three schemes are unconditionally energy stable rigorously. Various classical numerical experiments in 2D and 3D are performed to validate the accuracy and efficiency of the proposed schemes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

CTF-R: A novel residual-based thermal hydraulic solver

The traditional scientific process has been revolutionized by the advent of computational modeling, but the nuclear industry generally uses "legacy codes," which were developed early in the evolution of computers. One example of a legacy code, the thermal hydraulic subchannel code CTF, is modernized in this work through the development of a novel residual-based version, CTF-R. Unlike its predecessor, CTF-R is not limited by the strict computational limitations of the early 1980's, and can therefore be designed such that it is inherently flexible and easy to use. A case study is examined to demonstrate how the flexibility of the code can be used to improve simulation results. In this example, the coupling between the solid and liquid fields is examined. Traditionally, this coupling is modeled explicitly, which imposes numerical stability limits on the time step size. These limits are derived and it is shown that they are removed when the coupling is made implicit. Further, the development of CTF-R will enable future improvements in next generation reactor modeling, numerical methods, and coupling to other codes. Through the further development of CTF-R and other residual-based codes, state-of-the-art simulation is possible.

42 ENGINEERING↗
DOE OSTI2020

Durability of ionomer encapsulants in photovoltaic modules

Ethylene vinyl acetate (EVA) has for decades been the preferred material for photovoltaic (PV) module encapsulation. However, while it is relatively inexpensive and initially adheres well to module components, EVA discolors with age and - as interfacial adhesion degrades - becomes susceptible to delamination, ultimately resulting in reduced module efficiency and shortened service lifetimes. As potential replacements for EVA, ionomer thermoplastic materials cure faster, are more resistant to discoloration and potential induced degradation, and do not evolve corrosive acetic acid, making them compatible with new device materials such as perovskites. Since there is limited information on ionomer durability for PV module applications, a series of field and accelerated laboratory aging studies were conducted to assess ionomer interface stability in the presence of terrestrial environmental stressors. It is shown that adhesion to the glass and cell interfaces of PV modules is inferior to EVA, both before and after aging, rendering ionomers particularly susceptible to delamination after short timeframes. Potential solutions to improve ionomer adhesion are discussed.

14 SOLAR ENERGY↗
DOE OSTI2024

M IV /Co –I (M = Zr, Hf) Bis(phosphinoamide) Complexes with η 6 - and η 4 -Arenes

Despite their relevance to catalysis, low-/subvalent cobalt complexes are difficult to synthesize and isolate. Consequently, very few “cobaltate” complexes are known and there is a lack of architectural diversity in this field. Lewis acidic d 0 group IV metals have been demonstrated to stabilize Co –I centers via metal–metal bonds. Herein, we report the synthesis of bis(phosphinoamide) heterobimetallic M IV /Co –I arene complexes (M = Zr, Hf). The driving force to maintain the aromaticity of the arenes dictates the hapticity of the Co-bound arene ligands and influences the binding arrangement of the phosphinoamide ligands. Benzene and toluene were observed to bind η 6 to the Co –I center, forcing dissociation of one of the phosphinoamide ligands, whereas η 4 -coordination of anthracene allows both phosphinoamide ligands to remain bound to the Co center. The identity of both the arene and the group IV metal ion starkly influence the lability of the arene. For instance, the Co-bound benzene ligand in the Zr IV /Co –I benzene complex rapidly exchanges with C 6 D 6 in solution, whereas toluene/C 6 D 6 exchange is much slower and appreciable C 6 H 6 /C 6 D 6 exchange is not observed for the Hf analogue. The Zr IV /Co –I benzene complex loses benzene upon repeated exposure to vacuum to form an arene-free tetrametallic dimer.

aromatic compounds↗
DOE OSTI2014

How Robust are Semiconductor Nanorods? Investigating the Stability and Chemical Decomposition Pathways of Photoactive Nanocrystals

Anisotropic II–VI semiconductor nanostructures are important photoactive materials for various energy conversion and optical applications. However, aside from the many available surface chemistry studies and from their ubiquitous photodegradation under continuous illumination, the general chemical reactivity and thermal stability (phase and shape transformations) of these materials are poorly understood. Using CdSe and CdS nanorods as model systems, we have investigated the behavior of II–VI semiconductor nanorods against various conditions of extreme chemical and physical stress (acids, bases, oxidants, reductants, and heat). CdSe nanorods react rapidly with acids, becoming oxidized to Se or SeO2. In contrast, CdSe nanorods remain mostly unreactive when treated with bases or strong oxidants, although bases do partially etch the tips of the nanorods (along their axis). Roasting (heating in air) of CdSe nanorods results in rock-salt CdO, but neither CdSe nor CdO is easily reduced by hydrogen (H2). Another reductant, n-BuLi, reduces CdSe nanorods to metallic Cd. Variable temperature X-ray diffraction experiments show that axial annealing and selective axial melting of the nanorods precede particle coalescence. Furthermore, thermal analysis shows that the axial melting of II–VI nanorods is a ligand-dependent process. In agreement with chemical reactivity and thermal stability observations, silica-coating experiments show that the sharpest (most curved) II–VI surfaces are most active against heterogeneous nucleation of a silica shell. These results provide valuable insights into the fate and possible ways to enhance the stability and improve the use of II–VI semiconductor nanostructures in the fields of optics, magnetism, and energy conversion.

36 MATERIALS SCIENCE↗