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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 OSTI2022

Identity of the reversible hole traps in InP/ZnSe core/shell quantum dots

Density functional theory calculations are combined with time-resolved photoluminescence experiments to identify the species responsible for reversible trapping of holes following photoexcitation of InP/ZnSe/ZnS core/shell/shell quantum dots (QDs) having excess indium in the shell. Several possible assignments are considered, and a substitutional indium adjacent to a zinc vacancy, In 3+ /V Zn 2- , is found to be the most likely. This assignment is consistent with the observation that trapping occurs only when the QD has excess indium and is supported by experiments showing that the addition of zinc oleate or acetate decreases the extent of trapping, presumably by filling some of the vacancy traps. We also show that addition of alkyl carboxylic acids causes increased trapping, presumably by creation of additional zinc vacancies. The calculations show that either a single In 2+ ion or an In 2+ -In 3+ dimer is much too easily oxidized to form the reversible traps observed experimentally, while In 3+ is far too difficult to oxidize. Additional experimental data on InP/ZnSe/ZnS QDs synthesized in the absence of chloride demonstrates that the reversible traps are not associated with Cl - . Furthermore, a zinc vacancy adjacent to a substitutional indium is calculated to have its highest occupied orbitals about 1 eV above the top of the valence band of bulk ZnSe, in the appropriate energy range to act as reversible traps for quantum confined holes in the InP valence band. The associated orbitals are predominantly composed of p orbitals on the Se atoms adjacent to the Zn vacancy.

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

NEXAFS spectroscopy of alkylated benzothienobenzothiophene thin films at the carbon and sulfur K-edges

Alkylated benzothienobenzothiophenes are an important class of organic semiconductors that exhibit high performance in solution-processed organic field-effect transistors. In this work, we study the near-edge x-ray absorption fine-structure (NEXAFS) spectra of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene (C 10 -BTBT) at both the carbon and sulfur K-edges. Angle-resolved experiments of thin films are performed to characterize the dichroism associated with molecular orientation. First-principles calculations using the density functional theory-based many-body x-ray absorption spectroscopy (MBXAS) method are also performed to correlate the peaks observed and their dichroism with transitions to specific antibonding molecular orbitals. Interestingly, the dichroism of the dominant, lowest energy peak is opposite at the carbon and sulfur K-edges. While the low-energy peak at the carbon K-edge is assigned to carbon 1s → π* transitions with transition dipole moment (TDM) perpendicular to the planar BTBT core, the dominant low energy peak at the sulfur K-edge is assigned to sulfur 1s → σ* transitions with TDM oriented along the long axis of the BTBT core. These differences at the sulfur and carbon K-edges are understood through the MBAXS simulations that find a reordering of the energy of the lowest energy π* and σ* transitions at the sulfur K-edge due to the strong localization of the σ* orbital over the sulfur atom. This work highlights differences in the NEXAFS spectra of organic semiconductors at carbon and sulfur K-edges and provides new insights into peak assignment and x-ray dichroism relevant for studying the molecular orientation of organic semiconductor films.

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

First-Principles Treatment of Vibrational Broadening in X-Ray Excited Valence Band Spectra for n-SrTiO 3 (001)

The valence band maximum (VBM) is an important quantity for semiconductors as it locates the Fermi level relative to the band edge. Accurate measurement of this quantity in near-surface regions of semiconductors by photoemission is a first step toward determining the electronic properties of heterostructures involving these materials. While extrapolating the leading edge of the valence band to the energy axis in photoemission spectra is a widely used way to find the VBM, this method can be ambiguous if the leading edge exhibits multiple slopes. Another way to determine the VBM is to fit the leading edge to an appropriately broadened, cross-section modulated theoretical density of states (DOS). Three kinds of broadening that should be included for maximum accuracy are those due to: (1) finite instrumental resolution, (2) valence hole lifetime, and (3) vibrational excitations. While steps (1) and (2) are straightforward to implement, (3) is more difficult because the appropriate amount of broadening is not known a priori. Here, we demonstrate that explicit inclusion of vibrational broadening using ab initio molecular dynamics facilitates accurate VBM determination for n-SrTiO 3 (001). The total DOS is constructed by summing time-averaged projections at elevated temperature onto s - , p - , and d orbitals for the constituent atoms and modulating with the associated photoemission cross sections. Subsequent convolutions of the total DOS, first with a Gaussian of width equal to the experimental energy resolution and second with a Lorentzian to simulate valence hole lifetime effects, yield line shapes that reproduce the experimental leading edges rather well. The VBM is then given by the energy at which the vibrationally broadened total DOS (prior to the convolutions) goes to zero. The VBMs generated by this method quantitatively agree with those resulting from extrapolating from the middle of the measured leading edge for SrTiO 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2024

Monoatomic orbital-based one-dimensional topological crystalline insulator

The bulk-boundary correspondence in topological crystalline insulators (TCIs) links the topological properties of the bulk to robust observables on the edges, e.g., the existence of robust edge modes or fractional charge. In one dimension, TCIs protected by reflection symmetry have been realized in a variety of systems in which each unit cell has spatially distributed degrees of freedom (SDOF). However, these realizations exhibit sensitivity of the resulting edge modes to variations in edge termination and to the local breaking of the protective spatial symmetries by inhomogeneity. Here we demonstrate topologically protected edge states in a monoatomic, orbital-based TCI that mitigates both of these issues. By collapsing all SDOF within the unit cell to a singular point in space, we eliminate the ambiguity in unit-cell definition and hence remove a prominent source of boundary termination variability. The topological observables are also more tolerant to disorder in the orbital energies. To validate this concept, we experimentally realize a lattice of mechanical resonators where each resonator acts as an “atom” that harbors two key orbital degrees of freedom having opposite reflection parity. Finally, our measurements of this system provide direct visualization of the sp-hybridization between orbital modes that leads to a nontrivial band inversion in the bulk.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1984

Atomic-oxygen stimulated outgassing (A0034)

The objective of this experiment is to determine if the impingement of atomic oxygen in near-Earth orbit is a major factor in producing optically damaging outgassed products. The expected results will be to obtain samples which have been exposed to atomic oxygen for long durations. Analysis of these samples will determine if the impingement of atomic oxygen on the thermal control surfaces stimulates a significant amount of outgassed products. This experiment will give a clearer picture of the contamination problem and will assist in assuring that future Shuttle payloads, such as the Space Telescope and High-Energy Astronomy Observatory, will not experience Skylab contamination levels. Selected thermal control surfaces will be exposed to the atomic oxygen in near-Earth orbit. Passive collecting samples will collect any induced outgassing resulting from oxygen impingement. The optical condition of the passive samples will be measured using a ground-based integrating sphere reflectometer and a directional reflectometer.

Scott, R. L., Jr.↗
NASA NTRS1989

Oxygen evolution on a SrFeO3 anode - Mechanistic considerations from molecular orbital theory

Various pathways proposed in the literature for the evolution of O2 in electrochemical oxidations are explored using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory and the cluster models of the SrFeO3 surface as a prototype material. Calculations indicate that oxygen atoms can be easily formed on the (100) surface as well as on the edge cation sites of a SrFeO3 anode by the discharge of OH(-), followed by its deprotonation and electron transfer to the electrode. The O atoms can form O2 on the edge and corner sites, where the Fe(4+) is coordinated to four and three bulk oxygen anions, respectively. The calculations strongly disfavor mechanisms involving coupling of oxygen atoms adsorbed on different cations as well as a mechanism featuring an ozone intermediate.

Mehandru, S. P.↗
NASA NTRS1990

Orientation of Space Station Freedom electrical power system in environmental effects assessment

The orientation effects of six Space Station Freedom Electrical Power System (EPS) components are evaluated for three environmental interactions: aerodynamic drag, atomic oxygen erosion, and orbital debris impact. Designers can directly apply these orientation factors to estimate the magnitude of the examined environment and the environmental effects for the EPS component of interest. The six EPS components are the solar array, photovoltaic module radiator, integrated equipment assembly, solar dynamic concentrator, solar dynamic radiator, and beta gimbal.

Lu, Cheng-Yi↗
NASA NTRS1995

Integrated Logistics Support Analysis of the International Space Station Alpha: An Overview of the Maintenance Time Dependent Parameter Prediction Methods Enhancement

The objective of this publication is to introduce the enhancement methods for the overall reliability and maintainability methods of assessment on the International Space Station. It is essential that the process to predict the values of the maintenance time dependent variable parameters such as mean time between failure (MTBF) over time do not in themselves generate uncontrolled deviation in the results of the ILS analysis such as life cycle costs, spares calculation, etc. Furthermore, the very acute problems of micrometeorite, Cosmic rays, flares, atomic oxygen, ionization effects, orbital plumes and all the other factors that differentiate maintainable space operations from non-maintainable space operations and/or ground operations must be accounted for. Therefore, these parameters need be subjected to a special and complex process. Since reliability and maintainability strongly depend on the operating conditions that are encountered during the entire life of the International Space Station, it is important that such conditions are accurately identified at the beginning of the logistics support requirements process. Environmental conditions which exert a strong influence on International Space Station will be discussed in this report. Concurrent (combined) space environments may be more detrimental to the reliability and maintainability of the International Space Station than the effects of a single environment. In characterizing the logistics support requirements process, the developed design/test criteria must consider both the single and/or combined environments in anticipation of providing hardware capability to withstand the hazards of the International Space Station profile. The effects of the combined environments (typical) in a matrix relationship on the International Space Station will be shown. The combinations of the environments where the total effect is more damaging than the cumulative effects of the environments acting singly, may include a combination such as temperature, humidity, altitude, shock, and vibration while an item is being transported. The item's acceptance to its end-of-life sequence must be examined for these effects.

Sepehry-Fard, F.↗
NASA NTRS2001

MIR Solar Array Return Experiment: Power Performance Measurements and Molecular Contamination Analysis Results

A solar array segment was recently removed from the Mir core module and returned for ground-based analysis. The segment, which is similar to the ones the Russians have provided for the FGB and Service Modules, was microscopically examined and disassembled by US and Russian science teams. Laboratory analyses have shown the segment to he heavily contaminated by an organic silicone coating, which was converted to an organic silicate film by reactions with atomic oxygen within the. orbital flight environment. The source of the contaminant was a silicone polymer used by the Russians as an adhesive and bonding agent during segment construction. During its life cycle, the array experienced a reduction in power performance from approx. 12%, when it was new and first deployed, to approx. 5%, when it was taken out of service. However, current-voltage measurements of three contaminated cells and three pristine, Russian standard cells have shown that very little degradation in solar array performance was due to the silicate contaminants on the solar cell surfaces. The primary sources of performance degradation is attributed to "thermal hot-spotting" or electrical arcing; orbital debris and micrometeoroid impacts; and possibly to the degradation of the solar cells and interconnects caused by radiation damage from high energy protons and electrons.

Visentine, James↗
NASA NTRS2010

Environmental Impact Specification for Direct Space Weathering of Kuiper Belt and Oort Cloud Objects

The Direct Space Weathering Project of NASA's Outer Planets Research Program addresses specification of the plasma and energetic particle environments for irradiation and surface chemical processing of icy bodies in the outer solar system and the local interstellar medium. Knowledge of the radiation environments is being expanded by ongoing penetration of the twin Voyager spacecraft into the heliosheath boundary region of the outer heliosphere and expected emergence within the next decade into the very local interstellar medium. The Voyager measurements are being supplemented by remote sensing from Earth orbit of energetic neutral atom emission from this boundary region by NASA's Interstellar Boundary Explorer (IBEX). Although the Voyagers long ago passed the region of the Classical Kuiper Belt, the New Horizons spacecraft will encounter Pluto in 2015 and thereafter explore one or more KBOs, meanwhile providing updated measurements of the heliospheric radiation environment in this region. Modeling of ion transport within the heliosphere allows specification of time-integrated irradiation effects while the combination of Voyager and IBEX data supports projection of the in-situ measurements into interstellar space beyond the heliosheath. Transformation of model ion flux distributions into surface sputtering and volume ionization profiles provides a multi-layer perspective for space weathering impact on the affected icy bodies and may account for some aspects of color and compositional diversity. Other important related factors may include surface erosion and gardening by meteoritic impacts and surface renewal by cryovolcanism. Chemical products of space weathering may contribute to energy resources for the latter.

Cooper, John F.↗
NASA NTRS2011

A Laboratory and Theoretical Study of Silicon Hydroxide SiOH

The rotational spectrum of the triatomic free radical SiOH in its X 2A' ground electronic state has been observed in a supersonic molecular beam by Fourier transform microwave spectroscopy. The fundamental (10,1 yields 00,0) transition has been detected for normal SiOH and for three rare isotopic species: (30)SiOH, Si(18)OH, and SiOD. The same transition has also been observed in two of three excited vibrational states, v2 and v3, for the most abundant species. Precise spectroscopic constants, including those that describe the effective spin doubling and hydrogen hyperfine structure, have been derived for each isotopic species or vibrational state. To complement the laboratory work, theoretical calculations of the structure, dipole moment, and energies of the X 2A' and low-lying 1 2A'' states have also been undertaken at the coupled cluster level of theory. In agreement with theoretical predictions, we conclude from the hyperfine constants that SiOH is a best described as a pi-type radical, with the unpaired electron localized on a p orbital on the silicon atom. Assuming a bond angle of 118.5 deg, the Si-O bond length is 1.647(2) Angstroms and the O-H bond length is 0.969(4) Angstrom.

McCarthy, M. C.↗
DOE OSTI2025

Unveiling the covalency of versatile Pu(iii)-N bonds in a unique plutonium(iii) complex

A trivalent plutonium–pyrazinyl–tetrazolate complex Na 2 [Pu(Hdtp)(dtp) 2 (H 2 O) 4 ]·9H 2 O (Pu_dtp, H 2 dtp = 2,3-di-1H-tetrazol-5-ylpyrazine) was synthesized through metathesis reaction of plutonium bromide and Na 2 (dtp)·2H 2 O in water. This structure is particularly notable among complexes formed by trivalent f-elements and the dtp 2− ligand in aqueous media. In contrast to other trivalent f-elements, including all Ln 3+ (with the exception of Pm 3+ ) and Cm 3+ , preferentially coordinated with eight water molecules rather than the nitrogen donors of the dtp 2− ligand, Pu 3+ exhibits a distinct affinity for nitrogen coordination. This observation provides strong evidence that the 5f electrons in Pu 3+ are more delocalized than other studied trivalent f-elements. In Pu_dtp, three distinct Pu(III)–N bonds are present: one Pu(III)–N5 from pyrazinyl, one Pu(III)–N4 from the least electronegative nitrogen in the tetrazolate, and three Pu(III)–N1/N2/N3 from the most electronegative nitrogens in the tetrazolate. Experimental Pu(III)–N bond lengths, Wiberg bond indices (WBI), natural localized molecular orbitals (NLMO), quantum theory of atoms in molecules (QTAIM), and energy decomposition analysis (EDA), reveal a covalency trend: Pu(III)–N from the most electronegative nitrogen in tetrazolate > Pu(III)–N from the least electronegative nitrogen in tetrazolate > Pu(III)–N from pyrazinyl. This trend arises from the increased negative charge on the most electronegative nitrogen atoms in the tetrazolate ring, enhancing electrostatic Pu–N1/N2/N3 interactions. These stronger electrostatic interactions lead to shorter bond lengths, thereby enhancing orbital overlap and greater covalency, compared to the less electronegative nitrogen in tetrazolate (Pu–N4) and the neutral pyrazinyl nitrogen (Pu–N5).

Bai, Zhuanling [Colorado School of Mines, Golden, ↗
NASA NTRS1991

Atomic oxygen beam source for erosion simulation

A device for the production of low energy (3 to 10 eV) neutral atomic beams for surface modification studies is described that reproduces the flux of atomic oxygen in low Earth orbit. The beam is produced by the acceleration of plasma ions onto a negatively biased plate of high-Z metal; the ions are neutralized and reflected by the surface, retaining some fraction of their incident kinetic energy, forming a beam of atoms. The plasma is generated by a coaxial RF exciter which produces a magnetically-confined (4 kG) plasma column. At the end of the column, ions fall through the sheath to the plate, whose bias relative to the plasma can be varied to adjust the beam energy. The source provides a neutral flux approximately equal to 5 x 10(exp 16)/sq cm at a distance of 9 cm and a fluence approximately equal to 10(exp 20)/sq cm in five hours. The composition and energy of inert gas beams was diagnosed using a mass spectometer/energy analyzer. The energy spectra of the beams demonstrate energies in the range 5 to 15 eV, and qualitatively show expected dependences upon incident and reflecting atom species and potential drop. Samples of carbon film, carbon-based paint, Kapton, mylar, and teflon exposed to atomic O beams show erosion quite similar to that observed in orbit on the space shuttle.

Cuthbertson, J. W.↗
NASA NTRS1988

Velocity distributions of oxygen atoms incident on spacecraft surfaces

The angular distributions of oxygen atoms incident on surfaces in low earth orbit have been calculated for a number of ambient gas temperatures. Atom fluxes to surfaces were modeled by integrals over all permitted angles of incidence. Angles of incidence are limited by masking structures, and a number of types of mask were considered. Combustible surfaces exposed to the orbital atmosphere are heavily etched, creating profiles in mask shadows that are sensitive to ambient temperatures. The influence of the angular distributions on the characteristics of etched surfaces is discussed. Profiles measured for a September, 1983 flight were fitted to this model profile with a temperature of 750 + or - 50 K, which agrees with estimates based on solar activity at that time. Applications to sensing ambient temperatures and oxygen atom densities are discussed.

Peters, P. N.↗
NASA NTRS1987

Theoretical approach to oxygen atom degradation of silver

Based on available Rutherford backscattering spectrometry (RBS), proton induced X-ray emission (PIXE) and ellipsometry data obtained on silver specimens subjected to atomic oxygen attack in low Earth orbit STS flight 41-G, a theory was developed to model the oxygen atom degradation of silver. The diffusion of atomic oxygen in a microscopically nonuniform medium is an essential constituent of the theory. The driving force for diffusion is the macroscopic electrochemical potential gradient developed between the specimen surface exposed to the ambient and the bulk of the silver specimen. The longitudinal electric effect developed parallel to the gradient is modified by space charge of the diffusing charged species. Lateral electric fields and concentration differences also exist due to the nonuniform nature of the medium. The lateral concentration differences are found to be more important than the lateral electric fields in modifying the diffusion rate. The model was evaluated numerically. Qualitative agreement exists between the kinetics predicted by the theory and kinetic data taken in ground-based experiments utilizing a plasma asher.

Fromhold, Albert T., Jr.↗
NASA NTRSDate not supplied

Inflatable Softgoods Testing Capability Development for Habitation

The Habitation Systems Development Office (HP40) at NASA Marshall Space Flight Center supports systems engineering, integration, and project management for next generation space habitats. The office is responsible for formulation activities for Lunar Surface Habitat and Mars Transit Habitat. HP40 also manages commercial partnerships for exploration habitat development. The Lunar Surface Habitat is intended to initially support a minimum of two crew on the lunar surface for an approximately 30-day mission, with the ability to accommodate four crew for short durations. The Mars Transit Habitat will transport four crew to and from Mars orbit on a potentially 1,200 day mission. Both concepts are envisioned to leverage inflatable softgoods as the primary structural material for the habitation portion of the architecture. Inflatable softgoods are a complex, multi-material system consisting of multiple layers which perform specific functions. A bladder layer (polymer) contains the habitat’s atmosphere. The bladder is indexed to a woven restraint layer, usually Kevlar or Vectran, which serves as the structural layer. The remaining outer layers are for micro-meteoroid orbital debris (MMOD) shielding, protection from atomic Oxygen (if the material system is to be deployed in low earth orbit), and thermal insulation. The primary advantage of inflatable softgoods is that they can offer a larger volume per unit mass relative to a rigid metal structure. Inflatable softgoods have a strong development heritage within NASA, beginning with work in the 1960s in partnership with Goodyear Aerospace and continuing with Johnson Space Center’s TransHab project in the 1990s . Some development efforts subsequently transitioned into the private sector to mature the material technology for flight applications, including human habitation. In August 2022, NASA published its guidelines for certification of crewed inflatable softgoods structures (JSC-67721). This document outlines a series of tests which will be needed to certify inflatable softgoods material systems for use in crewed mission applications. It includes burst testing at the subscale and full-scale to determine the ultimate burst pressure of the softgoods architecture. Creep (long term deformation of the material under stress) is a key failure mode which must be characterized. Creep testing at the component and module levels is also recommended to generate time to failure curves and verify that the inflatable system has a predicted operational life which exceeds mission parameters. ET (Test Engineering) at MSFC and HP40 have worked together to establish softgoods testing capabilities for the center. This includes the ability to conduct module level burst tests and creep tests. The poster will highlight these capabilities and provide examples of testing which has been performed to date by ET. NASA MSFC anticipates that these test capabilities will continue to offer critical support for companies pursuing inflatable softgoods for exploration applications (lunar surface habitation, Mars transit) as well as commercial low earth orbit platforms.

habitation↗
NASA NTRS1994

A Comparison of Atomic Oxygen Erosion Yields of Carbon and Selected Polymers Exposed in Ground Based Facilities and in Low Earth Orbit

A comparison of the relative erosion yields (volume of material removed per oxygen atom arriving) for FEP Teflon, polyethylene, and pyrolytic graphite with respect to Kapton HN was performed in an atomic oxygen directed beam system, in a plasma asher, and in space on the EOIM-III (Evaluation of Oxygen Interaction with Materials-III) flight experiment. This comparison was performed to determine the sensitivity of material reaction to atomic oxygen flux, atomic oxygen fluence, and vacuum ultraviolet radiation for enabling accurate estimates of durability in ground based facilities. The relative erosion yield of pyrolytic graphite was found not to be sensitive to these factors, that for FEP was sensitive slightly to fluence and possibly ions, and that for polyethylene was found to be partially VUV and flux sensitive but more sensitive to an unknown factor. Results indicate that the ability to use these facilities for material relative durability prediction is great as long as the sensitivity of particular materials to conditions such as VUV, and atomic oxygen flux and fluence are taken into account. When testing materials of a particular group such as teflon, it may be best to use a witness sample made of a similar material that has some available space data on it. This would enable one to predict an equivalent exposure in the ground based facility.

Rutledge, Sharon K.↗