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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Corrections for Exchange and Screening Effects in Low-energy Beta Decays

The beta spectra of {sup 241}Pu and {sup 63}Ni have been recently measured using metallic magnetic calorimeters. This powerful experimental technique allows theoretical beta spectra calculations to be tested at low energy with an accuracy never before achievable. Their comparison with classical beta calculations exhibits a significant deviation below 4 keV for {sup 241}Pu and 8 keV for {sup 63}Ni. The atomic exchange effect explains the main part of this deviation in the {sup 63}Ni beta spectrum. This effect has a significant contribution, equivalent to the magnitude of the screening, in the {sup 241}Pu beta spectrum.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Screening effects in ferroelectric resistive switching of BiFeO{sub 3} thin films

We investigate ferroelectric resistive switching in BiFeO{sub 3} thin films by performing local conductivity measurements. By comparing conduction characteristics at artificially up-polarized domains with those at as-grown down-polarized domains, the change in resistance is attributed to the modification of the electronic barrier height at the interface with the electrodes, upon the reversal of the electrical polarization. We also study the effect of oxygen vacancies on the observed conduction and we propose the existence of a different screening mechanism for up and down polarized domains.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Final Report : Experiments and Theory on Nonlinear Wave Couplings and the Saltpeter Screening Effect

This Final Technical Report gives a brief overview of the apparatus upgrades, experimental results, and theory development which was funded by DOE grant DE-SC0008693 from 1-Aug-2012 to 31-Jul-2018. Experiments were performed on the "IV" apparatus, which contains un-neutralized Mg+ ion plasmas in a 3.Tesla magnetic field, with sophisticated laser diagnostics and control. The apparatus upgrade utilized a 4-channel absolute WaveMeter, with feedback control to stabilize the laser frequencies, enabling ultra-cold, highly-correlated plasma states. The Saltpeter enhancement of collisionality due to plasma correlations was verified experimentally, ruling out competing "dynamical screening" theories. This represents the only quantitative experimental tests of the Saltpeter enhancement, which is incorporated into all models of stellar nucleosynthesis. In closely related work, quantitative measurements of cyclotron modes for majority and minority ion species were obtained, verifying the theoretically-predicted dependence on the plasma electric field. Substantial experimental and theoretical work was devoted to understanding the (parametric) decay of large-amplitude plasma waves into longer-wavelength plasma waves. Here, there are simple "fluid theory" predictions, but the experimentally observed (dominant) process depends on subtle "kinetic" effects due to particles trapped in the wave-fields. The theory of this novel instability was augmented by several types of simulations, which are in broad agreement with the experiments. This research formed the basis for two Ph.D. theses, and also included undergraduate trainees. A list of journal publications is provided, in which the detailed scientific results are described.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Environmental Screening Effects in 2D Materials: Renormalization of the Bandgap, Electronic Structure, and Optical Spectra of Few-Layer Black Phosphorus

Few-layer black phosphorus has recently emerged as a potential 2D semiconductor, notable for its widely tunable bandgap, highly anisotropic properties, and theoretically predicted large exciton binding energies. To avoid degradation, it has become common practice to encapsulate black phosphorus devices. It is generally assumed that this encapsulation does not qualitatively affect their optical properties. In this work, we show that the contrary is true. We have performed ab initio GW and GW plus Bethe–Salpeter equation (GW-BSE) calculations to determine the quasiparticle (QP) band structure and optical spectrum of one-layer (1L) through four-layer (4L) black phosphorus, with and without encapsulation between hexagonal boron nitride and sapphire. We show that black phosphorus is exceptionally sensitive to environmental screening. Encapsulation reduces the exciton binding energy in 1L by as much as 70% and completely eliminates the presence of a bound exciton in the 4L structure. The reduction in the exciton binding energies is offset by a similarly large renormalization of the QP bandgap so that the optical gap remains nearly unchanged, yet the nature of the excited states and the qualitative features of the absorption spectrum change dramatically.

2D materials↗
DOE OSTI2019

Unveiling the Interfacial Effects for Enhanced Hydrogen Evolution Reaction on MoS 2 /WTe 2 Hybrid Structures

Using the MoS 2 –WTe 2 heterostructure as a model system combined with electrochemical microreactors and density function theory calculations, it is shown that heterostructured contacts enhance the hydrogen evolution reaction (HER) activity of monolayer MoS 2 . Two possible mechanisms are suggested to explain this enhancement: efficient charge injection through large–area heterojunctions between MoS 2 and WTe 2 and effective screening of mirror charges due to the semimetallic nature of WTe 2 . The dielectric screening effect is proven minor, probed by measuring the HER activity of monolayer MoS 2 on various support substrates with dielectric constants ranging from 4 to 300. Thus, the enhanced HER is attributed to the increased charge injection into MoS 2 through large–area heterojunctions. Based on this understanding, a MoS 2 /WTe 2 hybrid catalyst is fabricated with an HER overpotential of –140 mV at 10 mA cm –2 , a Tafel slope of 40 mV dec –1 , and long stability. Furthermore, these results demonstrate the importance of interfacial design in transition metal dichalcogenide HER catalysts. The microreactor platform presents an unambiguous approach to probe interfacial effects in various electrocatalytic reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Adverse physiological and psychological effects of screen time on children and adolescents: Literature review and case study

A growing body of literature is associating excessive and addictive use of digital media with physical, psychological, social and neurological adverse consequences. Research is focusing more on mobile devices use, and studies suggest that duration, content, after-dark-use, media type and the number of devices are key components determining screen time effects. Physical health effects: excessive screen time is associated with poor sleep and risk factors for cardiovascular diseases such as high blood pressure, obesity, low HDL cholesterol, poor stress regulation (high sympathetic arousal and cortisol dysregulation), and Insulin Resistance. Other physical health consequences include impaired vision and reduced bone density. Psychological effects: internalizing and externalizing behavior is related to poor sleep. Depressive symptoms and suicidal are associated to screen time induced poor sleep, digital device night use, and mobile phone dependency. ADHD-related behavior was linked to sleep problems, overall screen time, and violent and fast-paced content which activates dopamine and the reward pathways. Early and prolonged exposure to violent content is also linked to risk for antisocial behavior and decreased prosocial behavior. Psychoneurological effects: addictive screen time use decreases social coping and involves craving behavior which resembles substance dependence behavior. Brain structural changes related to cognitive control and emotional regulation are associated with digital media addictive behavior. A case study of a treatment of an ADHD diagnosed 9-year-old boy suggests screen time induced ADHD-related behavior could be inaccurately diagnosed as ADHD. Screen time reduction is effective in decreasing ADHD-related behavior.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2023

Effect of environmental screening and strain on optoelectronic properties of two-dimensional quantum defects

Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including the number of layers, substrates, and strain, influences SPEs has not been fully understood. Here, in this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that environmental screening causes a lowering of the quasiparticle gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the Bethe–Salpeter equation Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

36 MATERIALS SCIENCE↗
NASA NTRS2020

Nuclear Fusion Reactions in Deuterated Metals

Nuclear fusion reactions of D-D are examined in an environment comprised of high density cold fuel embedded in metal lattices in which a small fuel portion is activated by hot neutrons. Such an environment provides for enhanced screening of the Coulomb barrier due to conduction and shell electrons of the metal lattice, or by plasma induced by ionizing radiation (γ quanta). We show that neutrons are far more efficient than energetic charged particles, such as light particles (e−, e+) or heavy particles (p, d, α) in transferring kinetic energy to fuel nuclei (D) to initiate fusion processes. It is well-known that screening increases the probability of tunneling through the Coulomb barrier. Electron screening also significantly increases the probability of large- versus small-angle Coulomb scattering of the reacting nuclei to enable subsequent nuclear reactions via tunneling. This probability is incorporated into the astrophysical factor S(E). Aspects of screening effects to enable calculation of nuclear reaction rates are also evaluated, including Coulomb scattering and localized heating of the cold fuel, primary D-D reactions, and subsequent reactions with both the fuel and the lattice nuclei. The effect of screening for enhancement of the total nuclear reaction rate is a function of multiple parameters including fuel temperature and the relative scattering probability between the fuel and lattice metal nuclei. Screening also significantly increases the probability of interaction between hot fuel and lattice nuclei increasing the likelilhood of Oppenheimer-Phillips processes opening a potential route to reaction multiplication. We demonstrate that the screened Coulomb potential of the target ion is determined by the nonlinear Vlasov potential and not by the Debye potential. In general, the effect of screening becomes important at low kinetic energy of the projectile. We examine the range of applicability of both the analytical and asymptotic expressions for the well-known electron screening lattice potential energy Ue, which is valid only for E >> Ue (E is the energy in the center of mass reference frame). We demonstrate that for E ≤ Ue, a direct calculation of Gamow factor for screened Coulomb potential is required to avoid unreasonably high values of the enhancement factor f (E) by the analytical—and more so by the asymptotic—formulas.

Vladimir Pines↗
DOE OSTI2013

Charge exchange and ionization cross sections of H{sup +}+H collision in dense quantum plasmas

The plasma screening effects of dense quantum plasmas on H{sup +}+H charge exchange and ionization cross sections are calculated by the classical trajectory Monte Carlo method. For charge exchange cross sections, it is found that the screening effects reduce cross sections slightly in weak screening conditions. However, cross sections are reduced substantially in strong screening conditions. For ionization cross sections, with the increase of screening effects, cross sections for low energies increase more rapidly than those for high energies. When the screening effects are strong enough, it is found that ionization cross sections decrease with the increase of incident H{sup +} energy. In addition, the cross sections have been compared with those in weakly coupled plasmas. It is found that in weak screening conditions, plasma screening effects in the two plasmas are approximately the same, while in strong screening conditions, screening effects of dense quantum plasmas are stronger than those of weakly coupled plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1949

Effect of screens in wide-angle diffusers

An experimental investigation at low airspeeds was made of the filling effect observed when a screen or similar resistance is placed across a diffuser. The filling effect is found to be real in that screens can prevent separation or restore separated flow in diffusers even of extreme divergence and to depend principally on screen location and pressure-drop coefficient of the screen. Results are given for three different diffusers of circular cross section with a variety of screen arrangements. Effects of single screens and multiple screens are shown. The mechanics of the filling effect is explained, and possible efficiencies are discussed. Results of arrangements of multiple screens in wide-angle diffusers are given to show a possible application to damping screens as used in wind tunnels to reduce turbulence. (author)

Schubauer, G B↗
DOE OSTI2014

Dynamics of the fully stripped ion-hydrogen atom charge exchange process in dense quantum plasmas

The plasma screening effects of dense quantum plasmas on charge exchange processes of a fully stripped ion colliding with a hydrogen atom are studied by the classical trajectory Monte Carlo method. The inter-particle interactions are described by the exponential cosine-screened Coulomb potentials. It is found that in weak screening conditions, cross sections increase with the increase of the ionic charge Z. However, in strong screening conditions, the dependence of cross sections on the ionic charge is related to the incident particle energy. At high energies, cross sections show a linear increase with the increase of Z, whereas at low energies, cross sections for Z≥4 become approximately the same. The He{sup 2+} and C{sup 6+} impacting charge exchange cross sections in dense quantum plasmas are also compared with those in weakly coupled plasmas. The interactions are described by the static screened Coulomb potential. It is found that for both He{sup 2+} and C{sup 6+}, the oscillatory screening effects of dense quantum plasmas are almost negligible in weak screening conditions. However, in strong screening conditions, the oscillatory screening effects enhance the screening effects of dense quantum plasmas, and the enhancement becomes more and more significant with the increase of the screening parameter and the ionic charge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Effects of screening and pressure ionization on the electron broadening of spectral lines in dense plasmas

Collisions between electrons and radiating atoms broaden spectral absorption and emission lines in dense plasmas. High densities also introduce screening and pressure ionization effects that distort the wave functions of both bound and free electrons. In order to study how dense plasma effects influence the electron broadening of spectral lines, this paper incorporates electron wave functions from an average-atom (AA) model to calculate the linewidth of the B III 2⁢𝑝−2⁢𝑠 transition at 𝑇 = 10 eV for mass densities ranging from 𝜌 = 10 −4 to 0.4⁢ g⁡/cc. The calculation method uses the impact approximation, allowing the linewidth to be written in terms of electron-collision cross sections and an interference term. Compared to an otherwise identical calculation that uses Coulomb free wave functions, the AA method is found to modify both the cross sections and the resulting linewidth at sufficiently high density by introducing screening and pressure-ionized bound states. Screening lowers the cross sections at low energies and near electron excitation thresholds, while pressure-ionized bound states introduce resonances into the continuum. Thus, as the density increases, the relative linewidth between the AA and Coulomb calculations follows a general decrease because of screening, with sharp increases at various intervals due to pressure ionization. Finally, the AA results are also compared with a common approach to introduce screening through the interaction potential and reduced models that use the Bethe formula for the inelastic electron-collision cross sections.

electronic excitation & ionization↗
NASA NTRS2007

Arrays of Bundles of Carbon Nanotubes as Field Emitters

Experiments have shown that with suitable choices of critical dimensions, planar arrays of bundles of carbon nanotubes (see figure) can serve as high-current-density field emitter (cold-cathode) electron sources. Whereas some hot-cathode electron sources must be operated at supply potentials of thousands of volts, these cold-cathode sources generate comparable current densities when operated at tens of volts. Consequently, arrays of bundles of carbon nanotubes might prove useful as cold-cathode sources in miniature, lightweight electron-beam devices (e.g., nanoklystrons) soon to be developed. Prior to the experiments, all reported efforts to develop carbon-nanotube-based field-emission sources had yielded low current densities from a few hundred microamperes to a few hundred milliamperes per square centimeter. An electrostatic screening effect, in which taller nanotubes screen the shorter ones from participating in field emission, was conjectured to be what restricts the emission of electrons to such low levels. It was further conjectured that the screening effect could be reduced and thus emission levels increased by increasing the spacing between nanotubes to at least by a factor of one to two times the height of the nanotubes. While this change might increase the emission from individual nanotubes, it would decrease the number of nanotubes per unit area and thereby reduce the total possible emission current. Therefore, to maximize the area-averaged current density, it would be necessary to find an optimum combination of nanotube spacing and nanotube height. The present concept of using an array of bundles of nanotubes arises partly from the concept of optimizing the spacing and height of field emitters. It also arises partly from the idea that single nanotubes may have short lifetimes as field emitters, whereas bundles of nanotubes could afford redundancy so that the loss of a single nanotube would not significantly reduce the overall field emission.

Manohara, Harish↗
NASA NTRS1976

Accelerated test techniques for micro-circuits: Evaluation of high temperature (473 k - 573 K) accelerated life test techniques as effective microcircuit screening methods

The application of high temperature accelerated test techniques was shown to be an effective method of microcircuit defect screening. Comprehensive microcircuit evaluations and a series of high temperature (473 K to 573 K) life tests demonstrated that a freak or early failure population of surface contaminated devices could be completely screened in thirty two hours of test at an ambient temperature of 523 K. Equivalent screening at 398 K, as prescribed by current Military and NASA specifications, would have required in excess of 1,500 hours of test. All testing was accomplished with a Texas Instruments' 54L10, low power triple-3 input NAND gate manufactured with a titanium- tungsten (Ti-W), Gold (Au) metallization system. A number of design and/or manufacturing anomalies were also noted with the Ti-W, Au metallization system. Further study of the exact nature and cause(s) of these anomalies is recommended prior to the use of microcircuits with Ti-W, Au metallization in long life/high reliability applications. Photomicrographs of tested circuits are included.

Johnson, G. M.↗
NASA NTRS1960

Measurement of Screen-Size Effects on Intensity, Scale, and Spectrum of Turbulence in a Free Subsonic Jet

The results are reported of hot-wire anemometer measurements of the fluctuating longitudinal component of the turbulent velocities in the mean flow downstream of screens in an air jet. These measurements have been analyzed by well-established techniques to give the influence of tile screen mesh size on the turbulent intensity, scale, and the power-spectral-density. The results show a linear dependence of the intensity upon the screen mesh size for locations within the central core of the air jet. The spectral-density curves show that the screens redistribute the turbulent energy from the low frequencies (<1000 cps) to the high frequencies (>1000 cps). The effects of the screens are overwhelmed in the mixing region of the jet flow by the turbulence levels existing there. The large pressure drops occurring across the screens reduce the velocity of the jet as compared to the jet without screens by approximately one-third for the velocity and range of mesh sizes investigated and reported in this report. The turbulence scale is a linear function of distance from the nozzle exit and is somewhat greater than comparable jets without screens.

Howard, Charles D.↗