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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 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Co-doped sodium chloride crystals exposed to different irradiation temperature

Monocrystals of NaCl:XCl{sub 2}:MnCl{sub 2}(X = Ca,Cd) at four different concentrations have been analyzed. The crystals were exposed to different irradiation temperature, such as at room temperature (RT), solid water (SW), dry ice (DI) and liquid nitrogen (LN). The samples were irradiated with photon from {sup 60}Co irradiators. The co-doped sodium chloride crystals show a complex structure of glow curves that can be related to different distribution of traps. The linearity response was analyzed with the F(D) index. The F(D) value was less than unity indicating a sub-linear response was obtained from the TL response on the function of the dose. The glow curves were deconvoluted by using the CGCD program based on the first, second and general order kinetics.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Grain size softening effect in Al{sub 62.5}Cu{sub 25}Fe{sub 12.5} nanoquasicrystals

Inverse Hall-Petch (IHP) behavior in nano-quasicrystalline Al{sub 62.5}Cu{sub 25}Fe{sub 12.5} is reported. Powders with varying grain sizes were produced by mechanical milling of spray-formed quasicrystals. The hardness of the milled powders increased with decreasing grain size down to about 40 nm and decreased with further refinement, demonstrating the IHP behavior. This critical grain size was found to be larger compared to other metallic nanocrystalline alloys. This IHP behaviour has been attributed to the structural complexity in quasicrystals and to thermally activated shearing events of atoms at the grain boundaries.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

A variable-temperature nanostencil compatible with a low-temperature scanning tunneling microscope/atomic force microscope

We describe a nanostencil lithography tool capable of operating at variable temperatures down to 30 K. The setup is compatible with a combined low-temperature scanning tunneling microscope/atomic force microscope located within the same ultra-high-vacuum apparatus. The lateral movement capability of the mask allows the patterning of complex structures. To demonstrate operational functionality of the tool and estimate temperature drift and blurring, we fabricated LiF and NaCl nanostructures on Cu(111) at 77 K.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Magnetic reconnection in a comparison of topology and helicities in two and three dimensional resistive magnetohydrodynamic simulations

Through a direct comparison between numerical simulations in two and three dimensions, we investigate topological effects in reconnection. A simple estimate on increase in reconnection rate in three dimensions by a factor of √(2), when compared with a two-dimensional case, is confirmed in our simulations. We also show that both the reconnection rate and the fraction of magnetic energy in the simulations depend linearly on the height of the reconnection region. The degree of structural complexity of a magnetic field and the underlying flow is measured by current helicity and cross-helicity. We compare results in simulations with different computational box heights.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Delineating the magnetic field line escape pattern and stickiness in a poloidally diverted tokamak

We analyze a Hamiltonian model with five wire loops that delineates magnetic surfaces of tokamaks with poloidal divertor. Non-axisymmetric magnetic perturbations are added by external coils, similar to the correction coils that have been installed or designed in present tokamaks. To show the influence of magnetic perturbations on the field line escape, we integrate numerically the field line differential equations and obtain the footprints and deposition patterns on the divertor plate. Moreover, we show that the homoclinic tangle describes the deposition patterns in the divertor plate, agreeing with results observed in sophisticated simulation codes. Additionally, we show that while chaotic lines escape to the divertor plates, some of them are trapped, for many toroidal turns, in complex structures around magnetic islands, embedded in the chaotic region, giving rise to stickiness evidences characteristic of chaotic Hamiltonian systems. Finally, we introduce a random collisional term to the field line mapping to investigate stickiness alterations due to particle collisions. Within this model, we conclude that, even reduced by collisions, the observed trapping still influences the field line transport. The results obtained for our numerical estimations indicate that the reported trapping may affect the transport in present tokamaks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Dynamical analysis of an orbiting three-rigid-body system

The development of multi-joint-spacecraft mission concepts calls for a deeper understanding of their nonlinear dynamics to inform and enhance system design. This paper presents a study of a three-finite-shape rigid-body system under the action of an ideal central gravitational field. The aim of this paper is to gain an insight into the natural dynamics of this system. The Hamiltonian dynamics is derived and used to identify relative attitude equilibria of the system with respect to the orbital reference frame. Then a numerical investigation of the behaviour far from the equilibria is provided using tools from modern dynamical systems theory such as energy methods, phase portraits and Poincarè maps. Results reveal a complex structure of the dynamics as well as the existence of connections between some of the equilibria. Stable equilibrium configurations appear to be surrounded by very narrow regions of regular and quasi-regular motions. Trajectories evolve on chaotic motions in the rest of the domain.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Deviation from threshold model in ultrafast laser ablation of graphene at sub-micron scale

We investigate a method to measure ultrafast laser ablation threshold with respect to spot size. We use structured complex beams to generate a pattern of craters in CVD graphene with a single laser pulse. A direct comparison between beam profile and SEM characterization allows us to determine the dependence of ablation probability on spot-size, for crater diameters ranging between 700 nm and 2.5 μm. We report a drastic decrease of ablation probability when the crater diameter is below 1 μm which we interpret in terms of free-carrier diffusion.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Equivalent circuit for VO{sub 2} phase change material film in reconfigurable frequency selective surfaces

We developed equivalent circuits of phase change materials based on vanadium dioxide (VO{sub 2}) thin films. These circuits are used to model VO{sub 2} thin films for reconfigurable frequency selective surfaces (FSSs). This is important as it provides a way for designing complex structures. A reconfigurable FSS filter using VO{sub 2} ON/OFF switches is designed demonstrating −60 dB isolation between the states. This filter is used to provide the transmission and reflection responses of the FSS in the frequency range of 0.1–0.6 THz. The comparison between equivalent circuit and full-wave simulation shows excellent agreement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Three-dimensional piezoelectric vibration energy harvester using spiral-shaped beam with triple operating frequencies

This work has demonstrated a novel piezoelectric energy harvester without a complex structure and appended component that is capable of scavenging vibration energy from arbitrary directions with multiple resonant frequencies. In this harvester, a spiral-shaped elastic thin beam instead of a traditional thin cantilever beam was adopted to absorb external vibration with arbitrary direction in three-dimensional (3D) spaces owing to its ability to bend flexibly and stretch along arbitrary direction. Furthermore, multiple modes in the elastic thin beam contribute to a possibility to widen the working bandwidth with multiple resonant frequencies. The experimental results show that the harvester was capable of scavenging the vibration energy in 3D arbitrary directions; they also exhibited triple power peaks at about 16 Hz, 21 Hz, and 28 Hz with the powers of 330 μW, 313 μW, and 6 μW, respectively. In addition, human walking and water wave energies were successfully converted into electricity, proving that our harvester was practical to scavenge the time-variant or multi-directional vibration energies in our daily life.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Quasi-stable injection channels in a wakefield accelerator

The influence of initial position on the acceleration of externally injected electrons in a plasma wakefield is investigated. Test-particle simulations show previously unobserved complex structure in the parameter space, with quasi-stable injection channels forming for particles injected in narrow regions away from the wake centre. Particles injected into these channels remain in the wake for a considerable time after dephasing and as a result achieve significantly higher energy than their neighbours. The result is relevant to both the planning and optimisation of experiments making use of external injection.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Non-destructive evaluation of impact damage on carbon fiber laminates: Comparison between ESPI and Shearography

The aim of this paper is to investigate the ability of two different interferometric NDT techniques to detect and evaluate barely visible impact damage on composite laminates. The interferometric techniques allow to investigate large and complex structures. Electronic Speckle Pattern Interferometry (ESPI) works through real-time surface illumination by visible laser (i.e. 532 nm) and the range and the accuracy are related to the wavelength. While the ESPI works with the “classic” holographic configuration, that is reference beam and object beam, the Shearography uses the object image itself as reference: two object images are overlapped creating a shear image. This makes the method much less sensitive to external vibrations and noise but with one difference, it measures the first derivative of the displacement. In this work, different specimens at different impact energies have been investigated by means of both methods. The delaminated areas have been estimated and compared.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Modeling of reduced effective secondary electron emission yield from a velvet surface

Complex structures on a material surface can significantly reduce total secondary electron emission from that surface. A velvet is a surface that consists of an array of vertically standing whiskers. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at the bottom of the structure and on the sides of the velvet whiskers. We performed numerical simulations and developed an approximate analytical model that calculates the net secondary electron emission yield from a velvet surface as a function of the velvet whisker length and packing density, and the angle of incidence of primary electrons. We found that to suppress secondary electrons, the following condition on dimensionless parameters must be met: (π/2) DΑ tan θ >> 1, where theta is the angle of incidence of the primary electron from the normal, D is the fraction of surface area taken up by the velvet whisker bases, and A is the aspect ratio, A = h/r, the ratio of height to radius of the velvet whiskers. We find that velvets available today can reduce the secondary electron yield by 90% from the value of a flat surface. As a result, the values of optimal velvet whisker packing density that maximally suppresses the secondary electron emission yield are determined as a function of velvet aspect ratio and the electron angle of incidence.

36 MATERIALS SCIENCE↗
DOE OSTI2017

“Feathered” fractal surfaces to minimize secondary electron emission for a wide range of incident angles

Complex structures on a material surface can significantly reduce the total secondary electron emission from that surface. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at one point of the structure and intersecting another. We performed Monte Carlo calculations to demonstrate that fractal surfaces can reduce net secondary electron emission produced by the surface as compared to the flat surface. Specifically, we describe one surface, a “feathered” surface, which reduces the secondary electron emission yield more effectively than other previously considered configurations. Specifically, feathers grown onto a surface suppress secondary electron emission from shallow angles of incidence more effectively than velvet. Here, we find that, for the surface simulated, secondary electron emission yield remains below 20% of its un-suppressed value, even for shallow incident angles, where the velvet-only surface gives reduction factor of only 50%.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Modeling of reduced secondary electron emission yield from a foam or fuzz surface

Complex structures on a material surface can significantly reduce the total secondary electron emission yield from that surface. A foam or fuzz is a solid surface above which is placed a layer of isotropically aligned whiskers. Primary electrons that penetrate into this layer produce secondary electrons that become trapped and do not escape into the bulk plasma. In this manner the secondary electron yield (SEY) may be reduced. We developed an analytic model and conducted numerical simulations of secondary electron emission from a foam to determine the extent of SEY reduction. We find that the relevant condition for SEY minimization is $\bar{u}$≡AD/2>>1 while D <<1, where D is the volume fill fraction and A is the aspect ratio of the whisker layer, the ratio of the thickness of the layer to the radius of the fibers. As a result, we find that foam cannot reduce the SEY from a surface to less than 0.3 of its flat value.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Decoding the spectroscopic features and time scales of aqueous proton defects

Acid solutions demonstrate a variety of complex structural and dynamical features arising from the presence of multiple interacting reactive proton defects and counterions. Yet, disentangling the transient structural motifs of proton defects in the water hydrogen bond network and the mechanisms for their interconversion remains a formidable challenge. In this work, we use simulations treating the quantum nature of both the electrons and nuclei to show how the experimentally observed spectroscopic features and relaxation time scales can be elucidated using a physically transparent coordinate that encodes the overall asymmetry of the solvation environment of the proton defect. We demonstrate that this coordinate can be used both to discriminate the extremities of the features observed in the linear vibrational spectrum and to explain the molecular motions that give rise to the interconversion time scales observed in recent nonlinear experiments. This analysis provides a unified condensed-phase picture of the proton structure and dynamics that, at its extrema, encompasses proton sharing and spectroscopic features resembling the limiting Eigen [H 3 O(H 2 O) 3 ]+ and Zundel [H(H 2 O) 2 ]+ gas-phase structures, while also describing the rich variety of interconverting environments in the liquid phase.

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

Benchmarking semiclassical approaches to strong-field QED: Nonlinear Compton scattering in intense laser pulses

The recoil associated with photon emission is key to the dynamics of ultrarelativistic electrons in strong electromagnetic fields, as found in high-intensity laser-matter interactions and astrophysical environments such as neutron star magnetospheres. When the energy of the photon becomes comparable to that of the electron, it is necessary to use quantum electrodynamics (QED) to describe the dynamics accurately. However, computing the appropriate scattering matrix element from strong-field QED is not generally possible due to multiparticle effects and the complex structure of the electromagnetic fields. Therefore, these interactions are treated semiclassically, coupling probabilistic emission events to classical electrodynamics using rates calculated in the locally constant field approximation. Here, we provide comprehensive benchmarking of this approach against the exact QED calculation for nonlinear Compton scattering of electrons in an intense laser pulse. We find agreement at the percentage level between the photon spectra, as well as between the models' predictions of absorption from the background field, for normalized amplitudes a0 > 5. We discuss possible routes towards improved numerical methods and the implications of our results for the study of QED cascades.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Direct synthesis of biaxially textured nickel disilicide thin films by magnetron sputter deposition on low-cost metal tapes for flexible silicon devices

Nickel silicides are widely used as contact materials for electronic devices based on silicon (Si). However, they have been predominantly fabricated by annealing separate Ni and Si phases which leads to phase and structural complexity. In this letter, direct epitaxial growth of a single-phase nickel disilicide (NiSi 2 ) thin film by sputter deposition of NiSi 2 is achieved on low-cost and flexible Hastelloy tapes which offers a promising route to fabricate low-cost, flexible electronic devices. Biaxially textured titanium nitride (TiN) is applied as the seeding layer and the diffusion barrier under NiSi 2 . An epitaxial relationship of (001)$\langle$100$\rangle$NiSi 2 ∥ (001)$\langle$110$\rangle$TiN is observed with an extra-large lattice mismatch (~10.3%) between NiSi 2 and TiN. Both the bonding similarity and the passivation effect by hydrogen promote the epitaxial growth of NiSi 2 on TiN. The flat and smooth NiSi 2 thin film consists of grains with a size of 50–100 nm. An epitaxially grown Si film on NiSi 2 further demonstrates the potential of manufacturing high-performance Si flexible electronics with NiSi 2 /TiN/Hastelloy as the direct contact through this approach.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Perspective on ab initio phonon thermal transport

Coupling of the Peierls-Boltzmann equation with density functional theory paved the way for predictive thermal materials discovery and a variety of new physical insights into vibrational transport behaviors. Here, rapid theoretical and numerical developments have generated a wealth of thermal conductivity data and understanding of a wide variety of materials—1D, 2D, and bulk—for thermoelectric and thermal management applications. Nonetheless, modern ab initio descriptions of phonon thermal transport face challenges regarding the effects of defects, disorder, structural complexity, strong anharmonicity, quasiparticle couplings, and time and spatially varying perturbations. Highlighting recent research on these issues, this perspective explores opportunities to expand current ab initio phonon transport techniques beyond the paradigm of weakly perturbed crystals, to the wider variety of materials possible. Recent developments in phonon-defect interactions, complexity, disorder and anharmonicity, hydrodynamic transport, and the rising roles of molecular dynamics simulations, high throughput, and machine learning tools are included in this perspective. Lastly, as more sophisticated theoretical and computational methods continue to advance thermal transport predictions, novel vibrational physics and thermally functional materials will be discovered for improved energy technologies.

36 MATERIALS SCIENCE↗