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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 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

New perspectives on superparameterization for geophysical turbulence

This is a research expository paper regarding superparameterization, a class of multi-scale numerical methods designed to cope with the intermittent multi-scale effects of inhomogeneous geophysical turbulence where energy often inverse-cascades from the unresolved scales to the large scales through the effects of waves, jets, vortices, and latent heat release from moist processes. Original as well as sparse space–time superparameterization algorithms are discussed for the important case of moist atmospheric convection including the role of multi-scale asymptotic methods in providing self-consistent constraints on superparameterization algorithms and related deterministic and stochastic multi-cloud parameterizations. Test models for the statistical numerical analysis of superparameterization algorithms are discussed both to elucidate the performance of the basic algorithms and to test their potential role in efficient multi-scale data assimilation. The very recent development of grid-free seamless stochastic superparameterization methods for geophysical turbulence appropriate for “eddy-permitting” mesoscale ocean turbulence is presented here including a general formulation and illustrative applications to two-layer quasigeostrophic turbulence, and another difficult test case involving one-dimensional models of dispersive wave turbulence. This last test case has randomly generated solitons as coherent structures which collapse and radiate wave energy back to the larger scales, resulting in strong direct and inverse turbulent energy cascades.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Size effects on thermoelectric behavior of ultrathin Na{sub x}CoO{sub 2} films

Size effects in thermoelectric Na{sub x}CoO{sub 2} thin films are studied, focusing on the electrical resisitivity and Seebeck coefficient. For very thin films below 10 nm, we have observed an increase in resistivity, which is in agreement with theoretical models. In contrast to a predicted simultaneous suppression of the Seebeck coefficient for ultrathin films, we observe a constant Seebeck coefficient as a function of layer thickness due to changes in the structural properties as well as the presence of strong electron correlations. This preserved high Seebeck coefficient opens up new directions for Na{sub x}CoO{sub 2} ultrathin films as basic building blocks in thermoelectric superlattices with enhanced phonon scattering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Large magnetoresistance in current-perpendicular-to-plane pseudo spin-valves using Co{sub 2}Fe(Ga{sub 0.5}Ge{sub 0.5}) Heusler alloy and AgZn spacer

Fully epitaxial pseudo spin-valves (PSVs) using 10-nm-thick Co{sub 2}Fe(Ga{sub 0.5}Ge{sub 0.5}) (CFGG) ferromagnetic layers and a 5-nm-thick AgZn space layer annealed at 630 °C show a large current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) output with resistance-change area product, ΔRA, of 21.5 mΩ μm{sup 2} and MR ratio of 59.6% at room temperature. These values are substantially enhanced to ΔRA of 59.8 mΩ μm{sup 2} and MR ratio of 200.0% at 10 K. The large MR is attributed to the high spin polarization of the CFGG electrodes with the enhanced L2{sub 1} ordering induced by the atomic diffusion of Zn through the CFGG layers. The CPP-PSV shows relatively large ΔRA of 10.9 mΩ μm{sup 2} with the MR ratio of 25.6% for the low annealing temperature of 350 °C, which is a practically useful feature for read sensor applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Solvent effect on polystyrene surface roughness on top of QCM sensor

Quartz Crystal Microbalance (QCM) has been used as a basis for many chemical sensors and biosensor. Its sensitivity to mass change which can detect a mass change on its surface down to sub ng/cm2 is one of its interesting aspects. Another interesting feature is its ability to work in liquid environment. However, there are many aspects which influence QCM sensor properties in contact with liquid. One of the aspects is surface roughness of the matrix layer where on top of it a biological sensitive layer will be immobilized. One of matrix layers in the immobilizing biological sensitive layer was polystyrene. Polystyrene was coated on the QCM sensor by using the spin coating method. During the coating process, polystyrene was solved using non-polar solvent. It is known that the physical and chemical properties of the solvent affect a transition process from soluble polymer becoming rigid polymer layer. In this work, we show that polystyrene solved in chloroform has a higher surface roughness compare to one solved in toluene, xylene, or tetrahydrofuran. Surface roughness of the polystyrene coating were measured using a non-contact profilometer. However, we also found that there is no difference on the electrical impedance of the QCM sensor coated with polystyrene resulted from differing solvent when the sensor was in contact with air and water. Thus, all of the mentioned solvent can be used to solve the polystyrene as a coating material for QCM sensor without affecting the electrical performance of the sensor, but the choice of the solution can be used as a simple method to control the difference roughness of the polystyrene coating.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Hardening of the surface layers of commercial pure titanium VT1-0 under combined treatment

The treatment of VT1-0 titanium samples was carried out by concentrated energy fluxes. The combined treatment included surface carburizing with the joint use of powder samples of compounds with high physical and mechanical properties (namely, titanium diboride TiB{sub 2}, silicon carbide SiC and zirconium oxide ZrO{sub 2}) and subsequent electron beam treatment of surface layers formed in electroexplosive treatment. The combined treatment of surface layers resulted in the multifold increase in microhardness, which reduces depending on the depth of hardening zone. After electron-beam treatment, the depth of hardening zone is increased. During electron-beam treatment, the two-layer hardening zone forms.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Direct‐Write Printed Contacts to Layered and 2D Materials

Advancements in fabrication methods have shaped new computing device technologies. Among these methods, depositing electrical contacts to the channel material is fundamental to device characterization. Novel layered and 2D materials are promising for next-generation computing electronic channel materials. Direct-write printing of conductive inks is introduced as a surprisingly effective, significantly faster, and cleaner method to contact different classes of layered materials, including graphene (semi-metal), MoS 2 (semiconductor), Bi-2212 (superconductor), and Fe 5 GeTe 2 (metallic ferromagnet). Based on the electrical response, the quality of the printed contacts is comparable to what is achievable with resist-based lithography techniques. These devices are tested by sweeping gate voltage, temperature, and magnetic field to show that the materials remain pristine post-processing. This work demonstrates that direct-write printing is an agile method for prototyping and characterizing the electrical properties of novel layered materials.

2D materials↗
DOE OSTI2025

Halide perovskite-polymer composite film for bright and stable light-emitting devices

Stability is the primary hindrance for the application of halide perovskite material in light-emitting devices, solar cells, and other devices. In this work, halide perovskite and polymer composite film have been prepared for stable and bright light-emitting devices. Pure-phase Cs4PbBr6 crystals have been synthesized, and their photoluminescence (PL) properties and fluorescence lifetimes have been investigated. The Cs 4 PbBr 6 crystals exhibited high uniformity but underwent rapid photodegradation under light irradiation. To address this issue, we prepared bright light-emitting devices using composite of Cs 4 PbBr 6 crystals and polyethylene oxide (PEO) as the emission layer. The aim was to improve the optical and physical properties of halide perovskites, such as photodegradation and stability. PEO, with its excellent film-forming ability, created a uniform and dense film on the halide perovskite surface, filling microscopic defects and providing a protective barrier. FTIR, morphology, and PL analyses confirmed the protective role of the halide perovskite and polymer composite film. The composite film light-emitting devices demonstrated improved stability and higher PL brightness, with a peak brightness approaching 3 × 10 8 cd/m 2 , which was approximately 75% higher than the pure halide perovskite devices.

Composite film↗
DOE OSTI2020

Illuminating White Dwarf Spectra through Laboratory Experiments at Cosmic Conditions

We establish the work on white dwarf stars in the larger context of the experiments of the Wootton Center for Astrophysical Plasma Properties (WCAPP); these experiments are conducted at the plasma conditions found in the cosmos, without the need for scaling. We briefly summarize the results of these experiments to-date and their astrophysical and physical import, before focusing on the white dwarf experiments. Over 97% of stars either are, or will become, white dwarf stars, giving them broad relevance. We describe the astrophysical and physical problems associated with white dwarf photospheres, the plasma where the observed light arises. Here, the astrophysical questions include the age of the universe, the age and history of star formation in our Galaxy's varied morphological components and the evolution of stars. The compact nature of these ubiquitous stars means that the atomic physics is not well constrained even in the outermost layers. Further, it suggests that many important processes, including crystallization in dense Coulomb plasma, occur and have a significant effect on the evolution and structure of these stars and thus all their many applications.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

On the origins of hardness of Cu–TiN nanolayered composites

We investigated the mechanical response of physical vapor deposited Cu–TiN nanolayered composites of varying layer thicknesses from 5 nm to 200 nm. Both the Cu and TiN layers were found to consist of single phase nanometer sized grains. The grain sizes in the Cu and TiN layers, measured using transmission electron microscopy and X-ray diffraction, were found to be comparable to or smaller than their respective layer thicknesses. Indentation hardness testing revealed that the hardness of such nanolayered composites exhibits a weak dependence on the layer thickness but is more correlated to their grain size.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrafast Wavefront Shaping via Space-Time Refraction

A myriad of metasurfaces have been demonstrated that manipulate light by spatially structuring thin optical layers. Manipulation of the optical properties of such layers in both space and time can unlock new physical phenomena and enable new optical devices. Examples include photon acceleration and frequency conversion, which modifies Snell’s relation to a more general, nonreciprocal form. Here, we combine theory and experiment to realize wavefront shaping and frequency conversion on subpicosecond time-scales by inducing space-time refractive index gradients in epsilon-near-zero (ENZ) films with femtosecond light pulses. Furthermore, we experimentally tune wavefront steering by controlling the incident angle of the beams and the pump–probe delay without the need for nanostructure fabrication. As a demonstration of this approach, we leverage the ultrafast, high-bandwidth optical response of transparent oxides in their ENZ wavelength range to create large refractive index gradients and new types of nonreciprocal, ultrafast two-dimensional (2D) optics, including an ultrathin transient lens.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

Since the celebrated discovery of graphene, the family of two-dimensional (2D) materials has grown to encompass a broad range of electronic properties. Recent additions include spin-valley coupled semiconductors, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semi-metals with edge transport. Despite this progress, there is still no 2D crystal with intrinsic magnetism, which would be useful for many technologies such as sensing, information, and data storage. Theoretically, magnetic order is prohibited in the 2D isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. However, magnetic anisotropy removes this restriction and enables, for instance, the occurrence of 2D Ising ferromagnetism. Here, we use magneto-optical Kerr effect (MOKE) microscopy to demonstrate that monolayer chromium triiodide (CrI 3 ) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 K is only slightly lower than the 61 K of the bulk crystal, consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phases, showcasing the hallmark thickness dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, while in trilayer the interlayer ferromagnetism observed in the bulk crystal is restored. Our work creates opportunities for studying magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering for novel interface phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Altering surface fluctuations by blending tethered and untethered chains

"Partially tethering" a thin film of a polymer melt by covalently attaching to the substrate a fraction of the chains in an unentangled melt dramatically increases the relaxation time of the surface height fluctuations. This phenomenon is observed even when the film thickness, h, is 20 times the unperturbed chain radius, R g,tethered , of the tethered chains, indicating that partial tethering is more influential than any physical attraction with the substrate. Furthermore, a partially tethered layer of a low average molecular weight of 5k showed much slower surface fluctuations than did a reference layer of pure untethered chains of much greater molecular weight (48k), so the partial tethering effect is stronger than the effects of entanglement and increase in glass transition temperature, Tg, with molecular weight. Partial tethering offers a means of tailoring these fluctuations which influence wetting, adhesion, and tribology of the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2013

The liquid surface of chiral ionic liquids as seen from molecular dynamics simulations combined with intrinsic analysis

We present molecular-level insight into the liquid/gas interface of two chiral room-temperature ionic liquids (RTILs) derived from 1-n-butyl-3-methylimidazolium bromide ([bmim][Br]); namely, (R)-1-butyl-3-(3-hydroxy-2-methylpropyl)imidazolium bromide (hydroxypropyl) and 1-butyl-3-[(1R)-nopyl]imidazolium bromide (nopyl). We use our currently developed force field which was validated against the experimental bulk density, heat of vaporization, and surface tension of [bmim][Br]. The force field for the RTILs adopts the Chemistry at Harvard Molecular Mechanics (CHARMM) parameters for the intramolecular and repulsion-dispersion interactions along with the reduced partial atomic charges based on ab initio calculations. The net charges of the ions are around ±0.8e, which mimic the anion to cation charge transfer and many-body effects. Molecular dynamics simulations in the slab geometry combined with the intrinsic interface analysis are employed to provide a detailed description of the RTIL/gas interface in terms of the structural and dynamic properties of the interfacial, sub-interfacial, and central layers at a temperature of 300 K. The focus is on the comparison of the liquid/gas interface for the chiral RTILs with the interface for parent [bmim][Br]. The structure of the interface is elucidated by evaluating the surface roughness, intrinsic atomic density profiles, and orientation ordering of the cations. The dynamics of the ions at the interfacial region is characterized by computing the survival probability, and normal and lateral self-diffusion coefficients in the layers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Optical isolation via unidirectional resonant photon tunneling

We show that tri-layer structures combining epsilon-negative and magneto-optical material layers can exhibit unidirectional resonant photon tunneling phenomena that can discriminate between circularly polarized (CP) waves of given handedness impinging from opposite directions, or between CP waves with different handedness impinging from the same direction. This physical principle, which can also be interpreted in terms of a Fabry-Perot-type resonance, may be utilized to design compact optical isolators for CP waves. Within this framework, we derive simple analytical conditions and design formulae, and quantitatively assess the isolation performance, also taking into account the unavoidable imperfections and nonidealities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spin dependent transport properties of Mn-Ga/MgO/Mn-Ga magnetic tunnel junctions with metal(Mg, Co, Cr) insertion layer

We report a first principles theoretical investigation of spin polarized quantum transport in Mn{sub 2}Ga/MgO/Mn{sub 2}Ga and Mn{sub 3}Ga/MgO/Mn{sub 3}Ga magnetic tunneling junctions (MTJs) with the consideration of metal(Mg, Co, Cr) insertion layer effect. By changing the concentration of Mn, our calculation shows a considerable disparity in transport properties: A tunneling magnetoresistance (TMR) ratio of 852% was obtained for Mn{sub 2}Ga-based MTJs, however, only a 5% TMR ratio for Mn{sub 3}Ga-based MTJs. In addition, the influence of insertion layer has been considered in our calculation. We found the Co insertion layer can increase the TMR of Mn{sub 2}Ga-based MTJ to 904%; however, the Cr insertion layer can decrease the TMR by 668%; A negative TMR ratio can be obtained with Mg insertion layer. Our work gives a comprehensive understanding of the influence of different insertion layer in Mn-Ga based MTJs. It is proved that, due to the transmission can be modulated by the interfacial electronic structure of insertion, the magnetoresistance ratio of Mn{sub 2}Ga/MgO/Mn{sub 2}Ga MTJ can be improved by inserting Co layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Communication: Molecular simulation study of kaolinite intercalation with realistic layer size

Intercalation phenomena of kaolinite in aqueous potassium acetate and in hexyl-amine solutions are studied by large scale molecular dynamics simulations. The simulated kaolinite particle is constructed from ∼6.5 × 10{sup 6} atoms, producing a particle size of ∼100 nm × 100 nm × 10 nm. The simulation with potassium acetate results in a stable kaolinite-potassium acetate complex, with a basal spacing that is in close agreement with experimental data. The simulation with hexyl-amine shows signs of the experimentally observed delamination of kaolinite (the initial phase of the formation of nanoscrolls from the external layers)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

The density gradient effect on quantum Weibel instability

The Weibel instability plays an important role in stopping the hot electrons and energy deposition mechanism in the fast ignition of inertial fusion process. In this paper, the effects of the density gradient and degeneracy on Weibel instability growth rate are investigated. Calculations show that decreasing the density degenerate in the plasma corona, near the relativistic electron beam emitting region by 8.5% leads to a 92% reduction in the degeneracy parameter and about 90% reduction in Weibel instability growth rate. Also, decreasing the degenerate density near the fuel core by 8.5% leads to 1% reduction in the degeneracy parameter and about 8.5% reduction in Weibel instability growth rate. The Weibel instability growth rate shrinks to zero and the deposition condition of relativistic electron beam energy can be shifted to the fuel core for a suitable ignition by increasing the degeneracy parameter in the first layer of plasma corona.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Marangoni mixed convection flow with Joule heating and nonlinear radiation

Marangoni mixed convective flow of Casson fluid in a thermally stratified medium is addressed. Flow analysis has been carried out in presence of inclined magnetic field. Heat transfer analysis is discussed in the presence of viscous dissipation, Joule heating and nonlinear thermal radiation. The governing nonlinear partial differential equations are first converted into ordinary differential systems and then developed the convergent series solutions. Flow pattern with the influence of pertinent parameters namely the magnetic parameter, Casson fluid parameter, temperature ratio parameter, stratification parameter, Prandtl number, Eckert number and radiation parameter is investigated. Expression of local Nusselt number is computed and analyzed. It is found that the Nusselt number decreases by increasing magnetic parameter, temperature ratio parameter, angle of inclination and stratification parameter. Moreover the effect of buoyancy parameter on the velocity distribution is opposite in both the opposing and assisting flow phenomena. Thermal field and associated layer thickness are enhanced for larger radiation parameter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗