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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Contribution to the Study of Ferrite Nanobeads: Synthesis, Characterization and Investigation of Horizontal Low Gradient Magnetophoresis Behaviour

In this work we investigate the possibilities of the use of Horizontal Low Gradient Magnetic Field (HLGMF)(<100 T/m) for filtration, control and separation of the synthesized magnetic particles, considering, the characteristics of the suspension, the size and the type of nanoparticles (NPs) and focusing on the process scale up. Reversible aggregation is considered in the different steps of magnetic nanobeads synthesis. For these purpose, we synthesized Fe{sub 2}O{sub 3}-silica core-shell nanobeads by co-precipitation, monodispersion and silica coating. SQUID, TEM, XRD, and Zeta potential techniques were used to characterize the synthesized nanobeads. An extensive magnetophoresis study was performed at different magnetophoretic conditions. Different reversible aggregation times were observed at different HLGMF, at each step of the synthesis route: Several orders of magnitude differences where observed when comparing citric acid (CA) suspension with silicon coated beads. Reversible aggregation times are correlated with the properties of the NPs at different steps of synthesis.

36 MATERIALS SCIENCE↗
DOE OSTI2011

CdTe-Cu(OH){sub 2} nanocomposite: Aqueous synthesis and characterization

CdTe-Cu(OH){sub 2} nanocomposites were synthesized in aqueous solution by a seed-mediated growth approach. The effect of refluxing time and the concentration of Cu{sup 2+} on the preparation of these samples were measured using UV-visible absorption and photoluminescence analysis. The emission peak of the synthesized nanocomposites (CdTe-Cu(OH){sub 2}) was shifted from 605 (CdTe seed) to 621 nm. The size of CdTe nanoparticles were averaged about 3.22 nm, and the CdTe-Cu(OH){sub 2} nanocomposites were averaged as 5.19 nm. The synthesized CdTe-Cu(OH){sub 2} nanocomposite were characterized with XRD, EDAX, TEM, FT-IR, EPR, and thermal analysis (TG/DTG curves). The results indicate that as-prepared nanoparticles with core/shell structure exhibit interesting optical properties. -- Graphical Abstract: Schematic of aqueous synthesis route for CdTe-Cu(OH){sub 2} nanocomposite and The Stokes shift of CdTe nanocrystals and CdTe-Cu(OH){sub 2} Nanocomposites, (CdTe: emission at 605 nm, CdTe-Cu(OH){sub 2}: emission at 621 nm). Display Omitted Highlights: {yields} CdTe-Cu(OH){sub 2} nanocomposites were synthesized by a seed-mediated growth approach. {yields} The synthetic procedure is simple, and can be easily scaled up. {yields} The effect of refluxing time on the preparation of these samples was measured. {yields} The Cu(OH){sub 2} shell thickness was controlled by the amount of Cu in the solution. {yields} TEM images demonstrated homogeneous size distribution for these nanocomposites.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Tunable interaction between excitons and hybridized magnons in a layered semiconductor

The interaction between distinct excitations in solids is of both fundamental interest and technological importance. One example of such interactions is coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors1 provides a powerful platform for exploring these exciton-magnon interactions and their fundamental properties, such as strong correlation2, as well as their photo-spintronic and quantum transduction3 applications. Here we demonstrate precise control of coherent exciton-magnon interactions in the layered magnetic semiconductor CrSBr. We show that by controlling the direction of applied magnetic fields relative to the crystal axes, and thus the rotational symmetry of the magnetic system4, we can tune not only the exciton coupling to the bright magnon, but also to an optically dark mode via magnon hybridization. The exciton-magnon coupling and associated magnon dispersion curves can be further modulated by applying a uniaxial strain. At the critical strain, a dispersionless dark magnon band emerges. Our results demonstrate unprecedented control of the opto-mechanical-magnonic coupling, and a step towards the predictable and controllable implementation of hybrid quantum magnonics.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

A versatile apparatus for the fine-tuned synthesis of cluster-based materials

In this paper, a custom-designed experimental setup for the fine-tuned synthesis of various cluster–based materials is presented. Providing custom-designed deposition stages and special sample holders it offers a high degree of control over the sample characteristics such as the cluster size, cluster amounts, and sample homogeneity in combination with high sample purity. The system is capable of producing thin films of pure clusters and various cluster-matrix combinations with cluster sizes ranging from single atoms up to aggregates of several thousand atoms. Two custom-designed deposition stages are available, one utilizes the full cluster beam, yielding micrograms of pure clusters within a few hours and the second one uses mass-separated clusters (mass-resolution between 2% and 10%), yielding nanograms of pure clusters in the same timescale. Furthermore, at the second deposition stage, a variety of matrix materials can be co-deposited at a controlled sample temperature between 153 K and 673 K. In order to prove the capabilities of the apparatus, a series of experiments with Fe clusters embedded in Ag matrices with different volume fractions of clusters were carried out. Energy dispersive X-ray spectroscopy measurements revealed that the amount of deposited clusters as well as the sample homogeneity can be controlled with an outstanding accuracy of 10%. Additional measurements of the magnetic properties indicated the presence of separated clusters for volume fractions of clusters around 2 volume percent (vol. %), while above this concentration (10 vol. %) a partial aggregation of the clusters was observed. It was also shown that the resulting thin films are nearly oxygen free, which ascertains that oxygen sensitive materials can be safely handled in this newly developed apparatus.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Space dependent, full orbit effects on runaway electron dynamics in tokamak plasmas

The dynamics of RE (runaway electrons) in fusion plasmas span a wide range of temporal scales, from the fast gyro-motion, ∼10−11 s, to the observational time scales, ∼10−2→1 s. To cope with this scale separation, RE are usually studied within the bounce-average or the guiding center approximations. Although these approximations have yielded valuable insights, a study with predictive capabilities of RE in fusion plasmas calls for the incorporation of full orbit effects in configuration space in the presence of three-dimensional magnetic fields. We present numerical results on this problem using the Kinetic Orbit Runaway electrons Code that follows relativistic electrons in general electric and magnetic fields under the full Lorentz force, collisions, and radiation losses. At relativistic energies, the main energy loss is due to radiation damping, which we incorporate using the Landau-Lifshitz formulation of the Abraham-Lorentz-Dirac force. The main focus is on full orbit effects on synchrotron radiation. It is shown that even in the absence of magnetic field stochasticty, neglecting orbit dynamics can introduce significant errors in the computation of the total radiated power and the synchrotron spectra. The statistics of collisionless (i.e., full orbit induced) pitch angle dispersion, and its key role played on synchrotron radiation, are studied in detail. Numerical results are also presented on the pitch angle dependence of the spatial confinement of RE and on full orbit effects on the competition of electric field acceleration and radiation damping. Finally, full orbit calculations are used to explore the limitations of gyro-averaging in the relativistic regime. To explore the practical impact of the results, DIII-D and ITER-like parameters are used in the simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Interatomic interactions and thermally induced shifts and broadenings of energy levels of atoms in circular Rydberg states

Analytical expressions are obtained for the constants determining the energy of the long-range (at large distances exceeding the LeRoy radius R{sub LR} ≈ 4n{sup 2}) polarisation interaction of the multipole electric moments of two atoms in identical Rydberg states with large principal (n) and maximum orbital (l) and magnetic (m) quantum numbers (|m| = l = n − 1). Asymptotic expansions in powers of n are found for the components of the van der Waals interaction tensor, which determine the dependence of the dispersion energy on the orientation of the interatomic axis. The spontaneous and thermally induced broadenings of the energy levels of circular states are represented analytically as functions of the principal quantum number and temperature. It is found that analytical formulae for the sum of spontaneous and thermally induced line widths in the region of high temperatures and large principal quantum numbers (assuming n{sup 3}k{sub B}T >> 1) do not contain a temperature-independent contribution of the spontaneous part of the broadening. Closed analytical expressions are also obtained for the Farley − Wing function and for the n- and T-dependent corrections to the asymptotic behaviour of the thermally induced shift. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

MICROWAVE QUASI-PERIODIC PULSATIONS IN MULTI-TIMESCALES ASSOCIATED WITH A SOLAR FLARE/CME EVENT

Microwave observations of quasi-periodic pulsations (QPPs) in multi-timescales at the Solar Broadband Radio Spectrometer in Huairou (SBRS/Huairou) on 2006 December 13 are confirmed to be associated with an X3.4 flare/coronal mass ejection (CME) event. It is most remarkable that the timescales of QPPs are distributed in a broad range from hectoseconds (very long period pulsation, VLP, P>100 s), decaseconds (long period pulsation, LPP, 10 < P < 100 s), a few seconds (short period pulsation, SPP, 1 < P < 10 s), deciseconds (slow very short period pulsation, slow-VSP, 0.1 < P < 1.0 s), to centiseconds (fast very short period pulsation, fast-VSP, P < 0.1 s), and form a broad hierarchy. The statistical distribution of QPPs in logarithmic period-duration space indicates that all the QPPs can be classified into two groups: group I includes VLP, LPP, SPP, and some slow-VSPs distributed approximately around a line; group II includes fast-VSP and most of the slow-VSPs dispersively distributed away from the above line. This feature implies that the generation mechanism of group I is different from group II. Group I is possibly related to some MHD oscillations in magnetized plasma loops in the active region; e.g., VLPs may be generated by standing slow sausage mode coupling, resonating with the underlying photospheric 5 minute oscillation, with the modulation amplified and forming the main framework of the whole flare/CME process; LPPs, SPPs, and some slow-VSPs are most likely to be caused by standing fast modes or LRC-circuit resonance in current-carrying plasma loops. Group II is possibly generated by modulations of resistive tearing-mode oscillations in electric current-carrying flaring loops.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Modulation sensitive search for nonvirialized dark-matter axions

Nonvirialized dark-matter axions may be present in the Milky Way halo in the form of low-velocity-dispersion flows. In this paper, the Axion Dark-Matter eXperiment performed a search for the conversion of these axions into microwave photons using a resonant cavity immersed in a strong, static magnetic field. The spread of photon energy in these measurements was measured at spectral resolutions of the order of 1 Hz and below. If the energy variation were this small, the frequency modulation of any real axion signal due to the orbital and rotational motion of Earth would become non-negligible. Conservative estimates of the expected signal modulation were made and used as a guide for the search procedure. The photon frequencies covered by this search are 812–852 and 858–892 MHz, which correspond to an axion mass of 3.36–3.52 and 3.55 – 3.69 μ eV . Finally, no axion signal was found, and limits were placed on the maximum local density of nonvirialized axions of these masses.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1976

Hot interstellar tunnels. 1: Simulation of interacting supernova remnants

The theory required to build a numerical simulation of interacting supernova remnants is developed. The hot cavities within a population of remnants will become connected, with varying ease and speed, for a variety of assumed conditions in the outer shells of old remnants. Apparently neither radiative cooling nor thermal conduction in a large-scale galactic magnetic field can destroy hot cavity regions, if they grow, faster than they are reheated by supernova shock waves, but interstellar mass motions disrupt the contiguity of extensive cavities necessary for the dispersal of these shocks over a wide volume. Monte Carlo simulations show that a quasi-equilibrium is reached in the test space within 10 million yrs of the first supernova and is characterized by an average cavity filling fraction of the interstellar volume. Aspects of this equilibrium are discussed for a range of supernova rates. Two predictions are not confirmed within this range: critical growth of hot regions to encompass the entire medium, and the efficient quenching of a remnant's expansion by interaction with other cavities.

Smith, B. W.↗
NASA NTRS1988

Excitations of low-frequency hydromagnetic waves by freshly created ions in the solar wind

Low-frequency hydromagnetic waves excited by newborn ions in the solar wind plasma are studied. The freshly created ions appear in the solar wind frame with a ring beam distribution. Both Alfven and fast magnetosonic waves are made unstable by the presence of the newborn ions. The dependence of the growth rate of both waves on the newborn ion density, the angle between the interplanetary magnetic field (IMF) and solar wind flow, and the angle of wave propagation relative to the IMF is investigated. Analytic approximations for the growth rates are presented, and numerical solutions of the dispersion equation are shown. The approximations are quite close to the numerically determined growth rates. It is found that the waves grow preferentially in the direction parallel to the IMF, and that the growth rates increase with both newborn ion density and the angle between the IMF and the solar wind flow.

Price, C. P.↗
NASA NTRS1995

An intercomparison of plasma turbulence at three comets: Grigg-Skjellerup, Giacobini-Zinner, and Halley

The power laws are approximately f(exp -1.9), f(exp -1.9), and f(exp -2.1) respectively for the Grigg-Shjellerup (GS), Giacobini-Zinner (GZ), and Halley (H) comets. Other than similarities in the power spectra, the magnetic field turbulence is considerably different at the three comets. Phase steepening is demonstrated to occur at the trailing edges of the GS waves. This is probably due to nonlinear steepening plus dispersion of the left-hand mode components, i.e., the turbulence is whistler-mode. This too can be explained by nonlinear steepening plus dispersion of the magnetosonic waves. At the level of GS and GZ turbulence development when the spacecraft measurements were made, classical three-wave processes, such as the decay or modulation instabilities do not appear to play important roles. It is most likely that the nonlinear steepening and dispersive time scales are more rapid than three-wave processes, and the latter had not had time to develop for the relatively new turbulence. The wave turbulence at Halley is linearly polarized. The exact nature of this turbulence is still not well understood. Several possibilities are suggested, based on a preliminary analyses.

Tsurutani, Bruce T.↗
NASA NTRS1995

An intercomparison of plasma turbulence at three comets: Grigg-Skjellerup, Giacobini-Zinner, and Halley

We examine and intercompare the LF plasma wave turbulence at three comets: Grigg-Skjellerup (GS), Giacobini-Zinner (GZ), and Halley (H). All three have power spectral peaks at the local ion cyclotron frequency (the pump wave) at approx. 10(exp -2) Hz, and a power-law fall-off at higher frequencies that suggest the development of turbulent cascades. The power laws for the three comets are approximately f(exp -1.9), f(exp -1.9) and f(exp -2.1), respectively. However, other than the similarities in the power spectra, we find the magnetic field turbulence is considerably different at the three comets. Phase steepening is demonstrated to occur at the trailing edges of the GS waves. This is probably due to nonlinear steepening plus dispersion of the left-hand mode components. A coherency analysis of GZ turbulence indicates that it is primarily composed of righthanded mode components, i.e., the turbulence is 'whistlermode.' This too can be explained by nonlinear steepening plus dispersion of the magnetosonic waves. At the level of GS and GZ turbulence development when the spacecraft measurements were made, classical three-wave processes, such as the decay or modulation instabilities do not appear to play important roles. It is most likely that the nonlinear steepening and dispersive time scales are more rapid than three-wave processes, and the latter had not had time to develop for the relatively 'new' turbulence. The wave turbulence at Halley is linearly polarized. The exact nature of this turbulence is still not well understood at this time. Several possibilities are suggested, based on our preliminary analyses.

Tsurutani, Bruce T.↗
DOE OSTI2019

Chemical, Morphological, Structural, Optical, and Magnetic Properties of Transition Metal Titanium (Ti)-Doped LaFeO{sub 3} Nanoparticles

In the present work, pure and transition metal titanium (Ti)-doped LaFe{sub 1−x}Ti{sub x}O{sub 3}(x = 0.0, 0.2, 0.4, and 0.6) nanoparticles have been successfully synthesized by hydrothermal synthesis technique for various molar ratios of 0.2, 0.4, and 0.6. The prepared samples were grinned and annealed in furnace at 600 {sup ∘}C/4 h. The X-ray diffraction (XRD) analysis confirms the phase orthorhombic formation of LaFeO{sub 3} and Ti-doped LaFeO{sub 3}. The stoichiometric analysis was performed through energy-dispersive X-ray studies (EDX). The micro-structural features of surface morphology were examined by field emission scanning electron microscopy (FESEM). Further, surface nanocrystalline of the titanium (Ti)-doped LaFe{sub 1−x}Ti{sub x}O{sub 3} was also investigated through high-resolution transmission electron microscopic (TEM) analysis. The enhancement of room temperature magnetization value has been observed from the M–H curve which is measured using vibrating sample magnetometer (VSM).The weak ferromagnetic behavior was observed from hysteresis loop. From hysteresis loop, saturation takes place when the applied magnetic field intensity is 15 kO{sub e}.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Propagation speed of rotation signals for field lines undergoing magnetic reconnection

Reconnection is associated with two bending of the magnetic field lines. Considering the usual plane of a 2D reconnection simulation, the first bending is in-plane and produces the needed topological changes by bringing oppositely directed filed lines in proximity. The second is typical of fast reconnection and is out of plane, leading to the formation of the Hall magnetic field. This second rotation has recently been observed to proceed at superAlfvénic speeds and to carry substantial energy fluxes (Shay et al., Phys. Rev. Lett. 107, 065001 (2011)). We revisit these rotations with a new diagnostics based on dispersing a multitude of virtual probes into a kinetic simulation, akin the approach of multi spacecraft missions. The results of the new diagnostics are compared with the theory of characteristics applied to the two fluid model. The comparison of virtual probes and the method of characteristics confirm the findings relative to the out of plane rotation and uncover the existence of two families of characteristics. Both are observed in the simulation. The early stage of reconnection develops on the slower compressional branch and the later faster phase develops on the faster torsional branch. The superAlfvénic signal is only relevant in the second phase.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Enhanced Mechanical and Barrier Performance of Poly (Lactic Acid) Based Nanocomposites Using Surface Acetylated Starch Nanocrystals

Poly (lactic) acid (PLA) based nanocomposites reinforced with starch nanocrystals (SNC) extracted from high amylopectin waxy maize starch and acetylated starch nanocrystals (Ac-SNC), were prepared and their influence on the overall properties of PLA nanocomposites were studied. The two nanofillers (SNC and Ac-SNC) were incorporated in PLA at two different loadings (1 and 3 wt%) by solvent casting and evaporation technique. Surface acetylation of SNC was confirmed by Fourier Transform infra-red spectroscopy and nuclear magnetic resonance spectroscopy and the crystalline structure by X-ray diffraction. This paper reports the influence of incorporating unacetylated and surface acetylated starch nanocrystals on morphological, barrier, mechanical, and rheological properties of PLA based nanocomposites. While both nanofillers improved the properties of PLA, the PLA-Ac-SNC had superior properties to that of PLA-SNC nanocomposites, due to better filler dispersion and interaction with the polymer matrix. These sustainable nanocomposites with improved properties will expand the application of these bio-sourced starch nanocrystals. Graphical Abstract: .

36 MATERIALS SCIENCE↗
DOE OSTI2012

RESOLVED RADIO EMISSION FROM MODELS OF PHOTOEVAPORATED DISKS AROUND MASSIVE YOUNG STARS

We study the radio continuum and thermal hydrogen radio recombination line (RRL) emission from photoevaporated disk wind models around massive young stars. We applied the models of Lugo and coworkers to the source MWC 349A. The resolved synthetic radio continuum maps reproduce the observed hourglass morphology at low frequency but are more flattened than the observations at high frequency because the density in the model decreases too fast. These photoevaporated wind models naturally produce RRLs with FWHM {Delta}v {approx} 60 km s{sup -1}. Nevertheless, recent H66{alpha} line observations of MWC 349A by Loinard and Rodriguez have an FWHM {Delta}v {approx} 89 km s{sup -1}. We propose that such wide lines could be produced by an extra magnetocentrifugal acceleration of the flow due to a poloidal magnetic field anchored in the disk. Such fields could also prevent the flow divergence and the fast density drop of the photoevaporated disk wind model. To mimic this effect we include in this model a large non-thermal velocity dispersion {sigma}{sub nt} {approx} 70 km s{sup -1}. The width of the RRLs of this modified model increases with quantum number. This is in contrast with the observed H76{alpha} and H92{alpha} lines which are narrower than the H66{alpha} line. We argue that the low-frequency observations could have suffered from insufficient bandwidth and that new measurements of these lines would be very valuable to constrain the models. Finally, the resolved H66{alpha} and H53{alpha} line emission maps show the velocity asymmetry expected from flow rotation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Effects of Hall current and electron temperature anisotropy on proton fire-hose instabilities

The standard magnetohydrodynamic (MHD) theory predicts that the Alfvén wave may become fire-hose unstable for β{sub ∥}−β{sub ⊥}>2. In this study, we examine the proton fire-hose instability (FHI) based on the gyrotropic two-fluid model, which incorporates the ion inertial effects arising from the Hall current and electron temperature anisotropy but neglects the electron inertia in the generalized Ohm's law. The linear dispersion relation is derived and analyzed which in the long wavelength approximation, λ{sub i}k→0 or α{sub e}=μ{sub 0}(p{sub ∥,e}−p{sub ⊥,e})/B{sup 2}=1, recovers the ideal MHD model with separate temperature for ions and electrons. Here, λ{sub i} is the ion inertial length and k is the wave number. For parallel propagation, both ion cyclotron and whistler waves become propagating and growing for β{sub ∥}−β{sub ⊥}>2+λ{sub i}{sup 2}k{sup 2}(α{sub e}−1){sup 2}/2. For oblique propagation, the necessary condition for FHI remains to be β{sub ∥}−β{sub ⊥}>2 and there exist one or two unstable fire-hose modes, which can be propagating and growing or purely growing. For large λ{sub i}k values, there exists no nearly parallel FHI leaving only oblique FHI and the effect of α{sub e}>1 may greatly enhance the growth rate of parallel and oblique FHI. The magnetic field polarization of FHI may be reversed due to the sign change associated with (α{sub e}−1) and the purely growing FHI may possess linear polarization while the propagating and growing FHI may possess right-handed or left-handed polarization.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Revealing Site Occupancy in a Complex Oxide: Terbium Iron Garnet

Complex oxide films stabilized by epitaxial growth can exhibit large populations of point defects which have important effects on their properties. The site occupancy of pulsed laser-deposited epitaxial terbium iron garnet (TbIG) films with excess terbium (Tb) is analyzed, in which the terbium:iron (Tb:Fe)ratio is 0.86 compared to the stoichiometric value of 0.6. The magnetic properties of the TbIG are sensitive to site occupancy, exhibiting a higher compensation temperature (by 90 K) and a lower Curie temperature (by 40 K) than the bulk Tb 3 Fe 5 O 12 garnet. Data derived from X-ray core-level spectroscopy, magnetometry, and molecular field coefficient modeling are consistent with occupancy of the dodecahedral sites by Tb 3+ , the octahedral sites by Fe 3+ , Tb 3+ and vacancies, and the tetrahedral sites by Fe 3+ and vacancies. Energy dispersive X-ray spectroscopy in a scanning transmission electron microscope provides direct evidence of Tb Fe antisites. A small fraction of Fe 2+ is present, and oxygen vacancies are inferred to be present to maintain charge neutrality. In conclusion, variation of the site occupancies provides a path to considerable manipulation of the magnetic properties of epitaxial iron garnet films and other complex oxides, which readily accommodate stoichiometries not found in their bulk counterparts.

36 MATERIALS SCIENCE↗