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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Realizing one-dimensional quantum and high-frequency transport features in aligned single-walled carbon nanotube ropes

The superiority of the electronic transport properties of single-walled carbon nanotube (SWNT) ropes over SWNT mats is verified from low temperature and frequency-dependent transport. The overall change of resistance versus in nanotube mats shows that 3D variable range hopping is the dominant conduction mechanism within the 2–300 K range. The magneto-resistance (MR) is found to be predominantly negative with a parabolic nature, which can also be described by the hopping model. Although the positive upturn of the MR at low temperatures establishes the contribution from quantum interference, the inherent quantum transport in individual tubes is suppressed at elevated temperatures. Therefore, to minimize multi-channel effects from inter-tube interactions and other defects, two-terminal devices were fabricated from aligned SWNT (extracted from a mat) for low temperature transport as well as high-frequency measurements. In contrast to the mat, the aligned ropes exhibit step-like features in the differential conductance within the 80–300 K temperature range. The effects of plasmon propagation, unique to one dimension, were identified in electronic transport as a non-universal power-law dependence of the differential conductance on temperature and source-drain voltage. The complex impedance showed high power transmission capabilities up to 65 GHz as well as oscillations in the frequency range up to 30 GHz. The measurements suggest that aligned SWNT ropes have a realistic potential for high-speed device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Structural, Bulk Permittivity, and Magnetic Properties of Lead-Free Electronic Material: Ba{sub 1}Bi{sub 1}Cu{sub 1}Fe{sub 1}Ni{sub 1}Ti{sub 3}O{sub 12}

In this report, Ba{sub 1}Bi{sub 1}Cu{sub 1}Fe{sub 1}Ni{sub 1}Ti{sub 3}O{sub 12} (BBCFNTO termed further) ceramics were synthesized using a conventional ceramic processing route. The formation of multiple phases has been confirmed from by the XRD pattern at room temperature. The surface micrograph indicates the uniform distribution of grains with distinct grain boundary. The co-relation between the impedance and dielectric parameters was realized by using a phase sensitive meter over a good temperature and frequency range. The temperature- and frequency-dependent dielectric properties are linked to the conduction mechanism. The dielectric constant (K) and loss (tanδ) are increased sharply at high-temperature region, which is expected to be the onset of dipolar relaxation phenomena. The contribution of grain boundary (GBs) and grain (Gs) effects can be identified using the complex impedance spectroscopy. This synthesized material is investigated to model multifunctional devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

BiNb{sub 3}O{sub 9}, a metastable perovskite phase with Bi/vacancy ordering: Crystal structure and dielectric properties

The perovskite, BiNb{sub 3}O{sub 9}, is a metastable phase in the Bi{sub 2}O{sub 3}:Nb{sub 2}O{sub 5} system that forms only when cooled from a liquid phase. Crystals of BiNb{sub 3}O{sub 9} prepared in this way display pseudocubic 2a×2a×2a diffraction patterns, due to non-merohedral twinning of a tetragonal a×a×2a cell, with the doubled axis oriented along all three pseudocubic axes. The structure was refined using data collected on a twinned crystal to R{sub 1}=0.034 for 318 observed reflections. BiNb{sub 3}O{sub 9} has tetragonal symmetry, P4/mmm with a=3.9459(3) Å, c=7.8919(6) Å. Partial ordering of Bi atoms and vacancies occurs, with alternate (0 0 1) planes having 28% and 42% Bi occupancies. The Bi atoms are displaced from the A-site special positions by up to 0.4 Å. The compound exhibits a relatively high permittivity value of ∼230 at room temperature, and shows a sharp increase with increasing temperature towards an apparent diffuse phase transition above ∼180 °C, with an associated frequency dependent peak in the dielectric loss. - Graphical abstract: Structure for BiNb{sub 3}O{sub 9}, showing split Bi atoms. Highlights: ► First characterisation of a new perovskite phase with potentially useful dielectric properties. ► Solving of the structure using single crystal data on a multiply twinned crystal. ► Measurement of dielectric properties that show the phase has a high dielectric permittivity.

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

FOURIER ANALYSIS OF BLAZAR VARIABILITY: KLEIN–NISHINA EFFECTS AND THE JET SCATTERING ENVIRONMENT

The strong variability of blazars can be characterized by power spectral densities (PSDs) and Fourier frequency-dependent time lags. In previous work, we created a new theoretical formalism for describing the PSDs and time lags produced via a combination of stochastic particle injection and emission via the synchrotron, synchrotron self-Compton, and external Compton (EC) processes. This formalism used the Thomson cross section and simple δ-function approximations to model the synchrotron and Compton emissivities. Here we expand upon this work, using the full Compton cross section and detailed and accurate emissivities. Our results indicate good agreement between the PSDs computed using the δ-function approximations and those computed using the accurate expressions, provided the observed photons are produced primarily by electrons with energies exceeding the lower limit of the injected particle population. Breaks are found in the PSDs at frequencies corresponding to the cooling timescales of the electrons primarily responsible for the observed emission, and the associated time lags are related to the difference in electron cooling timescales between the two energy channels, as expected. If the electron cooling timescales can be determined from the observed time lags and/or the observed EC PSDs, then one could in principle use the method developed here to determine the energy of the external seed photon source for EC, which is an important unsolved problem in blazar physics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Giant dielectric response at low frequency of B-site modified Aurivillius oxide potassium bismuth titanate

Highlights: • K{sub 0.5}Bi{sub 4.5}Ti{sub 4-x}(Al{sub 0.5}V{sub 0.5}){sub x}O{sub 12} ceramics were synthesized by the conventional solid-state reaction method. • Giant dielectric response at low frequency were observed in the KBT-AlV ceramics. • A giant dielectric constant (5.4 × 10{sup 4} at 100 Hz) was obtained in the KBT-AlV specimens. - Abstract: K{sub 0.5}Bi{sub 4.5}Ti{sub 4-x}(Al{sub 0.5}V{sub 0.5}){sub x}O{sub 15} (KBT-AlV, x = 0.00, 0.04, 0.06 and 0.08) ceramics were prepared by using the conventional solid-state reaction method. Phase analysis was performed by X-ray diffraction, and micro-structural morphology was assessed by scanning electron microscopy. The Effect of aluminium and vanadium on the structures, morphologies and electric properties of the K{sub 0.5}Bi{sub 4.5}Ti{sub 4}O{sub 12} (KBT) ceramics were investigated. Frequency dependence of the dielectric constant exhibited that the dielectric constant of the KBT ceramic is enhanced gradually by the aluminium and vanadium modification and a giant dielectric constant (5.4 × 10{sup 4} at 100 Hz) can be observed in the KBT-AlV specimens. Moreover, the reasons for the giant dielectric constant of the KBT-AlV ceramics are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhanced dielectric nonlinearity in epitaxial Pb{sub 0.92}La{sub 0.08}Zr{sub 0.52}Ti{sub 0.48}O{sub 3} thin films

High quality c-axis oriented epitaxial Pb{sub 0.92}La{sub 0.08}Zr{sub 0.52}Ti{sub 0.48}O{sub 3} films were fabricated using pulsed laser deposition on (001) LaAlO{sub 3} substrates with conductive LaNiO{sub 3} buffers. Besides confirmation of the in-plane and out-of-plane orientations using X-ray diffraction, transmission electron microscopy study has revealed columnar structure across the film thickness with column width around 100 nm. Characterization of ferroelectric properties was carried out in comparison with polycrystalline Pb{sub 0.92}La{sub 0.08}Zr{sub 0.52}Ti{sub 0.48}O{sub 3} films to extract the effect of epitaxial growth. It is found that the ratio between the irreversible Rayleigh parameter and reversible parameter increased up to 0.028 cm/kV at 1 kHz on epitaxial samples, which is more than twice of that on their polycrystalline counterparts. While this ratio decreased to 0.022 cm/kV with increasing frequency to100 kHz, a much less frequency dependence was observed as compared to the polycrystalline case. The epitaxial Pb{sub 0.92}La{sub 0.08}Zr{sub 0.52}Ti{sub 0.48}O{sub 3} films exhibited a higher mobility of domain wall and the higher extrinsic contribution to the dielectric properties, as well as reduced density of defects, indicating that it is promising for tunable and low power consumption devices.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Analysis of Casimir forces with window functions: Kramers-Kronig general approach for real measured dielectric data

In this work we describe how the window method, which has been used with ideal Kramers-Kroning (K-K) consistent dielectric functions, is affected by real optical data where some degree of K-K inconsistency is always present. Analysis using measured data of the frequency-dependent dielectric function for gold and phase-change materials shows that the window method can in some cases amplify the errors introduced in the calculation of the Casimir force due to K-K inconsistency instead of minimizing them. The latter indicates that caution is required for the proper choice of the window function for dielectric data analysis and Casmir force calculations.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Impedance spectroscopy and conduction mechanism of LiAlP{sub 2}O{sub 7} material

Highlights: • X-ray diffraction analysis reveals the structure for LiAlP{sub 2}O{sub 7} compound. • Modulus study confirmed the grain relaxation. • The equivalent circuit consists of parallel combination of (R{sub g}//CPE{sub g}) series for region I and (R{sub g}//CPE{sub g}) series with CPE{sub e} for region II. • The conductivity is due to CBH model for region I and NSPT model for region II. - Abstract: LiAlP{sub 2}O{sub 7} compound has been obtained by standard solid-state reaction technique and found to crystallize in the monoclinic system with P2{sub 1/a} space group. The impedance plots are well fitted to two equivalent circuit models. The dc conductivity follows Arrhenius behavior with two different activation energies Ea(I) = 0.84 eV for T < 610 K and Ea(II) = 0.93 eV for T > 610 K. Actually, the values of activation energies obtained from the dc conductivity and the relaxation frequency are similar, and hence the transport in the titled compound can be described through a hopping mechanism, dominated by the motion of Li{sup +} ions. Besides, the frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. The NSPT model and the CBH model is the most applicable conduction mechanism in region (I) and (II), respectively. The ion transport is due to the hopping of Li{sup +} ions along [100] tunnels direction. The conduction mechanism is studied with the help of Elliot’s theory, and the Elliot’s parameters are determined.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Studies on structural, dielectric, and transport properties of Ni{sub 0.65}Zn{sub 0.35}Fe{sub 2}O{sub 4}

We report the crystal structure, dielectric, transport, and magnetic properties of Ni{sub 0.65}Zn{sub 0.35}Fe{sub 2}O{sub 4}. Rietveld refinement results of X-ray diffraction patterns confirm the phase formation of the material with cubic crystal structure (Fd3{sup ¯}m). The frequency dependent ac conductivity behavior obeys the Jonscher's power law and is explained using the jump relaxation model. The observed behavior of temperature dependent bulk conductivity is attributed to the variable-range hopping of localized polarons. The correlation of polaron conduction and high permittivity behavior of NZFO is established on the basis of long range and short range conduction mechanisms. The complex impedance spectra clearly show the contribution of both grain and grain boundary effect on the electrical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The polarization signature of local bulk flows

A large peculiar velocity of the intergalactic medium produces a Doppler shift of the cosmic microwave background with a frequency-dependent quadrupole term. This quadrupole will act as a source for polarization of the cosmic microwave background, creating a large-scale polarization anisotropy if the bulk flow is local and coherent on large scales. In the case where we are near the center of the moving region, the polarization signal is a pure quadrupole. We show that the signal is small, but detectable with future experiments for bulk flows as large as some recent reports.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Neutron noise calculations in a hexagonal geometry and comparison with analytical solutions

This paper presents the development of a neutronic and kinetic solver for hexagonal geometries. The tool is developed based on the diffusion theory with multi-energy groups and multi-groups of delayed neutron precursors allowing the solutions of forward and adjoint problems of static and dynamic states, and is applicable to both thermal and fast systems with hexagonal geometries. In the dynamic problems, the small stationary fluctuations of macroscopic cross sections are considered as noise sources, and then the induced first order noise is calculated fully in the frequency domain. Numerical algorithms for solving the static and noise equations are implemented with a spatial discretization based on finite differences and a power iterative solution. A coarse mesh finite difference method has been adopted for speeding up the convergence. Since no other numerical tool could calculate frequency-dependent noise in hexagonal geometry, validation calculations have been performed and benchmarked to analytical solutions based on a 2-D homogeneous system with two-energy groups and one-group of delayed neutron precursor, in which point-like perturbations of thermal absorption cross section at central and non-central positions are considered as noise sources. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2018

Study of Dissipative Losses in AC-Biased Mo/Au Bilayer Transition-Edge Sensors

We are developing kilo-pixel arrays of transition-edge sensors (TESs) for the X-ray Integral Field Unit on ESA’s Athena observatory. Previous measurements of AC-biased Mo/Au TESs have highlighted a frequency-dependent loss mechanism that results in broader transitions and worse spectral performance compared to the same devices measured under DC bias. In order to better understand the nature of this loss, we are now studying TES pixels in different geometric configurations. We present measurements on devices of different sizes and with different metal features used for noise mitigation and X-ray absorption. Our results show how the loss mechanism is strongly dependent upon the amount of metal in close proximity to the sensor and can be attributed to induced eddy current coupling to these features. We present a finite element model that successfully reproduces the magnitude and geometry dependence of the losses. Using this model, we present mitigation strategies that should reduce the losses to an acceptable level.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Dynamic Spectral Imaging of Decimetric Fiber Bursts in an Eruptive Solar Flare

Fiber bursts are a type of fine structure that is often superposed on type IV radio continuum emission during solar flares. Although studied for many decades, its physical exciter, emission mechanism, and association with the flare energy release remain unclear, partly due to the lack of simultaneous imaging observations. We report the first dynamic spectroscopic imaging observations of decimetric fiber bursts, which occurred during the rise phase of a long-duration eruptive flare on 2012 March 3, as obtained by the Karl G. Jansky Very Large Array in 1–2 GHz. Our results show that the fiber sources are located near and above one footpoint of the flare loops. The fiber source and the background continuum source are found to be co-spatial and share the same morphology. It is likely that they are associated with nonthermal electrons trapped in the converging magnetic fields near the footpoint, as supported by a persistent coronal hard X-ray source present during the flare rise phase. We analyze three groups of fiber bursts in detail with dynamic imaging spectroscopy and obtain their mean frequency-dependent centroid trajectories in projection. By using a barometric density model and magnetic field based on a potential field extrapolation, we further reconstruct the 3D source trajectories of fiber bursts, for comparison with expectations from the whistler wave model and two MHD-based models. We conclude that the observed fiber burst properties are consistent with an exciter moving at the propagation velocity expected for whistler waves, or models that posit similar exciter velocities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Structural and transport properties of double perovskite Dy{sub 2}NiMnO{sub 6}

Highlights: • Sol–gel citrate method is used to prepare the double perovskite Dy{sub 2}NiMnO{sub 6}. • Structure and dielectric relaxation of the sample are studied for nano and bulk phases. • The relaxation mechanism of the sample is modeled by Cole–Cole equation. • With increasing sintering temperature conductivity increases. • Electronic structures and magnetic properties have been studied by DFT calculations. - Abstract: The double perovskite oxide Dy{sub 2}NiMnO{sub 6} (DNMO) is synthesized in nano and bulk phase by the sol–gel citrate method. The Rietveld refinement of X-ray diffraction pattern of the sample at room temperature shows the monoclinic P2{sub 1}/n phase. Dielectric relaxation of the sample is investigated in the impedance and electric modulus formalisms in the frequency range from 50 Hz to 1 MHz and in the temperature range from 253 to 415 K. The Cole–Cole model is used to explain the relaxation mechanism in DNMO. The frequency-dependent maxima in the imaginary part of impedance are found to obey an Arrhenius law with activation energy of 0.346 and 0.344 eV for nano and bulk DNMO, respectively. A significant increase in conductivity of bulk DNMO has been observed than that of the nanoceramic. Electronic structures and magnetic properties of DNMO have been studied by performing first principles calculation based on density functional theory.

36 MATERIALS SCIENCE↗
DOE OSTI2016

MAXIMUM LIKELIHOOD FOREGROUND CLEANING FOR COSMIC MICROWAVE BACKGROUND POLARIMETERS IN THE PRESENCE OF SYSTEMATIC EFFECTS

We extend a general maximum likelihood foreground estimation for cosmic microwave background (CMB) polarization data to include estimation of instrumental systematic effects. We focus on two particular effects: frequency band measurement uncertainty and instrumentally induced frequency dependent polarization rotation. We assess the bias induced on the estimation of the B-mode polarization signal by these two systematic effects in the presence of instrumental noise and uncertainties in the polarization and spectral index of Galactic dust. Degeneracies between uncertainties in the band and polarization angle calibration measurements and in the dust spectral index and polarization increase the uncertainty in the extracted CMB B-mode power, and may give rise to a biased estimate. We provide a quantitative assessment of the potential bias and increased uncertainty in an example experimental configuration. For example, we find that with 10% polarized dust, a tensor to scalar ratio of r = 0.05, and the instrumental configuration of the E and B experiment balloon payload, the estimated CMB B-mode power spectrum is recovered without bias when the frequency band measurement has 5% uncertainty or less, and the polarization angle calibration has an uncertainty of up to 4°.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Cement paste prior to setting: A rheological approach

The evolution of cement paste during the dormant period is analyzed via small amplitude oscillation rheological measurements. Cement paste, from the very first moments after mixing cement and water, shows the formation of an elastic gel whose strength is rapidly increasing over time. Up to the onset of Portlandite precipitation G′(t) increases by more than 2 orders of magnitude and in the acceleratory period G′(t) continues steadily to increase. A microstructural modification is likely to occur between the dormant and the acceleratory period. At low deformations in the linearity domain the storage modulus G′(ω) exhibits a negligible frequency dependence. At higher deformations cement paste shows a yield stress which increases on increasing paste concentration. The presence of superplasticizers decreases the yield stress and increases the gelation threshold of the paste. Above the gelation threshold the evolution of cement paste with superplasticizers follows similar trends to the neat paste. -- Highlights: •The gelation of cement paste during the dormant period is analyzed via rheometry. •The observed evolution is proposed to be related to the pore structure refinement. •Similarities are observed with colloidal gels and colloidal glasses.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Lanthanide contraction effect on crystal structures, magnetic, and dielectric properties in ordered double perovskites LnPbCoSbO{sub 6} (Ln = La, Pr, Nd)

The crystal structures, magnetic, and dielectric properties for the ordered double perovskites LnPbCoSbO{sub 6} (Ln = La, Pr, Nd) have been investigated. The crystal structure has been solved by Rietveld refinements of X-ray diffraction data in the monoclinic space group P2{sub 1}/n (No. 14). The Co{sup 2+} and Sb{sup 5+} ions are almost fully ordered over the B-site, and the octahedral framework displays significant tilting distortion according to the Glazer's tilt system a{sup –}a{sup –}c{sup +}. As the result of lanthanide contraction from La{sup 3+} to Nd{sup 3+}, the B-site sublattice distortions become stronger accompanying with the reduction of the tolerance factor and coordination number. The magnetization measurements show an antiferromagnetic ordering with large effective magnetic moments (μ{sub eff}) suggesting that the orbital component is significant. The maximum values of isothermal magnetization increase with the decrease in radii of rare earth ions, which is attributed to the weakening of antiferromagnetic interaction via Co{sup 2+}–O–Sb{sup 5+}–O–Co{sup 2+} paths. The dielectric constants present frequency dependence and monotonically decrease with the ionic radii reduction from La{sup 3+} to Nd{sup 3+} due to the suppression of electron transfer. These results indicate that the magnetic and dielectric properties can be tuned by controlling the degree of lattice distortion, which is realized by introducing different Ln{sup 3+} ions at the A-site.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Contribution of an extrinsic mechanism for the electrical polarization in BiMn{sub 2}O{sub 5} ceramics

DC conductivity, frequency dependent dielectric constant and pyroelectric coefficients, obtained from thermal stimulated depolarization current curves, in BiMn{sub 2}O{sub 5} ceramics in the range of temperatures from 10 K to 320 K are reported. The data could be explained if it is assumed that a dipole defect is formed due to an oxygen vacancy and two manganese ions which have their valence changed to accept an electron.

36 MATERIALS SCIENCE↗