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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Giant dielectric permittivity and weak ferromagnetic behavior in Bi{sub 0.5}La{sub 0.5}Fe{sub 0.5}Cr{sub 0.5}O{sub 3} ceramic

A pervoskite (Bi{sub 0.5}La{sub 0.5})(Fe{sub 0.5}Cr{sub 0.5})O{sub 3} (BLFC) nanoparticles were synthesized by high energy ball milling. Rietveld refined X-ray diffraction studies revealed that this compound shows orthorhombic structure with Pbnm space group. The dielectric studies were investigated in wide frequency (10{sup 2}-10{sup 6}) range indicating giant dielectric permittivity behavior similar to LaFeO{sub 3} ceramic. The noted dielectric loss relaxation frequency dependent is as following the Arrhenius law can be ascribed as polaronic conduction. Further, magnetic transition at around 337 K and coexistence of weak ferromagnetic and antiferromagnetic behavior is observed below magnetic transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Polaritons of Triplet and Binary Hybrid Structures in a Microcavity

We analyze the properties of a triplet hybrid (semiconductor—organic layer–semiconductor) in a microcavity. Analysis is performed for excitation energies reduced in the triplet from the Wannier–Mott exciton with the highest energy to the Frenkel exciton with an intermediate energy and from the Frenkel exciton to the Wannier–Mott exciton with the lowest energy. The semiconductor components of the hybrid have the form of thin nanofilms; the organic component is represented by the layer of a finite thickness smaller than the diffusion length of the Frenkel exciton. We analyze the frequency dependence of polaritons formed during the general hybridization of the Wannier–Mott excitons, the Frenkel exciton, and the fundamental model of the microcavity. We have obtained the dispersion dependences of polaritons of reduced binary hybrids and the organic layer in the microcavity. Our results can be of interest for studying the energy transfer kinetics with a multiple change in the form of energy in triplet and binary hybrid systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Obtaining of SmS based semiconducting material and investigation of its electrical properties

Semiconductor samarium monosulfide polycrystals obtained by reaction between samarium trihydride (SmH{sub 3}) and its sesquisulfide were studied. The temperature, baric and frequency dependences of the resistivity and structural features of the samples were investigated. It is shown that the value of X-ray coherent scattering region is extremely small for SmS samples, 320 A; critical pressure of semiconductor-metal phase transition is higher than in the samples, obtained by other methods, 0.88 GPa; the temperature dependence of the resistivity has metallic behavior. Hopping mechanism of electron transport was found. All these features are explained by more defective structure of the polycrystalline SmS samples.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Thermodynamic Properties and Phase Transitions of Microcrystalline and Nanostructured SmFeO{sub 3} Ceramics

The heat capacity and dielectric properties of microcrystalline and nanostructured SmFeO{sub 3} ceramics obtained by solid phase synthesis are studied. The ceramics is synthesized by the treatment of the batch at room temperature in Bridgman anvils by forceful action combined with shear deformation followed by sintering. It is established that the mechanoactivation results in noticeable broadening antiferromagnetic–ferroelectric transition and shifting the temperature of phase transition in the low-temperature area. The phase transition having typical for relaxation oscillator frequency dependent character is found at 558 K. It is shown that the defect structure can take a dominant place in the formation of the physical properties of ceramics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Structural and dielectric properties of Nd/Ca co-doped bi-ferrite multiferroics

Single-phase polycrystalline samples of Bi{sub 0.8}Nd{sub 0.2−x}Ca{sub x}FeO{sub 3} (x = 0.0, 0.1) were synthesized by chemical coprecipitation method. X-ray diffraction patterns accompanied by Rietveld-refined crystal structure parameters reveal the phase transition of Bi{sub 0.8}Nd{sub 0.1}Ca{sub 0.1}FeO{sub 3} with rhombohedral R3c symmetry to triclinic structure of Bi{sub 0.8}Nd{sub 0.2}FeO{sub 3} sample. Frequency dependence of dielectric constant (ε') and dielectric loss (tanδ), infers enhancement of both ε' (tanδ) in narrow band gap of Nd/Ca co-doped BFO as compare to wide band pristine BFO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetic and magneto-dielectric studies in 0.9LaFeO{sub 3}-0.1BaTiO{sub 3} solid solution

Single phase perovskite 0.9LaFeO{sub 3}-0.1BaTiO{sub 3} ceramic was synthesized by the conventional solid-state reaction. Rietveld analysis of X-ray diffraction data clearly shows orthorhombic phase with Pbnm space group. The microstructure shows high density, uniform grains. The dielectric study shows the giant dielectric permittivity noted similar to that found in LaFeO{sub 3}. The observed frequency dependent dielectric loss relaxation was ascribed as polaranic conduction. The present ceramic shows the enhanced magnetic properties and positive MD effect. The suitable explanation was given in present paper such for large dielectric permittivity and origin of positive MD effect in present ceramic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Surface polaritons in a dielectric – anisotropic nanocomposite system

We have studied the peculiarities of propagation of surface polaritons at a planar interface between an isotropic dielectric and anisotropic nanocomposite with metal inclusions of ellipsoidal shape. For the case when the axes of all nanoellipsoids of revolution are perpendicular to the propagation direction and parallel to the medium interface, we have obtained frequency dependences of the propagation constant and transverse wave vector components, penetration depth and path length, longitudinal and transverse energy fluxes for surface polaritons. The shape of nanoellipsoids is shown to have an effect on the wave characteristics of surface polaritons. (nanostructures)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

SOLAR H{alpha} OSCILLATIONS FROM INTENSITY AND DOPPLER OBSERVATIONS

Chromospheric wave activity around flares and filaments has been a research focus for years, and could provide indirect measurements of local conditions that are not otherwise accessible. One interesting observed phenomenon is oscillations in filaments, activated by distant flares and the large-scale waves they produce. Characteristics of these oscillations, such as periods, amplitudes, and lifetimes, can provide unique information about the filament. We measure oscillation properties in flares and filaments from H{alpha} chromospheric data using a new method that provides important spatial and frequency content of the dynamics. We apply the method to two flare events where filaments are observed to oscillate and determine their properties. We find strong oscillatory signal in flaring active regions in the chromosphere over a range of frequencies. Two filaments are found to oscillate without any detectable chromospheric wave acting as an activation mechanism. We find that filaments oscillate with periods of tens of minutes, but variations are significant at small spatial scales along the filamentary region. The results suggest that there is a frequency dependence of the oscillation amplitude, as well as a spatial dependence along single filaments that is more difficult to quantify. It also appears that the strength of the oscillations does not necessarily depend on the strength of the trigger, although there are other possible effects that make this conclusion preliminary. Applications of this technique to other events and different data sets will provide important new insights into the local energy densities and magnetic fields associated with dynamic chromospheric structures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Dielectric relaxations and alternating current conductivity in manganese substituted cobalt ferrite

Manganese (Mn) substituted cobalt ferrites (CoFe{sub 2-x}Mn{sub x}O{sub 4}, referred to CFMO) have been synthesized by the solid state reaction method and their dielectric properties and ac conductivity have been evaluated as a function of applied frequency and temperature. X-ray diffraction measurements indicate that CFMO crystallize in the inverse cubic spinel phase with a lattice constant ∼8.38 Å. Frequency dependent dielectric measurements at room temperature obey the modified Debye model with relaxation time of 10{sup −4} s and spreading factor of 0.35(±0.05). The frequency (20 Hz–1 MHz) and temperature (T = 300–900 K) dependent dielectric constant analyses indicate that CFMO exhibit two dielectric relaxations at lower frequencies (1–10 kHz), while completely single dielectric relaxation for higher frequencies (100 kHz–1 MHz). The dielectric constant of CFMO is T-independent up to ∼400 K, at which point increasing trend prevails. The dielectric constant increase with T > 400 K is explained through impedance spectroscopy assuming a two-layer model, where low-resistive grains separated from each other by high-resistive grain boundaries. Following this model, the two electrical responses in impedance formalism are attributed to the grain and grain-boundary effects, respectively, which also satisfactorily accounts for the two dielectric relaxations. The capacitance of the bulk of the grain determined from impedance analyses is ∼10 pF, which remains constant with T, while the grain-boundary capacitance increases up to ∼3.5 nF with increasing T. The tan δ (loss tangent)-T also reveals the typical behavior of relaxation losses in CFMO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spin correlated dielectric memory and rejuvenation in multiferroic CuCrS{sub 2}

We report a rare consequence of memory effect in dielectric response (ϵ) and magnetic field induced rejuvenation in a relaxor-type multiferroic chalcogenide, CuCrS{sub 2}. Despite reasonably high conductivity, we are able to detect significant spontaneous polarization using an improvised technique verifying ferroelectric (FE) order. Concomitant appearance of both FE and antiferromagnetic orders authenticates multiferroicity. A smeared out FE transition and strong frequency dependence of the broadened peak in ϵ obeying Dynamical scaling law signify relaxor properties. We discuss the role of geometrical frustration in the antiferromagnetically coupled layered triangular lattice and metal ligand hybridization for these unusual properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Effect Of Neodymium Substitution In Structural Characteristics Of Magnesium Ferrite

The effect of Nd{sup 3+} substitution on the structural properties of Magnesium ferrite was studied in the series MgNd{sub x}Fe{sub 2-x}O{sub 4}, where x = 0 to 0.3 in steps of 0.05. The series was prepared by sol-gel technique which is one of the novel technique to prepare nanosized samples. Structural characterization was done using X-ray diffractometer and Fourier Transform Infrared Spectrometer. XRD analysis reveals the prepared samples are single phasic till x = 0.2. From x0 = .25, a secondary phase of iron neodymium oxide appears along with the spinel phase. Particle size calculation shows the prepared samples are in the 9nm to 11 nm regime. Lattice parameter was found to increase with concentration of Nd. XRD and FTIR analysis confirmed spinel structure of the prepared samples. XRF result shows the expected composition of prepared samples. The frequency dependence of the dielectric constant in the range 100 Hz--120MHz was also studied

36 MATERIALS SCIENCE↗
DOE OSTI2014

Highly conducting SrMoO{sub 3} thin films for microwave applications

We have measured the microwave resistance of highly conducting perovskite oxide SrMoO{sub 3} thin film coplanar waveguides. The epitaxial SrMoO{sub 3} thin films were grown by pulsed laser deposition and showed low mosaicity and smooth surfaces with a root mean square roughness below 0.3 nm. Layer-by-layer growth could be achieved for film thicknesses up to 400 nm as monitored by reflection high-energy electron diffraction and confirmed by X-ray diffraction. We obtained a constant microwave resistivity of 29 μΩ·cm between 0.1 and 20 GHz by refining the frequency dependence of the transmission coefficients. Our result shows that SrMoO{sub 3} is a viable candidate as a highly conducting electrode material for all-oxide microwave electronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Enhanced magnetism-generated ferroelectricity in highly frustrated Fe-doped Ho{sub 2}Ti{sub 2}O{sub 7}

We present careful experiments on the ferroelectric (FE), dielectric, and magnetic behaviors of Ho{sub 2−x}Fe{sub x}Ti{sub 2}O{sub 7} with Fe{sup 3+} substitution for Ho{sup 3+}. A remarkable enhancement of polarization up to 235 μC/m{sup 2} is obtained at a low level x = 0.08, accompanied with the FE transition up to ∼80 K. The ac susceptibility under magnetic fields shows an expected saturated maximum in the real part χ′, along with an unexpected frequency-dependent peak in the imaginary part χ″, indicating unusual slow spin relaxation. The coupled correlated spin domains through dipolar interaction are argued to give rise to nonzero electric-dipole via Dzyaloshinskii-Moriya interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

QUASI-PERIODICITIES AT YEAR-LIKE TIMESCALES IN BLAZARS

We searched for quasi-periodicities on year-like timescales in the light curves of six blazars in the optical—near-infrared bands and we made a comparison with the high energy emission. We obtained optical/NIR light curves from Rapid Eye Mounting photometry plus archival Small and Moderate Aperture Research Telescope System data and we accessed the Fermi light curves for the γ-ray data. The periodograms often show strong peaks in the optical and γ-ray bands, which in some cases may be inter-related. The significance of the revealed peaks is then discussed, taking into account that the noise is frequency dependent. Quasi-periodicities on a year-like timescale appear to occur often in blazars. No straightforward model describing these possible periodicities is yet available, but some plausible interpretations for the physical mechanisms causing periodic variabilities of these sources are examined.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Electrical and optical properties of gadolinium doped bismuth ferrite nanoparticles

Multiferroic bismuth ferrite (BFO) and gadolinium (Gd) doped bismuth ferrite had been synthesized by a sol-gel method. Particle size had been estimated by Transmission electron microscopy (TEM) and found to decrease with Gd doping. We studied the temperature and frequency dependence of impedance and electric modulus and calculated the grain and grain boundary resistance and capacitance of the investigated samples. We observed that electrical activation energy increases for all the doped samples. Optical band gap also increases for the doped samples which can be used in photocatalytic application of BFO.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Enhancement in dielectric behavior of (Ni, Zn)Fe{sub 2}O{sub 4} ferrite

In present work, NiFe{sub 2}O{sub 4}(NFO), ZnFe{sub 2}O{sub 4} (ZFO) and Ni{sub 0.5}Zn{sub 0.5}Fe{sub 2}O{sub 4} (NZFO) are synthesized by microwave assisted co-precipitation route. Their structural properties are confirmed by X-ray diffraction and data is fitted through Reitveld refinement using Full-Prof software suite. After refinement, the systematic crystal structures of NFO, ZFO and NZFO compounds are generated using output .cif file by VESTA (Visualization for Electronic and Structural Analysis) program. Structural parameters obtained after refinement and unit cell construction, are systematically tabulated. Room temperature frequency dependence dielectric properties are studied. We found enhanced values of dielectric constant for NZFO than individual NFO and ZFO phases. For NZFO sample, greater electron exchange between Fe{sup 2+} and Fe{sup 3+} which enhances polarization and dielectric constant.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Calculations of the thermal conductivity of National Ignition Facility target materials at temperatures near 10 eV and densities near 10 g/cc using finite-temperature quantum molecular dynamics

Using quantum finite-temperature density functional theory molecular dynamics (QMD), we performed simulations of several important materials in the Inertial Confinement Fusion-National Ignition Facility nominal target designs, comprising various mixtures of proposed ablator materials (Be or CH) with the DT fuel. Simulations were done over a range of temperatures between 5 eV and 20 eV, at densities between 7.5 and {approx}12.5 g/cc. From the QMD trajectories, we calculated the electrical and thermal conductivity. We estimated the number of free electrons per atom by fitting the frequency-dependent electrical conductivity to the Drude formula. The thermal conductivity of the fuel increases with density but that of the ablator material is insensitive to modest density variations. We find that the thermal conductivity depends strongly on the ablator/fuel mix fraction but a Faber-Ziman interpolation scheme provides a reasonable approximation. We also compare our QMD results to the Hubbard and Lee-More models.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Electronic structure and optical properties of the HoCoSi and ErNiSi compounds

The electronic structure and the optical properties of the HoCoSi and ErNiSi compounds are studied. Spin-polarized band calculations are performed in the local electron density approximation corrected for the strong electron–electron interactions in the 4f shell of a rare-earth ion (LSDA + U method [11]). The optical constants are measured by ellipsometry in a wide wavelength range, and the frequency dependences of a number of spectral parameters are determined. The calculated densities of states are used to interpret the structural features of the interband optical conductivities of the intermetallic compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗