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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 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Alternative current conduction mechanisms of organic-inorganic compound [N(CH{sub 3}){sub 3}H]{sub 2}CuCl{sub 4}

The [N(CH{sub 3}){sub 3}H]{sub 2}CuCl{sub 4} single crystal has been analyzed by X-ray powder diffraction patterns, differential scanning calorimetry (DSC), and electrical impedance spectroscopy. [N(CH{sub 3}){sub 3}H]{sub 2}CuCl{sub 4} crystallizes at room temperature in the monoclinic system with P2{sub 1}/{sub C} space group. Three phase transitions at T{sub 1} = 226 K, T{sub 2} = 264 K, and T{sub 3} = 297 K have been evidenced by DSC measurements. The electrical technique was measured in the 10{sup −1}–10{sup 7} Hz frequency range and 203–313 K temperature intervals. The frequency dependence of alternative current (AC) conductivity is interpreted in terms of Jonscher's law (developed). The AC electrical conduction in [N(CH{sub 3}){sub 3}H]{sub 2}CuCl{sub 4} compound is studied by two processes which can be attributed to a hopping transport mechanism: the correlated barrier hopping model in phases I, II, and III, the non-overlapping small polaron tunneling model in phase IV. The conduction mechanism is interpreted with the help of Elliot's theory, and the Elliot's parameters are found.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Direct writing of continuous and discontinuous sub-wavelength periodic surface structures on single-crystalline silicon using femtosecond laser

Laser-induced ripples or uniform arrays of continuous near sub-wavelength or discontinuous deep sub-wavelength structures are formed on single-crystalline silicon (Si) by femtosecond (fs) laser direct writing technique. Laser irradiation was performed on Si wafers at normal incidence in air and by immersing them in dimethyl sulfoxide using linearly polarized Ti:sapphire fs laser pulses of ∼110 fs pulse duration and ∼800 nm wavelength. Morphology studies of laser written surfaces reveal that sub-wavelength features are oriented perpendicular to laser polarization, while their morphology and spatial periodicity depend on the surrounding dielectric medium. The formation mechanism of the sub-wavelength features is explained by interference of incident laser with surface plasmon polaritons. This work proves the feasibility of fs laser direct writing technique for the fabrication of sub-wavelength features, which could help in fabrication of advanced electro-optic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of Ce doping on the electrocaloric effect of Sr{sub x}Ba{sub 1−x}Nb{sub 2}O{sub 6} single crystals

The electrocaloric effect (ECE) of Sr{sub x}Ba{sub (1−x)}Nb{sub 2}O{sub 6} (SBN100x) single crystals, with a tetragonal tungsten bronze structure and a high ECE near room temperature, is studied by direct measurements. It is shown that although the onset of the ECE peak is closer to room temperature in SBN80 than in SBN75, the effect of the increase of the strontium content is very detrimental to the ECE performances with a decrease to ΔT{sub EC} = 0.23 K for SBN80 from the reported value of ΔT{sub EC} = 0.42 K under 10 kV/cm for SBN75 [F. Le Goupil et al., “Anisotropy of the electrocaloric effect in lead-free relaxor ferroelectrics,” Adv. Energy Mater. (published online)]. However, when 1.40% of cerium is introduced in SBN61, the temperature of depolarisation is shifted below 30 °C, while an ECE above 0.6 K is maintained over more than 70 K for a low electric field of 28 kV/cm. The maximum ECE ΔT{sub EC} = 0.85 K is measured at 61 °C. In addition to having an ECE peak close to room temperature, the ECE measured in Ce-doped SBN61 is comparable with the best reported values for lead-free materials [Y. Bai et al., J. Appl. Phys. 110, 094103 (2011); X.-S. Qian et al., Adv. Funct. Mater. 24, 1300 (2014)], when linearly extrapolated to higher electric fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Vibrational states of a water molecule in a nano-cavity of beryl crystal lattice

Low-energy excitations of a single water molecule are studied when confined within a nano-size cavity formed by the ionic crystal lattice. Optical spectra are measured of manganese doped beryl single crystal Mn:Be{sub 3}Al{sub 2}Si{sub 6}O{sub 18}, that contains water molecules individually isolated in 0.51 nm diameter voids within the crystal lattice. Two types of orientation are distinguished: water-I molecules have their dipole moments aligned perpendicular to the c axis and dipole moments of water-II molecules are parallel to the c-axis. The optical conductivity σ(ν) and permittivity ε{sup ′}(ν) spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to ν = 7000 cm{sup −1}, at temperatures 5–300 K and for two polarizations, when the electric vector E of the radiation is parallel and perpendicular to the c-axis. Comparative experiments on as-grown and on dehydrated samples allow to identify the spectra of σ(ν) and ε{sup ′}(ν) caused exclusively by water molecules. In the infrared range, well-known internal modes ν{sub 1}, ν{sub 2}, and ν{sub 3} of the H{sub 2}O molecule are observed for both polarizations, indicating the presence of water-I and water-II molecules in the crystal. Spectra recorded below 1000 cm{sup −1} reveal a rich set of highly anisotropic features in the low-energy response of H{sub 2}O molecule in a crystalline nano-cavity. While for E∥c only two absorption peaks are detected, at ∼90 cm{sup −1} and ∼160 cm{sup −1}, several absorption bands are discovered for E⊥c, each consisting of narrower resonances. The bands are assigned to librational (400–500 cm{sup −1}) and translational (150–200 cm{sup −1}) vibrations of water-I molecule that is weakly coupled to the nano-cavity “walls.” A model is presented that explains the “fine structure” of the bands by a splitting of the energy levels due to quantum tunneling between the minima in a six-well potential relief felt by a molecule within the cavity.

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

Green emission in carbon doped ZnO films

The emission behavior of C-doped ZnO films, which were prepared by implantation of carbon into ZnO films, is investigated. Orange/red emission is observed for the films with the thickness of 60–100 nm. However, the film with thickness of 200 nm shows strong green emission. Further investigations by annealing bulk ZnO single crystals under different environments, i.e. Ar, Zn or C vapor, indicated that the complex defects based on Zn interstitials are responsible for the strong green emission. The existence of complex defects was confirmed by electron spin resonance (ESR) and low temperature photoluminescence (PL) measurement.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Pressure effect on elastic anisotropy of crystals from ab initio simulations: The case of silicate garnets

A general methodology has been devised and implemented into the solid-state ab initio quantum-mechanical CRYSTAL program for studying the evolution under geophysical pressure of the elastic anisotropy of crystalline materials. This scheme, which fully exploits both translational and point symmetry of the crystal, is developed within the formal frame of one-electron Hamiltonians and atom-centered basis functions. Six silicate garnet end-members, among the most important rock-forming minerals of the Earth's mantle, are considered, whose elastic anisotropy is fully characterized under high hydrostatic compressions, up to 60 GPa. The pressure dependence of azimuthal anisotropy and shear-wave birefringence of seismic wave velocities for these minerals are accurately simulated and compared with available single-crystal measurements.

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

Surface nanostructuring by ion-induced localized plasma expansion in zinc oxide

Creation of hillock-like nanostructures on the surface of zinc oxide single crystals by irradiation with slow highly charged ions is reported. At constant kinetic energy, the nanostructures were only observed after irradiation with ions of potential energies above a threshold between 19.1 keV and 23.3 keV. The size of the nanostructures increases as a function of potential energy. A plasma expansion approach is used to explain the nanostructures creation. The calculations showed that the surface nanostructures became taller with the increase of ionic temperature. The influence of charged cluster formation and the relevance of their polarity are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Neutron diffraction study on very high elastic strain of 6% in an Fe{sub 3}Pt under compressive stress

An Fe{sub 3}Pt alloy with degree of order 0.75 exhibits a second-order-like martensitic transformation from a cubic structure to a tetragonal one at about 90 K; its tetragonality c/a changes nearly continuously from 1 to 0.945 on cooling from 90 K to 14 K. We have investigated the change in lattice parameters in a single crystal of the Fe{sub 3}Pt alloy at 93 K under compressive stresses, σ, applied in the [001] direction by neutron diffraction. The tetragonality c/a has decreased continuously from 1 to 0.907 with an increase in |σ| up to |σ| = 280 MPa; the corresponding lattice strain in the [001] direction, due to the continuous structure change, increases from 0% to 6.1%. When the stress of 300 MPa is reached, c/a has changed abruptly from 0.907 to 0.789 due to a first-order martensitic transformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spectroscopic analysis of H{sub 2}/CH{sub 4} microwave plasma and fast growth rate of diamond single crystal

One of the best ways to increase the diamond growth rate is to couple high microwave power to the plasma. Indeed, increasing the power density leads to increase gas temperature the atomic hydrogen density in the plasma bulk, and to produce more hydrogen and methyl at the diamond surface. Experimental and numerical approaches were used to study the microwave plasma under high power densities conditions. Gas temperature was measured by optical emission spectroscopy and H-atom density using actinometry. CH{sub 3}-radical density was obtained using a 1D model that describes temperatures and plasma composition from the substrate to the top of the reactor. The results show that gas temperature in the plasma bulk, atomic hydrogen, and methyl densities at the diamond surface highly increase with the power density. As a consequence, measurements have shown that diamond growth rate also increases. At very high power density, we measured a growth rate of 40 μm/h with an H-atom density of 5 × 10{sup 17} cm{sup −3} which corresponds to a H{sub 2} dissociation rate higher than 50%. Finally, we have shown that the growth rate can be framed between a lower and an upper limit as a function depending only on the maximum of H-atom density measured or calculated in the plasma bulk. The results also demonstrated that increasing fresh CH{sub 4} by an appropriate injection into the boundary layer is a potential way to increase the diamond growth rates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Characteristics of intermediate state related to anti-Stokes luminescence of ZnO single crystals

Anti-Stokes luminescence from ZnO is supposed to be a two-step two-photon absorption process with an intermediate state. The intermediate state is assumed to be a localized state with two different excited and relaxed states. One of the localized states is believed to be the well-known 2.4 eV green luminescence; the other is difficult to observe experimentally. We found an interesting 2.25 eV deep luminescence from ZnO, which has been shown to relate to anti-Stokes luminescence. The 2.25 eV yellow luminescence was observable only below the band gap excitation and through a time-resolved observation after the excitation light was turned off. The intermediate states were found to be a photo-excited donor-acceptor pair and its lattice relaxation state. The characteristics and the role of the intermediate state of ZnO related to the anti-Stokes luminescence are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature dependent electrical transport studies of self-aligned ZnO nanorods/Si heterostructures deposited by sputtering

Self-aligned ZnO nanorods (NRs) were grown on n-Si(100) substrate by RF sputtering techniques. The NRs are uniformly grown on 2-inch wafer along [0001] direction. Single-crystalline wurtzite structure of ZnO NRs was confirmed by X-ray diffraction. The average diameter, height, and density of NRs are found 48 nm, 750 nm, and 1.26 × 10{sup 10} cm{sup −2}, respectively. The current-voltages (I-V) characteristics of ZnO NRs/Si heterojunction (HJ) were studied in the temperature range of 120–300 K and it shows a rectifying behavior. Barrier height (ϕ{sub B}) and ideality factor (η) were estimated from thermionic emission model and found to be highly temperature dependent in nature. Richardson constant (A{sup *}) was evaluated using Richardson plot of ln(I{sub o}/T{sup 2}) versus q/kT plot by linear fitting in two temperature range 120–180 K and 210–300 K. Large deviation in Richardson constant from its theoretical value of n-Si indicates the presence of barrier inhomogeneities at HJ. Double Gaussian distribution of barrier height with thermionic equation gives mean barrier heights of 0.55 ± 0.01 eV and 0.86 ± 0.02 eV for two different temperature regions 120–180 K and 210–300 K, respectively. Modified Richardson plot provided two values of Richardson constant for two temperature regions. However, for higher temperature range (210–300 K), the calculated value of Richardson constant ∼123 A cm{sup −2} K{sup −2} was close to the ideal Richardson constant for n-Si.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

In-situ neutron diffraction of LaCoO{sub 3} perovskite under uniaxial compression. II. Elastic properties

Calculations of elastic constants and development of elastic anisotropy under uniaxial compression in originally isotropic polycrystalline LaCoO{sub 3} perovskite are reported. The lattice strains in individual (hkl) planes as well as average lattice strain were determined both for planes oriented perpendicular and parallel to the loading direction using in-situ neutron diffraction. Utilizing average lattice strains as well as lattice strains along the a and c crystallographic directions, an attempt was made to determine Poisson's ratio of LaCoO{sub 3}, which was then compared with that measured using an impulse excitation technique. The elastic constants were calculated and Young's moduli of LaCoO{sub 3} single crystal in different crystallographic directions were estimated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Deep-level emission in ZnO nanowires and bulk crystals: Excitation-intensity dependence versus crystalline quality

The excitation-intensity dependence of the excitonic near-band-edge emission (NBE) and deep-level related emission (DLE) bands in ZnO nanowires and bulk crystals is studied, which show distinctly different power laws. The behavior can be well explained with a rate-equation model taking into account deep donor and acceptor levels with certain capture cross sections for electrons from the conduction band and different radiative lifetimes. In addition, a further crucial ingredient of this model is the background n-type doping concentration inherent in almost all ZnO single crystals. The interplay of the deep defects and the background free-electron concentration in the conduction band at room temperature reproduces the experimental results well over a wide range of excitation intensities (almost five orders of magnitude). The results demonstrate that for many ZnO bulk samples and nanostructures, the relative intensity R = I{sub NBE}/I{sub DLE} can be adjusted over a wide range by varying the excitation intensity, thus, showing that R should not be taken as an indicator for the crystalline quality of ZnO samples unless absolute photoluminescence intensities under calibrated excitation conditions are compared. On the other hand, the results establish an all-optical technique to determine the relative doping levels in different ZnO samples by measuring the excitation-intensity dependence of the UV and visible luminescence bands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Amorphization and recrystallization of single-crystalline hydrogen titanate nanowires by N{sup +} ion irradiation

We report on the phase transformation of hydrogen titanate (H{sub 2}Ti{sub 3}O{sub 7}) nanowires induced by 50 keV N{sup +} ion irradiation at room temperature with fluences of 1 × 10{sup 15} ions/cm{sup 2} and 1 × 10{sup 16} ions/cm{sup 2}, respectively. Using transmission electron microscopy, the internal structure of the ion irradiated nanowires is analyzed. At low fluence, a transformation from crystalline H{sub 2}Ti{sub 3}O{sub 7} to amorphous TiO{sub 2} is observed. However, at higher fluence, a remarkable crystalline-amorphous TiO{sub 2} core-shell structure is formed. At this higher fluence, the recrystallization occurs in the core of the nanowire and the outer layer remains amorphous. The phase transformation and formation of core-shell structure are explained using the thermal spike model, radiation enhanced diffusion, and classical theory of nucleation and growth under non-equilibrium thermodynamics. X-ray photoelectron spectroscopy and Raman scattering reveal further insight into the structure of the nanowires before and after ion irradiation.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

X-ray diffraction on the X-cut of a Ca{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14} single crystal modulated by a surface acoustic wave

The result of X-ray diffraction study on a single crystal of the calcium-gallogermanate family Ca{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14} (CTGS) modulated by a surface acoustic wave (SAW) is presented. The power flow angle for SAW propagating along the X{sub 2} axis of the X-cut in CTGS was measured. The rocking curves for the CTGS crystal were recorded at different amplitudes of an input high frequency electric signal on interdigital transducer used to excite a SAW. Based on the data obtained, intensity dependence of diffraction satellites on the amplitude of electric signal exciting a SAW was built. Numerical simulation of the crystal rocking curves and dependence of diffraction satellite intensities on the SAW amplitude enabled the selection of a set of material constants at which the most complete coincidence of experimental and calculated results is observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Grain boundary barrier modification due to coupling effect of crystal polar field and water molecular dipole in ZnO-based structures

Surface water molecules induced grain boundaries (GBs) barrier modification was investigated in ZnO and ZnMgO/ZnO films. Tunable electronic transport properties of the samples by water were characterized via a field effect transistor (FET) device structure. The FETs fabricated from polar C-plane ZnO and ZnMgO/ZnO films that have lots of GBs exhibited obvious double Schottky-like current-voltage property, whereas that fabricated from nonpolar M-plane samples with GBs and ZnO bulk single-crystal had no obvious conduction modulation effects. Physically, these hallmark properties are supposed to be caused by the electrostatical coupling effect of crystal polar field and molecular dipole on GBs barrier.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A diamond based neutron spectrometer for diagnostics of deuterium-tritium fusion plasmas

Single crystal Diamond Detectors (SDD) are being increasingly exploited for neutron diagnostics in high power fusion devices, given their significant radiation hardness and high energy resolution capabilities. The geometrical efficiency of SDDs is limited by the size of commercially available crystals, which is often smaller than the dimension of neutron beams along collimated lines of sight in tokamak devices. In this work, we present the design and fabrication of a 14 MeV neutron spectrometer consisting of 12 diamond pixels arranged in a matrix, so to achieve an improved geometrical efficiency. Each pixel is equipped with an independent high voltage supply and read-out electronics optimized to combine high energy resolution and fast signals (<30 ns), which are essential to enable high counting rate (>1 MHz) spectroscopy. The response function of a prototype SDD to 14 MeV neutrons has been measured at the Frascati Neutron Generator by observation of the 8.3 MeV peak from the {sup 12}C(n, α){sup 9}Be reaction occurring between neutrons and {sup 12}C nuclei in the detector. The measured energy resolution (2.5% FWHM) meets the requirements for neutron spectroscopy applications in deuterium-tritium plasmas.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Electron spin-lattice relaxation of low-symmetry Ni{sup 2+} centers in LiF

The spin-lattice relaxation times of charge-compensated Ni{sup 2+} centers in LiF single crystals were measured using the electron spin echo technique. An analysis of the results revealed a very high relaxation rate with a linear dependence on temperature within the intermediate temperature range of 5–15 K. This acceleration of the relaxation rate was found to be due to the inhomogeneous distribution of Ni{sup 2+} ions in the LiF lattice. It seems that the effective cross-relaxation mechanism through the exchange-coupled clusters of Ni{sup 2+} ions can play a dominant role. Two-phonon Raman type relaxation dominates at higher temperatures involving the Debye phonon spectrum of the LiF lattice.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗