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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Structural and electronic properties of Si{sub 1-x}Ge{sub x} binary semiconducting alloys under the effect of temperature and pressure

Based on the empirical pseudo-potential method which incorporates compositional disorder as an effective potential, the band structure of Si{sub 1-x}Ge{sub x} alloy are calculated for different alloy composition x. The effect of temperature and pressure on the electronic band structure of the considered alloy has been studied. Monotonic decreasing and increasing functions are obtained for the temperature and pressure dependent form factors respectively. Some physical quantities as band gaps, bowing parameters, and the refractive index of the considered alloy with different Ge concentration and under the effect of temperature and pressure are calculated. The results obtained are found in good agreement with the experimental and published data.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Phase transitions and photoinduced transformations at high pressure in the molecular donor-acceptor fullerene complex (Cd(dedtc){sub 2}){sub 2} · C{sub 60}

The Raman spectra of crystals of C{sub 60} fullerene-cadmium diethyldithiocarbamate molecular donor-acceptor complexes (Cd(dedtc){sub 2}){sub 2} · C{sub 60} were measured at pressures of up to 17 GPa, and the crystal lattice parameters of these complexes were determined at pressures of up to 6 GPa. An increase in pressure up to ∼2 GPa leads to changes in the Raman spectra, which are manifested by splitting of the intramolecular H{sub g}(1)-H{sub g}(8) phonon modes and by softening of the A{sub g}(2) mode of the C{sub 60} molecule. A further increase in pressure up to 17 GPa does not induce significant new changes to the Raman spectra, while a decrease is accompanied by the reverse transformation at a pressure of about 2 GPa. The pressure dependence of the lattice parameters also exhibits a reversible feature at 2 GPa related to a jumplike decrease in compressibility. All these data are indicative of a phase transition in the vicinity of 2 GPa related to the formation of covalent bonds between C{sub 60} molecules and, probably, the appearance of C{sub 120} dimers in fullerene layers. It was also found that, in the pressure interval from 2 to 6.3 GPa, the Raman spectra of complexes exhibit photoinduced transformations under prolonged exposure to laser radiation with a wavelength of λ = 532 nm and power density up to 5000 W/cm{sup 2}. These changes are manifested by splitting and softening of the A{sub g}(2) mode and resemble analogous changes accompanying the photopolymerization of C{sub 60} fullerene. The intensity of new bands exhibits exponential growth with increasing exposure time. The photopolymer yield depends on both the laser radiation power and external pressure. The A{sub g}(2) mode splitting under irradiation can be related to the formation of photo-oligomers with various numbers of intermolecular covalent bonds per C{sub 60} molecule.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Nonmonotonic pressure evolution of the upper critical field in superconducting FeSe

The pressure dependence of the upper critical field, H c2,c , of single crystalline FeSe was studied using measurements of the interplane resistivity, ρ c , in magnetic fields parallel to tetragonal c axis. H c2,c (T) curves obtained under hydrostatic pressures up to 1.56 GPa, the range over which the superconducting transition temperature, T c , of FeSe exhibits a nonmonotonic dependence with local maximum at p 1 ≈ 0.8 GPa and local minimum at p 2 ≈ 1.2 GPa. The slope of the upper critical field at T c ,(dH c2,c /dT)T c , also exhibits a nonmonotonic pressure dependence with distinct changes at p 1 and p 2 . For p < p 1 the slope can be described within a multiband orbital model. For both p 1 < p < p 2 and p > p 2 the slope is in good semiquantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov–de Haas measurements. Lastly, this finding indicates that Fermi surface changes are responsible for the local minimum of T c (p) at p 2 ≈ 1.2 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Importance of halogen---halogen contacts for the structural and magnetic properties of CuX{<_2}(pyrazine-N,N'-dioxide)(H{<_2}O){<_2} (x = Cl & Br).

The structural and magnetic properties of the newly crystallized CuX{sub 2}(pyzO)(H{sub 2}O){sub 2} (X = Cl, Br; pyzO = pyrazine-N,N{prime}-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) {angstrom}, b = 5.5560(2) {angstrom}, c = 10.4254(5) {angstrom}, {beta} = 115.400(2){sup o}, and V = 892.21(7) {angstrom}{sup 3} for X = Cl and a = 17.3457(8) {angstrom}, b = 5.6766(3) {angstrom}, c = 10.6979(5) {angstrom}, {beta} = 115.593(2){sup o}, and V = 950.01(8) {angstrom}{sup 3} for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu{sup 2+} ions linked through bidentate pyzO ligands. These chains are joined together through OH {hor_ellipsis} O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X {hor_ellipsis} X-Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu-X {hor_ellipsis} X-Cu supersuperexchange because the magnetic orbital of the Cu{sup 2+} ion is contained in the CuX{sub 2}(H{sub 2}O){sub 2} plane and the X {hor_ellipsis} X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J1D/kB = -11.1(1) (Cl) and -45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J1D is determined to be -11.5(2) (Cl) and -46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J1D/kB up to -51.2 K at 0.84 GPa, suggesting a shortening of the Br {hor_ellipsis} Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at 3.5 GPa. Muon-spin relaxation measurements indicate that CuCl{sub 2}(pyzO)(H{sub 2}O){sub 2} is magnetically ordered with TN = 1.06(1) K, while the signature for long-range magnetic order in CuBr{sub 2}(pyzO)(H{sub 2}O){sub 2} was much less definitive down to 0.26 K. The results for the CuX{sub 2}(pyzO)(H{sub 2}O){sub 2} complexes are compared to the related CuX{sub 2}(pyrazine) materials.

08 HYDROGEN↗
DOE OSTI2013

Generalized shock conditions and the contact discontinuity in the Hall-magnetohydrodynamics model

It is shown that shocks and contact discontinuities in the Hall-magnetohydrodynamics (HMHD) model must satisfy solvability conditions that replace some of the familiar Rankine-Hugoniot jump conditions when the latter do not apply due to singular behavior of fluxes of conserved quantities. Some of these conditions depend on the larger topology of the plasma and magnetic field and are not merely 'local.' The contact discontinuity which separates two adjoining plasma regions or plasma and vacuum regions is the simplest case where the new jump conditions are applicable and is discussed for a toroidal plasma with sheared magnetic field such as the tokamak, but with no initial mass flow. It is proven that a static discontinuous tokamak-like equilibrium is linearly stable in the HMHD model if it is linearly stable within the ideal magnetohydrodynamics model, provided that the electron pressure depends only on the density, and some other restrictions on the ratio of pressure to density gradients also apply. When the electron pressure does depend on two thermodynamic variables, a sufficient condition for Hall-MHD plasma stability is derived as well.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

The Effect of Pressure on NO x Entitlement and Reaction Timescales in a Premixed Axial Jet-In-Crossflow

This paper investigates the pressure dependency of a lean premixed jet injected into a lean vitiated crossflow with an experimentally verified detailed chemistry computational fluid dynamics (CFD) model and 53 species considered. Experimental data were taken in an axially staged combustor with an optically accessible test section, allowing the use of particle image velocimetry (PIV) and CH* chemiluminescence techniques as well as point measurement of species concentration, temperature, and pressure. The experimental data cases at one, three, and five atmospheres were selected to describe the flame stabilization dependency on pressure and gain the required knowledge for an extrapolation to engine condition. Simulated exit nitrogen oxide levels were validated with experimental emission data, and a global emission trend for the NO reduction at elevated pressure and constant turbine inlet temperature level was defined. The nitrogen oxide benefit at elevated operating pressure was justified with the significantly smaller flame surface area: the analysis of the simulated spanwise and top-view profiles showed a relatively short receded core flame with nitrogen oxide production in the center at high pressure relative to a longer and larger shear layer flame at atmospheric condition that produced NO toward the inner and outer side of the flame. Furthermore, decomposition of the Damköhler number revealed the strong influence of the reaction timescales with higher reaction rates at elevated pressure, along with a moderate influence of the turbulent timescales, showing higher turbulence intensity in the lee-side recirculation zone at lower pressure.

03 NATURAL GAS↗
DOE OSTI2010

Size-dependent amorphization of nanoscale Y 2 O 3 at high pressure

Y 2 O 3 with particle sizes ranging from 5 nm to 1 μm were studied at high pressure using x-ray diffraction and Raman spectroscopy techniques. Nanometer-sized Y 2 O 3 particles are shown to be more stable than their bulk counterparts, and a grain size-dependent crystalline-amorphous transition was discovered in these materials. High-energy atomic pair distribution function measurements reveal that the amorphization is associated with the breakdown of the long-rang order of the YO 6 octahedra, while the nearest-neighbor edge-shared octahedral linkages are preserved.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Optical breakdown threshold investigation of 1064 nm laser induced air plasmas

We present the theoretical and experimental measurements and analysis of the optical breakdown threshold for dry air by 1064 nm infrared laser radiation and the significance of the multiphoton and collisional cascade ionization process on the breakdown threshold measurements over pressures range from 10 to 2000 Torr. Theoretical estimates of the breakdown threshold laser intensities and electric fields are obtained using two distinct theories namely multiphoton and collisional cascade ionization theories. The theoretical estimates are validated by experimental measurements and analysis of laser induced breakdown processes in dry air at a wavelength of 1064 nm by focusing 450 mJ max, 6 ns, 75 MW max high-power 1064 nm IR laser radiation onto a 20 {mu}m radius spot size that produces laser intensities up to 3 - 6 TW/cm{sup 2}, sufficient for air ionization over the pressures of interest ranging from 10 to 2000 Torr. Analysis of the measured breakdown threshold laser intensities and electric fields are carried out in relation with classical and quantum theoretical ionization processes, operating pressures. Comparative analysis of the laser air breakdown results at 1064 nm with corresponding results of a shorter laser wavelength (193 nm) [M. Thiyagarajan and J. E. Scharer, IEEE Trans. Plasma Sci. 36, 2512 (2008)] and a longer microwave wavelength (10{sup 8} nm) [A. D. MacDonald, Microwave Breakdown in Gases (Wiley, New York, 1966)]. A universal scaling analysis of the breakdown threshold measurements provided a direct comparison of breakdown threshold values over a wide range of frequencies ranging from microwave to ultraviolet frequencies. Comparison of 1064 nm laser induced effective field intensities for air breakdown measurements with data calculated based on the collisional cascade and multiphoton breakdown theories is used successfully to determine the scaled collisional microwave portion. The measured breakdown threshold of 1064 nm laser intensities are then scaled to classical microwave breakdown theory after correcting for the multiphoton ionization process for different pressures and good agreement, regarding both pressure dependence and breakdown threshold electric fields, is obtained. The effect of the presence of submicron particles on the 1064 nm breakdown threshold was also investigated. The measurements show that higher breakdown field is required, especially at lower pressures, and in close agreement with classical microwave breakdown theory and measurements in air.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Flow strength of tantalum under ramp compression to 250 GPa

A magnetic loading technique was used to study the strength of polycrystalline tantalum ramp compressed to peak stresses between 60 and 250 GPa. Velocimetry was used to monitor the planar ramp compression and release of various tantalum samples. A wave profile analysis was then employed to determine the pressure-dependence of the average shear stress upon unloading at strain rates on the order of 10{sup 5} s{sup −1}. Experimental uncertainties were quantified using a Monte Carlo approach, where values of 5% in the estimated pressure and 9–17% in the shear stress were calculated. The measured deviatoric response was found to be in good agreement with existing lower pressure strength data as well as several strength models. Significant deviations between the experiments and models, however, were observed at higher pressures where shear stresses of up to 5 GPa were measured. Additionally, these data suggest a significant effect of the initial material processing on the high pressure strength. Heavily worked or sputtered samples were found to support up to a 30% higher shear stress upon release than an annealed material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Structural, Elastic, Thermodynamic, Electronic, and Magnetic Investigations of Full-Heusler Compound Ag{sub 2}CeAl: FP-LAPW Method

Structural, elastic, thermodynamic, electronic, and magnetic properties of the full-Heusler compound Ag{sub 2}CeAl were determined using generalized gradient approximation with exchange-correlation functional GGA (PBEsol) with spin-orbit coupling (SOC) correction. The elastic modulus and their pressure dependence are calculated. From the elastic parameter behavior, it is inferred that this compound is elastically stable and ductile in nature. Through the quasi-harmonic Debye model, in which the phononic effect is considered the effect of pressure P (0 to 50) and temperature T (0 to 1000) on the lattice constant, the elastic parameters, bulk modulus B, heat capacity and thermal expansion α, internal energy U, entropy S, Debye temperature 𝜃{sub D}, Helmholtz free energy A, and Gibbs free energy G are investigated. The thermodynamic properties show that the compound Ag{sub 2}CeAl is a heavy fermion material. The density of state (DOS), magnetic momentum, and band structure are computed, to investigate the magnetic and metallic characteristics. The calculated polarization of the compound is 77.34%. The obtained results are the first predictions of the physical properties for the rare-earth-based (Ce) full-Heusler Ag{sub 2}CeAl.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Pressure effects on phase equilibria and solid solubility in MgO-Y2O3 nanocomposites

We study the temperature and pressure dependence of phase evolution in the 0.5MgO-0.5Y{sub 2}O{sub 3} nanocomposite system using a diamond anvil apparatus in conjunction with in situ synchrotron energy dispersive x-ray diffraction at 7 GPa hydrostatic pressure. At (298 K, 7.0 GPa), structural transformations in the Y{sub 2}O{sub 3} phase are observed, giving rise to the co-existence of its cubic, hexagonal, and monoclinic polymorphs together with cubic MgO. An increase in temperature to 1273 K causes the crystallinity of the Y{sub 2}O{sub 3} hexagonal and monoclinic phases to increase. Isothermal and isobaric hold at (1273 K, 7.0 GPa) for 60 min results in yttrium dissolution in cubic MgO, causing {approx}1.0% expansive volumetric lattice strain despite the large differences in the ionic radii of the cations. Cooling the nanocomposite to (298 K, 0 GPa) after a 60 min soak yields four phase co-existence among cubic MgO and cubic, hexagonal, and monoclinic Y{sub 2}O{sub 3}. The residual MgO unit cell volume expansion is 0.69% at 298 K, indicating solid solution formation at room temperature despite large differences in the ionic radii of Mg{sup 2+} and Y{sup 3+}. The macroscopic shrinkage due to densification is 3% by volume. Thermodynamic considerations suggest that the relative molar partial volume of Y{sup 3+} in MgO is a negative quantity, indicating that the partial molar volume of Y{sup 3+} in the solid solution is smaller than its molar volume in the pure state. Aging of the nanocomposites for 240 h does not change the observed 4 phase co-existence. We propose a crystallographic model in which the observed volumetric expansion of the MgO unit cell is primarily attributed to two hydrostatic expansive strain components accompanying solid solution formation: (i) Coulomb repulsion among O{sup 2-} ions in the immediate vicinity of Mg{sup 2+} vacancies, and (ii) misfit strain due to differences in ionic radii upon Y{sup 3+} substitution on Mg{sup 2+} sites.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evolution of ferroelectricity in tetrathiafulvalene-p-chloranil as a function of pressure and temperature

The neutral-to-ionic phase transition in the mixed-stack charge-transfer complex tetrathiafulvalene-p-chloranil (TTF-CA) has been studied by pressure-dependent infrared spectroscopy up to p = 11 kbar and down to low temperatures, T = 10 K. By tracking the C=O antisymmetric stretching mode of CA molecules, we accurately determine the ionicity of TTF-CA in the pressure-temperature phase diagram. At any point, the TTF-CA crystal bears only a single ionicity; there is no coexistence region or an exotic high-pressure phase. Our findings shed new light on the role of electron-phonon interaction in the neutral-ionic transition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Correlation between superconductivity and structural properties under high pressure of iron pnictide superconductor Ce[subscript 0.6]Y[subscript 0.4]FeAsO[subscript 0.8]F[subscript 0.2]

We report here the pressure dependence of the electrical resistivity and magnetic susceptibility of polycrystalline Ce{sub 0.6}Y{sub 0.4}FeAsO{sub 0.8}F{sub 0.2} superconductor in the temperature range 4 K to 300 K up to 8 GPa. In-situ high pressure-low temperature x-ray diffraction was performed at 8 K up to 32 GPa using synchrotron x-rays with helium pressure medium. The results show that the applied pressure slightly increases the T{sub c} up to 1 GPa and then it decreases on further pressure increase. The reduction of superconducting transition temperature occurs with a transition to a collapsed tetragonal phase and may be associated with a possible valence change of Ce.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Voltage and pressure scaling of streamer dynamics in a helium plasma jet with N{sub 2} co-flow

Positive polarity applied voltage and gas pressure dependent scaling of cathode directed streamer propagation properties in helium gas flow guided capillary dielectric barrier discharge have been quantified from streamer velocity, streamer current, and streamer optical diameter measurements. All measurements of the non-stochastic streamer properties have been performed in a variable gas pressure glass cell with N{sub 2} co-flow and under self-consistent Poisson electric field dominated conditions to permit data comparison with 2-D streamer dynamics models in air/nitrogen. The streamer optical diameter was found to be nearly independent of both gas pressures, from 170 Torr up to 760 Torr, and also for applied voltages from 6 to 11 kV at 520 Torr. The streamer velocity was found to increase quadratically with increased applied voltage. These observed differences in the 2-D scaling properties of ionization wave sustained cathode directed streamer propagation in helium flow channel with N{sub 2} annular co-flow compared to the streamer propagation in air or nitrogen have been shown to be caused by the remnant ionization distribution due to large differences in the dissociative recombination rates of He{sub 2}{sup +} versus N{sub 4}{sup +} ions, for this 5 kHz repetition rate applied voltage pulse generated streamers.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Lattice dynamics and electronic structure of mixed halofluoride scintillators under high pressure

We report the structural, lattice dynamics and electronic structure of mixed halo-fluoride scintillators MClF (M = Ca, Sr) based on density functional theory within generalized gradient approximation. The pressure dependent structural parameters and zone centered phonon frequencies are in reasonable agreement with the available experimental data. Both the structures are dynamically stable up to 50 GPa. MClF compounds are found to have a direct band gap along Γ−Γ direction at ambient as well as under pressure. It is found that these materials can serve as good storage phosphors in the low pressure region from the calculated electronic structure and optical spectra.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Computational Fluid Dynamics Simulation of Compressible Non-Newtonian Biomass in a Compression-Screw Feeder

Compression-screw feeders play a critical role in biorefineries to transport lignocellulosic biomass feedstocks from the feeding hoppers to biomass-conversion reactors in order to pretreat and convert biomass to hydrocarbon liquid biofuels and other power and energy resources. One of the main challenges in the operation of screw feeder is plugging and jamming of compressed biomass with high concentration of insoluble solids. The focus of this paper is to numerically investigate the screw feeder at these challenging operating conditions and help with the optimization of the screw feeder design to avoid operation failure. In this work a customized CFD model based on open-source OpenFOAM package [1] was developed to simulate the concentrated biomass as a highly viscous non-Newtonian fluid in the screw feeder. The biomass is modeled as a single-phase compressible Bingham fluid with a plastic viscosity as well as a density-dependent yield stress. The compressibility formulation (pressure-dependent density) and the density-dependent yield stress formulation in the governing equations follow the suggestions from a recent study by Duncan et al. [2]. A pilot-scale hopper/screw feeding system at NREL [3] is used to compare the experimental observations with our simulation results. The auger is 280 mm long and tapered with outer diameter changing from 80 mm to 35 mm. The auger rotates from 10 to 60 rpm in a conical throat which contains anti-rotational bars. The simulations predicted the required torque for the screw feeder and the pressure increase at the exit for biomass feedstocks with various fluid viscosity properties and auger rotating speeds. The analysis of the stress forces helped to identify the critical conditions were the screw feeder excessive wear or jamming could occur.

biofuels↗
DOE OSTI2018

First-Principle Calculations on the Structural, Mechanical, and Electronic Properties of Mn{sub 2}RuSi and Mn{sub 2}RuGe Under Pressure

The second- and third-order elastic constants of the new Inverse Heusler compounds Mn{sub 2}RuZ (Z = Si, Ge) are determined from the first-principle calculations combined with homogeneous deformation methods. In terms of the second- and third- order elastic constants, the effective second-order elastic constants of Mn{sub 2}RuZ (Z = Si, Ge) are also presented. Besides, the pressure dependence of the brittle/ductile properties, elastic anisotropy factor, Young’s modulus, and Debye temperature is further investigated. It is found that both Mn{sub 2}RuSi and Mn{sub 2}RuGe display ductile nature at ground state and Mn{sub 2}RuGe has better ductility than Mn{sub 2}RuSi under different pressures. This phenomenon is improved by the Young’s modulus. Finally, the density of states of Mn{sub 2}RuZ (Z = Si, Ge) under different pressures is also calculated in order to reveal the origin of the effect of pressure on the the ductility of Mn{sub 2}RuZ (Z = Si, Ge).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Structural and chemical properties of the nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate under pressure

The structural and chemical properties of the bi-molecular, hydrogen-bonded, nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate C{sub 3}H{sub 12}N{sub 12}O{sub 2} (TAG-MNT) have been investigated at room pressure and under high pressure isothermal compression using powder x-ray diffraction and Raman and infrared spectroscopy. A stiffening of the equation of state and concomitant structural relaxation between 6 and 14 GPa are found to correlate with Raman mode disappearances, frequency discontinuities, and changes in the pressure dependence of modes. These observations manifest the occurrence of a reversible martensitic structural transformation to a new crystalline phase. The onset and vanishing of Fermi resonance in the nitrimine group correlate with the stiffening of the equation of state and phase transition, suggesting a possible connection between these phenomena. Beyond 15 GPa, pressure induces irreversible chemical reactions, culminating in the formation of a polymeric phase by 60 GPa.

08 HYDROGEN↗