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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Comparing linear ion-temperature-gradient-driven mode stability of the National Compact Stellarator Experiment and a shaped tokamak

One metric for comparing confinement properties of different magnetic fusion energy configurations is the linear critical gradient of drift wave modes. The critical gradient scale length determines the ratio of the core to pedestal temperature when a plasma is limited to marginal stability in the plasma core. The gyrokinetic turbulence code GS2 was used to calculate critical temperature gradients for the linear, collisionless ion temperature gradient (ITG) mode in the National Compact Stellarator Experiment (NCSX) and a prototypical shaped tokamak, based on the profiles of a JET H-mode shot and the stronger shaping of ARIES-AT. While a concern was that the narrow cross section of NCSX at some toroidal locations would result in steep gradients that drive instabilities more easily, it is found that other stabilizing effects of the stellarator configuration offset this so that the normalized critical gradients for NCSX are competitive with or even better than for the tokamak. For the adiabatic ITG mode, NCSX and the tokamak had similar adiabatic ITG mode critical gradients, although beyond marginal stability, NCSX had larger growth rates. However, for the kinetic ITG mode, NCSX had a higher critical gradient and lower growth rates until a/L{sub T} Almost-Equal-To 1.5 a/L{sub T,crit}, when it surpassed the tokamak's. A discussion of the results presented with respect to a/L{sub T} vs. R/L{sub T} is included.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Comparing Linear Microinstability of the National Compact Stellarator Expriment and a Shaped Tokamak

One metric for comparing con nement properties of di erent magnetic fusion energy con gurations is the linear critical gradient of drift wave modes. The critical gradient scale length determines the ratio of the core to pedestal temperature when a plasma is limited to marginal stability in the plasma core. The gyrokinetic turbulence code GS2 was used to calculate critical temperature gradients for the linear, collisionless ion tem- perature gradient (ITG) mode in the National Compact Stellarator Experiment (NCSX) and a prototypical shaped tokamak, based on the pro les of a JET H-mode shot and the stronger shaping of ARIES-AT. While a concern was that the narrow cross section of NCSX at some toroidal locations would result in steep gradients that drive instabilities more easily, it is found that other stabilizing e ects of the stellarator con guration o set this so that the normalized critical gradients for NCSX are competitive with or even better than for the tokamak. For the adiabatic ITG mode, NCSX and the tokamak had similar critical gradients, though beyond marginal stability, NCSX had larger growth rates. However, for the kinetic ITG mode, NCSX had a higher critical gradient and lower growth rates until a/LT ≈ 1:5 a/LT;crit, when it surpassed the tokamak's. A discussion of the results presented with respect to a/LT vs R/LT is included.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

SU-E-J-273: Skin Temperature Recovery Rate as a Potential Predictor for Radiation-Induced Skin Reactions

Purpose: To assess the potential of dynamic infrared imaging to evaluate early skin reactions during radiation therapy in cancer patients. Methods: Thermal images were captured by our home-built system consisting of two flash lamps and an infrared (IR) camera. The surface temperature of the skin was first raised by ∼ 6 °C from ∼1 ms short flashes; the camera then captured a series of IR images for 10 seconds. For each image series, a basal temperature was recorded for 0.5 seconds before flash was triggered. The temperature gradients (ε) were calculated between a reference point (immediately after the flash) and at a time point of 2sec, 4sec and 9sec after that. A 1.0 cm region of interest (ROI) on the skin was drawn; the mean and standard deviations of the ROIs were calculated. The standard ε values for normal human skins were evaluated by imaging 3 healthy subjects with different skin colors. All of them were imaged on 3 separate days for consistency checks. Results: The temperature gradient, which is the temperature recovery rate, depends on the thermal properties of underlying tissue, i.e. thermal conductivity. The average ε for three volunteers averaged over 3 measurements were 0.64±0.1, 0.72±0.2 and 0.80±0.3 at 2sec, 4sec and 9sec respectively. The standard deviations were within 1.5%–3.2%. One of the volunteers had a prior small skin burn on the left wrist and the ε values for the burned site were around 9% (at 4sec) and 13% (at 9sec) lower than that from the nearby normal skin. Conclusion: The temperature gradients from the healthy subjects were reproducible within 1.5%–3.2 % and that from a burned skin showed a significant difference (9%–13%) from the normal skin. We have an IRB approved protocol to image head and neck patients scheduled for radiation therapy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2014

Local CC2 response method based on the Laplace transform: Analytic energy gradients for ground and excited states

A multistate local CC2 response method for the calculation of analytic energy gradients with respect to nuclear displacements is presented for ground and electronically excited states. The gradient enables the search for equilibrium geometries of extended molecular systems. Laplace transform is used to partition the eigenvalue problem in order to obtain an effective singles eigenvalue problem and adaptive, state-specific local approximations. This leads to an approximation in the energy Lagrangian, which however is shown (by comparison with the corresponding gradient method without Laplace transform) to be of no concern for geometry optimizations. The accuracy of the local approximation is tested and the efficiency of the new code is demonstrated by application calculations devoted to a photocatalytic decarboxylation process of present interest.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

The structure and electronic properties of hexagonal Fe{sub 2}Si

On the basis of first principle calculations, we show that a hexagonal structure of Fe{sub 2}Si is a ferromagnetic crystal. The result of the phonon spectra indicates that it is a stable structure. Such material exhibits a spin-polarized and half-metal-like band structure. From the calculations of generalized gradient approximation, metallic and semiconducting behaviors are observed with a direct and nearly 0 eV band gap in various spin channels. The densities of states in the vicinity of the Fermi level is mainly contributed from the d-electrons of Fe. We calculate the reflection spectrum of Fe{sub 2}Si, which has minima at 275 nm and 3300 nm with reflectance of 0.27 and 0.49, respectively. Such results may provide a reference for the search of hexagonal Fe{sub 2}Si in experiments. With this band characteristic, the material may be applied in the field of novel spintronics devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Training material models using gradient descent algorithms

High temperature design requires accurate constitutive models to describe material inelastic deformation and failure behavior. Oftentimes, calibrating accurate models devolves into the problem of fitting the model parameters against experimental test data. Here, we present the pyopmat package, an open source framework for calibrating constitutive models against experiment data subjected to various loading conditions using machine learning techniques. The package calculates the exact gradient of the model response with respect to the parameters using a combination of automatic differentiation and the adjoint method. Given this exact gradient, we compare the performance of several gradient-based optimization techniques in fitting realistic constitutive models against data. Here, we demonstrate the efficiency and accuracy of our package through example problems using both synthetic data, generated using known parameter sets, under monotonic and cyclic loading conditions and also with an example applying the techniques developed here to actual high temperature creep-fatigue test data.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Efficient Molecular Dynamics Simulations of Multiple Radical Center Systems Based on the Fragment Molecular Orbital Method

The fully analytic energy gradient has been developed and implemented for the restricted open-shell Hartree–Fock (ROHF) method based on the fragment molecular orbital (FMO) theory for systems that have multiple open-shell molecules. The accuracy of the analytic ROHF energy gradient is compared with the corresponding numerical gradient, illustrating the accuracy of the analytic gradient. The ROHF analytic gradient is used to perform molecular dynamics simulations of an unusual open-shell system, liquid oxygen, and mixtures of oxygen and nitrogen. These molecular dynamics simulations provide some insight about how triplet oxygen molecules interact with each other. Timings reveal that the method can calculate the energy gradient for a system containing 4000 atoms in only 6 h. Therefore, it is concluded that the FMO-ROHF method will be useful for investigating systems with multiple open shells.

36 MATERIALS SCIENCE↗
DOE OSTI2015

An atomic orbital-based formulation of analytical gradients and nonadiabatic coupling vector elements for the state-averaged complete active space self-consistent field method on graphical processing units

We recently presented an algorithm for state-averaged complete active space self-consistent field (SA-CASSCF) orbital optimization that capitalizes on sparsity in the atomic orbital basis set to reduce the scaling of computational effort with respect to molecular size. Here, we extend those algorithms to calculate the analytic gradient and nonadiabatic coupling vectors for SA-CASSCF. Combining the low computational scaling with acceleration from graphical processing units allows us to perform SA-CASSCF geometry optimizations for molecules with more than 1000 atoms. The new approach will make minimal energy conical intersection searches and nonadiabatic dynamics routine for molecular systems with O(10{sup 2}) atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Infrared fixed point of the SU(3) gauge theory with 𝑁 𝑓 = 10 flavors

We use lattice simulations and the continuous renormalization group method, based on the gradient flow, to calculate the 𝛽 function and anomalous dimensions of the SU(3) gauge theory with 𝑁 𝑓 = 10 flavors of fermions in the fundamental representation. We employ several improvements to extend the range of available renormalized couplings, including the addition of heavy Pauli-Villars bosons to reduce cutoff effects and the combination of a range of gradient flow transformations. While in the weak coupling regime our result is consistent with those of earlier studies, our techniques allow us to study the system at much stronger couplings than previously possible. We find that the renormalization group 𝛽 function develops a zero, corresponding to an infrared-stable fixed point, at gradient-flow coupling 𝑔 2 = 15.0⁢(5). We also determine the mass and tensor anomalous dimensions: At the fixed point we find 𝛾 𝑚 ≃0.6, suggesting that this system might be deep inside the conformal window.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Electronic structures and ferromagnetism of SnO{sub 2} (rutile) doped with double-impurities: First-principles calculations

The electronic and magnetic properties of double-impurities-doped SnO{sub 2} (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed for double impurities (M1 and M2) from M1 = Cr, and M2 = Mn, and Re. The origins of ferromagnetism are shown to be different in the two cases. For Sn{sub 1-2x}Cr{sub x}Mn{sub x}O2, the hybridization between Cr-3d and O-2p results in Cr becoming ferromagnetic with a magnetic moment of about 5.0 μ{sub B} per supercell. The Cr-and Mn-doped SnO{sub 2} system exhibits half-metallic ferromagnetism. The strong ferromagnetic couplings between local magnetic moments can be attributed to p-d hybridization. In contrast, in (Cr, Re) codoped TiO{sub 2}, the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Re, which would lead to the ferromagnetic through the double-exchange mechanism in transition metal oxides. Since there are two possible couplings between the impurities, we studied both configurations (ferromagnetic and antiferromagnetic (AF)) for double-impurities-doped SnO{sub 2}. Our calculations show that a ferromagnetic alignment of the spins is energetically always more stable than simple AF arrangements, which makes these materials possible candidates for spin injection in spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Electronic transport, thermal transport, thermal expansion, and magnetization in the strongly correlated metal LaNi⁢O 3

Perovskite structured LaNiO 3 is a strongly correlated metal with intriguing thermal and magnetic properties. The volume dependence of calculated and measured physical properties can add additional critical information to develop a more in-depth understanding of this strongly correlated phenomenon. Taking advantage of recent single crystal LaNiO 3 growth using the floating-zone method, we have measured the thermal expansion, the magnetostriction, and the pressure dependence of the magnetic susceptibility, which then allows derivation of the Grüneisen parameters γ e = $\frac{dlnN(E_F)}{d lnV}$, γ χ = $\frac{dlnχ}{d lnV}$, as well as of electric and thermal transport properties. We simulate the volume dependence of structural and magnetic properties using Density Functional Theory calculations at the Generalized Gradient Approximation level. A large discrepancy between experimental values and calculated ones suggests that strong correlations are likely to be dynamic in nature. This study also provides a side-by-side comparison of measurements in single crystal and polycrystalline samples of LaNiO 3 to elucidate intrinsic materials properties. A broad hump at high temperatures in the temperature dependence of magnetization found in the single crystal sample of LaNiO 3 has been rationalized by a model that includes the influence of electron correlations on the Landau diamagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Crystal Structure and Cation Off-Centering in Bi(Mg1/2,Ti1/2)O3

We investigate the structure of Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3} using first-principles calculations. Electric field gradients are reported. The qualitative tilts of the perovskite octahedra are in accord with previously reported x-ray results, but we find significant differences in atomic positions. These amount to as much as 0.5 {angstrom}, with the primary differences from the reported structure being the presence of substantial cation off-centerings both on the A site and on the B site in the calculated structure. Implications for this compound as the end point of a solid solution with a morphotropic phase boundary are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Evaluation of INL Supplied MOOSE/OSPREY Model: Modeling Water Adsorption on Type 3A Molecular Sieve

The purpose of this study was to evaluate Idaho National Lab’s Multiphysics Object-Oriented Simulation Environment (MOOSE) software in modeling the adsorption of water onto type 3A molecular sieve (3AMS). MOOSE can be thought-of as a computing framework within which applications modeling specific coupled-phenomena can be developed and run. The application titled Off-gas SeParation and REcoverY (OSPREY) has been developed to model gas sorption in packed columns. The sorbate breakthrough curve calculated by MOOSE/OSPREY was compared to results previously obtained in the deep bed hydration tests conducted at Oak Ridge National Laboratory. The coding framework permits selection of various options, when they exist, for modeling a process. For example, the OSPREY module includes options to model the adsorption equilibrium with a Langmuir model or a generalized statistical thermodynamic adsorption (GSTA) model. The vapor solid equilibria and the operating conditions of the process (e.g., gas phase concentration) are required to calculate the concentration gradient driving the mass transfer between phases. Both the Langmuir and GSTA models were tested in this evaluation. Input variables were either known from experimental conditions, or were available (e.g., density) or were estimated (e.g., thermal conductivity of sorbent) from the literature. Variables were considered independent of time, i.e., rather than having a mass transfer coefficient that varied with time or position in the bed, the parameter was set to remain constant. The calculated results did not coincide with data from laboratory tests. The model accurately estimated the number of bed volumes processed for the given operating parameters, but breakthrough times were not accurately predicted, varying 50% or more from the data. The shape of the breakthrough curves also differed from the experimental data, indicating a much wider sorption band. Model modifications are needed to improve its utility and predictive capability. Recommended improvements include: greater flexibility for input of mass transfer parameters, time-variable gas inlet concentration, direct output of loading and temperature profiles along the bed, and capability to conduct simulations of beds in series.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

Linearity for Ca2+-Doped CeBr3 Scintillating Materials

The Remote Sensing Laboratory (RSL) developed an aliovalently calcium-doped cerium tribromide (CeBr3:Ca2+) crystal with 3.2% resolution. RSL completed a crystal assessment, and Sandia National Laboratories calculated the predictive performance and physical characteristics using proven density functional theory (DFT) formalism. Results are reported for the work done to map the detector performance, characteristics, calcium doping concentration, and crystal strength. Preliminary scintillation measurements for this aliovalently calcium-doped CeBr3 scintillator exhibit a slight blue shift in fluorescence emission at 371 nm excitation for CeBr3. The structural, electronic, and optical properties of CeBr3 crystals were investigated using DFT within generalized gradient approximation. The calculated lattice parameters are in good agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr3, including the dielectric function, were calculated. Specifically, we report excellent linearity with the aliovalently calcium-doped CeBr3 crystal. Proportionality of light yield is one area of performance in which Ce-doped and Ce-based lanthanide halides excel. Maintaining proportionality is key to producing a strong, high-performance scintillator. Relative light yield proportionality was measured for both doped and undoped samples of CeBr3 to ensure no loss in performance was incurred during doping. The light output and proportionality, however, appear to be similar to CeBr3. There was a reduced yield at low energy. Relative light yield proportionality measurements suggest that dopants do not significantly affect proportionality at higher energies. RSL completed additional testing and evaluation of the new crystal and assessed benchmark spectroscopy measurements. Results, which present energy resolution as a function of energy, are summarized. Typical spectroscopy results using a 137Cs radiation source are shown for our crystallite. We obtain 4.5% for the packaged crystallite. More spectra are presented for 241Am, 60Co, 228Th, and background to exemplify CeBr3:Ca2+ over a broader energy range.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Linearity for Ca2+-Doped CeBr3 Scintillating Materials

The National Security Technologies, LLC, Remote Sensing Laboratory (RSL) developed an aliovalently calcium-doped cerium tribromide (CeBr3:Ca2+) crystal with a FWHM resolution of 3.2% at the 137Cs 662 keV gamma energy. RSL completed a crystal assessment and Sandia National Laboratories calculated the predictive performance and physical characteristics using proven density functional theory (DFT) formalism. Results are reported for the work done to map the detector performance, characteristics, calcium doping concentration, and crystal strength. Preliminary scintillation measurements for this aliovalently calcium-doped CeBr3 scintillator exhibit a slight blue shift in fluorescence emission at 371 nm excitation for CeBr3. The structural, electronic, and optical properties of CeBr3 crystals were investigated using the DFT within generalized gradient approximation. The calculated lattice parameters are in good agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr3, including the dielectric function of photons per unit energy, were calculated. Specifically, we report excellent linearity with the aliovalent CeBr3:Ca2+ crystal. Proportionality of light yield is one area of performance in which Ce-doped and Ce-based lanthanide halides excel. Maintaining proportionality is the key to producing a strong, high-performance scintillator. Relative light yield proportionality was measured for both doped and undoped samples of CeBr3 to ensure no loss in performance was incurred by doping. The light output and proportionality, however, appear to be similar to CeBr3. There was a reduced yield at low energy. Relative light yield proportionality measurements suggest that dopants do not significantly affect proportionality at higher energies. RSL completed additional testing and evaluation of the new crystal as well as the assessment of benchmarking spectroscopy. Results, which present energy resolution as a function of energy, are summarized. Typical spectroscopy results using a 137Cs radiation source are shown for our crystallites with diameters <1 cm. We obtain 3.2% before packing the crystallite in a sealed detector container and 4.5% for the packaged crystallite. More spectra were obtained for 241Am, 60Co, 228Th, and background to exemplify CeBr3:Ca2+ over a broader energy range.

aliovalent substitution↗
DOE OSTI2015

First-principles study on the bulk and (1 1 1) surface half-metallicity of KS and RbS in CsCl structure

Graphical abstract: Spin-polarized total and atomic DOS at S-(1 1 1) terminated slab and bulk in CsCl-type RbS. - Highlights: • The half metallic properties of CsCl-type RbS and KS have been studied. • The RbS's and KS's (1 1 1) slabs have been investigated. • Surface energy of RbS's and KS's (1 1 1) slabs are calculated. - Abstract: The electronic and magnetic properties of RbS and KS in CsCl structure have been investigated by using the full-potential local-orbital minimum-basis method. Calculating the relation between the total energies and lattice parameters for RbS and KS, we find out that the equilibrium lattice parameters are 4.02 Å and 3.84 Å for RbS and KS, respectively. According to our calculations in generalized gradient approximation approximation, both RbS and KS are half-metallic ferromagnets with the magnetic moments of 1 μ{sub B} per formula unit, and band gap of 4.287 eV for RbS and 4.395 eV for KS. We also have studied the electronic and magnetic properties of (1 1 1) surfaces of RbS and KS, and have found out that the half-metallicity of their bulk is preserved in all of those surfaces. Finally, through the calculations of formation energy of RbS and KS, it is found that their thin films are stable in the equilibrium conditions, and the Rb-terminated (1 1 1) slab of RbS and the K-terminated (1 1 1) slab of KS are more stable than their S-terminated (1 1 1) slabs. All of the above properties lead the compounds of RbS and KS in CsCl structure to be promising candidates for spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2013

PROPERTIES OF M31. III. CANDIDATE BEAT CEPHEIDS FROM PS1 PANDROMEDA DATA AND THEIR IMPLICATION ON METALLICITY GRADIENT

We present a sample of M31 beat Cepheids from the Pan-STARRS 1 PAndromeda campaign. By analyzing 3 years of PAndromeda data, we identify 17 beat Cepheids, spreading over a galactocentric distance of 10-16 kpc. Since the relation between the fundamental mode period and the ratio of the fundamental to the first overtone period places a tight constraint on metallicity, we are able to derive the metallicity at the position of the beat Cepheids using the relations from the model of Buchler. Our metallicity estimates show sub-solar values within 15 kpc, similar to the metallicities from H II regions. We then use the metallicity estimates to calculate the metallicity gradient of the M31 disk, which we find to be closer to the metallicity gradient derived from planetary nebula than the metallicity gradient from H II regions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

High energy X-ray diffraction study of a dental ceramics–titanium functional gradient material prepared by field assisted sintering technique

A functional gradient material with eleven layers composed of a dental ceramics and titanium was successfully consolidated using field assisted sintering technique in a two-step sintering process. High energy X-ray diffraction studies on the gradient were performed at High Energy Material Science beamline at Desy in Hamburg. Phase composition, crystal unit edges and lattice mismatch along the gradient were determined applying Rietveld refinement procedure. Phase analysis revealed that the main crystalline phase present in the gradient is α-Ti. Crystallinity increases stepwisely along the gradient with a decreasing increment between every next layer, following rather the weight fraction of titanium. The crystal unit edge a of titanium remains approximately constant with a value of 2.9686(1) Å, while c is reduced with increasing amount of titanium. In the layer with pure titanium the crystal unit edge c is constant with a value of 4.7174(2) Å. The lattice mismatch leading to an internal stress was calculated over the whole gradient. It was found that the maximal internal stress in titanium embedded in the studied gradient is significantly smaller than its yield strength, which implies that the structure of titanium along the whole gradient is mechanically stable. - Highlights: • High energy XRD studies of dental ceramics–Ti gradient material consolidated by FAST. • Phase composition, crystallinity and lattice parameters are determined. • Crystallinity increases stepwisely along the gradient following weight fraction of Ti. • Lattice mismatch leading to internal stress is calculated over the whole gradient. • Internal stress in α-Ti embedded in the gradient is smaller than its yield strength.

36 MATERIALS SCIENCE↗