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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Characterization of the physical properties for solid granular materials

Accurate prediction of the behavior of a system is strongly governed by the components within that system. For multiphase systems incorporating solid powder-like particles, there are many different physical properties which need to be known to some level of accuracy for proper design, modeling, or data analysis. In the past, the material properties were determined initially as a secondary part of the study or design. In an attempt to provide results with the least level of uncertainty, a procedure was developed and implemented to provide consistent analysis of several different types of materials. The properties that were characterized included particle sizing and size distributions, shape analysis, density (particle, skeletal and bulk), minimum fluidization velocities, void fractions, particle porosity, and assignment within the Geldart Classification. In the methods used for this experiment, a novel form of the Ergun equation was used to determine the bulk void fractions and particle density. Materials of known properties were initially characterized to validate the accuracy and methodology, prior to testing materials of unknown properties. The procedures used yielded valid and accurate results, with a high level of repeatability. A database of these materials has been developed to assist in model validation efforts and future designs. It is also anticipated that further development of these procedures wil be expanded increasing the properties included in the database.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Laboratory Study Of Magnetic Reconnection With A Density Asymmetry Across The Current Sheet

The effects of an upstream density asymmetry on magnetic reconnection are studied systematically in a laboratory plasma. Despite a significant upstream density asymmetry of up to 10, the reconnecting magnetic field pro file is not signifi cantly changed. On the other hand, the out-of-plane magnetic field profile is considerably modified; it is almost bipolar in structure with the density asymmetry, as compared to the quadrupolar structure in the symmetric configuration. The in-plane ion flow pattern and the electrostatic potential pro file are also affected by the density asymmetry. Strong bulk electron heating is observed near the low-density-side separatrix together with electromagnetic fluctuations in the lower hybrid frequency range. The dependence of the ion outflow and reconnection electric field on the density asymmetry is measured and compared with theoretical expectations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1958

Heat-Transfer and Friction Measurements with Variable Properties for Airflow Normal to Finned and Unfinned Tube Banks

A single-line correlation of both the heat-transfer and pressure- drop data for electrically heated unfinned tubes is obtained by evaluating the density in the Reynolds number, specific heat, thermal conductivity, and viscosity at the film temperature, and the density in the friction coefficient at the bulk temperature. The heat-transfer data for finned tubes also exhibit an effect of physical-property variation which is removed by evaluating all properties, including density, at the primary surface temperature, and using k* = 0.015 square root of T/530 for the thermal conductivity of air where T is the absolute temperature. The pressure drop for finned tubes is correlated by the use of bulk density in both the Reynolds number and friction coefficient. The data reported are for Reynolds numbers from 2000 to 35,000, surface temperatures from 600 to 1400 R, and an air inlet temperature of 530 R.

Ragsdale, Robert G.↗
DOE OSTI2019

Biomass Grinding Process Optimization Using Response Surface Methodology and a Hybrid Genetic Algorithm

Biomass could be a key source of renewable energy. Agricultural waste products, such as corn stover, provide a convenient means to replace fossil fuels, such as coal, and a large amount of feedstock is currently available for energy consumption in the U.S. This study has two main objectives: (1) to understand the impact of corn stover moisture content and grinder speed on grind physical properties; and (2) develop response surface models and optimize these models using a hybrid genetic algorithm. The response surface models developed were used to draw surface plots to understand the interaction effects of the corn stover grind moisture content and grinder speed on the grind physical properties and specific energy consumption. The surface plots indicated that a higher corn stover grind moisture content and grinder speed had a positive effect on the bulk and tapped density. The final grind moisture content was highly influenced by the initial moisture content of the corn stover grind. Optimization of the response surface models using the hybrid genetic algorithm indicated that moisture content in the range of 17 to 19% (w.b.) and a grinder speed of 47 to 49 Hz maximized the bulk and tapped density and minimized the geomantic mean particle length. The specific energy consumption was minimized when the grinder speed was about 20 Hz and the corn stover grind moisture content was about 10% (w.b.).

09 BIOMASS FUELS↗
DOE OSTI2014

Effect of discharge current and deposition temperature on roughness and density of NbC films fabricated by ion beam sputtering technique

NbC films were prepared using Ion beam sputtering system at various discharges current from 0.4 amps to 1.2 amps at room temperature. Effect of temperature on NbC films were also studied by depositing NbC films at various temperatures from room temperature to 200,300,400 and 600°C. X-ray reflectivity (XRR) study shows that surface roughness of the film decreases with decrease in discharge current. The optimum lowest roughness 3.2Å having density 92% of bulk was achieved at discharge current 0.6 amps at 3.0 cm{sup 3}/min Ar gas flow. X-ray study also shows that film roughness decreases with increase in temperature of the film and after a certain temperature it increases with increase in temperature. The lowest surface roughness 2.1Å was achieved at 300°C with density 83% of bulk NbC at constant discharge current 0.6 amps.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Climatic controls on soil and saprock nitrogen distribution and persistence in the Sierra Nevada

Nitrogen (N) is an essential nutrient in soil that regulates plant growth, terrestrial sequestration of atmospheric carbon dioxide, and persistence of organic compounds. However, major knowledge gaps remain about how climate change may impact N accumulation and persistence, especially in deep soil and saprock (friable weakly weathered bedrock). Our objective was to understand how climate impacts the accumulation of N in soil and saprock and how climate impacts soil N distribution, persistence, and mechanisms of N persistence. We investigated N concentration in bulk soil and density fractions. We estimated N persistence along a bio-climatic sequence—sites range from a low-elevation oak savannah, mid-elevation pine-oak/mixed-conifer forest, to a high-elevation subalpine forest—in the southern Sierra Nevada in California. A combination of radiocarbon and elemental composition measurements along with a first-order kinetic model was used. The N concentration in the bulk soil and density fractions declined with depth, and there was a relatively greater mineral-associated heavy fraction (HF) N in deeper samples. The cooler/wetter mixed conifer site held 37% of profile N in saprock, which was greater than that of the entire soil profile at the drier/hotter oak savannah. The majority of N in soil, which was in the HF, was not influenced by climate proxies tested. However, both unprotected and occluded fractions of N were strongly influenced by climate. Soil N mean residence time (MRT) showed that drier/hotter climates have a shorter MRT, compared to mid-elevation sites with cooler/wetter climates. The effect of climate on deep saprock N storage might be indirect, primarily through climatic influence on the thickness of saprock. Overall, our findings suggest the mineral-associated HF N pool will not be vulnerable to changes in climate and will continue to contribute to the persistent soil N pool. The amount of topsoil and subsoil unprotected and occluded N can be explained by gross primary productivity and mean annual precipitation indicating that changes in climate can influence N partitioning. N stored in deep soil and saprock may be less vulnerable to climate than N stored in drier/hotter climates with less deeply stored N. It is critical to dig deeper to understand terrestrial ecosystems’ response to climate.

58 GEOSCIENCES↗
DOE OSTI2020

Nanoconfinement of Molecular Magnesium Borohydride Captured in a Bipyridine-Functionalized Metal-Organic Framework

The lower limit of metal hydride nanoconfinement is demonstrated through the coordination of a molecular hydride species to binding sites inside the pores of a metal-organic framework (MOF). Magnesium borohydride, which has a high hydrogen capacity, is incorporated into the pores of UiO-67bpy (Zr6O4(OH)4(bpydc)6 with bpydc2- = 2,2'-bipyridine-5,5'- dicarboxylate) by solvent impregnation. The MOF retained its long-range order, and transmission electron microscopy and elemental mapping confirmed the retention of the crystal morphology and revealed a homogeneous distribution of the hydride within the MOF host. Notably, the B-, N-, and Mg-edge XAS data confirm the coordination of Mg(II) to the N atoms of the chelating bipyridine groups. In situ 11B MAS NMR studies helped elucidate the reaction mechanism and revealed that complete hydrogen release from Mg(BH4)2 occurs as low as 200 °C. Sieverts and thermogravimetric measurements indicate an increase in the rate of hydrogen release, with the onset of hydrogen desorption as low as 120 °C, which is approximately 150 °C lower than that of the bulk material. Furthermore, density functional theory calculations support the improved dehydrogenation properties and confirm the drastically lower activation energy for B-H bond dissociation.

Nanoconfinement, Metal Hydrides, Metal-Organic Fra↗
DOE OSTI2020

Additive Manufacturing of Controlled Porous Elastomeric Nanocomposites for Enhanced Sensing Function

Sensors manufactured using piezoresistive elastomeric nanocomposites have a wide range of applications in many fields such as structural health monitoring, robotics, and biomedical industries. Additive manufacturing of these sensors, achieved using Direct Ink Writing (DIW), has provided numerous advantages including increased sensitivity and the fabrication of complex geometry. However, solid sensors manufactured either additively or using conventional methods, display a greater level of hysteresis, especially at strains higher than 10%, which limits their sensitivity. The solid sensors also display a greater level of piezoresistive relaxation where the resistance change with time either in a static or dynamic loading environment. Porosity can be introduced in the form of varying infill patterns and densities to reduce bulk material effects. In this paper, a lattice structure with four infill densities are investigated, and the samples are printed using a grid infill pattern. The fabricated samples are characterized using a scanning electron microscope (SEM) to validate the microstructural features and layer bonding. The pressure-sensing capability of each sensor is investigated using cyclic compression loading at various strain levels. Sensing results show that an increase in sensitivity, as well as a decrease in hysteresis with an increase in porosity, compared to solid samples (100% in-fill). The highest porosity was measured to be 62.3% with a corresponding 112% increase in the gauge factor compared to the solid bulk. These results indicate that introducing porosity using DIW is a sensible strategy to improve the piezoresistive performance of nanocomposites and to allow for the tunability of sensing capacity in pressure sensors

Sukati, Musa↗
DOE OSTI2013

A SURVEY OF THE PARAMETER SPACE OF THE COMPRESSIBLE LIQUID DROP MODEL AS APPLIED TO THE NEUTRON STAR INNER CRUST

We present a systematic survey of the range of predictions of the neutron star inner crust composition, crust-core transition densities and pressures, and density range of the nuclear 'pasta' phases at the bottom of the crust provided by the compressible liquid drop model in light of the current experimental and theoretical constraints on model parameters. Using a Skyrme-like model for nuclear matter, we construct baseline sequences of crust models by consistently varying the density dependence of the bulk symmetry energy at nuclear saturation density, L, under two conditions: (1) that the magnitude of the symmetry energy at saturation density J is held constant, and (2) J correlates with L under the constraint that the pure neutron matter (PNM) equation of state (EoS) satisfies the results of ab initio calculations at low densities. Such baseline crust models facilitate consistent exploration of the L dependence of crustal properties. The remaining surface energy and symmetric nuclear matter parameters are systematically varied around the baseline, and different functional forms of the PNM EoS at sub-saturation densities implemented, to estimate theoretical 'error bars' for the baseline predictions. Inner crust composition and transition densities are shown to be most sensitive to the surface energy at very low proton fractions and to the behavior of the sub-saturation PNM EoS. Recent calculations of the energies of neutron drops suggest that the low-proton-fraction surface energy might be higher than predicted in Skyrme-like models, which our study suggests may result in a greatly reduced volume of pasta in the crust than conventionally predicted.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Progress in Critical Current Density (J{sub c}) in Sintered MgB{sub 2} Bulks

The present paper focuses on methods of further improving the flux pinning and critical current density of disk-shaped MgB{sub 2} bulk superconductors by adding excess Mg metal in combination with an optimum silver content and optimized processing conditions. Bulk MgB{sub 2} samples were produced by in situ solid-state reaction in Ar gas ambient using high purity commercial powders of Mg metal and 1.5 wt% carbon-coated amorphous B powders mixed in a fixed ratio of Mg/B = 1.1:2. Further, 4 wt% silver was added to improve flux pinning as well as mechanical performance of the bulk MgB{sub 2} material. The magnetization measurements confirmed a sharp superconducting transition with T{sub c,onset} at around 37 K, which is only by 1 K lower than in bulk MgB{sub 2} material produced without carbon-coated amorphous boron. The critical current density (J{sub c}) values significantly improved in the MgB{sub 2} material with 4 wt% of silver and 1.5 wt% of carbon-coated amorphous boron, sintered at 775 {sup ∘}C for 3 h. At 20 K, this sample showed J{sub c} at around 500 and 350 kA/cm{sup 2} in the self-field and 1 T, respectively, which makes it suitable for several industrial applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

REMOVING BARYON-ACOUSTIC-OSCILLATION PEAK SHIFTS WITH LOCAL DENSITY TRANSFORMS

Large-scale bulk flows in the universe distort the initial density field, broadening the baryon-acoustic-oscillation (BAO) feature that was imprinted when baryons were strongly coupled to photons. Additionally, there is a small shift inward in the peak of the conventional overdensity correlation function, a mass-weighted statistic. This shift occurs when high-density peaks move toward each other. We explore whether this shift can be removed by applying to the density field a transform (such as a logarithm) that gives fairer statistical weight to fluctuations in underdense regions. Using configuration-space perturbation theory in the Zel'dovich approximation, we find that the log-density correlation function shows a much smaller inward shift in the position of the BAO peak at low redshift than is seen in the overdensity correlation function. We also show that if the initial, Lagrangian density of matter parcels could be estimated at their Eulerian positions, giving a displaced-initial-density field, its peak shift would be even smaller. In fact, a transformed field that accentuates underdensities, such as the reciprocal of the density, pushes the peak the other way, outward. In our model, these shifts in the peak position can be attributed to shift terms, involving the derivative of the linear correlation function, that entirely vanish in this displaced-initial-density field.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Anion Effects on the Interfacial Structure and Bulk Physical Properties in Choline-Based Hydrogen-Bonded Electrolytes

Electrolytes consisting of choline (Ch) salts with acetate (Ac) and oxalate (Ox) anions were studied in mixtures with ethylene glycol (EG), in comparison to chloride (Cl)- and bis(trifluorosulfonyl)imide (TFSI)-containing systems to understand the impact of anion size and charge density on the bulk electrolyte and interfacial properties. These represent hydrogen-bonded concentrated electrolytes where the Ch salts are the H-bond acceptors (HBAs) and EG is the H-bond donor (HBD) in 1:2 and 1:4 HBA:HBD ratios. Increased H-bonding in Ch 2 Ox with EG compacts the liquid compared to ChAc, consistent with the increased dissociation energies of the HBA:HBD clusters with Ox –2 calculated by density functional theory (DFT), thus leading to denser and more viscous liquids. Electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS) were used to investigate interfacial charge density and the electrode surface species. Differential capacitance curves demonstrate weak dependence on potential except for ChAc:EG, where Ac – is found to adsorb on the Au surface, in particular at +0.055 V vs Ag quasi-reference while no specific adsorption was observed on glassy carbon (GC). In situ SERS measurement confirms the formation of a surface layer in the ChAc:EG systems resulting from a Au–acetate interaction. Despite the structural similarities, Ox –2 did not show specific binding or any dependence of capacitance on potential as a result of strongly solvating EG molecules via H-bonds. Furthermore, this study shows that the charge density of the HBAs impacts both bulk properties and the electrical double-layer formation in concentrated H-bonded electrolytes that is relevant to electrochemical kinetics and morphology in electrodeposition processes and energy storage applications.

25 ENERGY STORAGE↗
NASA NTRS1994

Equatorial heating and hemispheric decoupling effects on inner magnetospheric core plasma evolution

We have extended our previous semikinetic study of early stage plasmasphere refilling with perpendicular ion heating by removing the restriction that the northern and southern boundaries are identical and incorporating a generalized transport description for the electrons. This allows investigation of the effects of electron heating and a more realistic calculation of electric fields produced by ion and electron temperature anisotropies. The combination of perpendicular ion heating and parallel electron heating leads to an equatorial electrostatic potential peak, which tends to shield and decouple ion flows in the northern and southern hemispheres. Unequal ionospheric upflows in the northern and southern hemispheres lead to the development of distinctly asymmetric densities and other bulk parameters. At t = 5 hour after the initiation of refiling with different source densities (N(sub north) = 100 cu/cm, N(sub south) = 50 cu/cm), the maximum potential drops of the northern and southern hemispheres are 0.6 and 1.3 V, respectively. At this time the minimum ion densities are 11 and 7 cu/cm for the northern and southern hemispheres. DE 1 observations of asymmetric density profiles by Olsen may be consistent with these predictions. Termination of particle heating causes the reduction of equatorial potential and allows interhemispheric coupling. When the inflows from the ionospheres are reduced (as may occur after sunset), decreases in plasma density near the ionospheric regions are observed while the heated trapped ion population at the equator persists.

Lin, J.↗
DOE OSTI2019

Density-Functional Tight-Binding for Platinum Clusters and Bulk: Electronic vs Repulsive Parameters

We present a general purpose Pt-Pt density-functional tight-binding (DFTB) parameter for Pt clusters as well as bulk, using a genetic algorithm (GA) to automatize the parameterization effort. First we quantify the improvement possible by only optimizing the repulsive potential alone, and second we investigate the effect of improving the electronic parameter as well. During both parameterization efforts we employed our own training set and test sets, with one set containing ~20,000 spin-polarized DFT structures. We analyze the performance of our two DFTB Pt-Pt parameter sets against density functional theory (DFT) as well as an earlier DFTB Pt-Pt parameters. Our study sheds light on the role of both repulsive and electronic parameters with regards to DFTB performance.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Role of quantum fluctuations in structural dynamics of liquids of light molecules

A possible role of quantum effects, such as tunneling and zero-point energy, in the structural dynamics of supercooled liquids is studied by dielectric spectroscopy. Our results demonstrate that the liquids, bulk 3-methyl pentane and confined normal and deuterated water, have low glass transition temperature and unusually low for their class of materials steepness of the temperature dependence of structural relaxation (fragility). Although we do not find any signs of tunneling in the structural relaxation of these liquids, their unusually low fragility can be well described by the influence of the quantum fluctuations. Confined water presents an especially interesting case in comparison to the earlier data on bulk low-density amorphous and vapor deposited water. Confined water exhibits a much weaker isotope effect than bulk water, although the effect is still significant. Here, we show that it can be ascribed to the change of the energy barrier for relaxation due to a decrease in the zeropoint energy upon D/H substitution. We observed a difference in the behavior of confined and bulk water demonstrates high sensitivity of quantum effects to the barrier heights and structure of water. Moreover, these results demonstrate that extrapolation of confined water properties to the bulk water behavior is questionable.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1972

Relationship of dislocation density of silicon to solar cell current loss at low temperature

Large decreases in short circuit current of silicon solar cells have been reported to occur as temperature is decreased below -60 C. Experimental results are presented which relate high dislocation density of the silicon bulk material of cells to the large current loss effect. Solar cells were made by the same processes from a variety of silicon materials, namely low-dislocation-density, high-dislocation-density float-zone, and Czochralski silicon. All cells were etched in a manner which revealed the dislocation density of the cell bulk silicon. It was found that every cell made from any of the various low-dislocation starting materials obtained from three suppliers still had a low-dislocation bulk after cell processing, and that all such cells belonged to category good. Cells made from float-zone materials showed high dislocation densities in their bulk and either fell into category poor, or had intermediate losses of short-circuit current at low temperature.

Mandelkorn, J.↗