Showing papers by "Sandia National Laboratories published in 1996"
TL;DR: In this article, the authors explore the interrelationships between the green 510 nm emission, the free-carrier concentration, and the paramagnetic oxygen vacancy density in commercial ZnO phosphors by combining photoluminescence, optical absorption, and electron paramagnetic resonance spectroscopies.
Abstract: We explore the interrelationships between the green 510 nm emission, the free‐carrier concentration, and the paramagnetic oxygen‐vacancy density in commercial ZnO phosphors by combining photoluminescence, optical‐absorption, and electron‐paramagnetic‐resonance spectroscopies. We find that the green emission intensity is strongly influenced by free‐carrier depletion at the particle surface, particularly for small particles and/or low doping. Our data suggest that the singly ionized oxygen vacancy is responsible for the green emission in ZnO; this emission results from the recombination of a photogenerated hole with the singly ionized charge state of this defect.
3,487 citations
TL;DR: In this paper, the authors examined the effect of ambient gas density and fuel vaporization on the penetration and dispersion of diesel sprays over a gas density range spanning nearly two order of magnitude.
Abstract: Ambient gas density and fuel vaporization effects on the penetration and dispersion of diesel sprays were examined over a gas density range spanning nearly two order of magnitude. This range included gas densities more than a factor of two higher than top-dead-center conditions in current technology heavy-duty diesel engines. The results show that ambient gas density has a significantly larger effect on spray penetration and a smaller effect on spray dispersion than has been previously reported. The increased dependence of penetration on gas density is shown to be the result of gas density effects on dispersion. In addition, the results show that vaporization decreases penetration and dispersion by as much as 20% relative to non-vaporizing sprays; however, the effects of vaporization decrease with increasing gas density. Characteristic penetration time and length scales are presented that include a dispersion term that accounts for the increased dependence of penetration on ambient density. These penetration time and length scales collapse the penetration data obtained over the entire range of conditions examined in the experiment into two distinct non-dimensional penetration curves: one for the non-vaporizing conditions and one for the vaporizing conditions. Comparison of the two nondimensional penetration curves to a theoretical penetration correlation for non-vaporizing sprays helped isolate and explain the effects of droplets and vaporization on penetration. The theoretical penetration correlation was derived using the penetration time and length scales and simple model for a non-vaporizing spray that has been previously presented in the literature. The correlation is in good agreement with the non-vaporizing data from this experiment and other commonly quoted penetration data sets. It also provides a potential explanation for much of scatter in the penetration predicted by various correlations in the literature.
965 citations
18 Jun 1996
TL;DR: A method for combining classifiers that use estimates of each individual classifier's local accuracy in small regions of feature space surrounding an unknown test sample that performs better on data from a real problem in mammogram image analysis than do other recently proposed CMC techniques.
Abstract: Combination of multiple classifiers (CMC) has recently drawn attention as a method of improving classification accuracy. This paper presents a method for combining classifiers that use estimates of each individual classifier's local accuracy in small regions of feature space surrounding an unknown test sample. Only the output of the most locally accurate classifier is considered. We address issues of (1) optimization of individual classifiers, and (2) the effect of varying the sensitivity of the individual classifiers on the CMC algorithm. Our algorithm performs better on data from a real problem in mammogram image analysis than do other recently proposed CMC techniques.
476 citations
TL;DR: The results of the laboratory and power plant tests which included: tracking and analyzing fuels and deposits by various methods; recording operating conditions; and extensive laboratory testing have advanced the understanding of the role of minerals in the combustion of biomass, and their occurrence in biofuels as discussed by the authors.
Abstract: Alkali in the ash of annual crop biomass fuels creates serious fouling and slagging in conventional boilers. Even with the use of sorbents and other additives, power plants can fire only limited amounts of these fuels in combination with wood. The National Renewable Energy Laboratory (NREL). U.S. Department of Energy (DOE), and the biomass power industry conducted eight full-scale firing tests and several laboratory experiments to study the nature and occurrence of deposits. The goal was to increase the quantities of these biofuels which can be used. This paper describes the results of the laboratory and power plant tests which included: tracking and analyzing fuels and deposits by various methods; recording operating conditions; and extensive laboratory testing. These analyses have advanced the understanding of the role of minerals in the combustion of biomass, and their occurrence in biofuels. Deposits occur as a result of the boiler design, fuel properties and boiler operation. The limited furnace volume and high flue gas exit temperatures of most biomass boilers promote slag or deposits from biofuels which contain significant amounts of alkali, sulfur or chlorine and silica. All annual growth, whether from urban tree trimmings, annual crops and residues or energy crops contains sufficient volatile alkali, 0.34 kg GJ − (0.8 lb MMBtu −1 ) or more, to melt in combustion or vaporize and condense on boiler tubes and refractory. Special boiler designs are required for annual crops, including grasses and straws. Addition of magnesium oxide and other additives may be necessary to inhibit alkali volatilization while burning these biofuels.
402 citations
399 citations
TL;DR: In this paper, the minimum Lp-norm solution is obtained by embedding the transform-based methods for unweighted and weighted least squares within a simple iterative structure, and the data-dependent weights are generated within the algorithm and need not be supplied explicitly by the user.
Abstract: We develop an algorithm for the minimum Lp-norm solution to the two-dimensional phase unwrapping problem. Rather than its being a mathematically intractable problem, we show that the governing equations are equivalent to those that describe weighted least-squares phase unwrapping. The only exception is that the weights are data dependent. In addition, we show that the minimum Lp-norm solution is obtained by embedding the transform-based methods for unweighted and weighted least squares within a simple iterative structure. The data-dependent weights are generated within the algorithm and need not be supplied explicitly by the user. Interesting and useful solutions to many phase unwrapping problems can be obtained when p< 2. Specifically, the minimum L0-norm solution requires the solution phase gradients to equal the input data phase gradients in as many places as possible. This concept provides an interesting link to branch-cut unwrapping methods, where none existed previously.
364 citations
TL;DR: No basal plane participation is required to explain the site densities determined from proton adsorption isotherms, and independent evidence from scanning force microscopy points to a higher percentage of edge surface area due to thicker particles and basal surface steps than previously assumed.
312 citations
TL;DR: In this article, the important changes produced on the electroluminescence characteristics of organic materials due to planar microcavity effects are examined in detail, and the design considerations for and device characteristics of a novel multiple emissive layer LED are also described.
Abstract: The important changes produced on the electroluminescence characteristics of organic materials due to planar microcavity effects are examined in detail. The photon density of states is redistributed such that only certain wavelengths, which correspond to allowed cavity modes, are emitted in a given direction. This enables us to realize color selectivity over a large wavelength (and color coordinate) range with broadband emitters such as 8‐hydroxyquinoline aluminum (Alq), and intensity enhancement in narrow band emitters. The intensity enhancement in Alq‐based cavity light emitting diodes (LEDs) is extensively evaluated both experimentally and theoretically. The design considerations for and device characteristics of a novel multiple emissive layer LED are also described.
309 citations
TL;DR: In this paper, the authors conducted depth of penetration experiments into grout and concrete targets with ogive-nose steel projectiles, and determined experimentally the striking velocities corresponding to maximum penetration depths.
289 citations
TL;DR: In this paper, a review of recent advances in understanding of the structure of turbulent non-premixed flames due to extensive data acquired from single-point and planar imaging experiments using the Raman, Rayleigh, and LIF diagnostic methods is presented.
283 citations
TL;DR: In this paper, the authors review the history and recent development of this theory, analysing various exact and approximate solutions and relating them to each other, and propose a nonlinear partial differential equation for the foam density as a function of time and vertical position.
Abstract: The drainage of liquid in a foam may be described in terms of a nonlinear partial differential equation for the foam density as a function of time and vertical position. We review the history and recent development of this theory, analysing various exact and approximate solutions and relating them to each other.
TL;DR: In this paper, the authors evaluated the performance of five popular DFT functionals, BH&HLYP, B3PW91, B 3P86, and BLYP, in characterizing a set of seven transition states.
TL;DR: In this paper, the effects of strain rate and curvature on the intermediate radical concentrations and heat release rate are evaluated, and the topology of the flame surface is interpreted in terms of its propagation and statistics.
TL;DR: In this article, the authors compared several methods of finding the surface heat flux using transient temperature measurements inside a heat-conducting body, including function specification, Tikhonov regularization, iterative regularization and specified functions over large time regions with Green's functions.
TL;DR: The diffusion of Si dimers on the Si(001) surface at temperatures between room temperature and 128{degree}C is measured using a novel atom-tracking technique that can resolve every diffusion event.
Abstract: The diffusion of Si dimers on the Si(001) surface at temperatures between room temperature and 128 \ifmmode^\circ\else\textdegree\fi{}C is measured using a novel atom-tracking technique that can resolve every diffusion event. The atom tracker employs lateral-positioning feedback to lock the scanning tunneling microscope (STM) probe tip into position above selected atoms with subangstrom precision. Once locked the STM tracks the position of the atoms as they migrate over the crystal surface. By tracking individual atoms directly, the ability of the instrument to measure dynamic events is increased by a factor of $\ensuremath{\sim}1000$ over conventional STM imaging techniques.
TL;DR: Using electron paramagnetic resonance, the motion of oxygen vacancies within the oxygen octahedron in perovskite BaTiO{sub 3} is observed via the alignment of oxygen vacancy-related defect dipoles induced by bias/heat combinations.
Abstract: Using electron paramagnetic resonance, the motion of oxygen vacancies within the oxygen octahedron in perovskite BaTiO{sub 3} is observed via the alignment of oxygen vacancy-related defect dipoles induced by bias/heat combinations. The vacancy motion is found to have an activation energy of 0.91 eV, in excellent agreement with the predicted. It is found that the onset of resistance degradation is also concurrent with oxygen vacancy motion. This result spectroscopically demonstrates that oxygen vacancy migration in the lattice is likely responsible for the observed degradation.
01 Jan 1996
TL;DR: In this article, the quantum chemical BAC-MP4 and BACMP2 methods have been used to investigate the reaction mechanisms leading to polycyclic aromatic hydrocarbon (PAH) ring formation, in particular the elementary reaction steps in the conversion of two cyclopentadienyl radicals to naphthalene.
Abstract: The quantum chemical BAC-MP4 and BAC-MP2 methods have been used to investigate the reaction mechanisms leading to polycyclic aromatic hydrocarbon (PAH) ring formation. In particular we have determined the elementary reaction steps in the conversion of two cyclopentadienyl radicals to naphthalene. This reaction mechanism is shown to be an extension of the mechanism occurring in the H atomassisted conversion of fulvene to benzene. The net reaction involves the formation of dihydrofulvalene, which eliminates a hydrogen atom and then rearranges to form naphthalene through a series of ring closures and openings. The importance of forming the -CR(·)-CHR-CR′=CR″- moiety, which can undergo rearrangement to form three-carbon atom ring structures, is illustrated with the C4H7 system. The ability of hydrogen atoms to migrate around the cyclopentadienyl moiety is illustrated both for methyl-cyclopentadiene, C5H5CH3, and dihydrofulvalene, C5H5C5H5, as well as for their radical species, C6H7 and C5H5C5H4. The mobility of hydrogen in the cyclopentadienyl moiety plays an important role both in providing resonance-stabilized radical products and in creating the -CR(·) CHR-CR′=CR″- moiety for ring formation. The results illustrate the radical pathway for converting five-membered rings to aromatic six-membered rings. Furthermore, the results indicate the important catalytic role of H atoms in the aromatic ring formation process.
TL;DR: A systematic experimental study of annular aluminum-wire Z-pinches on a 20-TW electrical generator shows that the measured spatial characteristics and emitted x-ray power agree more closely with rad-hydro simulations when large numbers of wires are used.
Abstract: A systematic experimental study of annular aluminum-wire Z-pinches on a 20-TW electrical generator shows that the measured spatial characteristics and emitted x-ray power agree more closely with rad-hydro simulations when large numbers of wires are used. The measured x-ray power increases first slowly and then rapidly with decreasing interwire gap spacing. Simulations suggested that this increase reflects the transition from implosion of individual wire plasmas to one of an azimuthally symmetric plasma shell. In the plasma-shell regime, x-ray powers of 40 TW are achieved.
TL;DR: This paper introduces a new algorithm called whisker weaving for constructing unstructured, all-hexahedral finite element meshes based on the Spatial Twist Continuum (STC), a global interpretation of the geometric dual of an all- hexahedral mesh.
Abstract: This paper introduces a new algorithm called whisker weaving for constructing unstructured, all-hexahedral finite element meshes. Whisker weaving is based on the Spatial Twist Continuum (STC), a global interpretation of the geometric dual of an all-hexahedral mesh. Whisker weaving begins with a closed, all-quadrilateral surface mesh bounding a solid geometry, then constructs hexahedral element connectivity advancing into the solid. The result of the whisker weaving algorithm is a complete representation of hex mesh connectivity only: Actual mesh node locations are determined afterwards.
The basic step of whisker weaving is to form a hexahedral element by crossing or intersecting dual entities. This operation, combined with seaming or joining operations in dual space, is sufficient to mesh simple block problems. When meshing more complex geometries, certain other dual entities appear such as blind chords, merged sheets, and self-intersecting chords. Occasionally specific types of invalid connectivity arise. These are detected by a general method based on repeated STC edges. This leads into a strategy for resolving some cases of invalidities immediately.
The whisker weaving implementation has so far been successful at generating meshes for simple block-type geometries and for some non-block geometries. Mesh sizes are currently limited to a few hundred elements. While the size and complexity of meshes generated by whisker weaving are currently limited, the algorithm shows promise for extension to much more general problems.
TL;DR: It is shown that frequency-resolved optical gating combined with spectral interferometry yields an extremely sensitive and general method for temporal characterization of nearly arbitrarily weak ultrashort pulses even when the reference pulses is not transform limited.
Abstract: We show that frequency-resolved optical gating combined with spectral interferometry yields an extremely sensitive and general method for temporal characterization of nearly arbitrarily weak ultrashort pulses even when the reference pulses is not transform limited. We experimentally demonstrate measurement of the full time-dependent intensity and phase of a train of pulses with an average energy of 42 zeptojoules (42 × 10−21 J), or less than one photon per pulse.
TL;DR: In this article, a reversible, temperature-induced surface charge density wave (CDW) localized at the lead-coated surface of a germanium crystal was observed and characterized, and the formation of this new phase is accompanied by significant periodic valence-charge redistribution, a pronounced lattice distortion and a metal-nonmetal transition.
Abstract: A CHARGE density wave (CDW) is a periodic symmetry-lowering redistribution of charge within a material, accompanied by a rearrangement of electronic bands (such that the total electronic energy is decreased) and usually a small periodic lattice distortion1,2. This phenomenon is most commonly observed in crystals of reduced symmetry, such as quasi-two-dimensional3 or quasi-one-dimensional4 materials. In principle, the reduction of symmetry associated with surfaces and interfaces might also facilitate the formation of CDWs; although there is some indirect evidence for surface charge density waves5–12,14, none has been observed directly. Here we report the observation and characterization of a reversible, temperature-induced CDW localized at the lead-coated (111) surface of a germanium crystal. The formation of this new phase is accompanied by significant periodic valence-charge redistribution, a pronounced lattice distortion and a metal–nonmetal transition. Theoretical calculations confirm that electron–phonon coupling drives the transition to the CDW, but it appears that some other factor—probably electron–electron correlations—is responsible for the ground-state stability of this phase.
TL;DR: The highly nonlinear dispersion of PBG materials near Brillouin zone edges is recognized and utilize the dispersion to achieve strong prism action in the millimeter-wave spectral regime.
Abstract: We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action Such a PBG prism is very compact if operated in the optical regime, ~20 mm in size for lambda ~ 700 nm, and can serve as a dispersive element for building ultracompact miniature spectrometers
TL;DR: Improvement associated with particle clustering is found for a number of optical processes, including four-wave mixing (FWM), third-harmonic generation (THG), Raman scattering, and nonlinear refraction and absorption in Kerr media.
Abstract: Strong fluctuations of local fields may result in very large optical nonlinearities in small-particle composites. Enhancement associated with particle clustering is found for a number of optical processes, including four-wave mixing (FWM), third-harmonic generation (THG), Raman scattering, and nonlinear refraction and absorption in Kerr media. Field fluctuations and optical nonlinear susceptibilities are especially large in fractal clusters. The enhancement of optical processes is expressed in terms of the resonant linear absorption by collective dipolar eigenmodes in a cluster, and quality factors, q, of the modes (q\ensuremath{\gg}1). It is shown that the susceptibility of a composite material consisting of random small-particle clusters is proportional to ${\mathit{q}}^{3}$ for Raman scattering and the Kerr optical nonlinearity, and to ${\mathit{q}}^{4}$ and ${\mathit{q}}^{6}$ for THG and FWM, respectively. For all of these processes, a spectral dependence of the effective susceptibility is found. Broad-scale numerical simulations of the optical response in small-particle composites are performed to complement the theory. The simulations are in reasonable agreement with available experimental data. \textcopyright{} 1996 The American Physical Society.
Patent•
08 Jul 1996TL;DR: In this article, a method and system for producing complex, three-dimensional, net shape objects from a variety of powdered materials is presented, which includes unique components to ensure a uniform and continuous flow of powder materials as well as to focus and locate the flow of powdered material with respect to a laser beam which results in the melting of the powdered material.
Abstract: A method and system are provided for producing complex, three-dimensional, net shape objects from a variety of powdered materials. The system includes unique components to ensure a uniform and continuous flow of powdered materials as well as to focus and locate the flow of powdered materials with respect to a laser beam which results in the melting of the powdered material. The system also includes a controller so that the flow of molten powdered materials can map out and form complex, three-dimensional, net-shape objects by layering the molten powdered material. Advantageously, such complex, three-dimensional net-shape objects can be produced having material densities varying from 90% of theoretical to fully dense, as well as a variety of controlled physical properties. Additionally, such complex, three-dimensional objects can be produced from two or more different materials so that the composition of the object can be transitioned from one material to another.
TL;DR: Advantages of this approach include its inherent ability for one-class generalization, freedom from characterizing the non-target class, and the ability to form closed decision boundaries for multi-modal classes that are more complex than hyperspheres without requiring inversion of large matrices.
TL;DR: In this paper, the effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated.
Abstract: The effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated. Specimens were initially sintered for 2 h at 1450 C to near theoretical density; some specimens were then heat-treated at 1550, 1650, 1750, or 1850 C to coarsen the microstructure. The average grain size increased with heat treatment from 1750 C. The maximum fraction of tetragonal phase that transformed during fracture corresponded with the largest tetragonal grain size of {approximately}5--6 {micro}m. Strength was on the order of 1 GPa, and was surprisingly insensitive to heat-treatment temperature and grain size, contrary to previous studies. The fracture toughness increased from 4 to 10 MPa{center_dot}m{sup 1/2} with increasing grain size, owing to an increasing transformation zone size. Grain sizes larger than 5--6 {micro}m spontaneously transformed to monoclinic phase during cooling. Such critical grain sizes are much larger than those found in past investigations, and may be due to the greater fraction of cubic phase present which decreases the strain energy arising from more » crystallographic thermal expansion anisotropy of the tetragonal phase. « less
TL;DR: In this paper, an implicit, pseudo-solid domain mapping technique is described that facilitates finite element analysis of free and moving boundary problems, based on an implicit full-Newton strategy, free of restrictions on mesh structure.
TL;DR: In this article, high-speed studies of oxide-confined, submilliampere threshold, oxide-constrained vertical cavity surface emitting lasers show modulation bandwidths > 16 GHz and very high modulation current efficiency factors occur at low bias but decrease as the modulation bandwidth and frequency of the relative intensity noise peak saturate at higher currents.
Abstract: High-speed studies of packaged, submilliampere threshold, oxide-confined vertical cavity surface emitting lasers show modulation bandwidths > 16 GHz. Very high modulation current efficiency factors occur at low bias but decrease as the modulation bandwidth and frequency of the relative intensity noise peak saturate at higher currents.
TL;DR: The Discontinuous Deformation Analysis (DDA) method was proposed by Shi et al. as mentioned in this paper, which is a new numerical method for the statics and dynamics of block systems.
TL;DR: In this paper, the authors present a comprehensive numerical model for vertical-cavity surface-emitting lasers that includes all major processes affecting cw operation of axisymmetric devices.
Abstract: We present a comprehensive numerical model for vertical-cavity surface-emitting lasers that includes all major processes affecting cw operation of axisymmetric devices. In particular, our model includes a description of the 2-D transport of electrons and holes through the cladding layers to the quantum well(s), diffusion and recombination of these carriers within the wells, the 2-D transport of heat throughout the device, and a multilateral-mode effective index optical model. The optical gain acquired by photons traversing the quantum wells is computed including the effects of strained band structure and quantum confinement. We employ our model to predict the behavior of higher-order lateral modes in proton-implanted devices and to provide an understanding of index-guiding in devices fabricated using selective oxidation.