Showing papers by "Sandia National Laboratories published in 1989"

Ionizing radiation effects in MOS devices and circuits

Abstract: Historical Perspective (H. Hughes). Electron--Hole Generation, Transport, and Trapping in SiO2 (F. McLean, et al.). Radiation--Induced Interface Traps (P. Winokur). Radiation Effects on MOS Devices and Circuits (P. Dressendorfer). Radiation--Hardening Technology (P. Dressendorfer). Process--Induced Radiation Effects (T. Ma). Source Considerations and Testing Techniques (K. Kerris). Transient--Ionization and Single--Event Phenomena (S. Kerns). Index.

A computational model of the structure and extinction of strained, opposed flow, premixed methane-air flames

Abstract: The application of laminar flamelet concepts to turbulent flame propagation requires a detailed understanding of strained laminar flames. Here we use numerical methods, including are-length continuation, to simulate the complex chemical kinetic behavior in premixed methane-air flames that are stabilized between two opposed-flow burners. We predict both the detailed structure and the extinction limits for these flames over a range of fuel-air mixtures. In addition to discussing the flame structure, a sensitivity analysis provides further insight on the chemical behavior near extinction. Finally, we discuss the comparison of the predictions with Law's experimental extinction data. An especially important aspect of this comparison is the recognition that fluid mechanical aspects of the traditional strained-flame analysis are deficient in representing experiments such as Law's. We develop and solve a new system of equations that is able to describe the experiments much more accurately.

Research opportunities on clusters and cluster-assembled materials—A Department of Energy, Council on Materials Science Panel Report

Abstract: The Panel was charged with assessing the present scientific understanding of the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties. The Panel was composed of both academic and industrial scientists from the physics, chemistry, and materials science communities, and met in January 1988.In materials (insulators, semiconductors, and metals) with strong chemical bonding, there is extensive spatial delocalization of valence electrons, and therefore the bulk physical properties which depend upon these electrons develop only gradually with cluster size. Recent research using supersonic-jet, gas-aggregation, colloidal, and chemical-synthetic methods indeed clearly establishes that intermediate size clusters have novel and hybrid properties, between the molecular and bulk solid-state limits. A scientific understanding of these transitions in properties has only been partially achieved, and the Panel believes that this interdisciplinary area of science is at the very heart of the basic nature of materials. In Sec. V (Future Challenges and Opportunities), a series of basic questions for future research are detailed. Each question has an obvious impact on our potential ability to create new materials.Present methods for the synthesis of useful amounts of size-selected clusters, with surface chemical properties purposefully controlled and/or modified, are almost nonexistent, and these fundamentally limit our ability to explore the assembly of clusters into potentially novel materials. While elegant spectroscopic and chemisorption studies of size-selected clusters have been carried out using molecular-beam technologies, there are no demonstrated methods for recovery and accumulation of such samples. Within the past year, the first reports of the chemical synthesis of clusters with surfaces chemically modified have been reported for limited classes of materials. Apparatus for the accumulation and consolidation of nanophase materials have been developed, and the first promising studies of their physical properties are appearing. In both the chemical and nanophase synthesis areas, clusters with a distribution of sizes and shapes are being studied. Progress on macroscopic synthetic methods for size-selected clusters of controlled surface properties is the most important immediate goal recognized by the Panel. Simultaneous improvement in physical characterization will be necessary to guide synthesis research.Assuming such progress will occur, the Panel suggests that self-assembly of clusters into new elemental polymorphs and new types of nanoscale heterogeneous materials offers an area of intriguing technological promise. The electrical and optical properties of such heterogeneous materials could be tailored in very specific ways. Such ideas are quite speculative at this time; their implementation critically depends upon controlled modification of cluster surfaces, and upon development of characterization and theoretical tools to guide experiments.The Panel concluded that a number of genuinely novel ideas had been enunciated, and that in its opinion some would surely lead to exciting new science and important new materials.

Semiempirical modified embedded-atom potentials for silicon and germanium

Abstract: Semiempirical potentials for silicon, germanium, and their alloys are derived with use of the modified-embedded-atom-method formalism. Following Baskes [Phys. Rev. Lett. 59, 2666 (1987)], it is found that the host electron density which is a linear superposition of atomic densities in the embedded-atom method (EAM) must have an angular modification in order to properly describe the bond-bending forces in the diamond-cubic structure. The angular dependence of this host electron density was found to be in qualitative agreement with the density of a first-principles calculation. As in the EAM, the potential functions are determined by using the measured lattice constants, sublimation energies, elastic constants, and alloying energies of silicon and germanium. In addition, first-principles calculations of structural energies are used. The potentials are used to calculate the energetics and geometrics of point defects, surfaces, metastable structures, and small clusters. In all cases, the calculations have been compared to first-principles calculations and experiment when available. The calculations predict that the vacancy mechanism is the dominant diffusion mechanism in both silicon and germanium. Surface energies and relaxations of the low-index faces of Si and Ge are compared with first-principles calculations.

Design of Exponentially Weighted Moving Average Schemes

Abstract: Plots of optimal smoothing parameters and control limit constants are given which make the design of exponentially weighted moving average charts simple. A design strategy is reviewed...

INCONEL 718: A solidification diagram

Abstract: As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to generate a constitution diagram for INCONEL 718* in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made using the Scheil solidification model, and the fraction of each phase in the eutectic is calculatedvia the lever rule. These are compared with experimentally determined values and found to be in good agreement.

Boron carbide structure by Raman spectroscopy

Abstract: We have obtained and analyzed Raman spectra of single-crystal, hot-pressed, and chemical-vapor-deposited boron carbide materials over their single-phase region (from {similar to}9 to {similar to}20 at. % carbon). These spectra provide insight into the substitutional disorder that characterizes these structurally ordered solids. In particular, although icosahedra and chain structures occupy regular lattice positions, there is local substitutional disorder resulting from the occupancy of certain sites within the icosahedra and chains by either boron or carbon atoms. Comparison of boron carbide Raman spectra with the Raman spectra of {alpha}-rhombohedral boron, boron phosphide, and boron arsenide has confirmed the following structural model derived from theoretical considerations and electrical and thermal transport data. The boron carbide composition with nearly 20 at. % carbon is composed of B{sub 11}C icosahedra linked by carbon-boron-carbon chains. As the carbon content is reduced toward approximately 13 at. %, carbon-boron-carbon chains are progressively replaced by carbon-boron-boron chains. Further reduction in the carbon content results in the replacement of B{sub 11}C icosahedra with B{sub 12} icosahedra.

Polymers, Fractals, and Ceramic Materials

Abstract: Concepts borrowed from polymer science have been applied to tailor the properties of inorganic materials, especially those derived from amorphous precursors. Fractal geometry can be used to characterize macromolecular precursors and to relate their structures to kinetic growth processes. Within the silica system, for example, it is possible to manipulate the conditions of solution polymerization to yield a variety of macromolecules from branched chains to smooth colloidal particles.

Self-diffusion and impurity diffusion of fcc metals using the five-frequency model and the Embedded Atom Method

Abstract: The activation energies for self-diffusion of transition metals (Au, Ag, Cu, Ni, Pd, Pt) have been calculated with the Embedded Atom Method (EAM); the results agree well with available experimental data for both mono-vacancy and di-vacancy mechanisms. The EAM was also used to calculate activation energies for vacancy migration near dilute impurities. These energies determine the atomic jump frequencies of the classic “five-frequency formula,” which yields the diffusion rates of impurities by a mono-vacancy mechanism. These calculations were found to agree fairly well with experiment and with Neumann and Hirschwald's “Tm” model.

Transition to chaos in a differentially heated vertical cavity

Abstract: We investigate numerically the transition from laminar to chaotic flow of a Boussinesq fluid with Pr = 0.71 in two-dimensional closed, differentially heated, vertical cavities having aspect ratios near unity. The cavities have rigid conducting sidewalls, and rigid insulating top and bottom walls. The physical nature of the resulting flow is a function of the aspect ratio and Rayleigh number.It is shown that an oscillatory approach to steady-state, oscillatory instabilities, quasi-periodic flow, and chaotic flow exist for the flow regimes investigated. We find that for aspect ratios of approximately three or larger the the first transition from steady-state is due to instability of the sidewall boundary layers, while for small aspect ratios, but larger than ½, it is due to internal waves near the departing corners. For both instabilities we obtain the critical Rayleigh number as a function of aspect ratio and write expressions relating the fundamental frequencies of the oscillatory flow to the Rayleigh number and aspect ratio. When Ra is increased significantly above the first critical value, the flow becomes complex since both types of instabilities can be present. With a further increase in Rayleigh number the flow becomes chaotic and eventually turbulent. The above results are illustrated for different Rayleigh numbers and aspect ratios using time histories, spectral analysis, and streamlines at different values of time.

Microscopic theory of the dynamics of polymeric liquids: General formulation of a mode–mode‐coupling approach

Abstract: A formally exact, nonlinear generalized Langevin equation (GLE) for a flexible probe polymer in a dense melt has been derived using molecular phase space kinetic theory and Mori–Zwanzig projection operator techniques. An approximate, linearized dynamic memory function is developed, and the resulting GLE is specialized to the problem of an overdamped liquid of uncrossable Rouse polymers. An analytically tractable, perturbative/short time evaluation of the projected force time correlation function matrix is proposed which accounts for uncorrelated intermolecular pair interaction effects in the polymer melt. The detailed predictions for transport coefficients and various time correlation functions are determined for linear chains, and compared with recent lattice Monte Carlo simulations. Significant slowing down of all dynamical processes relative to the Rouse behavior is found, but the molecular weight scaling is not correctly described. A nonperturbative approach based on a polymeric generalization of mole...

Model of metallic cohesion: The embedded-atom method.

Abstract: The embedded-atom method (EAM) [Phys. Rev. B 29, 6443 (1984)] has proven to be a significant improvement in simplified total-energy calculations for metallic systems. In the current work, the ansatz used in the EAM is derived from the local-density functional for the energy. The expression demonstrated here is most appropriate for simple metals and for transition metals with nearly empty or nearly full d bands. An embedding energy is defined as a function of an optimal constant background density, and an equation for that optimal background density is obtained. The cohesive energy is then related to the embedding energy and an electrostatic two-body interaction. It is shown that lowest-order electronic relaxations can be absorbed into the same ansatz. Model calculations are presented for fcc nickel within the Thomas--Fermi--Dirac--von Weizs\"acker model for the kinetic energy, with local exchange and correlation and frozen-electron distributions. The model is shown to provide a good description of the ground-state properties of nickel (e.g., energetics and structure of vacancies and surfaces) and also a good framework for evaluating the approximations used in justifying the EAM form. In particular, the model exhibits a simple relationship between the optimal constant background density and the background density at the atomic site. Corrections involving the gradient of the background density are shown to be important in the calculation of the surface energy. This work then provides a basis for the use of the EAM in semiempirical applications.

Altered-Stress Fracturing

Abstract: Altered-stress fracturing is a concept whereby a hydraulic fracture in one well is reoriented by another hydraulic fracture in a nearby location The application is in tight, naturally fractured, anisotropic reservoirs in which conventional hydraulic fractures parallel the highly permeable natural fractures and little production enhancement is achieved by conventional hydraulic fracturing Altered-stress fracturing can modify the stress field so that hydraulic fractures propagate across the permeable natural fractures A field test was conducted in which stress changes of 250 to 300 psi (17 to 21 MPa) were measured in an offset well 120 ft (37 m) away during relatively small minifracs in a production well These results show that stress-altered fracturing is possible at this site and others Analytic and finite element calculations quantify the effects of layers, stresses, and crack size Reservoir calculations show significant enhancement compared to conventional treatments 21 refs, 12 figs, 3 tabs

Speckle processing method for synthetic-aperture-radar phase correction.

Abstract: Uncompensated phase errors present in synthetic-aperture-radar data can have a disastrous effect on reconstructed image quality. We present a new iterative algorithm that holds promise of being a robust estimator and corrector for arbitrary phase errors. Our algorithm is similar in many respects to speckle processing methods currently used in optical astronomy. We demonstrate its ability to focus scenes containing large amounts of phase error regardless of the phase-error structure or its source. The algorithm works extremely well in both high and low signal-to-clutter conditions without human intervention.

Viscoelasticity near the sol-gel transition.

Abstract: Measurements of viscoelastic properties near the sol-gel transition demonstrate that viscoelastic phenomena are described by power laws. To describe these phenomena, we derive the distribution of relaxation times for branched polymers, both in the reaction bath and in the dilute solution. From this spectrum we can compute viscoelastic properties such as the shear relaxation modulus G(t) and the complex shear modulus G(\ensuremath{\omega}). Near the gel point we find G(t)\ensuremath{\sim}${t}^{\mathrm{\ensuremath{-}}\ensuremath{\Delta}}$, with \ensuremath{\Delta} a universal exponent, for times that are small compared to a divergent time ${\ensuremath{\tau}}_{z}$: For longer times the decay is a stretched exponential. The exponent \ensuremath{\Delta} is found to be sensitive to dilution. Likewise, the storage and loss parts of the complex shear modulus are found to scale as ${\ensuremath{\omega}}^{\ensuremath{\Delta}}$. These results for the dynamics lead to a theory for the critical growth of the equilibrium shear modulus E above the gel point, and the divergence of the steady-state creep compliance ${J}_{e}^{0}$ above the gel point. Finally, we discuss the concentration dependence of the viscosity of a solution of branched polymers.

The crystal structure of superconducting La2CuO4.032 by neutron diffraction

Abstract: The crystal structure of superconducting (37.5 K) La2CuO4.032 has been refined from single-crystal neutron-diffraction data at room temperature and 15 K. At both temperatures it exhibits the orthorhombic symmetry Cmca. The extra oxygen O4 atoms occupy the special positions ( 1 4 y 1 4 with y=0.243 at 15 K). They are located between two successive LaO layers and are surrounded by distorted cubes built up of two interpenetrated tetrahedra, one comprising four La atoms and the other four O1 atoms (the apical oxygen atoms of the CuO6octahedra). The O4 insertion causes a 0.75 A displacement of 4.8% of the O1 atoms towards new O3 positions (x=0.030(5), y=0.182(2), z=0.100(5)). From the refined values of the occupancy factors at 15 K, it is deduced that for each extra O4 three O1 are displaced to O3 with one short O3–O4 distance of 1.64(3) A. This value indicates the formation of a strong O-O covalent bond of peroxide type with a formal 2-valence. Since the La and Cu sublattices have been found to be fully occupied and the doping does not change the oxygen charge, the La, Cu and O sublattices have the formal valences 6+, 2+, and 8−, respectively. However, the increase in La coordination and the consequent La-O distance readjustment indicate, when compared to the undoped compound structure, that a charge transfer occurs in La2CuO4.032, with the excess positive charge going either to the La or to the O sublattice. In the latter case it would correspond to the formation of holes in the O 2p band.

Thermodynamic properties of fcc transition metals as calculated with the embedded-atom method

Abstract: Solid and liquid Gibbs free energies of Cu, Ag, Au, Ni, Pd, and Pt have been calculated through the use of the embedded-atom method, and the results are in good agreement with experiment. The melting points of the materials are calculated from the intersection of the solid and liquid free-energy curves. The values of the melting point are in excellent agreement for the noble metals and in reasonable agreement for Ni, Pd, and Pt. The thermal expansions of the solids are computed and the results are in excellent agreement with experiment over the entire temperature range.

Crystallization history of Obsidian Dome, Inyo Domes, California

Abstract: Samples obtained by U.S. Department of Energy research drilling at the 600-year-old Obsidian Dome volcano provide the rare opportunity to examine the transition from volcanic (dome) to plutonic (intrusion) textures in a silicic magma system. Textures in the lavas from Obsidian Dome record multiple periods of crystallization initiated in response to changes in undercooling (ΔT) related to variable degassing in the mag-ma. Phenocr)ysts formed first at low ΔT. A drastic increase in ΔT, related to loss of a vapor phase during initial stages of eruption, caused nucleation of microlites. All of the lavas thus contain phenocrysts and microlites. Extrusion and subsequent devitrification of the dry (0.1 wt% H2O) magma crystallized spherulites and fine-grained rhyolite at high ΔT. A granophyric texture, representing crystallization at a moderate ΔT, formed in the intrusions beneath Obsidian Dome. Textures in the intrusion apparently represent crystallization of hydrous (1–2 wt% H2O) rhyolitic magma at shallow depths.

Quiescent power supply current measurement for CMOS IC defect detection

Abstract: Quiescent power supply current (I/sub DDQ/) measurement is a very effective technique for detecting in CMOS integrated circuits (ICs). This technique uniquely detects certain CMOS IC defects such as gate oxide shorts, defective p-n junctions, and parasitic transistor leakage. In addition, I/sub DDQ/ monitoring will detect all stuck-at faults with the advantage of using a node toggling test set that has fewer test vectors than a stuck-at test set. Individual CMOS ICs from three different fabrication sites had a unique pattern or fingerprint of elevated I/sub DDQ/ states for a given test set. When I/sub DDQ/ testing was added to conventional functional test sets, the percentage increase in failures ranged from 60% to 182% for a sample of microprocessor, RAM, and ROM CMOS ICs. >

Buoyancy-driven motion of a deformable drop through a quiescent liquid at intermediate Reynolds numbers

Abstract: Numerical solutions have been obtained for steady streaming flow past an axisymmetric drop over a wide range of Reynolds numbers (0.005 [les ] Re [les ] 250), Weber numbers (0.005 [les ] We [les ] 14), viscosity ratios (0.001 [les ] λ [les ] 1000), and density ratios (0.001 [les ] ζ [les ] 1000). Our results indicate that at lower Reynolds numbers the shape of the drop tends toward a spherical cap with increasing We , but at higher Re the body becomes more disk shaped with increasing We . Unlike the recirculating wake behind an inviscid bubble or solid particle, the eddy behind a drop is detached from the interface. The size of the eddy and the separation distance from the drop depend on the four dimensionless parameters of the problem. The motion of the fluid inside the drop appears to control the behaviour of the external flow near the body, and even for cases when λ and ζ [Lt ] 1 (a ‘real’ bubble), a recirculating wake remains unattached.

Pdf Modeling of Turbulent Nonpremixed Methane Jet Flames

Abstract: Anexpanded model of turbulent nonpremixed combustion is herein presented. In the model, the scalar mixing and reactions are described by a probability density function (pdf) submodel capable or handling five scalars, while the turbulent velocity field is described by a second-order moment closure. Two plausible chemical reaction models are considered: a five-scalar, four-step, reduced reaction mechanism and a four-scalar constrainted equilibrium model. Detailed comparisons of model predictions with laser Raman experimental data provide a valuable evaluation of the model's ability in predicting nonequilibrium chemistry in turbulent nonpremixed flames. Overall, the model fails to predict greater departure from chemical equilibrium as mixing rates are increased. Interestingly, this failure is not due to the chemical model, both of which perform satisfactorily. Instead, the failure to predict greater departure from chemical equilibrium is a subtle artifact of the current Monte Carlo simulation of tur...

Integral equation theory of the structure and thermodynamics of polymer blends

Abstract: Our recently developed RISM integral equation theory of the structure and thermodynamics of homopolymer melts is generalized to polymer mixtures. The mean spherical approximation (MSA) closure to the generalized Ornstein–Zernike equations is employed, in conjunction with the neglect of explicit chain end effects and the assumption of ideality of intramolecular structure. The theory is developed in detail for binary blends, and the random phase approximation (RPA) form for concentration fluctuation scattering is rigorously obtained by enforcing incompressibility. A microscopic, wave vector‐dependent expression for the effective chi parameter measured in small angle neutron scattering (SANS) experiments is derived in terms of the species‐dependent direct correlation functions of the blend. The effective chi parameter is found to depend, ingeneral, on thermodynamic state, intermolecular forces, intramolecular structure, degree of polymerization, and global architecture. The relationship between the mean field Flory–Huggins expression for the free energy of mixing and our RISM‐MSA theory is determined, along with general analytical connections between the chi parameter and intermolecular pair correlations in the liquid. Detailed numerical applications to athermal and isotopic chain polymer blend models are presented for both the chi parameter and the structure. For athermal blends a negative, concentration‐dependent chi parameter is found which decreases with density, structural asymmetry, and increases with molecular weight. For isotopic blends, the effective (positive) chi parameter is found to be strongly renormalized downward from its mean field enthalpic value by long range fluctuations in monomer concentration induced by polymeric connectivity and excluded volume. Both the renormalization and composition dependence of the chi parameter increase with chain length and proximity to the spinodal instability. The critical temperature is found to be proportional to the square root of the degree of polymerization in stark contrast to the classical mean field prediction of a linear dependence. Comparison of the theoretical predictions with SANS measurements and computer simulations is presented, alongwith brief discussions of nonideal effects and lower critical solution temperature phenomena.

Resonant periodic gain surface-emitting semiconductor lasers

Abstract: A surface-emitting semiconductor laser structure with a vertical cavity, extremely short gain medium length, and enhanced gain at a specific design wavelength is described. The active region consists of a series of quantum wells spaced at one half the wavelength of a particular optical transition in the quantum wells. This special periodicity allows the antinodes of the standing-wave optical field to coincide with the gain elements, enhancing the frequency selectivity, increasing the gain in the vertical direction by a factor of two compared to a uniform medium or a nonresonant multiple quantum well, and substantially reducing amplified spontaneous emission. Optically pumped lasing was achieved in a GaAs/AlGaAs structure grown by molecular-beam epitaxy, with what is believed to be the shortest gain medium (310 nm) ever reported. >