Showing papers by "Sandia National Laboratories published in 1995"

A Multi-Level Algorithm For Partitioning Graphs

Abstract: The graph partitioning problem is that of dividing the vertices of a graph into sets of specified sizes such that few edges cross between sets. This NP-complete problem arises in many important scientific and engineering problems. Prominent examples include the decomposition of data structures for parallel computation, the placement of circuit elements and the ordering of sparse matrix computations. We present a multilevel algorithm for graph partitioning in which the graph is approximated by a sequence of increasingly smaller graphs. The smallest graph is then partitioned using a spectral method, and this partition is propagated back through the hierarchy of graphs. A variant of the Kernighan-Lin algorithm is applied periodically to refine the partition. The entire algorithm can be implemented to execute in time proportional to the size of the original graph. Experiments indicate that, relative to other advanced methods, the multilevel algorithm produces high quality partitions at low cost.

Bridged polysilsesquioxanes. highly porous hybrid organic-inorganic materials

Abstract: This contribution reviews a new family of inorganic-organic hybrid materials that are assembled by sol-gel polymerization of polyfunctional molecular building blocks. These bridged polysilsesquioxanes are three-dimensional network materials that are distinguished by incorporation of an organic fragment as an integral component of the network. The intimate association of the organic and inorganic phase, a true molecular composite, coupled with the variability of the organic component, permits engineering of both chemical and physical properties of the material. The paper reviews bridged polysilsesquioxanes, arylene-bridged polysilsesquioxanes, alkylene-bridged polysilsesquioxanes; and their applications.

The short-range structure of zinc polyphosphate glass

Abstract: 31 P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and Raman spectroscopy have been used to examine the polyhedral arrangements in x ZnO · (1 − x )P 2 O 5 (0.50 ≤ x ≤ 0.71) glasses. The depolymerization of P metaphosphate chains by the addition of ZnO is quantitatively described by the increase in the concentration of Q 1 -phosphate sites, determined from the 31 P MAS-NMR spectra. When x > 0.60, the NMR and Raman spectra exhibit peaks due to Q 0 and Q 2 tetrahedra, indicating that structures disproportionate in glass melts near the pyrophosphate composition. The splitting of the Raman peak due to the Q 1 terminal oxygen stretching mode indicates that a variety of P-O-Zn bonds participate in the polyphosphate glass structure. The complex mixture of P and Zn polyhedra contributes to the glass-forming tendency of the high ZnO (> 60 mol%) compositions.

Silica aerogel films prepared at ambient pressure by using surface derivatization to induce reversible drying shrinkage

Abstract: HIGHLY porous inorganic films have potential applications as dielectric materials, reflective and anti-reflective coatings, flat-panel displays, sensors, catalyst supports and super-insulating architectural glazings1–3. Aerogels4 are the most highly porous solids known, and can now be prepared from inorganic5 and organic6,7 precursors with volume-fraction porosities of up to 99.9% (ref. 8). Aerogels are normally prepared by supercritical extraction of the pore fluid from a wet gel1, which prevents the network collapse that is otherwise induced by capillary forces. But supercritical processing is expensive, hazardous and incompatible with the processing requirements of many potential applications,thus severely restricting the commercial exploitation of aerogels. Here we describe a means of preparing aerogels by a simple dip-coating method at ambient pressure without the need for supercriti-cal extraction. We add surface groups to the inorganic gel which make drying shrinkage reversible9: as the solvent is withdrawn, the gel springs back to a porous state. We can generate aerogel films with 98.5% porosity using this approach. We anticipate that it will greatly expand the commercial applications of these materials.

Genetic algorithms in optimization of system reliability

Abstract: After initial production, improvements are often made to components of a system, to upgrade system performance; for example, when designing a later version or release. This paper presents an optimization model that identifies the types of component improvements and the level of effort spent on those improvements to maximize one or more performance measures (e.g., system reliability or availability) subject to constraints (e.g., cost) in the presence of uncertainty about the component failure rates. For each component failure mode, some possible improvements are identified along with their cost and the resulting improvement in failure rates for that failure mode. The objective function is defined as a stochastic function of the performance measure of interest-in this case, 5/sup th/ percentile of the mean time-between-failure distribution. The problem formulation is combinatorial and stochastic. Genetic algorithms are used as the solution method. Our approach is demonstrated on a case study of a personal computer system. Results and comparison with enumeration of the configuration space show that genetic algorithms perform very favorably in the face of noise in the output: they are able to find the optimum over a complicated, high dimensional, nonlinear space in a tiny fraction of the time required for enumeration. The integration of genetic algorithm optimization capabilities with reliability analysis can provide a robust, powerful design-for-reliability tool. >

Selectively oxidised vertical cavity surface emitting lasers with 50% power conversion efficiency

Abstract: Index-guided vertical cavity top-surface emitting laser diodes have been fabricated from an all epitaxial structure with conducting mirrors by selective lateral oxidation of AlGaAs At low voltage, a 78% slope efficiency, and a 350 mu A threshold current in a single device combine to yield a maximum power conversion efficiency of 50% at less than a 2 mA drive current The device operates in a single mode up to 15 mW >

Bisimulation can't be traced

Abstract: In the concurrent language CCS, two programs are considered the same if they are bisimilar Several years and many researchers have demonstrated that the theory of bisimulation is mathematically appealing and useful in practice However, bisimulation makes too many distinctions between programs We consider the problem of adding operations to CCS to make bisimulation fully abstract We define the class of GSOS operations, generalizing the style and technical advantages of CCS operations We characterize GSOS congruence in as a bisimulation-like relation called ready-simulation Bisimulation is strictly finer than ready simulation, and hence not a congruence for any GSOS language

Effective index model for vertical-cavity surface-emitting lasers

Abstract: A new effective index optical model is presented for the analysis of lateral waveguiding effects in verticalcavity surface-emitting lasers. In addition to providing a concise formalism for reducing the dimensionality of the Maxwell equations describing the lasing mode, this model also provides new insights into waveguiding phenomena in vertical-cavity lasers. In particular, it is shown that the effective index responsible for waveguiding is dependent only on lateral changes in the Fabry-Perot resonance frequency. This concept leads naturally to new design methods for these lasers that are expected to result in more eff icient devices with superior modal characteristics.

Trapping of hydrogen to lattice defects in nickel

Abstract: This paper addresses the energy associated with the trapping of hydrogen to defects in a nickel lattice. Several dislocations and grain boundaries which occur in nickel are studied. The dislocations include an edge, a screw, and a Lomer dislocation in the locked configuration, i.e. a Lomer-Cottrell lock (LCL). For both the edge and screw dislocations, the maximum trap site energy is approximately 0.1 eV occurring in the region where the lattice is in tension approximately 3-4 angstroms from the dislocation core. For the Lomer-Cottrell lock, the maximum binding energy is 0.33 eV and is located at the core of the a/6(110) dislocation. Several low-index coincident site lattice grain boundaries are investigated, specifically the Sigma 3(112), Sigma 9(221) and Sigma 11(113) tilt boundaries. The boundaries all show a maximum binding energy of approximately 0.25 eV at the tilt boundary. Relaxation of the boundary structures produces an asymmetric atomic structure for both the Sigma 3 and Sigma 9 boundaries and a symmetric structure for the Sigma 11 tilt boundary. The results of this study can be compared to recent experimental studies showing that the activation energy for hydrogen-initiated failure is approximately 0.3-0.4 eV in the Fe-based superalloy IN903. From the results of this comparison it can be concluded that the embrittlement process is likely associated with the trapping of hydrogen to grain boundaries and Lomer-Cottrell locks.

Frequency‐domain modelling of airborne electromagnetic responses using staggered finite differences

Abstract: A 3D frequency-domain EM modelling code has been implemented for helicopter electromagnetic (HEM) simulations. A vector Helmholtz equation for the electric fields is employed to avoid convergence problems associated with the first-order Maxwell's equations when air is present. Additional stability is introduced by formulating the problem in terms of the scattered electric fields. With this formulation the impressed dipole source is replaced with an equivalent source, which for the airborne configuration possesses a smoother spatial dependence and is easier to model. In order to compute this equivalent source, a primary field arising from dipole sources of either a whole space or a layered half-space must be calculated at locations where the conductivity is different from that of the background. The Helmholtz equation is approximated using finite differences on a staggered grid. After finite-differencing, a complex-symmetric matrix system of equations is assembled and preconditioned using Jacobi scaling before it is solved using the quasi-minimum residual (QMR) method. The modelling code has been compared with other 1D and 3D numerical models and is found to produce results in good agreement. We have used the solution to simulate novel HEM responses that are computationally intractable using integral equation (IE) solutions. These simulations include a 2D conductor residing at a fault contact with and without topography. Our simulations show that the quadrature response is a very good indicator of the faulted background, while the in-phase response indicates the presence of the conductor. However when interpreting the in-phase response, it is possible erroneously to infer a dipping conductor due to the contribution of the faulted background.

Ruffling in a Series of Nickel(II) meso-Tetrasubstituted Porphyrins as a Model for the Conserved Ruffling of the Heme of Cytochromes c.

Abstract: Metalloporphyrins undergo remarkable nonplanar distortions of the macrocycle that perturb the chemical and photochemical properties of these important protein cofactors. Further, the tertiary structure of the surrounding protein can manipulate these distortions as a means of regulating biological function. For cytochromes c, for example, an energetically unfavorable, conserved nonplanar distortion of the heme exists and likely plays a role in its electron- transfer function. The heme distortion is primarily of the ruffling (ruf, type (corresponding to the lowest frequency BI,-symmetry normal mode) in which the pyrroles are twisted about the metal-Npyrrole bond. This BI.-symmetry nonplanar distortion is commonly observed in metalloporphyrin crystal structures, as are the saddling (sad) B2,- symmetry distortion, waving (wau) E,-symmetry distortions, and doming (dom) Az,-symmetry distortion. Each of these nonplanar distortions is expected to result in unique alterations of the chemical and physical properties of the nominally planar porphyrin macrocycle. Symmetrical porphyrin substitution with tetrahedrally bonded atoms at the four meso bridging carbons generally results in the BI, ruffling distortion; therefore, we investigated a series of meso-tetrasubstituted porphyrins for which the substituents vary in size (methyl, ethyl, propyl, pentyl, isopropyl, cyclopropyl derivative lla, cyclohexyl, apopinenyl (lo), tert-butyl, adamantyl), increasing the steric crowding at the periphery. Molecular mechanics calculations show increasing degree of ruffling (CaNNCa angle for opposite pyrroles varies from 0 to 57") for this series of porphyrins, generally agreeing with the X-ray structures that are available. In addition, the frequencies of the structure-sensitive Raman lines decrease nonlinearly with increasing ruffling angle. The localization of the Bl, nonplanar distortion in only the C,-C, bond torsion (not the case for the B2" sad distortion) suggests a means by which the B1, distortion might be distinguished from other types of nonplanar distortion by using resonance Raman spectroscopy. Also, the size of the red shifts in the n - n* absorption bands depends on Ca-Cm torsion angle in a nonlinear fashion and the shift is accurately predicted by INDO/s molecular orbital calculations when the nonplanar structures obtained from molecular mechanics are used.

Imaging the Pore Structure of Geomaterials

Abstract: Laser scanning confocal microscopy can be used to image the pore structure of geologic materials in three dimensions at a resolution of 200 nanometers. The technique involves impregnation of the void space with an epoxy doped with a fluorochrome whose fluorescent wavelength matches the excitation wavelength. Optical sections with a thickness of less than 1 micrometer can be sliced from thick polished sections and combined to produce three-dimensional reconstructions. Application of the technique to rocks with porosities from 1 to 20 percent reveals the geometric complexity of the pore space. The technique can also be applied to other brittle solids such as ceramics.

Image merging and data fusion by means of the discrete two-dimensional wavelet transform

Abstract: A new technique is developed for the merging and data fusion of two images. Two spatially registered images with differing spatial resolutions and color content are merged by combining multiresolution wavelet-decomposition components from each and then reconstructing the merged image by means of the inverse wavelet transform. The wavelet merger can employ a variety of wavelet bases, but in presentation of the concept, simple orthonormal sets—Haar and Daubechies wavelets—are explored. The wavelet technique is compared with the intensity–hue–saturation merging technique by means of multispectral and panchromatic test images. The results of the comparison show the wavelet merger performing better in combining and preserving spectral–spatial information for the test images.

Gain-dependent polarization properties of vertical-cavity lasers

Abstract: We show that the partitioning of power into the two orthogonal eigen polarizations of infra-red gain-guided vertical cavity lasers depends upon the relative spectral overlap of the nondegenerate polarization cavity resonances with the laser gain spectrum. Furthermore, at the condition where the polarization resonances and the peak laser gain are aligned, abrupt switching of power between the eigen polarizations is observed as the gain sweeps through the polarization resonances. The gain-dependence of the polarization requires spectral splitting between the eigen polarizations, which is found to be strongly influenced by local strain. The polarization of the fundamental and higher-order spatial modes can be selected and maintained for all InGaAs vertical-cavity lasers in a wafer simply by employing a 20 nm or greater blue-shift offset of the peak laser gain relative to the cavity resonances. >

Consistent structural properties for AlN, GaN, and InN.

Abstract: The plane-wave pseudopotential method is used to calculate structural properties for wurtzite and zinc-blende AlN and InN using large plane-wave basis sets and treating the indium 4[ital d] electrons as valence. These calculations, together with corresponding ones for GaN [A. F. Wright and J. S. Nelson, Phys. Rev. B 50, 2159 (1994)], yield a consistent set of well-converged structural properties for AlN, GaN, and InN. In particular, the measured lattice mismatch among these compounds---essential for an accurate description of alloy properties---is well reproduced.

Systematic wavelength selection for improved multivariate spectral analysis

Abstract: Methods and apparatus for determining in a biological material one or more unknown values of at least one known characteristic (e.g. the concentration of an analyte such as glucose in blood or the concentration of one or more blood gas parameters) with a model based on a set of samples with known values of the known characteristics and a multivariate algorithm using several wavelength subsets. The method includes selecting multiple wavelength subsets, from the electromagnetic spectral region appropriate for determining the known characteristic, for use by an algorithm wherein the selection of wavelength subsets improves the model's fitness of the determination for the unknown values of the known characteristic. The selection process utilizes multivariate search methods that select both predictive and synergistic wavelengths within the range of wavelengths utilized. The fitness of the wavelength subsets is determined by the fitness function F=f (cost, performance). The method includes the steps of: (1) using one or more applications of a genetic algorithm to produce one or more count spectra, with multiple count spectra then combined to produce a combined count spectrum; (2) smoothing the count spectrum; (3) selecting a threshold count from a count spectrum to select these wavelength subsets which optimize the fitness function; and (4) eliminating a portion of the selected wavelength subsets. The determination of the unknown values can be made: (1) noninvasively and in vivo; (2) invasively and in vivo; or (3) in vitro.

CTH: A software family for multi-dimensional shock physics analysis

Abstract: CTH is a family of codes developed at Sandia National Laboratories for modelling complex multi-dimensional, multi-material problems that are characterized by large deformations and/or strong shocks. A two-step, second-order accurate Eulerian solution algorithm is used to solve the mass, momentum, and energy conservation equations. CTH includes models for material strength, fracture, porosity, and high explosive detonation and initiation.

Four strikes against physical mapping of DNA.

Abstract: Physical mapping is a central problem in molecular biology and the human genome project. The problem is to reconstruct the relative position of fragments of DNA along the genome from information on their pairwise overlaps. We show that four simplified models of the problem lead to NP-complete decision problems: Colored unit interval graph completion, the maximum interval (or unit interval) subgraph, the pathwidth of a bipartite graph, and the k -consecutive ones problem for k ≥ 2. These models have been chosen to reflect various features typical in biological data, including false-negative and positive errors, small width of the map, and chimericism. Key words: physical mapping; NP-completeness; interval graphs; k-consecutive ones problem

An Inductively Coupled Plasma Source for the Gaseous Electronics Conference RF Reference Cell.

Abstract: In order to extend the operating range of the GEC RF Reference Cell, we developed an inductively coupled plasma source that replaced the standard parallel-plate upper-electrode assembly. Voltage and current probes, Langmuir probes, and an 80 GHz interferometer provided information on plasmas formed in argon, chlorine, and nitrogen at pressures from 0.1 Pa to 3 Pa. For powers deposited in the plasma from 20 W to 300 W, the source produced peak electron densities between 1010/cm3 and 1012/cm3 and electron temperatures near 4 eV. The electron density peaked on axis with typical full-width at half maximum of 7 cm to 9 cm. Discharges in chlorine and nitrogen had bimodal operation that was clearly evident from optical emission intensity. A dim mode occurred at low power and a bright mode at high power. The transition between modes had hysteresis. After many hours of high-power operation, films formed on electrodes and walls of one Cell. These deposits affected the dim-to-bright mode transition, and also apparently caused generation of hot electrons and increased the plasma potential.

Embedded micromechanical devices for the monolithic integration of MEMS with CMOS

Abstract: A flexible, modular manufacturing process for integrating micromechanical and microelectronic devices has been developed. This process embeds the micromechanical devices in an anisotropically etched trench below the surface of the wafer. Prior to microelectronic device fabrication, this trench is refilled with oxide, chemical-mechanically polished, and sealed with a nitride cap in order to embed the micromechanical devices below the surface of the planarized wafer. The feasibility of this technique in a manufacturing environment has been demonstrated by combining a variety of embedded micromechanical structures with a 2 /spl mu/m CMOS process on 6 inch wafers. A yield of 78% has been achieved on the first devices manufactured using this technique.

Surface micromachined microengine

Abstract: The design, fabrication, and preliminary testing of a polysilicon microengine are presented. In this early work, electrostatic comb-drive actuation is used to demonstrate the microengine. However, the basic gear/link element of the microengine can be driven by any suitably forceful linear actuators. This device has direct application as a drive and power source for micromachined mechanisms such as optical switches, electrical switches, micropositioners, or any other micro-sized device requiring mechanical power. This is the first device of its kind that is directly linked to an output gear and converts linear motion from comb-drive actuators to rotational motion. The microengine provides output in the form of a continuously rotating output gear (≈ 50 μm in diameter) that is capable of delivering torque to a micromechanism. The microengine can be operated at varying speeds and its motion can be reversed. Processing considerations address the elimination of natural interferences that arise when conformally deposited polysilicon films form the links, joints, and gears that comprise the microengine. The resultant device is completely batch fabricated without the need for piece-part assembly.

Strong quantum-size effects in a layered semiconductor: MoS 2 nanoclusters

Abstract: High-quality, size-selected, nano-size (2--15 nm\ifmmode\pm\else\textpm\fi{}10%) clusters of ${\mathrm{MoS}}_{2}$ have been successfully grown inside inverse micellar cages and their optical properties have been studied. These clusters exhibit (1) large blueshifts in the optical-absorption features with decreasing cluster size due to quantum confinement, affording great tailorability in the absorption thresholds and (2) a demonstration of the crossover from bandlike (solid) to moleculelike spectra as the size of the clusters becomes smaller than that of the exciton in the bulk.