Publications

Energy dispersive x-ray (EDX) spectrum imaging has been performed in a scanning electron microscope (SEM) on a metal/ceramic braze to characterize the elemental distribution near the interface. Statistical methods were utilized to extract the relevant information (i.e., chemical phases and their distributions) from the spectrum image data set in a robust and unbiased way. The raw spectrum image was over 15 Mbytes (7500 spectra) while the statistical analysis resulted in five spectra and five images which describe the phases resolved above the noise level and their distribution in the microstructure.

The microstructure of lateral composition modulation in InAs/AlAs superlattices grown by MBE on InP is examined. The use of x-ray diffraction, TEM, AFM, and STEM to characterize the modulations is discussed. Combining the information from these techniques gives increased insight into the phenomenon and how to manipulate it. Diffraction measures the intensity of modulation and its wavelength, and is used to identify growth conditions giving strong modulation. The TEM and STEM analyses indicate that local compositions are modulated by as much as 0.38 InAs mole fraction. Plan-view images show that modulated structures consists of short ({approx_lt}0.2 {micro}m) In-rich wires with a 2D organization in a (001) growth plane. However, growth on miscut substrates can produce a single modulation along the miscut direction with much longer wires ({approx_gt}0.4 {micro}m), as desired for potential applications. Photoluminescence studies demonstrate that the modulation has large effects on the bandgap energy of the superlattice.

The authors present a new technique for the design of approximation algorithms that can be viewed as a generalization of randomized rounding. They derive new or improved approximation guarantees for a class of generalized congestion problems such as multicast congestion, multiple TSP etc. Their main mathematical tool is a structural decomposition theorem related to the integrality gap of a relaxation.

The influence of the substrate on the translational and orientational ordering in sub-monolayer films of passivated multiply-twinned gold clusters has been investigated using high resolution and dark field transmission electron microscopy. Although clear differences were observed in the degree of translational ordering on amorphous carbon and etched silicon substrates, there was no corresponding variation in the crystallographic orientation of the nanocrystal cores. The results demonstrate that the orientation of passivated clusters with multiply-twinned cores is effectively random with respect to both the superlattice and the substrate.

This paper examines the impact of inserting Micro-Electro-Mechanical Systems (MEMS) into US defense applications. As specific examples, the impacts of micro Inertial Measurement Units (IMUs), radio frequency MEMS (RF MEMS), and Micro-Opto-Electro-Mechanical Systems (MOEMS) to provide integrated intelligence, communication, and control to the defense infrastructure with increased affordability, functionality, and performance are highlighted.

In this paper the authors present performance results from several parallel benchmarks and applications on a 400-node Linux cluster at Sandia National Laboratories. They compare the results on the Linux cluster to performance obtained on a traditional distributed-memory massively parallel processing machine, the Intel TeraFLOPS. They discuss the characteristics of these machines that influence the performance results and identify the key components of the system software that they feel are important to allow for scalability of commodity-based PC clusters to hundreds and possibly thousands of processors.

We report the growth and characterization of quaternary AlGaInN. A combination of photoluminescence (PL), high-resolution x-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) characterizations enables us to explore the contours of constant PL peak energy and lattice parameter as functions of the quaternary compositions. The observation of room temperature PL emission at 351nm (with 20% Al and 5% In) renders initial evidence that the quaternary could be used to provide confinement for GaInN (and possibly GaN). AlGaInN/GrdnN MQW heterostructures have been grown; both XRD and PL measurements suggest the possibility of incorporating this quaternary into optoelectronic devices.

We describe parallel simulations of viscous, incompressible, free surface, Newtonian fluid flow problems that include dynamic contact lines. The Galerlin finite element method was used to discretize the fully-coupled governing conservation equations and a ''pseudo-solid'' mesh mapping approach was used to determine the shape of the free surface. In this approach, the finite element mesh is allowed to deform to satisfy quasi-static solid mechanics equations subject to geometric or kinematic constraints on the boundaries. As a result, nodal displacements must be included in the set of problem unknowns. Issues concerning the proper constraints along the solid-fluid dynamic contact line in three dimensions are discussed. Parallel computations are carried out for an example taken from the coating flow industry, flow in the vicinity of a slot coater edge. This is a three-dimensional free-surface problem possessing a contact line that advances at the web speed in one region but transitions to static behavior in another part of the flow domain. Discussion focuses on parallel speedups for fixed problem size, a class of problems of immediate practical importance.

Al{sub 2}Cu thin films ({approx} 382 nm) are fabricated by melting and resolidifying Al/Cu bilayers in the presence of a {micro} 3 nm Al{sub 2}O{sub 3} passivating layer. X-ray Photoelectron Spectroscopy (XPS) measures a 1.0 eV shift of the Cu2p{sub 3/2} peak and a 1.6 eV shift of the valence band relative to metallic Cu upon Al{sub 2}Cu formation. Scanning Electron microscopy (SEM) and Electron Back-Scattered Diffraction (EBSD) show that the Al{sub 2}Cu film is composed of 30-70 {micro}m wide and 10-25 mm long cellular grains with (110) orientation. The atomic composition of the film as estimated by Energy Dispersive Spectroscopy (EDS) is 67 {+-} 2% Al and 33 {+-} 2% Cu. XPS scans of Al{sub 2}O{sub 3}/Al{sub 2}Cu taken before and after air exposure indicate that the upper Al{sub 2}Cu layers undergo further oxidation to Al{sub 2}O{sub 3} even in the presence of {approx} 5 nm Al{sub 2}O{sub 3}. The majority of Cu produced from oxidation is believed to migrate below the Al{sub 2}O{sub 3} layers, based upon the lack of evidence for metallic Cu in the XPS scans. In contrast to Al/Cu passivated with Al{sub 2}O{sub 3}, melting/resolidifying the Al/Cu bilayer without Al{sub 2}O{sub 3} results in phase-segregated dendritic film growth.

For the first time, we show that redox-sensitive metals, which are highly soluble in the oxidized state can be reduced and precipitated from aqueous solution using tin protoporphyrin and light in the presence of an electron donor. Hg{sup 2+} and Cu{sup 2+} were reduced to the metallic state, and Ub{sup 6+} precipitated as oxide with very low volubility, suggesting that removal of these metals via reductive photoreduction and precipitation may be an innovative way for wastewater treatment. Ag{sup 2+} and Au{sup 2+} were reduced to the metallic state and precipitated as nanoparticles. Finally, using tin porphyrins and light for a variety of purposes involving reactions that require a low redox potential may be a good step toward energy conservation and environmentally benign processing.

We report on reduction and precipitation of Se(VI), Pb(II), CU(II), U(VI), Mo(VI), and Cr(VI) in water by cytochrome c{sub 3} isolated from Desulfomicrobium baczdatum [strain 9974]. The tetraheme protein cytochrome c{sub 3} was reduced by sodium dithionite. Redox reactions were monitored by UV-visible spectroscopy of cytochrome c{sub 3}. Analytical electron microscopy work showed that Se(VI), Pb(II), and CU(II) were reduced to the metallic state, U(W) and Mo(W) to U(IV) and Mo(IV), respectively, and Cr(VI) probably to Cr(III). U(IV) and Mo(W) precipitated as oxides and Cr(III) as an amorphous hydroxide. Cytochrome c{sub 3} was used repeatedly in the same solution without loosing its effectiveness. The results suggest usage of cytochrome c{sub 3} to develop innovative and environmentally benign methods to remove heavy metals from waste- and groundwater.

An overview of the use of classical mechanical molecular simulations of porphyrins, hydroporphyrins, and heme proteins is given. The topics cover molecular mechanics calculations of structures and conformer energies of porphyrins, energies of barriers for interconversion between stable conformers, molecular dynamics of porphyrins and heme proteins, and normal-coordinate structural analysis of experimental and calculated porphyrin structures. Molecular mechanics and dynamics are currently a fertile area of research on porphyrins. In the future, other computational methods such as Monte Carlo simulations, which have yet to be applied to porphyrins, will come into use and open new avenues of research into molecular simulations of porphyrins.

Algorithms developed to enable the use of atomistic molecular simulation methods with parallel computers are reviewed. Methods appropriate for bonded as well as non-bonded (and charged) interactions are included. While strategies for obtaining parallel molecular simulations have been developed for the full variety of atomistic simulation methods, molecular dynamics and Monte Carlo have received the most attention. Three main types of parallel molecular dynamics simulations have been developed, the replicated data decomposition, the spatial decomposition, and the force decomposition. For Monte Carlo simulations, parallel algorithms have been developed which can be divided into two categories, those which require a modified Markov chain and those which do not. Parallel algorithms developed for other simulation methods such as Gibbs ensemble Monte Carlo, grand canonical molecular dynamics, and Monte Carlo methods for protein structure determination are also reviewed and issues such as how to measure parallel efficiency, especially in the case of parallel Monte Carlo algorithms with modified Markov chains are discussed.

The interfacial force microscope (IFM) was used to indent and image defect free Au(111) surfaces, providing atomic-scale observations of the onset of pileup and the excursion of material above the initial surface plane. Images and load-displacement measurements demonstrate that elastic accommodation of an indenter is followed by two stages of plasticity. The initial stage is identified by slight deviations of the load-displacement relationship from the predicted elastic response. Images acquired after indentations showing only this first stage indicate that these slight load relaxation events result in residual indentations 0.5 to 4 nm deep with no evidence of pileup or surface orientation dependence. The second stage of plasticity is marked by a series of dramatic load relaxation events and residual indentations tens of nanometers deep. Images acquired following this second stage document 0.25 nm pileup terraces which reflect the crystallography of the surface as well as the indenter geometry. Attempts to plastically displace the indenter 4-10 nanometers deep into the Au(111) surface were unsuccessful, demonstrating that the transition from stage I to stage H plasticity is associated with overcoming some sort of barrier. Stage I is consistent with previously reported models of dislocation nucleation. The dramatic load relaxations of stage II plasticity, and the pileup of material above the surface, require cross-slip and appear to reflect a dynamic process leading to dislocation intersection with the surface. The IFM measurements reported here offer new insights into the mechanisms underlying the very early stages of plasticity and the formation of pileup.

The failure of thermosetting polymer adhesives is an important problem which particularly lacks understanding from the molecular viewpoint. While linear elastic fracture mechanics works well for such polymers far from the crack tip, the method breaks down near the crack tip where large plastic deformation occurs and the molecular details become important [1]. Results of molecular dynamics simulations of highly crosslinked polymer networks bonded to a solid surface are presented here. Epoxies are used as the guide for modeling. The focus of the simulations is the network connectivity and the interfacial strength. In a random network, the bond stress is expected to vary, and the most stressed bonds will break first [2]. Crack initiation should occur where a cluster of highly constrained bonds exists. There is no reason to expect crack initiation to occur at the interface. The results to be presented show that the solid surface limits the interfacial bonding resulting in stressed interfacial bonds and interfacial fracture. The bonds in highly-crosslinked random networks do not become stressed as expected. The sequence of molecular structural deformations that lead to failure has been determined and found to be strongly dependent upon the network connectivity. The structure of these networks and its influence on the stress-strain behavior will be discussed in general. A set of ideal, ordered networks have been constructed to manipulate the deformation sequence to achieve different fracture modes (i.e. cohesive vs. adhesive).

The generalized method of Backus and Gilbert (BG) is described and applied to the inverse problem of obtaining spectra from a 5-channel, filtered array of x-ray detectors (XRD's). This diagnostic is routinely fielded on the Z facility at Sandia National Laboratories to study soft x-ray photons ({le}2300 eV), emitted by high density Z-pinch plasmas. The BG method defines spectral resolution limits on the system of response functions that are in good agreement with the unfold method currently in use. The resolution so defined is independent of the source spectrum. For noise-free, simulated data the BG approximating function is also in reasonable agreement with the source spectrum (150 eV black-body) and the unfold. This function may be used as an initial trial function for iterative methods or a regularization model.

The dynamical behavior of Cl{sup {minus}} and H{sub 2}O molecules in the interlayer and on the (001) surface of the Ca-aluminate hydrate hydrocalumite (Friedel's salt) over a range of temperatures from {minus}100 to 300 C is studied using the technique of isothermal-isobaric molecular dynamics computer simulations. This phase is currently the best available model compound for other, typically more disordered, mixed-metal layered hydroxides. The computed crystallographic parameters and density are in good agreement with available X-ray diffraction data and the force field developed for these simulations preserves the structure and density to within less than 2% of their measured values. In contrast to the highly ordered arrangement of the interlayer water molecules interpreted from the X-ray data, the simulations reveal significant dynamic disorder in water orientations. At all simulated temperatures, the interlayer water molecules undergo rapid librations (hindered hopping rotations) around an axis essentially perpendicular to the layers. This results in breaking and reformation of hydrogen bonds with the neighboring Cl{sup {minus}} anions and in a time-averaged nearly uniaxial symmetry at Cl{sup {minus}}, in good agreement with recent {sup 35}Cl NMR measurements. Power spectra of translational, vibrational, and vibrational motions of interlayer and surface Cl{sup {minus}} and H{sub 2}O were calculated as Fourier transforms of the atomic velocity autocorrelation functions and compared with the corresponding spectra and dynamics for a bulk aqueous solution. The ordered interlayer space has significant effects on the motions. Strong electrostatic attraction between interlayer water molecules and Ca atoms in the principal layer makes the Ca{hor_ellipsis}OH{sub 2} bond direction the preferred axis for interlayer water librations. The calculated diffusion coefficient of Cl{sup {minus}} as an outer-sphere surface complex is almost three times that of inner-sphere Cl{sup {minus}}, but is still about an order of magnitude less than that of Cl{sup {minus}} in bulk aqueous solution at the same temperature.

This paper reports on the development of a unified one-equation model for the prediction of transitional and turbulent flows. An eddy viscosity--transport equation for nonturbulent fluctuation growth based on that proposed by Warren and Hassan is combined with the Spalart-Allmaras one-equation model for turbulent fluctuation growth. Blending of the two equations is accomplished through a multidimensional intermittency function based on the work of Dhawan and Narasimha. The model predicts both the onset and extent of transition. Low-speed test cases include transitional flow over a flat plate, a single element airfoil, and a multi-element airfoil in landing configuration. High-speed test cases include transitional Mach 3.5 flow over a 5{degree} cone and Mach 6 flow over a flared-cone configuration. Results are compared with experimental data, and the grid-dependence of selected predictions is analyzed.

Many Navier-Stokes codes require that the governing equations be written in conservation form with a source term. The Spalart-Allmaras one-equation model was originally developed in substantial derivative form and when rewritten in conservation form, a density gradient term appears in the source term. This density gradient term causes numerical problems and has a small influence on the numerical predictions. Further work has been performed to understand and to justify the neglect of this term. The transition trip term has been included in the one-equation eddy viscosity model of Spalart-Allmaras. Several problems with this model have been discovered when applied to high-speed flows. For the Mach 8 flat plate boundary layer flow with the standard transition method, the Baldwin-Barth and both k-{omega} models gave transition at the specified location. The Spalart-Allmaras and low Reynolds number k-{var_epsilon} models required an increase in the freestream turbulence levels in order to give transition at the desired location. All models predicted the correct skin friction levels in both the laminar and turbulent flow regions. For Mach 8 flat plate case, the transition location could not be controlled with the trip terms as given in the Spalart-Allmaras model. Several other approaches have been investigated to allow the specification of the transition location. The approach that appears most appropriate is to vary the coefficient that multiplies the turbulent production term in the governing partial differential equation for the eddy viscosity (Method 2). When this coefficient is zero, the flow remains laminar. The coefficient is increased to its normal value over a specified distance to crudely model the transition region and obtain fully turbulent flow. While this approach provides a reasonable interim solution, a separate effort should be initiated to address the proper transition procedure associated with the turbulent production term. Also, the transition process might be better modeled with the Spalart-Allmaras turbulence model with modification of the damping function f{sub v1}. The damping function could be set to zero in the laminar flow region and then turned on through the transition flow region.

Certain applications for pulse power require narrow, high current pulses for their implementation. This work was performed to determine if MCTS (MOS Controlled Thyristors) could be used for these applications. The MCTS were tested as discharge switches in a low inductance circuit delivering 1 {micro}s pulses at currents between roughly 3 kA and 11 kA, single shot and repetitively at 1, 10 and 50 Hz. Although up to 9000 switching events could be obtained, all the devices failed at some combination of current and repetition rate. Failure was attributed to temperature increases caused by average power dissipated in the thyristor during the switching sequence. A simulation was performed to confirm that the temperature rise was sufficient to account for failure. Considerable heat sinking, and perhaps a better thermal package, would be required before the MCT could be considered for pulse power applications.

Inverter type thyristors were switched repetitively to failure with 1 {micro}s pulses at repetition rates of 10, 50 and 100 pps and at peak currents up to 12 kA. Millions of pulses could be obtained before failure if the peak current were held at around 6 kA.

The potential for cooperation between India and Pakistan is substantial. Topics as widely varying as national security, the environment and trade hold the potential for improved bilateral relations. This paper looks at a few areas in which monitoring technology could contribute to enhancing cooperative border agreements between the two nations. The goal of the paper is not to provide prescriptive solutions to regional problems, but to expand the number of options being considered for improving Indian-Pakistan relations. Many of the impediments to bilateral progress are a result of a history of conflict and mistrust. By utilizing technical monitoring and inspections, each side can begin to replace suspicion and doubt with knowledge and information useful in making informed political, economic and military decisions. At the same time, technical monitoring and inspections can build confidence through common interactions. India and Pakistan have pledged to resolve their disputes, including Kashmir, through dialogue. Implementation of that pledge is influenced by a number of factors, including changes in the political systems and the fortunes of the leadership. Events of the past year and a half have severely tested these two governments' ability to move forward along a constructive and positive path. Testing of new missile systems both preceded and followed testing of nuclear weapons in May 1998. Both countries disregarded subsequent international displeasure as they proceeded to openly declare their respective nuclear capability. Their brief engagement with each other in February 1999 and movement toward a rapprochement diluted international condemnation of their nuclear activity. Within a recent period of nine months however, progress in the dialogue has been stalled first by the Pakistani move in Kashmir in May 1999, then by the Indian election in the summer of 1999 and most recently by the military coup in Pakistan.

An assessment was made of the manufacturability of hybrid microcircuit test vehicles assembled using three Pb-free solder compositions 96.5Sn--3.5Ag (wt.%), 91.84Sn--3.33Ag--4.83Bi, and 86.85Sn--3.15Ag--5.0Bi--5.0Au. The test vehicle substrate was 96% alumina; the thick film conductor composition was 76Au--21Pt--3Pd. Excellent registration between the LCCC or chip capacitor packages and the thick film solder pads was observed. Reduced wetting of bare (Au-coated) LCCC castellations was eliminated by hot solder dipping the I/Os prior to assembly of the circuit card. The Pb-free solders were slightly more susceptible to void formation, but not to a degree that would significantly impact joint functionality. Microstructural damage, while noted in the Sn-Pb solder joints, was not observed in the Pb-free interconnects.

High-speed multiplication is frequently used in general-purpose and application-specific computer systems. These systems often support integer multiplication, where two n-bit integers are multiplied to produce a 2n-bit product. To prevent growth in word length, processors typically return the n least significant bits of the product and a flag that indicates whether or not overflow has occurred. Alternatively, some processors saturate results that overflow to the most positive or most negative representable number. This paper presents efficient methods for performing unsigned or two's complement integer multiplication with overflow detection or saturation. These methods have significantly less area and delay than conventional methods for integer multiplication with overflow detection and saturation.

The authors have demonstrated a functional MOCVD-grown AlGaAs/InGaAsN/GaAsPnP DHBT that is lattice matched to GaAs and has a peak current gain ({beta}) of 25. Because of the smaller bandgap (E{sub g}=1.20eV)of In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} used for the base layer, this device has a low V{sub ON} of 0.79 V, 0.25 V lower than in a comparable Pnp AlGaAs/GaAs HBT. The BV{sub CEO} is 12 V, consistent with its GaAs collector thickness and doping level.