Publications

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The thermochemistry of the transition-metal fluorides and chlorides MF{sub n} and MCl{sub n} (M = Cr, Mn, Fe; n = 1, 2) has been characterized by high-level ab initio electronic structure methods. Geometries and harmonic vibrational frequencies were computed at the B3LYP level of theory using triple-{zeta} basis sets including diffuse and polarization functions. Heats of formation were computed from isogyric reaction energies at the CCSD(T) level using high-quality basis sets, including corrections for core-valence correlation and scalar relativistic effects. To investigate the possible linearity of the ground states of CrCl{sub 2} and CrF{sub 2}, we performed geometry optimizations for these species at the CCSD(T) level using large basis sets. In both cases, a bent ({sup 5}B{sub 2}) minimum structure was located, but the bent structure is only slightly below the linear form, which was found to be a transition state. For all of the investigated halides, polynomial fits were carried out for the heat capacity and the standard enthalpy and entropy in the 300-3000 K temperature range.

The modeling of fluid/structure interaction is of growing importance in both energy and environmental applications. Because of the inherent complexity, these problems must be simulated on parallel machines in order to achieve high resolution. The purpose of this research was to investigate techniques for coupling flow and geomechanics in porous media that are suitable for parallel computation. In particular, our main objective was to develop an iterative technique which can be as accurate as a fully coupled model but which allows for robust and efficient coupling of existing complex models (software). A parallel linear elastic module was developed which was coupled to a three phase three-component black oil model in IPARS (Integrated Parallel Accurate Reservoir Simulator). An iterative de-coupling technique was introduced at each time step. The resulting nonlinear iteration involved solving for displacements and flow sequentially. Rock compressibility was used in the flow model to account for the effect of deformation on the pore volume. Convergence was achieved when the mass balance for each component satisfied a given tolerance. This approach was validated by comparison with a fully coupled approach implemented in the British PetroledAmoco ACRES simulator. Another objective of this work was to develop an efficient parallel solver for the elasticity equations. A preconditioned conjugate gradient solver was implemented to solve the algebraic system arising from tensor product linear Galerkin approximations for the displacements. Three preconditioners were developed: LSOR (line successive over-relaxation), block Jacobi, and agglomeration multi-grid. The latter approach involved coarsening the 3D system to 2D and using LSOR as a smoother that is followed by applying geometric multi-grid with SOR (successive over-relaxation) as a smoother. Preliminary tests on a 64-node Beowulf cluster at CSM indicate that the agglomeration multi-grid approach is robust and efficient.

The couple on a ball rotating relative to an otherwise quiescent suspension of comparably-sized, neutrally buoyant spheres is studied both experimentally and numerically. Apparent 'slip' relative to the analytical solution for a sphere spinning in a Newtonian fluid (based upon the viscosity of the suspension) is determined in suspensions with volume fractions c ranging from 0.03 to 0.50. This apparent slip results in a decrease of the measured torque on the spinning ball when the radius of the ball becomes comparable with that of the suspended spheres. Over the range of our data, the slip becomes more pronounced as the concentration c increases. At c = 0.25, three-dimensional boundary-element simulations agree well with the experimental data. Moreover, at c = 0.03, good agreement exists between such calculations and theoretical predictions of rotary slip in dilute suspensions.

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An aged polybutadiene-based elastomer was reacted with trifluoroacetic anhydride (TFAA) and subsequently analyzed via 19F NMR spectroscopy. Derivatization between the TFAA and hydroxyl functionalities produced during thermo-oxidative aging was achieved, resulting in the formation of trifluoroester groups on the polymer. Primary and secondary alcohols were confirmed to be the main oxidation products of this material, and the total percent oxidation correlated with data obtained from oxidation rate measurements. The chemical derivatization appears to be highly sensitive and can be used to establish the presence and identity of oxidation products in aged polymeric materials. This methodology represents a novel condition monitoring approach for the detection of chemical changes that are otherwise difficult to analyze.

In the present work the authors describe the adaptation of a standard SEM into a flexible microjoining tool. The system incorporates exceptional control of energy input and its location, environmental cleanliness, part manipulation and especially, part imaging. Beam energetics, modeling of thermal flow in a simple geometry, significant effects of surface energy on molten pools and beam size characterization are treated. Examples of small to micro fusion welds and molten zones produced in a variety of materials (Ni, tool steel, Tophet C, Si) and sizes are given. Future directions are also suggested.

High-resolution, variable temperature PL experiments were performed in the spectral region associated with recombination processes involving the ground and excited states of the neutral donor bound excitons. High-resolution infrared measurements in combination with high-sensitive SIMS unambiguously identified Si and O shallow donors and yield their ground state binding energies. These binding energies are in excellent agreement with values obtained by the analysis of the two-electron-satellite PL spectra considering the participation of ground and excited state donor bound excitons. This work clarifies conflicting aspects existing in donor identification and the binding energies of the impurities and excitons.

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Solid-state light sources emitting at wavelengths less than 300 nm would enable technological advances in many areas such as fluorescence-based biological agent detection, non-line-of-sight communications, water purification, and industrial processing including ink drying and epoxy curing. In this paper, we present our recent progress in the development of LEDs with emission between 237 and 297 nm. We will discuss growth and design issues of deep-UV LEDs, including transport in Si-doped AlGaN layers. The LEDs are designed for bottom emission so that improved heat sinking and light extraction can be achieved by flip chipping. To date, we have demonstrated 2.25 mW of output power at 295 nm from 1 mm x 1 mm LEDs operated at 500 mA. Shorter wavelength LEDs emitting at 276 nm have achieved an output power of 1.3 mW at 400 mA. The heterostructure designs that we have employed have suppressed deep level emission to intensities that are up to 330 x lower than the primary quantum well emission.

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A stereoscopic particle image velocimetry (PIV) instrument has been constructed for a transonic wind tunnel to study the interaction created by a supersonic axisymmetric jet exhausting from a flat plate into a subsonic compressible crossflow. Data have been acquired in the crossplane of the interaction at a single station in the farfield, in which the bulk particle motion is aligned with the out-of-plane velocity component. The resulting vector fields distinctly show the strength and location of the induced counter-rotating vortex pair as well as the remnant of the horseshoe vortex that wraps around the jet plume as it first exhausts from the nozzle. Data taken for four different values of the jet-to-freestream dynamic pressure ratio reveal the resulting change in vortex strength, size, and position. Vorticity fields were derived from the in-plane velocity data, but limited convergence of the present small data sets prevented any conclusions about the symmetry of the flowfield. Comparison of the present data is made with two-dimensional PIV data previously acquired in the streamwise plane.

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Powder phosphors of ZnS:Ag,Cl coated with SiO{sub 2} (22 or 130 nm nanoparticles), SnO{sub 2} or Al{sub 2}O{sub 3} showed different cathodoluminescent (CL) brightness versus time (temporal CL quenching) behavior as compared to noncoated phosphors. At high current density (e.g., 300-800 {micro}A/cm{sup 2}), the CL emission intensity of coated ZnS:Ag,Cl decayed over the first {approx}15 s of electron beam irradiation, which was postulated to result from a large concentration of nonradiative surface centers generated during surface modification of the phosphor, and from localization of generated electrons at the surface due to primary beam-induced internal electric fields. During the first {approx}15 s of excitation, generated electrons are postulated to be redistributed by this induced internal electric fields, resulting in increased nonradiative surface recombination between electrons and holes. The formation of a nonradiative surface layer either from electron-stimulated surface chemical reactions on coated or from heat treatment of noncoated ZnS:Ag,Cl powder phosphors were shown to affect temporal CL quenching.

Femtosecond differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found only for positive time delay in both DRS and FWM experiments. The rise time at negative time delay for the DRS was much slower than the FWM signal or differential transmission spectroscopy at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing, that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

The diagonal-mass-matrix spectral element method has proven very successful in geophysical applications dominated by wave propagation. For these problems, the ability to run fully explicit time stepping schemes at relatively high order makes the method more competitive then finite element methods which require the inversion of a mass matrix. The method relies on Gauss-Lobatto points to be successful, since the grid points used are required to produce well conditioned polynomial interpolants, and be high quality 'Gauss-like' quadrature points that exactly integrate a space of polynomials of higher dimension than the number of quadrature points. These two requirements have traditionally limited the diagonal-mass-matrix spectral element method to use square or quadrilateral elements, where tensor products of Gauss-Lobatto points can be used. In non-tensor product domains such as the triangle, both optimal interpolation points and Gauss-like quadrature points are difficult to construct and there are few analytic results. To extend the diagonal-mass-matrix spectral element method to (for example) triangular elements, one must find appropriate points numerically. One successful approach has been to perform numerical searches for high quality interpolation points, as measured by the Lebesgue constant (Such as minimum energy electrostatic points and Fekete points). However, these points typically do not have any Gauss-like quadrature properties. In this work, we describe a new numerical method to look for Gauss-like quadrature points in the triangle, based on a previous algorithm for computing Fekete points. Performing a brute force search for such points is extremely difficult. A common strategy to increase the numerical efficiency of these searches is to reduce the number of unknowns by imposing symmetry conditions on the quadrature points. Motivated by spectral element methods, we propose a different way to reduce the number of unknowns: We look for quadrature formula that have the same number of points as the number of basis functions used in the spectral element method's transform algorithm. This is an important requirement if they are to be used in a diagonal-mass-matrix spectral element method. This restriction allows for the construction of cardinal functions (Lagrange interpolating polynomials). The ability to construct cardinal functions leads to a remarkable expression relating the variation in the quadrature weights to the variation in the quadrature points. This relation in turn leads to an analytical expression for the gradient of the quadrature error with respect to the quadrature points. Thus the quadrature weights have been completely removed from the optimization problem, and we can implement an exact steepest descent algorithm for driving the quadrature error to zero. Results from the algorithm will be presented for the triangle and the sphere.

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Using in situ wafer-curvature measurements of thin-film stress, we determine the critical thickness for strain relaxation in Al{sub x}Ga{sub 1-x}N/GaN heterostructures with 0.14 {le} x {le} 1. The surface morphology of selected films is examined by atomic force microscopy. Comparison of these measurements with critical-thickness models for brittle fracture and dislocation glide suggests that the onset of strain relaxation occurs by surface fracture for all compositions. Misfit-dislocations follow initial fracture, with slip-system selection occurring under the influence of composition-dependent changes in surface morphology.

We report our development of terahertz (THz) quantum-cascade lasers (QCLs) based on two novel features. First, the depopulation of the lower radiative level is achieved through resonant longitudinal optical (LO-)phonon scattering. This depopulation mechanism is robust at high temperatures and high injection levels. In contrast to infrared QCLs that also use LO-phonon scattering for depopulation, in our THz lasers the selectivity of the depopulation scattering is achieved through a combination of resonant tunneling and LO-phonon scattering, hence the term resonant phonon. This resonant-phonon scheme allows a highly selective depopulation of the lower radiative level with a sub-picosecond lifetime, while maintaining a relatively long upper level lifetime (>5 ps) that is due to upper-to-ground-state scattering. The second feature of our lasers is that mode confinement is achieved by using a novel double-sided metal-metal waveguide, which yields an essentially unity mode confinement factor and therefore a low total cavity loss at THz frequencies. Based on these two unique features, we have achieved some record performance, including, but not limited to, the highest pulsed operating temperature of 137 K, the highest continuous-wave operating temperature of 97 K, and the longest wavelength of 141 {micro}m (corresponding to 2.1 THz) without the assistance of a magnetic field.

Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.