Publications

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Cracking was observed in the side walls of gold (Au) filled vias in low-temperature, co-fired ceramic (LTCC) substrates. Further analysis indicated the likely source as the constituents of the glassy phase component of the gold-platinum-palladium (Au-Pt-Pd) thick film used for the conductor traces and pads. The successful approach toward mitigating the cracking phenomenon was to place a Au thick film layer between the Au-Pt-Pd layer and the LTCC substrate, which significantly curtailed the diffusion of glassy phase components into the latter. However, it was necessary to determine the effects of the additional thick film layer on the microstructure and overall mechanical strength of tin-lead (Sn-Pb) solder joints made to device pads. Acceptable pull strengths were measured in the range of 3.5 - 4.0 lbs. The solder joint pull strength was sensitive to the number of firing steps as defined by the thick film layer construction. Both the solder/thick film and thick film/LTCC interface strengths had roles in this trend, thereby affirming the synergism between material, interfaces, and the firing processes The pull strength was optimized when the pad length ratio, 4596:5742, was 1.0:0.5, which was characterized by a reduced occurrence of the thick film/LTCC failure mode. © 2007 IEEE.

Sandia National Laboratories has world-class facilities for building and testing lithium and lithium-ion batteries. In this article we describe the in-house facilities for fabricating electrodes and cells in detail. Our in-house facility includes equipment for: 1) electrode coating, 2) electrode slitting, 3) electrode winding, 4) cell grooving, 5) electrolyte filling, 6) cell crimping and more. We also have a 48-channel Maccor tester and several impedance units for electrochemical characterization. These facilities provide flexibility for cell fabrication techniques which in turn allows us to continually improve cell performance. Under an internally funded program we are developing in-house capability to fabricate and evaluate 18650 Li/(CFx)n cells using "Li-ion" electrode fabrication methodologies to prepare the thin film (CFx)n electrodes. At a C/400 discharge rate cell delivered ~3.6 Ahrs capacity. We also evaluated cathodes of two different lengths for uniformity of loading. The loading along the electrode length was found to be extremely uniform, as the delivered capacity was proportional to cathode length. For example, a 0.91 meters long x 4.2 mil thick electrode gave 3.6 Ahrs capacity while a 0.72 meters long × 4.2 mil thick electrode (19.4% less length) gave 2.9 Ahr of capacity (19.4% less capacity). We also discharged the cells with 0.71 meters long electrodes at different temperatures. The cells delivered practically the same capacity over temperatures from 25 to 72°C. At -20°C the cells delivered 81% of the room temperature capacity at a C/200 rate; however, at -40°C the cells delivered close to 47% of the room temperature capacity under similar test conditions. The performance behavior of 18650 cells will be discussed in more detail in the paper. © 2007 by ESG.

The influence of the constant C3, which multiplies the mean flow divergence term in the model equation for the turbulent kinetic energy dissipation, is examined in a motored diesel engine for three different swirl ratios and three different spatial locations. Predicted temporal histories of turbulence energy and its dissipation are compared with experimentally-derived estimates. A "best-fit" value of C3 = 1.75, with an approximate uncertainty of ±0.3 is found to minimize the error between the model predictions and the experiments. Using this best-fit value, model length scale behavior corresponds well with that of measured velocity-correlation integral scales during compression. During expansion, the model scale grows too rapidly. Restriction of the model assessment to the expansion stroke suggests that C3 = 0.9 is more appropriate during this period. Copyright © 2007 SAE International.

Cyclic ethers, like tetrahydrofuran (THF), are formed during the autoignition of alkanes and subsequently influence their combustion chemistry. To learn more about the oxidation chemistry of these ether intermediates, a fuel-rich THF flame (π = 1.75) has been studied using the versatile technique of flame-sampling Molecular Beam Mass Spectrometry (MBMS) in combination with single-photon ionization. Several cyclic intermediates which are potentially formed by dehydrogenation of the fuel are identified by their ionization energies. Ethylene, propene, ketene and formaldehyde are major stable decomposition products of THF and their mole fraction profiles are presented. Detected oxygenated species include ethenol, acetaldehyde and propanal. Despite the fuel-rich conditions, the concentrations of benzene and other aromatic hydrocarbons are near the detection limit.

Modelling soot formation in turbulent nonpremixed combustion is a difficult problem. Unlike most gaseous combustion species, soot lacks a strong state relationship with the mixture fraction due to unsteady formation rates which overlap transport timescales, and strong differential diffusion between gaseous species and soot. The conditional moment closure model (CMC) has recently been applied to the problem of turbulent soot formation. A challenge in CMC modelling is the treatment of differential diffusion. Three-dimensional direct numerical simulation (DNS) of a nonpremixed ethylene jet flame with soot formation has been performed for the first time, using a nineteen species reduced ethylene mechanism and a four-step, three-moment, semi-empirical soot model. The DNS provides full resolution of the turbulent flow field and is used to perform a-priori analysis of a new CMC model derived from the joint scalar PDF transport equation. Unlike other approaches, this CMC model does not require additional transport equations to treat differentially diffusing species. A budget of the terms of the CMC equation for both gaseous species and soot is presented. In particular, exact expressions for unclosed terms are compared to typical closure models for scalar dissipation, cross dissipation, differential diffusion, and reactive source terms.

Direct numerical simulation of a nonpremixed, turbulent, ethylene jet flame is performed to investigate fundamental mechanisms of extinction and reignition processes. A reduced ethylene mechanism consisting of nineteen transported and ten quasi-steady state species, with 167 reactions was used, along with mixture averaged transport properties. The flow configuration is a temporally-evolving slot jet at a Reynolds number of 5,120. Extreme extinction of the nonpremixed flame occurs, followed by a period of intense turbulent scalar mixing between reactants and quenched products in which less than 2stratified mixture with nonhomogeneous composition and temperature. Various modes of reignition are analyzed-autoignition, edge flame propagation, and premixed flame propagation-by monitoring Takeno's flame index [H. Yamashitia, M. Shimada, and T. Takeno, Proc. Combust. Inst., 26 (1996) 27-34], homogeneous ignition delay times by sampling the mixture prior to reignition, and the turbulent displacement speed of the reaction front. The dominant reignition mechanism is found to be premixed flame propagation commencing from a few high temperature flame kernels which survive near global extinction.

Recent advances in electrode fabrication and tailoring electrolyte properties for numerous applications have generated wide-spread interest in (CFx)x chemistry since it has the highest theoretical capacity and hence longer life of the four well known Li-primary chemistries. We are applying "Li-ion technology" electrode fabrication methodologies and preparing thin film (CFx)n electrodes in-house for evaluation. In this program we have evaluated 4 different (CFx) n materials for performance in coin cells in the temperature regime -55 to 72°C. We continue to evaluate the top performer in 18650 cell configuration and obtained ∼3.6 Ahrs capacity at a C/400 rate. We also evaluated cathodes of different lengths for uniformity of loading. For example, a 36'' long × 4.2 mil thick electrode gave 3.6 Ahrs while a 29'' long × 4.2 mil thick electrode gave 2.9 Ahrs of capacity. We also measured impedance at different voltages and thermal abuse response of the 18650 cells. © The Electrochemical Society.

Accurate simulation of biological networks is difficult not only due to the computational cost associated with large-scale systems simulation, but also due to the inherent limitations of mathematical models. We address two components to improve biological circuit simulation accuracy: 1) feasible initial conditions, and 2) identification of critical yet unknown model parameters. For those parameters that may not be available from experimental data, we incorporate reachability analysis to enhance our optimization/simulation framework and estimate those parameters that are capable of creating behaviors consistent with known experimental data. We apply these techniques to a biological circuit model of tryptophan biosynthesis in E. coli, and quantify the improvement in simulation accuracy when reachability analysis is used. © 2008 IEEE.

Lithium-Ion batteries are being considered as a high-energy density replacement for Nickel Metal Hydride (NiMH) batteries in Hybrid Electric Vehicles (HEVs) and in the new Plug-In Hybrids (PHEVs). Although these cells can result in significant reduction in weight and volume, they have several safety related issues that still need to be addressed. We report here on the thermal response of Li-ion cells specifically assembled in our laboratory to test new materials, electrolytes and additives. Improvements in the thermal abuse tolerance of cells are reported and discussed in terms of the need for overall battery system safety. © The Electrochemical Society.

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Surface chemistry is one of the main factors that contributes to the longevity and compliance of cell patterning. Two to three weeks are required for dissociated, embryonic rat neuronal cultures to mature to the point that they regularly produce spontaneous and evoked responses. Though proper surface chemistry can be achieved through the use of covalent protein attachment, often it is not maintainable for the time periods necessary to study neuronal growth. Here we report a new and effective covalent linking approach using (3-glycidoxypropyl) trimethoxysilane (3-GPS) for creating long term neuronal patterns. Micrometer scale patterns of cell adhesive proteins were formed using microstamping; hippocampal neurons, cultured up to 1 month, followed those patterns. Cells did not grow on unmodified 3-GPS surfaces, producing non-permissive regions for the long-term cell patterning. Patterned neuronal networks were formed on two different types of MEA (polyimide or silicon nitride insulation) and maintained for 3 weeks. Even though the 3-GPS layer increased the impedance of metal electrodes by a factor of 2-3, final impedance levels were low enough that low noise extracellular recordings were achievable. Spontaneous neural activity was recorded as early as 10 days in vitro. Neural recording and stimulation were readily achieved from these networks. Our results showed that 3-GPS could be used on surfaces to immobilize biomolecules for a variety of neural engineering applications. © 2006 Elsevier B.V. All rights reserved.

X-ray powder diffraction data for a rhombohedral AlPt phase formed by self-propagating, high-temperature reactions of AlPt bi-layer films are reported. Multilayer AlPt thin film samples, reacted in air or vacuum, transformed into rhombohedral AlPt with space group R-3(148). Indexing and lattice parameter refinement of AlPt powders (generated from thin-film samples) yielded trigonal/hexagonal unit-cell lattice parameters of a=15.623(6) Å and c=5.305(2) Å, Z=39, and V=1121.5 Å3. © International Centre for Diffraction Data.

We present a new family of stabilized methods for the Stokes problem. The focus of the paper is on the lowest order velocity-pressure pairs. While not LBB compliant, their simplicity and attractive computational properties make these pairs a popular choice in engineering practice. Our stabilization approach is motivated by terms that characterize the LBB "deficiency" of the unstable spaces. The stabilized methods are defined by using these terms to modify the saddle-point Lagrangian associated with the Stokes equations. The new stabilized methods offer a number of attractive computational properties. In contrast to other stabilization procedures, they are parameter free, do not require calculation of higher order derivatives or edge-based data structures, and always lead to symmetric linear systems. Furthermore, the new methods are unconditionally stable, achieve optimal accuracy with respect to solution regularity, and have simple and straightforward implementations. We present numerical results in two and three dimensions that showcase the excellent stability and accuracy of the new methods. © 2006 Society for Industrial and Applied Mathematics.

Periodic processes are ubiquitous in biological systems, yet modeling these processes with high fidelity as periodic orbits of dynamical systems is challenging. Moreover, mathematical models of biological processes frequently contain many poorly-known parameters. This paper describes techniques for computing periodic orbits in systems of hybrid differential-algebraic equations and parameter estimation methods for fitting these orbits to data. These techniques make extensive use of automatic differentiation to evaluate derivatives accurately and efficiently for time integration, parameter sensitivities, root finding and optimization. The resulting algorithms allow periodic orbits to be computed to high accuracy using coarse discretizations. Derivative computations are carried out using a new automatic differentiation package called ADMC++ that provides derivatives and Taylor series coefficients of matrix-valued functions written in the MATLAB programming language. The algorithms are applied to a periodic orbit problem in rigid-body dynamics and a parameter estimation problem in neural oscillations.

Many large-scale computations involve a mesh and first (or sometimes higher) partial derivatives of functions of mesh elements. In principle, automatic differentiation (AD) can provide the requisite partials more efficiently and accurately than conventional finite-difference approximations. AD requires source-code modifications, which may be little more than changes to declarations. Such simple changes can easily give improved results, e.g., when Jacobian-vector products are used iteratively to solve nonlinear equations. When gradients are required (say, for optimization) and the problem involves many variables, "backward" AD in theory is very efficient, but when carried out automatically and straightforwardly, may use a prohibitive amount of memory. In this case, applying AD separately to each element function and manually assembling the gradient pieces - semiautomatic differentiation - can deliver gradients efficiently and accurately. This paper concerns on-going work; it compares several implementations of backward AD, describes a simple operator-overloading implementation specialized for gradient computations, and compares the implementations on some mesh-optimization examples. Ideas from the specialized implementation could be used in fully general source-to-source translators for C and C++.

Nanometric aluminum (123nm, spherical) was mixed with two different sieve-cut sizes of HMX (106-150 μm and 212-300 μm), and a series of gas gun tests were conducted to compare reactive wave development in pure HMX to that of aluminized HMX. In the absence of added metal, 4-mm-thick, low-density (68% of theoretical maximum density) pressings of the 106-150 μm HMX respond to modest shock loading by developing distinctive reactive waves that exhibit both temporal and meso-scale spatial fluctuations. Similar pressings of Al/HMX containing 10% aluminum (by mass) show an initial suppression of the usual wave growth seen in HMX samples. The suppression is then followed by an induction period where it is hypothesized that a phase change in the aluminum may occur. Data from VISAR, line-ORVIS, and 2-color pyrometry are given and discussed, and numerical modeling of inert sucrose is used to aid the explanation of the resulting data.

Impact-flash phenomenology has been known for decades, and is now being considered for missile-defense applications, in particular for remote engagement diagnostics. To technically establish this capability, we have conducted a series of experiments at impact velocities of ∼6, ∼11, and ∼25 km/s. Two- and three-stage light-gas guns were used for the lower two velocities, and magnetically-driven flyers on the Sandia Z machine achieved the higher velocity. Spectrally- and temporally-resolved flash output addressed data reproducibility, material identification, and target configuration analysis. Usable data were obtained at visible and infrared wavelengths. Standard atomic spectral databases were used to identify strong lines from all principal materials used in the study. The data were unique to the individual materials over the wide range of velocities and conditions examined. The time-varying nature of the signals offered the potential for correlation of the measurements with various aspects of the target configuration. Integrating the records over wavelength helped to clarify those time variations. © 2006 American Institute of Physics.

In conducting a probabilistic safety assessment of a nuclear power plant, it is important to identify unsafe actions (UAs) and human failure events (HFEs) that can lead to or exacerbate conditions during a range of incidents initiated by internal or external events. Identification and analysis of UAs and HFEs during a human reliability analysis can be a daunting process that often depends completely on subject matter experts attempting to divine a list of plant conditions and performance shaping factors (PSFs) that may influence incident outcomes. Key to this process of including the most important UAs and resulting HFEs is to speculate upon deviations of specific circumstances from a base case definition of a scenario that may present confusion regarding system diagnosis and appropriate actions (i.e., due to procedures, training, informal rules, etc.). Intuiting the location and impact of such system weaknesses is challenging and careful organization of analyst's approach to this process is critical for defending any argument for completeness of the analysis. Two dimensional distinguishability-confusability matrices were introduced as a tool to test symbol distinguishability for information displays. This paper expands on the tool by presenting multidimensional confusability matrices as a very helpful, pragmatic tool for organizing the process of combining expert judgment regarding system weaknesses, human performance and highly targeted experimentation in a manner that strengthens the quantitative justification for why particular UAs and HFEs were incorporated into a PSA. Furthermore, the particular approach presented here helps to strengthen the justification for specific likelihood determinations (i.e., human error probabilities) that end up being inserted into a probabilistic risk assessment (PRA) or other numerical description of system safety. This paper first introduces the multidimensional confusability matrix (MCM) approach and then applies it to a set of hypothetical loss of coolant accidents (LOCAs) for which a detailed human reliability analysis is desired. The basic structure of the MCM approach involves showing how actual plant states can be mapped to information available to the operators, and then mapping the information available to operator diagnoses and responses. Finally, there is a mapping of actual plant states to operator performance-each mapping is shown to vary along temporally grounded levels of dominant PSFs (e.g., stress, time available, procedures, training, etc.). MCM facilitates comprehensive analysis of the critical signals/information guiding operator diagnoses and actions. Particular manipulations of plant states, available information and PSFs and resulting operator performance may be experimentally gathered using targeted simulator studies, table top exercises with operators, or thought experiments among analysts. It is suggested that targeted simulator studies will provide the best quantitative mappings across the surfaces generated using the MCMs and the best aid to uncovering unanticipated pieces of 'critical' information used by operators. Details of quantifying overall operator performance using the MCM technique are provided. It is important to note that the MCM tool should be considered neutral regarding the issue of so-called 'reductionist' HRA methods (e.g., THERP-type) versus 'holistic' HRA methods (e.g., ATHEANA, MERMOS). If the analyst's support 'reductionist' approaches, then the MCM will represent more of a traditional interval-type, quantitative response surface in their analysis (i.e., more quantitative resolution and generalizability). If the analysis team places more emphasis on 'holistic' approaches, then the MCM will represent more of a nominal cataloging or ordinal ranking of factors influencing their specific analysis. In both types of analyses, the MCM tool helps in organizing, documenting and facilitating quantification of expert judgments and, when resources allow, targeted experimental data to support human reliability analyses. © 2006 by ASME.

A soft-landing actuation waveform was designed to reduce the bounce of a single-pole single-throw (SPST) ohmic radio frequency (RF) microelectromechanical systems (MEMS) switch during actuation. The waveform consisted of an actuation voltage pulse, a coast time, and a hold voltage. The actuation voltage pulse had a short duration relative to the transition time of the switch and imparted the kinetic energy necessary to close the switch. After the actuation pulse was stopped, damping and restoring forces slowed the switch to near-zero velocity as it approached the closed position. This is referred to as the coast time. The hold voltage was applied upon reaching closure to keep the switch from opening. An ideal waveform would close the switch with near zero impact velocity. The switch dynamics resulting from an ideal waveform were modeled using finite element methods and measured using laser Doppler vibrometry. The ideal waveform closed the switch with an impact velocity of less than 3 cm/s without rebound. Variations in the soft-landing waveform closed the switch with impact velocities of 12.5 cm/s with rebound amplitudes ranging from 75 to 150 nm compared to impact velocities of 22.5 cm/s and rebound amplitudes of 150 to 200 nm for a step waveform. The ideal waveform closed the switch faster than a simple step voltage actuation because there was no rebound and it reduced the impact force imparted on the contacting surfaces upon closure. © 2006 IEEE.

We demonstrate a robust approach to VCSEL polarization control using deeply-etched surface gratings oriented at several different rotational angles. A RCWA model is used to optimize the design for high polarization selectivity and fabrication tolerance. © 2006 Optical Society of America.

The 63Sn-37Pb (wt.%, designated Sn-Pb) solder interconnections made to copper (Cu) pads were examined on two printed wiring assemblies (PWAs) that had been in the field for 17 years and subsequently exposed to an accelerated aging test environment. A qualitative assessment of the solder joints indicated that there was excellent solderabiliry of the pins and Cu pads. Void formation was minimal or did not occur at all. Manufacturing defects were limited to minor Cu pad lifting with cracks in the underlying epoxy resin and local areas of Cu barrel separation from the laminate hole wall. Both defects would not have influenced the effects of the accelerated aging environment. A quantitative analysis examined the intermetallic compound (IMC) layer thickness of selected components on the PWAs. The IMC thickness data indicated that the PWAs were exposed to considerably lower, cumulative temperatures inside the product assembly than were present outside as a result of the accelerated aging environment. The quantitative analysis also evaluated the Pb-rich phase particle size in both fillets and the hole region of the PWA solder joints. The Pb-rich phase size confirmed that the temperature environment at the PWA level was significantly less severe than that of the accelerated aging environment. The Pb-rich phase size data indicated that the solder joints were exposed to a limited degree of thermal mechanical fatigue (TMF) that likely originated from the nominal temperature fluctuations coupled with the thermal expansion of the encapsulant as well as large expansion of the circuit board laminate in the z-axis (through-thickness) direction. This study demonstrated the methodology by which, IMC thickness and Pb-rich phase size were used to assess the temperature/time conditions experienced at the Sn-Pb/Cu interconnection level versus the external environment. Copyright © 2006 ASM International®.