Publications

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Strength, friction, and wear are dominant factors in the performance and reliability of materials and devices fabricated using nickel based LIGA and silicon based MEMS technologies. However, the effects of frictional contacts and wear on long-term performance of microdevices are not well-defined. To address these effects on performance of LIGA nickel, we have begun a program employing nanoscratch and nanoindentation. Nanoscratch techniques were used to generate wear patterns using loads of 100, 200, 500, and 990 {micro}N with each load applied for 1, 2, 5, and 10 passes. Nanoindentation was then used to measure properties in each wear pattern correcting for surface roughness. The results showed a systematic increase in hardness with applied load and number of nanoscratch passes. The results also showed that the work hardening coefficient determined from indentation tests within the wear patterns follows the results established from tensile tests, supporting use of a nanomechanics-based approach for studying wear.

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At Sandia National Laboratories, we have built 18650 Li-ion cells with Li reference electrode for in situ characterization of electrodes including impedance and other electrochemical properties. At a 200 mA ({approx}C/5 rate) discharge, the cell gave {approx}900 mAh. Impedance measurements indicate that the anode dominates the cell impedance. For example, at 0 C, the anode and cathode impedances at 10 mHz were around 149 and 53 m{Omega}, respectively, and the total cell impedance at 10 mHz was {approx}203 m{Omega}. The three-electrode configuration also permits measurement of individual electrode voltages during charge and discharge. During discharge, while the cell voltage drops from 4.1 to 3 V, the cathode and the anode voltages change from 4.1 to 3.7 and from {approx}0 to 0.7 V, respectively. During charge, the cathode and anode voltages trace back to their initial values before discharging. The voltage swing for the anode is higher than that for the cathode. This also indicates that the impedance for the anode is higher than for the cathode. Pulse measurements on the cells indicate the voltage drop of the full-cell is equal to the sum of the anode and cathode voltage drops for a 2 A discharge pulse.

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A classical mechanistic model was developed to capture the existence of pre-sliding tangential deflection (PSTD) in contacting polysilicon and coated polysilicon surfaces. For the purposes of modeling asperity friction, experiments have shown, and been supported through detailed finite element analyses, that frictional forces developed through tangential sliding scale linearly through a material parameter known as the junction strength. A junction strength model coupled with a discrete quasi-static contact mechanics analysis, using contacting surface descriptions sampled by AFM from actual polysilicon surfaces, predicts inelastic tangential displacements that are qualitatively consistent with observed PSTD response. The simulations imply that the existence of PSTD depends not only on the spatial characteristics of contacting surfaces, but also on the local loading characteristics.

This paper presents continuum simulations of polymer flow during nanoimprint lithography (NIL). The simulations capture the underlying physics of polymer flow from the nanometer to millimeter length scale and examine geometry and thermophysical process quantities affecting cavity filling. Variations in embossing tool geometry and polymer film thickness during viscous flow distinguish different flow driving mechanisms. Three parameters can predict polymer deformation mode: cavity width to polymer thickness ratio, polymer supply ratio and capillary number. The ratio of cavity width to initial polymer film thickness determines vertically or laterally dominant deformation. The ratio of indenter width to residual film thickness measures polymer supply beneath the indenter which determines Stokes or squeeze flow. The local geometry ratios can predict a fill time based on laminar flow between plates, Stokes flow, or squeeze flow. A characteristic NIL capillary number based on geometry-dependent fill time distinguishes between capillary- or viscous-driven flows. The three parameters predict filling modes observed in published studies of NIL deformation over nanometer to millimeter length scales. The work seeks to establish process design rules for NIL and to provide tools for the rational design of NIL master templates, resist polymers and process parameters.

Certain classes of dynamic network problems can be modeled by a set of hyperbolic partial differential equations describing behavior along network edges and a set of differential and algebraic equations describing behavior at network nodes. In this paper, we demonstrate real-time performance for optimization problems in drinking water networks. While optimization problems subject to partial differential, differential, and algebraic equations can be solved with a variety of techniques, efficient solutions are difficult for large network problems with many degrees of freedom and variable bounds. Sequential optimization strategies can be inefficient for this problem due to the high cost of computing derivatives with respect to many degrees of freedom. Simultaneous techniques can be more efficient, but are difficult because of the need to solve a large nonlinear program; a program that may be too large for current solver. This study describes a dynamic optimization formulation for estimating contaminant sources in drinking water networks, given concentration measurements at various network nodes. We achieve real-time performance by combining an efficient large-scale nonlinear programming algorithm with two problem reduction techniques. D Alembert's principle can be applied to the partial differential equations governing behavior along the network edges (distribution pipes). This allows us to approximate the time-delay relationships between network nodes, removing the need to discretize along the length of the pipes. The efficiency of this approach alone, however, is still dependent on the size of the network and does not scale indefinitely to larger network models. We further reduce the problem size with a subdomain approach and solve smaller inversion problems using a geographic window around the area of contamination. We illustrate the effectiveness of this overall approach and these reduction techniques on an actual metropolitan water network model.

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Adhesion is an important factor in controlling properties and performance of thin film devices. It is a critical factor in hybrid microcircuits with multilayer films and dissimilar metal interconnects where diffusion of copper from leads during processing and environmental effects during service can modify the adhesion strength of the gold conductive films. Previous work using gold and gold-copper alloy films to simulate different stages of processing and service showed that copper in solution improved film adhesion. More importantly, it took a combination of stressed overlayers and nanoindentation to trigger interfacial fracture of the gold-copper alloy films. The improvement in performance scaled directly with an increase in film strength. However, during two years air exposure telephone cord buckles formed at the gold-copper alloy film edges, grew slowing across the film surface, and eventually covered the sample. Formation of these buckles shows that a significant degradation in interfacial fracture strength had occurred in these films. We characterized the size and shape of the blisters that formed during nanoindentation of the as-deposited films and in the films following aging. These measurements were then combined with mechanics-based models to determine residual stresses and interfacial fracture energies. This analysis shows that air aging decreased the mode I interfacial fracture energy for the gold-copper alloy film from 3.2 J/m{sup 2} to 1.5 J/m{sup 2}. A similar decrease in fracture energy has been observed for many systems exposed to hydrogen from processing and environmental exposure, including copper films, beryllium films, steels and iron- and nickel-based superalloys. This paper describes the effect of environment on resistance of gold-copper alloy film systems to premature interfacial failure, and by comparison with previous studies shows it can be attributed to hydrogen embrittlement.

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Current methodologies for the production of meso- and nanoporous materials include the use of a surfactant to produce a self-assembled template around which the material is formed. However, post-production surfactant removal often requires centrifugation, calcination, and/or solvent washing which can damage the initially formed material architecture(s). Surfactants that can be disassembled into easily removable fragments following material preparation would minimize processing damage to the material structure, facilitating formation of templated hybrid architectures. Herein, we describe the design and synthesis of novel cationic and anionic surfactants with regularly spaced unsaturation in their hydrophobic hydrocarbon tails and the first application of ring closing metathesis depolymerization to surfactant degradation resulting in the mild, facile decomposition of these new compounds to produce relatively volatile nonsurface active remnants.

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Flow patterns with both recirculating and unidirectional characteristics are useful for controlled mixing and pumping within microfluidic devices. We have developed a fabrication process that converts injection-molded polymer chips into devices that demonstrate induced-charge electroosmosis (ICEO) effects (1,2) in AC fields. Polymeric insulating posts are coated with metal to produce a nonuniform zeta potential under an applied electric field. Induced flows are analyzed by particle image velocimetry. Stable, recirculating flow patterns are discussed, along with their potential to produce well-characterized and reversible streamlines for on-chip mixing in chemical separation and synthesis devices. Asymmetric conductive features can bias the flow direction, generating unidirectional pumping in an AC field. This pumping approach will be discussed in comparison with DC electrokinetic pumps we have studied.

Complex simulations (in particular, those involving multiple coupled physics) cannot be understood solely using geometry-based visualizations. Such visualizations are necessary in interpreting results and gaining insights into kinematics, however they are insufficient when striving to understand why or how something happened, or when investigating a simulation's dynamic evolution. For multiphysics simulations (e.g. those including solid dynamics with thermal conduction, magnetohydrodynamics, and radiation hydrodynamics) complex interactions between physics and material properties take place within the code which must be investigated in other ways. Drawing on the extensive previous work in view coordination, brushing and linking techniques, and powerful visualization libraries, we have developed Prism, an application targeted for a specific analytic need at Sandia National Laboratories. This multiview scientific visualization tool tightly integrates geometric and phase space views of simulation data and material models. Working closely with analysts, we have developed this production tool to promote understanding of complex, multiphysics simulations. We discuss the current implementation of Prism, along with specific examples of results obtained by using the tool.

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The objective of this study was to quantify the hydrogen-assisted fracture susceptibility of gas-tungsten arc (GTA) welds in the nitrogen-strengthened, austenitic stainless steels 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-5Mn (22-13-5). In addition, mechanisms of hydrogen-assisted fracture in the welds were identified using electron microscopy and finite-element modeling. Elastic-plastic fracture mechanics experiments were conducted on hydrogen-charged GTA welds at 25 C. Results showed that hydrogen dramatically lowered the fracture toughness from 412 kJ/m{sup 2} to 57 kJ/m{sup 2} in 21-6-9 welds and from 91 kJ/m{sup 2} to 26 kJ/m{sup 2} in 22-13-5 welds. Microscopy results suggested that hydrogen served two roles in the fracture of welds: it promoted the nucleation of microcracks along the dendritic structure and accelerated the link-up of microcracks by facilitating localized deformation. A continuum finite-element model was formulated to test the notion that hydrogen could facilitate localized deformation in the ligament between microcracks. On the assumption that hydrogen decreased local flow stress in accordance with the hydrogen-enhanced dislocation mobility argument, the finite-element results showed that deformation was localized in a narrow band between two parallel, overlapping microcracks. In contrast, in the absence of hydrogen, the finite-element results showed that deformation between microcracks was more uniformly distributed.

DNA-wrapped carbon nanotubes (DNA-CNT) have generated attention due the ability to disperse cleanly into solution, and by the possibility of sorting nanotubes according to size and conductivity. In order to learn more about the effects of DNA on the electrical transport characteristics of single wall carbon nanotubes, we fabricate and test a series of devices consisting of DNA-wrapped CNTs placed across gold, palladium, and palladium oxide electrodes. In addition, we look at how DNA functionalized CNTs react to presence of hydrogen, which has previously been shown to affect the conductivity of CNTs when in contact with palladium.