Sandia National Laboratories

About: Sandia National Laboratories is a facility organization based out in Livermore, California, United States. It is known for research contribution in the topics: Laser & Combustion. The organization has 21501 authors who have published 46724 publications receiving 1484388 citations. The organization is also known as: SNL & Sandia National Labs.

Modeling and simulating critical infrastructures and their interdependencies

Abstract: Our national security, economic prosperity, and national well-being are dependent upon a set of highly interdependent critical infrastructures. Examples of these infrastructures include the national electrical grid, oil and natural gas systems, telecommunication and information networks, transportation networks, water systems, and banking and financial systems. Given the importance of their reliable and secure operations, understanding the behavior of these infrastructures - particularly when stressed or under attack - is crucial. Models and simulations can provide considerable insight into the complex nature of their behaviors and operational characteristics. These models and simulations must include interdependencies among infrastructures if they are to provide accurate representations of infrastructure characteristics and operations. A number of modeling and simulation approaches under development today directly address interdependencies and offer considerable insight into the operational and behavioral characteristics of critical infrastructures.

Surface Step Effects on Nanoindentation

Abstract: Atomistic simulation is used to examine nanoindentation of a Au(111) crystal both near and far from a surface step. While the load needed to nucleate dislocations decreases significantly when indenting close to the step, the extent of the step's influence is not as great as seen experimentally. This behavior is explained by measuring the contact area from the simulation data. A new metric, the slip vector, shows material slip coinciding with the directions of a lowest unstable stacking fault barrier. The slip vector is used to calculate an atomic critical resolved shear stress, which is shown to be a good dislocation nucleation criterion.

Temperature and pressure dependences of the dielectric constants of semiconductors

Abstract: The effects of temperature and hydrostatic pressure on the static dielectric constant (epsilon) were investigated for a group of crystalline semiconductors chosen to be representative of III-V compounds (GaAs and GaP), II-VI compounds (ZnS and CdS), and group-IV (Si) materials. When combined with earlier results on the temperature and pressure dependences of the high-frequency optical (i.e., electronic) dielectric constants (epsilon/sub infinity/), the present results allow, for the compound semiconductors, determination of the lattice contribution to these effects. The results are discussed from both the microscopic and macroscopic points of view. For all the crystals studied, the pressure effects are dominated by the change in polarizability with volume, and the temperature effects by anharmonicities. The pressure dependences of the lattice contributions to the dielectric constants (epsilon/sub 11/ and epsilon/sub 33/) of hexagonal CdS are anomalous in that they increase with pressure. This behavior is most likely due to coupling between the appropriate TO modes and the TA modes which are known to soften on approaching the pressure-induced phase transition in this crystal. The transverse dynamic effective charge was calculated for the compounds, and its pressure dependence was determined for GaAs and GaP. The results and their implications are discussed.

The role of van der Waals forces in adhesion of micromachined surfaces

Abstract: Interfacial adhesion and friction are important factors in determining the performance and reliability of microelectro- mechanical systems. We demonstrate that the adhesion of micromachined surfaces is in a regime not considered by standard rough surface adhesion models. At small roughness values, our experiments and models show unambiguously that the adhesion is mainly due to van der Waals dispersion forces acting across extensive non-contacting areas and that it is related to 1/Dave2, where Dave is the average surface separation. These contributions must be considered because of the close proximity of the surfaces, which is a result of the planar deposition technology. At large roughness values, van der Waals forces at contacting asperities become the dominating contributor to the adhesion. In this regime our model calculations converge with standard models in which the real contact area determines the adhesion. We further suggest that topographic correlations between the upper and lower surfaces must be considered to understand adhesion completely.

Cold welding of ultrathin gold nanowires

Abstract: The welding of metals at the nanoscale is likely to have an important role in the bottom-up fabrication of electrical and mechanical nanodevices. Existing welding techniques use local heating, requiring precise control of the heating mechanism and introducing the possibility of damage. The welding of metals without heating (or cold welding) has been demonstrated, but only at macroscopic length scales and under large applied pressures. Here, we demonstrate that single-crystalline gold nanowires with diameters between 3 and 10 nm can be cold-welded together within seconds by mechanical contact alone, and under relatively low applied pressures. High-resolution transmission electron microscopy and in situ measurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electrical conductivity as the rest of the nanowire. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast surface-atom diffusion. Welds are also demonstrated between gold and silver, and silver and silver, indicating that the technique may be generally applicable.