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 & Thin film. The organization has 21501 authors who have published 46724 publications receiving 1484388 citations. The organization is also known as: SNL & Sandia National Labs.

Nanomechanical properties of Au (111), (001), and (110) surfaces

Abstract: Using the interfacial force microscope in an indentation mode, we have quantitatively investigated the mechanical properties for the (111), (001), and (110) surfaces of Au single crystals. Nanoscale indentations of wide, atomically flat terraces provide a measure of the nanomechanical properties of Au in the absence of bulk and surface defects. The elastic indentation modulus for the (111) surface was found to be 36% greater than for the (001) and 3% greater than for the (110) surfaces. These results are compared to earlier theoretical predictions of the effect of anisotropy on indentation based on continuum mechanics and atomistic simulations. Additionally, we have quantified the yield point of the three crystal orientations by measuring the stress at which initial plastic deformation occurs. By resolving the applied stresses on {111} slip planes, we have estimated maximum shear stresses at the yield point. For each orientation, plastic deformation occurred when the maximum resolved shear stress reached approximately 1.8 GPa on all {111} planes that appeared to contribute to deformation. Based on this estimate, we propose that the critical resolved shear stress for plastic indentation of Au is 1.8 GPa and that the yield criterion is that this stress be attained on all {111} slip planes noncoplanar with the surface.

Geometry of frictionless and frictional sphere packings.

Abstract: We study static packings of frictionless and frictional spheres in three dimensions, obtained via molecular dynamics simulations, in which we vary particle hardness, friction coefficient, and coefficient of restitution. Although frictionless packings of hard spheres are always isostatic (with six contacts) regardless of construction history and restitution coefficient, frictional packings achieve a multitude of hyperstatic packings that depend on system parameters and construction history. Instead of immediately dropping to four, the coordination number reduces smoothly from $z=6$ as the friction coefficient $\ensuremath{\mu}$ between two particles is increased.

Chloroaluminate-Doped Conducting Polymers as Positive Electrodes in Rechargeable Aluminum Batteries

Abstract: Demonstrated here is the use of conducting polymers as active materials in the positive electrodes of rechargeable aluminum-based batteries operating at room temperature. The battery chemistry is based on chloroaluminate ionic liquid electrolytes, which allow reversible stripping and plating of aluminum metal at the negative electrode. Characterization of electrochemically synthesized polypyrrole films revealed doping of the polymers with chloroaluminate anions. Cycling of the conducting polymer electrodes occurred via the electrochemical insertion and removal of these anions. Stable galvanostatic cycling of polypyrrole and polythiophene cells was demonstrated, with electrode capacities at near-theoretical levels (30–100 mAh g–1) and Coulombic efficiencies approaching 100%. The energy density of a sealed sandwich-type cell with polythiophene at the positive electrode was estimated to be 44 Wh kg–1 relative to the total mass of active components. This energy density is competitive with state-of-the-art bat...

H-Morph: an indirect approach to advancing front hex meshing

Abstract: H-Morph is a new automatic algorithm for the generation of a hexahedral-dominant finite element mesh for arbitrary volumes. The H-Morph method starts with an initial tetrahedral mesh and systematically transforms and combines tetrahedral into hexahedra. It uses an advancing front technique where the initial front consists of a set of prescribed quadrilateral surface facets. Fronts are individually processed by recovering each of the six quadrilateral faces of a hexahedron from the tetrahedral mesh. Recovery techniques similar to those used in boundary constrained Delaunay mesh generation are used. Tetrahedral internal to the six hexahedral faces are then removed and a hexahedron is formed. At any time during the H-Morph procedure a valid mixed hexahedral-tetrahedral mesh is in existence within the volume. The procedure continues until no tetrahedral remain within the volume, or tetrahedral remain which cannot be transformed or combined into valid hexahedral elements. Any remaining tetrahedral are typically towards the interior of the volume, generally a less critical region for analysis. Transition from tetrahedral to hexahedra in the final mesh is accomplished through pyramid shaped elements. Advantages of the proposed method include its ability to conform to an existing quadrilateral surface mesh, its ability to mesh without the need to decompose or recognize special classes of geometry, and its characteristic well-aligned layers of elements parallel to the boundary. Example test cases are presented on a variety of models.