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.

Evidence for graphene growth by C cluster attachment

Abstract: Using low-energy electron microscopy (LEEM), we have measured the local concentration of mobile carbon adatoms from which graphene sheets form on a Ru(0001) surface, and simultaneously, the growth rates of individual graphene islands. Graphene crystal growth on Ru differs strikingly from that of two-dimensional metal islands on metals: (i) C adatoms experience a large energy barrier to attaching to graphene step edges, so adatom diffusion does not limit growth. (ii) The supersaturations needed for appreciable growth rates are comparable to those required to nucleate islands. (iii) The growth rate is a highly nonlinear function of supersaturation, with a large activation energy (2.0±0.1 eV). Our analysis suggests that graphene grows by adding rare clusters of about five atoms rather than adding the abundant monomers (adatoms). Knowing the growth mechanism and monitoring the supersaturation, we can control the pattern of growing graphene sheets.

Phonon-assisted transition rates I. Optical-phonon-assisted hopping in solids

Abstract: An extensive calculation of the optical-phonon-assisted transition rates for non-adiabatic electronic hopping motion in a solid is presented. Holstein's Molecular Crystal Model is used as a basis for study and the computation involves no restrictions on either the magnitude of the electron-lattice coupling strength, the temperature, or the difference between the electronic energies of the initial and final sites. In the strong-coupling small-polaron regime, the jump rates, associated d.c. conductivity, a.c. conductivity, and electric-field dependence of the d.c. conductivity, for a crystal are all calculated. These transport properties manifest qualitatively distinct behaviours corresponding to whether the temperature is above or well below the optical-phonon temperature. In the low-temperature regime the energy-conserving processes which involve the absorption of the minimum amount of vibrational energy provide the dominant contribution to the thermally activated jump rates. At sufficiently high...

Raman Spectroscopy Study of the Structure of Lithium and Sodium Ultraphosphate Glasses

Abstract: Anhydrous binary phosphate glasses containing from 0 to 50 mol% Li2O or Na2O have been prepared and examined by Raman scattering spectroscopy. The unpolarized Raman spectrum of vitreous P2O5 has intense bands near 640 cm−1, attributed to the symmetric stretching mode of POP bridging oxygens, (POP)sym, between Q3 phosphate tetrahedra, and at 1390 cm−1 due to the symmetric stretch of the PO terminal oxygens, (PO)sym. With the addition of alkali oxide to P2O5, a new feature appears in the Raman spectra near 1160 cm−1 indicating the formation of Q2 phosphate tetrahedra with two bridging and two non-bridging oxygens. The increase in relative amplitude of this new (PO2)sym band with increasing modifier content is consistent with a simple depolymerization of the phosphate network. From 20 to 50 mol% alkali oxide, the position of the (PO)sym Raman band decreases by ∼ 130 cm−1 whereas the frequency of the (POP)sym band increases by ∼ 60 cm−1. These frequency shifts are the result of π-bond delocalization on Q3 species that effectively lengthens the PO terminal oxygen bond and strengthens the POP linkages with increasing alkali oxide content. The compositional dependence of the π-bond delocalization on Q3 tetrahedra is described by considering the interconnections between neighboring Q3 and Q2 tetrahedra. The onset of π-bond delocalization on Q3 species corresponds with the anomalous Tg minimum at 20 mol% alkali oxide in alkali ultraphosphate glasses. The increase in Tg between 20 and 50 mol% alkali oxide is attributed to the increased ionic interconnection of what becomes a chain-like phosphate network at higher alkali contents. Finally, the Raman spectra of several alkali ultraphosphate glasses show high frequency shoulders on the Raman bands attributed to the (PO2)sym and (PO2)asym vibrational modes. These shoulders represent the presence of strained structural units, possibly three- or four-membered rings.

A General Synthetic Procedure for Heteropolyniobates

Abstract: The heteropolyanions of W, Mo, and V, which have found numerous applications, are formed simply by acidification of solutions of their oxoanions. Under similar conditions, these oxoanion precursors are not available for Nb, and Nb-oxo chemistry is dominated by formation of the Lindquist ion [Nb6O19]8– only. However, heteropolyniobate formation is favored in hydrothermal reactions of aqueous, alkaline precursor mixtures. Here we give two examples of heteropolyniobates formed by this general reaction type: K12[Ti2O2][SiNb12O40]·16H2O [1], which contains chains of silicododecaniobate Keggin ions, and Na14[H2Si4Nb16O56]·45.5H2O [2], a new heteropolyanion structure type.

A resilience assessment framework for infrastructure and economic systems: Quantitative and qualitative resilience analysis of petrochemical supply chains to a hurricane

Abstract: In recent years, the nation has recognized that critical infrastructure protection should consider not only the prevention of disruptive events but also the processes that infrastructure systems undergo to maintain functionality following disruptions. This more comprehensive approach has been termed critical infrastructure resilience. Given the occurrence of a particular disruptive event, the resilience of a system to that event is the system's ability to reduce efficiently both the magnitude and duration of the deviation from targeted system performance levels. Under the direction of the U. S. Department of Homeland Security's Science and Technology Directorate, Sandia National Laboratories has developed a comprehensive resilience assessment framework for evaluating the resilience of infrastructure and economic systems. The framework includes a quantitative methodology that measures resilience costs that result from a disruption to infrastructure function. The framework also includes a qualitative analysis methodology that assesses system characteristics affecting resilience to provide insight and direction for potential improvements. This article describes the resilience assessment framework and demonstrates the utility of the assessment framework through application to two hypothetical scenarios involving the disruption of a petrochemical supply chain by hurricanes. © 2011 American Institute of Chemical Engineers Process Saf Prog, 2011