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.

Melting temperature and explosive crystallization of amorphous silicon during pulsed laser irradiation

Abstract: Measurements during pulsed laser irradiation indicate that amorphous Si melts at a temperature 200 \ifmmode\pm\else\textpm\fi{} 50 K below the crystalline value. Below energy densities required to melt the amorphous layer fully, the data are interpreted in terms of an explosive crystallization. The initial liquid layer solidifies to form coarse-grained polycrystalline Si. A thin, self-propagating liquid layer travels through the remaining amorphous Si at a velocity of 10-20 m/s, producing fine-grained polycrystalline Si.

Development of an Experimental Database and Kinetic Models for Surrogate Diesel Fuels

Abstract: Computational fluid dynamic (CFD) simulations that include realistic combustion/emissions chemistry hold the promise of significantly shortening the development time for advanced high-efficiency, low-emission engines. However, significant challenges must be overcome to realize this potential. This paper discusses these challenges in the context of diesel combustion and outlines a technical program based on the use of surrogate fuels that sufficiently emulate the chemical complexity inherent in conventional diesel fuel. The essential components of such a program are discussed and include: (a) surrogate component selection; (b) the acquisition or estimation of requisite elementary chemical kinetic, thermochemical, and physical property data; (c) the development of accurate predictive chemical kinetic models, together with the measurement of the necessary fundamental laboratory data to validate these mechanisms; and (d) mechanism reduction tools to render the coupled chemistry/flow calculations feasible. In parallel to these efforts, the need exists to develop similarly robust models for fuel injection and spray processes involving multicomponent mixtures of wide distillation character, as well as methodologies to include all of these high fidelity submodels in computationally efficient CFD tools. Near- and longerterm research plans are proposed based on an application target of premixed diesel combustion. In the near term, the recommended surrogate components include n-decane, iso-octane, methylcyclohexane, and toluene. For the longer term, n-hexadecane, heptamethylnonane, n-decylbenzene, and 1-methylnaphthalene are proposed.

Photoluminescence from nanosize gold clusters

Abstract: We have observed visible light emission from nanosize gold clusters. Liquid chromatographic analysis of the metal clusters shows that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm). The emission is strongly Stokes shifted and is assigned to radiative recombination of Fermi level electrons and sp- or d-band holes. The electron and/or hole states are perturbed by surface states, as indicated by the dependence of the emission spectrum on the nature of the cluster surface. Finally, we found that large, nonemitting gold clusters can also be made luminescent by partial dissolution using KCN.

Sol → gel → glass: I. Gelation and gel structure

Abstract: The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.

A computational model of the structure and extinction of strained, opposed flow, premixed methane-air flames

Abstract: The application of laminar flamelet concepts to turbulent flame propagation requires a detailed understanding of strained laminar flames. Here we use numerical methods, including are-length continuation, to simulate the complex chemical kinetic behavior in premixed methane-air flames that are stabilized between two opposed-flow burners. We predict both the detailed structure and the extinction limits for these flames over a range of fuel-air mixtures. In addition to discussing the flame structure, a sensitivity analysis provides further insight on the chemical behavior near extinction. Finally, we discuss the comparison of the predictions with Law's experimental extinction data. An especially important aspect of this comparison is the recognition that fluid mechanical aspects of the traditional strained-flame analysis are deficient in representing experiments such as Law's. We develop and solve a new system of equations that is able to describe the experiments much more accurately.