National Institute of Standards and Technology

About: National Institute of Standards and Technology is a government organization based out in Gaithersburg, Maryland, United States. It is known for research contribution in the topics: Laser & Scattering. The organization has 26667 authors who have published 60661 publications receiving 2215547 citations. The organization is also known as: National Bureau of Standards & NIST.

Properties of a silica-reinforced polymer for dental restorations

Abstract: An esthetic and permanent restorative material that could be placed directly into the cavities prepared in anterior teeth would be an important contribution to dental practice and public well-being. Such a material, which would have less solubility, sensitivity to desiccation, and brittleness than the silicate cements,1,2 and have greater dimensional stability than do the methyl methacrylate direct filling resins,3’4 has been in demand for many years. The present lack of a mate­ rial complying with these requirements is a problem worthy of extensive investiga­ tion. The tests reported here were under­ taken to determine whether the desired properties could be obtained with a re­ inforced organic polymer. In industry, plastics are commonly reinforced with fibers, particles, or a combination of these. The term “reinforcement” usually is reserved for instances where the physi­ cal properties are improved; the term “ filler” often implies no improvement of physical or chemical properties. Particles, rather than fibers, were selected for use in the material described iiere. Irregular shaped particles of vitreous silica (General Electric Co.’s Clear Fused Quartz Powder), with special surface treatment, were used for the reinforce­ ment. Except where otherwise specified, the particles were vinylsilane-coated and were fine enough to pass through a no. 100 sieve of the United States standard sieve series, having dimensions of ap­ proximately 150 microns (about 0.006 inch) and smaller. The organic binder for this reinforce­ ment was specially prepared for this study, because the commercially available monomers and comonomers did not ap­ pear to be as promising, with respect to polymerization shrinkage, as the comono­ mer system to be described. This system was especially designed to have low polymerization shrinkage.

Optical properties of coupled metallic nanorods for field-enhanced spectroscopy

Abstract: The optical properties of coupled metallic nanorods are studied to investigate the use of coupled plasmonic structures in field-enhanced spectroscopies. Light scattering by coupled nanorods is calculated with the boundary element method, including retardation. The modes of coupled nanorod systems are calculated by the boundary charge method and discussed in terms of their symmetry. Similar scattering behavior for isolated nanorods and pairs of nanorods can mask the very different local responses that produce near-field enhancement. The response of isolated rods redshifts with increasing rod length because intrarod restoring forces are reduced. The near- and far-field responses increase monotonically with increasing rod length increasing polarization along the rod. For coupled nanorods, coupling localizes charge at the gap between the rod ends and splits degenerate modes. The localized charge depolarizes the intrarod response and provides an additional redshift. Moreover, the near-field enhancement in the gap between the nanorods is dramatically increased by coupling-induced charge localization at the gap. For short nanorods, the near-field response in coupled systems is determined by the geometry of the rod ends that define the gap. For longer nanorods, the response in coupled systems is determined by the rod length. Changing the dimensions and geometry of the nanorods to modify the interrod coupling has a major effect on the local-field enhancement. The effects of the environment and the actual metallic material do not have as big an influence on the field enhancement.

Dependence of Mechanical Strength of Brittle Polycrystalline Specimens on Porosity and Grain Size

Abstract: An empirical equation form, approximating the apparent dependence of the strength of brittle polycrystalline specimens on the combined effect of porosity and grain size, has been developed from concepts and data in the literature. The form is shown to be applicable to data for thoria and chromium carbide specimens.