Publications

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An important step towards realizing the advantages of quantum dots in electro-optic applications is to understand the excitation dependences of optical properties. This paper discusses results obtained using a microscopic theory. The calculations uncovered complicated carrier density and electronic structure influences on absorption, gain and refractive index that can be attributed to a delicate balancing of electronic-structure and many-body effects in a coupled quantum-dot-quantum-well system.

The effect of sample microstructure on water dynamic wetting behavior was examined for superhydrophobic ZnO films. Surface morphology ranging from needle arrays to overlapping platelets was controlled through judicious choice of hydrothermal reaction conditions. Structure modification with alkyl and perfluoroalkyl chains yielded films characterized by advancing contact angles that ranged from 159° to 171°. Contact angle hysteresis was less than 2° with needles (tip diameter <30 nm) and less than 11° for rods (diameter <250 nm). Relatively thick (diameter ∼600 nm) structures were still characterized by advancing contact angles exceeding 165° and hysteresis <30°. Formation of nanometer-scale roughness on top of the microstructure via silica deposition significantly enhanced the surface superhydrophobicity. Similarly, following perfluoro-alkane treatment, all examined microstructures exhibited advancing contact angles > 169° and hysteresis < 7°. © 2008 American Chemical Society.

During the 1990's the Electric Power Research Institute (EPRI) developed methods for fire risk analysis to support its utility members in the preparation of responses to Generic Letter 88-20, Supplement 4, "Individual Plant Examination - External Events" (IPEEE). This effort produced a Fire Risk Assessment methodology for at-power that was used by the majority of US Nuclear Power Plants (NPPs) in support of the IPEEE program and several NPPs oversees. Although these methods were acceptable for accomplishing the objectives of the IPEEE, EPRI and the U.S. Nuclear Regulatory Commission (NRC) recognized that these methods require upgrades to support current requirements for Risk-Informed/Performance-Based (RI/PB) applications. In 2001 EPRI and the NRC Office of Nuclear Regulatory Research (RES) embarked on a cooperative project to improve the state-of-the-art in fire risk assessment to support this new risk-informed environment in fire protection. This project produced a consensus document, NUREG/CR-6850 (EPRI 1011989), entitled "Fire PRA Methodology for Nuclear Power Facilities" which addresses fire risk for at-power operations. This report developed: 1) the process for identification and inclusion of the post-fire Human Failure Events (HFEs), 2) the methodology for assigning quantitative screening values to these HFEs, and 3) the initial considerations of performance shaping factors (PSFs) and related fire effects that may need to be addressed in developing best-estimate Human Error Probabilities (HEPs). However, this document does not describe a methodology to develop these best-estimate HEPs given the PSFs and the fire-related effects. In 2007 EPRI and NRC's RES embarked on another cooperative project to develop explicit guidance for estimating HEPs for human error events under fire generated conditions, building upon existing human reliability analysis (HRA) methods. This paper will describe the progress to date on the development and testing of the fire HRA methodology, which includes addressing the range of fire procedures used in existing plants, the range of strategies for main control room abandonment, and the potential impact of fire-induced spurious electrical effects on crew performance. In addition to developing a detailed HRA approach, one goal of the project is to develop a fire HRA scoping quantification approach that allows derivation of more realistic HEPs than those in the screening approach from NUREG/CR-6850 (EPRI 1011989), while requiring less analytic resources than a detailed HRA. In this approach, detailed HRA will be used only for the more complex actions that cannot meet the criteria for the scoping approach.

This paper discusses the interplay between networks and control systems. As we gain more understanding about the structure and dynamics of physical networks, their effects on the performance of closed-loop control systems, as well as the ability to control such networks, provide fertile areas of research. The paper reviews such research with special emphasis on the connectivity and delays in the information transfer across networks. © 2008 Elsevier Ltd. All rights reserved.

A simple and effective approach is presented to construct coarse spaces for overlapping Schwarz preconditioners. The approach is based on energy minimizing extensions of coarse trace spaces, and can be viewed as a generalization of earlier work by Dryja, Smith, and Widlund. The use of these coarse spaces in overlapping Schwarz preconditioners leads to condition numbers bounded by C(1 + H/δ)(1 + log(H/h)) for certain problems when coefficient jumps are aligned with subdomain boundaries. For problems without coefficient jumps, it is possible to remove the log(H/h) factor in this bound by a suitable enrichment of the coarse space. Comparisons are made with the coarse spaces of two other substructuring preconditioners. Numerical examples are also presented for a variety of problems.

This paper investigates the use of strip transect sampling to estimate object abundance when the underlying spatial distribution is assumed to be Poisson. A design-rather than model-based approach to estimation is investigated through computer simulation, with both homogeneous and non-homogeneous fields representing individual realizations of spatial point processes being considered. Of particular interest are the effects of changing the number of transects and transect width (or alternatively, coverage percent or fraction) on the quality of the estimate. A specific application to the characterization of unexploded ordnance (UXO) in the subsurface at former military firing ranges is discussed. The results may be extended to the investigation of outcrop characteristics as well as subsurface geological features. © 2008 Springer-Verlag Berlin Heidelberg.

Several antennas with integrated high-impedance surfaces are presented. The high-impedance surface is implemented as a composite right/left-handed (CRLH) metamaterial fabricated from a periodic structure characterized by a substrate, filled with an array of vertical vias and capped by capacitive patches. Omnidirectional antennas placed in close proximity to the high-impedance surface radiate hemispherically with an increase in boresight far-field pattern gain of up to 10 dB and a front-to-back ratio as high as 13 dB at 2.45 GHz. Several TEM rectangular horn antennas are realized by replacing conductor walls with high-impedance surfaces. The TEM horn antennas are capable of operating below the TE{sub 1,0} cutoff frequency of a standard all-metal horn antenna, enabling a reduction in antenna volume. Above the cutoff frequency the TEM horn antennas function similarly to standard rectangular horn antennas.

This paper outlines a comparison between two cognitive modeling frameworks: Atomic Components of Thought - Rational (ACT-R; Anderson & Lebiere, 1998) and a framework under development at Sandia National Laboratories. Both frameworks are based on the cognitive psychological literature, although they represent different theoretical perspectives on cognition, with ACT-R being a production-rule-based system and the Sandia framework being a dynamical-systems or connectionist-type approach. This comparison involved a complex driving domain in which both the car being driven and the driver were equipped with sensors that provided information to each framework. The output of each framework was a classification of the real-world situation that the driver was in, e.g., being overtaken on the autobahn. Comparisons between the two frameworks included validation against human ratings of the driving situations via videotapes of driving sessions, along with twelve creation and performance metrics regarding the method and ease of framework population, processor requirements, and maximum real-time data sampling rate.