Advances in pulsed-laser atom probe: instrument and specimen design for optimum performance.

The performance of the pulsed-laser atom probe can be limited by both instrument and specimen factors. The experiments described in this article were designed to identify these factors so as to provide direction for further instrument and specimen development. Good agreement between voltage-pulsed and laser-pulsed data is found when the effective pulse fraction is less than 0.2 for pulsed-laser mode. Under the conditions reported in this article, the thermal tails of the peaks in the mass spectra did not show any significant change when produced with either a 10-ps or a 120-fs pulsed-laser source. Mass resolving power generally improves as the laser spot size and laser wavelength are decreased and as the specimen tip radius, specimen taper angle, and thermal diffusivity of the specimen material are increased. However, it is shown that two of the materials used in this study, aluminum and stainless steel, depend on these factors differently. A one-dimensional heat flow model is explored to explain these differences. The model correctly predicts the behavior of the aluminum samples, but breaks down for the stainless steel samples when the tip radius is large. A more accurate three-dimensional model is needed to overcome these discrepancies.     

Chromatographic separation of americium from europium using bis-2,6-(5,6,7,8-tetrahydro-5,9,9-trimethyl-5,8-methano-1,2,4-benzotriazin-3-yl) pyridine

The separation of americium(III) from europium(III) was achieved utilizing a bis-2,6-(5,6,7,8-tetrahydro-5,9,9-trimethyl-5,8-methano-1,2,4-benzotriazin-3-yl) pyridine (CA-BTP) chromatographic resin. The extraction chromatographic materials were prepared using various concentrations of CA-BTP. This new, hydrolytically stable extractant was impregnated on an inert polymeric support at 40% loading. The uptake of Am(III) and Eu(III) by this material from 0.1 to 4.0 M aqueous HNO₃ solutions was measured. The resulting dry weight distribution ratios, D _w , indicated a strong preference for Am(III) with little affinity for Eu(III). These results are similar to recently reported solvent extraction studies indicating a maximum uptake of Am(III) in the 0.5–1.0 M HNO₃ range. The resin preparation, performance, and characterization of the Am/Eu separation are reported herein.

     

A dynamic model for road protection against flooding

This paper focuses on the problem of identifying optimal protection strategies to reduce the impact of flooding on a road network. We propose a dynamic mixed-integer programming model that extends the classic concept of road network protection by shifting away from single-arc fortifications to a more general and realistic approach involving protection plans that cover multiple components. We also consider multiple disruption scenarios of varying magnitude. To efficiently solve large problem instances, we introduce a customised GRASP heuristic. Finally, we provide some analysis and insights from a case study of the Hertfordshire road network in the East of England. Results show that optimal protection strategies mainly involve safeguarding against flooding events that are small and likely to occur, whereas implementing higher protection standards are not considered cost-effective.     

Solution phase synthesis of a diverse library of highly substituted isoxazoles

The iodocyclization of O-methyloximes of 2-alkyn-1-ones affords 4-iodoisoxazoles, which undergo various palladium-catalyzed reactions to yield 3,4,5-trisubstituted isoxazoles. The palladium-catalyzed processes have been adapted to parallel synthesis utilizing commercially available boronic acid, acetylene, styrene, and amine sublibraries. Accordingly, a diverse 51-member library of 3,4,5-trisubstituted isoxazoles has been generated.     

Simultaneous quantitative flow measurement using PIV on both sides of the air–water interface for breaking waves

An experimental method for simultaneously measuring the velocity fields on the air and water side of unsteady breaking waves is presented. The method includes a novel technique for seeding the air flow such that the air velocity can be resolved in the absence of wind. Low density particles that have large Stokes drag and ability to respond to high-frequency flow fluctuations are used to seed the air flow. Multi-camera, multi-laser particle image velocimetry setups are applied to small-scale unsteady breaking waves, yielding fully time-resolved velocity fields. The surface tension of the fluid is altered and controlled to form spilling breaking waves. Results for the velocity and vorticity fields of representative spilling breakers, which show shedding of an air-side vortex and well-documented generation of water-side vorticity, are presented and discussed.     

Transport of airborne particulate matters originating from Mentougou, Beijing, China

In this study, a coupled regional air quality modeling system is applied to investigate the time spatial variations in airborne particulate matters （PM10）, originating from Mentougou to Beijing municipal area in the period of April 1-7, 2004, and the influences of complex terrain and meteorological conditions upon boundary layer structure and PM10 concentration distributions. An intercomparison of the performance with CALPUFF against the observed data is presented and an examination of scatter plots is provided. The statistics show that the correlation coefficient and STD between the modeled and observed data are 0.86 and 0.03, respectively. Analysis of model results illustrates that the pollutants emitted from Mentougou can be transported to Beijing municipal area along certain transport pathways, and PM10 concentration distributions show heterogeneity characteristics. Contributions of the Mentougou sources to the PM10 concentrations in Beijing municipal area are up to 0.1-15 μg/m^3.     

Reaction of acetylated carbohydrates with trimethylaluminum: concise synthesis of 1,2-O-isopropylidene d-ribofuranose

Treatment of β-d-ribose tetraacetate with trimethylaluminum gives α-3,5-O-acetyl-1,2-O-isopropylidene-d-ribofuranoside in excellent yield. This reaction allows for efficient and high-yielding installation of the 1,2-isopropylidene acetal (acetonide), which is difficult to prepare using more traditional acid-catalyzed methods. The reaction of trimethylaluminum with other per-acetylated carbohydrates is also described.