Synthesis of 1-(3-alkyl-2,3-dideoxy-D-pentofuranosyl)uracils with potential anti-HIV activity.

1-(3-Alkyl-2,3-dideoxy-alpha,beta-D-erythro-pentofuranosyl)uracils and 1-(3-alkyl-2,3-dideoxy-alpha,beta-D-threo-pentofuranosyl)uracils have been prepared from (E)-4,5-di-O-acetyl-2,3-dideoxy-aldehydo-D-glycero-pent-2-enose by a Michael addition reaction of the appropriate organocopper reagent followed in subsequent order by glycosidation of the resulting 3-alkyl-4,5-diacetoxypentanal with methanolic hydrogen chloride, protection with p-methoxybenzoyl chloride, and trimethylsilyl triflate catalyzed coupling with 2,4-di-O-(trimethylsilyl)uracil. The nucleosides were deprotected by treatment with 33% methylamine in absolute ethanol and separated by reversed-phase HPLC.     

Solution of the hyperbolic mild‐slope equation using the finite volume method

A finite volume solver for the 2D depth‐integrated harmonic hyperbolic formulation of the mild‐slope equation for wave propagation is presented and discussed. The solver is implemented on unstructured triangular meshes and the solution methodology is based upon a Godunov‐type second‐order finite volume scheme, whereby the numerical fluxes are computed using Roe's flux function. The eigensystem of the mild‐slope equations is derived and used for the construction of Roe's matrix. A formulation that updates the unknown variables in time implicitly is presented, which produces a more accurate and reliable scheme than hitherto available. Boundary conditions for different types of boundaries are also derived. The agreement of the computed results with analytical results for a range of wave propagation/transformation problems is very good, and the model is found to be virtually paraxiality‐free. Copyright © 2003 John Wiley & Sons, Ltd.     

Slow cyclotron instability in a high-power backward-wave oscillator

The linear dispersion relation of a backward-wave oscillator (BWO), derived earlier by the authors, is modified to include effects of RF surface current at the beam-vacuum interface. This modified dispersion relation results in an unstable interaction between the slow cyclotron mode (SCM) and the structure mode in addition to the conventional Cherenkov instability caused by the slow space charge mode. Numerical analysis is then carried out using parameters of a BWO experiment at University of Maryland. Fine structure of the SCM instability is elucidated. The analysis indicates that BWO radiation would not be suppressed near cyclotron absorption in an infinitely long system.     

Diffuse x-ray scattering study of the formation of microdefects in heat-treated dislocation-free large-diameter silicon wafers

Diffuse x-ray scattering (DXS) is used to study the formation of microdefects (MDs) in heat-treated dislocation-free large-diameter silicon wafers with vacancies. The DXS method is shown to be efficient for investigating MDs in silicon single crystals. Specific defects, such as impurity clouds, are found to form in the silicon wafers during low-temperature annealing at 450°C. These defects are oxygen-rich regions in the solid solution with diffuse coherent interfaces. In the following stages of decomposition of the supersaturated solid solution, oxide precipitates form inside these regions and the impurity clouds disappear. As a result of the decomposition of the supersaturated solid solution of oxygen, interstitial MDs form in the silicon wafers during multistep heat treatment. These MDs lie in the {110} planes and have nonspherical displacement fields. The volume density and size of MDs forming in the silicon wafers at various stages of the decomposition are determined.     

Non-destructive analysis of archeological samples by energy-dispersive X-ray fluorescence

Summary A non-destructive method is described for the determination of major and minor constituents in archeological specimens by energy-dispersive X-ray fluorescence. Homogeneity tests are made by measuring at various sites of the specimen. In the same way, mean values are obtained for inhomogeneous specimen without taking samples. For calibration, powder standards are used. In case of the determination of elements with numbers up to 14 (Si) a vacuum chamber is used and the dimensions of the specimens are limited by the dimensions of that vacuum chamber, whereas for the determination of elements from K up to U specimens of any size, form or weight are suitable.

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Zerstörungsfreie Analyse von archäologischen Proben mit Hilfe der Energie-dispersiven Röntgenfluorescenzanalyse

Zusammenfassung Eine zerstörungsfreie Methode für die Bestimmung von Haupt- und Nebenbestandteilen in archäologischen Proben mit Hilfe der Energie-dispersiven Röntgenfluorescenzanalyse wird beschrieben. Für Homogenitätstests wird an mehreren Stellen der Probe gemessen. In der gleichen Weise werden für inhomogene Proben Mittelwerte erhalten ohne Probenahme. Für die Eichung werden Pulverstandards verwendet. Im Falle der Bestimmung von Elementen mit Ordnungszahlen bis 14 (Si) wird eine Vakuumkammer eingesetzt, und die Dimensionen der Proben sind durch die Dimensionen dieser Vakuumkammer begrenzt, während für die Bestimmung der Elemente K bis U Proben jeder Größe, jeder Form oder jeden Gewichts verwendbar sind.

     

Korrosionstests an magnetischen Schichten und deren Beurteilung mit Hilfe ausgew?hlter oberfl?chenanalytischer Verfahren

Summary Besides functional behaviour of coatings their resistivity against corroding attack is a crucial criterion for quality. The application of special corroding and abrading tests yields after short times predictions about stability of the product in practical use. A number of tests have been applied to thin magnetic storage media. Besides tests of function and visual observations with light- and scanning electron microscopes, AES- and XPS-investigations were applied to discuss corrosion resistivity.