Backward Raman amplification in a plasma waveguide

Backward Raman amplification of a short laser pulse in a plasma waveguide is demonstrated. With a guided seed pulse of 0.8-microJ energy and a pump pulse of 345-mJ energy in a 9-mm-long optically preformed plasma waveguide, 910-fold energy amplification is achieved. Heating of the plasma by the long pump pulse is identified to be a key issue for plasma-waveguide-based backward Raman amplifiers.     

Kinetic analysis of curing of tetraepoxides and diamines in the presence of etherification side reactions

The theory of branching processes is used to describe the polymer network formation resulting from the reaction of tetraepoxides with diamines using various initial compositions. Differences in reactivities of primary and secondary amine groups and the reaction between the epoxide groups and reaction-generated hydroxyl groups are taken into account; however, intramolecular reactions in the pregel stage are neglected. Expressions are derived for the critical epoxide conversion at the gel point, molecular weight in the pregel stage, changes in sol and gel fraction in the post-gel stage, and the concentration of elastically active network chains as a function of reaction conditions. The analysis of the simulation results shows that etherification reactions significantly raise the concentration of elastically active network chains of the mixture under stoichiometric excess of epoxide groups.     

Effects of polymeric dispersants and surfactants on the dispersing stability and high‐speed‐jetting properties of aqueous‐pigment‐based ink‐jet inks

Good jetting performance and pigment‐dispersing stability are necessary for pigment‐based ink‐jet inks. This study investigates how the chemical structures of the polymeric dispersants affect the pigment‐dispersing stability and jetting performance of ink‐jet inks. A series of polymeric dispersants containing styrene (hydrophobic unit) and acrylic acid (hydrophilic unit) with different ratios have been synthesized and used to disperse different pigments. Because 3000–12,000 ink drops can be expelled from the nozzle of a thermal bubble‐type ink‐jet printer within 1 s, the jetting behavior is related to the rheology variation of a dilute solution under a high shear. An in situ drop formation system has been used to study the high‐shear‐rate rheology (dynamic surface tension and dynamic viscosity) of a solution with a viscosity lower than 3 cps, which no commercial rheometer can measure. Excellent ink‐jet inks can only be produced by the combination of pigments, dispersants, and surfactants with appropriate hydrophilicity. Moreover, the correlation of the pigment surface properties, the chemical structures of the polymeric dispersants, and ink formulation is discussed. The effects of the surfactants and firing conditions on the jetting behavior of the ink‐jet ink are also examined. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1909–1920, 2003     

Crystallization-induced diastereoselection: asymmetric synthesis of substance P inhibitors

A novel three-component condensation followed by a crystallization-induced asymmetric transformation is used to build this key substance P inhibitor intermediate in a short synthetic sequence.     

Outer-sphere oxidation of ascorbate with os(bpy)33+ incorporated in nafion coatings on graphite electrodes

The electro-oxidation of ascorbate proceeds very slowly at graphite electrodes coated with Nafion. Incorporation of Os(bpy)₃²⁺ (bpy = 2,2′-bipyridine) in the coating produces a catalyzed oxidation of ascorbate at potentials where Os(bpy)₃²⁺ is oxidized to Os(bpy)₃³⁺. Analysis of the kinetic data demonstrates that the reaction between catalyst (Os(bpy)₃³⁺) and substrate (ascorbate) proceeds only within the outermost layer of the coating at the coating/solution interface. As the substrate concentration is increased, the limiting oxidation currents at coated rotating disk electrodes do not increase as rapidly as expected on the basis of current models of the behavior of polymer coated electrodes. Some possible reasons for this deviant behavior of cast Nafion coatings are suggested. Some implications of the results on the general utility of Nafion-coated electrodes in electro-catalytic applications are presented.     

Isolation and structure elucidation of parnafungins, antifungal natural products that inhibit mRNA polyadenylation

The Candida albicans Fitness Test, a whole-cell screening platform, was used to profile crude fermentation extracts for novel antifungal natural products with interesting mechanisms of action. An extract with intrinsic antifungal activity from the fungus Fusarium larvarum displayed a Fitness Test profile that strongly implicated mRNA processing as the molecular target responsible for inhibition of fungal growth. Isolation of the active components from this sample identified a novel class of isoxazolidinone-containing natural products, which we have named parnafungins. These natural products were isolated as an interconverting mixture of four structural- and stereoisomers. The isomerization of the parnafungins was due to a retro-Michael ring-opening and subsequent reformation of a xanthone ring system. This interconversion was blocked by methylation of an enol moiety. Structure elucidation of purified parnafungin derivatives was accomplished by X-ray crystallography and NMR analysis. The biochemical target of these natural products has been identified as the fungal polyadenosine polymerase. Parnafungins demonstrated broad spectrum antifungal activity with no observed activity against gram-positive or gram-negative bacteria. The intact isoxazolidinone ring was required for antifungal activity. In addition, the natural products were efficacious in a mouse model of disseminated candidiasis.     

Different Reactivity of Rutile and Anatase TiO2 Nanoparticles: Synthesis and Surface States of Nanoparticles of Mixed-Valence Magnéli Oxides

Magnéli phases Ti_nO_2n−1 (3<n≤10) are mixed Ti⁴⁺/Ti³⁺ oxides with high electrical conductivity. When used for water remediation or electrochemical energy storage and conversion, they are nanostructured and exposed to various environments. Therefore, understanding their surface reactivity is of prime importance. Such studies have been hindered by carbon contamination from syntheses. Herein, this synthetic and characterization challenge is addressed through a new approach to 50 nm carbon-free Ti₄O₇ and Ti₆O₁₁ nanoparticles. It takes advantage of the different reactivities of rutile and anatase TiO₂ nanoparticles towards H₂, to use the former as precursor of Ti_nO_2n−1 and the latter as a diluting agent. This approach is combined with silica templating to restrain particle growth. The surface reactivity of the Magnéli nanoparticles under different atmospheres was then evaluated quantitatively by synchrotron-radiation-based X-ray photoelectron spectroscopy, which revealed oxidized surfaces with lower conductivity than the core. This finding sheds a new light on the charge transfer occurring in these materials.     

The impact of preventive activities on the economics of production systems: Modeling and application

In this paper, the impact of preventive activities on the economics of production systems has been discussed. A defective production system has been used to evaluate the financial return of a system with and without preventive activity. A mathematical model has been developed to simulate the situation. A practical case in the automotive industry has been used to verify the model. According to this research, it has been found that preventive activities will affect the return of the production system, and the effect depends on the cost of the action that has been taken.