Mechanistic aspects of chiral discrimination on modified cellulose

Cellulose and cellulose derivatives are biopolymers which are often used as stationary phases for the separation of enantiomers. Describing the mechanism of such separations is a difficult task due to the complexity of these phases. In the present study, we attempt to elucidate the types of interactions occurring between a diol intermediate for a LTD(4) antagonist and a tris(4-methylbenzoate)-derivatized cellulose stationary phase. Thermodynamic studies indicate that, at low temperatures, the enantioselectivity is entropy driven. At higher temperatures, the separation is enthalpy driven. DSC and IR experiments reveal that the transitions between the enthalpic and the entropic regions of the van't Hoff plots are a result of a change in conformation of the stationary phase. Investigation of chromatographic kinetic parameters reveals that, at low temperature, the second eluted enantiomer undergoes sluggish inclusion interactions. Subtle changes in the structure of the analyte indicates that π-π interactions do not contribute to enantioselectivity. Finally, molecular modeling of (R)- and (S)-diol and the stationary phase suggests that hydrogen bonding is a primary factor in the separation, and the calculated energy values obtained from the molecular modeling correlate well with the chromatographic elution order.     

Electrochemical reduction of carbon dioxide on flat metallic cathodes

The most important methods for the electrochemical reduction of carbon dioxide on flat metallic cathodes have been systematically summarized using a novel classification approach. In contrast to the usual classification systems that were based solely on the products of electrolysis, the electroreduction procedures have been grouped according to both the nature of the cathode (sp or d group metal electrodes) and the solvent used for the supporting electrolyte (aqueous or nonaqueous solutions). The new classification system allows the identity of the electroreduction product to be better related to the nature of the metallic electrode and the supporting electrolyte. Similar reduction products are formed by each of the four possible combinations of electrodes and supporting electrolytes (sp group metals in aqueous and nonaqueous electrolytes, and d group metals in aqueous and nonaqueous electrolytes, respectively). The discussion has included both synthetic aspects and mechanistic considerations. Of special interest in this review is the discussion of procedures for the selective preparation of formic acid and for the manufacturing of hydrocarbons and/or alcohols using carbon dioxide as the carbon source.     

FIB micro-milled sapphire for GaN maskless epitaxial lateral overgrowth: a systematic study on patterning geometry

Journal of Materials Science: Materials in Electronics - A systematic study of the GaN epitaxial lateral overgrowth (ELO) of the focused ion beam (FIB) patterned sapphire substrate is presented....     

Radionuclide contamination of foods imported into Iraq following the Chernobyl nuclear reactor accident

Since early 1986, a monitoring program for radionuclides in imported foods has been carried out by the Iraqi Atomic Energy Commission. After the Chernobyl nuclear reactor accident in the Soviet Union, the program was expanded; our laboratory was officially designated by the Iraqi Government to measure radionuclide activity concentrations in foodstuff imported from countries known to be severely contaminated by Chernobyl radioactive fallout. Gamma-spectrometric analysis was used. Food items such as powdered milk, lamb meat, poultry, cereals and grains imported into Iraq before the Chernobyl accident did not contain any detectable fission products. However, all lamb meat, 81% of the lentil, 44% of the powdered milk and chick-pea, and 17% of the roast beef samples were contaminated with 137Cs or 134Cs and 137Cs. The highest 137Cs contamination levels found were 82, 147, 420, 6 and 4 Bq kg-1, respectively. Contamination by 134Cs was approximately 50% of the values given above.     

Electron Beam Technologies for Preparation of Polymeric Materials Used for Waste Water Treatment, Agriculture, and Medicine

Radiation research results in the field of polymeric materials, obtained in the last few years by electron beam irradiation of aqueous solutions containing appropriate monomer mixtures, such as acrylamide, acrylic acid and vinyl acetate, are presented. Two types of polymeric flocculants for waste water treatment and three kinds of hydrogels for agriculture and medicine are described. The effects of radiation absorbed dose, radiation absorbed dose rate, and chemical composition of the irradiated solutions upon the polymeric materials characteristics are discussed. The required absorbed dose levels to produce the polymeric flocculants are in the range of 0.3 to 9 kGy and 4 kGy to 12 kGy for hydrogels. Some experimental results obtained by testing polymeric flocculants with waste water from food industry are given. Polymeric material processing was developed on a pilot plant level with ALID-7 electron linear accelerator of 5.5 MeV and 0.7 kW, built in the Electron Accelerator Laboratory of the Institute of Atomic Physics, Bucharest. A new facility permitting the application of simultaneous electron beam and microwave irradiation is presently under investigation. Preliminary results demonstrated that some polymeric flocculant characteristics, such as linearity, were improved by simultaneous electron beam and microwave treatment. Also, the absorbed dose levels decreased and intrinsic viscosity increased, respectively, by about two times by this new material processing method.     

Photoluminescence and intrinsic properties of MBE-grown InN nanowires

The influence of the growth parameters on the photoluminescence (PL) spectra has been investigated for samples with columnar morphology, either with InN columns on original substrates or as free-standing nanowires. Valuable information about band gap and electron concentration was obtained by line shape analysis. Optical band gaps between 730 and 750 meV and electron concentrations of 8 x 10(17) to 6 x 10(18) cm(-3) were derived from the fit of the PL spectra of different samples. The crystalline quality of the wires was investigated by high-resolution transmission electron microscopy.     

Polymetallated porphyrin ultrathin films as transducing elements for molecular devices and logic gates

Meso-tetrapyridylporphyrins peripherally coordinated to four ruthenium complexes, such as [Ru(bpy)2Cl] and [Ru(5-ClPhen)2Cl] (bpy = 2,2'-bipyridine; Phen = 1,10-phenanthroline), provide a versatile class of molecular materials in which the complexes act as co-factors, inducing electronic effects and acting as electron-transfer relays and electron pools or sinks, depending upon their oxidation state. These cationic porphyrins can be assembled into thin films by conventional methods, or into organized layer-by-layer structures by combining with negatively charged tetrasulfonated porphyrins or phthalocyanines. Their electrocatalytic and photoelectrochemical properties have been successfully exploited in chemical sensors. Their usefulness in molecular logic gates are being demonstrated by using modified transparent conducting electrodes in miniaturized flow injection cells. In such designs, the chemical, electrochemical, and light inputs can be readily combined to perform the basic logic functions, such as AND, OR, and NOT, for molecular computing.     

Robustness as a criterion for use of hollow clay masonry units in seismic zones: an attempt to propose the measure

In order to provide adequate seismic behavior of masonry walls, local brittle failure of masonry units in the most stressed zones of structural walls should be prevented. Although robust behavior is required by the code, no specifications are given regarding the criteria to fulfill this requirement. To propose such criteria, a series of 28 masonry walls, built with six different types of hollow clay masonry units, currently available on the market, have been tested by subjecting them to cyclic lateral load at two levels of constant precompression. Besides, the strength characteristics of the units, like compressive strength orthogonal and parallel to the bed joints and tensile and shear strength of the units have been determined by standardized and specifically designed testing procedures. By correlating the parameters of seismic resistance of the walls and strength characteristics of the units, no specific indicator for robustness could have been determined on the basis of the mechanical characteristics of the tested units. It has been found that in all cases the level of precompression, i.e. the ratio between the compressive stresses in the walls and the compressive strength of masonry, represents the governing parameter.     

A novel maximum power point tracker based on analog and digital control loops

In this paper, a new maximum power point tracking (MPPT) system for photovoltaic applications is presented. The proposed system consists of two analog loops, a current and a voltage loop, built around a boost converter and a digital loop for the computation of the maximum power point. The analog loops provide a fast response to sudden changes of irradiance conditions while the digital loop, enhancing the accuracy, allows the implementation of various MPPT algorithms and facilitates the integration of additional control and monitoring features. Different existing systems are simulated and compared. A small signal model for the proposed system is developed and a novel compensation method is found. The compensation is designed in such a way that it makes the system independent of the input panel used and also of the load connected at the output. In order to have fast simulation results, a macromodel is developed allowing comparison between different algorithms. Furthermore the “perturb and observe” (P&O) algorithm is improved to obtain a better response for changing irradiance conditions.