Deposition of aluminide and silicide based protective coatings on niobium

We compare aluminide and alumino-silicide composite coatings on niobium using halide activated pack cementation (HAPC) technique for improving its oxidation resistance. The coated samples are characterized by SEM, EDS, EPMA and hardness measurements. We observe formation of NbAl₃ in aluminide coating of Nb, though the alumino-silicide coating leads to formation primarily of NbSi₂ in the inner layer and a ternary compound of Nb-Si-Al in the outer layer, as reported earlier (Majumdar et al. [11]). Formation of niobium silicide is preferred over niobium aluminide during alumino-silicide coating experiments, indicating Si is more strongly bonded to Nb than Al, although equivalent quantities of aluminium and silicon powders were used in the pack chemistry. We also employ first-principles density functional pseudopotential-based calculations to calculate the relative stability of these intermediate phases and the adhesion strength of the Al/Nb and Si/Nb interfaces. NbSi₂ exhibits much stronger covalent character as compared to NbAl₃. The ideal work of adhesion for the relaxed Al/Nb and Si/Nb interfaces are calculated to be 3226 mJ/m² and 3545 mJ/m², respectively, indicating stronger Nb-Si bonding across the interface.     

Optical and electrical studies of as-prepared and annealed Se-Te-Bi thin films

The effect of thermal annealing on the electrical and optical properties of a thin film of Se-Te-Bi alloy has been studied. It has been found that the mechanism of optical absorption follows a non-direct transition. The optical band-gap decreases with an increase in the Bismuth concentration. The electrical conductivity of the as-deposited and annealed films has been found to be of Arrhenius type. The results are discussed on the basis of rearrangements of defects and disorders in the chalcogenide systems. PACS 71.23.An; 73.61.-r; 73.61Jc; 74.25Gz; 78.66Jg     

Kinetics and mechanism of the oxidation of diols by pyridinium bromochromate

The kinetics of oxidation of four vicinal diols, four nonvicinal diols, and one of their monoethers by pyridinium bromochromate (PBC) have been studied in dimethyl sulfoxide. The main product of oxidation is the corresponding hydroxyaldehyde. The reaction is first-order with respect to each the diol and PBC. The reaction is acid-catalyzed and the acid dependence has the form: k_obs=a+b[H⁺]. The oxidation of [1,1,2,2-²H₄]ethanediol exhibited a primary kinetic isotope effect (k_H/k _D=6.70 at 298 K). The reaction has been studied in 19 organic solvents including dimethyl sulfoxide and the solvent effect has been analyzed using multiparametric equations. The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical transition state in the rate-determining step. A suitable mechanism has been proposed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 285–290, 1998.     

Kinetics of nonisothermal crystallization in Sn<Subscript>10</Subscript>Sb<Subscript>20−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>70</Subscript> glassy semiconductors

The crystallization kinetics of Sn₁₀Sb_20−x Bi _x Se₇₀ (x=0,2,4,6,8) chalcogenide system has been studied. Crystallization studies using differential scanning calorimetry under nonisothermal conditions with different heating rates are reported. The glass transition temperature is found to increase with the increase in heating rate as well as with the addition of bismuth. The apparent activation energy for glass transition and that for crystallization have been determined using the Kissinger equation. Thermal stability and glass forming tendency have also been studied.     

Isolation and Crystal Structure of 6α,7α-Epoxy-5α,17α,27-trihydroxy-1-oxo-22R-witha-2,24-dienolide monohydrate-A Withasteroid from Withania somnifera Leaves

Abstract  

(5R,6S,7S,8S,9S,10R,13S,14S,17S,20R,22R)-6α,7α-epoxy-5α,17α,27-trihydroxy-1-oxo-22R-witha-2,24-dienolide has been isolated from Withania somnifera leaves. Three-dimensional X-ray intensity data were collected at 293 K on Bruker CCD area-detector diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 ?). The compound crystallizes in monoclinic space group P21 with unit cell parameters: a = 6.4540(2), b = 11.3656(4), c = 17.4982(5) ?, β = 90.730 ^o(2), Z = 2. The crystal structure was refined to a final R-value of 0.0419 for 6122 observed reflections. One water solvent molecule was observed. All the rings of the steroid skeleton are trans connected. Rings A and B exists in a half-chair conformation, ring C a chair, and five membered ring D is intermediate between half-chair and envelope. The δ-lactone ring E adopts a distorted sofa conformation. The characteristic pattern observed in the packing diagram is the appearance of twisted chains of molecules. The packing of the molecules in the crystal is stabilized by intra- and intermolecular O–H···O and C–H···O interactions.     

Continuous glucose detection using boronic acid-substituted viologens in fluorescent hydrogels: linker effects and extension to fiber optics

A fluorescent anionic dye and a viologen appended with boronic acids, which serve as glucose receptors, have been synthesized and immobilized into a poly(2-hydroxyethyl methacrylate) hydrogel for use as a continuous glucose monitor. The fluorescence of the dye is modulated by the quenching efficiency of the viologen-based receptor, which in turn is dependent on the glucose concentration. Two monomeric versions of the quencher/receptor unit were prepared and their performance within the hydrogel evaluated. By tethering the quencher/receptor to the hydrogel matrix using a single-point attachment, slightly improved glucose sensing was observed. The hydrogels were tested for their ability to continuously and reversibly detect glucose over the course of several hours. The tests were carried out using a cuvette-based system, as well as a fiber-optic-based configuration. Under physiological conditions (0.1 M phosphate buffer, pH 7.4, 37 degrees C), the fluorescent hydrogels display an excellent dynamic response to glucose concentrations within the biologically significant range (2.5-20 mM).     

Dihydroxyacetone variants in the organocatalytic construction of carbohydrates: mimicking tagatose and fuculose aldolases

Dihydroxyacetone variants have been explored as donors in organocatalytic aldol reactions with various aldehyde and ketone acceptors. The protected form of dihydroxyacetone that was chosen for in-depth study was 2,2-dimethyl-1,3-dioxan-5-one, 1. Among the catalysts surveyed here, proline proved to be superior in terms of yield and stereoselectivities in the construction of various carbohydrate scaffolds. In a fashion analogous to aldolase enzymes, the de novo preparation of L-ribulose, L-lyxose, D-ribose, D-tagatose, 1-amino-1-deoxy-D-lyxitol, and other carbohydrates was accomplished via the use of 1 and proline. In reactions using 2,2-dimethyl-1,3-dioxan-5-one 1 as a donor, (S)-proline can be used as a functional mimic of tagatose aldolase, whereas (R)-proline can be regarded as an organocatalytic mimic of fuculose aldolase.     

Evaluation of relative importance of ultrasound reactor parameters for the removal of estrogen hormones in water

The growing interest in sonochemistry as a tool for environmental remediation leads to the need for process optimization. Sonochemistry is a complex process, which depends on physical parameters and also on the process conditions. Physical parameters are interrelated and therefore a systematic approach has to be taken to optimize the process. The effect of physical parameters on the destruction of seven estrogen hormones (17α-estradiol, 17β-estradiol, estriol, 17α-ethinylestradiol, 17α-dihydroequilin, estrone and equilin) is reported in this study. Artificial neural networks (ANN) was used as a tool to identify the correlations between these process parameters. ANN enabled the establishment of relationship between sonication parameters such as power density, power intensity, ultrasound amplitude, as well as the reactor design parameters. The major significance was attributed to the area-specific power density and the volume-specific power intensity. The results of this work provide a sound basis to design pilot and full-scale ultrasound treatment systems. Process optimization lead to a 5-fold decrease in energy consumption as compared to the commercially available reactors, thereby making the process attractive for field applications.     

Electrochemical stripping voltammetry of gold ions for development of ultra-sensitive immunoassay for chlorsulfuron

The paper reports a highly sensitive enzyme free electrochemical immunoassay (EFEIA) for the detection of herbicide chlorsulfuron. The assay is based upon oxidative gold nanoparticle (GNP) dissolution in an acidic solution. The consequent release of large amounts of gold (Au) metal ions after dissolution of gold nanoparticles tagged to antibody leads to the development of sensitive stripping voltammetry based immunoassay. The detection is made possible by the reduction of Au^3 + ions at the screen printed electrode surface followed by metal analysis by using the square wave voltammetry technique. The sensitivity of chlorsulfuron detection by competitive assay procedure was 6.7 pg mL^− 1 for EFEIA in marked contrast to optical detection using Standard ELISA procedure that gives a sensitivity of 4.97 ng mL^− 1.