The global element method and its extension to parabolic problems

This article describes the derivation of the global element method, originally proposed by Delves and Hall, using the classical Euler–Lagrange theory. The method is then extended so as to be applicable to parabolic partial differential equations. The main advantages of the method, the ease of adjusting its accuracy and applying it to problems with discontinuous coefficients and point singularities, are also discussed.     

Synthesis, complete 1H and 13C NMR assignment and crystal structure of novel epoxide derivatives of cytochalasin B

Five novel epoxide derivatives of cytochalasin B were synthesized. Reaction of cytochalasin B with t-BHP and BuLi led to selective epoxidation of the C-21/22 double bond to give a single monoepoxide, while reaction with m-CPBA yielded two diepoxides. Reaction of the monoepoxide with m-CPBA yielded two triepoxides. The relative configurations of the epoxides were elucidated by analogy with the natural product by means of spectroscopic methods; full assignment of NMR signals was achieved, and the absolute configuration was confirmed by X-ray crystallography.     

Luminescence from Ce in sol–gel SiO2

The sol–gel process provides an attractive low temperature alternative to the melt process for producing Ce-doped silica, but reports of the emission wavelength have not been consistent. In this paper, luminescence measurements using a variety of excitation methods, including cathodoluminescence not yet reported by other researchers, are compared and evaluated in the light of previously published data. Several papers report luminescence around 350 nm but emission near this wavelength was not found from our samples. This luminescence originates from Ce that has not yet been incorporated in the silica and is found in samples that have not undergone high temperature annealing. Our photoluminescence results from samples annealed in a reducing atmosphere suggest that emission from Ce incorporated in the silica lattice occurs near 455 nm, and some indication of the emission from Ce in amorphous clusters at 400 nm is also found. However, our results also confirm earlier indications that intrinsic defects in silica can create photoluminescence near both these wavelengths, which can make identification of the luminescence due to Ce difficult. Finally, it has been found that samples which have been annealed in air, and therefore display poor photoluminescence because most of the Ce occurs in the tetravalent form, are luminescent under electron beam excitation. It is suggested that during cathodoluminescence measurements Ce⁴⁺ ions capture electrons to form excited Ce³⁺ ions from which the luminescence originates.     

Effect of precursor concentration on the growth of zinc oxide nanorod arrays on pre-treated substrates

Well aligned zinc oxide nanorod arrays (ZNAs) synthesized by a simple chemical bath deposition method were fabricated on pre-treated Si substrates. By keeping the molar VI/II ratio constant, the effect of precursor concentration on the growth and optical quality of the ZNAs was investigated. The as-synthesized ZNAs were characterized by field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD) and photoluminescence spectroscopy (PL). FESEM images show that both the diameter and aspect ratio of the ZNAs increase dramatically as the precursor concentration increases. The XRD analysis indicates that all the as-grown ZNAs are crystalline and are preferentially oriented along the c-axis. The high intensity ratio of the UV emission to visible emission in the room temperature PL spectra illustrate that high optical quality ZNAs were produced.     

Thermoluminescent and stuctural properties of BaAl2O4:Eu,Nd,Gdphosphors prepared by combustion method

BaAl₂O₄:Eu²⁺,Nd³⁺,Gd³⁺ phosphors were prepared by a combustion method at different initiating temperatures (400–1200 °C), using urea as a comburent. The powders were annealed at different temperatures in the range of 400–1100 °C for 3 h. X-ray diffraction data show that the crystallinity of the BaAl₂O₄ structure greatly improved with increasing annealing temperature. Blue-green photoluminescence, with persistent/long afterglow, was observed at 498 nm. This emission was attributed to the 4f⁶5d¹–4f⁷ transitions of Eu²⁺ ions. The phosphorescence decay curves were obtained by irradiating the samples with a 365 nm UV light. The glow curves of the as-prepared and the annealed samples were investigated in this study. The thermoluminescent (TL) glow peaks of the samples prepared at 600 °C and 1200 °C were both stable at ∼72 °C suggesting that the traps responsible for the bands were fixed at this position irrespective of annealing temperature. These bands are at a similar position, which suggests that the traps responsible for these bands are similar. The rate of decay of the sample annealed at 600 °C was faster than that of the sample prepared at 1200 °C.     

Role of VI/II ratio on the growth of ZnO nanostructures using chemical bath deposition

In this paper the growth process and morphological evolution of ZnO nanostructures were investigated in a series of experiments using chemical bath deposition. The experimental results indicate that the morphological evolution depends on the reaction conditions, particularly on OH⁻ to Zn²⁺ ratio (which directly affects the pH). For low VI/II ratios, quasi-spherical nanoparticles of an average diameter 30 nm are obtained, whereas for larger VI/II ratios, nanorods with an average diameter less than 100 nm are produced, which indicates that by systematically controlling the VI/II ratio, it is possible to produce different shapes and sizes of ZnO nanostructures. A possible mechanism for the nanostructural change of the as-synthesized ZnO from particle to rod was elucidated based on the relative densities of H⁺ and OH⁻ in the solution.     

Influence of Gas Chromatographic Parameters on Determination of Decabromodiphenyl Ether

In this paper, we present a study on the influence of chromatographic parameters on determination of decabromodiphenyl ether in an attempt to arrive at optimum conditions that enhance its detection in environmental samples. Gas chromatography?Celectron capture detector (GC?CECD) equipped with digital pressure and gas flow control, ZB-5 capillary column of 15 and 30 m length (0.25 mm id, 0.25 ??m d _f) are used. 1 ??L of 20 ng ??L^?1 commercial decabromodiphenyl ether mixture dissolved in toluene is injected into the column in splitless mode and each parameter varied. Injection temperature of 290 °C, initial and final oven temperatures of 90 °C and 300?C310 °C, respectively, 1.0 min splitless time and 2.5 mL min^?1 flow rate found to be optimum on 15 m column. Under optimum condition, entrance of BDE209 into the column is improved; retention time reduced by a factor of three and high repeatability with almost no degradation observed. Finally, the developed method is validated using standard reference material and applied to indoor dust samples. The average concentrations of decabromodiphenyl ether found from dust analysis in ng g^?1 are 118 ± 6.6, 103 ± 11 and 26 ± 1.6 in hotel, office and computer room, respectively.