Photoluminescence properties of BaMoO4 amorphous thin films

BaMoO₄ amorphous and crystalline thin films were prepared from polymeric precursors. The BaMoO₄ was deposited onto Si wafers by means of the spinning technique. The structure and optical properties of the resulting films were characterized by FTIR reflectance spectra, X-ray diffraction (XRD), atomic force microscopy (AFM) and optical reflectance. The bond Mo-O present in BaMoO₄ was confirmed by FTIR reflectance spectra. XRD characterization showed that thin films heat-treated at 600 and 200 °C presented the scheelite-type crystalline phase and amorphous, respectively. AFM analyses showed a considerable variation in surface morphology by comparing samples heat-treated at 200 and 600 °C. The reflectivity spectra showed two bands, positioned at 3.38 and 4.37 eV that were attributed to the excitonic state of Ba²⁺ and electronic transitions within MoO²⁻₄, respectively. The optical band gaps of BaMoO₄ were 3.38 and 2.19 eV, for crystalline (600 °C/2 h) and amorphous (200 °C/8 h) films, respectively. The room-temperature luminescence spectra revealed an intense single-emission band in the visible region. The PL intensity of these materials was increased upon heat-treatment. The excellent optical properties observed for BaMoO₄ amorphous thin films suggested that this material is a highly promising candidate for photoluminescent applications.     

Kinetics and mechanisms of gas phase elimination of ethyl 1‐piperidine carboxylate,ethyl pipecolinate,and (revisited) ethyl 1‐methyl pipecolinate

The kinetics of the gas‐phase elimination of the title compounds has been determined in a static reaction system over the temperature range of 340–420°C and pressure range of 45–96 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first‐order rate law. The estimated rate coefficients are represented by the following Arrhenius expressions: Ethyl 1‐piperidine carboxylate Ethyl pipecolinate Ethyl 1‐methyl pipecolinate The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The mechanism of this elimination reactions is suggested on the basis of the kinetic and thermodynamic parameters. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 383–389, 2005     

Photoluminescent BaMoO4 nanopowders prepared by complex polymerization method (CPM)

The BaMoO₄ nanopowders were prepared by the Complex Polymerization Method (CPM). The structure properties of the BaMoO₄ powders were characterized by FTIR transmittance spectra, X-ray diffraction (XRD), Raman spectra, photoluminescence spectra (PL) and high-resolution scanning electron microscopy (HR-SEM). The XRD, FTIR and Raman data showed that BaMoO₄ at 300 °C was disordered. At 400 °C and higher temperature, BaMoO₄ crystalline scheelite-type phases could be identified, without the presence of additional phases, according to the XRD, FTIR and Raman data. The calculated average crystallite sizes, calculated by XRD, around 40 nm, showed the tendency to increase with the temperature. The crystallite sizes, obtained by HR-SEM, were around of 40-50 nm. The sample that presented the highest intensity of the red emission band was the one heat treated at 400 °C for 2 h, and the sample that displayed the highest intensity of the green emission band was the one heat treated at 700 °C for 2 h. The CPM was shown to be a low cost route for the production of BaMoO₄ nanopowders, with the advantages of lower temperature, smaller time and reduced cost. The optical properties observed for BaMoO₄ nanopowders suggested that this material is a highly promising candidate for photoluminescent applications.     

Analysis of 5-hydroxy-7-methoxyflavones by normal-phase high-performance liquid chromatography

The separation and identification of flavones present in a chloroform extract of Baccharis trinervis leaves was investigated. The chromatographic system consisted of a amino-bonded column, gradient elution from hexane-chloroform (85:15) to chloroform-acetonitrile (40:60) and detection at 346 nm. Four flavones were found. From NMR and MS data they were identified as 5-hydroxy-7,4′-dimethoxyflavone (I), 5-hydroxy-7,3′,4′-trimethoxyflavone (II), 5,3′-dihydroxy-7,4′-dimethoxyflavone (III) and 5,4′-dihydroxy-7-methoxyflavone (IV). Flavone II and III have not been found in Baccharis trinervis before. The chromatographic system showed good selectivity for the separation of the flavones. The relation between t_R and the structure is discussed.     

Cytotoxic lignans from the stems of Styrax camporum (Styracaceae)

An ethanolic extract from the stems of Styrax camporum Pohl (Styracaceae), a plant traditionally used for gastrointestinal diseases, was fractionated and subjected to flash chromatography and afforded two benzofuran lignans, egonol and homoegonol, and one furofuran lignan, (+/-)syringaresinol, which were identified by spectral data interpretation. Their cytotoxic activities against Hep-2 (larynx epidermoid carcinoma), HeLa (human cervix carcinoma) and C6 (rat glioma) cell lines were evaluated using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) assay at several concentrations for 24h. Activities could be observed for egonol against C6 (IC50 = 3.2 microg/mL) and Hep-2 (IC50 = 3.6 microg/mL) cell lines, and for homoegonol against C6 (IC50 = 4.9 microg/mL) and HeLa (IC50 = 5.3 microg/mL) cells.     

Synthesis and structures of organoantimony compounds containing intramolecular Sb-N interactions

Stibines containing the pendant arm [2-(Me₂NCHR)C₆H₄] (where R = H or Me) were synthesized, and their reactions with CH3I and HBr have been carried out to obtain the diammonium salt {[2-(Me₂NCHR)C₆H₄] [2-(Me₃N⁺CHR)C₆H₄]}Sb 2[I]⁻ (where R = H 3; Me 4) and triammonium salt {2-[(Me₂HN⁺CH₂)C₆H₄]₃}Sb 3[Br]⁻5, respectively. A novel platinum complex 6 [PtCl₂ · 1] containing stibine 1 as a bidentate ligand has also been prepared. All these compounds show Sb-N interactions. A good conjunction was observed between semi-empirical method and ¹H and ¹³C NMR (at different temperature) data for the diammonium salts of compounds 3 and 4. The structures of all the synthesized compounds were determined by X-ray diffraction analyses. This appears to be the second molecular structure of a Pt stibine complex containing a Pt-Cl bond trans to stibine ligand, as few Pt-Sb X-ray structures are known.     

Thermal treatment of poly(ethylene oxide)-segmented copolyimide based membranes: An effective way to improve the gas separation properties

Copolyimide membranes with different poly(ethylene oxide) (PEO) content (from 28 to 68 wt percent, wt.%) have been thermally treated at different temperatures (from 200 to 300 °C) to evaluate the effect of the thermal protocol on the gas transport properties to O₂, N₂, CO₂ and CH₄. The permeability coefficients (P) for all gases increased after the thermal treatment of the membranes and were related to the PEO content, being this enhancement higher for membranes with lower PEO content. Thermal treatment at 300 °C of the membranes with 28 and 43 wt.% of PEO, yielded more productive materials for CO₂/N₂ separation since the permeability coefficients for CO₂ (PCO₂$P_{\text{C}{\text{O}}_2}$) increased 9.8 and 3.2 times, respectively, while the selectivity just suffered a small drop (less than 1.3 times in both cases). Overall, the membrane with 43 wt.% of PEO exhibited the best performance, with a PCO₂$P_{\text{C}{\text{O}}_2}$ of 78 Barrer and a CO₂/N₂ selectivity of 52. For CO₂/CH₄ separation, an increase on selectivity of 1.8 times was obtained in the copolyimide with 43 wt.% of PEO, reaching the selectivity a value of 18. This enhancement of productivity has been associated to an improvement of phase segregation.     

Pore morphology and interconnectivity in a mesoporous/macroporous polyhedral silica foam material

The pore system of a highly swollen, block-copolymer-templated, polyhedral silica foam material is investigated by a combination of transmission electron microscopy, nitrogen sorption, and NMR cryoporometry. The adsorption-desorption hysteresis and melting-freezing hysteresis data recorded by the respective methods provide pore volume and access channel sizes that virtually coincide for the two used methods. This provides a consistent picture where polyhedral foam cells of 60-70 nm diameter are interconnected by cylindrical access channels with several characteristic sizes for the latter.     

Transient mixed convection in a channel with two facing discretely heated semicircular cavities: Buoyancy,inclination angle,and channel aspect ratio effects

Space-averaged surface temperature distributions and overall Nusselt number measurements have been carried out to study the transient mixed convection heat transfer in a channel with two facing and symmetrically heated semicircular cavities. Effects of buoyancy, channel orientation, and channel aspect ratio on thermal behavior have been investigated from Re 500 to 1,500. Depending on the parametric set, steady, oscillatory, and irregular thermal regimes have been identified. The natural frequencies and time scales of the oscillatory regimes have been obtained using spectral analysis. Results show that with increase in channel aspect ratio, the heat transfer performance reduces for all inclination angles.