Study of the electrical properties of pulsed laser ablated (Ba0.5Sr0.5)TiO3 thin films

The laser ablated barium strontium titanate (BST) thin films were characterized in terms of composition, structure, microstructure and electrical properties. Films deposited at 300 degrees C under 50 mTorr oxygen pressure and 3 J cm(-2) laser fluence and further annealed at 600 degrees C in flowing oxygen showed a dielectric constant of 467 and a dissipation factor of 0.02. The room-temperature current-voltage characteristics revealed a space charge limited conduction (SCLC) mechanism, though at low fields the effect of the electrodes was predominant. The conduction mechanism was thoroughly-investigated in terms of Schottky emission at low fields, and bulk-limited SCLC at high fields. The change over to the bulk-limited conduction process from the electrode-limited Schottky emission was, attributed to the process of tunneling through the electrode interface at high fields resulting into the lowering of the electrode contact resistance and consequently giving rise to a bulk limited conduction process. The predominance of SCLC mechanism in the films suggests that the bulk properties are only revealed if the depletion width at the electrode interface is thin enough to allow the tunneling process to take place. This condition is only favorable if the him thickness is high or if the doping concentration is high enough. In the present case the film thickness ranged from 0.3 to 0.7 mu m which was suitable to show the transition mentioned above. (C) 1999 Elsevier Science S.A. All rights reserved.

Synthetic studies towards novel terpenic natural products kelsoene and poduran: Construction of the complete 4-5-5-fused tricarbocyclic core

A common synthetic approach to the recently reported sesquiterpene kelsoene 1 and the tetraterpene poduran 5, bearing a novel tricyclo[6.2.0.0(2,6)]decane framework, from commercially available 1,5-COD and leading to the first construction of the carbocyclic core present in these natural products is delineated.

Temperature dependence study of chlorine NQR frequency and spin-lattice relaxation time in 2-amino,-3,5-dichloropyridine

Chlorine-35 NQR frequency and spin-lattice relaxation time measurements as a function of temperature in the range 77-300 K were carried out on 2-amino-3,5-dichloropyridine. Two NQR signals were observed and were assigned to the two chlorines present in the molecule using the additive model for substituent effects. The temperature dependence of the NQR frequency was analysed in terms of the torsional oscillations of the molecule and the torsional frequencies and their temperature dependence were calculated numerically using a two-mode approximation. The temperature dependence of the NQR spin-lattice relaxation time was found to be mainly due to the torsional oscillations of the molecule, with anharmonicity effects showing up at higher temperatures. Copyright (C) 1999 John Wiley & Sons, Ltd.

Grain size effects on charge-ordering, phase segregation and related properties of rare-earth manganates, Nd(0.5)A(0.5)MnO(3) (A = Ca or Sr)

Grain size has marked effects on charge-ordering and other properties of Nd(0.5)A(0.5)MnO(3) (A=Ca or Sr). Thus, the anti-ferromagnetic (AFM) transition in Nd0.5Ca0.5MnO3 is observed distinctly only in samples sintered at 1273 K or higher. The sample with a small grain size (sintered at 1173 K) shows evidence for greater ferromagnetic (FM) interaction at low temperatures, probably due to phase segregation. The FM transition as well as the charge-ordering transition in Nd0.5Sr0.5MnO3 becomes sharper in samples sintered at 1273 K or higher. The sample sintered at 1173 K does not show the AFM-CO transition around 150 K and is FM down to low temperatures; the apparent T-c-T-co gap decreases with the increase in the grain size. The samples sintered at lower temperatures (<1673 K) show evidence for greater segregation of the AFM and FM domains. (C) 2002 Elsevier Science Ltd. All rights reserved.

Influence of HZSM-5 catalyst on the thermal degradation of poly(vinyl chloride) in solution

The kinetics of thermal degradation of poly(vinyl chloride) (PVC) in solution was investigated at various temperatures (210-250degreesC). The degradation rate coefficients were determined from the time evolution of the molecular weight distribution (MWD). The energy of activation, determined from the temperature dependence of the rate coefficient, was 26.6 kcal/mol. The degradation of PVC was also studied in the presence of a catalyst (HZSM-5 zeolite). The results indicated that increase of the degradation rate of PVC is first order with the HZSM-5 concentration up to 50 g/L and zero order at higher concentrations. The thermal degradation kinetics of PVC in the presence of 50 g/L of the catalyst was studied at various temperatures. The temperature dependency of the rate coefficient was used to calculate the activation energy (21.5 kcal/mol). This is consistent with the observation that the presence of a catalyst generally decreases the activation energy and promotes degradation. (C) 2002 John Wiley Sons, Inc.

Investigation of reversible and irreversible polarizations in thin films of SrBi2(Ta-0.5,Nb-0.5)(2)O-9

The reversible and irreversible components of the total polarization in a thin film of SrBi2(Ta-0.5,Nb-0.5)(2)O-9 were calculated. The C-V loop was integrated to obtain the reversible part of the total polarization. The reversible polarization was only 20% of the total polarization and showed almost no hysteresis. However, the dielectric constant due to the total polarization was almost the same as that for the reversible polarization in the saturation region of the large signal P-E hysteresis loop. The reversible part was subtracted from the total polarization to calculate the irreversible counterpart of it. The irreversible polarization showed a near-square shaped hysteresis loop, while the reversible polarization was obeying the Rayleigh law. The small signal hysteresis was simulated from the parameters obtained from the Rayleigh-curve fit with the experimental curve and then it was compared with the result obtained from direct measurement with small amplitude. (C) 2002 Elsevier Science B.V. All rights reserved.

Nanotubes of the disulfides of groups 4 and 5 metals

The layered chalcogenides, having structures analogous to graphite, are known to be unstable toward bending and show high propensity to form curved structures, thus eliminating dangling bonds at the edges. Since the discovery of fullerene and nanotube structures of WS2 and MoS2 by Tenne et al. [1-3], there have been attempts to prepare and characterize nanotubes of other layered dichalcogenides with structures analogous to MoS2. Nanotubes of MoS2 and WS2 were prepared by Tenne et al. by reducing the corresponding oxides to the suboxides followed by heating in an atmosphere of forming gas (5 % H-2 + 95 % N-2) and H2S at 700-900 degreesC [1-3]. Alternative methods of synthesis of MoS2 and WS2 nanotubes have since been proposed by employing the decomposition of the ammonium thiometallates or the corresponding trisulfide precursors. This alternative procedure was based on the observation that the trisulfide seems to be formed as an intermediate in the synthesis of the MoS2 and WS2 nanotubes [4]. Accordingly, the decomposition of the trisulfides of MoS2 and W in a reducing atmosphere directly yielded nanotubes of the disulfides MoS2 and WS2 [5]. In this article, we describe the synthesis, structure, and characterization of a few novel nanotubes of the disulfides of groups 4 and 5 metals. These include nanotubes of NbS2, TaS2, ZrS2, and HfS2. The study enlarges the scope of the inorganic nanotubes significantly and promises other interesting possibilities, including the synthesis of the diselenide nanotubes of these metals.

A comparison of the DNMR behaviour of methyl 5,6-bis(2-methoxyphenyl)-1,4-dimethyl-7-oxobicyclo[2.2.1]hept-5-en-2-endo-carboxylate and its 7-oxa analogue

Methyl 5,6-Bis(2-methoxyphenyt)-1,4-dimethyl-7-oxobicyclo[2.2.1]hept-5-en-2-endo-carboxylate, a moderately crowded norbornenone ester, exhibits complex VT-DNMR behaviour. A similar behaviour is not seen in its 7-oxa analogue, showing that conformational transmission from position 7 has a crucial influence on the distance parameters that govern the dynamic processes involving the substituents on the bicycloheptene framework.

Phenobarbitone-mediated translocation of the cytosolic proteins interacting with the 5 '-proximal region of rat liver CYP2B1/B2 gene into the nucleus

The positive element (PE) (-69 to -98 bp) within the 5'-proximal region of the CYP2B1B2 gene (+1 to -179 bp) of rat liver is essential for phenobarbitone (PB) response and gives a single major complex with the rat liver cytosol in gel shift analysis. This complex corresponds to complex I (top) of the three complexes given by the nuclear extracts. PB treatment of rats leads to a decrease in complex I formation with the cytosol and PE and an increase in the same with the nuclear extract in gel shift analysis. Both the changes are counteracted by simultaneous okadaic acid administration. The nuclear protein giving rise to complex I has been isolated and has an M-r of 26 kDa. The cytosolic counterpart consists of two species, 26 and 28 kDa, as revealed by Southwestern blot analysis using labeled PE. It is concluded that PB treatment leads to the translocation accompanied by processing of the cytosolic protein species into the nucleus that requires protein dephosphorylation. It is suggested that PB may exert a global regulation on the transcription of many genes by modulating the phosphorylation status of different protein factors involved in transcriptional regulation. (C) 2002 Elsevier Science (USA).

Synthesis and structure of La14V6CuO36.5: a transparent Cu(I) vanadate containing [OCuO](3-) sticks

The title compound, La14V6CuO36.5, was prepared from a stoichiometric mixture of La2O3,V2O5, and CuO at 1050-1080 degreesC. The compound forms transparent, pale green crystals and was characterized by wavelength dispersive spectroscopy and single crystal X-ray diffraction. The structure contains isolated VO43- tetrahedra and [OCuO](3-) sticks dispersed in a lanthanum oxide network. Films of La14V6CuO36.5 were grown on R-plane sapphire by using pulsed laser deposition. Rutherford backscattering spectroscopic and X-ray diffraction analyses of the films showed oriented growth of the title phase, a similar to5 eV optical band gap and n-type conductivity. The compound is an example of a transparent copper(I) oxide.

Thermodynamic investigation of the MOCVD of copper films from bis(2,2,6,6-tetramethyl-3,5-heptadionato)copper(II)

Equilibrium concentrations of various condensed and gaseous phases have been thermodynamically calculated, using the free energy minimization criterion, for the metalorganic chemical vapour deposition (MOCVD) of copper films using bis(2,2,6,6-tetramethyl-3,5-heptadionato)copper(II) as the precursor material. From among the many chemical species that may possibly result from the CVD process, only those expected on the basis of mass spectrometric analysis and chemical reasoning to be present at equilibrium, under different CVD conditions, are used in the thermodynamic calculations. The study predicts the deposition of pure, carbon-free copper in the inert atmosphere of argon as well as in the reactive hydrogen atmosphere, over a wide range of substrate temperatures and total reactor pressures. Thin films of copper, grown on SiO2/Si(100) substrates from this metalorganic precursor by low pressure CVD have been characterized by XRD and AES. The experimentally determined composition of CVD-grown copper films is in reasonable agreement with that predicted by thermodynamic analysis.

A convenient enantiospecific methodology for (-)-(1R,5S)-1,5-dimethylbicyclo[3.3.0]octa-2,7-dione: A formal total synthesis of (-)-ceratopicanol

An enantiospecific formal total synthesis of (-)-ceratopicanol starting from the readily and abundantly available monoterpene (R)-limonene is described. A combination of Claisen rearrangement-intramolecular diazo-ketone cyclopropanation-regiospecific reductive cyclopropane cleavage reactions are employed for the stereo- and regiospecific generation of the two vicinal ring junction quaternary carbon atoms.

Formation of amorphous xenon nanoclusters and microstructure evolution in pulsed laser deposited Ti(62.5)Si(37.5) thin films during Xe ion irradiation

As deposited amorphous and crystallized thin films of Ti 37.5% Si alloy deposited by pulsed laser ablation technique were irradiated with 100 keV Xe(+) ion beam to an ion fluence of about 10(16) ions-cm(-2). Transmission electron microscopy revealed that the implanted Xe formed amorphous nanosized clusters in both cases. The Xe ion-irradiation favors nucleation of a fcc-Ti(Si) phase in amorphous films. However, in crystalline films, irradiation leads to dissolution of the Ti(5)Si(3) intermetallic phase. In both cases, Xe irradiation leads to the evolution of similar microstructures. Our results point to the pivotal role of nucleation in the evolution of the microstructure under the condition of ion implantation.

Structural and dielectric properties of Bi2NbxVi−xO5.5 ceramics

Bi2Nbx V1−xO5.5 ceramics with x ranging from 0.01 to 0.5 have been prepared. The crystal system transforms from an orthorhombic to tetragonal at x 3= 0.1 and it persists until x = 0.5. Scanning electron microscopic (SEM) investigations carried out on thermally etched Bi2NbxV1−xO5.5 ceramics confirm that the grain size decreases markedly (18 μm to 4 μm) with increasing x. The shift in the Curie temperature (725 K) toward lower temperatures, with increasing x, is established by Differential Scanning Calorimetry (DSC). The dielectric constants as well as the loss tangent (tan δ) decrease with increasing x at room temperature.

Standard Gibbs energy of formation of Pb2Ru2O6.5

Lead ruthenate is used as a bifunctional electrocatalyst for both oxygen evolution and reduction and as a conducting component in thick-film resistors. It also has potential applications in supercapacitors and solid oxide fuel cells. However, thermodynamic properties of the compound have not been reported in the literature. The standard Gibbs energy of formation has now been determined in the temperature range from 873 to 1123 K using a solid-state cell incorporating yttria-stabilized zirconia (YSZ) as the electrolyte, a mixture of PbO + Pb2Ru2O6.5 + Ru as the measuring electrode, and Ru + RuO2 as the reference. The design of the measuring electrode is based on a study of phase relations in the ternary system Pb–Ru–O at 1123 K. For the reaction,S0884291400095625_eqnU1 the standard enthalpy of formation and standard entropy at 298.15 K are estimated from the high-temperature measurements. An oxygen potential diagram for the system Pb–Ru–O is composed based on data obtained in this study and auxiliary information from the literature