BaSnO3 fine powders from hydrothermal preparations

Attempts to prepare BaSnO3 by the hydrothermal method starting from SnO2·xH2O gel and Ba (OH)2 solution in teflonlined autoclaves at 150–260°C invariably lead to the formation of a hydrated phase, BaSn(OH)6·3H2O. On heating in air or on releasing the pressure Image at ≈260°C, BaSN (OH)6·3H2O converts to BaSnO3 fine powder which involves the formation of an intermediate oxyhydroxide, BaSnO(OH)4. TEM studies show that particle size of the resulting BaSnO3 ranges from 0.2–0.6 μm. Solid solutions of Ba(Ti, Sn) O3 were prepared from (TiO2+SnO2)·xH2O mixed gel and Ba(OH)2 solutions. Single-phase perovskite Ba(Ti, Sn)O3 was obtained up to 35 atom % Sn. Above this composition, the hydrothermal products are mixtures of BaTiO3 (cubic) and BaSn(OH)6·3H2O which on heating at ≈260°C give rise to BaTiO3+BaSnO3. Annealing at 1000°C results in monophasic Ba(Ti, Sn)O3, in the complete range of Sn/Ti. Formation of the hydrated phase is attributed to the amphoteric nature of SnO2·xH2O gel which stabilises Sn(OH)62− anions under higher H2O-pressures and elevated temperatures. The sintering characteristics and dielectric properties of ceramics prepared from these fine powders are presented.

Absorption of electromagnetic radiation by superconducting YBa2Cu3O7: an oxygen-induced phenomenon

In the superconducting state, YBa2Cu3O7 absorbs electromagnetic radiation over a wide range of frequencies (8 MHz-9 GHz). The absorption is extremely sensitive to temperature, particle size and the magnetic field and depends crucially on the presence of oxygen. A possible explanation for the phenomenon based on the formation of Josephson junctions is suggested.

Thermal polymerization of acrylamide in the solid state

The role of imperfections in thermal polymerization of acrylamide in the solid state was studied. The polymer yield and the degree of polymerization are highly dependent on the particle size and on the pressure to which the monomer is subjected prior to polymerization reaction. There is an enhancement in the rate of polymerization in air unlike in the case of radiation-induced polymerization. Thermal polymerization of acrylamide in pelletized form results in the formation of water-soluble linear polymer and water-insoluble cross-linked product with the evolution of ammonia. The activation energy (E) values obtained in the present investigation reveal that basically there are two processes taking place, one with E = 34–36 kcal/mole, corresponding to the initiation process, and the other with E = 19 ± 3 kcal/more for the propagation process.

Magnesium/Copper Nanocomposite through Digestive Ripening

Composing nanocomposites: Co-digestive ripening of as-prepared Mg and Cu colloids prepared by the solvated metal atom dispersion method results in a highly monodisperse colloid of Mg/Cu nanocomposite with an average particle size of 3.0 +/- 0.5 nm. Annealing of these samples at 300 degrees C gives the Cu/MgO nanocomposite.

Studies on vacuum pyrolysis of ammonium perchlorate and ammonium perchlorate/polystyrene propellant in the presence of transition metal oxides

Vacuum pyrolysis of ammonium perchlorate (AP) and ammonium perchlorate/polystyrene (PS) propellant has been studied by differential thermal analysis (DTA) in order to observe the effect of transition metal oxides on sublimation. Sublimation and decomposition being competitive processes, their proportions depend on the pressure of the pyrolysis chamber. The enthalpies for complete decomposition and complete sublimation are available from the literature and by using these data together with DTA area measurements, the extents of sublimation and decomposition have been calculated for AP and the propellant system. The effect of the metal ions on the extent and rate of sublimation depends on their nature. For AP the extent of sublimation increases with a decrease in particle size. For the propellants the powder sublimes more readily than the bulk material, but in the presence of metal ions the bulk material sublimes more readily than the powder. To substantiate this finding, the effect of MnO2 on AP sublimation as a function of particle size was examined, and it was observed that the extent of sublimation decreases as the particle size decreases.

Stream power criterion and design of sand bed channels at incipient motion

The critical stream power criterion may be used to describe the incipient motion of cohesionless particles of plane sediment beds. The governing equation relating ``critical stream power'' to ``shear Reynolds number'' is developed by using the present experimental data as well as the data from several other sources. Simultaneously, a resistance equation, relating the ``particle Reynolds number'' to the``shear Reynolds number'' is developed for plane sediment beds in wide channels with little or no transport. By making use of these relations, a procedure is developed to design plane sediment beds such that any two of the four design variables, including particle size, energy/friction slope, flow depth, and discharge per unit width in the channel should be known to predict the remaining two variables. Finally, a straightforward design procedure using design tables/design curves and analytical methods is presented to solve six possible design problems.

Defect Sensitization of Combustion and Thermal Decomposition of Ammonium Perchlorate: Effect of Pelletizing Pressure and Dwell Time

A systematic study was undertaken on the combustion and thermal decomposition of pelletized Ammonium Perchlorate (AP) to investigate the effects of pelletizing pressure and dwell time. At constant pressure, increasing the dwell time results in an increase in the burning rate up to a maximum and thereafter decreases it. The dwell time required for the pellets to have maximum burning rate is a function of pressure. The maximum burning rate is the same for all the pressures used and is also unaffected by increasing, to the range 90-250 μ, the particle size of AP used. In order to explain the occurrence of a maximum in burning rate, pellets were examined for their thermal sensitivities, physical nature and the changes occurring during pelletization with dwell time and pressure. The variations are argued in terms of increasing density, formation of defects such as dislocations leading to an increase in the number of reactive sites, followed by their partial annihilation at longer dwell times due to flow of material during pelletization.

Effect of experimental parameters on non-isothermal TG

The effect of some experimental parameters, namely sample weight, particle size and its distribution, heating rate and flow rate of inert gas, on the fractional decomposition of calcium carbonate samples have been studied both experimentally and theoretical. The general conclusions obtained from theoretical analysis are corroborated qualitatively by the experimental data. The analysis indicates that the kinetic compensating effect may be partly due to the variations in experimental parameters for different experiments.

Precipitation of rutile and anatase (Tio12) fine powders and their conversion to MTiO3 (M = Ba, Sr, Ca) by the hydrothermal method

Fine powders of TiO2 (rutile) with high degree of crystallinity are formed from aqueous titanium oxychloride solution under hydrothermal conditions at 160–230°C and 15–100 kg/cm2 for 1–2 hours. The anatase phase is produced from the same medium when sulfate ion impurity is present, with Image . Both these fine powders are converted to BaTiO3, SrTiO3 or CaTiO3 when suspended in Ba(OH)2 or Sr(OH)2 solution or in an aqueous slurry of carbonate-free CaO with Image , at 180–280°C and 12–65 kg/cm2 for 4–8 hours. The resulting fine powders contain monocrystallites of the perovskite phase with 0.1–1.5 μm particle size.

Kinetics of thermal decomposition of barium zirconyl oxalate

Kinetics of the thermal decomposition of anhydrous barium zirconyl oxalate and a carbonate intermediate have been studied. Decomposition of the anhydrous oxalate, though it could be explained based on a contracting-cube model, is quite complex. Kinetics of decomposition of the intermediate carbonate Ba2Zr2O5CO3 is greatly influenced by thermal effects during its formation. (agr-t) curves are sigmoidal and obey a power law equation followed by first order decay. Presence of carbon in the vacuum-prepared carbonate has a strong deactivating effect. Decomposition of the carbonate is accompanied by growth in particle size of the product barium zirconate.

Novel solid hypergolic fuels for hybrid propellants

A new class of compounds, viz., monothiocarbohydrazones, have been found to be hypergolic with anhydrous and red fuming nitric acids. A study of the ignition delays of the various thiocarbohydrazonenitric acid systems as a function of particle size and fuel/oxidizer ratio reveals no significant effect by these parameters. The observed ignition delays have been explained in terms of the chemical reactivity and structure of these compounds.

Spectrally Resolved Resonance Energy Transfer from ZnO:MgO Nanocrystals

Resonance energy transfer (RET) from the visible emission of core−shell ZnO:MgO nanocrystals to Nile Red chromophores, following band gap excitation in the UV, has been investigated for four different nanocrystal sizes. With use of steady state and time-resolved fluorescence spectroscopic measurements the wavelength dependent RET efficiencies have been determined. The RET process in ZnO:MgO nanocrystals occurs from emissions involving trap state recombination. There are two such processes with different RET efficiencies for the same particle size. This is shown to be a consequence of the fact that the recombination processes giving rise to the two emissions are located at different distances from the center of the particle so that the donor−acceptor distances for the two are different, even for the same particle size.

Thermal decomposition of titanyl oxalates II : Kinetics of decomposition of barium titanyl oxalate

Kinetics of the thermal decomposition of barium titanyl oxalate have been studied. Decomposition of the anhydrous oxalate is complex and deceleratory throughout. Kinetics of decomposition of the intermediate carbonate Ba2Ti2O5CO3 is greatly influenced by the thermal effects during its formation. The sigmoidal (α, t) curves obey a power law equation followed by first order decay. Presence of carbon in the vacuum prepared carbonate has a strong deactivating effect. Decomposition of the carbonate is accompanied by growth in particle size of the product, barium titanate.

Effect of pretreatment on the sublimation of ammonium perchlorate

The effects of pretreatments on the sublimation of pure ammonium perchlorate (AP) were studied by differential thermal analysis. The addition of inorganic salts (doping), or preheating, lead to desensitisation of the sublimation process, whereas it was sensitised by precompression. Sublimation increased with decrease in the particle size of the AP from 500 to 200 microns, but decreased with a further decrease in size from 200 to 100 microns. The results are interpreted in terms of gross imperfections and strain in the AP crystals.

Carbon-Supported Pt-TiO2 as a Methanol-Tolerant Oxygen-Reduction Catalyst for DMFCs

Carbon-supported Pt-TiO2 (Pt-TiO2/C) catalysts with varying at. wt ratios of Pt to Ti, namely, 1:1, 2:1, and 3:1, are prepared by the sol-gel method. The electrocatalytic activity of the catalysts toward oxygen reduction reaction (ORR), both in the presence and absence of methanol, is evaluated for application in direct methanol fuel cells (DMFCs). The optimum at. wt ratio of Pt to Ti in Pt-TiO2/C is established by fuel cell polarization, linear sweep voltammetry, and cyclic voltammetry studies. Pt-TiO2/C heattreated at 750 degrees C with Pt and Ti in an at. wt ratio of 2:1 shows enhanced methanol tolerance, while maintaining high catalytic activity toward ORR. The DMFC with a Pt-TiO2/C cathode catalyst exhibits an enhanced peak power density of 180 mW/cm(2) in contrast to the 80 mW/cm(2) achieved from the DMFC with carbon-supported Pt catalyst while operating under identical conditions. Complementary data on the influence of TiO2 on the crystallinity of Pt, surface morphology, and particle size, surface oxidation states of individual constituents, and bulk and surface compositions are also obtained by powder X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive analysis by X-ray, and inductively coupled plasm optical emission spectrometry.