Relaxation auto-oscillations in a fluidized bed

Oscillations of nonuniform fluidized bed in the slugging regime are considered. A nonlinear one-dimensional model of bed oscillations is developed and investigated, including the equations of the bed surface motion and the oscillations of the pressure drop in the bed for the phase of the surface ascent and for the phase of its fall. A quasi-discrete process of the gas escape from the layer with large bubble and the relaxation (discontinuous) type of oscillations of the bed surface and of the pressure, which are related to the above process, are shown based on the model analysis, the obtained numerical solutions, and the comparison with experiment.     

Spatial structure and stability of Mo<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>Si<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript> nanoparticles

By means of the ab initio DMol³ method Mo _n Si _m nanoparticles and fragments of Mo₃Si and MoSi₂ crystal lattices are theoretically modeled. For both crystals a few neutral Mo₄Si₆ and Mo₆Si₆ fragments of different shapes and symmetry are considered. In each case, after cluster separation its geometry is optimized, as a result of which the geometric structure noticeably changes and its stability increases. In order to theoretically search for the spatial configurations of Mo₄Si₆ and Mo₆Si₆ nanoparticle, two approaches are used: 1) in the most stable Fe₄C₆ and Fe₆C₆ isomers found previously, iron and carbon atoms are replaced by molybdenum and silicon respectively and then the geometry is optimized to obtain new equilibrium distances and angles; 2) the search for main Mo₄Si₆ and Mo₆Si₆ configurations is performed using the binominal scheme, starting from Mo₂, MoSi, and Si₂ dimers. The nanoparticle structures are found to contain metal atom chains and isolated pairs and triples of silicon atoms. In most cases, the nanoparticle stability proves to be higher than that of the crystal clusters.     

Optically and electrically pumped random lasing from ZnO films annealed at different temperatures

We have investigated the optically and electrically pumped random lasing (RL) actions in ZnO films annealed at low and high temperatures respectively. While the optically pumped RL threshold for the ZnO film annealed at a low temperature, which features stronger light scattering and larger optical loss, is far higher than that for the ZnO film annealed at a high temperature, the electrically pumped RL threshold currents for the two ZnO films are almost the same with the electrical pumping scheme of metal-insulator-semiconductor structure. It is suggested that if the lasing region within the ZnO film is narrow enough in the case of electrical pumping, the strong light scattering can substantially alleviate the adverse effect of large optical loss on the onset of RL.     

Electronic Structure and Chemical Bonding in Lead Hexacyanoferrate

This paper presents a cluster–DV study of the electronic structure of a large fragment of the crystal lattice of a new compound Pb₂Fe(CN)₆ having a trigonal structure. The electronic energy spectrum and electron density distribution between Fe–C, C–N, and Pb–N are discussed.     

Surface phases and nanostructures on silicon surface

The paper gives definitions of conventional terms related to silicon surface science, such as surface, surfaced phase, adatoms, “in phase” and “on phase” atoms. The formation methods of surface phases are illustrated as well as their role in physical processes on the surface. The influence of surface phases on diffusion, desorption, adsorption, and phase interface formation at the silicon surface is described.     

Electronic structure of Ni(II) centers in bismuth and thallium compounds

Conclusions From the results of our calculations, we can conclude that the replacement of copper by Ni(II) atoms in Bi₂Sr₂CuO₆ or Tl₂Ba₂CuO₆ produces a substantial change in the character of the filled and vacant states close to E_F and hence in the conduction characteristics of the original systems. On the other hand, the interaction of Ni–O and Bi(Tl)–O layers is not strong enough to produce any substantial change of charge distribution in these layers when Cu is replaced by Ni.Institute of Solid-State Chemistry, Urals Branch, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 33., No. 4, pp. 23–27, July–August, 1992.     

Synthesis of water and molecular oxygen highly enriched in <Superscript>17</Superscript>O and <Superscript>18</Superscript>O isotopes from carbon oxides

The reaction of carbon oxides and hydrogen in the presence of the Raney nickel catalyst has been used for water synthesis. A procedure has been developed for the recovery and collection of the synthesized water with minimal losses and without deteriorating the ¹⁷O or ¹⁸O isotope enrichment as compared to the initial CO₂ and CO. The recovery of oxygen with high concentrations of ¹⁷O and ¹⁸O isotopes is based on the reaction of xenon difluoride with water. The yield based on oxygen achieves 99% without reduction of isotope enrichment, which is confirmed by mass-spectral measurements of oxygen isotope concentrations in the initial reagents and final reaction products.     

Electrocatalytic cascade approach to the synthesis of dihydro-2'H,3H-spiro[1-benzofuran-2,5'-pyrimidines]

Chemistry of Heterocyclic Compounds - Electrocatalytic cascade assembly of benzylidenebarbiturates and cyclohexane-1,3-diones in methanol in the presence of sodium bromide as mediator in an...     

Exceptionally high decarboxylation rate of a primary aliphatic acyloxy radical determined by radical product yield analysis and quantitative 1H‐CIDNP spectroscopy

Symmetrical (RCO₂CO₂R; R = XCH₂CH₂) and asymmetrical (RCO₂CO₂R′; R = C₉H₁₉CH₂CH₂, R′ = CH₃ or m‐ClC₆H₄) primary diacyl peroxides were thermally decomposed under different conditions to analyze the decarboxylation rates of the thermally generated acyloxy radicals. Quantitative models of the geminate product yields, and qualitative and quantitative ¹H‐CIDNP spectroscopy were used to obtain the decarboxylation rate estimates. Results reported here suggest that, unlike short chain acyloxy radicals such as propanoyloxyl, long chain acyloxy radicals possess the highest decarboxylation rates of all known acyloxy radicals, estimated at (0.5–1.5) × 10¹² s^?1 between 80 and 140 °C. Given the nature of the dissociative state of acyloxy radicals, such rates appear to be the result of destabilization of the former by the steric bulk of the long chain substituents. Additionally, the rate of this order of magnitude suggests a nearly concerted decarboxylation of primary diacyl peroxides. Copyright © 2011 John Wiley & Sons, Ltd.