Direct current electrical resistivity of zinc borate glasses containing transition metal oxides

The resistivities of zinc borate glasses containing Fe2O3, V2O5, and Fe2O3 + V2O5 have been measured as a function of composition and temperature. The values of resistivity and activation energy decrease as the transition metal oxide content is increased. The conductivities of the glasses containing Fe2O3 + V2O5 are more than the sum of those of the glasses containing only Fe2O3 or V2O5 (i.e. the activation energies are less than the sum of those in the glasses containing only Fe2O3 or V2O5). The results are discussed in terms of existing theories.

Transition metal oxide perovskites by photoelectron and x-ray absorption spectroscopy

X-ray and ultraviolet photoelectron spectroscopy as well as x-ray absorption spectroscopy have been employed to investigate transition metal oxide perovskites of the general formula ABOs (A=La or rare-earth ion, B=trivalent transition metalion). Systematics in the core levels and in the valence bands in the series of LaBOa compounds have been discussed. Lanthanum chemical shifts in the x-ray absorption spectra in this series show interesting trends. Photoelectron spectra of the solid solutions, LaNil_x Coxes, LaNix_x FexO8 and LaFel_x Coxes show that the rigid band model is applicable to these systems. It is shown that x-ray photoelectron spectroscopy can be employed to identify multiple oxidation states of transition metal ions in oxide perovskites.

A study on boundary-layer transition induced by free-stream turbulence

Boundary-layer transition at different free-stream turbulence levels has been investigated using the particle-image velocimetry technique. The measurements show organized positive and negative fluctuations of the streamwise fluctuating velocity component, which resemble the forward and backward jet-like structures reported in the direct numerical simulation of bypass transition. These fluctuations are associated with unsteady streaky structures. Large inclined high shear-layer regions are also observed and the organized negative fluctuations are found to appear consistently with these inclined shear layers, along with highly inflectional instantaneous streamwise velocity profiles. These inflectional velocity profiles are similar to those in the ribbon-induced boundary-layer transition. An oscillating-inclined shear layer appears to be the turbulent spot-precursor. The measurements also enabled to compare the actual turbulent spot in bypass transition with the simulated one. A proper orthogonal decomposition analysis of the fluctuating velocity field is carried out. The dominant flow structures of the organized positive and negative fluctuations are captured by the first few eigenfunction modes carrying most of the fluctuating energy. The similarity in the dominant eigenfunctions at different Reynolds numbers suggests that the flow prevails its structural identity even in intermittent flows. This analysis also indicates the possibility of the existence of a spatio-temporal symmetry associated with a travelling wave in the flow.

Influence of cross-correlation between dipolar and chemical shift anisotropy relaxation mechanisms upon the transverse relaxation rates of 15N in macromolecules

Heteronuclear multiple-quantum coherence relaxation rate are calculated for the individual transitions of the S spin in an AIS nuclear spin system assuming that the heteronucleus (S spin) has relaxation contributions from both intramolecular dipole-dipole and chemical shift anisotropy relaxation. The individual multiplet components of the heteronuclear zero- and double-quantum coherences are shown to have different transverse relaxation rates. The cross-correlation between the two relaxation mechanisms is shown to be the dominant cause of the calculated differential line broadening. Experimental data are presented using as an example a uniformly 15N labelled sample of human epidermal growth factor.

Metal-Insulator Transition in the Hubbard Model on a Triangular Lattice

We discuss the results of an extensive mean-field investigation of the half-filled Hubbard model on a triangular lattice at zero temperature. At intermediate U we find a first-order metal-insulator transition from an incommensurate spiral magnetic metal to a semiconducting state with a commensurate linear spin density wave ordering stabilized by the competition between the kinetic energy and the frustrated nature of the magnetic interaction. At large U the ground state is that of a classical triangular antiferromagnet within our approximation. In the incommensurate spiral metallic phase the Fermi surface has parts in which the wave function renormalization Z is extremely small. The evolution of the Fermi surface and the broadening of the quasi-particle band along with the variation of the plasma frequency and a charge stiffness constant with U/t are discussed.

Nature of nitrogen adsorbed on transition metal surfaces as revealed by electron spectroscopy and cognate techniques

The nature of the chemisorbed states of nitrogen on various transition metal surfaces is discussed comprehensively on the basis of the results of electron spectroscopic investigations augmented by those from other techniques such as LEED and thermal desorption. A brief discussion of the photoemission spectra of free N2, a comparison of adsorbed N2 and CO as well as of physisorption of N2 on metal surfaces is also presented. We discuss the chemisorption of N2 on the surfaces of certain metals (e.g. Ni, Fe, Ru and W) in some detail, paying considerable attention to the effect of electropositive and electronegative surface modifiers. Features of the various chemisorbed states (one or more weakly chemisorbed gamma-states, strongly chemisorbed alpha-states with bond orders between 1 and 2. and dissociative chemisorbed beta-states) on different surfaces are described and relations between them indicated. While the gamma-state could be a precursor of the alpha-state, the alpha-state could be the precursor of the beta-state and this kind of information is of direct relevance to ammonia synthesis. The nature of adsorption of N2 on the surfaces of some metals (e.g. Cr, Co) deserves further study and such investigations might as well suggest alternative catalysts for ammonia synthesis.

Relation between effective atomic charge and chemical shifts in X-ray absorption spectra of transition-metal compounds

Chemical shifts of K absorption discontinuities, Delta E, of several manganese, iron and cobalt oxides with the metal in the formal oxidation states between +2 and +4, have been measured. These data, together with data in the literature on other compounds of these metals, can be fitted into the expression Delta E=aq+bq2, where q is the effective atomic charge on the metal. Theoretical considerations also support this functional relationship between Delta E and q.

Preparation and characterization of hydrazine derivatives. Part-II: Reaction of transition metal ammonium double sulphates with hydrazine hydrate

Transition metal ammonium double sulphates (NH4)2M(SO4)2· 6H2O, where M = Fe, Co and Ni react with hydrazine hydrate in air giving crystalline compounds of the general formula (N2H5) [M(N2H3COO)3] H2O. The reaction proceeds through (N2H5)2 M(SO4)2, · 3N2H4, (N2H5)2 [M(OH)4 · (N2H4)2], M(N2H3COO)2 · (N2H4)2 and N2H5 [M(N2 H3 COO)3] intermediates. The reaction sequence is followed by chemical analysis and infrared spectra. A possible reaction mechanism has been suggested.

Polymer flame inhibition in presence of transition metal oxides

Transition metal oxides like Fe2O3, Ni2O3, Co2O3 and MnO2 suppress the combustion of polystyrene. The effect has been explained on the basis of condensed-phase and gas-phase reactions.

Pes study of spin-state transitions in d6 transition metal complexes and oxides

Transitions from the low-to the high-spin state in Fe2+ and Co3+ compounds have been examined by X-ray and UV photoelectron spectroscopy. It has been shown that the core-level bands in XPES, in particular the metal 3s band, as well as the valence bands, are diagnosis in the study of spin-state transitions.

A model for antiferromagnetic to superconductor transition

A model system capable of exhibiting both superconductivity of conduction electrons and “antiferromagnetic” order of site localised electrons is studied. Coexistence of both types of order seems a possibility and the model predicts a re-entrant behaviour of the transition temperature.

In-Network Computation in Random Wireless Networks: A PAC Approach to Constant Refresh Rates with Lower Energy Costs

We propose a method to compute a probably approximately correct (PAC) normalized histogram of observations with a refresh rate of Theta(1) time units per histogram sample on a random geometric graph with noise-free links. The delay in computation is Theta(root n) time units. We further extend our approach to a network with noisy links. While the refresh rate remains Theta(1) time units per sample, the delay increases to Theta(root n log n). The number of transmissions in both cases is Theta(n) per histogram sample. The achieved Theta(1) refresh rate for PAC histogram computation is a significant improvement over the refresh rate of Theta(1/log n) for histogram computation in noiseless networks. We achieve this by operating in the supercritical thermodynamic regime where large pathways for communication build up, but the network may have more than one component. The largest component however will have an arbitrarily large fraction of nodes in order to enable approximate computation of the histogram to the desired level of accuracy. Operation in the supercritical thermodynamic regime also reduces energy consumption. A key step in the proof of our achievability result is the construction of a connected component having bounded degree and any desired fraction of nodes. This construction may also prove useful in other communication settings on the random geometric graph.