Mössbauer study of LaNiSn and NdNiSn compounds and their deuterides

Summary LaNiSn and NdNiSn compounds and their deuterides have been studied by variable temperature ¹¹⁹Sn M?ssbauer spectroscopy. The hyperfine parameters obtained experimentally are in good agreement with those derived from first principle calculations. The enlargement of quadrupole splitting observed for LaNiSn after deuteration confirms the lower symmetry of electron density around tin atoms indicated by the calculation of partial Sn-p density of states (DOS). Magnetic ordering is observed at low temperature in deuterided NdNiSn.     

Complexation and Oxygenation in Cobalt(II)–Diamine–Oxygen Systems

The general pattern of equilibria involved in the complexation and oxygenation in cobalt(II)–diamine (phenanthroline or bipyridine)–oxygen systems was suggested. The set of equilibria included the following reactions: dissociation of protonated diamine cations; the formation of cobalt(II) mono-, bis-, and tris-diamine complexes, a hydroxo aqua complex, a hydroxo dimer, and a dihydroxo complex (monomer); various pathways to the dihydroxo dimer; and oxygenation of the hydroxo dimer. A mathematical model of the process was derived based on the material balances for cobalt and diamine and on the electroneutrality equation. The data of pH-metric studies of the systems in air and in an inert atmosphere and manometric data were used to calculate the individual equilibrium constants for each step of the process.     

Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

Unified application of the coupling model to segmental, Rouse, and terminal dynamics of entangled polymers

The temperature dependence and relaxation function breadth of segmental dynamics (α-relaxation) for 1,2-polybutadiene and 1,4-polybutadiene are used to predict their respective temperature-dependent terminal relaxation times by unified application of the Ngai coupling model. Literature results for the terminal flow of near-monodisperse linear polybutadienes having widely varying molecular weights are successfully represented using the coupling model by variation of only a single parameter, C, which is the proportionality constant between the longest Rouse relaxation time and the primitive relaxation time which underlies the cooperative segmental process. The value of C varies with molecular weight (M) according to C ∝ M^b where b is found to range from 1.8 to 2.1, in close agreement with the expected exponent of 2. Contrary to experimental data and coupling model predictions, reptation theory predicts identical influences of temperature on Rouse and terminal relaxation processes; we suggest that invoking a temperature dependence for contour length fluctuations, and hence number of effective entanglements per chain, may resolve this deficiency of the tube model.     

On the Passivation of the Metallic Surfaces from the Nuclear Plant Water Circuits

Samples obtained from the hot conditioning and hot performance procedures have been investigated by CEMS. Various types of substrates were considered. The surface phases were assigned to magnetite, α-Fe, wüstite and to spinel solid solutions. The data have been analyzed taking into account the compound stoichiometry, the number of iron unequivalent positions appeared in each identified phase and the relative ratio of the formed products. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin configurations and interfacial diffusion in exchange bias and spin valve systems with Ir–Mn antiferromagnetic pinning layers

Exchange bias and spin valve structures with Fe as ferromagnetic layers selectively enriched in ⁵⁷Fe and Ir–Mn of different compositions as antiferromagnetic pinning layers have been prepared by r.f. sputtering. Conversion Electron Mössbauer Spectroscopy and Magneto Optic Kerr Effect have shown that the local structure and interactions in the whole ferromagnetic layers, with direct influence on their magnetic reversal processes, depend on growing order of the layers and composition of the AF layer. Interfacial atomic diffusion is the main reason of perturbing the local structure of the ferromagnet. Mössbauer Spectroscopy is a powerfool tool which allows a detailed analysis of local configurations, in order to optimize the magnetic and giant magnetoresistive parameters of such multilayers of important technological applications.     

On slow flows of the full nonlinear Doi–Edwards polymer model

The full Doi–Edwards model constitutive equation derived by Palierne (Phys Rev Lett 93:136001-1–136001-4, (2004) is discussed in detail. The corresponding configurational probability equation is next solved for slow flows, and the solution is used to calculate the material constants: zero-shear viscosity and the normal stress differences.     

Complexation of Cobalt(II) with Some Glycyl Dipeptides under Inert Atmosphere

The general pattern of equilibria involved in the complexation in cobalt(II)–dipeptide (glycylglycine, glycylalanine, glycylnorvaline, glycylnorleucine, glycylvaline, and glycylleucine) systems under inert atmosphere was suggested. The set of equilibria included the following reactions: acid dissociation of the dipeptide zwitter-ion, the formation of mono- and bis(dipeptide) cobalt(II) complexes with the single-charged dipeptide anion, acid dissociation of coordinated dipeptide anions, and the formation of the mixed-ligand cobalt complex containing one double- and one single-charged dipeptide anion. A mathematical model of the process was obtained by solving the material balance equations for cobalt and dipeptide together with the electroneutrality equation. The data of pH-metric studies under an inert atmosphere were used to calculate the individual equilibrium constants for each step of the process.     

Optical and electrochemical-mediated detection of ascorbic acid using manganese porphyrin and its gold hybrids

The developments concerning new hybrids based on porphyrin derivatives and colloids destined for the detection of ascorbic acid (AA) in the relevant range for medical investigations are presented. Mn(III) tetratolylporphyrin chloride (MnTTPCl), spherical gold colloid (n-Au), and their hybrid (MnTTPCl/n-Au) were chosen to be comparatively investigated by ultraviolet–visible spectroscopy in the presence of AA. The hybrid material (MnTTPCl/n-Au) has the best capacity to detect concentrations of AA in the range of 2.6 × 10^?6–4.38 × 10^?5 M. Modified glassy carbon (GC) electrodes were obtained by thin film deposition of MnTTPCl, n-Au alone, and in successive mixed thin films, comparing their response during the electrochemical oxidation of AA. The electrocatalytic effect of the MnTTPCl on the AA oxidation is justified both by the increase in the peak current density and by the shift toward more negative potentials (0.024 V). The GC/MnTTPCl electrode has the best electrocatalytic effect for the AA oxidation and is promising for sensor applications.