New method for the restoration of the continuous distribution function of hyperfine fields andits possibilities

A mathematical approach for the evaluation of hyperfine field distribution functions based on an analytical approximation of Mössbauer spectra was developed. The calculations were performed for different model spectra andfor the experimental spectra of crystalline andamorphous alloys. It was possible to obtain an adequate interpretation of complicated Mössbauer spectra of multicomponent heterophase amorpho-crystalline alloys of the Nd-Fe-B-type.     

Structure of trans-1,4-polybutadiene synthesized with a new catalyst system

Abstract

The structure, phase composition, and temperature behavior of two trans-1,4-polybutadienes (TPBs) were studied by means of x-ray scattering and differential scanning calorimetry (DSC) techniques. The two samples examined were (1) PB synthesized using an immobilized titanium-magnesium catalyst and (2) a random copolymer based on PB prepared with a homogeneous vanadium-containing catalyst used as a reference material. It was found that the nascent structure of the first PB involves three phases: crystalline, mesomorphic (low-temperature form), and amorphous. In the vicinity of 65°C, a first-order phase transition occurs. The system becomes biphasic and contains the high-temperature form of the mesophase, as well as the amorphous phase, component. Above 165°C, the polymer melts to form a single-phase isotropic melt with a structure typical of liquids. The lateral dimension of crystallites reversibly changes at the crystal-mesophase transition. It is suggested that during annealing of the mesophase formed by cooling of the isotropic melt, the chains acquire an extended conformation. Loss of regularity of the structure of macromolecules of TPB causes a reduction of phase transition temperatures, an increase of the imperfection of the crystalline phase, and a contraction of the temperature range of existence of the mesophase.     

Characterization of periodic domain structures in lithium niobate crystals by scanning electron microscopy and X-ray diffraction analysis

Congruent lithium niobate crystals with periodic domain structures formed by the method of thermoelectric postgrowth treatment have been investigated. Periodic domain structures in the samples of polar ZY-, YZ-, and YX-cuts were characterized by scanning electron microscopy, high resolution x-ray diffractometry, and topography. The evaluation and comparison of the secondary electron (SE) signal cycling, electron emission coefficient, and charge value to equalize the differences in the SE signal have allowed us to specify the features of ZY-and YZ-cut structures. A correlation between the crystal lattice distortions near domain boundaries and the type of domain walls has been found. The domain walls separating the areas with the tail-to-tail P _s vectors not only cause stronger crystal lattice distortions near them, but also are charged less quickly under electron irradiation.     

Quantitative characterization of the orientation of macromolecules in intercalated nanocomposites of polyolefins/layered silicates by Raman spectroscopy

Raman spectroscopy is used to study variations in the orientational order of macromolecules in the uniaxially drawn intercalated nanocomposites based on two polymer matrices (polyethylene (PE) and isotactic polypropylene (PP)) and a filler (modified clay (MC)). The orientation parameters of macromolecules measured using Raman spectroscopy are compared with the X-ray data. It is demonstrated that, for the uniaxially drawn PE-MC and PP-MC intercalated nanocomposites, the filler impedes the orientation along the draw direction for the macromolecules localized in the noncrystalline phase of the polymer matrix. The orientational ability of the PE and PP crystallites in nanocomposites is not affected by the filler.     

Effect of a Thermal-Vacuum Treatment and X-Ray Radiation on the Morphology and Electrical Properties of Titanium Oxide Nanocoatings

Russian Journal of Applied Chemistry - Effect of a simultaneous thermal treatment at 480°C and X-ray radiation (irradiation dose 10?3 C/kg (～4 R) in a vacuum (residual pressure...     

Evolution of Anolyte Composition in the Oxidative Electrolysis of Sodium Bromide in a Sulfuric Acid Medium

The oxidation of a 10 mM aqueous solution of sodium bromide in a sulfuric acid medium on the surface of a platinum electrode in a cell with separated spaces was studied. The process is important in view of the use of the bromine–bromide redox couple in redox flow batteries. The study was performed by cyclic voltammetry, potentiostatic chronoamperometry with optical absorption spectrum recording, and measurements of the potential of the redox reference electrode. A numerical procedure for processing the experimental spectra of the solution was developed to separate them into the spectrum of molecular bromine and the residual signal. The latter was attributed to the absorption of the tribromide anion based on the literature data. The experimental dependences of the Br₂ and Br₃^- concentrations for the oxidative electrolysis of the NaBr solution in the sulfuric acid medium agreed well with the theoretical predictions. The current efficiency of bromine formation was evaluated.

     

Maximum Current Density in the Reduction of the Bromate Anion on a Rotating Disk Electrode: Asymptotic Behavior at Large Thicknesses of the Diffusion Layer

The reduction of the bromate anion on a rotating disk electrode (RDE) in a steady-state mode due to the catalytic cycle consisting of a reversible bromine/bromide redox pair and irreversible counter-proportionation reaction was studied theoretically. As the cycle is autocatalytic (EC″ mechanism: Electrochim. Acta, 2015, vol. 173, p. 779), at high volume concentrations of bromate the passing current can reach huge values limited by the ultimate diffusion current of bromate through the diffusion layer even at very low volume concentrations of bromine. In contrast to previous theoretical studies of this process, for numerical solution of the set of nonlinear equations with boundary conditions for concentrations we used the COMSOL Multiphysics program package, with which the solution can be found for both the galvanostatic mode (at a given current density) and the maximum current density. This allowed us to study the behavior of the maximum current density for the case of very high thickness of the diffusion layer and very high reaction rate constant. In this mode, the ratio of the maximum current to the limiting diffusion current of the reduction of the bromate anion to bromine was found to exceed not only intuitively anticipated unity, but also the “critical” value of 1.2, which formally corresponds to the limiting diffusion current of its reduction to the bromide anion (though the real end product is bromine), and this limiting value depends on the volume concentrations of both bromate and bromine.