NMR study of the electric field gradient in the paramagnetic phase of M<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> (M = Co,Ni) compounds

The NMR spectra and the decay of a spin echo signal from ⁵¹V nuclei in Kagome-staircase Co₃V₂O₈ (CVO) and Ni₃V₂O₈ (NVO) single crystals are measured in the temperature range 30–300 K and a magnetic field H ₀ = 20 kOe. The orientation dependences of the ⁵¹V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency ν _Q and asymmetry parameter η. These parameters for NVO and CVO are ν _Q = 180(10) kHz, η = 0.5(1) and ν _Q = 130(10) kHz, η = 0.6(1), respectively. A comparison of the results of calculating EFG tensors with a point charge model and the NMR data indicates that the crystallographically equivalent vanadium atoms in the Ni₃V₂O₈ and Co₃V₂O₈ compounds differ in the EFG axis orientation. M₃V₂O₈ crystals are found to have vanadium positions (V1, V2) with different orientations of the z axis, which specifies the direction of the principal value of EFG (V _zz ): these orientations lie in the bc plane and make an angle of either +51(5)° (V1) or −51(5)° (V2) with axis c. In the temperature range 30–300 K, the EFG tensor components and the local symmetry of the charge surrounding of the vanadium positions in NVO and CVO oxides are found to change insignificantly.     

Ionization process in the humid air of the lower troposphere

This paper analyzes the local processes of ionization and electron loss in the early stages of an electric discharge in humid air at altitudes of 0–12 km. The processes of ionization in an external electric field, background ionization, attachment and detachment of electrons from atomic and molecular oxygen, charge transfer, and the conversion of negative ions are considered. The dependences of the rate constants on the pressure are taken into account. It is shown that the electric field strength at which the effective ionization of air begins decreases with increasing altitude.     

Mathematical simulation of anodic intercalation of graphite in dilute nitric acid

The article suggests a mathematical model for the process of anodic intercalation of the dense graphite suspension in dilute nitric acid, where the spontaneous intercalation is absent. The model is based on the idea of electric neutrality of a graphite particle, due to which the anion current NO₃⁻ entering the particle is precisely equal to the quantity of electrons, which are diverted from a particle to a current lead according to the mechanism of electronic conductivity provided by the outer constriction of the suspension. The anodic polarization is conditioned by the mass transfer of anions NO₃⁻ inside the equipotential graphite particle. The computational solution of the problem showed that even at a small suspension layer height (2 cm), there is a significant nonuniformity of intercalation, which corresponds to the data provided in the literature. A method for the estimation of an intercalation nonuniformity and a method for its reduction by using a multistep process were suggested. The results of the computation can be used for an estimation of the technical characteristics of the final product.     

Acoustic field generated by a beam of protons stopping in a water medium

In an experiment with a beam of protons accelerated up to an energy of 200 MeV, the space-time structure of the hydroacoustic field generated by protons stopping in the water medium was observed. The contributions of three components were separated: a cylindrical wave diverging from the middle part of the acoustic antenna and the signals from the ends of the antenna, namely, from the region of the maximal energy density release by protons (the Bragg peak) and from the other end corresponding to the beam entrance into water.     

Ethylene oxidation under conditions of the oxidative coupling of methane

Ethylene conversion under conditions of the oxidative coupling of methane has been investigated. In an empty reactor above 740°C, ethylene oxidation occurs at a higher rate and its main product is carbon monoxide. Filling the reactor with an inert material (quartz) or a NaWMn/SiO₂ catalyst leads to a marked decrease in the ethylene conversion rate. Addition of methane to the reaction mixture dramatically slows down ethylene conversion rate and increases the C₃ hydrocarbon content of the reaction products. The kinetics of ethylene oxidation in the presence of methane over the NaWMn/SiO₂ catalyst is reported.     

Stability estimates for linear stochastic systems with deviating argument of neutral type

We study linear stochastic differential equations with deviating argument of neutral type and establish sufficient conditions of stability. The functions determining the initial perturbations of solutions are found.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 45, No. 6, pp. 834–842, June, 1993.     

Reflection of electromagnetic waves from the surface of a cubic ferrite

The reflectivity of electromagnetic waves incident on the surface of a semi-infinite nonconducting cubic ferrite was found both analytically and numerically with inclusion of spin-wave damping. The frequency and field dependences of the reflectivity were found for various values of the damping parameter and the anisotropy and magnetostriction constants in a region far from the point of the orientational phase transition and at this point. It is shown that the reflectivity has peaks near the frequencies of ferromagnetic, magnetoacoustic, and magnetostatic resonances. The peak amplitude decreases with increasing spin-wave damping. At frequencies below the magnetoelastic gap, the reflectivity can take on anomalously small (down to zero) and anomalously large (up to unity) values. These frequencies can lie in the microwave range.