Amplification of magnetic fields by dynamo action in Gaussian-correlated helical turbulence

We investigate the growth and structure of magnetic fields amplified by kinematic dynamo action in turbulence with non-zero kinetic helicity. We assume a simple Gaussian velocity correlation tensor, which allows us to consider very large magnetic Reynolds numbers, up to one trillion. We use the kinematic Kazantsev-Kraichnan model of dynamo and find a complete numerical solution for the correlation functions of growing magnetic fields.Comment: 7 pages, 3 figure

Thermoelectric behavior of BaZr0.9Y0.1O3−d proton conducting electrolyte

BaZr0.9Y0.1O3-δ (BZY10), a promising proton conducting material, exhibits p-type conduction under oxidative conditions. Holes in BZY10 are of the small polaron type. However, there is no clear understanding at which places in the lattice they are localized. The main objectives of this work were, therefore, to discuss the nature of electronic defects in BZY10 on the basis of the combined measurements of the thermo-EMF and conductivity. Total electrical conductivity and Seebeck coefficient of BZY10 were simultaneously studied depending on partial pressures of oxygen (pO2), water (pH2O) and temperature (T). The model equation for total conductivity and Seebeck coefficient derived on the basis of the proposed defect chemical approach was successfully fitted to the experimental data. Transference numbers of all the charge carriers in BZY10 were calculated. The heat of transport of oxide ions was found to be about one half the activation energy of their mobility, while that of protons was almost equal to the activation energy of their mobility. The results of the Seebeck coefficient modeling indicate that cation impurities, rather than oxygen sites, should be considered as a place of hole localization. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.Russian Science Foundation, RSF: 18-73-00022Funding: This work was supported by the Russian Science Foundation (project No. 18-73-00022

PrBaCo 2 O 6−δ -Ce 0.8 Sm 0.2 O 1.9 composite cathodes for intermediate-temperature solid oxide fuel cells: Stability and cation interdiffusion

The single-phase oxide PrBaCo 2 O 6−δ and composites (100 − y)PrBaCo 2 O 6−δ -yCe 0.8 Sm 0.2 O 1.9 (y = 10–30 wt.%) were investigated as cathode materials for intermediate-temperature solid oxide fuel cells. The chemical compatibility, cation interdiffusion, thermal expansion and dc conductivity were studied. As a result, strong interdiffusion of Pr and Sm was found between PrBaCo 2 O 6−δ and Ce 0.8 Sm 0.2 O 1.9 . This leads to only insignificantly decreasing thermal expansion coefficient of composite with increasing fraction of Ce 0.8 Sm 0.2 O 1.9 and, thus, mixing PrBaCo 2 O 6−δ with Ce 0.8 Sm 0.2 O 1.9 does not improve thermal expansion behavior of the cathode material. Moreover, formation of poorly-conducting BaCeO 3 , caused by chemical interaction between the double perovskite and doped ceria, was shown to lead to pronounced drop in the electrical conductivity of the composite cathode material with increasing Ce 0.8 Sm 0.2 O 1.9 content. © 2019 by the authors

Magnetic reconnection with anomalous resistivity in two-and-a-half dimensions I: Quasi-stationary case

In this paper quasi-stationary, two-and-a-half-dimensional magnetic reconnection is studied in the framework of incompressible resistive magnetohydrodynamics (MHD). A new theoretical approach for calculation of the reconnection rate is presented. This approach is based on local analytical derivations in a thin reconnection layer, and it is applicable to the case when resistivity is anomalous and is an arbitrary function of the electric current and the spatial coordinates. It is found that a quasi-stationary reconnection rate is fully determined by a particular functional form of the anomalous resistivity and by the local configuration of the magnetic field just outside the reconnection layer. It is also found that in the special case of constant resistivity reconnection is Sweet-Parker and not Petschek.Comment: 15 pages, 4 figures, minor changes as compared to the 1st versio

Oxygen content and defect structure of the perovskite La0.5Ba0.5CoO3–δ

Received: 28.06.2018. Accepted: 25.07.2018. Published: 30.07.2018.Perovskite-type complex oxide La0.5Ba0.5CoO3–δ, promising cathode material for solid oxide fuel cells and precursor for synthesis of double perovskite LaBaCo2O6–δ, was prepared as a single-phase material. Its oxygen content was measured by two independent techniques in the temperature range 1000–1100 °C and at oxygen partial pressures corresponding to the stability field of cubic phase. The defect chemistry of this material was studied using the measured δ = f(pO2, T) dependences. The defect structure model based on the localized nature of the electronic defects was proposed and successfully verified

A model of Hall reconnection

The rate of quasi-stationary, two-dimensional magnetic reconnection is calculated in the framework of incompressible Hall magnetohydrodynamics (MHD). The calculation is based on the solution of Hall-MHD equations that include Hall and electron pressure terms for electric current. These equations are solved in a local region across the reconnection electron layer, including only the upstream region and the layer center. In the case when the ion inertial length d_i is larger than the Sweet-Parker reconnection layer thickness, the dimensionless reconnection rate is found to be independent of the electrical resistivity and equal to d_i/L, where L is the scale length of the external magnetic field in the upstream region outside the electron layer.Comment: 4 pages, 1 figur

Oxygen content and defect structure of the perovskite La0.5Ba0.5CoO3–δ

Perovskite-type complex oxide La0.5Ba0.5CoO3–δ, promising cathode material for solid oxide fuel cells and precursor for synthesis of double perovskite LaBaCo2O6–δ, was prepared as a single-phase material. Its oxygen content was measured by two independent techniques in the temperature range 1000–1100 °C and at oxygen partial pressures corresponding to the stability field of cubic phase. The defect chemistry of this material was studied using the measured δ=f(pO2,T) dependences. The defect structure model based on the localized nature of the electronic defects was proposed and successfully verified

Fast and slow two-fluid magnetic reconnection

We present a two-fluid magnetohydrodynamics (MHD) model of quasi-stationary, two-dimensional magnetic reconnection in an incompressible plasma composed of electrons and ions. We find two distinct regimes of slow and fast reconnection. The presence of these two regimes can provide a possible explanation for the initial slow build up and subsequent rapid release of magnetic energy frequently observed in cosmic and laboratory plasmas.Comment: 16 pages, 2 figures, 1 tabl

An evaluation of possible mechanisms for anomalous resistivity in the solar corona

A wide variety of transient events in the solar corona seem to require explanations that invoke fast reconnection. Theoretical models explaining fast reconnection often rely on enhanced resistivity. We start with data derived from observed reconnection rates in solar flares and seek to reconcile them with the chaos-induced resistivity model of Numata & Yoshida (2002) and with resistivity arising out of the kinetic Alfv\'en wave (KAW) instability. We find that the resistivities arising from either of these mechanisms, when localized over lengthscales of the order of an ion skin depth, are capable of explaining the observationally mandated Lundquist numbers.Comment: Accepted, Solar Physic