External Current Density Research Articles

Phenomena and microstructure at the aluminium-lead interface

An attempt has been made to study the precipitation behaviour of an Al-Pb alloy with respect to the unavoidable sedimented lead layer. The effect on the melt of Lorentz force on which is superimposed an external current is discussed briefly. This method could be further sophisticated to generate a higher volume fraction of heavier component in the matrix, by suitably adjusting the process parameters which influence the convection, and also by varying the applied external current density.

Collective flux creep in high-Tc superconductors.

We develop a scaling approach to flux-line pinning in high-{ital T}{sub {ital c}} superconductors. Our main result is a nonlinear relation {ital V}({ital j}{sub ex}){similar to}exp({minus}{ital Cj}{sub ex}{sup {minus}{mu}}) between the voltage {ital V} and the external current density {ital j}{sub ex}, where the exponent {mu} is related to the roughness exponent and has been estimated in an earlier paper (T. Nattermann, Phys. Rev. Lett. 64, 2454 (1990)). For the low-frequency ac resistivity we find two contributions {rho}{sub 1}({omega}){similar to}{omega}{sup 2}(1{minus}({ital T}/{ital T}{sup *})ln{omega}{tau}{sub 0}){sup 4/{psi}} and {rho}{sub 2}({omega}){similar to}{omega}(1{minus}({ital T}/{ital T}{sup *})ln{omega}{tau}{sub 0}){sup 2/{psi}}, where {psi}{approx}1 in three dimensions, and where {ital T}{sup *} is a characteristic temperature. Similar power laws are obtained for the dynamic susceptibility {chi}{prime}{prime}({omega}), whereas the magnetization due to the change of the applied field varies in time as (ln{ital t}){sup {minus}1/{mu}}.

The general solution of the time-dependent Maxwell’s equations in an infinite medium with constant conductivity

We solve the time-dependent Maxwell’s equations in all generality for an infinite medium in which the electric and magnetic permittivities and the conductivity are constant. In addition to the Ohmic current density, we include the effects of the presence of a charge density distribution and an external current density submerged in the medium. The longitudinal component of the magnetic field is shown to be zero, and the longitudinal component of the electric field is the gradient of the Coulomb potential, as in the case of zero conductivity. The longitudinal component of the external current density is given in terms of the charge density, as a generalization of the equation of continuity. The transverse electromagnetic fields are solved in terms of the initial values of the transverse fields and the transverse part of the external current density. Riemann functions are introduced for the problem. A critical wavelength is found such that fields associated with wavelengths shorter than this critical wavelength attenuate more rapidly in time than fields associated with wavelengths longer than the critical wavelength. Moreover, the fields with the shorter wavelengths are superpositions of standing waves that have a damping factor independent of the wavelength, whereas the fields whose wavelengths are longer than the critical wavelength are superpositions of ‘static’ helical fields, except for the damping factor which does depend on the wavelength. Stratton’s travelling and ‘diffusive’ plane wave solutions appear as special cases of these fields. There appear to be implications in biology and naval communications, which are discussed briefly.

Investigation of the coupling mechanism between superconducting grains in high- Tc superconductors

Electric field generation due to transport current was investigated experimentally in high- T c YBa 2Cu 3O 7−δ ceramic superconductors. The external magnetic field and transport current density dependence of resistivity-temperature characteristics, as well as the external magnetic field and temperature dependence of current-voltage characteristics, were investigated in the temperature range from 4.5 K up to T c and in magnetic fields of 0 to 6 T. The separation of intragrain and intergrain (grain boundary) contributions to the electric field was performed. The upper critical field of superconducting grains was calculated from the magnetic field dependence of resistivity-temperature curves. A power law dependence of voltage on ( I- I c) was found in up to 5 mV/cm electric fields. An exponential temperature dependence was also found.

Analysis of pre-breakdown current growth due to two emission mechanisms†

Abstract The transient electron avalanche which constitutes the primary pro-breakdown event of a gas medium is examined in a one-dimensional uniform field Townsend gas discharge system that is initiated by a single pulse of electrons at the cathode. Analytic solutions of other investigators which yield to stable numerical evaluation are presented. A convergent and stable finite-difference technique for systems of quasi-linear first-order hyperbolic partial differential equations is adapted to the Townsend system for two separate secondary electron emission mechanisms, due to either ion impact or incident photons. Numerical illustrations are given comparing the results of the finite-difference technique to the numerical evaluation of the given analytic solutions for the cathode electron current density, illustrating convergence of the numerical method with suggestion of adaptation of finite-difference techniques to more complicated discharge models. The total external current density is also computed for photon secondary emission with applied fields below, equal to, and above the breakdown threshold.

Spatial and temporal evolution of charge carrier distributions following pulsed photoinjection

The fundamental equations governing the charge carrier dynamics in photoinsulators are solved for the configuration usually employed in drift mobility measurements. The time dependent spatial distributions of the carrier density, electric field strength and conduction current density are obtained, and these internal distributions are related to the ordinaly studied external current density.

Stress‐corrosion cracking of austenitic stainless steels with applied E.M.F.

AbstractThe stress‐corrosion cracking of chromium‐nickel austenitic stainless steels in 42 wt.‐% aqueous magnesium chloride solution at 150–154°, saturated with air, has been studied under conditions of cathodic or anodic treatment of the metal during the induction period prior to crack initiation and/or during crack propagation.Mild cathodic polarization delays the onset of cracking and fracture, the time to fracture increasing with increasing degree of polarization. With plain 18–8 steel, polarization producing an external current density of 50 μa./cm.2delays for at least 200 h., probably indefinitely, the onset of cracking of material that fractures in about 2 h. in the absence of applied e.m.f. Polarization applied only after crack propagation has begun reduces its rate; propagation ceases immediately if the measured potential of the metal is reduced to ‐ 0.14 v (N‐hydrogen scale), which produces approx. 170 μa./cm.2external current, and after a finite period at slightly less polarization.Anodic polarization producing external current densities of up to 50 μa./cm.2greatly shortens the induction period and, although scarcely increasing the rate of crack propagation, increases the total number of cracks formed.The results are in harmony with the electrochemical mechanism of this type of stress‐corrosion cracking. They indicate that cathodic protection may be a promising practical method for its control in suitable cases.