Effect of microstructure on the magnetic-field dependence of the local critical current density inYBa2Cu3

Abstract

A contactless method for determining the local critical current density J,(B) and the efFective magnetic permeability p, of ceramic superconductors is presented. Using this method we have carried out systematic investigations of the magnetic field and temperature dependence of the local critical current density in YBapCu307 p samples with difFerent microstructures.The results show that the character of Josephson junctions in sintered ceramics changes as a function of heat treatment temperature. In spite of the differences in the values of J,(B) and in the types of Josephson junctions, the dependence of J,(B) for various kinds of ceramics has a common universal behavior. Using a scaling procedure it is possible to plot all experimental values of J,(B),obtained for samples prepared by solid-state sintering, on a single curve. At low magnetic fields this universal function is nearly constant with field and at higher fields it varies as B . The universal behavior of J,(B) breaks down as the field exceeds the first critical magnetic field of the grains, when the function J,(B) approaches a constant value. The universal behavior is completely defined by the critical current properties of individual Josephson junctions, a set of which forms the network of weak links in ceramics. Flux pinning therefore does not need to be considered when describing the current carrying capabilities of isotropic sintered ceramics which are weak linked. Analogous universal properties of J,(B) are observed in nontextured samples prepared by melt-processing technology.

The Low-Field Ac Impedance of YBa2Cu3O7-δSuperconducting Slab

“…(3) one can find a maximum at h ac /j c d = 4/3. The maximum value of the surface absorption of the infinite slab is given by the formula [6]:…”

“…We assume that the critical current density does not depend on ac field magnitude (j c (|H|) = j c (|H dc + h ac |) ≈ j c (|H dc |) for the case |H dc | |h ac |) and depends on frequency according to the Kim-Anderson creep model [2,8]. In the critical state model, the real part of the impedance of superconducting infinite slab, with the thickness 2d, R = Re(Z), is given by the following equations [5,6]:…”

“…In this case one can observe the generation of odd and even harmonics [3,4]. This behavior can be understood in frames of macroscopic critical state model [3][4][5][6][7]. In this model there are two critical current regimes: the Bean regime -for low magnetic field magnitude [5][6][7], and the Kim-Anderson regime -for high magnetic field magnitude [4].…”

“…This behavior can be understood in frames of macroscopic critical state model [3][4][5][6][7]. In this model there are two critical current regimes: the Bean regime -for low magnetic field magnitude [5][6][7], and the Kim-Anderson regime -for high magnetic field magnitude [4]. For these two regimes, measurements and simulations were carried out both for granular [4][5][6] and for melt-textured [3,5,7] ceramic bulk samples.…”

“…In this model there are two critical current regimes: the Bean regime -for low magnetic field magnitude [5][6][7], and the Kim-Anderson regime -for high magnetic field magnitude [4]. For these two regimes, measurements and simulations were carried out both for granular [4][5][6] and for melt-textured [3,5,7] ceramic bulk samples. It was shown that in the case of high temperature ceramics the measured critical current density is affected by thermal activation [2,7,8].…”