Grain-boundary Critical Current Densities Research Articles

Modeling the trapped field distribution in a polycrystalline bulk superconductor based on a critical state model

The trapped field distribution of a polycrystalline bulk superconductor induced by perpendicular magnetic field is theoretically analyzed based on the critical state model. A rational model is presented to describe the current pattern changes near a low-angle grain boundary (GB) that is observed in others’ experiment. The current pattern that varies with the ratio between GB critical current density and grain critical current density is analyzed based on the constant critical current density assumption. In addition, the magnetic field that trapped in the prism both on magnetization ascent and descent stage have been discussed in detail on the basis of the current distribution that we deduce above. The results of this work can also be useful for AC loss analysis and stress calculating of polycrystalline superconductors.

Flux-Pinning-Induced Stress and Deformation Analyses of a Long Rectangular Superconducting Bicrystal

The flux-pinning-induced stress and deformation of a long rectangular superconducting bicrystal with an arbitrary aspect ratio are analyzed based on the critical-state model and the finite element numerical method. The flux and current distributions in the superconductor with a low-angle grain boundary (GB) are obtained based on a constant GB critical current density assumption. The distributions of the stresses within the superconductor are obtained for different magnetization stages. The deformation and especially the shape distortion in the irreversible magnetostriction of the superconductor are analyzed. In addition, the relation between the maximum stress on the grain boundary and the ratio of the GB critical current density to the grain critical current density is discussed when the applied magnetic field is reduced to zero.

Simultaneous determination of grain and grain-boundary critical currents inYBa2Cu3O7-coated conductors by magnetic measurements

We present a complete description of a noninvasive inductive methodology developed to study the magnetic granularity inherent in coated conductors. The method is based on the analysis of coated conductor hysteresis loops and enables us to identify the presence of electromagnetic granularity from the appearance of a peak in the return branch of the irreversible magnetization. Minor hysteresis loop cycles reveal the evolution of this peak and allow us to separate and analyze independently grain, grain-boundary critical current densities, and grain size of coated conductors. We have performed different tests to the developed model in order to validate the main assumptions considered and better understand the key factors involved.

Grain and grain-boundary critical currents in coated conductors with noncorrelating YBa2Cu3O7 and substrate grain-boundary networks

The superconducting grain-boundary (GB) network of coated conductors (CCs) is usually assumed to be a replica of the substrate network. In this letter, we analyze IBAD and RABITS CCs, where such replica either do or do not exist. We have analyzed the effect of GB overgrowth on the critical currents by quantifying the average superconducting grain size and determining the intragrain and grain-boundary critical current densities, JcG and JcGB. We have employed a recently developed inductive methodology enabling the simultaneous determination of these three parameters. We show that the percolative JcGB may be reduced by 50% if the GB networks do not correlate, while JcG and the grain pinning properties appear unaffected.

Correlation between grain and grain-boundary critical current densities in ex situ coated conductors with variable YBa2Cu3O7−δ layer thickness

The dependence of the percolative critical current density at low magnetic fields on YBa2Cu3O7−δ (YBCO) layer thickness is studied by comparing grain, JcG, and grain-boundary, JcGB, critical current densities for a series of ex situ processed YBCO films on a RABiTS template. Both critical current densities decrease as a function of thickness and the values of JcG and JcGB show a clear correlation which suggests the existence of an interaction between Abrikosov–Josephson vortices on the grain boundaries and Abrikosov vortices in the bulk of the grains. This opens the possibility to improve JcGB by optimizing the pinning capabilities of the grains.

Effect of Strain on Grain and Grain-Boundary Critical Currents of YBCO Coated Conductors

We present a comparative analysis on tensile strain tolerance of critical currents for IBAD and RABiTS coated conductors. These measurements have been performed with a recently developed methodology based on dc-magnetization measurements, which enables us to analyze the grain and grain-boundary critical current densities simultaneously. We show that the percolative grain-boundary critical current density may be reduced by 50% with deformations of /spl epsiv//spl sim/0.32% and 1% for RABiTS and IBAD, respectively, while the grain critical current and thus the pinning properties of the grains are not affected by the deformation process.

Orientation dependence of grain-boundary critical current densities in high-Tc bicrystals.

The critical current density across grain boundaries of twinned thin-film ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{x}}$ bicrystals has been measured as a function of the tilt angle \ensuremath{\Theta}. For \ensuremath{\Theta}=0\ifmmode^\circ\else\textdegree\fi{} to 45\ifmmode^\circ\else\textdegree\fi{}, the ratio of intergrain to intragrain critical current density decreases exponentially with increasing tilt angle. Surprisingly, this orientation dependence is very similar to that observed for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ bicrystals. Microstructural investigations of plan view samples show a wavy grain boundary of the superconductor with a roughness of 100 nm to 1 \ensuremath{\mu}m which does not originate from the roughness of the substrate grain boundary (1--3 nm) but is caused by an island-plus-layer growth of twin domains. In general, one of the adjacent grains has a low-indexed habit plane at the boundary.

Superconducting transport properties of grain boundaries in YBa2Cu3O7 bicrystals.

Previous work on the superconducting transport properties of individual grain boundaries in thin-film bicrystals of YBa{sub 2}Cu{sub 3}O{sub 7} has been extended to provide a more comprehensive picture of their weak-link characteristics. Grain boundaries with three different geometries have been studied; the transport properties of all three types of boundaries are essentially identical, which implies that the poor superconducting coupling between grains is a result of the intrinsic structural disorder at the boundary. The grain-boundary critical current densities in bicrystal films prepared by evaporation and postannealing and by laser ablation are also in good agreement; this result demonstrates that the transport properties are insensitive to preparation technique and, thus, are not dominated by the diffusion of substrate impurities into the boundary region. High grain-boundary resistivities and low {ital I}{sub {ital c}R{ital n}} products imply that the boundaries act as strong barriers to current flow with locally depressed order parameters. Strong magnetic hysteresis, associated with trapped intragranular flux, is observed; this hysteretic behavior is also responsible for an increase in the grain boundary {ital J}{sub {ital c}} for {ital H}{sub {ital a}{ital p}{ital p}}{gt}300--500 Oe.

Orientation dependence of grain-boundary critical currents in YBa2Cu3O7- delta bicrystals.

The critical current densities across grain boundaries have been measured as a function of misorientation angle in the basal plane of bicrystals of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. For small misorientation angles, the ratio of the grain-boundary critical current density to the bulk critical current density is roughly proportional to the inverse of the misorientation angle; for large angles, this ratio saturates to a value of about $\frac{1}{50}$. These results imply that achieving a high degree of texture both normal to and within the basal plane is important for the obtaining of very high critical currents in pure polycrystalline samples.