Grain-boundary Critical Current Research Articles

The Use of Low Temperature Scanning Microscope for Estimating In-Plane Thermal Diffusivity in YBCO Thin Film

We have experimentally investigated thermal diffusion in a SrTiO3 based YBCO bicrystalline thin-film by means of one-dimensional low temperature scanning microscope (LTLSM). Grain boundary (GB) being a weak link is temperature sensitive, thus it can be used as a local thermal sensor. In experiments DC bias current was higher than the GB critical current, but much lower than the intra-grain one. At 10 Hz modulation frequency the detected GB trace was found to be about 300 micrometers. Upon the frequency increase, the trace diminishes and for 200 kHz was about 10 micrometers. Theoretical analysis shows that thermal length varies inversely as the square root of the frequency, and phase delay is in direct proportion. We have shown phase delay between modulating and response signals to be in a good agreement with theoretical frequency dependence. Effective thermal diffusivity coefficient calculated from the above dependence is close to that of the SrTiO3 and indicates that heat diffusion is much influenced by the substrate. Hence this result demonstrates the potential of the LTLSM for obtaining effective in-plane thermal diffusivity of multilayered superconducting tapes and films.

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.

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.

Grain boundary transport properties in YBa/sub 2/Cu/sub 3/O/sub x/ coated conductors

Critical current data obtained as a function of magnetic field on an isolated grain boundary (GB) of a coated conductor and two other types of bicrystal GBs of YBa/sub 2/Cu/sub 3/O/sub x/ show a peak in the critical current and an unusual hysteresis. These results provide support for a new mechanism for enhanced GB critical currents, arising from interactions of GB vortices with pinned Abrikosov vortices in the banks of a GB, as suggested by Gurevich and Cooley (1994). A substantial fraction of this enhancement, which can exceed a factor of ten, also occurs upon surpassing the critical current of the grains after zero field cooling. A bulk GB and thin film GBs show qualitatively identical results.

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.