MICROSTRUCTURAL ANALYSIS OF HIGH CRITICAL CURRENT-DENSITY AG-CLAD BI-SR-CA-CU-O (2/2/1/2) TAPES

The critical current density (J(c)) values of variously processed Ag-clad Bi2Sr2Ca1Cu2Ox tapes are related to their microstructures as observed by TEM, SEM, and X-ray pole figure analysis. The coupled microstructure-J(c) results confirm the general importance of a highly c-axis aligned microstructure for high critical current density. However, TEM experiments revealed a "colony" microstructure in all sample in which groups of several grains with nearly perfect c-axis alignment (colonies) are formed but where the c-axes of adjacent colonies are not parallel. SEM imaging of etched cross-sections showed that important features of this colony misalignment are not accurately reflected by stand alone X-ray pole-figure analysis or TEM imaging. Near-perfect alignment occurs within a few microns of the Ag cladding for all processing methods. However, the processing details control the macroscopic grain alignment within the bulk of the superconductor. In particular, second-phase particles disrupt the alignment of macroscopic colonies. These misalignments may control the transport critical current, yet they are not clearly evidenced by either macroscopic pole-figure analysis or by very local TEM examination.