Micro structural homogeneity and electromagnetic connectivity of YBa₂Cu₃O_7-δ grown on rolling-assisted biaxially textured coated conductor substrates

The electromagnetic connectivity and microstructure of three YBa₂Cu₃O_7-δ (YBCO) films grown on biaxially textured substrates were investigated by magneto optic (MO) imaging and scanning electron microscopy (SEM). The films were deposited by pulsed laser deposition (PLD) on yttria-stabilized zirconia (YSZ) and CeO₂-buffered, biaxially textured Ni tapes. The transport critical current density (J_c) values of the films were 0.3, 0.6 and 0.7 MA/cm² (77 K, 0 T). MO imaging revealed clearly granular electromagnetic behavior in the lowest J_c and one of the higher J_c samples, but considerably better connectivity in the sample with a J_c value of 0.6 MA/cm². High resolution SEM showed a dense and rather featureless microstructure in the YBCO of the most highly electromagnetically connected sample, whereas pores and/or second phase particles cluttered the YBCO layers of the granular samples. Thus, the granular behavior in these samples appears to be caused by pores and second phase particles that locally obstruct the superconducting current in the YBCO layer. Control of these types of defects clearly is important for raising the J_c value.