Distribution of second phases in the ceramic superconductor YBCO

The microstructure, composition, morphology and distribution of second phases in the YBCO superconductor have been investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning Auger microprobe (SAM) and optical polarising microscopy (OPM). The vast majority of 123 grains show the 'twin layer' structure (with 5-100 nm width). The twin boundary is usually a clean interface. A few second-phase domains are present between 123 grains (which have an average size of 1 mu m). Some microdomains ( approximately several tens of nm) of non-superconductive phase are present in a 123 grain. The majority of grain boundaries in the authors samples are free of second phases. A few of the boundaries usually contain second-phase particles and amorphous material and have a multilayer structure. SAM data suggest that these boundaries are rich in Ba and C and deficient in Cu and Y. SEM of the boundaries show that the second-phase particles contain barium carbonate, copper oxide and other carbides. The influence of second phases on the superconductivity of materials is also discussed.