No abstract

SEM, TEM, SAM and X-ray diffraction analyses have suggested that the bulk YBCO samples sintered with the co-precipitation powders are almost free of second phrases. However, a layer with non-123-stoichiometric compositions is always present near the grain boundaries, which forms the dominant zone of the weak links in these samples. The improved critical current density with reducing field dependence can be obtained by the melt-process. It has produced a multiphase microstructure with highly aligned plate-like 123 grains up to several mm in length and several mu m in thickness, which are parallel to the a-b plane, and the grain plate sometimes is the same width as the sample, and boundaries between parallel plates are usually discontinuous. Such structure is very favourable for the improvement of Jc. However, there are still various structure defects, such as second phases (CuO, BaCuO2, BaCO3 and 211 phase), cracks and some interfaces between adjacent superconducting regions with different c axis direction in the melt-processed samples. They would limit further improvement of the Jc of melt-processed samples.