Microstructures and critical currents in melt-processed YBCO

YBaCuO bulk superconductors prepared by the melt-powder-melt-growth process (MPMG) have a high critical current density. It has been found that non-superconducting Y₂BaCuO₅ (211) particles dispersed in the 123 superconducting phase can work as strong pinning centres, resulting in higher J_c values. Though 211 particles are considered to be the main pinning centres, defects such as twins and stacking faults are observed in melt-processed YBCO. In this study the effect of twin boundaries on the flux pinning was measured magnetically. The results indicate that twin planes contribute to flux pinning, but that their effect is not large in MPMG samples.