Study on microstructure of PMP-processed YBCO containing submicron Y₂BaCuO₅ particles

The microstructure of a PMP-processed YBCO single domain sample with a nominal composition of 123 + 15%211 + 0.5%CeO₂ fabricated by using 211 precursor powder smaller than 500 nm has been studied by SEM, TEM and HREM. The results show that the single domain contained a large amount of 211 particles with an average diameter of about 500 nm. The 211 particles were spherical, about one-fourth of which had one or more global cores smaller than 100 nm. Electron diffraction patterns proved that the cores were Y₂O₃. In addition, a tretragonal-shaped phase with a size about 1 nm was also observed. This phase was proved to be BaCeO₃ by ED and EDA. Therefore, the refinement mechanism of CeO₂ addition on 211 was believed to lie in a reaction between CeO₂ and part of 211 precursor yielding nanometer Y₂O₃ and by-product BaCeO₃. The nanometer Y₂O₃ acted as an inhomogeneous nucleation site for 211, providing more nucleation sites and suppressing the over-growth of 211. A high density of twins with spacing about 60 nm-80 nm was found. Additionally, a kind of twin domain boundary structure was observed. The formation mechanism of the twin domain boundary was discussed. Similar to the sample without CeO₂ addition, no impurity phases were found at the 211/123 boundaries by HREM.