Superconducting properties and growth mechanism of layered structure in MgB2 bulks with Cu/Y2O3 co-doping
Bulk MgB2 samples containing Cu and Y2O3 have been prepared by conventional solid state reaction at 850 °C, and the structure and superconducting properties have been investigated. Differing from the structure in previous studies, a type of novel, layered structure was obtained in Cu/Y2O3-doped MgB2 sample. Furthermore, the critical current density (Jc) at high field (>4 T) was improved compared to undoped, and Cu-doped MgB2 samples. After analyzing the phase composition, micro-structure, and sintering process, it was found that Y2O3 and Cu are independent in providing effective pinning centers, and YB4 impurity should be responsible for the enhancement of Jc as well as the increased irreversible magnetic field. However, Jc at low field was slightly worsened, but still maintaining 105 A cm-2 at 0 T, since the intercrystalline connectivity was not seriously deteriorated. Finally, a possible growth model was put forward to describe the formation sequences of the layered structure. It was supposed that the formation of steps at low temperature originated from the coherent relationship between Mg and MgB2, while the steps formed at high temperature were related to the pinning effect of secondary phase during the migration of grain boundary. Springer Science+Business Media New York 2012.