Structure and pinning centres in MgB2 bulk, wires and thin films and in MT-YBCO

Abstract

The structure and composition of MgB2-based materials (bulk, wires and thin films) prepared at different pressures (0.1 MPa–2 GPa) and temperatures (600–1050 oC), and melttextured YBa2Cu3O7–δ (MT-YBCO) oxidized in oxygen flow at 440 oC and under hydrostatic pressure of 16 MPa at 800 oC, which demonstrated high critical current densities jc, were analysed by X-rays, SEM–EDX and TEM. Correlations between the character of material inhomogeneities, which can be pinning centres and influence superconducting characteristics, are discussed. The  ffect of defects such as twins, dislocations, macrocracks and microcracks on critical current density and mechanical characteristics of MT-YBCO is considered. Regularly distributed nanostructural inhomogeneities responsible for pinning and connected with Mg, B and O content variation in the nanoscale were observed in all types of the MgB2-based materials. The effect of homogeneity of the matrix phase of MgB2 wires, with and without additions of carbon and dysprosium oxide, as well as the ratio of Mg to B on the critical current densities in the wires, were established.