Origin of the nanocrystalline interface in superconducting Bi-2223/Ag composites: a SEM/HREM study

Superconducting Bi-2223/Ag composites were prepared and the microstructure was thoroughly studied. A 20 nm thick amorphous-nanocrystalline phase was observed at the interface between the superconductor and the Ag. SEM/HREM/EDX observations were used to study this nanolayer and the results point to incongruent melting as the most probable cause of this phenomenon. The change in concentration of the Bi-2223 phase with the addition of Ag is elucidated in terms of the difference between the sintering temperature used and the incongruent melting. It is suggested that the frequently observed penetration of the ceramic phase into the Ag can be explained by the interfacial energy model