Structural Defects of Superconducting Core of the Single Fiber MgB₂/Nb,Cu Composite

The microstructure of the MgB₂ core of the single fiber composite consisting of MgB₂, the Nb barrier, and the Cu shell (MgB₂/Nb,Cu), which is synthesized by the powder-in-tube method with an ex situ option and by subsequent annealing, has been studied. It is shown that a dislocation microstructure that exhibits high thermal stability is formed in the MgB₂ core during cold deformation, in addition to powder compaction. A high dislocation density is observed inside MgB₂ grains. Dislocations form walls with low misorientation angles between subgrains. Annealing at a temperature of 900°C for 1 h leads to a higher density of MgB₂ ceramics, and the intergranular contact area increases. Moreover, MgO inclusions with a size of 10 nm or less are formed. Thus, various kinds of structural defects are formed, which can be considered as probable pinning centers for the magnetic flux.