Microstructure of undoped and C-doped MgB2 wires prepared by an internal magnesium diffusion technique using different B powders

MgB2/Ti/Cu single core wires were prepared by IMD or MIMD techniques using four different B powders. The MgB2 microstructure was analysed by XRD, SEM and TEM to correlate it to the purity and particle size of the B precursors and resulting wire superconducting properties. No direct relation was observed between the B powder particle size and MgB2 grain size. A small residual content of Cl of about 1 wt% in the B precursor did not influence the MgB2 phase as it did not change its critical temperature Tc. However, as a Cl-rich compound remained distributed in an inter-grain space, it significantly decreased the wire critical current density Jc and MgB2 compactness. A difference in the Jc of two similar Cl-free MgB2 wires was attributed to the presence of many stacking faults and low-angle boundaries in MgB2 grains, which can act as additional pinning centres.