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

Highlights: • B powder purity and particle size affect MgB₂ wires prepared by internal Mg diffusion. • No direct relation between B particle size and MgB₂ grain size were observed. • Residual Cl in B decreases Jc due to Cl-rich compound in MgB₂ inter-grain space. • B-rich amorphous grains can accommodate stress in MgB₂ layer. • a presence of stress-induced defects was correlated to a slightly increased Jc. 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.