The grain-size dependence of critical current and grain-boundary pinning in bulk MgB2 was reported. By combining polarized optical microscopy and electron back-scattering diffraction, evidence was obtained on special grain boundaries, having a high density of dislocations, which were able to provide high critical currents in MgB2 polycrystals. It was suggested that a reduction of the grain size to the nanoscale level was sufficient to provide the critical current densities required for large-scale applications at the boiling temperature of liquid H.

Grain Boundaries and Pinning in Bulk MgB2. P.Mikheenko, E.Martínez, A.Bevan, J.S.Abell, J.L.MacManus-Driscoll: Superconducting Science and Technology, 2007, 20[9] S264-70