The grain boundary was seen as being an amorphous region which extended from about 5 to 20nm and had a metallic character. This observation supported the concept of proximity coupling between the grains, and explained why the super-current density did not degrade in this material.

Grain Boundaries as Weak Links - the Case of MgB2 with Reference to YNi2B2C. S.B.Samanta, H.Narayan, A.Gupta, A.V.Narlikar, T.Muranaka, J.Akimitsu: Physical Review B, 2002, 65[9], 092510 (3pp)