High density nanocrystalline MgB/sub 2/ bulk superconductors with improved pinning

We report on the synthesis of high density nanocrystalline MgB/sub 2/ bulk samples with improved pinning via mechanical alloying of elemental Mg and B powders at ambient temperatures and hot pressing. The samples exhibit high j/sub c/ at 20 K and 1 T, and an irreversibility line strongly shifted toward higher fields resulting in H/sub irr/ of about 0.8H/sub c2/(T) compared to H/sub irr/(T) of about 0.5H/sub c2/(T) as typically observed for untextured conventional bulk specimens. The improved pinning of this material is attributed to the large number of grain boundaries in the nanocrystalline state.