Significant enhancement of irreversibility field in clean-limit bulk   MgB2
  Low resistivity ("clean") MgB2 bulk samples annealed in Mg vapor show an increase in upper critical field Hc2(T) and irreversibility field Hirr(T) by a factor of 2 in both transport and magnetic measurements. The best sample displayed Hirr above 14 T at 4.2 K and 6 T at 20 K. These changes were accompanied by an increase of the 40 K resistivity from 1.0 to 18 microohm-cm and a lowering of the resistivity ratio from 15 to 3, while the critical temperature Tc decreased by only 1-2 K. These results point the way to make prepare MgB2 attractive for magnet applications. 
V. Braccini, L.D. Cooley, S. Patnaik, P. Manfrinetti, A. Palenzona,   A.S. Siri, and D.C. Larbalestier