Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex situ tapes

Ex situ powder-in-tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H-*, the upper critical field H-c2, and the critical current density J(c)(H) together with the suppression of the anisotropy of all of these quantities. J(c) reached 10(4) A/cm(2) at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and H-c2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced J(c) and H-* values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, even if at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on J(c) is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases H-c2, especially H-c2 perpendicular to the Mg and B planes.