  The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2
wires, measured by d.c. transport and magnetization, is compared with that of
similar wires annealed at ambient pressure. The HIPed wires have a higher Jc
than the annealed wires, especially at high temperatures and magnetic fields,
and higher irreversibility field (Hirr). The HIPed wires are promising for
applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T
and 26.5 K, and Hirr ~ 17 T at 4 K. The improvement is attributed to a high
density of structural defects, which are the likely source of vortex pinning.
These defects, observed by transmission electron microscopy, include small
angle twisting, tilting, and bending boundaries, resulting in the formation of
sub-grains within MgB2 crystallites.
