  This work studies the influence of microstructures and crystalline defects on
the superconductivity of MgB2, with the objective to improve its flux pinning.
A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have
significantly increased critical current density (Jc) at high fields than its
un-HIPed counterpart. The HIPed sample had a Jc of 10000 A/cm2 in 50000 Oe (5
T) at 5K. This was 20 times higher than that of the un-HIPed sample, and the
same as the best Jc reported by other research groups. Microstructures observed
in scanning and transmission electron microscopy indicate that the HIP process
eliminated porosity present in the MgB2 pellet resulting in an improved
intergrain connectivity. Such improvement in intergrain connectivity was
believed to prevent the steep Jc drop with magnetic field H that occurred in
the un-HIPed MgB2 pellet at H > 45000 Oe (4.5 T) and T = 5 K. The HIP process
was also found to disperse the MgO that existed at the grain boundaries of the
un-HIPed MgB2 pellet and to generate more dislocations in the HIPed the
pellets. These dispersed MgO particles and dislocations improved flux pinning
also at H<45000 Oe. The HIPing process was also found to lower the resistivity
at room temperature.
