Influence of microstructures and crystalline defects on the superconductivity of MgB₂

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 (J _c) at higher fields than its un-HIPed counterpart. The HIPed sample had a J _c of 10000A/cm² in 50000 Oe (5T) at 5 K. This was 20 times higher than that of the un-HIPed sample, the same as the best J _c 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 J _c drop with magnetic field H that occurred in the un-HIPed MgB₂ pellet at H>45000Oe(4.5T) and T=5K. 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 pellets. These dispersed MgO particles and dislocations improved flux pinning also at H<45000Oe. The HIPing process was also found to lower the resistivity at room temperature. © 2002 American Institute of Physics.