Effect of Packing Density on Critical Current Density at High Magnetic Fields in Polycrystalline MgB2 Superconductors

Abstract

It is well understood that the critical current density in polycrystalline MgB2 superconductors is severely restricted by porosities and wet insulating layers that cover superconducting grains. The relationship between critical current density and electrical connectivity is well explained by the prediction of the percolation theory. Recently, it has been found that the irreversibility field that directly influences the high-field performance of the critical current density is also dependent on electrical connectivity. This behavior is explained by the finding that superconducting grains with the c-axis tilted by a small angle from the direction of applied magnetic field lose their superconductivity at relatively low magnetic fields owing to the low irreversibility field, resulting in a decrease in the effective packing density. This phenomenon is theoretically investigated using the percolation theory and the theory of flux creep and flow. The obtained result explains the experimental result well. This shows that the packing factor of MgB2 is also important for improving the critical current density at high magnetic fields.