Superior high-field current density in slightly Mg-deficient MgB_2 tapes

摘要

A series of Fe-clad Mg_xB_2 tapes with x varying from 0.5 to 1.2 was prepared by the in situ powder-in-tube method. Slightly Mg-deficient samples showed higher J_c in high magnetic fields, whereas samples with stoichiometric Mg or a slight excess of Mg exhibited better J_c in the low-field region. The sample with x = 0.9 showed the best J_c in the applied magnetic field. The MgB_2 core was porous in Mg-deficient tapes but with smaller grain sizes than the samples with a slight Mg excess due to insufficient grain growth. Some fine nanometer size grains were also observed in the B-rich samples. The enhanced grain boundary pinning due to the smaller grain size may explain the superior high-field J_c property of the slightly Mg-deficient MgB_2 tapes. Our results indicate that preparing MgB_2 samples with a slight excess of Mg may not be advantageous when developing devices for high-field applications.