Effect of Nanostructural Inhomogeneities on the Superconducting Characteristics of With Enhanced Grain Connectivity

MgB₂-based bulk materials synthesized under 30 MPa-2 GPa pressure demonstrated an enhanced connectivity A_F ~50%-98%,as compared tomostother reports (below50%), and a shielding fraction of 75%-100%. The materials inhibit high critical current densities j_c, but no strict correlation was found between A_F and j_c. On the other side, we found correlations between jc and the distribution of nanostructural inhomogeneities such as impurity oxygen in the form of oxygen-enriched Mg-B-O nanolayers or nanoinclusions and its solution in the MgB₂ matrix and as well with the amount and size of higher magnesium borides MgB_x, x ≥ 4 inclusions. The distribution of boron- and oxygen-enriched (as compared to stoichiometric MgB₂) inhomogeneities affects the type of pinning and, thus, jc, and to some extent, it can be regulated by synthesis temperatures and pressures and by Ti addition. Auger spectroscopy, scanning electron microscopy, and X-ray with Rietveld refinement were used for structural study.