Effect of Nanostructural Inhomogeneities on the Superconducting Characteristics of $\hbox{MgB}_{2}$ With Enhanced Grain Connectivity

MgB
₂
-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 MgB
₂
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 MgB
₂
) 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.