Understanding routes for high connectivity in ex situ MgB2 by self-sintering

Artificial promotion of self-sintering was studied to clarify the factors associated with higher intergranular connectivity in ex situ MgB2. Starting MgB2 powders with different magnesium precursor powder size and carbon additive in the form of B4C powder were prepared and used to synthesize sintered bulks to investigate the influences of interface impurities and intragrain lattice defects on self-sintering of MgB2. Laboratory-made high purity MgB2 powder with less oxidized surface and carbon-substituted MgB2 powder were effective to stimulate self-sintering at 900 degrees C by increasing the specific surface area and enhancing the driving force of mass transfer. Microstructural analyses showed that ex situ MgB2 bulks synthesized from such pretreated powders demonstrated increased sintered grain boundary area with the evolution of intergrain necks. Record high connectivity ( 35%) was achieved by a sintering period of 24 h, which is one-fourth of the conventional sintering time. The results suggest that carbon substitution stimulates sintering of grain boundaries and increases the upper critical field of the grains; this leads to quantitative and intensive enhancement of flux pinning strength in a high connectivity matrix.