Correlation between grain connectivity, packing density, and critical current density in MgB2 synthesized by in situ/ex situ combination technique.

Undoped magnesium diboride (MgB2) bulks were systematically synthesized by using in situ/ex situ combination method. Herein, using ex situ powder could act as a seed material to further improve adhesion on interface with MgB2 formed by in situ process and accelerate MgB2 phase formation. Crystallite size was decreased and microstrain increased from microstructure and X-ray analyses, while lattice parameters do not vary noticeably in the in situ/ex situ combined samples. Even if packing density is gradually enhanced with the increasing ex situ powder, connectivity became poorer. Under our experimental condition, all the critical current density (Jc), upper critical field (Bc2), and irreversibility field (Birr) were obviously enhanced in in situ MgB2 by adding 25 wt.% of ex situ MgB2.