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

摘要

Undoped magnesium diboride (MgB_2) bulks were systematically synthesized by using in situ/ex situcombination method. Herein, using ex situ powder could act as a seed material to further improve adhesion oninterface with MgB_2 formed by in situ process and accelerate MgB_2 phase formation. Crystallite size wasdecreased and microstrain increased from microstructure and X-ray analyses, while lattice parameters do notvary noticeably in the in situ/ex situ combined samples. Even if packing density is gradually enhanced withthe increasing ex situ powder, connectivity became poorer. Under our experimental condition, all the criticalcurrent density (J_c), upper critical field (Bc_2), and irreversibility field (B_（irr）) were obviously enhanced in in situMgB_2 by adding 25 wt.% of ex situ MgB_2.