Improved critical current density property in ex situ processed MgB2 tapes using filling powders with metallic particle addition

MgB2 powders milled with low-melting-point metallic particles, Bi and In, and Fe-sheathed tapes using those milled powders were sintered at various temperatures. With increasing the amount of those additives, the wear of WC jar and balls during milling process is promoted. Compared with In addition, Bi addition clearly causes the formation of some amorphous or poor crystalline phase in the as-milled state. Concerning the sintered tape samples, the optimal sintering temperature (T-s(in)), at which best performance in transport critical current density (J(c)) at 4.2 K in 10 T is obtained, is 680 degrees C for the tapes using the milled powders with In addition. This temperature is lower than that by 30 degrees C for the tapes without additives. Furthermore, In addition promotes the reaction at the interface between the MgB2 core and the Fe sheath. In contrast, Bi addition appreciably causes neither reduction of the optimal T-sin nor the promoted reaction at the interface. Although both additives hardly bring about critical temperature (T-c) reduction, only Bi addition can improve the transport J(c) property. This J(c) enhancement is mainly attributed to improved grain connectivity. In addition itself can be effective for grain connectivity improvement as well. However, reduced upper critical and irreversibility fields (H-c2 and H-irr) brought about by the addition degrade the J(c) property.