High transport critical currents in dense monofilamentary Fe- and Ni-clad MgB/sub 2/ tapes and MgB/sub 2//Fe wires with 7 filaments

Highly dense monofilamentary MgB/sub 2//Ni and MgB/sub 2//Fe tapes and multifilamentary MgB/sub 2//Fe wire with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. High density of the MgB/sub 2/ core and a fine starting powder appear to be decisive factors for reaching high critical current densities. After annealing, we measured transport j/sub c/ values up to 2.3/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K in a field of 1.5 T (corresponding to an I/sub c/ of 300 A) in a MgB/sub 2//Ni tape and up to 10/sup 4/ A/cm/sup 2/ at 4.2 K in 6.5 T in a MgB/sub 2//Fe tape. For higher currents these monofilamentary tapes quenched due to insufficient thermal stability. Thus, j/sub c/ at 4.2 K, 0 T can for instance only be extrapolated, yielding a value close to 1 MA/cm/sup 2/. The recrystallization during annealing leads to a densification and to j/sub c/ values which are more than a factor of /spl sim/10 higher than those measured in the as-deformed tapes. Ni shows a limited reaction with MgB/sub 2/, leading to the formation of Mg/sub 2/Ni reaction layers between the filament and the matrix. On the other hand, Fe stays chemically inert and turns out to be an excellent candidate material for preparation of multifilamentary MgB/sub 2/ tapes. A transport j/sub c/ value of 9/spl times/10/sup 4/ A/cm/sup 2/ at 4.2 K and 0.75 T in MgB/sub 2//Fe wire with 7 filaments produced by groove rolling was obtained.