High transport critical currents in dense monofilamentary Fe- and Ni-clad MgB2 tapes and MgB2/Fe wires with 7 filaments

Highly dense monofilamentary MgB₂/Ni and MgB₂/Fe tapes and multifilamentary MgB₂/Fe wire with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. High density of the MgB₂ core and a fine starting powder appear to be decisive factors for reaching high critical current densities. After annealing, we measured transport j_c values up to 2.3×10⁵ A/cm² at 4.2 K in a field of 1.5 T (corresponding to an I_c of 300 A) in a MgB₂/Ni tape and up to 10⁴ A/cm² at 4.2 K in 6.5 T in a MgB₂/Fe tape. For higher currents these monofilamentary tapes quenched due to insufficient thermal stability. Thus, j_c at 4.2 K, 0 T can for instance only be extrapolated, yielding a value close to 1 MA/cm². The recrystallization during annealing leads to a densification and to j_c values which are more than a factor of ∼10 higher than those measured in the as-deformed tapes. Ni shows a limited reaction with MgB₂, leading to the formation of Mg₂Ni 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 MgB₂ tapes. A transport j_c value of 9×10⁴ A/cm² at 4.2 K and 0.75 T in MgB₂/Fe wire with 7 filaments produced by groove rolling was obtained.