HIGH TRANSPORT AND INDUCTIVE CRITICAL CURRENTS IN DENSE Fe- AND Ni- CLAD MgB_2TAPES USING FINE POWDER

摘要

Highly dense monofilamentary MgB_2/Ni and MgB_2/Fe superconducting tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. High density of the MgB_2 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 x 10~5 A/cm~2 at 4.2 K in a field of 1.5 T (corresponding to an J_c of 300 A) in a MgB_2/Ni tape and up to 10~4 A/cm~2 at 4.2 K in 6.5 T in a MgB_2/Fe tape. For higher currents these monofilamentary tapes quenched due to insufficient thermal stability of filament thickness above 50μm. Thus, j_c at 4.2K, 0T can only be extrapolated, yielding a value close to 1 MA/cm~2. 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 MgB_2, leading to the formation of Mg_2Ni 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 multifilamentary tapes with self-field j_c values at 4.2 K in excess of 1 MA/cm~2.