Critical current densities of superconducting MgB2 tapes prepared on the base of mechanically alloyed precursors

Cu- and Fe-cladded monofilamentary tapes and 19-filamentary tapes with Fe matrix have been prepared by the powder-in-tube (PIT) method using mechanically alloyed precursor powders consisting of the constituents Mg, B, and MgB2. Despite of the low T-c values of about 31 K for monofilamentary tapes with Fe sheath maximum critical current densities (J(c)) of 29 and 6 kA/cm(2) at 4.2 K in external magnetic fields of 7.5 and 10 T, respectively, were achieved. The maximum J(c) value of 18.5 kA/cm(2) at 20 K in a field of 2 T was obtained for these conductors. The heat treatment of the monofilamentary tapes under uniaxial pressure of 0.5 GPa led to enhanced J(c) values compared to those of tapes annealed under normal pressure. Up to 27 and 11 kA/cm2 were achieved at 4.2 K in external fields of 8 and 10 T, respectively, for samples annealed under pressure. For the 19-filamentary tape J(c) values up to 35 and 9 kA/cm(2) in fields of 7.5 and 10 T, respectively, were measured at 4.2 K. Investigation of the microstructure by optical microscopy, SEM/WDX and XRD have shown that the high J(c) values may be mainly due to the remarkably small grain size of the MgB2 phase and defects particularly precipitates of magnesium oxide. J(c) of the Cu-cladded tapes is one order of magnitude below that of Fe-cladded ones. The main reason for this discrepancy is the higher porosity of the superconducting cores with Cu sheath compared to that with Fe sheath. At higher sintering temperatures strong interactions between the Cu sheath and constituents of the precursor contribute to the suppression of the superconducting properties of Cu-sheathed conductors. (C) 2004 Elsevier B.V. All rights reserved.