Enhanced critical current density in nanocrystalline mechanically alloyed MgB/sub 2/ bulk and Fe sheathed tapes

The application of the mechanical alloying (MA) technique for MgB/sub 2/ powder, tape and bulk preparation is a unique tool to obtain enhanced magnetic flux pinning by a combination of microstructure refinement and exact stoichiometry control of the MgB/sub 2/ compound. Additionally, variation of the stoichiometry as well as doping with MgO and SiO/sub 2/ lead to substantially increased critical current densities compared to the stoichiometric composition. MgB/sub 2/ bulk samples with a Mg excess of 5 wt.% show a critical current density J/sub c/ of about 1.5/spl middot/10/sup 6/ A/cm/sup 2/ at 7.5 K in self field. Doping with MgO, which improves J/sub c/ (1.6/spl middot/10/sup 6/ A/cm/sup 2/ at 7.5 K in self field), is more effective than doping with SiO/sub 2/. Stoichiometric and Mg-enriched partially reacted MA MgB/sub 2/ precursor powders were also used for powder-in-tube (PIT) tape fabrication with Fe sheath. By using this precursor the tapes can be annealed at relatively low temperatures of 500-600/spl deg/C. Despite reduced T/sub c/ values of 29-32 K, maximum critical current densities J/sub c/ of 35 kA/cm/sup 2/ and 9 kA/cm/sup 2/ in external magnetic fields of 7.5 T and 10 T, respectively, are achieved at 4.2 K. Microstructure investigations reveal that the high J/sub c/ values may be mainly due to the remarkably small MgB/sub 2/ grain size and defects, particularly MgO precipitates.