Enhanced critical current density in nanocrystalline mechanically alloyed MgB2 bulk and Fe sheathed tapes

The application of the mechanical alloying (MA) technique for MgB₂ 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 MgB₂ compound. Additionally, variation of the stoichiometry as well as doping with MgO and SiO₂ lead to substantially increased critical current densities compared to the stoichiometric composition. MgB₂ bulk samples with a Mg excess of 5 wt.% show a critical current density J_c of about 1.5·10⁶ A/cm² at 7.5 K in self field. Doping with MgO, which improves J_c (1.6·10⁶ A/cm² at 7.5 K in self field), is more effective than doping with SiO₂. Stoichiometric and Mg-enriched partially reacted MA MgB₂ 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°C. Despite reduced T_c values of 29-32 K, maximum critical current densities J_c of 35 kA/cm² and 9 kA/cm² 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_c values may be mainly due to the remarkably small MgB₂ grain size and defects, particularly MgO precipitates.