High Critical Fields and Currents in Mechanically Alloyed MgB$_2$

Abstract

The combination of high critical fields H_c2 and irreversibility fields H^irr and a critical current density j_c for example in thin films render MgB₂ promising for applications. However, in commercial powders and conventionally sintered polycrystalline MgB₂ samples and wires these properties are reduced due to weak pinning. In contrast, mechanical alloying of elemental Mg and B powders combined with hot pressing yields high density nanocrystalline bulk samples with grain sizes on the order of 40-100 nm and distinctly improved pinning. T_c, j_c and the H^irr to H_c2 ratio depend strongly on the milling parameters. Optimized preparation conditions render bulk specimens with critical current densities of about 7 ⋅10^5 A/cm^2 at 10 K and irreversibility fields of up to about 17 T. The improved pinning of this material is attributed to the large density of grain boundaries. The nanocrystalline MgB₂ can be used as starting material for the preparation of tapes with Cu or Fe as sheath material. So far, monofilamentary tapes annealed at 773 K yield j_c values of up to 2.2 ⋅10^4 A/cm^2 in external magnetic fields of 7.5 T at 4.2 K.