Critical Currents in Bulk MgB_2 Superconductors

摘要

Bulk superconducting MgB_2 materials were produced by (ⅰ) hot deformation of MgB_2 pellets prepared by solid-state reaction, (ⅱ) high-pressure high-temperature sintering of commercial MgB_2 powder, (ⅲ) hot pressing of mechanically alloyed MgB_2 powder and (ⅳ) preparation of wires and tapes by the powder-in-tube method using commercial and mechanically alloyed MgB_2 powders, respectively. The comparison of critical currents j_c of all the samples produced by the various methods shows that the highest j_c values (about 5·10~5 A/cm~2 at 10 K and 2 T) can be obtained for samples prepared from mechanically alloyed and hot pressed MgB_2 powders. For tapes, j_c values up to 2.2·10~4 A/cm~2 were measured at 4.2 K and 7.5 T. These values are higher than those reported in literature until now regarding materials without definite additives to MgB_2. It is assumed that the strong pinning of flux lines is (ⅰ) mostly due to the small grain size of MgB_2, but (ⅱ) also to fine WC particles found in mechanically alloyed powders, generated by abrasion of the used WC balls and containers during the milling process, and to nano-scaled oxide particles.