Fishtail effects and improved critical current density in polycrystalline bulk MgB₂ containing carbon nanotubes

Bulk, polycrystalline MgB₂ samples containing 2.5 wt.% multi-walled carbon nanotubes (CNTs) have been prepared by conventional solid state reaction at 800 °C. The effect of Mg precursor powders composed of two different particle sizes on the critical current density (J_c) of the as-sintered samples has been investigated. An enhancement of J_c at high field has been observed in MgB₂ samples containing CNTs prepared with fine Mg powders, whereas the values of J_c in the sample prepared using the coarser Mg powders was slightly decreased. These results contrast significantly with measurements on pure, undoped, MgB₂ samples prepared from the same Mg precursor powders. They suggest that carbon substitution into the MgB₂ lattice, which accounts for increased flux pinning, and therefore J_c, is more effective in precursor Mg powders with a larger surface area. Rather surprisingly, the so-called fishtail effect, observed typically in MgB₂ single crystals and in the (RE)BCO family of high temperature superconductors (HTSs), was observed in both sets of CNT-containing polycrystalline samples as a result of lattice defects associated with C substitution. Significantly, analytical fits to the data for each sample suggest that the same flux pinning mechanism accounts for the fishtail effect in polycrystalline MgB₂ and (RE)BCO.