The effect of refinement of carbon-substituted MgB₂ filling powder using various starting powders on the microstructure and critical current density of ex situ processed superconducting tapes

We have studied the effect of refinement of homemade carbon-substituted MgB₂ (MBC) filling powders prepared by using various commercially available Mg and B powders on the microstructure and critical current density (J_c) of Fe-sheathed MgB₂ tapes fabricated through an ex situ process in a powder-in-tube (PIT) technique. The homemade MBC powders contained a substantial amount of MgO as a main impurity phase when using fine Mg powder, whereas the use of coarse Mg powder significantly reduced the formation of MgO. This was independent of the starting B powder. The MgO was present as isolated particles and platelets inside MgB₂ crystallites. Weak acid could dissolve the MgO partially, whereas strong acid could dissolve it almost completely. When using the as-sintered MBC powders, the J_c properties of tapes were very poor. However, they were remarkably enhanced by the refinement of these powders with the acid solutions. The quality of the refined MBC powder prepared with low-cost B powder was comparable to that prepared with high-cost B. The J_c values of tapes using these powders reached about 6 kA cm⁻² at 4.2 K and 10 T. (paper)