Microstructure and high critical current of powder-in-tube MgB₂

We report dc transport and magnetization measurements of the critical current density, J_c , and microstructural analyses of MgB₂ wires fabricated by the powder-in-tube method (PIT) using commercial MgB₂ powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as a sheath material. We identify a weak-link behavior in the as-drawn wire associated with the presence of microcracks. By appropriate heat treatments, we can increase J_c  by more than one order of magnitude due to a recrystallization process promoted by the excess Mg, which results in the elimination of most of our microcracks. In contrast, inappropriate annealing conditions result in a deterioration of connectivity due to an inhomogeneous loss of Mg. Grain size and porosity play only a secondary role in determining J_c  in PIT MgB₂ .