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

We report dc transport and magnetization measurements of the critical current density, Jc, 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 Jc 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 Jc in PIT MgB2.