High critical currents in powder in tube MgB2 wires: influence of microstructure and heat treatments

MgB₂ wires were produced by the powder-in-tube method, using commercial MgB₂ powder with 5%at Mg powder added as an additional source of magnesium, and stainless steel as sheath material. Depending on the parameters of the heat treatments, the dc transport critical current can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire. Analysis of the zero-field-cooling dc magnetization demonstrates that these large variations are due to changes in the connectivity of the wires. We discuss the optimization of the annealing conditions in terms of the changes in the microstructure, as determined from TEM and SEM analysis. We show that annealing can strongly improve the connectivity by eliminating most of the micro-cracks present in the un-annealed wires. In contrast, inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg.