Influence of intermediate annealing on the microstructure of in situ MgB2/Fe wire

A two-step process with an intermediate annealing was used in an attempt to improve the critical current density (Jc) through a core densification of in situ MgB2/Fe wires. MgB2/Fe wires were first drawn to an outer diameter of 1.9mm, and heat-treated at 600º for 0.5, 1, 2 and 4 h in flowing Ar gas as an intermediate annealing (IA) step. In the cases of the wires heat-treated at 600ºC for 0.5-2h, un-reacted Mg and MgB2 phases coexisted. The sample heat-treated for 4 h showed a full MgB2 formation through a solid state reaction. The intermediate-annealed wires were then further drawn to an outer diameter of 1.42 mm as an attempt to reduce the porosity. This second mechanical deformation crushed the super-conducting grains into smaller pieces and mixed preformed MgB2 phase with un-reacted Mg and B powders. And then, the second heat-treatment was performed at 650ºC for 0.5 h. The Jc values of the MgB2 wires prepared with an intermediate annealing step were notably lower, over a wide range of magnetic fields, when compared to the reference sample heat-treated once at 650ºC for 0.5 h. The decrease in Jc is probably due to the introduction of a large amount of weak links between the grains.