Properties of superconducting MgB₂ wires: in situ versus ex situ reaction technique

We have fabricated a series of iron-sheathed superconducting wires prepared by the powder-in-tube technique from (MgB₂)_1−x:(Mg+2B)_x initial powder mixtures taken with different proportions, so that x varies from 0 to 1. It turned out that ex situ prepared wire (x = 0) has considerable disadvantages compared to all the other wires in which in situ assisted (0 < x < 1) or pure in situ (x = 1) preparation was used due to weaker inter-grain connectivity. As a result, higher critical current densities J_c were measured over the entire range of applied magnetic fields B_a for all the samples with x > 0. Pinning of vortices in MgB₂ wires is shown to be due to grain boundaries. J_c(B_a) behaviour is governed by an interplay between the transparency of grain boundaries and the amount of 'pinning' grain boundaries. Differences between thermo-magnetic flux-jump instabilities in the samples and a possible threat to practical applications are also discussed.