Properties of superconducting MgB2wires:in situversusex situreaction technique

We have fabricated a series of iron-sheathed superconducting wires prepared by the powder-in-tube technique from (MgB 2 ) 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 MgB 2 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.