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

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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 Jc were measured over the entire range of applied magnetic fields Bₐ for all the samples with x > 0. Pinning of vortices in MgB₂ wires is shown to be due to grain boundaries. Jc(Bₐ) behavior 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.