Properties of superconducting MgB_2 wires: "in-situ" versus "ex-situ" reaction technique

TL;DR

This study compares 'in-situ' and 'ex-situ' fabrication techniques for MgB_2 superconducting wires, showing that 'in-situ' methods yield better inter-grain connectivity and higher critical current densities, with vortex pinning mainly at grain boundaries.

Contribution

It demonstrates that 'in-situ' preparation improves inter-grain connectivity and critical current densities in MgB_2 wires compared to 'ex-situ' methods.

Findings

'In-situ' wires have higher critical current densities than 'ex-situ' wires.

Vortex pinning is primarily due to grain boundaries.

Differences in flux-jump instabilities are discussed.

Abstract

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) 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.