2 research outputs found
Overcritical state in superconducting round wires sheathed by iron
Magnetic measurements carried out on MgB_2 superconducting round wires have
shown that the critical current density J_c(B_a) in wires sheathed by iron can
be significantly higher than that in the same bare (unsheathed) wires over a
wide applied magnetic field B_a range. The magnetic behavior is, however,
strongly dependent on the magnetic history of the sheathed wires, as well as on
the wire orientation with respect to the direction of the applied field. The
behavior observed can be explained by magnetic interaction between the soft
magnetic sheath and superconducting core, which can result in a redistribution
of supercurrents in the flux filled superconductor. A phenomenological model
explaining the observed behavior is proposed.Comment: 9 pages, 7 figure
Properties of superconducting MgB_2 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 (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.Comment: To be published in Supercond. Sci. Technol. (2003), in pres