202 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
Direct visualization of iron sheath shielding effects in MgB 2 superconducting wires
Abstract Local magneto-optical imaging and global magnetization measurement techniques were used in order to visualize shielding effects in the superconducting core of MgB 2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can provide a Meissner-like state in the superconducting core in applied magnetic fields up to ∼1 T. The maximum shielding fields are shown to correlate with the saturation fields of magnetization in Fe-sheaths. The shielding has been found to facilitate the appearance of an overcritical state, which is capable of achieving a critical current density (J c ) in the core which is larger than J c in the same wire without the sheath by a factor of ∼2. Other effects caused by the magnetic interaction between the sheath and the superconducting core are discussed. New promising horizons for applications of superconducting wires were opened up by the discovery of the MgB 2 superconductor [1]. MgB 2 wires have been reported to have high values of the critical current density J c , in particular in wires sheathed by mechanically hard and magnetically soft materials such as iron (Fe) Fe-sheathed MgB 2 mono-core (MC) and multifilamentary (MF) superconducting wires were investigated. A detailed description of the wire manufacturing procedure and its characterization is given elsewher
Direct visualization of iron sheath shielding effect in MgB_2 superconducting wires
Local magneto-optical imaging and global magnetization measurement techniques
were used in order to visualize shielding effects in the superconducting core
of MgB_2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can
provide a Meissner-like state in the superconducting core in applied magnetic
fields up to ~1T. The maximum shielding fields are shown to correlate with the
saturation fields of magnetization in Fe-sheaths. The shielding has been found
to facilitate the appearance of an overcritical state, which is capable of
achieving a critical current density (J_c) in the core which is larger than J_c
in the same wire without the sheath by a factor of ~2. Other effects caused by
the magnetic interaction between the sheath and the superconducting core are
discussed.Comment: 4 pages, 3 figure
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