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Register a new userHigh-energy ball milling and synthesi temperature study to improve superconducting properties of MgB2 ex-situ tapes and wires
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MgB2 monofilamentary nickel-sheated tapes and wires were fabricated by means of the ex-situ powder-in-tube method using either high-energy ball milled and low temperature synthesized powders. All sample were sintered at 920 C in Ar flow. The milling time and the revolution speed were tuned in order to maximize the critical current density in field (Jc): the maximum Jc value of 6 x 10e4 A/cm2 at 5 K and 4 T was obtained corresponding to the tape prepared with powders milled for 144h at 180rpm. Vorious synthesis temperature were also investigated (730-900 C) finding a best Jc value for the wire prepared with powders synthesized at 745 C. We speculate that this optimal temperature is due to the fluidifying effect of unreacted magnesium content before the sintering process which could better connect the grains.
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In DC and AC practical applications of MgB2 superconducting wires an important role is represented by the material sheath which has to provide, among other things, a suitable electrical and thermal stabilization. A way to obtain a large enough amount of low resistivity material in to the conductor architecture is to use it as external sheath. In this paper we study ex-situ multifilamentary MgB2 wires using oxide-dispersion-strengthened copper (GlidCop) as external sheath in order to reach a good compromise between critical current density and thermal properties. We prepared three GlidCop samples differing by the content of dispersed sub-microscopic Al2O3 particles. We characterized the superconducting and thermal properties and we showed that the good thermal conductivity together the good mechanical properties and a reasonable critical current density make of GlidCop composite wire a useful conductor for applications where high thermal conductivity is request at temperature above 30K, such as Superconducting-FCL.
We report on the superconducting performance of the ex-situ SiC doped MgB2 monofilamentary tapes. Polycrystalline powders of MgB2 doped with 5 and 10 wt% SiC were synthesized by conventional solid-state reaction route and characterized for their superconducting performance. It is found that superconducting parameters i.e. upper critical field (Hc2), irreversibility field (Hirr) and critical current density (Jc) are all improved significantly with SiC addition. Also it was found that relatively lower synthesis temperature (700 C) resulted in further improved superconducting parameters. As synthesized powders are used for ex-situ powder-in-tube (PIT) monofilamentary tapes and superconducting parameters are determined. Albeit the superconducting transition temperature (Tc) is decreased slightly (2K) for SiC doped tapes, the superconducting performance in terms of critical current density (Jc), being determined from both magnetization and transport measurements, is improved significantly. In particular the SiC doped and 700 {deg}C synthesized MgB2 tapes exhibited the transport Jc of nearly 10^4 A/cm2 under applied fields of as high as 7 Tesla. Further it is found that the Jc anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB2 tapes are responsible for decreased anisotropy and improved Jc(H) performance.
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.
Ex-situ Powder-In-Tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H*, the upper critical field Hc2 and the critical current density Jc(H) together with the suppression of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced Jc and H* values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, although at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on Jc is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases Hc2, especially Hc2 perpendicular to the Mg and B planes.
The MgB2 superconductor has already demonstrated its applicative potential, in particular for DC applications such as MRI magnets, thanks to the low costs of the raw materials and to its simple production process. However further efforts have still to be made in order to broaden its employment also towards the AC applications such as SFCL, motors, transformers. The main issues are related to the reduction of the AC losses. Some of these can be faced by obtaining multifilamentary conductors with a large number of very fine filaments and, in this context, the powders granulometry can play a crucial role. We have prepared MgB2 starting powders with different granulometries and by the ex-situ P.I.T method we have realized multifilamentary wires with a number of filaments up to 361 and an average size of each filament lowered down to 30 microns. In particular we have studied the relationship between grain and filament size in terms of transport properties and show that the optimization of this ratio is possible in order to obtain suitable conductors for AC industrial applications.
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