High-performance dense MgB₂superconducting wire fabricated from mechanically milled powder

Abstract

Owing to the relatively high critical temperature and the low manufacturing cost, MgB2 superconducting wires are promising for liquid helium-free superconducting applications. Today, commercially available MgB2 wires are manufactured by either an in situ or ex situ powder-in-tube process, the in situ process being more effective to obtain high critical current density. In in situ-processed wires, however, the critical current density is seriously suppressed by the high porosity of MgB2 filaments. To resolve this problem, we propose an innovative method of using precursor powder prepared by mechanical milling of magnesium, boron, and coronene powders. This precursor powder has a metal–matrix–composite structure, in which boron particles are dispersed in a magnesium matrix. The plastic deformation of the precursor powder through wire processing leads to compact packing, and a dense MgB2 filament is generated after heat treatment. As a result, the limitation of critical current density that occurs for the typical in situ process is overcome, and the practical critical current density of 103 A mm−2 is obtained at 10 K and 6.1 T, at 15 K and 4.8 T, and at 20 K and 3.3 T.

Performance of MgB2 Superconductor Developed for High-Efficiency Klystron Applications

“…For using MgB 2 wire in such intermediate magnetic fields, a carbon additive should be mixed into MgB 2 filaments. The J c value at 20 K of the MgB 2 wire with 3% carbon additive made by Hitachi Ltd. is higher in the range of external magnetic fields of 2.3 T or more [12]. Fig.…”

“…The MgB 2 wire for the klystron magnet, with a unit length of 8 km, was made by the in situ method as follows. Magnesium powder (>99.8%) and boron powder (>98.5%, <250 nm; Pavezyum nano Boron) were mixed at a molar ratio of Mg: B = 1: 2 without a carbon dopant, because at high temperatures and in low magnetic fields a pure MgB 2 wire has higher J c than a carbon-doped MgB 2 wire [12]. Ten filaments with a Fe barrier sheath and a Cu bar as a stabilizer were embedded into a Monel sheath, then cold-worked to be 0.67 mm in outer diameter.…”

High-temperature post-annealing to improve Jc -B-T properties of MgB2 thin film synthesized via hybrid deposition combining thermal evaporation of magnesium and sputtering of boron

“…A high-J c MgB 2 wire synthesized via the PIT technique in a previous study exhibited a J c of 1000 A mm −2 at 10 K under 6 T, 900 A mm −2 at 15 K under 5 T, and 500 A mm −2 at 20 K under 4 T. 22) Sample (b) presented a J c of 32 000 A mm −2 at 10 K under 6 T and 13 000 A mm −2 at 15 K under 5 T. Therefore, the J c of sample (b) was several to several tens of times higher than that of the high-J c PIT wire. Figure 2 presents the J c -B-T properties around J c = 0 of the samples (a) and (b).…”

Superconducting MgB2 Wire Drawing Considering Anisotropic Hardening Behavior and Hydrostatic Effect

“…The weaknesses of the PIT-processed MgB 2 wire are its relatively low superconducting properties, such as the critical current (I c ) and critical current density (J c ) compared to those of MgB 2 bulks, which were simply made by the mechanical pressing and sintering. The reason for the low properties inherent in the wire is the low density of the MgB 2 core and poor grain connectivity, which results from the low packing density of the precursor powders before the heat treatment and volume shrinkage after the heat treatment [2,13,[16][17][18]. For this reason, most of the commercial PIT-processed wire exhibited less than 10% of the full grain connectivity [19], which is estimated by a function of packing density (or porosity) and nucleation of MgO grains or layers at grain boundaries [16].…”