Mechanism for high critical current density inin situMgB₂wire with large area-reduction ratio

Abstract

A comparative study of in situ MgB 2 wire and MgB 2 bulk was carried out to clarify the mechanism for the high critical current density, J c , in the practical in situ MgB 2 wires. The in situ MgB 2 wire was manufactured with an area-reduction ratio of 99.93%, which was one of the highest values in MgB 2 superconducting wires previously reported. The electrical connectivity, K , and the flux pinning strength, F p , which are important factors in explaining the behavior of J c , could be determined in the same manner as those for the bulk sample; K was well understood with the three-dimensional percolation model, and F p was effectively explained by the electron scattering mechanism by grain boundaries. On the other hand, the area-reduction process dramatically enhanced the value of K , leading to an increase in the value of J c . The respective values of K and J c (20 K, ∼0 T) reached 0.24-0.34 and 6.0 × 10 3 -8.4 × 10 3 A mm −2 , which were twice or three times higher than those of typical in situ bulks. This is because the plastic deformation of magnesium particles increased the packing factor of raw powders through a repetitive drawing process.

Enhanced trapped field in MgB2bulk magnets by tuning grain boundary pinning through milling

“…The transport measurements revealed that the residual resistivity (ρ 40 K ) increased, and the relative resistance ratio (RRR) decreased with milling (table 1). The connectivity-compensated residual resistivity ρ 0 = Kρ 40 K [9,28] and corresponding impurity parameter α i for the samples milled with 0, 2.1, and 17 MJ kg −1 were 2.1, 5.7, and 12.3 μΩ cm and 0.13, 0.35, and 0.76, respectively, suggesting enhanced electron scattering due to lattice distortion. However, the connectivity slightly increased with milling presumably due to improved microstructural uniformity, mainly by a reduction of large voids, which would increase the current percolation length.…”

“…MgB 2 exhibits unique superconducting characteristics, such as multi-gap superconductivity with Δ π (0) = 2.2 meV and Δ σ (0) = 7.1 meV [2,3], a high upper critical field μ 0 H c2 //ab (0) ∼ 48 T and μ 0 H c2 //c (0) ∼ 33 T with small anisotropy ∼1.5-4.5 [4,5], and a weak-link free transport critical current in polycrystalline forms [6]. A high critical current density J c has been developed in polycrystalline bulks, tapes, and wires, and currently over 10 5 A cm −2 is realized in the temperature range of 4-30 K using various fabrication processes [6][7][8][9][10][11]. Because MgB 2 is a simple binary line-compound composed of light and abundant elements, magnesium and boron, and can be operated at cryogen-free temperatures, MgB 2 is one of the most promising candidates for cryo-magnet applications.…”

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].…”

Transport Properties of Single MgB2 Filaments Extracted From Multifilamentary Conductors

“…Researchers have used the temperature dependent resistivity as a benchmark for the effective current path in bulk and monocore wires. More recently magnetic Jc or 3D tomography percolation models and parametrical indicators such as tortuosity have been used on multifilamentary conductors, [1][2][3][4]. The variations in density for in-situ wire are significantly more than for ex-situ and there is clear evidence for a more percolative current path.…”