Fine-filamentaryin situMgB₂wires

Abstract

Nineteen-filament MgB 2 /Ti/Cu/SS composites have been made by an in situ approach using only drawing deformation to a filament size of 16.6-58 μm. Circular cables containing 133 filaments were assembled from 1 + 6 strands of diameter 0.25 mm and 0.375 mm. All wires and cables were heat treated at 800 • C/0.5 h in pure Ar atmosphere. The critical current densities J c of the different wire sizes were measured and EDX element mapping was done for some of them. It was found that the critical current density decreases with reduced filament size, especially below the filament diameter of 27 μm due to copper penetration through the thin Ti barrier (<10 μm) and consequent contamination of MgB 2 . In spite of Cu contamination, J c > 10 4 A cm −2 was measured at 4.2 K and field 5.5 T for the thinnest 16.6 μm filaments published up to now.

EDX and ion beam treatment studies of filamentary in situ MgB2 wires with Ti barrier

“…Recently, 19 filaments in situ MgB 2 /Ti/Cu/SS wires of 0.86 mm in diameter have been reported with the best performance after annealing at 800 • C/0.5 h [6]. In our previous work we have found decreasing critical current density of this wire type with reduced diameter especially for diameters smaller than 0.4 mm [8]. This J c degradation was attributed to thin Ti barrier (<10 m), which is not able to protect MgB 2 filaments against the Cu penetration during annealing.…”

“…For improvement of in-field critical current density 5 wt% of SiC was added into a mixture of Mg and B powders [9,10]. Cold drawing deformation was applied from the composite wire diameters between 0.860 mm and 0.375 mm [8]. Short wire samples were heat treated at 800 • C/0.5 h in pure argon atmosphere.…”

“…1c), which reflects worse filament's quality and consequent transport current redistribution for thinner wire. The reduction of wire diameter (0.86 mm → 0.375 mm) leads to smaller filaments (58 m → 25.7 m) and also to thinner Ti barrier (see Table 1), which is not able to protect MgB 2 filaments against the Cu diffusion [8].…”

High temperature heat treatment on boron precursor and PIT process optimization to improve theJcperformance of MgB2-based conductors

“…In fact, more fine the precursor powders more chemically homogeneous the final superconducting product. Furthermore, very fine MgB 2 grain size plays a crucial role in the development of the multi-filamentary tapes [8,10,11].…”

“…Recently our group developed a new scalable method to synthesize boron [9] that permits to obtain an improved precursor for MgB 2 available at good prices. Main advantage of our process is production of nano-metric particle size boron powder (average grain size 100 nm [9]), which is very important to produce multi-filamentary and thinner conductors [8,10,11]. Recently by Brunauer-Emmet-Teller (BET) analysis an average grain size of 60 nm has been confirmed.…”