Development and Superconducting Properties of Practical 18- and 30-Filamentary MgB2 Long Wires with Twisting by the In Situ Way

In this work, practical kilometer-length 18- and 30-filament Monel/Nb/MgB2 wires have been prepared by in situ powder-in-tube (PIT) method, combing in the twisting of wires with twisting pitches (TP) of 5 mm, 10 mm, and 20 mm. Transport critical current density (Jc) properties at various temperatures and magnetic fields for MgB2 wires with different TP and the effects of the heat-treatment conditions and wire diameter size on the Jc of 30- filament wire have been investigated in detail. The results suggest the Jc of MgB2 wire without twisting at 4.2 K, 4 T, and 20 K, 3 T separately reaches 1.9 × 105 and 5.87 × 104 A/cm2 for 30-filament wire, and 1.48 × 105 and 4.4 × 104 A/cm2 for 18-filament wire. After twisting, as the TP reduction, the Jc degradation of MgB2 wire is much slower in 30-filament than that in 18-filament at various magnetic fields. At the TP of 5 mm, the Jc degradation comparing with the untwisted wires at 4.2 K and 4 T is up to 30% in 18-filament wire but only about 10% in the 30-filament. This work indicates that the twisting route is very promising to the engineering application of MgB2 superconductor.