Cu stabilized MgB2 composite wire with an NbTi barrier

Multi-core MgB2 wires with an NbTi barrier protecting the stabilized copper and reinforced with stainless steel have been made. A critical current density of 104 A cm−2 at 4.2 K was measured in the external field of 10 T. Though the filament size is reduced to 26 µm, the Jc values are not much decreased. Circular 1+6 cable containing 49 filaments 35 µm in size was also demonstrated. The NbTi barrier is still superconducting after annealing at 650–850 °C (up to 9 T), which allows easy current transfer and also contributes slightly to the wire Jc (maximally at 7 T). Inter-diffusion between the NbTi barrier and Cu stabilizer was analyzed by energy dispersive x-ray spectrometry. This is influenced by the annealing temperature and by the pressure stress induced by the stainless steel sheath. Ti diffusion worsens the conductivity of Cu stabilization and also changes the mechanical properties of composite wire. Lowered wire resistances to the axial tension were measured for annealing at 850 °C.

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