Improvement in microstructure and superconducting properties of single-filament powder-in-tube MgB₂ wires by cold working with a swaging machine

We investigated the influence of the mechanical deformation method of wire fabrication on the microstructure and superconducting properties of single-filament in situ powder-in-tube (PIT) MgB₂ wires. We employed three deformation methods to fabricate the wires: only swaging, groove rolling + roller drawing, and groove rolling + conventional drawing. We found that cold working by swaging has three advantages over the groove rolling + drawing method: (1) improved uniformity of the MgB₂ core along the longitudinal direction; (2) higher mass density of the Mg + B (MgB₂) core before (after) heat treatment (HT); and (3) well-developed fiber structures of Mg (MgB₂) before (after) HT. These three factors greatly enhanced the critical current density (J _c) values of PIT MgB₂ wires. The highest J _c values were obtained through mechanical deformation by swaging for both pure and carbon-doped wires. A J _c value of 3.5 ×10⁴ A cm^-2 and an engineering critical current density (J _e) of 1.1 ×10⁴ A cm^-2 were recorded at 4.2 K and 10 T for a swaged wire of 4.5%-carbon-coated boron powder heat-treated at 600 °C for 1 h.