In situ synthesis and superconducting properties of MgB₂ fibers

Superconducting MgB₂ fibers are grown by a diffusion method, in which B filaments are exposed to Mg vapor inside a folded Ta foil over a wide range of temperature and growth time. The as-grown wires with a diameter of about 110 μm are characterized by scanning electron microscopy and energy dispersive X-ray analysis. Surface morphology of the fibers turns out to be dependent on growth temperature and mixing ratio of Mg and B. Radial distribution of Mg ions into B is observed over the cross-sectional area. Transport properties of the MgB₂ fibers are investigated in magnetic fields from 0 to 8 T by use of a physical property measurement system. MgB₂ fibers grown at 900 °C for 2 h show a superconducting transition at 38.1 K with an onset temperature as 41.7 K and ΔT_c < 3.03 K. Resistance of the MgB₂ fiber at room temperature is 4 Ω and residual resistivity ratio (RRR) is estimated as 4.72. It is estimated that the upper critical field H_c2 at 4 K is more than 16 T. In addition, a small amount of magneto-resistance is detected at high magnetic fields.