Superconducting properties and microstructure in MgB₂ bulks, wires and tapes

We prepared a series of MgB₂ bulk samples under different temperatures, holding time and increasing rates in temperature by the solid state reaction. The thermodynamic behavior and phase formation in the Mg-B system were studied by using DTA, XRD and SEM. The results indicate that the formation of the MgB₂ phase is very fast and the high increasing rate in temperature is necessary to obtain high quality MgB₂. In addition, the effects of the Zr-doping in Mg_1-xZr_xB₂ bulk samples fabricated by the solid state reaction at ambient pressure on phase compositions, microstructure and flux pinning behavior were investigated by using XRD, SQUID magnetometer, SEM and TEM. Critical current density J_c can be significantly enhanced by the Zr-doping and the best data are achieved in Mg_0.9Zr_0.1B₂. For this sample, J_c values are remarkably improved to l.83 ×10⁶ A/cm² in self-field and 5.51 × 10⁵ A/cm² in 1 T at 20 K. Also, high quality MgB₂/Ta/Cu wires and tapes with and without Ti-doping, MgB₂/Fe wires and 18 filament MgB₂/NbZr/Cu tapes were fabricated by the powder-in-tube (PIT) method at ambient pressure. The phase compositions, microstructure features and flux pinning properties were studied. The results suggest that Fe is the best metal for these sheaths. MgB₂/Fe wires exhibit very high J_c at high temperatures and high fields. J_c values reach as high as 1.43 × 105 A/cm² (4.2K, 4 T) and 3.72 × 10⁴ A/cm²(15 K, 4 T).