Effects of B powder on the MgB₂ superconductor prepared by two-step PIT

MgB₂ superconductors were prepared by two-step PIT with B (crystalline or amorphous) and Mg as starting materials. The first step was mixing B (crystalline or amorphous) and Mg with the molar ratio of Mg:B=1:4. For the doping samples, 5wt% and 8wt% SiC were added to the mixture. Pressed pellets were treated at 900°C for 30 min or 800°C for 1 h. The second step was adding Mg to get the molar ratio of Mg:B=1:2. The tubes filled with the ground powders were treated at 750°C for 2.5 h. The phase identities and the microstructure were investigated by X-ray diffraction (XRD) and Scanning Electronic Microscopy (SEM), respectively. Voltage dependence of the current of MgB₂ wire was measured by a standard four-probe method. The results demonstrate that lots of MgB₄ in low energy is synthesized by crystalline B and Mg at the first step. The sample prepared from crystalline B exhibits an excellent grain connection and stable I-V characteristics.