Influence of atmosphere on the synthesis of MgB₂ superconductor

MgB₂ is an intermetallic compound and is a promising superconductor for practical applications due to its high T_C value of 39 K. The production cost of MgB₂ wires is expected to be low compared to the conventional superconducting wires, since it is composed of cheap and available elements. However, the critical current density J _c in applied magnetic fields is not yet at practical level and, hence, the improvement of high field performance remains one of the most important issue to be solved urgently. Recent advances are showing that the addition of SiC is effective to improve the high field J_c, while the use of MgH₂ instead of Mg as raw material is enhancing MgB ₂ crystal growth. In both situations the mechanisms of these effects are still unclear. This paper reports the synthesis of MgB₂ superconductor in high-pressure hydrogen-atmosphere, which is an extreme reducing atmosphere. The results are compared with those for inert atmosphere i.e. argon-atmosphere, in order to provide useful information for wire processing. Samples were prepared by reacting the mixture of Mg (or MgH ₂), B and nano-SiC powders in a high-pressure (up to 3.5 MPa) steel vessel, at temperatures below 900°C. Samples that are fabricated in the pressurized hydrogen atmosphere show higher T_C, bigger a-axis lattice parameter and larger grain size than for the samples synthesized in Ar-atmosphere, in the same conditions. It turns out that the pressurized hydrogen atmosphere is useful to produce MgB₂ with good crystallinity. Nevertheless, the J_c in magnetic field is higher for the samples synthesized in argon atmosphere and it is considered that the lattice distortion in a-plane, smaller grain size and fine precipitates, such as MgO, are responsible for the stronger flux pinning, SiC addition is effective regardless the atmosphere and the highest J_c was obtained for the sample starting from MgH₂ + 2B + SiC powder mixture heat treated in argon atmosphere.