MgB₂超伝導材料の現状と展望

抄録

Recent studies on MgB₂ superconductors are reviewed. MgB₂ single crystals show B_c2 anisotropy factors of 2.6—4.5. The B_c2 of a single crystal at 4.2 K is ∼15 T when the field is parallel to the ab plane. MgB₂ bulks, wires and tapes show a higher B_c2 than single crystals. This B_c2 enhancement can be explained by the introduction of defects that decrease the coherence length. MgB₂ thin films show a much higher B_c2 than bulk materials. The highest B_c2, 49 T at 0 K, was reported for highly resistive MgB₂ thin film. One of the effective pinning centers in MgB₂ seems to be grain boundary. Effective pinning centers are also introduced by nanometer-size impurity additions such as SiC. SiC addition significantly increased J_c values in the high field regions, and hence B_irr. Most tapes and wires are now fabricated by a powder-in-tube (PIT) method. The ex-situ PIT method employs MgB₂-reacted powder and hard sheath materials such as stainless steel. The J_c of ex-situ tapes sensitively depends on the packing density of the MgB₂ core. A maximum J_c of ∼4.5×10⁵ A/cm² is obtained at 4.2 K and zero fields. Ex-situ processed tapes show J_c anisotropy with respect to the field orientation due to the grain alignment. The in-situ PIT method employs a powder mixture of Mg and B. High-energy ball milling and mechanical alloying of the starting powder mixture enhanced the J_c values. The replacement of MgH₂ with Mg is effective for improving the reactivity and enhancing J_c values. The J_c at 4.2 K and zero magnetic field reached 2—3×10⁶ A/cm². The highest J_c, 2.5×10⁴ A/cm², was obtained at 10 T and 4.2 K when SiC was added to in-situ processed tape prepared with MgH₂+B powder. This J_c is comparable to that of a commercial Nb-Ti conductor.