超伝導体における磁束ピンニング[7]

-高温超伝導体のピンニング特性(2)-

抄録

The flux pinning properties of RE-123 bulk superconductors, RE-123 coated conductors, Bi-2223 tapes and MgB₂ superconductors are reviewed. In bulk superconductors, the pinning mechanism of the lower T_c region, such as the Ba sites substituted by RE elements or twin boundaries with oxygen deficiency, is considered to be kinetic energy interaction under a proximity effect with the superconducting matrix. The dependence of the irreversibility field on the superconducting layer thickness is very complicated for RE-123 coated conductors. This is partly attributed to the thickness dependence of the critical current density that arises from the deterioration of the superconducting layer structure in thicker films and partly to the thickness dependence of the flux creep. The reason for the dramatic improvement in the critical current properties is also discussed for Bi-2223 tapes fabricated using the over-pressure sintering technique. Theoretical analysis using the percolation theory clarifies that the low critical current density in conventional MgB₂ in situ wires is attributed to low electrical connectivity, which is due to the voids and wetting insulating layers between grains. It is clarified that the flux pinning strength of the grain boundaries in MgB₂ is significantly strong, and there is a room for a drastic increase in the flux pinning strength by dirtying the MgB₂ through C-doping. Finally, the flux pinning mechanism of columnar defects nucleated by heavy ion irradiation is discussed. It is shown that the columnar defects larger than the coherence length are desirable to increase the probability of flux lines being easily captured by the defects.