Clarification of Determining Factors of Irreversibility Fields in High-Tc Superconductors with Short Coherence Length

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Relationships among synthesis conditions, microstructure and critical current properties across the grain boundaries were studies for randomly oriented sintered bulks of cuprate and iron-based superconductors. Addition of reductive post-annealing process and achieving carrier overdoped state were found to be quite effective for dramatic enhancement of intergrain Jc of both Y123 and (Bi,Pb)2223 superconductors. This improvement is explained by the effect of an increase in condensation energy by a control of cation composition and an increase in coherence length along the c-axis direction. Irreversibility field of Y123 was also largely improved by adopting above method. For iron-based superconductors, improvement of intergrain Jc was succeeded by reduction of impurity phases and addition of Sn metal. Intergrain critical current properties achieved in this study are highest for each material as randomly oriented sintered bulks and further enhancement can be expected by densification.