Embedding of insulating nano-inclusions in Y-123 superconductor as pinning centers: The influence of size

The optimization of flux line pinning in superconductors is one of the most efficient ways to improve the transport properties of these materials. The generation of effective artificial pinning centers in a controlled way and with optimal dimensions can contribute to the enhancement of pinning capability and to the improvement of the critical current densities Jc. In this work, we examined the effectiveness of an insulating inclusion in a type II superconductor as a pinning center with a size close to the penetration depth λ instead of the coherence length ξ. To this effect, insulating nano-pinning centres (100-150 nm) have been successfully embedded into superconducting YBa2Cu3O7−d (Y-123) matrix by slightly doping with nano-particle alumina dispersions. Two alumina nano-particle dispersions with mean size diameters of about 130 nm and 150 nm which are considerably larger than the coherence length ξ of Y-123 were used. A systematic study of the relationship of Jc(H,T) with different amounts of D1 and D2-nanoalumina additions was performed to determine the optimum nano-particle doping contents. The results indicate that slight inclusions of D1 or D2-nanoalumina can effectively enhance the flux pinning capability of samples. The best flux pinning was observed in the sample with 10−2 wt.% D1-alumina and 3 × 10−2 wt.% D2-alumina. The present work suggests that the use of sufficiently large insulating inclusions in the nanometer sub-scale can stabilize the flux-line lattice. It also shows that the optimal size for an insulating inclusion, acting as a pinning center in bulk material, is more likely related to λ than to ξ.