Applications of superconducting oxides in magnetic field include many difficult problems to be solved. Especially, anisotropy both in the upper critical field Bc2 and the critical current density Jc, which originates from the lower symmetry of their crystal structures, will become crucial. Moreover, flux creep problems in relation to transport phenomena in magnetic field are under discussion. Recently, Jc values of the superconducting oxides have been rapidly improved with the progress in fabrication processes, for example, epitaxial and textured growth methods. In the case of Y1Ba2Cu3O7-δ films, the Jc values at 77.3K in magnetic field perpendicular to c-axis have been reported so far to be Jc(27T)=6.5×104A/cm2 for a specimen by CVD process, Jc(7.5T)=5×104A/cm2 by laser ablation, Jc (15T)≈6×105A/cm2 by electron beam evaporation, and Jc(20T)=1×104A/cm2 by sputtering. In this review paper, the problems on critical current densities of superconducting oxides are discussed and the fabrication processes which will attain higher Jc are summarized.
The authors have been studying the preparation of dense and highly oriented bulk sample of Bi based superconductor by the floating zone method. Recently, the effects of Ag doping on Jc and the related properties were investigated in FZ samples. The results obtained are summarized as follows. 1. Ag exists not only in grain boundary but also in 2212 grain. 2. Ag doping in the range of 0-10% does not significantly affect Jc. The doping of Ag over 10% degrades oriented structure resulting in decrease of Jc value. 3. The contact resistance at current terminal is drastically lowered by Ag doping. High contact resistance in undoped sample was suggested to potentially lower Jc measured value. 4. Measurement of complex magnetic susceptibility revealed FZ sample contains weak links. It was suggested that Ag doping improves weak link properties. Peak area in temperature dependence of imaginary part of complex magnetic susceptibility was proposed as an index for weak links in sample. Jc decreases with increase of this index value.
We report a novel melt process, or the MPMG (Melt-Powdering-Melt-Growth) process, for YBaCuO superconductors which promises high critical current density. It is important to disperse the Y2BaCuO5 (211) phase finely in the liquid in order to promote the continuous growth of the YBa2Cu3Ox (123) phase from the melt, because the 123 superconducting phase in YBaCuO system is produced by a peritectic reaction, Y2BaCuO5 (211)+L(liquid)→2YBa2Cu3Ox (123). We employed the process of pulverizing and well-mixing the oxides after cooled from partially melted region of 1, 200°C-1, 400°C, in order to achieve the fine dispersion of the 211 phase. The MPMG process has three advantages, 1) it can achieve the fine dispersion of the 211 phase, therefore the continuous growth of the 123 phase, resulting in high critical current density, 2) it is easy to form the samples in various shapes and sizes, and 3) it is possible to control the volume fraction of 211 inclusions by simply changing the starting compositions.