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.
The influence of an intermediate pressing process on the superconducting properties of Agsheathed Bi-Pb-Sr-Ca-Cu-O Superconducting tapes has been investigated. The critical Current density (Jc) was improved by the effect of a uniaxial pressing and sintering after the initial sintering. The maximum transport Jc at 77K under zero magnetic field was indicated as 2.9×104A/cm2, and that at 4.2K under zero magnetic field was indicated as 1.5×105A/cm2. The Jc at 4.2K and 12T has been shown to be 5×104A/cm2 when the magnetic field was applied parallel to the width of the tape, and 3×104A/cm2 under the condition of a perpendicular applied magnetic field. The reasons for these improvements are the formation of the high Tc (110K) single phase, a highly oriented texture with strong (00l) reflections and a compacted microstructure.
Superconducting properties were discussed in connection with microstructure for the grain oriented Bi-based high- and low-Tc tapes prepared by both Ag sheath method and doctor blade method. For the high-Tc tapes, grain oriented microstructure was obtained by applying the combination of cold rolling and sintering. For the low-Tc tapes, on the other hand, grain orientation was attained by the partial melting method. Critical current density Jc was significantly improved by the grain orientation for both high- and low-Tc tapes. Especially, the low-Tc tapes show excellent coupling properties of grains, leading to the large Jc above 104A/cm2 at 77K and zero magnetic field. However, at 77K, magnetic field dependence of Jc was very large both for high- and low-Tc tapes. At 4.2K, on the other hand, Jc was much less sensitive to magnetic field. Maximum Jc at 4.2K was 9×104A/cm2 at 10T and 6.8×104A/cm2 at 23T for low-Tc tapes prepared by the doctor blade method. The results indicate that grain oriented Bi-based oxide superconductor is very promising for the high field superconducting magnet.
Critical current characteristics of a bulk YBaCuO superconductor prepared by Quench and Melt Growth (QMG) process were investigated as a function of magnitude and direction of magnetic field and temperature. The QMG processed samples have highly oriented microstructure and are also free from grain boundaries which are considered to act as weak links. Magnetization curves were measured using a vibrating sample magnetometer. Critical current density (Jc) was estimated using the Bean type critical state model. When the magnetic field was applied along c-axis, the Jc perpendicular to c-axis was 1.5×104A/cm2 at 77K and 1T, while the Jc was 5×105A/cm2 at 4.2K and 4T in the same condition. The Jc exponentially increased with decreasing temperature. Resistive measurements were also done by using a four probe method at 77K. Large anisotropy of the Jc was observed with the direction of the applied field. It was found that Jc decreased with the field much more rapidly when the field was applied along c-axis and this was attributed to the small Bc2 (about 8T) in this direction.
Synthetic conditions of Pb-dopet Bi-Sr-Ca-Cu-O oxide superconductors produced by a technique of a uniaxial hot-press method were investigated. The quality of samples was evaluated by mean of X-ray, SEM, density and magnetic measurements. The transport Jc observed by resistive transition at 77.3K was 7, 000A/cm2 at H=10 Oe for the hot-pressed sample.
Ag-sheathed Bi0.8Pb0.2Sr1Ca1Cu1.5OyAgx(x=0 and 2.7) superconducting wires were prepared by solid state reaction method in order to examine effects of silver addition on critical current density (Jc). The critical transport current at 77K in a zero magnetic field was measured by the standard four-probe method. The maximum Jc of 5, 700A/cm2 was obtained with silver free specimens, but Jc's of silver added specimens prepared through the same conditions decreased to 2, 100A/cm2. Using XRD and SEM analysis it was found that the alignment of superconducting grains was obstructed by the silver grains.
In order to investigate the relationship between the oxygen deficiency and the weak links in sintered YBa2Cu3Oy, the inter- and intra-grain critical current densities (JcT and JcM) were measured for the samples with different oxygen contents. It is found that the JcT-B characteristics become worse with decreasing oxygen contents, while, the JcM-B characteristics, which are larger over four orders than the JcT'S, are almost unchanged. These results suggest that the oxygen deficiency exists at the grain boundaries and that is the origin of weak links in sintered YBa2Cu3Oy.
High Tc superconducting YBCO compounds have been prepared by Ar-F eximer laser ablation on SrTiO3 and on metalic substrates. Critical temperature (Tc) of the specimen formed on SrTiO3 (100) substrate was 88K. Critical current densities (Jc) at 77K over 2mm×2mm squared area at 1.0T and 8.0T were 1.5×104A/cm2 and 2.0×103A/cm2, respectively. The excellent Jc properties in magnetic field despite of the weak c-axis grain alignment encourages practical conductor fabrication by this method. YBCO on the substrate, Hastelloy c-276, with SrTiO3 thin buffer layer for one of the metalic substrates in view of long wire has been prepared by this method with high Tc of 83K and Jc of 1, 000A/cm2 at 77K.