A melt-infiltration process has been developed to make 123 superconductors with aligned grain structure in a relatively short time without temperature gradient. The process consisted of the preparation of 211 powder compact and Ba-Cu-O(liquid forming powder)powder, followed by infiltration of a Ba-Cu-O glass melt into the 211 compact below the peritectic temperature of 123. During the infiltration, the 123 phase began to form at the surface of the compact by the reaction between the two phases, and as a result, 123 grains grew directionally towards the center of the compact along the flow of the liquid. By this process, superconductor wires could be produced at shorter times and lower temperatures. This process may also be appropriate for various kinds of model, in particular, for that of controlling 211 particle size and liquid composition. In the present investigation, all 123 specimens were prepared by the developed melt infiltration process to study the microstructure and superconducting property of aligned 123 grains.
The formation mechanism of nonuniform distribution of 211 particles in 123 grains was investigated. In 123 grains prepared by melt involved processes, various type of patterns of 211 free-regions have been observed. In our case, it appeared as a butterfly-like pattern in a 2-dim section. By successive grinding and observation of infiltrated specimens, the 3-dim shape of the 211-free region was found to be a pair of vertex-shared pyramids and 211 entrapped region to be the rest of the bulk of the grain. The formation and growth of previously observed patterns had been explained by the difference in growth rate with crystallographic orientation of 123 grains. This explanation, however, did not appear to be appropriate for our case. The difference in entrapment depending on crystallographic planes was able to be well explained by the dihedral angles between 123 and 211. The dihedral angle between a(or b) plane and 211, which is believed to be greater than zero degree, causes the entrapment of 211 particles in a(or b) direction. In contrast, the dihedral angle of most probably zero degree between c plane and 211 inhibits the entrapment. The observed shape of 211 particles in front of a (or b) and c planes supported the above explanation of 211 entrapment to form butterfly-like patterns.
We also studied the effect of the chemical composition of infiltrated liquid on critical current density. Most of the 123 specimens prepared by melt- involved processes have remnant liquid phases at grain boundaries. In terms of chemical composition and microstructure two types of liquid was observed after the melt infiltration. In 123 grains adjacent to a Ba-rich remnant liquid, many cracks were observed. In contrast,123 grains adjacent to a Cu-rich liquid appeared to be more tightly bonded each other. From this microstructural difference, it was expected that the critical current density of the specimen infiltrated with a Cu-rich liquid was higher than that with a Ba-rich liquid. The highest Jc value was indeed obtained when a Cu-rich liquid was infiltrated. For the improvement of critical properties, the best way must be the complete elimination of remnant liquid phases which is practically impossible. An alternative may then be to use a Cu-rich liquid as infiltrant, as we found. Use of a melt with excess Cu would also be beneficial for any other melt-involved processes.