Enhanced granularity in Y₂Ba₄Cu₇O_14+x ceramics

We have investigated the dc magnetic susceptibility of a large number of 247 YBCO sintered samples. In specimens with optimized superconducting transition, Tcmax ≈ 93 K, the combined effects of intra- and inter-grain screening currents lead to a sharp shoulder at ≈80 K in the zero-field cooled magnetization curves M(T) (flux penetration). This shoulder is magnetic-field dependent and disappears upon pulverization of the samples. The granular effects are strongly enhanced in the 247 sintered samples as compared to the 123 or 124 phases. One explanation is found in the particularity of the phase diagram of the 247 compound. © 1994.