Inhomogeneous distribution of the intergranular pinning energy in polycrystalline Bi1.64Pb0.36Sr2Ca2Cu3Oy superconductors

Measurements of the electrical resistivity as a function of temperature, ρ(T), for different values of applied magnetic field, Ba (0 ⩽ Ba ⩽ 50 mT), were performed in samples extracted from different regions of the same Bi1.64Pb0.36Sr2Ca2Cu3Oy pellet in order to evaluate its homogeneity regarding the transport properties. Three pieces in form of slab were cut from the pellet. The first was extracted from the interface where the piston used in the compacting process has a direct mechanical contact with the powders (face A). The second, was taken from the interface parallel to the latter which is located in the base of the cylindrical sample (face B). Finally, the last piece was obtained by cutting an slab along the axis of the cylinder from the face A to face B. We have found appreciable differences in the grain orientation between these pieces by using X-ray diffractometry. Results indicate that the grains of the samples extracted from the interfaces A and B, are preferentially aligned with their c-axis parallel to the compacting direction. However, this feature is practically absent in the sample that was cut along the axis of the pellet, where the grains are more randomly oriented. The ρ(T) dependence at zero applied magnetic field shows a similar metallic-like behavior in the normal-state region in all samples, but the electrical resistivity at 200 K of the samples extracted from the interfaces A and B is ∼16% lower than the observed value for the other sample. In addition, the values of the so-called offset critical temperature, Toff, show an appreciable decrease with increasing applied magnetic field, Ba. In the case of the samples extracted from the faces A and B Toff falls ∼9%, while in the sample cut along the thickness of the pellets was ∼13%. The above transport experimental results are interpreted in terms of changes in the grain alignment and the effective intergranular pinning energy provoked by the inhomogeneous distribution of uniaxial compacting pressure in these ceramic samples.