Section area and self-field dependence of critical current density in homogeneous bulk textured Bi-2223

We report bulk ceramic Bi-2223 transport measurements for samples of various cross sections. The sample preparation process allows the synthesis of large and homogeneous samples. The measurements were carried out at 77 K in self-field (with currents up to 70 A) over a wide range of sample thickness and widths. 17 bars of various sizes, ranging from 0.5 to 3.5 mm width and 0.32 to 3.5 mm thickness, were prepared by sinter forging. The transport critical current density (JCT) clearly decreases as the sample cross section increases, and JCT-1 varies linearly with thickness. This JCT variation cannot be attributed to differences in sample quality (texture, phases, etc) because of the careful synthesis procedure, and implies that the current does not flow uniformly through the samples. The magnetic field dependence of the critical current density (JCM) was compared with the JCT values, allowing us to calculate the induced field, Bind, produced during the JCT measurement. This induced field contributes to a decrease in the measured JCT values. Therefore, the application of a correction factor obtained from Bind on the critical current density results in a value of JC independent of the cross section of the sample. These results question the significance of homogeneous bulk pinning and reveal the possible existence of surface and interface pinning processes. (author)