Critical currents in polycrystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$: Self-field and grain-size dependence

The variation of critical currents and their distributions with thickness has been investigated for two high-quality ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ samples with different average grain size (10 and 30 μm for samples ${\mathrm{S}}_1$ and ${\mathrm{S}}_2$, respectively) in the temperature range 78–90 K and in the applied magnetic field H<50 Oe. The critical-current density (${\mathit{J}}_{\mathit{c}}$) for ${\mathrm{S}}_1$ initially increased but later on leveled off on reducing the thickness, whereas for ${\mathrm{S}}_2$ remained essentially unchanged even after threefold reduction in thickness. Since the other parameters related to macroscopic homogeneity have not changed on reducing the thickness of the samples, the variations of ${\mathit{J}}_{\mathit{c}}$ are interpreted in terms of thickness and grain-size-dependent self-field effects. The same model explains well the changes of critical-current distribution curves with thickness and may also explain the variation of ${\mathit{J}}_{\mathit{c}}$ with the grain size, as reported recently for ceramic Y-Ba-Cu-O samples.