Superconducting shells in ceramic ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We demonstrate that, even in single-phase ceramic ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, superconductivity occurs in only thin shells surrounding normal grains. The shells comprise no more than 25 vol% of the material, independent of preparation technique and subsequent processing. This conclusion is based on comparison of magnetic flux exclusion, Meissner and resistivity data for bulk, compacted, and ground materials. We show that these data depend on the sample microstructure, but do not arise from pinning effects.