Microstructure and properties of Cu-rich 123. Part I: Copper at the grain boundaries

A range of copper-rich bulk YBa₂Cu_3+xo_7+y superconductors have been prepared by mixing excess copper oxide in the initial material and characterized for their magnetic properties and microstructure. The microstructure of the materials exhibits a high density of planar defects at the grain boundaries and a grain boundary amorphous phase. There is a small increase in the magnetic J_cat 4.5 K but a decrease compared to a conventional material at 77 K, and there is no correlation of the magnetic J_cwith the twin boundary density. The change in J_ccan be understood if the grain boundary pinning is strong at 4.5 K but weak at 77 K. Data obtained using a very wide range of different microstructure characterization techniques clearly indicate the dangers of relying on only one technique to obtain a full picture of the material.