GRAIN-GROWTH, MICROSTRUCTURE, AND SUPERCONDUCTING PROPERTIES OF PURE AND Y2BACUO5-DOPED YBA2CU3O7-X CERAMICS

Superconducting properties of YBa2Cu3O7-x ceramics have been investiga ted with the aim of avoiding the weak links that are the principal rea sons for low critical current densities in this compound. An efficient grain growth process should be useful in this respect. Superconductin g ceramics were prepared from a commercial precursor powder (Hoechst) with or without addition of Y2BaCuO5 and from a gel precursor powder. Grain growth kinetics and densification are first derived under such c onditions that control the liquid-phase contribution. Grain growth in the solid-state regime is anisotropic, possibly due to anisotropic gra in boundary energies. Grains in the fine powder (gel precursor) grow 1 0 times faster than in the commercial reference but grain size saturat es because of steric hindrance. Addition of Y2BaCuO5 particles inhibit s grain growth in the solid-state regime, 211 particles are incorporat ed into grains when a transient liquid phase is present, Superconducti ng properties were characterized by 50-Hz magnetization hysteresis cyc les at 77 K under H = 0.2 to 12 mT to probe weak-link behavior. Additi onal studies were performed at 5 K using higher fields. Weak-link cont ribution is decreased when samples from the gel route are processed in the solid-state regime (below the peritectic temperature). This may b e attributed to the propensity of gel powder to develop, naturally, pr eferential grain boundaries due to its intrinsic structural anisotropy .