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

Superconducting properties of YBa2Cu3O7-x ceramics have been investigated with the aim of avoiding the weak links that are the principal reasons for low critical current densities in this compound. An efficient grain growth process should be useful in this respect. Superconducting 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 conditions that control the liquid-phase contribution. Grain growth in the solid-state regime is anisotropic, possibly due to anisotropic grain boundary energies. Grains in the fine powder (gel precursor) grow 10 times faster than in the commercial reference but grain size saturates because of steric hindrance. Addition of Y2BaCuO5 particles inhibits grain growth in the solid-state regime, 211 particles are incorporated into grains when a transient liquid phase is present, Superconducting properties were characterized by 50-Hz magnetization hysteresis cycles at 77 K under H = 0.2 to 12 mT to probe weak-link behavior. Additional studies were performed at 5 K using higher fields. Weak-link contribution is decreased when samples from the gel route are processed in the solid-state regime (below the peritectic temperature). This may be attributed to the propensity of gel powder to develop, naturally, preferential grain boundaries due to its intrinsic structural anisotropy.