Synthèse et caractérisations mécaniques et supraconductrices de céramiques d'oxyde (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+x texturées par frittage-forgeage

摘要

The high-TC superconducting (110 K) BPSCCO-2223 oxide has been synthesized as a nearly single phase powder by a sol-gel route. The chemical process and sintering conditions are detailed herein, as is a study of the low activation energy which confirms that this route enhances the phase formation kinetics.The resultant powder has been sinter-forged under air into dense, monolithic, highly-textured discs by optimizing the processing conditions such as temperature (845°C), stress (~15 MPa) and time under load (~ 20 hours), that lead to a slow but high deformation, without decomposition.Bars cut from these discs exhibit reproducible critical current densities having values above 10000 A/cm2 at 77 K under self field but which strongly decrease with applied magnetic field, owing to the low angle c-axis misalignment of the platelet Bi-2223 grains and their weak vortice pinning along this axis. Bean critical current densities reach up to 105 A/cm2 at 10 K under low field and have a weak field dependence at this temperature (3 104 A/cm2 under 5 T, field parallel to c). Texture quality, heat treatment post-synthesis (air or 7.5% O2), and residual second phase amounts are clearly demonstrated to influence the critical current density. The highly oriented nature of the grains leads to very anisotropic mechanical properties. Hardness, elasticity and fracture toughness have been characterized and depend strongly on the sinter-forging conditions. A fracture initiated towards the sinter-forging axis preferentially propagates normal to it (i.e. along ab-direction of the grains), where cleaving is easier but fracture propagation is slowed by bridging effects between grains along the c-axis. This enhances the fracture toughness to 3 MPa.m½ for the sharpest textures. The density in such highly textured bulk ceramics (w/o silver) is above 95 % of the XRD theoretical one, and this leads to a Young’s modulus of ~100 GPa, a flexural strength of ~ 180 MPa. This material parameters combined with pseudo-metallic behavior of the Bi-2223 phase are responsible for the interesting mechanical properties which are useful in technological applications such as current leads and fault current limiters.