Anisotropic properties optimisation of bulk bismuth-based high temperature superconductors using sinter-forging texturing techniques

The high-Tc superconducting (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+x (Bi2223) oxide has been synthesised by a sol-gel route. The calcination and sintering conditions have been optimized in order to synthesise a nearly pure Bi2223 phase, around 94%. The resulting powder has been sinter-forged under air into highly textured discs by optimising the processing conditions. Bars cut from these discs exhibit reproducible transport critical current densities (J_c) having values around 10000 A/cm² (O T, 77 K) which are similar to the maximum ones present in the literature. A new way of synthesis, consisting in texturing pellets composed of the Bi2212 phase and secondary phases, was undertaken. The control of the particles size (diam. <63 μm) and of the sinter-forging parameters (845 deg. C/100 h) allowed to attain J_c around 12500 A/cm². Ball milling of the sol-gel powder is crucial for increasing its reactivity and in the same time the ratio of Bi2223 phase after sintering-forging. The great quantity of liquid phase produced during texturation allowed the improvement of texture quality via a better alignment of the grains and thus led to strongly increase the critical current densities (20000 A/cm²). In addition, the stacking of several discs was not only shown to be a possible and fast process, but also to produce a massive ceramic with critical current densities almost identical to that obtained on only one disc. The combined analysis (texture/structure/microstructure) treated via the program MAUD presents a great effectiveness to characterize our samples. Thanks to neutron diffraction, a direct relation between the rate of Bi2223 phase, the cell parameters, the crystallites size, the density of maximum distribution and the critical current density was established. (author)