STRUCTURAL AND MORPHOLOGICAL EVOLUTION FROM A FREEZE-DRIED PRECURSOR TO THE LA1.85SR0.15CUO4 SUPERCONDUCTOR DURING THERMAL-PROCESSING

The crystalline phases and morphology of solids with a composition of La1.85Sr0.15CUO4 were studied in terms of thermal processing conditions of freeze-dried acetates. Samples were characterized by X-ray diffraction, scanning electron microscopy and iodometric titration techniques. Results indicate that a full development of the superconducting phase only occurs after pressing sintering and annealing under oxygen of the powders treated at T-f=1253 K for 8 h. The structural evolution observed in this case involves a solid-state reaction between La1.67Sr0.33Cu2O5-delta, La(OH)3 and CuO. The existence of transient liquid phase at temperatures around 600 K generates local composition fluctations (lanthanum deficiency) where the non-superconducting ternary compound La1.67Sr0.33Cu2O5-delta develops. The selection of high heating rates (50 K min(-1)) below 773 K minimizes the deleterious effect of transient melting and, at the same time, ensures a high reproducibility in the morphology of the final material obtained.