Structural and morphological evolution from a freeze-dried precursor to the La1.85Sr0.15 CuO4 superconductor during thermal processing
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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 Tf=1253 K for 8 h. The structural evolution observed in this case involves a solid-state reaction between La1.67Sr0.33Cu2O5−δ, 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−δ 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.
KeywordsLanthanum Final Material Thermal Processing Structural Evolution Morphological Evolution
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