Characterization of freeze-dried precursors of La1.85Sr0.15CuO4

The production of acetate precursors of the La1.85Sr0.15CuO4 superconductor by freeze-drying was studied and compared with the solid obtained when the same starting solution was evaporated to dryness. The structural, morphological and thermal changes occurring during processing were followed using a battery of techniques such as X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), and differential thermal analysis (DTA). It was shown that freeze-drying leads to the production of homogeneous precursors which reflect the molecular dispersion of the starting solution. Evaporated precursors showed the opposite characteristics for they are inhomogeneous, exhibiting severe phase segregation. Several key factors in the thermal decomposition of the precursor to obtain the superconducting oxide were detected, the most important being the following: (i) The mechanism of decomposition of copper acetate is significantly affected by the gas atmosphere. Under vacuum or N-2, Cu(C2H3O2) sublimes and copper is lost. (ii) The melting of Cu(C2H3O2)(2) and its complex decomposition mechanism led to a transient liquid phase during calcination. This emphasizes the need for carefully designing and controlling the heating strategy and atmosphere composition used during the transformation of the precursor in the superconducting oxide.