PROCESSING AND PROPERTIES OF SUPERCONDUCTING LA1.85SR0.15CUO4 POWDER BY DOUBLE-STEP CALCINING

The preparation of single-phase tetragonal La1.85Sr0.15CuO4 powder usi ng two consecutive thermal cycles is described. Freeze-dried or evapor ated lanthanum, strontium and copper acetates were used as precursor m aterials. The effect of the reaction temperature (in the second cycle) on the characteristics of high-T-c superconductor was studied by X-ra y diffraction, idometric titration, temperature-programmed reduction, scanning electron microscopy and electrical resistivity measurements. Monophasic powder was obtained by reacting freeze-dried precursors at a final temperature of 1253 K for 8 h. The reduction of La1.85Sr0.15Cu O4 occurred in one step at 744 K. Thermal treatments in either N-2 or O-2 strongly affect the hydrogen consumption profile. Monophasic sinte red pellets were obtained from freeze-dried precursors calcined at fin al temperatures between 973 and 1253 K. The microstructure of sintered bodies was characterized by equiaxed grains of about 0.5 mu m. Evapor ated precursors did not yield single phase powder or sintered material , and gave a grain size of about 1 mu m. The critical temperature rang ed between 36.7 and 37.3 K and Delta T-c between 2.8 and 4.8 K.