PREPARATION OF STRONTIUM FERRITE PARTICLES BY SPRAY-PYROLYSIS

Crystalline, submicrometer strontium ferrite powders, including SrFeO2.97, SrFe2O4, Sr2FeO4, Sr3Fe2O6.16, and SrFe12O19, were prepared by spray pyrolysis of an aqueous solution of mixed metal nitrates. The Sr:Fe mole ratio in the precursor solution was retained in the final products. Phase-pure materials were typically obtained only at the highest temperatures investigated (>1100 degrees C) and powders prepared at lower temperatures frequently contained crystalline Fe2O3. The as-prepared particles were unagglomerated, polycrystalline, and hollow at lower temperatures, but densified in the gas phase at higher temperatures to give solid particles. The strontium ferrite (SrFe12O19) system was studied in detail as a representative example of the Sr-Fe-O system. At temperatures of 1200 degrees C, dense, phase-pure magnetoplumbite-structure material, SrFe12O19, was obtained, while at lower temperatures, small amounts of Fe2O3 were observed. The particles prepared at 800 degrees and 1100 degrees C were 0.1-1.0 mu m in diameter, and consisted of crystallites <100 mn, and were nearly solid. The difficulty in forming phase-pure SrFe12O19 was the different thermal decomposition temperatures of Sr(NO3)(2) (725 degrees C) and Fe(NO3)(3) . 9H(2)O (125 degrees C) as demonstrated by thermogravimetric analysis in the SrFe12O19 system.