Preparation of Strontium Ferrite Particles by Spray Pyrolysis

Crystalline, submicrometer strontium ferrite powders, including SrFeO_2.97, SrFe₂O₄, Sr₂FeO₄, Sr₃Fe₂O_6.16, 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°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°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° and 1100°C were 0.1-1.0 μm in diameter, and consisted of crystallites <100 nm, and were nearly solid. The difficulty in forming phase-pure SrFe₁₂O₁₉ was the different thermal decomposition temperatures of Sr(NO₃)₂ (725°C) and Fe(NO₃)₃9H₂O (125°C) as demonstrated by thermogravimetric analysis in the SrFe₁₂O₁₉ system.