Microwave hydrothermal preparation of submicron-sized spherical magnetite (Fe3O4) powders

Submicron-sized (0.15–0.2 μm) spherical agglomerates of magnetite (Fe3O4) powders have been prepared successfully by microwave hydrothermal (MH) reaction of ferrous sulphate and sodium hydroxide in the temperature range of 90–200 °C. X-ray powder diffraction patterns of all these powders indicated that the product is single-phase magnetite with cubic spinel structure having lattice parameter, a0=8.392 A. The Mossbauer spectra of these powders indicated that stoichiometric Fe3O4 particles are obtained only when molar ratio of Fe/NaOH≥0.133 is maintained in the solution. It is observed that Fe/NaOH ratio is an important parameter for the controlled oxidation of ferrous salts in alkaline media under MH condition to produce stoichiometric Fe3O4. Further, the kinetics of MH synthesis is one order faster than the reported conventional hydrothermal (CH) synthesis. The value of saturation magnetization M=70 emu/g is obtained in case of stoichiometric Fe3O4. However, when ferric salt is treated in alkaline medium single-phase α-Fe2O3 is obtained under the MH conditions of 200 °C (194 psi).