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

Abstract Submicron-sized (0.15–0.2 μm) spherical agglomerates of magnetite (Fe 3 O 4 ) 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, a 0 =8.392 A. The Mossbauer spectra of these powders indicated that stoichiometric Fe 3 O 4 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 Fe 3 O 4 . 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 Fe 3 O 4 . However, when ferric salt is treated in alkaline medium single-phase α-Fe 2 O 3 is obtained under the MH conditions of 200 °C (194 psi).