Effects of pH value on the microstructure and magnetic properties of solution combusted Fe3O4 powders

Magnetite (Fe3O4) powders were prepared by solution combustion synthesis method using conventional and microwave ignition at various pH values of starting solution, adjusted by NH4OH. The chelated species in dried gels were predicted by theoretical calculations and Fourier transform infrared spectroscopy. The combustion reaction rate strongly depended on pH values as investigated by thermal analysis. Phase evolution and structure characterized by X-ray diffraction method showed single phase and well-crystalline Fe3O4 powders which were achieved using conventional ignition at pH ≥ 7. However, the microwave ignition led to the formation of impure FeO phase together with Fe3O4. The microwave combusted powders exhibited the disintegrated structure in comparison with the bulky microstructure for conventionally combusted powders, as observed by scanning electron microscopy. Magnetic properties of the as-combusted powders studied by vibration sample magnetometry showed the highest saturation magnetization of 81.3 emu/g for conventional ignition at pH of 7, due to the high purity and large crystallite size.