Preparation and magnetic properties of hollow ferrite microspheres by a gas-phase diffusion method in an ionic liquid/H₂O mixed solution

In this work, hollow ferrite microspheres were prepared using a gas-phase diffusion method with cobalt nitrate and ferric nitrate as metal salt sources, an ionic liquid 1-butyl-3-methylimidazolium-tetrafluoroborate and water-mixed solvent as medium and ammonium carbonate as precipitant. Their structures and magnetization were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, thermogravimetry, infrared spectroscopy, and vibrating sample magnetometer. The effects of reaction time, reaction temperature, precipitant loading, and mole ratio of Co to Fe n(Co/Fe) on the structures and magnetization of the microspheres were studied. The results showed that ferrite hollow microspheres with uniform morphology and high-magnetic performance were obtained at 60-80 C for 12-16 h, while the (NH₄)₂CO₃ loading was 0.15 g/ml, n(Co/Fe) was 0.5:1, and calcination temperature was 550 C. The obtained products consisted of CoFe₂O₄ phase accompanied by ferric oxide phase, with an average particle size about 1 μm and magnetization intensity about 10 emu/g.