Flow method for rapidly producing barium hexaferrite particles in supercritical water

We propose a flow method to produce barium hexaferrite (BaO·6Fe₂O₃) particles with hydrothermal crystallization in supercritical water. Aqueous iron(III) and barium nitrate solution at room temperature was pressurized to 30 MPa and then mixed with potassium hydroxide solution (OH:NO₃ = 4) at the same conditions to generate metal hydroxides. This mixture was then rapidly heated to 400°C by mixing with supercritical water and then fed into a tubular reactor. Residence time was ∼1 min. The reaction was terminated by cooling at the exit of the reactor. The Ba:Fe mole ratio was varied over a range of 0.1-2. When the Ba:Fe ratio was ∼1/12, which is the stoichiometric ratio for BaO·6Fe₂O₃, the main products were α-Fe₂O₃. However, for the case of Ba:Fe > 0.5, fine particles of single-phase BaO·6Fe₂O₃ were produced. Batch experiments (380°C, 30 MPa) at Ba:Fe = 0.5 in supercritical water at a reaction time of 10 min produced a mixture of α-Fe₂O₃ and BaO·6Fe₂O₃. This product transformed to the equilibrium phase, BaO·2Fe₂O₃, in 4 h as the reaction time increased, which suggests that the BaO·6Fe₂O₃ that formed in supercritical water with our proposed flow method under nonstoichiometric conditions was an intermediate but stable product. Furthermore, the nonstoichiometric and nonequilibrium (dynamic) conditions are important for producing single-phase BaO·6Fe₂O₃ particles. The single-phase particles are highly stable and can be produced continuously in a reaction time of <1 min.