Preparation of NiFe₂O₄ powder by spray pyrolysis of nitrate aerosols in NH₃

To avoid the formation of hollow spheres during spray pyrolysis, NH₃ was employed to change the mechanism of forming NiFe₂O₄ from aerosols, containing Ni(II) and Fe(III) nitrates in the required stoichiometric ratio. Nearly spherical, solid submicron NiFe₂O₄ particles with narrow size distribution were produced in one step using a dilute aqueous solution at pyrolysis temperatures as low as 823 K. However, higher pyrolysis temperatures (≥1023 K) reduced the oxides to metallic alloy of Ni and Fe due to dissociation of NH₃. The forming steps and possible reaction mechanisms for aerosol droplets involved in the process were discussed.