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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.

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K k, r R = rate of reaction S T = temperature X = dimensionless pellet radius Y = dimensionless temperature = activity distribution function defined by Eq. 2 = effective thermal conductivity of catalyst pellet = radius of catalyst pellet = selectivity towards the desired reaction Greek Letters a P Y AH 1 (T = parameter defining activity profile according to Eq. 2 = heat of reaction parameter defined by Eq. 14 = dimensionless activation energy = standard enthalpy change of reaction = effectiveness factor = starvation parameter defined by Eq. 10 = modified Thiele modulus defined by Eq. 8 = dimensionless surface rate constant defined by Eq. 7 Subscripts 1,2 S = designates surface conditions = designate desired and undesired reactions

Aerosol droplets of a dilute aqueous solution of Ni(II) and Fe(III) nitrates were used to synthesize fine nickel ferrite powders by pyrolysis at temperatures ranging from 450 °C to 810 °C. The aerosol product was characterized by thermal analysis, x-ray diffraction, scanning electron microscopy, and BET surface area determination. At low temperature (480 °C), very active fine, hollow, noncrystalline spheres were formed. As the pyrolysis temperature increased, sintering occurred, larger particles with broader size distribution were obtained, and a higher degree of crystallinity of nickel ferrite was produced. The products at pyrolysis temperatures of 660 °C and above exhibited magnetic properties. The powders prepared from the aerosolization process at low temperatures had high reactivity and crystallized rapidly. The effect of the initial solution concentration was also studied, and dilute solutions produced spheres with fine size and narrow size distribution.