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Preparation of NiFe2 O4 powder by spray pyrolysis of nitrate aerosols in NH3 Hsuan-Fu Yu Chemical Engineering Department, Tamkang University, Taipei Hsien, Taiwan 25137, Republic of China

Ahmed M. Gadallaa) Chemical Engineering Department, Texas A&M University, College Station, Texas 77843 (Received 30 September 1994; accepted 25 October 1995)

To avoid the formation of hollow spheres during spray pyrolysis, NH3 was employed to change the mechanism of forming NiFe2 O4 from aerosols, containing Ni(II) and Fe(III) nitrates in the required stoichiometric ratio. Nearly spherical, solid submicron NiFe2 O4 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.

I. INTRODUCTION

Spray pyrolysis for ceramic powder preparation has attracted the attention of researchers. This technique is also known in the literature1–5 as the Ruthner process, evaporative decomposition of solution (EDS), and the thermal reaction of atomized solution process (TRAS). Recent studies and development showed that spray pyrolysis can produce fine spherical particles with narrow size distribution, high purity, and high homogeneity. However, the experimental results obtained from our previous studies6–8 and others2–5 indicated that in most cases the aerosol-derived particles were hollow spheres as a result of the following steps: (i) evaporation of solvent from the droplets, causing the size reduction and creating a concentration gradient, with the highest solute concentration on the droplet surface; (ii) precipitation of solute on the droplet surface; (iii) formation of solid crust with high porosity; (iv) decomposition of formed solute crust, accompanying the diffusions of gaseous product through the porous crust, and/or sealing of open-connected pores to form impermeable skins; and (v) crystallization and sintering of formed spheres whenever melting points were not reached. Although hollow particles with a thin crust are suitable for production of low density insulating materials or can be used as chemical catalyst supports, they have poor mechanical, electrical, and magnetic properties, and cause a great deal of shrinkage after sintering. Ingebrethsen et al.9 introduced water vapor into the aerosol process to react with the aerosol droplets of mixed aluminum-titanium alkoxides. The liquid droplets were formed, using the evaporation-condensation a)

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II. EXPERIMENTAL TECHNIQUES

Figure 1 is the spray pyrolysis process used in this study. Nickel nitrate hexahydrate of 99.7% purity and ferric nitrate nonahydrate of 98% purity were dissolved

FIG. 1. Schema of the spray pyrolysis process. J. Mater. Res., Vol. 11, No. 3, Mar 1996

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