Preparation of fine, hollow, spherical NiFe 2 O 4 powders
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INTRODUCTION
To achieve better control of homogeneity, purity, morphology, particle size, and size distribution for ceramic powder preparation, aerosol techniques have attracted a lot of attention. In aerosol techniques, the aerosol droplets are generated by the evaporationcondensation method or the atomization method and are then subject to thermal reactions: hydrolysis, decomposition, pyrolysis, etc., depending on the raw materials used. Each droplet acts as a separate reactor containing the constituents mixed in the atomic scale and minimizing the occurrence of chemical segregation. This process is known in the literature as the Ruthner process, evaporative decomposition of solutions (EDS), or, better, as spray pyrolysis.1"3 Matijevic and his co-workers4"6 carried out a series of works to examine the aerosolization process using metal alkoxides as starting materials. They used the evaporation-condensation method and obtained submicron spherical metal oxides, such as TiO2, A12O3, and the mixed metal oxides, with modal diameter below 1.0 ^tm. Messing's group1"3 used the atomization method to prepare several ceramic powders (e.g., MgO, ZnO, NiO, and NiMn2O4), and the powders obtained had surface areas between 6 and 60 m2/g. In our laboratory, a previous study combining the sol-gel method with aerosol techniques in the preparation of nickel ferrite powders7 was conducted. High purity powders with uniform chemical composition and submicron spherical particles were obtained. Our study also showed that after heating the aerosol product to 600 °C, pure crystalline nickel ferrite was obtained. Due to the limited solubility of the alkoxides in alcoa)
Address correspondence to this author. J. Mater. Res., Vol. 5, No. 12, Dec 1990
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hol, only a dilute solution was prepared and the effect of concentration was not studied. In the present work, an aqueous solution of the corresponding nitrates was used to produce NiFe2O4. Nickel and iron nitrates were selected in view of recent results8 on thermal analysis of nickel and iron salts as well as on mixtures to form the ferrite. These results showed that nitrates have high solubility in water and fully decompose to oxides at temperatures lower than other salts. II. EXPERIMENTAL TECHNIQUES
Ni(n) and Fe(m) nitrates, purchased from AESAR with a purity of 98%, were mixed in the stoichiometric ratio to form NiFe2O4 and dissolved in de-ionized water to prepare the required solution. A six-jet atomizer (Model 9306, Thermal System, Inc., St. Paul, MN) was used to aerosolize the solution. The pressure drop associated with a high-velocity jet stream (formed by passing dry, clean, compressed air through a 0.38 mm orifice) drew the starting solution up through a narrow tube and the solution then broke to droplets. The resultant large droplets impinged upon a spherical impactor and returned to the reservoir, but the small droplets failed to make contact and thus formed the aerosol. The aerosol was introduced to a streamlined tubular react
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