Preparation and Characterization of Ester-derived BaFe 12 O 19 Powder
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Ultrafine BaFe12O19 powder with crystallite sizes less than 200 nm was prepared via a citric acid precursor method. Citric acid was added into an aqueous solution, containing nitrates of Ba2+ and Fe3+ in a stoichiometric ratio to form barium ferrite, to chelate metallic ions in the solution. The pH of aqueous solutions was adjusted with NH4OH. After ethylene glycol was added into the solution and the system temperature was raised, esterification and dehydratation led to the formation of solid ester precursor. The distribution and contents of metallic ions in the ester are affected by the [citric acid]/[metallic ions] molar ratio used and the pH of starting solutions. When the ester-precursor obtained at pH ⳱ 9 with [citric acid]/[metallic ions] ⳱ 1.5 was used, crystalline BaFe12O19 appeared at temperatures as low as 923 K, and pure barium ferrite was obtained at 1073 K. According to the experimental results obtained, the reaction mechanism involved in the pyrolysis of esters is proposed and discussed.
I. INTRODUCTION
Hexagonal barium ferrite (BaFe12O19) is well established as a high-performance permanent magnetic material, 1 due to its fairly large magnetocrystalline anisotropy, high Curie temperature, and relatively large magnetization, as well as excellent chemical stability and corrosion resistivity. In recent years, barium ferrite has been extensively studied for advanced recording applications, such as disk drivers and video recorders.2,3 The characteristics of BaFe12O19 particles are very important for manufacturing permanent ceramic magnets because of their influence on the microstructure of final products. The conventional solid-state method for preparing BaFe12O19 is to fire an appropriate mixture of ␣-Fe2O3 and BaCO3 at very high temperatures (1423 to 1523 K). The resulting powder is then ground to reduce its particle size.4 Although high-temperature firing assures the formation of the required ferrite phase, larger particles (>1 m) are often obtained in this firing process. It has been shown that the theoretical intrinsic coercivities of ferrites can be approached only when the particle sizes are below 1 m.5 On the other hand, grinding may introduce impurities into the powder and cause strains in the crystal lattices, which all have unfavorable effects on the magnetic properties. Several unconventional techniques, such as the coprecipitation method,6–8 the glass crystallization method,9–11 the hydrothermal a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 1, Jan 2002
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technique,12 and the precursor method,13,14 have been used or are under development for preparing ultrafine barium ferrite particles. The most important feature of these methods is the use of liquid media to intimately mix the required metallic constituents on an ionic level. Mixing the required metallic ions homogeneously on the atomic scale can form the required oxide phase(s) at relatively low temperatures, resulting in smal
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