Monodispersed Sc 2 O 3 precursor particles via homogeneous precipitation: Synthesis, thermal decomposition, and the effe
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Monodispersed Sc2O3 precursor particles were synthesized by the urea-based homogeneous precipitation method, with an investigation into the effects of supporting anions (NO3−, Cl−, and SO42−) on powder properties. Characterizations of the powders were achieved by elemental analysis, x-ray diffractometry, Fourier transform infrared, differential thermal analysis/thermogravimetry, high-resolution scanning electron microscopy, and the Brunauer–Emmett–Teller method. Unlike other rare earths, Sc3+ does not precipitate as basic carbonate but instead forms hydrated ␥–ScOOH from either nitrate or chloride solution. Particles of the hydrated ␥–ScOOH are pumpkin-shaped (approximately 1.0 m) and are made up of thin-platelike crystallites emanating from a common axis. The presence of complexing SO42− changes the reaction chemistry toward Sc2O3 powders, leading to basic sulfate [Sc(OH)1.6(SO4)0.7 ⭈ H2O] precursor particles having hexagonal morphology (approximately 10 m in diameter and 0.5 m in thickness). The hydrated ␥–ScOOH directly converts to Sc2O3 by calcination at 400 °C or above, while the basic sulfate transforms to oxide at temperatures 艌900 °C via an amorphous state and a Sc2(SO4)3 intermediate. The effect of SO42− on powder morphologies and reaction chemistry is discussed. Nanocrystalline Sc2O3 powders comprising monodispersed particles were obtained via thermal decomposition of the precursors.
I. INTRODUCTION
A variety of ceramic powders with well-defined particle morphologies were synthesized via the so-called urea-based homogeneous precipitation method, which takes advantage of the slow hydrolysis of urea at elevated temperatures (艌83 °C).1,2 The in situ decomposition of urea releases precipitation-participating ligands (OH− and/or CO32−) slowly and homogeneously into the precipitation system, avoiding the localized distribution of reactants and making it possible to exercise control over the nucleation and growth of precursor particles.3 Among the many materials processed via this method, the rare earths have been studied more extensively and systematically, mainly by Matijevic et al.4–6 and Akinc et al.7,8 Previous work shows that the rare earths generally form basic carbonate particles, and the particle morphology is dependent on the ionic size of the element.4,6,7 It was demonstrated clearly that going from larger La3+ to a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 5, May 2003
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smaller Yb3+ (including Y3+) the particles steadily get smaller and corners are rounded under identical precipitation conditions.7,9 Such a phenomenon is a manifestation of the lanthanide contraction law, which predicts continuous property changes of the lanthanides along with a variation in the ionic size (Y3+ observes the lanthanide contraction law and shows properties close to Ho3+).10 Scandium has been one of the very few rare earths that have not been studied by this method. The element has the tiniest ionic s
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