Hydrothermal Coarsening of CeO 2 Particles
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Hydrothermal coarsening of CeO2 particles S. Lakhwani and M. N. Rahaman Department of Ceramic Engineering, University of Missouri, Rolla, Missouri 65409 (Received 1 June 1998; accepted 14 September 1998)
The effects of reaction temperature (150–300 ±C), chemical composition of the starting cerium salt (cerium nitrate and cerium chloride), and doping with trivalent cations (Sc31 and Y31 ) on the coarsening of CeO2 particles in dilute suspensions under hydrothermal conditions were investigated. The particle size was measured by x-ray line broadening and by transmission electron microscopy. The particle coarsening kinetics followed a parabolic law, indicating that the interfacial reaction (dissolution) was the rate-controlling step. Furthermore, the particle size distribution data can be well-described by the Lifshitz–Slyozov–Wagner theory of Ostwald ripening controlled by the interfacial reaction. Doping with 6 at.% Y31 produced a significant reduction in the coarsening rate but almost no change in the activation energy. At the same concentration, Sc31 was more effective than Y31 in reducing the coarsening rate. Particles synthesized from a starting solution of cerium(III) chloride coarsened at a markedly slower rate than that for particles synthesized from cerium(III) nitrate. The mechanisms controlling the coarsening of the particles are discussed.
I. INTRODUCTION
The method of precipitation from solution under hydrothermal conditions has been used for decades for the synthesis of fine, crystalline oxide particles.1 However, interest in the method has increased in recent years because of the need for fine, pure powders in the production of ceramics for electrical and dielectric applications. Commonly, the hydrothermal reactions are carried out in an autoclave at temperatures between the boiling and critical points of water (100–374 ±C) and at elevated pressures (up to 22.1 MPa). A benefit of the method is that the crystalline phase is produced directly during the reaction so that a calcination step is not required as in the case of other synthesis techniques such as solgel reactions and co-precipitation from solution under atmospheric conditions. However, an understanding of the mechanisms of particle formation and growth in hydrothermal reactions is limited. Cerium oxide, CeO2 , is an example of a material system where the hydrothermal synthesis route may provide important advantages in processing. The material is of considerable interest for several electronic applications because of its high oxygen ion conductivity. However, powders with particle sizes of the order of 1 mm or so are difficult to sinter to the high densities required for many applications. Fine powders with particle sizes less than , 100 nm, typical of the size range of hydrothermally synthesized powders, are normally required to provide the ease of sintering necessary for the achievement of high density. The synthesis of CeO2 particles under hydrothermal conditions has been the subject of several i
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