Temperature Effects on the Crystallization and Coarsening of Nano-CeO2 Powders

The effect of temperature on nano-CeO2 particle coarsening is investigated. The nanoceria powders were synthesized using the microemulsion method and then exposed to temperatures in the range of 373–1273？K. It was found that the nanoparticles exhibited a strong tendency to form agglomerates and through the application of ultrasound these agglomerates could be broken into smaller sizes. In addition average nanoparticle sizes were determined by powder X-ray diffraction (XRD). The outcome of this work indicates that the initial nano-CeO2 powders are amorphous in nature. Annealing promotes CeO2 crystallization and a slight shift in the (111) XRD intensity peaks corresponding to CeO2. Moreover, at temperatures below 773？K, grain growth in nano-CeO2 particles is rather slow. Apparently, mass transport through diffusional processes is not likely to occur as indicated by an estimated activation energy of 20？kJ/mol. At temperatures above 873？K, the measured activation energy shifted to 105？kJ/mol suggesting a possible transition to Ostwald-Ripening type mass transport mechanisms. 1. Introduction Nanocrystalline ceria possesses unique properties which enable it to be widely used in various industrial applications. Among the different uses of nanoceria are acting as coatings for high temperature oxidation protection in alloys [1–4], acting as catalysts and gas sensors [5, 6], being used for absorption and redistribution of UV radiation, [7]. Typical applications involve high temperature exposure above 873？K. Under these conditions mass transport mechanisms become active, particularly oxygen anions due to their inherently high mobility in the nanoceria crystal lattice [8]. In contrast, the diffusivity of cerium ions does not seem to be significant at these temperatures, but there is no data available on actual diffusivity values. Despite the lack of diffusivity data, it is expected that the nanosized CeO2 particles will exhibit appreciable coarsening upon exposure to elevated temperatures. Coarsening in nanoparticle dispersions at high temperatures is typically driven by chemical potential differences associated with curvature effects on the particle interfacial energies. This phenomenon, known as Ostwald-Ripening (O-R), has been widely investigated in conventional materials exposed to high temperatures [9, 10]. From the published literature [11], it is apparent that O-R also occurs in nanometallic and nanoceramic compounds. Eastman [12] reported grain growth exponents, of approximately 3 in nano-TiO2 or in yttria-stabilized nano-ZrO2 systems indicating that O-R