The phase stability of nanocrystalline ceria, zirconia, and their binary system.

摘要

Chapter 1. Introduction. A general introduction to the physical properties, redox reactions, and various applications of ceria is presented, highlighting that the nanocrystalline CeO₂ shows different properties from its micon-sized counterparts. Although possessing a simple cubic fluorite structure, cerium oxide is a complicate material because of the internal and/or external point defects in the system, especially in the nanometer scale. After being applied by industry for more than a decade, some basic science behind its applications of ceria remains unclear. It is of great interest to study nanocrystalline CeO₂.; Chapter 2. Synthesis and characterization of nanocrystalline cerium oxide. Nanoparticles of ceria oxide with a narrow size distribution were prepared by mixing cerium nitrate and hexamethylenetetramine solutions. The lattice parameter increases up to 0.45% as the particle size decreases to 6 nm, as observed with x-ray diffraction (XRD).; Chapter 3. Cerium oxidation state in ceria nanoparticles studied with x-ray photoelectron spectroscopy and absorption near edge spectrocopy . X-ray photoelectron spectroscopy and x-ray absorption near edge spectroscopy experiments were used to investigate the oxidation state of cerium ions in ceria nanoparticles. A comparison of results shows that XPS yields a higher concentration of Ce3+ ions, even after analysis with a core-shell model comprising an external oxide layer.; Chapter 4. Ceria nanoparticles: Size, distribution, and shape. Nanocrystalline ceria particles have been prepared by nixing aqueous solutions of cerium nitrate and hexamethylenetetramine at room temperature. The smallest size of nanoparticles synthesized is 2 nm.; Chapter 5. The stability of the cubic phase (c ′) in (1−x)CeO₂-xZrO ₂ nanoparticles. The stability of the cubic phase (c ′) of Ce_1−xZr_xO_2−y was studied by x-ray diffraction (XRD), time-resolved high temperature XRD, transmission electron microscopy, Raman spectroscopy, Ce L_III, and Zr L_III edge x-ray absorption spectroscopy. With decreasing particle size, the extended-cubic (c′)-tetragonal phase boundary was observed to shift to higher zirconium concentrations (i.e. 40–60% dependent on particle size).; Chapter 6. In-situ study of the crystallization transition from amorphous to cubic zirconium oxide: Rietveld and reversed Monte Carlo analysis. High-energy synchrotron x-eay with wavelength of 0.124123 Å was applied to study the amorphous-to-cubic phase transformation of nano-ZrO₂ in a reducing environment. (Abstract shortened by UMI.)