CeO2 Crystals Research Articles

Sintering of Sol—Gel‐Prepared Submicrometer Particles Studied by Transmission Electron Microscopy

We studied the sintering process of sol‐gel‐prepared monodisperse submicrometer CeO2 spheres by examining the microstructural changes of single spherical particles after successive heat treatment in oxygen up to 850°C using transmission electron microscopy. Steps of organic phase removal, CeO2 crystallization, grain growth, and particle condensation were clearly illustrated. Grain‐size and sphere‐diameter changes were measured quantitatively using this technique. CeO2 particles were found to be highly porous until collapse, which occurred at~850°C.

Local Structures around Ce Atoms in CeO<sub>2</sub>-Stabilized Tetragonal ZrO<sub>2</sub>

Local structures around Ce4+ ions in stabilized tetragonal zirconias with the chemical compositions of 86ZrO2⋅10CeO2⋅4YO1.5 (Ce 10) and 93ZrO2⋅4CeO2⋅3GdO1.5 (Ce 4) were studied by Ce LIII EXAFS (extended X-ray absorption fine structure) analysis. Using CeO2 crystals (Ce-O=2.343A) as a reference sample, the average Ce-O distances for both Ce 10 and Ce 4 samples were estimated to be 2.32A and average Ce-cation distances for Ce 10 and Ce 4 samples to be 3.62 and 3.61A, respectively.

Crystallization and crystal growth of CeO2 under hydrothermal conditions

CeO2 crystals have previously been grown by the flux method [1–4] due to their high melting point and high vapour pressure [5] at high temperatures. The flux method requires high temperatures and generally brings about greater contamination of the flux than does the hydrothermal method. This study deals with the effect of several mineralizers on the crystallization and the crystal growth of CeO2 under hydrothermal conditions.

ChemInform Abstract: OXIDATION OF SILICON NITRIDE‐HOT‐PRESSED WITH CERIA

AbstractDie Kurzzeitoxidation (30 h) von CeO2‐dotiertem heißgepreßtem Si3N4, die zwischen 773 und 1623 K im trockenen Luftstrom untersucht wird, liefert parabolische Gewichtszunahmekurven und Geschwindigkeitskonstanten, die fast unabhängig von der Menge an sekundärer Korngrenzenphase sind (Aktivierungsenergie 385 kJ · mol‐1).

Oxidation of Silicon Nitride Hot‐Pressed with Ceria

Short‐term (<30 h) oxidation of CeO2‐doped hot‐pressed Si3N4, studied at 773 to 1623 K in flowing dry air, resulted in parabolic‐weight gain curves and in oxidation‐rate constants nearly independent of the amount of grain‐boundary second phase. Exolution of CeO2 crystals occurs from the oxide layer; their morphology depends on oxidation temperature. Either a direct redox reaction between CeO2 and Si3N4 or solution of Si3N4 in the glassy silica‐rich oxide layer, followed by its oxidation by dissolved oxygen, have been proposed as possible oxidation mechanisms, both appearing strongly dependent on the low solubility of cerium oxides hi silicate melts. The value of the activation energy for oxidation of 385 kj·mol−1 suggests additive and impurity migration from the bulk to the Si3N4/oxide reaction interface as the most probable rate‐limiting step.

Spectroscopic studies on single crystals having the fluorite lattice. Part 1.—The fundamental absorption edge; Urbach's rule and the Debye temperature in CeO2

Measurements of the fundamental absorption edge of single crystal CeO2 up to absorption coefficients α of 120 cm–1 for 291 < T < 1235 K are presented. The results are analysed in terms of Urbach's rule, α=α0 exp[–σ(E0–hν)]/kT. Over this temperature range Urbach's rule is obeyed and its parameters are derived, viz., E0= 3.78 eV, α0= 7.6 × 107 cm–1, σ0= 0.90 and ħωp= 70.2 meV; σ increased with temperature more slowly than usual. Urbach's rule has not previously been obeyed up to such high temperatures or investigated for any fluorite lattice compound. Only optical photons are involved in the absorption tail formation. The characteristic Debye temperature for CeO2 was calculated as 630 and 606 K from specific heat and elastic data respectively. It was concluded that the ceria crystals retained their stoichiometry at all temperatures.

Growth and properties of CeO2 and ThO2 single crystals

Growth and properties of CeO2 and ThO2 single crystals