Fluorite structure oxides have a property of deviating from stoichiometry as a function of temperature and/or pressure. CeO 2 has a fluorite structure and a wide range of nonstoichiometry. 1 The material deviates from stoichiometry with increasing temperature and decreasing oxygen partial pressure, leading to a high concentration of defects. The concentration of defects can also be controlled by doping the oxide with impurities. Since CeO 2 exhibits a wide range of solubility 2 for rare earth elements, the concentration of defects may be controlled by dopant concentration. CeO 2 is a mixed conductor, and both electronic and ionic conductivities have been investigated in several studies. 3-10 It is generally agreed 11 that the electronic conductivity of undoped CeO 2 is of the n-type. Since the ionic conductivity in this material is directly related to oxygen diffusion, it is important to understand the diffusion characteristics. However, the characteristics of oxygen diffusion in CeO 2 remain unclear. In particular, the oxygen diffusion in undoped CeO 2 must be studied in detail, because it is useful in understanding the behavior of oxygen diffusion and electrical conductivity for doped CeO 2 .The oxygen self-diffusion coefficients in oxides having fluorite structure are summarized by Ando et al. 12 and Kamiya et al. 13 The characteristic in fluorite structure is that the absolute value of the oxygen diffusion coefficients in stoichiometric oxides, i.e., UO 2 , 14,15 PuO 2 , 16 ThO 2 , 17,18 and CeO 2 , 19 are lower than those of nonstoichiometric oxides, i.e., UO 2ϩx and CeO 2 doped with Y. In addition, the activation energy for oxygen diffusion in stoichiometric oxides is large (202 kJ mol Ϫ1 for PuO 2 and 248-273 kJ mol Ϫ1 for UO 2 ). However, the absolute values of the oxygen diffusion coefficients of nonstoichiometric oxides are higher than those of stoichiometric oxides, and the activation energy for oxygen diffusion in nonstoichiometric oxides is small (77-89 kJ mol Ϫ1 for CeO 2 doped with Y).In the literature, two results, one by Floyd 19 and the other by Kamiya et al., 13 have been presented for oxygen diffusion coefficients in undoped CeO 2 . The activation energy reported by Floyd for undoped CeO 2 was 104 kJ mol Ϫ1 and was close to the value for nonstoichiometric UO 2 and for CeO 2 doped with Y. In contrast, the activation energy indicated by the data by Kamiya et al. (322 kJ mol Ϫ1 ) was close to the value of stoichiometric UO 2 , ThO 2 , and PuO 2 and the absolute value of the oxygen diffusion coefficient of CeO 2.00 was found to be similar to the value of other stoichiometric oxides having fluorite structure. Consequently, Kamiya et al. concluded that their result corresponds to stoichiometric CeO 2 (Ce-1). In their study, the oxygen self-diffusion coefficient for stoichiometric cerium oxide was obtained using gas-phase analysis. Direct measurement of the diffusion coefficient can be performed via secondary ion mass spectroscopy (SIMS). One of the objectives of the present study was to investigate ...

An extensive X‐ray study of CeO2–Nd2O3 solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO1.5 were synthesized by coprecipitation from Ce(NO3)3 and Nd(NO3)3 aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO2–NdO1.5 solid solutions with 0–40 mol% NdO1.5, which changed to a rare earth C‐type structure at 45–75 mol% NdO1.5. The change in the lattice parameters of CeO2–NdO1.5 solid solutions, when plotted with respect to the NdO1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C‐type regions. The maximum solubility limit for NdO1.5 in CeO2 solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO1.5) of solid solution.

Polycrystalline Ce0.77Nd0.23O1.885having a relative density in excess of 98% was prepared. Oxygen diffusion experiments were performed for the temperature range from 750 to 1100 °C, in an oxygen partial pressure of 6.6 kPa. The concentration profile of18O in the specimens following diffusion annealing was measured by secondary ion mass spectroscopy (SIMS). The oxygen self-diffusion coefficient obtained using secondary ion mass spectrometry was expressed by D = 6.31 × 10−9exp(−53 kJ mol−1/RT) m2s−1and was in the extrinsic region. The oxygen diffusion coefficient of Ce0.77Nd0.23O1.885was larger than that of Ce0.8Y0.2O1.90; it was close to those of Ce0.6Y0.4O1.80and Ce0.69Gd0.31O2−δ. The oxygen diffusion coefficient obtained by the tracer method at 700 °C agreed with that calculated from the electrical conductivity in Ce0.77Nd0.23O1.885. The activation energy of the surface exchange coefficient was 94 kJ mol−1, and the values of the surface exchange coefficient were similar to those of stoichiometric CeO2and ThO2.

Mullite (3 Al 2 O 3 и2SiO 2 ) is the only compound in the Al 2 O 3 -SiO 2 binary system as seen in the phase diagram. 1 It has very good chemical stability at high temperatures and, due to its low thermal expansion coefficient, shows good thermal shock resistance. 2,3 Consequently, mullite has long been used as a refractory material in furnaces. Development of the sol-gel process has enabled stoichiometric mullite to be prepared with nearly theoretical density. 4,5 Subsequent studies have examined high temperature creep 6,7 and the mechanical properties 8-11 of polycrystalline mullite. Results have suggested that mullite exhibits superior creep resistance at high temperatures. Several studies have examined the electrical properties of mullite including electrical conductivity, electromotive force, and ionic transport number. [12][13][14] Despite widespread use of mullite in industry and studies of the kinetic properties in mullite, the basic characteristics of this material are not well understood, particularly the diffusion coefficient. Aksay et al. 15 studied the growth of a mullite layer between aluminum-silicate melts and sapphire and calculated the chemical diffusion coefficient in mullite, possibly under a compositional gradient. Okamoto et al. 7 studied the creep of polycrystalline mullite in air and calculated the effective diffusion coefficients with an apparent activation energy of 810 kJ/mol. These diffusion coefficients were estimated from the results of kinetic studies and may involve processes other than diffusion that influenced the apparent activation energy. Since there are no tracer diffusion data for mullite, the understanding of the kinetics of this material is limited. The tracer diffusion coefficient in mullite must be determined.The purpose of this study was to determine the oxygen self-diffusion coefficient in mullite and compare the results with those reported for similar materials.Experimental Specimen preparation.-If the diffusion sample should be examined by secondary ion mass spectroscopy, a large specimen with high density is required. Since mullite melts incongruently, 1 it is not easy to form a large single crystal of mullite. Consequently, we decided to produce a single-crystal powder of mullite and study the oxygen diffusion by gas analysis. Based on the oxygen diffusion coefficient in a similar material (forsterite), 16 the diffusion of oxygen in mullite was expected to be slow. Based on this hypothesis, we assumed that a large amount of single-crystal powder in a simple shape would be required for gas analysis. We manufactured singlecrystal mullite by making use of the fact that mullite coexists with a silica-rich liquid between 1585 and 1828ЊC. With this process, we can ensure that the composition of the resultant mullite is fixed and does not vary from powder to powder.We mixed 0.10 mol of aluminum alkoxide (Al(OC 3 H 7 ) 3 ) and 0.16 mol of silicon alkoxide (Si(OCH 3 ) 4 ). Excess deionized water was added to the mixture to produce the intimate mixture of aluminum hydroxide ...

No abstract