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 ...
A new method of scavenging highly resistive siliceous phase using two-stage sintering, named as "precursor scavenging," is suggested for improving the grain-boundary conductivity of 8 mol % yttria-stabilized zirconia (YSZ). The scavenging efficiency and mechanism were studied and compared with those of 8YSZ-Al 2 O 3 composites prepared by various methods using impedance spectroscopy and imaging secondary-ion mass spectroscopy. A heat-treatment at 1200ЊC for longer than 20 h before sintering increased grain-boundary conductivity remarkably. The forming of inclusions containing Si was considered to be the origin of scavenging. The grain-interior resistivity was not changed by precursor scavenging, while it increased more than 15% by adding 1 mol % Al 2 O 3 when sintered at 1600ЊC. Precursor scavenging, therefore, is a potential and promising way for improving the grain boundary conductivity without deteriorating the grain-interior one.
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.
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