Michiyo Kamiya

Intrinsic and Extrinsic Oxygen Diffusion and Surface Exchange Reaction in Cerium Oxide

Polycrystalline CeO 2 with a relative density in excess of 97% was prepared. The specimens contained a lower concentration of impurities than those examined previously. Oxygen diffusion experiments were performed for the temperature range from 800 to 1300°C, in an oxygen partial pressure of 6.6 X 10 3 Pa. The concentration profile of 18 O in the specimens following diffusion annealing was measured by secondary ion mass spectroscopy (SIMS). In the high-temperature region (intrinsic region, above 1000°C), the oxygen self-diffusion coefficient obtained using SIMS was observed to agree reasonably with that obtained by gas phase analysis in a previous study, but the activation energy was found to be slightly smaller. The present result, D = 3.16 X 10 -4 exp(-226 kJ mol -1 /RT) m 2 s -1 (T = I 100-1300°C), is thought to represent the intrinsic behavior of undoped CeO 2 . In contrast, in the low temperature region (extrinsic region, less than 1000°C), the activation energy was smaller than that in the high temperature region. Comparison with data reported in the literature for CeO 2 doped with Y and Gd, suggests that the low-temperature oxygen diffusion region is controlled by a trivalent impurity. The surface exchange coefficients obtained from gas phase analysis and SIMS agreed very well with each other and were represented by k = 1.93 X 10 -3 exp(-136 kJ mol -1 /RT) m s -1 (T = 800-1300°C). The data were also in good agreement with the surface exchange coefficient in ThO 2 , suggesting that the oxygen surface exchange reaction is insensitive to cation species.

Oxygen self-diffusion in cerium oxide doped with Nd

Polycrystalline Ce 0.77 Nd 0.23 O 1.885 having 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 of 18 O 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 −9 exp(−53 kJ mol −1 / RT ) m 2 s −1 and was in the extrinsic region. The oxygen diffusion coefficient of Ce 0.77 Nd 0.23 O 1.885 was larger than that of Ce 0.8 Y 0.2 O 1.90 ; it was close to those of Ce 0.6 Y 0.4 O 1.80 and Ce 0.69 Gd 0.31 O 2−δ . The oxygen diffusion coefficient obtained by the tracer method at 700 °C agreed with that calculated from the electrical conductivity in Ce 0.77 Nd 0.23 O 1.885 . 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 CeO 2 and ThO 2 .

Oxygen diffusion in single-crystal In2O3 and tin-doped In2O3

Single-crystal In 2 O 3 and Sn-In 2 O 3 (2.6% Sn) were grown by the flux method for use in an oxygen diffusion experiment. Diffusion profiles within the crystals were analyzed by secondary ion mass spectrometry. The concentration profile of 18 O in the solids could be explained very well by application of Ficks law. The oxygen diffusion coefficients in Sn-In 2 O 3 were higher than those in In 2 O 3 . However, the apparent activation energy for oxygen diffusion in Sn-In 2 O 3 was smaller than that in In 2 O 3 , probably due to the occurrence of a nonstoichiometric reaction in this oxide. Oxygen diffusion in single-crystal In 2 O 3 was slower than that reported for polycrystalline In 2 O 3 , although the activation energy was similar for both materials.

Crystal growth of CeO2 under hydrothermal conditions

Publisher Summary Ceria (CeO2) possesses the fluorite structure and is known to show both electric and ionic conduction mechanisms. The presence of a wide range of non-stoichiometry is largely responsible for the nature of electrical conductivity of this oxide. CeO2 powder precipitated by NH4OH was hydrothermally treated with LiOH as a mineralizer. After hydrothermal treatment, the samples were examined by X-ray diffraction, and observed by SEM and TEM to measure the particle sizes. The particle sizes of CeO2 powder after treatment grew bigger except for the conditions of 300-400oC, 100 MPa, for 0 h. All of the X-ray diffraction patterns obtained in this study were those of CeO2. As treatment temperature increased in the range of 500 to 800oC the peaks became gradually strong. This was closely related to the results obtained by electron microscopy.