The oxidation and hydration kinetics of a proton conductor oxide, SrCe(0.95)Yb(0.05)O(2.975), were examined via conductivity relaxation upon a sudden change of oxygen activity in a fixed water-activity atmosphere, and vice versa, in the ranges of -4.0 < log a(O(2)) < or = 0.01 and -5.0 < log a(H(2)O) < -2.0 at 800 degrees C. It was found that under an oxygen-activity gradient in a fixed water-vapor-activity atmosphere, the conductivity relaxation with time is monotonic with a single relaxation time (as usual), yielding a chemical diffusivity that is unequivocally that of the component oxygen. In a water-activity gradient in a fixed oxygen activity atmosphere, on the other hand, the conductivity relaxation appears quite unusual, exhibiting an extremum after an initial transient. The conductivity relaxation upon hydration or oxidation, in general, is quantitatively analyzed in terms of two apparent chemical diffusivities for component oxygen and hydrogen, respectively. The inner workings of hydration is discussed, and the as-evaluated chemical diffusivities are reported and compared with the conventional chemical diffusivity of water.

Electrical conductivity relaxations were measured at a fixed temperature ͑700°C͒ on proton-conducting BaCe 0.95 Yb 0.05 O 2.975 upon sudden changes of oxygen activity in fixed water-vapor activity atmospheres and vice versa, in the ranges of −4.5 Ͻ log a O 2 ഛ 0.02 and −5.0 Ͻ log a H 2 O Ͻ −2.0, respectively. It has been confirmed that the conductivity relaxes generally with two different relaxation times or twofold, but always nonmonotonically upon hydration/dehydration and monotonically upon oxidation/ reduction. This twofold relaxation is attributed to the superposition of the two chemical diffusion processes of component H and O, driven by either the water or oxygen chemical potential gradient. The analytic solution to the conductivity relaxation, be it monotonic or nonmonotonic, is presented in closed form and thereby evaluated the two chemical diffusivities governing the hydration and oxidation kinetics, respectively.In the process of measurement of the equilibrium ͑total͒ electrical conductivity on a proton-conducting oxide SrCe 0.95 Yb 0.05 O 2.975 against oxygen activity ͑a O 2 ͒ and water-vapor activity ͑a H 2 O ͒, it was observed ͑henceforth referred to as Ref. 1͒ that upon hydration or dehydration in fixed a O 2 atmospheres, the conductivity relaxes always nonmonotonically to a new equilibrium value, leaving an extremum ͑minimum for hydration and maximum for dehydration͒ in between, while the relaxation during oxidation or reduction is always monotonic, irrespective of a H 2 O in fixed a H 2 O atmospheres. The hydration kinetics of a proton conductor oxide used to be understood as being due to the chemical diffusion of H 2 O or ambipolar diffusion of oxide ions ͑or vacancies͒ and protons. 2,3 One thus expected a usual monotonic relaxation of conductivity as in chemical diffusion of oxygen in acceptor-doped BaTiO 3 under an oxygen potential gradient. 4 In this light, this nonmonotonic relaxation behavior was quite unusual.In Ref. 1, Yoo et al. interpreted the nonmonotonic relaxation upon hydration or dehydration as being due to the superposition of two chemical diffusion processes of chemical components H and O, or the ambipolar diffusion of protons ͑OH O · or equivalently H i · ͒ and holes ͑h · ͒ and of oxygen vacancies ͑V O ·· ͒ and holes, both down the water chemical potential gradient imposed. Upon hydration, for instance, in-diffusion of H reduces the concentration of holes via the reaction H + h · = H i · , but in-diffusion of O increases the hole concentration via O + V O ·· = O O x + 2h · , thus leading to a twofold nonmonotonic variation of the conductivity with time. On the basis of this picture, Yoo et al. presented a closed-form solution for conductivity relaxation upon hydration/dehydration, and thereby evaluated with sufficient accuracy the two chemical diffusivities, D iH and D vH , for H and O, respectively. Almost at the same time, Yu et al. 5 also confirmed experimentally by optical spectroscopic means these decoupled chemical diffusion processes upon hydration of Fe-doped SrTiO 3 .Th...

No abstract

Meaning as-calculated from the batch composition (irrespective of phase purity), which was formulated on the basis of the presumption ½La Ba 4½V 00 00 Ti .