Oxygen, apparent diffusion coefficient

Fig. 6.3. Apparent diffusion coefficient of oxygen as a function of separator saturation [1],...

Fig. 9.11. Apparent diffusion coefficient of oxygen as a function of AGM separator acid-saturation percentage. Dotted vertical lines represent the operational range for VRLA cell, 10%. In practice, this operational range is shifted to 85-95% [14].

Fig. 9. Apparent diffusion coefficients calculated from the volume kinetics portions of the shrinkage isotherms compared with the reported self-diffusion coefficients for aluminum and oxygen.

This model which associates the beginning of the distribution profile with an apparent diffusion coefficient has been recently discussed by Fielitz et al. [78] who proposed that the first part of the oxygen diffusion profile in polycrystalline mullite corresponded to the bulk diffusion coefficient in the B regime and not to the apparent diffusion coefficient. 

In the first place our understanding of factors that control and limit the interfacial kinetics is still rudimentary, and therefore should be a fruitful area for further investigation. The apparent correlation between the surface oxygen exchange coefficient and the tracer diffusion coefficient D for different classes of oxides, the fluorite-related and the perovskite-related oxides, as noted by Kilner et al. [73], clearly indicate the potential of isotopic exchange. 

The results of the study of diffusion rate as a function of temperature for sol-gel spheres, together with results from fused spheres run at approximately the same temperature, are presented in Table II. It is apparent that the diffusion coefficients for the sol-gel material are consistently slightly greater at the same temperature. In addition to fundamental differences in the nature of the samples, the oxygen pressure was slightly higher (30.6 mm versus... 

This flux will be dependent upon the surface vacancy concentration, the surface electron concentration, and the dissociation rate of the dioxygen molecule however, at present, the rate-limiting step has yet to be identified. Kilner et al. [11] have derived a simple relationship for the surface exchange coefficient in terms of the bulk and surface vacancy concentrations, in an attempt to explain the apparent correlation found between the activation enthalpy for the surface exchange coefficient and the diffusion coefficient, in a number of La-based perovskites. Adler et al. [12] have also arrived at a similar relationship for k, by consideration of the AC electrode behavior of symmetrical cells with a double cathode structure. As already mentioned, the exact mechanisms of oxygen surface exchange remain elusive however, the vacancy concentration is clearly a very important parameter. 

In addition to following the isothermal effects, it is also quite important to understand the changes that take place in the observed activation energy for diffusion as a function of the composition. The ease of oxygen ion diffusion in perovskite structure is usually attributed to the value of the apparent aetivation energy estimated from the Arrhenius plots of diffusion coefficient. This apparent activation energy, Ea, however, consists of several terms, as indicated in the following equation ... 

Marker experiments made with lead to the determination of oxygen diffusion coefficients via alumina grain boundaries and via the oxide bulk. Taking into consideration the plots of In [ 0] = f(x), the first part of the curve corresponds to the apparent diffusion of oxygen. From the three diffusion regimes, the B regime is usually chosen for the study of intergranular diffusion... 

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