Diffusion of oxidants

The diffusion of oxide ions may also dominate an oxidation reaction. In a similar fashion to that described for cations, the anions can be accompanied by a flow of holes or a counterflow of electrons (Fig. 5.22c and 5.22d). The arriving O2-anions extend the oxide film at the metal-metal oxide boundary. Once again, which of these mechanisms is theoretically possible will depend upon whether the oxide is naturally rc-type or p-typc. 

Since there is no oxidizer leaving the surface, the surface boundary condition for diffusion of oxidizer is... 

Fig. 11-8. Polarization curves for a corroding metallic electrode of which corrosion rate is controlled by diffusion of oxidants in aqueous solution solid curve = observable polarization curve.

In both cases of Eqns. 11-13 and 11-14, in which the corrosion reaction is controlled by the transfer of interfacial charge or by the diffusion of oxidants, the polarization resistance, Rf, can be used to estimate the metallic corrosion rate (corrosion current,... 

As shown in Table 10.5, non-metaUic fuels used as ingredients of pyrolants are boron, carbon, silicon, phosphorus, and sulfur. Similarly to metal particles, non-metal particles are oxidized at their surfaces. The processes of diffusion of oxidizer fragments to the surface of a particle and the removal of oxidized fragments therefrom are the rate-controlling steps for combustion. 

Harris, D.J., Lavrentiev, M.Y., Harding, J.H., Allan, N.L., Purton, J.A. Novel exchange mechanisms in the surface diffusion of oxides. J. Phys. Condens. Matter 2004, 16(13), L187-92. 

The concept proposed by us is pictured simply in Fig. 23. The level of water represents the chemical potential of active oxygen involved in the oxidation of propylene, and the vessels connected on the tank are involved in two kinds of active sites that activate molecular oxygen to atomic species and oxidize propylene to acrolein. If active sites expressed by vessels are isolated from each other, each site must do everything by itself to convert propylene to acrolein. This situation is less convenient than the preparation of the active catalyst system. When active species of oxygen can migrate rapidly through the bulk diffusion of oxide ion as shown in Fig. 23, equal-... 

III. Stability of the Multicomponent Bismuth Molybdate Catalyst Depending on the Bulk Diffusion of Oxide Ion... 

In the preceding section, we explained that the bulk diffusion of oxide ion plays an important role in the enhancement of the catalytic activity of the multicomponent bismuth molybdate systems. Here, another important role of the oxide ion migration in increasing the stability of the catalyst system is introduced. 

In the formation of metal oxides, both diffusion of oxygen inwards through the film on metal as well as diffusion of metal outwards may occur. Diffusion of oxide inwards to the metal/oxide interface is typified by titanium-oxygen and zirconium-oxygen systems. 

In a Figure 1 the theoretical dependences T5% on contents Z ( /. for cases of slow and rapid diffusion of oxidant and also experimental magnitudes Ty/ obtained in air and in helium atmosphere are shown. As follows from the data of this figure, the value Ts% for initial HDPE corresponds to slow diffusion case and for compositions HDPE+Z - to rapid diffusion. 

T. Ishigaki, S. Yamauchi, J. Mizusaki, K. Fueki and H. Tamura, Diffusion of oxide ions in LaFeOa single crystal. Solid State Chem., 55 (1984) 50-53. 

In the case of oxides with relatively slow diffusion of oxide inside crystals, and especially in the case of nonuniformity of surface oxygen, the determination of the predominant mechanism is more complicated. 

Thermal diffusivity of oxidation-resistant SiC/C compositionally graded graphite materials has been measured by using the laser flash method. In order to study the effect of the SiC/C graded layer on the diffusivity, the thickness of the graded layer and the SiC content were changed. In addition, the specific surface areas of the SiC/C materials have been measured. It is shown that the effect of the SiC/C graded layer on thermal diffusivity was small within SiC contents (0.27-8.52 mass%) used in this study. 

Robin R, Cooper AR, Heuer AH (1973) Application of a nondestructive single-spectrum proton activation energy to study oxygen diffusion in zinc oxide. J Appl Phys 44 3770-3777 Robertson WM (1969) Surface diffusion of oxides. J Nucl Mater 30 36-39... 

In one case, the diffusion of a2+ is much faster than b3+ and in the other the opposite is true. Note that charge-compensation of migrating species is maintained in aU cases by the diffusion of oxide species. 

The diffusion of oxide ion vacancies is affected hy the elastic strain energy, which is related to the size mismatch between the host and dopant cations [9]. Based on the earlier observations of Nowick [10], and Kilner and Steele [11], the importance of the defect pairs formed due to interaction between the oxide ion vacancies, Vo, and alivalent cations, Mce, in Ce02 should he emphasized. 

Tritium present in an oxide layer can be removed by acid dissolution of the oxide or more gently by isotopic exchange with normal water or activated hydrogen gas (plasma). Because diffusion of oxide- or bulk-dissolved tritium back to the surface of a material undergoing decontamination is often slow, exchange at an elevated temperature may be advantageous. 

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