Oxidants diffusion coefficient

The results presented by Bagley et al. [45] imply that the oxide diffusion coefficient is much smaller in the steady-state regime than in the diffusion-controlled regime where physical bombardment is absent. It may be possible to account for this effect in terms of the diffusive transport model presented earlier by using a smaller oxide diffusion coefficient in the steady-state regime. To explore this possibility, one may set dX/dt= 0 in Eq. 7 to obtain... 

FIGURE 11.12 Arrhenius plots tor different oxides diffusion coefficients varying with T". ... 

For simplification of the treatment, we may adopt the treatment described by Gough and Leypoldt and treat the electrode particles as a very thin equivalent electrode layer covered by an equivalent solid electrolyte membrane with a thickness of 5m, as shown in Figure 2.12(B). At the steady state of the reaction, the concentration of oxidant on the thin electrode layer surface is defined as Cq, the concentration at the interface of the equivalent electrode membrane/electrolyte solution at the membrane side is defined as Cm, and that at the solution side is defined as Co, respectively. The oxidant diffusion coefficient in the electrolyte solution is defined as Do and in the membrane as Dm, respectively. Note that the zero point of the x-axis is at the interface of the equivalent electrode membrane/electrolyte solution. In such an electrode configuration, the obtained steady-state diffusion current density was obtained as Eqn (2.81) ... 

For the primary study, the diffusion flame was assumed to be stationary, the fuel and oxidizer diffusion coefficients were assumed similar and equal to the mixture diffusivity (Le = 1). This allowed simplification of the energy equation and the continuity equation for components by introducing a new scalar variable named the Schwab-Zeldovich variable and the method itself is known as the Schwab-Zeldovich method [11]. The chemical reaction was considered as a single-stage, fast reaction. 

From the beginning of SOFC development, it was found that LaSrMnOs (LSM) electrodes had a high activity for oxygen reduction at high temperatures and were stable under SOFC operation conditions. These LSM cathodes have been improved over time and it has been seen that an yttria stabilization of the cathodes improves the performance [35]. Single-phase LSM cathodes show a low oxide diffusion coefficient, so it is better to use a two-phase cathode that results in a lower overpotential for the oxygen reduction reaction. 

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

The oxide solid elecU olytes have elecuical conductivities ranging from lO Q cm to 10 cm at 1000°C and these can be converted into diffusion coefficient data, D, for die oxygen ions by the use of the Nernst-Einstein relation... 

The kinetics of this Uansport, virtually of oxygen atoms tlrrough the solid, is determined by the diffusion coefficient of the less mobile oxide ions, and local... 

The diffusion coefficients of cations in metal oxides are usually measured through the use of radioactive isotopes. Because of the friable nature of oxides it is exU emely difficult to use the sectioning technique employed for metal samples. The need for this can be avoided by the application of radioisotopes which emit radiation having a well established absorption law in matter. Isotopes which emit y radiation are very useful when the cation has a relatively high diffusion coefficient because of the long-range peneU ation of y rays. The absorption law is... 

A number of metals, such as copper, cobalt and h on, form a number of oxide layers during oxidation in air. Providing that interfacial thermodynamic equilibrium exists at the boundaries between the various oxide layers, the relative thicknesses of the oxides will depend on die relative diffusion coefficients of the mobile species as well as the oxygen potential gradients across each oxide layer. The flux of ions and electrons is given by Einstein s mobility equation for each diffusing species in each layer... 

In part the parabolic law may also apply to multilayer oxide systems where the cation diffusion coefficient is much higher in the lower oxide tlran in the higher oxide, which, growing as a thin layer, undergoes plastic deformation at high temperatures, thus retaining the overall oxide layer as impervious to enuy of tire gas. 

The tlrermodynamic activity of nickel in the nickel oxide layer varies from unity in contact with tire metal phase, to 10 in contact with the gaseous atmosphere at 950 K. The sulphur partial pressure as S2(g) is of the order of 10 ° in the gas phase, and about 10 in nickel sulphide in contact with nickel. It therefore appears that the process involves tire uphill pumping of sulphur across this potential gradient. This cannot occur by the counter-migration of oxygen and sulphur since the mobile species in tire oxide is the nickel ion, and the diffusion coefficient aird solubility of sulphur in the oxide are both vety low. 

Why this large difference Well, whenever you consider an alloy rather than a pure material, the oxide layer - whatever its nature (NiO, Cr203, etc.) - has foreign elements contained in it, too. Some of these will greatly increase either the diffusion coefficients in, or electrical conductivity of, the layer, and make the rate of oxidation through the layer much more than it would be in the absence of foreign element contamination. 

We denote by x the distance from the metal surface, and by n x) and rip x) the concentrations of cation vancancies and positive holes in the oxide. Let and Vp be their mobilities, and and Dp their diffusion coefficients. Let F x) be the electrostatic field in the oxide. J, the flux of cation vacancies (number crossing unit area per second), will be expressed by... 

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