Abstract
The importance of the rate of the surface reaction on the kinetics of equilibration of an oxide with the surrounding atmosphere under isothermal conditions is examined by using a general expression of this reaction rate given by Wagner as a boundary condition to solve the relevant diffusion equation by numerical methods. Solutions are obtained for the ideal case of an oxide having a rather simple defect structure corresponding to that of cobalt oxide at high temperatures, but assuming a variable rate constant for the surface reaction, allowing it to change in a suitable range to obtain a change from a diffusion control to a mixed type of control. The effects of the surface reaction rate are discussed in relationship to the different factors affecting the final result, such as the value of the surface rate constant, the sample thickness, the oxygen pressure range examined, and the direction of the pressure change. The data calculated in this way are then analyzed according to the equation which applies for a simpler linear rate law, and it is shown that use of this procedure yields values of the chemical diffusion coefficient more accurate than the analysis made according to the assumption of apure diffusion control. Finally, the need of a more general type of regression analysis of relaxation data to obtain reliable estimates of the chemical diffusion coefficient in cases of mixed kinetic control in presence of a general type of surface rate law is pointed out.
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V. Dovì, F. Gesmundo, and F. Viani, to be published.
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Gesmundo, F., Viani, F. & Dovì, V. Kinetics of oxide equilibration in relaxation experiments with a partial degree of surface control under a Wagner-type rate law for the surface reaction. Oxid Met 23, 141–158 (1985). https://doi.org/10.1007/BF00659900
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DOI: https://doi.org/10.1007/BF00659900