ABSTRACT

Figure 6.46 Comparative plot of cation diffusion coefficients in Al20 3, MgO, CaO, Cr20 3, NiO, CoO, and oxygen diffusion in stabilized zirconia and Si02 [2].

NiO and CoO, which are characterized to possess appreciable nonstoichiometry, exhibit high rates of diffusion due to their high concentrations of point defects. Similarly, stabilized zirconia with a large concentration of oxygen point defects provides a poor diffusion barrier. To the contrary, the transport of molecular oxygen through Si02 is much lower than AI ions through Ah03 above 1773 K. Accordingly, in generalized form, it may be stated that simple oxides with large ionic character and small deviations from stoichiometry are expected to act as better protective layers. To the contrary, oxides like FeO, NiO, and CoO cannot act as effective diffusion barriers unless their point defect concentrations are decreased by doped element oxides.