Isothermal diffusion in multicomponent ionic solids

Stability of Mullite Protective Coatings for Silicon‐Based Ceramics

“…Thus, J 6 ∼0 in . Using this assumption along with charge conservation (− J 3 +2 J 4 + J 5 =0), lead to the following expression for the total entropy production, σ: The two terms bracketed in are charge‐free, and such groups are referred to as relative building units in the literature 19 . Following this approach, the units representing these groups are the diffusing “species” and are written out for clarity as: The quantity U I represents the correlated diffusion of Al and Si through the same vacancies in the lattice.…”

“…(4) are charge-free, and such groups are referred to as relative building units in the literature. 19 Following this approach, the units representing these groups are the diffusing ''species'' and are written out for clarity as: The quantity U I represents the correlated diffusion of Al and Si through the same vacancies in the lattice. In the absence of detailed data on relative Si and Al diffusivities in mullite, this U I formulation simplifies the problem by assuming that the two cations diffuse at comparable rates.…”

“…where J n is the volatilization flux given by Eq. (19), and c O 2 is the concentration of oxygen that is in equilibrium with the oxygen in the surrounding combustion atmosphere.…”