Equilibria for Reactive Solids and Vapors (Oxidation)

Consider a reaction where a solid is reacting with a gaseous component to produce a solid phase, such as the oxidation of aluminum metal to aluminum oxide:

$$\input{equations/alumina-rx}$$ (33-1)

or, for the case of pure silicon embedded to silica dissolved in alumina:

$$\input{equations/silica-rx}$$ (33-2)

In the most simple cases (e.g., Eq. 33-1), it is assumed that the reactions and products are pure (i.e., the solubility of oxygen is neglected in the solid phases.).

In more realistic cases (e.g., Eq. 33-2), it should be clear that, as well as the free energies of formation, considerations the free energy charges for forming a solution--such as those that have been considered in previous lectures--must also be applied.

In any case, it simplifies to divide a complex reaction into simpler steps.

For example Eq. 33-2, can be broken into two independent reactions:

$$\input{equations/silica-alumina}$$ (33-3)

In this way, free energy changes can be calculated (with respect to some standard state, usually taken as a pure component at a particular temperature).

Reaction (a) in Eq. 33-3 will involve the molar free energies of formation with respect to the pure components. These are usually tabulated in data books (e.g. the JANAF tables, the book by O. Kubaschewski and C.B. Alcock)

Reaction (b) in Eq. 33-3 will involve the free energies of mixing were considered in the construction of phase diagrams. Data is available for many practical systems and ThermoCalc (software) is a method for extrapolating such data from known values and phase diagrams.