Demixing of Oxides under a Temperature Gradient

The microscopic mechanism of a demixing of binary oxide solid solution in an oxidizing atmosphere under a temperature gradient is investigated by the path probability method of irreversibly statistical mechanics. The problem is reduced to a steady-state metal ion transport under the coexistence of an oxygen partial pressure gradient and a temperature gradient. Calculations are performed under two different conditions: (i) demixing in a steady-state diffusion process under the two driving forces and (ii) a redistribution of constituent metal ions in a closed system under a given temperature gradient (Soret effect). The results can be interpreted as the superposition of the contributions of two driving forces plus a cross term which tends to zero as the driving forces become small. The difference between these two cases is discussed in detail and some controversies in the past work with respect to the difference are pointed out.