Reactions in quasi-binary systems

If we were to permit a KCl crystal to react with a RbCl crystal, or a NiO crystal with a MgO or CoO crystal, or a C0AI2O4 crystal with a MgAl204 crystal, then different situations would be observed in each case, depending upon the relative mobilities of the ionic and electronic defects. First of all, since these systems form nearly ideal solid solutions [34], the thermodynamic factor (1 H- d In y /d In in eq. (5-28) can be set equal to unity, and Fick s first law applies in its simplest form (see eq. (5-29)). Essentially, the problem in this class of reactions is to determine the chemical diffusion coefficient as a function of composition. 

The chemical diffusion coefficient can be experimentally determined as described in section 5.5.6 with the aid of the Boltzmann-Matano analysis. The atomistic interpretation of this diffusion coefficient will be illustrated by two examples. 

A strongly asymmetric diffusion profile has been observed after a certain diffusion time by means of electron probe microanalysis. This profile is shown in Fig, 6-1 [11]. 

As always, electroneutrality of the volume elements and local thermodynamic equilibrium have been assumed. In the analogous systems MgO-CoO and NiO-CoO, it is knovm [12] that, at constant oxygen partial pressure, the mole fraction of vacancies depends upon the mole fraction of cobalt oxide, essentially according to  

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Reactions in quasi-binary systems in which the reactant partners no longer form a complete range of solid solutions can be treated in single-phase regions in exactly the same way as the diffusional processes just discussed above. This will be discussed in greater detail in connection with reactions in multiphase systems. 

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