Reequilibration kinetics of the heterogeneous system NiO-NiCr₂O₄

Abstract

Reequilibration kinetic data have been obtained for a model system consisting of an undoped NiO crystal covered with a thin layer of NiCr₂O₄. The data lead to following apparent chemical diffusion coefficients$\tilde D^*$ measured within the temperature range 900–1200°C when varying oxygen partial pressure from 1 to 10⁻⁵ atm:

$$\begin{gathered} \tilde D^* _{red} = 5.28 \times 10^{ - 4} exp\left( {\frac{{19,160 \pm 870}}{{RT}}} \right); \hfill \\ \tilde D^* _{oxid} = 1.82 \times 10^{ - 2} exp\left( {\frac{{28,270 \pm 2,020}}{{RT}}} \right) \hfill \\ \end{gathered} $$

. Values of the apparent activation energies for the reequilibration processes were independent of the thickness of NiCr₂O₄ surface layer from about 0.01 to 0.05 mm. As determined by microprobe, the thickness of the NiCr₂O₄ surface layer changed only slightly during long reequilibration experiments. The change is in agreement with diffusion coefficients of Cr in NiO. Oxygen pressure dependence of electrical conductivity leads to the parameter 1/n = 1/5.55 being the same as those for undoped NiO crystals. This indicates the values of electrical conductivity taken at equilibrium are determined by the equilibrated undoped NiO crystal while the dissimilar reequilibration kinetic data result from the presence of the NiCr₂O₄ surface layer. The results indicate the slower oxidation kinetics to be surface-controlled and faster reduction kinetics to be bulk-diffusion-controlled.