Kinetics of growth and lateral spreading of a spinel layer around NiO particles on Al₂O₃ substrate

The kinetics of thickening and lateral spreading of a thin layer of NiAl₂O₄ spinel around individual NiO particles, located on single crystalline Al₂O₃ substrate, have been analyzed theoretically and studied experimentally by SEM and AFM in the temperature range 1000-1100 °C. It was assumed that, due to small thickness of the forming layer, the kinetics of thickening is limited by the reaction rate at the spinel-substrate interface, whereas the lateral propagation of the layer is controlled by surface diffusion of NiO molecules. The reaction rate control at the spreading front of the growing compound layer was also taken into account. By comparison the rates of lateral spreading with relations derived, the diffusion coefficients of NiO on the NiAl₂O₄ spinel surface and the surface diffusion activation energy have been estimated.