Microstructural observation and reaction mechanism of ZnAI2O4 formation in the presence of various fluorides

The rates of ZnAI₂O₄ formation in the presence of various fluorides were controlled by the nucleation process. The rate constants of the nucleation process for various fluorides were in the order LiF > NaF and MgF₂ > CaF₂ < SrF₂ < BaF₂. This trend corresponds to the order of the promotion of ZnAI₂O₄ formation by these fluorides. In order to interpret the dependence of the kinetic constant of the nucleation of ZnAI₂O₄ from the intermediate phase on the kinds of counter cation of fluorides, it was assumed that the activated state for promotive formation of ZnAI₂O₄ under the presence of fluoride is similar to that of the following reaction: $$ZnO + Al_2 O_3 + 2MFleft( {M'F_2 } right) = ZnF_2 + M_2 Al_2 O_4 left( {M'Al_2 O_4 } right).$$ The order of the rate constant obtained in this study showed agreement with the order of the standard heats of formation for the above reaction. This relation was interpreted by applying the linear free energy relationship (LIFER).