Reactivity in the spinel formation reaction in $CuO/Al_2O_3$, $Fe_2O_3$, $Cr_2O_3$ systems

The reactivity of a solid in a solid state reaction is influenced by a number of factors like contact area [1], structure of reactants, structure of products, phase transition in reactants [2] and the ambient atmosphere [3]. A study of these factors has assumed considerable practical and fundamental importance in controlling the reaction rates and understanding the reaction mechanisms. In the present work an attempt has been made to identify the reactivity pattern in the formation of aluminate, ferrite and chromite of copper from the respective metal oxides. In an earlier study, Stone and Tilley [4] reported that among aluminates, gallates and indates of magnesium, zinc, nickel and copper, the formation of copper aluminate was associated with maximum reactivity. They attributed this to the Jahn-Teller effect of $Cu^{2+}$ ions on the surrounding oxide lattice in the product phase $(CuA1_20_4)$. However the Jahn-Teller effect in $CuCr_20_4$ and $CuFe_20_4$ is more pronounced [5, 6] than in $CuA1_20_4$ where no macroscopic distortion has been detected [7, 8]. Thus, the present study has been carried out on the formation of $CuCr_20_4$, $CuFe_20_4$ and $CuA1_20_4$ in order to gain better insight into the influence of the Jahn-Teller effect on reactivity