Reactivity of η‐, γ‐, and α‐Al 2 O 3 for ZnAl 2 O 4 Formation

The rate of ZnAl 2 O 4 formation was measured for η‐, γ‐; and α‐ Al 2 O 3 in order to distinguish the reactivity of them. The reactivity decreased as follows: η‐ > γ‐ > α‐Al 2 O 3 . The reaction rate fitted to Jander's equation and the activation energies calculated were 33, 47 and 113 Kcal/mol for η‐, γ‐ and α‐Al 2 O 3 systems, respectively. These differences are explained by an assumption that η‐ and γ‐Al 2 O 3 resulted in a ZnAl 2 O 4 with imperfect spinel structure, but α‐Al 2 O 3 gave the perfect spinel structure. This assumption is based on the theoretical consideration of the activation energy needed for the diffusion‐controlled reaction and date of lattice constant of each ZnAl 2 O 4 obtained from three aluminas. The fact that η‐Al 2 O 3 shows very high reactivity compared with that of γ‐Al 2 O 3 was found to be explained on the basis of Jander's equation, a comparison of specific surface area and the defect structures of the aluminas.