Role of Interfacial Energy and Crystallographic Orientation on the Mechanism of the ZnO + Al2O3 -> ZnAl2O4 Solid-State Reaction: I. Reactivity of Films Deposited onto the Sapphire (110) and (012) Faces

The initial steps of the reaction between ZnO and Al2O3 have been investigated with X-ray diffraction, atomic force microscopy, and X-ray absorption spectroscopy at the Zn-K edge starting from 45 nm thick zincite films deposited onto (110)- and (102)-oriented sapphire single crystals. The formation of nonequilibrium phase(s) has been detected for both orientations. For the (001)(zincite) parallel to (110)(sapphire) interface, the rate-determining step is the motion of the interface(s); the growth of the spinel layer is linear with time, with a rate constant k = 1.1(2) x 10(-9) cms(-1) at 1000 degrees C. At the (110)(zincite) parallel to (012)(sapphire) interface, the reaction shows dumped oscillations. The results are discussed along with a comparison with previous results on thinner films to clarify the role of interfacial free energy and crystallographic orientation.