Kinetics of thin-film reactions of nickel oxide with alumina: I, (0001) and {1120} reaction couples

Solid-state reactions between thin films of nickel oxide (NiO) that had been grown on four different orientations of α-alumina (α-Al2O3), each with an intermediate layer of spinel, have been studied by using electron microscopy. The starting reaction geometry consists of a nickel aluminate NiAl2O4) buffer layer 20 nm thick and a NiO overlayer 400 nm thick, both of which were grown via pulsed-laser deposition and epitactically aligned to the α-Al2O3. In the first of a series of three papers, the reaction on (0001)- and {1120}-oriented monocrystals of α-Al2O3 is discussed. When heat treatment is applied, the reaction layers thicken linearly as the time increases, which is indicative of kinetic control via an interfacial reaction. The activation energy for reaction on the (0001) face is 697 kJ/mol, whereas that for reaction on the {1120} reflection is 536 kJ/mol. Because the spinel and NiO have the same orientations (i.e., (111), parallel to the substrate surface plane) for both of these orientations, the NiO/spinel interfaces are the same. Thus, the difference in the kinetics is associated with the differences that occur at the spinel/alumina interface; hence, the kinetics of the interfacial reaction are dependent upon the interfacial structure. Observations of the location of the initial interfaces, relative to their final locations, lead to the conclusion that these reactions do not only occur via a counterdiffusion of cations, but rather must also involve the diffusion of oxygen.