Kinetics of thin-film reactions of nickel oxide with alumina: II, {1100} and {1102} reaction couples

The kinetics of the earliest stages of solid-state reactions between epitactic thin films of nickel oxide (NiO) and {1100}- and {1102}-oriented single crystals of alumina (Al2O3) have been studied. The starting reaction geometry consisted of a nickel aluminate (NiAl2O4) buffer layer 20 nm thick and a NiO overlayer that was 400 nm thick, both of which were grown via pulsed-laser deposition and were epitactic with the substrate. When heated, the reaction layers thickened mainly as the square root of time (i.e., parabolic kinetics) increased, which was indicative of reaction-controlled kinetics via diffusion through the product layer. The activation energy for the reaction was 461 kJ/mol for {1100} and 464 kJ/mol for the {1102}; both values are consistent with measurements made previously using thin-film and bulk reaction geometries. A smaller linear component to the kinetics has also been identified. The results of microstructural characterization and "marker" experiments will also be discussed.