Interfacial reaction of NiO with Al2O3(112̄0) and polycrystalline α-Al2O3

The reactivity of NiO with polycrystalline α-Al2O3 and Al2O3(112̄2) single crystals is compared. NiO/Al2O3 samples, prepared by vacuum vapour deposition of Ni layers (35–100 nm) onto both types of Al2O3 substrates, were oxidized and subsequently kept at 1020°C in a flow of 20 vol% O2 in balance N2. The lateral homogeneity of the samples was investigated using Scanning Electron Microscopy (SEM). Interdiffusion at the NiO/Al2O3 interface, due to formation of NiAl2O4, was assessed by Rutherford Backscattering Spectrometry (RBS). The formation of the spinel-type NiAl2O4 phase was confirmed by X-ray diffraction (XRD) and the appearance of the characteristic absorption triplet around 600 nm in Diffuse Reflectance spectra. In the initial stage of the reaction Al3+ ions rapidly diffuse along the boundaries of the NiO grains, as evidenced by the presence of some aluminium at the surface of the samples after relatively short annealing periods. The reaction rate of NiO with Al2O3(112̄0) to NiAl2O4 was significantly higher than with polycrystalline α-Al2O3 substrates (reaction rate constants at 1020°C: k = 0.66 ± 0.07 and 0.32 ± 0.03 nm2/s, respectively). This is attributed to a higher grain boundary density of the NiO layer supported by the smooth single crystal, as compared to NiO overlayers on the rougher polycrystalline support. Indications of an important role of the NiO grain boundaries were found with both RBS and SEM. It was also found on both single crystalline and polycrystalline substrates that the reaction product contained some excess Al2O3 after prolonged annealing (NiAl2O4·0.17Al2O3).