Interfacial reaction of NiO with Al 2O 3(112̄0) and polycrystalline α-Al 2O 3

Abstract

The reactivity of NiO with polycrystalline α-Al 2O 3 and Al 2O 3(112̄2) single crystals is compared. NiO/Al 2O 3 samples, prepared by vacuum vapour deposition of Ni layers (35–100 nm) onto both types of Al 2O 3 substrates, were oxidized and subsequently kept at 1020°C in a flow of 20 vol% O 2 in balance N 2. The lateral homogeneity of the samples was investigated using Scanning Electron Microscopy (SEM). Interdiffusion at the NiO/Al 2O 3 interface, due to formation of NiAl 2O 4, was assessed by Rutherford Backscattering Spectrometry (RBS). The formation of the spinel-type NiAl 2O 4 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 Al 3+ 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 Al 2O 3(112̄0) to NiAl 2O 4 was significantly higher than with polycrystalline α-Al 2O 3 substrates (reaction rate constants at 1020°C: k = 0.66 ± 0.07 and 0.32 ± 0.03 nm 2/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 Al 2O 3 after prolonged annealing (NiAl 2O 4·0.17Al 2O 3).