Abstract
The rates of metal aluminate formation inMeOx/Al2O3systems (Me=Ni, Co, Cu, Fe) at 500–1000°C in O2or N2atmospheres were compared, using bothα-Al2O3andγ-Al2O3substrates. Interfacial reaction toMeAl2O4was assessed using Rutherford backscattering spectrometry and X-ray diffraction. The lateral homogeneity of the samples was investigated using scanning electron microscopy. The spinel formation rate follows the sequence FeAl2O4<NiAl2O4<CoAl2O4<CuAl2O4. This is explained in terms of thermodynamic limitations (FeAl2O4) and site-preference energies (CoAl2O4, NiAl2O4). The fast reaction to copper aluminate was accompanied by a severe rupture of the top layer (∼1 μm) of the substrate. Penetration of transition metal ions intoγ-Al2O3substrates is notable at considerably lower temperatures than withα-Al2O3substrates, mainly due to grain boundary diffusion. Annealing conditions that favor the formation of transition metal cations of a low valency (Co2+, Cu+) are beneficial for reactions to aluminates.
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