Thin-Film Reactions

Structure evolution of h.c.p./c.c.p. metal oxide interfaces in solid-state reactions

The structure of crystalline interfaces plays an important role in solid-state reactions. The Al₂O₃/MgAl₂O₄/MgO system provides an ideal model system for investigating the mechanisms underlying the migration of interfaces during interface reaction. MgAl₂O₄ layers have been grown between Al₂O₃ and MgO, and the atomic structure of Al₂O₃/MgAl₂O₄ interfaces at different growth stages was characterized using aberration-corrected scanning transmission electron microscopy. The oxygen sublattice transforms from hexagonal close-packed (h.c.p.) stacking in Al₂O₃ to cubic close-packed (c.c.p.) stacking in MgAl₂O₄. Partial dislocations associated with steps are observed at the interface. At the reaction-controlled early growth stages, such partial dislocations coexist with the edge dislocations. However, at the diffusion-controlled late growth stages, such partial dislocations are dominant. The observed structures indicate that progression of the Al₂O₃/MgAl₂O₄ interface into Al₂O₃ is accomplished by the glide of partial dislocations accompanied by the exchange of Al³⁺ and Mg²⁺ cations. The interface migration may be envisaged as a plane-by-plane zipper-like motion, which repeats along the interface facilitating its propagation. MgAl₂O₄ grains can adopt two crystallographic orientations with a twinning orientation relationship, and grow by dislocations gliding in opposite directions. Where the oppositely propagating partial dislocations and interface steps meet, interlinked twin boundaries and incoherent Σ3 grain boundaries form. The newly grown MgAl₂O₄ grains compete with each other, leading to a growth selection and successive coarsening of the MgAl₂O₄ grains. This understanding could help to interpret the interface reaction or phase transformation of a wide range of materials that exhibit a similar h.c.p./c.c.p. transition.

Studying the surface reaction between NiO and Al2O3 via total reflection EXAFS (ReflEXAFS)

The reaction between NiO and (0001)- and (1102)-oriented Al2O3 single crystals has been investigated on model experimental systems by using the ReflEXAFS technique. Depth-sensitive information is obtained by collecting data above and below the critical angle for total reflection. A systematic protocol for data analysis, based on the recently developed CARD code, was implemented, and a detailed description of the reactive systems was obtained. In particular, for (1102)-oriented Al2O3, the reaction with NiO is almost complete after heating for 6 h at 1273 K, and an almost uniform layer of spinel is found below a mixed (NiO + spinel) layer at the very upmost part of the sample. In the case of the (0001)-oriented Al2O3, for the same temperature and heating time, the reaction shows a lower advancement degree and a residual fraction of at least 30% NiO is detected in the ReflEXAFS spectra.

On the Influence of Applied Fields on Spinel Formation

Interfaces play an important role in determining the effect of electric fields on the mechanism of the formation of spinel by solid-state reaction. The reaction occurs by the movement of phase boundaries but the rate of this movement can be affected by grain boundaries in the reactants or in the reaction product. Only by understanding these relationships will it be possible to engineer their behavior. As a particular example of such a study, Mgln2O4 can be formed by the reaction between single-crystal MgO substrate and a thin film of In2O3with or without an applied electric field. High-resolution backscattered electron (BSE) imaging and electron backscattered diffraction (EBSD) in a scanning electron microscope (SEM) has been used to obtain complementary chemical and crystallographic information.