GROWTH OF SPINEL PARTICLES ON ALUMINA THIN-FILMS .2. MORPHOLOGY AND CRYSTALLOGRAPHY OF THE INTERFACE

In the second of two papers on the growth of spinel particles on alumina thin films, the morphology and structure of the heterophase boundary has been examined. The phase boundaries are found to adopt planes which are low-index planes in the alumina and, whenever possible, they also facet parallel to the low-index planes in the spinel. These low-index planes are the planes with large d-spacings. The structure of the phase boundary appears to be such that the misfit along the interface plane is minimized. For some of the phase boundary planes examined, the density of near-coincident lattice points with respect to the oxygen sublattice of the spinel and alumina is particularly high. Each of the five different types of phase boundary produces a specific morphology as described in the companion paper [Acta metall. mater. 42, 2729 (1994)]. This morphology is related to the structure of the phase boundary. In the early stage of the thin-film reaction, the spinel growth consists of two stages; the spinel first grows out from the edge of the foil until it reaches a critical length, and then moves into the alumina substrate. It is proposed that the interface segment which moves fastest is the one along which the misfit is higher.