COARSENING AND SHAPE TRANSFORMATION OF NIFE2O4 PRECIPITATES IN A NIO MATRIX

Coarsening and shape transformation of individual NiFeO4 particles in a NiO matrix has been studied by transmission electron microscopy (TEM). A TEM sample of Fe-doped NiO containing NiFe2O4 particles was subjected to a series of heat treatments in air. By examining the same area of the TEM sample before and after each heat treatment, it was possible to observe the effects of the heat treatments on the morphology of many individual spinel particles. The spinel particles transformed from the initial dendritic morpbology to an octahedral morphology. Simultaneously, coarsening was observed to occur by two mechanisms: the shrinkage and eventual disappearance of small particles coupled with the growth of large particles and the coalescence of nearby particles which are similar in size. The kinetics of shape transformation were also monitored and occurred approximately 30 times more rapidly at 875-degrees-C than at 850-degrees-C.