Kinetics of NiFe<inf>2</inf>O<inf>4</inf> Precipitation in NiO

The kinetics of the precipitation of NiFe2O4 from an Fedoped NiO matrix has been studied. Polycrystalline samples were cooled at a constant rate through the solvus temperature, and the various stages of precipitation were monitored by periodically interrupting the cooling to observe the evolving size, distribution, and morphology of the precipitates. The precipitation occurred primarily by homogeneous nucleation and growth. The coherent precipitates have a dendritic morphology with a large interparticle spacing characteristic of fast precipitate growth. Reasonable rates for the nucleation of NiFe2O4 were observed to occur between 850° and 800°C, resulting in a 350° to 400°C undercooling. Preferential nucleation occurred at grain boundaries, but these particles were only twice as long as those nucleated homogeneously. The size and shape of the precipitates are independent of the distance from the surface, indicating that precipitation occurs by a local arrangement of the Ni and Fe cations and the preexisting cation vacancies. Copyright © 1992, Wiley Blackwell. All rights reserved