Growth and equilibrium morphology of metal precipitates formed inside an oxide matrix

Metal precipitation occuring during internal reduction of various mixed oxides, such as magnesia or alumina, doped with more noble transition metal oxides, is investigated for different doping contents, reaction temperature and driving forces. The precipitate morphology developing inside the solid oxide matrix and the corresponding orientation relationships between the metal and the oxide phase are analysed by transmission electron microscopy. The evolution of precipitate shape across the reduction scale is interpreted in terms of ageing of the metal inclusions; the various morphologies identified are ascribed to different stages of nucleation, diffusional growth and final equilibration.Precipitate morphology is analysed in terms of diffusion-controlled growth of small metal particles in their local and long range diffusion field. The influence of interfacial energy and related anisotropy is experimentally demonstrated for selected examples. The general role played by these parameters during growth and final equilibration is also discussed. The implications of the elastic strain fields building up during growth of the metal inside the oxide matrix are analysed in view of our experimental results. Shape changes taking place during the growth process are considered to be controlled by local matter transport properties. The influence of the reaction mechanism at an atomic scale and the relative fit of the crystallographic lattices are briefly summarised.