Effect of volume changes during precipitation on internal reaction kinetics

Internal reduction of mixed oxides, which results in the formation of a metal dispersion within the oxide, is generally accompanied by important local volume changes. Depending on the atomic mechanism (interstitial or substitutional formation of metal), compressive or tensile stresses build up during nucleation and growth of the metal precipitates. Depending on the nature of the oxide and the reaction temperature, these stresses can either be relaxed by plastic or brittle deformation or, if not relaxed, can inhibit further precipitation. A general model for stress generation during internal reduction and corresponding relaxation by different mechanisms is presented and illustrated by the study of two specific mixed oxides, (Mg,Ni)O and (AI,Cr)(2)O-3. Reduction experiments are performed for both oxides in a wide range of reaction conditions. Reduction kinetics are measured and reaction scales examined by transmission electron microscopy. The influence of stress relaxation, occurring at the reaction front and at the precipitate interfaces, on the global reaction kinetics is discussed.