Stability criteria for product microstructures formed on gaseous reduction of solid metal oxides

A range of different solid oxide and metal product morphologies can be formed on gaseous reduction of metal oxides. These product morphologies have been shown in previous studies to be critical in determining the rate limiting reaction mechanisms and the overall rates of reduction. By considering (1) established criteria for the stability of moving interfaces in a thermodynamic potential gradient, (2) the relative rates of chemical reactions on the oxide and metal surfaces, and (3) key process phenomena and physico-chemical properties of the solid phases, the conditions for the formation of various product morphologies are identified. The analysis also demonstrates the theoretical basis for the development of morphology maps that define the product morphologies as a function of thermodynamic driving force for reaction and reaction temperature. The methodology is shown to be general and can be applied to the analysis of any system involving the decomposition of metal compounds in reactive gas atmospheres. © The Minerals, Metals & Materials Society and ASM International 2009