Kinetic Analysis of Combustion Synthesis of Lead Magnesium Niobate from Metal Carboxylate Gels

A combustion synthesis route was developed for direct crystallization of perovskite lead magnesium niobate (PMN) from metal carboxylate gels. Direct PMN formation is attributed to a homogeneous cation distribution in the gel and the rapid heating during exothermic gel decomposition in oxidizing atmospheres. The maximum bed temperature and gel decomposition rate are determined by the combined influence of chemical kinetics, heat, and mass transfer. It is shown that there is a temperature window between 700° and 750°C which favors direct PMN formation, whereas pyrochlore formation is favored below 650°C and PbO volatilization occurs above 800°C. The effects of kinetic and transport parameters including oxygen partial pressure, gas flow rate, and bed geometry on PMN formation are discussed on the basis of a thermal ignition model for heterogeneous reactions.