Gas–solid reactions towards new frontiers

Abstract

Reactions between solids and a gas phase have interesting theoretical aspects as well as technological applications. Studies of the decomposition of alkaline earth carbonates have been carried out over the past decade by thermogravimetric analyses, using thin powder beds and isostatically pressed compacts with embedded thermocouples. The investigations revealed the complexity of the reactions and the impact of heat and mass transfer on the reaction kinetics. In the case of BaCO₃, the kinetics of decomposition is complicated by the formation of a liquid phase. For SrCO₃, the reaction kinetics could be simulated by theoretical modelling. Gas–solid reactions were also employed as a versatile process tool towards the production of alloys and intermetallics involving transition elements by hydrogen reduction of complex oxides involving transition metals. Thin powder beds were employed to examine the microkinetics, whereas fluidised beds were utilised to produce bulk alloys. Preliminary correlations could be drawn between the thermodynamic stabilities of the oxides and the activation energies of chemical reduction. By adjusting the process parameters, nanoscale products could be produced, the structural, mechanical and thermal properties of which were investigated. The potentialities of this method as a green route to produce industrially important alloys and intermetallics with unique properties are demonstrated.