Reactants Transport in Combustion Synthesis of Ceramics

Abstract

Diffusion in solids does not ensure the experimentally observed velocity of combustion wave propagation in the systems which are traditionally considered as gasless and burned in the mode of solid flames (gasless solid-state combustion). The phenomenology of indirect interactions, the thermochemistry and dynamics of the gas-phase carriers formation, as well as their participation in the reactants transport are studied in the systems Mo-B and Ta-C. The distributions of the main species in the gas phase of the combustion wave are measured in situ with the use of a dy namic mass-spectrometry (DMS) technique which allows for high tem poral and spatial resolution. The detailed chemical pathways of the proc esses were established. It was shown that the actual mechanism of com bustion in the systems under study is neither solid state nor gasless and the reactions are fully accomplished in a narrow front.For the last 30 years, the combustion synthesis of oxygenless ceramics has been drawing attention, first and foremost because of its evident technological advantages, i.e., sim plicity of the equipment, high productivity and low consumption of energy. In many cases, the combustion synthesis results in compounds, phases and materials that are im possible to be synthesized by other methods. These investigations were pioneered by A. Merzhanov and I. Borovinskaya (7). In later work they, together with their co-workers, created the essentials of the thermal theory of powder mixtures combustion and devel oped a well known method and various modifications of self-propagating hightemperature synthesis (SHS).SHS is based on layer-by-layer combustion of a metal powder mixture with an other reactant (reactants). Metal-reactant compositions can vary but have to meet the condition of adequate heat release to sustain the overall process. It has to be noted that processes of this kind have been known for many years (2), but up to the end of the I960's they were not used to synthesize materials of complex chemical composition.It is evident that SHS, by its chemical nature, is an exothermic redox reaction oc curring in the frontal mode. Metals (Ti, Zr, Cr, Mo, W and many others) are reductants,