A micromechanistic model of the combustion synthesis process: Influence of intrinsic kinetics
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A micromechanistic model of the combustion synthesis process: Influence of intrinsic kinetics Cheng He Institute for Aerospace Research, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada
Chantal Blanchetiere Institute for National Measurement Standards, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada
Gregory C. Stangle School of Ceramic Engineering and Sciences, New York State College of Ceramics at Alfred University, Alfred, New York 14802 (Received 12 March 1997; accepted 1 December 1997)
A micromechanistic model of the combustion synthesis of NbC has been developed by combining the results of an experimental study of the intrinsic, pore-level kinetic mechanism [C. He. and G. C. Stangle, J. Mater. Res. 10, 2829–2841 (1995)] and a theoretical model developed previously [Y. Zhang and G. C. Stangle, J. Mater. Res. 9, 2592–2604 (1994); 9, 2605–2619 (1994)], in order to account for the various physical and chemical processes that take place during the combustion synthesis process. Results of the present investigation are interpreted from both a macroscopic and a microscopic point of view. Moreover, the relationship between the microscopic processes and macroscopic features of the combustion synthesis process is discussed. The results show that the formation of a combustion wave in the Nb-C system corresponded to establishment of a proper balance between the rates of enthalpy redistribution within the sample. Furthermore, the pore size had a significant influence on the combustion synthesis process: smaller pores gave rise to a higher area of contact between the reactants, which in turn gave rise to a higher rate of enthalpy increase due to the enhanced rate of product formation. The influence of the pore size distribution on the process is also discussed.
I. INTRODUCTION
As a novel materials processing method, the self-propagating high-temperature synthesis (SHS), or combustion synthesis, process has various advantages, including the simplicity of the process, higher purity of the products, and relatively low energy requirements, when compared with the more conventional materials processing operations which use high-temperature furnaces.4,5 A large number of materials such as ceramics, intermetallics, and composites have been synthesized using this method.6–9 In a typical combustion synthesis process, a pressed mixture of powders, which are capable of undergoing an exothermic chemical reaction, is exposed to an external heat source for a short time. Once the mixture is ignited, a combustion wave forms and propagates through the entire sample in a self-sustaining manner. The combination of a highly exothermic reaction and an extremely high heating rate presents significant difficulties for conducting an experimental study of the reaction mechanism and the kinetics of the product formation process. On the other hand, a theoretical model of the combustion synthesis process can provide the basis J. Mater. Res., Vol. 13, No. 8, Aug 1998
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