A micromechanistic model of the combustion synthesis process: Modes of ignition
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A micromechanistic model of the combustion synthesis process: Modes of ignition Cheng Hea),b) and Gregory C. Stangle School of Ceramic Engineering and Sciences, New York State College of Ceramics at Alfred University, Alfred, New York 14802 (Received 26 July 1995; accepted 21 April 1997)
A theoretical model of the combustion synthesis process has been developed to study the ignition of a self-propagating combustion synthesis process in the Nb –C system. Compared with most of the previously published theoretical work on this subject, this model provides a much more detailed description of the combustion synthesis process from a microscale point of view, due to the fact that it takes into consideration the various microprocesses, such as the melting of reactants, the diffusion and mixing of reactants, and the formation of products. Different ignition modes, including constant-temperature ignition, constant-heat-flux ignition, and variable-temperature ignition, are considered in this work. The key parameters that influence the ignition process are also discussed. I. INTRODUCTION
The combustion synthesis [or self-propagating hightemperature synthesis (SHS)] process has been used to prepare a wide variety of materials, including a number of advanced ceramic, intermetallic, and composite materials,1–8 which makes it an important alternative to the more conventional routes to preparing such materials. Moreover, the combustion synthesis process is at once simple and complex. It is simple in the sense that, e.g., metal powders are simply mixed (and possibly shaped), after which the mixture is ignited and thus caused to react in a self-propagating manner. It is complex in the sense that ignition is not easily achieved in every instance, and that the reaction may not proceed in a spatially uniform manner (and thus may give rise to a spatially inhomogeneous product material). While both the ignition and the propagation processes have been studied in some detail,1–10 it is the ignition process that is the subject of the present paper. In addition, even though the results of both experimental and theoretical studies of the ignition process have been reported in the literature, this paper is focused on a theoretical study of that process, since a theoretical investigation of the ignition process can provide quantitative results that are based on sound physicochemical principles. A theoretical study of the ignition of the combustion synthesis process follows rather naturally from the theoretical models that have been developed to study the process in the self-propagating mode (i.e., once the sample has been successfully ignited). It is generally recognized that there are two classes of such theoretical models. The first class of combustion-synthesis moda)
Address all correspondence to this author. Present address: Institute for Aerospace Research, National Research Council Canada, Ottawa, Ontario K1A 0R6 Canada.
b)
J. Mater. Res., Vol. 13, No. 1, Jan 1998
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