A micromechanistic model of the combustion synthesis process: mechanism of ignition
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A micromechanistic model of the combustion synthesis process: Mechanism 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 12 March 1997)
A micromechanistic model of the combustion synthesis has been extended to study the detailed mechanism and influential parameters of a combustion synthesis process, as well as the development of ignition criteria in the Nb–C system. The case of constant heat-flux ignition conditions has been used to illustrate the details of the ignition process, in order to elucidate the various physical and chemical processes that take place during the initial stages of the combustion synthesis process; however, the results of this study can be generally extended to the other modes of the ignition process. The results showed that the ignition criteria for the Nb –C system corresponded to the establishment of a proper balance between the rates of enthalpy redistribution within the sample, and to the establishment of a kind of positive feedback loop during the ignition process that is necessary for self-propagation to occur. If the heat supplied from an external source to initiate the combustion synthesis process is less than a certain critical value, the combustion wave stops at a certain short distance from the ignition surface. Otherwise, the reaction proceeds in a self-propagating manner.
I. INTRODUCTION
It is well known that the combustion synthesis [or self-propagating high-temperature synthesis (SHS)] process begins with an ignition process, in which at least a portion of the sample’s surface is exposed to an external heat source for a short time, in order to generate a self-propagating combustion wave front that passes through the sample. Experimentally, there are several ways to ignite a sample, including the use of radiant thermal energy, laser irradiation, convection heat transfer, electrostatic energy, a so-called “chemical oven,” microwave irradiation, ohmic heating, and slow, linear heating.1–6 From a theoretical point of view, however, this array of ignition modes can, in a broad sense, be reduced to two types of ignition, based on the manner in which the boundary conditions are written. The first is the so-called “specified-temperature” mode, in which the temperature of the relevant boundary is given (and which may be a function of time). The second is the so-called “specified-heat-flux” mode, in which the heat flux to or from the relevant boundary is given (and which may also be a function of time). In other words, most of the ignition methods used in practice falls into one of these two categories.5–8 As a result, the theoretical approach takes a somewhat broader view, so that the
rather more general results of such a study are applicable to a number of experimental situations. In this paper, results of a study of the detailed mechanism and influential parameters involved in the
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