Modeling of wave configuration during electrically ignited combustion synthesis
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A model was developed to study the process of current-ignited combustion synthesis. In this process, Joule heating raises the temperature to the ignition point, at which the sample reacts to form a product. Two material systems were modeled: the synthesis of SiC and MoSi2. It was found that the mode of combustion is a function of the size (radius) of the sample. The anticipated volume combustion mode was only evident in small samples. At higher values of the radius, the mode becomes wavelike (selfpropagating high-temperature synthesis) in nature. The transition from volume to wave combustion mode also depended on the properties of the material. The results are interpreted in terms of thermal conductivity and heat-transfer conditions.
I. INTRODUCTION
There have been many studies aimed at elucidating the mechanisms of combustion synthesis of materials by reacting the entire solid compact at once.1,2 This method is referred to variously as simultaneous, volume, or furnace combustion. In the Russian literature it is commonly referred to as “thermal explosion,” a term that connotes, incorrectly, a lack of control. Volume combustion is accomplished by placing the compact in a furnace and heating it, usually at a definite heating rate, to the ignition temperature. This is in contrast to the traditional method known as self-propagating high-temperature synthesis (SHS), where a sample is ignited from one end and a combustion wave propagates through the material. The versatility of the SHS method has been demonstrated in the preparation of a large number of materials.3,4 Unfortunately, however, the method is limited by thermodynamic or kinetic considerations, a circumstance which excludes the preparation of some technologically important materials.5 Among the thermodynamic limitations, a low enthalpy of formation is the most common. We have developed a method through which these limitations can be overcome by activating the process. In our work, activation of SHS was accomplished by the application of an electric field in a direction perpendicular to that of the anticipated wave propagation direction.6 –11 Modeling of field-activated combustion synthesis (FACS) reactions has shown that the distribution of the current during wave propagation is dependent on the eleca)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 1, Jan 2001
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trical conductivity of the reactant and product phase.7,11 The role of the electrical conductivity as well as thermal conductivity and relative density in FACS has been modeled in a recent investigation.12 A more detailed description of the model used in FACS is provided elsewhere.13 As indicated above, volume combustion is typically done by placing the sample in a furnace and heating it up to the ignition point. Another way of accomplishing the same goal is by passing a current through the material. The advantage of the use of electric heating is its high rate relative to furnace heating. It ha
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