Investigation of propagation modes and temperature/velocity variation on unstable combustion synthesis
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Combustion synthesis/micropyretic synthesis is a technique in which material synthesis is accomplished by the propagation of a combustion front across the sample. In some cases, the combustion front may propagate in an unstable mode where the propagation velocity and combustion temperature of the combustion front are altered periodically. In this study, the processing conditions leading to unstable combustion reaction were first studied theoretically. The boundary temperatures separating stable and unstable reactions were then determined. The numerical analysis showed that the combustion temperature and the propagation velocity changed periodically during unstable combustion. As the combustion reaction became unstable, the average propagation velocity and the oscillatory frequency of front propagation decreased. The products of unstable combustion synthesis possessed the banded structures, implying the occurrence of the unstable oscillatory propagation, as demonstrated experimentally. In this study, high activation energy combustion (Ti + 2B reaction) and low activation energy combustion (Ni + Al reaction) were both chosen to illustrate the effect of unstable combustion. It is the first time the experimental and numerical results were combined to investigate the temperature and propagation velocity variations during unstable combustion.
I. INTRODUCTION
Micropyretic synthesis/combustion synthesis1–16 is a technique in which the synthesis of compounds and composites is accomplished by the propagation of a combustion front across the sample. The heat to propagate the combustion front is obtained from the heat of formation of the synthesized compound. The unreacted portion in front of the combustion front is preheated by the release energy, which initializes further reaction. A continuous cycle of reaction, heat transfer, and reaction initiation is reflected in propagation of the combustion front. Several numerical and analytical models of combustion/micropyretic synthesis of a composite system have been well developed.5,10–13 Lakshmikantha and Sekhar first explored the numerical model that includes the consideration of melting of each constituent of the reactants and products and the inclusion of considerations involving dilution and porosity.10,11 The analytical modeling of the propagation of the combustion front in solid–solid reaction systems is also reported.12 The analytical model gives good results when compared with the experimentally determined numbers and the numerically calculated values. In addition, a multidimensional numerical model and dynamic modeling of the gas and solid reaction have also been carried out to illustrate the various parameter effects on the micropyretic/combustion synthesis.5,13 J. Mater. Res., Vol. 17, No. 12, Dec 2002
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These numerical and analytical analyses provide the knowledge of the reaction sequence during micropyretic/ combustion synthesis reactions. It has been reported that the manner of combustion front propagation is
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