Numerical study of the second ignition for combustion synthesizing Ni-Al compounds
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8/8/03
5:07 PM
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Numerical Study of the Second Ignition for Combustion Synthesizing Ni-Al Compounds HUNG-PIN LI The propagation of a combustion front during Ni-Al combustion synthesis is often extinguished halfway through the reaction, due to the lower exothermic heat of the metallic reactions. To facilitate the complete propagation of the combustion front, the reaction is always ignited again during the experimental demonstration. The position and time of the second ignition have been found to influence the subsequent temperature profiles. In this numerical study, the different second-ignition positions in the combusted region, the reacting region, and the preheating region, as well as the different second-ignition times before and after the stop of the first combustion front, are chosen to study the effect of the second ignition. The stable propagation is only observed as the reaction is ignited again in the reacting region. When the reaction is ignited a second time in the combusted region or the preheating region, part of the specimens cannot be completely synthesized due to the low combustion temperature. In addition, the combustion temperature may be significantly enhanced in the other area and results in a heterogeneous microstructure. Delay of the second ignition time is also found to increase the initial propagation velocity of the new combustion front. From the results generated in this study, a process map of the second ignition is established. The process map provides the appropriate second-ignition conditions to propagate the combustion front completely and achieve a product of homogeneous microstructure.
I. INTRODUCTION
COMBUSTION synthesis[1–9] utilizes the propagation of a combustion front across the sample to achieve material processing. The energy for combustion-front propagation is obtained from the exothermic heat of the synthesis reaction. The unreacted portion in front of the combustion front is heated by this energy release and then again undergoes exothermic synthesis. Thus, this allows the combustion front to propagate in a continuing cycle of reaction and synthesis. Exothermic synthesis-reaction processing circumvents difficulties associated with conventional methods of time- and energy-intensive sinter processing and has been extensively studied for synthesizing ceramic and intermetallic compounds.[1–9] The combustion-synthesis technique provides rapid netshape processing and high product purity. Compared with conventional powder metallurgy operations, combustion synthesis not only offers shorter processing times but also excludes the requirement for high-temperature sintering. In addition, volatile contaminants or impurities can be eliminated as the high-temperature combustion front propagates through the sample, and, thus, the synthesized materials have high-purity products. The steep temperature gradient can also create metastable or nonequilibrium phases, which are not available in the conventional processing. Various numerical studies for the combustion temperature and t
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