Ignition criteria for self-propagating combustion synthesis
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Ignition criteria for gasless self-propagating combustion synthesis have been investigated through an ignition temperature analysis. The calculations were based on the dimensionless energy and mass continuity equations where the dimensionless parameters associated with the rate of local heat generation (/?), activation energy (y), the rate of surface heat loss by convection (co), the rate of surface heat loss by radiation (5), and the rate of reaction (A) were incorporated. The relative significance of each of these parameters on the ignition of the self-propagating combustion reaction was evaluated t o b e y > / ? > ( 5 > < y . The ignition region, transition region, and nonignition region were identified for selected conditions. The correlations between ignition behavior and the material properties, the thermodynamic and kinetic properties, as well as the experimental conditions were discussed. The calculations indicated that only those systems with AH/CP > 1.5 X 103 (K) will give rise to a self-propagating combustion reaction without external energy input. Thus, this value can be used as an approximate guide for the existence of self-sustaining combustion. The calculations provide a sound basis toward interpreting experimental observations and developing a fundamental understanding of the process.
I. INTRODUCTION Combustion synthesis has been successful in producing single-phase and composite monoliths, coatings, joints, and powders. The single-phase materials are typically ceramic or intermetallic.1"5 The composite materials may be ceramic-ceramic, ceramic-metal, ceramic-intermetallic, or intermetallic-metal.6"9 Combustion synthesis has also been used to synthesize fine single and multicomponent oxide ceramic powders.10"12 The precursor materials are typically mixed and formed into a compact. The compact was then ignited at an elevated temperature such that a solid-solid or solidgas chemical reaction takes place and proceeds rapidly in a self-propagating manner, converting the reactants into products. The self-propagating reaction continues until (i) all reactants are consumed, or (ii) conditions unfavorable to sustaining the reaction arise. Successful material synthesis using a selfpropagating combustion process calls for a quantitative understanding of the influence of each parameter important to the process. A systematic experimental study is difficult due to the intricate nature of the combustion process. Theoretical experiments provide a good approach to quantify many of the factors required to carry out the process successfully. Many investigations have modeled the self-propagating combustion processes. Emphasis has been on trying to associate the combustion wave with the chemical and physical characters of the system. Several types of propagation waves (constant-pattern profiles, oscillatory waves, and J. Mater. Res., Vol. 8, No. 7, Jul 1993
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spinning and fingering fronts) have been predicted.13 17 Correlations between the activation energy and the
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