AN INVESTIGATION OF THE EFFECT OF POROSITY AND DILUENTS ON MICROPYRETIC SYNTHESIS

A numerical model of combustion/micropyretic synthesis of a composite system is developed. The new features of the model include the consideration of melting of each constituent of the reactants and products and the inclusion of considerations involving dilution and porosity. The effect of porosity is to change both the thermal conductivity and density. The model for the porosity which is considered in this article shows the significance of its effect on the velocity and the type of the combustion front. Different values of porosities are considered for the reactants and products. For an illustrative analysis, a systematic study of the variation of porosity of reactants and products is carried out for the combustion synthesis of TiB2 and TiC. The numerical results indicate that as the reactant porosity values are decreased, the combustion velocity first increases because of an increase in the thermal conductivity. The combustion velocity, after reaching a maximum, decreases with a further decrease in the porosity. As the porosity is varied, there is a considerable effect on the nature of propagation of the combustion front, which may change from a steady state to an oscillatory mode. Results indicate that as the reactant porosity is decreased, the frequency of oscillations of the combustion front first increase and then decrease with a further decrease in the reactant porosity. The differences in the mechanism for the decrease in velocities at very low values of reactant porosities for the two systems TiC and TiB2 are identified. The results of considering different values of porosities in the reactants and the product are also presented. For the study of the effect of diluents, the product itself is considered as the diluent. The effect of adding the diluent to the initial reactants is to decrease the combustion temperature and the combustion velocity. The diluent also changes the mode of combustion. An increase in the amount of the diluent results in the decrease of the frequency of oscillations.