Numerical Study of the Effect of the Type of Refractory Particles on Product Formation under Conditions of Bulk Synthesis

Abstract

Methods of combustion for the synthesis of composites are widely used in modern chemical and metallurgical technologies. According to experimental data, the most homogeneous product can be obtained by bulk synthesis. Such modes are often used to produce composite materials. However, the high reaction rate of bulk synthesis significantly complicates the control of the formation of the product. One way to reduce the synthesis temperature consists in introducing inert particles into the reaction mixture. Additionally, the heating rate can be changed by changing the reactor wall thickness. However, the specifics of the conjugate heat exchange of the reaction mixture with the reactor walls under various synthesis conditions are rarely analyzed in publications. To demonstrate the role of these factors, a two-dimensional model of bulk synthesis of a composite from pure elements with the addition of inert particles is proposed in this work. The heating of the reactor walls by thermal radiation from a device whose temperature varies according to a given law is taken into account. The heat contact between the reactor walls and the powder mixture is assumed as ideal. The chemical reactions are described by a summary reaction scheme that corresponds to the synthesis of Ni₃Al from elements 3Ni+Al. The kinetic law takes into account the retardation of the reaction by the product layer and contains corresponding parameter. The effective thermal and physical properties of the mixture in the reactor depend on the properties of the initial components and the volume fraction of inert particles. The proposed model is implemented numerically. Process dynamics for WC, TiC, and Al₂O₃ particles is analyzed. Steel is used as the material of the reactor walls. It is established that the synthesis process in the bulk is inhomogeneous, that is directly related to the conjugate heat exchange conditions. In spite of this, for the chosen set of reactor parameters and particle fraction, the result is the same: there are almost no unspent reagents in the final products. The dynamics of temperature variation in different parts of the reactor may not correspond to the classical thermal explosion. This indicates that the transformation of the reagents into the product occurs in two stages. The first stage ends with an incomplete transformation. Then, due to the heat stored in the compact and in the reactor walls, the reactions are accelerated again, which leads to the formation of a final composite. Changing the fraction of inert particles in the mixture has an ambiguous effect on the dynamics of synthesis that is due to the thermal and physical properties changing with the fraction of particles and a decrease in the total heat release in the reaction.