V. Yu. Filimonov

Thermal explosion in mechanoactivated 3Ni + Al mixtures

Explored was thermal explosion in mechanoactivated 3Ni + Al mixtures. Mechanoactivation was found to result in an abnormal decrease in the effective activation energy E and ignition temperature Tign for thermal explosion. Analysis of reaction thermogram allowed us to find out the kinetic function. Mechanoactivation conditions for synthesis of Ni3Al in thermal explosion mode have been optimized. SHS reaction in 3Ni + Al mixtures mechanoactivated for 180 s was found to obey the first-order kinetics.

Anomalous decrease in the activation energy and initiation temperature of a thermal explosion in the mechanically activated composition 3Ni + Al

277 † In recent years, increasing interest in combination of the methods of self-propagating high-temperature synthesis (SHS) and mechanical activation (MA) has been observed [1–4]. This is explained by the fact that the preliminary MA of the powdered reaction mixtures provides a substantial expansion of the possibility of gasless combustion for the high-temperature synthesis of inorganic materials, specifically, to extend the concentration limits of combustion, to use for synthesis such compositions that do not combust under usual conditions, to avoid the necessity of pressing the starting samples [3, 4], and to realize the solid-phase combustion mode even in compositions with a low-melting reagent such as aluminum [4]. It should be noted that the synthesis in most of the investigated activated compositions was performed in the mode of frontal propagation of the SHS wave. However, the features of the dynamics of the self-heating process in the thermal explosion mode in activated systems and the corresponding interrelation of the processes of structure and phase formation with the MA modes are almost unknown. The thermograms of the synthesis of activated samples can give a lot of useful information with respect to the synthetic parameters and, first of all, the effective activation energy and critical conditions of interaction initiation.

Thermal explosion in Ti + 3Al mixture: Mechanism of phase formation

Synthesis of TiAl3 from Ti + 3Al powder mixtures in thermal explosion mode was found to proceed by the equilibrium mechanism in case of smaller Ti particles (55 μm) and by the non-equilibrium mechanism in case of larger Ti particles (130 μm). In the latter case, the product was found to contain the phases of TiAl3, TiAl, Ti3Al, Ti-based solid solution, and no unreacted Al.

Adiabatic thermal explosion in disperse condensed systems with limited solubility of the reactants in the product layer

A mathematical model of the self-heating of dispersed solid mixtures is considered taking into account the boundary kinetics of formation of the intermediate product phase. It is shown that decreasing the ratio of the characteristic diffusion time to the characteristic reaction time can lead to a transition from the diffusion regime to the kinetic regime, resulting in a change in the effective activation energy of the synthesis and the growth pattern of the product layer. This change may be due to decreases in the heterogeneity scale of the mixture and the ratio of the activation energy of the formation of the new phase to the activation energy of diffusion. An analytical model of the solid-state reaction in the kinetic regime was developed and used to obtain temperature dependences of the coordinates and velocities of the boundaries of the growing layer.

Microwave-assisted combustion synthesis in mechanically activated 3Ti + Al powder mixtures: Structure formation issues

MW-assisted combustion synthesis in mechanically activated 3Ti+Al powder mixtures in a thermal explosion mode was explored by thermography and XRD with special emphasis on the influence of mode/rate of MW heating. The main stages of the process were outlined as well as optimal conditions for preparation of single-phase Ti3Al.

Thermal explosion in closed systems: Criteria and critical conditions

A new criterion for thermal explosion in exothermically reacting systems for the case of arbitrary one-step conversions is proposed. The computation method is based on the analysis of the equation of maximum temperatures obtained for closed systems taking into account reactant consumption. The dependence of the maximum temperature on the Semenov and Todes parameters is bistable and the critical conditions are determined by the extremum conditions on the relevant parametric diagrams. The demarcation lines of ignition in the Todes criterion–Semenov criterion parametric plane are calculated for second-order exothermic reactions. Comparison of numerical and analytical calculations shows that they are in satisfactory agreement.

Thermal explosion in a binary homogeneous mixture

Self-heating processes in binary homogeneous mixtures are studied based on the analysis of phase trajectories on the heating rate–temperature plane. The proposed approach allows determining the characteristic regions of exothermic reactions in the parametric diagram of the Todes criterion versus the Semenov criterion. Generalized strict criteria for the degeneration of thermal explosion are determined for reactions with low activation energy and reactions with a low thermal effect for any reaction orders. An approximating formula describing the dependence of the Todes criterion on the Semenov criterion is derived for calculating the critical conditions of thermal explosion up to its complete degeneration for reaction orders higher (or equal to) one. A criterion for the applicability of the classical thermal explosion theory as a special case of the proposed model is found. A diagram of the critical parameters in the introduced new variables is considered to predict and control the self-heating kinetics of mixtures.

The method of phase trajectories in nonstationary thermal explosion theory

It is shown that consideration of the heating rate-temperature dependence, an analysis of self-heating for first-order exothermic reactions, and strict inclusion of kinetic deceleration puts forward fresh ideas about the diversity of reaction regimes on the Todes criterion-Semenov criterion parameter plane. From this point of view, classic Semenov theory is a particular case of the projection of two-dimensional regions onto the direction of Semenov criterion changes. An analysis of the mutual arrangement of reaction region boundaries with different kinetics and the shape of the representation point movement trajectory when the initial temperature of the system changed allowed us to determine the thermal explosion degeneration region, transition region, and regions of possible explosive reaction modes.

Thermal heating regimes during first-order homogeneous reactions

Qualitative features of self-heating regimes for first-order homogeneous reactions are considered by analyzing the structure of the phase trajectories. Consideration of the temperature dependence of the heating rate makes it possible to extend the main results of Semenov’s theory of thermal explosion and pass to a two-dimensional parameter diagram, which allows the determination of the critical ignition conditions taking into account kinetic inhibition. It is found that the diagram of the Todes criterion versus the reciprocal of the Semenov criterion has four characteristic regions with different kinetics, two of which are limiting and two transitional ones. The boundaries of the regions are calculated, and the classical theory of thermal explosion is shown to be a special case of the projections of these boundaries on the range of the Semenov criterion.

Critical Autoignition Conditions for Heterogeneous Condensed Systems in the Presence of Phase Transformations

The critical ignition conditions for a single particle which interacts with a melting medium to form an intermetallic compound on its surface were investigated using the model of a quasistationary melting front. For linear and parabolic drag laws, three characteristic regions, which transform with change of Biots criterion, are distinguished on the diagram of critical parameters. It is shown that for small values of this criterion, the conditions of self‐heating of the particle are identical to the ignition conditions in a medium with constant temperature. For values of Biots criterion greater than unity, the critical conditions are substantially affected by the parameters determining the melting kinetics. The characteristic features of self‐heating of a single particle can determine the qualitative pattern of self‐heating of the heterogeneous system as a whole.