Thermal Explosion Regime Research Articles

Preparation of an Al–Ti–Mg Composite by Self-Propagating High-Temperature Synthesis

A lightweight intermetallic alloy containing the ternary phase Al18Ti2Mg3 has been prepared for the first time in the Ti–Al–Mg system by self-propagating high-temperature synthesis. In the binary system Ti–Al, synthesis has been carried out in the thermal explosion regime, whereas in the ternary system Ti–Al–Mg layer-by-layer combustion synthesis has been performed. The porosity of the synthesized materials is 43.3%, and its density is 2.4 g/cm3.

Inhibition of the photoinduced ignition of chloromethane–chlorine mixtures by oxygen

The introduction of small amounts of oxygen inhibits the photoinduced ignition of chloromethane–chlorine mixtures. This is accompanied by an increase in the induction period and a decrease in the degree of heating of the reaction mixture. After the consumption of the entire inhibitor, the photochlorination reaction takes place again in the thermal explosion regime but with a lower degree of heating. In the dependence of the mixture temperature on the oxygen concentration, criticality is not pronounced. With an increasing concentration of oxygen, its effect on the induction period weakens.

Synthesis of Catalytically Active Cermet Membranes Using Combustion Processes for Small Power Plants

Porous cermet membranes with improved operating properties were synthesized by means of SHS with a reduction stage in the thermal explosion regime in vacuum from a mixture of powders of the PTYu alloy (95% N – 5% Al) + Co3O4. As a result porous cermet membranes with 10 – 20 nm Ni and Co nanoparticles in the surface layers of the pores were obtained. A prototype of a power plant with high mass/size cell-battery characteristics was developed: specific volume 3.3 liters/kW and 1.6 kg/kW.

Mathematical modeling of the thermal explosion in mechanically activated SiO2 + Al mixtures

A mathematical model of synthesis in a mechanically activated SiO2 + Al mixture is constructed in the macroscopic approximation. It is demonstrated that preliminary mechanical activation makes it possible to obtain solid-phase ignition and to ensure synthesis of Al2O3 and Si products in the thermal explosion regime. Based on experimental data, thermophysical and thermokinetic constants of the process are determined.

Thermal explosion in semi-batch reactors

The problem of thermal explosion in a semi-batch reactor is solved. The critical conditions for the thermal explosion are studied. It is shown that the ignition resulting from the chemical reaction is possible both during the stage of feeding of the second reactant solution and after its completion. The dynamic behavior of the reactor depending on the temperature of the heat exchanger and the volumetric feed rate of the second reactant is analyzed. It is shown that ignition at the feeding stage occurs at high temperature of the heat exchanger and involves two macroscopic stages. In the first stage, the temperature increases with increasing rate (thermal explosion regime), and in the second stage, its further increase occurs in the regime of combustion of the second reactant. The critical ignition conditions are found to be independent on the feed rate of the second reactant in a particular range of its values.

Molecular-dynamics simulation of the synthesis of intermetallic Ti–Al

In the present paper, results of a molecular-dynamics simulation study of the synthesis of intermetallic Ti–Al in the regime of thermal explosion at constant volume are reported. The structure of the polycrystalline intermetallic compound formed upon cooling was studied, and the dependence of crystallite sizes on the cooling rate was examined.

X-ray radiation in self-propagating high-temperature synthesis processes

Emission effects of heterogeneous combustion in the region of ionization radiation are studied. By an example of a Ti-B powder system, it is demonstrated that the processes of self-propagating high-temperature synthesis in the thermal explosion regime is accompanied by “soft” X-ray radiation with the quantum energy estimated as ≈5 keV. Key words: heterogeneous combustion, X-ray radiation.

Dynamics of phase-formation processes during self-propagating high-temperature synthesis in the thermal explosion regime

Specific features of formation of temperature and concentration fields during self-propagating high-temperature synthesis in the thermal explosion regime in a cylindrical reactor are studied by methods of mathematical modeling. The calculations are performed with allowance for melting of one (chemically active) component in the approximation with the high-melting component being non-soluble in the melt of the low-melting component. It is shown that the conditions of complete conversion of the original components in the volume of the reacting mixture depend on relations between the Biot criterion of the system, the ambient temperature, and the thermal effect of the reaction. After the thermal explosion, which occurs when the melting front reaches the geometric center of the reactor, a front of complete conversion is formed. This front moves from the cylinder centerline to the periphery with a gradually decreasing velocity. The diagram of the critical values of the Biot criterion at which the components burn down completely in the entire reaction volume is calculated.

The thermal radiation processes in the Sc–H system

The thermal-radiation processes in the system Sc–H2 have been investigated. The high-current 4 MeV electron accelerator LAE-5 has been used with a focused electron beam. As a result of experiments at different doses and dose rates (0.025–1.5 Mrad/s) in vacuum and in a hydrogen atmosphere, the main peculiarities of the thermal-radiation processes in Sc–H2 have been obtained. It was shown that at irradiation of Sc in H2 environ, the thermal-radiation synthesis (TRS) of Sc hydride, ScH1.92, occurred. It was established that on increase of irradiation dose, the temperature of the sample increases to 200–300°C, after which a sharp temperature jump was observed. Actually, the reaction Sc+H2 proceeded in a thermal explosion regime. It was shown that the characteristics of TRS depend essentially on irradiation dose rate.

Postinduction Processes During Thermal Explosion in the Systems “Porous Material–Reactive Gas–Solid Product”

A complete solution of the unsteady‐state filtration problem of thermal explosion incorporating the postinduction period is given for the first time. The paper describes a study of the temperature‐field dynamics, pore‐gas pressure, and the degree of condensed‐phase conversion versus the reactive‐gas deficiency in a reactive porous material. Focus is on the formation and propagation of frontal regimes of exothermic chemical reactions (their number, direction, and velocity of propagation, degree of condensed‐phase conversion at the front). The study revealed “double self‐ignition” phenomena and combustion‐wave propagation regimes with incomplete conversion at the front. A surface regime of thermal explosion limited by gas filtration from the outside was considered. The regularities in the dynamics of the exothermic chemical reaction found in the present study allow one to qualitatively control high‐temperature synthesis under thermal explosion conditions.

Dynamics of Phase Transitions in the SHS of a 3Cu–Al Powder Mixture in the Regime of Thermal Explosion

The paper reports results of dynamic x‐ray photography of self‐propagating high‐temperature synthesis of a 3Cu–Al powder mixture in the regime of thermal explosion. The stages of formation of the final product are considered, beginning with the heating of the starting mixture and interaction of its components and ending with phase transitions during cooling of the product — intermetallides Cu9Al4 and Cu3Al, which are the main components of Cu based tribotechnical materials. It is shown that upon heating in the temperature range of 550 – 590°C, the intensity of diffraction lines of Al drops to the background level. The exothermicreaction of synthesis of the intermetallide is initiated at a temperature of 610 – 630°C. In the period of sharp increase in temperature to 1040°C, the presence of the starting copper and the newly formed high‐temperature β′ phase with the Cu9Al4 tructure are detected simultaneously in the combustion wave. When cooled to 300$°C, the material contains two cubic phases — Cu9Al4 and an α‐solid solution of Al in Cu. Further cooling is accompanied by formation and increase of peaks of the Cu3Al phase with an orthorhombic lattice. The phase composition of aluminum bronze produced by SHS with combustion in the regime of thermal explosion is not in equilibrium.

High-temperature synthesis of a tungsten-free cermet

This paper presents the results of numerical modeling of high-temperature synthesis of cermets on the basis of titanium carbide with a metal binder in the regime of thermal explosion of a powder mixture of pure elements with a metal filler.

Numerical analysis on the hot spot in reactive chemical storage

Abstract In an exothermic reactive chemical storage tank, a thermal explosion can occur when the heat generation within the storage tank is greater than the heat removal from the storage tank. In a solid system, the existence of an inflection point in the temperature versus time curve is often used as a criterion for thermal explosion. In this study the applicability of this criterion to fluid storage is examined by investigating the hot spot in the system. Transient natural convection of an exothermically reactive fluid in a vertical, cylindrical storage tank with isothermal walls is investigated numerically. The axisymmetric 2-D Navier–Stokes equations governing the flow fields are reduced by introducing the stream function–vorticity formulation and solved by the alternating-direction-implicit (ADI) technique. The reactive heat sources are represented by a zeroth-order rate expression with an Arrhenius-type rate constant. It is found that the hot spot is no longer located in the center of the tank due to the buoyancy effect. In a particular range of Rayleigh number and Frank-Kamenetskii number, the hot spot is found to move periodically. A stable steady regime, stable oscillatory regime and thermal explosion regime can be specified based on the dependence of the critical Frank-Kamenetskii number on the Rayleigh number. In the stable oscillatory regime multiple inflections in temperature versus time are found. Therefore, the existing criterion, which defines thermal explosion by the appearance of an inflection in the temperature versus time curve, is no longer applicable in reactive fluid systems. The results of nonconvective systems do, however, provide a preliminary and conservative estimate for thermal explosion of a reactive storage tank.

A mathematical model of high-temperature synthesis of the intermetallic compound Ni3Al during ignition

The influence of intermetallic-phase nucleation at the nickel-aluminum interface on the ignition temperature and time of a powder mixture was considered in the framework of a mathematical model for the self-propagating high-temperature synthesis of the intermetallic compoundNi3Al in the regime of thermal explosion of a powder mixture of pure elements. The dependence of the formation rate of the intermetallide on the size distribution of the nickel particles and on the power of the external heating source was studied.

Mechanism of porous structure formation in intermetallic synthesis

The specific features of diffusional mixing of metals in synthesis of intermetallides, with one component being in the liquid state, are considered, and the nature of stresses in the diffusion zone is discussed. A model of a porous powder body is proposed to predict the behavior of mixtures in intermetallic synthesis in a thermal explosion regime. Analytical equations describing the sign and magnitude of volume changes of powder bodies in synthesis are obtained and experimentally checked. Examples are presented to illustrate the synthesis of highly porous materials.