Self-propagating High-temperature Synthesis Wave Research Articles

Mechanism of the ultrasound influence on combustion and structure formation at SHS in titanium-base systems

Using the developed experimental setup, the effect of ultrasonic oscillations (USO) on the temperature and combustion velocity during self-propagating high-temperature synthesis (SHS) in the Ti–C, Ti–C–Ni–Mo and Ti–B systems is studied. Basing on the analysis of data known in literature and our own measurements, theoretical explanation to the observed results is proposed. The effect of the intensity of ultrasonic oscillations on the composition and structure of the final synthesis products is established. It was found that along with changes in the parameters of the combustion wave, the completeness of the interaction increases, and changes in the microstructure and phase composition of the reaction products occur. The concept of separation of the effect of USO on SHS into thermal, or macroscopic, and non-thermal, or microscopic is proposed. The former is associated with forced convection of gas around an oscillating specimen and leads to a decrease in temperature and combustion velocity. The latter is connected with a change in the melt spreading conditions, the progress of heterogeneous reactions and mass transfer in the liquid phase in the high-temperature zone of the SHS wave, which lead to a change in the phase composition and structure of the final product.

Structural and phase states of titanium borides produced by the self-propagating high-temperature synthesis method in the field of ultrasound oscillations

The effect of ultrasound oscillations (USO) on the velocity and temperature of combustion during self-propagating high-temperature synthesis (SHS) in the Ti-B system and structural and phase states of the produced titanium borides is studied using the earlier developed experimental setup. The effect of USO on SHS is subdivided into thermal and physical (non-thermal). The thermal influence is connected with cooling of the specimen surface because of the occurrence of forced convection of the ambient gas, and the physical effect is due to the action of USO on complex interaction processes in the SHS wave such as melt spreading, heterogeneous reactions and mass transfer in the liquid phase. Imposition of USO on the SHS process brings about changes in the phase composition of the synthesis products. For charge composition Ti–1.0В the content of orthorhombic modification of phase TiB increases from 78.2 % without USO to 82.9 % at the USO amplitude ξ = 10 mm, while the content of the cubic modification of this phase decreases from 9.2 % at ξ= 0 to 6.8 % at ξ = 10 mm. For all the examined compositions, the amount of residual titanium and Ti3B4decreases and the content of TiB2increases. It is determined that carrying out SHS in the field of USO results in a change of the specific heat capacity of the target synthesis products: with raising the USO amplitude it increases by 4–5 %. Thereby it is shown that imposition of USO on SHS is an efficient physical method for purposeful regulation of structural and phase states and therefore properties of the synthesis products and can be used as a means for controlling the synthesis process.

Modeling Nonstationary Gas-Free Combustion Processes with Account of Sample Structure Heterogeneity

A model of nonstationary gas-free combustion has been formulated with account for the structure heterogeneity and for the temperature dependence of diffusion with the use of two models of reaction cells: plane and spherical ones. Numerical simulation of the propagation of the self-propagating high-temperature synthesis (SHS) wave and a comparative analysis of the SHS wave propagation velocity was carried out with the use of two models of reaction cells. The numerical investigations have shown that with the use of the model of spherical reaction cells the SHS wave propagation velocity is higher than for plane cells by 1.7 times. Dependences of the rate of combustion on the characteristic value of the heterogeneous structure of a sample have been obtained. The computational–theoretical values of the SHS wave propagation velocity coincide with good accuracy with experimental data in a wide range of the dimension of the heterogeneous structure of SHS.

Specific features of formation of bulk porous systems in the self-propagating high-temperature synthesis. Borides

The effect of ultrasound oscillations (USO) on the combustion velocity and temperature, phase composition and microstructure of compounds produced by the self-propagating high-temperature synthesis (SHS) in the Ti–B system is studied using the earlier developed experimental setup. It is shown that the effect of powerful ultrasound on SHS is connected with the thermal factor, viz. the enhancement of heat removal from the specimen surface due to forced convection, and the physical (non-thermal) factor that consists in the influence of USO on the melt spreading and crystallization of different phases in the SHS wave. It is demonstrated that for multiphase system Ti–B an increase of the boron content in the initial charge leads to grain refinement of the synthesized product and the imposition of USO on the SHS results in a change in the grain morphology: their shape becomes closer to equiaxial. In composition Ti+1.5B, under the action of USO cubic-shape product grains are observed on the inner surface of pores, and in composition Ti+2.0B the synthesized TiB2 grains become more vividly facetted.As a result of imposition of ultrasound on the synthesis process, grain refinement is observed for all the compositions along with the formation of a large number of borides and their redistribution in the volume. The use of ultrasound brings about changes in the quantitative phase composition of the synthesis products and the ratio between the orthorhombic and cubic modifications of phase TiB. The research has shown that the presence of pores in the initial charge plays an important role in the structure formation of the final product, and hence it appears impossible to obtain an equilibrium material by the SHS method. It is found that an optimal USO amplitude exists at which it is possible to obtain uniform fine-grained structure of the material. This permits controlling the structure formation at SHS.

Effect of powerful ultrasound on the combustion processes and phase composition of refractory compounds of titanium at the self-propagating high-temperature synthesis

The effect of ultrasound oscillations (USO) on the heat transfer conditions between a specimen and environment is examined using a specially developed experimental facility. The influence of the USO amplitude on the combustion temperature and velocity as well as on the phase composition and crystal lattice parameters of the synthesized compounds is studied for the self-propagating high-temperature synthesis (SHS) in Ti–С(Si,B) systems. The heat transfer coefficient on the surface of a specimen during its oscillations with an ultrasound frequency is assessed. Possible mechanisms of the effect of USO on the SHS process are considered. It is demonstrated that a decrease in the SHS temperature is connected with cooling the specimen due to forced convection of a surrounding gas, while a change in a phase composition of the synthesized material and the crystallographic parameters of the phases occurs due to changes in the conditions of high-temperature heterogeneous interactions in the SHS wave.

Chrono-topographic analysis of the fire focus dynamics in the SHS wave

The paper presents the means for measuring the spatial and temporal coherence of temperature in the micro regions of a wave of self-propagating high-temperature synthesis (SHS) and revealing the relationship between the events of ignition of individual foci. The method for determining the parameters of the thermal wave structure of the synthesis is based on the chronographical and topographical representation of micro thermal data in the form of maps that visualize the ergodicity of the SHS process and facilitate the recognition of individual foci of burning on thermal imaging images. The ergodicity of the phenomenon is used in the method to determine the time of induction of the combustion sites, the time of their growth, the growth rate of the foci tangentially to the front of the SHS wave, the size of the foci in the direction of the normal to the front. The error in measuring the velocity of the SHS front of the wave with the pro-posed technique was 0.05%.

Method of Differential Chronoscopy of Unstable Regimes of Combustion Wave Distribution

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MEANS OF THE ANALYSIS OF THE SHS PROCESS ON THE BASIS OF THE CHRONOGRAPHIC AND TOPOGRAPHIC PRESENTATIONS OF THERMAL IMAGING DATA

The paper presents the means for measuring the spatial and temporal coherence of temperature in the microregions of a wave of self-propagating high-temperature synthesis (SHS) and revealing the relationship between the events of ignition of individual foci. The method for determining the parameters of the thermal wave structure of the synthesis is based on the chronographical and topographical representation of micro- thermal data in the form of maps that visualize the ergodicity of the SHS process and facilitate the recogni- tion of individual foci of burning on thermal imaging images. The ergodicity of the phenomenon is used in the method to determine the time of induction of the combustion sites, the time of their growth, the growth rate of the foci tangentially to the front of the SHS wave, the size of the foci in the direction of the normal to the front. The technique is implemented in the form of software integrated into the original micro thermal imaging complex. Approbation of the method is represented by the results of experimental studies of the Ni + Al + NiAl system with a variation in the fraction and dispersity of the inert additive. The error in measuring the normal velocity of the SHS front of the wave with the proposed technique was 0.05 %.

The effect of mechanical activation of a reaction mixture on the velocity of the wave propagation of SHS reactions and microstructure of the TiC-Ni hard alloy

The effect of mechanical activation (MA) of the starting reaction charge of the Ti-C-Ni composition on the regularities of combustion and structure formation in the process of self-propagating high-temperature synthesis (SHS) is investigated. It is shown that the preliminary MA of the charge affects the sequence of physicochemical transformations in the SHS wave and the dynamics of the structure-formation of a product with finer grains, which are formed with a relatively narrow size distribution. The prospects for using MA to modify the structure of SHS materials are shown.

Effect of some mechanical activation parameters on the SHS characteristics

The effect of some mechanical activation parameters (duration, ball acceleration, atmosphere) on the self-propagating high-temperature synthesis (SHS) has been investigated by the example of the Ni + Al and Ti + Al systems. The main regularities of SHS wave propagation (gasless combustion) in mixtures subjected to preliminary mechanical activation significantly differ from the SHS regularities in traditional powder systems. This fact is explained primarily by the change in the reaction mixture microstructure and the formation of composite particles during mechanical activation.

On the Physicochemical Mechanism of the Influence of Preliminary Mechanical Activation on Self-Propagating High-Temperature Synthesis

The analysis of physicochemical mechanism of the influence of mechanical activation (MA) of a charge mixture on the subsequent self-propagating high-temperature synthesis (SHS) of intermetallic compounds is performed. Numerical estimates have revealed an insignificant role of the energy stored in solid reactants due to cold work during MA. The characteristic time of relaxation of non-equilibrium vacancies, which were generated in metals by MA, during heating in the SHS wave is estimated, and their insignificant influence on the reaction kinetics at high temperatures is demonstrated. It is shown that a strong effect of preliminary MA on SHS can be attributed only to deformation-enhanced solid-state diffusion during MA, which can lead to the formation of a supersaturated solid solution and thus affect the conditions for nucleation of a product phase (intermetallic compound) upon heating.

Causes of the Thermal Microheterogeneity of a Self-Propagating High-Temperature Synthesis Wave

Causes of the Thermal Microheterogeneity of a Self-Propagating High-Temperature Synthesis Wave

Regularities of self-propagating high-temperature synthesis in the solid 8-hydroxyquinoline—chloramine B system

The regularities of chemical reactions in solid 8-hydroxyquinoline—chloramine B mixtures were studied under conditions of organic self-propagating high-temperature synthesis (SHS), isothermal reaction, and thermal explosion in the 20–220 °C temperature range. Comprehensive physicochemical analysis and microstructural study of the reaction products were carried out. The temperature of SHS initiation (58 °C), the heat of the reaction (129±9 kJ mol−1), the stoichiometric coefficient (1), the maximum temperature (Tmax=98–140 °C), and the velocity of SHS wave propagation (u=0.15–0.55 mm s−1) were determined. Depending on the ratio of the reactants (n), a low-temperature non-degeerate stable gasless mode (n≤1,Tmax=115 °C,Ea=42 kcal mol−1) and a high-temperature mode (n>1,Tmax=140 °C,Ea=0.4 kcal mol−1) are possible for SHS. The SHS affords monohydroxy and monochloro derivatives of 8-hydroxyquinoline, benzenesulfonamide, NaCl, NaOH, and H2O. The mechanism of the solid-phase reaction at temperatures below 58 °C includes surface, solid-phase, and gas-phase diffusion; that for SHS is capillary spreading of the hydroxyquinoline melt.

Formation of self-propagating high-temperature synthesis products in a magnetic field

The combustion of compressed Ni+16% Al, Ni+50% Al, Co+30% S, Ti+58% FeB, Ti+20% C, and FeO+10% Fe+18% Al mixtures in a constant magnetic field is studied. The depth of conversion is found to increase and the geometric dimensions of the phase components of the reaction end products to coarsen under the influence of the field in the systems containing a ferromagnetic metal. The changes in the reaction product are related to additional mass transport behind the self-propagating high-temperature synthesis wave.

Mathematical modelling of structure formation during combustion synthesis of advanced materials

The synthesis of advanced materials by a wave of heterogeneous combustion propagating through a charge mixture, or the so-called self-propagating high-temperature synthesis (SHS) has many potential advantages over conventional techniques of synthesis. Because of high heating rates, steep temperature and concentration gradients, and fast accomplishment of reactions, the mechanisms of physical, chemical and structural transformations in the SHS wave are intricate and often not known. Further understanding of interaction mechanisms in SHS waves, relating the process parameters to structure and properties of the target material, and the application of SHS to producing final articles necessitates developing mathematical models for SHS-related phenomena. Various aspects of mathematical modelling of SHS are discussed in this paper. They include the analysis of novel factors influencing the structure formation, viz. the heating-to-reaction and mass transfer-to-reaction time ratios, autocatalysis and intrinsic stochasticity, which are unnoticed in traditional synthesis methods. The application of chemical thermodynamics and combustion theory to modelling SHS processes is outlined. Novel mathematical models are developed for SHS on condensed systems, which involve stochastic effects and autocatalysis. New models for solid-gas systems are worked out, which include the reaction kinetics and mass transfer of a gaseous reactant and permit predicting the structure formation pattern in the SHS wave. Application of mathematical modelling to producing porous final articles by means of SHS is discussed.

Influence of constant electric field on self-propagating high-temperature synthesis wave

A model of a heterogeneous medium allowing the action of a constant electric field (CEF) on a combustion wave in the condensed phase to be taken into account is proposed. It is shown that the influence of the CEF on the combustion temperature is of threshold type: At a CEF less than the threshold, the combustion temperature is constant; above the threshold, the combustion temperature falls with increase in CEF. The combustion rate falls with increase in CEF; this is associated with slowing of the reactive diffusion in the combustion wave. On passing through the CEF threshold, there is a discontinuity in the combustion-rate curve; this is due to additional slowing of the reactive diffusion on account of the reduction in combustion temperature.

Limits of the self-propagating high-temperature synthesis wave propagation in eutectic composite materials

Phase formation during the self-propagating high-temperature synthesis in the Ti-C-M (M = Ni, Co, Cr) eutectic-type ternary systems with and without preheating has been studied. It was found that the synthesis proceeds via origination of quasi-binary TiC-M eutectics, the principle reaction being that of TiC formation. The dependence of the combustion temperature on the preheating temperature and the initial charge composition has been evaluated by thermodynamic analysis.

Radiating power of a self-propagating high-temperature synthesis wave

Radiating power of a self-propagating high-temperature synthesis wave