A. S. Rogachev

Combustion Synthesis of Advanced Materials: Principles and Applications

Combustion synthesis is an attractive technique to synthesize a wide variety of advanced materials including powders and near-net shape products of ceramics, intermetallics, composites, and functionally graded materials. This method was discovered in the former Soviet Union by Merzhanov et al. (1971). The development of this technique by Merzhanov and coworkers led to the appearance of a new scientijc direction that incorporates both aspects of combustion and materials science. At about the same time, some work concerning the combustion aspects of this method was also done in the United States (Booth, 1953; Walton and Poulos, 1959; Hardt and Phung, 1973). However, the full potential of combustion synthesis in the production of advanced materials was not utilized. The scientijc and technological activity in thejeld picked up in the United States during the 1980s. The signijcant results of combustion synthesis have been described in a number of review articles (e.g., Munir and Anselmi-Tamburini, 1989; Merzhanov, I990a; Holt and Dunmead, 1991; Rice, 1991; Varma and Lebrat, 1992; Merzhanov, 1993b; Moore and Feng, 1995). At the present time, scientists and engineers in many other countries are also involved in research and further development of combustion synthesis, and interesting theoretical, experimental, and technological results have been reported from various parts of the world (see SHS Bibliography, 1996). This review article summarizes the state of the art in combustion synthesis, from both the scientijc and technological points of view. In this context, we discuss wide-ranging topics including theory, phenomenology, and mechanisms of product structure formation, as well as types and properties of product synthesized, and methods for large-scale materials production by combustion synthesis technique.

Gasless Combustion of Ti–Al Bimetallic Multilayer Nanofoils

Ti–Al multilayer foils were produced magnetron vacuum deposition. The microstructure period varied in the range of 5–110 nm, the number of layers was 150–4700, and the total thickness of a multilayer foil reached 15–20 μm. The gasless combustion of the foils was studied. Steady‐state and pulsating combustion regimes were revealed; combustion temperatures were determined for both regimes. It was shown that the most probable mechanism of the self‐propagating reaction is the diffusion of Al in β‐Ti at a temperature close to the temperature of the α → β transition.

Structural macrokinetics of SHS processes

Development of structural macrokinetics as a new field of science in the 80s and in the beginning of 90s is analyzed. Results having principal importance for working out procedures of controlling structure of products and materials prepared by self-propagating high-temperature synthesis is reviewed. Major attention has been paid to refractory products (carbides, borides, nitrides, silicides, etc. 1 and refractory base materials, viz. oxygenless ceramics, cermets, and hard alloys, as well as to high-temperature superconductors. It is shown that the structure of these materials produced in the combustion mode, in general, can be effectively controlled by changing relevant parameters of the synthesis. In connection with this, detailed studies on the mechanism of product structurization and the structure of combustion wave itself are necessary.

Self-sustained waves of exothermic dissolution in reactive multilayer nano-foils

The phenomenon of exothermic waves in reactive multilayer nano-foils demonstrates unusually high propagating rates (up to 100 m/s), which generate numerous applications and continuously motivate attempts for understanding its mechanism. In this work, based on the studies of the “quenched” exothermic waves, we have found that the driving mechanism of this phenomenon is a direct exothermic dissolution of one solid reactant in the molten layer of the other. The suggested mechanism of the exothermic wave in the Ni/Al multilayer nano-foil allows non-contradictory explanation of its main features.

On the mechanism of structure formation during combustion synthesis of titanium silicides

ABSTRACT The mechanism of structure formation during combustion synthesis of titanium silicides was investigated. It is shown that the propagation of an adiabatic combustion wave in a 5Ti + 3Si mixture follows the reaction coalescence mechanism, resulting in fine grains of Ti5Si3 crystallizing in the volume of melt. These grains grow rapidly behind the combustion front. Only the Ti5Si3 phase appears at the combustion temperature (2130°C), while other silicides (Ti5Si4, Ti3Si, etc.) may crystallize in the cooling zone. Time-resolved X-ray diffraction (TRXRD) analysis of the combustion process and layer-by-layer XRD analysis of the quenched combustion wave were utilized to study the sequence of phase formation. Interaction below the melting point of Ti is negligible in the self-propagating synthesis mode. A particle-foil technique was used to investigate the details of this lower temperature interaction.

Combustion Wave Microstructure in Gas-Solid Reaction Systems:Experiments and Theory

Using experimental techniques developed previously by the authors, the combustion wave microstructure in a gas-solid reaction system (specifically, the Ti-N2 system) was studied. A quantitative analysis of the location, shape and propagation of the combustion wave front was carried out. The relay-race mechanism of combustion wave propagation was observed for a wide range of experimental conditions. In conjunction with the experiments, a micro-heterogeneous cell model was developed that simulates the local propagation of the combustion wave in a random porous medium. For both experiments and calculations, the initial organization of the reaction medium (porosity, particle size, etc.) was found to affect the heterogeneity of the combustion wave. The fluctuations of the combustion wave, in terms of both shape and propagation, increased monotonically with increasing heterogeneity of the reaction medium (i.e., larger porosity or particle size).

Macrokinetics of structural transformation during the gasless combustion of a titanium and carbon powder mixture

ConclusionThe presented results provide a phenomenological description of the structural evolution of the wave of a Ti and C powder mixture combustion. They show that further progress in the understanding of the mechanisms of combustion and the structure formation is linked to the solution of two problems:1)What is the mechanism underlying the generation of carbide particles?2)Which kinetic laws and mechanisms govern the growth of carbide particles? It can be assumed that the noted characteristics of structural transformations are also observed in other systems of the type metal-carbon, metal-boron. This is indicated by the similarity of the morphology of products and of some combustion characteristics

Mechanisms of structure formation during combustion synthesis of materials

A study involving two distinct approaches, directed towards understanding the mechanisms of structure formation during combustion synthesis of materials, is described. The first approach consists of a novel technique which utilizes a microscopic high-speed video recording of the combustion front, followed by a quantitative framewise image analysis. Using this technique, it is discovered that combustion does not proceed uniformly but rather in a jump-wise manner, with the local front velocity exhibiting large fluctuations. The second approach isolates reactant interactions and involves a study of particles of the lower melting point reactant placed on the surface of a thin foil of the other reactant, heated at rates up to 10 4 K/s by passing electrical current through the foil. These heating rates simulate conditions of the combustion front. Simultaneous measurements of foil temperature coupled with microscopic observations provide insight into the spreading characteristics of melts over foils and subsequent product structure formation

Effects of Gravity on Combustion Synthesis in Heterogeneous Gasless Systems

The first systematic experimental investigation of combustion synthesis from elements conducted under low (10 -4 m/s 2 ) gravity conditions is reported. Several classes of gasless heterogeneous reaction systems were studied, e.g.,Ni-Al,Ti-C, and Ni-Al-Ti-B. A variety of gravity-related effects were observed. It was shown that the convection of inert gas, occurring in the reaction chamber during the combustion process in terrestrial conditions, leads to instability of combustion front propagation along the sample. Also, composite materials (Ni 3 Al-TiB 2 ) produced in microgravity have finer size of refractory phase (TiB 2 ) that is more uniformly distributed in the Ni 3 Al intermetallic matrix. The grains formed in terrestrial conditions were approximately 50% larger. The results obtained with quenched samples showed that this difference is related to the buoyancy enhanced coalescence process. For the Ti-C system, dynamics of sample expansion have been determined, and materials with final porosity up to 90% have been synthesized in microgravity conditions without the use of any gasifying additives. Finally, it was observed that the combustion velocity for loose Ni-Al mixtures in microgravity is about three times larger than in terrestrial conditions. This effect may be explained by a change in the primary mechanism of heat transfer through the reaction medium.

Combustion Wave Microstructure in Heterogeneous Gasless Systems

Abstract A novel method for studying the quantitative characteristics of combustion wave microstructure has been developed. Using the combustion of titanium and silicon powders as an example, the propagation of the combustion wave at the scale of microscopic heterogeneity is examined quantitatively for the first time. The variations in time and space of the combustion wave shape and propagation are studied experimentally. The sample porosity and the refractory reactant particle size are found to affect the heterogeneity of the combustion wave. For low porosities and small particle sizes, The shape and propagation of the combustion wave at the microscopic level approach that observed at the macroscopic level. However, For high porosities and large particle sizes, significant dispersion of the combustion front and local instantaneous velocities is observed at the microscopic level. A new relay-race mechanism of combustion wave propagation, that accounts for the heterogeneity of the reactant medium, is sugge...