Self-propagating exothermic reactions: the synthesis of high-temperature materials by combustion

Self-propagating high-temperature synthesis (SHS) of powder compacts is a novel processing technique currently being developed as a route for the production of engineering ceramics and other advanced materials. The process, which is also referred to as combustion synthesis, provides energy- and cost-saving advantages over the more conventional processing routes for these materials. At the same time, the rapid heating and cooling rates provide a potential for the production of metastable materials with new and, perhaps, unique properties. This paper reviews the research that has been, and is being, undertaken in this exciting new processing route for high-technology materials and examines the underlying theoretical explanations which will, eventually, lead to improved control over processing parameters and product quality.

Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials

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.

Combustion Synthesis of Advanced Materials: Principles and Applications

The formation of nickel aluminidms by the thermal explosion mode of gasless combustion synthesis was investigated for Ni-Al powders ranging in composition from 5 to 3O at% Al. Compound formation was found to take place sequentially starting with the most aluminium-rich and ending with AlNi3 as the predominant compound in the product. Compounds formed through both solid- and liquid-sate reactions, with the relative contribution of each depending on the rate of heating of the powders to the reaction temperature. The effect of the particle size of nickel on these reactions vvasalso investigated for powders with average diameters from 14 to 58μm.

An investigation of the synthesis of nickel aluminides through gasless combustion

The combustion synthesis of TiC, using the thermal explosion mode, was investigated by varying some of the process parameters including the reactant particle size, the pre-compaction pressure, and the heating rate. Based on these observations, a combustion model for the reaction was developed. When iron was added to titanium and carbon black powders, the ignition temperature was dictated by the eutectic temperature of the Fe-Ti system. Iron acted as a moderator for the reaction and led to a decrease in the combustion temperature.

Characteristics of the combustion synthesis of TiC and Fe-TiC composites

When a mixture of nickel and aluminum powders is sintered there is an exothermic effect which involves a rise of several hundred degrees in the temperature of the compact in a few seconds. The magnitude and nature of the exothermic effect depend on the aluminum content of the mixture, the degree of dispersion of the powders, and the initial porosity of the specimens. The temperature at which the exothermic effect begins moves towards lower temperatures with increase in the degree of dispersion, with increase in the concentration of aluminum in the mixture, and with decrease in the porosity. The magnitude of the heat effect increases with increase in the aluminum content. It is shown that the exothermic effect in the sintering process arises mainly from decomposition of the liquid phase and crystallization of chemical compounds with large heats of formation. A definite role is played by the metallothermic reduction of nickel oxide by aluminum present in the liquid phase and by the formation of intermetallic compounds at the boundaries of dissimilar particles of powder during sintering in the solid phase.

Exothermic effects during sintering of a mixture of nickel and aluminum powders. II

1.

During the sintering of mixed copper and aluminum powders at temperatures above the eutectic point, the compacts exhibit growth, which depends on the particle size of the aluminum powder and the rate of heating during sintering.




2.

The smaller the aluminum particle size of an alloy and the slower the heating of specimens to the temperature of isothermal holding, the greater is the extent of volume growth in the presence of a liquid phase.




3.

The observed growth is attributed to the preferential diffusion of the aluminum atoms from the liquid into the solid phase. When the diffusion cannot proceed to a sufficient extent, the compacts may exhibit shrinkage.

The effect of the particle size of aluminum powder on the sintering of a Cu-Al alloy in the presence of a liquid phase

1.

The methods of nonsteady-state thermal explosion theory have been used to describe the exothermic effects produced during the formation of a chemical compound at particle interfaces during the sintering of powders. Appropriate equations, determined by phase growth kinetics, have been derived and subjected to approximate analysis.




2.

An experimental study was made of the exothermic effect in the system Cu-Al. It is shown that heat evolution is observed at temperatures close to the melting point of the eutectic and is due to the formation of solid solutions and chemical compounds at the contact interfaces of dissimilar powder particles.




3.

For the case of solid-phase sintering resulting in the formation of the compound CuAl2 at the contact interfaces of copper and aluminum particles, theoretically calculated characteristics of the exothermic effect are compared with those determined experimentally. The results yielded by an approximate solution of the equations derived are in satisfactory agreement with experimental data.

Reactions at phase boundaries and their effects on the sintering process

The investigation has demonstrated that, when mixtures of copper and aluminum powders are sintered at temperatures exceeding the eutectic point, an increase in the specimen volume is observed; the higher the aluminum content of the alloys, the greater is this increase. The specimen growth is associated with the swelling of the copper particles as a result of the penetration of aluminum into them and the formation of voids at the sites of the aluminum particles.

K. V. Savitskii

Papers

Exothermic effects during sintering of a mixture of nickel and aluminum powders. II

The effect of the particle size of aluminum powder on the sintering of a Cu-Al alloy in the presence of a liquid phase

Reactions at phase boundaries and their effects on the sintering process

The mechanism of sintering of copper-aluminum powder alloys in the presence of a liquid phase

A study of the sintering of metalloceramic Ni-Al alloys