Combustion synthesis powders

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

Combustion synthesis of metal nitrides from a solid nitrogen source was first investigated by Petrov in 1975.32 He used barium azide (Ba2N6) powder for the synthesis of TiN and Mg3N2. Holt, in 1983,33 reported the synthesis of TiN by using sodium azide (NaN3)as the solid nitrogen source. 

In addition to conventional sintering, reactive powder processing, also called combustion synthesis or self-propagating high-temperature synthesis (SHS), can be used if the target compounds can be synthezised from the starting powder mixture (Stangle and Miyamoto, 1995). This process comprises a rapid and exothermic chemical reaction to simultaneously synthesize some or all of the constituent phases in the FGM and density the component. 

In order to carry out combustion synthesis, the powdered mixture of reactants (0.1-100 //m particle size) is generally placed in an appropriate gas medium... 

The combustion synthesis technique consists of bringing a saturated aqueous solution of the desired metal salts and a suitable organic fuel to the boil, until the mixture ignites a self-sustaining and rather fast combustion reaction, resulting in a dry, usually crystalline, fine oxide powder. By simple calcination, the metal nitrates can, of course, be decomposed into melt oxides upon heating to or above the phase transformation temperature. 

Historically, combustion synthesis (both SHS and VCS) is a direct descendant of classic works on combustion and thermal explosion (e.g.. Mallard and Le Chatelier, 1883 Semenov, 1929 Zeldovich and Frank-Kamenetskii, 1938 Williams, 1965 Glassman, 1977) see Hlavacek (1991) and Merzhanov (1995) for additional comments in this regard. We discuss later in Section IV how the theory of SHS grew directly from these works. The progress in combustion science made it possible to organize self-sustained exothermic reactions in powder mixtures that were controllable and predictable, hence avoiding the uncontrollable evolution of the reaction that is commonly associated with the terms combustion, fire, and explosion. 

Throughout this work, more emphasis is placed on the SHS mode of synthesis rather than the VCS mode because more information is available for SHS. Also, note that in this review, we do not consider production of powders by gas-phase combustion synthesis processes (e.g., Calcote et al, 1990 Davis et al, 1991). 

The main production technologies of combustion synthesis are presented in the second block of Fig. 4. Following Merzhanov (1990a), they may be classified into several major types powder production and sintering, densification, and casting and coating. 

The third main step of combustion synthesis technologies is postsynthesis treatment. This step is optional, since not all products require additional processing after synthesis. Powder milling and sieving are used to yield powders with a desired particle size distribution. Annealing at elevated temperatures (800-1200°C) removes residual thermal stress in brittle products. The synthesized materials and articles may also be machined into specified shapes and surface finishes. 

In some cases, several refractory compounds can result from two or more parallel reactions occurring simultaneously in the combustion wave. A typical example of this type is the Ti-C-B system, where both the Ti+C and Ti-I-2B reactions affect the combustion synthesis and structure formation processes (Shcherbakov and Pityulin, 1983). By adjusting the contents of carbon and boron powders in the reactant mixture, either carbide- or boride-based ceramics can be obtained. 

Combustion-synthesized MoSi2 powders have been used to produce high-temperature heating elements (Merzhanov, 1990a). The application of the SHS+extrusion method for one-step production of MoSi2 heaters has also been reported (Podlesov et al., 1992a). Further, combustion synthesis of molybdenum alu-mosilicide (Mo-Al-Si) was demonstrated (Hakobian and Dolukhanyan, 1994). 

Combustion synthesis of boron nitride powder, BN, was reported in one of the earliest works on SHS (Merzhanov and Borovinskaya, 1972). The mechanisms of combustion and product structure formation from elements were later investigated (Mukasyan and Borovinskaya, 1992). More recently, finely dispersed hexagonal boron nitride powder has been obtained from reduction-type reactions (Borovinskaya et fl/., 1991). 

Some comparisons of production costs by SHS and conventional processes have been reported. Analysis for SisNa, AIN, and SiC materials has shown that combustion synthesis is economically favorable to existing technologies, as shown in Tables XIII, XIV, and XV (Golubjamikov et al, 1993). For all materials, the total cost per kilogram of material produced is lower for SHS-based processing than for conventional methods, with a major contribution to savings from reduced fixed costs (i.e., capital equipment costs). The significant difference in the raw material cost for SiC tiles occurs due to the use of Si+C powder mixtures in the SHS case, compared with SiC powder for conventional methods. 

In Japan, several commercial projects have been reported in the literature. For example, at the National Research Institute for Metals, the NiTi shape-memory alloy is produced by combustion synthesis from elemental powder for use as wires, tubes, and sheets. The mechanical properties and the shape-memory effect of the wires are similar to those produced conventionally (Kaieda et ai, 1990b). Also, the production of metal-ceramic composite pipes from the centrifugal-thermite process has been reported (Odawara, 1990 see also Section III,C,1). 

In both the electrothermographic and foil assembly methods, the rapid heating rates associated with combustion synthesis are reproduced. However, the powder reactant contact found in a compacted green mixture of particulate reactants is not adequately simulated. One way to overcome this is to investigate interactions of particles of one reactant placed on the surface of the coreactant in the form of a thin foil. The physical simulation corresponds to the reaction of a powder mixture where the particle size of one reactant is small while that of the coreactant is relatively large. Two methods have been used to initiate the interaction. 

Kuroki, H., and Yamaguchi, K., Combustion synthesis of Ti/Al intermetallic compounds and dimensional changes of mixed powder compacts during sintering. Proceedings of the First US-Japanese Workshop on Combustion Synthesis, Tokyo, Japan, 23 (1990). 

The thermite process may be the original inspiration of combustion synthesis (CS), a relatively new technique for synthesizing advanced materials fl-om powder into shaped products of ceramics, metallics, and composites. Professor Varma and his associates at Notre Dame contributed the article Combustion Synthesis of Advanced Materials Principles and Applications, which features this process that is characterized by high temperature, fast heating rates, and short reaction times. 

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