Combustion Synthesis CS

For it to be self-sustaining, the SHS process must, perforce, be associated with high-temperature reactions. An important parameter in this regard is the adiabatic temperature of combustion, Tad- This thermodynamic parameter is the temperature to which the product is raised under adiabatic conditions as a consequence of the evolution of heat from the reaction. Reactions between particulate materials in a self-propagating mode present an attractive practical alternative to conventional methods of materials preparation for a variety of reasons [7, 8]. 

2) The higher purity of products obtained by this method. 

4) The possibility of simultaneous formation and densification of the desired materials. 

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Combustion synthesis (CS) can occur by two modes self-propagating high-temperature synthesis (SHS) and volume combustion synthesis (VCS). A schematic diagram of these modes is shown in Fig. 1. In both cases, reactants may be pressed into a pellet, typically cylindrical in shape. The samples are then heated by an external source (e.g., tungsten coil, laser) either locally (SHS) or uniformly (VCS) to initiate an exothermic reaction. 

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. 

Combustion synthesis (CS) is an attractive technique to produce 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 [13]. The development of this technique leads to the appearance of a new scientific direction that incorporates both aspects of combustion and materials science [14, 15]. CS can occur by two modes self-propagating high-temperature synthesis (SHS) and volume combustion synthesis (YCS). 

Fabrication of dense p-SiAlON by a combination of combustion synthesis (CS) and spark plasma sintering (SPS). IntermetalUcs, 18, 536—541. 

From the viewpoint of chemical nature, three main types of CS processes can be distinguished. The first, gasless combustion synthesis from elements, is described by the equation... 

The third main type of CS is reduction combustion synthesis, described by the formula... 

In this section, we discuss the laboratory techniques and production technologies used for the combustion synthesis process. The laboratory studies reveal details of the CS process itself, while the technologies may also include other processing, such as densification of the product by external forces. In both cases, it is necessary to control the green mixture characteristics as well as the reaction conditions. For the production technologies, however, optimization of parameters related to external postcombustion treatment is also necessary in order to produce materials with desired properties. 

The typical production technologies for combustion synthesis were described in Section II. It is evident that apparatus needed for synthesis is relatively simple, especially since no additional equipment is needed for bulk heating of the material (e.g., furnaces, plasma generators, etc.). In addition, it has been reported that replacing conventional furnaces with a CS apparatus leads to significant reduction in workspace requirements (Merzhanov, 1992). Finally, owing to low power requirements, combustion synthesis technology is one of the few methods of materials synthesis that is feasible in outer space (Hunter and Moore, 1994). 

Thermodynamic calculations have also been used to determine the equilibrium products (Mamyan and Vershinnikov, 1992 Shiryaev et ai, 1993), and to illustrate new possibilities for controlling the synthesis process, even for complex multicomponent systems. Correlating these calculations with the equilibrium phase diagram for each system provides a basis for predicting possible chemical interactions and even limits of combustion during CS of materials. Some examples are discussed in the following subsections. 

From the viewpoint of chemical nature, three main types of CS processes can be distinguished. The first is a gasless CS from elements. The second type, the so-called as-so//d combustion synthesis, involves at least one gaseous reagent in the main combustion reaction. And the third type of CS is reduction CS. In this chapter, we consider the gas-solid type of CS, which in general can be described by the following overall reaction formula ... 

Of the various synthetic methods, combustion synthesis (GS) has special significance because CS processes are characterized by high temperatures, fast heating rates and short reaction times. These features make CS an attractive method for the manufacture of materials. In this method, the exothermicity of the redox (reduction-oxidation or electron transfer) chemical reaction is used to produce useful materials. - ... 

The main characteristics of the green mixture used to control the CS process include mean reactant particle sizes, size distribution of the reactant particles reactant stoichiometry, j, initial density, po size of the sample, D initial temperature, Tq dilution, b, that is, fraction of the inert diluent in the initial mixture and reactant or inert gas pressure, p. In general, the combustion front propagation velocity, U, and the temperature-time profile of the synthesis process, T(t), depend on all of these parameters. The most commonly used characteristic of the temperature history is the maximum combustion temperature, T -In the case of negligible heat losses and complete conversion of reactants, this temperature equals the thermodynamically determined adiabatic temperature (see also Section V,A). However, heat losses can be significant and the reaction may be incomplete. In these cases, the maximum combustion temperature also depends on the experimental parameters noted earlier. 

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