Phenomenological Aspects

As stated above, the characteristic feature of combustion synthesis, as compared to conventional powder metallurgy, is that the process variables, such as combustion wave velocity, U, and temperature, Tc, are strongly related. For example, a small change in Tc may result in a large change in U (see, e.g., Eq 13) and hence the characteristic time of synthesis. The process parameters (e.g., green mixture composition, dilution, initial density, gas pressure, reactant particle characteristics) influence the combustion velocity and the temperature-time profile, and in turn can be used to control the synthesis process.

Based on the analysis of literature data and incorporation of additional details, some general relationships for gasless combustion synthesis of materials from elements have been outlined as shown schematically in Fig. 3. Both characteristic features of the process, U and Tc, have maximum values when the composition of the green mixture corresponds to the most exothermic reaction for a given system (Fig. 3a). In general, U and Tc decrease with increasing initial reactant particle size and with addition of an inert (nonreactive) diluent to the green mixture (Fig. 3b, c), while increasing significantly with increasing initial sample temperature (Fig. 3d). Different trends have been observed when the initial sample density is varied. With increasing P0, the combustion front velocity either increases monotonically or goes through a maximum, while the combustion temperature generally remains constant (Fig. 3e). A decrease in the sample size (e.g., sample diameter, D) does not influence U and Tc when the size is larger than a critical value D*, because heat losses are negligible as compared to heat release from the chemical reaction. Below the critical sample size, both the combustion velocity and temperature decrease due to significant heat losses (Fig. 3f). Many exceptions to the dependencies discussed above have been observed, however, even for the simplest case of gasless combustion synthesis from elements. The combustion wave behavior becomes more complicated in gas-solid and reduction-type reactions. All of these effects are discussed in greater detail in Ref 8, 10, and 12.

Fig. 3 Dependencies of combustion velocity, U, and maximum combustion temperature, Tc, on various combustion synthesis parameters

Within the region of optimal experimental parameters, the combustion wave velocity remains constant and the temperature profile T(t) has the same form at each point of the reaction medium. This regime is called steady propagation of the combustion synthesis wave, or steady SHS process. As the reaction conditions move away from the optimum, where the heat evolution decreases and/or heat losses increase, different types of unsteady propagation regimes have been observed. These include the appearance of an oscillating combustion synthesis regime, where macroscopic oscillations of the combustion velocity and temperature occur. The reaction may also propagate in the form of a hot spot that, for example, may move along a spiral pattern in cylindrical samples, and is called the spin combustion regime of CS. The combustion regime has great importance in the production of materials, because it influences the product microstructure and properties.

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