Process types, combustion synthesis

The processes of combustion synthesis differ considerably in the state of the substance in the reaction front (solids, melts, gases). Correspondingly, the transport processes responsible for chemical transformations in the combustion front are different as well. On the basis of the classifications presented in (J), four main types of processes can be recognized. 

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... 

Based on their analyses, and incorporation of additional details, we have outlined some general relationships for gasless combustion synthesis of materials from elements (type 1), as shown schematically in Fig. 3. Both characteristic features of the process, the combustion wave propagation velocity and maximum temperature, have maximum values when the composition of the green mixture corresponds to the most exothermic reaction for a given system (Fig. 3a). In gen-... 

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... 

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. 

The theory discussed here applies mainly to kinetic-controlled reactions. However, combustion synthesis reactions involve many processes, including diffusion control, phase transitions, and multistage reactions, which result in a complex heat release function, Z4> (r/,7). An analysis of the system of combustion equations for various types of heat sources was compared with experimental data, which led to the conclusion that in general, the combustion wave velocity can be represented as follows (Merzhanov, 1990a) ... 

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 ... 

Industrial analysis of hydrocarbon gases 25 years ago was limited almost to Orsat-type absorptions and combustion, resulting in crude approximations and inadequate qualitative information. The more precise method of Shepherd (56) was available but too tedious for frequent use. A great aid to the commercial development of hydrocarbon gas processes of separation and synthesis was the development and commercialization of high efficiency analytical gas distillation units by Podbielniak (50). In these the gaseous sample is liquefied by refrigeration, distilled through an efficient vertical packed column, the distillation fractions collected as gas and determined manometrically at constant volume. The operation was performed initially in manually operated units, more recently in substantially automatic assemblies. 

The application of combustion theory to gasless synthesis processes is based on a comparison of the mass and thermal diffusivities in the solid reactant mixture. In these types of systems, the Lewis number is very small ... 

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