Volume combustion synthesis

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. 

During volume combustion synthesis, the entire sample is heated uniformly in a controlled manner until the reaction occurs essentially simultaneously through-... 

The volume combustion synthesis mode is used primarily for the synthesis of weakly exothermic systems. Various types of heaters, mostly commercially available furnaces, in addition to spiral coil and foil heaters, are used to preheat the sample up to the ignition point. To date, VCS synthesized materials have been produced only in laboratories, and no industrial or pilot production by this mode of synthesis has been reported. 

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). 

CNTs can also be produced by diffusion flame synthesis, electrolysis, use of solar energy, heat treatment of a polymer, and low temperature solid pyrolysis. In flame synthesis, combustion of a portion of the hydrocarbon gas provides the elevated temperature required, with the remaining fuel conveniently serving as the required hydrocarbon reagent. Hence, the flame constitutes an efficient source of both energy and hydrocarbon raw material. Combustion synthesis has been shown to be scalable for a high volume commercial production. 

Another hypothesis was suggested by Kirdiyashkin et al. (1981) for the combustion synthesis systems characterized by melting of a reactant metal (e.g., Ti-C, H-B), where capillary spreading may control the combustion process (Shkiro and Borovinskaya, 1976). In these cases, it was suggested that an optimal density oc-ciffs where the volume of pores equals the volume of the molten metal. However, an analysis of the experimental data for the Ti-B system showed that this hypothesis may not be valid over the entire range of particle sizes investigated (Munir and Anselmi-Tamburini, 1989). 

Another example of synthesis of TiC-based compact cermets is reported by Xing et al. [90]. These authors describe the synthesis of dense NiAl-20 vol-% TiC composites through a reaction of a mixture of Ni, Al, Ti, and C powders in a hot press. The reaction was performed using the volume combustion mode heating the green mixture until a reaction is observed in the entire sample. The samples were heated up to 1500°C under the influence of an applied pressure. The products were 98.9% dense but the TiC particles (0.2-1 pm) were not uniformly dispersed in the matrix. The combustion process had similar ignition characteristics to the... 

The combustion technique appears to be controlled by the mass of the mixture and the volume of the container. Studies conducted by Kingsley and Patil [2] demonstrated that the mass/volume ratio is critical for the occurrence of combustion synthesis, as compositions with less than 5 g in containers of 300 mL did not undergo the ignition process. 

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. 

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

Oxygen is not estimated because there are no simple methods for it. Combustion in a closed system in which a measured volume of oxygen is circulated has been proposed and tried, but the reported results have not been satisfactory. If a polymer is known to contain only carbon, hydrogen, silicon, and oxygen, by virtue of its synthesis or by application of qualitative tests, it is customaiy to estimate oxygen by difference. 

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