Refractories combustion synthesis

In the field of high-temperature combustion synthesis, metals have been reacted with nitrogen, both in the gaseous and liquid phases, to form refractory nitrides [2], In most cases, this nitriding process is heterogeneous. 

A theoretical analysis of combustion synthesis of refractory nitrides was presented by Munir and Holt in 1987.37 They predicted the existence of an activation energy due to chemical reaction or mass-transport. Glassman et al. in 1987,38 in their thermodynamic analysis of TiN formation, examined the possibility of creating TiN by a self-sustained reaction of the metal particles and nitrogen gas in a rocket motor. They reported that for the stoichiometric ratio of 0.5 mole N2/mole titanium, the reaction has... 

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. 

S. Zhang and Z. A. Munir, The combustion synthesis of refractory nitrides Part II, the synthesis of niobium nitride, J. Mater. Sci. 1991, 26, 3380-3385. 

The plasma chemical method is relatively often used now for preparing many nanopowders for production of refractory compounds (nitrides, oxides, carboni-trides, and their compositions) [6]. The process of preparing aluminum nitride nanopowder by this technique is based on evaporation of aluminum in high temperature nitrogen flow, following chemical interaction, and subsequent condensation of reaction product. It means that this process could be considered as a kind of combustion synthesis. 

High-quality powders of refractory carbides and nitrides can be obtained by the more recent processing techniques of chemical-vapor deposition (CVD), RF-plasma torch, combustion synthesis, and sol-gel.l H l... 

For the CNTs growth from powder form catalysts, it is used as a support an inert and refractory oxide, and transition metals in the form of oxides, which will generate, during the reaction, the catalyst NP. To produce a homogeneous mixture between the oxide and metal oxide catalyst support, two methods are most commonly used solution combustion synthesis (SCS) [79] or the impregnation method [80]. Both use inorganic salts (usually nitrates) as the catalysts. 

It is not essential that, for combustion synthesis, the reactants are elements and initially solid (at least one of them). Refractory carbides and nitrides with complex chemical compositions (metal halogenides, organometallics, etc.) are also formed in combustion of gaseous systems. The special features of combustion synthesis in the gas phase were analyzed in the recent review by Brezinsky (5). The synthesis in a gaseous system proceeds at a stationary combustion front with moving flows of reactants and product. Consequently, these processes differ in essence from combustion of condensed systems by the conditions of nucleation and growth of the new phase, which manifest themselves in the product morphologies. 

Hlavacek and co-workers [32] have simulated the synthesis of refractory metal nitrides by adiabatic combustion. The reactions involved occur at 2500... 

In the case of hot-filament CVD, refractory metal filaments (e.g., W, Ta, Re, etc.) are electrically heated to very high temperatures (between 2000 and 2700°C) to produce the necessary amount of atomic hydrogen that is necessary for the reasons mentioned above for the synthesis of diamond. Except for combustion flame CVD, hot-filament CVD is considered the simplest of all of the methods and also the most inexpensive. Plasma-jet and laser-assisted CVD methods rely on a plasma torch or laser to attain the very high temperatures that are needed to... 

Autothermal reforming combines partial oxidation and adiabatic steam reforming for conversion of the hydrocarbon feedstock into synthesis gas free of soot and higher hydrocarbons. The ATR reactor design consists of burner, combustion chamber, and catalyst bed placed in a refractory lined vessel, as illustrated in Fig. 10. The hydrocarbon feedstock with steam is reacted with oxygen in a substoichiometric flame, often... 

The effect of dilution on the synthesis reaction of hard (refractory) materials has been examined experimentally and through modeling. Figure 16 shows the effect of adding TiC as a diluent on the adiabatic temperature and the fraction melted of the product [15]. The adiabatic temperature remains constant with the addition of TiC up to 20 mol%, at this point the product is entirely solid and further addition of the diluent decreases the adiabatic temperature as shown in the figure. The decrease in the combustion wave temperature is expected to result in a decrease in the wave velocity. Experimental confirmation of this is provided in Fig. 17 for the case of... 

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