Combustion synthesis borides

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. 

H. C. Yi and A. Petrie, Combustion synthesis of Ti-Al-Nb matrix composites reinforced by titanium borides, J. Mater. Synth. Proc. 1994, 2, 161-167. 

In order to carry out combustion synthesis, the powdered mixture of reactants (0.1-100 pm particle size) is generally placed in an appropriate gas medium that favours an exothermic reaction on ignition (in the case of oxides, air is generally sufficient). The combustion temperature is anywhere between 1500 and 3500K depending on the reaction. Reaction times are very short since the desired product results soon after the combustion. A gas medium is not always necessary. This is so in the synthesis of borides, silicides and carbides, where the elements are quite stable... 

Combustion synthesis (44) is an efficient method for making a wide variety of materials such as carbides (45,46), borides (47), and nitrides (48,49) and is superior to conventional methods with respect to higher purity and more reactivity (Fig. 3a). When the SHS reaction is carried out with powders containing nonvolatile contaminants, significant amounts of these impurities remain in the product (50). The initial reactant mixtures (chemical composition, shape and size of reactant particles, and shape, size, and density of samples) and combustion conditions (composition and pressure of the environment, initial temperature of the compact, the method and intensity of combustion initiation, or additional external effects) determine the properties of synthesized materials (45-49). 

The products of combustion synthesis can be sinters, melts or powders (ultrafine included) of carbides, nitrides, borides, silicides, halogenides and other compounds, solid solutions or mixed phases, depending on the parameters and conditions of the process. There are many preparative and technological procedures for producing various compounds, materials and items in the combustion mode. These are described in the scientific and patent literature and their consideration goes beyond the framework of the present paper. The same has to be said about the numerous theoretical and experimental results concerning various aspects of combustion, that are also considered in the above-mentioned reviews. 

It has been demonstrated that most of the transition metal borides can be synthesized in the SHS mode (Holt et al, 1985 Rice et al, 1987 Mei et al, 1992 Fu et al, 1993a Li, 1995). Early works reported synthesis of relatively pure (with residual unreacted boron less than 0.3 wt %) ZrB2, TiB2, HfB2, and MoB (Merzhanov and Borovinskaya, 1972 Hardt and Holsinger, 1973). Combustion-... 

Zenin, A. A., Merzhanov, A. G., and Nersisyan, G. A., Thermal wave structure in SHS processes (by the example of boride synthesis). Combust. Explos. Shock Waves, 17,63 (1981). 

In this book, we briefly examine the different types of reactions and methods employed in the synthesis of inorganic solid materials. Besides the traditional ceramic procedures, we discuss precursor methods, combustion method, topochemical reactions, intercalation reactions, ion-exchange reactions, alkali-flux method, sol-gel method, mechanochemical synthesis, microwave synthesis, electrochemical methods, pyrosol process, arc and skull methods and high-pressure methods. Hydrothermal and solvothermal syntheses are discussed separately and also in sections dealing with specific materials. Superconducting cuprates and intergrowth structures are discussed in separate sections. Synthesis of nanomaterials is dealt with in some detail. Synthetic methods for metal borides, carbides, nitrides, fluorides, sili-cides, phosphides and chalcogenides are also outlined. 

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