SH-process

Many SHS processes include an oxide as one of the reactant materials. As a convenience, all the elements to be discussed in the various SHS processes will be referred to as metals. The selection of a metal-metal oxide reacting combination is readily made without detailed thermodynamic considerations. Nevertheless, it is fruitful to examine the overall thermodynamics that govern the choice of a particular SHS combination. Consider the classic thermite reaction... 

Other manufacturing techniques include the SH-process invented by Schnurr, the K-process patented by Kndffler, the W-process patented by Wolfram, the E-process patented by Erbele, and the KA-method invented by Knoffler and Apel and also Bachmann. 

The SH-process involves continuous nitration of hexamethylenetetramine by concentrated nitric acid, with the production of nitrous gas. The RDX is filtered from the residual acid and stabilized by boiling in water under pressure and purified by recrystallization from acetone. 

Examples of products made by the SHS process include borides, carbides, chalcogenides, hydrides, intermelallic compounds, nitrides, silicides, carbonitrides, sulfides, cemented carbides (cermets), and various heterogeneous mixtures (microcomposiles). 

Nitrides such as TiN, AIN, ZrN, HfN, TaN, and Si3N4 and carbonitrides, such as TiCN, NbCN and ZrCN are among the important industrial products formed by the SHS process. Due to the high thermodynamic stability of titanium nitride and titanium carbide, their formation using the SHS method is highly favored even at relatively low pressures of nitrogen. 

Several different processes have been used, the simplest being by the reaction of hydrogen sulphide with molybdenum pentachloride, or the reaction of sulphur vapour with molybdic oxide or molybdenum metal. The last of these processes has been called the SHS process (Self-Propagating High-Temperature Synthesis) and Russian workers have reported that the product is less contaminated with impurities and has almost identical lubricating properties to natural molybdenum disulphide. The crystal structure is considered in more detail later, but it seems probable that the initial product of syntheses has a disordered... 

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... 

Because it is difficult to account for changes in the properties of the reaction medium (e.g., permeability, thermal conductivity, specific heat) due to structural transformations in the combustion wave, the models typically assume that these parameters are constant (Aldushin etai, 1976b Aldushin, 1988). In addition, the gas flow is generally described by Darcy s law. Convective heat transfer due to gas flow is accounted for by an effective thermal conductivity coefficient for the medium, that is, quasihomogeneous approximation. Finally, the reaction conditions typically associated with the SHS process (7 2(XX) K and p<10 MPa) allow the use of ideal gas law as the equation of state. 

Borovinskaya, I. P., Chemical classes of SHS processes and materials. Pure Appl. Chem., 64,919 (1992). 

Merzhanov, A. G., SHS-process Combustion theory and practice. Archivum Combustionis, 1, 24 (1981). 

Mukasyan, A. S., Structure- and phase-formation of nitrides in SHS processes. D.Sc. Dissertation, Institute of Structural Macrokinetics, Russian Academy of Science (1994). 

Osipov, E. Y., Levashov, Y. A., Chernyshev, V. N., Merzhanov, A. G., and Borovinskaya, I. R, Prospects for simultaneous use of vacuum-performed SHS processes and various hot rolling techniques for production of semifinished and finished items of ceramometallic or intermetallic composites. Int. J. SHS, 1,314 (1992). 

Sata, N., Nagata, K., Yanagisawa, N., Asano, O., Sanada, N., Hirano, T and Teraki, J., Research and development on functionally gradient materials by using a SHS process. Proceedings of the First US-Japanese Workshop on Combustion Synthesis, Tokyo, Japan, 139 (1990a). 

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). 

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