Self-propagating high-temperature synthesis
Self-propagating high-temperature synthesis (SHS) is a method for producing inorganic compounds by exothermic reactions, usually involving salts or pure metals. A variant of this method is known as solid state metathesis (SSM). Since the process occurs at high temperatures, the method is ideally suited for the production of refractory materials with unusual properties, for example: powders, metallic alloys, or ceramics with high purity, corrosion–resistance at high–temperature or super-hardnessity.
The modern SHS process was reported and patented in 1971, although some SHS-like processes were known previously.
In its usual format, SHS is conducted starting from finely powdered reactants that are intimately mixed. In some cases, the reagents are finely powdered whereas in other cases, they are sintered to minimize their surface area and prevent uninitiated exothermic reactions, which can be dangerous. In other cases, the particles are mechanically activated through techniques such as ball milling, which results in nanocomposite particles that contain both reactants within individual chemical cells. After reactant preparation, synthesis is initiated by point-heating of a small part (usually the top) of the sample. Once started, a wave of exothermic reaction sweeps through the remaining material. SHS has also been conducted with thin films, liquids, gases, powder–liquid systems, gas suspensions, layered systems, gas-gas systems, and others. Reactions have been conducted in a vacuum and under both inert or reactive gases. The temperature of the reaction can be moderated by the addition of inert salt that absorbs heat in the process of melting or evaporation, such as sodium chloride.
The reaction of alkali metal chalcogenides (S, Se, Te) and pnictides (N, P, As) with other metal halides produce the corresponding metal chalcogenides and pnictides. The synthesis of gallium nitride from gallium triiodide and lithium nitride is illustrative:
The process is so exothermic (ΔH = -515 kJ/mol) that the LiI evaporates, leaving a residue of GaN. With GaCl3 in place of GaI3, the reaction is so exothermic that the product GaN decomposes. Thus, the selection of the metal halide affects the success of the method.
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