Self-propagating high-temperature synthesis
Encyclopedia
Self-propagating high-temperature synthesis (SHS) is a method for producing inorganic compound
s by exothermic reactions, usually involving salts. A variant of this method is known as solid state metathesis (SSM). Since the process occurs at high temperatures, the method is possible to ideally suited for the production of refractory materials such unusual properties, for example powders, metallic alloys, 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.
.
s (S, Se, Te) and pnictides (N, P, As) with other metal halides produce the correspond 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 reside 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.
Other compounds prepared by this method include metal dichalcogenides such as MoS2
. The reaction is conducted in a stainless steel
reactor with excess Na2S.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
s by exothermic reactions, usually involving salts. A variant of this method is known as solid state metathesis (SSM). Since the process occurs at high temperatures, the method is possible to ideally suited for the production of refractory materials such unusual properties, for example powders, metallic alloys, 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.
Methodology
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. The 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 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 chlorideSodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
.
Examples
The reaction of alkali metal chalcogenideChalcogenide
A chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term is more commonly reserved for sulfides, selenides, and tellurides, rather than...
s (S, Se, Te) and pnictides (N, P, As) with other metal halides produce the correspond metal chalcogenides and pnictides. The synthesis of gallium nitride from gallium triiodide and lithium nitride
Lithium nitride
Lithium nitride is a compound of lithium and nitrogen with the formula Li3N. It is the only stable alkali metal nitride...
is illustrative:
- GaI3 + Li3N → GaN + 3 LiI
The process is so exothermic (ΔH = -515 kJ/mol) that the LiI evaporates, leaving a reside 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.
Other compounds prepared by this method include metal dichalcogenides such as MoS2
Molybdenum disulfide
Molybdenum disulfide is the inorganic compound with the formula MoS2. This black crystalline sulfide of molybdenum occurs as the mineral molybdenite. It is the principal ore from which molybdenum metal is extracted. The natural amorphous form is known as the rarer mineral jordisite. MoS2 is less...
. The reaction is conducted in a stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
reactor with excess Na2S.
External links
- ISMAN About SHS
- Combustion Synthesis Info by ESA
- Combustion Synthesis Bentham Ebook by Maximilian Lackner