View source for Self-propagating high-temperature synthesis

'''Self-propagating high-temperature synthesis''' (SHS) is a method for producing both [[inorganic compounds|inorganic]] and [[organic compounds|organic]] compounds by exothermic [[combustion]] reactions in solids of different nature.<ref>"Concise Encyclopedia of Self-Propagating High-Temperature Synthesis. History, Theory, Technology, and Products". 1st Ed., Editors: I.Borovinskaya, A.Gromov, E.Levashov et al., Imprint: Elsevier Science, 2017 [https://www.elsevier.com/books/concise-encyclopedia-of-self-propagating-high-temperature-synthesis/borovinskaya/978-0-12-804173-4]</ref> Reactions can occur between a solid reactant coupled with either a gas, liquid, or other solid. If the reactants, intermediates, and products are all solids, it is known as a solid flame.<ref>{{Cite journal|last1 = Mukasyan|first1 = Alexander S.|last2 = Shuck|first2 =Christopher E.|last3 = Pauls |first3 = Joshua M|last4 = Manukyan|first4 = Khachatur V.|date = 2018-12-02|title =The Solid Flame Phenomenon: A Novel Perspective|url = https://onlinelibrary.wiley.com/doi/full/10.1002/adem.201701065|journal=Advanced Engineering Materials|volume = 174|issue = 2–3|pages = 677–686|doi = 10.1016/j.cej.2011.09.028}}</ref> If the reaction occurs between a solid reactant and a gas phase reactant, it is called infiltration combustion. Since the process occurs at high temperatures, the method is ideally suited for the production of refractory materials including powders, metallic alloys, or ceramics.

The modern SHS process was reported and patented in 1971,<ref name="Merzhanov">"''Self-propagated high-temperature synthesis of refractory inorganic compounds''", A.G. Merzhanov, I.P. Borovinskaya. Doklady Akademii Nauk SSSR, Vol. 204, N 2, pp. 366-369, May, 1972</ref><ref>USSR Patent No. 255221, Byull. Izobr. No. 10</ref> although some SHS-like processes were known previously.

==Advantages and Disadvantages==
Self-propagating high-temperature synthesis is a green synthesis technique that is highly energy efficient, using little if any toxic solvents. There have been environmental analysis conducted to show that SHS has a lesser environmental impact than traditional solution-phase processing techniques.<ref>{{cite journal |last1=Pini |first1=Martina |last2=Rosa |first2=Roberto |last3=Neri |first3=Paolo |last4=Bondioli |first4=Federica |last5=Ferrari |first5=Anna Maria |title=Environmental assessment of a bottom-up hydrolytic synthesis of TiO nanoparticles |journal=Green Chemistry |date=2015 |volume=17 |issue=1 |pages=518–531 |doi=10.1039/C4GC00919C|hdl=11380/1074899 |hdl-access=free }}</ref> The technique uses less energy for production of materials, and the energy cost savings increase as synthesis batch sizes increase.

SHS is not a suitable technique for production of nanoparticles. Typically, the high-temperature nature of the process leads to particle sintering during and after the reaction. The high-temperatures generated during synthesis also lead to problems with energy dissipation and suitable reaction vessels, however, some systems use this excess heat to drive other plant-processes.

==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 [[Sintering|sintered]] to minimize their surface area and prevent uninitiated exothermic reactions, which can be dangerous.<ref name=Kaner/> In other cases, the particles are [[Mechanical Activation|mechanically activated]] through techniques such as high energy [[ball milling]] (e.g. in a planetary mill), which results in [[nanocomposite]] particles that contain both reactants within individual chemical cells.<ref>{{Cite journal|last1 = Mukasyan|first1 = Alexander S.|last2 = Khina|first2 = Boris B.|last3 = Reeves|first3 = Robert V.|last4 = Son|first4 = Steven F.|date = 2011-11-01|title = Mechanical activation and gasless explosion: Nanostructural aspects|journal = Chemical Engineering Journal|volume = 174|issue = 2–3|pages = 677–686|doi = 10.1016/j.cej.2011.09.028}}</ref><ref>{{Cite journal|last1 = Shuck|first1 = Christopher E.|last2 = Manukyan|first2 = Khachatur V.|last3 = Rouvimov|first3 = Sergei|last4 = Rogachev|first4 = Alexander S.|last5 = Mukasyan|first5 = Alexander S.|date = 2016-01-01|title = Solid-flame: Experimental validation|journal = Combustion and Flame|volume = 163|pages = 487–493|doi = 10.1016/j.combustflame.2015.10.025|doi-access = free}}</ref> 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]], or by adding "chemical oven"—a highly exothermic mixture—to decrease the ratio of cooling.<ref>{{cite journal|last1=Kurbatkina|first1=Viktoria|last2=Patsera|first2=Evgeny|last3=Levashov|first3= Evgeny|last4=Vorotilo|first4=Stepan|title=SHS Processing and Consolidation of Ta–Ti–C, Ta–Zr–C, and Ta–Hf–C Carbides for Ultra‐High‐Temperatures Application|journal=Advanced Engineering Materials|volume=20|issue=8|pages=1701065|date=2018|doi=10.1002/adem.201701065|doi-access=free}}</ref>

==Examples==
The reaction of alkali metal [[chalcogenide]]s (S, Se, Te) and [[pnictide]]s (N, P, As) with other metal halides produce the corresponding metal chalcogenides and pnictides.<ref>Richard G. Blair, Richard B. Kaner "Solid-State Metathesis Materials Synthesis" http://www.sigmaaldrich.com/sigma-aldrich/technical-documents/articles/chemfiles/solid-state-metathesis.html</ref>  The synthesis of [[gallium nitride]] from [[gallium triiodide]] and [[lithium nitride]] is illustrative:
:GaI<sub>3</sub>  +  Li<sub>3</sub>N   →  GaN  +  3 LiI
The process is so exothermic (ΔH = -515 kJ/mol) that the LiI evaporates, leaving a residue of GaN.  With GaCl<sub>3</sub> in place of GaI<sub>3</sub>, 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 [[molybdenum disulfide|MoS<sub>2</sub>]].  The reaction is conducted in a [[stainless steel]] reactor with excess Na<sub>2</sub>S.<ref name=Kaner>Philippe R. Bonneau, John B. Wiley, Richard B. Kaner "Metathetical Precursor Route to Molybdenum Disulfide" Inorganic Syntheses 1995, vol. 30, pp. 33–37. {{doi|10.1002/9780470132616.ch8}}</ref>

Self-propagating high-temperature synthesis can also be conducted in an artificial high gravity environment to control the phase composition of products.<ref>{{cite journal|last1=Yin|first1=Xi|last2=Chen|first2=Kexin|last3=Ning|first3=Xiaoshan|last4=Zhou|first4=Heping|title=Combustion Synthesis of Ti3SiC2/TiC Composites from Elemental Powders under High-Gravity Conditions|journal=Journal of the American Ceramic Society|date=2010|volume=93|issue=8|pages=2182–2187|doi=10.1111/j.1551-2916.2010.03714.x}}</ref>

SHS has been used to vitrify various nuclear waste streams including ashes from incineration, spent inorganic  ion exchangers such as clinoptilolite and contaminated soils.<ref>M.I. Ojovan, W.E. Lee. Self sustaining vitrification for immobilization of radioactive and toxic waste. Glass Technology, 44 (6) 218-224 (2003)</ref>

==Reaction Kinetics==
Due to the solid-state nature of SHS processes, it is possible to measure reaction kinetics ''in-situ'' using a variety of experimental techniques, including electrothermal explosion, [[differential thermal analysis]], combustion velocity approaches, among others.<ref>{{cite journal |last1=Mukasyan |first1=A. S. |last2=Shuck |first2=C. E. |title=Kinetics of SHS reactions: A review |journal=International Journal of Self-Propagating High-Temperature Synthesis |date=23 September 2017 |volume=26 |issue=3 |pages=145–165 |doi=10.3103/S1061386217030049|s2cid=139194446 }}</ref> There have been a variety of systems studied, including intermetallic, thermite, carbides, and others. Using SHS, it was shown that the particle size has a significant effect on the reaction kinetics.<ref>{{cite journal |last1=Hunt |first1=Emily M. |last2=Pantoya |first2=Michelle L.|author2-link=Michelle Pantoya |title=Ignition dynamics and activation energies of metallic thermites: From nano- to micron-scale particulate composites |journal=Journal of Applied Physics |date=August 2005 |volume=98 |issue=3 |pages=034909 |doi=10.1063/1.1990265}}</ref> It was further shown that these effects are related to the relationship between the surface area/volume ratio of the particles, and that the kinetics can be controlled ''via'' high-energy ball-milling.<ref>{{cite journal |last1=Shuck |first1=Christopher E. |last2=Mukasyan |first2=Alexander S. |title=Reactive Ni/Al Nanocomposites: Structural Characteristics and Activation Energy |journal=The Journal of Physical Chemistry A |date=February 2017 |volume=121 |issue=6 |pages=1175–1181 |doi=10.1021/acs.jpca.6b12314|pmid=28099018 }}</ref> Depending on the morphology of the reactants, it is possible to initiate a SHS reaction where a liquid phase occurs prior to phase formation or to directly result in solid-phase products without any melt.<ref>{{cite journal |last1=Mukasyan |first1=A.S. |last2=White |first2=J.D.E. |last3=Kovalev |first3=D.Y. |last4=Kochetov |first4=N.A. |last5=Ponomarev |first5=V.I. |last6=Son |first6=S.F. |title=Dynamics of phase transformation during thermal explosion in the Al–Ni system: Influence of mechanical activation |journal=Physica B: Condensed Matter |date=January 2010 |volume=405 |issue=2 |pages=778–784 |doi=10.1016/j.physb.2009.10.001}}</ref>

== References ==
{{reflist|30em}}

==External links==
*[http://ism.ac.ru/handbook/shsf.htm ISMAN] About SHS
*[https://web.archive.org/web/20100919000125/http://bentham.org/ebooks/9781608051557/contents.htm Combustion Synthesis] Info by ESA
*[http://www.spaceflight.esa.int/users/materials/research/combust/combust.html Combustion Synthesis] Bentham Ebook by Maximilian Lackner
*[http://www.vtnews.vt.edu/articles/2009/01/2009-2.html] Materials formed by SHS for needs of moon colonies.

[[Category:Solid-state chemistry]]