Editing Thermite (section)

== Types ==
[[Image:Thermite skillet.jpg|thumb|A thermite reaction taking place on a cast iron skillet]]
Red iron(III) oxide (Fe<sub>2</sub>O<sub>3</sub>, commonly known as [[rust]]) is the most common iron oxide used in thermite.<ref>{{cite web |url=https://news.google.com/newspapers?id=QKBQAAAAIBAJ&pg=6875,1422491 |title=Thermite Bombs used to Set Fires |publisher=The Milwaukee Journal |date=1 December 1939 |access-date=13 October 2011 }}{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }} (dead link 25 April 2020)</ref><ref>{{cite web|url=https://news.google.com/newspapers?id=lR8sAAAAIBAJ&pg=5630,1866720 |title=what it Means: Thermite Bombing |publisher=the Florence Times |date=31 August 1940 |access-date=12 October 2011}}</ref><ref>{{cite news|url=https://www.nytimes.com/1997/05/06/science/hydrogen-may-not-have-caused-hindenburg-s-fiery-end.html?pagewanted=all |title=Hydrogen May Not Have Caused Hindenburg's Fiery End |work=The New York Times |date=6 May 1997 |access-date=12 October 2011}}</ref> Black iron(II,III) oxide (Fe<sub>3</sub>O<sub>4</sub>, [[magnetite]]) also works.<ref name="amazingrust">{{cite web|url=http://amazingrust.com/experiments/how_to/thermite.html |title=Thermite |publisher=Amazing Rust.com |date=7 February 2001 |access-date=12 October 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110707122232/http://amazingrust.com/experiments/how_to/thermite.html |archive-date=7 July 2011 }}</ref> Other oxides are occasionally used, such as [[manganese(IV) oxide|MnO<sub>2</sub>]] in manganese thermite, [[chromium(III) oxide|Cr<sub>2</sub>O<sub>3</sub>]] in chromium thermite, SiO<sub>2</sub> (quartz) in silicon thermite, or copper(II) oxide in copper thermite, but only for specialized purposes.<ref name="amazingrust"/> All of these examples use aluminum as the reactive metal. [[Fluoropolymer]]s can be used in special formulations, [[Polytetrafluoroethylene|Teflon]] with magnesium or aluminum being a relatively common example. [[Magnesium/Teflon/Viton]] is another [[pyrolant]] of this type.<ref>{{cite journal |doi=10.1002/1521-4087(200211)27:5<262::AID-PREP262>3.0.CO;2-8 |title=Metal-Fluorocarbon-Pyrolants: III. Development and Application of Magnesium/Teflon/Viton (MTV) |journal=Propellants, Explosives, Pyrotechnics |volume=27 |issue=5 |pages=262–266 |year=2002 |last1=Koch |first1=Ernst-Christian}}</ref>

Combinations of dry ice (frozen carbon dioxide) and reducing agents such as magnesium, aluminum and boron follow the same chemical reaction as with traditional thermite mixtures, producing metal oxides and carbon. Despite the very low temperature of a dry ice thermite mixture, such a system is capable of being ignited with a flame.<ref>{{cite web |url=https://www.youtube.com/watch?v=_xCbal2YyaE |archive-url=https://ghostarchive.org/varchive/youtube/20211211/_xCbal2YyaE |archive-date=2021-12-11 |url-status=live |title=Burning magnesium in dry ice |date=September 2011 |publisher=Royal Society of Chemistry |via=YouTube}}{{cbignore}}</ref> When the ingredients are finely divided, confined in a pipe and armed like a traditional explosive, this cryo-thermite is detonatable and a portion of the carbon liberated in the reaction emerges in the form of [[diamond]].<ref>{{Cite web |url=http://brevets-patents.ic.gc.ca/opic-cipo/cpd/eng/patent/2710026/summary.html |title=Method For Creating Diamonds |last=Swanson |first=Daren |date=2007-12-21 |website=www.EnviroDiamond.com |publisher=Daren Swanson |access-date=17 October 2016 |archive-date=18 October 2016 |archive-url=https://web.archive.org/web/20161018221400/http://brevets-patents.ic.gc.ca/opic-cipo/cpd/eng/patent/2710026/summary.html |url-status=dead }}</ref>

In principle, any reactive metal could be used instead of aluminum. This is rarely done, because the properties of aluminum are nearly ideal for this reaction:

* It forms a [[passivation (chemistry)|passivation]] layer making it safer to handle than many other reactive metals.<ref>{{cite journal |doi=10.1023/B:JMSC.0000044879.63364.b3 |title=The role of the Al<sub>2</sub>O<sub>3</sub> passivation shell surrounding nano-Al particles in the combustion synthesis of NiAl |year=2004 |last1=Granier |first1=J. J. |last2=Plantier |first2=K. B. |last3=Pantoya |first3=M. L. |author3-link=Michelle Pantoya |journal=Journal of Materials Science |volume=39 |issue=21 |pages=6421 |bibcode=2004JMatS..39.6421G |s2cid=137141668}}</ref>
* Its relatively low [[melting point]] (660&nbsp;°C) means that it is easy to melt the metal, so that the reaction can occur mainly in the liquid phase, thus it proceeds fairly quickly.
* Its high [[boiling point]] (2519&nbsp;°C) enables the reaction to reach very high temperatures, since several processes tend to limit the maximum temperature to just below the boiling point. Such a high boiling point is common among transition metals (e.g., iron and copper boil at 2887 and 2582&nbsp;°C, respectively), but is especially unusual among the highly reactive metals (cf. magnesium and [[sodium]], which boil at 1090 and 883&nbsp;°C, respectively).
* Further, the low density of the aluminum oxide formed as a result of the reaction tends to leave it floating on the resultant pure metal. This is particularly important for reducing contamination in a weld.

Although the reactants are stable at room temperature, they burn with an extremely intense [[exothermic reaction]] when they are heated to ignition temperature. The products emerge as liquids due to the high temperatures reached (up to 2500&nbsp;°C (4532°F) with iron(III) oxide)—although the actual temperature reached depends on how quickly heat can escape to the surrounding environment. Thermite contains its own supply of oxygen and does not require any external source of air. Consequently, it cannot be smothered, and may ignite in any environment given sufficient initial heat. It burns well while wet, and cannot be easily extinguished with water—though enough water to remove sufficient heat may stop the reaction.<ref>{{cite journal|doi=10.1016/S0377-0273(01)00280-3 |title=Water/magma interaction: some theory and experiments on peperite formation|year=2002|last1=Wohletz|first1=Kenneth|journal=Journal of Volcanology and Geothermal Research|volume=114|issue=1–2|pages=19–35|bibcode = 2002JVGR..114...19W |url=https://zenodo.org/record/1260035}}</ref> Small amounts of water boil before reaching the reaction. Even so, thermite is used for [[hyperbaric welding|welding under water]].<ref>{{cite news|author=Sarah Lyall |url=https://www.nytimes.com/2006/10/27/world/europe/27camera.html?pagewanted=all |title=Cameras Catch Speeding Britons and Lots of Grief |work=The New York Times |date=27 October 2006 |access-date=12 October 2011}}</ref>

The thermites are characterized by almost complete absence of gas production during burning, high reaction temperature, and production of molten [[slag]]. The fuel should have high [[heat of combustion]] and produce oxides with low melting point and high boiling point. The oxidizer should contain at least 25% oxygen, have high density, low heat of formation, and produce metal with low melting and high boiling points (so the energy released is not consumed in evaporation of reaction products). Organic binders can be added to the composition to improve its mechanical properties, but they tend to produce endothermic decomposition products, causing some loss of reaction heat and production of gases.<ref name="pyrochem"/>

The temperature achieved during the reaction determines the outcome. In an ideal case, the reaction produces a well-separated melt of metal and slag. For this, the temperature must be high enough to melt both reaction products, the resulting metal and the fuel oxide. Too low a temperature produces a mixture of sintered metal and slag; too high a temperature (above the boiling point of any reactant or product) leads to rapid production of gas, dispersing the burning reaction mixture, sometimes with effects similar to a low-yield explosion. In compositions intended for production of metal by [[aluminothermic reaction]], these effects can be counteracted. Too low a reaction temperature (e.g., when producing silicon from sand) can be boosted with addition of a suitable oxidizer (e.g., sulfur in aluminum-sulfur-sand compositions); too high a temperature can be reduced by using a suitable coolant or slag [[Flux (metallurgy)|flux]]. The flux often used in amateur compositions is [[calcium fluoride]], as it reacts only minimally, has relatively low melting point, low melt viscosity at high temperatures (therefore increasing fluidity of the slag) and forms a eutectic with alumina. Too much flux, however, dilutes the reactants to the point of not being able to sustain combustion. The type of metal oxide also has dramatic influence to the amount of energy produced; the higher the oxide, the higher the amount of energy produced. A good example is the difference between [[manganese(IV) oxide]] and [[manganese(II) oxide]], where the former produces too high temperature and the latter is barely able to sustain combustion; to achieve good results, a mixture with proper ratio of both oxides can be used.<ref>{{cite web |url=http://developing-your-web-presence.blogspot.com/2008/07/manganese-thermite-based-on-manganese.html |title=Manganese thermite based on manganese (II) oxide |publisher=Developing your Web presence |date=10 July 2008 |access-date=7 December 2011}}</ref>

The reaction rate can be also tuned with particle sizes; coarser particles burn slower than finer particles. The effect is more pronounced with the particles requiring heating to higher temperature to start reacting. This effect is pushed to the extreme with [[nano-thermite]]s.

The temperature achieved in the reaction in [[adiabatic process|adiabatic conditions]], when no heat is lost to the environment, can be estimated using [[Hess’s law]] – by calculating the energy produced by the reaction itself (subtracting the [[enthalpy]] of the reactants from the enthalpy of the products) and subtracting the energy consumed by heating the products (from their specific heat, when the materials only change their temperature, and their [[enthalpy of fusion]] and eventually [[enthalpy of vaporization]], when the materials melt or boil). In real conditions, the reaction loses heat to the environment, the achieved temperature is therefore somewhat lower. The heat transfer rate is finite, so the faster the reaction is, the closer to adiabatic condition it runs and the higher is the achieved temperature.<ref>{{cite book|author=Gupta, Chiranjib Kumar |title=Chemical Metallurgy: Principles and Practice|url=https://books.google.com/books?id=Tq6MTFXk3cQC&pg=PA387 |date= 2006|publisher=John Wiley & Sons|isbn=978-3-527-60525-5|pages=387–}}</ref>

=== Iron thermite ===
The most common composition is iron thermite. The oxidizer used is usually either [[iron(III) oxide]] or [[iron(II,III) oxide]]. The former produces more heat. The latter is easier to ignite, likely due to the crystal structure of the oxide. Addition of copper or manganese oxides can significantly improve the ease of ignition.
The density of prepared thermite is often as low as 0.7&nbsp;g/cm<sup>3</sup>. This, in turn, results in relatively poor energy density (about 3 kJ/cm<sup>3</sup>), rapid burn times, and spray of molten iron due to the expansion of trapped air. Thermite can be pressed to densities as high as 4.9&nbsp;g/cm<sup>3</sup> (almost 16&nbsp;kJ/cm<sup>3</sup>) with slow burning speeds (about 1&nbsp;cm/s). Pressed thermite has higher melting power, i.e. it can melt a steel cup where a low-density thermite would fail.<ref>{{cite journal |last1=Elshenawy |first1=Tamer |last2=Soliman |first2=Salah |last3=Hawass |first3=Ahmed |title=High density thermite mixture for shaped charge ordnance disposal |journal=Defence Technology |date=October 2017 |volume=13 |issue=5 |pages=376–379 |doi=10.1016/j.dt.2017.03.005 |doi-access=free }}</ref> Iron thermite with or without additives can be pressed into cutting devices that have heat-resistant casing and a nozzle.<ref>{{Cite web|url=https://empi-inc.com/tec-torch/|title = TEC Torch - Energetic Materials & Products, Inc. - Central Texas}}</ref>
Oxygen-balanced iron thermite 2Al + Fe<sub>2</sub>O<sub>3</sub> has theoretical maximum density of 4.175&nbsp;g/cm<sup>3</sup> an adiabatic burn temperature of 3135 K or 2862&nbsp;°C or 5183&nbsp;°F (with [[phase transition]]s included, limited by iron, which boils at 3135&nbsp;K), the aluminum oxide is (briefly) molten and the produced iron is mostly liquid with part of it being in gaseous form - 78.4 g of iron vapor per kg of thermite are produced. The energy content is 945.4&nbsp;cal/g (3 956 J/g). The energy density is 16,516&nbsp;J/cm<sup>3</sup>.<ref name="osti.gov">{{Cite conference |url=https://www.osti.gov/servlets/purl/372665 |title=A survey of combustible metals, thermites, and intermetallics for pyrotechnic applications |date=August 1996 |last1=Fischer |first1=S. H. |last2=Grubelich |first2=M. C. |conference=32. AIAA/ASME/SAE/ASEE joint propulsion conference and exhibit}}</ref>

The original mixture, as invented, used iron oxide in the form of [[mill scale]]. The composition was very difficult to ignite.<ref name="pyrochem">{{cite book |author1=K. Kosanke |author2=B. J. Kosanke |author3=I. von Maltitz |author4=B. Sturman |author5=T. Shimizu |author6=M. A. Wilson |author7=N. Kubota |author8=C. Jennings-White |author9=D. Chapman |title=Pyrotechnic Chemistry |url=https://books.google.com/books?id=Q1yJNr92-YcC&pg=PA126 |access-date=9 January 2012 |date=December 2004 |publisher=Journal of Pyrotechnics |isbn=978-1-889526-15-7 |pages=126–}}</ref>

=== Copper thermite ===
Copper thermite can be prepared using either [[copper(I) oxide]] (Cu<sub>2</sub>O, red) or [[copper(II) oxide]] (CuO, black). The burn rate tends to be very fast and the melting point of copper is relatively low, so the reaction produces a significant amount of molten copper in a very short time. Copper(II) thermite reactions can be so fast that it can be considered a type of [[flash powder]]. An explosion can occur, which sends a spray of copper drops to considerable distances.<ref name="pyroguide">{{cite web |url=http://www.pyroguide.com/index.php?title=Thermite |title=Thermite |publisher=PyroGuide |date=3 March 2011 |access-date=6 December 2011 |archive-date=6 April 2012 |archive-url=https://web.archive.org/web/20120406224436/http://www.pyroguide.com/index.php?title=Thermite |url-status=dead }}</ref>
Oxygen-balanced mixture has theoretical maximum density of 5.109&nbsp;g/cm<sup>3</sup>, adiabatic flame temperature 2843 K (phase transitions included) with the aluminum oxide being molten and copper in both liquid and gaseous form; 343&nbsp;g of copper vapor per kg of this thermite are produced. The energy content is 974 cal/g.<ref name="osti.gov"/>

Copper(I) thermite has industrial uses in e.g., welding of thick copper conductors ([[cadwelding]]). This kind of welding is being evaluated also for cable splicing on the US Navy fleet, for use in high-current systems, e.g., electric propulsion.<ref>{{cite web|url=http://hts.asminternational.org/portal/site/hts/NewsItem/?vgnextoid=a7879c63e1681310VgnVCM100000621e010aRCRD |title=HTS > News Item |publisher=Hts.asminternational.org |date=1 August 2011 |access-date=6 December 2011}}</ref>
Oxygen-balanced mixture has theoretical maximum density of 5.280&nbsp;g/cm<sup>3</sup>, adiabatic flame temperature 2843&nbsp;K (phase transitions included) with the aluminum oxide being molten and copper in both liquid and gaseous form; 77.6&nbsp;g of copper vapor per kg of this thermite are produced. The energy content is 575.5&nbsp;cal/g.<ref name="osti.gov"/>

=== Thermates ===
{{main|Thermate}}
Thermate composition is a thermite enriched with a salt-based oxidizer (usually nitrates, e.g., [[barium nitrate]], or peroxides). In contrast with thermites, thermates burn with evolution of flame and gases. The presence of the oxidizer makes the mixture easier to ignite and improves penetration of target by the burning composition, as the evolved gas is projecting the molten slag and providing mechanical agitation.<ref name="pyrochem"/> This mechanism makes thermate more suitable than thermite for [[incendiary device|incendiary purposes]] and for emergency destruction of sensitive equipment (e.g., cryptographic devices), as thermite's effect is more localized.{{fact|date=March 2023}}