Thermite
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[[Image:Thermite mix.jpg|thumb|A thermite mixture using Iron (III) Oxide]]
'''Thermite''' is a [[pyrotechnic composition]] of [[aluminium]] powder and a [[rust|metal oxide]] which produces an [[aluminothermic reaction]] known as a '''thermite reaction'''. It is not explosive, but can create short bursts of extremely high temperatures focused on a very small target for a short period of time.
The aluminium reduces the oxide of another [[metal]], most commonly [[iron oxide]], because aluminium is highly combustible. The products are [[aluminium oxide]], free elemental [[iron]], and a large amount of [[heat]]. The reactants are commonly [[Powder (substance)|powdered]] and mixed with a binder to keep the material solid and prevent separation.
The reaction is used for [[thermite welding]], often used to join [[rail track]]s. Other metal oxides can be used, such as [[chromium oxide]], to generate elementary metal. [[Copper]] thermite, using copper oxide, is used for creating electric joints in a process called [[cadwelding]]. Some thermite-like mixtures are used as [[pyrotechnic initiator]]s such as [[fireworks]].
== History ==
[[Image:ThermiteFe2O3.JPG|thumb|right|A thermite reaction using Iron(III) Oxide]]
Thermite (Thermit) was invented in 1893 and [[patent]]ed in 1895 by [[Germany|German]] [[chemist]] [[Hans Goldschmidt]]. Consequently, the reaction is sometimes called the "Goldschmidt reaction" or "Goldschmidt process". Dr. Goldschmidt was originally interested in producing very pure metals by avoiding the use of [[carbon]] in smelting, but he soon realized the value in [[welding]].
The first commercial application was the welding of [[tram]] tracks in [[Essen]], in 1899. [[Degussa]], a corporate descendant of Goldschmidt's firm, is still today one of the world's largest producers of welding thermite.
== Types ==
[[Image:Thermite skillet.jpg|thumb|left|A thermite reaction taking place on a cast iron skillet.]]
[[Iron(II,III) oxide|Black or blue iron oxide]] (Fe<sub>3</sub>O<sub>4</sub>), produced by oxidizing iron in an oxygen-rich environment under high heat, is the most commonly used thermite oxidizing agent because it is inexpensive and easily produced. Red [[iron(III) oxide]] (Fe<sub>2</sub>O<sub>3</sub>, commonly known as [[rust]]) can also be used. 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, or [[copper(II) oxide]] in copper thermite, but only for highly specialised purposes. All examples use aluminium as the reactive metal. [[Fluoropolymers]] can be used in special formulations, [[Polytetrafluoroethylene|Teflon]] with [[magnesium]] or [[aluminium]] being a relatively common example. [[Magnesium/teflon/viton]] is another [[pyrolant]] of this type.
In principle, any reactive metal could be used instead of aluminium. This is rarely done, however, because the properties of aluminium are ideal for this reaction. It is by far the cheapest of the highly reactive metals; it also forms a [[passivation]] layer making it safer to handle than many other reactive metals. The [[melting point|melting]] and [[boiling point|boiling]] points of aluminium also make it ideal for thermite reactions. Its relatively low melting point (660 [[Celsius|°C]], 1221 [[Fahrenheit|°F]]) means that it is easy to melt the metal, so that the reaction can occur mainly in the liquid phase<ref name="liquid_phase">or rather, where the solid oxide particles meet the liquid metal</ref> and thus proceeds fairly quickly. At the same time, its high boiling point (2519 °C, 4566 °F) enables the reaction to reach very high temperatures, since several processes tend to limit the maximum temperature to just below the boiling point.<ref name="boiling_point">i.e. loss of fuel and heat due to [[vaporization]]</ref> Such a high boiling point is common among transition metals (e.g. iron and [[copper]] boil at 2887 °C and 2582 °C respectively), but is especially unusual among the highly reactive metals (cf. [[magnesium]] and [[sodium]] which boil at 1090 °C and 883 °C respectively). Further, the low density of the aluminium oxide formed as a result of the reaction tends to cause it to float on the iron, reducing contamination of the 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 °C (4500 °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 will burn well while wet and cannot be extinguished with water. Small amounts of water will boil before reaching the reaction. If thermite is ignited underwater, the molten iron produced will extract oxygen from water and generate hydrogen gas in a single-replacement reaction. This gas may, in turn, burn by combining with oxygen in the air.
== Ignition ==
[[Metals]] are capable of burning under the right conditions, similarly to the [[combustion]] process of wood or gasoline. In fact, [[rust]] is the result of [[oxidation]] of [[steel]] or [[iron]] at very slow rates. A thermite reaction is a process in which the correct mixture of metallic fuels are combined and ignited. Ignition itself requires extremely high temperatures.
Ignition of a thermite reaction normally requires supervision by a trained technician, and may require persistent efforts, as ignition can be unreliable and unpredictable. Thermite reactions require very high temperatures for initiation. These temperatures cannot be reached with conventional [[black powder]] [[fuse (explosive)|fuses]], [[nitrocellulose]] rods, [[detonator]]s, a suitable [[pyrotechnic initiator]], or other common igniting substances. Even when the thermite is hot enough to glow bright red, it will not ignite as it must be at or near white-hot to initiate the reaction. It is possible to start the reaction using a [[propane torch]] if done correctly. The torch can preheat the entire pile of thermite which will make it explode instead of burning slowly when it finally reaches ignition temperature.
Often, strips of [[magnesium]] metal are used as fuses. Because metals burn without releasing cooling gases, they can potentially burn at extremely high temperatures. Reactive metals such as magnesium can easily reach temperatures sufficiently high for thermite ignition. However, this method is notoriously unreliable: magnesium itself is difficult to ignite, and in windy or wet conditions the strip may be extinguished. Also, magnesium strips do not contain their own source of oxygen so combustion cannot occur unless the magnesium strips are exposed to air. A significant danger of magnesium ignition is the fact that the metal is an excellent conductor of heat; heating one end of the ribbon may cause the other end to transfer enough heat to the thermite to cause premature ignition. Despite these issues, magnesium ignition remains popular amongst amateur thermite users, mainly because it can be easily obtained.
The reaction between [[potassium permanganate]] and [[glycerine]] or [[ethylene glycol]] is used as an alternative to the magnesium method. When these two substances mix, a spontaneous reaction will begin, slowly increasing the temperature of the mixture until flames are produced. The heat released by the oxidation of glycerine is sufficient to initiate a thermite reaction. However, this method can also be unreliable and the delay between mixing and ignition can vary greatly due to factors such as particle size and ambient temperature.
Apart from magnesium ignition, some amateurs also choose to use [[sparkler]]s to ignite the thermite mixture. These reach the necessary temperatures and provide enough time before the burning point reaches the sample. However, this can be a dangerous method, as the iron sparks, like the magnesium strips, burn at thousands of degrees and can ignite the thermite even though the sparkler itself is not in contact with it. This is especially dangerous with finely powdered thermite.
Similarly, finely-powdered thermite can be ignited by a regular [[flint spark lighter]], as the sparks are burning metal (in this case, the highly-reactive [[rare-earth metals]] [[lanthanum]] and [[cerium]]). Therefore it is unsafe to strike a lighter close to thermite.
A [[stoichiometric]] mixture of finely powdered iron(III) oxide and aluminium may be ignited using ordinary red-tipped book matches by partially embedding one match head in the mixture, and igniting that match head with another match, preferably held with tongs in gloves to prevent [[flash burn]]s.
== Civilian uses ==
[[Image:Velp-thermitewelding-1.jpg|thumb|Thermite reaction proceeding for a railway welding. Shortly after this, the liquid iron flows into the mould around the rail gap.]]
[[Image:Thermite residues (railway welding).JPG|thumb|Residues of burnt thermite like these, left by railway workers, can be found along tracks.]]
[[Image:Utah-thermite.jpg|thumb|The violent effects of thermite demonstrated in the [[Utah]] desert]]
Thermite reactions have many uses. Thermite is not an explosive but instead operates by increasing a very small area of metal to extremely high temperatures. Intense heat focused on a small spot can be used to cut through metal or weld metal components together by melting a very thin film where the components meet.
Thermite may be used for repair by the welding in-place of thick steel sections such as [[locomotive]] [[axle]]-frames where the repair can take place without removing the part from its installed location. It can also be used for quickly cutting or [[welding]] steel such as [[rail tracks]], without requiring complex or heavy equipment. However, defects such as slag inclusions and holes are often present in such welded junctions and great care is needed to operate the process successfully.
A thermite reaction, when used to purify the [[ore]]s of some metals, is called the Thermite process, or [[aluminothermic reaction]]. An adaptation of the reaction, used to obtain pure [[uranium]], was developed as part of the [[Manhattan Project]] at [[Ames Laboratory]] under the direction of [[Frank Spedding]]. It is sometimes called the [[Ames process]].
Copper thermite is used for welding together thick copper wires for the purpose of electrical connections. It is used extensively by the electrical utilities and telecommunications industries ([[Exothermic Welded Connections]]).
Thermite's main ingredients were also utilized for their individual qualities, specifically reflectivity and heat insulation, in a paint coating or [[Aircraft dope|dope]] for the [[Hindenburg]], possibly contributing to its fiery destruction. This was a theory put forward by former [[NASA]] scientist [[Addison Bain]], and later tested in small scale by the [[Mythbusters]] with semi-inconclusive results (it wasn't proven to be the thermite reactions fault but instead was conjectured to be a mix between that and the [[Hydrogen]] filling the Hindenburg).
== Military uses ==
Thermite [[hand grenade]]s are used as [[incendiary device]]s to destroy enemy equipment quickly. Additionally, thermite grenades are used by friendly forces to destroy their own items and equipment when there is imminent danger of capture. Because standard iron-thermite is difficult to ignite, burns with practically no flame and has a small radius of action, standard thermite is rarely used on its own as an incendiary composition. It is more usually employed with other ingredients added to enhance its incendiary effects. [[thermate|Thermate-TH3]] is a mixture of thermite and pyrotechnic additives which have been found to be superior to standard thermite for incendiary purposes. Its composition by weight is generally 68.7% thermite, 29.0% [[barium nitrate]], 2.0% [[sulfur]] and 0.3% [[binder (material)|binder]] (such as [[PBAN]]). Addition of barium nitrate to thermite increases its thermal effect, creates flame in burning and significantly reduces the ignition temperature. Although the primary purpose of Thermate-TH3 is as an incendiary, it will also [[welding|weld]] metal surfaces together.
A classic military use for thermite is disabling [[artillery]] pieces and it has been used for this purpose since the [[World War II|Second World War]]. Thermite can permanently disable artillery pieces without the use of explosive charges and therefore can be used with a reasonable amount of stealth. There are several ways to do this. By far the most destructive method is to weld the weapon shut by inserting one or more armed thermite grenades into the [[Breech-loading weapon|breech]] and then quickly closing it. This makes the weapon impossible to load. An alternative method is to insert an armed thermite grenade down the muzzle of the artillery piece, fouling the barrel. This makes the piece very dangerous to fire. Yet another method is to use thermite to weld the traversing and elevation mechanism of the weapon, making it impossible to aim properly.
Thermite was also used in both German and Allied [[incendiary bomb]]s during [[WWII]]. Incendiary bombs usually consisted of dozens of thin thermite-filled canisters ([[bomblet]]s) ignited by a magnesium fuse. Incendiary bombs destroyed entire cities due to the raging fires that resulted from their use. Cities that primarily consisted of wooden buildings were especially susceptible. These incendiary bombs were utilized primarily during night time air raids. Bomb sights could not be used at night, creating the need to use munitions that could destroy targets without the need for precision placement.
== Hazards ==
Thermite usage is hazardous due to the extremely high temperatures produced and the extreme difficulty in smothering a reaction once initiated. The thermite reaction releases dangerous ultra-violet (UV) light requiring that the reaction not be viewed directly, or that special eye protection (for example, a welder's mask) be worn. Small streams of molten iron released in the reaction can travel considerable distances and may melt through metal containers, igniting their contents. Additionally, flammable metals with relatively low boiling points such as [[zinc]], whose boiling point of 907 °C (1665 °F) is about 1370 °C (2500 °F) below the temperature at which thermite burns, could potentially boil superheated metal violently into the air if near a thermite reaction, where it could then burst into flame as it is exposed to oxygen.
Preheating of thermite before ignition can easily be done accidentally, for example by pouring a new pile of thermite over a hot, recently-ignited pile of thermite [[slag]]. When ignited, preheated thermite can burn almost instantaneously, releasing a much greater amount of light and heat energy than normal and causing burns and eye damage at what would normally be a reasonably safe distance.
The thermite reaction can take place accidentally in industrial locations where abrasive [[grinding wheel|grinding and cutting wheels]] are used with [[ferrous metal]]s. Using aluminium in this situation produces an admixture of oxides which is capable of a violent explosive reaction.<ref>[http://www.hanford.gov/rl/?page=542&parent=506 Fireball from Aluminium Grinding Dust]</ref>
Mixing water with thermite or pouring water onto burning thermite can cause a [[phreatomagmatic]] explosion, spraying hot fragments in all directions.
== Notes ==
{{reflist}}
== References ==
{{refbegin}}
* {{cite journal | author = L. L. Wang, Z. A. Munir and Y. M. Maximov | title = Thermite reactions: their utilization in the synthesis and processing of materials | journal = Journal of Materials Science | year = 1993 | volume = 28 | issue = 14 | pages = 3693–3708 | doi = 10.1007/BF00353167 }}
* {{cite journal | author = M. Beckert | title = Hans Goldschmidt and the aluminothermics | journal = Schweissen und Schneiden | year = 2002 | volume = 54 | issue = 9 | pages = 522–526 }}
* {{ cite web | url = http://www.degussa-history.com/geschichte/en/inventions/thermite/ | title = DEGUSSA page on thermite }}
{{refend}}
==External links==
* [http://www.amazingrust.com/Experiments/how_to/Thermite_pics-videos.html Thermite Pictures & Videos (Including Exotic Thermite)]
* [http://www.theodoregray.com/PeriodicTable/Samples/026.17/index.video.s12.html Video - steel casting with thermite]
* [http://www.degussa-history.com/geschichte/en/inventions/thermite.html History of thermite]
[[Category:Welding]]
[[Category:Inorganic reactions]]
[[Category:Incendiary weapons]]
[[Category:Pyrotechnic compositions]]
[[Category:Aluminium]]
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