Thermite
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Image:Thermite mix.jpg A thermite reaction (a type of aluminothermic reaction) is one in which aluminium metal is oxidized by the oxide of another metal, most commonly iron oxide. The name thermite is also used to refer to a mixture of two such chemicals. The products are aluminium oxide, free elemental iron, and a great deal of heat. The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation.
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Types
Iron oxide (also known as "Rust") is the most commonly used oxide ingredient, because it is inexpensive, and molten iron is useful for welding. Other oxides are occasionally used for special purposes. In principle, the aluminium could also be replaced by any other reactive metal.
However this is almost never done because the properties of aluminium are ideal for this reaction. For one thing, it is by far the cheapest of the highly reactive metals. Also, many other possible candidates do not form a passivation layer as aluminium does, and consequently are much more dangerous to handle. For the purposes of this reaction the most important properties of aluminium are its relatively low melting point (660 °C, 1221 °F) and very high boiling point (2519°C, 4566°F). A low melting point means that it is easy to melt the metal, so that the reaction can occur mainly in the liquid phase (or rather, where the solid oxide particles meet the liquid metal) and thus proceeds fairly quickly. On the other hand, a high boiling point enables the reaction to reach very high temperatures, since several processes (such as loss of the fuel, and the heat it carries, as it boils away) would tend to limit the maximum temperature to just below the boiling point. This 1900 kelvin range for the liquid phase is quite broad for any common metals (e.g. iron and copper have a liquid phase of respectively 1320 and 1477 K), but is especially unusual among the highly reactive metals (c.f. magnesium and sodium, respectively 440 and 785 K)
Although the reactants are stable at room temperature, when they are exposed to sufficient heat to ignite (usually by igniting with a burning magnesium ribbon, but other methods are used as well, such as potassium permanganate and glycerine) they burn with an extremely intense exothermic reaction. The products emerge as liquids due to the high temperatures reached (with iron (III) oxide, up to 2500°C (4500°F)— 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 such as air. Consequently, it cannot be smothered and may ignite in any environment, given sufficient initial heat. It will burn just as well while underwater, for example, and cannot even be extinguished with water, as water sprayed on a thermite reaction will instantly be boiled into steam. This, combined with the extremely high temperatures generated, makes thermite reactions extremely hazardous even when appropriate precautions are taken.
Uses
Image:Utah-thermite.jpg Thermite reactions have many uses. It was originally used for repair welding in-place such things as locomotive axle-frames where the repair can take place without removing the part from its installed location. Thermite grenades are used in war to destroy sensitive equipment or documents when at imminent risk of capture by the enemy. Thermite grenades and bombs have been used in combat as incendiary devices, able to burn through heavy armor or other fireproof barriers. Thermite can also be used for quickly cutting or welding metal such as rail tracks, without requiring complex or heavy equipment. It also enables infantry to destroy enemy artillery without the use of loud explosive charges and therefore operate with stealth.
This type of reaction when used to purify the ores of some metals is called the Thermite process. An adaptation of the thermite 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.
The thermite reaction can take place by accident in industrial locations where abrasive grinding and cutting wheels are used with ferrous metals. Using aluminium in this situation produces an admixture of oxides which is capable of violent explosive reaction.
When thermite is made using iron (III) oxide, for maximum efficiency it should contain, by mass, 25.3% aluminium and 74.7% iron oxide. (This mixture is sold under the brand name Thermit as a heat source for welding.) The complete formula for the reaction using iron (III) oxide is as follows:
Thermite should not be confused with a thermal lance.
History
Thermite was invented in 1893 and patented in 1895 by German chemist Dr. Hans Goldschmidt, in consequence of which the reaction is sometimes called the "Goldschmidt reaction" or "Goldschmidt process". Dr. Goldschmidt was originally interested in producing high purity metals by avoiding the use of carbon in smelting, but 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.
See also
External links
- Thermit Welding LTD
- Video of a thermite reaction
- Video of thermite burning through car engine block
- History of thermite, from Degussa, successor of Dr. Goldschmidt's company
- Steel casting with thermite videode:Thermit
nl:Thermiet ja:テルミット法 pl:Termit (pirotechnika) pt:Termite ru:Термитная смесь tr:Termit reaksiyonu