A mixture of aluminium and iron oxide which, if ignited, undergoes a fierce chemical reaction producing a high temperature (c.2400°C) and yielding molten iron (or other metal if another oxide is used). The process was devised in 1895 by German chemist Hans Goldschmidt (18611923). It is useful for the preparation of intractable metals, or in welding, and has been used for incendiary bombs.
A thermite reaction is a type of aluminothermic reaction in which aluminium metal is oxidized by the oxide of another metal, most commonly iron oxide. The reaction is used for thermite welding, often used to join rails.
Types
Black or blue iron oxide (Fe3O4), 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 (Fe2O3, commonly known as rust) can also be used to make thermite but yields a less energetic reaction. Other oxides are occasionally used, such as in manganese thermite and chromium thermite, but only for highly specialized purposes. The melting and boiling points of aluminum also make it ideal for thermite reactions. 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.
Although the reactants are stable at room temperature, they burn with an extremely intense exothermic reaction when they are heated to ignition temperature. Thermite contains its own supply of oxygen and does not require any external source of air. If thermite is ignited underwater, the molten iron produced will extract oxygen from water and generate hydrogen gas in a single-replacement reaction.
Ignition
Conventional thermite reactions require very high temperatures for initiation. 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. The torch can preheat the entire pile of thermite which will make it explode instead of burning slowly when it finally reaches ignition temperature. Magnesium burns at approximately the temperature at which thermite reacts, around 2500 kelvin (4000 °F). 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.
The reaction between potassium permanganate and glycerine is used as an alternative to the magnesium method. The heat released by the oxidation of glycerine is sufficient to initiate a thermite reaction.
A stoichiometric mixture of finely powdered Fe(III) oxide and aluminum 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 burns.
Safety
The near-impossibility of smothering and high temperatures generated make thermite reactions potentially hazardous. Appropriate precautions must be taken before igniting thermite. Thermite should not be used near flammable materials; 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, should be kept away from thermite, as contact with such metals could potentially boil superheated metal violently into the air, where it could then burst into flame as it is exposed to oxygen. Thermite must be used with care in welding pipes or other items with air cavities, as thermal expansion of trapped gases may cause bursting. Generally, the ignition of thermite should be timed so individuals handling it have ample time to get away. As with any pyrotechnic composition, thermite that is not being used in a particular task should be kept far away from the site of ignition. When handled in a responsible manner by properly trained individuals, thermite can be reasonably safe.
The thermite reaction can take place accidentally in industrial locations where abrasive grinding and cutting wheels are used with ferrous metals.
Mixing thermite with water can be counterintuitively dangerous, causing a phreatomagmatic fragmentation explosion.
Military uses
Thermite grenades are used as incendiary devices to quickly destroy items or equipment when there is imminent danger of them being captured by enemy forces. Because of the difficulty in igniting standard iron-thermite, plus the fact that it burns with practically no flame and has a small radius of action, standard thermite is rarely used on its own as an incendiary composition. 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 thermite 68.7%, barium nitrate 29.0%, sulphur 2.0% and binder 0.3%. Addition of barium nitrate to thermite increases its thermal effect, creates flame in burning and significantly reduces the ignition temperature.
A classic military use for thermite is disabling artillery pieces. Thermite can be used to permanently disable artillery pieces without the use of explosive charges and therefore operate with a reasonable amount of stealth. The 2nd Ranger Battalion used thermite grenades against the Nazi artillery at Pointe du Hoc during the Allied invasion of Normandy. One method is to weld the breech of the weapon closed by inserting an armed thermite grenade into it and then quickly closing the breech. An alternative method is to insert an armed thermite grenade down the muzzle of the artillery piece, fouling the barrel. Yet another method is to use thermite to destroy the traversing and elevation mechanism on the cannon, making it impossible to properly aim the gun.
Civilian uses
Thermite reactions have many uses. Thermite was originally used for repair welding in-place thick steel sections such as locomotive axle-frames where the repair can take place without removing the part from its installed location.
A thermite reaction, when used to purify the ores of some metals, is called the Thermite process.
When thermite is made using iron (III) oxide, for maximum efficiency it should contain, by mass, 25.3% aluminum and 74.7% iron oxide. ΔH = -851.5 kJ/mol
When thermite is made using iron (II,III) oxide, for maximum efficiency it should contain, by mass, 23.7% aluminium and 76.3% iron oxide. gram of thermite mixture.
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. Degussa, a corporate descendant of Goldschmidt's firm, is still today one of the world's largest producers of welding thermite.
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