Thermite is a pyrotechnic composition of a metal powder and a metal oxide that produces an exothermic oxidation-reduction reaction known as a thermite reaction. If aluminium is the reducing agent it is called an aluminothermic reaction. Most varieties are not explosive, but can create short bursts of extremely high temperatures focused on a very small area for a short period of time. The thermite is simply a mixture of metal, often called the "fuel" and an oxidizer. Its form of action is very similar to other fuel-oxidizer mixtures like black powder.

Thermites can be a diverse class of compositions. Some "fuels" that can be used include aluminium, magnesium, calcium, titanium, zinc, silicon, and boron and others. One commonly-used fuel in thermite mixtures is aluminium, because of its high boiling point. The oxidizers can be boron(III) oxide, silicon(IV) oxide, chromium(III) oxide, manganese(IV) oxide, iron(III) oxide, iron(II,III) oxide, copper(II) oxide, and lead(II,III,IV) oxide and others.

Chemical reactions

The aluminium reduces the oxide of another metal, most commonly iron oxide, because aluminium is highly reactive:

: Fe₂O₃ + 2Al → 2Fe + Al₂O₃

The products are aluminium oxide, free elemental iron, and a large amount of heat. The reactants are commonly 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 tracks. Other metal oxides can be used, such as chromium oxide, to generate elemental metal. Copper thermite, using copper oxide, is used for creating electric joints in a process called cadwelding:

: 3CuO + 2Al → 3Cu + Al₂O₃

Some thermite-like mixtures are used as pyrotechnic initiators such as fireworks.

Thermites with nanosized particles are described through a variety of terms, such as metastable intermolecular composites, superthermite nanothermite, and nanocomposite energetic materials.

History

The thermite (thermit) reaction was discovered in 1893 and patented in 1895 by 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 discovered the value of thermite in welding.

The first commercial application of thermite was the welding of tram tracks in Essen, Germany, in 1899. Evonik, formerly Degussa, a corporate descendant of Goldschmidt's firm, is still today one of the world's largest producers of welding thermite.

Types

Red iron(III) oxide (Fe₂O₃, commonly known as rust) is the most common iron oxide used in thermite. Magnetite also works. Other oxides are occasionally used, such as MnO₂ in manganese thermite, Cr₂O₃ in chromium thermite, or copper(II) oxide in copper thermite, but only for specialized purposes. All of these examples use aluminium as the reactive metal. Fluoropolymers can be used in special formulations, 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 nearly 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 and boiling points of aluminium also make it ideal for thermite reactions. Its relatively low melting point (660 °C, 1221 °F) means that it is easy to melt the metal, so that the reaction can occur mainly in the liquid phase and thus proceeds fairly quickly. At the same time, its high boiling point () 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 and respectively), but is especially unusual among the highly reactive metals (cf. magnesium and sodium which boil at and 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 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 easily extinguished with water, although enough water will remove heat and stop the reaction. Small amounts of water will boil before reaching the reaction. Although thermite is used for welding underwater, in a haphazard ignition of thermite 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, similar 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 only a simple child's sparkler or easily obtainable magnesium ribbon, but may require persistent efforts, as ignition can be unreliable and unpredictable. These temperatures cannot be reached with conventional black powder fuses, nitrocellulose rods, detonators, pyrotechnic initiators, 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. Magnesium ignition remains popular among amateur thermite users, mainly because it can be easily obtained.

The reaction between potassium permanganate and glycerol 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 sparklers 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 burns.

Civilian uses

Thermite reactions have many uses. Thermite is not an explosive; instead it operates by exposing 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 both by melting metal from the components, and by injecting molten metal from the thermite reaction itself.

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.

Thermite can 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 voids (holes) are often present in such welded junctions and great care is needed to operate the process successfully. Care must also be taken to ensure that the rails remain straight, without resulting in dipped joints, which can cause wear on high speed and heavy axle load lines.

A thermite reaction, when used to purify the ores 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).

Military uses

Thermite hand grenades and charges are typically used by armed forces in both an anti-materiel role and in the partial destruction of equipment, the latter being common when time is not available for safer or more thorough methods. For example, thermite can be used for the emergency destruction of cryptographic equipment when there is a danger that it might be captured by enemy troops. Because standard iron-thermite is difficult to ignite, it burns with practically no flame, and it 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-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 about 68.7% thermite, 29.0% barium nitrate, 2.0% sulfur, and 0.3% of a binder (such as PBAN). The addition of barium nitrate to thermite increases its thermal effect, produces a larger flame, and significantly reduces the ignition temperature. Although the primary purpose of Thermate-TH3 by the armed forces is as an incendiary anti-materiel weapon, it also has uses in welding together metal components.

A classic military use for thermite is disabling artillery pieces, and it has been used for this purpose during and since World War II, (such as at Pointe du Hoc, Normandy). Thermite can permanently disable artillery pieces without the use of explosive charges, and therefore thermite can be used when silence is necessary to an operation. 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 and then quickly closing it. This makes the weapon impossible to be loaded. An alternative method is to insert an armed thermite grenade down the muzzle of the artillery piece, fouling the barrel. This does make the weapon 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 bombs during World War II. Incendiary bombs usually consisted of dozens of thin thermite-filled canisters (bomblets) 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 nighttime air raids. Bombsights 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 celsius (1,665 F.) is about 1,370 celsius (2,500 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 light and heat energy at a much higher rate 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 and cutting wheels are used with ferrous metals. Using aluminium in this situation produces a mixture of oxides which is capable of a violent explosive reaction.

Mixing water with thermite or pouring water onto burning thermite can cause a steam explosion, spraying hot fragments in all directions.

Thermite's main ingredients were also utilized for their individual qualities, specifically reflectivity and heat insulation, in a paint coating or dope for the German zeppelin Hindenburg, possibly contributing to its fiery destruction. This was a theory put forward by the former NASA scientist Addison Bain, and later tested in small scale by the scientific reality-TV show MythBusters with semi-inconclusive results. (It was not proven to be the fault of the thermite reaction, but instead was it conjectured to be a combination of that and the burning of hydrogen gas that filled the body of the Hindenburg). The MythBusters program also tested the veracity of a video found on the Internet, whereby a quantity of thermite was allowed to drop onto a block of ice of similar mass, causing a sudden explosion. They were able to confirm the results, and Jamie Hyneman conjectured this was due to the thermite mixture aerosolizing, perhaps in a cloud of steam, causing it to burn even faster. They found chunks of ice as far as from the point of explosion. Mr. Hyneman voiced skepticism about another theory explaining the phenomenon: one that the reaction somehow separated the hydrogen and oxygen in the water and then ignited them.

See also

ALICE (propellant)

Nano-thermite

References

;Notes

;Further reading

External links

Thermite Pictures & Videos (Including Exotic Thermite)

Video - steel casting with thermite

History of thermite

Category:Welding Category:Inorganic reactions Category:Incendiary weapons Category:Pyrotechnic compositions Category:Aluminium