Thermite

Thermite is a pyrotechnic composition of metal powder and metal oxide. When ignited by heat or chemical reaction, thermite undergoes an exothermic reduction-oxidation reaction. Most varieties are not explosive, but can create brief bursts of heat and high temperature in a small area. Its form of action is similar to that of other fuel-oxidizer mixtures, such as black powder.
Thermites have diverse compositions. Fuels include aluminum, magnesium, titanium, zinc, silicon, and boron. Aluminum is common because of its high boiling point and low cost. Oxidizers include bismuth oxide, boron oxide, silicon oxide, chromium oxide, manganese oxide, iron oxide, iron oxide, copper oxide, and lead oxide. In a thermochemical survey comprising twenty-five metals and thirty-two metal oxides, 288 out of 800 binary combinations were characterized by adiabatic temperatures greater than 2000 K. Combinations like these, which possess the thermodynamic potential to produce very high temperatures, are either already known to be reactive or are plausible thermitic systems.
The first thermite reaction was discovered in 1893 by the German chemist Hans Goldschmidt, who obtained a patent for his process. Today, thermite is used mainly for thermite welding, particularly for welding together railway tracks. Thermites have also been used in metal refining, disabling munitions, and in incendiary weapons. Some thermite-like mixtures are used as pyrotechnic initiators in fireworks.

Chemical reactions

In the following example, elemental aluminum reduces the oxide of another metal, in this common example iron oxide, because aluminum forms stronger and more stable bonds with oxygen than iron:
The products are aluminum oxide, 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.
Other metal oxides can be used, such as chromium oxide, to generate the given metal in its elemental form. For example, a copper thermite reaction using copper oxide and elemental aluminum can be used for creating electric joints in a process called cadwelding, that produces elemental copper :
Thermites with nanosized particles are described by a variety of terms, such as metastable intermolecular composites, super-thermite, nano-thermite, and nanocomposite energetic materials.

History

The thermite 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". 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 in 1899.

Types

Red iron oxide is the most common iron oxide used in thermite. Black iron oxide also works. Other oxides are occasionally used, such as MnO₂ in manganese thermite, Cr₂O₃ in chromium thermite, SiO₂ in silicon thermite, or copper oxide in copper thermite, but only for specialized purposes. All of these examples use aluminum as the reactive metal. Fluoropolymers can be used in special formulations, Teflon with magnesium or aluminum being a relatively common example. Magnesium/Teflon/Viton is another pyrolant of this type.
Combinations of dry ice 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. 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.
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 layer making it safer to handle than many other reactive metals.
Its relatively low melting point 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 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, but is especially unusual among the highly reactive metals.
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 with iron—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. Small amounts of water boil before reaching the reaction. Even so, thermite is used for welding under water.
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. 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.
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 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 can be boosted with addition of a suitable oxidizer ; too high a temperature can be reduced by using a suitable coolant or slag 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 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 oxide and manganese 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.
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-thermites.
The temperature achieved in the reaction in 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 and subtracting the energy consumed by heating the products. 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.

Iron thermite

The most common composition is iron thermite. The oxidizer used is usually either iron oxide or iron 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 g/cm³. This, in turn, results in relatively poor energy density, 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 g/cm³ with slow burning speeds. Pressed thermite has higher melting power, i.e. it can melt a steel cup where a low-density thermite would fail. Iron thermite with or without additives can be pressed into cutting devices that have heat-resistant casing and a nozzle.
Oxygen-balanced iron thermite 2Al + Fe₂O₃ has theoretical maximum density of 4.175 g/cm³ an adiabatic burn temperature of 3135 K or 2862 °C or 5183 °F, the aluminum oxide is 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 cal/g. The energy density is 16,516 J/cm³.
The original mixture, as invented, used iron oxide in the form of mill scale. The composition was very difficult to ignite.