page_title: Thermite
section_title: Types
hierarchical_section_title: Thermite / Types
caption_attribution_description: English: A thermite reaction, using about 110 g of the mixture, taking place. The cast-iron skillet was destroyed in the process.
context_page_description: Thermite is a pyrotechnic composition of metal powder and metal oxide. When ignited by heat, 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 aluminium, magnesium, titanium, zinc, silicon, and boron. Aluminium is common because of its high boiling point and low cost. Oxidizers include bismuth(III) oxide, 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, IV) oxide.
  The reaction, also called the Goldschmidt process, is used for thermite welding, often used to join 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.
context_section_description: 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, quartz in silicon 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.
  Combinations of dry ice (frozen carbon dioxide) and reducing agents such as magnesium, aluminium 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 aluminium. This is rarely done, because the properties of aluminium are nearly ideal for this reaction:
  It is by far the cheapest of the highly reactive metals. For example, in December 2014, tin was US$19,830/metric ton, zinc was US$2,180/t and aluminium was US$1,910/t.
  It forms a passivation layer making it safer to handle than many other reactive metals.
  Its relatively low melting point (660 °C) 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.
  Its high boiling point (2519 °C) 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 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 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 (up to 2500 °C 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 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 underwater.
  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 point (so the energy released is not consumed in evaporation of reaction products). Organic binders can be added to the composition to improve its mechanical properties, however 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 pr
caption: A thermite reaction taking place on a cast iron skillet
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