thermite

thermite

[from Thermit, a trade name], mixture of powdered or granular aluminum metal and powdered iron oxide. When ignited it gives off large amounts of heat. In wartime it has been used in incendiary bombs. A method for welding using thermite (invented by Dr. Hans Goldschmidt, a German chemist) is variously called the Goldschmidt process, the thermit process, or the aluminothermic process; it is used in welding large parts, e.g., castings, shafts, pipes, and steel rails. In the process the thermite, contained in a crucible, is ignited, e.g., by a strip of burning magnesium ribbon. The aluminum reduces the iron oxide to molten iron and forms a slag of aluminum oxide on its surface. The reaction is very exothermic; temperatures above 2,500°C; (4,500°F;) are often reached. Typically, the molten iron is poured into the joint to be welded, providing both heat for fusion and filler metal. Excess metal may be removed when the weld cools. Because thermite reacts with explosive violence once ignited, it cannot be heated as a mass to its kindling temperature (about 1,550°C;/2,800°F;); Goldschmidt was first to find a method for igniting thermite without explosion. He used a similar method to prepare various metals, e.g., chromium, manganese, and uranium, from their oxides.

Thermite

 

a mixture of powdered aluminum or, less frequently, magnesium and any of a number of metal oxides (iron, nickel, and so on). When thermite is ignited by a firing mixture, simultaneous, strongly exothermic oxidation and reduction reactions occur: the aluminum or magnesium is oxidized by oxygen from the oxide, and the oxide metal is reduced. The large quantity of heat evolved raises the temperature of the reaction products above 2000°C. The quantitative relationship between the components of the mixture is stoichiometric.

Iron-aluminum thermite, which is the most common, contains calcined scale or rich iron ore and is used for welding rails and in casting large machine parts. This type of thermite ignites at about 1300°C (the firing mixture ignites at 800°C), and the iron and slag that are produced may reach a temperature of 2400°C. Iron filings, alloying additives, and fluxes are sometimes added as constituents of iron thermite. The process is performed in a magnesite crucible. Special thermite mixtures are available for welding telephone and telegraph lines.

In military technology, thermites are used as incendiary compositions. In the production of ferroalloys, a thermite mixed with a flux is called a charge.

V. A. BOGOLIUBOV

thermite

[′thər‚mīt]

(materials)

A fire-hazardous mixture of ferric oxide and powdered aluminum; upon ignition by a magnesium ribbon, it reaches a temperature of 4000°F (2200°C), sufficient to soften steel; used for industrial purposes or as an incendiary bomb. Also spelled thermit.