thermite
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Thermite () is a
pyrotechnic composition A pyrotechnic composition is a substance or mixture of substances designed to produce an effect by heat, light, sound, gas/smoke or a combination of these, as a result of non-detonative self-sustaining exothermic In thermodynamics Thermodyna ...
of metal powder and
metal oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to oxygen. An oxide () is a chemical compound A chemical compound is a chemical substance composed of man ...
. When ignited by heat or chemical reaction, thermite undergoes an
exothermic In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is govern ...

exothermic
reduction-oxidation
reduction-oxidation
(redox) 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 Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, carbon (in the form of charcoal) and potassium nitrate (saltpeter). The ...
. Thermites have diverse compositions. Fuels include
aluminum Aluminium (aluminum in American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly known as the Unit ...

aluminum
,
magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

magnesium
,
titanium Titanium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same n ...

titanium
,
zinc Zinc is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbe ...

zinc
,
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

silicon
, and
boron Boron is a chemical element with the Chemical symbol, symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron g ...

boron
. Aluminum is common because of its high
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding enviro ...
and low cost. Oxidizers include
bismuth(III) oxide Bismuth(III) oxide is perhaps the most industrially important compound of bismuth Bismuth is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, a ...

bismuth(III) oxide
,
boron(III) oxide Boron trioxide (or diboron trioxide) is one of the Boron oxide, oxides of boron. It is a white, glassy solid with the formula B2O3. It is almost always found as the vitreous (amorphous) form; however, it can be crystallized after extensive Annealing ...
,
silicon(IV) oxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to oxygen. An oxide () is a chemical compound that con ...
,
chromium(III) oxide Chromium(III) oxide (or chromia) is an inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition ...
,
manganese(IV) oxide Manganese dioxide is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is not clearly define ...

manganese(IV) oxide
,
iron(III) oxide Iron(III) oxide or ferric oxide is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is ...
,
iron(II,III) oxide Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Gr ...
,
copper(II) oxide Copper(II) oxide or cupric oxide is an inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is not ...
, and
lead(II,IV) oxide Lead(II,IV) oxide, also called red lead or minium, is the inorganic compound with the formula Pb3O4. A bright red or orange solid, it is used as pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast ...
. The reaction, also called the Goldschmidt process, is used for thermite welding, often used to join
railway tracks.
railway tracks.
Thermites have also been used in metal refining, disabling munitions, and in
incendiary weapons Incendiary weapons, incendiary devices, incendiary munitions, or incendiary bombs are weapons designed to start fires or destroy sensitive equipment using fire (and sometimes used as anti-personnel weaponry), that use materials such as napalm, ...

incendiary weapons
. Some thermite-like mixtures are used as
pyrotechnic initiator A pyrotechnic initiator (also initiator or igniter) is a device containing a pyrotechnic composition used primarily to ignite other, more difficult-to-ignite materials, e.g. thermite Thermite () is a pyrotechnic composition of metal powder a ...
s in
fireworks Fireworks are a class of low explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion An explosion is a rapid expansion in volume associat ...

fireworks
.


Chemical reactions

In the following example, elemental aluminum reduces the oxide of another
metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts Electrical resistivity and conductivity, el ...

metal
, in this common example
iron oxide Iron oxides are chemical compounds composed of iron and oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, ...

iron oxide
, because aluminum forms stronger and more stable bonds with oxygen than iron: : Fe2O3 + 2 Al → 2 Fe + Al2O3 The products are
aluminum oxide Aluminium oxide is a chemical compound of aluminium Aluminium (aluminum in American English, American and Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a dens ...

aluminum oxide
, elemental
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...

iron
, and a large amount of
heat In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than Work (thermodynamics), thermodynamic work or Mass transfer, transfer of matter. The various mechanisms of energy transfer that define he ...

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 Copper is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same nu ...

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 (it may react violently): : 3 CuO + 2 Al → 3 Cu + Al2O3 Thermites with nanosized particles are described by a variety of terms, such as metastable intermolecular composites, super-thermite,
nano-thermiteNano-thermite or super-thermite is a metastable intermolecular composite (MICs) characterized by a particle size of its main constituents, a metal and a metal oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms ...
, and nanocomposite energetic materials.


History

The thermite (''thermit'') reaction was discovered in 1893 and
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of years in exchange for publishing an sufficiency of disclosure, enabling disclo ...

patent
ed in 1895 by German
chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European la ...

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 Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—making four electrons available to form covalent bond, covalent chemical bonds. ...

carbon
in
smelting Smelting is a process of applying heat to ore ore – psilomelane (size: 6.7 × 5.8 × 5.1 cm) ore – galena and anglesite (size: 4.8 × 4.0 × 3.0 cm) ore (size: 7.5 × 6.1 × 4.1 cm) File:OreCartPachuca.JPG, upMinecart on ...
, but he soon discovered the value of thermite in
welding Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...

welding
. The first commercial application of thermite was the welding of
tram A tram (also known as a streetcar or trolley in North America) is a train that runs on tramway track on public urban streets; some include segments of segregated Right-of-way (transportation), right-of-way. The lines or networks operated by ...

tram
tracks in
Essen Essen (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republ ...

Essen
in 1899.


Types

Red iron(III) oxide (Fe2O3, commonly known as
rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous ferric oxides, hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxid ...

rust
) is the most common iron oxide used in thermite.
Magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...

Magnetite
also works. Other oxides are occasionally used, such as
MnO2
MnO<sub>2</sub>
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 aluminum as the reactive metal.
FluoropolymersA fluoropolymer is a fluorocarbon-based polymer with multiple carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases. The best known fluoropolymer is polytetrafluoroethylene (Teflon). History In 1938, poly ...
can be used in special formulations,
Teflon Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymerA fluoropolymer is a fluorocarbon-based polymer with multiple carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases. The best known fluoropoly ...

Teflon
with magnesium or aluminum 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, 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 Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At Standard conditions for temperature and pressure, room temperature and pressure, another solid form of c ...

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 is by far the cheapest of the highly reactive metals. For example, in December 2014, tin was US$19,829/metric ton, zinc was US$2,180/t and aluminum 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 The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase exist in Thermodynamic equilibrium, equilibrium. ...

melting point
(660 °C) 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 The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding enviro ...
(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 and 2582 °C, respectively), but is especially unusual among the highly reactive metals (cf. magnesium and
sodium Sodium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...

sodium
, which boil at 1090 and 883 °C, respectively). * 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 reaction is famously exothermic. The reduction of iron(III) oxide by aluminium Aluminium (aluminum in American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the Uni ...
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 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 (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, 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 (above the boiling point of any reactant or product) 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 (e.g., when producing silicon from sand) can be boosted with addition of a suitable oxidizer (e.g., sulfur in aluminum-sulfur-sand compositions); too high a temperature can be reduced by using a suitable coolant and/or slag Flux (metallurgy), 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 (therefore increasing fluidity of the slag) 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(IV) oxide Manganese dioxide is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is not clearly define ...

manganese(IV) oxide
and manganese(II) 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 being heated to higher temperature to start reacting. This effect is pushed to the extreme with nanothermites. The temperature achieved in the reaction in adiabatic process, 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 (subtracting the enthalpy of the reactants from the enthalpy of the products) and subtracting the energy consumed by heating the products (from their specific heat, when the materials only change their temperature, and their enthalpy of fusion and eventually enthalpy of vaporization, when the materials melt or boil). 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(III) oxide Iron(III) oxide or ferric oxide is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is ...
or
iron(II,III) oxide Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite Magnetite is a mineral and one of the main iron ore Iron ores are rocks and minerals from which metal A metal (from Ancient Gr ...
. 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 (about 3 kJ/cm3), 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 (almost 16 kJ/cm3) with slow burning speeds (about 1 cm/s). 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 (with phase transitions included, limited by iron, which boils at 3135 K), the aluminum oxide is (briefly) 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 (3 956 J/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.


Copper thermite

Copper thermite can be prepared using either copper(I) oxide (Cu2O, red) or
copper(II) oxide Copper(II) oxide or cupric oxide is an inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. However, the distinction is not ...
(CuO, black). The burn rate tends to be very fast and the melting point of copper is relatively low, so the reaction produces a significant amount of molten copper in a very short time. Copper(II) thermite reactions can be so fast that it can be considered a type of flash powder. An explosion can occur, which sends a spray of copper drops to considerable distances. Oxygen-balanced mixture has theoretical maximum density of 5.109 g/cm3, adiabatic flame temperature 2843 K (phase transitions included) with the aluminum oxide being molten and copper in both liquid and gaseous form; 343 g of copper vapor per kg of this thermite are produced. The energy content is 974 cal/g. Copper(I) thermite has industrial uses in e.g., welding of thick copper conductors ( cadwelding). This kind of welding is being evaluated also for cable splicing on the US Navy fleet, for use in high-current systems, e.g., electric propulsion. Oxygen balanced mixture has theoretical maximum density of 5.280 g/cm3, adiabatic flame temperature 2843 K (phase transitions included) with the aluminum oxide being molten and copper in both liquid and gaseous form; 77.6 g of copper vapor per kg of this thermite are produced. The energy content is 575.5 cal/g.


Thermates

Thermate composition is a thermite enriched with a salt-based oxidizer (usually nitrates, e.g., barium nitrate, or peroxides). In contrast with thermites, thermates burn with evolution of flame and gases. The presence of the oxidizer makes the mixture easier to ignite and improves penetration of target by the burning composition, as the evolved gas is projecting the molten slag and providing mechanical agitation. This mechanism makes thermate more suitable than thermite for incendiary weapon, incendiary purposes and for emergency destruction of sensitive equipment (e.g., cryptographic devices), as thermite's effect is more localized.


Ignition

Metals, under the right conditions, burn in a process similar to the combustion of wood or gasoline. (In fact, rust is the result of oxidation of steel or iron at very slow rates.) A thermite reaction results when the correct mixtures of metallic fuels combine and ignite. Ignition itself requires extremely high temperatures. Ignition of a thermite reaction normally requires a 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 fuse (explosive), fuses, nitrocellulose rods, detonators,
pyrotechnic initiator A pyrotechnic initiator (also initiator or igniter) is a device containing a pyrotechnic composition used primarily to ignite other, more difficult-to-ignite materials, e.g. thermite Thermite () is a pyrotechnic composition of metal powder a ...
s, or other common igniting substances. Even when the thermite is hot enough to glow bright red, it does not ignite, as it must be at or near white-hot to initiate the reaction. Starting the reaction is possible using a propane torch if done correctly. Often, strips of
magnesium Magnesium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

magnesium
metal are used as Fuse (explosives), 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, but a piece of the burning strip can fall off into the mixture, resulting in premature ignition. 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 begins, slowly increasing the temperature of the mixture until it produces flames. The heat released by the oxidation of glycerine is sufficient to initiate a thermite reaction. 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. This can be a dangerous method, as the iron Spark (fire), sparks, like the magnesium strips, burn at thousands of degrees and can ignite the thermite, though the sparkler itself is not in contact with it. This is especially dangerous with finely powdered thermite. Match heads burn hot enough to ignite thermite. Use of match heads enveloped with aluminum foil and a sufficiently long viscofuse/electric match leading to the match heads is possible. Similarly, finely powdered thermite can be ignited by a 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.


Civilian uses

Thermite reactions have many uses. It is not an explosive; instead, it operates by exposing a very small area 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, so great care is needed to operate the process successfully. The numerical analysis of thermite welding of rails has been approached similar to casting cooling analysis. Both this finite element analysis and experimental analysis of thermite rail welds has shown that weld gap is the most influential parameter affecting defect formation. Increasing weld gap has been shown to reduce shrinkage cavity formation and cold lap welding defects, and increasing preheat and thermite temperature further reduces these defects. However, reducing these defects promotes a second form of defect: microporosity. 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 , 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 antimateriel 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, burns with practically no flame and has a small radius of action, standard thermite is rarely used on its own as an incendiary composition. In general, an increase in the volume of gaseous Chemical reaction, reaction products of a thermite blend increases the heat transfer rate (and therefore damage) of that particular thermite blend. It is usually used with other ingredients that increase its incendiary effects. thermate, Thermate-TH3 is a mixture of thermite and pyrotechnic additives that have been found 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 (material), binder (such as Polybutadiene acrylonitrile, 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 since World War II, such as at Pointe du Hoc, Normandy. Thermite can permanently disable artillery pieces without the use of explosive charges, so thermite can be used when silence is necessary to an operation. This can be done by inserting one or more armed thermite grenades into the breech-loading weapon, breech, and then quickly closing it; this welds the breech shut and makes loading the weapon impossible. Alternatively, a thermite grenade discharged inside the barrel of the gun fouls the barrel, making the weapon dangerous to fire. Thermite can also weld the traversing and elevation mechanism of the weapon, making proper aiming quite difficult. During World War II, both German and Allied incendiary bombs used thermite mixtures. Incendiary bombs usually consisted of dozens of thin, thermite-filled canisters (bomblets) ignited by a magnesium fuse. Incendiary bombs created massive damage in many cities due to fires started by the thermite. Cities that primarily consisted of wooden buildings were especially susceptible. These incendiary bombs were used primarily during Bombing of Tokyo#B-29 raids, 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

Image:Utah-thermite.jpg, The violent effects of thermite Thermite usage is hazardous due to the extremely high temperatures produced and the extreme difficulty in smothering a reaction once initiated. 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 (with a boiling point of 907 °C, which is about 1,370 °C below the temperature at which thermite burns) could potentially spray superheated boiling metal violently into the air if near a thermite reaction. If, for some reason, thermite is contaminated with organics, hydrated oxides and other compounds able to produce gases upon heating or reaction with thermite components, the reaction products may be sprayed. Moreover, if the thermite mixture contains enough empty spaces with air and burns fast enough, the super-heated air also may cause the mixture to spray. For this reason it is preferable to use relatively crude powders, so the reaction rate is moderate and hot gases could escape the reaction zone. 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 workers use abrasive grinding wheel, grinding and cutting wheels with ferrous metals. Using aluminum in this situation produces a mixture of oxides that can explode violently. 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 Aircraft dope, dope for the German zeppelin ''LZ 129 Hindenburg, 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 proven not to be the fault of the thermite reaction alone, but instead 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 in a metal bucket was ignited while sitting on top of several blocks of ice, causing a sudden explosion. They were able to confirm the results, finding huge chunks of ice as far as 50 m from the point of explosion. Co-host Jamie Hyneman conjectured that this was due to the thermite mixture aerosolizing, perhaps in a cloud of steam, causing it to burn even faster. Hyneman also voiced skepticism about another theory explaining the phenomenon: that the reaction somehow separated the hydrogen and oxygen in the ice and then ignited them. This explanation claims that the explosion is due to the reaction of high temperature molten aluminum with water. Aluminum reacts violently with water or steam at high temperatures, releasing hydrogen and oxidizing in the process. The speed of that reaction and the ignition of the resulting hydrogen can easily account for the explosion verified. This process is akin to the explosive reaction caused by dropping metallic potassium into water.


See also

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References


Further reading

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External links


Thermite Pictures & Videos (Including Exotic Thermite)


* {{Cite Americana, short=1, wstitle=Goldschmidt Process, year=1920 Welding Inorganic reactions Incendiary weapons Pyrotechnic compositions Aluminium