Thermite

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.

[19] 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.

It burns well while wet, and cannot be easily extinguished with water—though enough water to remove sufficient heat may stop the reaction.

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.

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.

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.

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.

These temperatures cannot be reached with conventional black powder fuses, nitrocellulose rods, detonators, pyrotechnic initiators, or other common igniting substances.

[citation needed] 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.

[36] This can be a dangerous method, as the iron 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.

[38] Thermite can be used for quickly cutting or welding steel such as rail tracks, without requiring complex or heavy equipment.

[39][40] 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.

[42] 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.

[44] As the reaction of thermite is oxidation-reduction and environmentally friendly, it has started to be adapted into use for sealing oil wells instead of using concrete.

Though thermite is usually in a powder-state, a diluted mixture can reduce damage to the surroundings during the process, though too much alumina can risk hurting the integrity of the seal.

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.

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.

[53] The addition of barium nitrate to thermite increases its thermal effect, produces a larger flame, and significantly reduces the ignition temperature.

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.

[56][57] Incendiary bombs usually consisted of dozens of thin, thermite-filled canisters (bomblets) ignited by a magnesium fuse.

Incendiary bombs created massive damage in numerous cities due to the fires started by the thermite.

Small streams of molten iron released in the reaction can travel considerable distances and may melt through metal containers, igniting their contents.

[citation needed] The thermite reaction can take place accidentally in industrial locations where workers use abrasive grinding and cutting wheels with ferrous metals.

[60] 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 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).

[61] 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.

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.

aluminum reacts violently with water or steam at high temperatures, releasing hydrogen and oxidizing in the process.