Thermite

Thermite is an energetic composition consisting of a metal powder and metal oxide. Thermites are a special type of single replacment reactions that do not take place in aqueous solution. They are capable of producing extreme heat, enough to melt or boil other metals. The products of this reaction are the oxide of the metal powder, and pure metal produced from the oxide.

Preparation

A sufficiently reactive metallic reducer and metal oxide oxidizer are required for a thermite.

The metal powder and metal oxide powder are mixed thoroughly, and most compositions can even be mixed in a blender, as the mixture is very stable.

Specific compositions

Aluminium-iron(III) oxide

This is the most commonly used composition. It combines slow burn rate with high temperature (2500 °C) and produces liquid iron, and is therefore used for thermite welding.

This composition usually uses an approximately 8:3 ratio Fe₂O₃ to Al by weight.

Aluminium-copper(II) oxide

This composition usually uses an approximately 4.4:1 ratio of copper(II) oxide to aluminum powder. This is one of the most energetic thermites commonly used, and shows properties similar to a high explosive if the materials are milled finely enough.

Aluminium-manganese dioxide

This composition uses approximately 2.42:1 MnO2 to Al. However, this is not a good mixture for recovery of manganese metal due to the metal itself boiling at the reaction temperature - this can be somewhat rectified by adding in calcium fluoride as a heat sink.

Aluminium-manganese(II,III) oxide

This is the preferred method of extracting manganese metal from its oxide, as manganese(II,III) oxide is less oxidizing.

Aluminium-silicon dioxide

This composition uses fine-grade aluminum powder and a source of silica, often sand, and produces pearls or lumps of elemental silicon, making it useful for element collectors. It is not energetic enough on its own to create a self sustaining reaction, so a common formula for this thermite includes extra aluminum powder and sulfur, often in the overall ratio 9:12:10 SiO2:S:Al. Because aluminum sulfide is produced, the slag reacts with water or acid to form hydrogen sulfide, which is highly toxic and can attract unwanted legal attention, sulfur can instead be replaced with an oxidizer such as sodium nitrate or potassium chlorate, which will both sustain and speed up the thermite reaction.

Aluminium-sulfur

This reaction produces aluminium sulfide, which in contact with water releases hydrogen sulfide.

Aluminium-bismuth(III) oxide

Due to the very low reactivity of bismuth as a metal, as well as its high density, a very energetic thermite is produced. Properly mixed, this thermite can act as a high explosive and produces a very loud retort.

Aluminium-vanadium pentoxide

This composition is highly explosive.

Calcium-neodymium fluoride

Calcium metal reacted with neodymium fluoride has been proposed as a method to reduce the neodymium metal extracted from neodymium iron boride magnets.

Lithium-neodymium fluoride

Lithium metal reacted with neodymium fluoride has been proposed as a method to reduce the neodymium metal extracted from neodymium iron boride magnets.

Magnesium-boric oxide

This composition can be used to isolate elemental boron. Care must be taken to cover the reaction after it begins to prevent re-oxidation of the boron. Afterwards, hydrochloric acid is used to dissolve the impurities, leaving boron undissolved. Some boron is reduced to magnesium boride in the reaction.

Magnesium-silicon dioxide

This reaction will also to produce magnesium silicide, especially if there's an excess of magnesium.

References

Relevant Sciencemadness threards

• Exotic thermites & analogs
• Questions about thermite