Delay Composition

Delay composition, also called delay charge or delay train, is a pyrotechnic composition, a sort of pyrotechnic initiator, a mixture of oxidizer and

fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...

that burns in a slow, constant rate that should not be significantly dependent on temperature and pressure. Delay compositions are used to introduce a delay into the firing train, e.g. to properly sequence firing of fireworks, to delay firing of ejection charges in e.g. model rockets, or to introduce a few seconds of time between triggering a

hand grenade A grenade is an explosive weapon typically thrown by hand (also called hand grenade), but can also refer to a shell (explosive projectile) shot from the muzzle of a rifle (as a rifle grenade) or a grenade launcher. A modern hand grenade genera ...

and its

explosion An explosion is a rapid expansion in volume associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Supersonic explosions created by high explosives are known ...

. Typical delay times range between several milliseconds and several seconds. A popular delay charge is a tube of pressed

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). Th ...

. The mechanical assembly prevents the outright detonation of the charge. While delay compositions are principally similar to other fuel-oxidizer compositions, larger grain sizes and less aggressively reacting chemicals are used. Many of the compositions generate little or no gas during burning. Typical materials used are: * Fuels: silicon,

boron Boron is a chemical element with the 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 group'' it has th ...

, manganese, tungsten, antimony, antimony trisulfide, zirconium, zirconium– nickel alloy, zinc, magnesium, etc. * Oxidizers: lead dioxide,

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...

s, barium chromate, lead chromate, tin(IV) oxide, bismuth(III) oxide,

barium sulfate Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium an ...

(for high-temperature compositions),

potassium perchlorate Potassium perchlorate is the inorganic salt with the chemical formula K Cl O4. Like other perchlorates, this salt is a strong oxidizer although it usually reacts very slowly with organic substances. This, usually obtained as a colorless, crysta ...

(usually used in small amount together with other oxidizers), etc. * Additives to cool down the flame and slow down the reaction can be employed; inert materials or coolants like titanium dioxide, ground glass, chalk,

sodium bicarbonate Sodium bicarbonate (IUPAC name: sodium hydrogencarbonate), commonly known as baking soda or bicarbonate of soda, is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cation ( Na+) and a bicarbonate anion ( HCO3−) ...

, etc. are common. The burn rates are dependent on

* nature of fuel - fuels that release more heat burn faster * nature of oxidizer - oxidizers that require less heat to decompose burn faster * the composition ratio - stoichiometric mixtures burn the fastest, also slight excess of metallic fuel also increases burn rate, probably due to heat transfer * particle sizes - smaller particles burn faster, but too small particles may lead to incomplete or interrupted burn due to too narrow heating zone * mechanical assembly and housing - charge diameter and thermal conductivity of housing influence lateral heat losses * ambient temperature - ideally this dependence is very low but extremely low or extremely high temperatures may have influence Examples of some compositions are

*

with addition of inert material, e.g. chalk or

* lead(II) oxide with silicon, burning at 1.5–2 cm/s * red lead with silicon, burning at intermediate rate * lead(IV) oxide with silicon, burning at 5–6 cm/s * potassium permanganate with antimony, very slow * Manganese Delay Composition: manganese with lead chromate and barium chromate (lead chromate is the principal oxidizer, barium chromate acts as burning rate modifier, the more of it the slower the reaction

* Tungsten Delay Composition: tungsten with barium chromate and

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*Zirconium Nickel Alloy Delay Composition: zirconium- nickel alloy with barium chromate and

. *

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References

Pyrotechnic compositions Pyrotechnic initiators {{chem-stub