Delay composition, also called
delay charge or
delay train, is a pyrotechnic composition, a sort of pyrotechnic initiator, a mixture of
oxidizer and
fuel 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
in e.g. model rockets, or to introduce a few seconds of time between triggering a
hand grenade and its
explosion. Typical delay times range between several milliseconds and several seconds.
A popular delay charge is a tube of pressed black powder. 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, manganese, tungsten, antimony, antimony trisulfide, zirconium, zirconium–nickel alloy, zinc, magnesium, etc.
-
Oxidizers: lead dioxide, , barium chromate, lead chromate, tin(IV) oxide, bismuth(III) oxide, barium sulfate (for high-temperature compositions), potassium perchlorate (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, etc. are common.
The burn rates are dependent on: [1]
-
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: [2]
