JWBK121-FM October 11, 2006 21:10 Char Count= 0Background
In the simplest terms, an explosive is defined as a substance, which on initiation by friction,
impact, shock, spark, flame, heating, or any simple application of an energy pulse, undergoes
a rapid chemical reaction evolving a large amount of heat and so exerting a high pressure on
its surroundings. The vast majority of explosives release gaseous products on explosion but
this is not an essential requirement as in the case of some metal acetylides.
Explosives, propellants and pyrotechnics belong to a broad group of compounds and com-
positions known as energetic materials. Many organic explosives consist of a carbon core
incorporating covalently bonded oxidiser groups such as nitro, nitramine, nitrate ester etc.
These groups, containing bonds like N-N and N-O, have two or more atoms covalently bonded
with non-bonding electrons present in p-orbitals. This creates electrostatic repulsion between
the atoms, and consequently, many explosives have a positive heat of formation. On explosion
an internal redox reaction occurs where these bonds break and form gaseous products, like N 2
and CO 2 , where the non-bonding electrons are tied up in stableπ-bonds.
All explosives can be classified as either low or high explosives. Low explosives, also
known as propellants, while still containing the oxygen needed for their combustion, are at
most combustible materials which undergo deflagration by a mechanism of surface burning.
Low explosives can still explode under confinement but this is a consequence of the increase
in pressure caused by the release of gaseous products. Some low explosives can also detonate
under confinement if initiated by the shock of another explosive. Low explosives include
substances like gunpowder, smokeless powder and gun propellants. High explosives, on the
other hand, need no confinement for explosion, for their chemical reactions are far more rapid
and undergo the physical phenomenon of detonation. In these materials the chemical reaction
follows a high-pressure shock wave which propagates the reaction as it moves through the
explosive substance. High explosives typically detonate at a rate between 5500–9500 m/s and
this velocity of detonation (VOD) is used to compare the performance of different explosives.
High explosives include compounds like TNT, NG, RDX and HMX.
Another way to classify an explosive is how sensitive it is to mechanical or thermal stimuli.
The sensitivity of explosives to stimuli is a broad spectrum, but those explosives that readily
explode from light to modest mechanical stimuli are designated as primary explosives, while
those explosives which need the shock of an explosion or a high energy impulse are known
as secondary explosives or simply high explosives. Primary explosives usually explode on
the application of heat, whereas some secondary explosives will simply burn in small enough
quantities. A number of sensitivity tests have been designed to determine the sensitivity of a
given explosive to thermal and mechanical stimuli. Some materials are very near the crossover
between a primary and secondary explosive. Primary explosives, also known as initiators,
are classified as to their effectiveness in causing the detonation of another explosive. Some
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