Explosions and Explosives
An explosion occurs after a chemical reaction when a large amount of energy is created in a short space of time. This energy comes in the form of heat and usually lots of gas. As the reaction occurs so quickly, the gases do not expand immediately but when they do create an enormous ‘blast wave’ which can cause a lot of damage to surroundings. The main steps of an explosion are described below.
Ignition is where an explosive compound is given energy. This could happen via friction, impact, heat, electrical impulse and through many other routes. The energy converts to heat and increases the temperature of the material. Once the heat generated from this extra input of energy is greater than the loss of energy that is naturally lost to the surroundings, the compound ignites. The temperature where this occurs is known as the ignition temperature, which is different for every explosive.
Deflagaration occurs once an explosive compound has been given enough energy to ignite. The process involves burning of the material at a faster rate than during combustion. Sometimes this burning can create sparks, cracking or hissing. The deflagaration is a very exothermic reaction and the heat generated starts decomposition of the compound and allows the reaction to continue. In other words, the reaction propogates itself. If deflagaration occurs in a confined space, the volatile products of the reaction, and the heat generated causes the pressure inside to increase. This is shown by the Ideal Gas Law. Once the pressure has reached a certain value, the material is detonated and an explosion occurs.
Detonation is where an explosive compound decomposes by releasing a shockwave rather than the heat generated by deflagaration. Although nearly all compounds decay via deflagaration, for sensitive explosives, this part of the pathway happens so rapidly that the delay between ignition and detonation is not noticeable. This occurs with primary explosives.
The advantage of detonation is that more stable materials like secondary explosives can be initiated by the shockwave released. If the primary explosive is in close contact with the secondary explosive, the shockwave compressions give it energy and cause it to increase its temperature to above its decomposition temperature and consequently detonate. This will only happen if the speed of the shockwave is greater than the speed of sound.
The detonation of velocity gets faster when the compaction density of the explosive compound increases which is why great care has to be taken when filling shells and bombs that no cracks or gaps are formed. The shockwave is also increased in velocity if the container of the bomb or shell is a certain diameter and shape, particularly that of a cylinder.
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